SOLID LUBRICANTS Robert J. Benzing

SOLID LUBRICANTS

Robert J. Benzing Fluid and Lubricant Materials Branch, Air Force Materials Laboratory,

Dayton, Ohio

Page I. Introduction 244

A. General Information 244 B. Advantages of Solid Lubricants 247

II. Theory of Lubrication with Solid Lubricants 247 A. Metallic Friction and Wear Theory 248 B. Theory of Lubrication with Laminar Solids 250 C. Theory of Lubrication with Plastics 258 D. Mechanism of Lubrication with Other Materials Acting as Solid

Lubricants 260 III. Resin-Bonded Solid Lubricants 261

A. General Information 262 B. Lubricating Pigments 266 C. Bonding Agents 268 D. Pretreatments 269 E. Application Processes 272

IV. Lubricating Plastics 274 A. General Information 274 B. Fluorocarbon Resins 275 C. Other Plastic Solid Lubricants 279 D. Application of Plastics to Wearing Parts 280

V. High-Temperature Bonded Films 281 A. General Information 281 B. Lubricating Solids 283 C. Substrates and Substrate Treatments 284 D. Bonding Agents 2 86

E. Application Processes 287 VI. Other Solid-Type Lubricants 287

A. General Information 287 B. Vapor-Deposited Solid Lubricants 288 C. Soft Metals 292 D. Wear-Resistant Nonmetals and Hard Metals 293 E. Flame-Sprayed Solid Lubricants 295 F. Powdered Solid Lubricants 296 G. Gas-Entrained Powders 298 H. Chemical and Electroplated Coatings 299

VII. Wear and Other Evaluation Techniques 299 A. General Information 299 B. Wear and Friction Tests 304

243

2 4 4 R O B E R T J . B E N Z I N G

I. Introduction

A. G E N E R A L I N F O R M A T I O N

Lubrication is generally thought of in terms of liquid and semisolid materials such as oils and greases. Conventionally these materials are used to reduce friction and wear in many types of mechanical devices such as gears, bearings, and other wearing components. Another area of lubrication which has a long history but which has received its major emphasis in the last twenty to thirty years is the use of solid lubricants. Because of the extensive research and development on solid lubricants during this period of time, the use of these materials is considered a "modern" or "new" area of lubrication. This chapter will discuss solid lubricants, techniques of solid lubrication, modern research and development, and application or use of solid lubricants.

The term "solid lubricants" might simply be defined as solid materials that provide lubrication to two relatively moving surfaces. For the purpose of this chapter this definition is much too broad in scope. Many areas not to be considered would fall under such a definition. An example of such an area would be solid lubricating surfaces deposited from an oil or grease through the action of an antiwear or extreme pressure additive with the metal surface being lubricated. Another example would be the use of lubricating solids suspended in an oil or grease.

To define the term more specifically for the purpose of this chapter, we shall refer to solid lubricants as materials that provide lubrication to two relatively moving surfaces under essentially dry conditions. This definition still includes many solid lubricants and solid lubrication techniques. The major areas of interest in this chapter as well as a brief description of each are listed in Table I .

C. Chemical Tests 307 D. Other Tests 308

VIII. Environmental Effects 308 A. General Information 308 B. Temperature Effects 309 C. Atmosphere 310 D. Radiation 311 E. Load and Other Effects 312

IX. Current and Future Applications in Industry 313 A. General Information 313 B. Metal-Processing Industry 313 C. Marine Industry 314 D. Aerospace Industry 315 E. Miscellaneous Industrial Uses 315 References 316

SOLID LUBRICANTS 245

T A B L E I S O L I D L U B R I C A N T S A N D T E C H N I Q U E S O F S O L I D L U B R I C A T I O N

Terminology Definition of terminology

1. Solid lubricant powders

2. Resin-bonded solid lubricants

3. Ceramic-bonded solid lubricants

4. Lubricating plastics

5. Soft metals

6. Flame-sprayed coatings

7. Wear- and friction-reducing metals

This is the simplest and earliest type of solid lubricant. It consists of a powdered material which provides friction and/or wear reduction to the surface being lubricated. In general, the method of application consists in rubbing the surface of the metal to be lubricated with the powder or simply in placing the powder between the two contacting surfaces. Graphite is an early example of such a material, but many others provide satisfactory results under the proper conditions of use.

This type of solid lubricant consists of a lubricating powder or pigment bonded to the bearing surface with an organic resin. The pigment would be one of the solid lubricant powders mentioned above, and the resin would serve solely as a means of improving the adhesion of the powder to the surface. Such a material might be compared to paint, where a resin is used to hold the pigment on the surface being coated.

This material is identical in concept to the resin-bonded material, except for the use of a ceramic adhesive in place of the resin material to obtain service at higher temperature.

These materials are organic plastics that exhibit friction and wear reduction characteristics. They can be coated on a bearing surface or be the material of construction themselves. The most common material of this type is polytetrafluoroethylene.

These materials are soft metals or metal alloys that possess low shear strengths and reduce wear and friction when introduced between two harder metals. One example of this type of material is a gold coating on a steel surface. Another is the use of lead to lubricate a bearing.

This is a technique of application of solid lubricants. A solid lubricating powder may be applied to a bearing surface by using plasma arcs or other such techniques. Another example is the use of the arc to apply a hard wear-resistant surface on the bearing as in the case of flame-sprayed ceramic coatings.

This type of solid lubricant can cover a broad class of materials. They are metals that provide friction and/or wear reduction when used as coatings or materials of construction for wearing components. In some cases they are not considered as solid lubricants but rather as bearing materials. In this chapter, however, they will be discussed as solid lubricants because their good behavior is not applicable over all environments and

246 ROBERT J . BENZING

T A B L E I (Continued)

Terminology Definition of terminology

8. Wear- and friction-reducing nonmetals

9 . Chemical coating and electroplating

10. Gaseous or vapor-phase lubrication

11. Gas-entrained powdered lubricants

therefore they must provide lubrication under certain conditions.

These materials serve in the same fashion as the metals but are nonmetallic in nature. This category includes the ceramics, which can possibly function also as liquids under the proper conditions of use.

These are applications of surfaces to wearing components to provide lubricating or wear-reducing characteristics. Examples include phosphate surfaces and hard chrome surfaces. Although mainly found in conjunction with liquid lubricants, they can serve as solid surfaces or as lubricants themselves.

This is a technique of solid lubrication whereby a lubricating solid film is formed on a bearing surface through the chemical reaction of the gas with the metal itself or through the formation of a lubricating polymer on the bearing surface.

This is also a technique of solid lubrication, in which the solid lubricant powders are carried to the wearing surface in a gas such as nitrogen. The powders provide the lubrication, and the gas serves simply as the carrier.

As shown in the table, solid lubricants represent a wide variety or selection of materials, as well as many techniques for their use. This can be best demonstrated by considering one solid lubricant (molybdenum disulfide) and how it may be used. Of the techniques listed in Table I, there are at least seven in which this material might be employed— numbers 1, 2, 3, 4, 6, 8, and 11. In all cases, except for number 4, the MoS2 would be the principal lubricant. In number 4 it would be used in conjunction with the lubricating plastic which would also provide lubrication.

An appreciable amount of literature has been published on solid lubricants. In the preparation of this chapter, approximately 1500 references were discovered and 400 abstracts obtained. Of this literature, well over 200 references were of pertinent interest and are included in the reference section. The majority of the literature is the result of work in this country and in England. This factor coupled with the great difficulty in obtaining literature from other countries is the reason that most of the literature cited will be from these two countries. Notation is made here of work from Sweden ( I ) , Russia (2) , Italy (3) , South Africa (4), and Australia (5) . These are only a few examples of the work conducted in each country.

Most of the work on solid lubricants has been in the area of lubrica-

S O L I D L U B R I C A N T S 247

tion with laminar solids and plastics. The main work on the laminar solids has been with graphite, which is one of the earliest solid lubricants, and with molybdenum disulfide. Both can be applied as either a powder or a resin-bonded powder to the metal surface being lubricated. Of the lubricating plastics, the most widely studied material is polytetrafluoro-ethylene. These two classes of materials are widely used in lubrication throughout many industries. The other groups of solid lubricants listed in Table I have been developed to serve in specific applications or to meet the extreme environmental requirements encountered in industries such as the aircraft, metals, and space fields.

B. A D V A N T A G E S O F S O L I D L U B R I C A N T S

As we have seen, there is a wide variety of solid lubricants and solid lubrication techniques. Each particular technique has specific advantages or conditions under which it will provide optimum performance as compared to other lubricating techniques. Specific performance factors will be discussed in Section VIII of this chapter. This section will present a brief introduction to the over-all advantages of solid lubricants as compared to other methods of lubrication so that the reader may be aware of the data of major importance for his area of lubrication.

Solid lubricants offer advantages both of stability under extreme environments (6-8) and of several desirable factors in relation to their use. Solid lubricants of one type or another have demonstrated exceptional performance under various conditions of extreme environments including high temperature (9) , nuclear radiation (JO), low temperature in the cryogenic region (11), very high vacuum (12), high load conditions (13), and reactive environments. From an operational aspect, solid lubricants are often employed for the lubrication of components that are diificult to reach with conventional materials. In such a case, as with more accessible components, solid lubricants can provide lubrication for the life of the part. Another area of importance is in applications in which contamination of the product being produced cannot be tolerated. It should not be thought, however, that all types or any one type of solid lubricant will meet all requirements. It is a complex lubrication engineering job to select the proper technique for the specific application. In many cases solid lubricants will not perform the best job, and other materials are required. The following sections will point out the uses of this type of material as well as the recent developments in the field.

II. Theory of Lubrication with Solid Lubricants The general theory of lubrication is an extensive and important field.

Rollins (Vol. 3, pp. 151-162) has covered the principles from the stand-

248 R O B E R T J . B E N Z I N G

point of lubricating oils; Forrester (Materials for Plain Bearings, this volume) has added further discussion in relation to bearings. In addition, a discussion of lubrication theory as applied to the various types of solid lubricants is necessary to an understanding of their behavior and application. First, however, the principles of metal friction and wear should be reviewed. This is essential, as much of the background and the basis for solid lubricant theory is related to that for the metal behavior. It is also important to understand this theory of metal behavior, as the prime functions of a solid lubricant are to reduce friction and to prevent wear. Of particular interest is the case of two solid surfaces in sliding contact with each other. In solid lubrication, sliding contact is the predominant application. There are also cases, however, of rolling contacts. Even in rolling contact uses the major problems are encountered in reducing friction and wear in the associated sliding. For the case of a ball bearing, this sliding occurs between the ball separator and both the balls and the inner and/or outer races.

A. M E T A L L I C F R I C T I O N A N D W E A R T H E O R Y

If one were to observe a flat metal surface on a microscopic or molecular scale, one would find a very rough surface. Even a finely polished surface has a roughness rating of several microinches. In a list of various finishing techniques it is reported (14) that a superfinish may range from 0.5 to 16 microinches, rms (root mean square), and a lapped surface may vary from 0.2 to 16 microinches, rms. Even these fine surface finishes are one to three orders of magnitude larger than the unit cell dimensions of most materials of interest. Rubbing two metal surfaces of such roughness together will result in a series of contacts between the peaks and the valleys of the metal surface. Figure 1 depicts two

F I G . 1. Representation of two contacting metal surfaces.

such contacting metal surfaces and the points of contact between the irregular peaks and valleys.

The most widely accepted theory of friction and wear is based on the principle that as any two sliding surfaces come in contact under load there will be a welding or adhesion of the two surfaces at the points of


contact of the irregular surfaces. The frictional force required to move the two different metal surfaces will then be a sum of the shearing force required to break the welded junctions and a ploughing term to include the movement of the harder metal through the softer material. This theory was developed independently both in England (15) and in the United States (16) in the early 1940's and will be reviewed in some detail in the following paragraphs.

If two different metal surfaces come in contact, they will have an apparent area of contact corresponding to their geometrical area of contact. In actuality, however, the area of contact (or real area of contact) will be much smaller than the geometrical area. The real area of contact will depend on the load applied and the yield pressure of the softer material. One may express this as follows:

A = W/p where A is the real area of contact, W is the applied load, and ρ is the yield or flow pressure of the softer material. This is explained by the fact that as the two surfaces are brought together they will contact at the asperities of the metal surface (that is, at the peaks and valleys). This contact under load will cause plastic flow in the metal until the area of contact is able to support the load. Except in extremely high loads the real area of contact is always less than the geometric area. The friction force is then the sum of the forces which include the shearing of these junctions, and the ploughing term is considered negligible. One then obtains the following equation for the friction force required to slide the two metals:

F = As where F is the friction force, A is the real area of contact, and s is the shear strength of either the bulk metal or the welded junction, whichever is smaller. The friction coefficient (μ) is then a ratio of the friction force to the applied load, or

μ = F/W which in turn gives

μ = S/p

Where the ploughing is appreciable, a term is required to account for it in the friction force. One then obtains

F = As + Α'ρ' where A' is the cross-sectional area of ploughing, and is essentially the flow pressure under ploughing conditions. In this theory wear can


be imagined to take place through several mechanisms. One, for example, would be the transfer of soft metal to the harder metal through the shearing of the junction in the bulk of the soft metal. Another mode of wear would be the generation of wear particles through the ploughing action of the harder metal in the soft metal. A third mechanism would be the breaking of the weld junction and subsequent generation of particles in the wear tract.

On the basis of the weld theory, one can devise methods of reducing friction and wear by either increasing the flow pressure or reducing shear stress. Since for most materials these properties are approximately equal, one might expect friction values close to the order of 1. In actual practice this is true for clean metals, where there is negligible contribution from the ploughing term. As we shall see in the case of some solid lubricants, however, friction force can be reduced by placing a thin film of a material of low shear strength on a metal possessing a higher yield pressure. This results in lubrication.

Several other theories of behavior have been proposed to explain the laws of friction as stated by Amontons: (1) Friction is independent of the area of sliding contact. (As we have seen, this is true for the apparent or geometrical area of contact.) (2) Friction is proportional to load. Although these other theories of friction and wear are less widely accepted, it is worth mentioning them here, as even the weld concept cannot satisfactorily explain all factors encountered. Feng (17) has proposed that the weld theory should be modified to include the possible interlocking of individual asperities when two metal surfaces are brought in contact. The subsequent movement of the two metal surfaces can result in generation of wear particles through a shearing in the softer metal, even if there is no welding between the two materials. One could thus obtain wear through either a breaking of the welded junctions or mechanical interlocking causing shear in the soft base metal.

Another theory, proposed by Tomlinson (18), attributes friction to the interlocking force and subsequent rupture by atomic particles of the two metal surfaces. That is, when the two surfaces are in contact the atoms from one penetrate the attractive force fields of the other, and when sliding commences the friction force is the pulling or separation of the interlocked field. The earliest theory of friction was that expressed by Coulomb in 1781 in which he attributed friction to the interlocking of the surface irregularities which resulted in the expenditure of force in lifting the mass up over these irregularities.

B . T H E O R Y O F L U B R I C A T I O N W I T H L A M I N A R S O L I D S

One of the most common classes of solid lubricants, if not the most common? is that of laminar solid materials. These materials are employed


in many solid lubrication techniques. Because of this, considerable emphasis will be placed on their behavior. This discussion will include the basic theory obtained with these materials in the bulk form. Later sections in this chapter will be particularly concerned with how this theory is employed to provide solid lubricants by using this type of material. Several general literature references are cited here (15, 19, 20).

Graphite and molybdenum disulfide are by far the two most common solid lubricants with laminar structures. The crystal structure for graphite is shown in Shobert's chapter on carbon and graphite; that for MoS2 is shown in Fig. 2. Graphite consists of layers of hexagonally

F I G . 2. Crystal structure of molybdenum disulfide.

bonded carbon atoms. The carbon in each such layer or lamella is strongly bonded to other carbon atoms. Between the layers or lamellae of carbon there is larger spacing and much weaker bonding. The spac-ings for the carbon-carbon atoms within each layer is 1.42 A, whereas that between layers is 3.40 A. The main bonding forces between the


layers are van der Waals forces. Molybdenum disulfide also consists of layers of material with strong intralayer bonding and weaker interlayer van der Waals forces; each layer consists of molybdenum sandwiched between layers of sulfur atoms. The MoS2 is arranged in hexagonal crystals. Other crystal structures reported (21) for synthetic MoS2 appear to give comparable friction values. The interlayer bonding is then primarily between the sulfur atoms of two adjacent MoS2 layers. Other such laminar-type materials have been investigated, some of which are listed in Table II. Referring to the theory of metal friction and wear,

T A B L E I I S O M E L A M I N A R - T Y P E S T R U C T U R E M A T E R I A L S T H A T H A V E B E E N I N V E S T I G A T E D

A S S O L I D L U B R I C A N T S

Material Chemical composition

Boron nitride BN Tungsten disulfide WSo Tungsten diselenide WSe2

Molybdenum diselenide MoSe2

Cadmium chloride CdCl2

Silver sulfate Ag 2S0 4

Nickel chloride NiCl2

Calcium sulfate CaS0 4

Titanium sulfide TiS2

Lead fluoride PbF 2

we see that these materials might serve as solid lubricants through low shear forces parallel to the laminae due to the weak interlaminae bonding. As will be seen in the following sections, the mechanism of lubrication with such materials is not that simple, and yet it is the basic principle for explaining their behavior.

1. Graphite Lubrication The original theory of lubrication with graphite was based on strong

intralayer bonding and weaker interlayer attraction. It was first expressed by Bragg (22), who attributed the low friction and wear to the weaker attraction of two parallel layers of graphite. Jenkins (23) demonstrated the beneficial effect of rubbing or polishing a graphite surface. He demonstrated preferred orientation of the graphite crystals with the main cleavage plane parallel to the direction of rubbing. Such explanations could satisfactorily account for the behavior of graphite in the applications of that time.

During the early 1940's graphite brushes in electrical equipment experienced high wear rates or "dusting" when used in aircraft flying


at high altitudes. This wear was related to the low water content of the atmosphere. During this period considerable work was conducted on the theory of graphite lubrication in an attempt to understand its behavior better and to alleviate this problem.

In a series of papers, Savage and co-workers (24-28) presented a new theory of lubrication of graphite which is currently the one most widely accepted. This theory states essentially that lubrication with graphite is due mainly to adsorption of a film on the surface of the graphite. One of the most effective films and one found most commonly in normal applications is water. Other materials will also provide effective lubrication, however, if they will condense and adsorb on the graphite surface. The lubrication is then attributed to a formation of surfaces with low cohesive energies, and not to the structure of the graphite itself. This mechanism of lubrication is highly dependent on atmospheric conditions, including temperature and vapor pressure. Such factors greatly affect the adsorption and evaporation of the surface films. Elaborate experiments were described by Savage (25) to illustrate the theory for carbon rubbing on copper and also for carbon on carbon. He showed the effectiveness of water vapor on a graphite^copper system at a pressure of only 3 mm Hg. The effectiveness of oxygen was also demonstrated but at much higher pressures, in the region of 200 to 400 mm Hg. For the graphite-graphite system a higher pressure was required at a speed transition point as the speed increased. This was explained as being due to the temperature increase, caused by poor conductivity of the graphite which resulted in higher evaporation rates of the adsorbed films. Savage and Schaefer (29) conducted additional work on the adsorption of gases of higher molecular weights on the graphite surface. This work showed that the vapor concentration of the gas can decrease with increased chain length and still give satisfactory lubrication. The area of study was for materials with chain lengths in the region of 5 to 15 A. Fullam and Savage (28) observed also the tilting of the graphite layers. They showed that the graphite particles had a directional overlap and upward tilting in the direction of motion. This resulted in initial increased friction with graphite when the direction of sliding was reversed, as some time would be required to reorientate the particles. Such an increase in friction is observed in practice when the direction of sliding is reversed.

In a parallel program on graphite wear, Campbell and Kozak (30) studied the effect of several gases (H 20, 0 2 , and C 0 2 ) . They showed that low wear was obtained when the cleavage surfaces were parallel to the direction of rotation. When the crystals were oriented so that the edges were perpendicular to the wear tract, however, the friction and wear increased unless a gas was available to form an adsorbed film


which would allow reorientation of the crystals parallel to the surface. Replying to comments by Savage on the paper, the authors stated that, because of the nature of their experiment, it was possible that their work was done at pressures above the vapor pressure of water indicated by Savage's work as being required for lubrication of the surfaces parallel to the wear tract. They further stated that the pressure required for the edge-on condition would probably be higher than for the parallel condition.

Several other theories have been suggested for the lubrication behavior of graphite. Holm ( 1 ) proposes the adherence of graphite scales to a metal surface through the sticking action of water. The graphite thus formed provides a low friction surface. When there is a scarcity of water, some of the graphite scales become unglued and peal up, causing high friction and wear due to loss of lubrication. Bollman and Spread-borough ( 3 1 , 32) propose that in the course of sliding the individual graphite crystallites orient themselves in the direction of sliding. Then, as motion continues, the graphite particles roll up into tubes much as a piece of paper rolls up when once started at one edge. These rollers give reduced friction due to the rolling aspect rather than the sliding behavior. The high friction encountered when the direction of sliding is changed is caused by the unrolling and subsequent rerolling of the cylinders in the opposite direction.

Other work on the lubrication behavior of graphite is too extensive to review in this chapter; however, some of the noteworthy studies will be briefly described. Bowden and Young (33) present data to show the effect of adsorbed gases observed by Savage, but they indicate a much lower pressure for satisfactory lubrication. This may be due to a lower sliding speed which allows more time for static equilibrium and also results in lower surface temperatures, causing lower vaporization of the adsorbed films. Bowden and co-workers (34) present further work in the same general area. Deacon and Goodman (35) show the orientation of graphite and molybdenum disulfide on rubbing. They state that the crystallites may protect the metal surface being lubricated by becoming embedded in the surface. Oxidation of both materials is obtained and results in increased friction. In later work (36) they point out reduced attraction and therefore reduced friction between crystallite edges and layers of graphite by adsorbed gases on the edges as well as the importance of orientation through rubbing. Rowe (37) presents elevated-temperature data on graphite and other materials. He shows the decrease in friction with increased temperature (800° to 1200°C) for graphite in vacuum and attributes it to a weakening of the interlayer bond strength. The normal effects of gaseous contamination are also discussed.


Braithwaite (38) proposes that the lubrication behavior of graphite is dependent on the orientation of the crystals and also on the size or state of their subdivision. He also states that it does not depend on the presence of adsorbed films.

Although Savage's theory on adsorbed films appears to be the best explanation of graphite lubrication, several questions relative to the complete mechanism remain. As can be seen, researchers do not agree on all aspects of the theory. One common point agreed on by most, however, is that adsorbed gases do have an effect. Bryant (39) in rather elaborate experiments in ultrahigh vacuum (10~13 mm Hg) has obtained data indicating the attack and reduction by gases of the bonding forces at the separation line of two lamellae. These data may lead to additional information in this area and could possibly combine and explain some of the contradicting information.

2. Molybdenum Disulfide

Experimentation on the theory of lubrication of molybdenum disulfide has not been as extensive or elaborate as that for graphite. Much of it has been conducted in the last ten years and has been stimulated mainly by the work on graphite. The current theories fall basically into two schools of thought. One is based on the theory that an adsorbed film is not required for proper lubrication with MoS2; the other takes the opposite approach and states that it is. Both schools have strong arguments to support their cause.

Feng (40) attributes the lubrication behavior of MoS2 to its laminar structure and states that it is independent of adsorbed layers of condensed gases. He bases this theory on work of his own and also of other investigators. Peterson and Johnson (41) present corroborative data by showing that an increase in water vapor content results in an increase in friction with the MoS2. They show this (42) plus the beneficial effects of small amounts (<10%) of oil on friction. Ross, however, challenged these observations in comments on a paper presented with Ballou (43) by stating that the hydrophilic nature of Mo0 3 contained as an impurity in the MoS2 results in better retention of water in the material and therefore could result in an effect from water. On the other hand, Deacon and Goodman (36) state that the presence of water as adsorbed gas causes increased friction due to increased hydrogen bonding from the hydrated edges of the crystals. This would explain the data of Peterson and Johnson (41).

The major work on adsorbed films and their effect on the lubrication behavior of MoS2 is presented by Johnson and Vaughn (44) and also reported by Lavik (45). They attribute the lubrication of MoS2 to an amorphous layer of sulfur generated during the sliding of compressed


pellets of MoS2 on a steel surface in vacuum. The theory is based mainly on high initial friction on start-up in vacuum with such a system. They show a drop in friction with running time and attribute this to the generation of the sulfur film. These data are demonstrated by Fig. 3.

T E M P . - 2 5 C. S Y M B O L STOP T I M E

• 15 HR. A 185 M I N . • 134 M IN . • 8 5 M I N .

16 HR.

• 9 0 MIN.

STEADY STATE V A L U E '

2 3 4 5

T I M E ( M I N . )

F I G . 3. Variation of coefficient of friction during run-in after various stop times. Reproduced with permission of the authors and Journal of Applied Physics from Fig. 1 of ref. (44).

p.04'

A S Y M P T O T I C B U I L D - U P VALUE>

STEADY STATE RUNNING VALUE

3 4 5 6 7 8 9

S T O P - T I M E ( H R S . )

F I G . 4. Coefficients of friction at initiation of sliding after various stop times. Reproduced with permission of the authors and Journal of Applied Physics from Fig. 3 of ref. (44).


They also show higher start-up friction with increased stop time (i.e., the period of no relative movement) in vacuum (Fig. 4 ) , which reaches an asymptotic value after about 3 hours. Effects of temperature and running or sliding speed are noted.

Recent work by Haltner (46) has challenged this inference by reporting improved lubrication with MoS2 in vacuum with increased vacuum. This work attributes the good performance to the inherent structure of the material, much in the way proposed by Feng (40). Earlier work of Haltner and Oliver (47) gave results of various vapors in a N2 environment and their effect on friction. In general, increased friction was obtained, and with water the presence of H2S was detected. No firm conclusions were drawn except that MoS2 did not react the same way as graphite did with such materials.

3. Other Laminar Solids

There has been little work on laminar materials other than that on graphite and molybdenum disulfide. The work that has been done is generally related to the theory developed for these two materials.

Johnson and co-workers (48) have suggested the similarity of tungsten disulfide to molybdenum disulfide and show curves similar to those of Fig. 4. Lavik and co-workers (49) develop this further and propose a mechanism similar to that developed by Johnson and Vaughn (44) for MoS2. They modify this somewhat by stating that factors involved in the experiment (such as the coefficients of friction between the sulfur film, of a sulfur film on WS2, and between WS2 crystals, as well as the sulfur build-up rate, and time of sliding) can have effects on the friction.

Lavik and co-workers (50) present the same type of data for molybdenum diselenide and attribute its lubrication behavior to the formation of a selenium-adsorbed film.

Deacon and Goodman (36) present data on talc and boron nitride in addition to data on graphite and molybdenum disulfide. This work was carried out mainly with platinum as the base metal being lubricated. The authors suggest that the low friction encountered with the talc and boron nitride is similar to the mechanism they propose for graphite. This consists essentially in the influence of adsorbed chemical layers. Their interpretation of the action, however, is different from that of other investigators. They state that on removal of the adsorbed gases there will be strong bonding of the crystallite edges to other layers of graphite, resulting in high friction. The addition of adsorbed layers results in a weakening of this bond and reduces friction. They also show data for orientation of the materials with the metal surface. Boron nitride in

2 5 8 R O B E R T J . B E N Z I N G

particular is suggested as a high-temperature lubricant because of its reduced friction at the elevated temperatures.

C . T H E O R Y O F L U B R I C A T I O N W I T H P L A S T I C S

A second class of solid lubricant that has found wide use is that of the lubricating plastics or solid polymers. As with the laminar solids, considerable effort has been devoted to the development of a theory to explain the mechanism of lubrication. Work on the plastics, however, has been even more recent than that on the laminar solids. The most significant work has been reported during the last decade. In addition to the work on the theory of lubrication with plastics, extensive work has been done on the friction and wear behavior of the polymeric type of textiles and fibers. This work is very specialized and will not be included here; it is mentioned because of its relation to plastic friction and wear.

The theory of friction and wear of plastics is quite analogous to that developed for metals, although not indentical. Shooter and Tabor ( 5 1 ) have investigated a series of linear polymers of various chemical compositions in sliding contact experiments. In these experiments they used a spherical rider sliding on a flat plate. They have concluded that the strong adhesion and shearing of the softer phase observed with plastics is similar to the welding and shearing action described as the mechanism of metal behavior. Extensive experiments have been reported for various combinations of plastics and metals. In the case of a plastic rubbing on a hard metal, such as steel, they show transfer of the plastic to the metal and friction values proportional to the ratio of shear strength to the yield pressure of the plastic. When a hard plastic slides on a soft metal, the friction is a function of the properties of the soft metal. When a hard metal slides on a soft plastic, there is an increase in the importance of the ploughing term, and friction increases. In the one case of polytetra-fluoroethylene these workers obtain values less than those calculated from the physical properties. They attribute this to lower adhesion which is not sufficient to cause shearing within the bulk of the plastic on sliding.

Rabinowicz and Shooter ( 5 2 ) present data to show metal transfer from a radioactive metal slider to a plastic material. This work supports the theory of the effect of strong adhesion in plastic behavior. King and Tabor ( 5 3 ) present data on the effect of temperature on the shear and yield strengths of four plastics and their effect on friction. The temperature range of interest was — 1 0 0 ° to + 8 0 ° C ( — 1 4 8 ° to 1 7 6 ° F ) . They offer the data in support of the adhesion theory for plastics. In general, they show the friction coefficient equal to a constant times the ratio of shear stress over yield pressure. For metals the constant would be 1, but


for the plastics used the constant was approximately 1 (1.1) for only one of the materials. For the other three it was greater than 1 (2.4 and 1.7) for two of the materials and less than 1 (0.3) for the third. The authors attribute the higher values to an increase in the shear stress under hydrostatic loading (or its brittleness). The low value for the one constant was that for polytetrafluoroethylene, and the authors attribute this to the low adhesion discussed before (51).

Flom and Porile (54) present several interesting observations for Teflon1 (polytetrafluoroethylene) sliding on itself. They show an increase in friction of this system under repeated high-speed sliding over the same track. The friction did not drop to a lower level with a reduction in speed. They also show an increase in the level of friction at about room temperature (20°C). This increase is attributed to a phase change in the material.

Pascoe and Tabor (55) hold to the theory based on the similarity of plastics to metals but present modifications on two points. They state that for plastics there is no junction growth as is reported for metals (56) and that the deformation of plastics is neither elastic nor plastic but is intermediate over a wide range of loads. The deformation properties of the bulk materials are used to predict friction values for crossed fibers, and the authors state that such correlation can be made over a wide range of conditions. They also state that there is a geometric consideration in the frictional properties of polymers.

Bueche and Flom (57, 58) have conducted extensive work on the effect of dynamical mechanical losses in polymers in rolling and sliding contact. Elastic losses in the material have been shown (58) to be important in sliding contacts. Curves relating friction to speed are shown to parallel closely those of dissipation factor versus stress frequency. These are presented over a range from 25° to 105°C for polymethylmethacrylate. This work also develops a preliminary analysis of rolling contacts in plastic material and states that such behavior can be closely related to the properties of the bulk materials. The authors also state that such a method might be used to predict these properties. The rolling studies were further investigated (57) with a series of plastic materials. This work supports the theory that rolling friction is highly dependent on mechanical properties but also shows that surface effects become important for sliding contacts.

Further work on polytetrafluoroethylene by Tabor and Williams (59) demonstrates the effect of molecular orientation on the friction value. This work shows higher friction when sliding is across the mole-

1 Registered trademark of Ε . I. du Pont de Nemours and Company.


cule than when it is along it. The effect is noted both for steel sliding on the plastic and for the plastic sliding on itself. For the metal it was concluded that the friction is composed of two terms, one due to adhesion and one due to deformation of the plastic. The adhesion term is the smaller, and for higher loads the deformation term may account for most of the friction. This is in accordance with the previously discussed factor of low adhesion with this plastic.

As can be seen, the friction of the plastics is similar but not identical to that for metals. Much of the friction can be attributed to adhesion, but there are cases for which other mechanisms have important effects. The plastics can vary considerably in structure, and thus these additional effects may be of significant consideration. Most of the work has been based on a small number of materials, and more studies on additional materials are required to define the theory further.

D . M E C H A N I S M O F L U B R I C A T I O N W I T H O T H E R M A T E R I A L S

A C T I N G A S S O L I D L U B R I C A N T S

Many of the other materials used as solid lubricants have not been developed as extensively as the laminar solids and the plastics. Therefore, the theory of how they lubricate has not been studied as fully. In many cases the fact that the materials give low wear or friction has been sufficient to warrant their use. The following discussion will be a brief review of the possible mechanisms of lubrication with the various materials.

1. Soft Metals

This type of solid lubricant can be imagined to lubricate through the reduction of s in the equation

μ = s/p (see p. 249)

If a thin film of a soft metal is coated on a hard bearing surface and the surface is loaded relative to another hard surface, the area of contact will be a function of the hardness or yield pressure of the hard materials. When sliding is initiated, the frictional force will be a function of the shear stress of the soft metal separating the two surfaces acting over the area of contact created by the deformation of the hard materials. Thus the explanation of lubrication with this material would be the reduction of shear stresses while a high yield pressure is maintained.

2. Nonfaminar Solids

Many nonlaminar solids serve as lubricants, with the same methods of application as for the laminar-type materials. These include materials such as PbS and PbO, used in the high-temperature ceramic-bonded


films to be discussed later. For the most part, these materials provide low friction and wear, but the mode of their behavior is not known. The fact that they provide a lubricating material between the surfaces is all that is really known.

3. Gaseous or Vapor-Phase Lubrication

The mechanism of lubrication of this type of solid lubricant is the formation of a lubricating film on the bearing surface (Table I, number 10). The type of film formed might be one in which the gas reacts with the metal surface to form a low-shear-strength metallic compound which reduces friction and wears preferentially rather than the bulk metal. The lubrication is achieved through the continuous replacement of the chemically reacted surface. The other possible method is the formation of a polymeric film on the metal surface with no chemical interaction or reaction with the metallic surface. Lubrication is also achieved in this method through the preferential wearing of such a film. In either case the reactive film or polymeric film is formed from the gas on contact with the bearing surface in a continuous manner. As long as there is a gaseous environment to provide the film and proper temperature and pressure to allow the reaction to take place, there will be lubrication.

4. Wear- and Friction-Reducing Hard Metals and Nonmetals

The mechanism of action for this type of solid lubricant can vary from material to material. With a ceramic material one can imagine the melting of the ceramic under conditions of pressure and load to form a fluid film. This film would act much in the same manner as a liquid lubricant. In other cases one might have hard materials, such as A1203, which give low wear but high friction. Here the material does not act solely as a solid lubricant but may be considered also as a bearing material possessing controlled wear. This type of material is encountered more in high-temperature environments where a balance is required between the amount of wear and the performance at temperature. Other materials include the various metal carbides. There are also hard metals which give reduced wear under certain conditions. For the most part these materials have been classified as solid lubricants by the aircraft and space industries where they are being considered for high-temperature requirements. They are also used in other industries but are not thought of so much as solid lubricants.

III. Resin-Bonded Solid Lubricants Resin-bonded solid lubricants are one of the most common and

widely used solid lubricant types found today. Most organizations working in this field have used this type of material at one time or another.


A . G E N E R A L I N F O R M A T I O N

The resin-bonded solid lubricant consists of a lubricating solid or "pigment"2 and a bonding agent. The pigment may be one material or it may be a mixture of several materials. Its function is to provide the wear reduction and low friction required by the system being lubricated. The binder serves mainly as a method of sticking the pigment to the metal surface so that the motion of the parts does not result in a throw-out or loss of the pigment. In addition to these two components of a resin-bonded film, other factors are important to the over-all performance in any given operating situation. One of these factors is the surface characteristics of the metal being lubricated. In most cases the surface is modified through some pretreatment to obtain optimum performance of the film. Other factors involve the variables directly associated with the application of the film to the surface to be lubricated.

The above paragraph has been directly concerned with the factors involved in obtaining a good resin-bonded solid lubricant film. The environmental conditions and the operating characteristics of the system being lubricated also can have a significant effect on the film. These effects can modify the desired characteristics of a film and must be considered in any final selection of material. For the scope of this chapter it is impossible to consider all such effects, but they will be briefly discussed in the sections on environmental effects and applications in industry. This section, as well as the sections on other solid lubricants, is concerned only with the average or ' rule-of-thumb" observations on what is involved in obtaining a good solid lubricant and a brief discussion of other variables of importance in their operation.

Resin-bonded solid lubricants are generally applied in thin films to the surface of the metal component to be lubricated. For most cases, the surface has received some pretreatment which will depend on the metal used and the service for which the component is intended. The mixture of the pigment and resin-forming material is applied prior to assembly of the components and is expected to operate without attention until failure. The resin-bonded solid lubricant is applied as a paste or liquid which on subsequent heating or curing forms a resin film on the metal surface. The use of thin films is beneficial, with thicknesses of 0.0003 to 0.0008 inch the optimum for most cases. It is thought that too thick a film will cause the film to peel or spew off with sliding, whereas

2 Pigment is not used in the sense of a coloring agent but rather as a con

venient notation for the component that provides the lubricating characteristics. It is referred to as a pigment because of the similarity of application to the pigment in a paint.


too thin a film will result in premature failure due to rubbing through of the film at a much earlier time.

The wear behavior and life of a resin-bonded solid lubricant are different from those of many other lubricants. In its initial performance it exhibits a relatively high rate of wear which tapers off with time. This initially high rate is attributed to the loss of loose material from the surface of the film. As rubbing continues, the film takes on a glossy or burnished appearance. The performance of the film is generally good during this period, and a burnished film is indicative of good operation. At failure the film is ruptured, and friction increases as metal-to-metal contact occurs. Since the film is a one-shot process or application, this high friction or failure generally occurs in a catastrophic manner.

Several investigators (60-62) have presented data on the effects of various conditions on the performance of a film. In addition to the beneficial effect of metal surface pretreatment, the hardness of the metal has received considerable attention. Crump (60) reports the beneficial effects of hard metal substrates on reduction of friction. He also shows beneficial effects of the hardness on wear. Lavik (61) reports data in an extensive statistical analysis of wear under various conditions of test with a block bearing on a rotating disk. The geometry of contact was either line or conforming area, depending on the block used. In the case of line contact for a commercially available film he finds little or no effect of hardness, whereas with the conforming geometry he shows a slight benefit with increased hardness. When he investigates the line

TABLE III F A C T O R S R E P O R T E D To A F F E C T W E A R A N D F R I C T I O N P E R F O R M A N C E

O F R E S I N - B O N D E D S O L I D L U B R I C A N T S

Factor Ref.

Beneficial Increased metal hardness on wear and friction (60) Increased rubbing speed (0.8 to 24 fpm) on wear (60) Pretreatment of metal surface on wear (60, 62) Increased load on friction (60) Careful handling (62)

Adverse Increased load on wear (60, 61, 63) High temperature on wear (61, 62) Contamination (62) Water (62) Sharp scraping edges (62) High resin-to-pigment content (60)


contact for several specially prepared films, he actually shows a decrease in life with increased hardness. H e also presents data on the commercial film to show an interaction between hardness and film thickness. Both

TABLE IV C O N D E N S E D R E Q U I R E M E N T S F O R R E S I N - B O N D E D S O L I D L U B R I C A N T S —

S P E C I F I C A T I O N MIL-L-25504 A (USAF)

1. Wear and friction properties Six coated test cups shall be run continuously on bare 4130 steel blocks for a minimum of 50 hours without an increase in friction coefficient to 0.1. The following conditions shall be used: speed—72 rpm or 26 fpm; load—630 lb; temperature—room temperature (75°-85°F); humidity—50 + 5%.

2. Temperature resistance high Two coated steel panels shall be placed in a circulating air oven at 500°F. The panels shall remain in the oven for 5 hours. They shall then be removed and allowed to return to room temperature, dried, and examined. Any blistering, flaking, softening, or other deterioration of the coating shall be cause for rejection. The test panels shall be able to pass the adhesion test below.

3. Temperature resistance low Two coated panels shall be cooled for 5 hours at — 100°F. They shall be removed and returned to room temperature. Any lifting, flaking, or other deterioration shall be cause for rejection. The panels must pass the following adhesion test.

4. Adhesion A piece of tape conforming to Specification PPP-T-60 shall be firmly pressed by hand onto a sample coated to a SAE 4130 steel panel. The tape shall be removed in one abrupt stripping motion and examined visually. The presence of traces of fine powdery material is acceptable; however, the presence of large flakes shall be cause for rejection.

5. Fluid resistance Aluminum panels with the solid lubricant film coating shall be immersed halfway into the fluids specified for a period of 120 hours. The panels shall be removed, cleaned, and examined visually. Any indication of discoloration due to corrosion, lifting, softening, or other deterioration shall be cause for rejection. They must also pass the adhesion test.

6. Corrosion resistance Lubricated anodized aluminum test panels shall be placed at right angles in direct contact with non-lubricated anodized aluminum panels. The entire assembly shall be preheated to 120°F after loading to a given load and placed in an environmental cabinet controlled at that temperature and 95% relative humidity. Panels must run for 500 hours without showing evidence of corrosion.


programs were conducted in laboratory simulative bench testers. In actual use it has been found that the harder bearings offer beneficial results, pointing out again that operating conditions must be considered. Table III lists other factors considered beneficial to the performance of resin-bonded films.

Probably one of the best methods of describing this type of film is to present a specification for the performance of the material and some typical friction and wear data on a series of films. Table IV gives the essential requirements of a specification issued by the Air Force and is the first known (64) to receive wide acceptance in industry. Previous to the issuance of this specification, most companies had their own individual versions. Table V gives friction and wear data for various resin-

TABLE V F R I C T I O N A N D W E A R D A T A F O R V A R I O U S R E S I N - B O N D E D S O L I D L U B R I C A N T S

I N B E N C H - T Y P E T E S T E R S

Average coefficient Wear

of life, Load, Speed, Temper-Source Type of geometry friction cycles lb fpm ature, ° F

Table II, Two flats on rotating 21,500 400 80 400 ref. (61) disk—Hohman A-3

geometry

Fig. 9, One flat on rotating 0.025 630 26 Room ref. (60) disk—MacMillan

geometry

Table I, Two flats on rotating 0.036 103,680 630 25.9 Room ref. (65) disk—Hohman A-3

geometry

Fig. 4, Two flats on rotating About About 630 25.9 300 ref. (65) disk—Hohman A-3 0.025 35,000

geometry

a Flat on rotating disk— 0.022 288,000 630 26 Room Alpha LFW-1 geometry



a Average of six runs.


bonded solid lubricants. These materials are expected to provide satisfactory lubrication for a considerable time, with friction values generally below 0.1. This low friction coupled with low wear and long life without relubrication are the attractive features of the material.

B . L U B R I C A T I N G P I G M E N T S

Graphite and molybdenum disulfide are the two most common pigments employed in resin-bonded solid lubricants. The theory of behavior of these two materials has already been reviewed. For normal air environments both materials have definite limitations. For graphite the limitation is the loss of the adsorbed layer of water with increased temperature, much as in the case of its loss with decreased pressure at high altitudes. At extremely elevated temperatures an oxide layer can provide a layer on the surface and give beneficial results. In the case of MoS2

the material oxidizes at about 750°F (66) to MoOs which is an abrasive and greatly reduces its performance in any film. The detrimental effects

T A B L E VI

R E S U L T S O F L U B R I C I T Y T E S T S O N T H E R M A L L Y S T A B L E M A T E R I A L S0

'6

Approximate friction coefficient

At room At 450°F Material temperature (232°C) Remarks

BN 0.3 0.15 Fairly steady CdCl2 0.6 0.17 CaS0 4 Pellets wore rapidly—brittle CrCl3 0.2-0 .3 Pellets wore rapidly PbF 2 0.6 0.6 Squeaky and unsteady MnCl2 0,35 0.17 Quiet NiCl2 0.45 0.19 Quiet SnS, 0 .9-0 .45 Pellets wore rapidly SnO 0.95 plus Pellets wore rapidly SnS 0.95 plus 0.63 Very unsteady Ta2S4 1.15 Pellets wore rapidly TiC 0.55 Fairly steady—quiet, rapid wear of

pellets, also crumbling TiB2 Could not form pellets TiSi2 Pellets wore very rapidly TiS2 0.7 0.6 Very unsteady—sulfur noticed W S 2 0 .7-1 .6 0.2 Heating produced steadiness and quiet

α The friction tests were run by compressing powders of the materials into pellets and running on a steel tract.

b Reprinted with permission of the author from ref. (67), Table II.


TABLE V I I F R I C T I O N V A L U E S F O R V A R I O U S P I G M E N T S

Coefficient Geometry and/or Pigment of friction conditions Ref.

NiCl2 0.03M).15 6 Three curved nodes on rotating (68)

M 0 S 2 0.017 α-0 .047 δ steel ring bearing on steel flat. WS2 0 .05ML08 6 Gives initial line contact, 40-lb CdCl2 0.03 β-0 .07 & load TiS2 0.20 α-ηο lubrication6

MoS2 0.05-0.095 Rubbed on steel surface, 40 fpm, (40) 900 gm

Graphite 0 .11-0.14 Rubbed on steel surface, 40 fpm, 900 gm

MoS2 pellets 0.20-0.22 Versus various metal flats a 1 minute of sliding. 6 30 minutes of sliding.

of elevated temperatures on these materials when used in the resin-bonded films are not of excessive importance, as the resins themselves also tend to break down at the elevated temperatures.

A wide variety of commercially available resin-bonded films contain these two pigments. For the most part the films contain a mixture of both graphite and MoS2. In general a mixture of 90% MoS2 and 10% graphite gives the best results when friction and wear are both considered. The small amount of graphite appears to improve the performance of the MoS2. Films are also available with varying concentrations of each of these materials.

The frictional behavior of a material is one important characteristic in its selection as a lubricating pigment for resin-bonded films. Tables VI and VII give various reported friction values for several materials that have received consideration. Feng (40) indicated higher values of friction for a bonded film than for the pigment by itself. Table V, when compared to Tables VI and VII, does not bear this out. The resin-bonded MoS2-graphite films can be seen to have friction values approaching those of the pure materials. This may be due to resin binders improved over those used by Feng. In the case of MoS2 and graphite, Crump (69) has studied the effect of various percentages of each in the mixtures on friction values of the resin-bonded films. He shows that higher MoS2 content reduces kinetic friction with little difference between 90% and 100% MoS2 content, which is in agreement with the better performance of a 90% MoS2-10% graphite material.


C . B O N D I N G A G E N T S

Although the bonding agent does not lend much to the lubricating ability of a resin-bonded solid lubricant, it is probably the heart of the over-all film. As was mentioned before, the use of solid lubricants such as graphite have been known for some time. One of the main problems with these materials when used by themselves, however, was that of maintaining a sufficient supply between the surfaces being lubricated. Norman (70) presented one of the early clues to providing better adherence of the solid lubricant powders. In a brief note on die lubrication, he described the use of a paste of MoS2 and corn syrup applied to a hot metal part to form an adherent film. Barwell and Milne (71) discuss the advantages of both corn syrup bonding and phosphated surfaces in the performance of MoS2-based materials. They did not get good adherence of the MoS2 by itself, as had been reported by others. In both cases the corn syrup forms a bonding resin.

One of the earliest reported investigations devoted to several types of bonding agents is that of Godfrey and Bisson (72, 73). They investigated the corn syrup type of binder as well as several others including

T A B L E V I I I V A R I O U S R E S I N - B O N D E D C O A T I N G S O V E R A M A N G A N E S E P H O S P H A T E D S U R F A C E

0 ,B

Kinetic Solid coefficient of Wear life,

Number Basic resin types lubricant friction cycles

1 Phenolic MoS2 0.034 103,680 2 Phenolic fluorocarbon MoS2 0.034 120,600 3 Phenolic vinyl copolymer MoS2 0.040 102,660 4 Phenolic vinyl acetate MoS2 0.040 96,120 5 Diisocyanate castor oil M0S2 0.060 96,750 6 Phenolic acyronitril M0S2 0.045 86,400 7 Diisocyanate phenolic MoS2 0.050 86,400 8 Corn syrup (72) MoS2 0.031 85,080 9 Phenolic rubber MoS2 0.064 69,120

10 Phenolic neoprene MoS2 0.035 68,000 11 Phenolic acrylic M0S2 0.060 50,400 12 Phenolic epoxy MoS2 0.063 36,000 13 Phenolic amide MoS2 0.074 23,760 14 Vinyl butyral M0S2 0.065 21,600 15 Vinyl chloride MoS2 0.070 21,600 16 Silicone MoS2 0.054 15,120

0 This table was reprinted with the permission of the author and Lubrication Engineering (6δ), where it appeared as Table 2.

6 Rotary speed constant at 72 rpm (25.9 fpm). Thickness of coating varied from 0.0003 to 0.0005 inch. All tests run at room temperature and no atmospheric control.


silicone-based and asphalt-based materials and glycerol. In their work, MoS2 was mixed with the liquid materials and applied as a paste with a brush to the metal surface. The specimens were then dried and heated to decompose or polymerize the liquid to a resin.

Two types of bonding agents are widely employed in this kind of film—thermosetting materials and air-dried materials. Thermosetting material requires baking at a given temperature to attain polymerization to the desired resin adhesive. Air-dried material can be cured by simple aging at room temperatures. Thermosetting material provides the best and most enduring film and can be one of several resin types. Those finding the widest use are the phenolic, epoxy, and silicone-based materials. The phenolic type is probably the most popular. The air-drying materials are generally used for field application of the materials or for repair use. They are sold in spray-type containers for general home or industrial use. Table VIII shows data presented by Stupp (65) on the effect of resin composition on lubricant performance. Mitchell and Fulford (63) also present data on various resins including polytetrafluoro-ethylene which is used not solely as a binder, as we shall see in later sections.

The bonding agents are mixed with the pigments to provide the material for application to the surface to be lubricated. The actual applications can be accomplished in several fashions. The three most common methods are brush, dip, or spray coating. In spray coating it is often necessary to dilute the mixture to the proper consistency for spraying by using some solvent such as dioxane. This method will be reviewed in more detail in the section on application of the films.

The content of the resin material in a film can be important to its over-all performance. If there is insufficient binder or resin, the film will not adhere properly. On the other hand, if the resin content is too high, the full benefit of the pigment's lubricating ability will not be realized. A lubricant-to-binder ratio of about 1:1 or 2:1 is used for many films. This ratio is based on the dried resin, as many of the materials used in commercial films have only a portion of their content as active binder material. The remaining portion may serve as a temporary binder or solvent. Mitchell and Fulford (63) show a decrease in friction for phenolic-bonded MoS2 from 0.20 to 0.15 as the MoS2 content is increased from 5% to 33%. Crump (60) also shows increased wear life with lower resin content.

D . P R E T R E A T M E N T S

The pretreatment of a metal surface can have a considerable effect on the performance of a resin-bonded solid lubricant. Proper modification of the metal surface greatly increases the wear life of the lubricant.


There are many methods of pretreating the metal surface (69, 74), including both chemical and mechanical processes. The type of pretreatment also depends on the base metal being employed. Such pre-treatments are required in at least one specification (75).

Table IX lists some of the methods of metal surface pretreatment

T A B L E I X S O M E P R E T R E A T M E N T S U S E D O N M E T A L S U R F A C E S T o I M P R O V E W E A R L I F E

O F R E S I N - B O N D E D S O L I D L U B R I C A N T S

Type of metal Pretreatment

Steel Phosphating Grit blasting or other mechanical process Sulfiding

Aluminum Anodizing Corrosion or chemically resistant Sandblasting with a vapor or grit blast process

metals such as stainless steel and titanium

Magnesium Sandblasting plus dichromate treatment

used in conjunction with various metals to improve the performance of resin-bonded solid lubricants. There are undoubtedly many others that might give acceptable behavior. The actual mode of behavior by which these treatments improve the wear life of the resin-bonded solid lubricant is not fully understood. One method could be the better adhesion of the lubricant to the surface of the metal due to the increased surface area for contact or bonding. This method is accepted by many in the field. Another mechanism, recently proposed by Devine et al. (76), is the formation of small "reservoirs" of lubricant which feed lubricant during the rubbing process. This theory is based on his observation that larger reservoirs machined in a metal surface also give improved performance.

The most common pretreatment is the formation of a manganese iron phosphate surface, or phosphating. This is because of its wide use on steel surfaces and the fact that steel is the most common metal found in the bearings and other components using resin-bonded solid lubricants. Phosphated surfaces have been widely used for providing wear-resistant surfaces in conjunction with oil lubrication. Roosa (77) discusses such applications of this surface pretreatment, and Gilbert (78) shows some of the complex considerations in obtaining a good phosphate coating. Midgley and Wilman (79) discuss the mechanism of behavior with liquids, which is not necessarily that found with solid lubricants.

Several authors have discussed the beneficial effects of phosphated


iron surfaces. Stupp (80) presents a detailed investigation of the factors involved in obtaining an optimum phosphate coating for use as a pretreatment with solid lubricant films. The effects of bath temperature, time of treatment, and bath acid strength on the thickness of the coating as well as on the structure are given. For the lubricants investigated in his experiments the optimum conditions were: temperature of bath, 205°F; strength of bath acid, 7 to 8 points (full acid); time of treatment 15 minutes (approximately).

Mitchell and Fulford (63) show the importance of phosphate thickness for unbonded films with little effect of thickness for bonded films as long as there is a phosphate surface. Grain pattern of the phosphate surface is also of importance. Some data of the grain structure effects have been reported by Stupp (80). Another important consideration in the use of phosphate treatments on the metal surface is the temperature of operation of the wearing surface. Phosphate coatings tend to break down thermally at around 600° to 700°F. This is shown (81) by Fig. 5,

6 7 0

6 6 0 1 L O A D 195 L B S . I SPEED 2 3 0 R P M Ο Ο PHOSPHATED D α N O T PHOSPHATED

T E M P E R A T U R E ( V. )

FIG. 5. Effect of temperature on the beneficial behavior of phosphated steel surfaces versus nonphosphated surfaces. Reproduced with permission of the author from ref. (81).

where the beneficial effects of a phosphated surface over a nonphosphated surface are lost at approximately 630°F. This is of particular interest for high-temperature materials, for which other techniques must be employed.


Work on other materials for surface pretreatments has also been reported. Wright and Scott (82) show the beneficial effect of anodized aluminum with conventional lubricants. Milne (83) shows the benefit of phosphated and sulfided steel with MoS2 which has been either bonded or carried in a liquid.

E . A P P L I C A T I O N P R O C E S S E S

The processes by which the resin-bonded solid film lubricants are applied to the metal surface can have considerable effect on the behavior of the lubricant. Brushing, spraying, and dip coating have been briefly discussed. These are used in only one step in the over-all process. Spraying and dipping are the two most common. The spraying is much like spraying of a paint. In the dip process the parts to be coated are simply dipped into a bath of the lubricant.

Figure 6 shows a general process for applying a resin-bonded solid

CLEANING OF COMPONENT INCLUDING DECREASING AND REMOVAL OF DIRT

AND CORRODED MATERIAL

I PHOSPHATE PRETREATMENT IN

ACCORDANCE WITH SPECIFICATION

I APPLICATION OF SOLID LUBRICANT BY BRUSH COATING, SPRAY COATING

OR DIP COATING

ί CURE STEP BY HEATING

IN AN OVEN CLOSE INSPECTION IS REQUIRED DURING THE VARIOUS STEPS IN THE PROCESS

F I G . 6. Resin-bonded solid lubricant application process for steel components.

film lubricant to a steel surface. Similar processes are outlined in the literature (74) and in specification MIL-L-25504 (75). In addition, Crump (69) shows processes for other materials. Many of the manufacturers have processes for their particular films. Most of these are similar in nature but vary as to condition of temperature, time, etc. This


is because of the variations in raw materials used in the preparation of the films.

Close control of variables is of importance. We have seen the importance of control of the phosphating bath for steel. The proper consistency of the material being sprayed on a part is also essential. This is varied by the addition of thinners to the resin-pigment mixture prior to coating of the part. If the mixture is too thick it will not properly coat the parts; if it is too thin it will run and drip off. Other variables of importance include baking or curing temperature. In addition to control at a given level it is essential that the curing temperature be not so high that the metal properties would be affected. Table X gives suggested

T A B L E X « S U G G E S T E D B A K I N G S C H E D U L E S F O R R E S I N - B O N D E D S O L I D L U B R I C A N T S

Baking schedule

Material Temperature, °F Time, min

Steel and alloys 400 60 Steel and alloys, case-hardened 300 60 Stainless steels (except 440) 400 60 Stainless steels (440) 300 60 Aluminum and magnesium 275 60 Aluminum and magnesium 300 25 Bronzes 300 60 Nickel and nickel alloys 400 60 Titanium 400 60 Monel and Inconel 400 60 Plated surfaces (except as limited by base metal) 400 60

« From Specification M I L - L - 2 5 5 0 4 A (USAF) .

(75) maximum curing temperatures and times for several metals. Size of pigment particles is just one of the other factors which must be considered.

The actual equipment used in the application process can be very simple or very complex. On a small scale one can use a batch process for cleaning and pretreatment. The application of the lubricant can likewise be simple, with curing being done in an air-flow oven. On the other hand, the process can be fully automated in a conveyor system with elaborate controls. Brown (84) discusses the similarity of application of this type of solid lubricant to that of industrial enamel finishes. The quantity and size of the parts being coated have a lot to do with the type of application process used.


IV. Lubricating Plastics


The solid lubricating plastics referred to in this chapter are all solid organic polymers. Because of the nature of polymeric materials, it becomes apparent that there are a wide variety and extremely large number of plastics which might be considered. Those plastics that are used as solid lubricants greatly narrow this field. Tables X I and X I I list some of the more common solid lubricant plastics along with their chemical composition.

T A B L E X I S O M E S O L I D L U B R I C A N T P L A S T I C S F O R M E D F R O M O N E M O N O M E R

Plastic Monomer

Polytetrafluoroethylene [ — C F 2— C F 2— ] X

Polyethylene [ — C H 2— C H 2— ] X

C 6H 5

I Polystyrene [ — C H 2— C H — ] X

Polychlorotrifluoroethylene [ — C F 2— C F C 1 — ] x

Polyvinylidene fluoride [ — C F 2— C H 2— ] X

T A B L E X I I S O M E S O L I D L U B R I C A N T P L A S T I C S F O R M E D B Y

C O P O L Y M E R I Z A T I O N O F Two D I F F E R E N T M O N O M E R S

Monomer number one Plastic Monomer number two

H 0 2C ( C H 2) 4C 0 2H Nylon type H2N(CH2)6NH2

Adipic acid [ — O C ( C H 2) 4C O N H ( C H 2) 6N H — ] x Hexamethylenedi amine

C F 3 C F = C F 2 Teflon« F E P C F 2 = C F 2

Hexafluoropropene Tetrafluoroethylene

a Registered trademark, Ε . I . du Pont de Nemours and Company.

The use of plastics in wearing devices can be broken down into two types of application. In the first, the plastics serve as actual lubricating films or materials for coating wearing surfaces. In the second, the plastics are used as the material of construction in various types of load-bearing device. This type of application has been covered by Forrester (Materials for Plain Bearings, this volume) but will be discussed further because of the interrelationship between the two types. In both applications the plastics may be used in conjunction with other materials in-


eluding reinforcing agents and other lubricating materials, and as impregnants in bearing surfaces. The section on theory has discussed the bulk friction and wear of this type of material. This section will show how these materials are used in actual lubrication techniques to take advantage of this lubricating ability.

B . F L U O R O C A R B O N R E S I N S

1. Introduction

One of the most widely used solid lubricant plastics is the class of fluorocarbon resins. Chemically, this class of compounds is composed mainly of fluorine and carbon. There can of course be other chemical constituents. Not all materials in this broad class of polymers are good lubricants. Therefore, one finds more specific terminology for the materials in the solid lubricant area. Examples would be either the chemical designation, an abbreviation of this designation, or even a manufacturer's trade name. A good example of this can be seen by the various designations in the literature for a common material in this class, polytetrafluoroethylene. In addition to this chemical term, reference is also made to P.T.F.E., T.F.E. resins, Teflon,3 Teflon3 TFE, T.F.E. fluorocarbon resins, and T.F.E.

The fluorocarbon resins have been a very recent development in the field of solid lubricants. The development of uses for polytetrafluoroethylene (P.T.F.E.) did not take place until the last twenty years, starting with the patent by Plunkett (85). The major studies of P.T.F.E. as a solid lubricant as well as other materials of a similar nature in this over-all class of plastics have been conducted during the last ten to fifteen years.

A discussion of fluorinated polymers as high-temperature plastics has been given by Precoplo, Cohen, and Zavist (Vol. 3, pp. 123-132).

2. Polytetrafluoroethylene

Polytetrafluoroethylene is unusual among the solid lubricating plastics. It has an extremely low coefficient of friction compared to all others. Table XIII presents values for the friction of P.T.F.E. as reported by several investigators. As was explained in Section II.C, the low friction of P.T.F.E. is attributed to its low adhesion properties. The coefficient of friction is one of the lowest reported for any solid material. It is not constant, however, for all conditions. At low loads the friction increases (51), whereas at higher loads the wear rate increases even though the

3 Registered trademark for P.T.F.E. of Ε. I. du Pont de Nemours and Company.

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friction drops off. Other factors affecting friction have already been discussed in the section on theory.

This material is limited in its applications by its mechanical properties. Load, speed, and temperature are very significant factors in its performance. These three factors can all interact on each other. As we shall see later in this section, the resistance of P.T.F.E. to these factors can be improved by the addition of various fillers. The temperature limit of P.T.F.E. as a solid lubricant is generally given as about 500°F (90, 91). Another significant factor in its use in plain bearings is the product of the load pressure and the speed of operation. This is designated as the PV relation, where Ρ is in pounds per square inch, and V is in feet per minute. An accepted limitation for this value is in the region of 1000 ( 92, 93), with higher values (2000 to 3000) quoted for intermittent use (93) or higher speeds in the region of 100 to 1000 fpm (92).

3. Other Fluorocarbon Resins

Several other fluorocarbon resins find use as solid lubricants. For the most part these materials do not have as good friction characteristics as P.T.F.E., but they possess other desirable features.

Polychlorotrifluoroethylene (P.C.T.F.E.) is a material quite similar to P.T.F.E., but it does not possess the low friction values. It does, however, give wear resistance. In general, friction coefficients for this material have been reported over a rather broad range from 0.25 to 0.55 (53, 86, 94). Bowers and co-workers (86) report the effect of chlorine substitution on friction of P.T.F.E. They show a rapid increase in friction up to 10% substitution of fluorine by chlorine atoms, with a leveling off in friction after that. The P.C.T.F.E. is equivalent to a 25% chlorine-substituted material. The increase in friction is accounted for by them as due to increased adhesion of the plastic to the metal which becomes larger than the bulk shear properties of the plastic and results in tearing in the plastic.

In general, P.C.T.F.E. is not stable at as high a temperature as is P.T.F.E., with a limit of about 390°F recommended (90). It has better mechanical properties of strength up to about 220° F (95).

Another material that has been developed and has potential as a solid lubricant is the copolymer of hexafluoropropane and tetrafluoro-ethylene. It is known as Teflon FEP. Like P.C.T.F.E., it has a reduced service temperature of about 400° F. Its PV values are lower than those of P.T.F.E., with 600 to 900 reported by O'Rourke (92). Gillespie and co-workers (93) also report higher coefficients of friction for this material. One major advantage is that the material is not so critical from a processing aspect as is P.T.F.E.


4. Filled or Impregnated Fluorocarbon Resins

Several methods have been employed to improve the mechanical properties of the fluorocarbon resins for applications as solid lubricants. One method is to fill the resin with some other material to improve its strength or other properties. Another method is to impregnate bearing surfaces with resin to provide a lubricating material in the bearing.

A wide variety of materials have been added to the various resins. Many of these materials were investigated by White (94) in a study of filled P.T.F.E. and P.C.T.F.E. He presents extensive tables of friction and wear behavior of the various compositions. For P.T.F.E. he shows the beneficial effect of molybdenum powder, graphite, glass fiber, and glass fiber with MoS2 on wear behavior. He also shows similar improved performance of P.C.T.F.E. The values he reports for the coefficient of friction of the filled and virgin resins are comparable. The values he reports for P.T.F.E. (0.12 to 0.26) are higher, however, than normally accepted figures and those shown in Table XIII (see p. 76).

Mitchell and Pratt (96) also present extensive data for filled P.T.F.E., including both mechanical and wear properties. They show reduced wear (in some cases by a factor of 1000) and no change in friction. Other properties were improved, including thermal expansion, thermal conductivity, and compressive strength. Twiss and co-workers (97) have investigated various filler materials for P.T.F.E. and obtained results similar to those of the above two studies. Barry (98) presents data on glass fiber-MoS2 fillers.

The use of fillers in fluorocarbon resins can provide beneficial results. Wear is reduced by a considerable amount with many materials. Reduction factors of 1000 and higher are not unusual. O'Rourke (92) shows limiting PV values for P.T.F.E. improved by a factor of about 10 at 10 fpm and about 18 at 1000 fpm with the proper selection of fillers. This particular characteristic is of extreme value in the design of journal-type bearings. Improvement in compressive strength and thermal properties by the use of fillers is also possible and of importance in bearing design.

The other method of taking advantage of the fluorocarbon resins is to use them as impregnating agents for metals and nonmetals such as graphite. In such a process the resin is incorporated in the porous structure of the base material. Friction and wear characteristics are improved by the smearing of a layer of the resin over the bearing surface.

Mitchell (99) presents data on the improvement of wear and friction through the process of impregnating bronze bearings with P.T.F.E. Bronze is compared to other base metals, including phosphated steel,


sintered stainless steel, and aluminum. It is considerably more receptive to the P.T.F.E. impregnation technique. The beneficial results of P.T.F.E. impregnation of graphite brushes for electrical sliding contacts are reported by Atkins and Griffiths (100). As for filled P.T.F.E., O'Rourke (92) shows increased PV values with P.T.F.E.-impregnated materials. Increases by as high as a factor of 20 are given at 10 fpm, and 14 at 1000 fpm.

C . O T H E R P L A S T I C S O L I D L U B R I C A N T S

I. Nylon Type The nylon solid lubricants are found in many applications. In most

cases the nylon is used in the bulk form as the material of construction for the bearing or other components. It may be used dry or with liquid lubricants. As with the fluorocarbon resins, it can also have fillers added to improve its friction and wear characteristics.

Nylon, in general, does not have an exceptionally low coefficient of friction when run dry. Various investigators (51, 89, 94, 101, 102) have reported values in the region of 0.2 to 0.5 over a range of conditions. These include nylon sliding against itself and against steel. The use of liquid lubricants in conjunction with nylon can reduce the friction coefficient appreciably (101). Values as low as 0.08 were reported for a steel-nylon combination lubricated with stearic acid solution. Similar improvement was noted for a nylon-nylon combination. Although many applications of nylon bearings are run either dry or with only initial lubrication (102), the PV values have been reported (103) to increase from 7000 to 40,000 with the use of a lubricant. Lower values for the unlubricated material of the order of 2000 or 3000 have also been reported (102).

The principal advantage of nylon over the fluorocarbon resins is its better mechanical properties (91, 102, 103). The better properties from a tensile and compression aspect allow for the design of stronger bearings. On the negative side are its lower temperature capability (102, 103) of about 300°F maximum. It also has a tendency to take up water, which can have detrimental results in many applications. In addition to the effect of its presence, the water can cause swelling of the nylon. As with other types of plastic, nylon has a higher coefficient of expansion and a lower thermal conductivity value than metals. This can create problems in the design of bearings used in conjunction with metal parts.

Fillers can be added to nylon to improve its properties as a solid lubricant or bearing material. MoS2 has been reported (104, 105) to improve the wear resistance and to reduce the friction coefficient. Fillers


can also be used to improve the thermal and expansion characteristics of the bulk material.

2. MisceUaneous Types

Many other types of plastic materials have been used as solid lubricants or wear-resistant materials. Some of these are listed in Tables XI and XII. Others that are not listed include various resin laminates and polymethylmethacrylate. There have been several good reviews on self-lubricating bearings using these materials as well as the previously discussed plastics. The review by Pinchbeck (106) is particularly worth noting. Further discussion of these materials is beyond the scope of this chapter, as they are used more as materials of construction. Mention is made because of their relation to the more common fluorocarbon resins and the nylon types which find broad application both as bearing materials and as solid lubricants. The reader is left to pursue any further interest in this area.

D . A P P L I C A T I O N O F P L A S T I C S T O W E A R I N G P A R T S

The various solid lubricant plastics can be employed in many ways to reduce friction and wear in a component. Many of the techniques to be discussed here are applicable to several types of plastic. Only a few examples will be given to indicate the various possible methods. In selecting a method, much depends on the eventual use of the plastic as a solid lubricant.

Unlike the resin-bonded solid lubricants, the plastics can be obtained in bulk form. Nylon, P.T.F.E., and others come in various shapes, including rod, bar, and sheet stock. As such, it is possible to machine the actual bearing or wearing component directly from the solid material. Because of this, the mechanical properties and machineability of the various materials are of importance to the user. Wide ranges of these properties can be obtained through modification with fillers, as has been previously discussed. It is impossible to discuss them here, and the user should consult the manufacturer of the bulk material to obtain the optimum material for his needs. Filled plastics come in many of the same forms as the pure bulk materials.

Plastics also come in the form of powdered material. Nylon can be sintered (107, 108) first under applied pressure and then with heat to provide coatings on metal surfaces. In general this can be done as a mixture with some other material. Powdered P.T.F.E. can also be used either by bonding it to the surface with a resin (97, 109) or by application in a liquid carrier such as water and subsequent curing at elevated temperatures around 700°F. The nylon materials can also be formed by various molding techniques.


V. High-Temperature Bonded Films

A . G E N E R A L I N F O R M A T I O N

The classification of high temperature has been used for various types of bonded solid lubricants. In this chapter the term will be used for the ceramic-bonded lubricating solids. These materials are intended for use at temperature ranges whose upper limit will generally exceed 1000°F. Some, however, are designed to operate at lower temperatures but still above that of the resin-bonded films. The high-temperature ceramic-bonded materials are a logical extension to the resin-bonded films. They employ ceramic binders to give greater temperature resistance than the resin materials and lubricating solids which are more thermally and oxidatively stable than graphite or MoS2. Most of the research and development on these films has been stimulated by interest in their application to aircraft and space vehicle components.

Early work on ceramic-bonded solid lubricants was not conducted until the middle and late 1950's. Some of the most promising films developed to date include:

1. Si02-bonded PbO reported by Sliney and Johnson (110) for 1250°F use in air.

2. B203-bonded PbS reported by Lavik (111) for 1000°F use in air. 3. A CoO-based ceramic bonded CaF 2 reported by Sliney for use to

1500°F (112) in air and later (113) for use to 1900°F. 4. Sodium silicate-bonded MoS2-graphite reported by Devine and

co-workers (114) for use at —300° to +750°F in air. 5. B203-bonded PbS and MoS2 reported by Lavik (115) for use to

1000°F in vacuum. Table XIV lists wear data for some of these films in various labora

tory wear and friction equipment. Because of the wide differences in configuration of this equipment, the behavior of these films cannot be directly compared. It should be noted, however, that most of the ceramic-bonded materials give better results at elevated temperature (115, 116) or under conditions of sliding that would generate hot spots at the interface. This does not hold for the films containing MoS2 (76, 115) when they are run in air. This is possibly due to its oxidation as well as other effects. The ceramic-bonded materials as a class, however, do not perform as well as the resin-bonded materials at lower temperatures but generally exceed the resin film's capabilities by a considerable amount at the higher temperatures. One exception to the low-temperature case occurs when the lubricants are run at high speeds resulting in high temperatures at the contact region. In such a case the ceramic-bonded films perform better than the resin-bonded films. These

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films are still basically in the development stage. Little experience has been gained in actual applications.

The state of the art for ceramic-bonded solid lubricants is mainly that of empirical studies. This is to be expected, owing to the complex nature of ceramic systems. One simply has to look at a series of ceramic phase diagrams (113) to appreciate the problems involved in this type of solid lubricant. The fundamentals are then further complicated by the various effects of temperature and pressure encountered in a rubbing system and the possibility of experiencing some hydrodynamic component of lubrication due to liquid formation in the solid body during various stages of operation. Another problem in the selection of materials is presented by the chemical reactions involved (110). The reactivity of the lubricant can affect its adherence to the surface being lubricated as well as its compatibility with the environment.

B. L U B R I C A T I N G S O L I D S

Most of the work on ceramic-bonded solid lubricants has been in the study of lubricating solids. This is because at first the lubricating solids were felt to pose the greatest block to obtaining component materials stable at elevated temperatures. As will be seen in the next section, the search for high-temperature lubricating solids is not limited to ceramic-bonded materials but is rather extensive. This section will discuss only those solids that have been investigated in conjunction with the ceramic-bonded films.

Peterson and Johnson (119) first investigated mixtures of graphite and various metallic compounds including PbO and CdO. They demonstrated the beneficial effects of such mixtures at temperatures to 1000°F over the friction of graphite by itself. In further work (120), they investigated a larger number of oxides as lubricants by themselves for temperatures to 1000°F. PbO was the best of the materials investigated and, as we have seen, has given promising results when bonded with Si0 2 (110). One problem encountered with PbO was the fact that it converts to Pb 3 0 4 at lower temperatures and results in an increase in friction.

In an extensive survey of lubricating pigments for use at elevated temperatures, Lavik ( 6 1 , 67, 111) has dealt with materials having melting points in excess of 1000°F. He reports friction and oxidative properties of a large number of materials (67, 111). The materials included many chlorides, oxides, and sulfides of metals in periods 3, 4, and 6 of the periodic table as well as other related materials including several selenides and tellurides. The B203-bonded PbS film was a result of this investigation. Many other pigments looked promising, but time did not


allow for their further evaluation. Some low-temperature friction data are presented in Table VI for a few of the materials from this program.

In later work, Sliney (112) further investigated a series of halogenated materials as possible lubricants for use at 75° to 1500°F. The CaF2

pigment was the most promising in this work, giving friction coefficients from about 0.3 at room temperature to less than 0.1 at 1500°F when bonded with a ceramic. Johnson and Sliney (121) also report data on oxides, sulfides, and halides for use at temperatures to 2000°F.

The selection of a high-temperature lubricating solid is a much more complex job than that required at low temperatures with the resin-bonded pigments. Many of the materials must be investigated from an oxidative and frictional aspect, as there are no firm rules to follow in selection of any one material. A further complicating factor is the compatibility of the pigment with the bonding agent. For films designed to work in vacuum, the oxidative properties can be neglected, and in some cases, as shown in Table XIV with MoS2, the low-temperature materials can be used at greatly elevated temperatures.

C. S U B S T R A T E S A N D S U B S T R A T E T R E A T M E N T S

In the discussion of resin-bonded solid lubricants it was shown that the hardness of the substrate and the pretreatment had a significant effect on the life of the film. It was also shown in Fig. 6 that one of the most common pretreatments deteriorated with temperature. The substrate material and the surface treatment also have an appreciable effect on the performance of ceramic-bonded solid lubricants. Sliney (118) has shown the beneficial effect of surface oxidation prior to application of the ceramic-bonded film for one type of material. Lavik has shown (115) the beneficial effect of a B 2 0 3 precoat on titanium when applying an MoS 2-B 20 3 film. Lavik and co-workers (117) also have shown the effect of various substrate metals on wear life. These are only a few examples of such effects for ceramic-bonded materials.

In general it has been shown that the substrate and/or substrate treatments can have an effect on wear life. These effects are probably due to several factors. The hardness of the base metal and the better adherence or bonding are two which were also observed with the resin-bonded materials. Another factor to be considered with the ceramic-bonded materials is a matching of the expansion characteristics of the base material and ceramic binder. This is necessary because of the wider temperature range experienced by this type of film and the more rigid film as compared to the resin-bonded materials. If the coefficient of expansion of the ceramic does not match that of the metal to some degree, cracking and spewing off of the coating will occur. With the ceramic


F I G . 7. Ceramic film showing no surface cracking. A ceramic ( N B S 3 3 2 ) on Rene 41 alloy applied by preheating the metal to form an oxide and curing the sprayed ceramic at 1 7 0 0 ° F .

materials some adjustments in their expansion characteristics can be made through modification of the composition. Figures 7 and 8 illustrate the effect of proper over-all consideration of surfaces and surface treatments. These two photographs show one ceramic composition as applied

286 ROBERT J. BENZING

F I G . 8. Ceramic film showing gross surface cracking. A ceramic on 440 C stainless steel applied by preheating the metal to form an oxide and curing the sprayed ceramic at 1 7 0 0 ° F .

to two different materials. The cracking in Fig. 8 would result in early rupture of the film and failure as a lubricant binder.

D . B O N D I N G A G E N T S

The early studies on ceramic-bonding agents were limited in nature because it was thought that many ceramic materials were stable at


elevated temperatures and could be used as binders. Although it is true that many materials are stable, we have seen that proper matching of the binder and the substrate can become a problem with this type of film.

The first films employing ceramic binders contained simple one-component materials such as Si0 2 and B 2 0 3 in low concentrations, usually less than 10%. Subsequent work has led to more complex systems (112, 118, 122) in attempts to match better the binder to the substrate and/or conditions of operation. Another approach has been to carry out rather detailed studies of the mode of failure (117) of the ceramic-bonded films. Still a third approach (123) has been to take a detailed look at one system, PbS-B2Ö3, in order to ascertain the phase relations of the system and correlate them with the friction and wear behavior. These studies have all had some promising results, but a considerable amount of effort is still required.

E . A P P L I C A T I O N P R O C E S S E S

Little can be reported on the application of ceramic-bonded solid lubricants to wearing components. Only a few commercial versions have been available, and the processes used have not been widely published. Crump (124) does present one, and others can be reviewed in references on the laboratory development work. These involve rather complex surface preparation, ceramic formulations, and application processes including the spraying techniques.

Although generally not reported, both good and poor results have been experienced in the actual use of this type of film. Some of the poor performance of ceramic materials in actual equipment can probably be attributed to inadequate matching of the ceramic binder and substrate materials. As with any laboratory development, the carryover to use requires additional work. This is then a major problem in the area of proper application to the component being lubricated.

VI . Other Solid-Type Lubricants


In addition to the three major classes of solid lubricants discussed in Sections III through V, several others may be of equal importance, but they can be reviewed in much less detail. In actual use they provide techniques that may find wide application in industry, such as the solid powders, as well as those such as the solid vapor-deposited materials, which are only in the stage of evaluation of feasibility. This section will discuss briefly the concept of the use of each class and will present references for further study. The length of discussion of each topic is not


indicative of its importance in industry, as some techniques can be reviewed briefly owing to their relationship to earlier information presented in this chapter. An example of this is the solid powders which can be directly related to the work on resin and high-temperature bonded materials and find wide use in industry.

B. V A P O R - D E P O S I T E D S O L I D L U B R I C A N T S

Solid lubricant films for protecting wearing components may be deposited in situ on metal surfaces from the surrounding atmosphere through the use of a reactive gas or film-forming vapor. One mode in which the solid lubricant may be formed is the direct chemical reaction of the gas with the metal surface to give a friction- and wear-reducing film which provides lubrication to the component. Another vapor-deposited lubricant may be obtained through a polymerization reaction of an organic vapor to form a lubricating film on the metal surface. This film must of course have lubricating characteristics. Both films are considered solid lubricants, as the lubrication is obtained from the solid film formed in situ, and the vapor is simply the technique of application.

The major work in this area of solid lubricants has stemmed from an interest in their possible application to the lubrication of aircraft, aerospace, and missile components. This interest is basically a result of the potential that these techniques offer for use at extremely elevated temperatures. As will be seen, the use of these techniques involves a thorough analysis of environmental conditions as well as the materials used for both the bearings and vapors. Success depends on the chemical reactivity under the conditions of operation, and this can be affected by temperature, load, speed of sliding or rolling, atmospheric pressure, etc. Therefore, proper performance can result only through a close match of the vapor, the metal used in the bearing, and the operating environmental conditions.

The effectiveness of lubrication from the vapor state is well demonstrated by the work initiated by the National Advisory Committee for Aeronautics (NACA) and later carried on by the National Aeronautics and Space Administration (NASA). This work will be discussed to show the complex balance of variables encountered in this type of solid lubrication. The objective of the program was to study vapors of halo-genated materials as possible lubricants. Initial studies were carried out on the lubrication of steel (SAE 1020) surfaces with vapors of fluorine and chlorine-substituted methane and ethane (125). This phase of the program was conducted at room temperature and also at 480° F. All the wear and friction studies were performed on an apparatus consisting of a hemispherical rider bearing on a rotating flat disk. This work


demonstrated that the materials could provide satisfactory lubrication of the metal surface. It was shown that the materials with the higher chlorine content (such as CF2C12) performed better under the test conditions, owing to the higher reactivity of the chlorine. A mechanism of the formation of iron chloride on the metal surface was postulated. Although the decomposition temperatures of the materials were above that found in the experiment, it was thought that the temperature generated at the rubbing interface was high enough to produce breakdown of the gases and provide chlorine for reaction with the metal. In such a case, the mode of lubrication would be very similar to that found in the use of extreme pressure additives in oils.

Continuation of this work was concerned mainly with the investigation of the technique for high temperature, and it provides a good example of the considerations involved in the development and eventual use of this type of system. Effective lubrication was obtained for M-l tool steel (126, 127) to 1000°F, by using the CF2C12 material found effective in the earlier work. The beneficial effects of SF6 as a catalytic additive in forming chloride surfaces on the metal were also demonstrated. When the temperatures were increased from 1000° to 1200° F, it was necessary to use a less reactive material (CF3C1) to reduce corrosion of the metal.

A third phase of the work (128) was concerned with lubrication over the range 75° to 1200°F. Because of the corrosion of the M-l steel at the higher temperatures, it was necessary to employ corrosion-resistant bearing materials. This also had an effect on the reactivity of the gas. A highly selective match of the various conditions of operation was required. Substitution of bromine for the chlorine proved beneficial for high-temperature nickel-based alloys. Cobalt alloys, on the other hand, continued to perform well with the CF2C12 with 1% SF6 added. Because of the good lubrication obtained with CF 2Br 2 at low temperatures and its high corrosion at elevated temperatures, it was mixed with CF3Br which gave lower corrosion and good lubrication at the high temperatures (129). A 1:1 mixture of the two gave friction data between that of the two and a lower wear rate than either by itself. This was true for various corrosion-resistant materials over the full temperature range of 75° to 1200°F.

In a further extension of the technique to even higher temperatures (1400°F), an investigation was made of still less reactive materials such as CF2C1CF2C1 and CF 2BrCF 2Br for the vapors and ceramics and cermets for the bearings (130). It was necessary, however, to use a metallic material for one of the bearing materials in order to obtain the lubricating metal chloride. The nonmetallic constituent reduced cor-


rosion, depending on proper selection of the material and on whether it was the disk or the rider. The best results were obtained when the rider was nonmetallic, owing to its ability to take the higher temperatures generated in this component and its lower reactivity. The ethane-substituted materials are not as thermally stable as the methane derivatives, but their breakdown products ( C F 2 X — ) do not release halogen as readily as the methane derivatives do. The effectiveness of lubrication of the dibrominated ethane material with a crystallized glass ceramic bearing on two nickel-based alloys was also demonstrated (131).

This series of investigations points out the important effects of temperature, chemical constitution of the vapor, bearing material reactivity, geometry of the mechanism being lubricated, and several other factors which account for the performance of this type of solid lubricant.

Another approach to vapor lubrication was carried out by Shell Development Company at about the same time as the above work. This study (132-135) was concerned with a slightly different approach to lubrication with vapor-deposited solid lubricants. Instead of reactive gases being used as lubricants, a reducing or "protective" atmosphere concept was developed. The atmosphere was formed by the vaporization of an organic liquid in a stream of hot air. Actual ball bearings were employed as the test specimens in this program. The mixture of the organic vapor and air was generally in the range of a normal combustion mixture, and reaction of the organic vapor with the oxygen in the air produced a reducing atmosphere. This reducing atmosphere prevented oxidation of the bearing metals and thus removed one component of wear. The organic material also contained small quantities of additives which formed solid lubricating films on the surfaces of the bearing components. These solid lubricants prevented mechanical wear of the bearing. The program was very detailed, and space does not allow a detailed discussion. Again proper selection and matching of materials and conditions of operation were necessary. Table XV presents some of the data from this program.

In addition to these two major programs, other work on vapor-deposited films or solid lubricants has been conducted. Campbell and Lee (136) discuss modes of polymer formation from organic vapors but do not show good lubrication. In a series of papers, Bowden and Rowe with other authors (137-140) discuss the reaction of various metals with gases to form known solid lubricants on the surface. For example, they present data to show the formation of MoS2 on the surface of molybdenum metal by reaction with H2S.

The above discussion presents methods of depositing solid lubricants of various types from a vapor atmosphere. The future potential of

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C. S O F T M E T A L S

The use of soft metal films as solid lubricants for harder substrates is readily explained in terms of the theory of metallic friction, as already discussed in Section II.D.l. In view of the low shear stress of a soft metal coating and the yield pressure of the hard substrate, one would expect a low coefficient of friction. In actual practice lower friction is obtained, but it is not always of the order of magnitude predicted. This can be attributed to the role of ploughing as well as other considerations.

Thin films of soft metals have been employed as solid lubricants by using the principle discussed above. Included in the list of these soft metals are gold, silver, platinum, lead, indium, and barium. An example of the low coefficient of friction that might be possible with such materials can be shown by a calculation with the equation μ = s/p for the case of a pure lead film on a 440C stainless steel surface. The shear stress of lead is reported (141) as 1825 psi. The same reference also gives a yield stress for the steel of about 90,000 to 100,000 psi. This results in a coefficient of friction of about 0.02. Of course, in actual sliding contact this would be modified by other considerations. One reference (142) gives a coefficient of friction of 0.3 for Inconel sliding on Inconel X with lead as the solid lubricant over a temperature range of 0° to 500°F. Inconel is softer than steel in general, which accounts somewhat for the higher value of friction.

Care must be taken in the application of the soft metal films to the bearing surfaces. If the film is too thick, the resulting yield pressure will not be that of the harder material, and higher friction will result. If the film is too thin, the solid lubricant will fail prematurely as the soft metal wears away. Two main techniques have been employed for applying the soft metal films—electroplating and vacuum deposition. The latter appears to provide a better film but is obviously more expensive.

The soft metal films have served as solid lubricants for both sliding contacts and rolling element bearings. They are mostly employed in lightly loaded situations. Other applications include electrical contacts, lubrication of bearings in vacuum tubes (143), lubrication of bearings for space applications (144, 145), and lubrication where a noble metal such as gold might be required for the lubricant. Another broad area of application for soft metals is found in the metal-bearing alloys, where the soft metal such as lead provides lubrication or assistance to lubrication of some other nature. The soft metal is actually one of the alloys in the bearing material and functions in a complex manner which can


include a smearing action on the bearing surface. This area is much too broad in scope for this chapter and is covered in considerable detail by Forrester in his chapter, Materials for Plain Bearings.

D . W E A R - R E S I S T A N T N O N M E T A L S A N D H A R D M E T A L S

These two classes of solid lubricants have been combined in one section because of their similar behavior and use. Table XVI lists several categories of these materials with specific examples of each and some wear and friction data. In general, these materials function more from the standpoint of construction of the component or bearing. Because of this, many engineers look upon them as bearing materials and not as lubricants. In the field of lubrication, however, they are considered as solid lubricants, as the environmental and operating conditions affect their performance. That is, they do not reduce friction and/or wear under all conditions and therefore cannot be considered as inherently exempt from the requirement of a lubricative medium. The fact that they do perform satisfactorily under some conditions indicates their ability to provide lubrication defined as friction reduction and/or wear prevention.

Table XVI presents only a few of these materials and a small amount of data. It does not cover all types of materials or even the ranges of operation. General but rather comprehensive reviews cover this area of solid lubricants (142, 145-147), and the reader is referred to them for further information. The data presented in Table XVI show, however, that both nonmetallic and metallic materials can provide satisfactory operation without other sources of lubrication. These materials, as will be seen, are meant to function in areas of extreme high temperatures and in cases where other lubricants will not suffice.

Although the physical properties of these materials cannot be fully classified, several properties apply to most of them. In general, they are relatively hard (the molybdates are one exception). They also have a high melting point, which probably is one reason they can resist the higher temperatures generated by dry sliding. They are resistant to oxidation in many cases, but this is not true for all the materials.

The mode of lubrication with these materials is not well defined. Because of the wide variety of materials it is not possible to define a mechanism that will account for the behavior of each and every material. Some have been postulated, however. One possible behavior would be the resistance of the material to chemical attack or chemical wear by the material it is rubbing against, much in the same manner as is discussed by Coes (148) for grinding materials. Another mode, suggested by Sibley and Allen (149) for refractory materials, is based on

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their ability to resist thermal stress caused by hot spots generated in sliding. These authors present equations for predicting wear, depending on the type of material. One is for cermets and ceramics, and the other for metal alloys. Both are based on thermal factors. In some materials, particularly the ceramics, lubrication may be obtained by the formation of a liquid film due to temperature and other conditions of operation. This has been shown to take place with B 2 0 3 (150). In the case of cermets, Brown and co-workers (151) also indicate the importance of oxidation of the surface of metal constituents and their sintered nonhomogeneous structure. In addition to the above possible modes of lubrication with these solids, there is the important area of alloying as typified by Buckley and Johnson's (152) work on addition of silicon to nickel alloys.

E . F L A M E - S P R A Y E D S O L I D L U B R I C A N T S

The use of various wear-resistant metallic and nonmetallic materials for high-temperature solid lubricants has been discussed. Most of the data presented have been for solid bodies of these materials. This section will present one technique for applying such materials to the surface of another material. The technique is that of flame spraying and also includes the plasma stream and other similar processes. These methods are not new, but considerable new interest has been stimulated by space applications for high temperature. Ault and Wheildon present a very good review of flame spraying of ceramic coatings in Vol. 2 of the Modern Materials series. They trace the development back to 1914.

In the area of lubrication, flame-spray processes have been used mainly for applying hard, wear-resistant surfaces to metals. These coatings were then used in conjunction with other lubricants to reduce wear of the components. Included in these coatings were various ceramics, such as A1203, and metallic materials, such as the hard-metal carbides. As we have seen, these materials now serve as hard, wear-resistant, solid lubricants for use in extreme environments, including high temperatures. In addition, there has been recent interest in the use of this technique to apply lower-temperature materials, such as MoS2. Other possible applications have also been reviewed (147).

Flame-sprayed coating of materials such as the ceramics is of interest as it provides a technique of application that does not grossly affect the properties of the substrate metal. Many of the ceramic materials can be prepared as a frit and coated on a surface by firing at elevated temperatures. The firing at the elevated temperature is often above the heat-treating temperature of the metal and thus drastically affects its mechanical properties. Flame spraying alleviates such problems. Like-


wise, for the other type of coating used at high temperatures it offers considerable advantages. These are not limited to the temperature effects.

Materials for use at lower temperatures (in the region of 1000°F) are now being considered for use with this method. This area of study is very new, and no publications have been presented for the reader. The method is of interest because of its versatility in applying coatings. Two examples of the future potential follow:

1. Many of the high-temperature ceramic adhesives which bond solid lubricants to a bearing must be fired at temperatures above the limits of the metal. The use of flame-sprayed materials consisting of a ceramic and a solid pigment might make such firing unnecessary.

2. Flame-sprayed solid lubricants in a mixture with particles of the base metal might allow a gradated coating to be formed on a metal surface which would range from pure metal at the metal-coating interface to pure solid pigment at the surface. This would solve problems associated with differential coefficients of expansion causing film or coating cracking.

This technique has been discussed because of its potential and future application. No data are available on its use for the lower-temperature materials. The response of the higher-temperature materials would be expected to be similar to their performance in the bulk form as long as the coating is not worn away and provided the coated material does not greatly affect the properties of the solid lubricant coating.

F. P O W D E R E D S O L I D L U B R I C A N T S

The oldest technique of solid lubrication is the application of powdered materials, a method still widely used in many pieces of equipment. It consists in applying solid powdered materials that possess lubricating characteristics to the surfaces requiring lubrication. The types of powdered solids used cover a wide spectrum of materials. The most common are some of the metallic and nonmetallic pigments already discussed, as well as certain organic solids. They are generally applied as a dry powder by simply dusting them on the surface or by rubbing them into the surface. With certain materials (35) the rubbing tends to give a better bond with the surface.

The use of powdered materials is limited mainly to areas where adhesion of the solid lubricant to the surface is not a major criterion. Actually, many of the techniques discussed previously were developed as a direct result of attempts to obtain better bonding of the materials initially used in powdered form. The properties of the various materials must be considered in the selection of any powdered lubricant.


One of the most important is compatibility with environmental conditions. The material must be stable at temperatures encountered as well as to the composition of the surrounding gases (air, vacuum, or some other gas). Another property which can be of importance is the reactivity of the solid material with the bearing material. Orcutt and co-workers (153) have shown that the reaction of the solid lubricant powder with the metal surface can greatly enhance its retention in the bearing and thus its over-all performance.

Graphite, one of the earliest solid lubricants, was first applied as a powdered material. It still is used in this manner, as is MoS2. Many of the materials listed in Tables VI and VII have been or could be used in the same fashion. The prime requirements for such a material are a low coefficient of friction and the ability to prevent wear. Selection of powdered materials should be based on such properties, once the chemical aspects have been satisfied. Because much of the performance of these materials has been covered in other sections in this chapter, the details will not be repeated here. The theory of performance is the same, as are such factors as lubricating characteristics. Of major difference would be life and retention in any given component. These, of course, would be less than for any other coated material.

The one broad class of solid lubricant in this area that has not been covered is that of the powdered organic materials. These materials are organics that remain solid under the conditions of operation of the system being lubricated. The outstanding example of such a material is the metal-free phthalocyanines. Considerable work has been conducted (154, 155) on their use for high-temperature applications. These materials are similar to the metal phthalocyanines used in dyes. The metal-free materials are complex organic compounds, as shown in Fig. 9. Their

F I G . 9. Metal-free phthalocyanine.


aromatic structure probably contributes to the thermal stability of the material. Although the mode of lubrication is not fully understood, it is postulated (155) that the planar structure gives reduced friction, and the chelating characteristics of the metal-free material provide better bonding to the metal surface. This is somewhat confirmed by the lesser performance of the metal-containing materials. Friction coefficients of the materials have been reported (155). They show superiority to other materials at temperatures in the region of 1000°F and higher. Values were reported of 0.04 for lubrication of a cermet at 1000° F in a four-ball tester at 700 fpm and initial contact stress of 250,000 psi in an atmosphere of nitrogen. Other complex organics have also been studied (156).

G. G A S - E N T R A I N E D P O W D E R S

This technique for application of solid lubricants consists in lubricating a component (mainly antifriction bearings) with a solid powder carried to the bearing in a gaseous stream. The gaseous streams are mainly inert gases, as the technique is intended for high-temperature use, and the inert gas prevents oxidation of the solid lubricating material. Most of the work in this area of solid lubricants has been conducted in relation to aircraft and space vehicle applications. It has not been used in any known system but is in the stage of feasibility research.

Early work in this field was conducted by Macks and co-workers (257) using MoS2 to lubricate 75-mm-bore roller bearings at speeds of 0.3 to 0.975 Χ 106 DN.4 The results of their work indicated feasibility of operation. They also studied smaller bearings (20-mm-bore) running at 0.04 to 0.09 χ 106 DN.

A more elaborate extension of this work (258, 259) demonstrated the wider potential of the technique. This was conducted almost completely with 20-mm ball bearings operating at speeds to 30,000 rpm and temperatures to 1200°F. Tables XVII and XVIII present data on various tests conducted with this technique. Mixtures of at least two powders were found to be essential because of the wide temperature range and other conditions of operation. The best results were obtained with a mixture of 76% MoS2 and 24% metal-free phthalocyanine carried in N2, and a mixture of 16%% CdO and 83y3% graphite carried in air. The mixtures were required, as no single powder operated over the full temperature range. This is likely due to the chemical aspects and was seen also in the vapor-deposited materials. The importance of chemical activity

4 DN is a representation common to the bearing industry standing for the product

of the bore of the bearing in millimeters times the speed in rpm.


was confirmed in laboratory work reported by Orcutt and co-workers (153).

One potential problem which can be anticipated with gas-entrained powders is bearing fatigue. This has been briefly investigated (160) with MoS2 initially carried in a liquid. At 450° F, where the liquid evaporated, there was much lower fatigue life, thought to be due to stress raisers formed by the solids in the regions of pure rolling contact.

H . CHEMICAL AND ELECTROPLATED COATINGS

These are two methods for application of solid surfaces which can reduce friction or prevent wear. Little attention will be devoted to the process of coating, as extensive literature is available. The main intent is to indicate the potential of such techniques for applying solid lubricants.

1. Chemical Coatings

We have already seen how phosphating can be used to improve the performance of resin-bonded solid lubricants. This technique can also be used as a surface coating or solid lubricant in conjunction with oils (77) to reduce wear. Other potential wear-reducing techniques include sulfurizing or nitriding or a combination of both. The latter has been reported by Waterfall (161). Most of these techniques are designed to increase hardness and are usually employed in conjunction with liquid lubricants. Chemical etching can also be done to provide porous surfaces for incorporation of lubricating solids.

2. Electroplated Coatings

This type of solid wear-resisting surface includes hard coatings such as chrome surfaces as well as soft metals such as gold. The hard coatings can be used in conjunction with liquids or as wear-resistant surfaces themselves. The soft metals provide lubrication as discussed earlier. The electroplated coatings can also be used for obtaining porous compacts.

VII. Wear and Other Evaluation Techniques

A. GENERAL INFORMATION

Most of the evaluation data on solid lubricants are concerned with the measurement of the friction and wear characteristics in either laboratory bench tests or mock-ups of full-scale bearings. Various chemical tests have been used for evaluating solid lubricant properties, but in general they serve more as specification controls (75, 162) than as re-

TA

BL

E X

VI

I G

AS

-E

NT

RA

IN

ED

P

OW

DE

R

LU

BR

IC

AT

IO

N

WI

TH

M

IX

TU

RE

O

F M

oS2

AN

D

ME

TA

L-

FR

EE

P

HT

HA

LO

CY

AN

IN

E

(MF

P)0

,6

Obj

ectiv

e

Inte

rnal

pl

ay,

inch

es

Lub

rica

nt

Lub

rica

nt

flow

, gm

/min

Out

er r

ace

tem

pera

tu

re,

°F

Cha

nge

Run

ning

in

pla

y,

time,

in

ches

ho

urs:

min

C

ycle

s R

emar

ks

25 h

ours

end

uran

ce,

1200

°F, 3

0,00

0 rp

m

25 h

ours

end

uran

ce,

100-

1200

°F,

5000

-30,

000

rpm

Tes

t to

det

erm

ine

tem

pera

ture

at

whi

ch lu

bric

ant

is le

ast

effe

ctiv

e,

30,0

00 r

pm

Tes

t to

ver

ify t

hat

M0S

2 in

lu

bri

can

t w

as n

ot o

xidi

zing

in

750

°-90

0°F

rang

e

0.00

34

70%

MoS

o,

0.44

and

11

80-1

120

+0.

0023

25

:36

30%

MF

P 0.

41

0.00

3 70

%

MoS

2,

30%

MF

P

0.00

31

0.35

8 (2

6Ji

hour

s)

0.38

9 (2

4 ho

urs)

0.00

32

70%

MoS

2,

0.27

30

% M

FP

76%

MoS

2, 24

% M

FP

0.34

7

100-

1215

+

0.00

48

50:3

0

600-

1200

+

0.00

02

6:35

550-

1200

+

0.00

19

14:0

0

All

surf

aces

hi

ghly

po

lis

hed.

B

eari

ng

clea

n.

Maj

or w

ear

on b

all

di

amet

ers.

Lub

rica

nt fl

ow

exce

ssiv

e.

10 t

empe

ra-

Hig

h to

rque

at

low

te

m-

ture

, pe

ratu

re.

Ran

w

ell

at

29 s

peed

hi

gh t

empe

ratu

re.

Tor

que

incr

ease

s to

ob

ta

in 3

0,00

0 rp

m a

t ea

ch

cycl

e.

Inne

r ra

ce

had

slig

ht l

ubri

cant

dep

osit.

So

me

ball

pock

et w

ear

Poor

lu

bric

ant

perf

orm

an

ce a

t 75

0° t

o 85

0°F.

L

ubri

cant

cl

ogge

d on

e in

let

tube

. E

xces

sive

w

ear

on

oute

r ra

ce.

Gen

eral

con

ditio

n go

od.

No

poor

lu

bric

ant

per

form

ance

in

the

oxid

iz

ing

tem

pera

ture

ra

nge.

In

ner

race

had

wav

e pa

tte

rn o

n ru

nnin

g tr

ack.

100

hour

s en

dura

nce,

0.

0030

12

00°F

, 50

00-

30,0

00 r

pm

100

hour

s en

dura

nce,

12

00°F

, 50

00-

30,0

00 r

pm

0.00

29

76%

MoS

2, 0.

352

24%

MF

P (4

8M

hour

s)

0.30

0

hour

s)

76%

MoS

2, 0.

327

24%

MF

P

150-

1200

+

0.00

4 56

:45

200-

1210

+

0.00

18

41:4

5

11%

tem

per-

ture

, 33

%)

spee

d

1%

tem

pera

tu

re,

28

spee

d

Ret

aine

r cr

acke

d ac

ross

po

cket

. O

ther

su

rfac

es

in g

ood

cond

ition

.

Hig

her

torq

ue a

t lo

w t

em

pera

ture

. L

ower

tor

que

at

high

te

mpe

ratu

re.

Inne

r ra

ce

had

wav

e pa

tter

n. O

ther

par

ts i

n go

od s

hape

.

Hig

her

torq

ue a

t lo

w t

em

pera

ture

. L

ower

tor

que

at

high

te

mpe

ratu

re.

Inne

r ra

ce

had

heav

y w

ave

patt

ern.

All

othe

r pa

rts

in g

ood

cond

ition

.

α All

test

s in

thi

s ta

ble

wer

e ru

n on

out

er-r

ace

doub

le la

nd r

idin

g be

arin

g m

ade

of R

ene

41.

b The

se d

ata

wer

e ta

ken

from

Tab

le I

of

a re

port

on

Air

For

ce C

ontr

act A

F 33

(616

)658

9 pr

epar

ed b

y A

lvin

L. S

chlo

sser

of

the

Stra

tos

Div

isio

n of

the

Fai

rchi

ld S

trat

os C

orpo

ratio

n. T

he r

epor

t was

dat

ed A

ugus

t, 19

62.

TA

BL

E X

VII

I G

AS

-E

NT

RA

IN

ED

P

OW

DE

R

LU

BR

IC

AT

IO

N

WI

TH

M

IX

TU

RE

S O

F C

dO

AN

D

GR

AP

HI

TE

Obj

ecti

ve

Inte

rnal

pl

ay,

inch

es

Lu

bri

can

t

Lub

rica

nt

flow

, gm

/min

Out

er-r

ace

tem

per

atu

re,

°F

Cha

nge

in p

lay,

in

ches

Run

ning

ti

me,

ho

urs:

min

C

ycle

s R

emar

ks

25 h

ours

end

uran

ce,

0.0

02

9 12

00°F

, 30

,000

rpm

25 h

ours

end

uran

ce,

0.0

03

2 12

00°F

, 30

,000

rp

m,

N2

atm

os

pher

e to

de

term

ine

its

effe

ct

on

bu

ild

-up

25-h

our

test

to

con-

0

.00

25

firm

th

at c

ycli

ng

of t

empe

ratu

re

bet

wee

n 10

00°

and

1200

°F

prev

ents

d

epos

it b

uil

d-u

p

3:1

rati

o gr

aphi

te

to C

dO i

n

15:1

rat

io

grap

hite

to

CdO

5:1

rati

o gr

aphi

te t

o C

dO i

n ai

r

0.1

74

1200

-124

0 +

0.0

00

6 11

:24

0.2

62

1190

-122

0 +

0.0

17

11:0

0

0.1

54

950-

1210

-0

.000

6 3

0:0

0 8%

te

mp

era

ture

, 18

sp

eed

Bea

ring

sl

owed

du

e to

lu

bric

ant

bu

ild

up

. B

alls

w

orn

due

to

clog

ging

. H

ard

blac

k d

epos

it

in

race

way

s an

d on

ot

her

surf

aces

.

Aft

er

5 ho

urs,

sp

eed

was

al

low

ed t

o fa

ll to

m

ain

ta

in

con

stan

t to

rque

. B

eari

ng

badl

y w

orn

in

reta

iner

, ba

ll po

cket

s,

and

guid

e di

amet

ers.

B

alls

0.

007

to

0.01

2 in

un

ders

ize.

N

o d

epos

its

or b

uild

up.

Bea

ring

ran

ver

y w

ell.

No

wav

e p

atte

rn

on

inne

r ra

ce.

Out

er

race

h

ad

slig

ht

ecce

ntr

ic

wea

r.

Gen

eral

ly

par

ts

and

wea

r su

rfac

es

in

good

co

ndit

ion.

100

hour

s en

dura

nce,

0.

0029

5:

1 ra

tio

0.10

9 10

00°F

, 50

00-

grap

hite

30

,000

rpm

to

CdO

in

air

100

hour

s en

dura

nce,

0.

0024

5:

1 ra

tio

0.25

7 10

00°F

, 50

00-

grap

hite

30

,000

rpm

to

CdO

in

air

150-

1080

+

0.00

14

65:5

8

150-

1060

+

0.00

13

22:5

9

Ιδ1^

tem

pera

- Bea

ring

ra

n ve

ry

wel

l, at

ure,

45J

^ In

ner

race

had

a w

ave

spee

d pa

tter

n on

run

ning

tra

ck

and

a bu

rr o

n th

e th

rust

sh

ould

er.

Res

t in

go

od

cond

ition

.

6 te

mpe

ra-

Bea

ring

sta

rted

with

rel

a-tu

re,

17

tivel

y hi

gh t

orqu

e. I

n-sp

eed

ner

race

ha

d a

wav

e pa

tter

n an

d bu

rr

on

thru

st

shou

lder

w

here

ba

lls r

an.

Res

t in

goo

d co

nditi

on.

° A

ll te

sts

wer

e ru

n on

bea

ring

s m

ade

from

Ren

e 41

with

out

er-r

ace

ridi

ng l

ands

. In

the

fir

st,

seco

nd,

and

four

th t

ests

the

y w

ere

doub

le l

and

ridi

ng b

eari

ngs;

in t

he o

ther

tw

o th

ey w

ere

sing

le.

& The

se d

ata

wer

e ta

ken

from

Tab

le I

I of

a r

epor

t on

Air

For

ce C

ontr

act A

F33

(616

)658

9 pr

epar

ed b

y A

lvin

L. S

chlo

sser

of

the

Stra

tos

Div

isio

n of

the

Fai

rchi

ld S

trat

os C

orpo

ratio

n. T

he r

epor

t was

dat

ed A

ugus

t, 19

62.


search and development tools. The various types of tests have not been standardized to the extent of those used in evaluation of liquid lubricants, mainly because no technical organization or society has taken the lead in such work. The lack of standard methods of evaluation often creates problems in comparing the results of various programs. It should also be noted, however, that many of the solid lubricants, by virtue of their different characteristics or types, do not lend themselves to standardization of test methods for their evaluation.

B . W E A R A N D F R I C T I O N T E S T S

A wide variety of wear and friction tests has been developed to evaluate solid lubricants. A recent survey in which the author participated included over thirty specific versions on an initial screening. There are undoubtedly many more. This section will not attempt to list all these methods but will only discuss the more common ones found in general use. Table XIX lists some of these along with their geometry and operating variables.

One of the most common types of configuration used for solid lubricant wear testers has been the flat block or shoe bearing on a rotating ring, disk, or cup as shown in Fig. 10. The configuration shown here is

L O A D

T E S T B L O C K

T E S T R I N G -

F I G . 10. Geometry of a block on rotating ring-type lubricant tester.

of one test block. Other versions employ two diametrically opposed rub blocks. Both give initial line contact, with area contact resulting from load and wear. These testers are generally used on resin- or ceramic-bonded solid lubricants.

Another unit that has found wide use is the Falex lubricant tester shown in Fig. 11. It consists of a rotating pin held between two V blocks in a nutcracker type of loading system, as shown in Fig. 12. This test gives initial line contact on loading, with eventual area contact as wear takes place. It has been used mainly for resin-bonded materials at room temperature but has also been modified for use with wear

TA

BL

E X

IX

S

OM

E

BE

NC

H

WE

AR

T

ES

TE

RS

FO

R

SO

LI

D

LU

BR

IC

AN

TS

Nam

e G

eom

etry

T

empe

ratu

re, °

F L

oad

Ref

.

Hoh

man

A-6

T

wo

bloc

ks o

n ro

tatin

g di

sk

To

1600

V

aria

ble

to a

t le

ast

600

lb

(80)

Hoh

man

A-3

Sa

me

as A

-6

To

1000

Sa

me

as A

-6

(65)

Mid

wes

t R

esea

rch

Inst

itute

Mar

k V

-B

for

10~6 to

rr

Tw

o bl

ocks

on

rota

ting

disk

T

o 15

00

Var

iabl

e to

at

leas

t 60

0 lb

(1

15)

Alp

ha L

FW

-1

One

blo

ck o

n ro

tatin

g di

sk

Am

bien

t V

aria

ble

to 6

30 lb

Alp

ha L

FW-2

Sa

me

as L

FW

-1

Am

bien

t V

aria

ble

to 1

000

lb

Alp

ha L

FW-3

A

nnul

us o

n fla

t bl

ock

To

1200

V

aria

ble

to 5

000

lb

Fale

x lu

bric

ant

test

er

See

Fig.

12

Am

bien

t V

aria

ble

to 3

000

lb

Mac

Mill

an

One

blo

ck o

n ro

tatin

g di

sk

Am

bien

t V

aria

ble

to a

t le

ast

630

lb

(163

)

Four

bal

l O

ne r

otat

ing

ball

on t

hree

st

atio

nary

bal

ls

To

1000

V

aria

ble

to a

t le

ast

40 k

g (1

64)

Var

ious

NA

CA

- an

d N

ASA

-des

igne

d fr

ictio

n an

d w

ear

devi

ces

^iQ

-inch

. he

misp

heri

cal

ride

r on

rot

atin

g fla

t di

sk

Var

iabl

e C

ryog

enic

to

200

0

Var

iabl

e (2

0,

88,

110,

11

8)

Mid

wes

t R

esea

rch

Inst

itute

Mar

k V

-A

for

10~6 to

rr

Tw

o bl

ocks

on

rota

ting

disk

T

o 10

00

Var

iabl

e to

at

leas

t 60

0 lb

(1

15)


F I G . 11. Falex lubricant tester as made by the Faville-LeVally Corporation.

L O A D

F I G . 12. Geometry of the Falex lubricant tester wear area.

specimens submerged in liquid nitrogen (165). A similar configuration has been used at temperatures above 1000°F (166).

The NACA and NASA have employed a series of testers based on a hemispherical rider pressing against the flat side of a rotating disk. Most units consist of one hemisphere, but several have had three bullet-type specimens. The various units have been employed with almost all types of solid lubricant including resin-bonded, ceramic-bonded, powdered, reactive gases, and hard and soft metals. Temperatures have ranged from the cryogenic region to 2000°F. Complete descriptions of the individual units can be found in the various technical reports.

Other sliding testers have been used. Cosgrove and co-workers (164)

< LOAD


used a four-ball tester for 1000°F studies. Johnson and co-workers (167) describe the first in a series of pellet machines. These units consist of compressed disks of powdered lubricants rotating on a flat stationary surface. The disks are held so that the curved part bears on the flat. This gives initial line contact with subsequent area contact. Hood and Campbell (168) describe a 3-inch-diameter cup bearing on a 14-inch by 3-inch-diameter flat for loads to 100,000 psi and speeds to 1000 rpm. Sonntag (169) describes a press-fit test.

In the area of bearing tests many units have been employed. For the most part, this type of tester is thought to be better in predicting the performance of the lubricant. Table XX lists some of these bearing

T A B L E X X

F U L L - S C A L E B E A R I N G T E S T S U S E D F O R E V A L U A T I O N O F S O L I D L U B R I C A N T S

Type bearing Type solid lubricant Temperature, °F Ref.

204 ball Ceramic-bonded To 750 (76) 204 ball Gas-entrained powders To 1200 (159) 206 ball Vapor-deposited To 900 (135) 209 ball Vapor-deposited To 900 (135) Instrument-sized ball Soft metal To 600 (143) 204 ball Gas-entrained powders To 1000 (157) 75-mm bore roller Gas-entrained powders To 1000 (157) Plain spherical Organic powders To 1200 (155) Plain spherical Ceramic-bonded To 750 (170) Spherical P . T . F . E — f a b r i c Ambient (171) Plain T . F . E . 550 (172) Cylindrical P . T . F . E . Ambient (92)

testers and references to their use. Standardization of these test methods has been lacking, even where

several laboratories have the same unit. One program, undertaken by the Coordinating Research Council, involved the Falex machine and a series of block-on-rotating shoe testers. This program showed promise, but additional work is required as the field of solid lubricant wear and friction evaluation grows with interest in this type of material.

C. C H E M I C A L T E S T S

Most chemical evaluation tests for solid lubricants are special procedures employed in the laboratory for studying various chemical properties. The properties of interest include chemical reactivity with the environment or bearing material, oxidative effects, and decomposition products. Since most of these tests are nonstandard in nature, they will not be discussed any further.


Some tests developed for resin-bonded solid lubricants can be considered standard by virtue of their use in specifications (75, 162). Some of these tests were presented in Table IV. Others may be found in the two specifications. In general, they are designed as empirical tests to evaluate or compare the properties of the solid lubricants, including resistance to corrosion, high temperature, low temperature, and peeling or cracking due to loss of adhesion. They are used more to establish quality control or acceptance than to determine absolute values.

D . O T H E R T E S T S

Various physical and mechanical properties such as tensile strength, hardness, and coefficient of expansion are measured by test methods common to other areas and will not be discussed here. One point worth noting is that many of these properties are required at elevated temperatures. Therefore some modifications are required in test methods to be used at temperatures above a few hundred degrees Fahrenheit.

VIII. Environmental Effects


Solid lubricants are generally selected because of their resistance to various environmental conditions or because they offer some other advantage over conventional lubricants. These other advantages include lubrication for the life of the component, lubrication without generation of dirt, essentially dry lubrication, and lubrication of components extremely difficult to reach. This section will be concerned mainly with the effects of various environmental conditions on solid lubricants rather than their use in specific wearing configurations. The section on applications in industry will discuss specific uses requiring solid lubricants.

The reader is cautioned in the use of the data presented in this section. Various environmental effects can have strong interactions, and therefore specific performance may vary from device to device for the same lubricant. Another complicating factor is that many of the data are based on observations in laboratory bench or simulative-type equipment, and such values do not correlate directly those derived from operational experience. Therefore, the data should be considered mainly as guides, and specific evaluation should be made in equipment known to correlate with the specific application. Resin-bonded solid lubricants, for example, have been criticized by some workers who attempted to use them for given applications on the basis of broad claims resulting from performance in bench tests. It is felt that this type of lubricant as well as any other requires careful consideration of its proper application.


Β . T E M P E R A T U R E E F F E C T S

The resistance of various solid lubricants to extremes of temperature has been the major stimulant in the development of these materials. Various types of solid lubricants have performed at temperatures in the cryogenic region and at temperatures to 2000°F or higher. Conventional petroleum-based fluids have upper temperature limits of 200° to 300°F. Advanced petroleum materials as well as the new synthetic fluids have an upper operating temperature of around 600° to 700°F. On the other end of the scale, only a few liquids are operational below —100°F. These limitations have led to greater interest in solid lubricants.

The resin-bonded solid lubricants, as we have seen, are generally limited to temperatures around 400° to 550°F. Kingsbury (163) has reported satisfactory operation at temperatures as high as 1200° F for resin-bonded materials, but only for a limited time. In equipment undergoing such high temperatures, the resin-bonded materials should be considered only if they are not expected to perform for more than a very short period of time. Good operating characteristics are limited to low temperatures, around —100° to —150°F. It had been thought that the resin-bonded materials would work in the cryogenic region, but data of McConnell and Merrill (165) show greatly reduced behavior (of the order of 90% loss in wear life) in a series of commercial films submerged in liquid nitrogen. The authors attributed this to loss of adhesion of the films, but is also could have been adverse effects on the adsorbed gases serving as colubricants, as we have seen for graphite at high altitudes.

The lubricating plastics all have relatively low temperature limits; even the impregnated materials are not much good at temperatures above 500° F. On the low-temperature end they offer considerable promise in the cryogenic region. Wisander and co-workers (88, 173-175) show the good performance of polytetrafluoroethylene and P.T.F.E.-based materials as well as some other plastics in the cryogenic region. Tanza also shows the use of plastic retainers in bearings for operation in that region (176).

The powdered solids, both by themselves and as gas-entrained materials, have a broad temperature spectrum. Their performance depends greatly on the environmental atmosphere as well as on the residence time at temperature. MoS2 is limited to 750°F in oxidizing atmospheres, for example, but can be used to 1200°F or higher when carried in a nitrogen gas which inhibits its oxidation. Other solids show similar changes in performance depending on the environment. It can generally be concluded that solid powders will perform at extremely low temperatures


to extremely high temperatures provided the chemical constituents that provide good lubrication are maintained in their proper state.

Soft metals are limited by their softening temperatures. Some, such as lead (melting point 327°C), will not operate at as high a temperature as others, such as gold (melting point 1063°C). These materials will not operate to their melting point but do perform in some cases at rather elevated temperatures. Barium, for example, has been reported (143) to work at 600°C. The noble metals also have the added advantage of being rather insensitive to environmental compositions. On the low-temperature side, some metals still perform satisfactorily from a wear aspect (177) at cryogenic temperatures.

The ceramic-bonded films are designed mainly for operation at 1000°F to 2000°F. Most experience is at the 1000° to 1200°F level. At temperatures below 700°F, the films show reduced performance and therefore cannot be considered as low-temperature lubricants. There are a few exceptions to this, such as the ceramic-bonded low-temperature pigments reported by De vine and co-workers (114) for use at —300° to +750°F. It is felt that ceramic-bonded films will work at the low-temperature region if properly designed with suitable materials and other conditions. Additional work is required.

The temperature limitations of several classes of solid lubricants have not been established. These include the reactive gases, hard metals, and nonmetals. Operation at greatly elevated temperatures (1000° to 2000° F ) has been demonstrated. Their performance is a function of the environment and chemical reactivity. They must be investigated first in an application before limits can be set.

C. A T M O S P H E R E

In the use of solid lubricants, the atmosphere in which they are expected to function must be considered. In addition to normal air environments, these lubricants may encounter high vacuums, reactive gases, inert gases, and other special conditions. Many solid lubricants will not provide lubrication under all such atmospheres. Others provide optimum lubrication under one particular atmosphere. The behavior of graphite, for example, relies very much on the composition of the atmosphere.

The effects of an oxidizing atmosphere can be directly related to the temperature in most cases. Except under cryogenic conditions or as noted, most of the temperature limits discussed in the preceding section were for oxidizing conditions. Although it is true in most cases, solid lubricants do not always provide poorer performance with increased temperature in the presence of oxygen. Two cases which stand out are


the PbO-Pb 30 4 example discussed in Section V.B and the beneficial effect of oxygen reported (178) for the performance of graphite at around 1000°F.

Vacuum environments encountered in outer space have stimulated considerable interest in solid lubricants. This can best be seen in a recent survey on aerospace lubrication (179), in which 62 of 91 organizations expressed interest or had done work on solid lubricants in relation to such environments. Conventional liquids and greases are not suitable for wearing components directly exposed to the vacuum of space because of the volatility problem. They can, however, be used in the closed portions of a vehicle. Solid lubricants offer one of the most promising techniques of lubrication for such systems. Very few data have been published, however, at the lower (10~6 torr or better) pressures.

Some soft metals have given acceptable results at pressures of 10~6

torr. The inherent lubricity of MoS2 at 10~9 torr have been reported (46). Polytetrafluoroethylene and P.T.F.E.-filled materials have been reported (12) to be good in vacuum. We have already seen the improved performance of a PbS-MoS2 system bonded with B 2 0 3 at 500°F (Table XIV). On the adverse side, graphite does not behave well in vacuum, owing to the absence of the adsorbed water layer. Buckley and coworkers (180) present further data on soft metals and certain solids to show improved performance in vacuum. Adamczak and co-workers (181) discuss lubrication in space and relate solid lubricants to other types of lubricating systems.

Various solid lubricants offer potential for lubrication in other atmospheres. The fluorocarbon resins as well as other plastics are chemically resistant to many reactive gases and therefore could be used under conditions not particularly suitable for other materials. Some reactive atmospheres might even be used in such techniques as the reactive or vapor phase form of solid lubrication. Here the proper choice of metals is critical. An example is the use of molybdenum metal for H2S environments, as has already been discussed.

Another consideration is the use of solid lubricants where environments of a vapor from a liquid or grease cannot be tolerated. In this application the solid lubricants are used to maintain a clean atmosphere.

D . R A D I A T I O N

Some work has been carried out on the effect of nuclear radiation on solid lubricant performance. The majority of this work has been conducted on the resin-bonded materials, although powders and plastics


have also been studied. In general, the solid lubricants are stable to much higher dosages than the organic materials that make up most other lubricants.

The results of several static and dynamic radiation programs have been summarized by Merrill (182) and by Rice and co-workers (10). They report the resin-bonded materials to be stable to dosages of about 109 to 10 12 ergs/gm C. The main weak point appears to be the binders. This compares well with data on pure phenolic resins which break down at about 10 12 ergs/gm C, and epoxy materials which degrade at 10 11

ergs/gm C. One of the programs included in these summaries was carried out by Lavik (67) on resin materials. Daniel (183) shows some effect of gamma irradiation on the performance of a resin-bonded material in an oscillating spherical bearing at 0.42 χ 10 10 ergs/gm C but attributes it to variation in wear life. The actual effect is of the order of magnitude of a one-third loss in life.

Daniel (183) also reports data for ceramic-bonded solid lubricants in an oscillatory spherical bearing tester. He found little effect in lubrication behavior of these materials in the region of 0.26 Χ 10 1 0 ergs/ gm C.

Pinchbeck (106) presents data on several plastics. He gives values in the region of 109 rads (about 10 1 1 ergs/gm C) for several thermosetting resins, and 10s rads (about 10 10 ergs/gm C) for nylon, P.T.F.E., and polyethylene.

Other solid lubricants such as the soft metals, hard metals, and ceramics would be expected to have better radiation resistance and to serve satisfactorily in radiation environments. No data on these materials have been presented in the literature other than on nonlubricative properties.

E. L O A D A N D O T H E R E F F E C T S

Several other effects that are not solely part of the surrounding environment include, for example, load, speed, and the effects of the mechanical design and operation of the equipment being lubricated.

The plastic solid lubricants generally perform best under lightly loaded conditions when compared to most other lubricants. We have seen the benefit achieved by additions of fillers. Even under such conditions the loading cannot be excessive. The plastic materials are best suited for extremely long service under the proper load and speed conditions.

Resin-bonded solid lubricants have the capability of supporting high loads, to 100,000 psi or higher. In general, such applications must be under slow-speed operation. These materials are also employed in condi-


tions where dirt contamination is expected because they are less affected than conventional lubricants. High-speed sliding with these materials as well as with ceramic-bonded materials must be carried out under lighter loads. Until recently, bonded solid lubricants were not considered suitable for rolling element bearings, but Devine and co-workers (76) have shown good performance with ceramic-bonded materials and thus have opened the door for further study of both bonded types.

Solid powders, such as MoS2, WS2, and graphite, have been shown (13) to give low friction with loads even up to 400,000 psi.

IX. Current and Future Applications in Industry


Solid lubricants are used in many industries, either on a wide scale or in limited applications. The applications range from lubrication of precise control mechanisms of an aircraft to the simple prevention of galling and seizing in a nut-and-bolt combination. The preceding section has considered the resistance of solid lubricants to various environments. This section will briefly discuss some of the various applications in the different industries in order to demonstrate the range of use of these materials. Not all applications will be reviewed, nor will any be covered in detail. As one reviews the various references for this section, it becomes apparent that solid lubricants have been used mostly in sliding applications. This has been previously discussed.

Several well-written articles have appeared on the use of various solid lubricants in industry. Rowe (184) presents a brief summary of solid lubricants and also cites various current and potential applications. He quotes temperature ranges from —200°C to +2000°C. Jost (185, 186) cites applications and case histories for MoS2-lubricated components including plain bearings, press fits, and threaded connectors. Some of the applications involve MoS2 in a liquid or paste carrier and as such cannot be considered as truly solid lubricants. DiSapio (187) and Crump (188) present informative articles on the proper use of bonded solid lubricants. In addition to the coating techniques they discuss the types of materials suitable for specific machine designs and operating conditions. Vineall discusses (189) the use of MoS2 materials in foundries as well as in other industries. Pinchbeck (106) discusses plastic solid lubricants as bearing materials.

B . M E T A L - P R O C E S S I N G I N D U S T R Y

Solid lubricants have found wide acceptance in various phases of the metal industry. In addition to true solid lubricants there has been wide


use of liquid- or paste-carried solid pigments such as MoS2. As we have seen, one of the early uses of a resin-bonded film in this industry was for lubricating dies, as disclosed by Norman (70).

Die lubricating practices have made wide use of solid lubricants. Liquid carriers are often involved for ease of application, but the solid provides the lubrication. Lang (190) and Hoagland (191) both report doubled life for graphite-lubricated forging dies.

Jost in a series of articles discusses the use of MoS2 in the iron and steel industry (192, 193) and in the sheet metal industry (194). Among the applications are bearings, conveyors, gears, worm gears, and threaded connectors. He gives several case histories in each article. In the one on sheet metal process lubrication (194) he describes MoS2 by itself and as a component in sintered-metal bearings. Cases are cited for lubrication of components in operations involving forming, drawing, and a wide variety of other applications.

Duwell and McDonald (195) present data on the use of reactive gases to reduce cutting forces in grinding processes. Although this is not a true lubrication process, it is quite analogous to vapor-phase solid lubricants. Jost and Winch (196) discuss the role of MoS2 in cutting and forming operations.

Sabroff and Frost (197) report the use of several coatings of graphite, BN, and MoS2 with a fluoride phosphate coating on cold extrusion of titanium. Another article (198) cites various forms for MoS2 and how they can be employed in given applications. Lubrication of high-temperature chain sprockets is one use.

Crewe and Crum (199) show reduction in the required power for a wire-drawing process when a mixture of MoS2 and calcium soap was used. Smigel and Miller (200) also show better performance with MoS2

and give data showing savings in power, increased drawing speeds, greater wire reductions, and greater die life. Magie (201) summarizes the use of MoS2 in the wire industry and states that 70% of the mills in the United States use this material in their plants.

Other uses of solid lubricants in the metal-processing industry include ceramic glasses as die lubricants and various materials for lubrication of equipment in dusty environments such as mines and mills.

C. M A R I N E I N D U S T R Y

The use of solid lubricants in naval equipment has been quite extensive, including lubrication of ship components and other equipment associated with the operation of civilian and military vessels.

Richards (202) discusses the role of graphite and MoS2 in marine applications. He concentrates on their use both as break-in materials for


steam plant components and for several full-time diesel engine components of a sliding nature.

FitzSimmons and Zisman (203) discuss the use of P.T.F.E. coatings by the U. S. Navy, including applications in submarines. Advantages were found over conventional lubricants for many pieces of equipment, due to resistance to the atmosphere, cleanliness, low maintenance, long life, and low friction and wear. Other areas associated with naval operation are discussed, such as coating of weapons and ammunition.

Devine and co-workers (76) discuss application of ceramic-bonded materials in ball screw jacks and liquid oxygen pumps. These applications are not necessarily associated with ships but may involve other naval uses.

D. A E R O S P A C E I N D U S T R Y

Solid lubricants of all types are used in the aerospace industry. Applications involve military and civilian uses in aircraft, missiles, and space vehicles. Much of the interest in their use has been stimulated by the extremes of environmental conditions and the lack of other materials.

Hood and Campbell (168) report about 150 to 200 uses in the Boeing 707 and related military aircraft, mainly for the resin-bonded solid lubricants. One example given is that of a thrust reverser pneumatic cylinder. Williams (172) and Craig (171) discuss the use of plastics for aircraft bearings. Williams (172) and Weisman (204) also present data on the use of resin-bonded films. Hegarty (205) describes the use of solid lubricant glasses in forming of metal parts from titanium for aircraft use.

Other aircraft applications include sliding hatches, screw actuators, pneumatic pistons, valves, rotating seals, and antiseize compounds for nuts and bolts.

In the area of space and missile vehicle lubrication the majority of the published work has been concerned with development studies. Macks (206) made an over-all survey including extensive solid lubricant work. Tanza (176) discussed plastics as self-lubricating bearing retainers for use in the region of 40°R to 560°R. Sliney (207) discussed ball-bearing operation to 1250°F. In related work Witherly (208) described the use of MoS2 and several other inorganic materials for lubricating high-speed sled tracts.

E. M I S C E L L A N E O U S I N D U S T R I A L U S E S

Fracalossi (209) describes the use of MoS2 in various railroad applications. Wheel wear on diesel locomotives was reduced greatly by providing continuous application to the wheel flanges. Other applications


involved expansion plates on bridges, switching gear, and nuts and bolts. Another article (210) discusses a 35% increase in curve rail life with application of MoS2.

Cosgrove and Jentgen (211) discuss plastic lubricants for automotive applications, including steering linkage bearings and ball joints. Potter and co-workers (212) also discuss plastics in conjunction with other lubricants for automobile applications. Use of various solid lubricants in the automotive industry is dictated by considerations of cost and life as well as convenience to the user. In this general area, Simon (213) discusses seven cases of improved results with MoS2 in the production and use of tractors.

Maries (214) discusses the use of bonded films in lubrication of antifriction bearings for nuclear applications where other materials cannot take the temperature and radiation environments.

The use of MoS2 is described (215) for the unusual case of lubrication of sliders for moving the 4000-ton Mangfall Bridge in Bavaria from one set of piers to another. Coefficients of friction of 0.08 to 0.13 were reported.

Use of P.T.F.E. for self-lubricating compression rings in radial gas engines is described by Sawyer and Pauli (216). An extremely large power generation system was involved in this application, and considerable savings of oil and maintenance time resulted.

Even the petroleum industry is bowing to use of solid lubricants. Among the applications involved is their use in valves (217) and in other components requiring resistance to fluids.

Many other applications, too extensive to discuss, include shaft seal lubrication, self-lubricating high-speed seals, dry pump bearings, journals, slides for doors, and plastic gears, and wide use for prevention of fretting corrosion (218, 219). This section has reviewed only a small portion of the literature. For possible applications in the various other areas, the reader is left to pursue the subject on his own.

A C K N O W L E D G M E N T S

I wish to acknowledge the valuable assistance and comments of Messrs. Vernice Hopkins, Karl Mecklenburg, Melvin Lavik, Bobby McConnell, and Dr. Tung Liu. The assistance of Miss Joyce Smith in the final typing and editing is also appreciated.

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96. D. C. Mitchell and G. Pratt, Friction, wear and physical properties of some filled P.T.F.E. bearing materials. Proc. Conf. Lubrication Wear, London, 1957 pp. 416-423 (1957) .

97. S. B. Twiss, P. H. Wilson, and E. J. Sydor, Friction of polytetrafluoroethylene dry bearings. Lubrication Eng. 14, 255-261, 273 (1958) .

98. R. C. Barry, Reinforced TFE and moly disulfide, Prod. Eng. 32, No. 12, 76-77 (1961) .

99. D. C. Mitchell, The wear of P.T.F.E. impregnated metal bearing materials. Proc. Conf. Lubrication Wear, London, 1957 pp. 396-404 (1957) .

100. B. R. Atkins and D. P. Griffiths, Electrical sliding contacts and their behaviour at high altitudes. Proc. Conf. Lubrication Wear, London, 1957 pp. 371-375 (1957) .

101. R. C. Bowers, W. C. Clinton, and W. A. Zisman, The friction and lubrication of nylon. Naval Research Laboratory Report 4389 (July, 1954).

102. G. S. Hudson and L. H. Gillespie, Design data for bearings of nylon, acetal, and TFE-fluorocarbon resins. 1959 Fall Convention of the Association of Iron and Steel Engineers.

103. Pauline Long, Self lubricating bearings. Eng. Mater. Design 3, 626-632 (1960) . 104. P. D. Mitchell, MoS2 enhances wear resistance of nylon. Eng. Mater. Design

4, 502-506 (1961) . 105. Thomas E. Powers, Molybdenum disulfide in nylon for wear resistance. Mod.

Plastics 37, No. 10, 148, 150, 153-154 (1960) . 106. P. H. Pinchbeck, A review of plastic bearings. Wear 5, 85-113 (1962) . 107. P. D. Mitchell, Sintered nylon. Eng. Mater. Design 5, 184-185 (1962) . 108. L. L. Stott and L. R. Hervey, Pressed and sintered nylon powder parts. Mater.

Methods 40, 108 (1952) . 109. V. G. FitzSimmons and C. M. Henderson, Resin bonded Teflon coatings as

dry film lubricants. Naval Research Laboratory Report 777 (February, 1958). 110. Harold E. Sliney and Robert L. Johnson, Bonded lead monoxide films as

solid lubricants for temperature up to 1250°F. National Advisory Committee for Aeronautics Research Memorandum E57B15 (May 7, 1957).

111. Μ. T. Lavik, Ceramic bonded solid-film lubricants. Wright Air Development Division Technical Report 60-530 (September, 1960).

112. Harold E. Sliney, Lubricating properties of some bonded fluoride and oxide coatings for temperatures to 1500°F. National Aeronautics and Space Administration Technical Note D-478 (1960) .

113. Ernest M. Levin, Howard F. McMurdie and F. P. Hall, "Phase Diagrams for Ceramists." The American Ceramic Society, Columbus, Ohio, 1956.

114. M. J. Devine, E. R. Lamson, and J. H. Bowen, Jr., Inorganic solid film lubricants. / . Chem. Eng. Data 6, No. 1, 79-82 (1961) .

115. Μ. T. Lavik, Ceramic bonded solid-film lubricants. Wright Air Development Division Technical Report 60-530 Part II (April, 1961).

116. R. L. Johnson and Η. E. Sliney, High-temperature friction and wear properties of bonded solid lubricant films containing lead monoxide. Lubrication Eng. 15, 487, 491, 496 (1959); Annual Meeting American Society of Lubrication Engineers, Buffalo, New York, April, 1959.

117. Melvin T. Lavik, Bruce Daniel, and Thomas M. Medved, Physical and chemical properties of ceramic bonded solid lubricant films. Wright Air Development Division Technical Report 60-530 Part III (February, 1962).


118. Harold E. Sliney, Lubricating properties of ceramic-bonded calcium fluoride coatings on nickel-base alloys from 75°F to 1900°F. National Aeronautics and Space Administration Technical Note D-1190 (1962) .

119. Marshall B. Peterson and Robert L. Johnson, Friction studies of graphite and mixtures of graphite with several metallic oxides and salts at temperatures to 1000°F. National Advisory Committee for Aeronautics Technical Note 3657 (1956) .

120. Μ. B. Peterson and R. L. Johnson, Solid lubricants for temperatures to 1000°F. Lubrication Eng. 13, 203-207 (1957) .

121. R. L. Johnson and Η. E. Sliney, Ceramic surface films for lubrication at temperatures to 2000° F. Am. Ceram. Soc. Bull. 41, 504-508 (1962) .

122. Harold E. Sliney, Lubricating properties of lead-monoxide-base coatings of various compositions at temperatures to 1250°F. National Aeronautics and Space Administration Memorandum 3-2-59E (1959) .

123. H. R. Thornton, Doris M. Krumwiede, J . F. Benzel, R. J. Forlano, and Dweight G. Bennett, Solid film lubricant-binder phenomena: PbS B 20 3 system. Aeronautical Systems Division TDR 62-449 (May 1962).

124. Ralph E. Crump, High temperature solid film lubricants for uses above 1000°F. SAE Annual Meeting, Detroit, Michigan, January, 1959.

125. S. F. Murray, Robert L. Johnson, and Max A. Swikert, Boundary lubrication of steel with fluorine- and chlorine-substituted methane and ethane gases. National Advisory Committee for Aeronautics Technical Note 3402 (1955) .

126. Gordon P. Allen, Donald H. Buckley, and Robert L. Johnson, Friction and wear with reactive gases at temperatures up to 1200°F, National Advisory Committee for Aeronautics Technical Note 4316 (1958) .

127. Donald H. Buckley and Robert L. Johnson, Friction and wear of corrosion resistant metals lubricated by reactive gases at temperatures to 1200°F. ACS Symposium on Chemistry of Friction and Wear, Chicago, Illinois, (1958) .

128. Donald H. Buckley and Robert L. Johnson, Halogen-containing gases as boundary lubricants for corrosion-resistant alloys at 1200°F. National Aeronautics and Space Administration Memorandum 2-25-59E (1959) .

129. Donald H. Buckley and Robert L. Johnson, Lubrication of corrosion-resistant alloys by mixtures of halogen-containing gases at temperatures up to 1200°F. National Aeronautics and Space Administration Technical Note D-197 (1959) .

130. Donald H. Buckley and Robert L. Johnson, Use of less reactive materials and more stable gases to reduce corrosive wear when lubricating with halogenated gases. National Aeronautics and Space Administration Technical Note D-302 (1960) .

131. Donald H. Buckley and Robert L. Johnson, Halogen-containing gases as lubricants for crystallized glass-ceramic-metal combinations at temperatures to 1500°F. National Aeronautics and Space Administration Technical Note D-295 (1960) .

132. S. S. Sorem and A. G. Cattaneo, High-temperature bearing operation in the absence of liquid lubricants. Lubrication Eng. 12, 258-260 (1955) .

133. Charles H. Bailey and Stanley S. Sorem, Research in high temperature bearing lubrication in the absence of liquid lubricants. Wright Air Development Center Technical Report 56-370 (May, 1956).

134. Charles H. Bailey and Stanley S. Sorem, Research in high temperature bearing lubrication in the absence of liquid lubricants. Wright Air Development Center Technical Report 56-370 Part II (October, 1956).

135. R. A. Coit, S. S. Sorem, R. L. Armstrong, and C. A. Converse, Research in


high temperature bearing lubrication in the absence of liquid lubricants. Wright Air Development Center Technical Report 56-370, Part III (December, 1957).

136. W. E. Campbell and R. E. Lee, Jr., Polymer formation on sliding metals in air saturated with organic vapors. ASLE/ASME Lubrication Conference, Chicago, Illinois, October, 1961.

137. F. P. Bowden and G. W. Rowe, Lubrication with molybdenum disulphide formed from the gas phase. Engineer 204, 667 (1957) .

138. G. W. Rowe, Vapour lubrication and the friction of clean surfaces. Proc. Conf. Lubrication Wear, London, 1957 pp. 333-338 (1957) .

139. F. P. Bowden, Recent experimental studies of solid friction. In "Friction and Wear" (Robert Davies, ed.), pp. 84-109. Elsevier, New York, 1959.

140. D. J . Baldwin and G. W. Rowe, Lubrication at high temperatures with vapor deposited surface coatings. / . Basic Eng. 83, 133-138 (1961) .

141. Samuel Hoyt (ed.), "ASME Handbook of Metals Properties" McGraw-Hill, New York, 1954.

142. Μ. B. Peterson, Investigation of solid film lubricants and sliding contacts at temperatures above 1000°F. USAF-AEC Report APEX-569 (1960) .

143. Z. J . Atlee, J . T. Wilson, and J. C. Filmer, Lubrication in vacuum by vaporized thin metallic films. / . Appl. Phys. 11, 611-615 (1940) .

144. Harold E. Evans and Thomas W. Flatley, Bearings for vacuum operation— Phase I. Society of Aerospace Material and Process Engineers; Symposium on The Effects of Space Environment on Materials, St. Louis, Missouri, May, 1962.

145. Μ. B. Peterson, S. F. Murray, and J. J . Florek, Consideration of lubricants for temperatures above 1000°F. ASLE Trans. 2, 225-234 (1960) .

146. Μ. B. Peterson, J. J. Florek, and R. E. Lee, Sliding characteristics of metals at high temperature. ASLE Trans. 3, 101-109 ( I 9 6 0 ) .

147. E. Haffner, P. Lewis, Μ. B. Peterson, and A. A. Schwartz, High temperature lubrication study. USAF-AEC Report APEX-672 (1961) .

148. Loring Coes, Jr., Chemistry of abrasive action. Ind. Eng. Chem. 47, 2493-2494 (1955) .

149. L. B. Sibley and C. M. Allen, Friction and wear behavior of refractory materials at high sliding velocities and temperatures. ASME Lubrication Symposium, Miami, Florida, May, 1961.

150. E. Rabinowicz and M. Imai, Boric oxide as a high-temperature lubricant. ASME Lubrication Symposium, Miami, Florida, May, 1961.

151. R. D. Brown, R. A. Burton, and P. M. Ku, Friction and wear characteristics of cermets at high temperature and high vacuum. ASLE/ASME Lubrication Conference, Chicago, Illinois, October, 1961.

152. Donald H. Buckley and Robert L. Johnson, The influence of silicon additions on friction and wear of nickel alloys at temperatures to 1000°F. ASLE Trans. 3, 93-100 (1960) .

153. F. K. Orcutt, Η. H. Krause, and C. M. Allen, The use of free-energy relationships in the selection of lubricants for high-temperature applications. ASME Lubrication Symposium, Miami, Florida, May, 1961.

154. Η. H. Krause, S. L. Cosgrove, and C. M. Allen, Phthalocyanines promise good 1000°F lubricants. Space/Aeronautics 34, 161-165 (October, 1960).

155. Η. H. Krause, S. L. Cosgrove, and C. M. Allen, Phthalocyanines as high-temperature lubricants. / . Chem. Eng. Data 6, No. 1, 112-118 (1961) .

156. A. J . Haltner and C. S. Oliver, Frictional properties of some solid lubricant films under high load. / . Chem. Eng. Data 6, No. 1, 128-130 (1961) .


157. E. F. Macks, Ζ. N. Nemeth, and W. J . Anderson, Preliminary investigation of molybdenum disulfide—Air mist lubrication for roller bearings operating to DN values of 1 X l(f and ball bearings operating to temperatures of 1000°F. National Advisory Committee for Aeronautics Research Memorandum E51G31 (1951) .

158. Stanley Gray, An accessory manufacturer's approach to bearing and seal development. AS LE/AS ME Lubrication Conference, Los Angeles, California, October, 1958.

159. Alvin L. Schlosser, The development of lubricants for high speed rolling contact bearings operating over the range of room temperature to 1200°F. Wright Air Development Division Technical Report 60-732, Part II (August, 1962).

160. Thomas L. Carter, Effect of temperature on rolling-contact fatigue life with liquid and dry powder lubricants. National Advisory Committee for Aeronautics Technical Note 4163 (1958) .

161. F. D. Waterfall, Reducing scuffing and wear of ferrous metals—Surface treatment by Sulfinuz process. Engineering p. 116 (January 23, 1959).

162. Specification MIL-L-22273 ( W E P ) , Lubricant, Solid Film, Dry (December 3, 1959).

163. E. P. Kingsbury, Solid film lubrication at high temperature. ASLE Trans. 1, 121-123 (1958) .

164. S. L. Cosgrove, L. B. Sibley, and C. M. Allen, Evaluation of dry powdered lubricants at 1000° F in a modified four-ball wear machine. ASLE Trans. 2, 217-224 (1959) .

165. Bobby D. McConnell and Charles F. Merrill, Investigation of wear of solid film lubricants in liquid nitrogen. Wright Air Development Division Technical Report 61-254 (April, 1961).

166. Ε. N. Klemgard, Fundamental processes in lubricating metal surfaces at 100°F to 1700°F. Lubrication Eng. 16, 468-476 (1960) .

167. Virgil R. Johnson, George W. Vaughn, and Melvin T. Lavik, Apparatus for friction studies at high vacuum. Rev. Sei. Instr. 27, 611-613 (1956) .

168. J . H. Hood and Μ. E. Campbell, Boeing experience with bonded solid film dry lubricants. Air Force-Navy-Industry Lubricants Conference, Dayton, Ohio, 1959.

169. A. Sonntag, Solid lubricants for extreme pressures. Prod. Eng. 30, No. 5, 64-66 (1959) .

170. Vernice Hopkins, Andrew St John, and Donnell Wilson, Lubrication behavior and chemical degradation characteristics of experimental high temperature fluids and lubricants. Wright Air Development Division Technical Report 60 -855, Part II (1962) .

171. W. D. Craig, Jr., Predicting spherical bearing life in airplane control system. Lubrication Eng. 18, 25-29 (1962) .

172. F. J . Wilhams, High temperature airframe bearings and lubricants. Lubrication Eng. 18, 30-38 (1962) .

173. D. W. Wisander and R. L. Johnson, A solid film lubricant composition for use at sliding velocities in liquid nitrogen. ASLE Trans. 3, 225-331 (1960) .

174. D. W. Wisander and R. L. Johnson, A solid film lubricant composition for use at high sliding velocities in liquid nitrogen. 1960 Annual Meeting ASLE, Cincinnati, Ohio, April, 1960.

175. D. W. Wisander, W. F. Hady, and R. L. Johnson, Friction studies of various materials in liquid nitrogen. Advan. Cryog. Eng. 3 , 390-406 (1960) .


176. G. F. Tanza, Development program for a non-lubricated 10,000 rpm bearing operating over a temperature range from 40°R to 560°R. Advan. Cryog. Eng. 2, 145-155 (1960) .

177. J . A. Russell, R. A. Burton, and P. M. Ku, Research on lubrication behavior under extreme low temperatures. Wright Air Development Division Technical Report 61-161, Part II (November, 1961).

178. Ε. E. Bisson, R. L. Johnson, and W. J . Anderson, On friction and lubrication at temperatures to 1000°F with particular reference to graphite. ASLE/ASME Lubrication Conference, Toronto, Ontario, October, 1957.

179. Aerospace Lubrication Survey, Office of The Director of Defense Research and Engineering, 1961.

180. D. H. Buckley, M. Swikert, and R. L. Johnson, Friction, wear, and evaporation rates of various materials in vacuum to 10"7 mm Hg. ASLE/ASME Lubrication Conference, Chicago, Illinois, October, 1961.

181. R. L. Adamczak, R. J. Benzing, and H. Schwenker, Lubrication in space environments. Society of Aerospace Material and Process Engineers' Symposium on The Effects of Space Environment on Materials, St. Louis, Missouri, May, 1962.

182. C. F. Merrill, Air Force dry film programs and requirements. Air Force-Navy-Industry Lubricants Conference, Dayton, Ohio, 1959.

183. Bruce Daniel, Solid film lubricants for high temperature nuclear environments. Wright Air Development Division Technical Report 60-823 (September, 1961).

184. G. W. Rowe, Solid lubricants. Research (London) 14, No. 4, 137-142 (1961) . 185. H. Peter Jost, New industrial applications of molybdenum disulphide. Mech.

World 135, 489-491 (1955) . 186. H. Peter Jost, Molybdenum disulphide a dry lubricant possessing remarkable

properties. Alloy Metals Rev. 9 , No. 88, 2-8 (1958) . 187. Alfred DiSapio, For high pressures, low velocities, bonded coatings lubricate

metal parts. Prod. Eng. 31, No. 36, 48-53 (1960) . 188. Ralph E. Crump, Where to use dry film lube. Plant Eng. 12, No. 12, 90-91

(1958) . 189. G. J. C. Vineall, Molybdenized lubricants and their uses in foundaries. Iron

Steel (London). 100-103 (March, 1962). 190. Walter E. Lang, Forging die lubrication. Metal Progr. 58, 337-339 (1950) . 191. C. R. Hoagland, How Harvester makes forging dies. Am. Machinist 9 2 , No.

25, 78-82 (1948) . 192. H. P. Jost, Applications of molybdenum disulphide in the iron and steel

industry. Iron Coal Trades Rev. 173 , 1063-1075 (1956) . 193. H. Peter Jost, Molybdenum disulphide in the iron and steel industry. Iron

Steel (London) 34 , No. 1, 24-28 (1961) . 194. H. Peter Jost, Pure molybdenum disulphide it's properties and uses in the

sheet metal industry. Sheet Metal Ind. 3 3 , 679^90 (1956) . 195. Ε. J . Duwell and W. J. McDonald, The effect of reactive gases on the dry

grinding of steel with aluminum oxide coated abrasives. Wear 4 , 384-386 (1961) .

196. H. Peter Jost and H. J. Winch, Molybdenum disulphide as an aid to cutting and forming operations. Sei. Lubrication 11 , No. 11, 72-76, 80-81 (1959) .

197. A. M. Sabroff and P. D. Frost, A comparison of lubricants and coatings for cold extruding titanium. ASLE/ASME Lubrication Conference, New York, October, 1959.

326 R O B E R T J. B E N Z I N G

198. Anon, Molybdenum disulfide does it. Power Eng. 62, No. 3, 60^-62 (1958) . 199. Leonard C. Crewe and E. Jefferson Crum, Improving wire draws through

proper lubrication, Tool Manuf. Eng. 48, No. 2, 67-68 (1962) . 200. Walter A. Smigel and R. H. Miller, Molybdenum disulfide in dry wire drawing.

Wire Wire Prod. 37, 331-332, 404-405 (1962) . 201. Peter M. Magie, Molybdenum disulphide as an additive to wire-drawing

compounds. Wire Ind. 29, 261-263 (1962) . 202. C. C. Richards, Marine applications of dry film lubrication—graphite and

molybdenum disulphide compared. Marine Eng. Naval Architect 81, 94-96 (1958) .

203. V. G. FitzSimmons and W. A. Zisman, Thin film of polytetrafluoroethylene resin (Teflon) as lubricants and preservative coatings for metals. Naval Research Laboratory Report 4753 (1956) .

204. Μ. H. Weisman, Use of dry films in plain bearings. Air Force-Navy-Industry Conference on Lubricants, San Antonio, Texas, 1956.

205. A. Hegarty, Enamel glasses as lubricants for forming sheet metal. Sei. Lubrication 10, No. 2, 12, 13, 34 (1958) .

206. Fred Macks, Lubrication reference manual for missile and space vehicle propulsion at temperatures above 700°F. WADC Technical Report 58-638, Vol. 1, Part I (1959) .

207. Harold E. Sliney, Bearings run at 1250°F with solid lubricant. Space Aeron. 35, No. 3, 91, 92, 94, 96, 98, 100 (1961) .

208. T. D. Witherly, Solid lubrication of metallic surfaces at very high sliding speeds. ASME/ASLE Lubrication Conference, Chicago, Illinois, October, 1961.

209. Ronald N. Fracalossi, Molybdenum disulfide in the plant lubrication field. Southern Power Ind. 72, No. 11, 70-73, 100, 102 (1954) .

210. Anon., Reading adds years to curve rail life with new lubricant. Railway Age 145, No. 19, 20-21 (1958) .

211. Stanley L. Cosgrove and Richard L. Jentgen, Recent developments in lubricants. Rattelle Tech. Rev. 10, No. 4, 9-14 (1961) .

212. R. I. Potter, E. J. Schanilec, and C. L. Knighton, 30,000 mile chassis lube experience points to permanent lubrication. SAE National West Coast Meeting, Los Angeles, California, August 13-16, 1962.

213. Harry Simon, Case for molybdenum disulfide. Steel 141, No. 27, 68-69 (1957) . 214. A. J . Maries, The lubrication of antifriction bearings in a nuclear power station.

Set. Lubrication Suppl. Issue pp. 33-37 (December, 1958). 215. Anon., Mangfall Bridge, Engineer 209, 32 (1960) . 216. D. W. Sawyer and D. A. Pauli, Polytetrafluoroethylene insert compression

rings in radial gas engines. Lubrication Eng. 16, 406-409 (1960) . 217. Anon, Solid film lubrication protects large valves. Gas Age 118, 16-17 (1956) . 218. J . R. McDowell, Fretting corrosion tendencies of several combinations of

materials. ASTM Symposium on Fretting Corrosion. ASTM Spec. Tech. Publ. 144, 24-35 (1952) .

219. Ε. E. Weismantel, Friction and fretting with solid film lubricants. Lubrication Eng. 11, 97-100 (1955) .

220. Barry L. Mordike, Lubrication of solids at high temperatures. ASLE Trans. 3, 110-115 (1960) .

Documents

SOLID LUBRICANTS Robert J. Benzing