Wear,50 (1978) 285 - 297 0 Elsevier Sequoia S.A., Lawanne - Printed in the Netherlands

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A PRACTICAL INT~R~RETATIUN OF U~LU3RIGATE~ WEAR DATA FUR SOME NUN-FERROUS METALS

(Received March 13, 1978)

Summary

An attempt is made to indicate how the results from simple unlubricat- ed pin-on-disc type wear tests can be meaningfully interpreted by a designer in his approach to the selection of materials for practical applications.

I. Introduction

The selection of materials for tribological applications must take into account the degree of wear likely to occur in case of total lubrication failure as well as under lubricated conditions. In at~rnp~~ to assess the wear prop- erties of bearing materials it is desirable to use the same running conditions as would be met in practice. Several problems become apparent.

(a) Field trials are expensive. (b) The values of wear rate obtained from a simple pin-on-disc machine

cannot be used directly in practical applications, This is mainly because of the geometrical differences between the pin-on-disc arrangement and the bearing configuration.

There are few industrial situations where dry sliding is desirable (except braking application), so consequently engineers use a lubricant to reduce both the frictional resistance to motion and the amount of wear that takes place. Even when the utmost cam and precision is used in setting up and performing such tests, the same operating conditions cannot be guaranteed.

(c) The presence of a lubricant complicates the assessment of wear properties by introducing a large number of variables, none of which can be easily controlled.

The efficiency with which a lubricant reduces wear depends on its ability to prevent metal-metal contact and to prevent shear taking place within the metals rather than at their interface. This efficiency depends on the ingredients in the oil and is likely to be modified by the alloying elements in the metals.

Ideally the designer would like to refer to data obtained from simple tests when selecting the best materials for his application. Unlubricated wear studies are the simplest form of test that can be carried out and these allow both the wear mechanisms and the load uersus speed regimesato be separated. The behaviour patterns observed in such tests have been shown to be operat- ing in field failures [l]

2. Experimental and results

A simple pin-on-disc type wear machine was used to carry out the un- lubricated sliding wear tests. Wear pins, 6.35 mm in diameter by 75 mm long, were made from five non-ferrous materials: copper, duraiumin (I&5), silicon bronze (DTD192), aluminium bronze (CA104) and phosphor bronze (PBl) (see Table 1.). These pins were slid against a rotating hardened steel disc (BS970 En31) under a variety of loads and sliding speeds. The weight loss versus the distance slid was automatically recorded. From such a wear trace the following information was obtained (see Fig. 1): (a) running-in wear; (b) steady state of equilibrium wear.

Equilibrium wear is further subdivided into mild, transitional and severe wear according to the type of wear debris produced and the surface condi- tion on the pin. However, materials such as duralumin and copper have greatly different densities so that the o&y way of comparing wear rates is by a volume-loss measurement. This is easily obtained by dividing the weight loss by the density of that particular material. Consequently equilibrium wear curves of volume loss ueraus pin load were plotted for various loads at constant speeds. Figure 2 shows the equilibrium wear curves for the five pin

Fig. 1. A schematic wear trace.

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Fig. 2. A comparison of equilibrium wear rates us. load at 200 cm s-l for five non-ferrous metals: X-X, PBl; ~---~, copper; 0-0, L65;A---A, DTD 192; Lo, CA 104.

materials, all obtained at a constant speed of 200 cm s- ‘. Results for other speeds are available but are not reproduced here.

Me~llography was carried out on selected wear pins of all materials. Photomicrographs for phosphor bronze only are reproduced here (Fig. 3). Figure 3(a) shows the general structure of PBI. The alloy is in the as-cast condition and shows cored Q: dendrites with interdendritic pools of LY + 6 eutectoid.

Mild wear of PBl is established when an oxide layer covers the whole of the pin surface. This oxide layer is supported by a flowed and work-harden- ed substrate (Fig. 3(b)). Wear causes the removal of the oxide but, providing the oxide growth rate is equal to or greater than its rate of removal, mild wear will continue. Mild wear for PBl extends up to 2‘7 kgf load (Fig. 2). Running temperatures will increase with both load and sliding speed. Thermal softening of PBl occurs around 400 “C and this coupled with the shear stresses due to sliding will cause subsurface movement. Consequently

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disruption of the oxide layer takes place. This marks the initiation of transi- tional wear which occurs between 27 and 31 kgf load (Fig. 2). The surface microstructure developed appears to be a layered mixture of oxide and homogenized 01 phase (Fig. 3(c)).

Severe metallic wear with the generation of metallic debris is produced at loads beyond the transitional value (greater than 32 kgf (Fig. 2)). Gross surface flow and homogenization of the surface layers (likely to be (Y phase) occurs (Fig. 3(d)).

(cl (d)

Fig. 3. Microstructure of PBl wear pins: (a) as-received showing cored dendrites with ~nterdendritic pools of (Y + 6 eutectoid (150x); (b) mild wear showing an oxide layer over a flowed substrate (75~); (c) transitional wear showing a layered mixture of oxide and homogenized Q (150X); (d) severe wear showing gross surface flow and homogenization of the surface layers(150x ).

3. Discussion

The mild wear regime is classified as the combination of load and speed for which the material develops a stable oxide film on its rubbing surface. In general the wear rate during the mild wear regime is low although exceptions to this can be quoted, e.g. silicon bronze, copper and duralumin. Conversely certain materials, e.g. ~uminium bronze, do not form an oxide coating under any combination of load and speed. By definition, therefore, such

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materials wear by a severe (metaflic) mechanism even though wear rates are comparable with the values obtained for mild (oxidative) wear. Whether wear takes place primarily by adhesive, abrasive or corrosive mechanisms or by a combination of these mechanisms, a low wear rate will be of major importance to the designer.

In addition, whether the material develops an oxide coating or not is of little consequence provided that both forms of rubbing produce the same wear rate. Thus the oxidative and metallic definitions of mild and severe wear are not particularly helpful to the designer and so he must seek other criteria by which he can make a rational approach to the selection of materials.

4. Practical interpretation of results

With hydrodynamic lubrication, in theory no wear takes place except when starting and stopping. However, if boundary lubrication conditions exist, the designer must accept that a certain amount of wear will occur, i.e. chemical wear is substituted for mechanical (adhesive) wear. Unfortunately even with pressure-fed bearings, the wholly hydrodynamic condition cannot always be guaranteed.

Whenever a new condition of runn~g is encountered, a bearing may have to recondition itself to withst~d the new operating stresses, i.e. it may have to run in again. This situation is easily envisaged for the case when a bearing which has been operating under a fully lubricated condition encounters a period of starved or even totally unlubricated running. As a rule the wear during running in is greater than during the equilibrium wear regime. However, it has been shown that running in is a process whereby sur- face flow and work hardening conditions the material, so that it will allow the development of an oxide film or hard phase at the surface [2] .

Even in the presence of a lubricant, running in produces flow and work hardening of the running surface. It is likely, however, that the amount of running in required to establish an oxide film on an already work-hardened bearing surface would be less than if the bearing had not been previously used. In the following in~rpre~tion no account has been taken of this re- conditioning process and the reader (designer) should be aware of this fact.

The following is an attempt to indicate how the results from simple un- lubricated wear tests may be meaningfully interpreted by a designer in his approach to the selection of materials. Several assumptions require clarifica- tion.

PBl should be used as a standard material In the engineering world PBl has been established as having good bearing properties. Therefore it would seem reasonable to use it as a standard by which to compare other test materials.

During boundary lubrication a designer must accept the fact that some wear takes place. Dry unlubricated running is the worst situation a bearing is

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likely to encounter, yet the mild wear condition is considered as the accep- table regime of running under unlubricated conditions. Ideally a designer would like a bearing to be self-protecting or at least to have a wear rate no greater than that obtained during unlubricated mild wear in the event of a lubrication failure. Reference to Fig. 2 indicates that PBl has a wear rate of 10-s cm3 cm-l or less when running in its mild oxidative wear regime. It is suggested therefore that the wear rate of lo’-* cm3 cm-’ should be used as the maximum value of mild (acceptable) wear. Similarly, the value of 10mm7 cm3 cm -’ should be used as the minimum value for the onset of severe (unacceptable) wear.

From a family of dry wear curves, it is possible to produce a series of load uersus speed graphs using the IO-* cm3 cm’-’ and lo-’ cm3 cm-l criteria for each material (Figs. 4 - 7). The designer can now compare materials on a cdmmon volume-loss basis without regard to wear mech- anisms. From such graphs, he will also be able to predict the expected behav- iour of the materials both under the design operating conditions and outside them.

1 , 4 8 12 16 30 31 33 33 36 40 44 %

c 1 I 300 400 600 8OQ 1000 1100 14w 1600 c 1800 3000 P*i

PMIDAD

Fig. 4. Load-speed-constant volume loss diagram for phosphor bronze (PBl) US. steel (En31): upper curve, 270 kg at 20 cm for lo-’ cm3 em-l; lower curve, 75 kg at 20 cm for 10V8 cm3 cm-l.

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WC

400

" % 2 300 0

w" w

D

gal0

Q

100

4 0 12

L,: I..

200 400 600 psi PlNlRAD

Fig. 5. Load-speed-constant volume loss diagram for aiuminium bronze (CA104) us. steel (En31).

500

400

1 300

"

D

g 200

3

100

L 1 I 1 1 I 1 L 4 8 12 16 ki: 20

I t ‘

200 400 600 800 !xl PINlOAD

Fig. 6. Load-speed- constant volume diagram for silicon bronze (DTD 19‘2) us, steel (En31).

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1 t 200 400 605 800

PI3S LOAD Psi

Fig. 7. Load--speed-constant volume loss diagram for duralumin (L65) us. steel (En31).

4.1. Load-speed-cons tan t volume loss diagrams Referring to Figs, 4 - 7, it will be noted that the 10m8 and lo-’ cm3

cm-’ lines divide the figures into three areas or regimes designated mild, transitional and severe. The following attempts to explain the meaning of these regimes.

4.1.1. ~i~d If the operating conditions fall within the mild wear regime, then in the

event of a total lubrication failure the bearing could reasonably be expected to cope with the situation incurring little damage or loss of service life.

4.1.2. Transitional If operating conditions fall into the transitional regime and lubrication

failure occurs, then an increase in wear rate and some damage to the compo- nents can be expected. Although the damage incurred whilst running in the transitional region may not be disastrous, it is likely to shorten the service life of the component. It must be emphasized that the closer to the lo- 7 cm3 cm-l curve the operating conditions lie the greater will be the wear rate and the more severe the damage, and hence a shorter service life will result in the event of a lubrication failure.

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4.1.3. Severe Operating conditions that fall into the severe region are dangerous, for

if a lubrication failure occurs then severe damage, short service life and even seizure can be expected. The further into the severe regime that the operating conditions go the more rapid will be the failure.

Table 2 list,s some applications where bush type bearings are used [ 31. A designer could compare these or his operating pressures with such load- speed-constant volume loss diagrams and deduce the wear regime that the bearing material would be operating in should it encounter the worst running conditions, i.e. unlubricated running. Preferably he would design his equip- ment and choose his materials such that they operated in the mild wear regime.

TABLE 2

Bearing and bush loadings -- _._..- ,_-l__l_ _____- .I__- .^ _-.--

Class of work and use Maximum pressure of projected area (lbf iK2)

--_- ~__.II__---- -__ ____~ .I_~--~

Automobile engines Main bearings 800~1800 Crank pin bearings 1000 - 2100 Wrist pin bushings 800 - 3500

Diesel engine Main bearings 800 - 2200 Crank pin bearings 1000 - 2700 Wrist, pin bushings 1000 - 5000

Marine diesel engine Main bearings 400 - X600 Crank pin bearings 1000 - 2000

Electric motors and generators 75- 200 Machine tooi thrust Colby 300 - 400 Turbine horizontal bearings 150- 400

Marine bearings 85 - 200 Hoisting machinery, bushings 70 - 200 Punch-press bushings, slow speed 3000 4oao Reduction gear bearings 150 - 300

---- _________.-_ ___._____p

From ref. 3.

Let us consider an example. An electric motor is working under constant operating conditions such that the load gives a bearing pressure of ilO0 lbf in ’ and the shaft peripheral speed is 200 cm s- r (400 ft min-’ ). The designer makes the initial choice of alurn~n~urn bronze rubbing against a steel motor shaft. Reference to Fig. 5 will show that under these operating conditions aluminium bronze is running in its severe regime, which is un- desirable. In order to allow this material to operate in the mild regime, a

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miI~imum increase in projected bearing area of a factor of 14 and probably a reduction in shaft speed would be required (Fig. 5).

Circumstances may be such that a change of this order would involve the complete redesign of the whole equipment and would probably not be an economical proposition. The designer therefore has three choices.

(a) The bearing material, or possibly both the shaft and bearing material combination, should be changed for one that will operate in the mild regime under the design conditions. For example PBl (Fig. 4) or perhaps aluminium bronze running on a chromium plated shaft would be reasonable choices. The latter combination has been used with success for small end bushes in marine diesel engines.

(b) Both the shaft/bear~g material combination and the design should be changed such that the equipment operates in the mild wear regime.

(c) The designer accepts the situation but designs the equipment to ensure an adequate supply of lubricant to the bearing.

In situation (c) the designer must realize that he is running the risk of serious damage in the event of a lubrication failure. Although the authors consider route (c) to be unsatisfactory, a manufacturer may design to this criterion in the case of low-cost equipment, especially when he expects to make most of his profit from the supply of spam parts. Routes (a) and (b) are likely to be the choice of the manufacturer who produces capital equip- ment which is expected to give continuous service for perhaps 20 years or more.

Let us consider another example where the operating conditions will be variable, the motor car engine. At a steady 30 mile h-l the crankshaft bear- ings may be operating in a mild regime, whilst at high constant speeds (say motorway driving) they may be operating in the transitional regime. Occa- sions will arise when the bearings will be operating in the severe regime, e.g. heavy acceleration or pulling hard up hills in high gear. Should the lubrica- tion system fail, particularly in the severe regime, then rapid failure of the bearings and probable destruction of the engine will occur.

The designer can assess from load-speed-constant volume loss diagrams how a particular material is likely to stand up to the imposed operating conditions. Let us suppose that the designer is faced with the choice of alu- minium bronze, silicon bronze or duralumin L65 as bigend bearing material (cc Figs. 5,6 and 7). Silicon bronze is likely to be chosen because of its greater load/speed tolerance in the transitional regime. During stopping and starting where full hydrodynamic lubrication is difficult to achieve, duralumm L65 would appear to be more promising than silicon bronze. However, duralumin is likely to enter the severe regime mom quickly at moderate load and speed fluctuations. Aluminium bronze would appear to be the least suitable.

4.1.4. Wear characteristics of material from load-speed-constant volume loss diagram A designer can deduce certain characteristics about the materials from

these Ioad-speed-constant volume loss diagrams. PBl (Fig. 4) is a speed-

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sensitive material. Thus at 500 cm s ’ PBl will change from a mild to a severe form of wear within the range 6.5 - 7.5 kgf (290 - 340 lbf in ‘). At 20 cm s *, although the load range for the transitional regime extends from 75 to 270 kgf (3370 - 12 120 lbf in ‘), a speed change from 20 to 40 cm s ’ at 75 kgf load is sufficient to transfer from the mild to the severe wear regime. PBl would appear to be best suited to high load low constant speed applications.

In contrast, copper (results not reproduced here) and to a certain extent aluminium bronze (Fig. 5) are load-sensitive but speed-insensitive materials. The lO_ 8 and 10 7 cm3 cm ml lines for both aluminium bronze and copper are almost parallel to the speed axis. It is suggested that these materials may be satisfactory for applications where the loading is both light and constant but the speed is variable.

A material such as silicon bronze may give good service in a bearing that is operating with both the load and speed varying (see Fig. 6). At 300 cm s ’ and up to 9 kgf (400 lbf in-‘) silicon bronze would appear to have a fair tolerance of both load and speed fluctuations.

Uuralumin (Fig. 7) would appear to be better suited to low speed work, where at 100 cm s ’ the transitional regime extends from 1 to 20 kgf. Its performance and tolerance of changing conditions are markedly decreased with increasing speed.

Overall, PBl has a greatly superior performance when rubbing against steel compared with the other materials and is likely to give a good performance under high load low speed conditions where hydrodynamic lubrication is difficult to achieve.

Of the remaining four materials, silicon bronze would give better performance at sliding speeds around 200 - 300 cm ss’. L65 may be best suited to low speed work where the likelihood of generating high temper- atures is reduced. Aluminium bronze shows poor performance.

5. Summarizing remarks

Before a designer can make an approach to the selection of materials, he must be quite clear as to the operating conditions (ranges of loads and speeds) t,he equipment is likely to encounter. In addition the type of lubrica- tion system he intends to employ will also be important from the point of view of poor or unlubricated operation. Consequently, the more the lubrica- tion system depends on the memory of the human being the more likely is the chance of starved or unlubricated running. At first sight the number of material combinations to choose from seems astronomical. However, if the designer were furnished with a series of load-speed-constant volume loss diagrams, the task would be somewhat simplified.

It, has been shown how such diagrams can be used to deduce certain tribological ~hara~~risti~s of various materials and how such diagrams can be used to develop an adequate design. In future it may be possible to produce

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a reference atlas for various conventional bearing alloys sliding against say four or five standard shaft (disc) materials, e.g. En31, En24, cast iron, chromium plating etc. From such an atlas the designer may then be able to select rationally suitable material combinations or at least be able to avoid disastrous combinations should a lubrication failure occur.

We should like to thank Professor Bodsworth for making laboratory facilities available. One of the authors (G. S.) would like to thank the Mobil Oil Co. Ltd. for their financial support,

References

1 T. S. Eyre and F. Wilson, Metallographic aspects of wear, Met. Mater., 3 (3) (1969) 86. 2 T. S. Eyre, A metallurgical study of frictional wear under dry sliding conditions, Ph.D.

Thesis, Brunei University, 1971. 3 W. E. Thill, Sleeve bearings and bushings, in Tool Engineer’s Handbook, McGra~~ill,

New York, 1st. edn., 1951, section 109, p. 1793.