STATIC FRICTION MEASUREMENTS ON STEEL AGAINST …

P. Andersson et al.: Static friction measurements on steel against uncoated and coated cast iron

5TRIBOLOGIA - Finnish Journal of Tribology 1-2 vol 34/2016

STATIC FRICTION MEASUREMENTS ON STEEL AGAINSTUNCOATED AND COATED CAST IRON

P. Andersson1, L. Kilpi1, K. Holmberg1, A. Vaajoki1 and V. Oksanen2

1 VTT Technical Research Centre of Finland, P.O.Box 1000, FI-02044 VTT (Espoo), Finland2 Tampere University of Technology, Department of Materials Science, P.O.Box 589, FI-33101

Tampere, Finland

ABSTRACT

Static friction is a phenomenon we may mainly consider as related to frictional joints within staticmechanics. The step from static friction to tribological phenomena is, however, rather short, since at theonset of sliding in a mechanical contact, the static friction determines the initial resistance against motion.Static friction furthermore plays a role in contacts subjected to traction and fretting. Although being aphenomenon of short duration, the tribological phenomena during the transition from static friction to slidingfriction may be of great importance for the operational life of the contact surfaces, particularly if theprocedure is repeated for a sufficient number of times.

The present paper describes the principles of static friction measurements, details of the employed staticfriction tribometer and the results of measurements with unlubricated and lubricated sliding couplesconsisting of steel against uncoated and coated cast iron.

Keywords: Static friction, friction transitions, sliding friction, tribometer.

*Corresponding author: Peter Andersson ([email protected])

INTRODUCTION

By definition, tribology is the science andtechnology of interacting surfaces in relativemotion and of the practices related thereto [1].Strictly taken, static friction as such is notcovered by this definition, since traditionallystatic friction is included in the staticmechanics, for example in the dimensioningof screw joints, shrink fits and pressure fits, inmore developed machine components likeclutches and parking brakes, and in beltdrives, rope drives, wheels, shoes and othertypes of traction drive systems.

However, before even the slightest relativemovement could commence in a mechanicalcontact intended for full or partial sliding

motion, the tangential force acting in thecontact has to pass the limiting friction forcethat forms the border between static andkinetic friction, and this again connects thestatic friction phenomenon to the field oftribology. A typical situation, in which thestatic friction precedes the onset of sliding ina contact, is when a shaft starts to rotate in ajournal bearing.

Under static friction, the contact temperatureequals the surrounding temperature, andhydrodynamic effects are absent. Thedifference between static and kinetic frictionfor a given sliding couple determines itstendency for stick-slip frictional behavior.Static friction is partly present in contactssubjected to fretting, furthermore in traction,



in which mechanical energy is transferredunder partly static and partly kinetic (sliding)friction from a driving component to a drivencomponent [2]. The dwelling time, i.e. thetime elapse from the formation of a staticcontact until the onset of relative motion inthe contact, can affect the magnitude of thecoefficient of static friction, particularly underlubricated conditions.

When a static contact, loaded by a normalforce Fn, evolves into a sliding contact, thelimiting static friction force Fµ has beenexceeded, and the coefficient of static friction(µs) can be established. The coefficient ofstatic friction between a body and a plane canbe experimentally determined, at certainaccuracy, by tilting the plane to an angle , atwhich the gravity starts to pull the bodydownwards along the inclined plane [2, 3].The expression µ=tan is valid for thelimiting frictional conditions at which a bodystarts to slide along the plane. The aboveexpression for the limiting friction has beendeveloped into a model called the cone offriction. Load conditions, in which the ratio ofthe tangential and normal forces give anglessmaller than , are located within the cone offriction and are secured against sliding.

Another method for the experimentaldetermination of the coefficient of staticfriction is to use a centrifugal force apparatus,in which the gravity causes the normal forceand the centrifugal force the tangential forceon the movable sample [4].

In the simplest measurements of the staticfriction force, a spring balance or anelectronic force recording system is used formeasuring the static friction force needed forthe initial mobilization of a loaded test samplealong a flat surface [2].

For a more precise determination of thecoefficient of static friction, the static frictionforce is measured at increasing load until the

onset of relative motion between the contactsurfaces, as recorded by force anddisplacement instrumentation. The normalforce Fn should act directly on the staticsample holder and sample, and if the staticsample is above the movable sample, thenormal force is preferably achieved by gravityand dead weights. The lower sample holderand sample must move along the plane thatgoes through the sliding contact, and forproviding accurate measurements, the motionmust take place without any friction. Bendingmoments of force on the movable sample areavoided by applying the tangential force at theplane that goes through the sliding contact,and by directing the static friction forcethrough the center of gravity of the contactsurface between the upper and lower sample.If the friction force is measured along a planethat lies above the contact surface, amomentum of force is formed from the forcesand the distance between their action points,and part of the contact pressure due to thenormal force is transferred from the trailingedge of the sample to the leading edge.Furthermore, the movement of the movablesample holder and sample must take placealong the plane through the contact interface,in the direction of the tangential force, and itmust be smooth and without external slidingfriction.

The present work is an attempt to support thedevelopment of rope drives, by performing aset of measurements on the static frictionbetween a steel wire slider and flat, smoothsurfaces of eleven different materials.

MATERIALS AND METHODS

EquipmentThe static friction measurements in thepresent work employed an in-house builtdevice called the VTT Static FrictionTribometer, see Figure 1. The tribometercasing consisted of an upper deck with two



support columns bolted onto a heavy platemade from cast iron. The upper deck hosedthe holder for the static sample, or the uppertest sample, which was loaded by deadweights and able to move in the verticaldirection. A horizontally movable framebeneath the upper deck hosed the lower ormovable test sample. For measuring the staticfriction force, the movable frame was pulledhorizontally by a pneumatic actuator equippedwith a force transducer.

Fig. 1. The VTT Static Friction Tribometer.Upper deck (1), movable frame with the lowersample (2), suspension springs for the uppersample holder (3), suspension springs for themovable frame (4), weights on a loading rod(5), pneumatic actuator (6) and forcetransducer (7). The displacement transducerwas located on the opposite side of thetribometer.

The upper test sample, which was a piece ofsteel wire that had been bent into U-shape,was mounted in a holder in a vertical channelthrough the upper deck. The holder wassuspended and guided by a pair ofhorizontally orientated leaf spring blades inthe direction of motion of the movablesample, see Figure 2. The leaf springs allowedthe holder to move vertically for the shortdistance needed when applying a load in atest, however providing determined,horizontal positioning of the upper and lowerends of the upper sample holder. The bladeshad been laser-cut from a 0.5 mm thick sheetof spring steel. One of the blades had beenrigidly attached by screw joints on the upperside of the upper deck and the upper sampleholder, and the other blade on the lower sideof the deck and holder. When loading theupper sample by the dead weights, the initialair gap between the upper and lower samplewas closed and the entire set-up was subjectedto minor compression, and due to this thespring blades were subjected to a weakdeflection. A pre-stress in the length directionof the spring blades removed any slacken inthe blades however still allowing thedeflection of the blades.

The lower test sample, which was a disc, wasmounted in a horizontally movable frame.The movable frame was suspended by fourvertical leaf spring blades (Fig. 1). Thesuspension blades allowed the frame to movehorizontally, and without internal friction, forthe short distance needed for the friction forcemeasurements, in the correct direction, andwithout frictional losses. The blades had beenrigidly mounted by screw joints to the upperdeck and to the movable frame, thus beingsubjected to deflection into a very weak S-shape when the frame was moved from itsresting position.



Fig. 2. Suspension leaf springs for the uppersample holder; the upper sample holder in theupper deck (upper left image), the springblade placed above the upper sample holder(upper right image) and the principle forspring blade suspension during loading by thenormal force (lower image).

In the friction force measurements, anincreasing tangential force was applied on themovable frame until motion could bedetermined by displacement measurements.The tangential force was achieved by apneumatic actuator without sliding friction.The function of this particular type ofpneumatic actuator was to create a tensileforce when pressurized by air. The tangentialforce was measured by strain gages on twothin steel sections mounted between twospherical joints, between the actuator and themovable frame. In order to allowmeasurements without interference byfrictional torque effects, the tangential forcefrom the actuator was designed tohorizontally intersect the contact spot of thefriction couple. The position of the movable

test disc frame in the direction of the motionwas recorded by means of a non-contactingdisplacement transducer (NCDT) based on theeddy-current effect.

Test specimens and sample holders

The tests were performed with a bent, U-shaped steel wire against an uncoated orcoated cast iron disc (Fig. 3).

Fig. 3. Contact geometry of the test set-upconsisting of a U-shaped steel wire and a castiron disc. The contact between the steel wireslider and the disc sample is seen above theletter “C”. In this image, the direction ofmotion of the disc sample in a test situationwas to the right.

The steel wire samples were produced from1.4 mm steel wire, which was pre-shaped on amandrel for an outer radius of curvature of 11mm, and mounted on a similarly curvedsupport on a holder. According to themanufacturer, the steel wire had been drawnfrom non-alloyed steel with a pearliticmicrostructure and had a tensile strength of1700 N/mm2. The wire was uncoated and hada surface roughness of Ra=0.30 µm andRsk=-1.0 when measured by VTT in thelength direction of the wire, using a diamondstylus profilometer. The direction of the steelwire in the contact area coincided with thedirection of the friction force and motion. Anew steel wire was used for each test series.



Table 1 presents the sliding surface materialsof the disc samples investigated, whichrepresent 11 different types of uncoated andthermal spray coated cast iron. The surfaceroughness Ra and Rsk values of the slidingsurfaces after flat grinding, lapping andpolishing are included in the Table. Thediameter of the uncoated discs was 24 mmand that of the coated ones 40 mm. In the test,

the cast iron disc was radially attached to thesample holder. For each test series, the discwas re-positioned in order to introduce a newtest spot.

The lubricant for the lubricated tests was anadditive-free, paraffinic vaseline (petrolatum)with a melting point around 80 ºC.

Table 1. Sliding surface materials of the disc samples investigated.Code Designation Tensile

strength1Surface roughness2

[N/mm2] Ra [µm] Rsk [ - ]Ferritic SGiron

Nodular cast iron - also known as spheroidal graphitecast iron (SG iron) or ductile cast iron - with a solutionstrengthened ferritic microstructure.

600 0.15 -1.1

Pearlitic SGiron

Nodular cast iron with a mainly pearliticmicrostructure.

700 0.244 -2.1

Grey cast iron Molybdenum-alloyed grey or lamellar cast iron. 330 0.36 -0.7ADI Austempered ductile cast iron (ADI). 1000 0.07 -2.0P/F-CDI(pearlitic-ferritic chill-cast SG iron)

Chill-cast nodular cast iron with a pearlitic matrix withdendritic, lamellar cementite precipitations and bull’s-eye ferrite.

N.A. 0.13 -1.1P-CDI(pearliticchill-cast SGiron)

Chill-cast nodular cast iron with a pearlitic matrix withdendritic, lamellar cementite precipititations and verylow ferrite content.

N.A.

0.11 -1.4CrC-TS1 Cast iron disc that had been thermal spray coated with

a commercial, thermal spray, chromium carbide (CrC)cermet based coating.

N.A.

0.02 -1.5WC-TS2 Cast iron disc that had been thermal spray coated with

a commercial, thermal spray, tungsten carbide (WC)cermet based coating.

N.A.

0.02 -1.9Mo-TS3 Cast iron disc that had been thermal spray coated with

a commercial, thermal spray, molybdenum (Mo) basedcoating.

N.A.

0.18 -2.3CrC-TS4 Cast iron disc that had been thermal spray coated with

an experimental, thermal spray, chromium carbide(CrC) cermet based coating.

N.A.

0.01 -2.0VC-TS5 Cast iron disc that had been thermal spray coated with

an experimental, thermal spray, vanadium carbide(VC) cermet based coating.

N.A.

0.07 -1.8

1 Tensile strength values obtained from the manufacturer.2 Surface roughness values as measured by VTT using a diamond stylus instrument.



Experimental procedure

The present experiments were carried out at atemperature of 22 1 C, in room air of50 5% relative humidity. At least two seriesof unlubricated tests and two series oflubricated tests with normal forces increasingfrom zero to 96 N and back were performedon each disc material, as shown in the testsequence of Fig. 4. Before testing, thesamples were ultrasonically cleaned inpetroleum ether. The thickness of thepetrolatum layer applied on the disc wasabout 0.5 mm. The petrolatum layer wasapplied before the first friction forcemeasurement on a specific contact spot, and itwas not renewed during the test series thatwas performed on the same contact spot.

When a pair of samples had been installed inthe tribometer, the clearance between thesamples was adjusted to be about 1 µm in thevertical direction, i.e. until the gap betweenthe upper and lower sample was almost closedafter a set of dead weights had been appliedon the upper sample holder. After this, thefriction force measurements were performedat the chosen normal force, which wasobtained by adding dead weights on the uppersample holder.

In each test cycle with a new steel wireagainst a new contact spot on the cast ironsample, the friction measurements wereperformed under loads increasing from zeroto 96 N, decreasing back to zero andincreasing to 96 N, for finally decreasing backto zero, all in steps of 9.6 N. Between eachmeasurement, the load was removed and thelower sample was allowed to return to itsresting position, see Fig. 4.

Within 20 seconds after the application of thechosen load, the air pressure to the tangentialforce actuator was slowly increased. Whenthe limiting friction had been exceeded andthe test disc frame begun to move, as

indicated by a change in the signal from theNCDT (Fig. 5), the air pressure was manuallylowered to zero.

Fig. 4. Illustration of the loading sequence inthe static friction measurements; the loadlevels were 9.6, 19.2, 28.8, 38.4, 48.0, 57.6,67.2, 76.8, 86.4 and 96.0 N.

Fig. 5: Example of force and displacementreadings in a static friction measurement. Thedotted, vertical line indicates the limitingfriction force, i.e. the value of relevance in thepresent study.

Calibrations

The spring constant for the leaf springs of theupper specimen holder was determined byloading the holder assembly, without a lowersample, by a set of dead weights and bymeasuring the corresponding verticaldisplacements by using a Heidenhain linear



variable displacement transducer (LVDT).The spring constant thus determined was 0.3N/µm and practically uniform within thepresent deflection ranges.

In order to evaluate the deflection of the leafsprings of the upper specimen holder, the testset-up was loaded by a set of dead weights,and the corresponding vertical displacementswere measured with the LVDT. At 100 Nnormal force, the leaf spring deflection due toelastic deformation of the samples andtribometer parts was 12 µm.

The reduction in normal force, by thedeflection of the leaf springs of the upperspecimen holder during the tests, wascalculated from the vertical displacements andthe spring constants; the normal force was94…96% of the load on the samples, and thiswas considered in the friction coefficientcalculations. Throughout the present work,only corrected normal forces are reported.Owing to the low magnitude of the verticaldeflection in comparison with the free lengthof the spring steel suspension blades, verticalforces due to the cosine rule can be neglected.

The tangential force transducer was calibratedin its horizontal working position by loadingit using a set of dead weights and a string viaa pulley with a ball bearing. Themeasurements showed a linear signalresponse of 1 mV / 0.52 N for forces up to 30N.

Using the actuator, the non-contactingdisplacement transducer, by which thehorizontal displacement of the movable framewas measured during the tests, was cross-calibrated against the pre-calibrated LVDT.The comparison showed that the NCDT gaveits nominal signal response, or 1 mV/µm.

In order to evaluate the influence of thedeflection of the four leaf springs carrying the

movable frame during the tests, on the frictionforce reading, the spring constant for the leafsprings was determined by loading the spring-and-frame assembly by a range of tangentialforces from the pneumatic actuator, and bymeasuring the corresponding horizontaldisplacements by using the LVDT. The springconstant of the set of blades was 0.015 N/µmhorizontal movement and practically uniformwithin the present deflection range.

The reduction in the friction force readings,by the deflection of the leaf springs of themovable lower specimen frame, wascalculated from the horizontal displacementsand the spring constant; the recorded frictionforce reading was more than 99% of theactual friction force on the samples. Owing tothe low magnitude of horizontal deflection incomparison with the free length of thesuspension blades, any horizontal reactionforces due to the cosine rule, from the normalforce on the test specimens, can be neglected.

RESULTS

Steel against ferritic SG iron

Graphs showing the coefficients of staticfriction for steel wire against the ferritic SGiron under unlubricated and petrolatum-lubricated conditions, respectively, arepresented in the Figs. 6 and 7. In these and thesubsequent friction graph images, “First up”refers to the first load increase from 0 N to 96N, “First down” refers to the first loaddecrease from 96 N to 0 N, “Second up”refers to the second load increase from 0 N to96 N, and “Second down” refers to the secondload decrease from 96 N to 0 N. Micrographsshowing wear scars on the samples after thetest cycles with steel against ferritic SG ironare presented in the Figs. 8 and 9.



Fig. 6. Coefficients of static friction for an unlubricated contact with steel wire against ferritic SGiron. The average value at 38.4...96 N load (26 measurement points) was µs=0.190±0.025.

Fig. 7. Coefficients of static friction for a lubricated contact with steel wire against ferritic SG iron.The average value at 38.4...96 N load (26 measurement points) was µs=0.073±0.016.

Fig. 8. Wear scar on a (left image) steel wire and the (right image) corresponding ferritic SG irondisc sample after the test cycle without lubrication. The direction of motion of the steel wire samplewas horizontal in the images.



Fig. 9. Wear scar on a (left image) steel wire and the (right image) corresponding ferritic SG irondisc sample after the test cycle with lubrication by petrolatum. The direction of motion of the steelwire sample was horizontal in the images.

Unlubricated contact of steel againstferritic nodular cast iron

With steel wire against ferritic nodular castiron of Rm=600 N/mm2 tensile strength, thecoefficient of static friction underunlubricated conditions was significantlyhigher at low loads than at higher loads. Thecoefficient of static friction decreased untilthe normal force increased to approximately70 N, above which the coefficient of staticfriction remained almost constant. Within thenormal force range 38.4...96 N, the averagevalue for the coefficient of static frictionunder unlubricated conditions was µs=0.18.With a disc made from the ferritic nodularcast iron, the difference between the frictionalvalues in the two load increasing cycles incomparison with those of the two loaddecreasing cycles was very small.

During the 38 dry measurements undernormal forces of 10…100 N, a sliding surfacehad been formed on the steel wire. The slidingsurface on the steel wire sample had becomeslightly smoothened and shining. Thecorresponding sliding surface on the ferriticcast iron disc was covered by tiny scratches inthe direction of motion of the steel wire,which means that during the unlubricated

measurements, neither of the two countersurfaces had suffered from extensivealterations on their sliding surfaces.

Lubricated contact of steel against ferriticnodular cast iron

When the steel wire and ferritic nodular castiron couple had been lubricated withpetrolatum, the average value for thecoefficient of static friction decreased toµs=0.07. Within the entire normal forcerange, the average values corresponding to theten load levels were significantly lower in thelubricated tests. With lubrication bypetrolatum, the difference between thefrictional values in the two load increasingcycles (µs=0.090 on an average) and the twoload decreasing cycles (µs=0.063 on anaverage) was significant, probably due tomore beneficial boundary lubrication whenthe load was reduced.

The sliding surface formed on the steel wiresample during the lubricated measurementswas more intensively polished, however on asmaller area, than in the dry tests with thesame material combination. The wear markon the ferritic cast iron disc only revealed tinyscratches or shallow grooves.



Steel against pearlitic nodular cast iron

Graphs showing the coefficients of staticfriction for steel wire against the pearliticnodular cast iron at 9.6…96 N normal force,under unlubricated and petrolatum-lubricatedconditions, respectively, are presented in theFigs. 10 and 11. Photomicrographs showing

wear scars on the samples after the test cyclesare presented in the Figs. 12 and 13. Twocross-sections across the sliding track of oneof the tested pearlitic SG iron disc samplesafter a test cycle without lubrication arepresented in Figure 14.

Fig. 10. Coefficients of static friction for an unlubricated contact with steel wire against pearliticSG iron at 10…100 N load. The average value at 38.4...96 N load (26 measurement points) wasµs=0.225±0.051.

Fig. 11. Coefficients of static friction for a lubricated contact with steel wire against pearlitic SGiron. The average value at 38.4...96 N load (26 measurement points) was µs=0.067±0.024.



Fig. 12. Wear scar on a (left image) steel wire and the (right image) corresponding pearliticnodular cast iron disc sample after the test cycle without lubrication. The direction of motion of thesteel wire sample was horizontal in the images.

Fig. 13. Wear scar on a (left image) steel wire and the (right image) corresponding pearlitic SGiron disc sample after the test cycle with lubrication. The direction of motion of the steel wiresample was horizontal in the images.



Fig. 14. Cross-section views of the sliding track of a pearlitic SG iron disc sample after a test cyclewithout lubrication. The direction of motion of the steel wire sample was horizontal in the image.The left image shows a deformed graphite sphere from which no graphite seems to have beenextruded. The right image shows a deformed graphite sphere from which part of the graphiteapparently has been extruded and has formed a solid lubricant film on the sliding surface.

Unlubricated contact of steel againstpearlitic nodular cast iron

The unlubricated measurements with steelwire and a disc made from the pearliticnodular cast iron with a tensile strength ofRm=700 N/mm2 showed that the averagevalue for the coefficient of static friction wasµs=0.22 at the normal force range 38.4...96 N.At lower loads, the coefficient of staticfriction decreased with an increase in theload, from µs=0.36 at about 20 N load, toµst=0.22 at about 60 N load. The frictionalvalues at the two load increasing cycles andthe two load decreasing cycles contain somescatter in the results rather than systematicdifferences between the friction force duringload increase and decrease.

The sliding surfaces, which had been formedon the steel wire and the pearlitic nodular castiron samples during the unlubricatedmeasurements, were rather rough. The contactsurface on the steel wire sample was shininghowever with scratches in the slidingdirection. The cast iron contact surface wasdark and probably covered by a mixture of

graphite and oxidized ferrous transfer layerfragments with scratches in the direction ofsliding. The extrusion of graphite from thegraphite spheres of the cast material isobvious from Fig. 14. With respect to thesliding distance elapsed, the surfacealterations were rather strong, which is inagreement with the high coefficient of frictionrecorded in the unlubricated conditions andthe relatively high initial surface roughness ofthe cast iron disc.

Lubricated contact of steel against pearliticnodular cast iron

In the lubricated measurements with steelwire and the pearlitic nodular cast iron, theaverage value for the coefficient of staticfriction was µs=0.07 at the normal force range38.4...96 N. The difference between frictionalvalues during the two load increasing cyclesand the two load decreasing cycles wassignificant, with a particularly low and stablelevel for the coefficient of static friction whengoing from higher normal forces towardslower ones; During the two cycles ofincreasing normal force, the average value for



the coefficient of static friction was µs=0.12for the entire normal force range, i.e. 9.6…96N. The corresponding value during the twocycles of decreasing normal force wasµs=0.06, which indicates beneficial boundarylubrication of the tribocouple during theunloading cycle.

Steel against grey cast iron

Graphs showing the coefficients of staticfriction for steel wire against grey cast iron at9.6…96 N load, under unlubricated andpetrolatum-lubricated conditions, res-pectively, are presented in the Figs. 15 and16, and photomicrographs showing wear scarson the samples after the test cycles arepresented in the Figs. 17 and 18.

Fig. 15. Coefficients of static friction for unlubricated contact with steel wire against grey cast iron.The average value at 38.4...96 N load (26 measurement points) was µs=0.233±0.038.

Fig. 16. Coefficients of static friction for a lubricated contact with steel wire against grey cast iron.The average value at 38.4...96 N load (26 measurement points) was µs=0.122±0.018.



Fig. 17. Wear scar on a (left image) steel wire and the (right image) corresponding grey cast irondisc sample after the test cycle without lubrication. The direction of motion of the steel wire samplewas horizontal in the images.

Fig. 18. Wear scar on a (left image) steel wire and the (right image) corresponding grey cast irondisc sample after the test cycle with lubrication by petrolatum. The direction of motion of the steelwire sample was horizontal in the images.

Unlubricated contact of steel against Mo-alloyed grey cast iron

In the dry tests, the level of the coefficient ofstatic friction for steel and grey cast iron wasclose to that in the tests with the nodular castirons. The decrease in the coefficient offriction with load was strong for normalforces up to 70 N, after which a stable leveloccurred. At the normal force range 38.4...96N, the average value for the coefficient ofstatic friction was µs=0.23, which is on the

same level as for the pearlitic nodular castiron and slightly higher than for the ferriticnodular cast iron. The relatively higharithmetic average surface roughness ofRa=0.36 µm of the cast iron disc, may bepartly responsible for the high coefficients ofstatic friction during the unlubricatedmeasurements. The influence of the load-increasing-and-decreasing cycles, i.e. thepreceding loads, on the coefficient of staticfriction at the respective load levels was smallduring the unlubricated measurements.



The wear scars formed on the steel wire andcast iron disc in the unlubricated testsrevealed irregular patches of transfer layersand a high density of tiny scratches in thedirection of the measurements.

Lubricated contact of steel against Mo-alloyed grey cast iron

Lubrication by paraffinic petrolatum had aclear effect on the coefficient of static frictionfor steel against lamellar cast iron, which wasonly µs=0.12 as an average value in thenormal force range 38.4...96 N. Thecoefficient of static friction showed adecrease with increasing normal force up to50 N. Certain degree of scatter in thecoefficient of static friction was recorded at

the different load levels, and this probablyindicates more or less beneficial boundarylubrication conditions, due to variations in thepreceding sliding events, and contactcondition changes arising thereof.

Steel against ADI

Graphs showing the coefficients of staticfriction for steel wire against the ADI at9.6…96 N load, under unlubricated andpetrolatum-lubricated conditions, res-pectively, are presented in the Figs. 19 and20, and photomicrographs showing wear scarson the samples after the test cycles arepresented in the Figs. 21 and 22.

Fig. 19. Coefficients of static friction for unlubricated contact with steel wire against ADI. Theaverage value at 38.4...96 N load (26 measurement points) was µs=0.083±0.013.

Fig. 20. Coefficients of static friction for a lubricated contact with steel wire against ADI. Theaverage value at 38.4...96 N load (26 measurement points) was µs=0.079±0.012.



Fig. 21. Wear scar on the (left image) steel wire and the (right image) corresponding ADI discsample after the test cycle without lubrication. The direction of motion of the steel wire sample washorizontal in the images.

Fig. 22. Wear scar on the (left image) steel wire and the (right image) corresponding ADI discsample after the test cycle with lubrication. The direction of motion of the steel wire sample washorizontal in the images.

Unlubricated contact of steel againstaustempered ductile cast iron

Under unlubricated conditions, the coefficientof static friction for steel against ADI wasamong the lowest within the present study, orµ=0.083, and quite stable over the load levelsand number of contacting events. The lowcoefficient of friction is in good agreementwith the low degree of surface alterations onthe steel wire and ADI surfaces, indicatingvery low degrees of plastic deformation and

adhesive wear. The low degree of plasticdeformation of the contact surfaces reflectsthe fairly high hardness of the two materials,while the low degree of adhesive wear reflectsa solid-lubricating effect of the graphitespheres in spite of the hard metallic matrix.

Lubricated contact of steel againstaustempered ductile cast iron

With lubrication, the coefficient of staticfriction (µ=0.079) was slightly reduced and



the wear or deformation of the steel and ADIsurfaces was reduced. Also in this case, thecoefficient of friction showed a fairly goodstability over the number of contact events,although a small decrease could be noticedwith higher loads. In terms of the coefficientof static friction, the steel-against-ADIcombination can be regarded as ratherinsensitive to the lubricating conditions withinthe range studied.

Steel against P/F-CDI

Graphs showing the coefficients of staticfriction for steel wire against the pearlitic-ferritic chill-cast SG iron at 9.6…96 N load,under unlubricated and petrolatum-lubricatedconditions, respectively, are presented in theFigs. 23 and 24, and micrographs showingwear scars on the samples after the test cyclesare presented in the Figs. 25 and 26.

Fig. 23. Coefficients of static friction for unlubricated contact with steel wire against pearlitic-ferritic chill-cast SG iron. The average value at 38.4...96 N load (26 measurement points) wasµs=0.234±0.032.

Fig. 24. Coefficients of static friction for a lubricated contact with steel wire against pearlitic-ferritic chill-cast SG iron. The average value at 38.4...96 N load (26 measurement points) wasµs=0.083±0.016.



Fig. 25. Wear scar on the (left image) steel wire and the (right image) corresponding pearlitic-ferritic chill-cast SG iron disc sample after the test cycle without lubrication. The direction ofmotion of the steel wire sample was horizontal in the images.

Fig. 26. Wear scar on the (left image) steel wire and the (right image) corresponding pearlitic-ferritic chill-cast SG iron disc sample after the test cycle with lubrication. The direction of motionof the steel wire sample was horizontal in the images.

Unlubricated contact of steel againstpearlitic-ferritic chill-cast SG iron

The coefficient of static friction for steelagainst the pearlitic-ferritic CDI showed somescatter and a decreasing trend at higher loads.The average value for the coefficient offriction, µ=0.234, was the highest among thecombinations of materials and lubricatingconditions studied in this work and close tothat of the unhardened cast irons inunlubricated contact with steel wire. The wire

and disc surfaces show traces of stronginteraction, which is in agreement with thehigh level of the coefficient of friction. Thesurface of the pearlitic-ferritic CDI discshows less clear graphite areas than what wasseen on the SG irons, hence the availability ofsolid lubricant in the interface can be assumedas poor.



Lubricated contact of steel againstpearlitic-ferritic chill-cast SG iron

The lubrication gave a strong reduction in thecoefficient of static friction of the steel andP/F-CDI couple. On the cast iron disc, almostno wear marks were found. On thecorresponding steel wire, a bright, polishedarea was found. The area of the wear surfaceon the steel wire slider from the lubricatedtest looks larger than that of the wire samplefrom the unlubricated test, while the latter hadbeen subjected to more extensive mutationsand appears to have been partially covered bya – possibly work hardened - transfer layer. Inthis test, the availability of graphite for thelubrication of the contact was as poor as

under the unlubricated conditions, andtherefore the reduction in the coefficient ofstatic friction has to be addressed to theparaffinic vaseline.

Steel against P-CDI

Graphs showing the coefficients of staticfriction for steel wire against the pearliticchill-cast SG iron at 9.6…96 N load, underunlubricated and petrolatum-lubricatedconditions, respectively, are presented in theFigs. 27 and 28, and micrographs showingwear scars on the samples after the test cyclesare presented in the Figs. 29 and 30.

Fig. 27. Coefficients of static friction for unlubricated contact with steel wire against pearlitic chill-cast SG iron. The average value at 38.4...96 N load (26 measurement points) was µs=0.072±0.008.

Fig. 28. Coefficients of static friction for a lubricated contact with steel wire against pearlitic chill-cast SG iron. The average value at 38.4...96 N load (26 measurement points) was µs=0.064±0.009.



Fig. 29. Wear scar on the (left image) steel wire and the (right image) corresponding pearlitic chill-cast SG iron disc sample after the test cycle without lubrication. The direction of motion of the steelwire sample was horizontal in the images.

Fig. 30. Wear scar on the (left image) steel wire and the (right image) corresponding pearlitic chill-cast SG iron disc sample after the test cycle with lubrication. The direction of motion of the steelwire sample was horizontal in the images.

Unlubricated contact of steel againstpearlitic chill-cast SG iron

Without lubrication, the pearlitic CDI showeda stable coefficient of friction of µ=0.072,which was the lowest value within the presentgroup of unlubricated couples and similar tothat obtained with the ADI and CrC materialswithout lubrication. Metal-to-metalinteractions between the steel wire slider andthe pearlitic CDI disc must, however, haveoccurred at some stage of the test cycle,

obvious from the surface images taken afterthe test. The transfer layer, which appears tohave been formed on the steel slider, seems tohave suppressed the wear of the steel surface,the area of which was not significantly largerthan after the corresponding lubricated test.

Lubricated contact of steel against pearliticchill-cast SG iron

The introduction of the chosen lubricant intothe interface between the steel wire and P-



CDI disc surfaces furthermore lowered thecoefficient of friction to the level µ=0.064,which was the lowest one in the present study.After the test cycle under the conditions oflubrication, no wear scars of significancewere found on the P-CDI disc, whilecorrespondingly the steel slider was mainlycharacterized by a shining contact surface.The coefficient of static friction for the steeland P-CDI combination was rather insensitiveto the present two lubricating conditions.

Steel against thermal spray coated CrC-TS1

Graphs showing the coefficients of staticfriction for steel wire against the thermalspray coated CrC-TS1 at 9.6…96 N load,under unlubricated and petrolatum-lubricatedconditions, respectively, are presented in theFigs. 31 and 32, and micrographs showing thewear scars on the samples after the tests arepresented in the Figs. 33 and 34. On thethermal spray coated disc, no wear scar wasfound after the lubricated test sequence.

Fig. 31. Coefficients of static friction for unlubricated contact with steel wire against the thermalspray coated CrC-TS1 coating. The average value at 38.4...96 N load (26 measurement points) wasµs=0.104±0.008.

Fig. 32. Coefficients of static friction for a lubricated contact with steel wire against the thermalspray coated CrC-TS1 coating. The average value, valid for 38.4...96 N load (26 measurementpoints), was µs=0.073±0.005.



Fig. 33. Wear scar on the (left image) steel wire and the (right image) corresponding CrC-TS1thermal spray coated sample after the test cycle without lubrication. The direction of motion of thesteel wire sample was horizontal in the images.

Fig. 34. Wear scar on the steel wire sample after the test cycle against the thermal spray coatedCrC-TS1 coating with lubrication. On the disc surface, no marks from the measurements werevisible. The direction of motion of the steel wire sample was horizontal in the image.

Unlubricated contact of steel against thecommercial CrC-based thermal spraycoating

For steel against the commercial CrC-TS1coating without lubrication, the coefficient ofstatic friction (µ=0.10) lied between thelowest and highest values for the unlubricatedcouples in this work, and was quite stableregardless of loads and number of slidingevents. On the CrC coated disc, some ferrous

deposits occurred after the test cycle, but noother surface alterations were observed on thedisc. The wire surface alterations were mainlylimited to polishing.

Lubricated contact of steel against thecommercial CrC-based thermal spraycoating

Lubrication decreased the level of thecoefficient of static friction for steel on the



commercial CrC-TS1 coating to µ=0.073 andthe level remained very stable during the testseries. The steel wire surface became slightlypolished by micro abrasion during the testcycle, but on the CrC coated disc the surfacealterations had been so small that no wearscars could be detected under the light opticalmicroscope.

Steel against thermal spray coated WC-TS2

Graphs showing the coefficients of staticfriction for steel wire against the thermalspray coated WC-TS2 at 9.6…96 N load,under unlubricated and petrolatum-lubricatedconditions, respectively, are presented in theFigs. 35 and 36, and micrographs showingthe wear scars on the test samples after thetests are presented in the Figs. 37 and 38. Onthe thermal spray coated disc, no wear scarwas found after the lubricated test sequence.

Fig. 35. Coefficients of static friction for unlubricated contact with steel wire against the thermalspray coated WC-TS2 coating. The average value at 38.4...96 N load (26 measurement points) wasµs=0.134±0.012.

Fig. 36. Coefficients of static friction for a lubricated contact with steel wire against the thermalspray coated WC-TS2 coating. The average value at 38.4...96 N load (26 measurement points) wasµs=0.075±0.006.



Fig. 37. Wear scar on the (left image) steel wire and the (right image) corresponding WC-TS2thermal spray coated sample after the test cycle without lubrication. The direction of motion of thesteel wire sample was horizontal in the images.

Fig. 38. Wear scar on the steel wire sample after the tests cycle against the thermal spray coatedWC-TS2 coating with lubrication. On the disc surface, no traces from the measurements werevisible. The direction of motion of the steel wire sample was horizontal in the image.

Unlubricated contact of steel against theWC-based thermal spray coating

When unlubricated, the tungsten carbidebased cermet coating WC-TS2 gave acoefficient of static friction that was higher(µ=0.13) than that of the two CrC cermetcoatings. The surface alterations on the steelwire slider and the WC coated disc stronglyresembled those of the two unlubricated steel-CrC combinations, in particular in terms of

the tiny scratches and the ferrous depositspots on the disc.

Lubricated contact of steel against the WC-based thermal spray coating

With lubricant added, the coefficient of staticfriction for steel and WC fell to almost thesame level (µ=0.075) as that of steel againstthe two CrC coatings when lubricated. On theWC disc, no wear marks were found by light



optical microscopy, and on the correspondingsteel wire slider only a shining wear scaroccurred.

Steel against thermal spray coated Mo-TS3

Graphs showing the coefficients of staticfriction for steel wire against the thermalspray coated Mo-TS3 coating at 9.6…96 N

load, under unlubricated and petrolatum-lubricated conditions, respectively, arepresented in the Figs. 39 and 40, andmicrographs showing the wear scars on thetest samples after the tests are presented in theFigs. 41 and 42. On the thermal spray coateddisc, no wear scar was found after thelubricated test sequence.

Fig. 39. Coefficients of static friction for unlubricated contact with steel wire against the thermalspray coated Mo-TS3 coating. The average value at 38.4...96 N load (26 measurement points) wasµs=0.183±0.023.

Fig. 40. Coefficients of static friction for a lubricated contact with steel wire against the thermalspray coated Mo-TS3 coating. The average value at 38.4...96 N load (26 measurement points) wasµs=0.075±0.002.



Fig. 41. Wear scar on the (left image) steel wire and the (right image) corresponding Mo-TS3thermal spray coated sample after the test cycle without lubrication. The direction of motion of thesteel wire sample was horizontal in the images.

Fig. 42. Wear scar on the steel wire sample after the tests cycle against the thermal spray coatedMo-TS3 coating with lubrication. On the disc surface, no traces from the friction measurementswere visible. The direction of motion of the steel wire sample was horizontal in the image.

Unlubricated contact of steel against theMo-based thermal spray coating

Under dry conditions, the coefficient of staticfriction for the steel and Mo-TS3 couple washigher (µ=0.183) than for steel against theother four thermal spray coatings withoutlubrication. This could be explained by thesurface roughness (Ra=0.18 µm) of the Mocoated disc, which was higher than that of theother four thermal spray coatings, but as willbe seen below, the surface roughness as such

cannot be the sole reason for the highcoefficient of static friction. As the result ofthe fairly strong tribological interactions, thecontact surfaces of the steel wire slider andthe Mo disc revealed tribolayers of adhesivelyattached material.

Lubricated contact of steel against the Mo-based thermal spray coating

The paraffinic vaseline reduced thecoefficient of static friction for the steel and



Mo-TS3 couple (µ=0.075) to approximatelythe same level as that of the four otherthermal spray coatings. The Mo coated discdid not reveal any marks from the slidingevents. During the test cycle, the contactsurface of the steel wire slider had beenpolished against the Mo coated disc surface.

Steel against thermal spray coated CrC-TS4

Graphs showing the coefficients of staticfriction for steel wire against the thermalspray coated CrC-TS4 at 9.6…96 N load,under unlubricated and petrolatum-lubricatedconditions, respectively, are presented in theFigs. 43 and 44, and micrographs showing thewear scars on the test samples after the testsare presented in the Figs. 45 and 46. On thethermal spray coated disc, no wear scar wasfound after the lubricated test sequence.

Fig. 43. Coefficients of static friction for unlubricated contact with steel wire against thermal spraycoated CrC-TS4. The average value at 38.4...96 N load (26 measurement points) wasµs=0.073±0.005.

Fig. 44. Coefficients of static friction for a lubricated contact with steel wire against thermal spraycoated CrC-TS4. The average value at 38.4...96 N load (26 measurement points) wasµs=0.073±0.006.



Fig. 45. Wear scar on the (left image) steel wire and the (right image) corresponding CrC-TS4thermal spray coated sample after the test cycle without lubrication. The direction of motion of thesteel wire sample was horizontal in the images.

Fig. 46. Wear scar on the steel wire sample after the tests cycle against the thermal spray coatedCrC-TS4 coating with lubrication. On the disc surface, no marks from the measurements werevisible. The direction of motion of the steel wire sample was horizontal in the image.

Unlubricated contact of steel against theexperimental CrC-based thermal spraycoating

The coefficient of static friction for steel wireagainst the experimental CrC-TS4 coatingwith no lubricant applied was as low asµ=0.073, and the value remained almost thesame during the entire test cycle. Tinyscratches and some ferrous depositscharacterized the sliding track on the CrC disc

surface, while the steel wire revealed ashining contact surface.

Lubricated contact of steel against theexperimental CrC-based thermal spraycoating

The introduction of the lubricant into the steeland CrC-TS4 interface did practically notaffect the coefficient of static friction, whichalso in this case was µ=0.073. The wear was,however, reduced by the lubricant, as on the



CrC disc surface no wear track was visibleunder the light optical microscope.

Steel against thermal spray coated VC-TS5

Graphs showing the coefficients of staticfriction for steel wire against the thermalspray coated VC at 9.6…96 N load, under

unlubricated and petrolatum-lubricatedconditions, respectively, are presented in theFigs. 47 and 48, and micrographs showing thewear scars on the samples after the test cycleare presented in the Figs. 49 and 50. On thethermal spray coated disc, no wear scar wasfound after the lubricated test sequence

Fig. 47. Coefficients of static friction for unlubricated contact with steel wire against thermal spraycoated VC. The average value at 38.4...96 N load (26 measurement points) was µs=0.136±0.009.

Fig. 48. Coefficients of static friction for a lubricated contact with steel wire against thermal spraycoated VC. The average value at 38.4...96 N load (26 measurement points) was µs=0.066±0.006.



Fig. 49. Wear scar on the (left image) steel wire and the (right image) corresponding VC-TS5thermal spray coated sample after the test cycle without lubrication The direction of motion of thesteel wire sample was horizontal in the images.

Fig. 50. Wear scar on the steel wire sample after the tests against the thermal spray coated VC-TS5coating with lubrication. On the disc surface, no marks from the measurements were visible. Thedirection of motion of the steel wire sample was horizontal in the image.

Unlubricated contact of steel against theVC-based thermal spray coating

Under unlubricated sliding conditions, thecoefficient of static friction for steel againstthe VC coating stayed on a fairly stable levelof µ=0.14 during the test cycle. The relativelyhigh coefficient of friction correlates with thetiny scratches and the patches of a thin,ferrous transfer layer that characterized thesliding surface on the VC-coated disc after thetest cycle.

Lubricated contact of steel against the VC-based thermal spray coating

The application of the lubricant into thesliding interface reduced the coefficient ofstatic friction for steel wire and the VCcoating to µ=0.066, which is among thelowest values within the present experimentalinvestigation. Under lubrication, no visiblepatches of a transfer layer were formed on thesliding track of the VC coated disc.



Overview of the measured coefficients ofstatic friction

Figure 51 summarizes the average values ofthe present, measured coefficients of staticfriction within the normal force interval 35-100 N. In the Figures 52 and 53, the

development in the average value for thecoefficient of friction within the normal forceinterval 35-100 N is shown for theunlubricated and lubricated sliding conditions,respectively.

Fig. 51. Graphical summary of the average values for the coefficients of static friction measuredunder 35-100 N normal force in the present work. The vertical bar at the upper end of each columnindicates the standard deviation of the average value.



Fig. 52. The development in the average value for the coefficient of friction within the normal forceinterval 35-100 N under unlubricated sliding conditions.

0,000

0,050

0,100

0,150

0,200

0,250

0,300Fe

rritic

SG

iron

Pea

rlitic

SG

iron

Gre

yca

stiro

n

AD

I

Fe-P

CD

I

P-P

CD

I

CrC

-TS

1

WC

-TS

2

Mo-

TS3

CrC

-TS

4

VC

-TS

5

First upFirst downSecond upSecond down

µAverage for Fn=35-100N, unlubricated

Coe

ffici

ento

fsta

ticfri

ctio

n,µ S

t[-]



Fig. 53. The development in the average value for the coefficient of friction within the normal forceinterval 35-100 N under lubricated sliding conditions.

DISCUSSION

High-friction and low-friction material pairs

A feature in common of all the tests with steelwire against cast iron was that the coefficientof static friction was higher at low loads thanat higher loads. The reason for the decrease inthe coefficient of static friction with loadwould require further studies. One possibleexplanation is that the surface roughness,abrasion and a tendency for plowing frictionplays a relatively greater role as long as thenormal force is low. Furthermore, at highercontact pressure and increasing elastic-plastic

deformation of the iron matrix that surroundsthe graphite spheres or lamellae, thesqueezing-out of near-surface graphite fromthe iron matrix and the formation ofdistributed graphite film patches can beassumed to offer friction reduction [5, 6].

The overviews of the static coefficients offriction for the present material pairs andlubrication conditions show a large variation,with average values ranging from µ=0.07 toµ=0.23. The lowest average static coefficientsof friction were obtained for steel wire in

0,000

0,050

0,100

0,150

0,200

0,250

0,300

Ferri

ticS

Giro

nP

earli

ticS

Giro

nG

rey

cast

iron

AD

IFe

-PC

DI

P-P

CD

IC

rC-T

S1

WC

-TS

2M

o-TS

3C

rC-T

S4

VC

-TS

5

First upFirst downSecond upSecond down

µAverage for Fn=35-100N, lubricatedC

oeffi

cien

tofs

tatic

frict

ion,

µ St[-]



contact with the ADI, the PCDI and the CrC-coated plate materials, both lubricated andunlubricated. The highest average staticcoefficients of friction were obtained in theunlubricated tests with grey cast iron, Fe-PCDI and SG iron plates, in the unlubricatedtests with the Mo, VC and WC coated platesand in the lubricated tests with grey cast ironplates.

The reason for the large differences in thestatic coefficients of friction is a complexcombination of chemical and physical factors.Among these, the chemical affinity, or thelack of it, between the steel wire and therespective plate materials is a dominatingeffect, often referred to as the adhesionbetween materials. According to early workby Bowen and Tabor [7], and by Rabinowicz[8], the tendency of the mating surfaces toform chemical joints largely determines thecoefficient of friction. In the case of theferrous metal plates and the metal-coatedplates, with the exception of the ADI andPCDI plates, iron-to-iron joints can beassumed as at least partly responsible for thehigh static coefficients of friction in theunlubricated and even in the lubricatedcontacts.

The dominating physical effect that affectsthe static coefficient of friction is theresistance against plowing (or the onset ofplowing, as the present work is dealing withstatic friction). Softer materials, like thepresent grey and SG irons, show a tendency tohigher friction forces due to plowing, whilethe hardest materials, like the present CrC-coatings, lead to lower plowing forces. Recentstatic friction tests by Dunn and co-workers,with steel against steel prepared for differentsurface roughness have shown that roughersurfaces lead to higher static coefficients offriction than do smoother surfaces, due tointerlocking of topographical features in therougher surfaces and the additional forcesneeded to shear the joints and to causescratches [9].

The standard deviation values of the averagestatic coefficients of friction seem to have atendency to increase with the level of thecoefficient of friction, most likely because thehigh average values are formed from bothhigh and low values, while the low averagevalues are mainly formed from low valuesonly. This may reflect the fact that a highcoefficient of friction causes more severechanges to the contact surface than does a lowcoefficient of friction in a similar situation.

When comparing the tribological behavior ofthe two present combinations of steel wireand CrC coated discs, with each other andwith the other present material combinations,it becomes obvious that this type of coatingcan offer a low coefficient of static frictionand contact surfaces that are not largelyaffected by the contacting events within theload and contact pressure levels studied.

Benefit from lubrication by paraffinicpetrolatum

For all the material pairs of the present study,except for that comprising the CRC-TS4coating, the static coefficient of friction waslower in the lubricated tests than in theunlubricated ones. As shown in the overviewsof the Figures 51-53, the lubricant not onlyreduced the average static coefficient offriction, but also reduced the differencesbetween the material combinations. Thereducing and equalizing effect most likelycomes from a reduction in the adhesion, dueto the interlayer that was physicallyintroduced between the steel wire and theplate sample surfaces. Due to the lack ofadditives in the petrolatum lubricant, and dueto the low sliding velocity in the slidingcontacts, chemical interactions between thelubricant or its constituents and the slidingsurfaces are less pronounced.



While a minute amount of the present,additive-free paraffinic lubricant wassufficient to prevent adhesive wear andtribolayer formation on all of the tribologicalcontact surfaces studied, the tests withoutlubrication caused the formation of tribolayerson the contact surfaces even at the presentlow sliding velocities and short slidingdistances.

The ability of the lubricant to suppress thealterations in the tribological contact surfacesduring the tests is reflected by the magnitudeof the standard deviation bars in the bargraph(Figure 51) for the average static coefficientsof friction, or the scatter in the measurements,which in general was lower in the lubricatedtests. The lowest standard deviation levelsoccurred with the wire/coating combinationswhen lubricated, probably because the wearand the surface roughness alterations in thesecases remained minute.

In the present petrolatum-lubricated tests, therecorded coefficients of static friction for steelagainst the three cast iron grades werepartially higher than the results presented byHwang and Zum Gahr for a hardened,polished steel ball in contact with a hardened,ground steel plate under lubrication by amineral oil [10]. However, the present initialvalue (µs=0.15) for the coefficient of staticfriction at lubricated conditions is close to thatpresented by Bowden and Tabor in theAppendix of Ref. [7], which states µs=0.21for a steel against a cast iron under lubricationwith mineral oil.

The benefit of the lubrication can be seen inthe influence of the subsequent loading cycleson the average value for the static coefficientof friction. For most of the unlubricatedsliding couples, the average value for thestatic coefficient of friction increased with thenumber of sliding cycles, as shown in Figure52. In the lubricated tests, however, theaverage value for the static coefficient offriction of most of the sliding couples was

decreased, see Figure 53, most likely owing toa beneficial running-in effect in the presenceof the lubricant.

Possibly because of the relatively high surfaceroughness of the grey cast iron disc, wearmarks of significant size were formed on thesteel wire and cast iron disc samples duringthe petrolatum lubricated measurements. Onthe steel wire surface, a shining flat withnumerous tiny scratches had been formed,probably due to an abrasive action by theroughness of the cast iron surface through thethin lubricant film. As a consequence of theinteraction, the peaks of the disc surfaceroughness seem to have been abraded by thesteel wire, in addition the formation ofscratches in the direction of themeasurements.

CONCLUSIONS

• For drawn, high-strength steel wireagainst the present counter materials,the coefficient of static friction withoutlubrication ranged from µ=0.07 toµ=0.23, and under petrolatum-lubricated conditions, the correspondingvalues were about µ=0.06 to µ=0.12. Aminor effect of the surface roughness onthe coefficient of friction was identified.

• Among the material combinationsstudied, the highest values for thecoefficient of static friction wereobtained with the two nodular cast irongrades, the grey cast iron and thepearlitic-ferritic chill-cast SG iron inunlubricated contact with the steel wire.

• Low values for the coefficient of staticfriction against steel wire were obtainedunder lubricated conditions with all thedisc materials except the grey cast iron.Consequently, the highest values for thecoefficient of static friction, bothlubricated and unlubricated, wereobtained when the steel wire was incontact with grey cast iron.



• Low and stable coefficients of staticfriction against steel wire, bothlubricated and unlubricated, wereobtained with ADI, pearlitic chill-castSG iron and CrC as counter material.The latter indicates the benefit of usingthis type of coating as a surfaceengineering method for cast iron.

• In the unlubricated tests, transfer ofmaterial from one counter-surface toanother occurred even at the present lowvelocities and short sliding distances.The performance of the additive-free,paraffinic lubricant was sufficient forpreventing adhesive wear and materialstransfer in this study.

ACKNOWLEDGEMENTS

The study was carried out as part of theFinnish joint industrial consortium strategicresearch action coordinated by FIMECC Ltd,within the program on BreakthroughMaterials called DEMAPP, in the Friction andEnergy Project. The authors gratefullyacknowledge the financial support of Tekes,the Finnish Funding Agency for Technologyand Innovation, the participating companies,and VTT Technical Research Centre ofFinland, and the cast iron samples from theComponenta Corporation. The authors arefurthermore grateful to the former colleaguesSenior Research Technician Sini Eskonniemiof VTT Technical Research Centre ofFinland, for the surface roughnessmeasurements, and Research Scientist AnderAmasorrain Urrutia of VTT TechnicalResearch Centre of Finland, for hiscontribution to the performing of the staticfriction measurements.

REFERENCES

1. Jost, H. P. (Ed.). Lubrication (Tribology),Education and research, A report on thepresent position and indystry's needs.London, UK, 1966, Her Majesty'sStationary Office, 80 p.

2. Bhusan, B. (ed.). Modern tribologyhandbook, volume one, principles oftribology. Boca Raton, FL, USA, 2001,CRC Press LLC, 765 p.

3. Rabinowicz, E. Friction coefficient ofnoble metals over a range of loads. Wear,1992, 159, 89-94.

4. Dunkin J.E. and Kim D.E. Measurementof static friction coefficient between flatsurfaces. Wear, 1996, 193, 186-192.

5. Quanshun Luo, Jingpei Xie and YanpeiSong, Effects of microstructures on theabrasive wear behaviour of spheroidal castiron. Wear, 1995, 184 (1), 1-10.

6. Ben Tkaya, M., Mezlini, S., El Mansori,M. and Zahouani, H. On some tribologicaleffects of graphite nodules in wearmechanism of SG cast iron: Finiteelement and experimental analysis. Wear,2009, 267 (1-4), 535-539.

7. Bowden, F. P. and Tabor, D. The frictionand lubrication of solids, Part I. Oxford,UK, 1971, at the Clarendon Press, 374 p.

8. Rabinowicz, E. Determination of thecompatibility of metals through staticfriction tests. ASLE Trans., 1971, 14 (3),198-205.

9. Dunn, A., Wlodarczyk, K.L., Carstensen,J.V., Hansen, E.B., Gabzdyl, J., et al.Laser surface texturing for high frictioncontacts. Appl. Surf. Sci, 2015, in press.

10. Hwang, D.-H. and Zum Gahr, K.-H.,Transition from static to kinetic friction ofunlubricated or oil lubricated steel/steel,steel/ceramic and ceramic/ceramic pairs.Wear, 2003, 255, 365–375.

Documents

STATIC FRICTION MEASUREMENTS ON STEEL AGAINST …