Friction and wear properties of black oxide surfaces in rollingsliding contacts

Friction and wear properties of black oxide surfaces in rolling/slidingcontactsCarl H.Hager Jr.n,Ryan D.EvansThe Timken Company,North Canton,OH44720,USAa r t i c l e i n f oArticle history:Received23March2015Received in revised form24June2015Accepted25June2015Available online3July2015Keywords:Black oxideBlack oxidizingFrictionWeara b s t r a c tBlack oxide chemical conversion treatment of steel surfaces was originally developed as a low-costmethod to improve the corrosion resistance of the base material.Over time,this surface treatment wasintroduced on tribological components amid claims that it improved galling and smearing resistance.Inrolling element bearings,smearing damage can occur during high transient slip events between rollerbodies and raceways,and black oxide surface treatments have recently been prescribed as a solution forthis mode of adhesive wear damage.Therefore,fundamental friction and wear experiments using uni-directional sliding and rolling/sliding contacts were conducted to explore the performance of black oxidesurfaces in tribological contacts.From this work,it was concluded that mated black oxide surfaces yield similar or lower friction ascompared to mated steel surfaces(in lubricated contacts),exhibit high wear in lowλconditions,and canmitigate adhesive wear damage as long as the oxide layer remains intact.&2015Elsevier B.V.All rights reserved.1.IntroductionThe service life of a rolling element bearing is often limited orreduced by component damage or material ponentdamage can result from wear,external contaminant ingress,severeapplication conditions,neglect,or misuse.One particular type oflife-limiting bearing damage is called“smearing”.Smearing is aterm that is often used to describe adhesive wear damage inrolling/sliding contacts when sliding is substantial and the lubri-cantfilm thickness is insufficient to adequately separate the con-tacting surfaces[1].Damage associated with smearing may occurwhen rolling elements slide along the raceway surface as theyenter or exit the loaded zone[2].In some cases,this type ofdamage can be mitigated through component and/or systemdesign.However,there are some instances where the damagecannot be avoided without modifying the tribological contactsurfaces.If smearing damage cannot be avoided from a component orsystem design approach,another way to mitigate adhesive weardamage is to change the chemical composition of one or both ofthe surfaces in contact.In1978,Gregory wrote a paper providing abrief summary of conversion coatings available to improve adhe-sive wear resistance[3].Gregory suggests that black oxide surfaces(when oiled or waxed)can yield low friction and resistance toadhesive wear damage.In1998,Philström and Ström published amaster’s thesis where black oxidizing was applied to sphericalroller bearings(Part number22222E/C3–110mm bore diameter–200mm outer diameter)[4].In their work,the authors con-ducted smearing tests in bearings by periodically stopping therollers while the shaft was still rotating.This was done for a totalof100cycles at eight different shaft speeds(100,200,400,800,1600,2400,3200,and4500rpm).If no smearing damage wasfound on the bearings after each shaft speed,the shaft speed wasraised to the next level.Upon the completion of this testing withand without black oxidized rollers,Philström and Ström foundthat black oxidized rollers yielded a small improvement insmearing resistance as compared to uncoated rollers[4].A few years later,Scherb and Zech developed a smearing testusing cylindrical bearings from which the authors concluded thatblack oxide can prevent smearing damage and reduce frictionaltorque by up to20%[5].In2010,Mihailidis et al.conductedsmearing tests with a two-disc machine where the entrainmentvelocity was held constant and the slide-to-roll ratio was increaseduntil severe adhesive wear occurred between the mated samples[6].These tests demonstrated that the black oxidized discs couldsurvive higher levels of slide-to-roll ratio compared to theuntreated discs.In addition,tests were also conducted with aconstant slide-to-roll ratio and demonstrated that the black oxi-dized discs exhibit higher wear rate and lower friction as com-pared to the untreated discs during a wear-in cycle.Although the aforementioned work found that black oxidesurfaces yield some benefit in reducing adhesive wear or smearingContents lists available at ScienceDirectjournal homepage:/locate/wearWear/10.1016/j.wear.2015.06.0130043-1648/&2015Elsevier B.V.All rightsreserved.n Corresponding author.Tel.:þ12342622077;fax:þ12342622282.E-mail address:carl.hager@(C.H.Hager Jr.).Wear338-339(2015)221–231damage compared to untreated surfaces,in2013Evans et al. concluded that black oxide treatment of rollers and raceways did not provide additional smearing protection in cylindrical roller bearing tests conducted under severe conditions[7].In this work, the authors conducted bearing tests where the load zone was shifted180°every15s.In three separate tests,the bearings with black oxidized raceways and rollers exhibited smearing damage after just one load cycle.Bearings that were untreated with ground raceways exhibited smearing after just one load cycle as well.This conclusion seems contrary to the previous bearing tests by Scherb and Zech[5]and the two-disc experiments by Mihailidis et al.[6], but seems to agree with the spherical bearing tests conducted by Philström and Ström[4].Although there are conflicting results in the literature(which may be due to the severity of the testing),black oxide surface treatments have recently been prescribed as a solution to mitigate various types of damage in rolling element bearings[8].In addi-tion,the literature also suggests that there may be other benefits to using black oxide such as low friction and high wear rates during break-in[5,6].Therefore,the focus of this work was to conduct bench-level tribological experiments to determine the friction and wear properties of black oxide surfaces in rolling/ sliding contacts.Several different experimental techniques and results are described in sequence and then drawn together with discussion and general conclusions.2.Black oxideAlthough black oxide or black oxidizing can be used to describe a number of surface coatings on steel,the term“black oxide”in this work refers to the conversion of a steel surface to Fe3O4or magnetite using a hot alkaline bath.This type of black oxide sur-face is created using a process that was developed in Germany in the early1900s and has been used to produce an attractive black finish,impart moderate corrosion resistance(with the addition of secondary oil impregnation or wax),enhance lubricity,and resist galling[9].It should be noted that the extent of these claimed benefits rely on a secondary process of oil or wax impregnation.In fact,many vendors state that the anti-galling properties of black oxide surfaces allow for the outer layer to be sacrificed during initial contact.Therefore,the black oxide layer is galling resistant in that it wears away without adhesive wear damage to the mated surfaces.The black oxide layer is generated by placing the steel com-ponents into a hot aqueous alkaline nitrate bath.The concentra-tion of the bath is controlled by boiling the solution at a desired temperature between285°F(141°C)and310°F(154°C)[10].This can be done using either a single-bath or double-bath process.The single-bath process typically uses one tank with a temperature range from285°F(141°C)to290°F(143°C)[10].The double-bath process typically uses a second tank with a temperature range from305°F(152°C)to310°F(154°C)[10].Depending on the process,the black oxide layer thickness can range from0.5to 3m m.A common standard for defining black oxide surface treat-ments on ferrous materials is DIN50938.Although the black oxide process is well documented,there is comparatively less data published on the properties and micro-structure of the black oxide surface layer.In this work,a through hardened AISI52100steel disk(HRC58-62)was metallo-graphically polished and then black oxidized using a double-bath process.The microstructure of the black oxide surface layer was analyzed using a focused ion beam to mill a trench followed by inspection of the cross section with a high-resolution scanning electron microscope(SEM).Fig.1shows a secondary electron SEM The oxide layer itself is dense,but there are pre-existing carbideparticles contained within the microstructure that exhibit voids atthe boundaries between the carbide and oxide materials.Mea-surements of the coating thickness varied from$1.4to2.2μm for this sample.In comparison,Evans et al.[11]published a trans-mission electron micrograph of a single-bath black oxide surfacecross section.The view shown has a thinner(200–500nm)blackoxide layer that did not exhibit voids at the carbide/oxide interfacelike those observed for the double-bath layer in Fig.1.The mechanical properties of the black oxide layer were mea-sured with a nanoindenter using the continuous stiffness techni-que[12]and a Berkovich tip.The indentation parameters includeda depth limit of2500nm,a strain rate of0.1sÀ1,a harmonicdisplacement target of1nm,a frequency of45Hz,and an assumedPoisson’s ratios for black oxide(0.25)and untreated steel(0.3).Fifteen replicate tests were performed on each sample.In additionto measurements on the black oxide surface,a second throughhardened AISI52100steel disk was metallographically polishedand analyzed along with the black oxidized sample.The mea-surement results from both surfaces are summarized in Table1.Based on these measurements,the black oxide surface layer hasapproximately25%of the hardness and27%of the modulus of thebase steel surface.These results were also published by Evans et al.in2014[11].3.Experimental detailsAll sliding and rolling sliding wear experiments were con-ducted using a Wedeven Associates Inc.manufactured WAMsc4test machine.This machine is able to run block-on-ring experi-ments with a stationary block and a rotating cylinder,and can alsoperform rotating cylinder-on-cylinder or rotating ball-on-cylinderrolling/sliding wear experiments.The machine has two opposingspindles that can be controlled independently.One of the spindlesis mounted on three load cells and an actuator that allows for it tomove toward the opposing spindle in the contact plane.Movingone spindle toward thefixed second spindle allows for a normalFig.1.High-resolution scanning electron microscope image of black oxide surface layer.Table1Nanoindentation mechanical property comparison for untreated AISI52100steel and black oxide layer[11].Sample Avg.hard-ness(GPa)Depth rangefor avg.hard-ness(nm)Avg.elasticmodulus(GPa)Depth rangefor avg.mod-ulus(nm) Untreated AISI52100steel10.670.5100–200230710100–200Double-bathblack oxide2.670.3200–300617580–100C.H.Hager Jr.,R.D.Evans/Wear338-339(2015)221–231 222rotating.The friction between the test samples was measured by the vertical load cells mounted underneath the test spindle,with the actuator system,and/or by a torque sensor on the same spindle.All of the cylinder,block,and ball geometry test samples used for this work were manufactured from hardened AISI52100steel (HRC58-62).The cylinders were fabricated with a35mm outside diameter and a25.4mm width.All the cylinders were ground to yield a457–558nm Ra surfacefinish.The block test samples were 19.05mm squares with a7.874mm thickness.The edges of the blocks were chamfered so that the block-on-ring test configuration yielded a6.5mm contact width.The test surfaces of the blocks were ground to yield a356–457nm Ra surfacefinish.The ball samples were20.6375mm diameter ABMA Grade25balls with a 6.5mm diameter hole in the center.The center hole permitted the balls to be bolted into a collet and driven by a spindle for rolling/ sliding wear experiments.The cylinder samples had a taper on the inner diameter so that they could be held onto a spindle nosepiece with a tapered outer diameter.All black oxidized test samples were manufactured similarly to the untreated samples,and then conversion coated using the double-bath process.3.1.Sliding friction test descriptionSimple lubricated sliding wear experiments were conducted to compare the lubricated sliding friction of the black oxidized sur-faces to that of the base steel material.For these experiments, 35mm diameter through hardened AISI52100steel cylinders were loaded against a through hardened AISI52100steel block with a contact width of6.5mm.The sliding experiments were conducted with a linear sliding velocity of0.5m/s and a contact stress of200MPa.The friction force was measured with three vertical load cells that support the stationary block sample.The tests were lubricated with an ISO VG68poly-alpha-olefin(PAO) oil,without extreme pressure(EP)or anti-wear(AW)additives,and an inlet temperature of40°C.Fig.2shows a picture of the samples in contact.Three experiments were conducted with untreated steel rings mated with untreated steel blocks,and three experiments were conducted with a steel ring that was black oxidized and mated with an untreated steel block.The test duration was20min to establish a steady-state friction measurement.The resolution on the normal load and friction force measurements allowed for the friction coefficient to be measured within70.009.3.2.Sliding friction test resultsFig.3shows a picture of the black oxidized test ring after all three sliding wear tests were completed.The picture shows that the black oxide surface was still intact after each sliding wear test. Fig.4shows a plot of the measured friction coefficients from all six experiments.Interestingly,the untreated steel surfaces yielded more variation in measured friction as compared to the black oxide/untreated steel surfaces.Due to the larger error bars from the untreated steel experiments,the data suggest that the lubri-cated sliding of a black oxide surface against an untreated steel surface will yield sliding friction similar to that of two untreated steel surfaces.3.3.Rolling/sliding friction test descriptionAlthough the black oxide and ground steel surfaces yielded similar friction in simple sliding wear experiments,friction in rolling element bearings results from rolling/sliding contact at various entrainment velocity levels.For this work,entrainment velocity is defined as the mean of the linear velocities at the sur-faces of two contacting bodies.The sliding velocity is defined as the difference between the linear velocities of two contacting bodies.Therefore,the slide-to-roll ratio(SRR)is defined as the ratio of the sliding velocity to the entrainment velocity(as defined above).In order to accomplish the task of measuring the lubricated rolling/sliding coefficient of friction between mated black oxide surfaces and mated steel surfaces,a ball-on-ring contact geometry was utilized,as shown in Fig.5.The tests were lubricated by continuously pumping ISO VG220PAO oil without EP or AW additives into the contact between the ball and the cylinder with a 60°C inlet temperature.The Hertzian contact pressure between the samples was maintained at a constant1.95GPa(283ksi)forFig.3.Black oxidized test ring after all three sliding wear tests werecompleted.Fig.4.Measured friction from ring-on-block sliding wear experiments.C.H.Hager Jr.,R.D.Evans/Wear338-339(2015)221–231223ball and cylinder test samples were accelerated to the same sur-face speed (SRR ¼0%)such that the entrainment velocity was 9m/s.Once the samples reached the initial test speed,with oil flowing between the contacts to create a fully flooded condition,the samples were brought into contact and loaded to 200N.After the test load was achieved,the slide-to-roll ratio was ramped to 5%by slightly slowing down the cylinder and speeding up the ball while maintaining the 9m/s entrainment velocity.Finally,the entrainment velocity was decreased from 9m/s to 0.1m/s over a period of 150s,while maintaining a constant 5%SRR.The friction force between the ball and cylinder surfaces was measured by a torque sensor on the shaft that was rotating the test cylinder.The resolution on the normal load and torque measurements allowed for the friction coef ficient to be measured within 70.01.Two experiments were conducted with untreated steel test samples,and two experiments were conducted with mated black oxide balls and black oxide cylinders.Prior to conducting each test,the ball surface roughness was measured in 10random locations around the circumference with a white light interferometric microscope.Similarly,the cylinder surfaces were measured in 15random locations around the circumference.After conducting each test,the surface roughness within the wear track on the ball sur-faces and the cylinder surfaces was measured at 10and 15random locations,respectively,around the circumference of the wear track.The pre-test and post-test measurements were then used to calculate the root mean square (RMS)composite surface rough-ness by first calculating all the possible combinations of the 25measurements from each surface (150combinations in total).The mean and standard deviations from the RMS composite roughness after a short duration test,the mated black oxide samples exhib-ited a signi ficant reduction in composite surface roughness.The untreated steel samples did not yield a statistically signi ficant reduction in composite surface roughness.The post-test RMS composite surface roughness from each pair of test samples was used to plot the measured friction coef ficient with respect to the non-dimensional film thickness (λ),as shown in Fig.7.In this work,λis de fined as the ratio of the calculated isothermal elastohydrodynamic central lubricant film thickness [13]to the RMS composite surface roughness.This allows for the friction coef ficient to be associated with the appropriate λvalue so that the effect of surface roughness and oil film thickness on the measured friction can be analyzed.3.4.Rolling/sliding friction test resultsFig.7shows that both the mated black oxide samples and the untreated steel samples yielded similar coef ficients of friction when λr 0.4.When λ40.4,the mated black oxide samples yiel-ded a friction coef ficient that was $0.01lower than that of the untreated steel samples.Although 0.01is the calculated resolution of the experiment,the replicate experiments show repeatability that is better than the calculated uncertainty.Therefore,it is possible that the mated black oxide friction coef ficient in the mixed lubrication regime is signi ficantly lower than the untreated steel pair.One explanation for this reduction could be associated with the fact that the mated black oxide surfaces yielded a smoother surface finish at the end of the test,as compared to the mated steel samples.Kang et al.[14]found that in similar rolling/sliding experiments,the measured friction in the mixed regime can be reduced due to a lower composite surface roughness without reducing the measured friction in the boundary lubrica-tion regime.3.5.Rolling/sliding wear test descriptionAfter conducting sliding wear and rolling/sliding friction tests,it became apparent that lubrication conditions govern black oxide surface wear.Evidence of this was demonstrated by the signi ficant reduction in surface roughness after short duration (150s)rolling/sliding wear tests.Because the tests were conducted by measuring the friction at various λconditions,it is impossible to determine which factors had the most in fluence on the wear of the black oxide surfaces.Therefore,a full factorial design of experiments was used to determine the effect of Hertzian contact pressure,slide-to-roll ratio and λon the wear rate of mated black oxide surfaces.Tables 2and 3summarize the variables and experimental testFig.5.Black oxide ball-on-black oxide ring test con figuration.Fig. 6.Measured surface roughness from ball-on-ring rolling/sliding wearexperiments.Fig.7.Measured friction from ball-on-ring rolling/sliding wear experiments.C.H.Hager Jr.,R.D.Evans /Wear 338-339(2015)221–231224with a repeated center point for improved error estimation.The tables describe a total of 10tests that can be modeled by Eq.(1).The test lubricant was a fully formulated ISO VG 320PAO gear oil with EP and AW additives and an oil inlet temperature of 80°C.The first 10tests were run until the mated surfaces had been subjected to a total of 1000m of total sliding distance.Upon the completion of these experiments,each of the tests was repeated (except Test 2)with increased sliding duration.The repeat experiments were run until the black oxide appeared to be visually removed,or until a maximum of 10,000m of total sliding distance had been accumulated.Wear Rate B1SRR B2Hertz B3B4SRR Hertz B5SRR B6Hertz 1λλλ=*+*+*+**+**+**()Wear analysis of each experiment focused on the ball wear track only.This was due to the relative ease of identifying the wear scars on the smooth Grade 25balls compared to the rougher cylinders.The wear volume was calculated from three-dimen-sional interferometric microscope surface measurements made on each ball surface.The first step in the calculation required making a measurement on the nominal ball surface (outside of the wear track),followed by a measurement in the wear track.This allowed for the nominal ball surface geometry to be subtracted from the ball wear track measurement,as shown in Fig.8.Once the wear track was flattened,the data set was leveled and the wear track was isolated from the surface roughness by identifying the area below À3.3*Sq (Sq is the RMS surface roughness of the nominal ball surface with the spherical radius removed).Fig.9shows an example of the isolated wear area in a two-dimensional view.The isolated wear area was integrated and then multiplied by the circumference of the ball to get an approximate wear volume.The wear rate was determined by dividing the measured wear volume by the total sliding distance from the test.3.6.Rolling/sliding wear test resultsTable 4lists the calculated wear volumes,wear depths,and wear rates from each of the rolling/sliding wear tests.Linear least squares regression [15]was used to solve for the coef ficients from Eq.(1)using the wear rate values in Table 4.Once the coef ficients were calculated,the signi ficance of each variable and each variable interaction was determined using an analysis of variance (ANOVA)for the regression components [15].Table 5lists the calculatedTable 2Design of experiments variable de finitions.Parameter xyz Level SRR (%)Hertz pressure (GPa)λÀ150.820.30055 1.240.8711051.661.44Table 3Experimental test conditions.Test #x (SRR)y (GPa)z (λ)SRR (%)Load (N)Ue (m/s)Us (m/s)1À1À1À15150.40.0221À1À1105150.40.423À11À151230.50.03411À11051230.50.535À1À11515 4.250.2161À1110515 4.25 4.467À1115123 5.310.278111105123 5.31 5.5890005552 2.3 1.271055522.31.27Fig.9.Method for flattening the determining wear area from a 2D slice of the 3D measurement.Table 4Calculated wear volume and wear rate from each experiment.Test Wearvolume (mm 3)Depth (mm)Sliding speed (m/s)Sliding dis-tance (m)Wear rate(mm 3)/m19.9*10À3 1.0*10À30.0210009.9*10À62 2.4*10À2 1.5*10À30.421000 2.4*10À53 1.4*10À29.0*10À40.031000 1.4*10À54 2.4*10-2 1.1*10À30.531000 2.4*10À55 2.9*10À5 4.0*10À40.211000 2.9*10À86 2.1*10À4 5.0*10À4 4.461000 2.1*10À77 1.5*10À37.0*10À40.271000 1.5*10À68 2.0*10À37.0*10À4 5.581000 2.0*10À69 1.3*10À2 1.2*10À3 1.271000 1.3*10À5109.1*10À48.0*10À4 1.2710009.1*10À711 1.8*10À2 1.2*10À30.0225007.3*10À612 1.7*10À2 1.0*10À30.035000 3.4*10À613 2.0*10À2 1.1*10À30.535000 4.1*10À614 1.5*10À3 1.1*10À30.2110,000 1.5*10À715 3.2*10À4 5.0*10À4 4.4610,000 3.2*10À816 1.7*10À3 1.0*10À30.2710,000 1.7*10À717 1.5*10À2 1.4*10À3 5.5810,000 1.5*10À618 6.1*10À3 1.2*10À3 1.275000 1.2*10À6192.4*10À22.5*10À21.2710,0002.4*10À6C.H.Hager Jr.,R.D.Evans /Wear 338-339(2015)221–231225coef ficients,the regression sum of squares values,and the residual sum of squares (error).It should be noted that there is a total of only 19°of freedom because Test 2was not replicated.The regression sum of squares for each coef ficient,the residual sum of squares,and the residual degrees of freedom can be used to cal-culate the F value (which is also listed in Table 4).In order for a variable or interaction to be considered statisti-cally signi ficant with 95%con fidence,the F value must be greater than F 1,13¼4.67.Based on these experiments and the analysis of the wear surfaces,only the λvalue had a statistically signi ficant effect on the wear rate of the black oxide surfaces.Fig.10shows images of the wear on black oxidized balls from tests 1through 6.These images show the impact that λhad on the black oxide wear.Although neither is statistically signi ficant,the slide-to-roll ratio had the next highest impact,followed by the interaction between λand the slide-to-roll ratio.The Hertzian contact pressure had almost no impact on the wear rates in these experiments.3.7.Rolling/sliding adhesive wear test descriptionAs stated in the introduction,smearing is a term that is often used to describe adhesive wear damage in contacts when sliding isTable 5Linear least squares regression analysis of wear rate test data.VariablesBCoef ficient value (1e À5)Degrees of freedom Regression sum of squares (1e À9)FSRR B10.183410.05050.5766Hertz B2À0.006410.00010.0007λB3À0.573010.4925 5.6285SRR*hertz B4À0.030710.00140.0161SRR*λB5À0.160010.03840.4386Hertz*λB60.065510.00640.0736Error ––13 1.1375–Total ––191.7268–Table 6Calculated λvalues based on pre-test surface roughness measurements.Contacts Entrainment velocity (m/s)λMated steel30.23Mated black oxide 30.30Mated steel70.41Matedblack oxide70.45Fig.11.Measured friction from ball-on-ring rolling/sliding wear experiments with 3m/s entrainment velocity and increased slide-to-roll ratio (scaling in %).C.H.Hager Jr.,R.D.Evans /Wear 338-339(2015)221–231226substantial and the lubricantfilm thickness is insufficient to ade-quately separate the contacting surfaces[1].With respect to black oxide and smearing,the literature must be interpreted with respect to the severity of the test methods.Evans et al.[7]and Philström and Ström[4]concluded that black oxide treatments in bearings provided minimal or no additional smearing protection under severe contact conditions.Scherb and Zech[5]and Mihai-lidis et al.[6]suggest that there is a benefit to applying black oxide to bearing surfaces for smearing protection under milder test conditions.In an attempt to clarify these varied results and con-clusions,ball-on-cylinder rolling/sliding wear experiments were conducted with mated steel and mated black oxide surfaces.The tests were lubricated by continuously pumping an ISO VG 68PAO oil without EP or AW additives into the contact between the ball and the cylinder with an inlet temperature of80°C.The Hertzian contact pressure between the samples was maintained at a constant1.95GPa(283ksi)for the duration of the test.Tests were conducted at two different entrainment velocities,3m/s and 7m/ing the composite RMS roughness from pre-test surface roughness measurements and the calculated isothermal elasto-hydrodynamic central lubricantfilm thickness[13],the pre-testλvalues were calculated and are listed in Table6.Each test was conducted by initially loading the samples in a pure rolling condition(SRR¼0%).Once the samples reached the test load of200N,the slide-to-roll ratio was increased to10%, maintaining the entrainment velocity at either3m/s or7m/s. Then every150s,the slide-to-roll ratio was increased an addi-tional10%(while keeping the entrainment velocity constant),until the samples exhibited severe adhesive wear or a maximum of 170%slide-to-roll ratio was reached.As soon as either of these conditions was met,the test was suspended.A minimum of two tests were conducted for the mated steel and mated black oxide samples at each entrainment velocity.The friction force between the ball and cylinder surfaces was measured by a torque sensor on the shaft that was rotating the test cylinder. The resolution on the normal load and torque measurements allowed for the friction coefficient to be measured within70.01.3.8.Rolling/sliding adhesive wear test resultsFig.11shows the measured friction coefficient from the experiments conducted with3m/s entrainment velocity.In the background of the plot is a stepped curve that is associated with the y-axis on the right,and shows the slide-to-roll ratio at that point in the test.Both of the untreated steel tests exhibited excessive friction and adhesive wear damage at the very beginning of the140%and130%slide-to-roll ratio stages,respectively.TheFig.13.Measured friction from ball-on-ring rolling/sliding wear experiments with7m/s entrainment velocity and increased slide-to-roll ratio(scaling in%).C.H.Hager Jr.,R.D.Evans/Wear338-339(2015)221–231227。