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Ž . Surface and Coatings Technology 133]134 2000 376]382 Ball crater testing for the measurement of the unlubricated sliding wear of wear-resistant coatings M.G. Gee U , M.J. Wicks Centre for Materials Measurement and Technology, Queens Road, Teddington, Middlesex TW11 0LW, UK Abstract This paper describes a simple modification of the conventional ball-cratering test equipment that enables sliding wear and friction experiments to be performed. Tests can be carried out either dry or with lubricants. In the modified test no abrasive is added, and friction is measured by means of strain-gauged flexure elements. Friction and wear are recorded throughout the test. The utility of the test system is illustrated by the results of a number of preliminary experiments on a range of thin, hard coatings, Ž . including TiN, diamond-like carbon DLC , and CrN. The test results are augmented by optical and SEM observations of the wear mechanisms that occurred. The nitride coatings that were tested all showed relatively high values of friction coefficient, in the range 0.8] 1.2. This was accompanied by the formation of large quantities of oxide debris, which changed in colour with the composition of the oxide. It was thought that most of the wear had taken place by abrasion from the oxide that had been generated at the interface. In many of the tests, the high contact pressure at the centre of the wear scar prevented ingress of debris at this point and reduced wear, leaving a ribbon of unworn coating across the wear scar. In the tests on DLC coatings no obvious debris was formed and the values of friction coefficients were much lower at 0.2] 0.3 for one DLC coating, and 0.5 for the second. Perforation of the coating took place for all the coatings tested at a load of 4.7 N, but the coatings remained intact for most of the coatings tested at 0.47 N. Q 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Wear testing; Friction; Coating; Ball cratering 1. Introduction Although the take-up of thin, hard wear-resistant coatings in engineering applications has been good, it would be further enhanced by the provision of robust methods for assessing their wear and friction perfor- mance in the laboratory. Considerable progress has been made in the last few years towards this goal, with the development of the ball-cratering or micro-abra- w x sion test 1 ] 4. In this test, a ball is rotated and pressed against the coated test sample. An abrasive slurry is introduced between the ball and the test sample, and after a fixed number of revolutions the size of the crater that is formed can be used to calculate the wear that occurs. If U Corresponding author. a series of craters is made for a range of test durations, then wear rates for both the substrate and the coating can be derived. This test is proving to be suitable for testing the resistance of thin, hard coatings to wear by fine abra- sives. The test has advantages that it is quick, conve- nient, it can be performed on small samples, and uses relatively inexpensive equipment. There is still, how- ever, a need for tests that can be used to evaluate the sliding wear and friction performance of coatings. Al- though conventional tests, such as pin-on-disc tests, have been used many times in the investigation of wx friction and wear of thin, hard coatings 5 , there are a number of problems with their use and interpretation. This paper describes a modification of the ball- cratering test system to enable sliding wear and friction tests to be performed without the presence of added abrasive. Preliminary results are presented from tests 0257-8972r00r$ - see front matter Q 2000 Published by Elsevier Science B.V. All rights reserved. Ž . PII: S 0 2 5 7 - 8 9 7 2 00 00966-X

Ball crater testing for the measurement of the unlubricated sliding wear of wear-resistant coatings

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Ž .Surface and Coatings Technology 133]134 2000 376]382

Ball crater testing for the measurement of the unlubricatedsliding wear of wear-resistant coatings

M.G. GeeU, M.J. WicksCentre for Materials Measurement and Technology, Queens Road, Teddington, Middlesex TW11 0LW, UK

Abstract

This paper describes a simple modification of the conventional ball-cratering test equipment that enables sliding wear andfriction experiments to be performed. Tests can be carried out either dry or with lubricants. In the modified test no abrasive isadded, and friction is measured by means of strain-gauged flexure elements. Friction and wear are recorded throughout the test.The utility of the test system is illustrated by the results of a number of preliminary experiments on a range of thin, hard coatings,

Ž .including TiN, diamond-like carbon DLC , and CrN. The test results are augmented by optical and SEM observations of thewear mechanisms that occurred. The nitride coatings that were tested all showed relatively high values of friction coefficient, inthe range 0.8]1.2. This was accompanied by the formation of large quantities of oxide debris, which changed in colour with thecomposition of the oxide. It was thought that most of the wear had taken place by abrasion from the oxide that had beengenerated at the interface. In many of the tests, the high contact pressure at the centre of the wear scar prevented ingress ofdebris at this point and reduced wear, leaving a ribbon of unworn coating across the wear scar. In the tests on DLC coatings noobvious debris was formed and the values of friction coefficients were much lower at 0.2]0.3 for one DLC coating, and 0.5 for thesecond. Perforation of the coating took place for all the coatings tested at a load of 4.7 N, but the coatings remained intact formost of the coatings tested at 0.47 N. Q 2000 Published by Elsevier Science B.V. All rights reserved.

Keywords: Wear testing; Friction; Coating; Ball cratering

1. Introduction

Although the take-up of thin, hard wear-resistantcoatings in engineering applications has been good, itwould be further enhanced by the provision of robustmethods for assessing their wear and friction perfor-mance in the laboratory. Considerable progress hasbeen made in the last few years towards this goal, withthe development of the ball-cratering or micro-abra-

w xsion test 1]4 .In this test, a ball is rotated and pressed against the

coated test sample. An abrasive slurry is introducedbetween the ball and the test sample, and after a fixednumber of revolutions the size of the crater that isformed can be used to calculate the wear that occurs. If

U Corresponding author.

a series of craters is made for a range of test durations,then wear rates for both the substrate and the coatingcan be derived.

This test is proving to be suitable for testing theresistance of thin, hard coatings to wear by fine abra-sives. The test has advantages that it is quick, conve-nient, it can be performed on small samples, and usesrelatively inexpensive equipment. There is still, how-ever, a need for tests that can be used to evaluate thesliding wear and friction performance of coatings. Al-though conventional tests, such as pin-on-disc tests,have been used many times in the investigation of

w xfriction and wear of thin, hard coatings 5 , there are anumber of problems with their use and interpretation.

This paper describes a modification of the ball-cratering test system to enable sliding wear and frictiontests to be performed without the presence of addedabrasive. Preliminary results are presented from tests

0257-8972r00r$ - see front matter Q 2000 Published by Elsevier Science B.V. All rights reserved.Ž .PII: S 0 2 5 7 - 8 9 7 2 0 0 0 0 9 6 6 - X

( )M.G. Gee, M.J. Wicks r Surface and Coatings Technology 133]134 2000 376]382 377

made on a range of coatings, with the intention ofshowing the potential of the measurement methodrather than exploring the detail of coating perfor-mance.

2. Test system

The conventional abrasive ball-cratering test systemis shown in Fig. 1a. A rotating ball is clamped in a splitshaft, which is driven at constant speed. The test sam-ple is clamped in place on a lever arm, which is loadedagainst the rotating ball. Abrasive slurry is drip fedonto the interface between the sample and the ball.Wear is assessed by measuring the size of the craterformed on the sample.

Fig. 1b shows the test system modified for unlubri-Ž .cated sliding wear without abrasive slurry . A friction

measurement element has been added between thesample and the lever arm. This uses strain-gauged leafsprings to measure the frictional force generated. Acapacitance displacement probe is positioned above thefriction device. This gives a continuous measure ofdisplacement wear as the test proceeds.

3. Materials and test conditions

Thin, hard coatings from two different manufactur-ers were tested. The coatings are listed in Table 1. Allthe coatings were deposited on tool steel substrateswith the exception of the chromium nitride coating,which had been deposited on a titanium alloy substrate.The test ball used in these experiments was a 25-mmdiameter AISI 52100 ball bearing. These provide acheap and easily obtained source of test samples, butballs of other materials can also be obtained relativelyeasily. A new ball was used for each test.

The test conditions are listed in Table 2. All thesamples were tested using an applied load of 4.7 N, buta reduced number of samples were tested with a load

Ž .Fig. 1. Schematic diagram of: a original, abrasive ball-crateringŽ .system; and b modified dry sliding ball-cratering system.

of 0.47 N. The two DLC samples were additionallytested for an extended test-duration of 28 800 s.

During the tests, friction and displacement signalswere logged by the NPL wear-acquisition and real-timeanalysis system. This captures complete rotations of therequired signals at predefined intervals, and in addition

Table 1Materials used for wear testing

aDesignation Coating Substrate Coating ManufacturerŽ .thickness mm

Ž .TiN a Titanium nitride M42 tool steel 4.1 1Ž .TiN b Titanium nitride HSS 1.5 2Ž .TiN c Titanium nitride HSS 4.6 2

TiCN Titanium carbon nitride M42 tool steel 4.2 1TiAlN Titanium aluminium nitride M42 tool steel 2.3 1

Ž .DLC a Diamond-like carbon M42 tool steel 4.1 1Ž .DLC b Diamond-like carbon M42 tool steel 2.5 1

CrN Chromium nitride TiAl V 6.0 24

a Manufacturer 1 used an unbalanced magnetron PVD coating process, and manufacturer 2 used an arc evaporation process.

( )M.G. Gee, M.J. Wicks r Surface and Coatings Technology 133]134 2000 376]382378

Table 2aMatrix of test conditions

Material Load 4.7 N Load 0.47 NDuration 3600 s Duration 3600 s Duration 28 800 s

U UŽ .TiN aUŽ .TiN bU UŽ .TiN cUTiCNUTiAlNU U UŽ .DLC aU U UŽ .DLC bU UCrN

a Note a speed of 0.1 m sy1 was used for all tests.

to the mean friction and displacement values, alsocalculates the spread in friction and displacement con-tinuously and stores them as standard deviations aboutthe respective means.

Ž . Ž .Fig. 2. Wear and friction traces for TiN a sample tested at 4.7 N: aŽ .wear displacement; and b friction. msmean, mqssmeanqS.D.,

myssmeanyS.D.

Ž .Fig. 3. Friction traces for DLC a sample tested at 4.7 N. msmean,mqssmeanqS.D., myssmeanyS.D.

4. Results

4.1. Wear and friction traces

Typical wear and friction traces for the 4.7-N loadtests are shown in Figs. 2 and 3. In all cases there waslittle observable wear seen on the displacement tracesŽ .Fig. 2a . The final wear displacements are given inTable 3. The maximum displacement was 7.8 mm for

Ž .the TiN c and the minimum was 0.96 mm for theŽ .DLC b sample. The spread in displacement results

Ž .shown as the mean"S.D. lines on the graphs iscaused by residual misalignment of the test ball, i.e. itis inevitably mounted slightly eccentrically. This spreadis thus an artefact of the test system, and has a magni-tude of approximately 20]24 mm. There is also a rapidŽ .order of a few seconds fluctuation in the displace-ment value, which has a magnitude of approximately 4mm. The source of this fluctuation is not known.

The mean friction, in most cases, initially rises quicklyto a peak value, and then starts to drop slowly over a

Ž .period of time Fig. 2b . An exception to this behaviourŽ .is given by the DLC a friction trace, which shows an

Ž .unusually slow increase in friction coefficient Fig. 3 .After a certain period of time in many tests, there is amarked increase in the variability in the friction. This issometimes accompanied by a small increase in theaverage friction.

The wear displacement traces for the 0.47-N testsshow that even less overall change in displacement has

Ž .occurred Fig. 4a .There is less sign of the increase in variation in

Ž . Ž .friction Fig. 4b and Fig. 5 , but for the DLC b testthere is a sharp rise in friction coefficient after 1500 s.

( )M.G. Gee, M.J. Wicks r Surface and Coatings Technology 133]134 2000 376]382 379

Table 3Friction and displacement values

Designation Overall Displacement Initial Final Period beforedisplacement speed friction friction friction instabilityŽ . Ž .mm coefficient coefficient s

4.7 NŽ .TiN a 4.7 12.3 0.97"0.04 1.10"0.1 233Ž .TiN b 7.3 10.5 1.23"0.11 1.23"0.10 230Ž .TiN c 7.8 10.15 1.10"0.45 0.98"0.32 Unstable from start

TiCN 6.2 12.1 0.62"0.25 0.98"0.37 Unstable from startTiAlN 4.2 12.3 0.98"0.08 0.92"0.19 180

Ž .DLC a 2.8 12.0 0.59"0.16 0.47"0.16 734Ž .DLC b 0.96 12.9 0.24"0.04 0.31"0.22 Unstable from start

CrN 2.8 10.6 0.83"0.07 1.04"0.21 62

0.47 NŽ .TiN a 1.2 9.8 0.98"0.13 0.90"0.21 730Ž .TiN c 2.4 12.4 1.17"0.17 0.91"0.32 StableŽ .DLC aŽ .DLC b 0.4 11.2 0.19"0.11 0.39"0.26 1580

CrN y0.4 11.5 1.00"0.23 0.91"0.17 Stable

0.47 N extended durationŽ .DLC a 1.0 11.2 0.52" 0.51"0.43 22 000Ž .DLC b 0 12.0 0.81"0.21 0.81"0.45 10 000

Values of friction were calculated from analysis ofthe data and are reported in Table 3 and Fig. 6. Thereported values are the mean and spread in friction atthe end of the test, and maximum initial friction and

Žspread either peak friction or friction just before the.increased variability in friction .

In the normal duration tests, the friction coefficientfor the Ti-containing coatings was normally quite high,with values from 0.8 to 1.2. The exception was theTiCN sample, which initially showed a friction coeffi-cient of 0.62. The CrN sample also showed a high

Ž .friction coefficient, in the range 0.8]1.0. The DLC acoating showed much lower friction coefficients, ofapproximately 0.5. The lowest friction coefficient was

Ž .found for the DLC b sample, which was in the range0.2]0.3.

In the extended duration tests, the friction coeffi-Ž .cient was approximately 0.5 for the DLC a sample and

Ž .0.8 for the DLC b sample.

5. Examination of worn samples

Optical micrographs of the craters on the samplesand the wear tracks on the balls tested at 4.7 N areshown in Figs. 7 and 8. The coatings were perforated inevery case. Iron oxide debris was generated for all theTi-containing coatings and for the CrN coatings. Thecolour of this debris varied as the coating changed.

Ž . Ž .Thus, the TiN b , c samples showed dark brownrŽ .almost black debris, the TiAlN, TiN a and CrN sam-

ples showed a brown-red debris, and the debris fromthe TiCN was bright orange. The debris was normallypushed into two ‘butterfly wings’ on either side and tothe rear of the crater. By comparison, the DLC sam-ples showed only a small amount of white or yellowdebris.

There was evidence on many samples that the cen-tral band of the crater had undergone less wear than

Ž .for the outside part of the crater Fig. 7 .This reduced wear is also evident when the micro-

graphs of the balls are considered. Thus, a metallicband of reduced wear runs around the equator of theball. The wear tracks on the ball also show an orangecoloration, presumably from the presence of iron oxide,for all of the samples where oxide debris was produced.In the case of the DLC samples, only a narrow weartrack was produced on the ball.

None of the samples tested at 0.47 N had perforated,and there was a considerably reduced volume of debrisproduced.

In the extended duration tests at low load on theDLC samples, a small volume of coloured oxide wasproduced, and the coatings had perforated during thetest.

Scanning electron microscopy was used to examinethe wear craters. An example of a high-load TiN crateris shown in Fig. 9. The band of reduced wear can beseen across the centre of the crater. The perforation ofthe coating can be seen more clearly in the SEMmicrograph, due to the atomic number contrast in theSEM.

( )M.G. Gee, M.J. Wicks r Surface and Coatings Technology 133]134 2000 376]382380

Ž .Fig. 4. Wear and friction traces for TiN a sample tested at 0.47 N:Ž . Ž .a wear displacement; and b friction. msmean, mqssmeanqS.D., myssmeanyS.D.

In the TiN coatings tested at 0.47 N perforation ofthe coating had not occurred, but the surface of thecrater is covered with adherent material, which isthought to contain iron oxide.

There was no evidence for the formation of theribbon of reduced wear in the DLC samples.

6. Discussion

The results that have been presented here show thatthe micro-abrasion or ball-cratering test can easily bemodified to perform sliding wear tests. This consider-ably extends the capability of the test system to allowfor the investigation of the tribological properties of

Žcoatings under dry sliding conditions there is no rea-son why tests cannot be performed under lubricated

Ž .Fig. 5. Friction traces for DLC b sample tested at 0.47 N. msmean,mqssmeanqS.D., myssmeanyS.D.

.conditions; but this was not attempted in this study .The modified test has the advantages that it is cheap,readily available and easy to use, and can generate aconsiderable volume of results in a reasonable time.

Although perforation of the coating occurred in manyof the tests carried out here, it is also clear that testconditions can easily be varied so that perforation doesnot occur.

Tribological coatings are often used in applicationswhere friction is an important performance parameter,such as in dies, pressing tools, valve components andmechanical seals. The addition of friction measurementto the ball-cratering test provides a cheap and conve-nient method for measuring friction. This informationcan be used in the selection or development of coatingswith the requisite frictional properties for the applica-tion.

All of the high-load tests, except for the DLC sam-

Fig. 6. Comparison of friction for different materials tested at 4.7 N.

( )M.G. Gee, M.J. Wicks r Surface and Coatings Technology 133]134 2000 376]382 381

Ž .Fig. 7. Optical micrographs of wear scar from TiN c test under 4.7Ž . Ž .N: a crater; and b wear track on ball.

ples, were dominated by the formation of iron oxides.ŽThis was formed from wear of the ball and substrate,

.once perforation had occurred and caused furtherwear due to abrasion by oxide particles. This abrasivewear explains the formation of the raised bands inmany of the samples. These were caused by the highcontact-pressure at the centre of the ball-contact withthe sample, which prevented the debris from beingcarried into the ball]sample contact at this point, andthus reduced wear here. Very similar effects have beenseen in the intentional abrasive wear of coatings withthe ball-cratering rig.

With the exception of the tests on DLC coatings, thefriction coefficients were very high. The reason for thisis not understood, but it is perhaps significant that thehigh friction coefficient was always associated withspecimens where large quantities of oxide debris weregenerated. Thus, the high friction may be due to thepresence of the oxide at the wear interface.

A major difference between this test and conventio-nal pin-on-disc or reciprocating testing of coated sam-ples is that, in the ball cratering test presented here,

Ž .the coated sample flat is continuously in contact withthe rotating ball. In conventional tests, any point on thecoated sample is brought into repeated contact with

Ž .the opposing sample pin or ball . This alters the ther-

Ž .Fig. 8. Optical micrographs of wear scar from DLC a test under 4.7Ž . Ž .N: a crater; and b wear track on ball.

mal conditions of the coated sample and also alters thedynamics of how the debris is removed from the sys-tem, with the effects discussed above.

The coatings that showed the lowest friction andwear in these tests were the DLC coatings. There wasalso little formation of oxide in these tests. This isperhaps a function of the reduced friction observed inthese tests.

Ž .Fig. 9. Scanning electron micrograph of crater on TiN a sampletested under a load of 4.7 N.

( )M.G. Gee, M.J. Wicks r Surface and Coatings Technology 133]134 2000 376]382382

7. Conclusions

Modifications have been made to a ball-cratering ormicro-abrasion rig to develop it into a sliding weartribometer. A number of coatings were tested in thenew test system. The test system proved to be easy touse, giving useful information on the wear and frictionperformance of tribological coatings that could be usedto give guidance in the application of the coatings insliding wear applications.

The nitride coatings that were tested all showedrelatively high values of friction coefficient in the range0.8]1.2. This was accompanied by the formation oflarge quantities of oxide debris, which changed in colourwith the composition of the oxide. It was thought thatthe most of the wear had taken place by abrasion fromthe oxide that had been generated at the interface. Inmany of the tests, the high contact-pressure at thecentre of the wear scar prevented ingress of debris atthis point and reduced wear, leaving a ribbon of un-worn coating across the wear scar.

In the tests on DLC coatings no obvious debris wasformed, and the values of friction coefficients weremuch lower at 0.2]0.3 for one DLC coating, and 0.5 forthe second.

Perforation of the coating took place for all thecoatings tested at a load of 4.7 N, but the coatingsremained intact for the coatings tested at 0.47 N.

Acknowledgements

The authors would like to acknowledge the supportof the UK Department of Trade and Industry throughthe Characterisation and Performance of MaterialsProgramme for the work carried out in this study.

References

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w x2 K.L. Rutherford, I.M. Hutchings, Theory and application of aŽ .micro-scale abrasive wear test, J. Test. Eval. 25 1997 250]260.

w x3 M.G. Gee, A. Gant, Ball Cratering for the Measurement ofŽ . Ž Ž . .Coating Wear, 1998 NPL Report CMMT D 162 .

w x4 D.N. Allsop, R.I. Trezona, I.M. Hutchings, The effect of ballsurface condition in the micro-abrasive wear test, Tribol. Lett.Ž .5 1998 259]264.

w x5 H. Vetters et al., Development and Validation of Test MethodsŽ . Žfor Thin Hard Coatings, 1998 FASTE, final report, EU.Contract, MAT1-CT 940045 .