An investigation into the tribological behaviour of DLC coatings deposited on sintered ferrous alloy substrate

WEAR ELSEVIER

An investigation into the tribological behaviour of DLC coatings deposited on sintered ferrous alloy substrate J i a r e n J i a n g ~'*, R . D . A r n e l l ~, J i n T o n g h , R~.~earch ht~titutc Ibr Ih'.~ign. Mamllhcture trod ,ffarketing. I.'niverxity ql.~allbr~L .~dlbrd M5 4WT. UK " Jilin Unil'er.~ily ~] Terhn~b.~,y. Changchtm..lilh~ Prol'it~n,. Chhta

Received 17 February 1997:accepted 15 September I*)t)7

Abstract In thi~. ~.tudy. graded DLC coatings have been deposited on a ~intered ferrous alloy substrate using the combined CFUBMS-PACVD technique. Tribological behaviour of the composite coatings has been investigated under various conditions in a continuous ball-on-disk sliding wear machine in dr3.'air. A significant dependence of specific wear rate on diameter of wear tracks has been observed under nominally identical conditions of Ioado sliding speed and total sliding distance. The specilic wear rate increased slightly with load below some critical load. Sm~n~th load-bearing areas developed frum the compaction of wear debris particles were observed. However. when a transition load of approximately 51) N wa~.exceeded at a ball diameter of 6.3 ram. the specilic wear rate increased dramatically. This wear transition with load was accompanied by a ~.evere spallation of the coatings. Wear of the balls was negligible under all the experimental conditions studied fi~r both ,,ted and tungsten carbide balls. A considerable am~unt of material was translerred from the DLC coating to the ball surfaces. A maximum friction ',*.'asshown in the variation of friction c~veflicient as a function of sliding time. The friction c~efficient in the initial stages of sliding ,.,.as in the range of 1).2 to 0.24 while in the later steady-state stages this was in the range of 0 2 to 0.29. The maximum fi'iction coefficient rarely exceeded 11.32.A physical mtvdel tot wear processes during sliding of a coating system is presented ba:~ed~m SEM observati~as. The experimental results are well interpreted according to this mt:,del. ;~ 1998 Elsevier Science S.A. gt'x ~v,,v,h:

131.("coatings:Trib4+logicalbeha~i,ur: Sliding wear: Sinlcred I;~rrt~usalto~

!. Introduction

Due to their very attractive mechanical, chemical, electrical and optical properties, hard carlm)n coatings have been increasingly studied I I-7 ] since the publication of the lirst rel~rt by Aisenburg and C h a l ~ t 181 on production of extremely hard carl~)n lilm in a vacuum environment by an ion beam sputtering method. Various techniques have been u ~ d lor the deposition of hard carbon cnatings from cracking of hydrocarbon ga.~s, for example, dc or rf discharge 19121. ior. plating II 31. magnetn~n sputtering 114.151. dual ion beam ~,puttering I 16 I. micn~wavc discharge 16.17-19 I and closed lield unbalanced magnetron sputter ion plating 1211-221- Pulsed la~,er sanrces have also been applied tbr the depor, ition of hydr~gen-free carbon lilms [ 23 I. A ~ery wide range of properties, varying from pl~lymer-like to very hard diamond-like, ha~e been obtained. Fnnn the limited number ~l ,,tudie,, on their tribological pn~pcrties 16.24-261 and due - C,trc,p~ndlt!g Juth~,r oo4.l-lt4S/t#s/',l,l.lXl, II~t~ I~l~:~lcr Scxnce S.A. All rlgla~ rc.,cncd I'll SIal43-164Sl It71nu221)-2

to their high hardness, it appears that DLC coating, ,re very promising for applications in wear protection, nt~= only for tools but also for mechanical components. Howc~cr. since very high internal stresses are usually present within the coalings [ 2 7 - 3 0 I. industrial applications seemed to be unrealistic until the introduction of a hybrid technique which deposits transition layers between substrate and the top DLC layer using combined closed field unbalanced magnetron sputter ion plating ( C F U B M S ) and plasma assisted chemical vapour deposition ( P A C V D ) [20-221. By using this technique. ,=dhesion of the DLC coating to substrate is considerably enhanced. Compared with the vast number of researches carried out on the production and structural characterisation of the carbon coatings, detailed tribological studies are still lacking. In this study, graded DLC coatings have been deposited on a sintcred I~crrous alloy substrate using the combined C F U B M S PACVD technique. Tribological behaviour of the composite coatings has been investigated under various conditions in a continuous ball-on-disk sliding wear machine. A physical model for wear processes during sliding of a coating system

J. Jiane el ul. / Wear 21411t)98/ 14 -22

is presented based on SEM observations, and experimental results are interpreted according to this model.

2. Experimental methods 2. I. Speeimen.~"

Specimens were coated using the combined C F U B M S PACVD technique [20--221. The substrate was a sintercd ferrous alloy which has a hardness of Hv 1270 MPa. The surface of the substrate was linished by grinding and had a roughness. Ra. of 0.56/.Lm. Before coating, the specimens were etched in 2c,~ nital for5 rain. Porosity was clearly visible at this stage. Alter coating, the roughness was marginally decreased. Fig. I shows the surface of the specimens after coating. The coating was composed of muhilayers of. from the substrate outwards. Ti/TiN/transitional T i I N . C ) / T i C / D L C The thicknesses of the TiN. TiC and DLC layers, measured using a ball crater method, were approximately 0.8. 0.7. and 1.4 pro. respectively. The hardness and elastic modulus of the DLC coating, measured using a nanoindeuter, were 70t)0 MPa and 180 GPa. respectively. Hydrogen content of the DLC coating, detected using elastic recoil detection ( E R D ) method, was in the range of 17 to 23 at.5~- ( D. Giersch. Private communication. BMW Group. December 1995 ). 2.2. W e a r test

Wear tests were carried out on a continuous ball-on-disk sliding wear machine in dry air. The relatively humidity of the environment was approximately 7F/f controlled by passing compressed air through a saturated NaOH solution before introducing it into the specimen chamber. T w o types of polished ball materials, tungsten carbide and 52100 steel, were tested. The two types of ball materials produced similar friction and wear results. In this paper, when unspecified, a tungsten carbide ball was used. The diameter of the ball was 6.35 ram. In all the tests, sliding speed was kept at a constant value

m/ wJx

Fig. I, The coating ~,urlhcebefore ',,,cartest. Grinding mark~,;lruduplicated by the ctl;~tillg.

15

of 3 m min * by adjusting the rotating speed of the disk specimen and the diameter of wear track simultaneously. The total sliding time was 2 h. giving a total sliding distance of 361) m. Before each sliding test. the ball and the disk were ultrasonically cleaned in acetone for I0 rain and then rinsed in acetone to remove residual dust. grease and other solid contaminants so as to keep the surface conditions as identical to each other as possible. Wear volumes of the disk specimens were evaluated using a profilometer. Talysurf 10. attached to a PC. The average cross-section area of a wear track was calculated from at least 15 measurements on each specimen. The standard deviation of cross-section areas on a specimen was generally in the range of approximately I0 to 18% that of their average value. This relatively large deviation resulted mainly from the uneven wear along the circumference of the wear track, uncertainties resulting from the measurement technique itselfheing much less than this. Wear results rclx)rted in this paper are in the form of Archard's specific wear rate. K. in m ~ N * m *. Wear scars on balls were examined under a m i c r o ~ o p e to measure the two dominant diameters of the ellipse scar. However. detailed examination of the scars in SEM showed that a significant amount of material was transferred from the coating to the ball surface and little wear from the ball material had actually cvccurred: therefore, wear of the balls is not reported here. The variations of friction h)rce as a function of time were measured and recorded using a load cell and Handyscope software at intervals of 5 s: these data were then transformed into hiction coeflicient loci. 2.3, S E M ol~.~'errathuts

Specimens alter sliding test were directly observed in an SEM without any cleaning so that the surface conditions during the sliding process could he observed.

3. Experimental results 3 1 Wear behaviour r~l'the DLC ~oating~

During the experiments, several wear tracks were made on each disk specimens, sliding speed being kept constant. Intuitively, under identical conditions of load. speed, and sliding distance, the specific wear rate. K. should be independent of the diameter of the wear track. However. it was found that the specific wear rate. K. increased almost linearly with the diameter of wear track~,, as is shown in Fig. 2. Wear data obtained using 52100 steel balls are also incorporated in Fig. 2. The use of either steel balls or tungsten carbide balls did not produce siglfificantly different wear results from each other. Very similar frictional behaviour wa~, also produced. The effect of load on specific wear rate of the coatings is shown in Fig, 3. A transition load of appn)ximately 5¢) N can

J. Jiant, 't d. / Wear 21411'49,~' 14-22

Ib

1.40 ---

t 1-201

~r ~

•

1.30

.............."~" .....-

. . . . y " .....

~~ "° 1 1.00

.,

•

. . . . - ....

0.70 0.60O 10

. 12

. . . . 14 16 18 2'0 22 D i a m e t e r of w e a r track, m m

Steel ball

•

WC ball

24

26

.......... Unear fitting

3.3. S E M observations

Fig 2. The variation of spccilie ',,,carrate a~,a function o f '.','cartrack diameter Ol| the disk at a toad ol 211 N. 530

410 370 29O

~25o 210 17 0 O9O

05O

to

~

2o

25

3o 3s Load. N

~

~

go

55

Fig. 3. Variation of swcilic v,ear rale v.ilh load (We hall: ',',car track dianteler~ ~`.ere III to 25 nun ;It 21) N and v,ere IS altd 211it|in at the othel Ioad~I.

o20

o 015 olo

Due to the high w e a r resistance o f the D L C coatings, w e a r occurred mainly at the top o f asperities. A l t e r a sliding w e a r test. the original grinding marks were still observable, e v e n after sliding at a load close to the transition load o f 50 N ( Fig. 5). The coatings started to break-down at the centre o f the w e a r track, some striations along the sliding direction being noticeable ( Fig. 5 ). One o f the m a j o r tolx)graphic features o f w e a r surfaces below the transition load is that m a n y w e a r debris particles were accumulated and c o m p a c t e d in the t r o u g h s / g r o o v e s o f asperities in the surfaces ( Fig. 6a--c). These c o m p a c t layers had smooth surfaces. At the higher load. 50 N. coating spallation in a small scale was observed at the centre part o f the w e a r track (Fig. be). T h e spallatioa o f the coating and the c o m p a c t particle layers showed features o f brittle fracture. When load was above the transition load. severe break-down and spallation of the coating layers occurred ( Fig. 6d ). Ripples covered almost the entire w e a r track. T h e presence o f porosities in the ~.ubstrate surface was n e v e r noticed to have initiated cracks o,r to have promoted spallation o f t b e coatings. Instead, they might have acted as sinks for the accumulation o f wear debris particles to fi)rm c o m p a c t particle layers l Fig. 7 I. The size of wear debris particles was very line. in the order of O. I In (1.3/.tin ( Fig. 8 ). and was even liner at lower loads.

)

oo5 o5o

coefficient increased relatively rapidly with sliding time and reached a m a x i m u m value alter a period o f sliding. It then decreased gradually with further sliding and finally, alter longer-sliding, a steady-slate was reached and a stable value o f friction coefficient was maintained. T h e initial valnes of friction coefficient under most o f the experimental conditions were generally in the range of 0.20 to 0.23. The steady-state values o f friction coefficient ranged approximately from 0.2 to 0.28. this being slightly lower when a steel ball was used. At a low load. 5 N, the initial friction coefficient was approximately 0.15 to 0.20. The friction coefficient at Ihe m a x i m u m in the friction loci rarely exceeded 0.32.

1~

2ooo

3ooo 4OO0 5OO0 6OOO 7OOO 8OOO Sliding lime, sec

Fig. 4. Varialion ol Irb.'tio:lct~lhcient v,itil ~liding tinlc fl)r tilt' DI.(" colni'lo,qle coaling sliding agaill.I a ttlllgn|cn earbitlc hall t21) N. I~ lint) :'.ear track dianP..'ter). be observed a b o ~ e which '.'.'ear rate increases swiftly with load. Belt)',', this transititm load. the spccilic '.,,'ear rate increases slightl) v, ith load. 3.2. ['rit'lion.I hcltat'io.r ~y the I)1.(" coalhtgs ~ho'ing sibling w~'.r A typical *,ariation of friction cc,cflicient with sliding time b, sho,.vn in Fig. 4. ht the early stages o f sliding, friction

Fig. 5. Appearance of the wear track after sliding against a tungsten carbide ball Ibr 2 Ii at 5n N. Grinding marks are still clearl) oh~,ervablee',en after slidill~ ullder ~uch se`.ere conditiom,.

J. Jiang et al. / Wear 214 11991¢j 14-22

17

Fig. 6. Majortopographic featuresof wearscar surfacesat the variousloads. ( a ) 20 N: (b) 50 N; (c) 50 N: (d) 54 N.

Fig. 7. The appearanceof wear surl~ce near porosities in Ihe sintered alloy surlhce. They seemedto have acted it., sinks fi)r the accumulaliouof wear debris particle~,and prmm,lcdthe devclopn!entof c~,mpact ',,,ear debris, particle layers. Wear o f the balls sliding against the DLC coatings was very slight and showed very similar features under the various conditions. A large amount of wear debris accumulated at the edges of and in the front end of the ~ a r ( Fig. 9a). Many wear debris particles had been compacted on to the ball surlace. especially in the front edge of the scar. and acted as load-hearing areas (Fig. 9b). A significant amount of material transferred from the coating to the ball surface to form a uniform smooth surface layer ( Fig. 9c). The original ball surface was not worn. Pits present on the original tungsten carbide ball surface have been filled with transferred

Fig. 8. Wear debris particles accumulatedaround the wear track. Load= 51)N. materials, as is shown in Fig. 9c by the black arc.'~5. At areas immediately next to the edges of the wear m a r on the ball. very fine wear debris particles were shown to have accumulated and strongly adhered to the surface of the ball ( Fig. 9d). Some degree of pressure might have been experienced by these particles. This is probably the preceding step lot the formation of compact layers from the transferred materials on the ball scars ( Fig. 9c ). The easy formation of such compacted layers on the wear surface of the uncoated ball is a very useful property of DLC coatings lbr providing both low friction and g o ~ l protection to the uncoated counterpart wear components.

18

,I. Jiang et aL I Wear 214 f 1998~ 14-22

Iqg. 9. Son~e features of wear ~ars formed on the balls. ( a ) Overall view of a typical .,;caron the ball surface; ( b ) magnified view at the front edge of the scar: I c ~compact layer fi~rmedfrom transferred material at the centre of the scar; (d) strongly adhered wear debris particles to the ball surface at the area immediately next to the side edge of the scar. 4. D i s c u s s i o n

4. I. W e a r processes o f dte composite D L C ~'~mthzg system M o s t o f the e x i s t i n g w e a r theories are c o n c e r n e d with the process o f generation o f the w e a r debris f r o m a bulk material. O n c e formed, it is a s s u m e d to be r e m o v e d f r o m the r u b b i n g interface to cause wear. T a k i n g into account the widely o b s e r v e d fact that tribo-particulates can have a significant effect on w e a r and friction b e h a v i o u r o f the sliding s y s t e m , J i a n g et al. 131.321 proposed a w e a r m o d e l for sliding w e a r and w e a r transitions o f metals at various temperatures. In this model, the variation o f w e a r and w e a r m e c h a n i s m s v, ith sliding time w a s considered. G o o d a g r e e m e n t between theory and e x p e r i m e n t s w a s observed. H o w e v e r , no m o d e l has been presented for w e a r o f coated materials. A c c o r d i n g to the experimental o b s e r v a t i o n s in this study, the following w e a r p r o c e s s e s Ior sliding w e a r o f the D L C c o m p o s i t e c o a t i n g s c a n be e n v i s a g e d ( Fig. 10). in the early stages o f sliding, w e a r debris particles are g e n e r a t e d mainly f r o m the tips o f asperities on the coating surface. Since the hardness o f the coating is very high. the particles will be very line c o m p a r e d with metals 133.341 if the load is not high e n o u g h to cause spallation o f the coating. W i t h the p r o g r e s s o f sliding, m o t e and more w e a r debris panicle.,, li'om the coating are generated. T h e s e particles, instead o f b e i n g r e m o v e d from the contact area. can be entrapped within the rubbing interlace, preferentially in the

(a)

(b)

(c) I:ig. lU. A ~,chematic diagram ~.howing the wear pr~s:es~s during ~liding wear of a hard q )ating in which the wear of the ball i~, negligible due to the fi~rmation ~Jf~.om::kind of protective laycr~ tTom tran~,terred material on it: ( a ~generation and accumulafilm of wear debris panicles: ( b ~development of compact wear debris, panicle: (c) inifiatitm of cracks at the ct)ating/ sub~tratc interlace and ~pallation of the coating and the compact load-hearing layers.

J. Jiang et aL / Wear 214 (199H) 14-22

-I L

Focmationof comap,:t layers. 'mild' wear

~~ I / /

,, '. N,,

;o_o: , o,>o,, NiD) )

N(D2)

Sliding cycles, n Fig. I I. A schematicdiagramshowingthe variationof pear depth, h. of the coaling as a funclionof number. N. of contact cycles of the disk (coating satiate ) with the ball. as predictedby the ',,.'earn'mdel. troughs/grooves in the coating surface ( Fig. IOa). They can also be adhered and/or transferred to the ball surface, depending on adhesion energy and size of the particles [ 35 I. When the asperities of the coatings are worn down enough, the accumalated wear debris particles are subjected to compression and are compacted and/or sintered to form solidcompact load-hearing layers ( Fig. lob and Fig. 6a). The formation of such layers should produce better wear protection to the wear surface due to two major reasons: (a) real area of contact is increased since the lbrmation of such layers allows the rubbing surfaces to conform more closely; (b) such layers are composed of similar materials to those from the coating ( there might he some translormation for the DLC coating from amorphous carbon into graphite, but this should only cover a very small proportion, if any. of the generated wear debris particles ) but with relatively lower stresses, at least at the initial stages of their formation, than in the original coatings. As a result, a wear transition from high rate to low rate will be observed following the development of such layers. During the subsequent sliding, both the coating layer(s) and such compact particle layers are subject to further "mild" wear. Eventually. after a long time of sliding, depending on load and residual thickness and properties of the coating layers. plastic deformation at or near the interface between the coating and the substrate may (v.:cur and spallatiou of the layers, including the compact layers and the coatings, via a fatigue failure mechanism, is observed (Fig. IOc and Fig. 6c ). Wear rate may increase rapidly again after this stage due to the spallation of the coating layer(s). According to the above discussion, the variation of wear depth of the coating. h o t ) . with sliding cycles, n. will have the shape as shown in Fig. I I. 4.2. The dependence ~ [ w e a r on diameter r?l'wear tracks

in Fig. 2. a significant dependence of specific wear rate. K. on diameter of wear tracks. D. has been shown. This phenomenon cannot be explained by the existing wear theories such as adhesion wear theory (the Archard's wear law) and the

19

delamination theory hecause most of them predict a direct dependence of wear on sliding distance. S. and the specific wear rate should be independent of the diameter of wear tracks and sliding distance. On the other hand. according to the fatigue wear theory, wear should decrease, instead of increase, with increase in wear track diameter. D. because the total number of contact. N. between the ball and a given point on the disk surface decrea~s inversely with the increase in wear track diameter, i.e.. N 0t I / D. Therefore. the experimental results cannot be explained by the fatigue wear theory. either. However. on the basis of the above proposed wear processes for coating specimens ( Section 4. I ). this can he explained as follows. According to the proposed wear model, the number of contact cycles, n. at a given point of the wear surface is the dominant parameter in controlling wear (Fig. II ): i.e.. the depth of wear into the wear track, h. is a function of contact cycles, n: h = hi n ). Obviously. the function hi n ) depends on properties of the rubbing materials and the loading conditions ( load. speed and contact geometry between specimen surfaces), but does not vary with wear track diameter. This effect has been observed by Skinner 1361 and Newman and Skinner 1371 in the sliding wear of stainless steels at elevated temperatures. Assume the cross-~ction area of the wear track alter n cycles of sliding is A l n ) . A l n ) is a function of the wear depth, h i n ) . at the corresponding time. depending on the instant contact geometry between the ball and the disk specimens. The total wear volume of the disk specimen. V. for a given diameter of wear track. D. after a total number of N cycles of sliding can he expressed by ....~". . . .

Jf"[dAVdl,~

o

o N,,

<,..,t), dA

dh

q dA

dh

J

where N,, is the number of sliding cycles to the wear transition (Fig. II ). According to the present rondel wear rate is decreased to a very low value alter a critical number of cycles of sliding. N,,. If it is assumed that the wear rate after the wear transition, k,, = ( d h / d n L,,. is negligible compared to the wear rate in the "severe" wear regime, k, = {dh/dn),. during the early stages of sliding then the total wear volume. V(N). can be attributed to wear occurred during the sliding to the transition. N,,. by

,,,N)=~,,f(--~--~d .... o, dh ~dnl, ,v,, dA

dh

(2)

where I represents the integration tern',. The specific wear rate. K. at a given load. L. after a sliding distance of S is. thus. equal to: K= V

rrl

=~-~D

(3)

J. Jiang ,'t aL / Wear 214 ~1998) 14-22

2o

As has been shown by SEM observations (Fig. 9). the balls were almost unworn during the sliding wear test. As an order-of-magnitude estimation, it is assumed that the crosssection of the wear track eonfi~rms with the ball surface. The cross-section area of the wear track.. A( n ). after n cycles of sliding is. thus. related to the depth, h( n ). by:

,,h= 2¢~h,,

da :~_,¢--~h,~,

14~

where R is the ~ d i u s of the ball. By substituting Eq. ( 4 ) into Eq. ( 2 ). the total wear volume. V ~ N I . after ,;liding for longer than N,, cycles of contact is obtained.

The specific wear rate. K. is. thus. equal to: K=

2,.n-D~_RIhl N,, ) I ~'' "5LS

~-trD

a cot ~elation coefficient of 0.9463. Statistical variance analysis using the F distribution parameter to test the significance oftbe fitted equation showed that the value of the F parameter for the fitting/regression of the equation is equal to 28.36: this is much higher than the critical value for the Fdistribution with I and 7 degrees of freedom at a confidence level 0f99%. Fo ,~,, ( I. 7 ). which is equal to 12.2. This means that the linear relationship between the specific wear rate. K. and the diameter of wear tracks. D, is highly significant. If a power law. K ctlY", is assumed, the fitting produced a power ofm = 0.82 with a correlation coefficient of 0.9497 and a value of 30.5 lbr the F parameter. Only very marginal improvement in the fitting results is achieved by the power law. Considering the limited data available and the experimental errors involved. it is not appropriate to assume a delinite form for the curve. But it is significant to notice that the specific wear rate. K. is stl'ongly dependent on the diameter of wear tracks and the agreement between the present model ( Eq. (6) or Eq. ( 3 ) ) and experiment is rather good. 4.3. The variathm of/~'iction coefficient with sliding time

(61

where, since the relationship between h(ll) and the number of cycles, n. does not vaD' with weax ~rack diameter, a is a constant independent of the diameter of the wear truck. A linear relationship is obtained according to the model between the specific wear rate. K. and the diameter of the wear track. D. for a constant load. L. and a constant sliding distance. S. In tact. the assumptions made in Eq. 14 ) for the derivation of Eq. ~6 } on the variation of the cross-section area. A ( n ). with the depth of the wear track, hi n). are not necessa U to get a linear relationship between the specific wear rate. K. and the diameter. D. oftbe wear track. Under the experimental conditions used in producing the data for Fig. 2 where all the experimental parameters except the diameter of the wear track are identical, the contact geometry and stress conditions at a position on the wear track after a given number of cycles of sliding contact are essentially identical for any diameter of the wear track. At each cycle of contact, the same amount of material is removed from a given point on the wear track surface. Therefore. the relationship between the wear track depth, hi n 1. and the number of contact cycles, n. is the same at an)' wear track diameter, as indicated in Fig. I I. Following the ~,ante argument, the function ( d A / d h H d h / d n ) and the transition cycles. N,,. are ah,o independent of diameter of the wear track. As a result, the integration term./, in Eqs. (2) and ( 31 is a constant lor a given set of experimental conditions and is independent of the diameter of wear track. D. According to Eq. 13 ~. a linear variation of the specific wear rate. K. with the diameter of the ~ear track. D. is to be expected if the sliding is carried out long enough so that the wear transition occurs t N > N,, ). Fitting of the data in Fig. 2 showed that the specific wear rate. K. is proportional to the diameter of wear tracks. D. with

The frictional behaviour of various sliding systems in the running-in period is one of the most complicated topics in tribological studies. Blau [38] summarised eight types of typical running-in friction loci under various conditions. These are far from being exhaustively listed. Suh and Sin 139] identified two types of running-in curves for sliding between steels containing different carbon and copper: these curves had similar shapes to the ones obtained in this study for the DLC coatings (Fig. 4). it was found [39] that the initial friction coefficient was approximately equal to 0. I to I).2. irrespective of the combinations of materials and with or without lubricant. Friction coefficient increased slightly with sliding time initially and then increased rapidly to reach a maximum value. When the combinations of materials were such that mirror surl:aces were produced, friction coefficient reduced to some lower stable value in the fi~llowing sliding. Two major components of friction, i.e.. adhesion (/.t,,) and ploughing ( P'o ~" have generally been identilied. The overall friction coeflicient, p.. is a weighted sum of these contributions: 9. = p.~,+ p- p The adhesion component of friction./.t,,, depends critically on ductility of the materials in contact, which determines growth of asperities, and on surface energy of the contacting pair 140-421. The ploughing component of friction, p.p. is very sensitive to the slopes of penetrating conical asperities 1421 or. for spherical particles or asperities, to the ratio of the width of the formed grooves, wr. to the radius of curvature of entrapped spherical hard particles, r 1431. For penetrations, w~/r. less than 0. I. the ploughing component of friction tends to zero and when the penetration, wg/r. is higher than 0.1. the ploughing friction coefficient increases swiftly towards I 1431.

J. Jiang et aL / Wear 214 q1'19,~ 14-22

For the sliding of hard DLC coatings below the transition load in the present study, significant material transfer from the coating surface to the ball occurred ( Fig. 9). Therefore. the friction behaviour of the sliding system is mainly dominated by the properties of the coatings and the compacted transferred layers. In the early stages of sliding, since contamination of the surfaces is usually inevitably present, adhesion is the dominant contributor to friction: this is very low under present conditions ( Fig. 4). For DLC coatings, the presence of a contamination layer and its influence on friction is probably particularly important because the surface is chemically reactive. It has been widely observed in this laboratory that in most cases the friction coefficient at the start of sliding is approximately 0.2. This value is the typical theoretically predicted value for adhesion friction coefficient without growth of adhesion joints. As sliding proceeds, wear debris particles are generated ( Fig. 10) and the contribution from ploughing by entrapped wear debris particles increases gradually. However. since the size of wear debris particles is very fine ( Fig. 8 ) and the rubbing surfaces are hard. the penetration parameter. w J r . will he relatively low compared with what it would he for metal to metal contact. Therefore. friction will not rise substantially before a maximum friction is attained I Fig. 4). and the maximum friction is much less than that obtained hw sliding of most metals. When wear debris particles are compacted ( Fig. I 0b). the rubbing surfaces are burnished during further sliding, forming smooth surfaces. The generation rate. as well as the number, of loose wear debris particles within the rubbing interlace, decreases. As a result, the phmghing component of friction is gradually reduced, leading to a gradual decrease in friction ( Fig. 4). Adhesion becomes the dominant source of friction. As has been reasoned by Jahanmir et al. 144 I. graphitization might occur at the real area of contact during the sliding of DLC coatings due to frictional heating. This soft lubricant on top of a hard DLC supporting surface makes the adhesion component of friction and the overall friction c(eefficient remain low in steady-state sliding ( Fig. 4). 4.4. The ~]'ect cy'h,ud on w e a r

As ca~, he expected fiw wear of coated materials, a transition load existed ('or sliding wear of the DLC coatings on sintered ferrous alloy, above which the coatings were rapidly destroyed due to substrate plastic deformation and tensile fracture of the coatings (Fig. 6d). specific wear rate being increased dramatically (Fig. 3). The slight increase in specific wear rate. K. with load in the region below the transition load ( Fig. 3 ) means that the total wear volume. V. increased with load, L. in a power law of Vet L" with n > I. This type of relationship has been observed for sliding wear of nickelbased alloys at various temperatures and has been shown to be mainly a result of wear transition from high rate to low rate with sliding 132 I. The transition of wear with load for metals has been widely studied 145~1-7 ] and has been attributed to the sustaining of

21

severe metal-metal contact wear at high loads. However. the failure mechanisms for coatings at/above the transition load is not well understood yet. Transition presumably results from some kind of low cycle fatigue and the transition load presumably corresponds to the load at which significant plastic deformation at the interface between the coating and the substrate occurs, resulting in severe spallation of the coating ( Fig. IOc ). The value of the transition load is a very complicated function of adhesion of the coating to the substrate. mechanical properties of the coating and the substrate, and contact geometry between the ball and the disk specimens. Further studies on the relationship between the transition load and the above parameters and on the transition mechanisms are required. These are being carried out using various techniques including Finite Element Analysis ( FEA 1.

5. C~mclusions I. A significant dependence of specific wear rate on diameter of wear tracks has been observed under nominally identical conditions of load. sliding speed and total sliding distance. This unexpected phenomenon can be explained ifa transition in wear from high rate to low rate is assumed to occur alter a certain number of cycles of contact. 2. A transition load of approximately 50 N was o b . ~ , e d for the composite DLC coating under present experimental conditions using balls with a diameter of 6.3 mm. Below the transition load. wear of the coating surfaces was slight, in the order of the surface rougfiness. Compact ,.,,'ear debris particle layers were developed at the roughness grooves in the coating surface which acted as load-hearing areas. Above the transition load. severe spallation of the coating occurred due to severe plastic deformation in the substrate and tensile stresses in the coating. 3. A peak was shown in the variation of friction coefficient as a function of sliding time for the DLC c,~atings sliding against steel or tungsten carbide balls. Friction coefficient in the initial stages of sliding was in the range of 0.2 to 0.3. The friction coefficient initially quickly increased with sliding time to a maximum value: this maximum friction coefficient rarely exceeded 0.32. Alter that. friction coefficient decreased gradually to reach some lower stable value: this was in the range of 0.2 to 0.29. 4. A mndel describing the wear processes of coatings has been presented on the basis of SEM observations. The tribological behaviour of the DLC coatings is well explained according to this model.

Acknowledgements The authors are grateful fiw the financial support provided by the DTI/EPSRC LINK Surface Engineering SchemeThey also wish to thank Mr. G. France and Mr. H. Pendlebury for their technical assistance.

J. Jhm.~ el al, I Wear 214 t IO~),v,I 14-22

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