Paper XI (iii) Wear Debris Action in Sliding Friction of Ceramics

Wear Pariicles - D. Dowson et al. (Editors)

0 1992 Elsevier Science Publishers 6.V. All rights reserved.

453

Paper XI (iii)

Wear Debris Action in Sliding Friction of Ceramics J. Denape

The wear behaviour of four commercially structural ceramics (Silicon Carbide, Silicon nitride, alumina and partially stabilized zirconia) was invertigated under various conditions of applied loads (2 to 40 N) and sliding Speeds (0.1 to 4 m / s ) in dry and wet environment. Primary emphasis was placed on the mechanical action of wear debris. The quantity of entrapped wear debris and their circulation in the sliding interface simultaneously control the wear rate and the friction response of the examined ceramics. Friction increases and wear rate decreases when an accumulation of wear debris occurs. On the contrary, the elimination of wear debris leads to a lower coefficient of friction while wear enhances. The Parameters which control the accumulation and the elimination of wear debris were identified. 1. INTRODUCTION

The friction and wear studies on ceramic materials represent quite a recent investigation field, which explains t h e still limited fundamental works on the subject. However, the clear knowledge of the friction Performances and the wear behaviour of these materials, has a great importance with regard to their applications as mechanical components. Friction and wear have been studied at first in the case of metallic materials where plastic flow and adhesion mechanisms of asperities within the sliding surfaces dominate the friction and wear processes. T h e main properties Controlling the tribological behaviour are thus the yield Stress and the hardness of the meta1 : a high yield Stress and hard material must give a good wear resistance (1,2).These properties are all to be found in ceramic materials and justis. their use t o prevent wear. However, ceramics are affected by specific properties such as a very pronounced brittle fracture behaviour and structural defects which concentrate Stresses. The studies on mechanical aggression modes on a ceramic surface have started with the static contact mode. In that field, a great number of works are devoted t o static indentation and Hertzian contact (3-5). The degradations observed a r e relevant t o deformation mechanisms and macroscopic fracture, which are quite different mechanisms than those generated in metals. Hence, models stemming from the analysis of metallic material degradations do not fit any more and new models, based on fracture mechanic, have

proved to be more suitable for ceramic case (69). Developped in static contact, these models have been applied successfully to punctual abrasion where material removal occurs by chipping the edges of the groove (10). The surface damaging depends then on the material mechanical properties such as elastic modulus, hardness or fracture toughness. However, the static contact does not give a satisfactory basis for surfacing friction studies. Actually, these latter models come up against a scale factor : the wear particules observed in sliding friction prove to be of microscopic size, far smaller than grain size, t h a t is out of the macroscopic cracking range which characterizes the static models(l1-13). Moreover, the contribution of the wear particles must be taken into account as a component of the frictional force (14). To meet this scale requirement, a different approach called the third body model has been performed. This model takes into account not only the surface damaging induced by friction Stresses, but also the effect of the debris moving through t h e contact (15). T h i s l a t t e r consideration is a n important step of generalisation in t h e evaluation of the tribological behaviour of a material because the implied mechanisms are actually common t o many metals and Polymers and can be applied to ceramics as well as we are going to prove it. The wear process can be analysed into three Stages with their own cinetic in g r e a t interaction between one another (16).The first Stage is concerned with the debris generation and deals with various mechanisms of material removal from a sliding surface. The

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second one is devoted to the evolution of the debris generated in the first stage such as their mechanical and physicochemical changes (grinding, Oxidation...I inside the contact. The third and last stage refers to the own behaviour of the debris which can be either eliminated out of the contact (case of lubricated or Open systems), or accumulated between the sliding surfaces (case of dry closed systems). Its accumulation produces a bed of powder which separates the contact surfaces. The result is a load-carrying phenomenon, as occurs in lubrification : it is as if the debris was acting as a fluid, reducing the interaction between the two surfaces. In this way, a reduction in wear can be interpreted as a gain in the load-carrying capacity of the debris. In this study, we shall deal mainly with the last stage of the debris life and its interaction with the first two stages. Hence, we shall show that the presence of debris implies not only a modification of t h e friction characteristics (friction coefficient, wear rate) b u t also a contribution to material removal.

2.2.Experimental apparatus The experimental apparatus used was specially developed for this study. I t consists of a rolleron-plate tribometer, which allows dry o r wet sliding at ambient temperature. The apparatus comprises a curved roller rotating against a flat sample of the same material. This sample is attached to an arm equiped with two stain-gauge bridges giving continuous recording of the applied laod and the friction force, so as t o follow the Variations in the friction coefficient . Friction produces on the sample a n ellipsoidal track, on which one measures the long and small axes 2a and 2b (in mm) and the wear depth h (in pm) in two perpendicular planes, using a profilometer. This method allows direct calculation of the wear volumes V using the approached formula defined as :

V = d 2 a.b.h The tests are regularly stopped t o record the wear track as a function of sliding distance. Several specimens are used per test conditions.

2. EXPERIMENTAL STUDY

2.3. Experimental procedure

The experimental study considers the wear phenomenon as the production of fine particles. A first series of experiments attempts to keep the debris in the contact interface by means of a tribometer working dry and for which the geometry of t h e specimens favours the concentration of debris. A second series of experiments are conducted with the same tribometer but under distilled water to eliminate wear debris from the sliding interface.

The normal load and the sliding speed are kept constant throughout each test. The test program consists in exploring a load range of between 2 and 40 N for a constant sliding speed of 0.25 d s , as well as a speed range of 0.1 to 4 m/s at a constant load of 5 N. The total sliding time corresponds to a covered distance of 4000 m.

2.1.Tested materials

We first of all studied the evolution of the wear volumes and t h e friction coeficient as a function of the distance covered.

The materials used in the study are four commercial ceramics : a sintered Silicon Carbide SSC, a Silicon nitride of the SiAlON family, a partially stablilized zirconia PSZ and an alumina 99.7% Ai203 which represent all of the so-Ca11 thermomecanical ceramics. Their mains properties are summarised in table 1. material SSC SiAlON PSZ Al203 density 3.15 3.2 5.7 3.9 E (GPa) 420 2 9 0 2 0 0 2 4 0 H(GPa) 29 16 U 13 KIC(MPadm) 3.5 4.8 7 3.5 1-5 35-50 2-5 grain size (pm) 3-8

Table 1 : Main properties of the tested ceramics.

3. DRY FRICTION BEHAVIOUR

The wear volume curves as a function of distance (at constante load and speed) have the following general shape : at the beginning of sliding, one can observe a rapid and short increase in wear volume (transient stage) followed by linear growth (permanent stage). This behaviour is similar for the four ceramics studied, whatever the load-speed values. There are also two stages with the friction coefficients as a function of distance : the transient stage is independent of the experimental conditions but specific to each material : for PSZ, Al203 and SiAlON at high speed the friction coefficient increases, for SiAlON at low speed it decreases and for SSC it begins to decrease then increases.

455

10 1.2

1.2 4

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g

8

8

.m

1.0

3 8

2 zd

B 6

T

0 friction coefficients 0 wearrates

8

.CI

3

1.0

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Q)

8

0

2 .r(

0.8

cr 0.6

0.6

0.4

0.4

0.2

0.2

0

0

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4

2

0 10

20

10

3 0 4 0 Load (NI 20,

1.2

I

3 0 4 0 Load (NI

20

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I

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1

1.2 U

U

G

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1.0

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1.0

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8

8

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2

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0.8

30 0.6

0.6

20

1

0.4

10

0

0.4

0.2

0.1 0.2 0.5

1

2 5 10 Speed(m/s)

0.1 0.2 0.5

1

2 5 10 Speed(m/s)

Figure I : Wear rate (in 10-6 rnm3N.m) and friction coefficient of Al203 and SSC plotted (a) et (b) against applied load at a constant Speed of 0.25 m/s and (c) et (d) against sliding Speed at a constant load of 5 N.

456

The transient Stage is followed by a steady-state Stage depending on both the load and Speed and the material. At this point in the results, a wear rate W is introduced, defined as : W =VLD where V is the wear volume , L the applied load and D the distance covered. This wear rate expressed in 10-6mm31N.m, is justified by the linearity of the curves V(D) and by the regular increase in their slope for increasing loads. It is determined by linear regression and its value is characteristic of a given test, thus allowing easy comparison of results. Despite ~

more or less significant scatter of the results obtained with several consecutive tests, displayed in the form of uncertainty bars, good stability of this rate as a function of the load is observed. It is therefore perfectly justifiable t o examine all behaviour curves as a function of the load and the Speed, only in tems of the wear rate, with which a friction coefficient is associated. 3.1. Influence of the applied load

Low influence of the load L is observed both on the wear rate and on the friction coefficient : they remain appreably constante within the load range studied (figure 1 ab). One can however note the exitence of an accelerated wear regime

~

testing conditions

SEX

loads

2

2.6 +I_1.9

15.5+1- 7.5

(NI

5

3.7 +I_1.2

11.7+/- 4.9

10

4.6+1- 1.5

10.5+/_3.5

20

4.2+1- 1.5

l O . l + l - 2.3

40

5.2+1- 0.2

134+/_22

speeds

0.1

6.3+1- 1.5

36.6+1, 8.7

(mls)

0.25

4.0+/_ 1.4

12.2 +I_4.5

0.5

2.8+1- 0.6

4.5 +I_0.9

1

3.3+1- 1.4

2 4

+ 12.9 +I_ 2.2

2.5 +I- 1.2

11.8+1, 1.0

3.7+/- 2.2

106 +I_5.3

18.1 +I- 4.1

71.2+1- 9.5

10.3+/- 3.1

13.2+1- 1.7

2.7+/- 1.1

6.4+1- 0.8

2.2+/_ 0.5

6.4+/- 1.3

11.0+/- 1.4

3.2 +I- 1.5

5.3+/- 1.4

23.1 +I- 5.5

47.8+/- 6.1

15.2+/- 4.5

6.9+1- 3.1

47.2+/- 11

90.4+1_ 11

I

40.2+1- 10

Table 2 : Wear rates (in 10-6 mm3lN.m) obtained during dry sliding test testing conditions

.

SSC

SiAlON

PSZ

Al203

loads

2

0.76 +I-0.07

0.80 +I_0.10

0.90 +I- 0.02

0.75 +I- 0.15

(NI

5

0.74 +I- 0.05

0.80 +I- 0.05

0.88 +I- 0.03

1.05 +I_ 0.04

10

0.70 +I_ 0.05

0.76 +I-0.06

0.85 +I_ 0.02

1.10 +I- 0.02

20

0.72 +I-0.03

0.86 +I_ 0.04

0.90 +I-0.02

1.05 +I- 0.02

40

0.71 +I_ 0.03

0.88 +I- 0.01

0.84 +I- 0.02

1.03 +I- 0.01

speeds

0.1

0.60 +I_0.05

0.82 +I_ 0.07

0.04 0.86 +I_

0.95 +I- 0.15

(mls)

0.25

0.73 +I_ 0.04

0.82 +I_ 0.04

0.87 +I_ 0.02

1.06 +I-0.03

0.5

0.69 +I_ 0.04

0.88 +I-0.05

0.86 +I_ 0.08

1.10 +I-0.05

1

0.64 +I_0.03

0.91 +I-0.03

0.75 +I-0.02

1.05 +I-0.03

2

0.60 +I_0.02

0.80 +I_0.02

0.70 +I-0.02

0.90 +I_0.02

4

0.56 +I_ 0.02

0.63 +I_0.03

0.56 +I-0.01

0.62 +I- 0.05

Table 3 : Steady-state friction coeficients obtained during dry sliding test

.

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with a load of 40 N using SiAlON, PSZ and, to alesser extent, Al2O3. This wear regime does not afYect the stability of the friction coefficient.

ground and accumulate in a large number of dense films of weak area, firmly adhesive to the worn surface (figure 44.

3.2. Influence of the sliding speed

The worn surface of SiAlON Shows two damage Patterns according to the load level. At low load and speed, a mechnism of abrasion become apparent through the accumulation of debris in the porosity (figure 4d). At high load and speed, particle removal a n d debris accumulation occur in the Same way as observed on Al203 (figure 4e).

Variations in the wear rate and the friction coefficient with speed v, show opposing developments, as well as the existence of a critical speed, located at about 0.5 m/s for all materials. This critical speed corresponds, on one hand, to the speed of least wear in the k(v) curves : on either side of this speed, wear increases greatly, and on the other hand, at the speed of maximum friction coefficient in the f(v) curves : on either side of this speed the friction coefficient is reduced (figure 1 dd). The friction coeficients recorded depend on the type of ceramics. At the critical speed, Al203 has a coefticient of about 1.10 ; SSC a coeficient of 0.69 and SiAlON and PSZ have intermediate values, 0.88 and 0.86 respectively. The Overall experimental results are given in tables 2 and 3. SSC, with the lowest friction coefficient, a steady wear rate for all loads studied, and low sensibility to speed is therefore able to withstand greater loads and receives less damage than the other three ceramics in dry conditions of sliding. 3.3. Microscopic Observations The friction tracks show a large quantity of debris ejected out of the contact, at both the entrance and exit, which implies that the debris is recycled. Finer debris is also observed inside the contact itself (figure 2). These debris have a whitish colour, whatever the original colour of the ceramic. Two types of debris were identified throughout the tests : free individual rounded particules, with a size of 0.1 to 0.5 pm, and more or less compact agglomerates forming powder, flakes, rolls, or platelets with more or less larger particles resulting from brittle fracture (figure 3). Examination of the surface damage requires prior cleaning of the samples in Order t o remove the bed of debris masking the subjacent material. The worn surface of the SSC is particularly smooth, indicating material removal by polishing (figure 4a). Filling in ofthe porosity by very fine debris is observed. At more severe loads, abrasion grooves and microchipping also appear (figure 4b).The surface of the Al203 is damaged by the pulling out of grains throughout the range of loads and speeds studied. These torn-out grains are then finely

The surface of PSZ Shows the Same mechanism of particle removal whatever the sliding test studied. The debris gathers in large compact films which mask the all worn surface (figure 40. Microscopic Observations confirm the presence of d e b r i s ( i n d i v i d u a l p a r t i c l e s a n d agglomerates) as well as its relationship with the various surface damage types : polishing involving free individual particles only, abrasion via the accumulation in the pores, and finally material tear-Off o r pulling out associated with compacted debris films. 3.4. Interpretation of dry friction results Knowledge of the role played by the debris is the key t o interpreting the dry friction behaviour curves. I t is recognised that the accumulation of debris in the contact interface leads t o a n increase i n load carrying capacity, t h u s reducing wear (basis of t h e third body approach). Moreover the presence of particles wedged in the contact interface entails a n increase in the friction coefficient. However, removal of the debris causes a reduction in the friction coefficient b u t also a drop of load carrying capacity which increased wear. The friction coefficient therefore reflects t h e quantity of debris in the contact at any given moment. Variations in the friction coefficient thus show the evolution of the amount of debris in the contact (the effect of recycling) : stability of the fiction coefticient indicates a constant amount of debris in the contact interface, its increase reflects a debris build-up phase and i t s reduction a debris removal phase. The friction coefficient Variations are therefore used t o interpret the dry friction behaviour curves. According t o the speed, the friction coefficient initially increases : this reflects a debris accumulation phase and the reduction in wear rate is clearely noted. The friction coefficient

458

Figure 2 : Fine debris observed inside the contact interface before cleaning (a) on SSC (5N, l d s , 4000m) and (b) on Al203 (5N, l d s , 4000m)

Figure 3 : Electron micrographs of various wear debris observed in the sliding tests (a) free individual particles (SiAlON, 5N, 4 d s ) , (b) compact agglomerates of wear particles (PSZ, 5N, 4 d s ) (c) fine debris and larger agglomerates forming flakes (SSC, 5N,0.25ds) and (4roll debris (SSC,40N, 0 . 5 d s )

459

then drops : debris are removed and the wear rate increase as expected. The common critical Speed of 0.5 d s then corresponds to the Optimum load capacity of the debris, depending on the kinetic characteristics of the tribometer. The Same interpretation can be followed to explain the transient Stages of the friction coefficient of each ceramic. As a function of the load, the friction coefficient remains constant, as does the wear rate, thus implying removal of debris in relationship to the number of particles produced in Order to maintain a constant quantity of debris in the contact interface, whatever the wear volume involved in the test concerned. The role of debris can be confirmed by modifying their distribution in the interface by means of tests using distilled water which will partially or totally remove the debris from the contact. 4. EFFECT OF DEBRIS REMOVAL WITH DISTILLED WATER The influence of distilled water on the behaviour of the four ceramics studied Shows a marked increase in wear and a reduction in the friction coefficient. Removal of the debris confirms the reduction of their load carrying role. The case of SiAlON does not follow this rule and, furthermore, PSZ Shows no marked reduction in its friction coeficient in relation t o the dry tests (table 4). Microscopic Observations will once again shed light and enable us t o interpret these results. Microscopic obsewation of the wear morphology of SSC tracks Shows a polished surface with appearance of t h e microstructure, t h u s implyinganisotropic wear on each grain. A few Shorts Cracks approximately perpendicular t o the sliding direction, plus micro-chipping at the level of the pores, are also observed (figure 5a). SiAlON Shows an extremely finely polished material SSC SiAlON PSZ 111203

I

I

wearrate 8.3 (2.6) 1.5 (4.5) 206 (6.4) 6 (2.2)

I friction [

coef. 0.30 (0.69) 0.45 (0.88) 0.80 (0.86) 0.25 (1.10)

Table 4 : Wear rates (in 10-6 mrn3/N.m) and friction coefficients obtained with tests conducted under distilled water (5 N - 0.5 d s ) . Corresponding values obtained in dry tests are given in brackets.

surface with filling in of the pores by very fine debris (figure 5b). Al203 also Shows a polished surface but without filling in the pores (figure 512).The surface of PSZ is more regular than in dry tests, b u t large numbers of debris accumulation areas still exist, and explain the small Variation in the friction coefficient (figure 5d). SiAION is a particular case : it Shows a damage regime under distilled water different from that in the dry tests. The dry wear mechanism consists in abrasion on debris accumulation in the pores. Under water, the pores a r e obstructed and material removal takes place by polishing, which results in less severe wear in t h a t case. These Observations Show that the debris trapped in the pores are responsible for the abrasion mechanism. Microscopic observations thus clearly Show the dual role of the debris. The first is a protective role : its removal leads t o a reducting in the load-carrying capacity a n d t h e friction coefficient, as well as an increase in wear. The second role is one of attacking the surface : the debris itself contribute t o the material removal and also determine the wear mechnism. The dual role of debris should be considered in the context of two categories of debris obsewed in the contact interface in dry tests. The bed of free individual particles can thus be associated with the load-carrying effect (protective aspect) and the agglomerations of particles adhesive in the wear track a r e responsable for the mechanism of abrasion of the opposit surface.

5. PHYSICO-CHEMICAL ASPECTS OF DEBRIS Physico-chemical analyses conducted on wear debris Show profound changes in t h e cristallographic s t r u c t u r e a n d chemical composition in relation t o t h e initial bulk materials. ,These changes reflect the high reactivity of the debris in the contact under the effect of Stresses, the oxidising environment, and tmperature. This sensibility is increased even further by a large specific surface of the debris powder. Thus, certain tests carried out with SSC produce roll shape wear debris generated by rolling in the contact and moving penpendicularly to the direction of movement. This debris proved t o consist of amorphous silica SiO2, the carbone having completely disappeared. SiAlON shows a high degree of debris Oxidation. PSZ, a multiphase structure, produces only cubic phase debris, with no sign of the monoclinic or tetragonal phases present in the bulk material.

460

Figure 4 : Electron micrographs of worn surfaces of the ceramics (dry friction test) showing various wear mechanisms of material removal (a) polishing of SSC at low and intermediate loads, (b) abrasion and microchipping of SSC at more severe loads, (c) grain torn-out and debris accumulation on Al203 , (d) abrasion of SiAlON at low and intermediate loads, ( e ) particle removal and debris accumulation on SiAlON at more severe loads, (0 particule removal and dense films of debris on PSZ.

46 1

However, these various changes did not modie the Overall behaviour of the sliding ceramics : the wear Parameters, wear rate and friction coefficient only depend on the mechanical behaviour of the bed of particles and not influenced by the physico-chemical changes in these debris. 6. CONCLUSION

The wear behaviour of four commercially ceramics representing a broad spectrum of available structural ceramics (Silicon Carbide SSC, SiAlON, alumina Al203 and partially stabilized zirconia PSZ) was investigated at room temperature, with the aim t o examine the mechanical action of wear debris upon both

friction and wear response. For this purpose, two different testing conditions were selected involving either the accumulation of the wear debris or its elimination from the sliding interface. The wear sensitivity of the examined ceramics depends to a large degree on the testing conditions. In dry conditions involving the contribution of the wear debris in the sliding interface, SSC gave the highest wear resistance, followed by Al2O3. SiAlON and PSZ exhibited the lowest wear resistance. This trend is strongly affected in the presence of distilled water involving the elimination of debris where SiAlON showed the highest wear resistance followed by Ai2O3, SSC and PSZ.

Figure 5 : Electron micrographs of worn surfaces of ceramics (tests under distilled water) showing changes in wear mechanisms (a) polishing and fracture of SSC associated with anisotropic wear, (b) fine polishing of SiAlON with filling in of the porosity, (c) polishing of Ai203 without filling in of the porosity, (d) accumulation of debris still present on PSZ.

462

The circulation of wear particles is reflected by the friction coefficient, which increases when particles are accumulated and decreases when particles a r e removed from the sliding interface. T h e accumulation a n d t h e elimination of debris are governed by the sliding Speed. Regardless of the material, a critical sliding Speed provides the maximum quantity of wear debris in the sliding interface, which induces a maximum friction coefficient and a minimum wear rate. The debris determines the wear mechanism. Polishing is due to fine individual wear particles smaller than 1 pm, circulating in the sliding interface. Abrasion and grain pull-out are associated with dense accumulations of particles adherent t o the sliding surfaces. The fine individual debris has a dual action. It acts as a lubricant with a load-carrying effect, forming layers separating the sliding surfaces. I t also contributes t o material removal by polishing. The agressive or protective action of fine debris depends essentially on the quantity entrapped in the sliding contact. SiAlON exhibited specific features, such as a very important sensitivity to the presence of particles which strongly degraded the surface by abrasion. Moreover it showed a very good behaviour in lubricated conditions where surface Oxidation is bound t o occur in a larger extent under water. The results of this study are in correlation with the principles of the third body theory. Therefore, this shows t h a t sliding friction should not be approached exclusively from the material side, but rather through a n Overall framework into which t h e individual Observations can be fitted. This should be an important step towards a unifying theory of wear. ACKNOWLEDGMENTS This study have been conducted in the Centre des Materiaux de 1'E.N.S des Mines de Paris (Evry-France). The SSC and SiAlON samples were prepared by the Ceraver Company (Tarbes-France) and those of PSZ and Al203 by the Desmarquest Company (Trappes-France). REFERENCES (1) H.CZICHOS, "Tribology", Tribology Series 1, ED. Elsevier (1978).

(2) E.RABINOWICK, "Friction and wear of materials", ed Wiley Inc. (1965). B.R.LAWN and T..WILSHAW, (3) "Indentation fracture : principles and applications" J.Mater.Sci. 10 (1975) 10491081. (4) A.G.EVANS and T.R.WILSHAW, "Quasistatic solid particle damage in brittle solids" Acta.Met.24 (1976) 939-956. (5) B.R.LAWN and M.V.SWAIN, "Microfracture beneath point indentations in brittle solids" J.Mater.Sci. 10 (1975) 113122. (6) J.T.HAGAN, "Micromechanisms of crak i n d e n t a t i o n s" d u ring n u cl e a t i o n J.Mater.Sci. 14 (1979) 2975-2980. (7) B.R.LAWN and A.G.EVANS, "A model for Crack i n i t i a t i o n i n elastic-p lastic indentation fields" J.Mater.Sci. 12 (1977) 2 195-2199. (8) B.R.LAWN, A.G.EVANS and D .B .MARS HALL , " E 1a s ti c - p 1a s t i c indentation damage in ceramics : the median-radial Crack system" J.Am. CeramSoc. 63 (1980) n"9-10,574-581. ( 9 ) D.B.MARSHALL, B.R.LAWN a n d A.G.EVANS, " Elasric-plastic indentation damage in ceramics : the lateral Crack system" J.Am.Ceram.Soc 65 (1982) n"l1, 561-566. (10) D.B.MARSHALL, "Surface damage in ceramics : implications for strengh degradation, erosion and wear" Progress in nitrogene ceramics, NATO ASI Serie E : applied sciences 65 (1983) 635-656. (11) D.C.CRANMER, "Friction and wear properties of monolithic Silicon based ceramics" J.Mater.Sci. 20 (1985) 2029-2037. (12) 0.O.ADEWOYE a n d T.F.PAGE, "Frictional deformation and fracture in polycristalline SiC and Si3N4" Wear 70 (1981) 37-51. (13) E.BREVAL, J.BREZNAK a n d N.H. McMILLAN , "Sliding friction and wear of structural ceramics Part 2 : Analysis of room-temperature wear debris"J.Mater.Sci. 2 1 (1986) 93 1-935. (14) N.P.SUH and M.C.SIN, "On the genesis of friction and its effect on wear" Wear of materials (1981) 167-183. (15) M.GODET, "The fhird body approach : a mechanical view of wear" Wear 100 (1984) 437-452. (16) Y.BERTHIER, C.COLOMBIE, M.GODET, G.LOFFICIAL and L.VINCENT, "L'usure par petits debattements" Eurotrib '85, ed. Soc.Fr.Tribologie (Elsevier, Amsterdam) ~01.11,section 5.5.1 (1985).