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Cumulative deformation and fracture of sliding surfaces
149
Wear, 126 (1988) 149 - 165
CUMULATIVE SURFACES
DEFORMATION
AND FRACTURE
OF SLIDING
M. P. SHAW Department of Materials Engineering, (South Africa)
University of Cape Town, Rondebosch
7700
C. A. BROOKES Department of Engineering Design and Manufacture, North Humberside HU6 7RX (U.K.) (Received February 3,1988;
University of Hull,
accepted March 30,1988)
Summary The influence of cumulative deformation introduced by softer materials during slow sliding on harder lubricated surfaces is investigated for a range of different solids. Fragmentation of the harder surfaces is demonstrated for non-metallic crystals, polycrystalline metals and amorphous sodium silicate glass. The accommodation of strain, sometimes with subsequent work hardening, is discussed in relation to the relative hardness of the material couples.
1. Introduction Surface damage and wear of harder materials, due to the cumulative effect of repeated sliding contacts with softer solids, is a subject that has received relatively little systematic attention when considering the purely mechanical aspects contributing to eventual surface failure. Investigations into the wear of such hard-soft couples have usually been based on the extensive interaction of thermal, chemical and mechanical effects pertaining typically to tool-workpiece situations during the machining of metals (e.g. ref. 1). In such cases, dominant mechanisms involve diffusion and solubility criteria resulting from high local temperatures generated at the interface between the materials in question. However, more widespread use of hard materials (particularly ceramics) in demanding engineering situations (see, for example, ref. 2) has led to the increasing possibility that cumulative mechanical effects (which may be of little significance individually) will have a dominant influence on eventual surface degradation. In the past, relatively isolated studies [3 - 51 have been devoted to these aspects; the exception being the wear of diamond in which the situation is inescapable [6-81. 0043-1648/88/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
150
Recent work by Brookes et al. [9] has, however, demonstrated that repeated sliding contact between hard non-metallic surfaces and considerably softer conical metal sliders can, under lubricated conditions, produce significant work hardening and subsequent fracture of the hard crystal surfaces. In the above work, the nominal contact pressure transmitted to the hard crystal was directly related to the flow stress of the softer metal slider via the area of the blunted tip of an initially sharp cone, and was thus dependent on the metal used for the conical slider. It was shown that single crystals of MgO, NiO and TiO absorbed significant amounts of strain, through dislocation production and multiplication beneath the wear tracks, before eventual surface failure by fracture was produced by a fatigue mech~ism. The conditions thought to be necessary for the operation of the proposed mechanism can be summarized as follows. (a) A nominal contact pressure produced by the softer slider which was sufficient to exceed the critical resolved shear stress of the harder crystal but not sufficient to produce cracking. (b) Repeated traversals over the same contact area to increase the density of dislocations, thus causing work hardening and the production of dislocation debris in the surface layers of the crystal. (c) Initiation of cracks, at or close to the surface, as a result of a fatigue process. The purpose of the present work was to investigate a more general application of the above methodology to materials representing a wider range of experimental conditions, i.e. the same crystal structure, bonding (ionic) and slip systems but higher homologous temperature (LiF); a different crystal structure but the same bonding and slip systems (strontium titanate); the same bonding but different crystal structure and unknown slip systems (yttrium aluminium garnet); different (metallic) bonding, various crystal structures and slip systems (polycrystahine metals); and an amorphous glass (sodium silicate)_ Various materials, of wide-ruing hardness, are used as the softer cones to impart specific nomindl mean pressures to the surface of the harder solids.
2. Experimental details The reciprocating sliding apparatus used has been described previously in detail 193. The essential features only are given here together with a schematic illustration in Fig. 1. All experiments were performed at slow sliding speeds (approximately 5 mm s-i) in the presence of a plentiful supply of lubricant. The lubricant employed was a saturated solution of stearic acid in mineral oil. The apparatus consisted of a reciprocating specimen stage, moving in a horizontai plane, with a stationary softer slider attached to the end of a pivoted and counterbalanced beam, A pyrophylite weight platform was attached to the end of the beam, via a load transducer, and a removable slider was secured into the centre of its lower face. Thus the normal load on
151
Weight platform \ SLider
Counterbalance weight
Load transducer
If71
1 Reciprocating \
/
yble
Drive crank
Fig. 1. A schematic illustration of the reciprocating sliding apparatus.
the weight platform was transmitted directly through the axis of the slider on to the horizontal specimen. The frictional force between the sliding surfaces could be measured by feeding the output from the load transducer through an amplifier into a continuous pen recorder. The specimen was clamped to a movable stage which was mounted on a table that could be rotated about both a vertical and, to a limited extent, a horizontal axis. This arrangement allowed single-crystal specimen surfaces to be aligned in specific crystallographic directions, whilst remaining horizontal, when necessary. The specimen stage was split, so that the specimen could be removed for examination and replaced in exactly the same position relative to the slider. Normal loads of 4.9 N were used in all the experiments. All of the softer sliders were machined into cones having a total included angle of 136”. Each experiment resulted in the cones being flattened, to a degree dependent upon the hardness of the materials used. Knoop microhardness measurements were obtained from both the blunted flat on the cone tip and the surfaces of wear tracks on the harder crystal, using a Leitz “Miniload” microhardness tester with a 0.98 N normal load. Specimen preparation procedures for the materials studied are given below in Table 1, together with the Knoop microhardness data from surfaces before sliding tests were performed. The softer sliders were examined in profile microscopically before each experiment to ensure sharpness and integrity and, immediately prior to the tests, were thoroughly cleaned ultrasonically in an organic solvent. The materials used, together with microhardness values before testing (initial) and after approximately 20 traversals from the flat surface of the blunted slider tips (ultimate), are given in Table 2. Results of reciprocating sliding experiments were taken from at least three tests for any given sliding couple, and reproducibility was usually within +5% unless stated otherwise.
Crystal plane
0 001
(1001
{lOOI
Polycrystalline
Polycrystalline
Polycrystalline
Amorphous
Material
Strontium titanate
Yttrium aluminium garnet
LiF
cu
MO
co
Sodium silicate
(100):5.36,
Conventional mechanical polishing down to l/4 pm diamond paste
(Knoop)
As received
As above
As above
Conventional mechanical polishing down to l/4 pm diamond paste
6.63
2.96
3.06
0.94
(100):1.01,
Cleaved in air, followed by conventional mechanical polishing down to l/4 pm diamond paste. Then annealed in argon for 3 h at 400 “C to remove dislocations from the surface
(110):1.07
1.47 in both (100) and (110)
(GN m-*)
As above
(110):8.97
Microhardness
Surface preparation
Specimen preparation procedures and microhardness (0.98 N load) before deformation
TABLE 1
z to
153
3. Results
and discussion
3.1. Lithium fluoride LiF has the rock salt crystal structure and its cleavage characteristics and slip systems are directly comparable with MgO (see ref. 9). However, room temperature corresponds to 0.26 T, (as opposed to 0.17 T, for MgO) and there is a greater degree of plastic deformation possible. Repeated sliding experiments were performed on the (001) plane, in the (100) direction using zinc, cadmium, poly(methy1 methacrylate), polystyrene and high density polyethylene sliders (see Table 2). Knoop indentations, used to measure the hardness of the blunted tips of the polymer sliders, listed in Table 2, gave only approximate hardness values because of the difficulty encountered in measuring the indentations accurately. The problems arose because the indentations were difficult to see and also the size of the impressions tended to change, during measurement, owing to the elastic recovery of the polymers. Therefore, in order to obtain a more meaningful estimate of the relative hardness of the three polymers, the effective mean pressure (at 4.9 N normal loading) was measured by loading the conical polymer sliders against a glass microscope slide and focusing on to the contact surface. The mean pressure values were obtained by dividing the applied load by the apparent area of contact and values between 0.1 and 0.15 GN me2 were measured for the three polymers in question. The zinc slider was the only one of the five sliders used to give reproducible results on LiF. Fragmentation was produced after approximately 500 traversals and the degree of work hardening on the wear track surface is shown in Fig. 2. As in MgG, the fracture pattern was of “chevron” type and fragmentation appeared suddenly with a corresponding increase in friction.
TABLE 2 Initial and ultimate microhardness (0.98 N load) of slider materials
Material
Martensitic steel EN58J stainless steel EN42 steel Phosphor bronze (95/5) Brass (70/30) Al Zn Cd Poly( methyl methacrylate) Polystyrene High density polyethylene
Knoop hardness (GN me2) Initial
Ultimate
10.82 3.92 3.72 2.45 1.96 0.90 0.41 0.20 0.05 0.06 0.05
12.75 6.37 4.61 3.04 2.45 1.27 0.41 0.20 0.05 0.05 0.05
154
I 100
200 NUMBER OF TRAVERSALS
Fig. 2. Surface hardening of LiF (001) (110) sliding direction).
300
GO
as a function of number of traversals (zinc slider,
Both the cadmium and the poly(methy1 methacrylate) sliders gave inconsistent results. Fragmentation occurred in the case of both sliders but the number of traversals needed to promote fracture varied between 10 and 20. It was not possible to detect any work hardening of the wear track surfaces, in either case, but the number of hardness measurements taken was limited because of the erratic nature of the fragmentation. It is curious that fracture should occur after a fewer number of traversals than was the case for zinc, as poly(methy1 methacrylate) sliders are considerably softer (see Table 2). It is possible that this may be due to an increased tendency for adhesion to occur between LiF and cadmium and the possible influence of the lubricant on the surface properties of poly(methy1 methacrylate). Wear tracks produced by the polystyrene and high density polyethylene slider did not exhibit any fracture, even after 10 X lo4 traversals, and there was no evidence of work hardening. However, dislocations were revealed in
Fig. 3. Cross-section of wear track on LiF (001) after 25 X lo3 traversals with a polystyrene slider, showing dislocation density beneath the track after etching (sliding direction (100)) (bar, 400 pm).
155
the bulk of the solid (by cleaving the crystal and subsequently etching in H,Oz) beneath the wear tracks (see Fig. 3). 3.2. Strontium titanate Strontium titanate has the cubic- perovskite crystal structure at room temperature, which consists of a close-packed arrangement of oxygen ions with titanium occupying the lattice sites at the centre of oxygen octahedra. It is a crystal that is frequently used as a diamond substitute in the gemstone industry because of its optical properties. Very little data exist on the mechanical properties of these crystals. However, Bumand [lo] has investigated the Knoop hardness anisotropy of both the (001) and the (110) planes as a function of temperature and has shown that its behaviour is similar to MgO, which is a well-characterized material and has been the subject of detailed study in the experimental situation described here, as reported [9]. Thus, the mechanical properties and slip systems are directly comparable with MgO, except that strontium titanate does not have a specific cleavage plane and tends to fracture conchoidally .
Experiments were performed on the (001) plane of strontium titanate, using the EN58J, EN42 and 95/5 phosphor bronze sliders (see Table 2) in both the (100) and the (110) sliding directions. In the initial stages of the test, work hardening of the harder crystal occurred and the degree of hardening was dependent upon the ultimate hardness of the softer slider used (see Fig. 4). There appears to be no significant difference in the rate or degree of hardening in the two different sliding directions. The initial stages of the wear process produced plastic deformation, which resulted in a smooth-sided groove being formed by the tip of the slider (see Fig. 5). Although the crystal surface had been originally prepared by mechanical polishing, slip lines consistent with slip on the (110) (110) system were discernible within the wear track when the crystal was examined under Nomarsky interference contrast conditions on an optical microscope. Similar features were visible in tracks produced by both (100) and (110) sliding directions on the (100) plane. Cracks, within the contact area of the slider and crystal, were eventually produced by all three sliders for both (100) and (110) sliding directions. The results are shown in Fig. 6 and it can be seen that significant anisotropy in wear resistance is evident; the (110) sliding direction being more resistant to cracking, as was shown for MgO [9]. The nature of the fracture observed varied, depending upon the slider and the direction of sliding. For experiments involving (100) sliding the phosphor bronze slider produced cracks after approximately 6 X lo4 traversals, and these did not greatly increase in degree or severity even after 10 X lo4 traversals. The fracture was intermittent along the length of the wear track and complete fragmentation was not observed, in contrast to the behaviour of MgO. The cracks produced by the EN42 and EN58J sliders, when sliding in (100) directions, took the initial form of semicircular “ring cracks” (see Fig. 7). On increasing the number of
156
NUMBER
OF TRAVERSALS
(al
14.0
100
200 NUMBER
300 OF
LOO
500
TRAVERSALS
(b)
Fig. 4. Surface hardening of strontium titanate (0013 as a function of number of traversals. (a) Sliding direction (100). (b) Sliding direction (110). (a, EN58J; 0, EN42 ; 0, 95/5 phosphor bronze.)
traversals, beyond the point at which cracks first became detectable, eomplete fragmentation of the wear track occurred and the fracture pattern was of the “chevron” form, as observed for MgO [9] (see Fig. 8). The fracture patterns observed from experiments in the (110) sliding direction were somewhat different. In particular, the fracture seemed less
157
Fig. 5. Strontium titanate {OOl) after 100 traversals with 95/5 phosphor bronze slider (4.9 N load, (100) sliding direction) showing smooth-sided groove before fracture. Nomarsky interference contrast (bar, 200 pm).
c~s~lo~aphi~ in nature. All three sliders produced fracture, initially in the form of a semicircular “ring crack” (see Fig. 9), which eventually produced wear by the “plucking out” of particles from the wear track surface. The fracture was intermittent along the wear track, and did not exhibit the “chevron” cracking observed in the (100) direction (see Fig. 10). The degree of cracking did increase, on increasing the number of traversals beyond the point of initial fracture, but only in the case of the EN58J slider did fragmentation occur over the whole area of the wear track. An interesting aspect of fracture, observed when sliding in both directions on the (001) plane, was the presence of a certain degree of timedependent subsurface cracking associated with the wear process. Wear tracks that showed no sign of fracture, on initial examination, occasionally developed lateral vent-type cracks after a certain length of time, e.g. 2 or 3 h, had elapsed. This time-dependent phenomenon was only observed in some of the wear tracks produced by the EN58J stainless steel slider and it appears to be associated with the relief of residual stresses produced by the softer slider. This effect was also observed in Knoop indentations, on {OOl) planes, during measurement of the impression size whilst the specimen was under microscopic examination. 3.3. Yttrium aluminium garnet Yttrium aluminium garnet (YAG) is used mainly in connection with solid state lasers and also as a gemstone. Although its optical properties have been well characterized, little is known about its mechanical behaviour. The crystal structure is basically b.c.c. [ 111 with no simple close-packed planes and the relevant slip systems have not yet been identified. Reciprocating sliding tests were performed on the mechanically polished {OOl) plane surface using the martensitic steel, EN58J and EN42 steel sliders. Time~ependent fracture was again observed during the measurement of Knoop indentations. Conchoidal subsurface fracture was seen to occur, beneath the surface, around the edges of the indentations. These cracks joined with the specimen surface on occasions, forming a detached particle
158
% z 9
9515phosphorbronze 2.0-
i c
L
2000
4OM)
6000
8000
I\ "
60X
CRITICAL NUMBEh OF TRAVERSALS t0 FRACTURE (aI 7.0
2000
4000 6cQO i3000 CRITICALNUMBER OF TRAVERSALS TO FRACTURE
I
_I
100K
#I
Fig. 6. Critical number of traversals to fracture as a function of slider hardness for strontium titanate (001) (4.9 N load). (a) (100) Sliding direction. (b) (110) Sliding direction.
around the impression. However, these me-decadent cracks were not observed during subsequent experiments involving the softer sliders. All three sliders produced fracture, when sliding in both (100) and (110) directions, which was preceded by the formation of a smooth and apparently plastically deformed groove {see Fig. 11). However, no evidence of work
Fig. 7. Strontium titanate (001) after four traversals with EN58J stainless steel slider, showing the presence of semicircular “ring cracks” prior to complete fragmentation, sliding direction (100) (bar, 200 pm). Fig. 8. As Fig. 5, after 10 traversals, showing “chevron”-type after complete fragmentation (bar, 200 pm).
fracture pattern observed
Fig. 9. Strontium titanate (001) after 2500 traversals with EN42 steel slider, showing the formation of the first observable crack, sliding direction (110). Nomarsky interference contrast (bar, 100 pm). Fig. 10. Strontium titanate (001) after 10 traversals with EN58J stainless steel slider, showing intermittent fragmentation of the wear track, sliding direction (110) (bar, 200 Pm).
Fig. 11. Yttrium aluminium garnet (001) after 800 traversals with a martensitic steel slider, showing plastic deformation, sliding direction (100) (bar, 100 pm). Fig. 12. As Fig. 11, after 100 traversals, showing semicircular “ring cracks” (bar, 100 pm).
hardening could be detected in any of the experiments involving YAG and no slip lines were evident in any of the wear tracks. When cracks were eventually produced, they took the form of semicircular “ring cracks” (see Fig. 12); the harder the slider, the fewer the
160 14.0 l
hardenedmortensittc steel
12.0 ;; : 10.0
I CO,OCKl
30.000 20,000 10,000 CRITICALNUMBEROF TRAVERSALS TO FRACTURE
Fig. 13 Critical number of traversals to fracture as a function (4.9 N load, (100)sliding direction}.
of slider hardness for YAG
number of traversals required to produce cracking (see Fig. 13). Once “ring cracks” had formed they did not appear to increase in severity or degree, to any significant extent, with increasing number of traversals. However, partial pension of the wear track was observed after 10 X lo4 traversals (see Fig. 14). The fracture was non~~s~lo~aphic in nature and it appeared that wear particles were formed by the joining up of two adjacent semicircular “ring cracks” produced earlier in the wear process.
Fig. 14. YAG (001) after 10 X lo4 traversals with a martensitic partial fragmentation of the wear track (bar, 100 pm).
steel slider, showing
Three polycrystahine metals, i.e. copper (f.c.c.), molybdenum (b.c.c.) and cobalt (h.c.p.) were chosen to examine the cumulative effect of softer sliders (see Table 1).
161
3.4.1. Copper A zinc slider was used on the copper specimen, but transfer of material from the slider to the wear track occurred after approximately 10 traversals and this tended to obscure the deformation in the copper. However, there were areas of the wear track which were free from adhered zinc and hardness values were taken on a number of wear tracks after between 1 and 2 X lo4 traversals. No work hardening could be detected in any of the tracks, although obvious signs of fairly extensive deformation were observed. The deformation took the form of a smooth-sided groove with slip lines being produced within the area traversed by the slider. Transfer of zinc obscured the groove in tracks produced by more than 10 traversals and no sign of fracture was observed after 2 X lo4 traversals. Reciprocating sliding experiments were performed using a poly( methyl methacrylate) slider in order to avoid problems associated with adhesion and transfer. No sign of fracture could be detected after 10 X lo4 traversals but plastic deformation of the harder metal surface was observed (see Fig. 15). Again, hardness measurements on the wear track surface revealed no sign of hardening.
Fig. 15. Polycrystalline copper surface after approximately 30000 traversals with a poly(methy1 methacrylate) slider, showing evidence of plastic deformation (bar, 400 pm).
3.4.2. Molybdenum Sliding experiments were performed on polycrystalline molybdenum specimens using aluminium and 70/30 brass sliders (see Table 2). There was marked adhesion and transfer associated with the wear tracks produced by the aluminium slider and no useful information was obtained. No fracture could be detected after 10 X lo4 traversals and the adhesion on the wear track surface masked any hardening that may have been present. However, the 70/30 brass slider gave wear tracks that were free of adhesion and transfer. No work hardening or fracture could be detected, even after 10 X lo4 traversals, but a limited degree of plastic deformation was visible. 3.4.3. Cobalt A polycrystalline cobalt surface was tested with a 70/30 brass slider and a copper slider (see Table 2). No work hardening was measurable in wear tracks produced by either slider but plastic deformation and eventual frag-
162
mentation did occur in both cases. In the early stages of the wear process a smooth-sided groove was formed (Fig. 16) but no slip lines could be detected. Partial fra~en~tion followed after approximately 500 and 800 traversals respectively for the brass and copper sliders. Eventually, complete fragmentation occurred after approximately 1000 traversals with brass and 1200 traversals with copper. Evidence of slip in the individual grains could be seen at the edges of the completely fragmented wear tracks (see Fig. 1’7).
Fig. 16. Polycrystalline cobalt surface after 100 traversals with a copper slider, showing a plastically deformed groove. Nomarsky interference contrast (bar, 200 pm).
Fig. 17. Evidence of slip in individual grains at the edge of the fragmented wear track after 1000 traversals in polycrystalline cobalt using a 70130 brass slider (bar, 50 pm).
3.5. Sodium silicate glass As an example of an amorphous solid, a conventional sodium silicate glass microscope slide was used as the harder specimen with ENMJ, EN42, 9515 phosphor bronze and ‘IO/30 braas sliders (see Table 2). Fracture was produced in the glass by all of the sliders and the results are presented in Fig. 18. It can be seen that the curve produced is similar to those produced by the crystalline solids that gave reproducible results (see also ref. 9). The fracture initially took the form of semicircular “ring cracks” (see Fig. 19) and the points in Fig. 18 represent the formation of the first observable “ring crack”. As the number of traversals was increased, past the point of initial cracking, partial fragmentation of the wear tracks was observed. The wear fragments appeared to be produced by the merging of two of the “ring cracks” (see Fig. 20), whereupon further traversals produced more
163 7.0 1
2 3.0 -
. 9515 phosphorbronze 70130brass .
B = 2.0 B x
1.0-
I
I SOW
10,000
15,aM
20,ooo
25,OM)
CRITICALNUMEIEROF TRAVERSALS TO FRACTURE
Fig. 18. Critical number of traversals to fracture as a function sodium silicate glass, 4.9 N load.
of slider hardness for
Fig. 19. Sodium silicate glass after 7000 traversals with a 96/5 phosphor bronze slider, showing semicircular “ring cracks” (bar, 400 pm). Fig. 20. Sodium silicate glass after 8000 traversals with a 95/5 phosphor bronze slider, showing the start of fragmentation (bar, 200 pm).
severe fragmentation. Marked transfer track surface, once cracking had been change in hardness of the glass prior to deformation could be seen within the four sliders.
of metal occurred, on to the wear initiated. There was no measurable fracture, although evidence of slight wear track region in the case of all
4. Concluding discussion The materials in which cumulative deformation, induced by softer sliders, has resulted in surface fragmentation fall into four categories.
164
(1) Non-metallic single crystals which exhibit plasticity, work hardening, some degree of crystallographic fracture and anisotropy in wear resistance, e.g. strontium titanate. (2) Non-metallic single crystals which exhibit apparent plasticity, no measurable work hardening, no anisotropy in wear resistance and conchoidal fracture, e.g. yttrium aluminium garnet. (3) Polycrystalline metals which exhibit plasticity but no measurable work hardening, e.g. cobalt. (4) Amorphous materials which exhibit no measurable work hardening, e.g. sodium silicate glass. In all these categories it has been established that the wear process initially identified in ref. 9 is germane, although the detailed mechanism leading to eventual fragmentation must vary, particularly with respect to amorphous solids. A surprising feature of the results is the absence of measurable work hardening in the metal specimens, despite clear evidence of plastic deformation. However, it is possible that initial mechanical polishing of the metal surface has introduced a sufficient degree of plastic deformation to mask the influence of the softer sliders. It has been shown previously [9] that work hardening does occur, to a significant extent, below the wear track surface and it would be reasonable to assume that ductile metals do possess a deformation zone resulting from sliding that penetrates further into the bulk than that resulting from mechanical polishing. However, without an initially pristine chemically polished surface this is difficult to investigate experimentally. In addition, Knoop microhardness measurements from the polycrystalline metal specimens did exhibit a much greater degree of scatter than in the single crystals, no doubt partially due to variable orientation effects within individual grains. The results obtained from sodium silicate glass are directly comparable, in terms of the form of the fragmentation curve, with those obtained for crystalline solids both here and in ref. 9. Clearly, a mechanism based on dislocation interaction and conventional work hardening is inapplicable in this case but a surface fatigue mechanism based on the propagation of preexisting surface cracks may be expected to lead to a similar conclusion. In all cases, where reproducible fragmentation has occurred, it would appear that the attainment of a critical strain prior to fracture is significant (as suggested by Ball [ 121 and in ref. 9). The form of the fragmentation curves applicable to these cases suggests a limiting slider hardness below which cumulative deformation would not produce eventual fracture, although a wider range of sliders would need to be investigated before the limiting hardness could be accurately identified for any particular crystal. Relatively large strains appear to be produced by this type of experimental geometry in inherently brittle materials and the relevance of cumulative deformation in harder materials may well be of greater significance in practical situations than is commonly supposed.
165
References 1 E. M. Trent, Metal Cutting, Butterworths, London, 1977. 2 E. A. Almond, C. A. Brookes and R. Warren (eds.), Science of Hard Materials, Proc. Znt. Conf. (Rhodes), Inst. of Phys. Conf. Ser. No. 75, Adam Hilger, Bristol, 1986. 3 K. F. Dufrane and W. A. Glaeser, Rolling contact deformation of MgO single crystals, Wear, 37 (1976) 21. 4 C. A. Brookes and M. P. Shaw, Cumulative deformation of magnesium oxide crystals by softer sliders, Nature, 263 (1976) 760 - 762. 5 C. A. Brookes and P. Green, Deformation of magnesium oxide crystals by softer indenters and sliders, Nature, 246 (1972) 119 - 120. 6 D. Crompton, W. Hirst and G. W. Howse, The wear of diamond, Proc. R. Sot. London, Ser. A, 333 (1973) 435 - 454. 7 M. Casey and J. Wilks, The friction of diamond sliding on polished cube faces of diamond, J. Phys. D, 6 (1973) 1772 - 1781. 8 C. A. Brookes, in J. E. Field (ed.), Properties of Diamond, Academic Press, London, 1979, pp. 348 - 402. 9 C. A. Brookes, M. P. Shaw and P. E. Tanner, Non-metallic crystals undergoing cumulative work-hardening and wear due to softer lubricated metal sliding surfaces, Proc. R. Sot. London, Ser. A, 409 (1987) 141 - 159. 10 R. P. Burnand, Ph.D. Thesis, University of Exeter, 1974. 11 K. H. G. Ashbee and G. Thomas, Electron microscopy of yttrium aluminium garnet (YAG), J. Appl. Phys., 39 (8) (1968) 3778 - 3780. 12 A. Ball, On the importance of work-hardening in the design of wear-resistant materials, Wear, 91 (1983) 201 - 207.
Wear, 126 (1988) 149 - 165
CUMULATIVE SURFACES
DEFORMATION
AND FRACTURE
OF SLIDING
M. P. SHAW Department of Materials Engineering, (South Africa)
University of Cape Town, Rondebosch
7700
C. A. BROOKES Department of Engineering Design and Manufacture, North Humberside HU6 7RX (U.K.) (Received February 3,1988;
University of Hull,
accepted March 30,1988)
Summary The influence of cumulative deformation introduced by softer materials during slow sliding on harder lubricated surfaces is investigated for a range of different solids. Fragmentation of the harder surfaces is demonstrated for non-metallic crystals, polycrystalline metals and amorphous sodium silicate glass. The accommodation of strain, sometimes with subsequent work hardening, is discussed in relation to the relative hardness of the material couples.
1. Introduction Surface damage and wear of harder materials, due to the cumulative effect of repeated sliding contacts with softer solids, is a subject that has received relatively little systematic attention when considering the purely mechanical aspects contributing to eventual surface failure. Investigations into the wear of such hard-soft couples have usually been based on the extensive interaction of thermal, chemical and mechanical effects pertaining typically to tool-workpiece situations during the machining of metals (e.g. ref. 1). In such cases, dominant mechanisms involve diffusion and solubility criteria resulting from high local temperatures generated at the interface between the materials in question. However, more widespread use of hard materials (particularly ceramics) in demanding engineering situations (see, for example, ref. 2) has led to the increasing possibility that cumulative mechanical effects (which may be of little significance individually) will have a dominant influence on eventual surface degradation. In the past, relatively isolated studies [3 - 51 have been devoted to these aspects; the exception being the wear of diamond in which the situation is inescapable [6-81. 0043-1648/88/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
150
Recent work by Brookes et al. [9] has, however, demonstrated that repeated sliding contact between hard non-metallic surfaces and considerably softer conical metal sliders can, under lubricated conditions, produce significant work hardening and subsequent fracture of the hard crystal surfaces. In the above work, the nominal contact pressure transmitted to the hard crystal was directly related to the flow stress of the softer metal slider via the area of the blunted tip of an initially sharp cone, and was thus dependent on the metal used for the conical slider. It was shown that single crystals of MgO, NiO and TiO absorbed significant amounts of strain, through dislocation production and multiplication beneath the wear tracks, before eventual surface failure by fracture was produced by a fatigue mech~ism. The conditions thought to be necessary for the operation of the proposed mechanism can be summarized as follows. (a) A nominal contact pressure produced by the softer slider which was sufficient to exceed the critical resolved shear stress of the harder crystal but not sufficient to produce cracking. (b) Repeated traversals over the same contact area to increase the density of dislocations, thus causing work hardening and the production of dislocation debris in the surface layers of the crystal. (c) Initiation of cracks, at or close to the surface, as a result of a fatigue process. The purpose of the present work was to investigate a more general application of the above methodology to materials representing a wider range of experimental conditions, i.e. the same crystal structure, bonding (ionic) and slip systems but higher homologous temperature (LiF); a different crystal structure but the same bonding and slip systems (strontium titanate); the same bonding but different crystal structure and unknown slip systems (yttrium aluminium garnet); different (metallic) bonding, various crystal structures and slip systems (polycrystahine metals); and an amorphous glass (sodium silicate)_ Various materials, of wide-ruing hardness, are used as the softer cones to impart specific nomindl mean pressures to the surface of the harder solids.
2. Experimental details The reciprocating sliding apparatus used has been described previously in detail 193. The essential features only are given here together with a schematic illustration in Fig. 1. All experiments were performed at slow sliding speeds (approximately 5 mm s-i) in the presence of a plentiful supply of lubricant. The lubricant employed was a saturated solution of stearic acid in mineral oil. The apparatus consisted of a reciprocating specimen stage, moving in a horizontai plane, with a stationary softer slider attached to the end of a pivoted and counterbalanced beam, A pyrophylite weight platform was attached to the end of the beam, via a load transducer, and a removable slider was secured into the centre of its lower face. Thus the normal load on
151
Weight platform \ SLider
Counterbalance weight
Load transducer
If71
1 Reciprocating \
/
yble
Drive crank
Fig. 1. A schematic illustration of the reciprocating sliding apparatus.
the weight platform was transmitted directly through the axis of the slider on to the horizontal specimen. The frictional force between the sliding surfaces could be measured by feeding the output from the load transducer through an amplifier into a continuous pen recorder. The specimen was clamped to a movable stage which was mounted on a table that could be rotated about both a vertical and, to a limited extent, a horizontal axis. This arrangement allowed single-crystal specimen surfaces to be aligned in specific crystallographic directions, whilst remaining horizontal, when necessary. The specimen stage was split, so that the specimen could be removed for examination and replaced in exactly the same position relative to the slider. Normal loads of 4.9 N were used in all the experiments. All of the softer sliders were machined into cones having a total included angle of 136”. Each experiment resulted in the cones being flattened, to a degree dependent upon the hardness of the materials used. Knoop microhardness measurements were obtained from both the blunted flat on the cone tip and the surfaces of wear tracks on the harder crystal, using a Leitz “Miniload” microhardness tester with a 0.98 N normal load. Specimen preparation procedures for the materials studied are given below in Table 1, together with the Knoop microhardness data from surfaces before sliding tests were performed. The softer sliders were examined in profile microscopically before each experiment to ensure sharpness and integrity and, immediately prior to the tests, were thoroughly cleaned ultrasonically in an organic solvent. The materials used, together with microhardness values before testing (initial) and after approximately 20 traversals from the flat surface of the blunted slider tips (ultimate), are given in Table 2. Results of reciprocating sliding experiments were taken from at least three tests for any given sliding couple, and reproducibility was usually within +5% unless stated otherwise.
Crystal plane
0 001
(1001
{lOOI
Polycrystalline
Polycrystalline
Polycrystalline
Amorphous
Material
Strontium titanate
Yttrium aluminium garnet
LiF
cu
MO
co
Sodium silicate
(100):5.36,
Conventional mechanical polishing down to l/4 pm diamond paste
(Knoop)
As received
As above
As above
Conventional mechanical polishing down to l/4 pm diamond paste
6.63
2.96
3.06
0.94
(100):1.01,
Cleaved in air, followed by conventional mechanical polishing down to l/4 pm diamond paste. Then annealed in argon for 3 h at 400 “C to remove dislocations from the surface
(110):1.07
1.47 in both (100) and (110)
(GN m-*)
As above
(110):8.97
Microhardness
Surface preparation
Specimen preparation procedures and microhardness (0.98 N load) before deformation
TABLE 1
z to
153
3. Results
and discussion
3.1. Lithium fluoride LiF has the rock salt crystal structure and its cleavage characteristics and slip systems are directly comparable with MgO (see ref. 9). However, room temperature corresponds to 0.26 T, (as opposed to 0.17 T, for MgO) and there is a greater degree of plastic deformation possible. Repeated sliding experiments were performed on the (001) plane, in the (100) direction using zinc, cadmium, poly(methy1 methacrylate), polystyrene and high density polyethylene sliders (see Table 2). Knoop indentations, used to measure the hardness of the blunted tips of the polymer sliders, listed in Table 2, gave only approximate hardness values because of the difficulty encountered in measuring the indentations accurately. The problems arose because the indentations were difficult to see and also the size of the impressions tended to change, during measurement, owing to the elastic recovery of the polymers. Therefore, in order to obtain a more meaningful estimate of the relative hardness of the three polymers, the effective mean pressure (at 4.9 N normal loading) was measured by loading the conical polymer sliders against a glass microscope slide and focusing on to the contact surface. The mean pressure values were obtained by dividing the applied load by the apparent area of contact and values between 0.1 and 0.15 GN me2 were measured for the three polymers in question. The zinc slider was the only one of the five sliders used to give reproducible results on LiF. Fragmentation was produced after approximately 500 traversals and the degree of work hardening on the wear track surface is shown in Fig. 2. As in MgG, the fracture pattern was of “chevron” type and fragmentation appeared suddenly with a corresponding increase in friction.
TABLE 2 Initial and ultimate microhardness (0.98 N load) of slider materials
Material
Martensitic steel EN58J stainless steel EN42 steel Phosphor bronze (95/5) Brass (70/30) Al Zn Cd Poly( methyl methacrylate) Polystyrene High density polyethylene
Knoop hardness (GN me2) Initial
Ultimate
10.82 3.92 3.72 2.45 1.96 0.90 0.41 0.20 0.05 0.06 0.05
12.75 6.37 4.61 3.04 2.45 1.27 0.41 0.20 0.05 0.05 0.05
154
I 100
200 NUMBER OF TRAVERSALS
Fig. 2. Surface hardening of LiF (001) (110) sliding direction).
300
GO
as a function of number of traversals (zinc slider,
Both the cadmium and the poly(methy1 methacrylate) sliders gave inconsistent results. Fragmentation occurred in the case of both sliders but the number of traversals needed to promote fracture varied between 10 and 20. It was not possible to detect any work hardening of the wear track surfaces, in either case, but the number of hardness measurements taken was limited because of the erratic nature of the fragmentation. It is curious that fracture should occur after a fewer number of traversals than was the case for zinc, as poly(methy1 methacrylate) sliders are considerably softer (see Table 2). It is possible that this may be due to an increased tendency for adhesion to occur between LiF and cadmium and the possible influence of the lubricant on the surface properties of poly(methy1 methacrylate). Wear tracks produced by the polystyrene and high density polyethylene slider did not exhibit any fracture, even after 10 X lo4 traversals, and there was no evidence of work hardening. However, dislocations were revealed in
Fig. 3. Cross-section of wear track on LiF (001) after 25 X lo3 traversals with a polystyrene slider, showing dislocation density beneath the track after etching (sliding direction (100)) (bar, 400 pm).
155
the bulk of the solid (by cleaving the crystal and subsequently etching in H,Oz) beneath the wear tracks (see Fig. 3). 3.2. Strontium titanate Strontium titanate has the cubic- perovskite crystal structure at room temperature, which consists of a close-packed arrangement of oxygen ions with titanium occupying the lattice sites at the centre of oxygen octahedra. It is a crystal that is frequently used as a diamond substitute in the gemstone industry because of its optical properties. Very little data exist on the mechanical properties of these crystals. However, Bumand [lo] has investigated the Knoop hardness anisotropy of both the (001) and the (110) planes as a function of temperature and has shown that its behaviour is similar to MgO, which is a well-characterized material and has been the subject of detailed study in the experimental situation described here, as reported [9]. Thus, the mechanical properties and slip systems are directly comparable with MgO, except that strontium titanate does not have a specific cleavage plane and tends to fracture conchoidally .
Experiments were performed on the (001) plane of strontium titanate, using the EN58J, EN42 and 95/5 phosphor bronze sliders (see Table 2) in both the (100) and the (110) sliding directions. In the initial stages of the test, work hardening of the harder crystal occurred and the degree of hardening was dependent upon the ultimate hardness of the softer slider used (see Fig. 4). There appears to be no significant difference in the rate or degree of hardening in the two different sliding directions. The initial stages of the wear process produced plastic deformation, which resulted in a smooth-sided groove being formed by the tip of the slider (see Fig. 5). Although the crystal surface had been originally prepared by mechanical polishing, slip lines consistent with slip on the (110) (110) system were discernible within the wear track when the crystal was examined under Nomarsky interference contrast conditions on an optical microscope. Similar features were visible in tracks produced by both (100) and (110) sliding directions on the (100) plane. Cracks, within the contact area of the slider and crystal, were eventually produced by all three sliders for both (100) and (110) sliding directions. The results are shown in Fig. 6 and it can be seen that significant anisotropy in wear resistance is evident; the (110) sliding direction being more resistant to cracking, as was shown for MgO [9]. The nature of the fracture observed varied, depending upon the slider and the direction of sliding. For experiments involving (100) sliding the phosphor bronze slider produced cracks after approximately 6 X lo4 traversals, and these did not greatly increase in degree or severity even after 10 X lo4 traversals. The fracture was intermittent along the length of the wear track and complete fragmentation was not observed, in contrast to the behaviour of MgO. The cracks produced by the EN42 and EN58J sliders, when sliding in (100) directions, took the initial form of semicircular “ring cracks” (see Fig. 7). On increasing the number of
156
NUMBER
OF TRAVERSALS
(al
14.0
100
200 NUMBER
300 OF
LOO
500
TRAVERSALS
(b)
Fig. 4. Surface hardening of strontium titanate (0013 as a function of number of traversals. (a) Sliding direction (100). (b) Sliding direction (110). (a, EN58J; 0, EN42 ; 0, 95/5 phosphor bronze.)
traversals, beyond the point at which cracks first became detectable, eomplete fragmentation of the wear track occurred and the fracture pattern was of the “chevron” form, as observed for MgO [9] (see Fig. 8). The fracture patterns observed from experiments in the (110) sliding direction were somewhat different. In particular, the fracture seemed less
157
Fig. 5. Strontium titanate {OOl) after 100 traversals with 95/5 phosphor bronze slider (4.9 N load, (100) sliding direction) showing smooth-sided groove before fracture. Nomarsky interference contrast (bar, 200 pm).
c~s~lo~aphi~ in nature. All three sliders produced fracture, initially in the form of a semicircular “ring crack” (see Fig. 9), which eventually produced wear by the “plucking out” of particles from the wear track surface. The fracture was intermittent along the wear track, and did not exhibit the “chevron” cracking observed in the (100) direction (see Fig. 10). The degree of cracking did increase, on increasing the number of traversals beyond the point of initial fracture, but only in the case of the EN58J slider did fragmentation occur over the whole area of the wear track. An interesting aspect of fracture, observed when sliding in both directions on the (001) plane, was the presence of a certain degree of timedependent subsurface cracking associated with the wear process. Wear tracks that showed no sign of fracture, on initial examination, occasionally developed lateral vent-type cracks after a certain length of time, e.g. 2 or 3 h, had elapsed. This time-dependent phenomenon was only observed in some of the wear tracks produced by the EN58J stainless steel slider and it appears to be associated with the relief of residual stresses produced by the softer slider. This effect was also observed in Knoop indentations, on {OOl) planes, during measurement of the impression size whilst the specimen was under microscopic examination. 3.3. Yttrium aluminium garnet Yttrium aluminium garnet (YAG) is used mainly in connection with solid state lasers and also as a gemstone. Although its optical properties have been well characterized, little is known about its mechanical behaviour. The crystal structure is basically b.c.c. [ 111 with no simple close-packed planes and the relevant slip systems have not yet been identified. Reciprocating sliding tests were performed on the mechanically polished {OOl) plane surface using the martensitic steel, EN58J and EN42 steel sliders. Time~ependent fracture was again observed during the measurement of Knoop indentations. Conchoidal subsurface fracture was seen to occur, beneath the surface, around the edges of the indentations. These cracks joined with the specimen surface on occasions, forming a detached particle
158
% z 9
9515phosphorbronze 2.0-
i c
L
2000
4OM)
6000
8000
I\ "
60X
CRITICAL NUMBEh OF TRAVERSALS t0 FRACTURE (aI 7.0
2000
4000 6cQO i3000 CRITICALNUMBER OF TRAVERSALS TO FRACTURE
I
_I
100K
#I
Fig. 6. Critical number of traversals to fracture as a function of slider hardness for strontium titanate (001) (4.9 N load). (a) (100) Sliding direction. (b) (110) Sliding direction.
around the impression. However, these me-decadent cracks were not observed during subsequent experiments involving the softer sliders. All three sliders produced fracture, when sliding in both (100) and (110) directions, which was preceded by the formation of a smooth and apparently plastically deformed groove {see Fig. 11). However, no evidence of work
Fig. 7. Strontium titanate (001) after four traversals with EN58J stainless steel slider, showing the presence of semicircular “ring cracks” prior to complete fragmentation, sliding direction (100) (bar, 200 pm). Fig. 8. As Fig. 5, after 10 traversals, showing “chevron”-type after complete fragmentation (bar, 200 pm).
fracture pattern observed
Fig. 9. Strontium titanate (001) after 2500 traversals with EN42 steel slider, showing the formation of the first observable crack, sliding direction (110). Nomarsky interference contrast (bar, 100 pm). Fig. 10. Strontium titanate (001) after 10 traversals with EN58J stainless steel slider, showing intermittent fragmentation of the wear track, sliding direction (110) (bar, 200 Pm).
Fig. 11. Yttrium aluminium garnet (001) after 800 traversals with a martensitic steel slider, showing plastic deformation, sliding direction (100) (bar, 100 pm). Fig. 12. As Fig. 11, after 100 traversals, showing semicircular “ring cracks” (bar, 100 pm).
hardening could be detected in any of the experiments involving YAG and no slip lines were evident in any of the wear tracks. When cracks were eventually produced, they took the form of semicircular “ring cracks” (see Fig. 12); the harder the slider, the fewer the
160 14.0 l
hardenedmortensittc steel
12.0 ;; : 10.0
I CO,OCKl
30.000 20,000 10,000 CRITICALNUMBEROF TRAVERSALS TO FRACTURE
Fig. 13 Critical number of traversals to fracture as a function (4.9 N load, (100)sliding direction}.
of slider hardness for YAG
number of traversals required to produce cracking (see Fig. 13). Once “ring cracks” had formed they did not appear to increase in severity or degree, to any significant extent, with increasing number of traversals. However, partial pension of the wear track was observed after 10 X lo4 traversals (see Fig. 14). The fracture was non~~s~lo~aphic in nature and it appeared that wear particles were formed by the joining up of two adjacent semicircular “ring cracks” produced earlier in the wear process.
Fig. 14. YAG (001) after 10 X lo4 traversals with a martensitic partial fragmentation of the wear track (bar, 100 pm).
steel slider, showing
Three polycrystahine metals, i.e. copper (f.c.c.), molybdenum (b.c.c.) and cobalt (h.c.p.) were chosen to examine the cumulative effect of softer sliders (see Table 1).
161
3.4.1. Copper A zinc slider was used on the copper specimen, but transfer of material from the slider to the wear track occurred after approximately 10 traversals and this tended to obscure the deformation in the copper. However, there were areas of the wear track which were free from adhered zinc and hardness values were taken on a number of wear tracks after between 1 and 2 X lo4 traversals. No work hardening could be detected in any of the tracks, although obvious signs of fairly extensive deformation were observed. The deformation took the form of a smooth-sided groove with slip lines being produced within the area traversed by the slider. Transfer of zinc obscured the groove in tracks produced by more than 10 traversals and no sign of fracture was observed after 2 X lo4 traversals. Reciprocating sliding experiments were performed using a poly( methyl methacrylate) slider in order to avoid problems associated with adhesion and transfer. No sign of fracture could be detected after 10 X lo4 traversals but plastic deformation of the harder metal surface was observed (see Fig. 15). Again, hardness measurements on the wear track surface revealed no sign of hardening.
Fig. 15. Polycrystalline copper surface after approximately 30000 traversals with a poly(methy1 methacrylate) slider, showing evidence of plastic deformation (bar, 400 pm).
3.4.2. Molybdenum Sliding experiments were performed on polycrystalline molybdenum specimens using aluminium and 70/30 brass sliders (see Table 2). There was marked adhesion and transfer associated with the wear tracks produced by the aluminium slider and no useful information was obtained. No fracture could be detected after 10 X lo4 traversals and the adhesion on the wear track surface masked any hardening that may have been present. However, the 70/30 brass slider gave wear tracks that were free of adhesion and transfer. No work hardening or fracture could be detected, even after 10 X lo4 traversals, but a limited degree of plastic deformation was visible. 3.4.3. Cobalt A polycrystalline cobalt surface was tested with a 70/30 brass slider and a copper slider (see Table 2). No work hardening was measurable in wear tracks produced by either slider but plastic deformation and eventual frag-
162
mentation did occur in both cases. In the early stages of the wear process a smooth-sided groove was formed (Fig. 16) but no slip lines could be detected. Partial fra~en~tion followed after approximately 500 and 800 traversals respectively for the brass and copper sliders. Eventually, complete fragmentation occurred after approximately 1000 traversals with brass and 1200 traversals with copper. Evidence of slip in the individual grains could be seen at the edges of the completely fragmented wear tracks (see Fig. 1’7).
Fig. 16. Polycrystalline cobalt surface after 100 traversals with a copper slider, showing a plastically deformed groove. Nomarsky interference contrast (bar, 200 pm).
Fig. 17. Evidence of slip in individual grains at the edge of the fragmented wear track after 1000 traversals in polycrystalline cobalt using a 70130 brass slider (bar, 50 pm).
3.5. Sodium silicate glass As an example of an amorphous solid, a conventional sodium silicate glass microscope slide was used as the harder specimen with ENMJ, EN42, 9515 phosphor bronze and ‘IO/30 braas sliders (see Table 2). Fracture was produced in the glass by all of the sliders and the results are presented in Fig. 18. It can be seen that the curve produced is similar to those produced by the crystalline solids that gave reproducible results (see also ref. 9). The fracture initially took the form of semicircular “ring cracks” (see Fig. 19) and the points in Fig. 18 represent the formation of the first observable “ring crack”. As the number of traversals was increased, past the point of initial cracking, partial fragmentation of the wear tracks was observed. The wear fragments appeared to be produced by the merging of two of the “ring cracks” (see Fig. 20), whereupon further traversals produced more
163 7.0 1
2 3.0 -
. 9515 phosphorbronze 70130brass .
B = 2.0 B x
1.0-
I
I SOW
10,000
15,aM
20,ooo
25,OM)
CRITICALNUMEIEROF TRAVERSALS TO FRACTURE
Fig. 18. Critical number of traversals to fracture as a function sodium silicate glass, 4.9 N load.
of slider hardness for
Fig. 19. Sodium silicate glass after 7000 traversals with a 96/5 phosphor bronze slider, showing semicircular “ring cracks” (bar, 400 pm). Fig. 20. Sodium silicate glass after 8000 traversals with a 95/5 phosphor bronze slider, showing the start of fragmentation (bar, 200 pm).
severe fragmentation. Marked transfer track surface, once cracking had been change in hardness of the glass prior to deformation could be seen within the four sliders.
of metal occurred, on to the wear initiated. There was no measurable fracture, although evidence of slight wear track region in the case of all
4. Concluding discussion The materials in which cumulative deformation, induced by softer sliders, has resulted in surface fragmentation fall into four categories.
164
(1) Non-metallic single crystals which exhibit plasticity, work hardening, some degree of crystallographic fracture and anisotropy in wear resistance, e.g. strontium titanate. (2) Non-metallic single crystals which exhibit apparent plasticity, no measurable work hardening, no anisotropy in wear resistance and conchoidal fracture, e.g. yttrium aluminium garnet. (3) Polycrystalline metals which exhibit plasticity but no measurable work hardening, e.g. cobalt. (4) Amorphous materials which exhibit no measurable work hardening, e.g. sodium silicate glass. In all these categories it has been established that the wear process initially identified in ref. 9 is germane, although the detailed mechanism leading to eventual fragmentation must vary, particularly with respect to amorphous solids. A surprising feature of the results is the absence of measurable work hardening in the metal specimens, despite clear evidence of plastic deformation. However, it is possible that initial mechanical polishing of the metal surface has introduced a sufficient degree of plastic deformation to mask the influence of the softer sliders. It has been shown previously [9] that work hardening does occur, to a significant extent, below the wear track surface and it would be reasonable to assume that ductile metals do possess a deformation zone resulting from sliding that penetrates further into the bulk than that resulting from mechanical polishing. However, without an initially pristine chemically polished surface this is difficult to investigate experimentally. In addition, Knoop microhardness measurements from the polycrystalline metal specimens did exhibit a much greater degree of scatter than in the single crystals, no doubt partially due to variable orientation effects within individual grains. The results obtained from sodium silicate glass are directly comparable, in terms of the form of the fragmentation curve, with those obtained for crystalline solids both here and in ref. 9. Clearly, a mechanism based on dislocation interaction and conventional work hardening is inapplicable in this case but a surface fatigue mechanism based on the propagation of preexisting surface cracks may be expected to lead to a similar conclusion. In all cases, where reproducible fragmentation has occurred, it would appear that the attainment of a critical strain prior to fracture is significant (as suggested by Ball [ 121 and in ref. 9). The form of the fragmentation curves applicable to these cases suggests a limiting slider hardness below which cumulative deformation would not produce eventual fracture, although a wider range of sliders would need to be investigated before the limiting hardness could be accurately identified for any particular crystal. Relatively large strains appear to be produced by this type of experimental geometry in inherently brittle materials and the relevance of cumulative deformation in harder materials may well be of greater significance in practical situations than is commonly supposed.
165
References 1 E. M. Trent, Metal Cutting, Butterworths, London, 1977. 2 E. A. Almond, C. A. Brookes and R. Warren (eds.), Science of Hard Materials, Proc. Znt. Conf. (Rhodes), Inst. of Phys. Conf. Ser. No. 75, Adam Hilger, Bristol, 1986. 3 K. F. Dufrane and W. A. Glaeser, Rolling contact deformation of MgO single crystals, Wear, 37 (1976) 21. 4 C. A. Brookes and M. P. Shaw, Cumulative deformation of magnesium oxide crystals by softer sliders, Nature, 263 (1976) 760 - 762. 5 C. A. Brookes and P. Green, Deformation of magnesium oxide crystals by softer indenters and sliders, Nature, 246 (1972) 119 - 120. 6 D. Crompton, W. Hirst and G. W. Howse, The wear of diamond, Proc. R. Sot. London, Ser. A, 333 (1973) 435 - 454. 7 M. Casey and J. Wilks, The friction of diamond sliding on polished cube faces of diamond, J. Phys. D, 6 (1973) 1772 - 1781. 8 C. A. Brookes, in J. E. Field (ed.), Properties of Diamond, Academic Press, London, 1979, pp. 348 - 402. 9 C. A. Brookes, M. P. Shaw and P. E. Tanner, Non-metallic crystals undergoing cumulative work-hardening and wear due to softer lubricated metal sliding surfaces, Proc. R. Sot. London, Ser. A, 409 (1987) 141 - 159. 10 R. P. Burnand, Ph.D. Thesis, University of Exeter, 1974. 11 K. H. G. Ashbee and G. Thomas, Electron microscopy of yttrium aluminium garnet (YAG), J. Appl. Phys., 39 (8) (1968) 3778 - 3780. 12 A. Ball, On the importance of work-hardening in the design of wear-resistant materials, Wear, 91 (1983) 201 - 207.