Wear, 133 (1989)
SLIDING WEAR OF CERAMIC-CERAMIC, CERAMIC-STEEL AND STEEL-STEEL PAIRS IN LUBRICATED AND UNLUBRICATED CONTACT* K.-H.
Unirwrsity of Karlsruhe, Institute of Materials Science and Karlsruhe Nuclear Research Center, Institute of Materials and Solid State Research, D-7500 Karlsruhe (F.R.G.)
Summary Sliding wear tests were carried out on different materials in air, water and oil at ambient temperatures by using a pin-on-ring tribometer. The tests in water and oil were run under conditions of boundary lubrication. The investigated ceramics could be divided into oxides‘(Al,O,, ZrO,, A12TiOs) and non-oxides (SiSiC). Steels with 0.2 - 0.9 wt.% C were heat treated to obtain spheroidized, normalized, and quenched and tempered structures. The steels and ceramics were studied with different metallic, ceramic and ceramic-metallic combinations. Scanning electron microscopy and energy-dispersive X-ray analysis revealed different wear mechanisms, depending on the materials and experimental conditions. Plastic deformation, material transfer, tribochemical reaction, grooving and microfracture were observed on the worn surfaces. The results show that ceramics may be more wear resistant than steels but their tribological behaviour is strongly influenced by operating conditions.
1. Introduction Ceramic materials are of increasing interest for many engineering applications. High hardness and high resistance to thermal and corrosive loadings combined with relatively low densities offer substantial advantages over metallic or polymeric materials. Monolithic, oxide, nitride and carbide ceramics such as A1203, ZrO,, Si,N,, Sic and others have been recognized as potential candidates for use in structural applications. A general problem of the ceramic materials is their inherent brittleness at ambient temperatures. Internal and surface flaws are introduced during fabrication and machining of these materials. Therefore their mechanical behaviour is strongly controlled by the size, volume, shape and distribution of flaws such as pores, cracks, inclusions, glassy phases at grain boundaries etc. [l]. Trans*Paper presented at the International U.S.A., April 8 14, 1989.
on Wear of Materials,
in The Netherlands
formation toughening (i.e. a metastable phase which undergoes a stressinduced martensitic transformation), whisker or fibre strengthening (microcrack toughening) and grain refinement can lead to enhanced toughness ]21. Ceramics are being increasingly used for wear-resistant components. e.g. seal rings, valve seats, draw-cones, dies for extrusion, guides, watermixing taps, valve train components, bearing parts or cylinder liners. Studies on ceramics in unlubricated sliding have reported a wide range of values of the coefficient of friction depending on the experimental conditions. Values between 0.4 and 0.8 were measured for various ceramics in vacuum, compared with about 0.2 for all these materials in air at room temperature [S]. In dry atmospheric tests , using a pin-on-disc tribometer, values of the coefficient of friction ranging from about 0.6 to 0.8 occurred for SiSiC, Al,O, and Mg-PSZ (magnesia-partially stabilized zirconia). With increasing sliding speed, friction increased for [email protected]
,O, and exhibited a maximum for (Mg-PSZ)-(Mg-PSZ) pairs. The effect of temperature on the coefficient of friction was more complicated. At 800 “C a coefficient of friction higher than that at room temperature but lower than that at 400 “C was reported. A decreasing coefficient of friction was observed with increasing temperatures for M-2 tool steel-A120, sliding pairs [ 31. From several studies [ 5 - 91 it can be concluded that, in unlubricated sliding, friction may be greater for ceramic than for metallic materials, e.g. hardened steels or steel-ceramic pairs. Wear rate can be enhanced with ceramic-ceramic  and reduced with ceramic-steel [lo] pairs in dry contact by increasing the sliding speed. A strong environmental effect on friction and wear behaviour of ceramics has to be expected. The influence of environment, e.g. of humidity, can be a result of (1) change in the mechanical properties of the ceramic, (2’) adsorption of environmental atoms or molecules such as water, and/or (3) formation of surface films by chemical reaction. The presence of water can reduce the coefficient of friction 16, 11 - 131 but increase the wear rate by several orders of magnitude. The effect on wear behaviour depends on the type of ceramic. Because of the presence of water in sliding pairs the wear rates of ZrO, [ 121 and Al,O, 1141 were substantially increased, but wear was reduced for Si3N4 [ll, 151. The different influence of water on wear behaviour was explained for non-oxide ceramics by tribochemistry and for oxide ceramics by stress corrosion cracking . Under the action of friction, Si3N4 reacts with water to form SiO,. ZrOz, by contrast, is chemically stable in water and hexadecane containing steric acid. With Sic no formation of a friction-reducing adsorption layer takes place on the sliding surface in the presence of a lubricating oil, in contrast to cx-Al,O, . This is a result of the different crystalline structures and atomic bonds of the ceramics. Plastic deformation, brittle fracture and tribochemical processes are reported to be the main mechanisms involved in wear of ceramic materials [8, 10, 15, 181. Plastic deformation as a result of rubbing contact can occur
also on brittle ceramics such as Al*O,. X-Ray analysis  of worn surfaces of PSZ revealed the occurrence of phase transformations as a result of plastic deformation and localized heating in the area of contact. We compare here the friction and wear behaviour of ceramics, steels and their combinations in water- and oil-lubricated and dry sliding contact. The tests were run at ambient temperature, using a pin-on-ring tribometer.
2. Materials and experimental
2.1. Ma terids The ceramic and metallic materials studied are listed in Table 1. All ceramics were used as delivered. Mg-PSZs were produced by using synthetic (Z96VlO) or natural (Z96V20) powder, respectively. ZA50, Z95P and AT(p) contained an open porosity whereas all other ceramics were dense. SiSiC was a reaction-sintered silicon carbide containing free metallic silicon. A O.Gwt.% carbon steel was hardened by austenitizing, oil quenching and tempering to a Vickers hardness of 470 HV 30. This hardened condition was called C6OV and the detailed heat treatment is given in Table 1. For
Type, composition and properties (density, apparent the engineering ceramics and metallic materials Sample
Ceramics A90 A99 ZA50 296VlO Z96V20 Z95P AT(P) SiSiC
90% A120a, Ker 706 99.7% AlTOa, Ker 710 50% ZrOz, 50% AlzOz 96% ZrOz, Mg-PSZ, Var. 10 96% ZrOz, Mg-PSZ, Var. 20 95% ZrOz, Mg-PSZ AI,TiOS, pressed 90% SIC, balance Si
Density (g cm-“)
3.6 3.9 4.4 5.6 5.6 5.0 3.0 3.0
950 1373 323 767 748 220 200 1450
4 0 0 10 16 0
HV HV HV HV HV HV HV HV
30 50 30 50 50 30 30 10
Metals C6OV ASt52-3 Ck45 Ck45 SOMnCrV8 90MnCrV8 cr
Steel 0.61% C, hardeneda Steel 0.20% C, as delivered Steel 0.45% C, hardened Steel 0.45% C, normalized Steel 0.94% C, hardened Steel 0.94% C, spheroidized Cr plating 100 pm thick Substrate Ck45 (209 HV 30)
a~~ustenitizing for 60 min at 840 “C, oil quenching 60 min at 250 “C, air cooling.
7.85 7.85 7.85 7.85 7.85 6.90
470 180 567 209 714 193 843
HV 30 HV 30 HV 30 HV 30 HV 30 HV 30 HVli,
comparison, a chromium coating 100 pm thick plated on a normalized structure (209 HV 30) of the steel Ck 45 was studied in some wear tests. The ceramic materials were delivered as plates 5 mm thick or as rings with diameter of 14/37 mm and raceways width of 14 mm. Pin specimens of 5 mm X 5 mm X 20 mm were cut from the plates by diamond sawing. The ground raceways of Zr0, rings showed values of surface roughness of R, (c.1.a.) = 0.32 - 0.58 pm and .&120, rings of 1.04 - 1.36 ,~m. The C6OV steel rings were ground to a surface roughness of R, = 0.25 I 0.36 pm. Before wear testing, all pin specimens were run-in by sliding against 220 mesh Sic abrasive paper under a normal load of 20 N. A smfaue roughness of about R, = 0.3 pm was produced by this procedure and pull-outs of a few grains occurred with pins of brittle ceramics. 2.2, Wear tests Sliding wear was studied in air, distilled water and oil by using a pin-onring tribometer. Before wear testing all specimens were cleaned with alcohol and dried in air. The pin specimens of 5 X 5 mm cross-section were loaded to 10 N in air or 200 N in distilled water and oil, respectively. Figure 1 shows schematically the tribometer and the lubrication equipment. It used a mineral oil (Shell KS 111) without additives, a viscosity of 2.0 mm2 s ’ at 20 “C and a density of 0.759 g cm-~” at 15 “C. The test conditions used resulted in bound~y lubrication for both water and oil as interfacial substances. All tests were carried autr at a sliding speed of 0.77 m s -’ and for a wear path of at Least 8.7 km.
Fig. 1. Pin-on-ring t.ribometer
Wvis was evaluated
W, Wv/s = _PS where W, is the mass loss as a result of wear, p is the density of the wearing material and s is the length of the wear path. Mass loss of the specimens was measured by using an electronic balance with a scale of lop5 g. Before measuring mass loss, all specimens were put in vacuum of 1 mPa for 5 h. Wear intensity of a sliding pair was defined as the sum of wear intensity of the pin and ring specimen. As a lower load was applied during the wear tests in air, the quotient of volumetric wear intensity divided by normal load W,,,/F, was used for comparison of wear behaviour under different environments. Friction torque was measured continuously during the tests, and from the average values the coefficient of friction was evaluated as the quotient of frictional force divided by normal load. All reported values of wear intensity and coefficient of friction are averages of at least two runs. Additional runs were carried out if large scatter of the results occurred. All wear tests were run in normal laboratory air at ambient temperatures between 17 and 21 “C and relative humidities of 42% - 56%. 3. Results 3.1. Microstructures The investigated ceramic and metallic materials are listed in Table 1. With the exception of SiSiC, all ceramics were oxides based on Al,O, (A90, A99), Al,O, and ZrO, (ZA50), Mg-PSZ (Z96V10, Z96V20, Z95P) and A12Ti0, (AT(p)). The Z96VlO ceramic was produced by using a synthetic and the Z96V20 by using a natural powder. SiSiC was a reaction-sintered, free-silicon-containing silicon carbide. The oxide ceramics AT(p), Z95P and ZA50 contained an open porosity of 16%, 10% and 4%, respectively. The steels varied in the carbon content between 0.2 and 0.94wt.70 C. ASt52-3, Ck45 and C60 were plain carbon steels, whereas 90MnCrV8 was an alloyed die steel. The ASt52-3 was used as received with a normalized, ferritic-pearlitic structure. The steel C60 was austenitized, oil quenched and tempered that resulted in a tempered martensitic microstructure. Similar heat treatments led to tempered martensitic structures with hardened conditions of the steels 90MnCrV8 and Ck45. In addition, softer conditions were studied, i.e. a normalized (ferrite-pearlite) structure with Ck45 and a spheroidized (ferrite-carbide) structure with 9OMnCrV8. Average values of Vickers hardness are presented in Table 1. Figure 2 shows the structures of SiSiC, ZA50, Z96VlO and A99. The Al,Os (A99) contained grain sizes ranging from 2 to 20 pm. ZA50 was a mixture of about 50% Al,O, (dark phase) and 50% ZrO, (bright phase). The Z96VlO (Mg-PSZ ceramic) exhibited a finer-grained structure than did the Z96V20 modification produced from natural powder. With the latter
Fig. 2. Scanning electron micrographs ZrOz (ZA50), (c) ZrO? (Z96VlO) and
of the microstructures (d) A1203 (A99).
ZrOz the magnesium-rich phase (stabilizing MgO) was reticulated at the grain boundaries. Both ZrO,s consisted of a mixture of cubic and monoclinic phases with very small tetragonal precipitates. The reaction-sintered SiSiC contained about 12% free silicon. 3.2. Wear intensity
3.2.1. Sliding wear in air Figure 3 shows wear intensities of ceramic-ceramic pairs as a function of hardness of the pin specimens divided by hardness of the ring specimens. The largest wear intensity of all pin specimens (Fig. 3(a)) was measured for an Al,TiOs (AT(p)) pin sliding against a ZrO, (Z96VlO) ring. However, the ring of this sliding pair showed the lowest wear intensity of all ring specimens (Fig. 3(b)). Relatively low wear of both pm and ring occurred for self-mated Al,O, (A99) or ZrO, (Z96VlO or Z96V20). Figure 3 shows the strong effect of the hardness ratio of pin to ring on wear intensity. Wear of the pins decreased by several orders of magnitude with increasing hardness ratio. However, the wear intensity of the rings increased with increasing hardness ratio, i.e. the harder the pin the greater the wear of the ring. The apparent deviation of the pair ZA50-Z96VlO from this general tendency can easily be understood if we consider that ZA50 contained 50% Al,O,. This hard phase caused a wear intensity about equal to that of the A120,-ZrO, (A99-Z96VlO) sliding pair. It follows that bulk hardness
Pin Spectmenl'hckersHOWhess of hg
“AT,‘-“‘* 02 iI4 -~-Weir
06 08 ID 12 14 I6 18 Hordners al PIP Spec~~~~l~rk~rs Hardness o! Rq Specimen-
Fig. 3. Votumetric wear intensity of pin (a] and ring fb) pairs in air us. the ratio of pin-to-ring hardness. Nomenclature
specimens of sliding
of ceramic slidirmg pin/ring.
of a multiphase dispersion ceramic can fail in predicting wear intensity. The dependence of wear on the effective hardness ratio of pm to ring points to a contribution of abrasion to wear process. Figure 4 shows wear intensities of metallic (steels and hard chrom~um~ and ceramic pins sliding against hardened steel rings (C6OV, 470 HV 30) as a function of hardness of the pin specimens. Wear loss of the pins decreased with increasing hardness to a first approximation (Fig. 4(a)). The Z95P ZrO, pin containing an open porosity of 10% suffered the greatest wear loss of all ceramics. With this exception, all other ceramic materials showed lower wear loss than did the steel pins. Whereas the wear intensities of the pins differed by more than three orders of magnitude, those of the rings differed only by about one order of magnitude (Fig. 4(b)). Pins of the hard ceramics caused larger wear than did those of galvanic chromium or
,,Rmg C6OViL70HV 30) Shdq Speed 07lm/s
Vlckers Hardness ofPm Speomen
5 E x 5 y 10'0 * $ E Y
GlO"._ 5 E D r"
ASt52-3 %--9OfhCrVB Z9TP
107.. 8 #I
SiSlC v a_ A99 -
Vtckets Hardness of Pm Spectmen
Fig. 4. Voiumetrie wear intensity of pin (a) and ring (b) specimens of metals or ceramics sliding against C6OV rings in air as a function of hardness of pin specimens.
the hardened steels 90MnCrV8 and Ck45. High wear intensities of the steel ASt52-3 and the spheroidized structure of 90MnCrV8 were a result of scuffing, i.e. adhesion effects. Relatively high values of the coefficient of friction were measured under the experimental conditions used, as a result of surface roughening, scuffing or pull-out of grains with the brittle ceramics. Cleaning of the raceways of ring specimens for about 2 s, using a soft cloth, resulted in a substantial drop in friction. A value of 1.48 of the coefficient of friction was
V!ckersHardness of PinSpecimeniVlckersHardnessof Ring8peclmen --
Ibl' Ailpii296Y!O 0
Fig. 5. Minimal coefficient of friction (a) and wear intensity of sliding pairs (b) of ceramic-ceramic, ceramic-CGOV and met&-CGOV couples in air Us. the ratio af pin-to-ring hardness. Nomene~ature of sliding pairs: pin/ring.
measured for the [email protected]
, fA90)-C6OV pair, which dropped to 0.8 because of the cleaning procedure. Figure 5(a) shows the lowest values of the coefficient of friction measured during tests on the different sliding pairs as a function of the hardness ratio of pin to ring. It is obvious that no distinct effect of the hardness ratio on friction values was found. Abrasion contributed substantially to the high friction of the ceramic-ceramic and ceramic-steel (e.g. SiSiC-C6OV) pairs. Scuffing was a main mechanism for ASt52-3-C6OV. Comparison of the data of Z96Vl0, A90, A99 and SiSiC sliding against C6OV rings leads to the conclusion that friction values of ceramic-steel couples increased with increasing hardness ratio of pin to ring, It becomes clear, however, that friction values could not be explained
by the hardness ratio only. High friction of the pair Z95P-C6OV was caused by pull-out of grains of the porous ceramic, which resulted in abrasion. Comparing the pairs Z96V10-A99 and A99- Z96V10, the lower coefficient of friction was measured with the first pin-ring combination where the pin was softer than the ring specimen. Figure 5(b) shows wear intensities of sliding pairs, i.e. the sum of wear intensities of pin and ring, us. the hardness ratio of pin to ring. Lowest wear intensities occurred on self-mated Ai,O, (A99) and ZrO,, (Z96VlO). These ceramics resulted in substantially greater wear intensities of the pairs, when sliding against C6OV rings. Unlike ceramic pairs (A99-296VlQ and 296VlO-,499) led to greater wear loss than like-on-like ceramics and ceramic-steel pairs. Wear intensity of the pair Z95P-C6OV was almost one order of magnitude greater than that of Z96VlO-C6OV, because of the open porosity of Z95P. A low wear loss was measured with a ehromium pin sliding against a hardened C6OV ring. Scuffing led to high wear of the metallic ASt52-3-C60V pair.
3.2.2. Riding wear in water Figure 6 shows wear intensities of pin and ring specimens measured under lubrication by water as a function of hardness of the pins. The applied normal load of 200 N resulted in conditions of boundary lubrication. Wear intensities of the pins tended to decrease with increasing hardness to a first approximation (Fig. 6(a)). SiSiC pins displayed the lowest and ASt52-3 pins the higkest wear in sliding contact with hardened C6OV rings. With the exception of ,[email protected]
, (A99) sliding against itself, ceramic-ceramic pairs exhibited greater wear losses than ceramic-CGOV pairs. This was very obvious for ZrO, (Z96V20)-C6OV compared with self-mated ZrOz, where a difference of wear loss of about three orders of magnitude occurred. The lowest wear intensity of all ring specimens (Fig. 6(b)) was measured for the self-mated Al,O, (A99). Wear loss of C6OV rings sliding against Al,O, (A99) was about two orders of magnitude greater than that of A&O, lA99) rings with self-mated couples. A high wear intensity was observed for the ring OT like-on-like ZrOz (Z96V20). Figure 7 shows wear intensities of the sliding pairs as a function of hardness of the pm specimens. Wear loss of self-mated ZrO,, (Z96V20) was about three orders of magnitude larger than that of self-mated Af,O, (A99) and more than two orders of magnitude larger than that of ZrOz (Z96V20) sliding against a C6OV ring, In contrast, Al,O,--Al,O, pairs showed substantially lower wear than did Al,O, (A99)-C60V pairs. This difference in wear behaviour of AlzO, and ZrO, points to the effect of thermal conductivity, which is substantially lower for ZrO,. Wear of the ceramic-CGOV pairs increased with hardness of the ceramic pins and with the amount of open porosity to a first approximation. Because of a lower tendency for scuffing, the wear intensity of the steel-CGOV pairs decreased with increasing hardness of the pins.
Fig. 7. Volumetric wear intensity sliding pairs with water lubrication clature of sliding pairs: pin/ring.
1000 1200 of Pm Specimen _.._._._ _.~~._
i&M ._... -
of ceramic--ceramic, ceramic-CGOV and steel.-CGOV as a function of hardness of pin specimens. Nomen-
greater wear intensities than did pins of dense ZrOz, e.g. Z95P compared with Z96VlO or Z96V20 in contact with C6OV. The lowest wear intensity of all pin specimens was measured for ZrO, (Z96VlO or Z96V20) sliding against C6OV rings. Figure 8(b) shows wear intensities of the ring specimens us. hardness of the pins. Wear of C6OV rings was lower in sliding contact with ZrO, (296VlO or Z96V~O) than with harder A1203 (A99, A90). Wear intensities of C6OV rings increased with increasing hardness of the mated ceramic pins to a first approximation. Figure 9 shows wear intensities of the sliding pairs plotted over hardness of the pin specimens. ZrOz (Z96VlO or Z96V20) mated with C6OV rings resulted in the lowest wear intensities of all. Wear was enhanced by a factor of about ten because of 10% open porosity with ZrO* (295P). Self-mated dense ZrOz showed about 200-fold greater wear loss than did ZrO, paired with C6OV steel. In contrast, wear of self-mated AlzO, (A99) was lower than in sliding contact with C6OV rings. This different wear behaviour can be explained by a lower thermal conductivity of ZrOz than of A120,. Because of the applied high load and resulting friction substantial heating occurred in the contact area of self-mated Zr02. As a result of the lubricating effect of the mineral oil, no marked difference in wear intensity was observed between the spheroidized and hardened structure of the steel 90MnCrVS sliding against C6OV rings.
V~ckws Hardness of Rn Specimen
Fig. 8. Volumetric wear intensity of pin (a) and ring (b) specimens of ceramic-ceramic, ceramicC6OV and steel-CGOV couples with oil lubrication us. hardness of pin specimens. Nomenclature of sliding pairs: pin/ring.
Fig. 9. Volumetric sliding pairs with pairs: pin/ring.
Vlckers Hardness of PIN Swclmen -
wear intensity of ceramic--ceramic, ceramic-CGOV and oil lubrication U.S. hardness of pin specimens. Nomenclature
steel-CGOV of sliding
Figure 10 shows the surfaces of ZrOz (Z96VlO) pin and C6OV ring specimens worn in air, water or oil. During dry sliding, material was transferred from the steel ring to the ceramic pin (Figs. IO(a) and 10(b)). Strong adhesion occurred between the transferred material and the surface of the steel ring, which resulted in back transfer, mutual transfer and finally in loose wear fragments (Fig. 10(b)). In the presence of water, tribochemical reactions were promoted and material transfer was reduced. Because of the 20-fold larger load than in the dry tests, microspalling occurred locally on the ceramic pin (Fig. 10(c)) and the steel ring (Fig. 10(d)). Wear by delamination was observed on the raceways of rings. Oil as an interfacial substance resulted in wear of the ceramic pins about two orders of magnitude lower than in water (Figs. 6 and 8). Worn pins showed smooth raceways with dark brown surface layers (Fig. 10(e)) locally. Detachment of thin reaction layers was observed on the raceways of the rings (Fig. 10(f)). Dark reaction layers were also observed with the self-mated %rOl sliding pairs. Because of very low wear loss with these couples, former grooves from the grinding procedure were seen on the ring raceways after the wear tests. Wear loss of the rings was lower by a factor of about 30 in oil than in water.
pins Fig. 10. Scanning electron micrographs of ZrO 2 (Z96VlO) sliding against C6OV rings (b,d,f) in air (a.b), water (c,d) and oil (e,f).
Figure 11 shows surfaces of self-mated ZrOz (296VZO) worn during sliding in water and oil. Plastic deformation, material transfer and detachment of surface layers by delamination were observed on surfaces worn in water (Figs. 11(a) and 11(b)). Raceways of the ZrO, rings looked milky, which pointed to tribochemically induced modifications of the surface layer during sliding contact in water. Single sparks were observed from time to time in the contact area. This means that frictional heating resulted in high flash temperatures in the m~gin~ly lubricated contact. Local heating to high temperatures was also observed for self-mated ZrO, under oil lubrication. Worn surfaces were substantially smoother in the presence of oil as lubricant (Figs. 11(c) and II(d)). Shallow spalls and loose wear debris were found on surfaces of worn pins (Fig. 11(c)). Raceways of both pins and rings were partially covered by dark reaction layers. Wear occurred on the rings because of detachment of thin, highly deformed surface layers (Fig. 11(d)). Oil resulted in wear loss for the pins more than two orders of magnitude lower than in water. Wear loss of rings was reduced by about one order of magnitude in the presence of oil (Figs. 6 and 8). Microcracking is an important wear mechanism for porous ceramics. Pull-outs of grains led to surface roughening and increased wear
Fig. 11. Scanning electron micrographs of worn pin (a,c) and ring (b,d) surfaces mated 2X02 (Z96V20) with water (a,b) and oil (c,d) lubrication.
Fig. 12. Scanning electron micrographs of worn Alit), cated in air against (b) Al203 (A99) and (d) CfjOV rings.
(A99) pins (a,~) sliding
loss of the counterbody. Mating of ceramic materials of different hardnesses promoted wear grooves by abrasion. Figure 12 shows surfaces of [email protected]
,O, and Al,O,-CGOV pairs worn under unlubricated sliding. Self-mating of A1203 caused pull-outs of individual grains on the pin surface (Fig. 12(a)). The resulting rough surfaces were locally smoothed by filling of valleys with wear debris. Plastic deformation, pull-outs of grains and detachment of deformed surface layers were observed on the AlzO, rings (Fig. 12(b)). Al,O, pins mated with C6OV rings exhibited a substantial amount of one-way transfer of material from the ring to the pin (Fig. 12(c)). This material transfer smoothed the rough pin surface locally. As a result of metal and metal oxide transfer, adhesion between pin and ring was enhanced and back transfer and/or mutual transfer occurred (Fig. 12(d)). Wear loss of the A120, pins was smaller by about one order of magnitude when mated to the steel than in self-mated pairs (Figs. 3 and 4). However, wear loss of the steel rings was about one order of magnitude greater than that of the A120, rings. Adhesion resulting in scuffing was a main wear mechanism of the soft steel-steel pairs. Figure 13 shows surfaces of ASt52-3-C60V couples worn during sliding in air, water or oil. Third body layers were formed because of mutual transfer of material in air. These relatively thick layers covered t.he surfaces of both pin and ring (Figs. 13(a) and 13(b)). Detachment of these layers by grooving and/or delamination resulted in very large wear intensities
Fig. 13. Scanning C6OV rings (b,d,f)
electron micrographs of ASt52-3 pins (a,c,e) in air (a,b), water (c,d) and oil (e,f).
(Fig. 4). Wear loss was substantially greater on the ASt52-3 pins than on the C6OV rings. The presence of water reduced the effect of adhesion. Compared with the tests in air, the very enhanced load led to high wear intensities for the soft ASt52-3 pins. Flaking and grooving (Fig. 13(c)) were combined with high plastic deformation. The deformed and therefore work-hardened flakes promoted abrasion and, as a result, grooving. The harder C6OV rings showed a type of delamination wear (Fig. 13(d)). Oil as lubricating substance lowered wear intensities of the ASt52-3 pins by more than two orders of magnitude (Figs. 6(a) and 8(a)). Worn surfaces of the pins (Fig. 13(e)) showed evidence of plastic deformation, formation of third body layers and a type of delamination wear. Delamination of highly deformed surface layers, formation of a third body layer and local grooving (Fig. 13(f)) were observed on the mated relatively smooth C6OV rings.
4. Discussion In this study sliding wear tests under dry and boundary lubricated conditions were carried out using a model tribometer at ambient temper-
Fig. 14. Coefficient
in air, water
atures. Some important experimental results are shown in Fig. 14. The measured values of coefficient of friction were very high with unlubricated sliding contact. These values, measured under stationary conditions, were drastically reduced by removing loose wear debris from the area of contact (indicated by the hatched blocks in Fig. 14). For example, values of the coefficient of friction of 1.35 for self-mated 296VlO dropped to 0.80, for A90-C60V from 1.48 to 0.80 or for Z96VlO- A99 from 1.75 to 0.68, as a result of cleaning with a soft cloth for 2 s. After a few minutes, friction increased again to greater values, i.e. from the low (hatched) to the high level in Fig. 14. This means that, under the high applied surface pressure, pull-outs of grains of brittle ceramics and very intense adhesion effects of steel-steel pairs (scuffing combined with abrasion) contributed to high friction. Frictional heating resulted in high flash temperatures, particularly for ceramics such as ZrO,. Further, relatively rough surfaces were produced on the pin specimens because of the running-in procedure with abrasive paper. The geometry of pin and ring produced mostly line contact at the beginning of the wear tests. Therefore the absolute values of the coefficient of friction can be seen to be strongly influenced by the tribosystem used. Values of the friction coefficient as high as about 1.4 and 1.5 were also reported  for self-mated SIC and Si,N, in dry multipass sliding tests. Other studies  showed the effect of surface texture on friction and wear. The friction coefficient of A1$IJ~A120, in dry and oil-lubricated sliding contact decreased with increasing roughness and decreasing bearing ratios of the mated surfaces . However, abrasion can enhance friction and wear, depending on surface roughness, properties of the mated materials
and the tribosystem. At a given load in sliding contact, the wear intensity of ceramics such as Al,O, and ZrOz can be substantially enhanced by increasing the size of hard abrasively acting particles such as SIC [X2]. Friction of ceramics can depend strongly on sliding speed [ 4, 91, load [ 91 and temperature [ 41. The friction coefficient of self-mated ceramics or ceramic-steel pairs dropped by about half when the sliding speed in dry contact was reduced from 0.77 to about 0.1 m s ’ . However, the friction coefficient decreased with increasing load in dry sliding contact. This apparent contrast can be explained by the effect of load and sliding speed on plastic deformation and loading rate. At low temperatures brittle events such as cracking or pull-outs of grains play an important role in wear of ceramics. These brittle events are enhanced by increasing sliding speeds (e.g. from about 0.1 to 0.77 m ss’) at low applied loads because of increasing loading rate. As result, friction values decrease with decreasing sliding speed at ambient temperature [ 91. Plastic deformation occurs in the contact area as a result of high applied loads and/or high temperatures. Increasing the load from 10 to 120 N at a sliding speed of 0.77 m so ’ produced an enhanced amount of plastic deformation and fewer brittle effects, i.e. contributions of cracking and pull-outs of grains by inter-granular fracture were reduced. Therefore increasing loads reduced the friction values of like-on-like ceramic couples [ 91. In dry sliding contact (Fig. 14) ZrO, (Z96VlO or Z96V20) showed substantially lower friction coefficients than did Al,O, (A99 and A90) in self-mated pairs or mated with C6OV steel. This was caused by a greater contribution of abrasion for Al,O, than for ZrO, because of the greater brittleness of the former. Hard ceramic pins sliding against softer ceramic rings caused high friction and high wear of the rings. Water reduced friction substantially despite a 20-fold larger load than in air. Self-mated AU,O, (A99) showed a very great reduction in friction in water. It may be concluded that both Al,Os-Al,O, and [email protected]
sliding pairs should be used preferentially under lubricated conditions. Lower friction values were measured with oil than with water lubrication. Self-mated ZrO, (Z96V20) offered lower friction coefficients than did self-mated Al,O, (A99) in unlubricated contact but in lubricated contact the lower friction was measured for Al,Os. Porous ZrO, (Z95P) led to high friction under unlubricated conditions. Detrimental events such as cracking and pull-outs of grains because of inherent brittleness or low resistance to thermal shock can be reduced by lubricants. A favourable effect of lubricants on friction was also measured for SiSiC. Figure 15 shows a summary of volumetric wear intensities of sliding pairs divided by normal load. The differences in wear intensities of the couples were more marked in the presence of water than in air or oil. Despite a very high friction coefficient, self-mated A120, (A99) showed low wea intensities in dry contact. Wear of self-mated A120, was lower than that of self-mated ZrO, (Z96V20). [email protected]
couples showed the lowest wear intensities of all materials tested under water lubrication. Compared
Fig. 15. Volumetric air. water and oil.
load of sliding
with air the presence of water substantially reduced the wear intensity of almost all pairs; exceptions were ASt52-3-C60V, 90MnCrV8 (spheroidized)C6OV, ZA50-C60V and self-mated Z96V20. The 20-fold enhanced normal load in the lubricated wear tests can increase frictional heating locally at the solid-solid contact, and hence changes wear mechanisms. A change in wear mechanism when a critical load is exceeded cannot be included in the related wear intensity as defined in Fig. 15. Wear intensities of the metallic pairs were much lower with oil than with water lubrication. Whereas AlzO, (A99-A99) showed the lowest wear intensity in water, the pairs Z96VlO-C6OV and Z95P-C6OV showed lower wear intensities in oil. Wear of self-mated ZrO?, can be larger than that of ZrOz-steel pairs under severe loads because of frictional heating and the low thermal conductivity of ZrOz. This disadvantage of ZrOz, compared, for example, with A1203 or Sic, can be compensated by mating ZrOz with steel. Ceramic-steel couples may also show a more effective lubricant film because of greater wettability of metals than ceramics. ZrOz has a greater fracture toughness than A1203, which can result in lower wear intensities for ZrOzC6OV pairs in oil (Fig. 15). In contrast to the present study, larger wear and friction were reported  for like-on-like ZrOz (Mg-PSZ) couples in water than in air at very low sliding speeds (less than 10 mm s-l) and loads. This difference may be caused by the occurrence of different plastic deformation, loading rates, temperatures and chemical effects in the tribosystems.
Ceramic materials can offer a substantially greater resistance to sliding than steels, particularly in lubricated contact. Under severe loading
the low thermal conductivity of ZrO, may be a disadvantage compared with Al,Os, but it can be compensated by using a steel counterbody. A1203 showed the lowest wear of the self-mated ceramics tested in air, water and oil. Al,Os sliding against steels resulted in greater wear loss than in the selfmated A1203 pairs. Increasing brittleness of ceramics increased wear intensities under the severe test conditions used. ZrO, and SiSiC mated to steel may offer greater wear resistance under oil lubrication than that of selfmated Al,O, or Al,O,-steel couples. A relatively high wear resistance with oil lubrication was also measured on 20, containing 10% open porosity in sliding contact with hardened steel.
Acknowledgments The author is grateful to Th. Degen, A. Nowak, A. Graf, J. Weidmann and L, Wurm for carefully carrying out the wear tests.
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