Friction and wear characteristics of mullite, ZTM and TZP ceramics

Friction and wear characteristics of mullite, ZTM and TZP ceramics

WEAR ELSEVIER Wear 218 {1998} 159-166 Friction and wear characteristics of mullite, ZTM and TZP ceramics Yongfa K o n g a.b.* Z h e n g f a n g Ya...

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WEAR ELSEVIER

Wear 218 {1998} 159-166

Friction and wear characteristics of mullite, ZTM and TZP ceramics Yongfa

K o n g a.b.* Z h e n g f a n g

Yang b Guangyin

Z h a n g ~, Q i m i n g

Yuan b

"Department of Ph)~vics. Nankai Univerl'ity. Ttanjin 30f~)7L China "Advanced Curator" Institute of Tianfin Universitxz Tian]in 300072. China

Received26 January 1998:accepted2.-I.April 19q8

The friction and wear of self-mated mullite-zirconia-toughened mullite ( ZTM )-TZP ceramics have been investigated using a block-onring tribometer in different lubricants at varying loads. Load-dependent wear transitions were observed for these ceramics. The wear transition was usually accompanied by the abrupt change of friction coefficient and v,ear rate. The main pretransition wear mechanisms are plastic deformation, ploughing and sometimes microl'ractures, while fracture is the dominant wear mechanism of posttransition. Compared with mullite, T"ZPhas the distinctive feature of having various cracks on its worn surfaces, while no large crack was obse~ed on the worn surfaces of mullite even when wear transition had occurred. Phase transformation of zirconia was expected to have a two-sided influence to the wear of ceramics: it not only restrains the propagation of preexited cracks, but also induces microcracks and causes high residual stress. In ZTM composites, the wear rate is usually smaller than that of mullite at low loads and large cracks occur more easily on the worn surfaces as the volume content of zirconia is increased. © 1998 Elsevier Science S.A. All rights reserved. Ke)a4"ords: Wear-Friction:Mullite:TZP; ZTM

1. Introduction Ceramics are increasingly being used in sliding components such as journal bearings, cylinder liners and piston rings in automotive engines, mechanical seals and chemical properties. Most research on ceramics for tribology application has focused on AI20~, SiC, Si3N4, and ZrO2 ceramics I I-8 I. However, the friction and wear characteristics of a most important ceramic, mullite, has been rarely studied [9,101. During the last twenty years, mullite in particular has received significant attention as a potential matrix for high-temperature structural applications, principally because it retains a significant portion of its room temperature strength at elevated temperatures and displays very high creep and thermalshock resistance. Furthermore, recent studies have shown that phase-pure mullite can also retain its strength at temperatures as high as 1500°C, and that a noticeable increase in strength occurs above the softening temperature of the glassy inclusions when a polycrystailine mullite is not phase pure I I I I. Combined with its intrinsic thermal stability under oxidizing conditions, mullite then stands out as a unique material for high-temperature applications not only as a single-phase material but also as a matrix material in the development of * Correspondingauthor.NankaiUniversity,Dept.of Physics.300071Tianjin, China. E-mail:[email protected]

high-temperature composites. What restrains mullite to be used widely is that the room temperature mechanical properties are low. Zirconia-toughened mullite (ZTM) increases the room temperature mechanical properties of mullite greatly [ 12 I. Thus, mullite and mullite-based ceramics have attracted serious interest recently. The main objective of the present study was to investigate the friction and wear hehaviour of self-mated mallite and ZTM ceramics. For comparison, TZP ceramic was also investigated. Though the tribologicai properties of TZP ceramics have been widely investigated I 1,6,13--151, little attention has been paid to the friction and wear characteristic of zirconia in composites [ 16,171, especially in mullite-hased composites. 2. E x p e r i m e n t a l procedure All powders used in this study are commercial. Samples were prepared by uniaxial cold pressing and then isostatic pressing at 200 MPa. Mullite ceramic with 2 tool% YzO3 added as sintering aid was sintered pressure less at 1600°C for 4 h, ZTM 15 ( 2 mol% YzO3 stabilized) and ZTM45 ( 3 tool% Y_,O~ ) at 1570°C for 3 h, and TZP ( 3 tool% Y_,O3) at 1450°{2 for 2.5 h. The properties of these sintered ceramics are given in Table I. Specimens used for the measurements were prepared in

0043-1648/98/$ - see front matter © 1998ElsevierScienceS.A. All rights reserved. PII S0043-1648 ( 98 )00212-9

E Kong er al. /Wear 218 (19981 159-166

160 Table I Mechanical and physicalpropertiesof ceramicsamples Material

Mullite y-'I~p

ZTMI5 ZTM45

Elastic modulus (GPa) Flexural strenglh (Mpa) Fracture toughness (Mpa mI/'1 Hardness (GPa) Porosity ( ~ ) Grain size (v.m)

213 182 2.5 9.3 0.40 3.04

203 318 4.7 11.4 0.19 1.96

203 419 7.8 11.5 0.28 0.337

210 286 4.8 I 1.0 0.13 2.26

the form of rectangular bars. 25,0 cm x 5.0 cm X 2.5 mm. The flexural strength was measured using a three-point bending test with a crosshead speed of 0,368 ram/rain. The fracture toughness was determined by single-edge notch beam method with a notch about 0.4-0.6 height ( 5.0 ram) of the specimen and a crosshead speed of 0.05 m m / m i n . The hardness of the materials was measured by the Vickers indentation technique on mirror polished surfaces with a 100 N load at room temperature. The porosity was measured using Archimedes method. The microstructures of the ceramics were studied by scanning electron microscopy ( S E M ) after polishing and thermal etching of the surfaces. The average grain sizes were calculated from data of quantitative image analysis. The tribological tests were carried out using a block-onring tribometer. The block and ring were made of the same material. The ring (about 40 m m outer diameter, 16o +°°'s m m internal diameter) was 10+0.01 m m thick. The block was machined to the dimensions 25+o.5 +L° m m x 7 _ n t ° m m × 7 _ o l ° mm. Prior to testing, all samples were heat

treated at 800°C for I h to relax the stress induced by machining, cleaned in an ultrasonic bath with pure ethyl alcohol and deionized water for 10 min respectively, and dried in hot air. The tests were run at normal force from 10-100 N for water lubricant and 100-1000 N for machine oil lubricant, sliding speed of 0.42 m / s and for wear path lengths of 0.5 km. The tangential friction force was measured continuously and the values of the coefficient of friction were determined. The diameter of the worn block was measured by an optical microscope with an accuracy of +0.01 mm. The net volume loss and wear rate ( m m " / m ) were then calculated. Wear scars, ultrasonic cleaned, were further examined using SEM to determine the dominant wear phenomena and surface deg)adation processes involved.

3. Results 3. I. Coefficient o f friction The friction coefficients of these sliding pairs are listed in Table 2 for deionized water and Table 3 for machine oil. In the tables,/z n, P,~oand/x2o correspond to friction coefficients for sliding times of I min, 10 min and 20 min, respectively. Table 3 shows that the friction coefficients of all sliding couples decrease with increasing sliding distance and becomes more stable when the normal load is increased. Using p,,) as the average friction coefficient, Table 3 also shows that the average friction coefficients of mullite are greater than that of ZTM but smaller than that of TZP. In all sliding pairs, ZTM 15 has the smallest coefficients of friction

Table 2 Friction coefficientsof self-matedceramicpairs in deionizedwater Load iN)

10 20 30 40 50 60

MuIlite

Y-'I"ZP

ZTM15

ZTM45

#I

~.Llqi

~.~.O

~.LI

ILl, )

~.~2H

~.~I

~H,

~'~2c)

1.14 0.79 0.78 0.73 0.71

0.84 0.59 0.79 0.73 0.69

0.64 0.57 0.78 0.73 0.68

0.49 0.41 0.42 0.52

0.45 0.55 0.53 0.54

0.44 0.55 0.51 0.51

1.23 0.86 0.82 0.48 0.74

0.74 0.79 0.84 0.77 0.75

0.64 0.54 0.84 11.77 0.74

0.74

0.73

11.73

I00

~-LI

~.&lq)

~211

0.89 0.84 0.77 0.78 0.75

0.48 0.44 0.41 0.83 0.81

0.46 0.39 0.38 0.82 0.82

0.76

0.81

0.82

Table 3 Friction coefficientsof self-matedceramicpairs in machineoil Load (N)

IO0 200 300 400 50O 600 70O 1000

Mullite

Y-TZP

ZTMI5

ZTM45

~'LI

)ILll )

p.21i

~.L I

#-IO

/.L2I)

j[~l

~-LIt)

~.2t)

)~I

lJ.lqi

,U.2{)

O.144 0.142 11.138

0.109 0.119 11.I18

0.090 0.097 0, I04

O.134 0.136 0.142 0.148

0.136 0.141 0.142 0.150

0.131 0.132 0.142 0.144.

1).130 0.111 0.144

0.038 0.024 0.082

0.024 0.019 11.1134

0.137 11.127 11.122

0.076 0.063 0.008

0.056 0.048 0.042

0.152 0.166

0.13O 0.131

{kIO9 0.109

0.147

0.(187

0.(M2

0.132

0. I 12

0.092

0.138

0A21

0.099

o. 158

0.1I)6

0.086

K K,mt~ et cal./ Wear 2IN */Vt~ 15V-/6t5

at the low normal load range. For mullite and TZP. coefficients of friction are quite stable when the normal loads var~ among our experimental range. However. the phenomena of ZTM are quite different, the friction coeflicient of ZTM 15 has a sudden increase from aboul 0.03 to 0.08 when Iho normal load changes from 200 N m 300 N. and another increase from 0.07 to 0.1 l occurs fi~r ZTM45 as the load varies from 300 N to 5(]{) N. With deionized water as the lubricant. Table 2 ~hox~> that the friction coefficients of all sliding conples are much greater than those for machine oil. though the normal load is one order of magnitude lower. The lubrication effect of machine oil for ceramics is apparently belier than deionized water. Table 2 also shows sudden changes of friction coefficients from about 0.59 to 0.79 for mullite and from 0.41 It) l).81 for ZTM45. However. the increase from 0.45 to 0.55 for TZP i,, not abrupt when the normal load changes from 10 N to 20 N and no apparent change appears fl)r ZTM 15 at lhe h)ad range in our experiments. As a function of time. the friction coeflicients of ceramics in deionized water arc much more ~lable compared to that in machine oil.

161

I.OE-I

- ~ :~ 1

: r- : Y-rZP

-

I 0E-3

.o _

tO

tO0

Normal Load (N) 1.00E-2 = ~: Y-'lzP I OOE-3

~

-

~

1.00E-5

100

3.2. W e a r rate

Wear rates of the blocks plotted as a function of nornlal loads are shown in Fig. ht fi)r deionized ~ater and Fig. Ib for machine oil. From Fig. la, it is shown that the wear rate of mullite is low and steady when the normal load is belov. 20 N. and that an abrupt increase in wear rate ( named ~ e a r transition) occurs when the load exceeded 20 N. Noting the slope change ( w e a r rate) pre- and posttransition, the wear transition of T Z P is more abrupt than that of mullite. The wear rate of T Z P is lower than that of mullite before wear transition but higher after transition. The wear transition load of Z T M 4 5 is much higher than those of T Z P and mullite. Before and alter transition, the wear rate of ZTM45 is lower than that of mullite, so the wear resistance of ZTM45 is better than mullite. In double logarithmic coordinates, the wear rate of ZTM 15 changes nearly as a linear function of normal load. that is, much different from that of the other three ceramics. The interpretation of this phenomenon will be premntcd latter. With machine oil is the lubricant, Fig. Ib shows that the wear rates of mullite. TZP and ZTM45 are linearly increased with the normal load. However, wear transitions arc also found in our experiments. Since the thermal conductivity coefficients of machine oil and ceramics arc very low, and the normal load is one order of magnitude higher than that in deionized water, too much friction heat cannot di fl'um in time. The temperature of machine oil was so high to cause carbonization of the oil and subsequent loss of lubrication effect. Experiments had to bc stopped, otherwise, the intensive heat shock caused by friction would induce very high stress that would cause the fracture of the ceramic rings. Wear rate values for this situation are not obtained, so plots shown in Fig. Ib are belbre wear transition only. The wear rate of Z T M I 5 has a sudden change between 250 N and 3(X) N, which is different from that of the other ceramics. It is

1000

Nonma Load(N) [.Ig i. v. car rate~,,I hlock~,in ( a ) dcionizcd water and (b) machineoil vs. normal h,ad. believed that this phenomenon is related to the phase transformation of zirconia in ZTM 15. 3.3. tl,'onz .sut?t~we

Fig. 2 shows the SEM microgmphs of worn surfaces of mullite blocks in deionized water. Fig. 2a and c are typical pictures for different parts o f worn surface at low normal load. From Fig. 2a, it is ~ e n that a film covers the worn surt:ace. Fig. 2b. the enlargement of Fig. 2a, shows that this film is a mixture c o m p o ~ d o f debris and some other substances. The trilx~chemieal reaction between water and ceramics has been widely studied [ 7 . 1 8 - 2 0 ] . For this situalion, it is believed that the following reaction may occur: 3A I : O , . 2SiO_, + 12H,O

(I)

" ' ~ ' " 6 A I ( OH ) ~+ - 3i( O H ).~. Fig. 2c shows that the wear mechanism also includes ploughing and plastic deformation. As the normal load is high, Fig. 2d shows thin film on some p a n s of worn surface a l ~ , but fracture becomes the main wear mechanism apparently and fragments are produced. Fig. 3 shows the SEM micrographs of worn surfilces of mullite in machine oil. From Fig. 3a. it is seen that the worn surface of mullite is "cleaner" than that in water at low load, that is, no tribochemical reaction like Eq. ( I ) occurs. The worn surface is rather smooth; only slight ploughing and plastic deformation can he.seen. Fig. 3b shows the worn surface alter wear transition. It shows "black" sliding traces and layer-on-layer fragmentation among different

162

Y. Kong et al. I Wear 218 f l998~ 159-166

Fig. 3. SEM micrographsof worn sur[~es of rnunitcblocks ~sled in machineoil: ( a ) at I00 N: ( b ~and ( c ~at 750 N. ranges. Fig. 3c, the enlargement of the black trace in Fig. 3b. shows that the black trace is composed of clusters of little debris with different sizes. This phenomenon is believed to proceed through surface fracture followed by fragmentation and milling o f the debris between the mating surfaces. The wear fragments are spread over the surfaces where they

become mechanically (crushing, self-grinding) and thermally (sintering) treated to reform clusters of debris and "black" trace. Fig. 4 shows the surfaces of TZP worn in water: there are various cracks in these micrographs. As shown in Fig. 4a, cracks are small and wear occurred mainly through large

Y. Kong et al. / Wear 2181199~¢~ / ~9-16h

:r

Fig. 4. SEM micrographsof worn surfaces of Y-TZP blocks tested in dcionized water: (a) and (b) at In N: (c) and (d) at 40 N.

pieces of fragments while the normal load is low, Fig. 4b is the enlargement of the relatively smooth place in Fig. 4a, showing little cracks are also formed. From Fig. 4c and d. it is seen that very large cracks formed" they propagate and intersect or circle then great fragments are produced. It is shown that the wear mechanism of T'ZP is quite different from that of mullite• For mullite ceramics, there are no large cracks on the worn surfaces and the main wear mechanism is ploughing and plastic deformation with the normal load in a low range. At a high load. though fracture becomes the dominant wear mechanism, no large crack was found on the SEM micrographs. In other words, cracks are nucleated and propagate very fast. and the range of fracture is small, so small fragments are produced fast, causing the layer-upon-layer

163

fragmentation shown in Figs• 2d and 3b. As for TZP, whether the load is high or low. cracks always formed on the worn surfaces. It is considered that the sample used in our tests does not contain 100% t-phase zirconia, little content of mphase zireonia was, formed in sintering and (or) machining (especially on the surface) process [ 2 1 ] . The t ~ m transform ation will introduce stress and microcracks, where cracks are easy to nucleate under the high stress induced by load, so the worn surfaces of T Z P have many cracks. When restrained by the stress-induced t ~ m transformation, cracks are difficult to propagate at low normal load. and the fragments shown in Fig. 3a are probably due to the preexiting flaws in grain boundaries. At high normal load. excessive stress can overcome the restraint o f stress-induced transformation, cracks begin to propagate. When restricted by the stress-induced transformation, the propagating speed of cracks in T Z P is slower than that of mullite, and cracks have to propagate through a . ,rag distance and large fragments are induced. Fig. 5a and b are SEM micrographs of worn surfaces of ZTM 15 in machine oil. As compared to that o f mullite, no large difference is o b ~ r v e d in Fig. 5a, and short cracks appear and fracture is more ~ r i o u s as shown in Fig. 5b. The worn surface of ZTM 15 in deionized water is also almost similar to that of mullite, so micrographs are not presented here. Fig. 6 shows the worn surfaces of Z'TM45 in deionized water. From Fig. 6a• it is ~ e n that the main wear mechanism is ploughing and plastic deformation, and large fragments occasionally appear. Fig. 6b shows large cracks and fragments at high load, they are apparently similar to that o f ' i Z P . So the wear characteristic of Z T M 4 5 is similar to that o f ' l ' Z P rather than that of mullite at high load. Fig. 7a shows the worn surface of ZTM45 in machine oil at low IoncL the w e a r i s very slight, only shallow ploughing traces and slightly plastic deformation are seen. Wear surface at high load is shown in Fig. 7b. Fractures and fragments can be ,seen in the right side of this picture, and a net o f cracks in the left side; an enlargement of this place is shown in Fig. 7c. Compared to the situation in water, cracks were not so large, intersecting with each other and composing a great crack net in machine oil environment• This difference is thought to he related with the effect of water, especially the stress-induced conrosion cracking and the chemical reaction with Y20~.

Fig. 5. S E M micro~dpbs of worn surfacesof Z T M 15 blocks testedin machine oil:(a) at 200 N: (b) at I000 N•

164

E Kon g et aL / Wear 218 t 1008) 159-166

Fig. 0. SEM micmgrq~s of worn sufftges ofZ'l'lVl45blogks test~ in dcionized water: (a) at 20 N; (b) at 100 N. .......

Fig. 7. SEM micrographsof worn surfaces of munite blocks tested in deionized waler: (a) al 2(g)N: ( b ) and
4. Discussion As mentioned above+ the friction and wear characteristics of zirconia ceramics have been widely studied. Now+ two main problems, phase transformation of zireonia (from tetragonal to monoclinic ) and the effect of water lubrication. have been paid much attention. Firstly, is there exited phase transformation of zirconia (in particular the stress-induced t - ~ m transformation) on the contacted sliding surfaces? Three conflicting answers: transformation occurs [ 2 2 , 2 3 ] . does not occur [ 15+24], and occurs only a m o n g the debris and not on the wear tracks 125,26], were reported. These apparently contradictory results were explained as resulting from the fact that testing conditions and characteristics of the coupled materials heavily influence the tribological behavior of the zirconia ceramics. To the tribometer used in our experiments, transformation of zirconia has been reported by other

researchers [ 27 I. From Fig. I a and b. it is seen that the wear transition load of T Z P is lower than that of mullite, that was thought to be related with the transformation ofzirconia, since the elastic modulus of mullite is close to that of Z r O : ( see data in Table I ) and the lattice structure of mullite is loose and has a lot of intrinsic defects. The restraint of mullite to the t ~ m transformation of zirconia is low in ZTM composites. The sudden increase of wear rate for Z T M I 5 between 250 N and 300 N was interpreted to be induced by the trans+ formation of zirconia. Furthermore+ how does this phase transformation influence the wear rate of zirconia ceramics'? Conflicting results+ increases and decreases, have also been reported [ 1,6,281. From Fig. Ib, it is seen that the wear rate of T Z P is lower than that of mullite before wear transition. When the load is below 250 N. the wear rate of ZTM 15 is about one order of magnitude lower than that of mullite and close to that of

Y. Kon.e et aL / W e a r 2181109.~)

Z T M 4 5 and TZP. Thus, transformation of zirconia can apparently decrease the wear rate of ceramics at the low normal load range. It is considered that stress-induced transformation of zirconia can effectively restrain the propagation of cracks at low load. However, as the normal load is above 300 N. the wear rate o f Z T M I 5 is almost similar to that of mullite and one order of magnitude higher than that of ZTM45 and TZP. So. when the normal load is above a threshold, in other words. the stress-induced t--, m transformation cannot restrain the propagation o f cracks, the wear rate of ZTM apparently does not decrease while the content of zirconia in the composite is relatively low, for example, 15 vol%. As to ZTM45. it is considered that the main toughening mechanism is microcracking rather than stress induced transformation. As mentioned above, the restraint of mullite to the transformation of zirconia is low, when cooling from the sintering temperature to room temperature in ceramic processing, t ---,m transformation has occurred for the main part of zirconia, so the main phase in as-sintered ZTM45 is not t-phase but m-phase usually. When the content of zirconia in ZTM45 is very high, there are not enough mullite grains to relax the stress caused by this transformation: as a result, many intergranutar and intragranlar microcracks induce. In the process of friction, the high load for machine oil environment induces very high stress. Though this stress can be partly reduced through stress induced t--o m transformation, too many microcracks are easily interconnected to cause large cracks induced by the loaddependent stress. Propagation of large cracks will induce fracture in the contacting surfaces, then debris is produced. This process is mainly related to the value of normal load. so it is seen in Fig. Ib that the wear rate of ZTM45 has a linear relationship with the normal load in the double logarithmic coordinates. Fig. I b also shows that the wear rate of ZTM45 is about one order of magnitude smaller than that of mullite and Z T M I 5 at the middle load range. It is considered that microcracks play a main role in this phenomenon. The second problem about the tribological characteristics of zirconia is the effect o f water lubricant on the wear rate of ceramics. Conflicting results, increase and decrease, have been published 11.4.7,291. Tribochemical reaction 118-201 and stress induced corrosion cracking (in particular, transformation of t--* m was also caused for zirconia ceramics) [ 301 have been reported as the main effect of water on ceramics expecting lubrication and cooling. For Y-TZP, the most important effect of water is considered to be the chemical reaction [ 20,311: Y203 + 3 H 2 0 - , 2 Y ( O H ) 3

159-/66

165

compared with other ceramics. The wear rate of Y-TZP at a very low normal load is several times smaller than that of mullite and Z T M I 5 but very high after wear transition. So water lubricant is apparently harmful to Y-TZP ceramics while the normal load is not too low. The linear relationship between the wear rate of Z T M i 5 and the normal load is considered to be depended on the transformation of zirconia caused by the stress-induced corrosion cracking and the loss of stabilization of Y203. For having more Y203 (3 reel%) than Z T M I 5 (2 m o l % ) , t-phase zirconia of ZTM45 can maintain its stability at a high load range. This phenomenon also indicated that the loss of stabilization of Y203 is the main wear mechanism for Y-T'ZP ceramic and zirconia composites in water environment. 5. C o n c l u s i o n s The friction and wear characteristics of ~elf-mated mullite. ZTM and TZP in different lubricants at varying loads have been studied. Load-dependent wear transitions were found for these materials. The wear transitions are not only accompanied by the abrupt change of wear rate more than one or two orders of magnitude but also apparent change of friction coefficient. The main pretransition wear mechanism is ploughing, plastic deformation and occasionally microfracture, while fracture is the dominant mechanism of posttransition. Compared to mullite, Y-TZP showed lower wear rate at low load. lower wear transition load, more abrupt wear transition, high wear rate at high load and more and larger cracks on the worn surface. The t -* m phase transformation of zirconia is considered to have a two-sided influence on the wear of T Z P and zirconia composites- thus, it not only restrains the propagation of pmexited cracks, but also induces microcracks and high residual stress. Wear rate of ZTM composites is lower than that o f mullite at low load. Friction and wear properties of ZTM are related to the zirconia content. They are more similar to mullite at low load, and more similar to real life also as the content o f zireonia is 15 vol% but to T Z P for 45 v o l ~ zirconia at high load. Compared to deionized water, machine oil more effectively reduces wear rate and friction coefficient, and increases the wear transitton load. Water has some harmful effect on zirconia in Y-TZP and Z ' r M ceramics, such as stress induced corrosion cracking and chemical reaction with Y203. Conclusively. ZTM45 has the best tribological properties among these experimental materials.

(2)

that causes the loss of Y20~ and the t-phase zirconia unstable: t---, m transformation occurred, and the property degradation in the surface o f Y-T'ZP ceramics. This chemical reaction of Y203 is well known in the heat treating process at the temperature range 200°-400°C in high humidity environment. The reaction temperature is easily reached through the sliding friction though lubricant, water, has been employed. Fig. I shows that the wear transition o f Y-TZP is much abrupt

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130] T.A. Michalske. S.W. Fre2man. A molecular mechanism for stress corrosion in vitreous silica. J. Am. Ceram. Soc. 66 (1983) 284-288. [ 311 M. Yosbimum. T. Noma. K. Kawabata. S. Somiya. The role of H20 on the degradation process of Y-TZP. J. Mater. Sci. Lett. 68 (1985) 356-359. Biographies Y o n g f a K o n g , born on M a y 25, 1968. received his P h D d e g r e e f r o m Materials Science and Engineering in Tianjin University in 1997. He has been i n v o l v e d in research activities in the fields o f synthesis and processing o f c e r a m i c s (including mullite, zirconia, zirconia toughening mullite and a l u m i n a reinforced zirconia toughening mullite ), c e r a m i c tribology, growth o f crystals (especially in doped lithium niobate ), and photorefraction resistance. He is n o w a postdoctoral fellow with the D e p a r t m e n t o f Physics o f Nankai University. His primary interests and responsibilities include c e r a m i c sliding w e a r modelling, testing and d e v e l o p i n g h e a v y - d u t y engine v a l v e s d o p e d lithium niobate crystals, photorefraction, nonlinear optical effectsand photonics. Z h e n g f a n g Yang, 10ore on N o v e m b e r 17, 1936, r e c e i v e d her BS in C e r a m i c T e c h n o l o g y at Tianjin University in 1961. She is a Professor o f Materials Science and Engineering and the Vice Director for A d v a n c e d C e r a m i c Institute at Tianjin University. H e r research has focused on c e r a m i c p o w d e r preparation, mullite-based c o m p o s i t e s and their n e w application, pilot production o f c e r a m i c cylinder liner for oildrilling p u m p s , etc. She r e c e i v e d an a w a r d in 1991 f r o m T h e State C o m m i s s i o n o f Science and T e c h n o l o g y in C h i n a for her contribution to the d e v e l o p m e n t o f c e r a m i c e n g i n e components. G u a n g y i n Zhang, born on O c t o b e r 30, 1932, r e c e i v e d his P h D d e g r e e from the D e p a r t m e n t o f Physics o f M o s c o w University in 1962. H e is a Professor o f Physics at Nankai University. He has been i n v o l v e d in research activities in the fields o f solid e n e r g y spectra and laser physics. Presently, his p r i m a r y interests include nonlinear optical n e w materials, crystal growth, photorefraction and photonics. H e r e c e i v e d a w a r d s form T h e State Education C o m m i s s i o n and T h e State C o m m i s s i o n o f Science and T e c h n o l o g y o f China several times for his contribution to the photorefraction o f d o p e d lithium niobate crystals. Q i m i n g Yuan, born on O c t o b e r 9, 1932. receive his M S in C e r a m i c T e c h n o l o g y at Tianjin University in 1956. H e w a s a postdoctoral Fellow at the University o f Sheffield. He is a Professor o f Materials Science and Engineering, the Chairm a n for the Materials Research Center and the Director for the A d v a n c e d C e r a m i c Institute at Tianjin University. He has been the Academician. A c a d e m y o f C e r a m i c s since the 4th Election, 1992. He is the present C h a i r m a n for the Tianjin C e r a m i c Society and also for the Tianjin Materials Research Society. His research has contributed to the d e v e l o p m e n t o f c e r a m i c engine c o m p o n e n t s , toughening and strengthening o f a d v a n c e d oxide structural ceramics, and surface modification o f zirconia-toughened ceramics.