Ceramic tribology in Japan

Tribdogy

Ceramic

tribology

Inrernafional

Vol. 28. No. 1, pp. 1-6, 1995 Elsevier Science Ltd Printed in Great Britain 0301-679X/95/$10.0 +O.OO

in Japan

Yuji Enomoto

This paper reviews recent fundamental work in Japan on the tribology of engineering ceramics. These ceramics show a characteristic tribological response depending on the materials and environmental species in a vacuum, in various gases, in oils, in water and in low-viscosity fluids and at elevated temperatures. The unique tribological interactions, in some cases, significantly affect both the friction and the wear behaviour of ceramics. Keywords:

ceramics,

friction,

wear,

tribochemistry,

solid lubricant

Introduction During the last decade, especially since the National R&D programme on ‘Fine ceramics’ was initiated in Japan in 1981 to promote basic studies on the practical application of ceramics, engineering ceramics have attracted the attention of many researchers as promising tribomaterials for mechanical components operating under severe or hostile environments. Practical applications, however, have been slow to materialize in comparison with initial expectations. Nevertheless, silicon nitride ball bearings have now been in use since 1986, and mechanical parts for car engines, such as valves, turbocharger rotors and rocker-arms made of silicon nitride. have followed.

generated in a plastically deformed zone act as luminescent centres’. Observation of cross-sections of MgO crystals scratched by a diamond hemisphere shows that the zone deformed by sliding contact consists of three characteristic zones whose structure depends on a/R, which is a measure of the contact strain*. CL observation of alumina sliding surfaces also reveals fatigue-type subsurface cracking, because lateral cracks suppress the penetration of the electron beam and/or the escape of luminescence, thus reducing the CL intensity2. The load (W) dependence of the friction coefficient (p) could be explained, provided that all the fractional work was expended in the CL zone.

The subsequent national project on the ‘Ceramics gas turbine engine’ has been underway since 1990 to develop engines for 300 kW power plants. Further R&D in the car industry is working on the development of a ceramic turbocompound engine and an alcoholfuelled two-cycle ceramic engine. Further research work is still needed to understand the friction and wear behaviour of engineering ceramics especially under hostile conditions, and many presentations on related topics were made at the recent JAST Tribology Conference. This paper reviews recent research work in Japan on the fundamental tribological responses of ceramics.

Observation of the wearing surface of S&N4 using an SEM reveals that cracks, initiated at preferential sites of subsurface defects, cause the removal of fragments, which are dragged along the sliding surface while being crushed to create two-dimensional plastically deformed grooves. There is a geometrical resemblance between this microtopography and that often found on metallic sliding surfaces in a steady state3.

Deformation

and fracture

in sliding

contact

Although ceramics are typically brittle solids, ductile deformation can occur when a contact stress is concentrated on the real-contact area of the sliding surfaces. This deformed zone of oxide ceramics can be seen using the cathodoluminescence (CL) mode of a scanning electron microscope, because point defects Mechanical 305, Japan

Engineering

Laboratory,

Namiki

I-2,

Tsukuba,

Ibaraki

Tribology

The brittle behaviour of ceramics under a concentrated sliding contact has been analysed based on fracture mechanics. Kimura and Shima employed a longitudinal contact-point model to evaluate stress intensity factors at wear cracks by three-dimensional finite element analysis. They showed that the amplitude of the mode II stress-intensity factor at an L-shaped crack reaches as high as 2.8 MPaml’*, which exceeds the threshold value for crack extension, as shown in Fig 1. This suggests that horizontal parts of L-shaped cracks may grow in the direction of sliding. On the other hand, Hokkirigawa et a1.5J’ employed a transverse contact model, and analysed the crack extension conditions of a vertical crack at the rear of the contact zone. Using the crack propagation conditions as a basis they deduced the dimensionless parameter S (or S*), which is a function of contact stress or

International

Volume 28 Number 1 February

1995

1

Ceramic

tribology

in Japan:

longitudinal

contact

Y. Enomoto

Silicon nitride ball bearings with a solid lubricant film were developed by Ogawa et aL9 for high-vacuum, high-temperature, high-speed application. All S&N4 bearings with an MoS, film and hybrid bearings (S&N4 ball + steel races) with a Pb or an MoS2 film have good durability if lamellar solid/metal composite retainers are used.

model

L-shaped

crack

To develop a dust-free bearing system, Ito et a1.l’ recently conducted experiments in a vacuum to count the number of particles emitted from a bearing system. Of the various bearings, including soft metal-coated steel bearings, hard-coated steel bearings and ceramics, they found that the best combination for a dust-free system is a ZrOz ball with a steel inner and outer race and a PTFE retainer.

Tribochemistry environments

Fig 1 Stress intensity factor KI1 of S&N, at the subsurface craclp

hardness, the fracture toughness, friction coefficient and crack size. These parameters accurately describe the transition behaviour from severe wear due to microfracture to mild wear resulting from ploughing.

Friction and wear of ceramics in a vacuum The friction and wear of various engineering ceramics on themselves in a vacuum were investigated in a vacuum by Sasaki’. The friction coefficients k of A1203, PSZ and SIC were relatively high at /.~=0.4-0.8. Although the friction coefficient of S&N4 showed ~=0.3 in a vacuum, the coefficient was p=O.7 at atmospheric pressure’. The wear behaviour of ceramics depends on the nature of the materials and the surrounding gas. The wear of Sic on itself is minimum at pressures of 10-l Pa to 1 Pa. Recently, Hiratsuka et a1.8 found that when a ceramic pin is slid in a vacuum against a metal disk, which has the lower absolute standard heat of formation, both friction and wear were low. Figure 2 shows the friction coefficient of A1203, ZrOz and Si02 rubbed against various metals. The friction coefficient of an A1203 pin rubbed against a platinum disk was as low as 0.005, whereas in oxygen it was 0.45.

0

Standard

of ceramics in gaseous

It is well established that tribochemical interactions between ceramic surfaces and environmental species have an important effect on both friction and wear. A notable technique has been employed by Mori et al.ll to determine the dynamic behaviour of the adsorption of gas species on wearing surfaces using a scratch tester in a vacuum chamber equipped with a Q-mass spectrometer. They evaluated the adsorption rate and adsorption activity of molecules on nascent ceramic surfaces and found that? 1. 2.

Even chemically stable saturated hydrocarbons chemisorb on nascent ceramic surfaces, and Polar compounds, such as water, amines and carboxylic acids that are barely chemisorbed on nascent metal surfaces, are chemisorbed well on nascent ceramic ones. As shown in Fig 3, the absorption activity of the polar compounds propyl amine and propionic acid rises as the ionic character of the ceramic bond increases12. This indicates that such polar compounds are adsorbed onto the ceramic surfaces by electrostatic interaction.

In experiments by Sasaki’, ceramics were slid against themselves in nitrogen (carrier gas) + organic gases saturated at 2o”C, ethanol, acetic acid, acetone, hexane, benzene and water. As shown in the results for Zr02 and S&N4 in Fig 4, both friction and wear are strongly affected by the environmental species. The marked bars in Fig 4 indicate that a grease-like reaction product (the so-called friction polymer) was

tt tt

-1000 t Mg St Fe heat of oxide

-2000

TlAl

fromation

(kJ/mol) Amount

Fig 2 Friction coeficient of Alz03, ZrOz and SiO, rubbed against various metals in a vacuum as a function of standard heat of oxide formations 2

Tribology

International

Volume

28 Number

of ionic

character,

Fig 3 Adsorption activity of non-oxide function of ionic character” 1 February

1995

4b

ceramics

as a

Ceramic

GZiSG3

Friction

CoeffiCient

0.2

a)

0.4

specific

P

0.6

0.8

,o”’

1.0

H

H2a C&OH

rate

mm2lN 10-7 ,0-s 7

I

H

Nz

wear

,O’O ,0-v 104

a

B

I H

CHJCOOl

l-l

(CH312CO

C&4 C&6

b)

H

I*

H

m Y

NZ H20

C&OH

L

tally 0.1250.6, 0.1420.2, 0.08?0.3 and 0.11?0.4, respectively. To further improve the lubricity of ceramics, some additives are needed. Tsuya et all3 showed that engineering ceramics and ceramic coatings are very sensitive to the base oils with and without SP-type additives. Ceramic coatings such as Ti02, Cr203 and ZrO, showed better tribological responses in the oils at elevated temperatures of around 300°C.

Lubrication

u I*

II

(CH3)2CO

m*

.i

I*

H

c6b

Y. Enomoto

l-4

CHjCOOH

C6H14

in Japan:

Imazumi and Hatal showed that a kind of phosphate and alkyl-type phosphite additive in paraffin oil reduced the wear of a S&N4 block in combination with a steel ring, whereas an aryl-type phosphite did not, as shown in Fig 5. Formation of FeP04, FeSi03 and a friction polymer acts as a wear inhibitor.

!

I+

tribology

Fig 4 Friction and wear of (a) A&O, and (b) Si,N, in an organic compound + nitrogen vapour7. *A grease-like friction polymer was visible by eye on the sliding surface

a) .,“’ 35,

in low-viscosity

fluids

Since the pioneering work on water lubrication of S&N4 by Tomizawa and Fischeris, the lubrication of ceramics in various low-viscosity process fluids has been studied in detail. According to their work, when S&N4 is slid in water hydrodynamic lubrication exists above a certain critical speed. Further studyi showed that this behaviour strongly depends on the sintering additives in the materials. For a lower sintering additive content in Si,N4, the critical velocity at which hydrodynamic lubrication sets in is reduced. However, at velocities below the critical value, both the friction and wear of S&N4 are relatively high. To improve this behaviour, the effect of an aqueous solution of additive fluids, such as fatty acids, silane coupling agents, acids, etc. was examined7J6.“. Hibi and Enomoto showed that an aqueous solution of a silane coupling agent with an amino group greatly reduced both friction and weari’, as shown in Fig 6. This is due to the fact that a polysiloxane film forms on oxyhydrated S&N4 and acts as a lubricant. Sliding tests of S&N, in a diluted aqueous solution of different pH values by Mizuhara and Hsu’* showed that both friction and wear increase with increasing pH, and fall to a minimum in a pH = 3 solution. The

TEP. 1BP,mP, 1DS/70.2, 1

AlkYl

LYP

.I[.

1’Y’

type

1

Fig 5 Wear of SI,N, in oils containing and (b) phosphate compound additivesI

(a) phosphite

visible by eye on the surface after sliding. In this case, both friction and wear were relatively low. W

Lubrication

x iU8m&kg~mm

in oils

Ceramics may also be used for the mechanical sliding parts of high-precision machine tools, etc. where lubrication with oil is needed. The friction coefficients of Si3N4, Sic, ZrO,, A1203 under boundary lubrication in poly-phenylether found by Kawamural* were typiTribology

silane

3 aq.

0.001 0.01 0.1

mol/l molll molll 5

I 10

15

2b

Fig 6 (a) Friction and (b) wear of S&N, in an aqueous solution of 3-aminopropyltriethoxysilane (silane 3)16 International

Volume

28 Number

1 February

1995

3

Ceramic

tribology

in Japan:

Y. Enomoto

hydrophilic silicic acid film, formed on the sliding surface, easily dissolves in a solution of higher pH values. On the other hand, dissolving of the film is suppressed in a solution of a lower pH value, and thus a thin, tight film forms on the sliding surfaces, helping to reduce the friction coefficient. Lubrication tests of S&N4 with water and water dissolved in oil by Kimura and Okada19 showed that the friction of S&N4 in water and water + polyalkylene glycol was high in the low Hersey number region, and transition to very low friction took place at a threshold value, forming Stribeck-like curves in some cases, but no such transition was found with the emulsion with a paraffinic base oil. The presence of water considerably increased the amount of wear. Kitaoka et al.*O investigated the tribological characteristics of a-alumina and S&N4 in a high-temperature water vapour up to 300°C. They showed that the wear of these ceramics significantly increases at elevated temperatures. The wear of alumina is attributed to the solution of intergranular impurities and corrosion cracking*‘, whereas the tribochemical dissolution into water in the following reaction scheme is the predominant wear mechanism for Si3N4*l: Si3N4 + 6H2O + 3Si0,

+ 4NH3

Hibi and Enomoto22,23 found that, as shown in Fig 7, purified n-alcohols effectively reduce the friction and wear of Si3N4 even at a lower sliding velocity; i.e. the coefficient of friction is 0.1 for lower alcohols and 0.07 for higher ones. As the carbon number ZV, in lower n-alcohols increases, wear decreases. Hibi and Enomoto showed that silica, amines and polymeric compounds are formed due to tribochemical interaction between Si3N4 and lower n-alcohols according to the following reaction scheme24: a-Si3N4 + R*OH+Si02 R*: H, CH3, where R **: H, H,CH,

+ R**NH, C2H5,

C3H7,

H,

H,

+ Polymer C4J&

H

Hisakado et a1.25 showed that the friction and wear of Si3N4, Sic and Zr02, Al2O3, lubricated in ethanol, decrease as the mean depth of fine roughness of the ceramic surfaces increases. Triboelectronic The sliding Furthermore,

lead to the emission of charged particles, the socalled tribo-emission or fracto-emission. Although this behaviour has been known since ancient times, the relationship between tribo-electrification and triboinduced physiochemistry is not yet fully understood. The effect of environmental gas pressure of triboelectrification of various ceramics under sliding contact was measured using a vibrating-reed type electrostatic voltmeter26. As seen in Fig 8, the surface electrostatic potential in a Zr02 pin on a ZrO,, disk configuration reached as high as 3 kV at a N2 pressure of less than 10 Pa, and decreased quickly to the level of 0.5 kV around the N2 pressure range of 0.1-l Pa. Although the wear of Zr02 was so small that it could not be measured, it is interesting to note that the wear of Sic was minimum at a similar pressure range. Further detailed investigation is needed to clarify the mechanism responsible for the correlation between triboelectrification and the friction and wear behaviour of ceramics. Intense fracto- or tribo-emission of charged particles from fracturing brittle solids has been successfully measured under atmospheric conditions using a newly developed fast operating charge amplifier”. It was suggested that the fracto-emission of electrons is caused by the separation of charges on the fracturing surfaces, which leads to the formation of an electric field and the liberation of exo-electrons28. Recently this technique has been used to investigate the electrical activity of fracturing rocks for studying the source mechanism of seismic electromagnetic signals29 and the role of charged particles under boundary lubrication conditions30. High-temperature

solid

lubrication

There is an increasing need to lubricate mechanical components in advanced applications such as ceramic gas-turbine engines and combined jet engines for hypersonic transportation to show operation in air over a wider temperature range, say from room temperature up to 1000°C. Conventional solid lubricants such as MoS2, graphite, etc. however, do not

phenomena of ceramics causes triboelectrification. the sliding and fracture of ceramics also aI 'G 5

7!!!2i

----\ .

0.6-

.-a

ew

.\

E c 0.4.-0 z

0

Fig 7 Friction and wear map of S&N, and Sic in water and n-alcoholti2. m; S&N,, 0; Sic 4

Tribology

International

Volume

- 1 ‘t z

10-4

,jL, 100

N2 pressure

Coefficient

28 Number

z 9

,

0e

/ -2a

0.2-

lk

Friction

/'. ','

1 3 5

lo4

(Pa)

Fig 8 Electrostatic potential of Sic due to frictional electrification as a function of N2 pressure26 1 February

1995

Ceramic

have a stable friction coefficient over such wide temperatures due to oxidation degradation. Yamamoto and Ura31 showed that when a Sic surface was oxidized by heating to 1000°C in air, it exhibited excellent friction and wear characteristics in argon, but not in air. Metallic thin films such as Mn, Fe, Co and Ti on ceramics effectively reduce both friction and wear up to 1000°C as long as the film lasts long enough3”,33. These results are shown in Fig 9. The metallic films

1.0 Co fflm(0.2

T I film

(1 vam)/AlzO3~

T I fllm(0.2

pm)/AlzO3

NaZrO3

t 8

I

T

pm)lA12Og

[

+ 23.3%CrzO3/Al~O3

I

1

0

I

I

I

I

1

I

500

Temperature,

1000

‘C

Fig 9 Friction coefficient of metal-coated ceramics and oxide lubricants as a function of temperature in air Table 1 (a) Effect of metal film on wear reduction surfaces after tests at 1000°C by X-ray analysis (a)

S&N,

SIC 40s 30,

tribology

in Japan:

Y. Enomoto

were subjected to oxidation during sliding and served to prevent adhesion between the ceramics. Shimura and MizutanP3 found that the first transition metals having a high oxidation number show better lubrication properties, as seen in Table l(a). Their results indicate that films which effectively prevent adhesion have a form of complex oxides rather than a stoichiometric oxide, as indicated in Table 1(b). In general, oxide substances are naturally stable under oxidizing environments, even at higher temperatures. Solid oxide lubricants have been investigated among double, tertiary and quaternary oxides and their mixtures3j. As a result of a series of experiments, Na*ZrO, + 28.8 wt% Crz03 shows good lubrication properties from room temperature up to lOOO”C, as illustrated in Fig 9. Since the complex oxide, however, is a deliquescent substance, we need further research into more stable solid oxide lubricants. Recently, a solid oxide lubricant, in which the Na in the above solid oxide lubricant was replaced by Ba or Ca, showed greater chemical stability and stable lubrication of p = 0.3-0.4 up to 1oOO”c”-5. Takebayashi et al. 36 confirmed that S&N4 bearings with self-lubricating retainers made of a graphiteboron-chromium composite perform better at 500°C in air and at a rotational speed of 15 000 rpm.

of ceramics.

(b) Metallic

oxides

as detected

on sliding

Cr

Mn

Fe

co

Ni

cu

&I

Ni-Cr

C A A B

A A A C

A A A C

A A A C

A B B C

C C A A

C C A C

A A C B

A: fair W, i 3 x lo-’ mgim, very little damage B: good W, < 3 x 10m6 mg/m, slight damage C: no good W, > 3 x 10ee mg/m, substantial damage lb)

Cr

Mn

Fe

co

cu

Ag

Ni-Cr

Si,N,

CrAL

MM3 Mn304

cu-Fe,O,

Co,SiO, cowo,

&I SiO,

SIC

CrA

MnJL MM,

cu-Fe,O, SiO,

Co,SiO, SiO,

cue SiO, cue cu,o SiO,

NiO NiCr,O, NiO CrA NiCr,O, Ni,Si

ZrO,

Cr203

wFe203 wFe,O, (Fe,AI 1203 FeAI,O, cy-Fe,O,

co304 co3Ql CoAI,O, coo co,o*

cue cue CuAIO, CuAI,O, cue cu,o

Ag

40,

(700°C -+I Mn,O, MnAI,O, CrJL

Tribology

International

MM,

Volume

(Ag) SiO,

Ag Ag

28 Number

NiO NiCr,O, NiAI,O, NiO NiCr,O,

1 February

1995

5

Ceramic

tribology

in Japan:

Y. Enomoto

Conclusions Recent studies on the tribological behaviour of engineering ceramics have demonstrated that unique tribological interactions with environmental species often significantly affect the friction and wear characteristics of engineering ceramics. These findings suggest that such characteristics of engineering ceramics may be utilized for tribomaterials under certain special conditions. Newly developed micro-analytical tools are expected to reveal the nature of tribo-induced physiochemical interactions at the atomic and molecular level, thus increasing the reliability of tribomaterial design.

16.

17.

18.

19.

20.

Ceram.

Acknowledgments The author wishes to thank Professor Y. Kimura the opportunity to present this review paper.

for

References 1. Enomoto Y., Yamanaka K. and Saito K. Some applications of cathodoluminescence mode in a scanning electron microscope for studying the wear behavior of ceramics. Wear 1986, 110, 239-250 2. Enomoto Y. Studies of deformation and fracture behavior of brittle solids under sliding contact. Reports of Me&. Eng. Lab. 1992, No. 158 3. Kimura Y., Okada K. and Enomoto Y. Sliding damage of silicon nitride in plane contact. Wear 1989, 133, 147-161 4. Kimura Y. and Shima M. A fracture-mechanics approach to ceramic wear. To be published in Tribology Transactions 5. Hokkirigawa K., Kato K., Kitsunai I-I. and Mizumoto M. Microscopic wear mechanism of alumina and silicon nitride observed in the FE-SEM tribosystem. Proc. Japan Internat. Tribology Conf., Nagoya, 1990, pp. 1413-1418 6. Hokkirigawa K. Wear mode map of ceramics. Wear 1991, 151, 219-228 I. Sasaki S. Effects of environment on friction and wear of ceramics. Bull. Mech. Eng. Lab. 1992, No. 58 H., Enomoto A. and Sasada T. Friction and wear of 8. Hiratsuka A1203, ZrO, and SiOz rubbed against pure metals. Wear of Materials

1991,

Tribology

International

Sot.

1992,

75, 3075-3080

21. Kitaoka S., Tsnji T., Kato T. and Yamaguchi Y. Tribological characteristics of S&N, ceramics in high temperature and high pressure water. Proc. JAST Tribology Conf., Tokyo, May 1993, pp. 90-94 22. Hibi Y. and Enomoto Y. Tribochemical wear of silicon nitride in water, n-alcohols and their mixture. Wear 1989, 13, 133-145 23. Hibi Y. and Enomoto Y. Friction and wear of silicon nitride in water, n-alcohok, water-methanol and water-gIyco1. Bull. Mech. Eng. Lab. 1990, 53, 1-15 24. Hibi Y. and Enomoto Y. Tribochemistry of alcohol/silicon-based ceramics. Proc. of JAST Tribology Conf., Morioka, October 1992, pp. 353-356 25. Hisakado T. and Motoi M., Oguro T. and Ariyoshi H. Friction and wear mechanisms of ceramics in ethanol. 26. Sasaki S. Effects of tribo-electrification on wear particle behavior of ceramics, Wear particles: from the cradle to the grave. Proc. 18th Leeds-Lyon Symp. Tribology, 1992, pp. 469-475 27. Enomoto Y. and Hashimoto H. Fractoemission during indentation fracture of brittle solids. J. Mater. Sci. Lett. 1989, 3, 1107-1109 28. Enomoto Y. and Chaudhri M.M. Fracto-emission during fracture of engineering ceramics. J. Amer. Ceram. Sot. 1993, 76, 2583-2587 29. Enomoto Y. et al. Exoelectron emission: possible relation to seismic geo-electromagnetic activities as a microscopic aspect in geotribology. Wear 1993, 168, 135-142 30. Nakayama K and Hashimoto H. Triboemission of charged particles and photons from wearing ceramics in various gases. Tribology

1, 375-381

9. Ogawa T., Konishi K., Aihara S. and Sawamoto T. Development of ball bearing with solid film for high-vacuum, high-temperature, high-speed application. ASLE Preprint No. 92-TC-lC-3, 1992 10. Itoh H., Yatutani K. and Naka M. Particle-dust emission characteristic from ceramic and film-coated bearings (translated from the Japanese). Proc. JAST Tribology Co&, Tokyo, 1992 May, pp. 339-342 1991, 11. Mori S. Adsorption and lubrication. Jap. .I. Tribology 36, 161-168 12. Kawamura H. J. Ceram. Sot., Internat. edn 1989, 97, R13-R20 13. Tsuya, Y. et a/. Compatibility of ceramics with oils. Proc. JSLE Internat. Conf., Tokyo, 1985, pp: 167-170 14. Imazumi N. and Hata H. Effects of phosphorus compound additives on the lubrication between silicon nitride and steel. Proc. Internat. Conf. Advanced Materials, Tokyo, 1993, to be published 15. Tomizawa H. and Fischer T.E. Friction and wear of silicon nitride and silicon carbide in water: hydrodynamic lubrication

6

at low sliding speed obtained by tribochemical wear. ASLE 1986, 30, 41-46 Hihi Y. and Enomoto Y. Lubrication of S&N, and Al,O, in water with and without addition of silane coupling agents in the range of 0.05-0.10 molll, Tribology International, to be published Hibi Y. and Enomoto Y. Improvement of water lubricity of silicon nitride by using silane coupling agents containing amino groups. Jap. J. Tribology 1990,’ 35, 719-732 Mizuhara K. and I&u S.M. Tribochemistry of silicon-based ceramics under aqueous solution, Proc. EUROTRIB Conf. Budapest, 1993. Kimura Y. and Okada, K. Tribological behaviors of silicon nitride in the presence of water. Proc. 5th Nordic Symp. on Tribology, 1992, pp. 98-106 Kitaoka S., Yamaguchi Y.and Takahashi Y. Tribological characteristics of a-alumina in high temperature water. J. Amer. Trans.

Trans.

1992,

35, 643-650

31. Yamamoto Y. and Ura A. Influence of interposed wear particles on the wear and friction of silicon carbide in different dry atmospheres. Wear 1992, 154, 141-150 32. Wang H., Kimura Y. and Okada K. Sliding friction and wear of ceramics at elevated temperatures up to 1000°C. Proc. Japan Internat. Tribology Conf., Nagoya, 1990, pp. 1389-1394 33 Shimura Y. and Mizutani Y. Wear of ceramics at high temperatures and its improvement by metallic coatings. In: Wear of Materials, Ludema, K.C. and Bayer R.G. (Eds.), 1991, pp. 405-410 34. Umeda K. and Enomoto Y. Search for oxide solid lubricants usable from room temperature to 1000°C in air. Proc. EUROTRIB, Budapest, 1993, pp. 222-227 35. Umeda K. and Enomoto Y. Solid lubrication of double and tertiary oxides in air at room/elevated temperatures. Proc. JAST Tribology Conf. Nagoya, 1993, pp. 415-468 36. Takebayashi H., Yuine T. and Yoshioka T. Performance of ceramic ball bearings at high temperature. Jap. Sot. Tribologbts 1993, 38, 83-90

Volume 28 Number 1 February 1995