Lubrication of silicon nitride sliding contact

Ceramics International 20 (1994) 277-282

Lubrication of Silicon Nitride Sliding Contact J. E. Lovern & T, A. Stolarski Department of Mechanical Engineering, Brunel University, Uxbridge, Middlesex, UK, UB8 3PH (Received 24 July 1993; accepted 17 September 1993)

A~tract: Engineering ceramics are perceived as very hard, wear-resistant

materials able to operate at elevated temperatures. However, there are cases where the friction in a ceramic sliding contact is the most important factor. Therefore, the method of securing low friction through effective lubrication of ceramic sliding contacts is of practical importance. The results of studies into the effectiveness of lubricants normally used for metallic contacts in lowering the friction of a silicon nitride sliding contact are presented. The results indicate that ceramic contacts, to be effectively lubricated, might require specially formulated lubricants.

INTRODUCTION

Most of the publications on tribology of engineering ceramics concern their wear characteristics. Much less attention has been given to friction properties of ceramics and, even less has been done regarding their lubrication. Papers by Studt ~2 and Erdemir e t aL 13 are among the few publications devoted exclusively to the problem of ceramic lubrication. However, there are numerous engineering applications where ceramics operate in the presence of a fluid in the form of a lubricating oil or a process liquid, the main function of which is to lower the friction--usually high in a dry ceramic sliding contact. Therefore, the effectiveness of lubrication of ceramics by some commonly used lubricants is of interest not only from the fundamental point of view but also from a practical one. The objective of the work reported in this paper was to investigate whether lubricants used for metal lubrication are suitable for ceramic lubrication. This was prompted by the study conducted by Erdemir e t aL,~3 which indicated that there is a need for the development of lubricants formulated specifically for the lubrication of ceramics. The studies reported here were carried out on both ceramic-ceramic and ceramic-steel contacts using three different liquid lubricants.

In recent years, there has been a growing interest in the use of engineering ceramics as a material for machine components. These materials offer substantial advantages compared with more established materials such as metals and polymers. Useful mechanical, thermal and chemical properties make ceramics an attractive material for applications where extreme loads, temperatures or aggressive agents are the main consideration. In many cases the tribological properties of ceramics, such as wear and corrosion resistance, make them particularly suitable for contacting elements in relative motion. An increasing number of investigations into the tribological properties of ceramics have been published recently. Basic tribological tests on ceramics were carried out by Buckley and Miyoshi, 1 who found that abrasive wear of ceramics increases with grit size. Silicon nitride (Si3N4) was studied by a number of researchers, among them Adewoye,2 Fisher and Tomizawa, 3 Cranmer, 4 and Enomoto e t al. 5 Alumina (A1203) has been studied by Czichos e t al., 6 Swain 7 and Libsch e t al. 8 Wear of silicon nitride resulting from surface fatigue caused by cyclic loading has been extensively studied by Hadfield and coworkers. 9-11 277

Ceramics International 0272-8842/94/$7.00 © 1994 Elsevier Science Limited, England and Techna S.r.l. Printed in Great Britain

278

J. E. Lovern, T. A. Stolarski

EXPERIMENTAL PROCEDURE

for this technique are magnesium and yttrium oxides. Other additives, namely titanium, tantalum and zirconium, may also be used to customise material properties. Preformed 'green' ball blanks were produced by die compaction or cold isostatic pressing after blending, milling and agglomeration operations. The geometry of preformed blanks may be improved at this stage by soft machining using standard operations. Densification of ball blanks was achieved by HIP at 200-300 MPa and 1750-1900°C. This method of silicon nitride densification is preferred to other methods such as hot-pressing, gas pressure sintering and pressureless sintering, because greater control and superior quality of components can be achieved. The stoichiometric molecular weight of silicon nitride used was 140 g/mol with 60-40% of silicon to nitrogen. Typical mechanical and physical properties of the silicon nitride used are summarised in Table 1. The dimensional precision and the accuracy of shape of the balls used were the same as those specified for balls used in commercial hybrid ceramic rolling contact bearings. By turning every ball by a small angle before each test run, the contact between them was established at different points. In this way, it was possible to use the same set of balls for all friction measurements for a given lubricant.

The friction between three stationary balls and one rotating ball in contact with them under specified load and speed was the main parameter used to assess the effectiveness of various lubricants considered. For ceramic--ceramic contact all four balls were made of silicon nitride, whereas for ceramic-steel contact the stationary balls were made of silicon nitride and the rotating ball was a steel ball.

Apparatus A high-speed four-ball apparatus previously used to study rolling contact fatigue of ceramic balls 9 was modified tO create sliding contact. This was achieved by immobilising the three lower balls while the fourth, upper ball, fixed into the spindle, was driven by a thyristor-controlled electric motor. The cup accommodating the three stationary balls and the lubricant was supported by a thrust rolling contact bearing. The force required to rotate the cup was taken as being equivalent to the force necessary to overcome the friction between the four balls in contact. This rotational force was measured using a torque arm and a system of strain gauges. Figure 1 is a schematic diagram of the essential elements of the apparatus. A comprehensive description of the apparatus has been given elsewhere. 9

Lubricating liquids Three test lubricants were used, as described in Table 2. They are: (1) a highly refined straight naphthenic mineral oil, Shell Talpa 20; (2) a low-viscosity synthetic oil, Exxon 2389; (3) a high-viscosity hydrocarbon research oil, BP HiTec 174 with a package of additives.

Specimens The specimens used were silicon nitride balls 25.4 mm in diameter, with a surface finish, R a of 0.008/~m. The balls were prepared by hot isostatic pressing (HIP) followed by low-pressure sintering of the 'green' powder. The basic additives required

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Lubrication of silicon nitride sliding contact

279 Table 2

Table I Property

Unit

Density Elastic modulus Poisson's ratio Compressive strength Hardness (HV5) Toughness Thermal expansion

g/cm 3

GPa -MPa kg/m 2 MPa/m 10°C

Range

3.0-3.5 280-330 0.23-0.29 2500-4000 1400-1900 3-7 2.8-3.8

Recommended foruse

Lubricant

3.25 310 0.26 3500 1600 6.5 3-20

Talpa 20 Exxon 2389 HiTec 174

Test procedure Before each test run, all balls and the cup containing them were cleaned using an ultrasonic cleaner filled with the laboratory-grade solvent Genklene. Next, the cup was press fitted into the housing and the lower three balls were placed inside it. The cover (see Fig. 1) was fitted to the cup and screwed down tightly to immobilise the balls. The cup was then filled with test lubricant to a level that was just above the point at which the lower balls contacted the upper ball. The upper ball was fitted into the collet and placed in the spindle. The housing with the cup and the friction measuring arm attached was placed on the rolling contact thrust bearing inside the apparatus, and the strain gauges were then connected to the strain gauge bridge. The driving motor was switched on, the required speed set, and the load in the form of a dead weight was applied to the loading arm. A strain recording was taken every 250 s. A typical test duration was 750 s. On completion of the test run, the balls were removed from the cup

Specific gravity at 15°C

Flash point (°C)

0.899 0.955 0.950

216 220 255

Kinematic viscosity (cSt)

40°C

100°C

94.6 12-5 200-0

8.8 3.2 40.0

and thoroughly cleaned. The cup and collet were also cleaned and made ready for another test run. Each of the three lubricants was tested over a range of speeds starting at 100 rev/min with increments of 50 rev/min up to 300 rev/min. Throughout these tests, the load was kept constant at 2 N. Each test lubricant was used to lubricate three different ball material combinations. One combination was with all ceramic balls, another was with one upper ceramic ball and three lower steel balls (hybrid contact). Finally, a combination with four steel balls was used for comparison. A further set of tests was carried out using the base oil (Talpa 20), where the load was increased from 2 N in steps of 1 N up to 6 N while keeping the speed constant at 200 rev/min. All three combinations of ball materials were tested. EXPERIMENTAL

RESULTS

Friction curves for the investigated material combinations, lubricants and test conditions are given in Figs 2-5. Figure 2 shows the coefficient of friction as a function of rotational speed at a constant load of 2 N for Talpa 20. In all cases of material combinations tested, the relatively high value of friction

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280

J. E. Lovern, T. A. Stolarski 0,14 -

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Fig. 3. Coefficientof friction as a function of speed at constant load. coefficient initially decreases with speed. This is especially true for contacts consisting of four ceramic balls and one ceramic ball plus three steel balls, where the minimum of friction is reached in the region of 200-250 rev/min. Friction increases rapidly with speed. There is no pronounced minim u m in the value of friction coefficient for the four steel balls contact. In this case, the friction is continuously decreasing with speed. Figure 3 presents the coefficient of friction as a function of rotational speed at a constant load of 2 N for HiTec 174. As in the previous case, friction decreases with increasing speed, reaching a minimum at about 250 rev/min for the four ceramic balls contact and one ceramic plus three steel balls

contact. Beyond that speed, there is increase in friction with speed. Again, the four steel balls contact does not have any pronounced friction minimum as it is continuously decreasing with speed. Friction coefficient vs rotational speed for Exxon 2389 is shown in Fig. 4. The same trend as for the other lubricants can be observed. In the case of the four ceramic balls and one ceramic plus three steel balls contacts, friction decreases as the speed increases and, after reaching a minimum at 200 rev/min, starts to increase with further increase in speed. The four steel balls contact behaves differently this time. There is a continuous decrease in friction with speed until 250 rev/min; it then starts to increase very slightly with speed.

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281

Lubrication of silicon nitride sliding contact 0.14

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The three lubricants tested do not give significantly different magnitudes of friction coefficient, although there are subtle variations. The most effective lubrication of the four ceramic balls contact is provided by Exxon 2389. The next in the ranking of effectiveness is HiTec 174, and the last one is Talpa 20. The relationship between coefficient of friction and load at a constant speed of 200 rev/min is shown in Fig. 5. Under these test conditions, the friction in contact of four ceramic balls lubricated with Talpa 20 steadily increases with increasing load. At a load of 5 N it stabilises, reaching a plateau. The one ceramic plus three steel balls and four steel balls contacts behave similarly. The friction coefficient initially increases with load, attaining a maximum at a load of 4 N; it then decreases with further increase in load. An interesting observation is the fact that at the low load (2 N), the friction in the four ceramic balls contact is almost three times less than that in the other two contacts investigated. DISCUSSION The friction curves shown in Figs 2-4 all exhibit the same general trend of decreasing friction with increasing speed. The curves for the all-ceramic and hybrid contacts reach a minimum point, then the friction rises again. This rather peculiar friction characteristic of the contacts studied can be explained as follows. At low velocities the predominant mode of lubrication is boundary regime. With the increase in speed, the separation of contacting surfaces improves, and all initial direct

micro-contacts between surface asperities are eliminated. The observed effect is a decrease in friction. As speed continues to increase, a change from thin film boundary regime to a thicker fluid film lubrication mode takes place. At this point, friction starts to rise with speed because of the increasing viscous shearing within the increasingly thick fluid film. However, an alternative explanation accounting for the rise in friction in the allceramic contact after the minimum friction point has been reached can also be offered. It is well known that the thermal conductivity of ceramics is much lower than that of steel and, as a consequence, the heat generated within the contact zone cannot be sufficiently dissipated by the ceramic balls. Therefore, the surface temperature may increase substantially, leading to the breakdown of lubricating film in localised regions of the contact zone. This, in itself, could cause the increase in friction observed during the experiments. It is clearly seen from the graphs presented that, at speeds lower than 150 rev/min, the all-ceramic contact produces the highest friction, followed by the hybrid contact. The friction in the all-steel contact is consistently the lowest, in the conditions used during the investigations. This is true for all three lubricants tested. The friction may be lower in the hybrid contact than in the all-ceramic contact, because there is more affinity between the steel surface and the lubricants. In other words, a stronger adsorbed boundary layer is presumably produced on a steel surface in hybrid contact, which accounts for the lower observed friction. Another factor which might have affected friction in the hybrid contact is the

J . E . Lovern, T. A. Stolarski

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extent of deformation of contacting materials. Steel is a softer material than silicon nitride and is, therefore, able to deform more under a given load, producing favourable contact conditions for effective lubrication. As was mentioned above, friction decreases with speed for all types of contacts studied. However, the friction in the all-ceramic and hybrid contacts reaches a pronounced minimum and then it starts to rise again, whereas that in the all-steel contact continues to decrease. The friction coefficient of the steel contact is always larger than that of the ceramic contact at the point where it reaches a minimum. This may be due to the steel ball surface finish being poorer than that of the ceramic ball, and hence a greater film thickness being required to prevent asperity contact. The effect of load on the friction coefficient is shown in Fig. 5. It is apparent that the hybrid and steel contacts have a very similar friction characteristic, whereas the friction characteristic of the ceramic contact is completely different. The change of friction coefficient with load is most pronounced for the ceramic contact, whereas the opposite is true for the steel contact. The hybrid contact follows closely the steel contact in that respect. The change in friction of the steel contact caused by increasing load seems to point to the fact that surface physico-chemistry of steel balls plays an important role in the process. As the load increases, the lubricant film becomes thinner generally, but in the case of the steel contact the strength of the boundary layer is sufficient to prevent direct metal-metal contact and thus increase in friction. The ceramic contact has a low friction coefficient initially and this means that a boundary layer has been formed on a very smooth surface. However, the strength of this boundary layer is apparently not adequate. As the load increases and the lubricant film becomes thinner, the boundary layer must become fragmented, allowing for ceramic-ceramic contact and consequently for the increase in friction. The hybrid contact behaves in a similar way to the steel contact, but the friction in this case is slightly higher than that in the steel contact. However, the differences are small and practically insignificant. This is, in a way, confirmation of the importance of the affinity of the lubricant to the surface lubricated. Clearly, ceramics are less effectively lubricated by a lubricant which is effective on a steel surface. The decrease in friction for both the steel and the hybrid contacts after the point of maximum friction can be attributed to the deformation of

the steel surface under the load applied. With the increase in load, the contact area grows rapidly, owing to elastic-plastic deformations. The initial point contact changes to surface contact, and this creates favourable conditions for the generation of a lubricating film. As a result, friction decreases although the load increases. It is not known how far this process could go, but presumably a load could be reached at which friction would start to rise again.

CONCLUSIONS The results reported in this paper allow the following conclusions to be reached: (1) under the test conditions adopted, the steel contact, as testified by the magnitude of the friction coefficient, is more effectively lubricated than the ceramic contact; (2) the base oil was found to be the least effective in the lubrication of the ceramic contact; (3) low-viscosity synthetic oil proved to be the most effective for lubrication of the ceramic contact; (4) the frictional characteristic of the hybrid contact is similar to that of the steel contact.

REFERENCES 1. BUCKLEY, D. AND MIYOSHI, K., Friction and wear of ceramics. Wear, 100 (1984) 333-53. 2. ADEWOYE, O., Frictional deformation and fracture in polycrystalline SiC and Si3N4. Wear, 70 (1981) 37-51. 3. FISCHER, T. & TOMIZAWA, H., Interaction of tribochemistry and microfracture in the friction and wear of silicon nitride. Wear, 105 (1985) 29--45. 4. CRANMER, D., Friction and wear properties of monolithic silicon-based ceramics. J. Mat. Sci., 20 (1985) 2029-37. 5. ENOMOTO, Y., KIMURA, Y. & OKADA, K., Wearing behaviour of silicon nitride in plane contact. Proc. Int. Conf. Tribology--50 years Anniv., London, 1-3 July, 1987. IMechE, pp. 173-8. 6. CZICHOS, H., BECKER, S. & LEXOW, J., Multilaboratory tribotesting. Wear, 114 (1987) 109-30. 7. SWAIN, M., Microscopic observations of abrasive wear of polycrystalline alumina. Wear, 35 (1975) 185-9. 8. LIBSCH, T., BECKER, P. & RHEE, S., Dry friction and wear of toughened zirconias and toughened aluminas against steel. Wear, 110 (1986) 263-83. 9. HADFIELD, M., Rolling contact fatigue of ceramics. PhD thesis, Brunel University, 1993 10. HADFIELD, M. & STOLARSKI, T. A., Delamination of ceramic balls in rolling contact. Ceramics Int., 19(3) (1993) 25-34. 11. HADFIELD, M., STOLARSKI, T. A. & TOBE, S., Residual stresses in failed rolling contact balls. Ceramics Int., 19(5)(1993) 131-42. 12. STUDT, P., Influence of lubricating oil additives on friction of ceramics under conditions of boundary lubrication. Wear, 115 (1986) 185-91. 13. ERDEMIR, A., AJAYI, O., FENSKE, G., ERCK, R. & HSIEH, J. H., The synergistic of solid and liquid lubrication on the behaviour of toughened ZrO2 ceramics. Triboloszv Trans.. 35 (1992~ 287-97