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The superior qualities of ceramics such as high hardness, chemical stability and wear resistance make them promising tribological materials for machine elements, for example, pumps, bearings and heat engines. Ceramics are hard and brittle, therefore, machining such materials is time-consuming, difficult and expensive. A low cost machining process that can remove material rapidly while maintaining a good surface finish is required. The general aim of the studies presented is to find a correlation between the wear mechanisms and surface modifications induced by interactions of tribochemical nature and to identify the most effective combination of parameters involved in producing high material removal rates of ceramics. This study concentrates on silicon nitride, as this material has most potential for use in industry. The influence of lubricant chemistry on the friction and wear of silicon nitride is investigated using a ball-on -plate tribotester and a modified four-ball machine. A wide variety of liquid lubricants and additives supplied by Castrol International are tested to evaluate the role of the chemical characteristics of the lubricant on the friction and wear of silicon nitride. Surface and chemical analysis results reveal that by using different chemistries of liquids, the material removal rate and the surface finish of the silicon nitride can be significantly altered. The highest material removal rate is obtained when using the ester base fluid T80884 + 0.3% triethanol amine. This gives an increase by four fold when compared to the material removal rate obtained with the industrial reference slurry Kemet. The topography of the silicon nitride ball after the grinding test is found to be very smooth indicating that the predominant mechanism of material removal rate could be due to a tribochemical reaction occurring at the contact interface.

Ceramic/ceramic and ceramic/steel contacts under lubricated rolling conditions are studied. This work is of interest to ball bearing manufacturers as the use of ceramics in the design of these components has some advantages over traditional bearing-steel materials. Low density and increased stiffness are the mechanical properties which gas-turbine and machine tool manufacturers are most likely to realise. Much research over the past two decades on material structure, quality control and manufacturing techniques has produced a material which can seriously challenge bearing steel in ball-bearing design. This is especially the case for hybrid ball-bearings, ie ceramic balls with steel bearing races which are now used as standard components. The purpose of this study is to examine the rolling contact fatigue failure modes of ceramics. This study concentrates on silicon nitride as this material has most potential for use by industry. The primary reason for studying ceramic balls is because of interest in ball-bearing applications, hence a modified four-ball machine is employed which correctly models ball motions and precisely defines ball load. Experimental and theoretical kinematic analysis of ball motion during modified four ball machine tests is presented. The kinematic analysis reveals that in practice, lower ball tracking exists at high speeds. Test conditions of lubricated contacts under high compressive stress show delamination type failures. Delamination failures are classified in terms of propagation and initiation from scanning electron microscope observations. Residual stresses are measured on delaminated surfaces, which implies plastic deformation of the ceramic. Also, chemical analysis implies that disruption of silicon, nitrogen and oxygen levels may take place on delaminated surfaces. Experiments illustrating various fatigue failure modes using artificially pre-cracked ceramic balls in contact with a steel upper ball are presented.

Two methods of measuring the dielectric properties of materials - matched termination and coaxial line resonance- have been developed and used to study the properties of two groups of nitrogen ceramics, namely, silicon nitrides and oxynitride glasses. In these methods advantage is taken of the wide frequency range, from 500 MHz up to about 9 GHz, covered by a single apparatus - the General Radio slotted coaxial line. Previous measurements in this Department have indicated the difficulties in the determination of the loss tangent of low to medium loss samples (tan Ỏ ~ 10(^-3) - 10(^-2)). The two methods developed reduced these difficulties. The applicability of these methods was assessed using known materials including the high loss liquids water and chlorobenzene, medium loss solutions of chlorobenzene in cyclohexane and the low loss solids polymethyl methacrylate and polytetrafluoroethylene. The silicon nitride ceramics were in various degrees of nitridation given by the weight gain which ranged from 38% for partially-nitrided to a maximum of 63.2% for the fully-nitrided samples. The dielectric constant at 1 GHz increased from 4.51 for fully-nitrided to about 9.9 for the 38% weight gain samples. The fully-nitrided material has a loss factor of 7.6 x 10(^-3); this increased to 1.85 x l0(^-1)as the weight gain decreased to 38%. These values have been extrapolated to 'zero weight gain' and compared with pure silicon for which the dielectric constant is 11.7 and the loss factor approximately 0.2. The oxynitrides have dielectric constants between 6.5 and 7.5 depending on the cation present and the percentage of nitrogen substituted for oxygen in the glasses. Є' increased in the cation order Mg, Y, Ca and increased with increasing nitrogen substitution in each cation series. The loss factor, however, depends not only on these cation types but also on the other constituents of the glasses. The dielectric constant of both the silicon nitride ceramics and oxynitride glasses fitted the Jonscher universal law of dielectric response (Є' - Є) w(^n-1), where the exponent n - 1 for all the samples. Similarly, the loss factor for these materials showed a frequency dependence Є" w(^n-1), again with n approximately 1. This is a limiting case of dielectric behaviour corresponding to a frequency independent loss where most dipolar processes have been eliminated.