Engineering Ceramics for Tribological Systems. Manufacture, Properties and Testing

Doctoral thesis, 1997

A broad range of commercial and experimental grades of ceramics and ceramic composites was tested in various model tests and simplified component tests. The tests were done to establish important microstructural features, friction and wear mechanisms. Important tribological systems used in the tests included face seals, abrasive-waterjet nozzles and hybrid bearings. The importance of mechanical and physical properties at the micro-level has been highlighted in the work, as they play vital roles in the tribology of ceramics and are less commonly investigated than the bulk properties. Novel ceramic whisker composites were produced by hot-isostatic-pressing. The composites were based on a silicon nitride matrix reinforced with silicon carbide whiskers. The matrix of silicon nitride showed high strength and fracture toughness. But the composites showed no improvements in the bulk mechanical properties when compared with the matrix. This was mainly due to impurities in the whisker batch, causing defects and lower fracture toughness. A new type of silicon carbide material was made dense using liquid-phase sintering, and its wear behaviour was established and compared with conventionally solid-state sintered silicon carbides. The presence of grain boundary phases was both an advantage and a disadvantage regarding wear rates. At high wear rates the grain boundary phase limits the severity, probably due to crack deflection of the micro-fractures that occur. However, in mild wear the limited high-temperature properties of the grain boundary phases can be a disadvantage. Monolithic ceramics and composites with similar mechanical and physical properties to the bulk showed dramatically different wear behaviour in the tests. A newly developed, more homogeneous grade of whisker reinforced alumina showed superior wear resistance to the other monolithic ceramics and composites that were considered. A high-speed abrasive-waterjet test was developed in the work for wear testing, and was used to investigate the resistance of ceramics to thermal wear. The wear was caused by thermal spalling, oxidation, creep and plastic deformation. Abrasive-waterjet tests were done at low inclination angles, and newly commercialized machines were used. The tests highlighted the influence of thermal wear in the mixing nozzle for the abrasive and the waterjet, and the thermal mechanisms present while machining engineering ceramics. It was previously not known that an abrasive waterjet creates a thermal zone when machining ceramics at the wear front and in the mixing nozzles. The temperatures reached in the wear zone of the test specimens were at least 1280° C for the garnet abrasive, and 2050° C for the alumina abrasive. In scratch testing with diamond tips the presence of four different wear regimes was established for ceramics. On the tested materials a transition was seen from entirely ductile deformation at low loads, towards catastrophic brittle fractures at high loads. In scratch testing some materials, such as a hot-isostatic-pressed silicon nitride composite, showed much higher resistance to the initiation of brittle fracture. Ceramic composites reinforced with long fibres were studied to rank their sensitivity to particle impact. The test showed that composites with silicon carbide spheres on the surface were more resistant to the impact of abrasive particles. The spheres were present from the manufacturing process (chemical vapour infiltration). A study of thrust bearings was performed to compare all-steel bearings with hybrid ones. The hybrid bearing was of the same type as the all-steel one except the balls were of silicon nitride. The influence of contaminants in the bearings was investigated for fairly soft micron-sized (0.1-5.my.m titania) and macro-sized (75 100 .my.m quartz) ceramic contaminants. The hybrid bearing showed greatly superior performance when contaminants were present compared to the traditional all-steel type of bearing. It was shown that it is even possible to reduce friction and wear in a hybrid bearing system by the introduction of micron-sized titania particles.