During the last few years, ceramics have become interesting for use in various tribological applications, such as roller bearings, cam roller followers and valve engines. In hybrid ball bearings ceramic balls of silicon nitride are used together with steel races and steel cages. Important properties of the ceramic balls are low porosity, a uniform microstructure and high fracture boughness. The objective of this work was to study cyclic contact fatigue of silicon nitride and the influence of different environments, such as water, air and ethylene glycol. In an MTS 880 Mechanical Test Machine a tungsten carbide ball was cycled on a disc of silicon nitride. During the experiments two commercial silicon nitrides for ball bearings were tested. The applied load was between 120 and 3500 N, and the number of cycles was varied between 100,000 and 5,000,000, whereas the frequency was kept constant at 10 Hz. The materials were characterised regarding their microstructure, porosity, hardness and fracture toughness. Crack initiation, crack propagation, different types of cracks and possible volume losses were studied in a light optical microscope (LOM), a scanning electron microscopy (SEM) and an interference microscope (IM). Tests performed in water resulted in deep and large indentations with few cracks on the silicon nitride specimens, whereas when testing in air the wear scars were smaller but there were more and longer cracks. A test environment of ethylene glycol resulted in very deep indentations but no cracks.
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