Effect of fretting conditions on the wear of silicon nitride against bearing steel

Lubricated and unlubricated wear tests were conducted with silicon nitride against bearing steel couples under fretting conditions. The effect of fretting amplitude (5-50 mu m), test duration at constant frequency and load on the tribological behaviour was evaluated according to wear volume, coefficient of friction, as well as the wear mechanisms involved. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were employed to determine the detailed nature of the friction and wear processes. It is suggested that wear under lubricated conditions is primarily related to the tribochemical reaction between silicon nitride and iron, which is the main cause of cratering on the steel counterpart, while wear of the ceramic part is relatively low. The wear rate is controlled by the nature of the tribolayer which is created, and this is a function of the sliding speed which influences the temperature at contact. At lower amplitudes the layer is brittle, while above 25 mu m it becomes more plastic which hinders the wear process. Under unlubricated conditions, the wear is much more severe and increases exponentially with amplitude.