Wear of silicon nitride ceramics under fretting conditions

The wear behaviour of isostatically hot pressed silicon nitride under lubricated fretting conditions was analysed in ball-on-flat geometry. The coefficient of friction and wear rates were measured as a function of time and amplitude in the range of 5 to 50 mu m. Wear scars were examined by SEM and TEM in order to identify the wear mechanisms involved. Wear rate and the coefficient of friction were found to be dependent on test duration as well as on amplitude. Accordingly, evidence for different wear mechanisms was obtained. Under lubricated conditions the highest wear rate was observed during the initial stage of the test and was caused by brittle fracture of the silicon nitride grains. In the next period tribochemical reaction of the mechanically damaged surface layer dominated. The mechanism of further wear depended on sliding speed and consequently on the temperature at contact. At lower amplitudes spalling of the created tribofilm prevailed, while at amplitudes of above approximately 30 mu m plastic flow of the layer seems to be the mechanism responsible for the lower wear rate.