Phase Transformations in Silicon Under Dry and Lubricated Sliding

Sliding friction and wear mechanisms of silicon/silicon nitride test pairs were investigated under conditions of both dry and lubricated sliding. High-resolution surface topography mapping and electron microscopy studies revealed that microfracture was the predominant wear mechanism under dry and grease-lubricated sliding conditions. Raman spectroscopy suggested that in certain areas of the sliding contact, silicon underwent phase transformation and reached a metallic state because of high contact pressures. The extent of phase transformation was greater during the very early stages of the run-in period than during steady-state sliding regimes. The use of grease and oil as lubricants led to a substantial reduction in friction and greatly diminished wear due to microfracture. Furthermore, almost all areas on Si surfaces subjected to lubricated sliding contact underwent pressure-induced phase transformation. Both amorphous material and crystalline Si phases were identified by Raman spectroscopy. The experimental observations suggested that the wear process in lubricated sliding contacts was mainly dominated by the formation of a ductile metallic Si phase and subsequent removal of the transformed layers. The results of this study demonstrate that pressure-induced phase transformation must be taken into account when considering possible wear mechanisms of silicon in contact with other hard materials, inasmuch as it contributes notably to the wear of silicon under lubricated sliding.