Wear-Induced Changes in the Raman,Infrared,and Fluorescence Spectra of Silicon Nitride Surfaces

Commercial α-silicon nitride plates and balls used in simulated sliding bearing tests were found to fluoresce under 514.5-nm argon-ion laser excitation after heating to above 350 °C in a stream of argon. Wear track areas fluoresced much more intensely than their surroundings. When small concentrations of ethylene or other carbonaceous gas were added to the argon stream, carbonaceous deposits formed primarily in the wear track in amounts roughly paralleling the fluorescence intensity. Some of the deposits were lubricating carbon and some nonlubricating silicon oxycarbide. Continuous high-temperature lubrication was possible by balancing deposit removal by wear with surface-chemical deposit formation rates. Raman, infrared, fluorescence, and other spectroscopies helped explain the process. Defect energy states within the large silicon nitride energy gap and dangling bonds appear to play a role.