Instability in the tribochemical wear of silicon carbide in unlubricated sliding contacts

The present study is based on tests with sintered silicon carbide sliding unlubricated on itself in point (pin-on-disc), line (journal bearing) and plane (mechanical face seal) contacts. Tribo-oxidation and surface fracture were identified as the dominating deterioration mechanisms. The oxidation products formed were silicon dioxide and, within narrow operational regimes, silicon monoxide. Part of the silicon dioxide wear debris was compacted under frictional heating to form smooth tribofilms on the mating surfaces, providing protection against excessive wear; the corresponding specific wear rates ranged from 10⁻⁶ to 10⁻⁵ mm³ N⁻¹ m⁻¹. The silicon monoxide, when formed, appeared as a loosely attached powder which provided no protection against wear, as indicated by the one order of magnitude increase in the wear rates. The tribochemical instability has a potential to disturb the otherwise favourable SiC/SiC applications. The highest wear rates occurred in the pin-on-disc configuration, while the lowest rates were obtained in the journal bearing tests.