Tribological properties and hardness of silicon nitride ceramics after ion implantation and subsequent annealing

Silicon nitride ceramic (beta-Si3N4) with Y2O3 and Al2O3 as sintering additives was implanted with Cr or Ti ions to a fluence of 10(17) cm(-2) at energies ranging from 200 keV to 2 MeV. Changes in the phase composition in the near surface layer due to the implantation and subsequent annealing at 1000 and 1200 degrees C and their correlation with the behaviour of hardness and wear were investigated. Implantation results in amorphization of the near surface layer, which is buried for MeV implantations. For Ti-implanted samples, annealing leads to oxidation, resulting in the formation of gamma-YSi2O7 and cristobalite besides the beta-Si3N4. In the case of Cr implantation with MeV energies, the amorphous layer recrystallizes to alpha-Si3N4 already at 1000 degrees C catalyzed by the chromium. This causes a significant suppression of the oxidation. Consequently, the hardness and the tribological behaviour of Ti- and Cr-implanted samples, respectively, are affected differently by the annealing process. For chromium, the improved wear behaviour due to the high-energy implantation remains after annealing while the reduced hardness as result of the amorphized surface is partly recovered. For titanium, the oxide formation leads to a poorer hardness and tribological behaviour. (C) 1998 Elsevier Science S.A. All rights reserved.