Alteration of oxidation behaviour of silicon carbide by aluminium implantation

Abstract

Surface modification of ceramics by ion implantation is a fruitful area of research, giving the prospect of improving their surface-sensitive properties such as oxidation and wear resistance, hardness, and fracture toughness, while retaining the desirable bulk properties. In the case of oxidation, the surface chemistry of ceramics can be so tailored as to favour conditions for enhanced oxidation stability. These conditions include formation of an oxide barrier layer, neutralization of detrimental effects of bulk or environmental impurities, and modification of oxide defect concentration and conductivity. The effects of ion implantation on the oxidation behaviour of ceramics, mainly silicon nitride and silicon carbide, have been shown to be complex [1, 2]. Noda et al. [1] investigated the oxidation behaviour of sintered silicon nitride implanted with chromium to doses in the range 1 × 10175 × 1017 ions/cm 2 at 200 keV. It was shown that, at temperatures up to 1100 °C, a stable chromia layer was preferentially formed and acted as a good diffusion barrier to inward diffusion of oxygen and outward diffusion of impurity cations present in the sintered specimens. Chromium implantation markedly increased the oxidation resistance of silicon nitride. Above 1100 °C, little difference was observed in the oxidation behaviour between the implanted and unimplanted specimens due to rapid vaporization of chromium oxide. Studies of chromium-implanted silicon carbide by McHargue et al. [2] yielded an opposite outcome, however. It was revealed that silicon carbide implanted with chromium to doses in the range 6.2 × 1015-2.7 × 1016 ions/cm 2 at 95-280 keV exhibited significantly faster chemical-etching and oxidation rates than unimplanted samples~ These results were attributed to the amorphization of SiC by chromium implantation [2]. The adverse effect of implantation-induced structural changes on oxidation was also shown by Due t al. [3] in their oxidation studies of self-implanted single crystal silicon carbide at 1100 °C. Documented studies which investigate the potential of ion implantation to improve the oxidation resistance of silicon carbide and silicon nitride ceramics have been focused on using chromium as the dopant [1, 2]. In principle, any dopant may be considered to have a potentially beneficial effect on the oxidation stability if the dopant preferentially