Oxidation behaviour and strength degradation of a Yb₂O₃−SiO₂−doped hot-pressed silicon nitride between 1200 and 1500°C

Abstract

The oxidation behaviour and strength degradation of a Yb₂O₃−SiO₂−doped hot-pressed silicon nitride ceramic were investigated in air for up to 100 h at temperatures between 1200 and 1500°C. The parabolic weight gain versus oxidation time was measured. The oxidation products consisted of Yb₂Si₂O₇ and SiO₂ (α-cristobalite). The apparent oxidation activation energy Q was determined to be approximately 285kJ mol⁻¹. The high oxidation resistance observed was attributed to the raised refractoriness of the grain-boundary phase and the minimization of the amorphous phase by extensive devitrification during the sintering process. After oxidation, it was found that the room-temperature flexural strength of the material degraded and this degradation occurred during the early stage of oxidation. The decrease in the flexural strength was associated with the formation and coarsening of defects at and/or near the interface between the oxide layer and the Si₃N₄ bulk.