Effects of Active Oxidation on the Flexural Strength of α-Silicon Carbide

The effects of oxygen partial pressure ( P _o2) on the oxidation behavior and room-temperature flexural strength of sintered α-SiC were investigated. Groups of flexure bars were exposed at 1400°C to flowing Ar containing various levels of oxygen ( P _o2 ranging from 7.5 × 10⁻⁷ to 1.5 × 10⁻⁴ MPa). The changes in weight, flexural strength, and surface morphology of the samples were strongly influenced by the P _o2 level. When the P _o2 was higher than 3 × 10⁻⁵ MPa, SiO₂ was formed on the surface (i.e., passive oxidation occurred) and the strengths of the samples were not significantly affected. However, when the P _o2 was lower than 2 × 10⁻⁵ MPa, material loss occurred (active oxidation), decreasing the weight and strength of the samples. Both the reduction in strength and the weight loss resulting from active oxidation were proportional to the P _o2. An approximately 50% reduction in strength was observed in the SiC after oxidation for 20 h at a P _o2 of 1.5 × 10⁻⁵ MPa, a level that is slightly lower than the P _o2 at which the transition from active to passive oxidation occurs. Large pits formed during exposure were responsible for the reduction in strength.