EFFECTS OF ACTIVE OXIDATION ON THE FLEXURAL STRENGTH OF ALPHA-SILICON CARBIDE

The effects of oxygen partial pressure (Po2) 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 (Po2 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 Po2 level. When the Po2 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 Po2 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 Po2. An approximately 50% reduction in strength was observed in the SiC after oxidation for 20 h at a Po2 of 1.5 × 10−5 MPa, a level that is slightly lower than the Po2 at which the transition from active to passive oxidation occurs. Large pits formed during exposure were responsible for the reduction in strength. Copyright © 1990, Wiley Blackwell. All rights reserved