Oxidation and Strength Retention of Monolithic Si ₃ N ₄ and Nanocomposite Si ₃ N ₄ ‐SiC with Yb ₂ O ₃ as a Sintering Aid

The oxidation behaviors of monolithic Si 3 N 4 and nanocomposite Si 3 N 4 ‐SiC with Yb 2 O 3 as a sintering aid were investigated. The specimens were exposed to air at temperatures between 1200° and 1500°C for up to 200 h. Parabolic weight gains with respect to exposure time were observed for both specimens. The oxidation products formed on the surface also were similar, i.e., a mixture of crystalline Yb 2 Si 2 O 7 and SiO 2 (cristobalite). However, strength retention after oxidation was much higher for the nanocomposite Si 3 N 4 ‐SiC compared to the monolithic Si 3 N 4 . The SiC particles of the nanocomposite at the grain boundary were effective in suppressing the migration of Yb 3+ ions from the bulk grain‐boundary region to the surface during the oxidation process. As a result, depletion of yttribium ions, which led to the formation of a damaged zone beneath the oxide layer, was prevented.