Role of Si 2 N 2 O in the Passive Oxidation of Chemically‐Vapor‐Deposited Si 3 N 4

The results of two‐step oxidation experiments on chemically‐vapor‐deposited Si 3 N 4 and SiC at 1350°C show that a correlation exists between the presence of a Si 2 N 2 O interphase and the strong oxidation resistance of Si 3 N 4 . During normal oxidation, k p for SiC was 15 times higher than that for Si 3 N 4 , and the oxide scale on Si 3 N 4 was found by SEM and TEM to contain a prominent Si 2 N 2 O inner layer. However, when oxidized samples are annealed in Ar for 1.5 h at 1500°C and reoxidized at 1350°C as before, three things happen: the oxidation k p increases over 55‐fold for Si 3 N 4 , and 3.5‐fold for SiC; the Si 3 N 4 and SiC oxidize with nearly equal k p 's; and, most significant, the oxide scale on Si 3 N 4 is found to be lacking an inner Si 2 N 2 O layer. The implications of this correlation for the competing models of Si 3 N 4 oxidation are discussed.