High temperature oxidation kinetics of amorphous silicoboron carbonitride monoliths and silica scale growth mechanisms determined by SIMS

The highly dense, amorphous silicoboron carbonitride (Si-B-C-N) monoliths provide superior oxidation resistance above 1600 C-omicron, but the in-depth oxidation kinetics are unknown. Here oxidation kinetics of amorphous Si-B-C-N monoliths was investigated by one-step oxidation (in flowing O-16-rich air) over 1500-1700 C-omicron. Coincidently, the growth mechanisms of oxide scales on ceramic surfaces were studied by oxidizing ceramics in O-18-rich air followed by 160 where 160 was used as an equivalent isotopic tracer and the distributions of 160 and 180 in oxide scales were determined directly by secondary ion mass spectrometry (SIMS). The thickness changes of silica scales fit well to parabolic rules at 1500 C-omicron (32.5 mu m(2) h(-1)) and 1600 C-omicron (86.1 mu m(2) h(-1)) within 16 h, but are irregular at 1700 C-omicron. The calculated Arrhenius activation energy is ca. 116 kJ mol(-1) for 1500-1600 C-omicron/16 h oxidation. The relative concentration profiles of 160 from SIMS analysis indicate growth of oxide scales is mainly by oxygen lattice diffusion inside scales during oxidation of the amorphous Si-B-C-N ceramics.