Dependence of Oxidation Modes on Zirconia Content in Silicon Carbide/Zirconia/Mullite Composites

Two basic oxidation modes of silicon carbide/zirconia/mullite (SiC/ZrO 2 /mullite) composites were defined based on the plotted curve of the gradient of the silica (SiO 2 ) layer thickness (formed on individual SiC particles) versus depth. Mode I, where oxygen diffusivity was much slower in the matrix than in the SiO 2 layer, exhibited a relatively large gradient and limited oxidation depth. Mode II, where oxygen diffusivity was much faster in the matrix than in the SiO 2 layer, displayed a relatively small gradient and an extensive oxidation depth. When the volume fraction of ZrO 2 was below a threshold limit, the composites exhibited Mode I behavior; otherwise, Mode II behavior was observed. For composites with a ZrO 2 content above the threshold limit, the formation of zircon (ZrSiO 4 ), as a result of the reaction between ZrO 2 and the oxidation product (i.e., SiO 2 ), might change the oxidation behavior from Mode II to Mode I.