Oxidation of Polymer‐Derived HfSi CNO up to 1600°C

Oxidation behavior of HfSi CNO ceramics for Hf / Si ratio of 0.09 at 1400°C–1600°C in ambient air is reported. Quantitative X ‐ray analysis of oxidized powders shows crystalli‐zation of the amorphous phase into tetragonal hafnia, hafnon, and cristobalite (carbides, seen in inert atmosphere heat treatments are absent). Cross‐sectional SEM shows the oxide overgrowth on the particles to contain precipitates of hafnia/hafnon, while the interior of the particles is decorated with nanoscale grains of hafnia in a necklace‐like formation. The oxidation kinetics of these materials, determined both from weight‐change measurements and from direct observation of oxide overgrowth, are shown to be comparable to the oxidation of SiC single crystals. Oxidation of SiC – SiC minicomposites (straight fiber bundles infiltrated with a SiC matrix), coated with thin films of HfSi CNO prepared by dip‐coating was studied. The overgrowth thicknesses for oxidation time of 1000 h at 1600°C are compared for uncoated, SiCN ( O )‐coated, and HfSi CNO ‐coated minicomposites.