Oxidation of Hafnium Diboride-Silicon Carbide at 1500 degrees C in Air; Effect of Compressive Stress

The long-term oxidation behavior of HfB2 and of HfB2-20 vol.% SiC was studied. Test samples of each material were oxidized at 1500 degrees C in air using a box furnace. The exposure times were 0, 0.5, 1, 2, 3, 6, 9, 12, 15, 30, 45 and 90 h. Weight gain, oxide scale composition and oxide scale thickness were characterized for both materials. Crystal structure of the surface scales was analyzed using x-ray diffraction. Oxide scales were further characterized via scanning electron microscopy with energy dispersive spectroscopy analysis. For HfB2 the oxide scale consists predominantly of porous HfO2. For HfB2-20 vol.% SiC, the oxide scale is composed of a borosilicate glass outer layer and a porous HfO2 layer. Weight gain and the growth of oxide scale with exposure time were measured. The oxidation kinetics were determined using the weight gain as well as the scale thickness measurements, and the parabolic rate constants were calculated for both materials. The addition of SiC dramatically inhibited the oxidation of HfB2. The effects of compressive stress on oxidation of HfB2-20 vol.% SiC were also examined. Samples were oxidized while being subjected to compressive stress of 50-150 MPa for up to 30 h at 1500 degrees C in air. Compressive stress was found to have little effect on the growth of oxide scale with time. The oxidation data were analyzed in terms of mechanistic models for the oxidation of monolithic and SiC-containing refractory diborides. For HfB2-20 vol.% SiC, the model predictions agreed well with experimental data. For HfB2, the model significantly under-predicted the scale thickness, but accounted for weight gain reasonably well except for the longest exposure time of 90 h.