OXIDATION OF HAFNIUM CARBIDE IN THE TEMPERATURE-RANGE 1400-DEGREES TO 2060-DEGREES-C

After hafnium carbide has been oxidized at temperatures in the range of 1400-degrees to 2060-degrees-C, three distinct layers are present in the film cross section: (a) a residual carbide layer with dissolved oxygen in the lattice, (b) a dense-appearing oxide interlayer containing carbon, and (c) a porous outer layer of hafnium oxide. Experimental measurements of layer thicknesses and oxygen concentrations are combined with an extended formulation of moving-boundary diffusion theory to obtain the diffusion constants of oxygen in each of the three layers. The results indicate that the oxide interlayer is a better diffusion barrier for oxygen than either of the other layers. Based on X-rav microanalysis, X-ray diffraction, and resistance measurements, the interlayer is an oxygen-deficient oxide of hafnium with a carbon impurity. The interlayer hardness equals that of the residual carbide layer.