The oxidation of zirconium: An electron microscopy study of zirconia formed in the thin-film region

Transmission electron microscopy of stripped oxide films formed by oxidation of zirconium in the interference color region, 250 to 450°C has shown that uniform film growth does not occur. Single crystal monoclinic zirconia films replicated the general grain structure of the metal substrate but also contained a large volume of structural heterogeneities which were independent of the substrate. Regions of finegrain, about 0.1 μ dia., polycrystalline oxide occupied about 5 % of the film area. Preferential oxidation of these areas resulted in massive mounds of oxide at longer times. Pseudo-amorphous patches also existed at lower temperatures and shorter times ranging in size from about 0.1 μ, to about 10 μ,. Continued oxidation of the amorphous regions gave rise to pustule formation upon breakdown of the patches. The pseudo-amorphous patches contained substructure in the form of "grain boundaries" and small disc-like crystals which sometimes changed contrast upon tilting in the microscope. Crystallization of the pseudo-amorphous regions by heating in the microscope occurred by the formation of a single layer of crystal plates, having a thickness less than the oxide film. The new crystals formed independently of the "grain boundaries," and the preexisting, disc-like crystals did not act as nuclei for crystallization. Prolonged heating eventually removed the boundaries. A tentative explanation for the presence of the boundaries is presented. The non-uniform nature of the thin films can readily account for the numerous rate laws for oxidation reported in the literature and the variations in oxidation behavior for supposedly identical conditions.