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Titanium oxidation kinetics and TiO2 growth effects Rose, Daniel Joseph

Abstract

Titanium oxidation was studied between 400 and 800°C with oxygen partial pressures over the range 760 to 10⁻⁵ mm Hg and times up to 20,000 minutes. Oxidation rates were measured with a Cahn electrobalance after sophisticated metal preparation and experimental procedures. Growth effects in the oxide film were thoroughly examined at time intervals by removing specimens for metallographic observation. Films were stripped from these specimens for transmission electron microscope examination. The morphology development was then related to the rate curves. In all cases the rate curves were complex functions of temperature, pressure, specimen preparation, and oxidation procedure. In spite of the numerous rate transitions with time, reproducibility under a specific set of conditions was excellent. As the oxide grows from a thin film to a thick scale, an intermediate period of peculiar network formation, most pronounced at 500°C, is observed. The networks consist of thousands of small crystallites, approximately 100 to 5000 Angstroms in size, and arranged in random and oriented patterns of accelerated and suppressed growth areas. Two different types of whisker formation were observed in addition to the network phenomena. The complex rate data associated with periods of network growth do not conform to any of the simple rate laws reported by previous investigators. Since the network structures violated all of the assumptions associated with current transport theories regarding planar homogeneous films, it was concluded that these theories were inadequate to describe the growth kinetics. Surface diffusion is probably the main growth mechanism during network formation although orientation and epitaxial effects may have some influence.

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