Microtopography of Oxide Films Formed on Tantalum

Abstract

Optical and electron microscopy were used to characterize the surface of tantalum specimens oxidized at temperatures ranging from 300° to 700 °C. At the lower temperatures, the initial stages of oxidation were shown to consist of an aggregation of oxide platelets visible on the surface as crystallographically dependent, acicular structures extending from the grain boundaries. At higher temperatures, nodular growths were observed whose nucleation was not a grain‐boundary phenomenon. Continued oxidation at temperatures of 500 °C and above produced blistering and cracking of the oxide layer. The oxide platelets were shown to have grown during oxidation and not during cooling from the reaction temperature. At higher temperatures, both platelets and nodules served as preferred sites for blister and crack formation in the oxide and thus played an important role in the transition from the protective to the nonprotective stage of oxidation of tantalum.