Oxygen tracer studies of the oxidation of niobium

Abstract

SIMS was used to locate O¹⁸ in oxide scales grown on niobium specimens oxidized at 600, 700, 780, and 825°C. Analyses were performed in depth profile from the oxide-gas interface and in step-scan across polished sections. The measured distributions confirm that oxygen is the most mobile species in T -Nb₂O₅ and gave no evidence that pore diffusion was important in the growth of compact scales. In these scales, which grow essentially at a parabolic rate, O¹⁸ had a sigmoidal profile across the scale, plateauing near the oxide-gas interface. The O¹⁸ content of the plateau was typically less than 90% of the O¹⁸ content of the oxidizing atmosphere. At temperatures below 720°C the measured O¹⁸ distributions are compatible with a paralinear growth mode of the layered scales. In this mode the reaction rate is controlled by the diffusional properties of the oxide layer in contact with the specimen.