The growth and structure of oxide films formed on single crystal (100) and polycrystalline Cr between 550 and 900 °C

The oxidation of polycrystalline and (100) Cr at temperatures from 550–900°C and 5 × 10−3 to 5 × 10−1 torr oxygen pressure, choosing surface pretreatments such that only adherent, uniform, pore‐free polycrystalline α‐Cr2O3 is produced, show long term parabolic growth kinetics with an activation energy ∽ 51 kcal mol−1. Oxide layers, produced by sequential oxidation in 18O2 and 16O2, were suitable for the application of polyatomic SIMS to elucidate the extent of oxygen transport in the growing oxides and the mechanism for their growth. The data show that the predominant growth mechanism is by outward cation transport, with a small yet significant amount of inward oxygen diffusion down oxide grain boundaries. ∽ 1% of the oxide is created within the film as a result of this inward diffusion. Oxygen diffusion coefficients were found to be orders of magnitude higher than oxygen self diffusion coefficients in Cr2O3 but much too low to account for the observed oxidation rates.