The Effect of Stress on the Low-Temperature Oxidation of Niobium

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Abstract
Oxidation, cooling, dissolution, and annealing flexure measurements were performed on thin coupons of niobium presaturated with oxygen. Oxidation was done near 425°C in dry oxygen, dissolution in hydrofluoric acid, cooling and annealing in vacuum. From curvature measurements during each process, bending stresses were determined and correlated with oxidation rate and surface morphology. A thin‐film model was applied to the bending due to niobium pentoxide formation. The resulting calculated compressive stress of about 123,000 psi agreed well with the known compressive strength of . Application of the thin‐film model to an “effective” film of niobium suboxide gave compressive stresses of 125, 150, and 177 thousand psi for , , and , respectively, the reported formulas for . Suboxide formation was found to cause bending during early stages of the oxidation process. An oxygen solution model did not adequately explain bending during early oxidation times for oxygen‐presaturated niobium specimens. The resulting tensile stress on the substrate surface caused by oxidation was found to be significant, possibly approaching the yield stress.

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