Oxidation behavior of tantalum in high temperature molecular and dissociated oxygen

Abstract

Tantalum (Ta) was oxidized in a new resistive heating system equipped with a DC microplasma capable of heating samples to 2400 °C via Joule heating in a molecular or dissociated oxygen environment. Oxidation was conducted at 1300 °C, 1400 °C, and 1500 °C for 2.5 and 5 min in molecular or partially-dissociated 1 % O₂ – balance Ar. The presence of atomic oxygen is verified through emission spectroscopy and quantified via Kapton HN erosion studies. The degree of dissociation is estimated as ∼40 %. In molecular oxygen, the thickness of the oxide grown on Ta decreased as temperature increased, suggesting the formation of a more protective scale. Gas-phase diffusion through cracks in the oxide is identified as the rate-limiting process. For oxidation in atomic oxygen at 1300 °C and 1400 °C, the highly cracked oxide provided no barrier to atomic oxygen ingress and the extent of oxidation increased ten-fold. At 1500 °C, the more protective oxide scale prevented rapid atomic oxygen ingress, and the oxide thickness is unchanged relative to that observed in molecular oxygen. Atomic oxygen exposures are effective for identifying non-protective scales.