A scanning electron microscope study of the surface morphology of TD-NiCr oxidized at 800°C to 1200°C

Abstract

Scanning electron microscopy was used to study the surface morphology of oxides formed on TD-NiCr at 800, 1000, and 1200° C for times up to 64 hr. Surfaces abraded with 120 grit oxidized to form Cr ₂O₃ which gradually developed large crystals of Cr₂O₃ (R~5 μm). The shape of the crystals suggested vapor growth. Surfaces polished through 0.5 μm diamond paste and oxidized had NiO on the surface except over grain boundaries and scratches which had Cr₂O₃ covering them. The NiO was faceted but showed little evidence of vapor growth. Wire samples of TD-NiCr 0.005 in. diam were oxidized at 1200°Cfor up to 50 hr at pressures from 10⁻⁵ to 700 Torr. The only oxide formed was Cr₂O₃. Extensive large surface crystals ofCr₂O₂ resulting from vaporization and condensation were observed at pressures of 10Torr and above, but not at lower pressures. Humid air oxidation altered the oxide morphology markedly.