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

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 Cr₂O₃ which gradually developed large crystals of Cr₂O₃ (R~5 m). The shape of the crystals suggested vapor growth. Surfaces polished through 0.5 m 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.