Microstructural observation of the ZrC/ZrO2 interface formed by oxidation of ZrC

The oxidation of ZrC single crystals having (100) faces was performed at a temperature of 600 degrees C at an oxygen pressure of 2.6 kPa. A polished cross section of oxide scale was observed by backscattered electron imaging (BEI) in a scanning electron microscope (SEM). The oxide scale was observed to consist of two subscales, zones 1 and 2; zone 1 is adjacent to the ZrC. The thickness of zone 1 was kept constant (about 2 mu m), independent of oxidation time. The interfacial microstructure between ZrC and cubic ZrO2 (c-ZrO2) phase in zone 1 was observed by high resolution transmission electron microscopy, by using an extremely thin foil of an oxidized crystal. The c-ZrO2 crystallites of 2 to 10 nm in size showing the (111), (200), and (220) lattice fringes were aggregated and distributed in an area about 10 to 20 nm away from the interface, with an amorphous layer observed adjacent to the ZrC. Electron dispersive x-ray analysis (EDX) indicated that carbon is concentrated at the interface; a decreasing oxygen concentration gradient in the oxide phase and away from the interface in the ZrC suggests the formation of oxygen-deficient ZrO2-x and oxycarbide on the respective sides of the interface. A black coating layer appeared, resulting from detachment or dissolution of zone 2, when a crystal oxidized for 1 h was treated in a HF solution. The layer was shown by the Raman spectrum to be amorphous carbon.