Microstructural observation of the ZrC/ZrO 2 interface formed by oxidation of ZrC
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Microstructural observation of the ZrCyZrO2 interface formed by oxidation of ZrC Shiro Shimadaa) and Michio Inagaki Department of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan
Mikio Suzuki JEOL, Akishima 196, Japan (Received 25 February 1996; accepted 11 April 1996)
The oxidation of ZrC single crystals having (100) faces was performed at a temperature of 600 ±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 mm), 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 ZrO22x 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.
I. INTRODUCTION
In a previous paper, the oxidation of ZrC powders was studied from a kinetic viewpoint, and the retention of carbon in the oxide was inferred from observation that the degree of oxidation exceeds 100%.1 The retention of carbon during oxidation of ZrC powders was also reported by Dufour et al.,2 who explained that the presence of carbon stabilizes c-ZrO2 formed by oxidation of ZrC. Barnier and Thevenot investigated oxidation of zirconium oxycarbide powders of different compositions and reported a high carbon retention,3 but did not determine how much carbon was formed nor where it was deposited. The present authors have already reported the microstructure of c-ZrO2 scale and determined the amount of carbon formed in the oxide upon oxidation of ZrC single crystals, using electron microscopy, x-ray microanalysis, and Raman spectroscopy.4 It was demonstrated that the oxide scale consisted of two regions, zones 1 and 2, which contained 14 to 23 at. % and 7 to 10 at. % carbon, respectively. However, the microstructure of the ZrCyzone 1 (ZrO2 ) interface could not be determined, because of the difficulty of preparing an extremely thin a)
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2594
http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 10, Oct 1996
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