FORMATION AND MICROSTRUCTURE OF CARBON-CONTAINING OXIDE SCALES BY OXIDATION OF SINGLE-CRYSTALS OF ZIRCONIUM CARBIDE

The isothermal oxidation of ZrC single crystals with (100) orientation was carried out at temperatures of 500 degrees, 550 degrees, and 600 degrees C at an oxygen pressure of 2.6 kPa for times up to 240 h. A polished cross section of the oxidized crystal was observed by backscattered electron imaging in a scanning electron microscope. Quantitative chemical analysis for Zr, O, and C and their elemental profiles by the linescan method in the ZrC and oxide scale were performed by wavelength dispersive X-ray microanalysis. A thin foil of a crystal oxidized at 600 degrees C was examined by transmission electron microscopy. It was found that the oxide scale was divided into two regions, zones 1 and 2, which contained 14 to 23 and 7 to 10 at. % carbon, respectively. Zone 1 exhibited an almost compact, pore-free matrix of c-ZrO2. In zone 2, some growth and aggregation of the c-ZrO2 occurred, producing 5- to 20-nm-sized particles between which carbon should have been present. The thickness of zone 1 increased parabolically up to 240 h at 500 degrees C and probably in an early period at 550 degrees and 600 degrees C, reaching a constant (about 2 mu), in contrast to the thickness of zone 2, which increased linearly with time.