Thermal oxidation of sintered beta-sialon (z=3) ceramics in atmospheres with water vapor

Thermal oxidation of beta-sialon (Si6-zAlzOzN8-z, z = 3) ceramic was investigated at 1300 to 1600degreesC in dry (Ar/O-2= 90/10 kPa), wet O-2 (Ar/O-2/H2O = 80/10/10 kPa) and wet (Ar/H2O = 90/10 kPa) atmospheres. Oxidation was followed from weight gains using thermogravimetry and the oxidized sample was characterized by scanning electron microscope, electron probe microanalysis and X-ray powder diffraction. Oxidation kinetics in the three atmospheres were represented by a parabolic rate model except for at 1600degreesC. The wet and wet 02 oxidation rates were greater than the dry oxidation, the degree decreasing with rising temperature from 1300 to 1500degreesC, while wet and wet O-2 oxidation at 1600degreesC was even faster again than the dry oxidation. It was found that the scale formed by the dry, wet and wet O-2 oxidation consisted of mullite and cristobalite at 1300 - 1500degreesC and of mullite and SiO2 rich aluminosilicate glass at 1600degreesC.