Oxidation processes in a high substitution β'-sialon

The oxidation of a z = 3 β'-sialon in 1 atm. air at a temperature of 1380°C is reported. After a short incubation period the oxidation kinetics follow a linear regime for 3 1 2 h and then a parabolic regime for the remainder of the time period investigated (80 h). Distinct changes occur as the oxidation proceeds. The incubation period is characterised by the formation of a liquid by the preferential oxidation of the grain boundary glass phase and partial oxidation of the sialon to X-phase, silicon oxynitride and silica also occurs. During the linear period, more of the liquid phase forms and completely covers the sialon surface. Furthermore, the amounts of X-phase and silicon oxynitride in the surface layers decrease as their oxidation leads to the formation of further silica and mullite. Once this fully oxidised surface layer has formed the oxidation kinetics become parabolic giving rise to a duplex scale. The scale layer adjacent to the sialon is depleted in aluminium, but enriched in impurity cations (calcium and potassium) and also yttrium from the pressureless sintering aid. The oxide layer adjacent to the air atmosphere comprises mullite crystals in an almost pure silica matrix. The results obtained are compared with other similar oxidation studies and possible reaction mechanisms are discussed.