Effect of the amount of excess oxides on the densification of a-SiAlON fabricated via a reaction-bonding process

Y-a-SiAlONs with elongated grains were fabricated via a reaction-bonding process using starting compositions containing excess oxides in the form of Y2O3 and SiO2. The density of post-sintered specimens reached a maximum value with compositions containing 2 mass% excess oxides, although conventionally sintered materials of this composition were not fully dense. However for compositions containing smaller amounts of excess oxides, the density of the specimen fabricated via a reaction-bonding process was lower than that via a conventional process. In these samples it is thought that liquid phase sintering was difficult to be achieve during the post-sintering process, since the amount of SiO2 contained in the starting powder was lower and the Al2O3 in the starting composition was consumed for the production of b-SiAlON during nitridation. There was also a decrease in density of the reaction-bonded materials with further increases in the amounts of additional oxides. For these samples the compacts could not be densified uniformly, because of non-uniformity of the phase composition in the nitrided compacts. The dense reaction bonded a-SiAlON with elongated grains, fabricated from compositions with 2 mass% excess oxides exhibited both high fracture toughness and high hardness.