Effects of α-Si 3N 4 and AlN addition on formation of α-SiAlON by combustion synthesis

Abstract

Preparation of Yb α-SiAlON was investigated by self-propagating high-temperature synthesis (SHS) from α-Si 3N 4- and α-Si 3N 4/AlN-diluted powder compacts under nitrogen of 2.17 MPa. For the AlN-free samples, the molar ratio of Si 3N 4/Si varies between 0.22 and 0.5. The starting stoichiometry of the AlN-added samples comprises a constant proportion of Si 3N 4/Si equal to 0.22, but a broad range of AlN/Al from 0.33 to 1.0. The self-sustaining combustion wave propagated in the spinning mode on account of highly diluted samples adopted in this study. The overall reaction exothermicity increases with Si 3N 4/Si ratio for the AlN-free samples, while decreases with AlN/Al ratio for the AlN-added powder compacts. As a result, the amount of unreacted Si left in the final product was significantly reduced and the formation of nearly single-phase Yb α-SiAlON was achieved in the sample with Si 3N 4/Si = 0.5. Moreover, the growth of elongated α-SiAlON grains was enhanced in the samples with high contents of Si 3N 4. In contrast, the nitridation of Si was only improved to a certain extent with the addition of AlN and no further improvement was attained by increasing the AlN content. Due to the lack of sufficient liquid phases during combustion and the weak reaction exothermicity, the samples with high contents of AlN were inclined to produce α-SiAlON grains in a fine equiaxed form.