DENSIFICATION OF SILICON-NITRIDE CERAMICS UNDER SINTER-HIP CONDITIONS

The influence of several sinter-HIP variables on the densification behaviour of silicon nitride-based ceramics has been investigated. The processing conditions were studied for Si3N4 powder mixtures containing controlled amounts of Y2O3+Al2O3 or Y2O3+MgO. The specimens were subjected to sinter-HIP cycles under argon or nitrogen atmospheres at various temperatures and pressures. The final density of the powder compacts exhibited a strong dependence not only on the applied pressure, the composition and the processing temperature, but also on the pressurization rate and the initial pressurization time. The microstructural changes induced by the application of high pressure were followed by transmission electron microscopy. On examination by TEM, large concentrations of dislocations, generated inside some β-Si3N4 grains, were observed. Characterization of these dislocations showed that b=〈0001〉 is their most frequently found Burgers vector. Also, two relaxation mechanisms, tending to release the stored energy of deformation in the β-Si3N4 grains, namely grain penetration (a form of strain-induced boundary migration) and grain fragmentation (the formation of subgrains due to rearrangement of dislocations into low energy configurations), have been identified. The intergranular phases formed were characterized by energy dispersive X-ray spectroscopic analysis, X-ray diffraction and electron diffraction. The influence of different sinter-HIP cycles on the α →β transformation of silicon nitride without additives was also investigated by X-ray diffractometry. © 1990 Chapman and Hall Ltd.