Densification behaviour of oxide-coated silicon nitride powders

The effectiveness of the powder coating technique has been evaluated by comparing the powder properties, densification behaviour, microstructure and mechanical properties of coated Si3N4 powders with identical powders in which the additive oxide has been added in particulate form. It is concluded that the powder coating technique is an excellent method of homogeneously incorporating minor amounts of sintering additive into a powder. The coated powder exhibited improved homogeneity, and gave good green compact density, high green strength, and faster densification rate. Moreover, coated powders densified more easily by pressureless sintering and showed a more homogeneous microstructure, higher strength and faster densification rates, compared with materials prepared using mixed oxide powders. Significant improvements in hardness and fracture toughness were observed for the coated powders.