Pressureless sintering and processing of silicon nitride powder coated with yttria and alumina

A Si3N4 powder coated with Y2O3 and Al2O3 as sintering aids was prepared by a co-precipitation method, using urea. as precipitate reagent. The electrophoretic measurements revealed that the coated powder possessed the surface character of Al2O3 or Y2O3 instead of Si3N4. A well dispersed suspension with 20 vol% coated powder was prepared at pH = 9, which exhibits a low slip viscosity and yield a high green density. Green body shaped by slip casting shows less agglomerate formation than that prepared by dry pressing. High densities were achieved by pressureless sintering these powder compacts at 1750 degreesC for 2 hours under I atm nitrogen. The microstructure observations of the sintered samples show homogeneous distribution of additives in the Si3N4 matrix in both cases. However, more homogeneous distribution of the glassy phase and less porosity was observed in the samples shaped by the slip casting technique.