Abstract
Particle size effects on HIP sintering were studied by using three different silicon nitride powders (specific surface areas of 2.0, 4.9 and 12.0m2/g) with yttria and alumina additives up to 6mol% in total. The experimental conditions of HIP were temperatures between 1723-1973K and pressures of 60-200MPa. The influences of particle size of raw materials, additive ratio and HIP condition on the HIP sintering were investigated. The effects of specific surface area (or particle size), temperature, and pressure on the relative density of the sintered body by a specially developed dimensionless analysis were also evaluated. Within the frame of the present work the following results were obtained:
(1) HIP sintering temperature is about 7-25 times more effective than other factors by the dimensionless analysis.
(2) Increasing the specific surface area of silicon nitride from 2.0 to 12.0m2/g the sintering temperature could be lowered to about 200K.
(3) The specific surface area of silicon nitride and the HIPping pressure have almost the same effect on the achieved relative density. This implies that liquid phase sintering was dominant.
(4) The particle size of silicon nitride has a more profound effect than the additive ratio.
(5) The second important variable affecting the relative density is the specific surface area of yttria.
(6) Alumina seems to be better than yttria as sintering aid. The difference is remarkable at low sintering temperatures (below 1873K).
(7) Within the range of this study the effect of oxygen content in the raw materials is smaller than that of the particle size.
