Effect of two-step sintering on optical transmittance and mechanical strength of polycrystalline alumina ceramics

Abstract

The effect of two-step sintering (TSS) process on optical and mechanical strength of polycrystalline alumina ceramic was investigated. High purity alumina compacts with a ppm level of impurities were prepared by a ceramic injection molding technique for the two-step pressureless sintering under vacuum atmosphere. The first sintering step was carried out by a fast heating rate to reach higher sintering temperatures within a short time, followed by cooling and holding at a lower temperature for the subsequent second step. The temperature of 1780°C was effective for the first step sintering (FSS) showing the highest densification rate. The second isothermal sintering was then carried out at 1300-1600°C (TSS) for various hours in order to avoid the grain boundary migration and to enhance segregation of secondary phases in the grain boundary. The two-step sintering temperature and time increase, the grain size tends to gradually increase, which leads to promote homogeneous grain microstructure without any significant grain growth and to enhance transparency of polycrystalline alumina without degradation of mechanical strength.