Sintering mechanism of fine zirconia powders with alumina added by various ways

Small amounts of Al₂O₃ were added to fine zirconia powder by different ways: powder mixing, hydrolysis of alkoxide, and homogeneous precipitation. During a constant rate heating process, the Al₂O ₃ addition slightly raised the starting temperature of densification of powder compact, and the densification was remarkably stimulated by Al ₂O₃ at temperatures above about 1100°C. According to an isothermal analysis of densification, the densification rate was retarded by Al₂O₃ addition just after the start of sintering and then the densification rate increased significantly during sintering compared to Al₂O₃-free powder. These results mean that Al ₂O₃ particles pinned the shrinkage of zirconia powder compact at the initial stage, and diffuse toward zirconia surface to enhance the sintering. The sintering mechanism was explained by the grain-boundary diffusion for the Al₂O₃-free powder and the volume diffusion for Al₂O₃-added powder. When the Al ₂O₃ was added to zirconia powder by homogeneous precipitation and alkoxide methods, the densification rate was more stimulated compared to powder mixing method. The sintering mechanism did not change by the way for Al₂O₃ addition. The Al₂O₃ addition by the chemical process tended to enhance the grain growth of zirconia, while the uniform microstructure was achieved because of homogeneous addition of Al₂O₃ by these chemical processes.