Contamination of planetary milled alumina powder has been studied and correlated to sinterability. Ground powders contain Si and N contaminations derived from the grinding media and mill pot made of Si₃N₄. Strong shear stress generated during planetary milling induces a mechanochemical reaction of worn-out Si₃N₄ with H₂O contained in methanol used as a liquid medium forming hydrated SiO₂ and provides an amorphous layer consisting of Si₃N₄, hydrated SiO₂ and A1₂O₃ on the surface of alumina particles. This suggests a possibility of applying planetary milling to surface modification of ceramic powders despite the problem of contamination. The surface layer containing the contaminations results in the formation of a mullite phase at the grain boundary during sintering. The low diffusion coefficient of mullite degrades sinterability even at a contamination as low as 400 ppm-Si. When the contamination increases to 0.5 mass%-Si, irregular grain growth and the formation of closed pores take place, suggesting the degradation of mechanical properties.

^† This report was originally printed in J. Soc. Powder Technology, Japan. 30(8), 548-555 (1993) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.