Kinetics of rare earth incorporation and its role in densification and microstructure formation of α-SiAlON

The reproducible sintering and microstructure formation of alpha- and alpha/beta-SiAlONs is a precondition for the production of such materials with reproducible properties. The Rietveld method was used for quantitative analysis of the phase composition and the kinetics of incorporation of the rare earth into the alpha-SiAlON structure. The analysis showed that in the early stages of sintering more rare earth cations were incorporated into the grains than would be expected from the equilibrium conditions. This was observed for all investigated rare earths (Y, Nd and Yb) with different ionic radii. The consequences for sintering and microstructure formation are the fast disappearing of the transient liquid and the formation of polytypes. The use of excess of rare earth in comparison to the stoichiometric SiAlON composition results in a stable residual liquid. It allows reproducible densification, anisotropic grain growth and materials with high fracture toughness. (C) 2011 Published by Elsevier Ltd.