Reaction Densification of α′‐SiAlON: 1, Wetting Behavior and Acid‐Base Reactions

Phase development during reaction hot pressing of α‐Si 3 N 4 , AlN, Al 2 O 3 , and M 2 O Z powder mixtures (M = Li, Mg, Ca, Y, Nd, Sm, Gd, Dy, Er, and Yb) forming α′‐SiAlON has been studied. The wetting behavior of the ternary eutectic melt of M 2 O z ‐Al 2 O 3 ‐SiO 2 was found to control the reaction sequence during hot pressing. Li, Ca, Mg, Nd, Sm, and Gd were found to preferentially wet and react with Si 3 N 4 first, whereas Dy, Er, and Yb preferentially wetted and reacted with AlN first. The intermediate phases are Si rich in the former and Al rich in the latter case. Pearson's principle of acid‐base theory, which predicts decreasing basicity of oxides of Li, Ca, Mg, Nd, Gd, Sm, Dy, Er, and Yb and decreasing acidity of oxides of Si and Al, is used to understand these reactions.