Fabrication of pressureless sintered dense β-SiAlON via a reaction-bonding route with ZrO2 addition

A reaction-bonding and post-sintering process was applied to fabricate pressureless sintered beta-Si6-ZAlZOZN8-Z (Z = 1-3) ceramics with monoclinic ZrO2 added to the starting powder. Samples with ZrO2 showed enhanced nitridation and achieved near theoretical density following post-sintering, although this could not be achieved in the samples produced without ZrO2. ZrO2 was effective in both enhancing the nitridation of Si and as a sintering additive for densification of beta-SiAlON. Part of the added monoclinic ZrO2 was transformed to tetragonal ZrO2, and the ZrN phase was also formed due to the reaction with nitrogen during the reaction-bonding process. After the post-sintering process, the ZrN phase remained only in the Z = 3 composition. In the other compositions the ZrN reacted with SiO2 to form both tetragonal and monoclinic ZrO2 phases. These differences are explained in terms of the increasing densification and grain growth for Al2O3 rich compositions and the ZrN being trapped inside such grains in the case of the Z = 3 sample.