In situ formation of CA₆ platelets in Al₂O₃ and Al₂O₃/ZrO₂ matrices

Abstract
Al₂O₃ and Al₂O₃/ZrO₂ compacts containing CaO as a dopant have been sintered under different conditions and atmospheres: air, high vacuum (> 10^-6torr). SEM observations have been made on the polished surfaces of sintered and also of annealed samples. Only after the annealing treatment in air at temperatures ranging from 1400° to 1 500°C, a massive formation of CA₆ platelets was detected in samples sintered in low oxygen partial pressure atmospheres.ln order to clarify the mechanism of formation of this secondary phase at the grain boundaries, CaO has been introduced in the form of either plaster of Paris (to reproduce a possible contamination provided by the molds in slip casting) or CaCO₃. The obtained results indicate the important role of the firing atmosphere on the precipitation of secondary phases at grain boundary.