Effect of the pressureless post-sintering on the hot isostatic pressed Al₂O₃ prepared from the oxidized AlN powder

The effect of the pressureless post-sintering in hydrogen on the structural and mechanical properties of the hot isostatic pressed Al₂O₃ prepared by oxidized AlN powder has been studied. The micrometer size AlN powder has been oxidized in air at 900° C and sintered by hot isostatic pressing (HIP) at 1700 °C, 20 MPa nitrogen atmosphere for 5 h. Pressureless sintering (PS) has been applied for all HIP sintered samples in H₂ gas at 1800° C for 10 h. It has been shown that the oxidation caused a core–shell AlN/Al₂O₃ structure and the amount of Al₂O₃ increased with increasing of the oxidation time of the AlN powder. For the first time, the green samples obtained from oxidized AlN powder have been successfully sintered first by HIP followed by post-sintering by PS under hydrogen without adding any sintering additives. All post-sintered samples exhibited the main α-Al₂O₃ phase. Sintering in H₂ caused the full transformation of AlN to α-Al₂O₃ phase and their better densification. Therefore, the hardness values of post-sintered samples have been increased to 17–18 GPa having apparent densities between 3.11 and 3.39 g/cm³.