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 degrees C and sintered by hot isostatic pressing (HIP) at 1700 degrees C, 20 MPa nitrogen atmosphere for 5 h. Pressureless sintering (PS) has been applied for all HIP sintered samples in H-2 gas at 1800 degrees 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 alpha-Al2O3 phase. Sintering in H-2 caused the full transformation of AlN to alpha-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/cm(3).