Sinter-HIP of α-alumina powders with sub-micron grain sizes

The densification behaviour of different nanoscale pure alumina powders by sinter-HIP to obtain specimens with sub-micron grain size has been investigated. The sinterability of these powders to determine the minimum temperature to obtain close porosity has been determined by dilatometry and by pressureless sintering in air. A good correspondence between minimum temperature to obtain closed porosity and minimum temperature to obtain full density by sinter-HIP has been determined. Different green performing routes has been analysed, uniaxial pressing and colloidal pressure filtration, being the green density obtained of 56% TD and 62% TD respectively. From the powders analysed Taimicron TM-DAR is the powder with its maximum sinterability and can be fully densified by sinter-HIP at temperature as low as 1250 °C and 150 MPa. The specimens obtained at the lowest temperature presented a grain size of 0.45 μm, a hardness of 23–24 GPa and fracture toughness KIC=3.5 MPa m1/2.