Abstract
Hydrothermal reaction sintering of hafnia (HfO2) was studied under 100MPa at temperatures up to 1300°C. HfO2 formed from Hf and H2O on heating was sintered above 900°C for 3h. The density and crystallite size of sintered bodies increased monotonically with temperature in the range between 900°C and 1200°C, then saturated at the maximum relative density of 98%. The hydrothermal reaction sintering of HfO2 proceeded through three stages with the following features; (1) a large increase in crystallite size was observed, while the density did not increase appreciably, (2) a large increase in density was observed, while crystallite size increased a little, (3) a small increase in density and a small variation of crystallite size were observed. The growth of crystallite size was influenced by the presence of high-temperature high-pressure water and the temperature of heat-treatment. The densification was thought to take place on the second and third stages in the hydrothermal reaction sintering mainly by pore elimination due to the effect of hot isostatic processing. Greyish sintered bodies were sometimes obtained, having a smaller crystallite size and larger density than white sintered bodies fabricated under the same experimental conditions. The disper sion of fine Hf hydride_particles was considered to be the cause of greyish color, smaller crystallite size and denser body, which would be expected to suppress the grain growth and to be effective for the densification.
