High pressure densification of nanocrystalline mullite powder

Investigations of the high-pressure sintered nanocrystalline mullite powder are presented. The synthesized mullite powder with crystallite size of 51 nm was densified by using high-pressure “anvil-type with hollows” apparatus at 4 GPa over the temperature range of 1100-1500 °C in 100 °C steps. The phase composition and structural parameters of the densified samples were studied as a function of densification temperature. The XRD analysis revealed the appearance of new phases, such as kyanite and corundum, whose development affected the densities of the sintered samples. High relative densities of the sintered samples were obtained because of the application of high pressure. The needle-like microstructure was developed owing to the anisotropic grain growth of mullite. The elongated mullite grains reached the length of approximately 5 µm at 1400 °C, whereas the grains treated at 1500 °C became thicker preserving the same needle length. The Vickers microhardness of the developed microstructures increased with the increase of temperature up to 1400 °C, while at 1500 °C it was slightly reduced due to the grain coarsening.