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 degrees C in 100 degrees 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 mu m at 1400 degrees C, whereas the grains treated at 1500 degrees C became thicker preserving the same needle length. The Vickers microhardness of the developed microstructures increased with the increase of temperature up to 1400 degrees C, while at 1500 degrees C it was slightly reduced due to the grain coarsening. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved