Microstructure and Mechanical Properties of Sol-Gel Derived Mullite Containing Needle-Like Grains

Synthesis of mullite whiskers in F− atmosphere was carried out by means of vapour-phase reaction. A mullite precursor sol was prepared by mixing aluminium nitrate and ethyl silicate in propanol medium. The addition of HF triggers the gelation of mullite. The powders was uniaxialiy pressed into bars and was heat-treated at 1100°C in an airtight container. During this process, mullite whiskers were formed and they were used as growth centres for needle like grains. The sapmles when sintered at 1600°C for 4 h in air, showed needle morphology and the fracture toughness was 2.3 MPa.m½.