Mullite (Nextel 720) fibre-reinforced mullite matrix composites exhibiting favourable thermomechanical properties

A mullite matrix containing homogeneously distributed ultra-fine (70-350 nm) pores was reinforced with NdPO4-coated woven mullite fibre mats (Nextel(TM) 720) leading to damage-tolerant composites with good high temperature (1300 degreesC strength and thermal cycling resistance. Electrophoretically deposited fibre preforms were placed in a high-load pressure filtration assembly, leading to formation of consolidated compacts with high green densities. After sintering at 1200 degreesC for 3 h, the compacts had a density of 86.4% of theoretical density and showed damage-tolerant behaviour up to 1300 degreesC, with flexural strength values of 235 MPa and 224 MPa at room temperature and 1300 degreesC, respectively. No significant microstructural damage was detected after thermal cycling the samples between room temperature and 1150 degreesC for up to 300 cycles. The thermomechanical test results combined with detailed electron microscopy observations indicate that the overall composite behaviour in terms of damage-tolerance, thermal capability and thermal cycling resistance is mainly controlled by two microstructural features: (1) the presence of a dense NdPO4 interphase but weak bonding with the matrix or fibre and (2) the presence of homogeneously distributed nano pores (