AlPO4-coated mullite/alumina fiber reinforced reaction-bonded mullite composites

A precursor for reaction-bonded mullite (RBM) is formulated by premixing Al2O3, Si, mullite seeds and mixed-rare-earth-oxides (MREO). An ethanol suspension thereof is stabilized with polyethyleneimine protonated by acetic acid. The solid in the suspension is infiltrated into unidirectional mullite/alumina fiber-preforms by electrophoretic infiltration deposition to produce fiber-reinforced, RBM green bodies. Crack-free composites with ≤25% porosity were achieved after pressureless sintering at 1300 °C. Pre-coating the fibers with AlPO4 as a weak intervening layer facilitates significant fiber pullout on composite fracture and confers superior damage tolerance. The bend strength is ∼170 MPa at 25 °C ≤ T ≤ 1100 °C. At 1200 °C, the composite fails in shear due to MREO-based, glassy phase formation. However, the AlPO4 coating acts as a weak layer even after thermal aging at 1300 °C for 100 h.