AlPO 4 Coating on Alumina/Mullite Fibers as a Weak Interface in Fiber‐Reinforced Oxide Composites ‡

A sol of amorphous aluminum orthophosphate (AlPO 4 ) nano particles (30–100 nm) was synthesized by co‐precipitation of Al 3+ and HPO 4 2− with urea at 95°C. A narrow particle size distribution was achieved when [Al 3+ ]≤0.1 M . The isoelectric point of the synthesized powder is pH∼4.7. Alumina/mullite fibers (Nextel™ 720) were pretreated with cationic polyelectrolyte to render them of opposite charge to the AlPO 4 particles, and the latter were then coated by electrostatic‐attraction dipcoating. A uniform, continuous coating formed after several cycles, and its effectiveness as the weak layer between the fibers and the Al 2 O 3 matrix of a hot‐pressed composite was reported. Significant fiber pullout was noted.