AlPO sub(4) Coating on Alumina-Mullite Fibers as a Weak Interface in Fiber-Reinforced Oxide Composites

A sol of amorphous aluminum orthophosphate (AlPO sub(4)) nano particles (30-100 nm) was synthesized by co-precipitation of Al super(3+) and HPO sub(4) super(2-) with urea at 95C. A narrow particle size distribution was achieved when [Al super(3+)],0.1M. The isoelectric point of the synthesized powder is pH64.7. Alumina-mullite fibers (Nextel+ 720) were pretreated with cationic polyelectrolyte to render them of opposite charge to the AlPO sub(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 sub(2)O sub(3) matrix of a hot-pressed composite was reported. Significant fiber pullout was noted.