AlPO4 coating on alumina/mullite fibers as a weak interface in fiber-reinforced oxide composites

A sol of amorphous aluminum orthophosphate (AlPO4) nano particles (30-100 nm) was synthesized by co-precipitation of Al3+ and HPO42- with urea at 95 degrees C. A narrow particle size distribution was achieved when [Al3+]<= 0.1M. The isoelectric point of the synthesized powder is pH similar to 4.7. Alumina/mullite fibers (Nextel (TM) 720) were pretreated with cationic polyelectrolyte to render them of opposite charge to the AlPO4 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 Al2O3 matrix of a hot-pressed composite was reported. Significant fiber pullout was noted.