Control of aluminum phosphate coating on mullite fibers by surface modification with polyethylenimine

Aluminum phosphate (AIPO(4)) is a promising oxidation-resistant and weak interface for ceramic-matrix composites. In this research, AIPO(4) coating was deposited on mullite fibers by an improved liquid-phase method based on electrostatic attraction. A cationic polyelectrolyte, polyethylenimine (PEI), was used for surface modification of mullite fibers. The formation process, phase evolution and microstructure of the coating were studied. The zeta potential of AIPO(4) particles, PEI-adsorbed AIPO(4) particles, and PEI-adsorbed mullite particles was characterized to find the proper pH value for improving electrostatic attraction. The obtained AIPO(4) coating was porous and continuous, whose thickness could be controlled by multiple coating cycles. The relatively low calcination temperature (600 or 1000 degrees C) was a useful heat treatment method to develop bonding between coating and fiber as well as reduce the fiber strength degradation. The phase transformations of A1PO4 have little volume change, and cristobalite AIPO(4) is thermal compatible with mullite. Therefore, the coating structure was preserved after calcining at 1200 degrees C. The technique is also applicable for other fibers contained mullite phase to fabricate high-performance AIPO(4) coated mullite/mullite composites.