Microstructure and properties of monazite (LaPO4) coated saphikon fiber/alumina matrix composites

The objective of this research was to engineer a weak interfacial bond in single crystal α-alumina (Saphikon) fiber/polycrystalline alumina matrix composites by incorporating a monazite (lanthanum phosphate, LaPO4) coating onto Saphikon fibers via sol–gel dip process. Uniaxial hot pressing was used to densify LaPO4-coated Al2O3 fiber in an Al2O3 matrix composites. Characterization of the composites was done by optical microscopy, SEM (scanning electron microscopy), EDS (energy dispersive spectrometer), indentation tests, three-point bend and fiber pushout tests. The results showed that the Saphikon fiber/monazite interface was weaker than the polycrystalline alumina/monazite interface. Crack deflection, interfacial debonding and fiber pullout occurred at this interface. This was attributed to the fact that the Saphikon fiber/monazite interface was smoother than the monazite/polycrystalline alumina matrix interface. Monazite coating obtained by sol–gel dip coating method withstood high fabrication temperatures (1400°C) and was conducive to the toughness properties of the composites.