Formation and evaluation of highly uniform aluminate interface coatings for sapphire fiber reinforced ceramic matrix composites (FRCMCs) using carboxylate-alumoxane nanoparticles

Sapphire fibers have been dip-coated in aqueous and CHCl₃ solutions of carboxylate-alumoxane nanoparticles and calcium-, lanthanum-, and yttrium-doped carboxylate-alumoxane nanoparticles and fired up to 1400°C to form uniform, conformal and contiguous, aluminate coatings. Optimum solvent, dip/dry, and firing sequences were determined for the formation of crack-free coatings. Both carboxylate-alumoxane and ceramic coated fibers were examined by field emission scanning electron and transmission electron microscopy, microprobe analysis and optical microscopy. Coatings produced were stable to thermal cycling in air up to a temperature of 1400°C. The ability of the carboxylate-alumoxanes to provide crack infiltration and repair was demonstrated. Sapphire fiber/alumina matrix FRCMCs have been prepared with calcium-, lanthanum-, and yttrium-aluminate interphase layers. Microscopy and fiber push-out data confirm that the calcium- and lanthanum-aluminate coatings provide a means for controlling failure properties at the fiber-matrix interface. However, FRCMCs containing YAG-coated fibers failed catastrophically before interfacial debonding and/or sliding occurred.