Textured magnetoplumbite fiber-matrix interphase derived from sol-gel fiber coatings

A calcium hexaluminate (CaAl12O19, magnetoplumbite structure) sol was used to coat alumina and yttrium-aluminum-garnet (YAG) single-crystal fibers and single-crystal alumina plates. When the coated substrates were either annealed or hot-pressed in polycrystalline alumina and YAG matrices, the calcium hexaluminate basal cleavage planes were aligned parallel with the fiber-matrix interface. A complex series of reactions and phase transformations contributed to texture formation on alumina substrates, The alumina fibers and plates seeded the phase transformation of sol-derived transition aluminas to alpha-Al2O3. CaAl12O19 and CaAl4O7 formed between the seeded alpha-Al2O3, and CaAl4O7 later reacted with the seeded alpha-Al2O3 to form CaAl12O19, resulting in a single-phase coating. Several different mechanisms may be responsible for the texture. The microstructure, phase evolution, and possible mechanisms for texture formation of CaAl12O19 powders, sol-derived thin films, and coated plates and fibers, with and without hot-pressed matrices, were studied and are discussed. Deflection and propagation of cracks within the fiber-matrix interphase in thin foils suggests that such an interphase may protect fibers from matrix cracks.