Textured Magnetoplumbite Fiber‐Matrix Interphase Derived from Sol‐Gel Fiber Coatings

A calcium hexaluminate (CaAl 12 O 19 , 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 α‐Al 2 O 3 . CaAl 12 O l9 and CaAl 4 O 7 formed between the seeded α‐Al 2 O 3 , and CaAl 4 O 7 later reacted with the seeded α‐Al 2 O 3 to form CaAl 12 O 19 , 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 CaAl 12 O 19 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 inter‐phase may protect fibers from matrix cracks.