Hexaluminates as a cleavable fiber–matrix interphase: synthesis, texture development, and phase compatibility

The current state of research on hexaluminates as a potential cleavable oxide fiber–matrix interphase is reviewed. Calcium hexaluminate was used initially to produce highly textured fiber coatings and interphases in single-crystal alumina fiber-based ceramic–matrix composites. Cracks were shown to deflect and propagate within the interphase by cleavage. Critical strain-energy release rates of 2.2 J/m2 were measured for highly textured polycrystalline CaAl12O19 interphases. Subsequent work has focused on lowering the temperature for synthesis and texturing of both calcium- and lanthanum-based hexaluminates. Doping of hexaluminates, primarily with transition-metal oxides, allows for their formation at temperatures as low as 1000°C. Grain-growth rates are about an order of magnitude greater than for undoped powders. Textured coatings have been grown on single-crystal YAG plates at 1200°C. However, there does not seem to be an adequate driving force for grain growth and texturing of the coatings on polycrystalline alumina fibers (Nextel™ 610) at 1200°C, the maximum processing temperature for these fibers. The lack of a more refractory, commercially available fiber that is phase compatible with the hexaluminates currently limits further development of a hexaluminate fiber–matrix interphase.