Alumina reinforced tetragonal zirconia (TZP) composites. [Annual report, February 16, 1994--February 15, 1995]

Objective is to develop high-strength and high-toughness ceramic composites by combining mechanisms of platelet, whisker or fiber reinforcement with transformation toughening. The approach being used includes reinforcement of ceria or yttria-partially-stabilized zirconia (CE-TZP or Y-T-ZP) with platelets, whiskers or continuous filaments of alumina. Critical stresses for extension of filament-bridged matrix cracks were measured as a function of crack length in a model composite system, SiC (filament)-reinforced epoxy-alumina (matrix). The crack-length dependence of the crack extension stress at short crack lengths followed the prediction of a fracture-mechanics analysis that employed a new force-displacement law for the crack-bridging filaments developed in this study. At large crack lengths, the measured matrix-cracking stress was close to the prediction of the steady-state theory of Budiansky, Hutchinson and Evans. Optical fluorescence was employed to measure stresses in crack-bridging sapphire filaments and assess interfacial properties in a model sapphire-epoxy composite. Composites of yttria-partially-stabilized zirconia (Y-TZP) matrix dispersed with either single crystal A1{sub 2}0{sub 3} platelets or particulars were fabricated by a hybrid suspension/powder processing route to densities greater than 99.0% of theoretical. Both transformation toughening and platelet reinforcement contribute to the high fracture toughness of the A1{sub 2}0{sub 3} (platelet)-Y-TZP composites