THE USE OF SOLID-STATE REACTIONS WITH VOLUME LOSS TO ENGINEER STRESS AND POROSITY INTO THE FIBER-MATRIX INTERFACE OF A CERAMIC COMPOSITE

The effect of the 11 vol% loss during reaction of yttrium-aluminum gar net (YAG) and zirconia was observed in zirconia coated single-crystal alumina fiber-YAG matrix composites. The reaction caused plastic defor mation in the alumina fibers, and possibly a minor amount of porosity at fiber-matrix interfaces that was usually indistinguishable from mat rix porosity. The results were analyzed by models for diffusive cavita tion modified to use reaction self-stress. Crack-healing, tensile stre ss states along the reaction front that approach plane stress, and the small volume of self-stressed material make crack-like pores unlikely at the high temperatures required for reaction. Smaller matrix grains might promote formation of smaller cavities but are also incompatible with high temperature. Both modeling and experiment suggest that suff icient porosity for crack deflection and fiber pullout cannot form unl ess processing methods that form dense composites at lower temperature s are used.