Examination of Several Potential "Transformation Weakeners" for Ceramic-Composite Interfaces

Abstract

In an effort to broaden the use of transformable interphases in ceramic composites, investigations were conducted towards identifying several displacive phase transformations which could potentially be used in composites with alumina, YAG, and/or mullite. Phase transformations were examined in four materials: the rare earth pyroaluminates, e.g. $\rm Y\sb4Al\sb2O\sb9$; leucite, $\rm KAlSi\sb2O\sb6$; $\rm KAlSiO\sb4$; and hexacelsian, BaAl$\rm\sb2Si\sb2O\sb8.$ Two previously unreported phase transformations were observed, a presumably second order transformation at $\sim$1500$\sp\circ$C in $\rm Gd\sb4Al\sb2O\sb9$ between orthorhombic and monoclinic phases and another first order transformation in hexacelsian at ${\sim}700\sp\circ$C. Unfortunately, none of the transformations studied appear capable of causing transformation weakening of composite interfaces due to phase compatibility problems, difficulties stabilizing the high temperature phase, or lack of distinctive character in the transformation. (A potentially useful transformation was noted in $\rm KAlSiO\sb4$ above 1500$\sp\circ$C, but the transformation temperature was beyond the capability of available equipment.) Experiments with composites of BaAl$\rm\sb2Si\sb2O\sb8$ and alumina, however, revealed a new mechanism for achieving debonding in composites. This mechanism, called reconstructive transformation toughening, uses a volume reducing reconstructive phase transformation to generate tensile stresses at interfaces in composites. Excellent debonding behavior was observed in $\rm BaAl\sb2Si\sb2O\sb8/Al\sb2O\sb3$ composites tested at room temperature and at 850$\sp\circ$C. The most significant advantage of this new mechanism is that it can be made insensitive to changes in temperature.