INTERFACE ENGINEERING IN ALUMINA GLASS COMPOSITES

Oxide-fiber/oxide-matrix composites, such as alumina-fiber/glass-matri x composites, represent an important class of ceramic matrix composite s because of their inherent stability in air at high temperatures. Alu mina and glass, however, form a very strong chemical bond, which is un desirable from a toughness point of view. We present an interface engi neering approach, which involves the incorporation of an interphase be tween the matrix and the fiber in order to produce energy dissipating processes such as interface debonding, crack deflection, and fiber pul l-out in this system. We first examined the efficacy of tin dioxide as a barrier coating between alumina and glass bars. We confirmed by mic roprobe analysis that alumina and tin dioxide were mutually insoluble but there was some solubility between silica and tin dioxide. This was followed by coating continuous PRD-166 (alumina + 15 wt.% zirconia) f iber with SnO2 and analyzing the microstructure and mechanical behavio r of coated fiber composites. We observed that although the SnO2 coati ng provided the intended diffusion barrier and the thermal stress dist ribution was of the desirable kind, a neat and clean fiber pull-out wa s absent because of the roughness of the PRD-166-SnO2 interface. Some fiber/matrix debonding, crack deflection, and crack bridging occurred. The roughness-induced radial clamping stress was too large to allow f iber pull-out. To reduce this radial clamping effect, we then used a r elatively smooth fiber, Saphikon, a single-crystal alumina fiber. As e xpected, the SnO2-coated-Saphikon-fiber/glass composite showed a much larger fiber pull-out length than the coated-PRD-166-fiber/glass compo site. Thus, a judicious interplay of thermal stress distribution and i nterfacial roughness in a ceramic matrix composite with an interphase can result in the deformation micromechanisms required for enhanced to ughness.