INTERFACE ENGINEERING IN ALUMINA GLASS COMPOSITES

Oxide-fiber/oxide-matrix composites, such as alumina-fiber/glass-matrix composites, represent an important class of ceramic matrix composites because of their inherent stability in air at high temperatures. Alumina and glass, however, form a very strong chemical bond, which is undesirable from a toughness point of view. We present an interface engineering approach, which involves the incorporation of an interphase between the matrix and the fiber in order to produce energy dissipating processes such as interface debonding, crack deflection, and fiber pull-out in this system. We first examined the efficacy of tin dioxide as a barrier coating between alumina and glass bars. We confirmed by microprobe 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) fiber with SnO2 and analyzing the microstructure and mechanical behavior of coated fiber composites. We observed that although the SnO2 coating provided the intended diffusion barrier and the thermal stress distribution was of the desirable kind, a neat and clean fiber pull-out was 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 fiber pull-out. To reduce this radial clamping effect, we then used a relatively smooth fiber, Saphikon, a single-crystal alumina fiber. As expected, the SnO2-coated-Saphikon-fiber/glass composite showed a much larger fiber pull-out length than the coated-PRD-166-fiber/glass composite. Thus, a judicious interplay of thermal stress distribution and interfacial roughness in a ceramic matrix composite with an interphase can result in the deformation micromechanisms required for enhanced toughness.