An experimental study of toughening and degradation due to microcracking in a ceramic composite

Abstract

An experimental investigation has been carried out to study the effects of controlled microcracking on the fracture resistance of brittle solids. The material chosen as a model system for the experimental study is an aluminum oxide reinforced with 33 vol.% SiC whiskers. The experimental program involves the determination of fracture toughness at room temperature on four-point flexure specimens containing sharp, through-thickness precracks where different amounts of microcrack damage are introduced a priori at different elevated temperatures and tensile load levels. The room temperature fracture initiation toughness of the pre-damaged material with a controlled amount of small-scale microcracking ahead of the stationary macrocrack is compared and contrasted with that of the undamaged material and the enhancement or reduction in fracture initiation toughness is estimated. Detailed transmission electron microscopy of the crack-tip damage zone has been conducted in an attempt to examine the mechanisms of permanent degradation and of microcrack formation. These observations reveal that the mechanism of microcracking by the coalescence of intergranular/interfacial cavities in the ceramic composite exposed to the high-temperature environment dominates over any other possible source of permanent deformation such as dislocation plasticity for the conditions of the experiments of this study. The fracture test results suggest that some toughness gains can be achieved, in certain cases, despite the creation of damage which primarily involves microcracking ahead of the crack-tip; however, significant increases in microcrack density, in fact, lead to a deterioration in the resistance of the material to fracture. The experiments of this study are discussed in the context of available theories of microcrack shielding, material degradation, and crack-microcrack interactions, as well as possible effects of dislocation plasticity and residual stresses.