TOUGHNESS-CURVE BEHAVIOR OF AN ALUMINA-MULLITE COMPOSITE

Toughness-curve (T- or R-curve) behavior of a composite of 30 vol%, po lycrystalline, coarse-grained, spherical alumina agglomerates disperse d throughout a fine-grained, 50/50 vol% alumina-mullite matrix, and th at of its microstructural end-members (100% matrix and 100% alumina), mere studied using the indentation-strength-in-bending technique. T-cu rves mere deconvoluted from indentation-strength data using an indenta tion fracture mechanics model, The monolithic matrix and alumina exhib ited an invariant toughness and a moderate T-curve, respectively. In c omparison, the composite exhibited a pronounced T-curve. The T-curve o f the composite is best explained as deriving from the interaction of a propagating crack with the alumina agglomerates: crack propagation e xperiments revealing two possible toughening mechanisms-intra-agglomer ate frictional grain bridging and elastic bridging ligaments in the ma trix that appeared to be associated with alumina agglomerates, Rule-of -mixtures toughness calculations indicated that intra-agglomerate brid ges could account for only a fraction of the toughening exhibited by t he composite. It is suggested that the extra toughening arises from th e elastic bridging ligaments.