Toughness-curve behavior of an alumina-mullite composite

Toughness-curve (T- or R-curve) behavior of a composite of 30 vol%, polycrystalline, coarse-grained, spherical alumina agglomerates dispersed throughout a fine-grained, 50/50 vol% alumina-mullite matrix, and that of its microstructural end-members (100% matrix and 100% alumina), were studied using the indentation-strength-in-bending technique T-curves were deconvoluted from indentation-strength data using an indentation fracture mechanics model. The monolithic matrix and alumina exhibited an invariant toughness and a moderate T-curve, respectively. In comparison, the composite exhibited a pronounced T-curve. The T-curve of the composite is best explained as deriving from the interaction of a propagating crack with the alumina agglomerates: crack propagation experiments revealing two possible toughening mechanisms - intra-agglomerate frictional grain bridging and elastic bridging ligaments in the matrix that appeared to be associated with alumina agglomerates. Rule-of-mixtures toughness calculations indicated that intra-agglomerate bridges could account for only a fraction of the toughening exhibited by the composite. It is suggested that the extra toughening arises from the elastic bridging ligaments.