Toughness‐Curve Behavior of an Alumina‐Mullite Composite

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Abstract
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.

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