Toughening of an Alumina—Mullite Composite by Unbroken Bridging Elements

Abstract

The toughness‐curve (T‐curve) behavior of a composite that is composed of 30 vol% spherical, polycrystalline, fine‐grained alumina agglomerates dispersed throughout a constant‐toughness 50‐vol%‐alumina—50‐vol%‐mullite matrix was compared to that for a coarse‐grained agglomerate version of the composite, with the intent of identifying the operative toughening mechanisms in the latter. The T‐curve behavior was evaluated using the indentation‐strength method, and the underlying T‐curves were deconvoluted from experimental data using an indentation‐fracture‐mechanics model. Compared to the T‐curve of the coarse‐grained composite, the toughness of the “fine‐grained” composite increased from a similar initial toughness and over a similar crack‐length range but to a lower saturation value. The T‐curve of the fine‐grained composite can be explained as being derived predominantly from unbroken bridging elements in the crack wake, set up by agglomerate‐induced crack‐path deflections in the matrix. This mechanism is proposed to function to the same degree and account for the majority of the observed toughening in the coarse‐grained composite, with the remaining toughening increment being derived from intra‐agglomerate grain bridging. Calculations suggest that unbroken bridging elements do not behave as simple elastic cantilevered beams that bend uniaxially with increasing crack‐opening displacement.