Fracture Mechanisms in Laminates in the Alumina - Titania System

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Abstract
Alumina (Al2O3)-aluminium titanate (Al2TiO5) composites present higher toughness than alumina materials but rather low strength due to microcracking. Laminates in which a flaw tolerant material is located between high strength layers is one way to overcome this problem. In this work, the fracture behaviour of a laminated structure constituted by five layers, where low residual stresses are expected, is studied. In this system, external and central layers of monophase alumina with high strength were combined with intermediate layers of alumina with 10vol.% of aluminium titanate. In the monophase alumina layers, an additional "in situ" formed layer of about 200 µm, constituted by large (@10µm) grains was found, close to the composite layers. The laminated structure presented semistable behaviour during SENVB tests for conditions in which stable crack propagation is not predicted for small grain sized alumina materials. Toughening mechanisms related to thermal expansion mismatch between matrix and second phase in the composite layers and crack bridging in the large grain sized alumina layer were identified.

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