Microstructural design, characterisation and indentation responses of layer-graded alumina/aluminium–titanate composites

A study has been conducted on the depth-profiling of composition, residual strains, mechanical characteristics and the evaluation of indentation responses in a layer-graded material (LGM) of alumina/aluminium–titanate. An infiltration route fabricates LGM samples with a homogeneous layer of alumina and a graded layer of heterogeneous alumina/aluminium–titanate. Depth profiling of Vickers hardness shows that the hardness of the LGM is depth dependent with a relatively soft graded layer but a hard homogeneous layer. The microhardness of the graded layer is load dependent with 5.6 GPa as the asymptotic value at high loads. Similarly, the elastic modulus and residual strains are depth-dependent. The graded layer exhibits a distinctive ‘softening’ in the stress–strain curve, indicating a microscale quasi-plasticity which can be associated with grain debonding, grain sliding, diffuse microcracking, grain push-out, and grain bridging. No contact-induced cracks are observed in the graded layer and the microdamage is widely distributed within the shear–compression zone around and below the contacts. The capability of the LGM to absorb energy from the loading system and to distribute damage is strongly influenced by the existence of residual strains, which is somewhat akin to that of ceramics with heterogeneous microstructures. q 2006 Published by Elsevier Ltd.