Fracture toughness enhancements through stress shielding in functionally graded SiC-TiC composites

It is often the case that the low fracture toughness of polycrystalline ceramics limit the load-bearing capabilities of components made from such materials. In an effort to improve the fracture toughness of a SiC- TiC composite material, a novel stress-shielding approach was adopted so as to reduce the effective mode I stress intensity around a crack tip within the composite body. Using this methodology, for an approximately pure LPSSIc region of the material, an enhancement in the fracture toughness of up to 50 % was achieved, resulting in an effective K(ind Ic) value of about 7.5 MPa.m(exp. 0.5), which is at least as god as the values obtained for such materials through the action of crack bridging and deflection mechanisms. To this end, a series of functionally graded materials were produced, with samples having a preselected spatial SiC- TiC composition variation. The spatial composition variation within each sample was set up according to one of several exponential mathematical functions. In this way the resultant spatial variation in thermal expansion coefficients was also controlled through the thickness of the material, and a particular stress and strain distribution could be tailored within the body. Samples with tailored near-surface compressive stress states showed a significant improvement in K(ind lc), as measured by means of a flexure test, in comparison with stress-free samples of a homogeneous composition corresponding to the composition found at the test-point within the gradient materials.