Fracture of Ceramics with Near Surface Gradient Residual Stresses

The fracture toughness and strength of ceramics can be improved with respect to monolithic ceramics by developing graded materials as laminates composed of periodic alternating layers of one material separated by layers of a second material. The second layer must contain residual compressive stresses which are induced during densification because of differential thermal contraction of the layers. The overall residual stresses increase the apparent fracture toughness of the laminate. However, most deleterious natural flaws and most of the damage induced in service by the environment, contact loading, wear, etc, are small cracks on the surface of the outer layer, so that the effect of the laminate residual stresses on these cracks should be rationalised to understand their behaviour. This work presents an analysis of the influence of the gradient residual stresses on the behaviour of surface cracks under bending and indentation in materials with outer layers either with tensile or compressive residual stresses. Introduction The contact of indenters on the surface of brittle materials is usually performed by means of either sharp or blunt indenters. Sharp indenters, like the Vickers indenter, are extensively used to study the fracture and deformation properties of polycrystalline ceramics. Sharp contacts provide deep insight into the fundamental nature of damage modes in brittle solids. It can be also used to induce controlled flaws from which different properties can be evaluated. Increase in crack length of these flaws in service by thermal shock, static or cyclic fatigue, etc, conducts to strength degradation from which the damage induced by these mechanisms can be assessed [1]. Indentations can be used to estimate the fracture toughness of intrinsically brittle ceramics with routine simplicity. In particular, Vickers indentation is now the most widely used test method for evaluating local qualitative fracture toughness in brittle materials. The toughness is calculated from the lengths of surface radial cracks at the indentation corners [2]: 2 / 3 0 c P Kc χ = , (1)