On the Most Suitable Equation for Structural Ceramics Fracture Toughness Determination

In present paper 20 published models/equations for fracture toughness determination were analyzed by the following parameters: crack length after penetration, microhardness and fracture toughness of the ceramics. The following criteria were chosen to meet the practical use of the formulas for structural ceramics: (i) material hardness belongs to the range of 16–20 GPa with the maximum coverage of this range, (ii) c/a ratio lies in the range of 2.5–3, (iii) equation contains explicit and simple to measure parameters. Equations that meet all of these criteria were tested on composite ceramics based on alumina, zirconia and silicon nitride and it turned out that equation, proposed by Antstis et al. being simple, most cited and accounting the elastic-plastic characteristics of the material, suits best for such ceramics. At the same time equation by Evans and Wilshaw can be recommended when it is impossible to determine, for example, the elastic modulus of the material. Equation, derived by Niihara, Morena and Hasselman, appeared to fit best at low loads (<20 N) only where Palmquist crack system is formed.