Relation between Dependence of Fracture Surface Area of Specimen on Flexural Strength and Fracture Toughness for WC-Co Cemented Carbide and Si<SUB>3</SUB>N<SUB>4</SUB>-Base Ceramics

In our previous study on the flexural strength (σm) of Si3N4-base ceramics, it was found that the number of fragments (Nf) of the broken test piece increased with increasing σm, and the analysis of this phenomenon suggested that Nf had a correlation with the fracture toughness (KIC). In this study, we examined first whether such a correlation between Nf and σm is found for WC-10 mass%Co cemented carbides which has higher KIC than Si3N4-base ceramics, and second whether a correlation is observed between the total macroscopic area of fracture surfaces (Smf) and σm for both specimens, and third whether a correlation is observed between the dependence of Smf on σm and KIC.The results obtained were as follows. (1) A positive correlation was found between Nf and σm, also for the cemented carbides. Between Smf and σm, a more strong positive correlation was observed for both specimens. The σm at a given Nf or a given Smf was higher in the cemented carbides than the Si3N4. (2) Taking into consideration the microscopic and macroscopic energy balance in the propagation of cracks or the formation of fracture surfaces, the equation of σm=ψ·KIC·Smf1⁄2(ψ was a shape factor) was derived for general hard materials. In fact, it was confirmed for both specimens that σm was proportional to Smf1⁄2. And the ratio (3.9) of the slopes of two regression lines for both specimens coincided well with the ratio (3.7) of KIC which was obtained by SEPB method. (3) There is a possibility that the KIC of a hard material can be estimated in general from its σm−Smf1⁄2 relation, by using the σm−Smf1⁄2 relation and KIC of above two kinds of hard materials.