Relation between fracture surface area of a flexural strength specimen and fracture toughness for WC-10mass%Co cemented carbide and Si3N4 ceramics

The relation between flexural strength (σm) and number of fragments (Nf) of a broken test piece was first investigated for WC-10mass%Co cemented carbide with a higher fracture toughness (KIC) than Si3N4-base ceramics that had been studied in our previous report. Then the relation between σm and total macroscopic fracture surface area (Smf), and also the relation between KIC and the dependence of Smf on σm were investigated for both specimens. It was found that Nf of the cemented carbide also tended to become larger for higher σm in a similar manner to the ceramics, and a strong positive correlation was observed between σm and Smf for each specimen. σm under a given Nf or Smf was higher for the cemented carbide with higher KIC, compared with the ceramics. An equation of σm = ΦKICSmf1/2 (Φ is a shape factor) was theoretically derived. The experimental results supported this equation; a good linear correlation was observed between σm and Smf1/2 for each specimen, and the ratio of the slopes of two σm—Smf1/2 regression lines for both specimens was in good accordance with the ratio of KIC measured by the SEPB method.