Relationship between fracture toughness determined by surface crack in flexure and fracture resistance measured by indentation fracture for silicon nitride ceramics with various microstructures

The reliability of the Vickers indentation fracture (IF) method for various types of silicon nitride (Si3N4) ceramics was assessed by comparing the fracture resistance, KR obtained from the IF test with the fracture toughness, KIc from the surface crack in flexure (SCF) technique in the same crack depth region. The KR of a fine-grained and equiaxed Si3N4 matched with the KIc from the SCF test when Miyoshiʹs equation was used, while the KIc of a bearing-grade Si3N4 was found to lie between KR values calculated with Niiharaʹs equation (higher side) and Miyoshiʹs equations (lower side). In the case of coarse Si3N4 with elongated grains, the KR determined using Niiharaʹs equation gave the best fit with KIc. The inconsistent outcomes were explained by the probable mechanisms, indicating that the KR from the IF test cannot be correlated directly with the KIc unless the effective crack length for the IF test was clarified.