Refined measurements of indentation fracture resistance of alumina using powerful optical microscopy

A round-robin of the indentation fracture (IF) method using two alumina ceramics was performed in 12 laboratories to confirm the significantly improved reproducibility of indentation fracture resistance K_IFR, using powerful optical microscopy. Powerful optical microscopy with both an objective lens of 40× or 50× and a traveling stage was employed to reduce the error in reading crack length. Indentations at 98 N for the two samples had moderate between-laboratory standard deviations of 0.3 and 0.2 MPa m^1/2 for K_IFR of 4.3 and 3.6 MPa m^1/2, respectively, which indicates the effectiveness of this measurement technique to improve the reliability of the IF method. The deviations of the grand average K_IFR reported by the laboratories from those re-measured by the authors using the returned samples were only ca. 0.4 MPa m^1/2, which was attributed to the slight misreading of the crack length by the participant laboratories. Thus, the reliability of the IF method seems reasonable by this advanced approach because our recent round-robins, together with this study, have confirmed that the precision for the three major structural ceramics, SiC, Si₃N₄ and alumina, could meet the necessary condition of reproducibility.