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 KIFR, 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 m1/2 for KIFR of 4.3 and 3.6 MPa m1/2, respectively, which indicates the effectiveness of this measurement technique to improve the reliability of the IF method. The deviations of the grand average KIFR reported by the laboratories from those re-measured by the authors using the returned samples were only ca. 0.4 MPa m1/2, 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.