Improved accuracy of the measurements of indentation fracture resistance for silicon nitride ceramics by the powerful optical microscopy

Thirteen laboratories participated a round robin to investigate the reproducibility of indentation fracture resistance, KIFR, of silicon nitride ceramics. When the usual optical microscope with an object lens of ∼10× was employed for the crack-length measurements, KIFR varied from 7.7 to 8.8 MPa m1/2, whereas those re-measured by authors using the measuring microscopy with an objective lens of 50× exhibited a constant value of 7.5±0.2 MPa m1/2. The inaccurate KIFR of each laboratory was attributed mainly to the misreading of the crack length ∼30 μm shorter. By contrast, powerful microscopy with both an object lens of 40× and a traveling stage for the crack-length reading enabled each lab to measure more accurate KIFR of 7.8±0.3 MPa m1/2. The usefulness of the new measuring technique for the higher reliability of KIFR was also confirmed for the Si3N4 ceramics just like those reported for SiC ceramics previously.