Crack-Growth Resistance of in Situ-Toughened Silicon Nitride

The fracture toughness of a commercial, hot-pressed, in situ -toughened silicon nitride with an elongated grain structure is determined by four different testing methods. The fracture toughness is found to be 5.76 ± 0.27, 8.48 ± 0.50, 10.16 ± 0.66, and 10.68 ± 0.39 Mpa.m^1/2, respectively, by indentation crack size measurement, indentation strength, single-edge-precracked-beam, and chevron-notched-beam methods. The discrepancy in fracture toughness between the testing methods is related to R -curve behavior, as measured using the indentation strength technique. These results indicate that there is no unique fracture toughness value and that a fracture toughness testing method with appropriate qualifiers is needed for rising R -curve materials. Therefore, care should be taken in interpreting and utilizing fracture toughness values evaluated from different testing methods if a material exhibits a rising R -curve. Complete characterization of the R -curve may be a prerequisite.