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 fo ur 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.m1/ 2, respectively, by indentation crack size measurement, indentation st rength, 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 strengt h technique. These results indicate that there is no unique fracture t oughness value and that a fracture toughness testing method with appro priate qualifiers is needed for rising R-curve materials. Therefore, c are should be taken in interpreting and utilizing fracture toughness v alues evaluated from different testing methods if a material exhibits a rising R-curve. Complete characterization of the R-curve may be a pr erequisite.