Crack‐Growth Resistance of in Situ‐Toughened Silicon Nitride

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Abstract
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.m1/2, 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.