R-curve behaviour of sintered silicon nitride

R-curves for two in situ reinforced silicon nitrides A and B of different microstructures were determined by three different characterization methods. The saturated crack growth resistance was found to be 5.2, 7.2 and 9.2 MPa m^1/2 for silicon nitride A and 5.8, 8.0 and 10.0 MPam^1/2 for silicon nitride B, respectively, by indentation, indentation-crack growth, and indentation-strength methods. The rising behaviour of R-curves was also found to depend on the characterization method. These results indicate that care should be taken in interpreting and utilizing R-curves evaluated from different characterization methods and an R-curve characterization method with appropriate qualifiers is needed for rising R-curve materials.Visiting scientist, on leave from Korea Institute of Science and Technology, Seoul, Korea.