R-CURVE BEHAVIOR OF INDENTATION CRACKS IN SILICON-NITRIDE BASED CERAMIC COMPOSITE

R-curves for two types of Si3N4-based ceramic are established by direct measurements of the change in lengths of Vickers indentation cracks during bending test. The residual stress due to indentation is eliminated by annealing before the test in order to use the empirical equations proposed by Newman and Raju to calculate the crack resistance. An exponential relation is selected to describe the experimental results. Some material properties which characterize the cracking behavior, such as fracture toughness, range of the crack extension over which toughening effects develop and saturate, and the threshold crack resistance of a pre-existing, traction-free crack, are shown to be deduced with the parameters occurring in the exponential relation.