Critical Evaluation of Indentation Fracture Toughness Measurements with Vickers Indenter on Ceramic Matrix Composite Tools

The evaluation of fracture toughness of advanced ceramic matrix composites (CMC) may be expensive if performed using conventional techniques (notch, long and short pre-cracks methods using the three or four point bending fracture test). The use of cracking during the indentation technique is ideally suited for toughness evaluation on a comparative basis and its result appears to provide a method for classifying CMC tools, according to their potential, as cutting tool materials having both high hardness and good crack resistance.The mechanism that leads to the higher fracture toughness of the composite are not yet fully understood. A number of toughening mechanisms such as crack deflection, crack branching, whiskers pullout, bridging (governed by interracial properties), and microcracking have been suggested in the literature, but there is still an ongoing debate about the relative importance of each of these mechanisms. Some authors believe that crack deflection is the main toughening mechanism: it occurs in materials containing anisotropically shaped grains or reinforcements. Crack deflection results when a crack front interacts with a second phase and changes its propagation direction, altering the fracture mode from pure mode I. Other authors suggest that in the crack wake bridging zone, whisker pull out is the major contributor in toughening.In this paper, the effective weight of indentation size effect (ISE), post indentation slow crack growth, material anisotropy and toughening mechanisms are critically evaluated.The CMC tools investigated were: SiCw/Al2O3; TiC/Al2O3; ZrO2/Al2O3. The fracture toughness measurements were performed considering the total crack length (summing all cracks visible on the surface) and the results were compared with those obtained in accordance with the Japanese Industrial Standard JIS R 1607.