Quantifying Local Microcrack Density in Ceramics: A Comparison of Instrumented Indentation and Thermal Wave Techniques

Instrumented indentation and thermal wave techniques are used as local, quantitative probes of microcrack density in a silicon nitride that has been subjected to severe Hertzian contact stress. The techniques act as local probes, sampling volumes of material <10⁻³ mm³. Microcracked regions are created using a tungsten carbide spherical indenter, as a result of high shear stress in the compressive zone beneath the indenter. The microcracked zones, studied in cross section using a "bonded interface" technique, increased both in size and in degree of damage with increasing Hertzian load. Within the zones, Young's modulus is measured using instrumented indentation, and the thermal diffusivity using a thermal wave technique; both quantities are lower in the damaged regions than in the undamaged regions. The shifts in modulus and diffusivity are used independently to calculate microcrack densities using related models of the effect of microcracks on each property. The two techniques show the same functional relationship between Hertzian contact load and microcrack density, and yield densities that agree to within a factor of approximately 2.