Microcantilever investigation of fracture toughness and subcritical crack growth on the scale of the microstructure in Al2O3

Knowledge of the fracture characteristics of individual microstructural features such as grains and grain boundaries is essential for a full understanding of the macroscopic behaviour of ceramics. We have investigated the fracture of single crystal, bicrystal and polycrystalline Al2O3 on the micrometre scale by testing microcantilevers containing notches and artificial flaws introduced at features of interest (e.g. grain boundaries) using focused ion beams (FIB). The specimens were loaded to fracture using a nanoindenter and tests were performed under air, water and silicone oil. The results were influenced significantly by ion implantation at the notch tip, moisture assisted slow crack growth and finite notch tip radius. Methods of avoiding these effects or correcting for them are proposed. It is shown that using these methods, the fracture toughness (Kc) and threshold stress intensity for subcritical crack growth (K0) can be measured in individual grains and grain boundaries of typical microstructures.