In situ characterization of fracture toughness and dynamics of nanocrystalline titanium nitride films

We designed a clamped beam bending test using a nanoindentation holder with help of transmission electron microscopy (TEM) and focused ion beam specimen fabrication. The microstructure evolution and crack propagation in nanocrystalline TiN were studied by electron imaging and load-displacement measurements during mechanical loading. By measuring the loads under which the crack starts and stops propagating and the time, we obtained the film's fracture toughness using the finite element method and crack propagation speed. Among these, we identified three types of crack propagation pathways, namely bridging, intergranular and a mixed mode of transgranular and intergranular fracture, and the associated microstructure changes. The measured fracture toughness is in agreement with the reported values. Thus, our in situ TEM bending test provides the first direct measurement of fracture toughness in a TEM and a correlation of fracture toughness with fracture toughening mechanisms in nanocrystalline TiN. The method is general and can be applied to other nanocrystalline materials.