In situ nanoindentation in the TEM

Nanoindentation is the most widely applied technique for mechanical probing of materials with limited spatial extent, such as thin films, individual phases of multiphase materials, and lithographic structures. It is also used to examine length-scale effects in bulk samples by varying the size of the indentations. Most investigations can be categorized either as a quantitative assessment of a material's elastic modulus and hardness or a phenomenological study of the underlying deformation physics. In the latter case, the literature is replete with examples of curves exhibiting multiple, sharp discontinuities during their initial loading segment. Such discontinuities correspond to discrete deformation events; however, conventional ex situnanoindentation's frustrating inability to establish a one-to-one relationship between load-depth characteristics and stress-induced microstructure evolution often renders the proposed deformation mechanism a reasonable speculation at best. Coupling TEM to nanoindentation can monitor the material's microstructure throughout the test. However, severe TEM-related challenges have hampered the realization of in situ TEM nanoindenters giving rigorous load-depth curves. The greatest barrier is the extremely cramped space afforded by a TEM holder, but the requirement for a horizontally aligned indenter to be perpendicular to the electron beam, the electron beam impinging the indenter, the presence of a strong magnetic field, and the high-vacuum environment also complicate matters.