| Instrumented indentation is widely used to extract material elastoplastic properties from the measured force-displacement curves, for bulk materials, thin films, coatings, and other small material structures. Compared with other techniques, indentation test is easy to perform with minimum sample preparation and handling. However, the measured indentation load-displacement curves are implicitly related with material elastoplastic properties, and the mechanics and mechanisms of the indentation must be sufficiently understood (especially when microstructure presents), before the mechanical properties of materials can be accurately derived from the indentation measurements.; Based on a comprehensive numerical and theoretical analysis, we develop several novel indentation techniques (e.g., improved spherical indentation and film indentation with substrate effect) to effectively measure the elastic and plastic properties of bulk materials, thin films, and small material systems. Explicit relationships are established to relate the material properties with indentation response, and effective reverse analysis algorithms are proposed. The formulations are valid for a very wide material space covering essentially all engineering materials, and the material elastoplastic properties solved from reverse analysis are validated through systematic numerical experiments, specific case studies, experiments in the literature, as well as parallel nanoindentation experiments of our group. The error sensitivity and uniqueness of the indentation analyses are also discussed in detail. These main contributions of the thesis provide useful guidelines to measure the material properties via nanoindentation experiments.; The thesis begins with a thorough investigation of the uniqueness of indentation test on bulk materials, where we show the existence and explicitly derived the "mystical materials", which have distinct elastoplastic properties yet they yield almost identical indentation behaviors, even when the sharp indenter angle is varied in a large range. These mystical materials are therefore indistinguishable by most existing indentation analyses unless extreme and impractical indenter angles are used.; Two alternative novel techniques are then proposed: an improved spherical indentation method (with moderately deep penetration) and film indentation method (with a sharp indenter), both can be used to effectively distinguish the mystical materials. The spherical indentation method utilizes the dissimilarity of indenter geometry whereas the film indentation technique utilizes the substrate effect. Both methods show high accuracy in terms of the determined elastoplastic parameters, and they may receive wide potential applications for measuring the mechanical properties of bulk materials and thin films, respectively.; In addition, selected applications of nanoindentation are also presented, where it is applied to measure the residual stress and material properties of bond coat in a thermal barrier coating system (TBC), as well as to probe the material properties of gold nanocrystal beam and gold nanoporous beam, where strong size effects are discovered.; The thesis is concluded with main findings and examples of future works. Nanoindentation is a wide-open area and more endeavors are needed to make this technique eminently useful. |
