Spherical nanoindentation: insights and improvements, including stress-strain curves and effective zero point determination

Alexander J. Moseson

doi:10.17918/etd-1868

Instrumented nanoindentation is a valuable method for mechanical characterization. Typically, sharp tips are used to indent surfaces and wellestablished techniques used to determine the hardness and moduli values of a wide range of materials. Spherical indentation tips, though less common, offer the distinct advantage of providing useful insight into the elasto-plastic transition region. In this thesis the results of continuous stiffness measurements with spherical indenters - with radii of 1 [mu]m and/or 13.5 [mu]m - and Hertzian theory are used to convert indentation load/depth curves to their corresponding stress-strain curves. We applied the technique to a wide range of materials, including fused silica, aluminum, iron and single crystals of sapphire and ZnO. In all cases, the stress-strain curves clearly showed the elastic, plastic, and elastoplastic regions. The modulus and hardness obtained by our method show, for the most part, a strong correlation with bulk and Vickers values obtained on the same surface, respectively. When both the 1 [mu]m and 13.5 [mu]m indenters were used on the same material, for the most part, the indentation stress-strain curves traced one trajectory. Furthermore, accurate determination of the "zero point", first contact between an indenter tip and sample surface, has to date remained elusive. Herein a relatively simple, objective procedure by which that zero point can be determined accurately and reproducibly using a nanoindenter equipped with CSM option and a spherical tip is described. The method relies on applying a data shift, which insures that stiffness versus contact radius curves are linear and go through the origin. The method was applied to fused silica, sapphire single crystals and polycrystalline iron with various indenter sizes, to a zero point resolution of 2 nm. Errors of even a few nm can drastically alter plots and calculations which use the data, including stress vs. strain curves. The method is the first to use a parameter inherently not affected by zero point to correct the displacement and all subsequent uses thereof, which is highly sensitive to zero point. The applications of this method range from increased accuracy for all tests including stress vs. strain, to sample leveling, to individual grain characterization, and beyond. Finally, I herein present our most recent work, including further insights into the characterization of individual grains.

Spherical nanoindentation: insights and improvements, including stress-strain curves and effective zero point determination

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Spherical nanoindentation: insights and improvements, including stress-strain curves and effective zero point determination

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