Multiscale viscous and non-viscous deformation of bulk & thin film systems via improved nanoindentation methodology

Shah, Abdul Khaliq (2020) Multiscale viscous and non-viscous deformation of bulk & thin film systems via improved nanoindentation methodology. Doctoral thesis, University of Central Lancashire.

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Nanoindentation is a technique for studying multiscale contact deformation. Over the years, different investigators have made improvements to it’s methodologies. In this thesis, the current Oliver and Pharr’s (1992), Fujisawa and Swain (2006) and Feng’s (2002) unloading methodologies are examined, and novel robust characterisation methodologies proposed.

For any nanoindenter machine, the exact configuration including; actuator/transducer electronics and control strategies, machine calibrations etc., and the corrections associated with thermal drift, initial contact, sink-in and pile- up still limit the acquisition of consistent data. Additionally, variations in the output data are related to material characteristics and geometrical effects such as; indenter area function, surface roughness, thin film thickness, the tilt of the surface, indentation size effects etc. The main goal of the work described in the thesis was to address these factors in the characterisation methods, as for all materials, reliability and reproducibility becomes inherent, including for more challenging viscous/polymeric materials and at low load responses, where the measured properties, such as hardness and elastic modulus, can be time-dependent.

In seeking to unify the different approaches, for viscous/polymeric materials and low load testing, the well- established unloading methodologies were further developed. Previous methods treat the unloading to be fully elastic. However, a plastic correction is proposed at the point where the delayed plasticity cease. For low load testing, a datum correction is implemented according to a parameter, the Roughness Depth Limit (RDL), which aids in
splitting the deformation for determining the correct contact area. The author’s DU method is demonstrated to characterise various material types at any test condition in one-cycle. Above RDL, the method compared remarkably well to tensile testing, for several material types. Results for viscous/polymeric materials, were consistent with the well- established unloading methodologies and also with hold- time methodologies. Other novel models, for thin film characterisation, were applied and found to fit precisely the DU method data, i.e. the elastic modulus as a function of depth, for substrate independent properties. With further work the method has the potential to form the basis of a formal standard for polymer characterisation which is still needed in today's industry.

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