Identification of elastic-plastic anisotropic parameters using instrumented indentation and inverse analysis

Mechanical responses of thin films or coatings often display anisotropic behaviors because of their unique microstructures. However, their small size scales can also make determination of material properties difficult. The present paper introduces a simple yet versatile procedure with advanced data interpretation scheme to identify key anisotropic parameters. This procedure utilizes instrumented indentations and an inverse analysis to extract unknown parameters of elastic-plastic transversely isotropic materials. In particular, it post-processes load-displacement records of depth-sensing indentations to obtain best estimates of Young's moduli and yield stresses along longitudinal and transverse directions, respectively. Major advantages of this method are the minimal specimen preparations and the straightforward testing procedure. To enhance the accuracy, the method utilizes two differently profiled indenter heads, spherical and Berkovich. Prior to actual testing, detailed simulations were performed to verify the method's applicability and robustness. In the experiment, a thermally sprayed NiAl coating which possesses process-induced anisotropic features is considered. The load-displacement records of spherical and Berkovich nano-indentations are post-processed with the proposed inverse analysis scheme. The estimated results predict dissimilar responses along the longitudinal and transverse directions. Separate tests are also conducted with micro-indenter heads under larger loads. They demonstrate lesser anisotropic effects but with more compliant responses. These results are attributed to the unique morphology of thermally sprayed coatings, which inherently exhibit size and anisotropic effects.