Characterization of elastic-plastic coated material properties by indentation techniques using optimisation algorithms and finite element analysis

Determination of the material properties of coated thin films on substrates has always been complicated due to the influence of the substrate deformation on the thin film behaviour. In this study, indentation tests are used to continuously capture the indentation load, P, as a continuous function of the depth of penetration, h, into the indented specimen. Numerical optimisation schemes are used to extract elastic-plastic material properties of thin film coating materials from an indentation loading–unloading curve. Material parameters with non-linear isotropic hardening are determined by minimizing the least squares error (LSE) between target data and the results obtained from FE analysis of the indentation test. Lagrange linear interpolation polynomial (LLIP) and Cubic B-Spline approach are used for axisymmetric conical model and three-dimensional Berkovich model, respectively, to obtain the relationship between the applied load and penetration depth in the loading–unloading curve. This study also considers a global optimisation approach using a load–displacement curve based on a sharp indenter tip applied to HA coating on Ti–6Al–4 V substrate, Al2O3 coating on Al substrate and Cr coating on SS substrate. Additionally, in order to maintain FE analysis accuracy, multiple starting points are performed to obtain the correct material properties. The proposed optimisation algorithm is shown to achieve excellent convergence even when the initial guess values are much greater or less than the target values and hence is potentially considered for characterising intrinsic material properties of coated thin films on substrates that exhibit significant elastic-plastic deformation.