Coupled friction and roughness surface effects in shallow spherical nanoindentation

When nanoindentation is used for thin film characterization, the penetration of the indenter is usually limited to shallow indents to avoid the spurious effect of the substrate. However, the surface effects stemming from the sample surface topography and friction are the most pronounced in shallow indentation depths, potentially resulting in variations of the evaluated material parameters. A numerical study is conducted to understand the coupled influence of friction and sample surface roughness in nanoindentation of pure nickel. Results show a strong interaction between these two contributions of surface effects, and their cumulative effect leads to significant variations in the load-displacement curves. Two experimentally used post-treatment methods are applied to obtain the elastic properties from the raw numerical data and a large variation in the evaluated elastic modulus is observed comparable to experimental dispersion levels, independently of the post-treatment method. This suggests that the coupled effect of roughness and friction may at least partially explain the dispersion otherwise attributed to the tested material behavior. Surface effects are found to contribute significantly to the energy balance of shallow nanoindentations, raising the question of the interpretation of the evaluated plastic material properties with work of indentation based methods. (C) 2009 Elsevier B.V. All rights reserved.