Finite element simulation of the effect of surface roughness on nanoindentation of thin films with spherical indenters

Abstract

The effect of the surface roughness on nanoindentation results was investigated instancing a series of CrN thin films deposited by unbalanced magnetron sputtering. The arithmetic roughness (Ra) of the films ranged between 2 and 10 nm and was measured by atomic force microscopy. The measured surface topography was incorporated into a finite element model, which allowed simulating the indentation of an axisymmetric sample by a rigid spherical indenter. For the applied conditions it was found that plastic deformation could be neglected and thus purely elastic material behavior was assumed. For roughness values of Ra ≈ 2, 5, and 10 nm, 100 indents each were simulated. Subsequently, the software Elastica and the approach by Oliver and Pharr were used to evaluate Young's modulus of the CrN thin films from the simulated load-displacement curves. Under the applied conditions, the increasing roughness causes a reduction of the contact area and leads to an underestimation of Young's modulus. The mean Young's modulus of all simulated indents on the rough surfaces lies 5–14% below the Young's modulus determined for a perfectly smooth surface. This deviation seems to be independent of Ra, although the data scatter increases significantly with increasing roughness. Additionally, an influence of the lateral extension of the surface texture on the data scatter was observed which is not accounted for in roughness measures such as Ra.