Modeling Effect of Surface Roughness on Nanoindentation Tests

Surface roughness is a commonly used criterion for characterization of surface quality in a machining operation. In the study of micro-scale mechanical properties of machined surface and cutting tool with nanoindentation, prefect surface finish on the test specimen is often required for the reliable result. However, the prefect surface finish is often difficult to obtain from the machining operation due to the limitation of the cutting tool geometry and machining dynamics. In presented paper, the effect of surface roughness on nanoindentation measurement was investigated by using finite element method. A 3D finite element model with three levels of surface roughness was developed to simulate the load-displacement indentation process with a Berkovich indenter. The material used in the simulation is AISI 316L stainless steel and it was modeled as an elastic-plastic von Misses material. Three levels of surface roughness, Ra, are used in the simulation, including 2 nm, 20 nm and 37 nm. The mechanical properties were calculated by combined simulation with the Oliver-Pharr method. The hardness and reduced modulus from the simulation was found to decrease with an increase of roughness. The scatter of load-depth curves and deviation of hardness and reduced modulus are affected by the changing of roughness. The height of pile-up was little affected by the surface roughness from the simulation. Combined effect of indenter tip radius and surface roughness was also investigated.