Micro hardness determination on a rough surface by using combined indentation and topography measurements

Micro hardness determination on rough surfaces is topic of interest e.g. in industrial applications where the component surface is of functional relevance. Even when the indentation depth is large enough to overcome indentation size effects, surface topography signicantly affects the indentation testing result. In most approaches hardness on a rough surface is determined by simply including profile roughness parameters like R a (e.g. [1-5]) to adjust the measured hardness value or to get the minimum indentation depth value where the influence of surface topography is assumed to become negligibly small. In the present study, local surface topography data were used instead to enable precise micro hardness measurements on a sample with arbitrary surface topography. Three samples made of 1.4571 stainless steel with different surface states were face milled by using an end mill with different feed rates. Instrumented indentation tests were performed on these samples as well as on comparative samples with polished surfaces. From the resulting load-indentation depth (Fd)-curves the averaged indentation hardness was calculated for all surface states. Then, a method was applied to manipulate and to average the F-d-curves in order to eliminate large deviations, mainly occurring at the very beginning of indentation. Indentation hardness was calculated from the resulting modied F-d-curves and compared to the indentation hardness from the actually measured F-d-curves of the polished samples with feasible results. Using surface topography measurements is considered to enable deriving more accurate indentation hardness values directly and to put the investigations to another level. Surface topography of the samples was evaluated by confocal microscope measurements before and after the indentation tests. From local surface topography data at the location of indentation, four parameters were calculated: volume, projected contact area and depth of indentation mark, as well as the curvature of the surface topography before indentation. The four parameters were correlated with the hardness value from the respective indentation and compared to the hardness of the polished sample. The results of the present study are the basis for combining optical imaging techniques like confocal microscopy or white light interferometry and indentation testing equipment to broaden the field of application.