Improvements in the indentation method with a surface force apparatus

On the nanometre scale, the actual indenter-material contact area must be carefully determined to obtain reliable values of mechanical properties from an indentation test. On this scale, the contact area is greatly affected by the geometrical tip defect and by the possible formation of plastic pile-up (or sink-in) around the indent. Parameters such as local surface roughness and heterogeneity in surface and in thickness make it difficult to conduct and to interpret nanoindentation tests. A new method, which couples nanoindentation experiments and imaging procedures, has been developed. Nanoindentation tests and topographic images are performed with a surface force apparatus developed in our laboratory. The important point of our method is the ability of our three-axis device to generate topographic images without having to move the sample. This allows us to determine precisely the actual tip-sample contact area, while performing accurate continuous quantitative quasistatic load measurement and simultaneous dynamic measurement of the contact stiffness against penetration depth. In this paper, the method is described. Results obtained on a gold layer are presented as an illustration of the method. They show how we solve some problems and try to push forward the limits of indentation tests.