Verification of selected key assumptions for the analysis of depth-sensing indentation data

Methods for indentation analysis involve assumptions that may be problematic for visco-elastic materials. Three assumptions are discussed: (1) the unloading is predominantly elastic, (2) tip/sample adhesion and friction are negligible, and (3) no cracking occurs around the indent. A series of copolymers is investigated by indentation at room temperature, just below their glass transitions. Closer to the glass transition assumptions (1) and (2) were violated. Erroneous analysis due to extensive creep was identified by comparison of the creep rate at the end of the hold period with the unloading displacement rate and from the power of the fit to the unloading response. Assumption (3) was studied by scanning the indenter tip over the residual indent in polystyrene, which is known to be susceptible to stress localization