The influence of indenter-surface misalignment on the results of instrumented indentation tests

A quantitative study is presented on the influence of the sample misalignment on load-displacement curves measured in instrumented indentation. Three different tip geometries are considered: a Berkovich tip, a spherical tip and a flat-ended punch. A special alignment tool was developed that allowed us to perfectly align the sample surface perpendicular to the loading-axis of the tip, regardless of the tip geometry and the origin of the misalignment. Moreover, this tool enabled us to systematically vary the angle of misalignment and study its effect on the indentation results.It is shown that sample-misalignment angles smaller than 1.2¿ have no effect for the Berkovich and spherical tips, whereas flat-ended punch indentations are extremely sensitive to these small alignment errors. The strongest influence is observed in the linear elastic region, where the contact stiffness decreases markedly with increasing misalignment. In the plastic regime, the sensitivity to misalignment disappears.Finally, we present a simple method to correct the influence of sample misalignment on the load-displacement curves obtained in flat-ended punch indentation