In this work, we performed nanoindentation studies on polymers with different moduli in the range of several millipascals up to several gigapascals. The focus was on the initial contact identification during indentation testing. Surface-detection methods using quasi-static loading as well as methods employing the dynamic forces associated with the continuous stiffness measurement technique were compared regarding their practicability and accuracy for the testing of polymeric materials. For the most compliant material with a modulus of 1 MPa, where contact identification is most critical, we used load-displacement curves obtained from finite element modeling analysis as a reference for the evaluation of experimental techniques. The results show how crucial the precise surface detection is for achieving accurate indentation results, especially for compliant materials. Further, we found that surface detection by means of dynamic testing provides mechanical-property values of higher accuracy for all polymers used in this study. This was due to smaller errors in surface detection, thus avoiding a significant underestimation of the contact area.
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The authors thank C. Greiner and E. de Souza for providing the SU8 and PDMS samples. We gratefully acknowledge the opportunity to use the facilities of the Institut fuer Statik und Dynamik at the University of Stuttgart for performing the FE simulations, and thank M. Deuschle for his support. For the measurements using the Hysitron indenter, we are grateful to Dr. Z. Burkhard. Helpful discussions with Dr. H. Pfaff are very much appreciated.
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Deuschle, J., Enders, S. & Arzt, E. Surface detection in nanoindentation of soft polymers. Journal of Materials Research 22, 3107–3119 (2007). https://doi.org/10.1557/JMR.2007.0394