Surface detection in nanoindentation of soft polymers
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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.
I. INTRODUCTION
Over the past decade, nanoindentation testing equipment and methods have continuously been improved to allow a fast and convenient determination of mechanical material properties such as modulus and hardness. This technique is now widely used to study the behavior of metallic and ceramic engineering materials. Nano-indentation offers the possibility to probe mechanical properties in confined volumes and with sub-micronewton and subnanometer resolution. For this reason, it can also be beneficial for the mechanical characterization of polymeric materials and biological tissues, which are of increasing importance for technological and medical purposes.1,2 Polymers are often used as wear- and scratch-resisting coatings, electrically insulating or optically functional films, and in medical applications. The use of polymeric material in reduced dimensions, however, requires adequate test methods that capture the time-dependent viscoelastic behavior of polymers and the small forces associated with the mechanical testing of such materials. Several have reported in the literature have indicated that nanoindentation can be successfully applied to measure the mechanical properties of polymers3–9 and of biological materials,10–13 but there is still considerable uncertainty associated with the accuracy of these measurements. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0394 J. Mater. Res., Vol. 22, No. 11, Nov 2007
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The high compliance of polymers in comparison with metals or ceramics leads to very low forces between several nanometers and micronanometers during the indentation experiment.3,5,7 For most commercially available nanoindentation instruments, the theoretical load resolution is better than 1 nN. Thus, they are theoretically capable of detecting the initial cont
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