Keywords: nanoindentation, adhesion, soft polymers, JKR, AFM
Summary:Although nanoindentation (or instrumented indentation) was originally developed for characterization of hard, stiff materials such as metals and silicon, this tool has recently seen increased use with compliant polymers and hydrogels. One challenge associated with nanoindentation of these soft materials is adhesion. Although commonly neglected, adhesion between the tip and the sample has been shown to lead to an overestimation of modulus values when data are analyzed using the traditional Oliver-Pharr method used by most nanoindentation software. Our research group has been investigating an alternative data analysis method, the nano-JKR method, to address adhesion and accurately measure modulus values for soft materials. In the nano-JKR method, force curves that capture the whole interaction between the tip and the sample during tip approach, indentation, and tip retraction are collected using large diameter (200 microns) spherical tips and analyzed using a Johnson-Kendall-Roberts (JKR) contact mechanics model that accounts for adhesion. We have validated the nano-JKR method for characterization of compliant silicone samples (Dow Corning Sylgard 184, Smooth-On Ecoflex 10 and 50, and Smooth-On Dragonskin 10 samples) and hydrogels (UV-cured polyethylene glycol (PEG) gels) with nominal elastic moduli between 50 and 2000 kPa. Modulus values from nano-JKR analysis were validated by comparison with modulus values for the same materials measured using either nanoindentation in the presence of a surfactant, which eliminates adhesion and allows data analysis using the Oliver-Pharr method, or unconfined compression. A comparison of modulus results from different methods demonstrates that the nano-JKR method results in comparable moduli to other measurement techniques (within 10%). In contrast, analyzing the same force curve data using the Oliver-Pharr method leads to significant overestimation of the moduli for each material. This overestimation of modulus is typically larger for softer materials, for materials with higher surface energy (i.e., increased adhesion force), and for shallower indentation depths. The nano-JKR method provides a practical method for calculating modulus values from nanoindentation analysis of compliant materials in the presence of adhesion. This technique would be equally applicable to analysis of indentation force curves collected using an atomic force microscope (AFM), which often show visible evidence of adhesion. The nano-JKR method would be easy to implement in commercial software, as it can be implemented either using a curve-fitting algorithm or using a simplified “two-point method” that relies on only two data points from the unloading curve. Broad adoption of the nano-JKR method through integration into commercial nanoindentation and AFM software packages would improve the quality of nanoindentation data reported in the literature and would facilitate comparisons of results between different studies or different measurement techniques.