Local mechanical properties by Atomic Force Microscopy nanoindentations

The analysis of mechanical properties on a nanometer scale is a useful tool for combining information concerning texture organization obtained by microscopy with the properties of individual components- Moreover, this technique promotes the understanding of the hierarchical arrangement in complex natural materials as well in the case of simpler morphologies arising from industrial processes. Atomic Force Microscopy, AFM, can bridge morphological information, obtained with outstanding resolution, to local mechanical properties. When performing an AFM nanoindentation, the rough force curve, i.e., the plot of the voltage output from the photodiode vs. the voltage applied to the piezo-scanner, can be translated into a curve of the applied load vs. the penetration depth after a series of preliminary determinations and calibrations. However, the analysis of the unloading portion of the force curves collected for polymers does not lead to a correct evaluation of Young’s modulus. The high slope of the unloading curves is not linked to an elastic behavior, as would be expected, but rather to a viscoelastic effect. This can be argued on the basis that the unloading curves are superimposed on the loading curves in the case of an ideal elastic behavior, as for rubbers, or generally in the case of materials with very short relaxation times. In contrast, when the relaxation time of the sample is close to or even much larger than the indentation time scale, very high slopes are recorded. Where AFM nanoindentations are concerned, one observes a dependence of the penetration, i.e., the relative motion between the sample and the tip (indenter), on the elastic properties of a material when using equivalent loads. This relationship becomes visible on samples that are homogeneous down to the scale of nanoindentation. The elastic modulus can be obtained by applying Sneddon’s elastic contact mechanics approach, since the contact between the tip and the sample is dominated by an elastic behavior with negligible plastic deformation. Under such circumstances, the dependence of the penetration on the load follows an exponent of 1.5, consistent with elastic contact mechanics and justified on the basis of the large elastic range exhibited by polymers, on the constraints due to the geometry of the deformation during indentation and to the critical yielding volume needed in order to induce plasticity. As a result, elastic moduli taken from AFM force curves show a very good agreement with bulk values obtained by macroscopic tensile testing. This is true for a broad range of polymers, from materials with rubbery to semicrystalline, or even glassy behaviors. This result confirms that AFM nanoindentations in polymers take place mostly in the elastic range and opens the possibility of characterizing the mechanical behavior of polymers on an unparalleled small scale as compared to commercial depth sensing instruments (DSIs), which use much blunter indenters. A further application is discussed where, upon decreasing the load, and consequently the penetration depth, the scale becomes comparable to that of the underlying texture which is probed as opposed to the bulk material. Although this apparently presents a limitation on the resolution of the scale that can be mapped, this feature is discussed and shown to open the possibility of identifying properties of individual phases with their surroundings as well as the role of the connectivity among the phases.