Mapping Nanomechanical Properties near Internal Interfaces in Biological Materials
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Mapping Nanomechanical Properties near Internal Interfaces in Biological Materials Igor Zlotnikov,1 Haika Drezner,2 Doron Shilo,2 Barbara Aichmayer,1 Yannicke Dauphin,3 Emil Zolotoyabko1,4 and Peter Fratzl1 1
Department of Biomaterials, Max Planck Institute of Colloids and Interfaces, Potsdam 14476, Germany 2 Faculty of Mechanical Engineering, Technion, Haifa 32000, Israel 3 Micropaléontologie, UFR TEB, Université P. & M. Curie, 75252 Paris cedex 05, France 4 Faculty of Materials Engineering, Technion, Haifa 32000, Israel ABSTRACT Modulus mapping using nanoDMA (Dynamic Mechanical Analysis) is a recently developed technique based on a nanoindentation instrument equipped with an AFM-like piezoscanner and dynamic force modulation system. The surface properties, storage and loss moduli are quantified based on the Hertz model for the contact mechanics of the sample-tip configuration. In this approach, the applied load, topography features, and their size may have a pronounced effect on the obtained results. In order to demonstrate that, internal interfaces of deep sea sponge (Monorhaphis chuni), which comprises alternating layers of relatively thick (4 µm in average) biosilica and thin (60 nm) organic material, were characterized using the nanoDMA modulus mapping technique. Experimental data were analyzed in tight interrelation with finite element simulations. This combination allowed us to evaluate elastic modulus of a 60 nm wide organic layers in M. chuni. INTRODUCTION A recently developed nanoDMA-based (Dynamic Mechanical Analysis) modulus mapping technique [1] has been shown to be efficient in quantitatively mapping the mechanical properties of submicron features in several different materials [2,3]. This technique is based on a nanoindentation instrument equipped with an AFM-like piezoscanner and dynamic force modulation electronics. A sinusoidal force (FAC) with predetermined frequency is applied during topography scanning utilizing a constant set-point force (FDC). Local storage and loss moduli are then related to displacement amplitude and phase shift relative to applied force and calculated based on Hertzian contact mechanics of the sample-tip configuration [4]. Smooth surface, pure elastic contact and material homogeneity are the requirements of the Hertzian approach (see Figure 1a). Hence, the topography features may have a pronounced effect on the obtained results and the accumulated experimental data could be far away from reflecting the true mechanical nature of the studied material. Most common origin of topography features in composite structures is the sample preparation procedure, in particular the surface polishing. Softer components are polished faster than harder ones which results in step formation that eventually leads to deviations from pure Hertz contact and, hence, inaccurate analysis of mechanical characteristics. This effect is even stronger in case of inclusions, having the sizes on a hundred nanometer scale (i.e. close to the radius of the indenter tip). Polishing of such structures results
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