International audienceNacre (the pearly internal layer of molluscan shells) is an attractive nanocomposite displaying high mechanical properties, low density and a good biocompatibility with human bones. It is currently studied for both the prosthesis design and the creation of new organic/inorganic hybrid materials by mimicking biomineralisation processes. These exceptional mechanical properties are ascribed to its highly ordered layered 'bricks and mortar' microstructure and more particularly to the energy absorption ability of the mortar during crack propagation. However, this ability appears to be drastically reduced in presence of nano-shocks generated during friction by the dynamic solicitations. This paper compares two Finite Element simulations – a quasi-static compression test and a dynamic impact test – in order to consider the fracture mechanisms induced by friction. It reveals that cracks migrate from the mortar to the bricks, involving in the latter case, the formation of wear nano-debris. These numerical results are confronted with experimental results during friction

Sheet nacre is a natural biocomposite with a multiscale structure including a mineral phase of calcium carbonate (97 wt.%)

International audienceMany tribological properties and wear mechanisms occurring on the micro-and nanoscale are strongly controlled by the so-called real contact area (Ar) which is a small fraction of the nominal or apparent contact area (Aa). The determination of Ar is often based on either (i) a geometrical approach describing the real geometry of contacting surfaces or (ii) a mechanical approach involving contact mechanics and physical-mechanical properties. In addition some experimental methods have also been attempted but they generally do not take into account the presence of third body at the interface--i.e. the wear debris trapped within the contact. In this paper we propose an experimental approach to estimate the dynamic real contact area from the operating parameters (Fn, v, T) and the tribological responses (μ, Ft) in presence of third body. A scanning thermal microscope (SThM) is used for determining both the thermal conductivity of the third body and the relationship between the contact temperature and the thermal power really dissipated at the micro-asperity level. These results are combined with a thermal model of the macro-tribocontact for computing the real contact area and the real contact pressure. Validation of these results is carried out using a classical Greenwood Williamson model and finite element models built from the real AFM maps

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The load-penetration curve in elastic nanoindentation of an elastic micropolar flat by a diamond spherical punch is analyzed. The presented results are obtained by a specifically developed numerical tool based on a judicious combination of the conventional boundary element method and strong form local point interpolation method. The results show that the usual linear relationship between the material depression and the square of the radius of the contact area is also valid in this case of micropolar elastic material. It is also shown that the relation between the indentation stress (applied load over the contact surface) and the indentation strain (ratio of contact radius by the punch radius) is linear. The proportionality coefficient which is none other than the indentation stiffness varies with the coupling factor of the micropolar elastic medium. A relation between the indentation stiffness of a micropolar solid and that of a conventional solid with the same Young modulus and Poisson ratio is derived.

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