Evaluation of the real contact area in three-body dry friction by micro-thermal analysis

Many tribological properties and wear mechanisms occurring on the micro-and nanoscale are strongly controlled by the so-called real contact area (A_r) which is a small fraction of the nominal or apparent contact area (A_a). The determination of A_r 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 (F_n, v, T) and the tribological responses (μ, F_t) 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.