| Friction may cause considerable mechanical stress in microvolumes adjacent to the contact spot. Relaxation of this stress is often associated with plastic deformation and structural changes in the surface layer. A layer of secondary structures appears [1]. In the literature, this is also referred to, for example, as the third body [2], the frictional layer [3], and the friction-induced deformation layer [4]. Its thickness characterizes the penetration depth of plastic deformation into the frictional-contact zone. The output characteristics of the frictional system (wear rate, frictional coefficient, surface temperature, etc.) depend on the properties of the layer of secondary structures. Severe friction appears, for example, at high pressure or high slip rate, with limited heat transfer from the frictional zone, in the absence of lubrication, and with the passage of electrical current through the contact spot. The mean surface temperature in those conditions may exceed 200°C [5–۸]. It is of interest to determine the mean temperature of the slip surface, since the temperature affects the strength of the surface layer more powerfully than factors such as the load, the slip rate, or the number of loading cycles [7]. Known methods of assessing the mean contact surface temperature do not always present a satisfactory picture of the influence of temperature on surface failure in conditions of cyclic plastic deformation [5–۱۰]. It follows from general principles that the wear resistance will decline with increase in mean surface temperature. However, we need better information regarding the mutual dependence of these characteristics and their relation with the layer thickness of the secondary structures. The model frictional pair may be the slipping electrical contact of steel 3 and steel 45, by analogy with [11]. In that case, the contact current density is the main factor responsible for the increase in temperature and the failure of the surface layer. Therefore, the dependence of the mean surface temperature on the contact current density is also of interest. In the present work, we obtain an initial idea of the relations among the contact current density, the layer thickness of the secondary structures, the mean surface temperature, and the wear rate of steel 3 in slipping electrical contact without lubrication. The model sample is produced from carbon steel 3 (Fe + 0.2% C) of hardness HB = 2740 MPa. Metallographic data regarding the cross section of the secondary structures is obtained on a Neophot-21 optical microscope. The wear rate of steel 3 and the electrical conductivity of the frictional zone are determined in conditions of slipping electrical contact without lubrication in the presence of 50-Hz alternating current, at a pressure p = 0.13 MPa and slip rate v = 5 m/s on an SMT-1 frictional machine in a pin-on-ring configuration (Fig. 1a). The counterbody is made of steel 45 (50 HRC). The slipping distance is 9 km at each current density. |
