Study of frictional properties, long-term (cyclic) strength of materials of brake pads of motor vehicles

Abstract

Fractogram analysis shows that deep scratches, particles of titanium and chromium carbides and other elements were found on the friction surfaces. Moreover, in the braking devices there was damage to the large size of the brake pads of trucks. The transfer of particles of borides and carbides to the surface of the wheels can be explained by the processes of metal flooding with their subsequent embrittlement, which inevitably leads, as a rule, to the destruction of friction wheel pairs. It is established that such types of wear as fretting corrosion significantly (by 1.5-2 times) reduce the fatigue limit of parts. Also significantly reduce the cyclic strength of metal friction pairs oxide films on their surface in the absence of lubricant. The service life of friction wheel pairs has a particularly strong impact on fatigue strength. The main reason for the decrease in endurance due to the processes of setting on the working surfaces of friction units is a high concentration of stresses caused by deep tears, cuts, microcracks. The process of destruction of brake pads from fatigue begins from the surface of the part. In this regard, the quality of the surface, its structural-phase composition, physical and mechanical properties of the surface layer in most cases are decisive for the intensity of the development of wear processes of parts from fatigue of the tribosystem (friction wheel pairs), which are operated under cyclic loads. The peculiarity of the influence of friction and wear processes on the fatigue strength of metal is that at the time of running-in there is a change in surface roughness, structure and properties of surface layers. As the analysis of literature sources has shown, the effectiveness of the influence of friction and wear processes on the characteristics of fatigue resistance in the case of repeatedly alternating (cyclic) loads is essential, and therefore ignoring this effect during the traditional assessment of the reliability of parts by individual criteria, for example, wear resistance, often leads to an incorrect assessment of the operational durability of the elements of the tribological system of road or rail transport. The long-term (cyclic) strength of brake pads was determined on a specialized unit model 1251 by Instron company (Great Britain). The basis for spraying and surfacing of different types of coatings was normalized steel 35. Tensile-compressive deformations at zero average stress and a cycle frequency of 20 Hz were studied on the laboratory unit. Most of the tests were carried out in salt solutions (NaCℓ of industrial purity was used).The process of destruction of brake pads from fatigue begins with the surface of the part. In this regard, the quality of the surface, its structural-phase composition, physical and mechanical properties of the surface layer in most cases are decisive for the intensity of the development of wear processes of parts from fatigue of the tribosystem (friction wheel pairs), which are operated under cyclic loads. Endurance limits in the case of simultaneous exposure to friction forces and cyclic loads will depend on the sliding speed of the tangent surfaces of the normal contact load, which determines the friction force, and the composition of the environment.

Keywords

• fractograms, friction, destruction, wear, structural-phase composition, physical and mechanical properties, wheel pair, tribology, cyclic strength, microcracks