Microstructural Changes in Surface Layers of Metal During Running-in Friction Processes

Studies of the dislocation structure of surface layers of some FCC and BCC metals, such as copper and steel, after sliding-friction tests for various lengths of time are reported. The results of the transmission electron microscopy and X-ray investigations show that each material is characterized by its own definite state of surface layer structure, which corresponds to the conditions of the friction, such as load, materials of counterbody and others. A sequence of microstructural changes for increasing test time consists of an increase in dislocation density and formation of dislocation pileups and a fragmented structure. It is established that under constant conditions of friction, the characteristics of the microstructure only vary in the running-in period. The kinetics of microstructure formation in the surface layers during friction predetermine the hardening and negative hardening processes and the wear of material. Comparison of the results of investigation of the structure with wear data and the data on roughness changes, shows that the time of transition to stable wear and equilibrium roughness of the surface coincides with the time of stabilization of the structure. The changes of the surface roughness in the running-in period occur simultaneously with those of the microstructure. These two processes are interrelated. First, when the equilibrium roughness is reached, the pressure on the contact spots decreases. Second, the fragmented structure, formed during the running-in period, strengthens the surface layers. Both these phenomena lead to a reduction in the wear rate. The results of this investigation can be used for development of structural methods of tribosystem diagnostics. They show that the running-in period is characterized by the change of microstructural state of surface layers. Consequently, the parameters of the microstructure can be used to determine this period.