Evolution Mechanism of Tribo-Magnetization Effect for Ferromagnetic Materials

The origin of tribo-magnetization correlates to the plastic deformation of subsurface caused by interfacial tribological sliding, the plastic deformation of subsurface affects the friction coefficient and friction state, in turn. This allows to establish the relationship among the wear state of the interface, the plastic deformation of the subsurface layer and the magnetic field upon the sliding surface, which helps to develop a new detection and prediction technology of wear state so as to make up for the shortcomings of the existing technologies. According to the theory of ferromagnetism, the elastic-plastic deformation of ferromagnetic materials in the process of tension or compression can lead to the change of magnetic domain structure in varying degrees. Different from the simple tension or compression condition, the effect of plastic deformation caused by interfacial tribological sliding on the magnetic domain structure is more complex. So far, it is not very clear about the specific microstructure changes and corresponding domain structure changes of the subsurface region under the complex tribological sliding. In this paper, the plastic deformation underneath the sliding interface and the changes of magnetic domain structure caused by the interfacial tribological sliding were deeply studied to reveal the evolution of tribo-magnetization of ferromagnetic materials. The friction tests of non-magnetic 316L stainless steel/commercial pure iron were carried out at room temperature under the geomagnetic field. The number of reciprocating sliding of each specimen was set to 3 000 times. The magnetic field signal upon the surface of commercial pure iron was detected before and after friction. The friction during sliding was monitored in real time. The results indicated that when the friction variance increased, the mean value of the magnetic field increment upon the sliding surface also increased, indicating that there was a certain relationship between the tribological behavior of the sliding interface and the tribo-magnetization. Here, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used to obtain the microstructure of the subsurface region of commercial pure iron after sliding, then calculated the GOS value of the grain. The results showed that the friction variance was 3.26×10⁻⁵, 1.64 ×10⁻³, 7.02×10⁻³ and 7.23×10⁻² respectively, the corresponding GOS peaks were 5.76°, 6.69°, 8.60° and 5.89°, and the depth of deformation area were 2.33×10², 2.65×10², 2.80×10² and 3.24×10² μm. The depth of deformation area was defined by the depth of GOS value from large fluctuation to relatively stable fluctuation. Concurrently, the magnetic domain structure of the subsurface region underneath the sliding interface was observed by using the Bitter method. The results showed that at the depth of 162.5 μm underneath the sliding interface, the domain structures of the two grains present strip-shaped with different directions before sliding. After sliding, their width increased from 1.6 µm to 3.7 µm, their spacing increased from 3.1 µm to 10.7 µm, and some magnetic domains were in circular arc shape. Interestingly, the phenomenon that the magnetic domain crossed the grain boundary appeared, which can be speculated from the change in grain boundary misorientation under the tribological contact. Moreover, scanning electron microscopy (SEM) image and geometrically necessary dislocation (GND) density map showed that many sub-grains were formed, especially near the grain boundary. The geometric necessary dislocation density near the grain boundary increased obviously. The changes of magnetic domain structure were closely related to the changes of microstructures as mentioned above. The results mentioned above can provide the theoretical basis for using the effect of tribo-magnetization to detect and predict the friction and wear state.