Exploring the Evolution on Fretting Behaviors of DLC Film

Fretting refers to the relative motion of two close contact parts on the order of micron under the action of alternating load, including air flow fluctuation, thermal cycle stress, fatigue load and so on. Wear refers to the process of continuous loss of material on the contact surface in the process of relative motion. Fretting damage is a form of damage with strong concealment and great harmfulness, which directly threatens the maneuverability and reliability of mechanical equipment in the whole service cycle. At present, the most commonly used method to reduce or restrain fretting damage is to plating solid film on the interface or introducing fluid lubrication to achieve protection. The most widely used solid lubricating material is carbon-based material. Diamond-like carbon (DLC) films are widely used in various lubrication because its good tribological properties. Therefore, it is necessary to study the fretting wear characteristics and fretting damage mechanism of DLC films, which can not only enrich the fretting theory of DLC films, but also provide some reference for practical engineering applications. There are many influencing factors in the process of fretting friction, which are mainly summarized as three aspects: mechanical parameters, environmental factors and material properties. In this paper, by adjusting the normal load and displacement amplitude, the fretting friction test of DLC film was carried out on a multi-function fretting wear testing machine, the ball-on-plate configuration was adopted. The effect of normal load and displacement amplitude on the fretting friction behavior of DLC films was investigated, and the damage and wear mechanism of DLC films were studied by scanning electron microscope (SEM), electron spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (Raman). The results showed that the magnitude of normal load and displacement amplitude would directly affect the fretting contact state in fretting test. With the fixed displacement amplitude, the load increased and the friction coefficient decreased. When the load was constant and the displacement amplitude increases, the fretting operation region changed from partial slip zone to slip zone, and the friction coefficient increased during the stabilization period, but the increase decreased. And the fretting damage mechanism in the partial slip regime is different from that in the sliding regime, adhesive wear is dominant in partial slip regime, and abrasive wear and oxidation wear are dominant in the sliding regime. Normal load and displacement amplitude determine the anti-friction/wear mechanism of DLC films by regulating the oxidation degree of friction interface, the formation of carbonaceous transfer film and the degree of graphitization of DLC films. In the process of fretting, under the action of extrusion and shear of two contact bodies, wear debris gradually forms a uniform and dense third-body transfer film, which transforms the friction and wear of dissimilar materials into friction and wear of the same material, which plays a good role in lubricating and reducing wear. After increasing the load, the carbon transfer layer on the surface of the wear mark is more compact and uniform, which can effectively prevent the direct contact between the film and the counterpart ball, thus reducing the friction coefficient. The friction of DLC film is further reduced by graphitization. This work provides important guiding significance and engineering value for the application of DLC films in actual fretting conditions.