| Fretting wear is a surface degradation process invariably observed when two contacting bodies are subjected to small amplitude oscillatory movements, which has been one of main causes leading to tight fit assembly failures in aeronautical application, nuclear powder, railway industries and so on. In nuclear power plants, due to the special condition of high temperature, the fretting wear damage of steam generator tube materials is now one of the most severe degradation mechanisms, which is caused by flow-induced vibration between tube and support materials. Thanks to their attractive properties such as high hardness, good wear and chemical stability, hard ceramic coatings, as a important protection to resist the wear damage, have been widely developed and studied for various industrial applications. However, at present several studies on the fretting behavior of hard ceramic coatings were conducted generally at room temperature and almost never at high temperatures. Therefore, it is very important to investigate and understand the fretting wear behavior of coatings at high temperatures to mitigate the wear damage.The fretting wear behaviors of TiAIN coatings and Ti-Al-Zr titanium alloy were studied experimentally on a high precision hydraulic fretting wear test rig at elevated temperatures from room temperature (25℃) to 400℃in ambient air. In the tests, a sphere (Si3M4 ceramic ball) was designed to rub against a plane (Ti-Al-Zr titanium alloy with or without TiAIN coatings). An experimental layout was designed to perform analysis of the friction coefficient, wear volume and morphology of the worn surface. The worn scars were examined with Laser Confocal Scanning Microscope (LCSM). Optical Microscopy (OM), Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and profilemeter. The fretting wear behavior of the coatings and substrate were investigated systematically under different displacement, normal load and temperature. And the mechanism of high temperature fretting wear was studied. The conclusions are drawn as follows:1. The friction coefficient of the coatings and substrate is directly correlated with the normal load, which rises with the increasing of the normal load. However, there is a slight incidence of the amplitude on the friction coefficient, especially at steady stage. At 400℃, the friction coefficient was subject to considerable fluctuation with a sharp phenomenon.2. The variation of normal load and amplitude have a significant influence on the fretting wear damage of the coatings, and the wear loss increases with the rise of normal load and amplitude respectively. At ambient, the morphology of the coatings worn surface get rougher with the increasing of normal, yet inverse at 400℃. At elevated temperature, the morphology of worn surface is nearly independent of variation of the amplitude.d3. Two opposite acts temperature plays on the fretting wear were existed:①a rise in temperature will cause adhesive junctions and plastic deformation at localized regions of the contact surface, and the different thermal expansivity between the coatings and substrate. Under alternating stress, cracks nucleation and propagation occur, which lead to the fracture and delamination of the coatings. Accordingly, it causes a severe damage.①the oxidization rate of the surface layers increase with temperature increasing, which results in the formation of the oxide films on the worn scar surfaces. Once a compacted and denser oxide films formation, it can play a lubrication and protection, and mitigate damage. The former plays a leading role between 25℃and 200℃, which causes the friction coefficient and wear loss increased with temperature rise; at 200℃～400℃, however, the latter is the leading one, which causes a inverse variation.4. The wear mechanism of the coatings at 25℃and 200℃were mainly a combination of abrasive wear, delamination and oxidization wera, yet oxidization wear and delamination at 400℃. At ambient, the damage mechanism of Ti-Al-Zr alloy was mainly delamination and abrasive wear. The damage mechanism at 200℃is mainly delamination, as well as adhesive wear, while at 400℃was primarily the combination of delamination, oxidative and adhesive wear; and SEM photographs of a longitudinal cross-section of the substrate fretting scar shows three different areas:oxidised debris layer, tribologically transformed structure layer and substrate layer.5. Compared with the Ti-Al-Zr titanium alloy substrate, the stabilized and maximal friction coefficient of the coatings were lower than that of the substrate at ambient, yet inverse at 400℃. At all temperatures, the wear loss of the coatings was remarkably less than that of the substrate, and the wear scar damage of the coatings was slight, compared with the substrate. It presents that the coatings have excellent fretting wear-resistance, and improve the surface wear resistance. |
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