Solid lubricant coatings are widely applied in tribological contact parts working under extreme environments, such as high-temperature and high-vacuum conditions, where traditional liquid lubricants fail to provide effective lubrication. At temperatures above 400℃, most conventional solid lubricants such as graphite and MoS2 lose their lubrication properties due to oxidation. In order to reduce high-temperature friction and wear, many efforts have been made on various materials. Recently, adaptive self-lubricating coatings have been considered promising materials for their good tribological properties over a wide range of temperatures. These coatings usually employ at least two components as solid lubricants to adjust the surface chemical compositions with the changes of temperature and reduce the friction. At present, one trend in developing adaptive self-lubricating coatings is focused on a Ag-Mo composite solid lubricant and on the study of its synergistic lubrication effect. Huet al. prepared a yttria-stabilized zirconia (YSZ) nanocomposite coatings by combining filtered vacuum arc, magnetron sputtering and pulsed laser deposition with addition of Ag and Mo as the solid lubricants. These coatings maintained a friction coefficient of about 0.4 from 25 to 700℃. The diffusion and coalescence of Ag played an important role in reducing friction below 500℃ while the formed molybdenum oxides above 500℃ were found to be effective high-temperature lubricants. The tribological properties of plasma-sprayed NiCrAlY-Ag-Mo coating were investigated by Chen et al. The results showed that the silver molybdate and molybdenum oxide produced through tribo-chemical reaction above 600℃ effectively reduced the friction coefficients to about 0.3. HVOF-sprayed NiMoAl-Ag coating also exhibited excellent self-lubricating properties from 20 to 800℃, and the nearly molten Ag on the wear surface at 800℃ had a synergistic effect with Ag2MoO4 on reducing the friction. However, some problems may occur during the plasma spraying process by using mechanically blended Ni-based alloy, Ag and Mo feedstock powders. Firstly, the general structure of conventional plasma spraying devices is based on a single-powder feeding pipe. It is practically impossible to feed the traditional blended powders into the center of the plasma plume due to the large differences of density among them. Therefore, different constituents become separated in the flame resulting in poor uniformity of the coating’s microstructure. Secondly, these blended powders have significant differences in melting point, specific heat capacity and other thermo-physical properties. For example, Ag has a melting point of about 960℃ and a boiling point of about 2210℃, while Mo exhibits a much higher melting point of about 2610℃. If the spraying parameters meet the melting point of Mo, Ag will have a significant vaporization loss. But if only the deposition condition of Ag is met, Mo particles will not achieve sufficient adhesion in the collision onto the substrate surface. Besides, it is unavoidable to have some oxidation of pure Mo and the consequent volatilization of MoO3 during the spraying process, inducing a big loss of the spray powders and a negative effect on the coating’s constituents. These problems have deeply influenced the microstructure, hardness and triblogical properties of the self-lubricating coatings. In our previous study, coated powders have been proved to enhance the uniformity of the feedstock and reduce the volatile loss of the feedstock during plasma spray process. In this paper, NiCr-coated Ag-Mo composite powders were prepared as the feedstock powders. The self-lubricating properties as well as the formation mechanisms of high-temperature lubrication films on this NiCr/Ag-Mo composite coating obtained by air plasma spraying were further explored.
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