The multiphase high temperature solid-lubricating coatings were pioneered by USA National Aeronautics and Space Administration (NASA) laboratories. In the past 40 years, the PS100, PS200, PS300 and PS400 families of plasma sprayed coatings with solid-lubricating behavior were developed at NASA Lewis Research Center (now named NASA Glenn Research Center). Comparison of NASA plasma spray coatings is shown in Table. The chief design concept of PS high temperature solid-lubricating coatings is to employ the combination of solid lubricants in obtaining acceptable lubricity over a wide temperature range because the single solid lubricant cannot achieve low friction coefficient from low temperature to elevated temperature. It is found that the joint action of silver and fluorides is a most effective combination. The PS100 family consisting of nickel-glass-solid lubricant coatings is the first generation PS high-temperature solid-lubricating coating. As PS100 coating worked on the bearing for a long time, it exposed the following shortcoming: low hardness, high wear rate, and easy oxidation of above 700 °C. Subsequently, to remedy the weakness, PS200 coating system were developed, which are composed of a hard nickel-cobalt-bonded chrome carbide matrix and solid lubricants of Ag and BaF2/CaF2 eutectic. However, PS200 have some drawbacks. The hard nickel-cobalt bonded chrome carbide requires diamond grinding prior to service. Furthermore, at high temperatures above 800 °C in air, chrome carbide oxidizes to cause slight dimensional swelling of the coatings. To overcome these disadvantages, PS300 coatings supplanted the nickel-cobalt bonded chrome carbide of PS200 coatings with the nickel-chrome bonded chrome oxide. This coating system was not very hard but had desirable wear resistance and friction coefficient, especially at elevated temperature up to 650 °C. Recently, a new coating system, PS400 containing a nickel-molybdenum-aluminum matrix with chrome oxide hardeners combined with silver and fluoride solid lubricants, has been developed due to several defects of PS300, namely the need to undergo a heat treatment for dimensional stabilization and poor initial surface finish. These four distinct families of coatings have engineering application over the last four decades. PS200 has solved a wide range of lubrication problems in prototype hardware applications such as process control valve stems, foil air bearings, rotating face valves and butterfly valve stems. Additionally, PS200 has been used successfully in a Stirling engine as a cylinder wall lubricant coating and also as a gas bearing journal back up lubricant. PS304, one of the best performing PS300 coatings, is comprised of 20 wt% Cr2O3, 10 wt% Ag, 10 wt% BaF2/CaF2 eutectic, and 60 wt% NieCr matrix, which is successfully applied to foil gas bearings and other high temperature sliding contacts. NASA PS coatings firstly realize the excellent lubricity over a wide temperature range. Base on NASA PS coating idea and strategy, some new solid-lubricating coatings developed by adjusting composition or altering preparation technique attempt to improve the overall performance, and some evolutions of solid-lubricating coating and preparation technology are presented as shown in Table. The plasma spraying composite coatings usually consist of porous microstructure and low cohesive strength, which causes the reduction in the hardness and bonding strength of the coatings. These disadvantages can be overcome by an advanced surface modification technology-laser cladding, which uses a high power laser beam to form a coating metallurgically bonded to the substrate. Referring to PS212 coatings, a NiCreCr3C2eAgeBaF2/CaF2 high temperature solid-lubricating coating was prepared by laser cladding. The coating exhibits high microhardness and metallurgically bonding strength, and low friction coefficient of 0.28–0.36 from room temperature to 500 °C when rubbing against Si3N4 ball.
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