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Self-adaptive high temperature solid-lubricating coatings

Time:2020-05-22 21:14:30  From:  Author:

 USA Air Force Research Laboratory (AFRL) has synthesized and evaluated a number of adaptive high temperature solid-lubricating coatings, also named “chameleon” coatings, which are as a new class of smart materials that are designed to adjust their surface chemical composition and structure in the various working environment to reduce friction and wear between contact surfaces. Adaptive oxidebased and nitride-based high temperature solid-lubricating coatings were developed progressively. The hard matrices were incorporated into soft metal for low-moderate temperatures and dichalcogenide and carbon phases for low temperature lubrication, while lubricous oxides provide low friction coefficient at high temperature. Tribological properties of the typically adaptive coatings are given. Adaptive oxide matrix high temperature solid-lubricating coatings, such as YSZ-Au, YSZ-AgeMo, YSZ-AgeMoeMoS2, YSZ-Au-DLC-MoS2 and Al2O3-DLC-AueMoS2 coatings, were produced by hybrid magnetron sputtering and pulsed laser deposition processes. The friction coefficients are in the range from 0.10 to 0.40 for 5000 to 10000 cycles from room temperature to 500 °C. However, soft metal diffusion on the surface reduces the lifetime of coatings. By tailoring the pattern in the TiN diffusion barrier, the diffusion of silver to the wear scar is restricted, therefore providing a larger supply of soft metal. The coating longevity is extended without sacrificing low friction properties. Adaptive nitride-based high temperature solid-lubricating coatings was explored continuously, which include Mo2NeAg,Mo2NeMoS2eAg, Mo2NeCu, TiNeAg, CrNeAg, ZrNeAg, CrAlNeAg, VNeAg, NbNeAg, and Ta-NeAg. At low-moderate temperatures, soft metal with an easy shear can effectively lubricate for nitride-based hard coatings using metal diffusion on the worn surface. When the working temperature exceeds 500 °C in air, tribo-oxidation is a dominating lubricating mechanism. The frictional surfaces of the composite coatings form the lubricious ternary oxides such as silver molybdates, vanadates, niobates, and tantalates. It is noted that the lubricious ternary oxides are found to be associated with the layered atomic structure with the weak interplanar bonds. When tested against silicon nitride at high temperatures, the friction coefficient for silver molybdates in Mo2NeMoS2eAg coating is found to be in the 0.10–0.20 range in the vicinity of 600 °C, silver vanadates in VN-Ag coating has the friction coefficient of 0.15–0.20 in the 700–1000 °C range, while silver niobates in NbN/Ag and silver tantalates in TaN/Ag coating has the average friction coefficient of 0.27 and 0.23 at 750 °C, respectively. Based on the find of lubricious ternary oxides, the individual lubricant coatings like the Ag3VO4 coating and the AgTaO3 coating were also investigated. It was found that the silver tantalate coating against Si3N4 counterface reveals very low friction coefficients in the 0.04–0.15 range at 750 °C. Self-adaptive coatings provide an inspiring clue to explore smarting high temperature lubricating materials. At a wide temperature range, adaptive solid-lubricating coatings offer low friction coefficient in the 0.10–0.40 range, especially at elevated temperatures, providing friction coefficient below 0.20. The broad temperature range adaptation are attributed to adaptive mechanisms using metal diffusion, adaptive mechanisms using tribo-oxidation, and adaptive mechanisms using structural transitions. The synergistic action of these mechanisms can cover the full range. Special emphasis will be placed on modern practices that are aimed at enhancing the properties of these coatings and expanding their uses in practical applications.


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