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Adaptive tribological coatings are materials capable to retain their tribological properties in varying environment due to an adjustment of the surface composition and micro-structure to the changing conditions.
The adaptive tribological coatings are developed mainly for space vehicles (sattelites, shuttles), which operate in widely changing ambient conditions: at temperatures from -233 to +212°F (-147 to +100°C), humid to dry atmosphere or vacuum, normal to carbon dioxide atmosphere (on Mars), cosmic radiation.
In addition some systems of the space vehicles may heat up even higher than 212°F (100°C) reaching 1472°F (800°C) during entry to the atmosphere.
Liquid lubricants are not able to serve in such broad environmental range. Some Solid lubricants may withstand the extreme conditions but their effectivenes as lubricants changes drastically.
Graphite has excellent lubrication properties in a humid atmosphere however its tribological characteristics are low in vacuum or dry atmospere. Additionally it oxidizes at temperatures higher than 900°F (482°C).
Molybdenum disulfide in contrast to graphie demonstrates low friction in dry atmosphere and in vacuum where its coefficient of friction is even lower than in the presence of water vapor. In non-ixidizing environment and in vacuum molybdenum disulfide is stable up to 2100°F (1150°C). However in open air at elevated temperatures it oxidizes at temperatures above 700°F (371°C).
PTFE provides coefficient of friction which does not depend on the environment. It is as low in vacuum as in oxidizing, non-oxidizing moist and dry atmospheres. However applications of polytetrafluoroethylene in open air at elevated temperatures is limited to 500°F (260°C).
Soft metals (lead, tin, bismuth, silver, gold) may serve as solid lubricants due to their low shear strength. Their lubrication characteristics do not depend on the atmosphere and they are particularly high at elevated temperatures close to the melting point. Noble metals such as silver and gold have additional advantage since they oxidize much slower at elevated temperatures than other soft metals.
Adaptive tribological materials have a composite nano-structure combining different solid lubricants incorporated in the coating matrix. The coating structure provides selective action of solid lubricants depending on the temperature, atmospheric and loading conditions. The environmental conditions determine the composition and the structure of the tribo-layer forming on the coating surface. When the conditions change the the tribo-layer composition change adapting itself to the new environment. Due to such behavior adaptive coatings are sometimes called “chameleon” or smart coatings.
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The following concepts are proposed by A.A. Voevodin and J.S. Zabinski [1] for a design of adaptive tribological nanocomposite coatings:
Adaptive tribological coating of yttria-stabilized zirconia (YSZ), gold (Au), Dimond-Like Carbon (DLC) and Molybdenum disulfide (MoS2) was obtained by magnetron assisted pulsed laser deposition. Nano-crystalline YSZ provided a combination of high hardness and toughness. Nano-crystalline and amorphous Au, MoS2 and DLC were added to impart lubricity under different ambient conditions. They are sealed in the coating and protected from an oxidation at high temperatures. When a friction starts the lubricants behave differently. At low temperature and in a humid atmosphere DLC partially converts into graphite and forms a transfer tribo-film with low coefficient of friction (0.1-0.15). Molybdenum disulfide is not active in this environment. However when the atmosphere becomes dry or changes to vacuum the inclusions of amorphous mlybdenum disulfide transforms into crystalline form operating as lubricant in the transfer layer. Graphite is removed from the surface because of its high friction in vacuum. When the temperature rises the lubrication properties of graphite and MoS2 decrease but the atoms of soft metal (Au) diffuse to the coating surface, form crystalline grains of 20-80 nm, which form an easy-to-shear transfer film.
Other examples of adaptive tribological coatings:
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