Chameleon coatings are adaptive surface layers made of nanoscale materials that can change surface texture and respond to extreme conditions. They often use tungsten, yttrium, gold, silver, and graphitic carbon. Yttrium-stabilized zirconia is an example of a second-generation coating that can withstand high temperatures and friction cycles. The coatings must have reversible morphology and are important for hypersonic vehicles and other high-stress applications. A 2011 US Army research yielded a durable chameleon-like coating composed of nitride, silver, and MoS2 compounds.
A chameleon coating is a type of surface layer applied to various artifacts that has adaptive qualities. Usually, it is composed of materials engineered at the nanoscale around the size of single atoms or small molecules. Coatings are designed to have properties such as the ability to change surface texture or dynamically respond to humidity, extreme temperatures, or vacuum environments, such as for reusable launch vehicle coatings in rockets. Composite linings are often used that consist of multiple layers of different materials in the lining so that it can be self-lubricating and have a lower than normal melting point.
The very first types of nanoscale engineered wear control coatings were based on the element tungsten as it is a metal widely used in the manufacturing of high stress metal parts. Compounds such as WC, tungsten fused with diamond-like carbon (DLC), and tungsten disulfide, WS2, have been investigated. Metals such as tungsten and yttrium are often part of a chameleon coating because their lubricating quality is provided by dry lubricants in the form of carbon, sulfur or other nanoparticles which interact favorably with them. Gold and silver are other frequently used metallic elements, as they have the ability to withstand a high level of temperature gradient where they can exhibit lubricating qualities when incorporated within a matrix structure. Graphitic carbon is also included in some formulations as a useful sliding surface in the presence of water or high humidity.
An example of what is considered a second-generation chameleon coating as of 2004 is yttrium-stabilized zirconia (YSZ), a form of ceramic, which is embedded within a gold grid structure that encapsulates nanoparticles of molybdenum disulfide, MoS2 and diamond-like carbon. This particular chameleon coating composition can withstand sliding friction cycles of 10,000 repetitions or more without degrading and temperatures up to 932° to 1,112° Fahrenheit (500° to 600° Celsius). This has led to the interest of various manufacturing companies in the aerospace sector. Such chameleon coatings are designed to have tribological properties, where tribology refers to the ability of the coating to resist friction, wear and also have self-lubricating aspects.
The key aspect of any chameleon coating is that it has a reversible morphology in its composition. The coating must be able to change friction and wear in response to conditions and then return to a latent state without chemical degradation. Arenas where this is important include building structural and aerodynamic elements for hypersonic vehicles subjected to extreme physical stresses at high altitudes and speeds.
A chameleon coating is designed to have a large change in material properties from a scale of one-trillionth of a meter down to one-tenth of a millimeter (0.0000000001 to 0.0001 meters). March 2011 research for the US Army yielded a chameleon-like coating composed of nitride, silver, and MoS2 compounds that set a world record for durability. The material has been cycled over 300,000 times through a temperature range from room temperature up to 1,292° Fahrenheit (700° Celsius) with no noticeable degradation in material properties.
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