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A chameleon coating is a type of surface layer applied to various manufactured goods that has adaptive qualities. Usually, it is composed of materials engineered at a nanotechnology scale around the size of individual atoms or small molecules. The coatings are designed to have properties such as the ability to change surface texture or dynamically respond to moisture, temperature extremes, or vacuum environments such as for reusable launch vehicle coatings in rocketry. Composite coatings are often used that are made up of several layers of different materials in the coating so that it can be self-lubricating and have a lower-than-normal melting point.
The very first types of wear control coatings engineered at a nano scale were based on the element tungsten since it is such a widely-used metal in manufacturing for high-stress metal parts. Compounds like WC, tungsten merged with diamond-like carbon (DLC), and tungsten disulfide, WS2, were researched. 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 powered carbon, sulfur or other nanoparticles that interact favorably with them. Gold and silver ar,e other frequent metal elements used, as they have the ability to withstand a high level of temperature gradient where they can exhibit lubricating qualities when embedded within a matrix structure. Graphitic carbon is also included in some formulas as a useful sliding surface in the presence of water or high humidity.
An example of what is considered to be a second-generation chameleon coating as of 2004 is yttria stabilized zirconia (YSZ), a form of ceramic, that is embedded within a gold grid-like structure that encapsulates nanoparticles of molybdenum disulfide, MoS2, and diamond-like carbon. This particular chameleon coating composition can endure 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 brought it to the interest of various manufacturing concerns in the aerospace industry. Such chameleon coatings are engineered to have tribology properties, where tribology refers to the coating's ability to resist friction, wear, and to also have self-lubricating aspects.
The key aspect of any chameleon coating is that it has a reversible morphology to its composition. The coating must be able to change its friction and wear in response to conditions and then change back to a latent state without chemical degradation. Arenas where this is important include in the building of structural and aerodynamic elements for hypersonic vehicles which undergo extreme physical stress at high altitudes and speeds.
A chameleon coating is engineered to have a wide-ranging change in material properties from a scale of one-trillionth of a meter on up to one-tenth of a millimeter (0.0000000001 to 0.0001 meters). Research in March 2011 for the US Army has produced a chameleon coating composed of nitride compounds, silver, and MoS2 that has set a world record for durability. The material was cycled over 300,000 times through a temperature range from room temperature up to 1,292° Fahrenheit (700° Celsius) without any noticeable degradation in material properties.
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