Tetrahedral amorphous carbon
Diamond-like carbon (DLC) is a class of amorphous carbon material that displays some of the typical properties of diamond. DLC is usually applied as coatings to other materials that could benefit from some of those properties.
DLC exists in seven different forms. All seven contain significant amounts of sp3hybridized carbon atoms. The reason that there are different types is that even diamond can be found in two crystalline polytypes. The more common one has its carbon atoms arranged in a cubic lattice, while the less common one, lonsdaleite, has a hexagonal lattice. By mixing these polytypes in various ways at the nanoscale level of structure, DLC coatings can be made that at the same time are amorphous, flexible, and yet purely sp3 bonded "diamond". The hardest, strongest, and slickest is such a mixture, known as tetrahedral amorphous carbon (ta-C). For example, a coating of only 2 μm thickness of ta-C increases the resistance of common (i.e., type 304) stainless steel against abrasive wear; changing its lifetime in such service from one week to 85 years. Such ta-C can be considered to be the "pure" form of DLC, since it consists only of sp3 bonded carbon atoms. Fillers such as hydrogen, graphitic sp2 carbon, and metals are used in the other 6 forms to reduce production expenses or to impart other desirable properties.
The various forms of DLC can be applied to almost any material that is compatible with a vacuum environment. In 2006, the market for outsourced DLC coatings was estimated as about 30,000,000 € in the European Union. In October 2011, Science Daily reported that researchers at Stanford University have created a super-hard amorphous diamond under conditions of ultrahigh pressure, which lacks the crystalline structure of diamond but has the light weight characteristic of carbon.
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