Diamond-like carbon
Diamond-like carbon (DLC) is a class of amorphous carbon materials that display some of the typical properties of diamond. DLC is usually applied as coatings to other materials that could benefit from such properties.
DLC exists in seven different forms. All seven contain significant amounts of sp3 hybridized carbon atoms. The reason that there are different types is that even diamond can be found in two crystalline polytypes. The more common one uses a cubic lattice, while the less common one, lonsdaleite, has a hexagonal lattice. By mixing these polytypes at the nanoscale, 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 tetrahedral amorphous carbon (ta-C). Ta-C can be considered to be the "pure" form of DLC, since it consists almost entirely of sp3 bonded carbon atoms. Other materials notably hydrogen and graphitic sp2 carbon are present in the other 6 forms usually as a result of the specific production process. The various forms can also be doped with elements such as Silicon to impart other desirable properties.
The various forms of DLC can be applied to almost any material that is compatible with a vacuum environment.