What is DLC?

	DLC is an acronym for diamond-like carbon. As the name implies, diamond-like carbon has some of the valuable properties of diamond. It can be applied as a coating on almost any material that is compatible with the vacuum in which it is usually produced. When applied in pure form it is as hard as natural diamond - or even harder. In pure form these diamond coatings offer extraordinary protection against abrasive wear and attack from atmospheric moisture and chemical vapors. Although smooth when seen with visible light, diamond like carbon actually has the form of a cobblestone street. Little known is the fact that in nature there are two different crystalline geometries for diamond. "Common" diamond known from jewelry has carbon atoms arranged in 3-dimensional cubic lattices. In a rare form called lonsdaleite the lattice would be hexagonal, like cells in a beehive. In DLC the cobbles are not crystalline; they are amorphous because they are made from random alternations between cubic and hexagonal lattices. The cobbles have no long-range order and so they have no fracture planes along which to break. The result is a very, very strong material.		Polished germanium (Ge) infra-red optical window protected by a coating of "pure" DLC.
	By stacking more and more layers of these nodules, DLC coatings can be made that at the same time are amorphous, flexible, and yet purely 3-dimensionally (sp3 ) bonded "diamond". The hardest, strongest, and slickest is known as tetrahedral amorphous carbon, or ta-C. For example a coating of only 2 μm thickness of ta-C increases the resistance of common (ie. 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 often used in 6 other known forms of "impure" DLC to reduce production expenses. Such economies decrease the service lifetimes of the articles being coated; sometimes by drastic amounts. A more extensive review of the current status and technical aspects of DLC and its 7 forms can be found in the online "Encyclopedia" article at: Diamond-like carbon at Wikipedia Back to Table of Contents		Nanostructure of pure DLC showing the nodules ("cobblestones") in which carbon atoms are arranged in 3-dimensional diamond bonding. The scale marker is about 1% of the thichness of a human hair. Image made with electron microscopy.