Types of Diamond like carbon (DLC)

Diamond-like carbon (DLC) is an umbrella term that refers to 7 forms of amorphous carbon materials that display some of the unique properties of natural diamond. The German Fraunhofer - IST institute has organized them into the chart form seen in this page background. 

In 2006 the Association of German Engineers, VDI, the largest engineering association in Western Europe issued an authoritative report VDI_2840 in order to clarity the existing multiplicity of confusing terms and trade names. It provides a unique classification and nomenclature for diamond-like-carbon (DLC) and diamond films. It succeeded in reporting all information necessary to identify and to compare different DLC carbon films that are shown in the Fraunhofer-IST chart and which are offered on the market.

	The hardest, strongest, and slickest DLC is known as tetrahedral amorphous carbon, or ta-C.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, but at the cost of decreasing the service lifetimes of the articles being coated.
	The authorative German VDI 2840 standards report affirms the superiority of ta-C.		These [sp3] bonds can occur not only with crystals - in other words, in solids with long-range order - but also in amorphous solids where the atoms are in a random arrangement. In this case there will be bonding only between a few individual atoms and not in a long-range order extending over a large number of atoms. The bond types have a considerable influence on the material properties of amorphous carbon films. If the sp2 type is predominant the film will be softer, if the sp3 type is predominant the film will be harder. A secondary determinant of quality was found to be the fractional content of hydrogen. Some of the production methods involve hydrogen or methane as a catalyst and a considerable percentage of hydrogen can remain in the finished DLC material. When it is recalled that the soft plastic, polyethylene is made from carbon that is bonded purely by the diamond-like sp3 bonds, but also includes chemically bonded hydrogen, it is not surprising to learn that fractions of hydrogen remaining in DLC films degrade them almost as much as do residues of sp2 bonded carbon.
	To identify which of the forms is on a particular sample, the fraction of hydrogen and the fraction of sp3 bonded carbon atoms (not graphite) must be measured. Knowing those two numbers enables a user to plot the "location" of the sample on the VDI-map. The closer to the upper left corner that a material plots, the better (and more) pure is the DLC.		Dilutions with hydrogen and graphitic carbon degrade the DLC.