Diamond-like Carbon - Properties

Properties

As implied by the name, diamond-like carbon (DLC), the value of such coatings accrues from their abilities to provide some of the properties of diamond to surfaces of almost any material. The primary desirable qualities are hardness, wear resistance, and slickness (DLC film friction coefficient against polished steel ranges from 0.05-0.20).

However, which properties are added to a surface and to what degree depends upon which of the 7 forms are applied, and further upon the amounts and types of diluents added to reduce the cost of production. In 2006 the Association of German Engineers, VDI, the largest engineering association in Western Europe issued an authoritative report VDI2840 in order to clarify 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 which are offered on the market. Quoting from that document:

These 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. The VDI2840 report confirmed the utility of locating a particular DLC material onto a 2-dimensional map on which the X-axis described the fraction of hydrogen in the material and the Y-axis described the fraction of sp3 bonded carbon atoms. The highest quality of diamond-like properties was affirmed to be correlated with the proximity of the map point plotting the (X,Y) coordinates of a particular material to the upper left corner at (0,1), namely 0% hydrogen and 100% sp3 bonding. That "pure" DLC material is ta-C and others are approximations that are degraded by diluents such as hydrogen, sp2 bonded carbon, and metals. Valuable properties of materials that are ta-C, or nearly ta-C follow.