DLC (diamond-like carbon) coatings are basically diamond, as their name suggests, but most of the coatings marketed as DLC are not these crystalline diamond coatings. Many people labour under the impression that DLC is one coating, but it is in fact a wide range of coatings which are based on carbon – hundreds of commercially available coatings are marketed as DLC.
There are two types of bonds in carbon coatings, known as sp2 and sp3 bonds. Carbon composed solely of sp2 bonds is what we would called graphite; carbon composed of sp3 bonds is what we know as diamond. Graphite is soft and lubricious, whereas diamond is one of the hardest substances known. Commercially available DLC coatings have a mixture of sp2 and sp3 bonds, and control over the ratio of these has a strong effect on the properties of the coating. You may see some of them referred to as a-C coatings, which stands for amorphous carbon.
Many coatings are not solely carbon, but have a large proportion of hydrogen, as high as 50% in some cases. You will often see these referred to as a-C:H, which signifies that they are amorphous hydrogenated carbon coatings. If you see ta-C:H, the ‘t’ signifies tetrahedral, showing that the carbon bonding is predominantly tetrahedral sp3 bonds.
It is also common to tailor the coatings’ composition and properties further by ‘doping’ with metals or ceramics, and there are at least a dozen metallic doping agents in use. These agents modify various factors, from wear and friction to coating adhesion, and can play an important role in making the coating suitable for use in certain environments. For example, the coefficient of friction for many DLC coatings is very sensitive to relative humidity, and some of the doped coatings make the coating less sensitive in this respect. A Czech study (1) of both ‘plain’ and hydrogenated zirconium-doped DLC coatings showed a marked difference in frictional behaviour, with the doped coatings having a coefficient of friction about 50% lower than their undoped counterparts when running against an unlubricated steel ball.
With such a huge number of available coatings, the difficulty in making the correct choice is not easy, and is further complicated by the fact that the coatings, when used in an engine or transmission, are generally used in an oil-lubricated environment. A specific study (2) into the application of DLC coatings in automotive applications found that different DLC formulations can affect the interaction of lubricant additives with any uncoated parts in a sliding contact. One common example of this might be the use of an uncoated camshaft with a DLC-coated cam follower. The way each coating affected the wear behaviour was not constant, but also depended largely on the composition of the lubricant, in particular detergent and anti-wear additives.
1. Vitu, T., Pimentel, B., Escudeiro, A., Cavaleiro, A., and Polcar, T., “Zr-DLC coatings – analysis of the friction and wear mechanisms”
2. Renondeau, H., Papke, B., Pozebanchukz, M., and Parthasarathy, P., “Tribological properties of diamond-like carbon coatings in lubricated automotive applications”, Proc. of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 2009
Written by Wayne Ward