The Science Behind Diamond like Coatings (DLCs)
DLC is a term used to cover a wide range of different types of coatings, all of which are comprised of carbon chains in an amorphous structure. The coatings are usually classified according to the proportion of Sp2 and Sp3 bonds together with the hydrogen content. Sp2 is the triagonal form of graphite offering low friction, sp3 is the tetrahedral form of diamond offering very high hardness.
When combined, they form an amorphous (i.e. non-crystalline) low shear strength structure which in essence acts as a boundary lubricating solid film.
DLC coatings can be classified on the tertiary diagram shown below, showing the proportions of sp2, sp3 and hydrogen.
PCS instruments offer two types of DLC coatings on the standard (52100) and tool steel MTM specimens. Graphit-IC and Dymon-IC. The relevant PCS part numbers and coating characteristics are shown below:
*Depending on application
Frequently asked questions
- Q. Why are the coatings available on two different materials?
A. Because of the relatively high application temperature used to apply DLC coatings, a reduction in hardness of the standard (52100 specimens) occurs. This reduction is relatively small however (50-100 on the Vickers hardness scale). High speed tool steel (M2) specimens are available, since their hardness is not affected by the high temperatures used during coating deposition.
- Q.Why are two different coatings available?
- A. Recent studies have shown that certain lubricant additives such as friction modifiers (MoDTC) can adversely affect the performance of some coatings, showing increased friction and wear . This has been reported primarily with hydrogenated DLCs. The precise cause of this reduction in performance is still unclear.
- Q. What does hydrogenated mean?
- A. Hydrogenation refers to the treatment of substances with molecular hydrogen (H2). Therefore, hydrogenated DLCs contain a proportion of hydrogen and non-hydrogenated DLC coatings are hydrogen free.
Hydrogenated DLC coatings consist of amorphous DLC (a-C:H) and tetrahedral amorphous DLC (ta-C:H).
Non-hydrogenated DLC coatings consist of hydrogen free amorphous DLC (a-C) and tetrahedral amorphous DLC (ta-C).
- Q. Is a bonding layer used when the coatings are applied?
- A. Both coatings use a very thin bonding layer of chromium between the substrate and the DLC layer.
- Q. Are the coatings doped? And what does doping mean?
- A. The Graphit-ic coating is chromium doped. The Dymon-ic coating is not doped.
Doped coatings have different elements incorporated in their structure. The purpose of this is to achieve multifunctionality and to further improve the film properties. Many different doping materials are used, to modify properties such as hardness, friction, wear resistance, internal stress, adhesion, electrical conductivity and biocompatibility.
- Q. What is the surface roughness of the coating?
- A. The coatings are very thin (less than 2 microns), so the coating have the same roughness as the ball and disc substrate (less than 0.02 microns Ra).
- Q. Which of the two coatings available is recommended for high humidity conditions?
- A. Graphit-ic is more suited to high humidity environments. Hydrogenated DLC coatings perform poorly in water since hydrogen bonding occurs between the oxygen in the water and the hydrogen in the DLC coating.
Hydrogenated coatings with unsaturated -CH bonds shown unsatisfactory performance in water, however some hydrogenated coatings with saturated -CH bonds perform well.
- Q. Which coating is considered to be a standard DLC?
- A. Dymon-ic.
- Q. Are other types of DLC or different coatings available?
- A. Yes, depending on the commercial availability. Please contact your distributor or PCS Instruments.
- Q. Are coatings available on other test specimens?
- A. Yes, we are always happy to discuss this. Please contact your distributor or PCS Instruments.
Please feel free to contact PCS Instruments, or your local distributor if you have any further questions.
 Wear mechanisms of hydrogenated DLC in oils containing MoDTC, Kosarieh et al, Tribology letters, 64, 4 (2016)