Friction and wear characteristics of DLC coatings with different hydrogen content lubricated with several Mo-containing compounds and their related compounds

• Wear acceleration of hydrogenated DLC by MoDTC was confirmed.
• Hydrogen-free DLC did not show significant wear acceleration by MoDTC.
• MoO3 formation was not a major factor in the wear acceleration of hydrogenated DLC.

Although the molybdenum dithiocarbamate (MoDTC) additive results in superior friction-reducing performance for both steel and diamond-like carbon (DLC) surfaces, it sometimes causes wear acceleration of DLC surfaces. This study confirmed the wear acceleration of a DLC surface lubricated with MoDTC-formulated oil. Three types of DLCs with different hydrogen contents were used. The nominal hydrogen contents of DLCs were hydrogen-free (ta-C), 10 at% (a-C:10H) and 30 at% (a-C:30H). The DLC coatings were deposited on a tool steel substrate. It was confirmed that MoDTC accelerated the wear of hydrogenated DLCs, although the formulation of MoDTC showed good friction-reducing performance for every specimen. To confirm the tribological characteristics of the other Mo-containing compounds for DLC lubrication, friction tests were carried out with several types of Mo-containing additives and their related Mo-free organic compounds. When the hydrogenated DLCs were lubricated with Mo-containing additives, the wear of the hydrogenated DLCs was accelerated. However, the hydrogenated DLCs did not show wear acceleration when lubricated with the related Mo-free organic compounds. The DLC surfaces after the friction test with Mo-containing additives were analyzed by X-ray photoelectron spectroscopy, and the MoO3 formation was confirmed on every specimen. Although the wear behavior was considered in relation to MoO3 formation on the worn surface to determine whether MoO3 plays a key role in accelerating the wear of DLC, apparent relationship between the wear acceleration and the MoO3 formation was not obtained.