MoDTC lubrication of DLC-involving contacts. Impact of MoDTC degradation

Abstract

Recently Diamond-Like Carbon (DLC) coatings have attracted considerable attention due to their low friction, high hardness, good wear and corrosion resistance, high thermal and chemical stability. Although considerable research has been conducted on the effect of molybdenum-based additive, e.g. molybdenum dithiocarbamate (MoDTC), on the boundary lubrication of DLC coatings, the wear mechanisms leading to the coating removal are not fully understood to date. Moreover, the impact of degradation of MoDTC-containing base oil on the tribological properties of DLC coatings has not been investigated. This study could help to elucidate the mechanisms by which the coating is worn out in presence of MoDTC-containing base oil. In this work, the friction and wear performances of different types of DLC coatings with different hydrogen contents and doping elements have been investigated in presence of MoDTC-containing base oil. To enable the correlation between these changes and the modification of contact surface chemistry, tribofilms generated during friction were investigated by SEM combined with Energy Dispersive X-ray Spectrometry (EDX) and X-ray Photoelectron Spectroscopy (XPS). A new mechanism explaining the severe wear experienced by DLC coatings when lubricated with a MoDTC-containing base oil is proposed.

Wear Mechanisms of Hydrogenated DLC in Oils Containing MoDTC

“…The obtained results do not contradict the findings by Sugimoto et al [6] but suggest that graphitization/surface modification of DLC did not occur when the counterpart was ceramic. Experiments with the hard ceramic counterpart in this study rule out the effect of increase in the hardness of the steel counterpart (by formation of molybdenum compounds on the steel Based on the observations of this study and the published literature [4][5][6][7][8][9][10][11][12][13], it is reasonable to assume that MoDTC decomposition products are playing a great role in giving this high wear to the hydrogenated DLC and that the presence of a steel counterpart is essential for such high wear.…”

“…Recently, the effect of MoDTC in increasing wear of the DLC coating in a DLC/steel contact has been reported by the current authors and others [4][5][6][7][8][9][10][11][12][13]. Different mechanisms by which lubrication by MoDTC leads to high wear to tribological systems including DLC have been proposed.…”

“…Reaction of MoDTC-derived MoO 3 with the DLC coating [5], graphitization of the DLC followed by the formation of hard Mo compounds on the DLC/CI interface [6] and the role of the counterpart in a ''pro-wear process'' [11] are some of the potential mechanisms for the occurrence of this phenomenon. Based on the published literature [4][5][6][7][8][9][10][11][12][13], it appears that MoDTC-derived products are key to this detrimental effect and that the type of counterpart may also play a part in high wear of DLC imposed by MoDTC. However, the origin of MoDTC-induced wear of DLC and the mechanisms by which the type of counterpart could affect this wear are not fully understood.…”

Tribocatalytic behaviour of a TiO2 atmospheric plasma spray (APS) coating in the presence of the friction modifier MoDTC: a parametric study

“…The only difference in the tting procedure used in this case for Mo3d is the addition of a new contribution in the Mo spectra at the binding energy of 227.8 AE 0.2 eV which was assigned to the carbide species (Mo x C). 11 Since Mo x C and Mo metal contributions have similar binding energies, it is possible that both of them are present. Therefore, from the above results it can be conrmed that Mo x C species is formed in the contact involving ZrO 2 and Al 2 O 3 balls.…”

“…7 For the reduction reaction of MoDTC to occur (from Mo +V to Mo +IV), a reducing agent seems to be needed in the form of negatively charged particles (ions, electrons, free radicals) or additives (electron donor). 11,14 The change in the oxidation state of Mo is also needed in order to lower the energy barriers leading to a linkage isomer of the MoDTC that has been identied as a good candidate to be easily decomposed to MoS 2 . We theoretically studied a possible reaction pathway for reduction of MoDTC to the linkage isomer (LI MoDTC).…”