The role of surface roughness and slide-roll ratio on the decomposition of MoDTC in tribological contacts

Abstract

In this study, the role of surface roughness and slide-roll ratio in the decomposition and friction performance of molybdenum dialkyldithiocarbamate (MoDTC) has been investigated. Tribotests were carried out in a minitraction machine (MTM) using steel discs of varying roughness rubbing against smooth steel balls in a sliding/rolling contact. Tests were conducted at slide-roll ratio (SRR) values of SRR = 100% and 200%. Raman spectroscopy was used to perform chemical characterisation on the resulting wear scars. The friction performance of rough discs was not affected by the SRR. On the other hand, increasing the SRR from 100% to 200% in tests with smooth discs resulted in higher friction with large instabilities. Raman analysis showed significant differences in chemical composition of the wear scars generated after tests with smooth and rough discs. Wear scars generated using rough discs were mainly composed of MoS 2 indicating complete MoDTC decomposition while those generated using smooth discs were composed of a mixture of MoS 2 , MoS x (x > 2) and FeMoO 4 indicating partial MoDTC decomposition. Numerical simulation of the contact revealed that under similar loading conditions rough surfaces have higher local pressures than smoother surfaces. It is proposed that higher local pressures in rough surfaces promoted complete MoDTC decomposition. The novel finding from results presented in this study is that at similar temperature and MoDTC concentration, the degradation of MoDTC within tribocontacts is highly dependent on the roughness of the tribopair. This is because surface roughness determines the local pressure at the asperity-asperity contact.

The effect of clearance between tappet insert and camlobe on the tribological and tribochemical performance of cam/follower surfaces

“…Molybdenum Dialkyl Dithiocarbamate (MoDTC) additive has been extensively used in engine oils as a friction modifier since the 1960s. MoDTC produces MoS 2 friction sheets which result in low friction under boundary lubrication [17][18][19]. This additive was also reported to form MoO 3 compounds in the tribofilm [18,20].…”

Reactive Pulsed Laser Deposition of Clustered-Type MoSx (x ~ 2, 3, and 4) Films and Their Solid Lubricant Properties at Low Temperature

“…For the MoS 2 coating, in the regions of the sliding track that were not subject to substantial wear (area A2, Figure 8), the peak intensity grew significantly at 410 cm −1 ; this points to an increased content of lamellar material and/or increased crystallinity in these regions. During the activation of the development of the layered MoS 2 structure, the sliding friction caused the formation of Mo-O and Fe-Mo-O compounds, which was evidenced by the emergence of a broadened band in the Raman spectrum, at 800-980 cm −1 [42,43]. An Fe-Mo-O compound (e.g., with Mo = O stretching in FeMoO 4 , the Raman shift was 925 cm −1 ) was probably formed during a reaction between molybdenum-based compounds and iron oxides on the surface of the steel ball.…”

Friction Modifier Additives, Synergies and Antagonisms

“…However, some MoFMs contain no sulphur and are only effective when used in conjunction with sulphur-containing additives such as an extreme pressure additives or some detergents. An important feature of MoFMs is that they only form MoS 2 at quite severe contact stresses, which in practice means that they only form when asperity-asperity contact takes place [12,13]. This has the advantage that MoS 2 is formed only where it is needed to reduce boundary friction, on the load-supporting high spots of surfaces [11].…”