Interplay of mechanics and chemistry governs wear of diamond-like carbon coatings interacting with ZDDP-additivated lubricants

Abstract

Friction and wear reduction by diamond-like carbon (DLC) in automotive applications can be affected by zinc-dialkyldithiophosphate (ZDDP), which is widely used in engine oils. Our experiments show that DLC’s tribological behaviour in ZDDP-additivated oils can be optimised by tailoring its stiffness, surface nano-topography and hydrogen content. An optimal combination of ultralow friction and negligible wear is achieved using hydrogen-free tetrahedral amorphous carbon (ta-C) with moderate hardness. Softer coatings exhibit similarly low wear and thin ZDDP-derived patchy tribofilms but higher friction. Conversely, harder ta-Cs undergo severe wear and sub-surface sulphur contamination. Contact-mechanics and quantum-chemical simulations reveal that shear combined with the high local contact pressure caused by the contact stiffness and average surface slope of hard ta-Cs favour ZDDP fragmentation and sulphur release. In absence of hydrogen, this is followed by local surface cold welding and sub-surface mechanical mixing of sulphur resulting in a decrease of yield stress and wear.

Superlow Friction of a-C:H Coatings in Vacuum: Passivation Regimes and Structural Characterization of the Sliding Interfaces

“…More in general, the limited time scale of the simulations is not the only factor affecting the comparison between simulations and experiments. In spite of the accuracy of quantummechanical MD simulations, the sliding speeds remain orders of magnitude larger than the experimental ones, although this aspect does not affect the outcome of the mechanically driven chemical reactions we observe under shear [4,12]. Moreover, the limited size of the simulation box poses other limits to the comparison to experiments.…”

“…For instance, shear rates can be very high, unless the thickness of real shear bands is in the order of some nm, and surface roughness features above the nm scale are necessarily neglected. These can, however, be considered indirectly by calculating nm-scale asperity-asperity contact pressures via contact mechanics and then applying these pressure values in the MD simulation [4].…”

“…We note that, for an infinitely long sliding time, all metastable cold-welded interfaces would reach the thermodynamically stable graphitic state (i.e., aromatic or partially aromatic passivation). However, as the typical size of an asperity on a a-C:H surface is of the order of 100 nm [4], the typical duration of asperity-asperity collisions in DLC is less than a µs for sliding speeds of ~1 m s −1 . Thus, some cold-welded interfaces can be expected to persist for the duration of the asperity-asperity collisions.…”

“…The extraordinary thermal, chemical, and mechanical properties of diamond-like amorphous carbon (DLC) coatings make them suitable materials for tribological applications, such as camshafts, piston rings, and valves in automotive 2 of 15 engines [2]. DLC coatings are classified as amorphous carbon (a-C), tetrahedral amorphous carbon (ta-C), and hydrogenated amorphous carbon (a-C:H) according to the hydrogen content and sp 2 /sp 3 ratio [3,4]. Hydrogenated amorphous carbon is one of the most technologically relevant classes of DLC coatings.…”

A Combined Experimental and Atomistic Investigation of PTFE Double Transfer Film Formation and Lubrication in Rolling Point Contacts

“…they predominantly depend on the sliding distance s rather than on speed v x or time t [27]. In other words, simulations with high sliding velocity deliver the same results regarding shear-induced structural transformations as simulations with lower sliding velocity as long as the sliding distance ( s = v x t ) remains the same [28,29] (see Appendix).…”