Abstract
The application of surface coatings has been shown to reduce friction in elastohydrodynamic lubrication (EHL), not only in the mixed and boundary regime when asperity interactions occur, but also in the full film regime. Several studies suggest that the full film friction reduction is due to a violation of the no-slip boundary condition and thus slip is taking place between the solid and the liquid. Another hypothesis proposes that the full film friction reduction is due to the low thermal conductivity of diamond-like carbon (DLC) coatings. In this work, two DLC coatings with the same composition, but different thicknesses, are investigated with uncoated steel specimens as a reference, all with the same surface roughness. Friction tests in a ball-on-disk machine show that both coatings reduce friction compared to the uncoated reference case in full film EHL. The thicker coating is significantly more effective at reducing friction than the thinner one at a maximum friction reduction of 41 % compared to 29 % for the thinner coating. Moreover, contact angle measurements, surface energy measurements, and spreading parameter calculations show no statistically significant differences between the two coatings, suggesting that the friction reduction capabilities of coatings in full film EHL cannot be described by solid–liquid interactions alone. The difference in friction reduction between the specimens in this work is mainly attributed to different thermal properties.
Highlights
Surface engineering has emerged as an important part in reducing friction in the field of elastohydrodynamic lubrication (EHL)
The matter of interest in this paper is the reduction that is achieved by Diamond-like carbon (DLC) coatings in full film EHL where there is no contact between the surfaces and the lubricant carries all of the load
The tribological tests performed in this investigation clearly show the friction-reducing effect of a DLC coating
Summary
Surface engineering has emerged as an important part in reducing friction in the field of elastohydrodynamic lubrication (EHL). Smoother surfaces in contact have the advantage of pushing the transition from full film lubrication to mixed lubrication toward lower speeds and will lead to reductions in both friction and wear. DLC coatings generally reduce friction in boundary and mixed lubrication regimes. The matter of interest in this paper is the reduction that is achieved by DLC coatings in full film EHL where there is no contact between the surfaces and the lubricant carries all of the load. Several authors have experimentally observed a reduction in friction with DLC-coated surfaces in full film EHL [6, 7, 14, 27, 48]. The friction reduction has been explained by several authors as an effect of boundary slip, or solid–liquid interface slip [14, 27, 29] a phenomena thoroughly discussed in literature [12, 25, 37, 38, 41, 42, 46, 47, 49], where some of the work is based on atomically smooth surfaces
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