Effect of Degree of Unsaturation in Vegetable Oils on Friction Properties of DLC Coatings

Abstract

Recently, it has been reported that combinations of various types of vegetable oils containing organic acids and DLC coatings are effective for reducing friction, but there are few reports of detailed investigations into the relationship between differences in the chemical structures of vegetable oil lubricants and friction reduction. Therefore, the authors investigated the influence on friction properties of two types of DLC coatings, a-C:H and ta-C under lubrication with vegetable oils which have different degrees of unsaturation. The ta-C coating displayed markedly lower friction coefficients than the a-C:H coating and the uncoated steel with all of the vegetable oil lubricants used. It was found that the low friction properties of the ta-C coating showed even lower friction coefficients with vegetable oils containing a higher content of monounsaturated fatty acids. Additionally, the sliding surfaces of ta-C coated discs and cylinders were analyzed by XPS and ToF-SIMS analysis. The results confirmed the formation of a surface layer consisting of C-OH bonds, and a lower friction coefficient was seen as the detected intensity of these bonds increased.

Tribocatalysis Induced Carbon-Based Tribofilms—An Emerging Tribological Approach for Sustainable Lubrications

“…Along with the severe removal of carbon-based tribofilm from CPCa, the tribotest of this combination resulted in a continuous increase in the frictional outcome and an unstable antiwear performance. Adapting to the modern trends of low-viscosity lubricant development where the phosphorus and sulphur compounds are minimized [29], the DLC coatings were been studied to optimize their interactions with other "green" additives like Glycerol Mono-Oleate [159][160][161], fatty acid [162][163][164][165], etc. Noticeably, ultralow-friction outcomes originating from organic molecules and DLC coating interaction can be achieved by probing their hydroxyl-terminated tribolayers [166,167].…”

Influence of Molecular Structure on the Physicochemical and Tribological Properties of Biolubricants: A Review

“…It has also been shown that the effect of increased unsaturation on tribological properties is more pronounced at higher temperatures [112]. Yoshida et al [111] also investigated the effect of double bonds on the COF of six vegetable oils with different levels of unsaturation in relation to the presence of mono-, di-, and tri-unsaturated FA. The oil with the highest content (%) of triunsaturation showed the highest COF results (linseed oil), followed by the oils with the highest content (%) of monounsaturation (rapeseed and olive oils).…”