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Carbon-based tribofilms from lubricating oils
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Carbon-based tribofilms from lubricating oils
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Journal Article
Carbon-based tribofilms from lubricating oils
2016
Overview
Moving mechanical interfaces need to be lubricated to ensure long life and easy slippage; here, a new type of coating is described—comprising nitrides of either molybdenum or vanadium, together with a copper or nickel catalyst—that generates protective tribofilms from lubricating oils.
Low-friction tribofilms created
in situ
This paper describes a novel approach to lubrication between moving mechanical interfaces that relies on the extraction of carbon tribofilms
in situ
directly from the base-oil molecules on catalytically active, sliding nanometre-scale crystalline surfaces. The newly developed coatings — comprising molybdenum or vanadium nitrides plus a copper or nickel catalyst — deposited on steel substrates generate protective tribofilms from lubricating oils that virtually eliminate wear and provide lower friction than films produced from formulated lubricants.
Moving mechanical interfaces are commonly lubricated and separated by a combination of fluid films and solid ‘tribofilms’, which together ensure easy slippage and long wear life
1
. The efficacy of the fluid film is governed by the viscosity of the base oil in the lubricant; the efficacy of the solid tribofilm, which is produced as a result of sliding contact between moving parts, relies upon the effectiveness of the lubricant’s anti-wear additive (typically zinc dialkyldithiophosphate)
2
. Minimizing friction and wear continues to be a challenge, and recent efforts have focused on enhancing the anti-friction and anti-wear properties of lubricants by incorporating inorganic nanoparticles and ionic liquids
3
,
4
. Here, we describe the
in operando
formation of carbon-based tribofilms via dissociative extraction from base-oil molecules on catalytically active, sliding nanometre-scale crystalline surfaces, enabling base oils to provide not only the fluid but also the solid tribofilm. We study nanocrystalline catalytic coatings composed of nitrides of either molybdenum or vanadium, containing either copper or nickel catalysts, respectively. Structurally, the resulting tribofilms are similar to diamond-like carbon
5
. Ball-on-disk tests at contact pressures of 1.3 gigapascals reveal that these tribofilms nearly eliminate wear, and provide lower friction than tribofilms formed with zinc dialkyldithiophosphate. Reactive and
ab initio
molecular-dynamics simulations show that the catalytic action of the coatings facilitates dehydrogenation of linear olefins in the lubricating oil and random scission of their carbon–carbon backbones; the products recombine to nucleate and grow a compact, amorphous lubricating tribofilm.
