Carbon-based tribofilms from lubricating oils

Abstract

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. 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). 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. 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. 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.

Influence of tribology on global energy consumption, costs and emissions

“…The thin coatings can be further improved by processing nanostructures and nanolayered coatings [14,39,[81][82][83][84]. As a new concept, researchers have also designed smart catalytically active nano-composite layers with an ability to crack long-chain hydrocarbon molecules of base oils and turn them into diamond-like carbon tribofilms and other forms of carbon nanostructures on rubbing surfaces [41]. The resultant tribofilms were proven to be very slick and highly protective against wear and if and when worn away, they were shown to self-heal by a catalytic reaction with the lubricant.…”

“…As a result of concerted efforts, very low friction, even as low as a coefficient of friction of 0.005, has been measured in the presence of friction modifier additives like glycerol mono-oleate (GMO) or pure glycerol when lubricating tetragonal amorphous carbon coatings [33][34][35]. Nanomaterials with very promising tribological properties under investigation are carbon-based additives including nano-diamonds, onion-like carbons, carbon nanotubes, graphene, graphite as well as some inorganic fullerenes of transition metal dichalcogenides, like MoS 2 and WS 2 , and copper, polymeric and boron-based nanoparticles [36][37][38][39][40][41][42][43].…”

Ionanocarbon Lubricants. The Combination of Ionic Liquids and Carbon Nanophases in Tribology

“…Very recently, Erdemir et al [23] have demonstrated that, under operating conditions, lubricating oils form tribofilms similar to DLC (diamond-like carbon). The proposed mechanism consists of the catalytic dehydrogenation of the linear olefins present in the lubricating oils by nanocrystalline coating materials, followed by carbon-carbon bond cleavage and recombination to grow the amorphous anti-wear lubricating films.…”

Tribological properties of Ti‐doped diamond‐like carbon coatings under dry friction and PAO oil lubrication

“…With the development of industrial technology, some engine components have to serve in harsh conditions, such as high speeds and rigorous loads . To improve the stability and safety of components, liquid lubricants are usually adopted to solve the wear replenishment problem and remove the wear debris during the friction process.…”