The Role of Oleophilic Surface in Lubrication

Abstract
Surfaces of MoS2 and graphite are composed of ‘oleophilic’ regions, which have a strong affinity for long-chain normal hydrocarbons, and polar regions. The oleophilic regions adsorb hydrocarbons in preference to polar compounds, such as water and alcohols. Conversely, water and alcohols easily displace hydrocarbons adsorbed in the polar sites. A study of MoS2 and graphite having different proportions of oleophilic and polar sites shows that the lubricating effectiveness of these solids increases with the proportion of oleophilic sites on their surfaces. For MoS2 the sites consist of regularly disposed sulphur atoms in the basal planes of individual crystallites, and it is postulated that they can become chemically bonded to steel surfaces. When one basal plane side of a MoS2 crystallite is bonded to the steel surface, the other side presents an oleophilic surface to the surroundings, which has a low surface energy and high affinity for mineral oils. The oleophilic ‘sites’ produced in this way on the metal surface are believed to be instrumental in increasing the load-carrying capacity of the steel surfaces in sliding and rolling contact, especially in the presence of mineral oils.