Friction and Film-Formation Properties of Oil-Soluble Inorganic Nanoparticles

Many vehicle and engine test studies have shown that the fuel efficiency of automobiles can be improved by reducing friction between moving parts. Typically, organic friction modifiers such as glycerol monooleate (GMO) or metal containing friction modifiers such as molybdenum dithiocarbamate (MoDTC) have been added to engine oils to reduce boundary friction and improve fuel efficiency. These traditional friction modifiers act by forming either a self-assembled organic film (in the case of GMO) or a Mo-disulfide chemical film (in the case of MoDTC). More recently, the ability of inorganic tungsten disulfide (WS₂) nanoparticles to reduce boundary friction has been described. Martin has proposed that WS₂ nanoparticles are transported into a contact zone where they are compressed and peel open like an onion to form a film. In this study, oil-soluble inorganic nanoparticles containing cerium (Ce) and zinc (Zn) have been synthesized. These nanoparticles reduce friction when solubilized in base oil or fully-formulated engine oil. In addition, boundary films formed in or around the contact zone by oils containing the nanoparticles contain the metal in the nanoparticle.