Controlled synthesis of CeO2 nanoparticles and their tribological effects on micro-pitting and sliding wear

In this paper, the good disperse and particle size adjustability of cerium oxide (CeO₂) nanoparticles were achieved by solvothermal secondary growth method. The micro-pitting resistance of three CeO₂ nanoparticles (1.85 nm, 2.85 nm, 4.22 nm) in polyalphaolefin (PAO) and gear oil was evaluated by reciprocating step loading method. The results showed that micro-pitting corrosion occurred in base oil lubrication from low load to high load (150 N–350 N). After adding CeO₂ nanoparticles, at low load (150 N), 2.85 nm and 4.22 nm CeO₂ nanoparticles formed a discontinuous CeO₂ tribofilm through shear sintering to reduce the surface shear force and produce significant anti-wear effect. The 1.85 nm CeO₂ has a friction-reducing and anti-wear effect through direct contact with isolated friction pairs. The formation of micro-pitting corrosion is inhibited by the three particle sizes, and the anti-wear performance decreases with the increase of particle size. Under the condition of high load (350 N), the discontinuous single CeO₂ tribofilm spillage due to its limited load capacity, which leads to increased wear and inhibits the formation of micro-pitting. Under the condition of high load (550 N), the formula oil has a micro-pitting phenomenon. After adding CeO₂ nanoparticles, a composite tribofilm containing cerium phosphate and CeO₂ is formed through tribochemical reaction with phosphorous additives, which greatly improves the bearing capacity and anti-wear ability of the tribofilm. Even under the condition of 550 N high load, the three particle sizes of CeO₂ nanoparticles have significant anti-wear effect and low friction properties, and all inhibit the formation of micro-pitting corrosion, among which 2.85 nm particle size of CeO₂ has the best anti-wear ability.