Tribological Enhancement Features of Various Nanoparticles as Engine Lubricant Additives: An Experimental Study

This study presents tribological enhancement properties of 40–80 nm copper (II) oxide (CuO, nanopowder, ~ 50 nm, 99.9% trace metal basis), copper iron oxide (CuFe₂O₄, nanopowder < 100 nm, 98.5% trace metal basis), and copper zinc iron oxide (CuZnFe₂O₄, nanopowder < 100 nm, 99.5% trace metal basis) nanoparticles incorporated (0.1 wt%) in SAE 5W-40 engine lubricant (base oil). Friction and wear evaluations were performed on the sample discs (62–65 HRC, 100 Cr6 steel) using pin-on-disc tribotester. The samples were immersed in the base oil and the nano-oil (oil with each nanoparticle) ambients one after another. Stribeck parameter was analyzed to determine the effects of oil bath temperature and pin-sliding speed on friction and wear within different lubrication regimes. Due to excess increment in an average coefficient of friction (ACOF) above the nanoparticle concentration of 0.1 wt%, the aforementioned amount was used for all experiments. The physical structure of the nanoparticles was examined via scanning electron microscopy and atomic force microscopy analyses were conducted to observe the topography of the worn surfaces. Thermogravimetry (TG)–differential scanning calorimetry measurements were also carried out in order to determine the nanoparticle accumulation on the worn surfaces (thermal stability) and to have an idea about probable surface chemical decomposition. A maximum reduction of 50.8% in ACOF was observed for 5W-40 + CuZnFe₂O₄ suspension under the temperature of 40 °C and sliding speed of 1 m/s. Furthermore, minimum surface roughness was also determined for the sample processed in 5W-40 + CuZnFe₂O₄ under the temperature of 70 °C and sliding speed of 0.5 m/s.