Scuffing resistance of polyalphaolefin (PAO)-based nanolubricants with oleic acid (OA) and iron oxide nanoparticles

Abstract

The development and application of nanolubricants has shown great potential to reduce friction and wear, especially in the automotive field for systems operating under boundary and mixed lubrication regimes. The search for additives that do not attack the environment has motivated the use of nanoparticles, especially metallic oxides, because of their characteristics such as low toxicity, high surface area, high wettability, and easy dispersion. The objective of this work was to evaluate the addition of iron oxide nanoparticles as additive to pure PAO 8 base oil. The nanoparticle action mechanisms, lubrication efficiency, friction, and wear rates were evaluated. The concentration of nanoparticles in the oil and the addition of oleic acid as a dispersing agent were varied. The synthesis of the nanoparticles was done by aqueous precipitation; characterization was performed by XRD, FTIR, Raman and SEM analyses. Tribological tests were performed in a reciprocating tribometer (Plint TE 67) using AISI 1020 prismatic samples and AISI 52100 chrome steel balls of 5 mm diameter. Test conditions resulted in boundary lubrication regime, for a starved contact with limited lubricant supply (0.5 µL). Concentrations of 0.4 and 1.6 wt% of nanoparticles were added to the base oil with and without the presence of oleic acid. Friction coefficient was not sensitive to the presence of nanoparticles, with statistically similar values for all the fluids tested. On the other hand, the presence of nanoparticles increased scuffing resistance and reduced wear rates. The lubricants with nanoparticles proved able to withstand relatively high loads, keeping the friction coefficient low for longer test durations due to the formation of protective tribolayers between the worn surfaces. The addition of nanoparticles reduced wear rates by around 27% when compared to tests using pure PAO 8.