The tribological properties of nanofluids are influenced by multiple factors, and the interrelationships among the factors are deserving of further attention. In this paper, response surface methodology (RSM) was used to study the tribological behavior of reduced graphene oxide-Al2O3 (rGO-Al2O3) nanofluid. The interaction effects of testing force, rotational speed and nanoparticle concentration on the friction coefficient (μ), wear rate (Wr) and surface roughness (Ra) of steel disks were investigated via the analysis of variance. It was confirmed that all the three input variables were significant for μ and Wr values, while testing force, nanoparticle concentration and its interaction with testing force and rotational speed were identified as significant parameters for Ra value. According to regression quadratic models, the optimized response values were 0.088, 2.35 × 10-7 mm3·N-1·m-1 and 0.832 μm for μ, Wr and Ra, which were in good agreement with the actual validation experiment values. The tribological results show that 0.20% was the optimum mass concentration which exhibited excellent lubrication performance. Compared to the base fluid, μ, Wr and Ra values had a reduction of approximately 45.6%, 90.3% and 56.0%. Tribochemical reactions occurred during the friction process, and a tribofilm with a thickness of approximately 20 nm was generated on the worn surface, consisting of nanoparticle fragments (rGO and Al2O3) and metal oxides (Fe2O3 and FeO) with self-lubrication properties.
Keywords: lubrication mechanism; nanofluid; reduced graphene oxide; response surface methodology; tribology.