Tribological characterisation of graphene hybrid nanolubricants in biofuel engines

The rapid development of nanotechnology allows further enhancement of the tribological performance of lubricants by utilizing nano-additives. This study used a four-ball tribometer to examine the tribological properties of three different oils, namely 15 W-40 mineral oil, 5 W-30 completely synthetic polyalphaolefin with ester oil and pongamia oil with additional graphene nanoplatelets. The experiment model was constructed using a mathematical technique known as Response Surface Methodology (RSM), with the experimental design developed by using Optimal Custom Design. The extent of the influence to which various operating parameters, including load, speed and concentration of nanoparticles, were assessed by analysis of variance (ANOVA) and regression analysis. The simulation results are used for optimization purposes to determine the optimum concentration of nanoparticles that provides excellent tribological properties. The surface morphology was analysed using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy to explore the mechanisms that improve the tribological performance. The optimum concentration of graphene nanoplatelets (GNP) was determined to be 0.126 wt%, 0.15 wt%, and 0.096 wt% for mineral oil, synthetic polyalphaolefin with ester oil and pongamia oil respectively. The optimization of graphene nanoplatelets (GNP) concentration on mineral oil (MO), synthetic oil (SO) and Pongamia oil (PO) exhibits 5.78, 15.63 and 6.82% friction reduction respectively and 17.68, 29.46 and 97.32% wear reduction respectively compared to base oils. The dispersion stability results show that GNP is more stable in MO and SO than PO in the absence of surfactant. Finally, the improvement on worn surface was observed with optimization of concentration due to the significant polishing effect of GNP.