In recent years, there has been growing concern regarding the use of petroleum-based lubricants. This concern has generated interest in readily biodegradable fluids such as vegetable oils. The present work evaluated the rheological and tribological characteristics of sunflower oil modified with silicon dioxide (SiO2) and titanium dioxide (TiO2) nanoparticles as lubricant additives at different concentrations. A parallel plate rheometer was used to evaluate the effects of concentration and shear rate on the shear viscosity, and the experimental data was compared with conventional models. The wear protection and friction characteristics of the oil-formulations were evaluated by conducting block-on-ring sliding tests. Surface analysis-based instruments, including scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and profilometry, were used to characterize the morphology and structure of the worn surfaces. The experimental results showed that the coefficient of friction decreased with the addition of SiO2 and TiO2 nanoparticles by 77.7% and 93.7%, respectively when compared to base sunflower oil. Furthermore, the volume loss was lowered by 74.1% and 70.1%, with the addition of SiO2 and TiO2 nanoparticles, respectively. Based on the experimental results, the authors conclude that modified sunflower oil enhanced with nanoparticles has the potential for use as a good biodegradable lubricant.

In metal-forming processes, the use of lubricants for providing desirable tribological conditions at the tool-workpiece interface is critical to increase the material formability and prolonging tool life. Nowadays, the depletion of crude oil reserves in the world and the global concern in protecting the environment from contamination have renewed interest in developing environmentally-friendly lubricants derived from alternative sources such as vegetable oils. In the present study, the rheological and tribological behavior of coconut oil modified with nanoparticle additives was experimentally evaluated. Two different nanoparticle additives were investigated: Silicon dioxide (SiO 2 ) and copper oxide (CuO). For the two conditions, nanoparticles were dispersed at different concentrations within the coconut oil. The effects of concentration and shear rate on the viscosity were evaluated and the experimental data was compared with conventional models. A custom-made tribotester was used to evaluate the effect of concentration on the tribological performance of the nano-lubricants. The experimental results showed that wear volume loss was lowered by 37% and 33% using SiO 2 and CuO nanoparticles, respectively. Furthermore, the addition of SiO 2 and CuO nanoparticles decreased the coefficient of friction (COF) by 93.75% and 93.25%, respectively, as compared to coconut oil without nanoparticles.Lubricants 2019, 7, 76 2 of 12 of the lubricant. Silica and copper oxide nanoparticles are efficient at room temperature, therefore no induction period is needed to see improvement in tribological properties. Peng and co-workers analyzed the tribological properties of SiO 2 in liquid paraffin using a ball-on-ring wear tester, and they demonstrated that SiO 2 helped lower wear and friction when compared to the base paraffin [7]. The main issue is compatibility of the nanoparticles and the base oil. Over long periods of time, the nanoparticles tend to sediment, making their tribological properties diminish [8].Lubricants 2019, 7, x FOR PEER REVIEW 2 of 12 lubricant. Silica and copper oxide nanoparticles are efficient at room temperature, therefore no induction period is needed to see improvement in tribological properties. Peng and co-workers analyzed the tribological properties of SiO2 in liquid paraffin using a ball-on-ring wear tester, and they demonstrated that SiO2 helped lower wear and friction when compared to the base paraffin [7]. The main issue is compatibility of the nanoparticles and the base oil. Over long periods of time, the nanoparticles tend to sediment, making their tribological properties diminish [8].

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