Toward sustainable tribological systems: Copper nanoparticle-enhanced vegetable oil nanolubricants

This study explores the possibilities of replacing mineral oil by copper nanoparticle (CuNP)-added vegetable oil-based nanolubricants for their use as environmentally friendly lubricants from a tribological perspective. Vegetable oils, being biodegradable and renewable in nature, have been reported to serve as potential bio-lubricants. However, such bio-lubricants still suffer from low oxidative stability and suboptimal viscosity characteristics because of their unsaturated fatty acid chains. The literature indicates that CuNPs added to vegetable oils can enhance wear resistance, reduce the coefficient of friction (COF), and improve thermal stability, thereby mitigating these impacts. The tribological properties of CuNP-reinforced vegetable oils are evaluated by the coefficient of friction (COF), wear scar diameter (WSD), and stability at different loads and temperatures. Published studies indicate that CuNPs were able to notably enhance the performance of vegetable oils, particularly in boundary and mixed lubrication conditions. CuNPs can form protective tribofilms in the presence of metal-metal contact and decrease wear depth; therefore, they improve wear resistance and reduce friction. Additionally, the morphology, size, concentration, dispersion stability, and surface functionalization of copper nanoparticles (CuNPs) are also indispensable to lubrication performance. For example, spherical nanoparticles have been shown in many instances to behave as better lubricants than non-spherical nanosolids; however, the enhancement strongly depends on tribological operating conditions and specific system configurations (e.g., homogeneous dispersions or reduction of abrasiveness) - including additive concentration, base oil chemistry, and test parameters, among others. For example, palm oil, soybean oil, and coconut oil are ideal base oils due to their high viscosity index, non-toxicity, etc. Lastly, it pinpoints the research gaps: the need to optimize CuNP dispersion, ensure long-term colloidal stability of such nanolubricants, and assess their life-cycle and environmental impacts. In conclusion, the CuNP-based vegetable oil nano-lubricants are highly promising for eco-friendly lubrication with the potential to replace the conventional petroleum-based oils in industrial applications.