Experimental evaluation of the tribological behaviour of CeO₂ nanolubricants under extreme pressures

Lubrication of moving components is critical for lowering friction and reducing energy consumption. Nanoparticles have been recently studied as additives for improving the load-carrying capacity and overall tribological performance of lubricants employed for metal-mechanic applications. Cerium oxide is a lanthanide metal oxide (rare earth) with environmentally friendly characteristics that is widely used for consumer products, and most recently for enhancing the tribological performance of composite materials and lubricants. In this work, formulated polymeric lubricants employed for metal forming processes with dispersed cerium oxide (CeO₂) nanoparticles were characterized under extreme pressure (EP) conditions. Nanoparticle concentrations of 0.01, 0.05, and 0.10 wt.% were dispersed by homogenization and ultrasonication in the selected lubricant. Tribological characterization under EP was performed with a T-02 four-ball tribometer according to the ITeE-PIB Polish method for testing lubricants under conditions of scuffing. This method applies and increasing load of 0-7200 N to the tribopair in a very short amount of time (18s) and is employed to find the maximum pressure a lubricant film can withstand. The scuffing load (P _t), and pressure loss limit or load carrying capacity (p_oz) were determined after each test. Wear scar diameters (WSDs) of worn ball materials were measured with an Alicona optical 3D measurement system. The anti-wear (AW) and friction-reducing characteristics of CeO₂ nanolubricants were determined with a T-05 block-on-ring test. No seizure was found for all lubricants likely due to the presence of additives in the polymeric lubricant. Nanoparticles were found to delay scuffing initiation at concentrations up to 0.05 wt.%, and the load required for scuffing increased by 35 %. The load-carrying capacity was enhanced by up to 84% with only 0.01 wt.% CeO₂. Higher concentrations (0.10 wt.%) had a detrimental effect, most likely due to nanoparticle agglomeration. The coefficient of friction (COF) was lowered from 0.035 for the polymeric lubricant to 0.006 with a concentration of 0.01 wt.% CeO₂.The tribological results obtained by both tests demonstrate that CeO₂ nanoparticles have friction-reducing and EP characteristics, making them suitable as lubricant additives in the metal-mechanic industry. (paper)