Lubricant properties of a polyalphaolefin oil additivated with zirconia and ceria-zirconia nanoparticles

• Nanoparticles do not affect the volumetric behaviour of PAO8 base oil. • Zirconia and ceria-zirconia nanoparticles increase PAO8 viscosity by up to 13%. • PAO8′s friction coefficient was reduced by 13% with 0.05 wt% ZrO 2 or 0.075 wt% CeO 2 -ZrO 2 . • Wear resistance of PAO8 improved by 76% with low concentrations of ZrO 2 . • Tribo-film formation and rolling effect enhance tribological performance of PAO8. In this work zirconia (ZrO 2 ) and hybrid ceria-zirconia (CeO 2 -ZrO 2 ) nanoparticles are used for the first time as lubricant additives of the PAO8 base oil (low-viscosity polyalphaolefin oil) at concentrations from 0.05 wt% to 0.20 wt%. Thermophysical properties of the eight formulated nanolubricants were experimentally determined observing that the presence of these nanoadiditves not significantly alter the volumetric behaviour of neat PAO8. On the contrary, the presence of CeO 2 -ZrO 2 in formulation increases the viscosity of the base oil by up to 13 %. Tribological tests were performed under pure sliding conditions at 393.15 K at a load of 20 N using a ball-on-three-pin rotational tribometer. Moreover, the wear was quantified with a 3D optical profilometer. Nanolubricants with zirconia nanoparticles exhibited superior characteristics in terms of stability, antifriction and anti-wear properties than those formulated with hybrid ceria-zirconia nanoparticles. The best tribological behaviour was obtained with 0.05 wt% ZrO 2 , thus reductions of 13 % in friction coefficient and of 73 % in the worn area were obtained. The results obtained have been compared with those previously reported for the same base oil (PAO8) and other types of nanoadditives. Finally, surface roughness analysis, Raman microscopy and SEM-EDX were used to analyse the wear mechanism produced by zirconia and hybrid ceria-zirconia nanoparticles.