Thermophysical and Tribological Properties of Highly Viscous Biolubricants

Abstract

Two new highly viscous biodegradable oils are investigated for use in wind turbine gearboxes (BIO-G00) and in mechanical transmissions of agricultural tractors (BIO-G02). Studies on their thermophysical and tribological properties were performed. High-pressure–viscosity measurements were obtained up to 250 MPa and 363.15 K using a falling-body apparatus. The viscosity of BIO-G00 and BIO-G02 reaches maximum values of 14720 and 7072 mPa·s, respectively. The film thickness and the tribological performance, from boundary to full-fluid lubrication regimes, under a slide-to-roll ratio of 5% obtained in an EHD2 ball-on-disk test rig are reported. Film thickness has also been computed through the Hamrock and Dowson equation, considering the inlet shear heating (thermal correction factor) due to the high viscosity of both biolubricants. Differences between the experimental and theoretical film thicknesses are around 4% at 353.15 K and 14% at 303.15 K for both oils. The universal pressure–viscosity coefficients, αfilm, for both oils are lower than those of other mineral and synthetic oils. Higher friction coefficients are obtained for BIO-G00 in all of the studied lubrication regimes for the different rough disks and in the entire temperature range. A suitable wetting behavior on steel surfaces is observed for both selected oils.