Lubricant properties of trimethylolpropane trioleate biodegradable oil: High pressure density and viscosity, film thickness, Stribeck curves and influence of nanoadditives

Abstract

Lubricant properties of trimethylolpropane trioleate synthetic base oil (TMPTO) were experimentally determined under different temperatures, pressures and rolling-sliding conditions. With the aim to obtain the viscosity-pressure coefficient, density and viscosity measurements were performed up to 150 MPa with a falling-body viscometer and a vibrating tube densimeter, respectively. Film thickness and friction properties were determined with a ball-on-disc apparatus from temperatures of 303.15 to 353.15 K, from slide-roll ratios from 5 to 50% at 50 N (applied load). Finally, it has also been evaluated if the use of nanoparticles as additives could involve changes on film thickness and Stribeck curves of TMPTO base oil. For this aim, hexagonal-boron nitride nanoparticles (h-BN) and graphene nanoplatelets (GnPs) were used at mass concentrations of 0.25, 0.5 and 1.0 wt%. The viscosity of TMPTO increases from 15 mPa s (at 10 MPa and 353.15 K) to 525 mPa s (at 150 MPa and 303.15 K). The Stribeck curves for TMPTO are placed between elastohydrodynamic and mixed lubrication. All nanolubricants show very similar Stribeck curves, being the lowest friction coefficient obtained for 0.25 wt% of GnP. It has been found that for most of the experimental conditions the addition of the GnP promotes an increase of the film thickness