Pressure-viscosity behaviour and film thickness in elastohydrodynamic regime of lubrication of ionic liquids and other base oils

Abstract

Viscosities at high pressures for 16 base oils (six polyolesters, three polyglycols, six ionic liquids and squalane) were used to analyze the influence of their chemical structure on the pressure-viscosity coefficient, α. Comparisons with literature α values for vegetable bases (canola, soybean and jojoba oils) are also performed. For the molecular lubricants, we found trends that are coherent with the literature, whereas for ionic liquids (ILs), new trends were found. ILs based on tris(pentafluoroethyl)trifluorophosphate anion [FAP]−are those which present higher α values. The role of the lubricants in the central thickness in the isothermal elastohydrodynamic regime of lubrication was evaluated through the product η00.69 α0.56, η0 being the viscosity at 0.1 MPa. One of the [FAP]− ILs provides film thicknesses close to the polyglycols with similar viscosity. Bis(trifluoromethylsulfonyl)imide ILs and the shorter polypropyleneglycoldimethyl ethers are the lubricants that better avoid the changes on the film thicknesses with the temperature owing to a high viscosity index.