Thermophysical and tribological properties of ionic liquids as neat lubricants and as additives of biodegradable base oils for renewable energies

Resumo

This PhD Thesis is devoted to the study of ionic liquids (ILs) as neat lubricants or lubricant additives for ester and vegetable base oils, through the measurement of several thermophysical and tribological properties. Some of the ILs, base oils and blends (base oil + 1 wt% IL) were characterized in terms of density, viscosity, viscosity index and thermal behaviour. The friction coefficient was evaluated in a reciprocating ball-on-plate tribometer for a steel-steel contact pair under different loads. Then, the wear volumes of the tracks formed on the surface of the plates were determined using a profilometer. Besides, some of the wear tracks were analysed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). In general, as neat lubricants, the lowest friction coefficients were found for tri(butyl)ethylphosphonium diethylphosphate [P4,4,4,2][C2C2PO4], whereas the best anti-wear abilities were shown by the ILs based on tris(pentafluoroethyl)trifluorophosphate [(C2F5)3PF3]-. Complex tribofilms composed of adsorbed IL and also new compounds tribochemically originated were identified on the worn surface through XPS analyses. In the case of the study of ILs as additives, firstly miscibility essays were performed with eight ILs and four base oils. As a result it was found that only two ILs were miscible with some of the base oils, leading to seven homogeneous blends containing 1 wt% of IL. The thermophysical properties of the base oils were only slightly affected by the addition of an 1 wt % of IL as additive, whereas the tribological performance was especially improved using [P4,4,4,2][C2C2PO4] for two base oils. For this IL as additive, desirable protective tribofilms with phosphorous compounds were found in the XPS analyses on the worn surface despite of the low IL concentration in the blend. Nevertheless, in the same conditions, a [(C2F5)3PF3]- IL seems not to be able to reach the surface.