Estudio del comportamiento tribológico y de las interacciones de superficie de nuevos nanofluidos iónicos

Abstract

Ionic liquids are molten salts which are liquid at room temperature, as they have a melting point below 100ºC. They present a unique combination of properties that make them interesting for a wide range of industrial applications. In the present work, we focus on their use as lubricants in complex tribological problems such as the lubrication of metals that slide against themselves, the development of water based lubricants and new self-lubricated surfaces. When it is difficult to reduce friction coefficients and wear by lubrication, as in the case of magnesium alloys, ionic liquids are studied as protective coatings precursors. Surface interactions and corrosion processes with protic and aprotic ionic liquids on copper and steel have been determined in order to develop new lubricants and lubricant additives. The tribological performance of new protic ammonium ionic liquids and aprotic imidazolium ionic liquids as lubricants and additives of a polyalphaolefin has been evaluated in the copper/copper contact. All ionic liquids present better tribological performance than the synthetic oil, except for the oleate derivative. Protic ionic liquids show an outstanding tribological behaviour as neat lubricants and as additives. New protic ionic liquids are not only exceptional lubricants of the steel/sapphire contact as neat lubricants, but when they are used as additives in water, the formation of a boundary layer after water evaporation occurs, thus reducing friction and wear with respect to neat water. The formation of this boundary layer on steel under static conditions is described in order to prevent the running-in period with respect to water and the solution of ionic liquid in water, and to reduce the volume of liquid used with respect to neat ionic liquid. The best lubricating behaviour for the copper/copper contact and also for the steel/sapphire contact is obtained for the diprotic ammonium dianionic adipate, that has two carboxylate groups in its anion. A higher polarity and a higher number of ammonium protons, carboxylate and hydroxyl groups would give rise to stronger surface interaction with the metal surfaces and more stable boundary films. The tribological performance of new aprotic thiazolium ionic liquids and commercial aprotic imidazolium ionic liquids has been compared as lubricants of the steel/sapphire contact, obtaining the best results for the bis(trifluoromethanesulfonyl)imide derivatives, and also preventing tribocorrosion processes. The formation of a coating layer on magnesium alloys from phosphonate imidazolium ionic liquids by immersion and by chronoamperometry has been described. The new coatings reduce the abrasive wear in the magnesium-aluminium alloy but they are not effective in the magnesium-zinc alloy, which prevent the formation of continuous coatings. Electronic microscopy (SEM), energy dispersive (EDX), infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies have been used to study the surface interactions, wear mechanisms and tribochemical processes that take place in the contacts