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

[SPA] Los líquidos iónicos son sales fundidas a temperatura ambiente, dado que su punto de fusión es menor de 100ºC. Presentan excelentes propiedades físico-químicas que los convierten en sustitutos de otros compuestos convencionales en numerosas aplicaciones industriales. En el presente trabajo se analizan dichas aplicaciones y se centra el interés en su uso como lubricantes en problemas tribológicos complejos como la lubricación de metales contra sí mismos, el desarrollo de lubricantes base agua y de nuevas superficies autolubricadas. Cuando no es posible reducir los coeficientes de fricción y desgaste mediante lubricación, como es el caso de las aleaciones de magnesio, los líquidos iónicos se han estudiado como precursores de recubrimientos protectores. Se han determinado las interacciones superficiales y los procesos de corrosión sobre cobre y sobre acero con diferentes líquidos iónicos próticos y apróticos para desarrollar nuevos lubricantes y aditivos. Se ha evaluado el comportamiento tribológico de nuevos líquidos iónicos próticos derivados de amonio y de líquidos iónicos apróticos derivados de imidazolio como lubricantes y como aditivos del lubricante comercial polialfaolefina 6 en el contacto cobre/cobre. Con excepción del líquido iónico prótico derivado del oleato, todos los líquidos iónicos estudiados presentan mejor comportamiento tribológico que el lubricante comercial. Los líquidos iónicos próticos muestran un excelente comportamiento tribológico tanto en estado puro, como en forma de aditivos. Los nuevos líquidos iónicos próticos no sólo son buenos lubricantes del contacto acero/zafiro cuando se utilizan en estado puro, sino que, como aditivos en agua, generan películas adsorbidas sobre la superficie del metal reduciendo los coeficientes de fricción y las tasas de desgaste tras la evaporación del agua. Para evitar el periodo de alta fricción inicial y desgaste severo en presencia de agua, se han generado películas de líquido iónico adsorbido sobre la superficie del acero en condiciones estáticas. El mejor comportamiento lubricante tanto en el contacto cobre/cobre como en el contacto acero/zafiro se obtiene para el líquido iónico prótico derivado del anión adipato, con dos grupos carboxílicos. Las interacciones de los grupos hidroxilo y carboxilato con la superficie metálica y los puentes de hidrógeno entre el catión y el anión podrían ser los responsables del buen comportamiento tribológico. Se ha comparado el comportamiento tribológico de los líquidos iónicos apróticos derivados de imidazolio y tiazolio como lubricantes del contacto acero/zafiro, obteniéndose los mejores resultados para los derivados del anión bis(trifluorometanosulfonil)imida, que evita a su vez los procesos de tribocorrosión. Se han generado recubrimientos superficiales sobre aleaciones de magnesio a partir de tres líquidos iónicos derivados del anión fosfonato, tanto por inmersión como mediante cronoamperometría. Los nuevos recubrimientos reducen el daño superficial por deslizamiento o abrasión de la aleación magnesio-aluminio pero no de la aleación magnesio-cinc, que impide la formación de recubrimientos continuos. Las interacciones superficiales y los procesos triboquímicos y tribocorrosivos que tienen lugar en las interfases entre los líquidos iónicos y las superficies se han analizado mediante microscopía electrónica de barrido (SEM), espectroscopía fotoelectrónica de rayos-X (XPS), microanálisis por energías dispersivas de rayos-X (EDX) y espectroscopía infrarroja (FTIR). [ENG] 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.[ENG] 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.Universidad Politécnica de CartagenaPrograma de doctorado en Tecnologías Industriale