Tribochemical and chemical reactions of potential ionic liquid lubricants in contact with metal surfaces

Abstract

Since the first mentioning of ionic liquids in the field of tribology in 2001, research concerning the applicability of these fluids as lubricants or additives experienced a constant growth. Through properties like low vapour pressure, low flammability, good conductivity and the possibility of tailor made design, due to the variability of the anionic and cationic structures, ionic liquids became such interesting objects to study in this field. One essential aspect in this field of research is to investigate the compatibility of these novel substances with different kinds of tribosystems. In this respect the main focus of this thesis was to study the interaction of the ionic liquids with different kinds of metal materials commonly used in tribosystems e.g. bearings and to learn more about the influence of the respective IL structural components on this interaction. Therefore two different approaches were followed. As starting point of the study static corrosion experiments were performed which were rather simple to execute, hence a large sample matrix, consisting of different ionic liquids structures, metal materials and variations of the experimental conditions, were possible to work off. All ionic liquids investigated throughout this thesis were based on the bis(trifluorosulfonyl)imide (TFSI) anion and the cationic moieties applied were based on phosphonium, sulfonium, imidazolium, ammonium and pyrrolidinium. These liquids were investigated in contact with metals such as e.g. CuSn8P, 100Cr6 and AISI 440C. Due to the high temperatures applied throughout the corrosion experiments (from 150 C to 190 C) and water added as contaminant (in the case of closed tests), harsh operating conditions were intended to be simulated. In some of the cases severe corrosion was possible to be determined by the measuring of dissolved metals e.g. Fe or Cu, by ICP-OES analysis. Also the surface of the metals, investigated by SEM-EDS and XPS after finishing the corrosion experiment, showed significant evidence for corrosive processes. Amendment was achieved by adding benzotriazole and stearic acid as corrosion inhibitors to the ionic liquids. The next step was to investigate the ionic liquid behavior under dynamic conditions, hence in the tribological contact. For elucidating the tribological properties of the selected ionic liquids a well established bench test machine, the SRV tribometer was used. The tribometrical experiments were performed in the ball on disc assembly. The respective wear tracks generated were analyzed by XPS in order to determine possible reaction products (tribochemical reaction products) formed in the metal/IL contact. With XPS the top surface of the tribolayer was characterized by spot analysis and by imaging experiments, further the depth distribution of the elements detected were acquired by performing depth profiling. Although the XPS is a strong surface analysis technique, it lacks in providing molecular chemical information about the compounds on the surface. Thus it was necessary to broaden the analytical range, by applying a method which was capable of providing this information by directly measuring the wear tracks after the tribometer experiments. The method of choice was laser desorption ionization reflectron time of flight mass spectrometry (LDI-RTOF-MS). This technique was selected because ionic liquids are known to be easily desorbed by the LDI, hence the sample preparation was, with just cleaning the tribometer disc, straight forward. Challenging was, to adapt a common MALDI-TOF target by milling, in a way that it was possible to be used as sample holder for a tribometer disc. By applying this method after the tribo experiment, it was possible to determine chemically modified IL cations modified on the surface and their lateral distribution.