Friction and wear mechanisms of metal surfaces with ionic liquids as lubricant

Lubricants have been playing a critical role in improving the energy efficiency and lifespan of machinery by minimizing friction and wear. Accordingly, the development of advanced lubrication techniques has become more and more important. Ionic liquids (ILs) have attracted great attention in the field of surface engineering since ILs were first proposed as a potential lubricant in 2001. ILs are molten salts at room temperature that have unique properties such as high thermal stability, non-volatility, and non-flammability. And these special properties make ILs a future candiate for new lubricants under extereme environmental conditions such as ultra high vacuum and severe temparature. When an IL is applied onto metal surfaces, an ordered layered structure is formed: anions are strongly bonded and anchored to the metal surface through polar interaction thus enabling efficient separation of counteracting metal surfaces, with the cations forming a layer over anions, thereby providing a mobile characteristic with lower shear and friction. At light load conditions, the adsorbed layered structure keeps the moving parts separated, preventing direct solid contacts. Under high load conditions, the layered IL can be broken down leading to chemical reactions with metal surfaces promoted by the high contact pressure and temperature at the interface. Depending on the chemical compositions, these reactions can help to develop tribofilm on the metal surfaces that favorably reduces the friction and wear. In this study, different types of ILs and metal samples are prepared to investigate in-situ friction coefficient using a reciprocating ball on disc tribometer under controlled sliding speed and applied normal load. Then the wear mechanism and material transfer behavior were examined by 3D surface profilometer and Scanning Electron Microcopy with Energy Dispersive Spectroscopy (SEM/EDS). Also the mechanism of the tribofilm formation at the contacting interface was investigated by X-ray photoelectron spectroscopy (XPS) with depth profiling technique.