Nanostructure and composition of tribo-boundary films formed in ionic liquid lubrication

Since the idea of using ionic liquids (ILs) as lubricants was raised in 2001, many studies have been conducted in this area and results have demonstrated superior lubricating performance for a variety of ILs. It is widely believed that a protective tribo-boundary film is formed on the contact area by tribochemical reactions between the metal surface and the IL during the wear process and, as a result, reduces friction and wear. However, the study of this critical boundary film in the literature has been limited to two-dimensional topography examination and chemical analysis from the top surface. This study demonstrates a multi-technique three-dimensional approach to characterize the boundary films on IL-lubricated metallic surfaces. The complementary characterizations at the top surface, cross-section, and different layers of the boundary film provide direct measurement of the film thickness, visualization of the nanostructure, and analysis of the composition change. The boundary films observed on different alloys are substantially distinct from both physical and chemical perspectives. The measured mean film thicknesses for cast iron, steel, and aluminum worn surfaces are 300, 60, and 200 nm, respectively. The boundary films on ferrous alloys are dominated by amorphous phase mixing with well-dispersed very fine (a few nm) nanocrystals, while the film on aluminum contains many larger size (tens of nm) metallic particles in a less organized manner.