Understanding the lubrication mechanisms of ionic liquids when used as additives or as neat lubricants

The promising properties of ionic liquids have attracted many tribologists to use them as lubricants. They have been used as either neat lubricants or as anti-wear additives. The results show that ionic liquids reacted/adsorbed with the lubricated surfaces and form a protective layer (tribofilm). A large number of previous studies have examined the tribological performance of ionic liquids. However, the relationship between the tribological performance and ionic liquids’ chemistry is still not fully understood. The previous studies focused on the suggestion of various ionic liquids as lubricants and study their tribological behaviour. The effect of ionic liquids’ chemistry, including anion and cation chain lengths, anion type and cation type, on their lubrication mechanism is still not fully understood. This lack of knowledge is the motivation to conduct the first part of this study (using neat ionic liquids as lubricants).
In terms of effective anti-wear additives, Zinc Dialkyl Dithio Phosphate (ZDDP) is one of the most common. However, the production of ash when it is decomposed and the emission of harmful gases push researchers to identify alternatives which can provide comparable anti-wear performance. Therefore, ashless additives are proposed as anti-wear additives instead of ZDDP. Soluble ILs are also suggested to be an effective alternative to ZDDP, especially when lubricating light alloys. Phosphonium phosphate (PP) ionic liquid is oil-soluble. Recent studies showed that this ionic liquid provided a superior anti-wear performance than ZDDP at some conditions (high temperature and sliding distance over 40 m). However, the reason behind this superior performance is still vague. This lack of knowledge is the motivation to conduct the second part of this study. It compares the reactivity and the tribological performance of both anti-wear additives (ZDDP and PP).
The lubrication mechanism is studied by considering the formation mechanisms of both thermal and tribo films. The durability of both films are studied. A nanotribometer is used to conduct the tribological experiments. Two tribopairs are utilized; borosilicate against silicon to simulate the working of Micro Electro Mechanical Systems.
Silicon against steel to simulate the working of the internal combustion engines (piston ring versus cylinder). Al-Si is commonly used as a cylinder liner material and the counter face is steel based piston rings. Previous studies showed that tribochemical studies using Si can be a good analogue for the Al-Si.
PP ionic liquid is used as a sample of oil-soluble ionic liquid and compared with ZDDP. Five ionic liquids are used as neat lubricants which are ethyl methyl imidazolium ethyl sulfate, ethyl methyl imidazolium octyl sulfate, butyl methyl imidazolium octyl sulfate, ethyl methyl imidazolium ethyl phosphate and butyl methyl phosphonium ethyl sulfate. Surface morphology of the thermal and tribo films is analysed by using Scanning Electron Microscopy, Atomic Force Microscopy and white light interferometry. Besides, surface chemistry is analysed using Energy Dispersive Spectroscopy, Fourier-transform infrared spectroscopy and X-ray Photoelectron Spectroscopy.
The results show that the formation rate of thermal/tribo films for Zinc Dialkyl Dithio Phosphate is higher than the formation rate for phosphonium phosphate. The absence of the positive cation after the decomposition of phosphonium phosphate leads to a self-limited formation of its thermal/tribo film. In contrast, the presence of zinc cation maintains the formation of thermal/tribo film. This causes a higher consumption of Zinc Dialkyl Dithio Phosphate (faster depletion). This leads to phosphonium phosphate can provide higher protection against wear at the highest sliding distance.
The results of the use of ionic liquids as neat lubricants show that their chemistry strongly affects their tribological performance. The reactivity of ionic liquids and hence the thickness of the thermal/tribo films is strongly affected by their chemistry. Further, the ionic liquids’ chemistry also affects the viscosity and thermal stability. All the examined ionic liquids reacted with silicon surfaces and formed thermal films. Lastly, the formation of tribofilms by ionic liquids enhances the tribological behaviour of the lubricated surfaces.