Nowadays the demand for intelligent control of tribological interactions is strongly increasing in various applications. We often strive to minimize friction but there are also many situations where high friction is desirable. In some cases, something in between, i.e. optimum friction, is attractive. Driven by the broad application prospects, many controllable friction systems regulated with external stimuli such as solvent, pH, temperature, electric potential, and magnetic field have been designed and fabricated. When external stimuli are imposed on the smart materials, the macroscopic physicochemical properties of the materials are dramatically changed, making controllable friction behavior to become possible. However, most of these exploratory works are in nano/micro size and it’s difficult to use these incredible methods in macroscale directly due to that macroscopic laws of friction do not generally apply to nanoscale contacts. This thesis attempts to find more versatile methods of friction control and try to find the possibility to achieve friction control at macro-size.

Firstly, since viscosity plays an important role in elastohydrodynamic lubrication (EHL) at macro-size, it is instigated if it would be possible to adjust friction by controlling viscosity in a lubricated contact. By exploiting the ability to adjust the viscosity of the switchable ionic liquids, 1,8-Diazabicyclo (5.4.0) undec-7-ene (DBU)/ glycerol mixture via the addition of CO₂, the friction could be controlled in the EHL regime (Paper Ⅰ). In order to understand more about the lubricating mechanism of DBU/glycerol/CO₂ mixture, the central film thickness of the lubricants as a function of the entrainment speed was investigated.

Secondly, due to that adhesion could have influence on boundary lubrication (BL) friction at macro-size, it is investigated if it would be possible to adjust friction in a lubricated contact by controlling environmental humidity, which can alter the H-bond types, leading the change of adhesion. By exploiting the ability to adjust the environmental humidity by various saturated salt solutions, friction behavior lubricated by Choline L-Proline ([Cho][Pro]) could be modulated in a wide range of relative humidities (RH) (Paper Ⅱ).

We often strive to minimize friction but there are also many situations where high friction is desirable. In some cases, something in between, i.e. optimum friction, is attractive. Driven by the broad application prospects, many controllable friction systems regulated with external stimuli such as solvent, pH, temperature, electric potential, and magnetic fields have been designed and fabricated. When external stimuli are imposed on the smart materials, the macroscopic physicochemical properties of the materials are dramatically changed, making controllable friction behavior to become possible. However, most of these exploratory works are in nano/micro size, which cannot be applied in macroscale for industry applications. 

Lubrication is mainly classified as three regimes: boundary, mixed and full film lubrication. If considering molecular size as well, there exists thin film lubrication between boundary lubrication and elastohydrodynamic lubrication, with the lubricating film thickness of a few nanometers to tens of nanometers. This thesis attempts to find more versatile methods of friction control and tries to find the possibility to achieve friction control at macroscale in all these lubrication regimes.

In this thesis, we investigated the possibility of adjusting friction by controlling viscosity in a lubricated contact and it was found that friction of switchable ionic liquids could be controlled in the EHL regime enabled by CO2 absorption and desorption in Paper 1. Glycerol solution formulated with ionic liquid was employed in Paper 2 to obtain superlubricity in thin film lubrication, and it was observed that the lubrication state could be switched between superlubricity and non-superlubricity by adjusting humidity. Friction control of multi-functional green lubricant in mixed lubrication was evaluated in Paper 3. Finally, the macroscopic friction control of ionic liquids in boundary lubrication enabled by environmental humidity was described in Paper 4, and stimuli responsive hydrogel also could be used for achieving friction control in boundary lubrication, studied in Paper 5.