In this work, superlubricity between steel surfaces lubricated by mixtures of [Choline][Proline] ([Cho][Pro]) ionic liquid and glycerol aqueous solution has been reached by using a rotating tribometer. Stable superlubricity could be obtained even under the humidity between 7 to 9% RH. The lowest friction is observed when the lubricant contains 3 wt.% ionic liquid. It is found that adding 3 wt.% [Cho][Pro] is helpful to maintain enough water in the steady period to retain a low viscosity. According to the calculation, the superlubricity achieved in thin film lubrication region, which is attributed to the stern layer formed by [Cho][Pro] and hydrogen-bond network that enabled a thin water layer at the interface. Interestingly, it is observed that humidity can be used to control lubrication state between superlubricity and non-superlubricity. This study provides a new method to accomplish switchable superlubricity under low humidity.

The tunable friction behavior of Chitosan (CS)-Ag hydrogel enabled by altering metal ions is evaluated. Friction control could be achieved under boundary lubrication. When adding Ag+ into a CS solution, the formed gel provided lower friction. The difference in friction coefficient between the two phases can be reversibly switched by adding Cl- or excessive Ag+ ions. It also can be found that the gel phased lubricant has a better anti-wear ability under boundary lubrication conditions. Both solution and gel typed lubricants could achieve superlubricity under elastohydrodynamic lubrication. The switchable and tunable frictional hydrogels can extend the application in the design of smart control equipment.

Nowadays, the awareness for the importance of green lubricants and green lubricating additives is increasing. In this work, [Choline][Proline] ([Cho][Pro]) was added into glycerol aqueous solution to receive a high-performance green lubricant. The effect of environment condition, e.g., CO₂ and water, on the green lubricant was studied. It is found that the properties of the green lubricant could be modulated by CO₂ and water content. As CO₂ was absorbed by the liquid, the viscosity of the liquid increased, while the viscosity of liquid diminished after adding more water. The presence of CO₂ led to an obvious increase of friction. At the same time, it is also found that the friction could be altered by water content. Thus, it is possible to control friction by changing CO₂ and water content.

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.

Intelligent control of friction is an attractive but challenging topic. In this work, it is investigated if it would be possible to adjust friction in a lubricated contact by controlling environmental humidity. By exploiting the ability to adjust the environmental humidity by various saturated salt solutions, friction behavior of contacts lubricated with Choline l‐Proline ([Cho][Pro]) is modulated in a wide range of relative humidity (RH). The friction increases when the environmental humidity is increased and decreases when water is partially evaporated to a lower RH. It is thus possible to control friction by environmental humidity. The addition of water in ionic liquids (ILs) causes a decrease in viscosity, but as the tests are calculated to be performed in boundary lubrication the viscosity change is not the main factor for the change in friction. The friction sensitivity of RH can be explained by the effect of adhesion on the water uptake from humid air by [Cho][Pro]. Furthermore, the reversible changes of H‐bond types determined by the water content could be another explanation to the altered friction.

Monitoring human motion and harvesting its energy is an interesting and important topic. Triboelectric nanogenerators (TENGs) have shown their potential for signal detecting and energy harvesting. In this paper, a noncontact paper-based TENG is creatively proposed to be used as a self-powered human motion monitoring sensor and also to be used for human motion energy harvesting. The noncontact mode avoids the requirement of direct physical contact between the TENG and the human subject, which fascinates the application of TENG and can increase its service life. It is found that the walking gait cycle (leg raising and falling), moving direction, walking or running speed and moving path can be reflected by the output voltage signals directly and clearly. Noncontact human motion energy harvesting is also achieved by using this TENG with a rectification circuit. It is also surprisingly found that this TENG is able to detect human motion even through a building wall, and the output voltage is strong enough to be captured by a normal multimeter without any additional amplification circuit. This study opens up a new method for self-powered noncontact human movement detecting and monitoring.

Intelligent control of friction is an attractive but challenging topic and it has rarely been investigated for full size engineering applications. In this work, 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 elastohydrodynamic lubrication (EHL) regime. The friction decreased with increasing the amount of CO2 to the lubricant and increased after partial releasing CO2. As CO2 was absorbed by the liquid, the viscosity of the liquid increased which resulted in that the film thickness increased. At the same time the pressure-viscosity coefficient decreased with the addition of CO2. When CO2 was released again the friction increased and it was thus possible to control friction by adding or removing CO2.

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).

Diamond-like carbon (DLC)–steel contacts become more and more popular in the industry now. Since the surface chemical properties of DLC are quite different from those of iron, traditional formulated lubricants have problems to form tribo-chemical films, which are effective to improve the tribological performance for steel-steel contacts, on the surface of DLC. Thus, new lubricants formulation strategies are needed to be considered for steel-DLC applications. A kind of green lubricant (lignin-[Choline][L-Proline] (L-[CH][Pro])) without any traditional tribo-chemical active element, i.e., free of P, S, B, etc., was studied in this paper for the steel-DLC contact. To find the difference between this new ILs and the traditional lubricants, a commercially available fully formulated lubricant was used as a reference. An Optimol SRV-III oscillating friction and wear tester was used to evaluate the tribological performance. Three different kinds of commercially available DLC coatings (Tribobond 40(Cr + a-C:H:W), Tribobond 43 [(Cr+) a-C:H), and Tribobond 44(a-C:Cr)] were investigated. The results show that the ILs exhibit an obviously lower friction coefficient than that of the traditional commercially available fully formulated lubricant. Among those three DLC coatings, the (Cr+) a-C:H DLC coating exhibits the biggest improvement of wear resistance lubricated with the new ILs than that of the commercially available fully formulated lubricant. It's expected that its excellent tribological properties are attributed to the affinity of the ILs to the metal surface and the strength of the ionic liquids interactions by hydrogen bonding. Thus, forming strong physical adsorption strategy, instead of forming chemical tribo-films, is recommended to enhance the lubricating performance of lubricants for DLC.

Lignin strengthened ionic liquids (ILs) have shown high potential to be used as high performance green lubricants. Strengthened lignin-ILs molecular interaction is an effective approach to improve their lubrication properties. The molecular interactions of ILs’ cation and anion containing different functional groups with lignin and efficiency on the lubricating properties have rarely been studied yet. In this work, a series of novel green lubricants with dissolved lignin in [Choline][Amino Acid] ([CH][AA]), [Tetramethylammonium][Glycine] ([N 1111 ][Gly]) and [Tetrabutylammonium][Glycine] ([N 4444 ][Gly]) ILs have been synthesized and their tribological properties were systematically investigated. The longer alkyl chain in cation without reciprocal H-bond interaction between ILs’ cation and anion has the positive effect on the anti-wear properties. In addition, the less steric effect and more negative natural charges of amino acid anion synergistically contribute to the stronger H-bond interaction between lignin and choline base ILs, which enhances lubrication film strength and thus resulting in the better tribological property of ILs/lignin green lubricants. Specifically, the wear volume loss of the steel disc lubricated by [N 4444 ][Gly] with the addition of 15% lignin is only 12% of the one lubricated by pure [N 4444 ][Gly]. This work presents a method to tune molecular interaction between lignin and ILs via the structural design of ILs’ cation and anion, which are revealed as the key factor that bridges the individual components and improves overall lubricating properties.