**1. Introduction**

Lubricants are very important materials for human and society due to their applications from "mobility" in ancient era to durability in modern times and then most recently in enhancement of "energy efficiency process". Petroleum-based lubricants are popular and used as the standard materials in transportation, manufacturing, and power generation industries etc. [1]. From economic point of view, 1.0–1.4% of a country's GDP may be achieved through lubrication R&D, which has provoked the relentless quest of advances in lubricants in order to increase both energy efficiency and durability [2]. Generally, commercial lubricant contains a combination of base oils and additives including antioxidants, detergents, dispersants, friction modifiers, antiwear and/or extreme-pressure additives, and viscosity modifiers.

As energy and environment play an important role in our life, there need for energy efficient systems, and utilization/conversion of energy in environmentally benign practices have been increasing immensely because of high volatility in fuel prices, stringent environmental regulations and global awareness on the sustainability of fuels. High fuel consumption is arisen due to high friction and wear in the transportation system during energy conversion process [3, 4]. Due to high friction and wear, failure of engine parts is often happened with large amount of discharge of partially oxidized fuels and greenhouse gas emission etc. For reducing the production of these hazardous materials, low friction and wear are required for energy conversion process. Lowering the friction and wear are important to reduce the production of hazardous materials during energy conversion process to the mating surfaces of the engine. Only an efficient lubricant can solve the problem related to energy conversion process and global awareness on the sustainability of fuels. Zinc dialkyldithiophosphate (ZDDP) is well-known as efficient antiwear and friction-reducing additive for iron-based components. Presently, it is observed that ZDDP is an efficient antiwear and friction-reducing additive but has shown toxic nature to aquatic wildlife, human-health issues and poisonous automotive exhaust gas as catalyst components.

Ionic liquids (ILs) have been known as new ionic materials and great important of applications in organic chemistry to as electrolytes in alternative energy generation/storage devices etc. (**Figure 1**). ILs have been known for their stability, wellestablished structural characterization and low viscosity etc. The choice of cation and anion is an important parameter for IL to determine the desirable physical properties. The tunable physical properties of the ILs make also an important material for the application in lubricant industries [5]. The length of side chain of the cation is responsible for making ILs as tailor-made lubricants and lubricant additives. Due to presence of unique physical and chemical properties of ILs, strong surface adsorption, high thermal stability, and low sensitivity in rheological behavior are observed compared to conventional oil lubricants. In early 2012, exploring the feasibility of ILs as lubricant additives was limited due to very low solubility in common nonpolar hydrocarbon lubricating oils [6–10]. The efficient oil-miscible ILs were discovered and reported as promising antiscuffing/antiwear functionalities [11, 12]. Since then, ILs is used as efficient lubricant additives in oil-based lubricant to increase both energy efficiency and durability due to improved solubility property [13, 14]. Hydrophobic cation or anions of ILs is responsible for showing good lubricant properties and making significantly stable thermo-oxidative materials.

Recently, ILs have been studied as versatile lubricants and lubricant additives for various engineering surfaces. The solid surfaces mediated thin films of ILs have shown more efficient lubricating properties compared to conventional non-polar hydrocarbon liquids due to presence of hydrophobic character, change of geometry of cation and charge characteristics of ILs. The dynamic conformation changes of cation and anion play important role to show the lower shear stress and friction than conventional non-polar molecular lubricant. ILs have also been studied as lubricating additives in water and lubricating oils due to their unique polar and

**5**

BF4 −

*Tribological Properties of Ionic Liquids DOI: http://dx.doi.org/10.5772/intechopen.94024*

topic in lubrication processing.

still under research and development.

good boundary lubrication properties.

properties depending upon cations and anions [19–21].

−

imidazolium, etc. as cations and X−

tribological test of BF4

of this book chapter.

non-polar domain solutions and miscibility with polar and non-polar solvents. Now, ILs as lubricant and lubricant oil additives have become the new central research

This book chapter starts with the tribological performance of ILs as lubricant additives. The physicochemical properties of ILs have been correlated with their nature of cation and anion. Future research directions are also suggested at the end

Generally, lubricants are used for extend the device life cycle and reduced parasitic energy loss by reducing friction. For these purposes, the lubricant must be high non-flammable and thermal stable with safer transportation and storage. ILs have shown interesting application in tribological studies due to their unique characteristic physical features [15]. It is also observed that addition of ILs to grease has shown substantially improved tribological performance. Similarly, IL-additive has shown to reduce more friction and wear compared to synthetic oil additives in base oil. Interestingly, imidazolium cation based ILs with long side-chain substituted cation and different anions have reduced more the friction and wear of steel-steel sliding pairs compared to base oil without additives. The excellent tribological properties of ILs as additives are due to their formation of physically adsorbed films and antiwear

The purity of IL is also key factor for improving wear and friction properties of ILs with additives. The highly purified IL has shown excellent friction reduction, antiwear performance and high load carrying capacity [18]. Further, lubricating performance of ILs depends on thermal stability, polarity, ability to form ordered adsorbed films and antiwear boundary film at the interface. Specially, polar nature of ILs can able to facilitate interactions in engineering surfaces forming the boundary thin film. The formation of unique protective thin film of ILs can able to avoid the direct contact between mating surfaces and is believed to be responsible for showing the antiwear property. ILs can provide an effective surface separative film at wide temperature ranges compared to conventional oils due to higher thermal stability. The area of functional fluids for lubricants and hydraulic oils is

Literature survey reveals that tribological study has been examined in ILs consisted of ammonium, phosphonium, pyrolidium, pyridinium, imidazolium cations as the cation and tetrafluoroborate (BF4), hexafluorophosphate (PF6), bis(trifluoromethanesulphonyl)imide (NTf2), for the anion (**Figure 2**). On the other hand, ILs containing halogen exhibit have shown low friction and wear with

> , PF6 −

Last one decade, several types of ILs like ammonium, phosphonium, pyridinium,

been extensively studied as lubricant and lubricant additives for wide range of application in surface engineering. ILs have also exhibited structure dependent lubrication

The halogenated ILs are used over the steel surface for avoiding direct contact between tribo interfaces, consequently reduction in both friction and wear. During

tribo-thin film is composed of FeF2 and B2O3 [22]. Phillips et al. have reported that

 anion based ILs can under go into several reaction with product of FeF2, and lead to deduction of lubricant properties and corrosion of the substrate surface [22]. Metal fluorides (Like FeF2) are formed on a boundary lubricating layer of

, CF3SO3

−

anion based ILs, it is observed that the developing a

, (CF3SO2)2N−

etc. as anions have

**2. Tribological performance of halogenated ionic liquids**

boundary film to reduce the friction and antiwear performance [16, 17].

*Various application of ILs in different fields.*

#### *Tribological Properties of Ionic Liquids DOI: http://dx.doi.org/10.5772/intechopen.94024*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

gas as catalyst components.

the production of these hazardous materials, low friction and wear are required for energy conversion process. Lowering the friction and wear are important to reduce the production of hazardous materials during energy conversion process to the mating surfaces of the engine. Only an efficient lubricant can solve the problem related to energy conversion process and global awareness on the sustainability of fuels. Zinc dialkyldithiophosphate (ZDDP) is well-known as efficient antiwear and friction-reducing additive for iron-based components. Presently, it is observed that ZDDP is an efficient antiwear and friction-reducing additive but has shown toxic nature to aquatic wildlife, human-health issues and poisonous automotive exhaust

Ionic liquids (ILs) have been known as new ionic materials and great important of applications in organic chemistry to as electrolytes in alternative energy generation/storage devices etc. (**Figure 1**). ILs have been known for their stability, wellestablished structural characterization and low viscosity etc. The choice of cation and anion is an important parameter for IL to determine the desirable physical properties. The tunable physical properties of the ILs make also an important material for the application in lubricant industries [5]. The length of side chain of the cation is responsible for making ILs as tailor-made lubricants and lubricant additives. Due to presence of unique physical and chemical properties of ILs, strong surface adsorption, high thermal stability, and low sensitivity in rheological behavior are observed compared to conventional oil lubricants. In early 2012, exploring the feasibility of ILs as lubricant additives was limited due to very low solubility in common nonpolar hydrocarbon lubricating oils [6–10]. The efficient oil-miscible ILs were discovered and reported as promising antiscuffing/antiwear functionalities [11, 12]. Since then, ILs is used as efficient lubricant additives in oil-based lubricant to increase both energy efficiency and durability due to improved solubility property [13, 14]. Hydrophobic cation or anions of ILs is responsible for showing good lubricant properties and making significantly stable thermo-oxidative materials. Recently, ILs have been studied as versatile lubricants and lubricant additives for various engineering surfaces. The solid surfaces mediated thin films of ILs have shown more efficient lubricating properties compared to conventional non-polar hydrocarbon liquids due to presence of hydrophobic character, change of geometry of cation and charge characteristics of ILs. The dynamic conformation changes of cation and anion play important role to show the lower shear stress and friction than conventional non-polar molecular lubricant. ILs have also been studied as lubricating additives in water and lubricating oils due to their unique polar and

**4**

**Figure 1.**

*Various application of ILs in different fields.*

non-polar domain solutions and miscibility with polar and non-polar solvents. Now, ILs as lubricant and lubricant oil additives have become the new central research topic in lubrication processing.

This book chapter starts with the tribological performance of ILs as lubricant additives. The physicochemical properties of ILs have been correlated with their nature of cation and anion. Future research directions are also suggested at the end of this book chapter.
