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

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

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 still under research and development.

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 good boundary lubrication properties.

Last one decade, several types of ILs like ammonium, phosphonium, pyridinium, imidazolium, etc. as cations and X− , PF6 − , CF3SO3 − , (CF3SO2)2N− etc. as anions have been extensively studied as lubricant and lubricant additives for wide range of application in surface engineering. ILs have also exhibited structure dependent lubrication properties depending upon cations and anions [19–21].

The halogenated ILs are used over the steel surface for avoiding direct contact between tribo interfaces, consequently reduction in both friction and wear. During tribological test of BF4 − anion based ILs, it is observed that the developing a tribo-thin film is composed of FeF2 and B2O3 [22]. Phillips et al. have reported that BF4 − 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

**Figure 2.**

*Structures and abbreviations of cations and anions of the halogenated ILs used as lubricant additives.*

friction surfaces by a tribochemical reaction. It is also known that ILs containing a halogen such as fluorine has been known to cause corrosion in steel aluminum alloy, bronze, and titanium alloy sliding materials [23, 24]. The corrosion of alloy sliding materials has been reported to be the formation of hydrogen fluoride (HF) due to the decomposition of halogenated ILs [25–28]. The formation of hydrogen fluoride is accelerated due to presence of water impurity in halogenated ILs. The change in color of the friction surface for steel bearings is observed using the hydrophobic IL as the lubricant in air at higher humidity [29]. The corrosion products are containing mainly metal fluorine and metal oxide on the surface which are experimentally verified [30].

After detailed investigation, halogenated ILs have hazardous and toxic effects to the environment and corrosive nature towards the engineering surfaces. The halogenated ILs can produce toxic and corrosive products after decomposition under different tribo-chemical reaction conditions for environment and the surface-engineering. High cost of halogens, particularly, fluorine-based precursors and disposal/discharge of halogenated ILs are big challenges for their penetration to the industrial applications.

Thus, halogen-free IL have been attracted more interest for developing the new type of lubricant for the energy efficient and environmentally-friendly processess.

### **3. Tribological performance of halogen-free ionic liquids**

Accordingly, developing environmentally friendly ILs from renewable and biodegradable resources to diminish or avoid corrosion and toxicity has been becoming an inevitable strategy. A great effort has been devoted to searching for new halogenfree ILs. From literature surveys, halogen-free bioactive ILs such as saccharin [31–33], amino acid [34, 35] and ibuprofen [36, 37] ILs have been reported to replace traditional corrosive or hazardous halogenated ILs. Unfortunately, these halogen-free bioactive ILs are very poor thermal stability. However, low thermal stability and high cost of precursors cause less usable from application perspective. Interestingly, physicochemical properties and nontoxicity of these ILs can be regulated and customized by building precursor units from active pharmaceutical ingredients and biomass [38–40]. Literature survey reveals that tribological study of halogen-free ILs has been on the boundary lubricating capacity. Examined ILs are mainly consisted of ammonium, phosphonium, pyrolidium, pyridinium and imidazolium as the cation and phosphonate, dicyanamide, tricyanomethanide for

**7**

**Figure 3.**

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

mance compared to synthetic lube oils.

reduction property to ZDDP [50].

contact, [51–53].

friction tests (**Figure 4**).

anion (**Figures 3**-**5**). The halogen-free ILs have showed good tribological perfor-

Phosphonate-based halogen-free ILs has shown good thermal stability [41–43]. Phosphorus-containing ILs have been used for tribological study and shown effective lowering friction and wear reductions ability [12, 13, 44, 45]. The effectiveness of lowering friction and wear reductions ability, from high to low, was observed in phosphonium-phosphate, phosphonium-carboxylate, and phosphonium-sulfonate [46]. Experimental studies suggest that [P8,8,8,8][DEHP], [N8,8,8,H][DEHP], and [P6,6,6,14][BTMPP] provide similar surface protection for both steel−steel and steel−iron contacts to ZDDP compound [47–49]. In choline based ILs, [choline] [DEHP], [choline][DBDP], [P6,6,6,14][BTMPP], [P6,6,6,14][Tf2N], [P6,6,6,14][DMP], and [P6,6,6,14][DEP]) have showed higher wear reduction to compared with the base oil, but only [choline][DEHP] and [P6,6,6,14][Tf2N] have only shown similar wear

Phosphonium based ILs are used as additives in ester base oils and a VO, but only [P2,4,4,4][DEP] and [P6,6,6,14][FAP] have showed a stable >1% oil-solubility. At the same concentration, [P2,4,4,4][DEP] and [P6,6,6,14][FAP] ILs have showed comparable wear protection to ZDDP under low loads for a steel−steel ball-on-flat

Further, halogen-free ILs have been used for extended tribological properties of the steel-steel and DLC–DLC tribo-pairs. Lubricating and additive properties of bmimDCA and bmimTCM have been tested on the steel and DLC surfaces after the

A chemical reaction film is observed on the sliding surface of the steel-steel tribo-pair. It is considered that a corrosive attack of ILs to the metal surface is also

*Structures and abbreviations of cations and anions of the nonhalogenated ILs used as lubricant additives.*

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

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

friction surfaces by a tribochemical reaction. It is also known that ILs containing a halogen such as fluorine has been known to cause corrosion in steel aluminum alloy, bronze, and titanium alloy sliding materials [23, 24]. The corrosion of alloy sliding materials has been reported to be the formation of hydrogen fluoride (HF) due to the decomposition of halogenated ILs [25–28]. The formation of hydrogen fluoride is accelerated due to presence of water impurity in halogenated ILs. The change in color of the friction surface for steel bearings is observed using the hydrophobic IL as the lubricant in air at higher humidity [29]. The corrosion products are containing mainly metal fluorine and metal oxide on the surface which are experimentally

*Structures and abbreviations of cations and anions of the halogenated ILs used as lubricant additives.*

After detailed investigation, halogenated ILs have hazardous and toxic effects to the environment and corrosive nature towards the engineering surfaces. The halogenated ILs can produce toxic and corrosive products after decomposition under different tribo-chemical reaction conditions for environment and the surface-engineering. High cost of halogens, particularly, fluorine-based precursors and disposal/discharge of halogenated ILs are big challenges for their penetration to

Thus, halogen-free IL have been attracted more interest for developing the new type of lubricant for the energy efficient and environmentally-friendly processess.

Accordingly, developing environmentally friendly ILs from renewable and biodegradable resources to diminish or avoid corrosion and toxicity has been becoming an inevitable strategy. A great effort has been devoted to searching for new halogenfree ILs. From literature surveys, halogen-free bioactive ILs such as saccharin [31–33], amino acid [34, 35] and ibuprofen [36, 37] ILs have been reported to replace traditional corrosive or hazardous halogenated ILs. Unfortunately, these halogen-free bioactive ILs are very poor thermal stability. However, low thermal stability and high cost of precursors cause less usable from application perspective. Interestingly, physicochemical properties and nontoxicity of these ILs can be regulated and customized by building precursor units from active pharmaceutical ingredients and biomass [38–40]. Literature survey reveals that tribological study of halogen-free ILs has been on the boundary lubricating capacity. Examined ILs are mainly consisted of ammonium, phosphonium, pyrolidium, pyridinium and imidazolium as the cation and phosphonate, dicyanamide, tricyanomethanide for

**3. Tribological performance of halogen-free ionic liquids**

**6**

verified [30].

**Figure 2.**

the industrial applications.

anion (**Figures 3**-**5**). The halogen-free ILs have showed good tribological performance compared to synthetic lube oils.

Phosphonate-based halogen-free ILs has shown good thermal stability [41–43]. Phosphorus-containing ILs have been used for tribological study and shown effective lowering friction and wear reductions ability [12, 13, 44, 45]. The effectiveness of lowering friction and wear reductions ability, from high to low, was observed in phosphonium-phosphate, phosphonium-carboxylate, and phosphonium-sulfonate [46]. Experimental studies suggest that [P8,8,8,8][DEHP], [N8,8,8,H][DEHP], and [P6,6,6,14][BTMPP] provide similar surface protection for both steel−steel and steel−iron contacts to ZDDP compound [47–49]. In choline based ILs, [choline] [DEHP], [choline][DBDP], [P6,6,6,14][BTMPP], [P6,6,6,14][Tf2N], [P6,6,6,14][DMP], and [P6,6,6,14][DEP]) have showed higher wear reduction to compared with the base oil, but only [choline][DEHP] and [P6,6,6,14][Tf2N] have only shown similar wear reduction property to ZDDP [50].

Phosphonium based ILs are used as additives in ester base oils and a VO, but only [P2,4,4,4][DEP] and [P6,6,6,14][FAP] have showed a stable >1% oil-solubility. At the same concentration, [P2,4,4,4][DEP] and [P6,6,6,14][FAP] ILs have showed comparable wear protection to ZDDP under low loads for a steel−steel ball-on-flat contact, [51–53].

Further, halogen-free ILs have been used for extended tribological properties of the steel-steel and DLC–DLC tribo-pairs. Lubricating and additive properties of bmimDCA and bmimTCM have been tested on the steel and DLC surfaces after the friction tests (**Figure 4**).

A chemical reaction film is observed on the sliding surface of the steel-steel tribo-pair. It is considered that a corrosive attack of ILs to the metal surface is also

**Figure 3.** *Structures and abbreviations of cations and anions of the nonhalogenated ILs used as lubricant additives.*

**Figure 4.**

*Structures and abbreviations of cations and anions of the carbon-nitrogen atom based ILs used as lubricant additives.*

**Figure 5.**

*Structures and abbreviations of cations and anions of amino acid ILs used as lubricant additives.*

occurred because the chemical reaction film was mainly composed of the elements of the halogen-free ILs [54, 55]. The appearance of the chemical reaction film is similar to reported literature for tribo-films originating from zinc dialkyldithiophosphates (ZDDP) [56–59]. Additional analysis of chemical reaction film is also needed to identify the species generated on the steel surface. On the other hand, the chemical reaction film formation is not observed on the DLC surfaces. As DLC films have high chemical stability, the inhibition of the chemical interaction between the DLC surfaces and the halogen-free ILs is observed. The bmimDCA has showed better reducing frictional properties than bmimTCM for the steel-steel tribo-pair, whereas bmimTCM has showed better reducing frictional properties than bmim-DCA for the DLC-DLC tribo-pairs. For explain the above phenomena, different lubrication mechanism is employed for DLC-DLC and steel–steel tribo-pairs [60].

A new family of green fluid lubricants (AAILs) have been designed for the lubrication of steel/steel, steel/copper and steel/aluminum contacts at room temperature (**Figure 5**). These AAILs can be obtained by simply neutralizing amino acids, which can be easily obtained in large quantities at low cost with the corresponding onium hydroxide. Use of natural amino acids as component ions makes the AAILs environmentally friendly with good biodegradability and reduced toxicity, making the AAILs as good potential green lubricants. The degree of hydrolysis of these AAILs are much higher than that of bmimBF4 and the anti-corrosion properties of the AAILs are also far better than bmimBF4 and hmimNTf2, due to their halogen-free characters. The tribological properties of the AAILs (**Figure 5**) have been tested on steel-steel contacts as steel is the most widely used material in various machines in our everyday life. Generally, AAILs produce a lower friction coefficient value than hmimNTf2 and prove the better friction-reducing performances where commercial oil PAO and a conventional IL hmimNTf2 are chosen for comparison purposes.

From experimental results, the wear volume losses of the steel discs lubricated by all AAILs are lower than that of the hmimNTf2 but higher than that of the PAO. The anti-wear properties of the AAILs should be improved compared with PAO. For

**9**

**Figure 6.**

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

contact [62].

metal surface contact.

their intrinsic environmentally friendly characters.

improving anti-wear properties of the AAILs, [TBA][Ser] and [TBA][Thr] have been synthesized and exhibit the higher anti-wear properties due to attribution of their anionic moiety. Due to presence of hydroxyl groups in the anion structures of [Ser] and [Thr] effective protection films is formed on the metal surfaces. The effect of hydrogen bonding in [TBA][Ser] and [TBA][Thr] ILs provides effective separation of the steel surfaces, further reducing friction and wear. Besides, from an application point of view, [TBA][Ser] and [TBA][Thr] are more useful as lubricants than hmimNTf2, because of the lower cost associated with their preparation, and

The AAILs are used to lubricate Cu alloys to change in friction coefficients and the wear volume losses of the copper discs under lubrication process [61, 62]. The evolution of friction coefficients shows that hmimNTf2 and [TBA][Leu] start at a moderately high value and then tend to become lower and more stable. From experimental results, AAILs have good lubricating effects for steel/copper

The lubrication of aluminum alloys has shown relatively poor wear-resistance, makes them especially difficult to be lubricated at a modest load [63]. It is also observed that the halogen-containing IL (like hmimNTf2) is not an efficient lubricant for aluminum, and that severe wear may be caused by its tribo-corrosion during the sliding process [64]. On the contrary, the AAILs are effective lubricants for aluminum alloy, and their tribological properties are comparable to PAO.

The friction-reducing and anti-wear mechanism of the AAILs are explored by XPS analysis. However, characteristic peaks of N1s, which provide important information regarding the occurrence of a tribochemical reaction on a metal surface, are not detected. Besides, the lubricated metal surfaces by the AAILs and nonlubricated surface have shown similar binding energies of C1s, O1s, Fe2p, Cu2p and Al2p [65]. An AAIL adsorbed layer is formed via adsorption of cations and amino acid anions through an electrostatic attraction. The physical adsorption films by several AAIL adsorbed layer prevent close contact of metal–metal and further reduce the friction and wear on metal–metal surface [65]. The AAILs can substitute PAO and especially halogen containing ILs for use as neat lubricants for metal–metal contact. Additionally, the environmentally friendly and outstanding anti-corrosion properties of the AAILs also confirm that they are suitable for the lubrication of metal–

Boron containing ILs are also class of non-halogenated ILs (**Figure 6**). Recently, boron containing ILs are reported as an efficient lubricant and additive [66, 67]. Development of halogen-free orthoborate anions based phosphonium ILs has been attracted research for tribological studies [68]. It is also reported that the boron constituted materials are well-known for exhibiting excellent friction-reducing

*Structures and abbreviations of cations and anions of boron based ILs used as lubricant additives.*

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

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

occurred because the chemical reaction film was mainly composed of the elements of the halogen-free ILs [54, 55]. The appearance of the chemical reaction film is similar to reported literature for tribo-films originating from zinc dialkyldithiophosphates (ZDDP) [56–59]. Additional analysis of chemical reaction film is also needed to identify the species generated on the steel surface. On the other hand, the chemical reaction film formation is not observed on the DLC surfaces. As DLC films have high chemical stability, the inhibition of the chemical interaction between the DLC surfaces and the halogen-free ILs is observed. The bmimDCA has showed better reducing frictional properties than bmimTCM for the steel-steel tribo-pair, whereas bmimTCM has showed better reducing frictional properties than bmim-DCA for the DLC-DLC tribo-pairs. For explain the above phenomena, different lubrication mechanism is employed for DLC-DLC and steel–steel tribo-pairs [60]. A new family of green fluid lubricants (AAILs) have been designed for the lubrication of steel/steel, steel/copper and steel/aluminum contacts at room temperature (**Figure 5**). These AAILs can be obtained by simply neutralizing amino acids, which can be easily obtained in large quantities at low cost with the corresponding onium hydroxide. Use of natural amino acids as component ions makes the AAILs environmentally friendly with good biodegradability and reduced toxicity, making the AAILs as good potential green lubricants. The degree of hydrolysis of these AAILs are much higher than that of bmimBF4 and the anti-corrosion properties of the AAILs are also far better than bmimBF4 and hmimNTf2, due to their halogen-free characters. The tribological properties of the AAILs (**Figure 5**) have been tested on steel-steel contacts as steel is the most widely used material in various machines in our everyday life. Generally, AAILs produce a lower friction coefficient value than hmimNTf2 and prove the better friction-reducing performances where commercial oil PAO and a conventional IL

*Structures and abbreviations of cations and anions of amino acid ILs used as lubricant additives.*

*Structures and abbreviations of cations and anions of the carbon-nitrogen atom based ILs used as lubricant* 

From experimental results, the wear volume losses of the steel discs lubricated by all AAILs are lower than that of the hmimNTf2 but higher than that of the PAO. The anti-wear properties of the AAILs should be improved compared with PAO. For

**8**

**Figure 4.**

*additives.*

**Figure 5.**

hmimNTf2 are chosen for comparison purposes.

improving anti-wear properties of the AAILs, [TBA][Ser] and [TBA][Thr] have been synthesized and exhibit the higher anti-wear properties due to attribution of their anionic moiety. Due to presence of hydroxyl groups in the anion structures of [Ser] and [Thr] effective protection films is formed on the metal surfaces. The effect of hydrogen bonding in [TBA][Ser] and [TBA][Thr] ILs provides effective separation of the steel surfaces, further reducing friction and wear. Besides, from an application point of view, [TBA][Ser] and [TBA][Thr] are more useful as lubricants than hmimNTf2, because of the lower cost associated with their preparation, and their intrinsic environmentally friendly characters.

The AAILs are used to lubricate Cu alloys to change in friction coefficients and the wear volume losses of the copper discs under lubrication process [61, 62]. The evolution of friction coefficients shows that hmimNTf2 and [TBA][Leu] start at a moderately high value and then tend to become lower and more stable. From experimental results, AAILs have good lubricating effects for steel/copper contact [62].

The lubrication of aluminum alloys has shown relatively poor wear-resistance, makes them especially difficult to be lubricated at a modest load [63]. It is also observed that the halogen-containing IL (like hmimNTf2) is not an efficient lubricant for aluminum, and that severe wear may be caused by its tribo-corrosion during the sliding process [64]. On the contrary, the AAILs are effective lubricants for aluminum alloy, and their tribological properties are comparable to PAO.

The friction-reducing and anti-wear mechanism of the AAILs are explored by XPS analysis. However, characteristic peaks of N1s, which provide important information regarding the occurrence of a tribochemical reaction on a metal surface, are not detected. Besides, the lubricated metal surfaces by the AAILs and nonlubricated surface have shown similar binding energies of C1s, O1s, Fe2p, Cu2p and Al2p [65]. An AAIL adsorbed layer is formed via adsorption of cations and amino acid anions through an electrostatic attraction. The physical adsorption films by several AAIL adsorbed layer prevent close contact of metal–metal and further reduce the friction and wear on metal–metal surface [65]. The AAILs can substitute PAO and especially halogen containing ILs for use as neat lubricants for metal–metal contact. Additionally, the environmentally friendly and outstanding anti-corrosion properties of the AAILs also confirm that they are suitable for the lubrication of metal– metal surface contact.

Boron containing ILs are also class of non-halogenated ILs (**Figure 6**). Recently, boron containing ILs are reported as an efficient lubricant and additive [66, 67]. Development of halogen-free orthoborate anions based phosphonium ILs has been attracted research for tribological studies [68]. It is also reported that the boron constituted materials are well-known for exhibiting excellent friction-reducing

**Figure 6.**

*Structures and abbreviations of cations and anions of boron based ILs used as lubricant additives.*

and antiwear properties [69–72]. Boron-containing ILs have been attracted great interest in recent time. Chelated orthoborate anion [BScB]<sup>−</sup> with different cations provides large number of ILs [73]. With Cation bmim+ , [BScB]− has shown lower friction than [FAP]− or [DBP]− , but [DBP]− has shown the most wear reduction [74]. Further, wear and friction are significantly reduced when [BScB]− anion paired with dicationic [bis(imidazolium)]2+ and [bis-(ammonium)]2+ [75]. For cation [TBA]+ , anions [BScB]<sup>−</sup> , [BMIB]− , and [BOxB]<sup>−</sup> show 50% or more in wear reduction under similar testing conditions [73–76].

The scope of chelated orthoborate anions based ILs are further extended with imidazolium, bis-imidazolium and pyrrolidinium cations for their application in tribological studies. The ILs with aromatic and aliphatic structures (**Figure 6**) which are reported recently with an aim to probe their structural effects on corrosion and tribo-physical properties compared with the halogenated analogue TBA-BF4 [73]. It is also observed that TBA-BMdB, TBA-BOxB and TBA-BScB ILs exhibited higher thermal stability due to the presence of aromatic rings in their chelated structure and presence of various intermolecular interactions and rigidity to their anionic moieties.

Presence of halogen, phosphorus, and sulfur constituent components in the lubricant system facilitates the corrosion events and damages the engineering surfaces. Khatri and co-worker have investigated the corrosion property of boron based ILs (**Figure 6**) probed by copper strip test meth by optical and electron microscopic techniques [73]. It is also reported that the copper strip, exposed to TBA-BOxB, exhibited corrosion pits distributed throughout the substrate. The surface features of copper strips remain intact without any damage, exposed to TBA-BMdB, TBA-BScB and TBA-ILs. These experimental results suggested that TBA-BMdB, TBA-BScB and TBA-BMlB ILs (halogen-free), do not corrode the copper strips surface, whereas, presence of fluorine in TBA-BF4, corrosive events on copper strips surface are facilitated. Furthermore, TBA-BOxB IL has poor thermal stability and its decomposed acidic (oxalic acid) product leads to corrosive events. As a result, TBA-BOxB showed higher friction and WSD compared to other chelated orthoborate ILs. Most of chelated orthoborate ILs has shown noncorrosive properties and can be tested for their lubrication properties.

Among all boron based ILs (**Figure 6**), maximum antiwear property is achieved by TBA-BMdB IL due to compact, rigid and stable structure of BMdB anion. To understand the effect of halogen, the friction and wear properties of fluorine constituted TBA-BF4 ILs are examined under identical condition. It is observed that TBA-BF4 has showed poorer tribo performance compared to the chelated orthoborate ILs. Poor tribo-performance and corrosion results suggest that corrosive products generation by BF4 anion constituted ILs could be further facilitated by trapped water molecules in the lubricant [28].

The exact mechanism and role of boron based ILs in tribo-chemical thin film formation is believed to be complex because of their inherent polarity. Recently, Oganov et al. have revealed that boron containing ILs can generate partial negative charge and facilitate the interaction of chelated orthoborate anions with steel surfaces and forms the tribo-thin film under the high pressure [77]. Usually, under the tribo-stress, the positive charge is induced on metal surfaces. Chelated orthoborate anions are adsorbed on induced positive charge surface with counter cations. The layering structure on metal surface is formed through electrostatic attractions and generates the physico-chemically adsorbed tribo-thin film [78]. Furthermore, the very hard nature of boron is understood to provide durable tribo-thin film, which protects the steel interfaces and reduces wear significantly.

It has been suggested that the dangling bonds of carbon atoms on the metal surface are terminated by lubricant additives or the decomposition of lubricant

**11**

**Table 1.**

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

utility of ILs as lubricant.

additives and the formation of a monomolecular layer, which results in ultralow friction. These results suggest that an adsorbed film derived from the halogen-free ILs formed on the surfaces, which led to the ultralow friction. Moreover, a soft, thin layer on hard substrate materials is important for achieving an ultralow friction regime under boundary lubrication in accordance with the adhesion theory of friction [60]. The tribo-chemical thin film developed by chemical interaction of ILs and their decomposed products with steel interfaces could be an alternate to justify the tribo-mechanism [6, 10, 13]. Comparison of halogenated and non-halogenated ILs with conventional lubricants is listed in **Table 1** for better understanding the

Oil-soluble ILs, when used as lubricant additives, have repeatedly exhibited effective wear and friction reductions in tribological bench tests and demonstrated improved engine mechanical efficiency in engine dynamometer tests. The lubricating performance has shown a strong correlation with the ILs chemistry, concentration, compatibility with other oil additives, material compositions of the contact surfaces, and rubbing conditions. While some results simply showed improvement over the base oils, others have direct comparisons with commercial antiwear additives. Phosphonium based ILs with halogenated and non-halogenated anions are also used as additive for different contact surfaces [14]. Further, tribological study

**Lubricants COF Wear Contact Reference**

/m

/m

/m

/m

/m

/m

/Nm

/Nm

/Nm

/Nm

/Nm

/Nm

/Nm

/Nm

/Nm

/m Titanium-Steel [79]

/m Steel-Steel [80]

/Nm Copper-Si3N4 [81]

/Nm Engine inner ring [5]

/Nm Nickel-Steel [82]

/Nm Copper-Copper [83]

/Nm Steel-Si3N4

/Nm Crystalline

emimBF4 0.56 3.11x10−3 mm3

bmimBF4 0.17 0.02x10−3 mm3

bmimCl 0.17 0.02x10−3 mm3

hmimPF6 0.19 0.08x10−3 mm3

omimBF4 0.18 0.1x10−3 mm3

Mineral Oil 0.45 1.9x10−3 mm3

hmimPF6 0.065 9.3x10−3 mm3

PAO 0.105 9x10−3 mm3

bmimBF4 0.045 230x10−9 mm3

Diesel oil 0.07 210x10−9 mm3

bmimBF4 0.041 73.1x10−9 mm3

Diesel oil 0.105 80.2x10−9 mm3

bmimBF4 0.035 75x10−9 mm3

(C8H17)3NHNTf2 0.05 29.1x10−9 mm3

dmimNTf2 0.07 24.5x10−9 mm3

Mineral Oil 0.11 44.8x10−9 mm3

15w40 Engine oil 0.11 36.9x10−9 mm3

hmimPF6 0.085 3x10−9 mm3

omimPF6 0.1 9x10−9 mm3

PFPE 0.145 37x10−9 mm3

DSa 0.3 0.26x10−9 mm3

PAO 0.1 4.54x10−9 mm3

*Comparison of ionic liquids (ILs) and conventional lubricants.*

Cr- Si3N4 Diesel oil 0.075 34x10−9 mm3

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

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

interest in recent time. Chelated orthoborate anion [BScB]<sup>−</sup>

provides large number of ILs [73]. With Cation bmim+

or [DBP]−

reduction under similar testing conditions [73–76].

ties and can be tested for their lubrication properties.

trapped water molecules in the lubricant [28].

protects the steel interfaces and reduces wear significantly.

, anions [BScB]<sup>−</sup>

friction than [FAP]−

to their anionic moieties.

cation [TBA]+

and antiwear properties [69–72]. Boron-containing ILs have been attracted great

paired with dicationic [bis(imidazolium)]2+ and [bis-(ammonium)]2+ [75]. For

The scope of chelated orthoborate anions based ILs are further extended with imidazolium, bis-imidazolium and pyrrolidinium cations for their application in tribological studies. The ILs with aromatic and aliphatic structures (**Figure 6**) which are reported recently with an aim to probe their structural effects on corrosion and tribo-physical properties compared with the halogenated analogue TBA-BF4 [73]. It is also observed that TBA-BMdB, TBA-BOxB and TBA-BScB ILs exhibited higher thermal stability due to the presence of aromatic rings in their chelated structure and presence of various intermolecular interactions and rigidity

Presence of halogen, phosphorus, and sulfur constituent components in the lubricant system facilitates the corrosion events and damages the engineering surfaces. Khatri and co-worker have investigated the corrosion property of boron based ILs (**Figure 6**) probed by copper strip test meth by optical and electron microscopic techniques [73]. It is also reported that the copper strip, exposed to TBA-BOxB, exhibited corrosion pits distributed throughout the substrate. The surface features of copper strips remain intact without any damage, exposed to TBA-BMdB, TBA-BScB and TBA-ILs. These experimental results suggested that TBA-BMdB, TBA-BScB and TBA-BMlB ILs (halogen-free), do not corrode the copper strips surface, whereas, presence of fluorine in TBA-BF4, corrosive events on copper strips surface are facilitated. Furthermore, TBA-BOxB IL has poor thermal stability and its decomposed acidic (oxalic acid) product leads to corrosive events. As a result, TBA-BOxB showed higher friction and WSD compared to other chelated orthoborate ILs. Most of chelated orthoborate ILs has shown noncorrosive proper-

Among all boron based ILs (**Figure 6**), maximum antiwear property is achieved

The exact mechanism and role of boron based ILs in tribo-chemical thin film formation is believed to be complex because of their inherent polarity. Recently, Oganov et al. have revealed that boron containing ILs can generate partial negative charge and facilitate the interaction of chelated orthoborate anions with steel surfaces and forms the tribo-thin film under the high pressure [77]. Usually, under the tribo-stress, the positive charge is induced on metal surfaces. Chelated orthoborate anions are adsorbed on induced positive charge surface with counter cations. The layering structure on metal surface is formed through electrostatic attractions and generates the physico-chemically adsorbed tribo-thin film [78]. Furthermore, the very hard nature of boron is understood to provide durable tribo-thin film, which

It has been suggested that the dangling bonds of carbon atoms on the metal surface are terminated by lubricant additives or the decomposition of lubricant

by TBA-BMdB IL due to compact, rigid and stable structure of BMdB anion. To understand the effect of halogen, the friction and wear properties of fluorine constituted TBA-BF4 ILs are examined under identical condition. It is observed that TBA-BF4 has showed poorer tribo performance compared to the chelated orthoborate ILs. Poor tribo-performance and corrosion results suggest that corrosive products generation by BF4 anion constituted ILs could be further facilitated by

, and [BOxB]<sup>−</sup>

, but [DBP]−

[74]. Further, wear and friction are significantly reduced when [BScB]−

, [BMIB]−

with different cations

show 50% or more in wear

has shown lower

anion

, [BScB]−

has shown the most wear reduction

**10**

additives and the formation of a monomolecular layer, which results in ultralow friction. These results suggest that an adsorbed film derived from the halogen-free ILs formed on the surfaces, which led to the ultralow friction. Moreover, a soft, thin layer on hard substrate materials is important for achieving an ultralow friction regime under boundary lubrication in accordance with the adhesion theory of friction [60]. The tribo-chemical thin film developed by chemical interaction of ILs and their decomposed products with steel interfaces could be an alternate to justify the tribo-mechanism [6, 10, 13]. Comparison of halogenated and non-halogenated ILs with conventional lubricants is listed in **Table 1** for better understanding the utility of ILs as lubricant.

Oil-soluble ILs, when used as lubricant additives, have repeatedly exhibited effective wear and friction reductions in tribological bench tests and demonstrated improved engine mechanical efficiency in engine dynamometer tests. The lubricating performance has shown a strong correlation with the ILs chemistry, concentration, compatibility with other oil additives, material compositions of the contact surfaces, and rubbing conditions. While some results simply showed improvement over the base oils, others have direct comparisons with commercial antiwear additives. Phosphonium based ILs with halogenated and non-halogenated anions are also used as additive for different contact surfaces [14]. Further, tribological study


#### **Table 1.**

*Comparison of ionic liquids (ILs) and conventional lubricants.*


#### **Table 2.**

*Tribological properties of ILs as lubricant additives.*

of oil-miscible quaternary ammonium phosphites ILs as Lubricant additives in PAO is also investigated in different surface environment and shows efficient reduction of wear [53]. Biodegradable fatty-acid-constituted halogen-free ILs are efficient for renewable, environmentally friendly, and high-performance lubricant additives [76]. Halogen-free imidazolium/Ammonium-bis(salicylato)borate ILs act as highperformance lubricant additives and lower wear values on metal surfaces [74]. For better understanding the utility of ILs as lubricant additive in oils, COF and wear properties are for few ILs and listed in **Table 2**.

### **4. Conclusion**

For ILs as lubrication, the major concerns included corrosion, thermal oxidation, oil-miscibility, toxicity, and cost. The recent successful development of noncorrosive, thermally stable, and oil-soluble ILs has largely been addressed and discussed in technical barriers and application point of views. The mainstream research of IL involved lubrication has been shifted from using ILs as neat or base lubricants to using them as lubricant additives. The development of ILs as new lubricating systems are encouraging and still challenging issues in present day. There must be considered the disintegration and corrosion problems of ILs related to their applications as lubricant. However, these fundamental issues can help us to the understanding of fundamental mechanisms of tribology. Now, the focus is to develop halogen and phosphorus-free ILs as energy efficient and

**13**

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

**Acknowledgements**

Conflict of Interest.

**Conflict of interest**

**Acronyms and abbreviations**

BF4 tetrafluoroborate

BScB bis(salicylato)borate

COF coefficient of friction DEHP bis(2-ethylhexyl)phosphate DLC diamond Like Carbon DOSS dioctyl sulfosuccinate DOP dioctyl phosphite

DEHP bis(2-ethylhexyl)phosphate emim 1-ethyl-3-methylimidazolium

hmim 1-hexyl-3-methylimidazolium

ILs ionic liquids Leu leucine Lys lysine MO mineral oil

PAO poly-α-olefin PE polyester

POE polyolester PAO poly-α-olefin Ser serine Thr threonine

PEG poly(ethylene glycol) PF6 hexafluorophosphate

AW anti-wear

AAILs amino acid ionic liquids

bmim 1-butyl-3-methylimidazolium BMP 1-butyl-1-methylpyrrolidinium

BTAG3 methoxy tris-ethoxy methylene benzotriazole BTMPP bis(2,4,4-trimethylpentyl) phosphinate

FAP tris(pentafluoroethyl)trifluorophosphate

MIm5 1,1′-(pentane-1,5-diyl)bis(3-methylimidazolium) MMIm5 1,1′-(pentane-1,5-diyl)bis(2,3-dimethylimidazolium)

publication.

for application as lubricant in near future.

environment-friendly lubricant additives for the steel-based engineering surfaces, and to establish the correlation between structure of anion and tribo-physical properties of ILs. Halogen free ILs (mainly borate based ILs) are more important

SKP acknowledges Department of Chemistry, Uka Tarsadia University, Maliba Campus, Gopal Vidyanagar, Bardoli, Mahuva Road, Surat-394350, Gujrat, India.

The author confirms that he has no conflict of interest to declare for this

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

environment-friendly lubricant additives for the steel-based engineering surfaces, and to establish the correlation between structure of anion and tribo-physical properties of ILs. Halogen free ILs (mainly borate based ILs) are more important for application as lubricant in near future.
