Recent developments and performance review of metal working fluids

Tribology International 114 (2017) 389–401

Contents lists available at ScienceDirect

Tribology International journal homepage: www.elsevier.com/locate/triboint

Recent developments and performance review of metal working fluids a

a

a,⁎

MARK

b

Mohamed Osama , Amarpreet Singh , Rashmi Walvekar , Mohammad Khalid , Thummalapalli Chandra Sekhara Manikyam Guptac, Wong Wai Yind a

Energy Research Group, School of Engineering, Taylor's University Lakeside Campus, Subang Jaya, Malaysia Faculty of Engineering, The University of Nottingham, Semenyih, Malaysia Research and Development, Apar Industries Limited, Mumbai, India d Fuel Cell Institute, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia b c

A R T I C L E I N F O

A BS T RAC T

Keywords: Metal working fluids Vegetable oil Fatty acid methyl ester Ionic liquid Nanofluid

There have been continues efforts in developing novel metal working fluids (MWF) to replace the conventional mineral oil based MWF. This paper reviews recent developments in cutting fluids performance and tribological studies of different MWF formulation including the application of vegetable oils, fatty acid methyl ester, ionic liquids and nanolubricants. It was concluded that more studies should be focused on obtaining theoretical models which can predict the performance of a MWF based on its physical properties. In order to have a holistic view on the overall feasibility and possibility of large scale industrial application, further studies on the stability and life cycle assessment of the novel MWF are required.

1. Introduction

operations can be found in the writings of Biringuccio in the early 1500s. The widespread use of MWFs can be attributed to the industrial revolution in the late 18th century. It has been reported in the year 1868 that the use of oils as MWFs improved significantly the cutting speeds and the tool life. In addition, the power consumption was reduced and smoother cuts were produced. Like many fields, the limitations of the fluids used manifest itself as the equipments develop, for instance the development of high-speed cutting tools. There are four major types of MWFs which are straight oils, soluble oils, synthetic and semisynthetic. The straight oils are mainly derived from petroleum fractions, however vegetable and animal oils can be used. Regardless of their origin, they are usually formulated with various boundary and extreme pressure additives without being diluted with water. On the other hand, soluble oils are oil in water emulsion. The oil to water ratio in these fluids is about 1–5, respectively. In contrast, the synthetic lubricants are chemical solutions that contain no petroleum oils and form true solutions when mixed with water. However, synthetic MWFs can include various esters and polyolefin as well which can be regarded as synthetic oils. Finally, semisynthetic are mixture liquids whose concentrates contain less than 20% of petroleum oil along with watersoluble and emulsifying additives. Therefore, it is designed to get the best of both classes [4]. Metalworking processes are intended to have at least some metal to metal contact. Full separation will render the process inefficient. As such the quality of the lubricant film formed is very critical. Since they are related to the thickness of the film formed, viscosity, viscosity-

Since before recorded history, the making of things have been an essential feature of human civilization. Nowadays, the making of things is known as manufacturing. In particular, metalworking is a process whereby a bulk metal is converted into a component or a part. Two modes of operations are identified for metalworking processes which are the metal debris producing and the metal debris free operations. The former is a material removal process, while the latter is known as metal forming operation [1,2]. Metalworking processes involve elevated temperatures (375 − 750℃), high pressure (up to 1379 MPa), with considerable friction and tool wear [3]. As such, metalworking lubricants play a major role during the process to control these parameters through lubrication, cooling and protection against corrosion in order to increase the efficiency and productivity. During the cutting process, metal surface is plastically deformed at the front of the cutting edge. At this position, the estimated temperature at the cutting tool edge, chip and the workpiece are 900℃, 500℃ and 200℃, respectively. About 25% of the heat generated is due to the friction between the chip and tool face; the remaining 75% of the heat is generated due to deformation of the metal. Extensive tool wear and rough finishing of the workpiece are commonly observed in the cutting zone due to high temperature [1]. The problem of high temperature can be resolved by applying lubricant fluids. Lubricants that are applied in metalworking operations are known as metalworking fluids (MWFs). One of the earliest recorded uses of lubricants in metalworking ⁎

Corresponding author. E-mail address: [email protected] (R. Walvekar).

http://dx.doi.org/10.1016/j.triboint.2017.04.050 Received 23 January 2017; Received in revised form 27 April 2017; Accepted 28 April 2017 Available online 02 May 2017 0301-679X/ © 2017 Elsevier Ltd. All rights reserved.

Tribology International 114 (2017) 389–401

M. Osama et al.

(aliphatic) and cyclic carbon chains. Moreover, mineral oils are impure which results in a range of harmful and useful properties. Detailed examination of crude oils shows that they contain 125 different compounds, only 45 compounds have been analyzed thoroughly [16]. Due to this, it is not possible to give detailed analysis of mineral oils. As a consequence, the quality of mineral oils highly depends on the source of the crude oil and the refining process that they were exposed to. Fundamental differences of minerals oils are based on the chemical forms, sulfur content and viscosity. On the basis of chemical forms, minerals oils can be classified as paraffinic, naphthenic or aromatic. Paraffinic means straight chain hydrocarbons, naphthenic implies saturated cyclic carbon molecules and aromatic contains benzene type compounds [16]. However, the classification on the basis of the chemical form is rather vague, since mineral oils basestock are manufactured based on viscosity and not on structural composition. A consequence of this is that there is no a basestock which is purely paraffinic, naphthenic or aromatic. Mineral oils exist as a blend of all of the three types [1]. As a result, the chemical form based classification indicates which chemical type makes the major proportion of the lubricant. Synthetic lubricants in contrast to mineral oils have well defined structures and predictable properties. Mineral oils on the other hand due to their complex mixture, they have less defined properties. Synthetic liquid lubricants cover nonpetroleum base stock which may be manmade or biological in origin [1]. In this paper, the term synthetic is used exclusively to refer to manmade lubricants. Synthetic lubricants were developed by countries that lack reliable supply of mineral oil. Though mineral oils are cheap relatively and abundant, they suffer from many drawbacks which are linked to oxidation performance, high temperature viscosity-loss, explosion or combustion by strong oxidizing agents and low temperature solidification [16]. These effects are prohibited in a number of specialized applications such as gas turbine engines. The use of synthetic lubricants has grown in the area of specialized applications where the use of mineral oil is not suitable [16]. It was found that synthetic fluids possess better chemical and thermo-oxidative stabilities as compared to mineral oils. A detailed classification of synthetic lubricants will divide them into seven classes which are synthetic hydrocarbon polymers, carboxylate esters, phosphate esters, polyalkyleneglycol, silicon compounds, polyphenyl ethers and halogenated hydrocarbons [1]. Nonetheless, a simpler classification will divide the synthetic lubricants into three classes which are synthetic hydrocarbons, silicon analogous of hydrocarbons and organohalogens [16]. The lubricants in the latter broad classification have distinct characteristics which are attributed to each classification group. For example, synthetic hydrocarbon lubricants provide superior performance at prices as low as mineral oils prices [16]. In principle, the main attribute of synthetic hydrocarbon is their low temperature fluidity. On the contrary, the main attribute of silicones is their excellent viscosity-temperature properties [15]. In addition, silicones group are resistant to extreme vacuum and temperatures, but they are expensive and they are not good for adsorption or extreme pressure lubrication application. In contrast, organohalogens provide effective lubrication for adsorption, extreme pressure and extreme temperature applications, however they are also expensive. It is to be noted that not all the available synthetic lubricants conform to the three broad categories, for instance polyphenyl phosphate deviates from the pattern. Besides this, on the basis of performance, synthetic lubricants can be classified as either superior lubricants at ambient or elevated temperature, or lubricants for extreme pressure or chemical attack [16]. On the other hand, vegetable oils and animal fats are the sources for the biological lubricants. Both are renewable and sustainable resources that are easier to be replenished relative to petroleum resources. In addition to this, they are biodegradable and pose less threat to the environment [1]. Triacylglycerol forms the backbone structure of most of the vegetable oils, furthermore the triacylglycerol structure is

temperature and viscosity-pressure behaviors are very important to cutting fluids. For metal forming operations, the higher the operation speed and the viscosity are, the higher the film thickness is. The operation speed affects the pressure and the temperature which have an impact on the viscosity. Since they have lower viscosity index and greater viscosity response to pressure, naphthenic base mineral oils are more preferred over paraffinic basestock. On the other hand, the requirements of metal removal processes are more diverse and therefore the optimum lubricant viscosity must be estimated for each operation [1]. Other important lubricant properties include thermal conductivity and specific heat. Generally, selection of cutting fluids for a particular application is a complex process and many factors such as machine type, operation type, operation severity, ease of disposal, ease of use and performance-cost tradeoff must be considered [5]. The rise in awareness of occupational health problems caused by conventional MWFs and the limitations of fossil fuel resources have encouraged the investigation for alternative MWFs [6]. In recent years, novel classes of lubricants have emerged as an alternative to conventional lubricants, namely raw or chemically modified vegetable oils, ionic liquids, nanolubricants, etc. These fluids are described to provide superior enhancements that cannot be attained by conventional fluids. Chemically modified vegetable oils are developed to remove or correct deficiencies that are inherent to vegetable oils, such as poor thermosoxidative stability and higher pour point. Meanwhile, the chemical modification retains their positive attributes such as exceptional lubricity, high viscosity index, good corrosion behavior and low evaporation losses [7]. Relative to this, ionic liquids are composed entirely of ions and have melting points below 100 °C. Asymmetric ions lower the melting points even further, ionic liquids of melting points lower than 30 °C are called room temperature ionic liquids [8]. Since 2001, ionic liquids have been tested for lubricant applications as straight fluids and additives. Their results have shown promising performance in terms of lubricating properties [9]. On the other hand, fluids that contain dispersed nanoparticles are known as nanofluids. Nanofluids were first proposed by Choi in 1995 as heat transfer fluids. Since then, they gained popularity in the field of research [10,11]. Owing to this, the use of nanoparticles as additives in the lubricating oil has gained a lot of attention. The nano-approach strategy is based on the direct feed of the sliding interfaces (tribofilm) with nanoparticles or precursors of tribo-active phases that are dispersed in the lubricating base oil. The use of nano-particles overcomes the disadvantages of conventional friction-reduction and antiwear additives where chemical reaction is required to produce the tribofilm on the sliding surfaces. During the induction period, the wear rate is as fast as additive-free oil [12]. As such, nanolubricants appear to be one of the promising alternative lubricants for cutting applications. These novel and conventional types of MWFs will be covered in this paper. 2. Classification of conventional lubricants The modern lubricants are most often the products of the petroleum chemistry. By the physical state, lubricants can be divided into liquids, solids, gaseous and plastics. Among these, liquid lubricants are the most widely used. Liquid lubricants can be classified based on their origins into two types, mineral based which is derived from petroleum also known as mineral and synthetic based lubricants [13]. Hydrocarbons or mineral oils that are derived from crude oil are the most conventionally used lubricants [14]. The principle attribute of mineral oils fractions is their low cost [15]. Mineral oils are derived from crude oils which in turn are mined from different locations around the world. The cost of mineral oils is low and despite the significant development in wear resistant polymers, synthetic oils and solid lubricants, the use of mineral oils is still very common [16]. Mineral oil structure is very complex and the major part of it contains hydrocarbon compounds of approximately 30 carbon atoms in each molecule. Each molecule structure consists of several of straight 390

Tribology International 114 (2017) 389–401

M. Osama et al.

presence of the oil and heat dissipation due to the presence of water [3]. Despite the fact that mineral oils are widely used, it suffers from a number of limitations and therefore other type of fluids were developed to meet the needs of newly developed high speed cutting tools. Thus, the conventional cutting fluids showed limitations as the cutting speeds increased. One of the problems faced by the use of neat mineral oils is the fact that their cooling capacity is limited. As a result, their use at higher speed operations resulted in reducing tool life and producing irritating fumes and smokes. In order to response to this, soluble oils were used. Soluble oils proved to be cleaner, safer in terms of fire hazard and easier in terms of residue removal from the workpiece and the machinery. Another major advantage is that the cooling and lubricating properties can be varied by altering the dilution ratio or changing the additives used. Furthermore, while the soluble oil concentrate is more expensive than the neat mineral oil, the actual cost is reduced since the concentrate is diluted with water. Along with this, the soluble oil is not prone to losses from dragout as much as the straight mineral oil. However, soluble oils, due to the presence of water are prone to a bacterial attack [4]. Besides, mineral oils pose environmental problems due to its poor biodegradability. For these reasons and more, attention was shifted to developing and applying other type of fluids for metalworking applications.

associated with different fatty acids chains. As a result, vegetable oils are composed of a complex association of fatty acids molecules with a single triacylglycerol structure [7]. Fatty acids are mainly aliphatic long chained unbranched acids which are composed of carbon and carbon attached atoms. The latter can be hydrogen or other group atoms. The chain is terminated with a carboxylic acid. In nature, fatty acids are more often composed of an even number of carbon atoms. The even range for the number of carbon atoms in the fatty acid chain is 14–22. Of all the even fatty acids, fatty acids with 16 or 18 carbon atoms occur more often. The shortest fatty acid chains are water-soluble due to the presence of the polar –COOH group. This is because the oily or fatty characteristic is directly proportional to the chain length of the fatty acid. For the points where the hydrogen atoms are missing, the corresponding carbon atoms will be double bonded. Such a phenomenon, if it occurs at multiple sites (up to a maximum of about six sites), the fatty acid is polyunsaturated, otherwise it is monounsaturated. The unsaturated fatty acids have relative to saturated fatty acids lower melting points. Besides, saturation results in a better thermal stability when compared to unsaturated structures. Oleic acid (mono-saturated acid) is one of the most common fatty acid that is found in vegetable oils and it is used as a lubricant. It is known that oleic acid is more thermally stable than polyunsaturated fatty acids, therefore it is highly desirable [7]. Moreover, triacylglycerol is composed of three fatty acids which are bonded to each of the three –OH sites of the alcohol in the glycerol compound. Triacylglycerol is a fully alkylated derivative of glycerol. In a similar fashion, monoacylglycerol and diacylglycerol are formed when one and two of the –OH sites are esterified with alcohol, respectively [17]. The chemical structure of vegetable oils has an influence on their physical and chemical properties. Bent structure is a direct consequence of unsaturation; thus the degree of linearity can be linked to degree of saturation of fatty acids. There is a direct tradeoff between the performance and properties of the fatty acid and the degree of linearity. For example, vegetable oils with high proportion of linear saturated fatty acid are generally liquid or solid at near room temperature. This limits their application as a lubricant, as wide range of ambient temperature is usually needed [17]. Generally, vegetable oil's lubricity and antiwear properties are superior to mineral oils and many synthetic lubricants [18].

3.2. Synthetic lubricants Synthetic oil based lubricant emerged to overcome the lack of mineral oil based lubricant supply. Synthetic lubricants were not initially favored due to their high cost of production. However, as time passes synthetic oils have gained more attention and applications. Synthetic oil can be manufactured for more specific and high performances applications such as operating at extreme temperatures, low toxicity requirements and low vapor pressure conditions [16]. Synthesis based metalworking fluids also known as chemical metalworking fluid are made up of discrete compounds which distinguish them from refined petroleum oils. Synthetic lubricants include polyalkylene glycol (PAG), various esters and synthetic hydrocarbons such as polyalphaolefins. In addition, there are other types of synthetic lubricants that have been used in metalworking applications such as polyacrylonitriles [4]. One of the synthetic based metalworking fluids is polybutene which is non-toxic and widely used for aluminium metal. The favored quality of polybutenes is the ability to easily depolymerise which results in low deposit formation or non-staining of the metal surface at high temperature. Besides that, the polybutene forms stable emulsion for high temperature application with higher resistance to biodegradation and less susceptible to microbial attack which increases the application duration of the metalworking fluid [19]. Generally, PAGs are ideally used as water soluble metalworking fluids where water acts as the coolant. The lubrication property of PAG is not affected by the addition of water as water has little effect on the good lubricating property of PAG. Furthermore, PAG has good hydrolytic stability. The water-miscible PAG has several other properties such as good wetting and penetrating, non-corrosiveness towards most metals, low toxicity and good resistance against bacteria. At high temperature, PAG becomes less soluble in water and tends to come out of the solution to form a coat on the surface which lubricates the metalworking operation [1]. In one study, it has been shown that the blend of alpha olefins and paraffinic base had a better lubricity and improved surface finish relative to pure paraffin base [4]. On the other hand, Katsuki [20], studied the hobbing operation using four water-soluble formulations of PAGs and one excellent chlorinated fatty oil. The results show that chlorinated fatty oil obtained lower wear than any of the PGAs tested. In contrast, another study has showed that PAG added at low concentrations have obtained lower corner wear than water and chlorinated fatty oil [4].

3. Performance of lubricants as cutting fluids 3.1. Mineral oil based lubricants Cutting fluids can be mineral oil, fatty oil or a blend of these. Extreme pressure additives such as sulfur, chlorine or phosphorous are used to enhance the antiweld properties for heavy-duty applications. The mineral based cutting oils can generally come as straight, fattymineral blend, sulfurized mineral oil, sulfurized fatty-mineral blend, sulfo-chlorinated mineral oil and soluble oils. The straight mineral oil contains no additive and it is used for light duty machining of ferrous and nonferrous metals. On the other hand, the fatty-mineral blend as the name suggests contains fatty oil which acts as a wetting agent to improve the lubrication properties of the mineral oil. They are applied when high surface precision and finish are required. Besides, the sulfurized mineral oil has sulfur dissolved in it. The sulfur forms an iron sulfide film which helps to reduces friction, built-up edge and welding during machining. This form is useful for applications where ductile and tough metals are involved. Similarly, the sulfurized fattymineral blend contains sulfur which is added as a sulfurized fat. This form stains less than sulfurized oil due to the strong chemical bonding of the sulfurized fat. The sulfo-chlorinated mineral oil contains chlorine and sulfur additives. This results in impressive antiweld properties over extended temperature range. The soluble form or the emulsifier oils contain a mixture of mineral oil and emulsifiers. These are prepared as concentrates which can be mixed with water at a ratio of 5–20 parts of water to 1 part of concentrate. Soluble oils offer lubricity due to the 391

Tribology International 114 (2017) 389–401

M. Osama et al.

3.3. Vegetable based lubricants In the middle ages, vegetable oils such as palm, olive, castor and other seed oils along with animal fats were used to compose MWFs. However, the increased availability of mineral oils by the 19th century led to the replacement of vegetable oils and animal fats with by-product of refining kerosene due to its low cost [6]. The growing interest on the use of vegetable oils as a base fluid for lubricating application led to more interest on researching the potential of vegetable oils as MWFs. This is due to the pressure that governmental regulations and societies place on industries to increase the use of renewable and biodegradable lubricants such as vegetable based lubricants. Vegetable oils such as soybean oil, sunflower oil, coconut oil, castor oil, etc. are of high interest for lubricants and industry applications [21]. In recent literature, the use of vegetable oil as a cutting fluid has been investigated in some studies. Clarens et al. [22], used a tapping torque test to study the performance of five base oils, three of them were based on bio fluids. The results indicate that the bio based fluids had much higher torque efficiency relative to petroleum based straight oils, soluble oils and semisynthetic formulations. It was found as well that bio based soluble and semisynthetic formulations have lower torque efficiency than straight bio oils. In order to verify the trends, the experiment was repeated using three soluble oils which were soybean, TMP ester and mineral oil. Again, the bio based fluids exhibited higher torque efficiency, however the differences between the bio based fluids efficiency and that of the mineral oil was reduced. This reduction in differences of the performance was attributed to the coating of the tool. Fig. 1 shows the tapping torque efficiency of the soluble and semisynthetic formulations for the uncoated tool run. In another study, Belluco and Chiffre [23], have studied the drilling of austenitic stainless steel using commercial mineral oil, commercial vegetable oil and four different formulations of rapeseed oil blended with an ester oil. The vegetable based cutting fluids were found to generally behave better than the mineral based fluid. The best relative increase in tool life with respect to the mineral based oil was found to be 177%, meanwhile the best reduction of the thrust force was no more than 7%. Furthermore, Alves et al. [24], studied the performance of sulfonate vegetable oil based cutting fluid with high concentrations of water for application of grinding wheels using cubic boron nitrite (CBN). Based on identification of corrosion with respect to cast iron, the new cutting fluid showed no presence of corrosion. Biodegradability assessment of the new cutting fluid showed that it is easily biodegradable and not aggressive to the environment. The 21% new cutting fluid achieved the lowest roughness value. On the other hand, the wear characteristic was presented using the G ratio (material removed volume/wheel worn volume). It was found that the commercial cutting oil exhibited the lowest wear or highest G ratio. The 21% concentration achieved the highest G ratio among different formulation of the new cutting fluid. Fig. 2 shows the G ratio trend for the cutting oils tested in their experiment. The results show that the 21% concentration of the new

Fig. 2. G Ratio Values for Different Cutting Oils [24].

cutting fluid is the optimum formulation. Lawal et al. [25] have tested different cutting fluid emulsions of about 10% concentrations as coolants. The base oils used in the study were commercial base oil, palm oil, palm kernel oil and groundnut oil. All these oils were compared against cutting without coolant. Each formulated cutting fluid contained an emulsifier, a disinfectant and an extreme pressure agent. The results showed that groundnut oil emulsion performed the best in terms of reducing the contact point temperature. In another study, Ojolo et al. [26], investigated the effect of vegetable oils on the cutting force while cylindrical turning of copper, aluminium and mild steel. The vegetable oils used in the study were shear butter oil, coconut oil, palm kernel oil and groundnut oil. In the case of aluminium machining, groundnut oil was found to result in the least cutting force irrespective of the feed rate selected. For the case of copper machining, it was found that the vegetable oils tested except for groundnut oil have resulted in increasing the cutting force as the feed rate is increased. Thus, groundnut oil was found to decrease the cutting force as the feed rate is increased for copper. Furthermore, it was found that groundnut oil resulted in the highest reduction of the cutting force of aluminium at the cutting speed of 8.25 m/min . In contrast, Roegiers et al. [27], used ionized vegetable oils as a lubricity component in metalworking fluids. The results of their study show that the addition of ionized vegetable oil increases the lubricity. Furthermore, it has been observed that by the addition of ionized vegetable oil on the top of two different extreme pressure additives, the average scar diameter can be reduced by an average of 33%. Along with this, the observed wear reduction can be increased by an average of 22%. In terms of surface cleanliness and sludge control, it was found that mineral oil that is formulated with ionized vegetable oil forms no sludge after oxidation. In contrast, pure mineral oil produces a lot of sludge upon oxidation. Sharif et al. [28], investigated the performance of fatty alcohol and three different formulations of palm oil by the application of the minimum quantity lubrication (MQL) during the end milling of stainless steel with TiAlN and AlTiN coated carbide tools. The results show that the tool wear development depends on the coolant conditions. Relative to dry and flood coolant conditions, the palm oil and the fatty alcohol showed slower rate of wear progress. Furthermore, the palm oil obtained the highest tool life at a value of 160.27 min. In addition, the palm oil obtained the highest cutting length value among all the tested coolant conditions. Table 1 shows the tool life for the tested coolant conditions as reported in the study. The surface roughness results showed that the fatty alcohol behaved better than the palm oil. The cutting force analysis showed that the palm oil resulted in the Table 1 Tool Life and Cutting Length [28].

Fig. 1. Tapping Torque Efficiency for Soluble and Semisynthetic Emulsions [22].

392

Coolant Condition

Tool Life (min)

Cutting Length (mm)

Dry Flood Fatty Alcohol Palm Oil

35 40 138 160

8057 9099 31,788 35,885

Tribology International 114 (2017) 389–401

M. Osama et al.

Fig. 3. SEM View of the Auxiliary Flank Wear [29].

lowest cutting force. Besides, Khan et al. [29], studied the turning of AISI 9310 alloy steel using a lathe machine under three different conditions. The conditions tested are dry, wet and MQL using a vegetable oil. MQL by vegetable oil showed a better performance by maintaining lower interface temperature for different feed rates and cutting speeds. Furthermore, MQL by vegetable oil was found to produce chips with smoother and brighter back surface which indicates favorable chip-tool interaction. Furthermore, it was found that cutting by MQL by vegetable oil produced the lowest chip reduction coefficient. Lower chip reduction coefficient indicates improved surface finish and tool life [30]. Fig. 3 shows scanning electron microscope (SEM) image of auxiliary wear after machining for 43 min. The image shows that the MQL by vegetable oil has a better reduction of groove wear growth. Relative to this, cutting using MQL by vegetable oil resulted in the lower growth rate of flank wear as when it is compared to the wet and dry environments. The surface roughness results show that vegetable oil MQL maintained a lower growth rate of surface roughness as the machining time is increased [29]. Xavior and Adithan [31], studied the turning of AISI 304 austenitic stainless steel using three cutting fluids which were coconut oil, neat cutting oil and an emulsion. The analysis shows that variation of the type of the cutting fluid has more influence on surface roughness than tool wear. Fig. 4 shows the surface roughness as a function of feed rate at a constant cutting speed and depth of cut for different oils. The results show that coconut oil outperforms the other two cutting fluids when the surface roughness is considered. On the other hand, Fig. 5 shows the tool wear as function of cutting speed for different combinations of feed rates at a constant depth of cut of 0.5 mm. Similarly, the coconut oil outperformed the other two cutting fluids by maintaining a lower tool wear at different cutting speeds. In another study, Kuram et al. [32], studied the milling of AISI 304 austenitic stainless steel using two different vegetable based cutting fluids and semisynthetic oil. The two vegetable oils used were canola oil and

Fig. 4. Surface Roughness as a Function of Feed Rate for 1) Coconut Oil, 2) Emulsion and 3) Neat Cutting Oil [31].

Fig. 5. Tool Wear as Function of Cutting Speed for 1) Coconut Oil, 2) Emulsion and 3) Neat Cutting Oil at Feed Rates of a) 0.2 mm/rev, b) 0.25 mm/rev and c) 0.28 mm/rev and Constant Depth of Cut 0.5 mm [31].

sunflower oil. To each vegetable oil, 8% of extreme pressure additive were added. It was found that the optimal conditions with respect to the tool wear and the cutting forces can be achieved more considerably 393

Tribology International 114 (2017) 389–401

M. Osama et al.

Fig. 6. (a) Viscosity at 40 °C and (b) Flash Point of Cutting Fluids [33].

Fig. 7. Wear Progress Under Various Coolant-Lubricating Conditions [34].

Rahim and Sasahara [34], studied the performance of palm oil as a minimum quantity lubricant (MQLPO) in a high speed drilling application of Ti-6Al-4V. The performance of palm oil was compared with the action of air blower, flood condition (water-soluble coolant) and synthetic ester as a minimum quantity lubricant (MQLSE). The results show that the air blow achieved the lowest tool life at a value of 48 s. In comparison, flood condition, MQLSE and MQLPO had a higher tool life by a percentage increase of 55.4%. Measurements of the thrust force and torque shows that air blow achieved the highest values which is undesired. The second highest thrust force was produced by the MQLSE; in comparison the MQLPO reduced the thrust force by 30% relative to the air blow. Similarly, the MQLPO reduced the torque by 32% relative to the air blow. The flood coolant condition achieved the lowest thrust force and torque. Considering the workpiece temperature, the highest temperatures was recorded at the air blow condition. The second highest temperatures were achieved by MQLSE at reduction percentages of 15% and 6.5% relative to the air blow. The MQLPO performance was found to be comparable to that of the flood conditions at reduction percentages of 16.5% and 21.6%. Fig. 7 shows the flank and corner wear rates of air blow, flood coolant, MQLPO and MQLSE. It can be seen from Fig. 7 that MQLPO generally had the lowest wear rates in comparison to other modes lubricating. In summary, MQPLO achieved the best results when the wear rates are considered. Meanwhile, the flood coolant condition achieved the best results when the thrust force, torque and the workpiece temperature are considered. Furthermore, the study revealed that at equal conditions of feed rate and cutting speed, the palm oil serves as a better MQL as when compared to the tested synthetic ester. Ojolo and Oshunakin [35], have investigated the cutting speed,

by using the vegetable cutting oils than using the semisynthetic cutting fluid. Furthermore, Kuram et al. [33], studied the drilling of AISI 304 austenitic stainless steel using HSSE tool. They applied three different vegetable oils that were developed from crude sunflower oil. In addition, the drilling characteristics under sunflower oils group were compared to that of two commercial vegetable and mineral based cutting fluids, (CVCF) and (CMCF) respectively. The sunflower oils group consists of crude sunflower oil cutting fluid (CSCF), sunflower oil cutting fluid mixed surfactant Tween 85 (SCF-I) and sunflower oil cutting fluid mixed with surfactants Tween 85 and Peg 400 (SCF-II). Fig. 6 shows the viscosity and flash point characteristics of the lubricants tested in their study. From Fig. 6 it can be seen for the crude sunflower oil that the viscosity increases slightly as the number of surfactants increases. It cab seen as well that the commercial vegetable oil had the largest viscosity, meanwhile the mineral oil had the smallest viscosity. SCF-I was found to achieve the least thrust force among the fluids tested under constant feed rate and depth of cut while varying the spindle speed. The surface roughness results show that under conditions of variable spindle speed and variable feed rate, CVCF and SCF-II achieved the least surface roughness. The results of this study show that the vegetable oils had a superior performance and characteristics when they are compared to the commercial mineral oil. The commercial vegetable oil achieved the best performance at different conditions, except for thrust behavior under variable spindle speed. Finally, the surfactant modified sunflower oils had a better performance than the crude sunflower oil. The results of this study can be correlated to viscosity as well, since better performance was obtained with cutting fluids of higher viscosity. 394

Tribology International 114 (2017) 389–401

M. Osama et al.

used to chemically break the vegetable oils molecules into their methyl or ethyl esters [7]. It has been reported that synthesized esters that are based on vegetable oils are the most common renewable fluids and the most suitable alternative to traditional lubricants when switching to environmentally adapted lubricants [6,38]. The fatty acid methyl esters (FAMEs) were investigated in a few studies as a base fluid or an additive. Masjuki and Maleque [39], used palm oil methyl ester (POME) as an additive in application of sliding wear of cast iron against mild steel. In the study pure diesel oil, pure POME and formulated diesel oil blended with different percentages of POME as an additive were studied. The results showed that the use of pure diesel oil produced moderate wear rate, meanwhile pure POME produced the highest wear rate. It was observed that at a percentage of 5%, POME acts as an antiwear additive, thereby reducing the wear rate to a minimum. In another study by the same researchers [40], POME was assessed using the four ball test. The load was varied in the study and the speed was constant. Similarly, the addition of POME at a percentage of 5% acted as an additive which reduced the friction coefficient. Micrograph study shows that wear scar surface when 5% of POME was used is much smoother which indicates low material transfer. In another study, Dayou et al. [41], evaluated the use of palm oil methyl ester (POME) as an additive in milling and four ball tests. In their study, liquid paraffin oil with cyclomethicone (LPOC), mixture of 5 vol% of POME with remaining 95 vol% of LPOC and water based coolant were tested as lubricants for the four ball and milling tests. From the four ball tests, it was found that presence of the POME in the mineral oil results in a shorter running-in process time and lower steady state frictional coefficient. Running-in process time is a state in which a lot of materials are lost and therefore shortening of this period is desired. The worn surfaces under the presence of POME were found to appear softer, unlike the mineral oil without POME and the water based coolant for which the worn surfaces were found to be rougher. The water based coolant was found to suppress frictional coefficient more effectively than the other tested fluids. Furthermore, the water based coolant was found to have the lowest wear scar diameter. This can be attributed to its lowest frictional coefficient and the shortest running-in process time. On the other hand, POME mineral based lubricant was found to attain lower scar diameter than the POME-free mineral based lubricant. It was found as well that at low loads, the frictional coefficients for all the tested fluids were found to be the same. Finally, it was found that the presence of POME in the oil-mist lubrication delayed cracking occurrence and fracture of the cutting tool as when compared to the oil-mist lubrication. In another study, Shashidhara and Jayaram [42], studied the performance of two raw nonedible vegetable oils along with their modified versions as straight cutting fluids for the turning and drilling of AA 6061. The vegetable oils tested were pongam raw oil (PRO) and jatropha raw oil (JRO). The former was modified to pongam methyl ester (PME) and epoxidized pongam raw oil (EPRO). On the other hand, the later was modified to jatropha methyl ester (JME) and epoxidized jatropha raw oil (EJRO). Further modification was carried on the PME and JME samples through epoxidization. The resultant modified vegetable oils are epoxidized pongam methyl ester (EPME) and epoxidized jatropha methyl ester (EJME). The performance of these vegetable oils and their modified versions were examined against the performance of commercial mineral oils through utilization of MQL as a mean to supply cutting fluids. The results of the turning application show that the PRO and its modified versions had a lower cutting power in comparison to the mineral oil for the complete range of cutting speeds, feed rates and depths of cut. The maximum drop of cutting power that was achieved is 30%. Furthermore, the turning results show that the JRO family has a higher reduction of cutting power when compared to the PRO family, however the difference between them is not very significant. Furthermore, the epoxidized methyl ester of each family shows higher cutting power reduction when

depth of cut, rake angle and feed rate on the cutting force during the cylindrical turning of brass, aluminium rod and mild steel. Palm kernel oil was used as a cutting fluid and its performance was compared with dry cutting. The study shows that the lubricated mode of experimentation using palm kernel oil generally resulted in decreasing the cutting force relative to the dry mode of cutting. The palm kernel oil was found to reduce the friction coefficient by 33.3% when the aluminium is machined. On the other hand, Lawal et al. [36], studied the turning of AISI 4340 steel with a carbide tool using vegetable and mineral oils in water emulsion cutting fluids. The oils used in this study were palm kernel oil, cottonseed oil and mineral oil. The characteristics of the two vegetable oils in water emulsion were found to be corrosion resistant. In addition, the two vegetable oils in water emulsion were found to have higher viscosity than the mineral oil in water emulsion. The surface roughness results show that palm kernel oil in water emulsion achieved the least surface roughness. Based on the ANOVA techniques, the type of the cutting fluid was found to contribute 1.8% of changes of the surface roughness. Similarly, the palm kernel oil based cutting fluid was found to be the best cutting fluid in terms of reducing the cutting force. The mineral oil based cutting fluid was found to produce the largest cutting force just after the cottonseed oil based cutting fluid. Herrmann et al. [37], performed life cycle assessment of plant seed oil, rapeseed oil, used cooking oil and animal fat that are intended to be used for grinding application. The results showed that all these oils had much lower global warming potential relative to the mineral oil. The rapeseed oil was found to have higher global warming potential than palm oil. In terms of the life cycle costing, it was reported that even though that plant oil has higher market price, the additional cost can be compensated through savings that can be realized by extending the tool life and reducing lubricant losses. In summary, there is moderate number of researchers who have investigated the role of vegetable oil as a base cutting fluid for metalworking applications. Their studies involved comparisons with mineral oil, synthetic oil, wet condition, air cutting condition and emulsion oil. Generally, vegetable oils showed promising results and outperformed other conventional modes of lubrication for cutting applications. The vegetable oil was applied as crude, formulated cutting oil and emulsion. Generally, formulated vegetable based oil was found to outperform its crude version. Nonetheless, the aforementioned conclusion needs more studies to confirm it. The reason for this is that the majority of the studies did not compare the performance of the formulated vegetable oil with its crude version. The studies show that the cutting parameters such as cutting temperature and cutting force depend not only on the cutting fluid, but also on the parameters of operation, workpiece and cutting tool materials. Comparisons of the performance of one particular vegetable oil for different cutting applications were not reported in most studies. Therefore, it cannot be asserted that the type of cutting application is a factor of influence. However, by intuition it can be deduced that the performance of particular oil will be different for different cutting applications. The results of these studies should attract more research on the use vegetable oils as cutting fluids. Table 2 shows the summary for the papers reviewed in this section. 3.4. Fatty acid methyl ester Despite the fact that vegetable oils possess excellent lubricity and other properties, they have poor oxidative stability and low-temperature characteristics relative to mineral oils [7]. The deficiencies of vegetable oils can be corrected by blending with fluids that have higher performance in the properties that are targeted for correction, for example, blending with fluids that have good low-temperature properties. Alternatively, the poor performance can be corrected by additives, for example oxidation inhibitors or through chemical modification of the oil [1]. One of the chemical modifications routes is the transesterification using an alcohol such as methanol or ethanol. The process is 395

Tribology International 114 (2017) 389–401

M. Osama et al.

Table 2 Summary of Vegetable based Oils Studied as MWFs. Research Group

Cutting/ Lubricating Test

Clarens et al. [22]

Tapping Torque Test

Belluco and Chiffre [23]

Drilling Application

Research Group

Alves et al. [24]

Cutting/ Lubricating Test Grinding Process

Lawal et al. [25]

Turning Operation

Ojolo et al. [26]

Cylindrical Turning

Roegiers et al. [27]

Pad-on-disc Tribometer

Sharif et al. [28]

Milling Application

Khan et al. [29]

Turning Application

Xavior and Adithan [31]

Turning Application

Research Group

Kuram et al. [32]

Cutting/ Lubricating Test Milling Application

Kuram et al. [33]

Drilling Application

Rahim and Sasahara [34]

High Speed Drilling

Ojolo and Oshunakin [35]

Cylindrical Turning

Lawal et al. [36]

Turning Application

Herrmann et al. [37]

Grinding Application

Materials Tested

Tested Fluids

Main Findings

All Vegetable Based Stocks Performed Significantly better 1018 & 4140 Steel • WP: • Naphthenic Mineral Oil than Mineral Oil MicroTap Mega G8 • Paraffin Mineral Oil • CT: Machine Tool Oil • Soybean Oil • Canola TMP Easter • AISI 316 L Commercial Oil • All Vegetable Based Oils Performed Better than the • WP: • Mineral Based Mineral Reference Oil Based Commercial • CT: HSS-Co ø8.8 DIN1897 • Vegetable Oil Maximum Enhancements of tool life and thrust force • were 177% increase and 7% decreases, respectively Formulated Rapeseed Oil • Materials Tested Tested Fluids Main Findings SAE 8640, tempered • WP: and quenched, 52 HRc •NACT: CBN

Mild Steel, Aluminium • WP: & Copper Tungsten Carbide • CT: Cutter Steel on Steel

Sulfonate Based Vegetable Oil Cutting Fluid Cutting Fluid • Commercial Emulsion Oil Emulsion • Groundnut Kernel Oil Emulsion • Palm Oil Emulsion • Palm without Coolant • Cutting Oil • Groundnut Butter Oil • Shear Kernel Oil • Palm Coconut Oil •Elektrionized Vegetable Oil

Alcohol • WP: • Fatty ESR Mould Steel Palm Olein • STAVAX • Coated Carbide Tool • CT: AISI 9310 Oil Based Cutting • WP: • Vegetable as an MQL Lubricant • CT: Uncoated Carbide Tool Fluid Machining • Dry Wet Machining • AISI 304 • WP: • Soluble Oil Sandvik's carbide • Coconut Oil • CT: CNMG 12 04 08 Straight Cutting Oil •Tested Materials Tested Fluids AISI 304 • WP: • CT: NA AISI 304 • WP: DIN 338 HSSE 130° • CT: Silver Series drill

Titanium Alloy • WP: • CT: Carbide Drill Mild Steel, Brass & • WP: Aluminium Rod Steel • CT: AISI 4340 • WP: • CT: Coated Carbide Tool 100 Cr6, 62 HRC • WP: CBN, vitrified bonded, • CT: • B126 M8 VD49

The new vegetable based cutting fluid reduced wheel wear workpiece roughness. Groundnut oil achieved the lowest cutting temperature

Biological oils reduced cutting force and improved product quality. Wear Protection and Extended Tool Life • Enhanced Surface Finish • Excellent Lubrication Consumption • Reduced by palm oil resulted in a better performance in • MQL terms of tool life, wear and cooling. by fatty alcohol resulted in a better surface finish. •TheMQL MQL provided significant improvements in the tool wear, surface finish and chip interface-temperature

Coconut oil obtained lower surface roughness and tool wear than mineral oils Main Findings

Sunflower Cutting Formulated vegetable oils had more effects on the tool • Formulated wear and force components with respect to optimal Fluid Canola Cutting machining conditions. • Formulated Fluid Semisynthetic • Commercial Sunflower Cutting Fluid • Formulated sunflower cutting obtained the highest • Crude reduction of the cutting force. Sunflower Cutting • Formulated Fluid Commercial vegetable cutting fluid resulted in the • lowest surface roughness. Vegetable Cutting • Commercial Fluid Mineral Cutting • Commercial Fluid Palm Oil as a MQL Lubricant

Palm Kernel Oil Kernel Oil Emulsion • Palm Oil Emulsion • Cottonseed Oil Emulsion • Mineral Oil • Mineral Oil • Rapeseed Oil • Palm Fat • Animal • Used Cooking Oil

compared to the epoxidized raw oil of each group. Finally, the raw oil of each group achieved higher cutting power reduction when they are compared to the modified version. On the other hand, the drilling

Palm Oil produced better results than synthetic ester by obtaining lower cutting force and work-piece temperature. Palm kernel oil improved cutting parameters.

Vegetable oil in water-emulsion has improved cutting force and surface roughness behavior. oils have lower environmental impact, but • Vegetable with higher purchase relative to mineral oils force and workpiece roughness were found to • Grinding fluctuate within a normal range.

application results show that the vegetable oils and their modified versions had a higher material removal rate as when compared to the material removal rate of mineral oil. The maximum material removal 396

Tribology International 114 (2017) 389–401

M. Osama et al.

Fig. 8. Variation of Material Removal Rate (MRR) Against Thrust Force (a) Pongam family; (b) Jatropha family [42].

3.5. Ionic Liquid based lubricants

rate of 50% was achieved by EJME. Fig. 8 shows the material removal rate for the PRO and JRO families as a function of thrust force. It can be seen from the graphs that the epoxidized methyl ester version of each family achieves higher material removal rate. At lower thrust forces, the raw oil of each version had a higher material removal rate than that of the epoxidized raw oil. It can be concluded from the study that in turning and drilling applications, the JRO family has better a performance when they are compared to that of the PRO family. Furthermore, the raw oils of each family is better for turning application, meanwhile the epoxidized methyl ester of each family is better for drilling application. In summary, little attention was given to researching the potential of FAMEs in metalworking applications. This could be due to that the cost of fatty acid methyl esters being higher than crude vegetable oils. From the literature reviewed, epoxidized methyl esters showed better performance than the epoxidized raw oils. It was found that the type of cutting applications influence the performance of the cutting fluids. In three different studies, POME was used as an additive and proved to be good friction and antiwear additive at a low percentage of 5%. Similar to vegetable oils literature, the parameters of operation were found to affect the performance. Table 3 shows the summary for the papers reviewed in this section.

Ionic Liquid (IL) is relatively new type of chemical that has been discovered and found to be applicable for different applications. In addition, it has promising potentials which are rapidly being explored for other implications. Currently, ILs have been applied in food science, separation, nanomaterial processing and many other industries. The asymmetrical cations and anions structure of the IL results in lower melting point. The type of cations and anions also determines the physical as well as the chemical properties of the resultant IL. In the field of tribology, IL has the potential of being used as an additive or as neat lubricant [43]. The ability of ionic liquid to perform as a lubricant is heavily researched where various type of ILs are being synthesized and tested. Table 4 shows summary of the literature reviewed of ILs. From Table 4, it can be summarized that ILs have good ability to perform as lubricants. The type of anion and the length of the alkyl chain heavily affect the outcome of ILs as lubricants. When the alkyl chain length is increased, ILs have better lubricity. As an additive, ILs are capable of improving the viscosity of base fluid and reducing the coefficient of friction. There is another study which was conducted by Singh et al. [64]. The study specifically focused on the metal working fluid using 1-methyl-3-butylimidazolium hexafluorophosphate (BMIMPF6), 1-methyl-3-butylimidazolium tetrafluoroborate (BMIMBF4) and 1-methyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide (BMIMTFSI) ILs. At weight percentage of one, these ILs

Table 3 Summary of Fatty Acid Methyl Esters Studied. Research Group

Cutting/Lubricating Test

Materials Tested

Tested Fluids

Masjuki & Maleque [39]

Sliding Wear Testing

Research Group

• POME Mineral Oil •Tested Fluids

Masjuki & Maleque [40] Dayou et al. [41]

Cutting/Lubricating Test Four Ball Test Milling & Four Ball Test

Cast Iron and Mild Steel Materials Tested

Shashidhara and Jayaram [42]

Turning & Drilling Application

EN31 Steel

Modified AISI • WP: 420 Coated Carbide • CT: AA 6061 • WP: Cemented • CT: Carbide

POME

• POME Oil • Mineral Based Coolant • Water Oil • Mineral Raw Oil • Pongam Methyl • Pongam Easter Pongam • Epoxidized Oil Raw Oil • Jatropha Methyl • Jatropha Easter Jatropha • Epoxidized Oil 397

Main Findings

POME at 5 wt% acts as an anti-wear additive. Main Findings POME at 5% improved the lubricant performance. Small amount of POME as an additive can result in a higher weld load, lower steady state friction coefficient and smaller degree of adhesion. Pongam and Jatropha performed better than mineral oils by reducing cutting forces and power.

Tribology International 114 (2017) 389–401

M. Osama et al.

Table 4 Summary of type of ILs and the respective performance outcome. Author

Type of Ionic Liquid

Experimental Detail

Comments

[44]

Trihexyltetradecylphosphonium bis(2 ethylhexyl)phosphate

1.0 wt% was added to base oil and compared with the zinc dialky-dithiophosphate additive (ZDDP)

• Anti-scuffing • Anti-wear conducted at room temperature and • Test elevated temperature. At 100 °C, the ILs

[45]

Tributyl(methyl)phosphonium diphenylphosphate, P1444DPP

[46]

tetrafluoroborate • 1-butyl−3-methylimidazolium hexafluorophosphate) • 1-butyl−3-methylimidazolium phosphonium bis(2,4,4-trimethylpentyl) • Trihexyltetradecyl phosphinate phosphoniumbis (2-ethylhexyl) phosphate • Trihexyltetradecyl bis(trifluoromethylsulfonyl) • 1-ethyl−3-methylimidazoliumm imide (low-viscosity ionic liquid) • 1-butyl−3iodide [BMIM] (high viscosity ionic liquid) •Typemethylimidazolium of Ionic Liquid N-Methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) • imide bis(trifluoromethylsolfonyl) • Trihexyl(tetradecyl)phosphonium imide tetrafluoroborate • 1-Butyl−3-methylimidazolium hexafluorophosphate • 1-Butyl−3-methylimidazolium bis(trifluoromethanesulfonyl) • 1-Butyl−3-methylimidazolium • imide iodide • 1-Butyl−3-methylimidazolium dicyanamide • 1-Ethyl−3-methylimidazolium tricyanomethane • 1-Butyl−3-methylimidazolium tris(pentafluoroethyl) • 1-Butyl−1-methylpyrrolidinium trifluorophosphate ethyl-dimethyl−2-methoxyethylammoniumtris(pentafluoroethyl) • trifluorophosphate 1-Butyl−1-methylpyrrolidinium bis(trifluoromethylsulfonyl) • imide and Ammonium ILs. • Imidazolium imidazolium tetrafluoroborates • Alkyl tetrafluoroborate ([EMIM]BF ) • 1-ethyl−3-methylimidazolium chloride ([BzMIM]Cl) • 1-methyl−3-benzylimidazolium hexafluorophosphate ([HMIM] • 1-hexyl−3-methyl-imidazolium PF ) 1-(3′-O,O-diethylphosphonyl-n-propyl)−3-alkylimidazolium • tetrafluoroborate

Anti-wear additive and compared against Amine Phosphate (AP) when mixed with biodegradable base stocks gel was formed by mixing 0.7 5 wt% MWCNTs with ILs

performed better than ZDDP

[47]

[48]

Author [49]

[50]

[43] [51]

Alkyl imidazolium Ils

Added as additives in diesel engine lubricant

Tested with other two conventional lubricants and water

• Excellent anti-wear additives

conductivity, • High stability • Good tribological performance • Excellent in friction and anti-wear for • Improvement used lubricant (additive) was noted for fresh lubricant • Wear high viscosity IL exhibit better • The machined surface compared to other lubricant

Experimental Detail

Comments

based ILs shows good boundary • Fluorine lubrication but has the tendency to cause corrosion

has higher coefficient of friction • Phosphorus but lower wear and prevents corrosion

coefficient of friction • Good temperature increases • Asperformance decreases

the anti-wear

friction and wear • Reduce withstand temperature • ItIt canshows tribological properties • depending ongood the type of material used

4

6

Author [52]

[53]

[54]

[55]

Type of Ionic Liquid

trihexyltetradecylphosphonium cation (P • salicylate and saccharinate anions

666,14+)

1-alkyl−3-methylimidazolium and • tetraalkylammonium, tetraalkylphosphonium as cations and • bis(triflurometanesulfonyl)imide trifluorotis(pentafluoroethyl)phosphate as hydrophobic anions tributylborane • 1-Allylimidazole azole tributylborane • 1-Ethylimid bis(trifluoromethanesulfonyl) • 1-Butyl−3-methylimidazolium imide Tributylmethylphosphonium dimethylphosphate •Blending an appropriate molar ratio of bis(trifluoromethylsulfonyl)imide (LiTFSI) with • Lithium Tri(methoxyethoxyethoxyethoxy)triazine (C N (OR) ) or hexa • (methoxyethoxyethoxyethoxy) cyclotriphosphazene 3

Author [56]

combined with

3

Pendulum-type and ball-on-reciprocatingflat type tribo testers were used

steel to steel test was conducted

Si3N4/steel contact was used for testing

Comments

liquid crystal structure • Lamellar-like improving lubricity and wear resistance based ILs have lower coefficient of • Salicylate wear and saccharinate Ils have lower • Salicylate wear rate any other lubricants and better

• • •

tribological properties than 1-decyl−3methylimidazolium bis[(trifluoromethyl) sulfonyl]imide (C10mimTf2N,) Imidazolium and phosphonium derivatives showed good lubricity whereas ammonium derivatives showed the less lubricity As the 1-alkyl−3-imidazolium chain increases the friction reduces Two imidazolium borane ILs selected allowed the replacement of hetero-elements such as fluorine and sulfur that are usually found in ILs

new ILs performed better • The conventional alkylimidazolium ILs at

than

certain condition

3

(P3N3(OR)6) Type of Ionic Liquid 1-hexyl-2-methylpyridinium bromide [C6–2-Mepyr][Br] 1-hexyl-3-methylpyridinium bromide [C6–3-Mepyr][Br] 1-hexyl-2-methylpyridinium tetrafluoroborate [C6–2-Mepyr] [BF4]

• • •

Experimental Detail Pin-on-tests was conducted to investigate the tribological properties

Experimental Detail Steel-on-steel and steel-on-PTFE systems was assessed

Comments

[C6–2-Mepyr][BF ] has the best tribo• logical performance which lead to low 4

friction coefficients (continued on next page)

398

Tribology International 114 (2017) 389–401

M. Osama et al.

Table 4 (continued) Author

Type of Ionic Liquid

Experimental Detail

[57]

hexafluorophosphate • 1-hydroxyethyl−3-hexylimidazolium together with Multiwalled Carbon Nanotube (RTIL/MWNTs) ) • (HEHImPF tris(pentafluoroethyl) • 1-butyl−2,3-dimethylimidazolium trifluorophosphate[C C C Im][(C F ) PF ] tris(pentafluoroethyl) • trihexyl(tetradecyl)phosphonium trifluorophosphate [P ][(C F ) PF ] dimethyl phosphate • Tributylmethylphosphonium (2-ethylhexyl) phosphonium bis(2-ethylexyl) phosphate • Tributyly (2-ethylhexyl) phosphonium bis92-ethylhexyl) phosphate • Tetra tetradecylphosphonium bis92-ethylhexyl) • Tri(2–3thylhexyl) phosphate X− • 1-n-alkyl−3-methylimidazolium n = 6] • [X=PF6; n = 2, 6, 8] • [X=BF4; n = 2] • [X=CF3SO3; n = 2] •Type[X =of(4-CH3C6H4SO3); Ionic Liquid

Room Temperature Ionic Liquid

The RTIL/MWNTs composite was dispersed in 1-methyl−3Hexylimidazolium hexafluorophosphates

6

[58]

4 1 1

6,6,6,14

[59]

[60]

Author [61]

[62]

[63]

2 5 3

2 5 3

3

Neat lubricants for the sliding pair of stainless steel

3

Room-temperature ionic liquids (ILs), 1-hexyl−3-methylimidazolium tetrafluroborate [HMIM][BF4] 1-hexyl−3-methylimidazolium hexafluorophosphate [HMIM] [PF6], Protic ionic liquids (PILs) Di-[bis(2-hydroxyethyl)ammonium] adipate (DAd), Bis(2-hydroxyethyl) ammonium salicylate (DSa) Bis(2-hydroxyethyl) ammonium oleate(DO)

• • •

• • • and pyrrolidinium-based cations combined with • Ammoniummethylsulphate, • Methylsulphonate and/or (CF3SO2)2N- anions

In vacuum four-ball tribometer

Comments coefficient of friction and wear • Decreases volume IL [P ][(C F ) PF ] showed better • lubricating properties than [C4C1C1Im] 6,6,6,14

•

2 5 3

3

[(C2F5)3PF3] in terms of friction coefficient and wear volume Improved tribological performance with high loading capacity of ILs

the alkyl chain length reduces • Increasing friction and wear

Experimental Detail 1%wt. additives of a mineral hydrocracking oil for steel–steel contacts

Compared with PAO 6, aprotic ionic liquid (APIL) 1-hexyl−3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIM](CF3SO2)2 N) Investigate the ecotoxicity and biodegradability of ILs

Comments

increased the viscosity of • ILs base oil and decreased friction as well as • the wear order of coefficients of friction and wear • The rates: DAd < DSa < ([HMIM](CF SO ) N) 3

2 2

< PAO6 low acute toxicity and biological • Exhibit activity

In another study done by Wan et al. [66], heaxagonal shaped Boron nitride nanoparticles were added to commercial SE 15W-40 with concentrations of 0.1, 0.5 and 1.0 wt% with 25% of oleic acid to produce a stable suspension of the nanoparticles. During the experiment, negligible difference was observed for the viscosity of the base oil and nanofluids as the temperature was increased from 20 to 60 °C with shear rate of 100 s−1. However, the lowest average friction coefficient was obtained for the 0.1% of boron nitride with 76.9% reduction. On the other hand, concentrations of 0.5% and 1.0% obtained 53.8% and 27.7% reduction of frictional coefficient, respectively. It was found as well that as the concentration of boron nitride increases, the friction coefficient increases. Besides, a concentration of 0.1% showed least surface roughness after the test and all the nanofluids were found to have improved thermal conductivity. Moreover, nanocomposite also aids in enhancing the lubricating property of base fluid. Al2O3 and TiO2 nanocomposites were dispersed in a lubricating oil with concentrations of 0.05 wt%, 0.1 wt%, 0.5 wt% and 1.0 wt%. The most optimum concentration was found to be 0.1 wt % of Al2O3-TiO2 in lubricating oil. This optimum concentration achieved 20.51% reduction in friction coefficient and 44% reduction in wear scar diameter. The coefficient of friction and the wear scar diameter at 1.0 wt% of the nanocomposite were found to be higher than base lubricating oil [67]. This could be due to higher concentration of nanocomposite resulting in the agglomeration and chemical condensation. If too little of nanoparticles are added, the friction is reduced but not significantly. Therefore, the amount of nanoparticle or nanocomposite added is essential factor as to convert the sliding friction to rolling friction which eventually reduces the coefficient of friction. The thermal property of the nanolubricant plays an important role as to improve life span of the base fluid lubricant as well as the thermal conductivity of the medium. Rasheed et al. [68], found that by adding graphene nanoflakes at concentrations of 0.01 wt% to paraffin oil graded API SJ/CF 20 W 50 (introduced at 2001) and API SN/CF 20 W 50 (used after 2011) engine oil, stable suspension was formed.

were added to vegetable oil MQL machining and tested with orthogonal milling operations using the CNC vertical milling machine and plain carbon steel of grade AISI 1045. The testing revealed that small traces of ILs are sufficient to reduce the cutting forces required as well as the surface roughness of the workpiece. There are several challenges that need to be considered for ionic liquids to be commercially used as a neat metal working fluid. One of the challenges would be the stability of ILs with respect to temperature. The performance of IL at elevated operational temperature is an essential property to be considered. The lithium based ILs were found to be effective lubricant at high temperatures and had onset decomposition temperature at about 220 °C. Comparing lithium based IL to imidazolium based IL, the coefficient of friction and wear volume were found to be relative low [55]. In another study by Qu et al., the trihexyltetradecylphosphonium based IL was stable at temperature of about 200 °C and it only decomposes completely at 300 °C [44]. It can be deduced that the ILs have the potential of performing at elevated temperatures as per the requirement of most metalworking fluids. Selection of ILs must take into consideration the right combination of cation and anion which deeply influences the stability of IL with respect to temperature.

3.6. Nanolubricants Nanolubricants are base fluids in which nanoparticles are dispersed to enhance the resultant properties of the lubricant. In the study that was conducted by Xiang et al. [65], magnetite nanoflakes (Fe3O4) was added to #40 base oil. The friction coefficient was found to be reduced. The hexagonal shaped nanoflakes were added with mass percentage of 0.5, 1.0, and 2.0 wt% with oleic acid to improve the distribution of additives in the base fluid. The optimum concentration was found to be 1.5% which reduced the average friction coefficient by 18.06% and the wear scar diameter of friction pair by 11.21% compared to pure base fluid. 399

Tribology International 114 (2017) 389–401

M. Osama et al.

particular lubricant on the basis of relevant selected properties. Thus, time and resources can be saved and the selection of the lubricants to be tested for specific cutting application can be narrowed down.

Images of SEM showed the graphene with size 60 nm are freely suspended. The API SN/CF 20 W 50 nanolubricant showed thermal conductivity enhancement of 23% when tested at 80 °C. In the study by Liu et. al. [69], graphene with concentrations 0.03% and 0.06% were added to 1-hexyl-3-methylimidazolium tetrafluoroborate to form ionanofluids. It was found that the thermal conductivity enhancement ratio with respect to the pure ionic liquid increased with the increase of mass fraction of graphene. The enhancement ratio was 8.4–11.5% for 0.03%, and 15.2% to 22.95 for 0.06% at temperature increment from 20 °C to 200 °C. The viscosity of both ionanolfuid decreased compared to the pure ionic liquid when the temperature was increased. The reduction in viscosity can be related to the selflubrication of graphene which favors the medium about of 140 °C to 210 °C to high range temperature application. In general, the addition of nanoparticle leads to alteration of physical properties such as tribology, viscosity and thermal conductivity. These properties were determined to establish the capabilities of the nanolubricant to perform as cutting fluid. From the results found, there are high possibilities for nanolubricantto to perform well as cutting fluid. However, in depth analysis and more cutting fluid experiments need to be carried out to obtain overall analysis of the outcome of these lubricants. From the results, it is found that nanolubricants were able to reduce friction even when the contact involves in tight space due to nanosize of the particles used. Besides that, the nanolubricant has the ability to act as coolants to remove heat built during operations. Another factor that needs to be studies would be the type of cutting process and the materials involved. Each type of nanolubricant fluid would interact differently to different material due to the interaction between surfaces and different types of cutting processes such as grinding, milling or others. Moreover, the addition of nanoparticles should be at an optimum concentration depending on the desired outcome or application. Taking into account all the conditions, the nanolubricant actually would be a great addition to the cutting fluids family that can be expanded to industrial scale.

5. Conclusion Cutting applications play a major role in manufacturing operations. The process parameters and the final workpiece can be improved through the application of lubricants. Conventionally, mineral oils were used as cutting fluids. However, the mineral oils cooling capacity is limited and therefore soluble oils were seen as a good alternative. Nevertheless, soluble oils contain water which is susceptible to bacterial attack. Synthetic lubricants are superior in many regards, but cost is higher. On the other hand, vegetable oils though more expensive, have good lubricating properties and pose less threat to the environment. Based on the literature reviewed, vegetable oils provided better results than mineral oil and synthetic lubricants. However, vegetable oils suffer from many drawbacks, in particular with their high pour point and low oxidative stability. These problems can be overcome and corrected through chemical modification of the vegetable oils structure into its FAME. Attempts were made to use FAME in cutting applications, the results of application of these modified vegetable oils showed positive outcomes. In particular, POME has shown to improve the cutting parameters and lubricating performances as an additive at low concentrations. Other possibilities of MWF can be seen in ionic liquids and nanofluids. Both novel lubricants have shown good results and enhancements of the cutting process parameters. For future research, there should be more focus on applying novel lubricants to non-ferrous metals and super alloys. The oxidation, thermal and nanosuspension stabilities must be investigated and reported as well. It is very essential that these novel lubricants prove to be resistant to degradation at various conditions and to be stable long enough. Long stabilities can prove that the application of these novel lubricants is feasible and economical. It is also noted that little attention was given to theoretical modelling of the performance of lubricants as cutting fluids. Theoretical models, if they are proved to be reliable and accurate can help to predict the performance of the lubricants before testing. This can help the researchers to narrow down the lubricants to be researched and discard those that are not seen worthy of testing. The theoretical models should be able to predict the performance of a particular lubricant based on the knowledge of its relevant physical properties such as viscosity and thermal conductivity. By doing this, the potential fluids for a particular cutting application can be characterized and subsequently narrowed down based on the values of their properties. This can save time and resources, additionally blends of lubricants or additives can be devised without a need for trial and error testing.

4. Research development and challenges From the literature reviewed, it can be seen that the application of vegetable oils, FAMEs and ILs have resulted in positive outcomes in terms of reducing cutting temperature, cutting forces, surface roughness and many other properties. Nanolubricants results reveal good potential of this type of lubricants to be applied in metalworking applications. Nevertheless, research groups need to focus more on reporting oxidation and thermal stabilities of these novel lubricants. For the case of nanolubricants, the nanosuspension stability must be evaluated and reported as well. Based on the criteria of environmentally adapted lubricants, candidate lubricants must be evaluated considering their environmental impact, economical and technological performances [6]. Therefore, for these novel lubricants to be applied at a large scale, it must prove to be economically and environmentally superior or at least competent. Consequently, long term stabilities must be insured so that the lubricant serves its intended duration. Another aspect to look at is the materials to be cut. According to Lawal et al. [70], most of the research on the application of vegetable oils as metalworking lubricants was on ferrous metals. Little attention was given to non-ferrous metals such as copper, brass and aluminium. Super alloys as well were not researched enough. Super alloy materials have good properties which allow them to be applied in different sector of the industry. Therefore, more research should focus on applying vegetable oils and other novel lubricants on these materials. The authors note that few attempts such as in [32,71,72], were made to theoretically model the performance of the liquid lubricants as cutting fluids. More attempts can be made to correlate the properties of the tested lubricant with the process parameters. For instance, attempts to find correlations between thermal conductivity and cutting temperature. Such a correlation can allow predications of the performance of a

References [1] Rizvi SQA. Comprehensive review of lubricant chemistry, technology, selection, and design. ASTM International; 2009. [2] Groover MP. Fundamentals of modern manufacturing materials, processes, and systems, fourth edition. USA: John Wiley & sons, Inc; 2010. [3] Ralph K, Gregory F. "Cutting Fluids" in CRC handbook of lubrication. CRC Press; 1988. p. 357–69. [4] Richard GB, William LB. "Metalworking Fluids" in synthetics, mineral oils, and biobased lubricants. CRC Press; 2005. [5] Kelly R, Foltz G. "Cutting Fluids" in CRC handbook of lubrication. CRC Press; 1988. p. 357–69. [6] Brinksmeier E, Meyer D, Huesmann-Cordes AG, Herrmann C. "Metalworking fluids—Mechanisms and performance" in CIRP Annals - Manufacturing Technology, vol. 64; 2015: p. 605–628. [7] Sevim ZE, Atanu A, Brajendra KS. "Chemically functionalized vegetable oils" in synthetics, mineral oils, and bio-based lubricants. CRC Press; 2005. [8] Castro CND, Ribeiro APC, Vieira SIC, França JMP, Lourenço MJV, Santos FV, et al., Synthesis, Properties and Physical Applications of IoNanofluids; 2013. [9] Somers A, Howlett P, MacFarlane D, Forsyth M. A review of ionic liquid lubricants. Lubricants 2013;1:3. [10] Taylor SCR, Otanicar T, Phelan P, Gunawan A, Lv W, Rosengarten G, Prasher R,

400

Tribology International 114 (2017) 389–401

M. Osama et al.

[11]

[12] [13] [14] [15] [16] [17] [18] [19] [20]

[21] [22]

[23] [24]

[25] [26]

[27] [28]

[29]

[30]

[31]

[32]

[33]

[34] [35] [36]

[37]

[38]

[39] [40] [41]

[42]

Tyagi H. "Critical Review of the Novel Applications and Uses of Nanofluids" presented at the ASME 2012 In: Proceedings of the Third International Conference on Micro/Nanoscale Heat and Mass Transfer, USA; 2012. Kostic MM. "Critical Issues and Application Potentials in Nanofluids Research" presented at the ASME 2006 Multifunctional Nanocomposites International Conference, USA; 2006. Mansot JL, Bercion Y, Romana L, Martin JM. Nanolubrication. Braz J Phys 2009;39:186–97. Stepina V, Vesely V. Lubricants and special fluids. Czecho-Solvakia: Elsevier; 1992. Stepina V, Vesely V. Chapter one the definition and classification of lubricants in lubricants and special fluids, 23. Elsevier; 1992. p. 1–8. Klaus EE, Tewksbury EJ. Liquid lubricants in CRC HANDBOOK of LUBRICATION (Theory and Practice of Tribology), II. CRC Press; 1983. p. 229–54. Stachowiak GW, Batchelor AW. "3 - Lubricants and Their Composition" in Engineering Tribology (Third Edition); 2006, p. 51–101. Joseph MP, Leslie RR, Sevim ZE. "Natural oils as lubricants" in synthetics, mineral oils, and bio-based lubricants. CRC Press; 2013. p. 375–84. Bartz W, Rudnick L. "Comparison of synthetic, mineral oil, and bio-based lubricant fluids" in synthetics, mineral oils, and bio-based lubricants. CRC Press; 2005. Mortier Roy M, Orszulik , Orszulik S. Chemistry and technology of lubricants. Netherlands: Springer; 2010. Katsuki A, Ueno T, Matsuoka H, Kohara M. Research on soluble cutting fluids for gear cutting: 2nd Report, the influence of dilution ratio and the effect of synthetic fluids. Bull JSME 1985;28:735–43. Lawal SA. A review of application of vegetable oil-based cutting fluids in machining non-ferrous metals. Indian J Sci Technol 2013;6:3951–6. Clarens AF, Zimmerman JB, Landis HR, Hayes KF, Skerlos SJ. "Experimental Comparison of Vegetable And Petroleum Base Oils In Metalworking Fluids Using The Tapping Torque Test" in Japan-USA Symposium on Flexible Automation; 2004. Belluco W, Chiffre LD. Performance evaluation of vegetable-based oils in drilling austenitic stainless steel. J Mater Process Technol 2004;148:171–6. Alves SM, Oliveira JFGD. Development of new cutting fluid for grinding process adjusting mechanical performance and environmental impact. J Mater Process Technol 2006;179:185–9. Lawal SA, Abolarin MS, Ugheoke BI, Onche EO. Performance evaluation of cutting fluids developed from fixed oils. Leon- Electron J Pract Technol 2007. Ojolo SJ, Amuda MOH, Ogunmola OY, Ononiwu CU. Experimental determination of the effect of some straight biological oils on cutting force during cylindrical turning. Matér (Rio De Jan) 2008;13:650–63. Roegiers M, Zhang H, Zhmud B. Elektrionized™ vegetable oils as lubricity components in metalworking lubricants. FME Trans 2008;36:133–8. Sharif S, Yusof NM, Idris MH, Ahmad ZA, Sudin I, Ripin A, Zin MAHM. Feasibility study of using vegetable oil as a cutting lubricant through the use of minimum quantity lubrication during machining; 2009. Khan MMA, Mithu MAH, Dhar NR. Effects of minimum quantity lubrication on turning AISI 9310 alloy steel using vegetable oil-based cutting fluid. J Mater Process Technol 2009;209:5573–83. Mohanty T. Study of chip reduction coefficient in boring operation using metal laminates at the tool holder interface. Int J Innov Res Sci Eng Technol 2014;3:16712–9. Xavior MA, Adithan M. Determining the influence of cutting fluids on tool wear and surface roughness during turning of AISI 304 austenitic stainless steel. J Mater Process Technol 2009;209:900–9. Kuram E, Simsek BT, Ozcelik B, Demirbas E, Askin S. "Optimization of the Cutting Fluids and Parameters Using Taguchi and ANOVA in Milling" in The World Congress on Engineering 2010, Imperial College London, UK; 2010. Kuram E, Ozcelik B, Demirbas E, Şık E. Effects of the cutting fluid types and cutting parameters on surface roughness and thrust force" in the world congress on engineering. UK: Imperial College London; 2010. Rahim EA, Sasahara H. A study of the effect of palm oil as MQL lubricant on high speed drilling of titanium alloys. Tribol Int 2011;44:309–17. Ojolo SJ, Ohunakin OS. Study of Rake face action on cutting using palm-kernel oil as lubricant. J Emerg Trends Eng Appl Sci 2011;2:30–5. Lawal SA, Choudhury IA, Nukman Y. Evaluation of vegetable and mineral oil-inwater emulsion cutting fluids in turning AISI 4340 steel with coated carbide tools. J Clean Prod 2014;66:610–8. Herrmann C, Hesselbach J, Bock R, Dettmer T. Coolants made of native ester — technical, ecological and cost assessment from a life cycle perspective in Advances in Life Cycle Engineering for Sustainable Manufacturing Businesses. London: Springer; 2007. p. 299–303. Pettersson A. Environmentally adapted lubricants – properties and performance, department of applied physics and mechanical engineering, [Doctoral Thesis]. Sweden: University of Technology; 2006. Masjuki HH, Maleque MA. The effect of palm oil diesel fuel contaminated lubricant on sliding wear of cast irons against mild steel. Wear 1996;198:293–9. Masjuki HH, Maleque MA. Investigation of the anti-wear characteristics of palm oil methyl ester using a four-ball tribometer test. Wear 1997;206:179–86. Dayou S, Liew WYH, Ismail MAB, Dayou J. Evaluation Of palm oil methyl ester as lubricant additive using milling and four-ball tests. Int J Mech Mater Eng 2011;6:374–9. Shashidhara YM, Jayaram SR. Experimental determination of cutting power for

[43] [44] [45]

[46] [47]

[48]

[49] [50]

[51] [52]

[53] [54] [55]

[56]

[57] [58]

[59]

[60]

[61]

[62] [63]

[64]

[65] [66]

[67] [68] [69]

[70]

[71]

[72]

401

turning and material removal rate for drilling of AA6061-T6 using vegetable oils as cutting fluid. Adv Tribology 2013;13:1–7. Rensselar JV. Unleashing the potential of ionic liquids. Tribol Lubr Technol 2010;66. Qu J, Luo H, Chi M, Ma C, Blau PJ, Dai S, et al. Comparison of an oil-miscible ionic liquid and ZDDP as a lubricant anti-wear additive". Tribology Int 2014;71:88–97. Khemchandani B, Somers A, Howlett P, Jaiswal AK, Sayanna E, Forsyth M. A biocompatible ionic liquid as an antiwear additive for biodegradable lubricants. Tribol Int 2014;77:171–7. Fan X, Wang L. Ionic liquids gels with in situ modified multiwall carbon nanotubes towards high-performance lubricants. Tribol Int 2015;88:179–88. Anand M, Hadfield M, Viesca JL, Thomas B, Battez AH, Austen S. Ionic liquids as tribological performance improving additive for in-service and used fully-formulated diesel engine lubricants. Wear 2015;334–335:67–74. Pham MQ, Yoon HS, Khare V, Ahn SH. Evaluation of ionic liquids as lubricants in micro milling – process capability and sustainability. J Clean Prod 2014;76:167–73. Kondo Y, Koyama T, Sasaki S. Tribological properties of ionic liquids in ionic liquids - new aspects for the future; 2013. García A, González R, Hernández Battez A, Viesca JL, Monge R, FernándezGonzález A, et al. Ionic liquids as a neat lubricant applied to steel–steel contacts. Tribol Int 2014;72:42–50. Bermúdez MD, Jiménez AE, Sanes J, Carrión F-J. Ionic liquids as advanced lubricant fluids. Molecules 2009;14:2888. Reeves CJ, Garvey S, Menezes PL, Dietz M, Jen TC, Lovell MR. "Tribological Performance of Environmentally Friendly Ionic Liquid Lubricants" in ASME/STLE 2012 International Joint Tribology Conference IJTC2012, USA; 2012. Minami I, Nanao H, shigeyuki M. Tribol Ion Liq 2007;40. Totolin V, Minami I, Gabler C, Dörr N. Halogen-free borate ionic liquids as novel lubricants for tribological applications. Tribol Int 2013;67:191–8. Song Z, Liang Y, Fan M, Zhou F, Liu W. Lithium-based ionic liquids functionalized by sym-triazine and cyclotriphosphazene as high temperature lubricants. Tribol Int 2014;70:136–41. Tiago G, Restolho J, Forte A, Colaço R, Branco LC, Saramago B. Novel ionic liquids for interfacial and tribological applications. Colloids Surf A: Physicochem Eng Asp 2015;472:1–8. Yu B, Liu Z, Zhou F, Liu W, Liang Y. A novel lubricant additive based on carbon nanotubes for ionic liquids. Mater Lett 2008;62:2967–9. Otero I, López ER, Reichelt M, Fernández J. Friction and anti-wear properties of two tris(pentafluoroethyl)trifluorophosphate ionic liquids as neat lubricants. Tribol Int 2014;70:104–11. Zhang S, Hu L, Qiao D, Feng D, Wang H. Vacuum tribological performance of phosphonium-based ionic liquids as lubricants and lubricant additives of multialkylated cyclopentanes. Tribol Int 2013;66:289–95. Jiménez AE, Bermúdez MD, Iglesias P, Carrión FJ, Martínez-Nicolás G. 1-N-alkyl -3-methylimidazolium ionic liquids as neat lubricants and lubricant additives in steel–aluminium contacts. Wear 2006;260:766–82. Battez AH, González R, Viesca JL, Blanco D, Asedegbega E, Osorio A. Tribological behaviour of two imidazolium ionic liquids as lubricant additives for steel/steel contacts. Wear 2009;266:1224–8. Espinosa T, Sanes J, Jiménez AE, Bermúdez MD. Protic ammonium carboxylate ionic liquid lubricants of OFHC copper. Wear 2013;303:495–509. Stolte S, Steudte S, Areitioaurtena O, Pagano F, Thöming J, Stepnowski P, Igartua A. Ionic liquids as lubricants or lubrication additives: an ecotoxicity and biodegradability assessment. Chemosphere 2012;89. Seliger G, Yusof NM, Goindi GS, Chavan SN, Mandal D, Sarkar P. et al., "In: Proceedings of the 12th Global Conference on Sustainable Manufacturing – Emerging PotentialsInvestigation of Ionic Liquids as Novel Metalworking Fluids during Minimum Quantity Lubrication Machining of a Plain Carbon Steel", Procedia CIRP, 26; 2015, p. 341–345. Xiang L, Gao C, Wang Y, Pan Z, Hu D. Tribological and tribochemical properties of magnetite nanoflakes as additives in oil lubricants. Particuology 2014;17:136–44. Ge W, Han Y, Wang J, Wang L, Liu X, Zhou J, et al. New paradigm of particle science and technology proceedings of the 7th world congress on particle technology tribological behaviour of a lubricant oil containing boron nitride nanoparticles. Procedia Eng 2015;102:1038–45. Luo T, Wei X, Zhao H, Cai G, Zheng X. Tribology properties of Al2O3/TiO2 nanocomposites as lubricant additives. Ceram Int 2014;40:10103–9. Rasheed AK, Khalid M, Walvekar R, Gupta TCSM, Chan A. Study of graphene nanolubricant using thermogravimetric analysis. J Mater Res 2016;31:1939–46. Liu J, Wang F, Zhang L, Fang X, Zhang Z. Thermodynamic properties and thermal stability of ionic liquid-based nanofluids containing graphene as advanced heat transfer fluids for medium-to-high-temperature applications. Renew Energy 2014;63:519–23. Lawal SA, Choudhury IA, Nukman Y. Application of vegetable oil-based metalworking fluids in machining ferrous metals—A review. Int J Mach Tools Manuf 2012;52:1–12. Daniel CM, Rao KVC, Olson WW, Sutherland JW. "Effect of cutting fluid properties and application variables on heat transfer in turning and boring operations" in Japan/USA Symposium on Flexible Automation; 1996, p. 1119–1126. Smith T, Naerheim Y, Lan MS. Theoretical analysis of cutting fluid interaction in machining. Tribol Int 1988;21:239–47.