Tribological investigations on the application of oil-miscible ionic liquids additives in modified Jatropha-based metalworking fluid

Accepted Manuscript Tribological investigations on the application of oil-miscible ionic liquids additives in modified Jatropha-based metalworking fluid Amiril Sahab Abdul Sani, Erween Abd Rahim, Zaidi Embong, Syahrullail Samion PII:

S0301-679X(18)30030-6

DOI:

10.1016/j.triboint.2018.01.030

Reference:

JTRI 5061

To appear in:

Tribology International

Received Date: 8 September 2017 Revised Date:

6 December 2017

Accepted Date: 12 January 2018

Please cite this article as: Sani ASA, Rahim EA, Embong Z, Samion S, Tribological investigations on the application of oil-miscible ionic liquids additives in modified Jatropha-based metalworking fluid, Tribology International (2018), doi: 10.1016/j.triboint.2018.01.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Tribological investigations on the application of oilmiscible ionic liquids additives in modified Jatrophabased metalworking fluid

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Amiril Sahab Abdul Sani a,b; Erween Abd Rahim a,1; Zaidi Embong c; Syahrullail Samiond

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Faculty of Applied Science and Technology, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia d

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Faculty of Manufacturing Engineering, Universiti Malaysia Pahang, 26600 Pekan, Pahang, Malaysia

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Precision Machining Research Centre (PREMACH), Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia

Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

Abstract

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This paper studies the applications of modified Jatropha oil-based (MJO) lubricant as potential vegetable-based metalworking fluids, containing additives of two oil-miscible ionic liquids; [P66614][Phosphinate] (PIL) and [N1888][NTf2] (AIL) at 1%, 5%, and 10% weight concentration. The lubricant samples are validated for corrosion, four ball tribology and tapping torque experiments. Using optical microscope, profilometer, AFM, SEM, EDS and XPS analysis, worn surfaces were investigated. The lubrication performance of MJO+AIL10% and MJO+PIL1% samples provide competitive lubrication performance to that other lubricant samples used herein. They have shown improved corrosion inhibition, superior friction reduction, lower worn surface area, excellent surface finish and increased tapping torque efficiency. These superior tribological results correspond to the metal oxide tribofilm formation and anti-corrosion behavior of MJO+AIL10% sample.

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Keywords: Ionic liquids; Jatropha oil; metalworking fluid; sustainable manufacturing

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1. Introduction

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Petroleum-based lubricants in 2016 have increased tremendously on high global consumption, showing at least 1% annual increments with 13,726 million tons of oil equivalent [1]. This has therefore, prevail another viewpoint of bad impact on the environmental pollution, the danger of large loss proportion (13-50 %) of the lubricants in the aquatic and terrestrial ecosystems, including continuous depletion of global energy and natural resources [2–4]. In the conduct of manufacturing industries, in working towards achieving the optimal situation of maximum benefit for minimum risk, many researchers have investigated the use of metalworking fluids (MWFs) from biodegradable lubricants and alternative lubrication methods [5,6] in order to reduce the dependency on traditional petroleum-based lubricants [4,5].

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Corresponding author

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However, the high dependency on the applications of petroleum-based MWFs has raised health and environmental concern, causing huge liability to the industrial worker. The potential health impacts through exposure of the cutting fluids include the inhalation of aerosols (spray mist) transported by the wind, resulting in breathing obstruction and skin contact with the fluid that induce allergic reactions [7,8]. Development of innovative future of manufacturing industries is therefore vital for promoting value-added sustainable manufacturing, for instance, to efficiently improve the environmental impact, global energy, and resources, to generate a minimum quantity of wastes, and to provide operational safety and personal health while maintaining or improving the product quality [7]. This can be achieved by limiting or eliminating the extensive use of the environmentally hazardous chemicals.

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Biodegradable lubricants from either edible or non-edible vegetable oils have been introduced, being environmentally friendly and safe for operators, exposing less volatile compound with less toxicity, also preventing skin irritation and breathing obstruction [3,9]. As such, there is perceived potential for the non-edible oil for the biodegradable lubricant to augment or perhaps replace the existing edible oil, providing no competition between industrial usage, thus is seen to be an ideal feedstock for biolubricant production [9,10]. Bork et al. in 2014 [11] has proven the high efficiency of Jatropha oil as non-edible cutting fluids in comparison to Canola oil as an alternative plant-based cutting fluid in the production of structured aeronautical parts of aluminum alloys. A recent study by Talib et al. [12] has revealed the good tribological characteristics of the newly formulated Jatropha oils and their emulsions, containing boron nitride particles. These lubricant mixtures presented good antiwear and antifriction improvements, which correspond to the selected formula of modified Jatropha oil and the addition of a low amount of boron nitride solid additives. The modification process made on the Jatropha oil via transesterification process has been successfully improved its thermal oxidative stability as being affirmed by a study in [13]. Research has pointed to improvements in the machining tests on medium carbon steel discs, utilizing three newly formulated modified Jatropha-based MWFs, namely MJO1, MJO3 and MJO5 [14]. The results have determined MJO5 oil-type generated lowest cutting force and cutting temperature compared to those two samples, leading to lowest calculated specific cutting energy value. Further evidence for this has been shown in the results of MJO5 to pose high viscosity, indicating the high formation of TMP-ester, high viscosity index of the base oil and thicker formation of lubricant layer on the machined surfaces.

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While a lubricant mixture of Jatropha oils with boron nitride offers the basis of tribological characteristics, these have several acknowledged drawbacks, including high immiscibility of boron nitride solid particles that created emulsion type of lubricant and may contaminate the equipment during the machining applications [15]. Investigation is made on ionic liquids in improving the chemical and physical characteristics of the base oil. Ionic liquids (ILs) are organic salts consisting of cations and anions with low molten temperatures, possessing unique characteristics such as wide range of viscosity levels, inherent polarity, low vapor pressure, low volatility, low melting point, superior thermal stability, high combustible temperature and highly miscible with organic compounds [8,16]. A recent review has highlighted the ability of ILs to build thin lubricant layer fast enough during the short contact time of the metalworking process by forming a molecular film that may prevent direct metal contact at boundary lubrication regime compared to the lubrication ability of mineral oil and water [8]. Studies have suggested that the ILs tribofilm was composed of oriented, density‐packed molecules which reacted by adsorbing on the friction surfaces especially in

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boundary lubrication condition. This tenacious tribolayer depending mostly on the type of anionic and cationic structures of the ILs that helps in enhancing the friction and wear reduction capability of the lubricating fluids on the sliding materials [17–19].

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In this work, two low toxic, biocompatible and fully oil-miscible ILs, trihexyltetradecyl phosphonium bis (2,4,4-trimethylpentyl) phosphinate, [P66614][Phosphinate] (PIL) and methyltrioctylammonium bis(trifluoromethylsulfonyl)imide, [N1888][NTf2] (AIL) are chosen as the phosphonium- and ammonium-based liquid additives, respectively. The former displayed noncorrosiveness on cast iron surface and cast aluminium 319 at 135 °C for seven days [20]. AIL is another oil-miscible ammonium-based IL (highly soluble in biodegradable ester; > 30% miscibility) [21,22] being investigated that does not corrode on several metal substrates (steel, TiN, CrN and ZrN) [23]. Moreover, both ILs were found to be fully miscible to the nonpolar hydrocarbon oils [20–22,24] and considered biocompatible due to their low toxicity level [25–30]. Several works have also reported high solubility of phosphonium- and ammonium-based ILs in many polar based oils due to their molecular shapes that have three-dimensional quaternary structures with long hydrocarbon chains [17,18,31,32,15]. Other than that, hydrogen bonding between the cation and anion with long alkyl chains being similar to the base oil geometry, reduces the IL’s polar nature and thus, contribute to the high miscibility of ILs in polar base oil [8]. With the above factors, these ILs’ categories are therefore considered favorable lubricant additives to improve the lubricity performances of the base oils in many research works as well as for industrial applications [8,33– 37].

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In present study, the main objective has been to evaluate the effectiveness of modified Jatrophabased lubricant with the presence of oil miscible ILs additives, namely PIL and AIL at three nominal 1%, 5% and 10% weight concentration. In support of this, and to ascertain the rheological properties and lubrication performances, four ball tribology test and tapping operation have been undertaken as a benchmark against the performance of neat base oil and conventional cutting fluid, e.g. a synthetic ester. Corrosion test have also been performed on all lubricant samples prior to tribological experiments in order to establish their oxidative stability. This study adds further crucial information to explore the synergistic effects of the media under investigation in polar oil at different additive treat rate. These results are expected to help in understanding the lubrication capabilities for greener metalworking operations that benefit the environment, also involving conserving energy and pave the way towards the development of “green” metalworking fluids for sustainable manufacturing activities.

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2. Methodology

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2.1

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The modified Jatropha-based MWF was synthesized in the laboratory through two stage acid/base chemical modification processes from the crude Jatropha oil (CJO) [12–14]. At first stage, an acid esterification process was carried out on the CJO being mixed with methanol at 22:1 weight to weight ratio, 0.5 wt. % sulphuric acid as catalyst and heated at 60 °C for 4 hours to produce an esterified Jatropha oil with low fatty acid percentage. At the second stage, a base transesterification process was carried out on the esterified Jatropha oil with methanol at 6:1 ratio, sodium hydroxide as the catalyst and heated at 60 °C for 3 hours to produce Jatropha fatty acid methyl ester (FAME). Next, another base transesterification process was carried out on FAME with trimethylolpropane

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Lubricant samples preparation

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(TMP) at 3.5:1 ratio, with sodium methoxide as the catalyst and heated at 120 °C for ca. 24 hours in a vacuum condition to produce the modified Jatropha oil (MJO), a polar base oil. A detailed explanation of the synthesis process of vegetable-based TMP-ester is also reported in published articles [10,13,38]. All chemicals mentioned in this work were of analytical reagent grades obtained from various chemical manufacturers.

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Fig. 1. Chemical structures of AIL and PIL.

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The ionic liquids were supplied by IoLitec GmbH, Germany and used directly as received without further purification. Two oil-miscible ILs, PIL and AIL were reported to be sufficiently biocompatible because of their low toxicity levels that can allow the growth of different microorganisms including human coronary artery endothelial cell as well as Escherichia coli, respectively [27,30]. The chemical structures demonstrating the cations and anions of both ILs are presented in Figure 2. The ILs obtained were mixed into the modified Jatropha oil (MJO) as lubricant additives. There are three levels of mixture concentration for each type of ILs. They were mixed based on ‘weight to weight’ ratio of base oil to IL and the selected levels are 1, 5 and 10 wt. %. The additives and the MJO were both heated at ca. 70 °C before being mixed and stirred for 30 minutes to ensure homogeneity of the mixtures [39]. The mixtures were visually monitored for any occurrences of precipitation and separating layers both at -15 °C and 100 °C. All blend mixtures were monitored visually at the controlled temperatures and even after few months of storage. The inspection revealed that no precipitation or separated layer in the base oils were found [15], thus indicating the high solubility of the small concentrations of ILs additives in the polar oil of Jatropha-based TMP-ester, MJO [22,40].

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The density of each lubricants is measured with a pycnometer between room temperature to 100°C and is confirmed according to the ASTM D4052. The resistance to flow of a lubricant or lubricant viscosity with temperature dependence is measured following the ASTM D445 method by using a viscometer (Viscolite 700). The viscometer probe was immersed into the lubricant and the dynamic viscosity was recorded. The kinematic viscosity was then calculated with Equation (1), with ν is the kinematic viscosity in mm2/s, η is the dynamic viscosity in mPa·s and ρ is the density in g/ml. The index of viscosity-temperature change was calculated by using the kinematic viscosity of the lubricant measured at 40 °C and 100 °C based on ASTM D2270.

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The temperature at which the lubricant vapors start to ignite is termed as the lubricant flash point (FP). It was measured by using a closed cup method (Pensky-Martens Closed Cup Tester - PMA4) based on ASTM D93. The amount of lubricant used for every test was 70 ml and it was poured into the test cup before the equipment was set to run and measure the FP automatically.

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2.2

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Corrosion tests have been performed to analyze the corrosion level of the lubricant samples on copper strips which correspond to the ASTM D130 method. The copper strips were prepared with a length, width and thickness of 25, 15 and 4 mm, respectively. They are polished, cleaned and dried prior to the immersion in the lubricant samples. The copper strips were dipped into the lubricant samples and maintained at 100 °C for 24 hrs. At the end of the test, the copper strips were taken out,

Corrosion tests

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washed with acetone and dried at room temperature before determining the corrosion level by comparing with the blank copper strip as well as the ASTM copper strip corrosion standard board.

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Four ball tribology test

The tribological performance of the lubricant samples was evaluated by using a four ball tribomachine based on ASTM D4172 on a steel/steel surface. The experiment was conducted by using four chrome steel balls, AISI 52100 with a diameter of 12.7 mm, a hardness value of 62 HRC, extra polished of grade 25 and with an initial surface roughness of 0.015 µm. Steel balls and the ball pots were smeared with acetone and then wiped clean by using a lint free industrial tissue to avoid surface and lubricant contamination prior to the tribology test. Three stationary balls were clamped together in the pot and then secured with a lock ring before being covered with approximately 10 ml of the lubricant for each test. The fourth ball, which was fastened on a collet and referred as the rotating ball was pressed against the three stationary balls with a stationary load of 392 N. The chemical composition of the chrome steel balls and the ball pot assembly prior to the four ball wear test experiments is shown in Figure 2. The lubricant temperature was controlled automatically and set at a constant temperature of 75 °C. The sliding time and rotation speed were set to an hour and 1200 rpm respectively.

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Fig. 2. Four-ball wear test assembly and the chemical composition of the steel ball bearings.

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After one hour sliding period, the machine was stopped and the thermocouple was detached to stop heating the ball pot. The stationary balls were taken out from the pot after the lubricant was drained off from the oil cup. Prior to the wear scar analysis, the three stationary balls were again wiped clean with acetone and dried by using the industrial tissue. The diameter of the wear scar on the stationary ball-bearings was observed and examined under an optical microscope. Beneath the optical lens, the image of the scar was magnified for an image analysis and the diameter of the scars was measured directly by using an image acquisition program. Each scar was measured both in their vertical and horizontal direction with the line of sight is perpendicular to the worn surface being measured. The mean wear scar diameter (WSD) were obtained from the three stationary balls of each test and recorded for result analysis.

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The friction coefficient of the lubricants during the steel/steel sliding process was determined through the four-ball tribology tests. Equation (2) shows the correlation between the frictional torque, T (kg·mm), the test load, W (kg), the radial distance between the rotation axis to the center of the contact surface on the lower balls, r, which was 3.67 mm, and the dimensionless friction coefficient, µ. The frictional torque on the lower balls was acquired automatically by the computer through a specific data acquisition system from the four-ball tribo-machine and the results of the friction coefficient were displayed and analyzed in a PC.

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µ=

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Surface roughness was measured with a profilometer, Mahr Marsurf PGK-120 measuring station. At the surface roughness measurements setup, cut-off length of 0.08 mm and a traverse length of 0.56 mm were selected based on DIN EN ISO 4288 standard. The measurements were performed on all three stationary balls from each sample of the tested lubricants and the mean value was used for analysis. Measurement stylus was made to directly touch the wear surface perpendicular to the groove direction prior to traverse movement of the stylus. Atomic force microscopy (AFM) analysis

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was conducted on the wear scars to obtain 3D topography maps of the worn surface. Each image was taken at the center of the scar with a scanning area of 100 µm x 100 µm.

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Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-Ray photoelectron spectroscopy (XPS) were employed to analyze and evaluate the interaction of the lubricant samples with the worn surfaces. The surface morphologies were first examined by using SEM on Hitachi SU1510 and JEOL JSM-6380LA equipped with EDS system. The acceleration voltage used for the SEM-EDX analysis was at optimum setting for this sample which is between 10–15 kV for generating the best spatial resolution of the micrographs. XPS is an advanced surface analytical technique that was used to characterize the surface elemental composition of the lubricated worn scar surfaces. The method was favorable for several nanometer thickness oxides or deposition [41]. The XPS equipment used is one of the type of Auger Electron Spectrometer (AES) with X-ray photoelectron spectrometer (XPS), a brand of Kratos / Shimadzu (Model Axis Ultra DLD). It is equipped with a monochromatic Al Kα (1486.6 eV), dual x-ray sources (Al & Mg), argon etching system (ion gun) for sample cleaning and depth profiling analysis. XPS wide scan and narrow scan data acquisition are controlled by a software called VISION. X-ray source, Al Kα is powered at 300W (15kV, 20 mA) and operated under ultra-high vacuum (UHV) condition at 10-11 Torr. The kinetic energies of the photoelectrons were measured using a hemispherical electron analyzer working in the fixed analyzer transmission (FAT) pass energy mode of 180 eV and 20 eV for wide and narrow scanning respectively. Spectrum analysis of Fe2p and O1s narrow scan were deconvoluted using commercial software, Casa XPS. This software was capable to manipulate the spectrum iteration for photoelectron and background signal. The synthetic spectrum for the photoelectron peaks and its background signal was optimized using a specific model. Here, the appropriate background model used was Shirley method, while the synthetic photoelectron signal assigned using mixed of a Gaussian-Lorentzian method. The ratio for both Gaussian and Lorentzian had been assigned using 70% and 30% respectively. In order to avoid carbon charging effect, each narrow scan is calibrated using adventitious carbon binding energy at 284.8 eV. Further discussion for this narrow scan deconvolution will be discussed in the result and analysis section.

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2.4

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The performance evaluation of the lubricant samples was carried out by using the tapping torque test method. This method exhibits the tapping energy efficiency which is a sensitive and accurate method to evaluate the cutting performance which considers both the tapping torque and the effect of chip expulsion during metal cutting [42]. The force moment of a tapping tool during tapping process of a low carbon AISI 1215 steel was investigated in the tapping test method according to ASTM D5619 as depicted in Figure 3.

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Fig. 3. Setup assembly of tapping test and the chemical composition of the carbon steel specimen.

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A laboratory setup of the tapping process was conducted on a vertical CNC machine (Mazak Nexus 410-A II). A dynamometer (Kistler 9345A) was attached underneath the steel specimen, which was used to measure and record the cutting force and torque. The steel specimens were initially prepared by cutting the steel rod to a cylindrical shape of 35 mm diameter and 12 mm height. A new uncoated high-speed steel tapping tool of M6 x 1.0 was used for each tested lubricants and the measurements were repeated five times for each lubricant samples. A pilot hole, diameter 5 mm, was made on the steel specimen in situ prior to the tapping process. An approx. 20 ml of lubricant was used prior to the tapping test and covered the drilled hole and flooded the specimen to ensure

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enough lubrication during the cutting process. The tapping tool rotation speed was set at 400 rpm with a feed rate of 1 mm/rev through the specified hole of the steel specimen. An amplifier (Kistler 5070) was used during the data acquisition process and the results were displayed and analyzed in a PC. The measured cutting force and torque are depicted for result comparisons and further data analysis. The torque efficiency of the lubricants is determined by using Equation (3): Efficiency =

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Reference Fluid Torque ⋅100% Test Fluid Torque

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According to ASTM D5619, the higher efficiency number represents a lower wear rate of the tapping tool and thus reflected on the good lubricity as well as lower friction and good wear performance of the lubricants during the tapping process.

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3. Results and discussion

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3.1

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The physicochemical properties of all lubricant samples are presented in Table 1. A conventional cutting fluid, synthetic ester (SE) and the modified Jatropha oil (MJO) as the base oil were used as the benchmark oils to the newly refined lubricant mixtures being studied. The rheological properties of the highly viscous ILs are also shown in Table 1. The PIL has a lower density than AIL which has the highest VI of 457. The increasing number of carbon in the alkyl substituent chains attached to the cationic structure is believed to be responsible to the decrement of the density and to the increment of the viscosity of the ILs [43–46]. The ILs present the highest FP among other lubricants, with PIL, which has greater carbon number than AIL, having the highest FP of 320 °C.

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Table 1 Physical and chemical properties of the lubricant samples

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MJO shows a comparable index value with the SE. The FP of neat MJO as shown in Table 1 is 12 % lower than SE. The composition of the fatty acids in MJO was dominated by linoleic acid which has 18:2 cis-9,cis-12 double bonds [47]. The number of double bonds is responsible for the oxidation rate of the polyunsaturated fatty acid methyl esters and thus, higher oxidation rate posed by MJO may reduce its FP temperature [48].

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The kinematic viscosity of all the blended MJOs was shown to increase steadily with the increase of ILs’ concentrations. The higher number of carbons in the cationic quaternary structure of the ILs might play a significant role to the increment of carbon number in the carbon chain length of the blended MJOs and the subsequent increase of their viscosity values [46]. In general, the addition of highly viscous ILs into the MJO increases the kinematic viscosity of the mixtures depending on the treat rate. The number of VI represents the ratio of logarithmic values between the kinematic viscosities of the lubricants at 40 °C and 100 °C. A small increase in viscosity value will affect the calculated VI number as in the case of MJO+PIL1%.

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In Table 1, the FPs of the lubricant mixtures were shown to be comparable to the neat MJO. It is also shown that the different weight concentration of ILs mixed in the MJO does not significantly change (ca. 1% increase) the FPs compared to the neat MJO. The increase in the number of carbon chains of the blended MJO that reacted with the small concentration of ILs (1 – 10 wt. %) could become the reason on the small and negligible increment of the FP. The low amount of ILs blended in the MJO may require the same equivalent heat energy to break the chemical bonds that hold the

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intermolecular structures together in order to produce enough flammable vapor readily to be ignited during the closed cup test [8,49].

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Corrosion test analysis

In this test, the copper sheets were immersed in the lubricant samples and kept under the condition of 100 °C for 24 hours. Figure 4 presents the photographs of the copper sheet after the experiment. It is clearly seen that the neat lubricants; viz. MJO and SE samples had slightly tarnished the copper sheet surfaces, while AIL which has double bonds in its anionic structure is also prone to oxidizing the metal surface. PIL which was reported to inhibit corrosion [8], has excellently prevented the occurrence of surface corrosion on the copper sheet after the test. It is also anticipated that the longer chain length of the PIL anion enhances its corrosion protection capability compared to the AIL [50].

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Fig. 4. Results of the corrosion tests on copper specimen immersed in all lubricant samples. From top left: SE; AIL; MJO+AIL1%; MJO+AIL5%; and MJO+AIL10%. Bottom from left: MJO; PIL; MJO+PIL1%; MJO+PIL5%; and MJO+PIL10%.

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In contrast to the corrosion inhibition performance of neat AIL, the addition of AIL in the MJO shows a good result, whereby less corrosion occurred on the copper surfaces compared to the surface lubricated with neat MJO and AIL. The increments of weight percentage of AIL in the MJO had increased the corrosion inhibition performance of the base oil, with the highest treat rate of 10 wt. % AIL shows almost no significant color changes on the copper surface. The presence of a thicker protecting layer on the copper sheet surface could explain the impediment to corrosion process compared to the other two lubricant mixtures. Therefore, MJO+AIL10% has successfully improved the corrosion behavior of the modified Jatropha oil.

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The addition of PIL additive in the MJO exhibits excellent performance of corrosion prevention. The increment of PIL treat rate in MJO did not significantly improve the corrosion inhibition of the lubricant mixtures on the copper sheet after the test. Therefore, the lowest treat rate of 1 wt. % of PIL is determined sufficient to improve the corrosion protection capability of the MJO. From this result, it is shown that all lubricant mixtures are having higher oxidative stability than the neat SE, MJO, and AIL. The trend of corrosion in ascending order when compared to the copper strip corrosion standard chart is expressed as: PIL (1a) > MJO+PIL10% (1a) ≥ MJO+PIL5% (1a) ≥ MJO+PIL1% (1a) ≥ MJO+AIL10% (1a) > MJO+AIL5% (1a) > MJO+AIL1% (1a) > AIL (1a+b) > SE (1b) > MJO (1b). This result also indicates the feasibility of applying AIL and PIL additives at low weight concentrations into the neat bio-based lubricant, MJO to improve its corrosion protection capability which is in agreements with the results shown in studies using the same ILs groups [20,23,33,34,37].

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3.3

Four ball wear test analysis

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3.3.1

Evaluation of coefficient of friction (COF)

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Figure 5 shows the distribution of the friction coefficient with the sliding time of all lubricant samples. The lubrication regime that occurred during the four ball wear tests was suggested to be between mixed lubrication and boundary lubrication state [9,51]. Referring to the Stribeck curve [52], the general behavior of fluid lubrication is shown as a function of viscosity, velocity, load, and contact area. As the viscosity was also inversely proportional to the temperature of the lubricating

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oil, film thickness parameter may decrease and the decrease in viscosity due to high temperature will cause a shift along the Stribeck curve into the boundary lubrication regime [53,54]. This will lead to contact increments between surface asperities and by which may explain the different progression of the friction coefficient. Various chemical compositions of the lubricant samples with the presence of additives at different weight concentrations also interact with the steel surfaces. These lubrication effects play a significant role in determining the chemical reactions, physisorption and chemisorption processes of the lubricant molecules with the metal substrates [54–56].

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Fig. 5. Friction coefficient of the tested lubricants after the four ball tribotest.

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The neat lubricant samples of SE, AIL and PIL present high friction coefficient with the mean COF values are greater than 0.1. It has been reported that the high number of carbon in anionic and cationic substituents of ILs increases the viscosity value [8]. The highly viscous neat AIL and PIL decreases lubricant flow, thus increases drag and friction and in turn, as shown in Fig. 5 (a), exhibits a high coefficient of friction compared to the MJO. Different from ILs, the poor lubrication ability of SE is anticipated by the existence of thinner lubricant films on the steel surfaces [31,57]. Lubricant film breakdown is also inevitable at a very high contact pressure and high impact speed, thus leading to the direct metal contacts that produce metal friction and scuffing, which resulted in high COF value [44,52,58].

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The vegetable based-oil, MJO poses COF value lesser than the other neat lubricants with the mean COF values of below than 0.6. Sudden peaks are seen within the first 1000 s of the sliding period for the steel surfaces lubricated with the vegetable-based oils, which implies to the metal scuffing due to the abrasive wear mechanism. It was predicted from the presence of a long chain of fatty acids such as linoleic, oleic and palmitic acids in the vegetable-based oils, that these fatty acids components formed multi and mono lubricant films on the steel/steel mating pairs which corroborated with the studies carried out in [59,60]. A stable oxide film was formed to prevent direct contact between the steel surfaces. MJO contains approximately 36% linoleic acid, C18H33COOH [61], a long covalently bonded hydrocarbon chain, that plays an important role in reducing the coefficient of friction. The long hydrocarbon chain helps in forming a protective layer between the steel mating pairs and becomes a barrier between the metal asperities to directly resist the shear forces and decreases the COF [53].

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The ILs additives in the MJOs also reacted on the rubbing zones to form boundary lubricant films on the contact surfaces, which suggests tribochemical reactions of the lubricant mixtures with the steel ball bearings. Figure 5(b) shows the interesting effect on the improvement of antifriction performance by the addition of AIL and PIL additives into the base oil, MJO. High peaks are seen in Figure 5(b), from which abrasive wear mechanisms are expected to increase the worn surface area. Different from MJO+AIL1% and MJO+AIL5%, no peak is seen for the steel surface lubricated with MJO+AIL10%. A great reduction of up to 50% improvement of antifriction ability compared to the neat AIL is corresponding to the good lubrication ability and the presence of tenacious lubrication layer on the steel surfaces. The addition of PIL additive has shown a comparable result to the lubrication ability of MJO with AIL additive. However, the increment of PIL volume in it has resulted in the presence of a sudden peak as shown by MJO+PIL10%. The lowest treat rate of 1 wt. % of PIL is therefore seen to be adequately sufficient to produce a protective layer on the steel surfaces that reduces scuffing effect during to the continuous sliding operation. The tenacious lubrication film provides adequate load carrying capacity that reduces the shear stress and consequently lowers the friction and wear [21,15,34].

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Evaluation of wear scar diameter and wear volume

The results of the mean wear scar diameter (WSD) with the appropriate error bars and the calculated wear volume are presented in Figure 6 and Table 2 respectively. It is found that the ball bearings lubricated with MJO have a higher WSD with an increment of approx. 57 % than SE. In boundary lubrication regime, the formation of a solid-like film that adsorbed on the contact surfaces is the result of intense liquids suppression under higher load or when the viscosity and sliding speed are very low [54,62]. Furthermore, the double bonds existed in the unsaturated fatty acids of MJO are prone to bond breaking at high temperature, thus although the long covalently bonded hydrocarbon chain is capable in reducing the COF, the increment of wear is inevitable [48,54]. By comparing the neat SE with the base oil, MJO, the wear volume depicted in Table 2 shows that MJO poses highest volume of worn surface at 76 x 10-4 mm3, an increase of more than 500 % of wear volume loss, thus revealed its needs for major improvement in antiwear and antifriction performances before it can be used as a bio-based metalworking fluid.

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Fig. 6. The average wear scar diameter after the four ball tribotest.

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The steel surfaces lubricated with AIL and PIL shows a decrement of approximately 42% and 47% of the mean WSD respectively, compared to the neat MJO. The ionic moieties of the ILs that reacted on the fresh steel ball surfaces by the chemical processes formed oxide layers. These boundary films acted as sacrificial layers that protect the metal surfaces from direct contact and thus, reduces the WSD. The neat PIL provided the lowest scar size amongst all tested lubricants with a 17 % decrement of WSD or 54 % reduction of wear volume against SE, while AIL presents an 8 % difference of WSD or 29 % lower wear volume compared to the same conventional synthetic ester. The surface erosion or corrosion during the chemical attacks is the most eligible reason suggesting the high wear volume of AIL compared to PIL as well as producing a higher coefficient of friction. The chemical attacks on the wear surface by the presence of [NTf2] anion dissolved in the neat AIL were reported to create two competing processes of tribo-film formation and material removal due to wear in the boundary lubrication regime, which leads to severe abrasive wear of the steel surfaces [21,63,64].

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Table 2 Wear volume of the steel ball bearings

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The addition of AIL additive in the MJO had improved the antiwear performance of the base oil. The highest concentration of AIL treat rate in MJO+AIL10% has shown approx. 28 % decrement of WSD compared to the neat MJO. The physical adsorption and chemical reaction of the base oil and AIL molecules on the steel surfaces during the sliding processes might be responsible for these improvements. The reduction of WSD by changing the additive concentration can be explained by the ability of this additive to create stronger inter- and intra-molecular interactions between the oil and additive molecules with the metal surfaces to form a tenacious tribo-layer. Thus, by increasing the AIL volume into the MJO, sufficient amount of AIL molecules will increase the physical proximity between them and the metal surfaces which subsequently creates stronger and thicker lubricant films that prevent direct contact between metal asperities [21,56]. In case of PIL additive, it is conceivable that 1 wt. % of the additive is sufficient to improve the antifriction and antiwear performance of the MJO [15]. Compared to the base oil, MJO+PIL1% poses approx. 36 % of WSD reduction. In terms of wear volume, MJO+PIL1% retains a lower volume of material removed of approx. 84 % reduction than the neat MJO and 0.5 % decrease compared to SE.

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Surface morphology analysis

The optical and SEM images of the scars after being lubricated with the lubricant samples are illustrated in Figure 7. It was found out from the tribotests that for ball bearings lubricated with MJO, blended MJOs and neat AIL, the wear scars were circular in shape. For SE, the scars had an irregular circular shape with the edge of the wear scar was a little bit ragged, while PIL formed a small elliptical scar shape on the ball bearings worn surface. Underneath the optical microscope, it is observed that the neat SE, AIL and PIL exhibited many black lines on the worn surfaces compared to the MJO and the lubricant mixtures. The surface irregularities produced after the tribology tests might contribute to the appearance of these black lines (deep grooves), where a mild scuffing could be interpreted as the abrasive wear mechanism [35].

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Fig. 7. Optical and SEM images of the worn scar surfaces after lubrication with all lubricant samples; (a, a’) SE; (b, b’) MJO; (c, c’) AIL; (d, d’) PIL; (e, e’) MJO+AIL1%; (f, f’) MJO+AIL5%; (g, g’) MJO+AIL10%; (h, h’) MJO+PIL1%; (j, j’) MJO+PIL5% and (k, k’) MJO+PIL10%.

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Figure 8 depicts the result of the mean arithmetic roughness, Ra value of all sliding surfaces together with their corresponding mean COF values after the tribology tests. The results revealed a correlation between COF values and the surface quality produced after the tribology tests. The highest COF value of 0.11 pose by SE, does correspond to the production of a poor surface quality of 0.64 µm. The roughness profile on the worn surface lubricated by MJO showed an improvement in surface roughness by 53 % decrease of Ra value compared to the SE, whilst the addition of AIL and PIL in the MJO at different weight concentrations exhibited a reduction of roughness value between 40 to 75 %. It is also shown in the diagram that the surface quality by adding 10 wt. % AIL and 1 wt. % PIL into the MJO presented the lowest roughness value among other lubricant mixtures of below than 0.2 µm, an improvement of approx. 68 % and 74 % respectively compared to the neat SE. The chemical properties of the ILs added into the MJO may have affected the nature of the base oil depending on the added weight concentration, thus increasing the surface quality of the worn scar lubricated with MJO+AIL10% and MJO+PIL1%. The tribochemical reaction (adsorptiondesorption) of the ILs additives on the metallic surfaces may explain on the reduction of material removal process as well as the increased roughness profile when mixed into the neat MJO [21,36].

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Fig. 8. Surface roughness profile of the worn scars and their corresponding mean friction coefficients after the four ball tribotest.

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The characteristics of the worn scar surfaces were further investigated by using AFM. Figure 8 shows the SEM images and the AFM results of the selected steel ball bearings used in the tribology experiments. It is obvious that for the wear of the surfaces lubricated with the neat SE, deep and shallow grooves with a presentation of adhesive wear and surface oxidation are expressed. The surfaces of the balls lubricated with neat ILs show the lowest wear followed by strong corrosion [35]. For AIL additivated MJOs, the increase in AIL concentration, the worn surface became smoother, while 1 wt. % PIL is sufficient in producing smooth worn surface. The surface of the balls lubricated with MJO+AIL10% and MJO+PIL1% were very smooth. The lowest wear among these group mixtures is also detected for these two lubricants. This was caused by the presence of adequate ILs within the mixtures, which react on the sliding surfaces and directly influenced the friction and wear reduction capabilities of the lubricant mixtures [65]. A surface smoothening phenomenon was also detected in the SEM and AFM pictures, Figures 7 and 9.

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Fig. 9. SEM images and atomic force microscopy three-dimensional surface topography after lubrication with SE; AIL; PIL; MJO+AIL10%; and MJO+PIL1% of the four ball wear test.

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Fig. 10. EDS spectrum of the worn scar surfaces lubricated with (a) MJO+AIL10% and (b) MJO+PIL1%.

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The presence of chemical elements on the scar surfaces was analyzed using an EDS machine. The surface lubricated with MJO+AIL10% indicates the presence of metal oxides (1.15% C and 0.07% O) by the adsorption of the lubricant molecules on the steel ball interfaces as shown in Figure 10(a) [37]. The worn surface lubricated with MJO+AIL10% presents a high percentage of iron elements (> 96%) similar to the worn surface areas lubricated with MJO+PIL1% (Figure 10(b)), which suggested on the formation of chemically reacted sacrificial tribolayers [66]. The addition of 10 wt. % of AIL shows an adequate synergistic effect in the base oil, promoting the formation of lubricant layer which prevented the metal corrosion behavior and subsequently prohibited direct metal contacts. This is also true for MJO+PIL1%, whereby it presents a very smooth worn surface with shallow parallel grooves. The formation of metal oxide layers suppresses the material transfer and adhesive wear mechanisms and hence increases the ability of these lubricant mixtures to form strong tenacious oxide layers, which prevent direct metal contacts and resulting in good surface lubrication at the boundary contact condition. The reduction of the friction coefficient, wear scar diameter and excellent surface quality are the direct consequences of these results [8,39,67].

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A narrow scan of Fe2p3/2 and O1s for the steel surfaces lubricated with MJO+AIL10% and MJO+PIL1% are presented in Figure 11. XPS narrow scan spectrum deconvolution was performed for both Fe2p and O1s narrow scan using commercial software, CasaXPS. The background model and photoelectron synthetic spectrum have been assigned using Shirley and Gaussian-Lorentzian method respectively. The ratio between Gaussian and Lorentzian for each peak was fixed at 70%:30%. However, as XPS binding energy (BE) position for C1s is normally shifting to lower binding energy due to carbon charging effect. Hence, prior to the narrow scan deconvolution, carbon charging factor correction is calibrated based on adventitious carbon binding energy at 284.8 eV. The BE position for each synthetic component in the narrow scan of Fe2p3/2 and O1s were referred to their electron binding energy (eV). For surface lubricated with MJO+AIL10%, the XPS peaks of Fe2p3/2 appear at 706.7, 709.93 and 711.97 eV [68,69]. On the other hand, a narrow scan of O1s for surface lubricated with MJO+AIL10% exhibited three main oxide species involved in this O1s envelope which are at 529.82, 531.50 and 532.73 eV and are attributed to the formation of FeO, and Fe2O3 and hydroxyl compound, OH– [17,18]. For surface lubricated with MJO+PIL1%, the deconvolution of Fe2p3/2 photoelectron was contributed by pure metal Fe, FeO and Fe2O3 species with binding energy position of 706.7, 709.23 and 711.51 eV [68,69]. Whilst, the deconvolution of the O1s narrow scan exhibited three main oxide species, which are FeO, and Fe2O3 and hydroxyl compound, OH– at 530.02, 531.52 and 532.66 eV respectively [21,41].

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Fig. 11. XPS spectra deconvolution for (a) Fe 2p and (b) O 1s narrow scan for MJO+AIL10% and MJO+PIL1% lubricants. The surface atomics percentages of oxide species for both Fe 2p and O 1s from the quantitative analysis of post deconvolution are tabulated in a (iii) and b (iii).

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The quantitative analysis of surface concentration percentage of the Fe metal signals from Fe2p3/2 of both lubricant samples (Figure 11(a)(iii)) indicate that the Fe metal signals are much intense for

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MJO+AIL10% compared to MJO+PIL1%. This implies that MJO+AIL10% provides better corrosion resistance property compared to MJO+PIL1%. The oxide species (Figure 11(b)(iii)) from O1s narrow scan reveals that MJO+AIL10% adsorbs less water vapor (hydroxyl component, OH–) than MJO+PIL1%. This indicates that the rate of oxidation and physisorption mechanism of MJO+PIL1% is much higher than MJO+AIL10%. Since the amount of hydroxyl component is higher in MJO+PIL1% compared to MJO+AIL10%, we can consider that the reactivity of the MJO+PIL1% with water component is much faster than MJO+AIL10%. As a result, the existence of more water vapor component in the MJO+PIL1% might be contributed to the induction of corrosion on the metal surface of the base metal, Fe.

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Additionally, the Fe2O3 concentration on the surface lubricated with MJO+AIL10% is dominant than FeO species, while the surface lubricated with MJO+PIL1% presented high percentage concentration of FeO species than the Fe2O3 oxide layer. These oxide layers, which are derived from the reaction of the lubricant mixtures with the fresh steel surface, are responsible for generating protective layers which contribute to the surface separation and hence reduces wear. Less FeO oxide formation on the worn scar surface has provided evidence that the 10 wt. % AIL additive is capable of slowing down the FeO oxide formation growth on the sample surface compared to 1 wt. % PIL [68,69]. This indicates that MJO+AIL10% lubricant exhibits anticorrosion behavior on the metal surface and very significant in improving the metal surface quality.

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3.4

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Figure 12 depicts the results of thrust force from the tapping process on AISI 1215 low carbon steel lubricated with each of all the lubricant samples. The tests were carried out at a constant tapping speed and feed rate. MJO poses the highest mean thrust force of 70.4 N. The addition of AIL into the MJO decreases the thrust force gradually with the increment of AIL concentrations, while the addition of PIL with increasing weight concentration increases the thrust force marginally. Generally, the increase in the length of carbon chains in both fatty acids and alkyl groups of ILs did increase the viscosity and the load carrying capacity of the mixtures, which is very important for a lubricant to perform especially during boundary lubrication state. The adequate amount of viscosity had lowered the friction force of the lubricated regions. MJO+AIL10% and MJO+PIL1% are seen to significantly reduce the thrust force required during the tapping process compared to the base oil and the conventional cutting fluid, SE. Protective layers are formed on the metal substrates of the sliding surfaces by the adsorption of the additives and the base oil molecules due to their high polarity structures, which helps in providing effective lubrication layer that reduces the real contact area between the metal asperities [39,70].

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Fig. 12. Results of thrust force after the tapping torque tests.

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Based on ASTM D5619, a higher torque efficiency number depicts a lower tapping torque produced against the benchmark oil. Figure 13 presents the results of the tapping torque and the torque efficiency of all lubricant samples. The tapping torque results are benchmarked against the neat SE to predict their lubrication efficiency. The reduction of wear rate of the tapping tool and a good lubricity performance at the contact surfaces during the tapping process which subsequently minimizes the friction force as well as improves the antiwear ability of the lubricants are the direct implication of high torque efficiency value. It is clearly seen that the MJO, provided by its strong intermolecular interactions between the long fatty acids with the ferrous metal substrates, retained a high efficiency compared to the SE. During the short metalworking period, the fatty acid chains of

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MJO helps resist the lubricant flow, thus provides effective boundary lubrication at the tool–chip interfaces, which reduces the friction and subsequently decreases the thrust force and torque. Additionally, the MJO also provides an excellent lubrication due to the increase in the number of ester groups [12,14]. The longer covalently bonded carbon chain of the fatty acids contained in the MJO compared to the SE does also increase the durability of the metal contacts as well as increases the adsorbed film thickness on the metal substrates [12,14].

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Fig. 13. Tapping torque and lubricant efficiency results of all lubricant samples.

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As neat lubricants, PIL and AIL provide good lubrication capacity as compared to SE (improvement of 15%). The temperature distribution of a cutting process was reported to be within 100 to 1000 °C [71], which directly correlates with the tribowear mechanism on the sliding pairs [65,72]. Thus, with the decrease in viscosity value by such high temperature range, the ILs lubrication capacities were dominated by the ability of the compounds in their anion or cation structures to react with the metal surface [73]. Within the relatively short contact time of the tapping process, the high polarity of the ILs molecules are capable to form a firm boundary lubricant layer on the steel surface, leading to a decrease in friction and wear [8]. However, in the state of boundary lubrication regime during the material removal process, the wear rate increases by orders of magnitude, thus destroying the tribofilm and exposed the fresh cut surface again to a new tribofilm formation and surface damage propagation. As a result, although relatively thin, the sustained tribofilm in ILs lubrication implies a stronger tribofilm forming process, which may explain the higher resistance to scuffing and surface damage propagation of ILs compared to the conventional SE [74].

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As expected, the MJO+AIL10% and MJO+PIL1% recorded high efficiency against SE and surpasses the performance of the base oil, MJO. These results also proved that the MJO+AIL10% and MJO+PIL1% provided a significant lubricity advantage compared to their base oil. The presence of branched and long carbon chains of neat MJO (C16-C18) [12] including the polar structure of the ILs additives provide effective lubrication layers on the surface being machined [34,37]. By increasing the polar nature of MJO, the interaction between ester molecule chains and ILs moieties becomes stronger. The high polarity of the mixture increases the adsorption potential of the molecules on the metal surfaces and forms an effective molecular barrier that reduces wear and friction.

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This evidence, together with the thermal, mechanical (e.g., abrasion and adhesion) and chemical (corrosion) stresses that occurred during the material removal process are contributing to the detrimental tribological interactions between the metal substrates and the ILs additives present in the MJO. The wear results obtained are in correspondence with the metal oxide tribofilm formed on the sliding surfaces. The lowest wear result and smooth surface finish are also related to the anticorrosion behavior of the lubricating fluids on the metal surface as shown by the XPS analysis, whereby MJO+AIL10% poses better corrosion resistance property than MJO+PIL1% during metal sliding contacts. In conclusion, the improved tribological reactions of the MJO+AIL10% and MJO+PIL1% have successfully strengthened their potential usage in both hydrodynamic and boundary lubrication regimes.

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4. Conclusion

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The presence of polyol esters, fatty acids and polar structure of AIL and PIL in the newly refined biodegradable lubricants play a significant role to their tribological performances on the metal sliding pairs, which had greatly reduced the friction and wear as well as producing an excellent surface finish. The addition of the adequate amounts of oil-miscible and biocompatible ionic liquids, AIL & PIL into the MJO resulted in the good synergistic effect on the tribological performance of the MJO. The adequate amount of AIL additive was found at 10 wt. % treat rate, while 1 wt. % of PIL was enough to improvise the lubrication capacity of the MJO. Excellent tribological performances were exhibited with improved rheological properties and oxidative stability as well as superior antifriction and antiwear performance. MJO+AIL10% & MJO+PIL1% have shown good corrosion inhibition, superior friction reduction, lower worn surface area, excellent surface finish improvement and increased tapping torque efficiency than the neat base oil as well as when compared to a commercially available synthetic ester, SE. From the EDS and XPS data, we concluded that the 10 wt. % of AIL additive was more prominent than 1 wt. % of PIL in preventing surface corrosion which leads to the production of a smooth machined surface with superior antiwear performance. The amount of additive being mixed into the MJO has successfully inhibited water vapor adsorption on the machined surface and is found to be better by using MJO+AIL10% than MJO+PIL1%. The performance of an MWF is the result of the lubricant interactions with the involved metal surfaces. The physical adsorption (physisorption), chemical adsorption (chemisorption) and chemical reaction are the possible working mechanisms of MWFs on the metal surface due to the tribomechanical excitations.

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The lubrication performances of the newly refined bio-based cutting fluids from Jatropha oil containing IL additives were studied in this research. The addition of fully oil-miscible and low toxic phosphonium-based and ammonium-based ILs with long alkyl chains have been successfully evaluated for their corrosion inhibition behaviors, antifriction and antiwear abilities and tapping torque efficiencies. The following conclusions are drawn according to the results obtained:

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From these observations, it can be concluded that the potential use of the oil-miscible and biocompatible AIL and PIL at lower treat rate as lubricant additives can promote the development of the new advanced biodegradable MWFs. MJO+AIL10% and MJO+PIL1% have great potential to be used as sustainable cutting fluids for metalworking applications in terms of high energy efficiency and good environmental impact. In further promoting ‘greener’ manufacturing activities, it was found that the MJO+AIL10% and MJO+PIL1% can become a good alternative as a reference to replace the industrial dominating mineral oil based lubricants and produce significant cost and waste savings.

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Acknowledgement

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The authors sincerely acknowledge the financial contribution from the Ministry of Higher Education Malaysia (MOHE) for the financial support via the Fundamental Research Grant Scheme under Vot. 1467 and Vot. 1591.

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Mordukhovich G, Qu J, Howe JY, Bair S, Yu B, Luo H, et al. A low-viscosity ionic liquid demonstrating superior lubricating performance from mixed to boundary lubrication. Wear 2013;301:740–6.

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Table 1. Physical and chemical properties of the tested lubricants

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0.99 0.96 1.12 0.92 0.96 0.98 0.98 0.97 0.96 0.97

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Synthetic Ester (SE) 257 TMP-Triester (MJO) 226 [MOA][NTF2] (AIL) 313* 320* [P6,6,6,14][Phosphinate] (PIL) MJO+AIL1% 233 MJO+AIL5% 229 MJO+AIL10% 229 MJO+PIL1% 229 MJO+PIL5% 229 MJO+PIL10% 229 * = Test based on D93A (high viscous oil)

Kinematic viscosity, ν in mm2/s (ASTM D445) 40 °C 100 °C 21.70 5.80 17.65 4.80 277.80 123.60 596.40 190.10 20.67 6.08 20.83 6.30 21.05 6.41 20.00 5.00 21.94 6.20 24.48 7.21

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Density, ρ at 25 °C in g/ml (ASTM D4052)

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Lubricant

Flash point in °C (ASTM D93)

Viscosity Index, VI (ASTM D2270) 230 215 457 403 274 288 292 192 260 288

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Table 2. Wear volume of the steel ball bearings

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COF

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Wear volume, WV in 10-4 mm3 12 76 9 6 44 22 21 12 14 15

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Synthetic Ester (SE) TMP-Triester (MJO) [MOA][NTF2] (AIL) [P6,6,6,14][Phosphinate] (PIL) MJO+AIL1% MJO+AIL5% MJO+AIL10% MJO+PIL1% MJO+PIL5% MJO+PIL10%

WSD in mm 0.634 0.996 0.582 0.523 0.869 0.733 0.718 0.633 0.651 0.663

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0.112 0.055 0.104 0.101 0.060 0.056 0.054 0.060 0.062 0.063

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Manuscript Figures

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Fig. 1. Chemical structures of AIL and PIL.

Fig. 2. Four-ball wear test assembly and the chemical composition of the steel ball bearings.

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Fig. 3. Setup assembly of tapping test and the chemical composition of the carbon steel specimen.

Fig. 4. Results of the corrosion tests on copper specimen immersed in all lubricant samples. From top left: SE; AIL; MJO+AIL1%; MJO+AIL5%; and MJO+AIL10%. Bottom from left: MJO; PIL; MJO+PIL1%; MJO+PIL5%; and MJO+PIL10%.

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a)

SE MJO AIL MJO+AIL1% MJO+AIL5% MJO+AIL10%

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0.15

0.10

0.05 0

500

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Coefficient of Friction, COF

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b)

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SE MJO PIL MJO+PIL1% MJO+PIL5% MJO+PIL10%

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Coefficient of Friction, COF

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0

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2000

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Sliding time (s)

Fig. 5. Friction coefficient of the tested lubricant samples after the four ball tribotest. (a) Comparison between SE, MJO, neat AIL and AIL added MJOs, and (b) comparison between SE, MJO, neat PIL and PIL added MJOs.

1000 800 600

200 0 SE

O MJ

Lubricants

AIL IL1% IL5% L10% I A A O+ O+ JO+A MJ MJ M

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400

PIL IL1% IL5% L10% I P P O+ JO+P O+ MJ MJ M

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Wear Scar Diameter, WSD in µm

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Fig. 6. The average wear scar diameter after the four ball tribotest.

Fig. 7. Optical and SEM images of the worn scar surfaces after lubrication with all lubricant samples; (a, a’) SE; (b, b’) MJO; (c, c’) AIL; (d, d’) PIL; (e, e’) MJO+AIL1%; (f, f’) MJO+AIL5%; (g, g’) MJO+AIL10%; (h, h’) MJO+PIL1%; (j, j’) MJO+PIL5% and (k, k’) MJO+PIL10%.

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0.12 Ra COF

0.8

0.10 0.08

0.6 0.06 0.4

0.2

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0.04 0.02 0.00

SE

AIL IL1% IL5% L10% I A A O+ O+ JO+A MJ MJ M

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Lubricants

PIL IL1% IL5% L10% I P P O+ JO+P O+ MJ MJ M

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0.0

Coefficient of Friction, COF

Roughness, Ra in µ m

1.0

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Fig. 8. Surface roughness profile of the worn scars and their corresponding mean friction coefficients after the four ball tribotest.

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Fig. 9. SEM images and atomic force microscopy three-dimensional surface topography after lubrication with SE; AIL; PIL; MJO+AIL10%; and MJO+PIL1% of the four ball wear test.

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Fig. 10. EDX spectrum of the worn scar surfaces lubricated with (a) MJO+AIL10% and (b) MJO+PIL1%.

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a) Fe 2p 5000

5000

4500

Fe2p 3/2

Fe2p 3/2

4500

4000

4000

2+

2+ 3500

3+

3500

3+

3000

3000

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Fe 2000

1500

1500

1000

Fe

1000

(i) MJO+AIL10%

500

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(ii) MJO+PIL1%

500

718

716

714

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706

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Binding Energy (eV)

712

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706

MJO+AIL10% MJO+PIL1%

(iii)

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Binding Energy (eV)

30 20 10 0

Fe 2p (metal)

b) O 1s

Fe 2p (FeO)

Fe 2p (Fe2O3)

Fe photoelectron signal

7000

7000

6000

5000

4000

529.82eV

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EP

532.73eV

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531.52eV

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532.66eV

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532

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Binding Energy (eV)

Concentration %

534

530.02eV

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536

(ii) MJO+PIL1% 6000

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Binding Energy (eV)

80

MJO+AIL10% MJO+PIL1%

(iii) 70 60 50 40 30 20 10 0 O 1s (substrate, FeO)

O 1s (Fe2O3)

O photoelectron signal

O 1s (OH)

528

700

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Fig. 11. XPS spectra deconvolution for (a) Fe 2p and (b) O 1s narrow scan for MJO+AIL10% and MJO+PIL1% lubricants. The surface atomics percentages of oxide species for both Fe 2p and O 1s from the quantitative analysis of post deconvolution are tabulated in a (iii) and b (iii).

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Thrust Force, Fz (N)

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0 SE

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Lubricants

AIL IL1% IL5% L10% I A A O+ JO+ JO+A MJ M M

PIL IL1% IL5% L10% P P +PI O+ O+ MJ MJO MJ

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Fig. 12. Results of thrust force after the tapping torque tests.

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Torque Efficiency

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SE

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Lubricants

AIL IL1% IL5% L10% I A A O+ JO+A O+ MJ MJ M

PIL IL1% IL5% L10% I P P O+ JO+P O+ MJ MJ M

Fig. 13. Tapping torque and lubricant efficiency results of all lubricant samples.

Efficiency (%)

Mean Torque, Mz (Nm)

Mean Torque, Mz

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Newly refined Jatropha oil-based (MJO) lubricant containing 10 wt. % [N1888][NTf2] (AIL) or 1 wt. % [P66614][Phosphinate] (PIL) additives showed best tribological performances with similar friction behavior. MJO+AIL10% & MJO+PIL1% pose good corrosion inhibition, excellence lubrication quality, superior friction and wear reduction, high surface quality and tapping torque efficiency. Friction and wear reduction on the steel surfaces is related to metal oxide tribofilm formation and anti-corrosion behavior of the lubricant mixtures on the steel surface with MJO+AIL10% performed better than MJO+PIL1%.

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