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Two phosphonium cation-based ionic liquids used as lubricant additive
Tribology International 107 (2017) 233–239
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Two phosphonium cation-based ionic liquids used as lubricant additive Part I: Film thickness and friction characteristics
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A. Hernández Batteza,e, , Carlos M.C.G. Fernandesb, Ramiro C. Martinsb, M. Bartoloméc, R. Gonzálezc,e, Jorge H.O. Seabrad a
Department of Construction and Manufacturing Engineering, University of Oviedo, Asturias, Spain INEGI, Universidade do Porto, Campus FEUP, Rua Dr. Roberto Frias 400, 4200-465 Porto, Portugal c Department of Marine Science and Technology, University of Oviedo, Asturias, Spain d FEUP, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal e Department of Design and Engineering, Bournemouth University, Poole BH12 5BB, UK b
A R T I C L E I N F O
A BS T RAC T
Keywords: Ionic liquids Additive Film thickness Coefficient of friction
The lubricant film thickness and friction properties of a mineral base oil and its mixtures with two phosphonium cation-based ionic liquids: [P66614][(iC8)2PO2] and [P66614][BEHP ] at 0.5 and 1 wt% concentrations were studied under different slide-to-roll ratios (SRR) and temperatures in an EHD2 ball-on-disc test rig. All lubricant samples showed similar lubricant film thickness in full film lubrication and behaved similarly in friction tests with polished discs at SRR of 5%, but the mixtures outperformed the neat base oil at SRR of 50%. The mixtures were also better in tests with rough discs. Tribological improvements with mixtures were achieved under mixed and boundary lubrication regimes. Real application such as rolling bearings will be presented in the second part of this work.
1. Introduction Ionic liquids (ILs) have become an increasing research topic in tribology since 2001 [1]. The interest of the lubrication field for the ionic liquids is due to their exceptional properties such as high thermooxidative stability, non-flammability, non-volatility, ashless character and controlled miscibility with organic compounds [2–7]. The good lubricating properties of the ionic liquids are linked to their high polarity [8] and the adsorbed tribofilms formed on the metal surfaces, which contribute to reducing friction and wear [9–13]. In general, the ionic liquids have a low solubility in non-polar hydrocarbon oils. Due to this fact, many research works have been developed in the last decade using the ionic liquids as a lubricant additive or as a component of emulsions [14–17]. Since 2009, the commercial availability of phosphonium cation-based ionic liquids has increased interest for their potential use in lubrication [18–20]. Some of these phosphonium based ILs have a good solubility in oils, so their application as a lubricant additive has become a current research topic [15–17,21–29]. Most of the above-mentioned works studied the tribological behaviour of these ILs using reciprocating sliding tests under a mixed or boundary lubrication regime. However, the study of the lubricant film thickness of IL-containing mixtures and their friction
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properties under different testing conditions should also be addressed. About 33% of the fuel energy is wasted by cars due to friction losses and from that amount the engine and the transmission systems represent the 35% and 15%, respectively [30]. These systems work under the three general lubrication regimes: boundary, mixed and elastohydrodynamic/hydrodynamic [15]. Among the potential mechanisms to reduce friction in these systems are the use of low-viscosity and low-shear lubricants as well as the development and use of novel additives [30,15]. Recently, the improvement of mechanical efficiency during engine cold start by increasing the oil temperature more quickly has been proved by using different solutions under the New European Driving Cycle (NEDC) [31]. For all the studied solutions, the oil temperature reached values from 95 and 105 °C. Roberts [32] reported that 11 min was the average trip duration and two-thirds of the trips did not go over the line of 11 min, and for this duration, the oil temperature was between 70 and 95 °C under the NEDC. This work studies the film forming and friction properties of a mineral oil used in the formulation of fuel economy motor oils and its mixtures with two phosphonium cation-based ionic liquids used as additive. These lubricant samples were tested at room temperature in [29]. However, the main goal of this study is to understand the influence of the type and concentration of the ionic liquid on the
Corresponding author at: Department of Construction and Manufacturing Engineering, University of Oviedo, Asturias, Spain. E-mail address: [email protected] (A. Hernández Battez).
http://dx.doi.org/10.1016/j.triboint.2016.10.048 Received 27 July 2016; Received in revised form 4 October 2016; Accepted 28 October 2016 Available online 24 November 2016 0301-679X/ © 2016 Elsevier Ltd. All rights reserved.
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Table 1 Physical and chemical properties of the base oil and its mixtures with the ionic liquids.
Table 2 Main characteristics of the ball and disc.
Physical properties
Unit
Yubase4
05IL1
10IL1
05IL2
10IL2
Parameters
Ball
Disc
Density at 21 °C Viscosity at 40 °C Viscosity at 70 °C Viscosity at 100 °C Refractive index
g/cm3 mPas mPas mPas –
0.829 12.79 4.87 2.48 1.4606
0.829 12.81 4.78 2.43 1.4606
0.829 13.03 4.92 2.49 1.4603
0.830 12.95 4.89 2.54 1.4603
0.830 12.99 4.92 2.55 1.4603
Elastic modulus – E (GPa) Poisson coefficient – ν (–) Radius – R (mm) Surface roughness – Ra (nm) Spacer layer thickness – (nm) Spacer layer refractive index – (–)
210 0.29 19.05 20 – –
64 0.2 50 ≈5 ≈500 ≈1.4785
lubricant film thickness and coefficient of friction under different temperature and sliding/rolling conditions. Stribeck curves will be used to characterise the tribological performance of the lubricant samples.
Table 2. The lubricant film thickness tests were made under fully flooded lubrication (120 ml of lubricant sample) for the base oil and all mixtures described in Table 1. A load of 50 N (corresponding with a maximum Hertz pressure of p0=0.66 GPa) and three operating temperatures (40, 70 and 100 °C) were used. The tests were performed at 5% slide-to-roll ratio (SRR), defined by
2. Experimental details 2.1. Base oil and ionic liquids
(Udisc − Uball ) × 100 (Udisc + Uball )
The base oil used in this work was a hydrocracked mineral oil (Yubase4/Group III) provided by Repsol S.A. and the two ionic liquids trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate or [P66614][(iC 8)2 PO2] (coded as IL1) and trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate or [P66614][BEHP ] (coded as IL2) were provided by Ionic Liquid Technologies GmbH. The base oil was mixed with [P66614][(iC 8)2 PO2] at concentrations of 0.5 wt% (coded as 05IL1) and 1.0 wt% (coded as 10IL1). Also, mixtures of the base oil with [P66614][BEHP ] at concentrations of 0.5 wt% (coded as 05IL2) and 1.0 wt % (coded as 10IL2) were prepared. The most relevant physical and chemical properties of the lubricant samples were measured before tribological testing and are shown in Table 1.
SRR[%] = 2 ×
2.2. Film thickness measurements
US =
A ball-on-disc test rig (PCS Instruments, model EHD2) equipped with optical interferometry, see Fig. 1, was used for measuring the lubricant film thickness in the contact formed between a 3/4 in (19.05 mm) diameter steel ball and a rotating glass disc. The glass disc is coated with a chromium (20 nm) and silica (500 nm) layer and the load-applying system is based on moving the ball against the disc. The disc and the steel ball are controlled by two electric motors for performing tests under rolling/sliding conditions. The system can accurately calculate the central film thickness from measuring the wavelength of the light returned from the central plateau of the contact. The standard ball specimen with a high grade surface finish is made from carbon chrome steel and the glass disc can be tested up to approximately 0.7 GPa of maximum Hertz pressure. The ball and disc characteristics (provided by the manufacturer) are presented in
The lowest ball speeds were 0.097, 0.243 and 0.487 m/s for temperatures of 40, 70 and 100 °C, respectively, and the highest ball speed was consistently 1.950 m/s. On the other hand, the lowest disc speeds were 0.102, 0.256 and 0.512 m/s for 40, 70 and 100 °C, respectively, and the highest disc speed was consistently 2.049 m/s. The equipment automatically adjusts the disc and ball speeds, making the disc speed faster to obtain positive SRR and then making the ball faster to obtain negative SRR, while keeping the entrainment speed constant. The result is the average of both measurements. This procedure is performed for each entrainment speed.
(1)
where Udisc and Uball are the speed of the disc and ball on the contacting surfaces, respectively. Different entrainment (or mean) speed values were used for each operating temperature. The lowest value was 0.1 m/s for 40 °C, 0.25 m/s for 70 °C and 0.5 m/s for 100 °C. These conditions avoid working with very thin lubricant film thickness protecting the glass disc. The highest entrainment speed used was always limited to 2 m/s because the optical device has a maximum film thickness measurement range of around 1000 nm. The entrainment (or mean) speed is defined as:
(Udisc + Uball ) 2
(2)
2.3. Coefficient of friction measurements The coefficient of friction measurements were also performed on
Fig. 1. EHD2 ball-on-disc test rig (from PCS Instruments).
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Table 3 Disc properties.
Table 4 Pressure–viscosity coefficient determined based on film thickness measurements.
Parameters
Disc
Temperature (°C)
α × 108 (1/Pa) Polished Elastic modulus - E (GPa) Poisson coefficient – ν (–) Radius – R (mm) Surface roughness – Ra (μm)
210 0.29 50 0.023
40 1.447
70 1.265
100 1.157
Rough
Table 5 Pressure–viscosity coefficient (α × 108 [1/Pa]) determined based on film thickness measurements.
0.35
the EHD2 ball-on-disc described in the previous section. In this case, the ball runs against a steel disc and the applied load generates contact pressures up to 1.11 GPa. The ball and disc used were manufactured from carbon chrome steel, with 3/4 in (19.05 mm) and 100 mm diameters, respectively. The ball is the same used in the lubricant film thickness measurements, Table 2, and has an initial roughness (Ra) of 0.02 µm. The main disc properties were provided by the manufacturer and roughness was measured by the authors for both polished and rough samples (Table 3). The coefficient of friction for all lubricant samples, described in Table 1, were measured at the same three operating temperatures used in the previous section: 40, 70 and 100 °C. Using a polished disc, the friction properties were studied through Stribeck curves determined for two SRR values (5% and 50%) and entrainment speeds varying from 0.01 to 3 m/s. With a rough disc, the Stribeck curves were determined for an SRR value of 50% with entrainment speeds also varying from 0.01 to 3 m/s. For this test, the ball speed is made constant and the disc speed increases and decreases in order to obtain the same positive and negative SRR. The result is the average of both measurements. The ball speed changed from 0.01 to 3 m/s for both SRR values (5% and 50%). On one hand, the disc speed changed from lowest values of 0.009 and 0.01 m/s for both SRR values and highest values of 3.150 and 2.850 m/ s for SRR of 5% and 4.500 and 1.500 m/s for SRR of 50%.
Oil (°C)
Yubase4
05IL1
05IL2
10IL1
10IL2
40 70 100
1.9749 1.9685 1.7153
2.0316 1.9862 1.6953
2.0803 1.881 1.6661
2.2809 2.0368 1.5678
2.1545 2.2427 1.7872
α = s·ν t × 10−8
(3)
where s=9.904, t=0.139 and ν is the kinematic viscosity in (cSt). Using the procedure presented in [36], the pressure–viscosity coefficient was determined for each operating temperature based on the measurements. The results are presented in Table 5. It is interesting to note that all mixtures have higher pressure–viscosity coefficient than Yubase4 at 40 °C. The difference is higher for higher concentrations of ionic liquid. When temperature increases only the mixture with 1 wt% of [P66614][BEHP ] always presented higher pressure–viscosity than Yubase4. 3.2. Coefficient of friction The Stribeck curves for all lubricant samples were measured at temperatures of 40, 70 and 100 °C and SRR of 5% and 50%. The coefficient of friction was plotted against the modified Hersey number proposed by Brandão [37]. The modified Hersey number is given the Eq. (4), while the original Hersey number (H) is given by Eq. (5). Brandão suggested that the contact is under boundary film lubrication for a modified Hersey number Hp < 10−9 and under full film lubrication for Hp > 10−7.
3. Results and discussion
Hp =
3.1. Film thickness All lubricant samples showed similar film thickness values during the tests performed under full film lubrication (see Figs. 2a and b) according to their similar rheological properties listed in Table 1. Although the viscosity and the pressure–viscosity coefficient have a similar influence on the film thickness, different tribological behaviours were observed under mixed and boundary lubrication regimes for low viscosity base oils and their mixtures with substances with higher polarity and viscosity [33,34]. According to Gold's [35] equation (3), the pressure–viscosity coefficient (α) for a mineral base oil is given in Table 4.
H=
η·US ·α1/2 F1/2
η·US F
(4) (5)
where η is the oil dynamic viscosity at the inlet of the contact, US is the mean speed (Eq. (2)), α is the pressure–viscosity coefficient and F is the normal force. 3.2.1. Polished disc Under a SRR of 5% (see Figs. 3a–c and Fig. 4a–c), it can be observed that the addition of the ionic liquids at two concentrations (0.5 and 1 wt%) hardly changed the coefficient of friction of the
Fig. 2. Film thickness of the base oil and mixtures with the ionic liquid (a) [P66614][(iC 8)2PO2] and (b) [P66614][BEHP ] at SRR5%.
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Fig. 3. Friction characteristics of the base oil and mixtures with the ionic liquid [P66614][(iC 8)2PO2] tested with polished discs at SRR5% and SRR50%.
varied from full film lubrication to mixed lubrication under the entrainment speed range used for a SRR of 5%. Also, the toughest lubrication condition (i.e. highest temperature, 100 °C) shifted the contact to a mixed lubrication regime for all modified Hersey number values. However, for an SRR of 50% the tests always performed under a mixed lubrication regime.
mixtures in comparison with the base oil. Only at 100 °C did the mixtures with 1 wt% of [P66614][BEHP ] clearly show better friction behaviour at the lowest values of the modified Hersey number (i.e. low speeds), which is an interesting fact in order to reduce friction during start–stop cycles (boundary conditions). It should be stressed that the Stribeck curve for the neat base oil became erratic at SRR of 50%, 100 °C and lowest speed conditions (Fig. 3c or 4c). With increasing values of the modified Hersey number (i.e. higher speeds) the coefficient of friction tends to reach a similar value for all lubricant samples, which is related to their similar viscosity (see Table 1). However, under full film lubrication, the ionic liquid mixtures present a slightly higher coefficient of friction, mainly at 40 and 70 °C, which is in agreement with the highest pressure–viscosity coefficients extracted from film thickness measurements. According to the literature [37], higher pressure–viscosity coefficient means higher coefficient of friction under full film lubrication. On the other hand, the tribological behaviour of the base oil and the mixtures for a SRR of 50% was quite different (see Figs. 3d–f and 4d– f). In all cases the mixtures outperformed the tribological behaviour of the base oil showing a lower coefficient of friction at decreasing values of the modified Hersey number. This better tribological behaviour of the mixtures is clearly influenced by the SRR; therefore, the friction modifier action of the ionic liquids is more evident for higher SRR values. In general, it can also be observed that lubricant samples are operating in the full film lubrication regime when Hp > 10−7 and under mixed lubrication regime when 10−9 < Hp < 10−7. This conclusion fully agrees with Brandão [37] and Hammami [38]. The lubrication regime
3.2.2. Rough disc As expected the friction tests performed with rough discs generated higher friction values than those made with smooth discs. The composite roughness is σ ≈ 0.35 μm and the contact was always working under a boundary lubrication regime (λ < 1). The mixtures with the ionic liquid [P66614][(iC 8)2 PO2] at an SRR of 50% behaved similarly to the base oil with the exception of the test at 100 °C when both mixtures had a lower coefficient of friction than the base oil at decreasing values of the modified Hersey number (Figs. 5a– c). Meanwhile, the mixture at 0.5 wt% of the ionic liquid [P66614][BEHP ] showed the best tribological behaviour for all test conditions (Figs. 6a– c), and the mixture at 1.0 wt% only outperformed the base oil at highest temperature for low Hersey number values (i.e. low speeds). In this case, it seems that two simultaneous processes (faster roughness reduction and tribochemical) contributed to the better tribological behaviour of the mixtures. As temperature increases, it is clear that the ionic liquid activation with temperature clearly improves the friction behaviour of the base oil. This is particularly interesting for oil engine application usually operating around 100 °C [31] and under different sliding/rolling ratio conditions. Taking into account these facts, the tribological behaviour 236
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Fig. 4. Friction characteristics of the base oil and mixtures with the ionic liquid [P66614][BEHP ] tested with polished discs at SRR5% and SRR50%.
can be drawn:
and tribochemical interactions of these lubricant samples in a real application (rolling bearings) will be presented in the second part of this work.
•
4. Conclusions The film forming properties and friction characteristics of a hydrocracked mineral oil and its mixtures with two phosphonium cation-based ionic liquids used as additive at concentrations of 0.5 and 1 wt% were studied under two SRR values and three different temperatures. From the results obtained, the following conclusions
•
The addition of the ionic liquids at low concentrations hardly influenced the rheology of the base oil used; therefore, all the lubricant samples (base oil and its mixtures with both ionic liquids) showed similar lubricant film thickness in a full film lubrication regime. This behaviour was proved for all temperatures tested. The friction properties of the lubricant samples containing ionic liquids, as well as the base oil, are strongly influenced by testing conditions (temperature, speed, SRR) and the surface finish of the specimens.
Fig. 5. Friction characteristics of the base oil and mixtures with the ionic liquid [P66614][(iC 8)2PO2] tested with rough discs at SRR50%.
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Fig. 6. Friction characteristics of the base oil and mixtures with the ionic liquid [P66614][BEHP ] tested with rough discs at SRR50%.
•
• •
Tests made with polished discs took place under full film and mixed lubrication regimes for all tested temperatures at an SRR of 5% and the lubricant samples showed similar friction characteristics. The SRR value of 50% shifted the contact to a mixed lubrication regime showing the mixtures lower coefficient of friction than the base oil at low values of the modified Hersey number (i.e. low speeds), which is important during start–stop periods of machines. The friction tests made with rough discs were all performed under boundary and mixed lubrication regimes leading to different results according to the temperature values. In general, the mixtures showed better tribological behaviour than the base oil. In general, the mixtures showed a tribological improvement in comparison with the neat base oil. Testing them in a real application (e.g. rolling bearings) will be very interesting as the next research idea to be presented in the second part of this work.
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Acknowledgements The authors want to thank to the Spanish Ministry of Education, Culture and Sport for funding (ref: PR2015-00458) the research stay of Antolin Hernández Battez at Universidade do Porto (Portugal) within the programme ‘Salvador de Madariaga’. They also thank to the Spanish Ministry of Economy and Competitiveness and the Foundation for the Promotion in Asturias of Applied Scientific Research and Technology (FICYT) for supporting the research projects DPI2013-48348-C2-1-R and GRUPIN14-023, respectively. The authors gratefully acknowledge the funding supported by National Funds through Fundação para a Ciência e Tecnologia (FCT), under the project EXCL-II/SEM-PRO/0103/2012; NORTE-01-0145FEDER-000022 - SciTech – Science and Technology for Competitive and Sustainable Industries, cofinanced by Programa Operacional Regional do Norte (NORTE2020), through Fundo Europeu de Desenvolvimento Regional (FEDER) and LAETA under the project UID/EMS/50022/2013.
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Two phosphonium cation-based ionic liquids used as lubricant additive Part I: Film thickness and friction characteristics
crossmark
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A. Hernández Batteza,e, , Carlos M.C.G. Fernandesb, Ramiro C. Martinsb, M. Bartoloméc, R. Gonzálezc,e, Jorge H.O. Seabrad a
Department of Construction and Manufacturing Engineering, University of Oviedo, Asturias, Spain INEGI, Universidade do Porto, Campus FEUP, Rua Dr. Roberto Frias 400, 4200-465 Porto, Portugal c Department of Marine Science and Technology, University of Oviedo, Asturias, Spain d FEUP, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal e Department of Design and Engineering, Bournemouth University, Poole BH12 5BB, UK b
A R T I C L E I N F O
A BS T RAC T
Keywords: Ionic liquids Additive Film thickness Coefficient of friction
The lubricant film thickness and friction properties of a mineral base oil and its mixtures with two phosphonium cation-based ionic liquids: [P66614][(iC8)2PO2] and [P66614][BEHP ] at 0.5 and 1 wt% concentrations were studied under different slide-to-roll ratios (SRR) and temperatures in an EHD2 ball-on-disc test rig. All lubricant samples showed similar lubricant film thickness in full film lubrication and behaved similarly in friction tests with polished discs at SRR of 5%, but the mixtures outperformed the neat base oil at SRR of 50%. The mixtures were also better in tests with rough discs. Tribological improvements with mixtures were achieved under mixed and boundary lubrication regimes. Real application such as rolling bearings will be presented in the second part of this work.
1. Introduction Ionic liquids (ILs) have become an increasing research topic in tribology since 2001 [1]. The interest of the lubrication field for the ionic liquids is due to their exceptional properties such as high thermooxidative stability, non-flammability, non-volatility, ashless character and controlled miscibility with organic compounds [2–7]. The good lubricating properties of the ionic liquids are linked to their high polarity [8] and the adsorbed tribofilms formed on the metal surfaces, which contribute to reducing friction and wear [9–13]. In general, the ionic liquids have a low solubility in non-polar hydrocarbon oils. Due to this fact, many research works have been developed in the last decade using the ionic liquids as a lubricant additive or as a component of emulsions [14–17]. Since 2009, the commercial availability of phosphonium cation-based ionic liquids has increased interest for their potential use in lubrication [18–20]. Some of these phosphonium based ILs have a good solubility in oils, so their application as a lubricant additive has become a current research topic [15–17,21–29]. Most of the above-mentioned works studied the tribological behaviour of these ILs using reciprocating sliding tests under a mixed or boundary lubrication regime. However, the study of the lubricant film thickness of IL-containing mixtures and their friction
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properties under different testing conditions should also be addressed. About 33% of the fuel energy is wasted by cars due to friction losses and from that amount the engine and the transmission systems represent the 35% and 15%, respectively [30]. These systems work under the three general lubrication regimes: boundary, mixed and elastohydrodynamic/hydrodynamic [15]. Among the potential mechanisms to reduce friction in these systems are the use of low-viscosity and low-shear lubricants as well as the development and use of novel additives [30,15]. Recently, the improvement of mechanical efficiency during engine cold start by increasing the oil temperature more quickly has been proved by using different solutions under the New European Driving Cycle (NEDC) [31]. For all the studied solutions, the oil temperature reached values from 95 and 105 °C. Roberts [32] reported that 11 min was the average trip duration and two-thirds of the trips did not go over the line of 11 min, and for this duration, the oil temperature was between 70 and 95 °C under the NEDC. This work studies the film forming and friction properties of a mineral oil used in the formulation of fuel economy motor oils and its mixtures with two phosphonium cation-based ionic liquids used as additive. These lubricant samples were tested at room temperature in [29]. However, the main goal of this study is to understand the influence of the type and concentration of the ionic liquid on the
Corresponding author at: Department of Construction and Manufacturing Engineering, University of Oviedo, Asturias, Spain. E-mail address: [email protected] (A. Hernández Battez).
http://dx.doi.org/10.1016/j.triboint.2016.10.048 Received 27 July 2016; Received in revised form 4 October 2016; Accepted 28 October 2016 Available online 24 November 2016 0301-679X/ © 2016 Elsevier Ltd. All rights reserved.
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Table 1 Physical and chemical properties of the base oil and its mixtures with the ionic liquids.
Table 2 Main characteristics of the ball and disc.
Physical properties
Unit
Yubase4
05IL1
10IL1
05IL2
10IL2
Parameters
Ball
Disc
Density at 21 °C Viscosity at 40 °C Viscosity at 70 °C Viscosity at 100 °C Refractive index
g/cm3 mPas mPas mPas –
0.829 12.79 4.87 2.48 1.4606
0.829 12.81 4.78 2.43 1.4606
0.829 13.03 4.92 2.49 1.4603
0.830 12.95 4.89 2.54 1.4603
0.830 12.99 4.92 2.55 1.4603
Elastic modulus – E (GPa) Poisson coefficient – ν (–) Radius – R (mm) Surface roughness – Ra (nm) Spacer layer thickness – (nm) Spacer layer refractive index – (–)
210 0.29 19.05 20 – –
64 0.2 50 ≈5 ≈500 ≈1.4785
lubricant film thickness and coefficient of friction under different temperature and sliding/rolling conditions. Stribeck curves will be used to characterise the tribological performance of the lubricant samples.
Table 2. The lubricant film thickness tests were made under fully flooded lubrication (120 ml of lubricant sample) for the base oil and all mixtures described in Table 1. A load of 50 N (corresponding with a maximum Hertz pressure of p0=0.66 GPa) and three operating temperatures (40, 70 and 100 °C) were used. The tests were performed at 5% slide-to-roll ratio (SRR), defined by
2. Experimental details 2.1. Base oil and ionic liquids
(Udisc − Uball ) × 100 (Udisc + Uball )
The base oil used in this work was a hydrocracked mineral oil (Yubase4/Group III) provided by Repsol S.A. and the two ionic liquids trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate or [P66614][(iC 8)2 PO2] (coded as IL1) and trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate or [P66614][BEHP ] (coded as IL2) were provided by Ionic Liquid Technologies GmbH. The base oil was mixed with [P66614][(iC 8)2 PO2] at concentrations of 0.5 wt% (coded as 05IL1) and 1.0 wt% (coded as 10IL1). Also, mixtures of the base oil with [P66614][BEHP ] at concentrations of 0.5 wt% (coded as 05IL2) and 1.0 wt % (coded as 10IL2) were prepared. The most relevant physical and chemical properties of the lubricant samples were measured before tribological testing and are shown in Table 1.
SRR[%] = 2 ×
2.2. Film thickness measurements
US =
A ball-on-disc test rig (PCS Instruments, model EHD2) equipped with optical interferometry, see Fig. 1, was used for measuring the lubricant film thickness in the contact formed between a 3/4 in (19.05 mm) diameter steel ball and a rotating glass disc. The glass disc is coated with a chromium (20 nm) and silica (500 nm) layer and the load-applying system is based on moving the ball against the disc. The disc and the steel ball are controlled by two electric motors for performing tests under rolling/sliding conditions. The system can accurately calculate the central film thickness from measuring the wavelength of the light returned from the central plateau of the contact. The standard ball specimen with a high grade surface finish is made from carbon chrome steel and the glass disc can be tested up to approximately 0.7 GPa of maximum Hertz pressure. The ball and disc characteristics (provided by the manufacturer) are presented in
The lowest ball speeds were 0.097, 0.243 and 0.487 m/s for temperatures of 40, 70 and 100 °C, respectively, and the highest ball speed was consistently 1.950 m/s. On the other hand, the lowest disc speeds were 0.102, 0.256 and 0.512 m/s for 40, 70 and 100 °C, respectively, and the highest disc speed was consistently 2.049 m/s. The equipment automatically adjusts the disc and ball speeds, making the disc speed faster to obtain positive SRR and then making the ball faster to obtain negative SRR, while keeping the entrainment speed constant. The result is the average of both measurements. This procedure is performed for each entrainment speed.
(1)
where Udisc and Uball are the speed of the disc and ball on the contacting surfaces, respectively. Different entrainment (or mean) speed values were used for each operating temperature. The lowest value was 0.1 m/s for 40 °C, 0.25 m/s for 70 °C and 0.5 m/s for 100 °C. These conditions avoid working with very thin lubricant film thickness protecting the glass disc. The highest entrainment speed used was always limited to 2 m/s because the optical device has a maximum film thickness measurement range of around 1000 nm. The entrainment (or mean) speed is defined as:
(Udisc + Uball ) 2
(2)
2.3. Coefficient of friction measurements The coefficient of friction measurements were also performed on
Fig. 1. EHD2 ball-on-disc test rig (from PCS Instruments).
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Table 3 Disc properties.
Table 4 Pressure–viscosity coefficient determined based on film thickness measurements.
Parameters
Disc
Temperature (°C)
α × 108 (1/Pa) Polished Elastic modulus - E (GPa) Poisson coefficient – ν (–) Radius – R (mm) Surface roughness – Ra (μm)
210 0.29 50 0.023
40 1.447
70 1.265
100 1.157
Rough
Table 5 Pressure–viscosity coefficient (α × 108 [1/Pa]) determined based on film thickness measurements.
0.35
the EHD2 ball-on-disc described in the previous section. In this case, the ball runs against a steel disc and the applied load generates contact pressures up to 1.11 GPa. The ball and disc used were manufactured from carbon chrome steel, with 3/4 in (19.05 mm) and 100 mm diameters, respectively. The ball is the same used in the lubricant film thickness measurements, Table 2, and has an initial roughness (Ra) of 0.02 µm. The main disc properties were provided by the manufacturer and roughness was measured by the authors for both polished and rough samples (Table 3). The coefficient of friction for all lubricant samples, described in Table 1, were measured at the same three operating temperatures used in the previous section: 40, 70 and 100 °C. Using a polished disc, the friction properties were studied through Stribeck curves determined for two SRR values (5% and 50%) and entrainment speeds varying from 0.01 to 3 m/s. With a rough disc, the Stribeck curves were determined for an SRR value of 50% with entrainment speeds also varying from 0.01 to 3 m/s. For this test, the ball speed is made constant and the disc speed increases and decreases in order to obtain the same positive and negative SRR. The result is the average of both measurements. The ball speed changed from 0.01 to 3 m/s for both SRR values (5% and 50%). On one hand, the disc speed changed from lowest values of 0.009 and 0.01 m/s for both SRR values and highest values of 3.150 and 2.850 m/ s for SRR of 5% and 4.500 and 1.500 m/s for SRR of 50%.
Oil (°C)
Yubase4
05IL1
05IL2
10IL1
10IL2
40 70 100
1.9749 1.9685 1.7153
2.0316 1.9862 1.6953
2.0803 1.881 1.6661
2.2809 2.0368 1.5678
2.1545 2.2427 1.7872
α = s·ν t × 10−8
(3)
where s=9.904, t=0.139 and ν is the kinematic viscosity in (cSt). Using the procedure presented in [36], the pressure–viscosity coefficient was determined for each operating temperature based on the measurements. The results are presented in Table 5. It is interesting to note that all mixtures have higher pressure–viscosity coefficient than Yubase4 at 40 °C. The difference is higher for higher concentrations of ionic liquid. When temperature increases only the mixture with 1 wt% of [P66614][BEHP ] always presented higher pressure–viscosity than Yubase4. 3.2. Coefficient of friction The Stribeck curves for all lubricant samples were measured at temperatures of 40, 70 and 100 °C and SRR of 5% and 50%. The coefficient of friction was plotted against the modified Hersey number proposed by Brandão [37]. The modified Hersey number is given the Eq. (4), while the original Hersey number (H) is given by Eq. (5). Brandão suggested that the contact is under boundary film lubrication for a modified Hersey number Hp < 10−9 and under full film lubrication for Hp > 10−7.
3. Results and discussion
Hp =
3.1. Film thickness All lubricant samples showed similar film thickness values during the tests performed under full film lubrication (see Figs. 2a and b) according to their similar rheological properties listed in Table 1. Although the viscosity and the pressure–viscosity coefficient have a similar influence on the film thickness, different tribological behaviours were observed under mixed and boundary lubrication regimes for low viscosity base oils and their mixtures with substances with higher polarity and viscosity [33,34]. According to Gold's [35] equation (3), the pressure–viscosity coefficient (α) for a mineral base oil is given in Table 4.
H=
η·US ·α1/2 F1/2
η·US F
(4) (5)
where η is the oil dynamic viscosity at the inlet of the contact, US is the mean speed (Eq. (2)), α is the pressure–viscosity coefficient and F is the normal force. 3.2.1. Polished disc Under a SRR of 5% (see Figs. 3a–c and Fig. 4a–c), it can be observed that the addition of the ionic liquids at two concentrations (0.5 and 1 wt%) hardly changed the coefficient of friction of the
Fig. 2. Film thickness of the base oil and mixtures with the ionic liquid (a) [P66614][(iC 8)2PO2] and (b) [P66614][BEHP ] at SRR5%.
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Fig. 3. Friction characteristics of the base oil and mixtures with the ionic liquid [P66614][(iC 8)2PO2] tested with polished discs at SRR5% and SRR50%.
varied from full film lubrication to mixed lubrication under the entrainment speed range used for a SRR of 5%. Also, the toughest lubrication condition (i.e. highest temperature, 100 °C) shifted the contact to a mixed lubrication regime for all modified Hersey number values. However, for an SRR of 50% the tests always performed under a mixed lubrication regime.
mixtures in comparison with the base oil. Only at 100 °C did the mixtures with 1 wt% of [P66614][BEHP ] clearly show better friction behaviour at the lowest values of the modified Hersey number (i.e. low speeds), which is an interesting fact in order to reduce friction during start–stop cycles (boundary conditions). It should be stressed that the Stribeck curve for the neat base oil became erratic at SRR of 50%, 100 °C and lowest speed conditions (Fig. 3c or 4c). With increasing values of the modified Hersey number (i.e. higher speeds) the coefficient of friction tends to reach a similar value for all lubricant samples, which is related to their similar viscosity (see Table 1). However, under full film lubrication, the ionic liquid mixtures present a slightly higher coefficient of friction, mainly at 40 and 70 °C, which is in agreement with the highest pressure–viscosity coefficients extracted from film thickness measurements. According to the literature [37], higher pressure–viscosity coefficient means higher coefficient of friction under full film lubrication. On the other hand, the tribological behaviour of the base oil and the mixtures for a SRR of 50% was quite different (see Figs. 3d–f and 4d– f). In all cases the mixtures outperformed the tribological behaviour of the base oil showing a lower coefficient of friction at decreasing values of the modified Hersey number. This better tribological behaviour of the mixtures is clearly influenced by the SRR; therefore, the friction modifier action of the ionic liquids is more evident for higher SRR values. In general, it can also be observed that lubricant samples are operating in the full film lubrication regime when Hp > 10−7 and under mixed lubrication regime when 10−9 < Hp < 10−7. This conclusion fully agrees with Brandão [37] and Hammami [38]. The lubrication regime
3.2.2. Rough disc As expected the friction tests performed with rough discs generated higher friction values than those made with smooth discs. The composite roughness is σ ≈ 0.35 μm and the contact was always working under a boundary lubrication regime (λ < 1). The mixtures with the ionic liquid [P66614][(iC 8)2 PO2] at an SRR of 50% behaved similarly to the base oil with the exception of the test at 100 °C when both mixtures had a lower coefficient of friction than the base oil at decreasing values of the modified Hersey number (Figs. 5a– c). Meanwhile, the mixture at 0.5 wt% of the ionic liquid [P66614][BEHP ] showed the best tribological behaviour for all test conditions (Figs. 6a– c), and the mixture at 1.0 wt% only outperformed the base oil at highest temperature for low Hersey number values (i.e. low speeds). In this case, it seems that two simultaneous processes (faster roughness reduction and tribochemical) contributed to the better tribological behaviour of the mixtures. As temperature increases, it is clear that the ionic liquid activation with temperature clearly improves the friction behaviour of the base oil. This is particularly interesting for oil engine application usually operating around 100 °C [31] and under different sliding/rolling ratio conditions. Taking into account these facts, the tribological behaviour 236
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Fig. 4. Friction characteristics of the base oil and mixtures with the ionic liquid [P66614][BEHP ] tested with polished discs at SRR5% and SRR50%.
can be drawn:
and tribochemical interactions of these lubricant samples in a real application (rolling bearings) will be presented in the second part of this work.
•
4. Conclusions The film forming properties and friction characteristics of a hydrocracked mineral oil and its mixtures with two phosphonium cation-based ionic liquids used as additive at concentrations of 0.5 and 1 wt% were studied under two SRR values and three different temperatures. From the results obtained, the following conclusions
•
The addition of the ionic liquids at low concentrations hardly influenced the rheology of the base oil used; therefore, all the lubricant samples (base oil and its mixtures with both ionic liquids) showed similar lubricant film thickness in a full film lubrication regime. This behaviour was proved for all temperatures tested. The friction properties of the lubricant samples containing ionic liquids, as well as the base oil, are strongly influenced by testing conditions (temperature, speed, SRR) and the surface finish of the specimens.
Fig. 5. Friction characteristics of the base oil and mixtures with the ionic liquid [P66614][(iC 8)2PO2] tested with rough discs at SRR50%.
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Fig. 6. Friction characteristics of the base oil and mixtures with the ionic liquid [P66614][BEHP ] tested with rough discs at SRR50%.
•
• •
Tests made with polished discs took place under full film and mixed lubrication regimes for all tested temperatures at an SRR of 5% and the lubricant samples showed similar friction characteristics. The SRR value of 50% shifted the contact to a mixed lubrication regime showing the mixtures lower coefficient of friction than the base oil at low values of the modified Hersey number (i.e. low speeds), which is important during start–stop periods of machines. The friction tests made with rough discs were all performed under boundary and mixed lubrication regimes leading to different results according to the temperature values. In general, the mixtures showed better tribological behaviour than the base oil. In general, the mixtures showed a tribological improvement in comparison with the neat base oil. Testing them in a real application (e.g. rolling bearings) will be very interesting as the next research idea to be presented in the second part of this work.
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Acknowledgements The authors want to thank to the Spanish Ministry of Education, Culture and Sport for funding (ref: PR2015-00458) the research stay of Antolin Hernández Battez at Universidade do Porto (Portugal) within the programme ‘Salvador de Madariaga’. They also thank to the Spanish Ministry of Economy and Competitiveness and the Foundation for the Promotion in Asturias of Applied Scientific Research and Technology (FICYT) for supporting the research projects DPI2013-48348-C2-1-R and GRUPIN14-023, respectively. The authors gratefully acknowledge the funding supported by National Funds through Fundação para a Ciência e Tecnologia (FCT), under the project EXCL-II/SEM-PRO/0103/2012; NORTE-01-0145FEDER-000022 - SciTech – Science and Technology for Competitive and Sustainable Industries, cofinanced by Programa Operacional Regional do Norte (NORTE2020), through Fundo Europeu de Desenvolvimento Regional (FEDER) and LAETA under the project UID/EMS/50022/2013.
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