The Lubrication Ability of Ionic Liquids as Additives for Wind ... - MDPI

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The Lubrication Ability of Ionic Liquids as Additives for Wind Turbine Gearboxes Oils Miguel A. Gutierrez 1 , Michael Haselkorn 2 and Patricia Iglesias 1, * 1 2

*

Mechanical Engineering Department, Kate Gleason College of Engineering, Rochester Institute of Technology, Rochester, NY 14623, USA; [email protected] Golisano Institute for Sustainability, Rochester Institute of Technology, Rochester, NY 14623, USA; [email protected] Correspondence: [email protected]; Tel.: +1-585-475-7710

Academic Editors: Antolin Hernández Battez and Rubén González Rodríguez Received: 22 March 2016; Accepted: 22 April 2016; Published: 5 May 2016

Abstract: The amount of energy that can be gained from the wind is unlimited, unlike current energy sources such as fossil and coal. While there is an important push in the use of wind energy, gears and bearing components of the turbines often fail due to contact fatigue, causing costly repairs and downtime. The objective of this work is to investigate the potential tribological benefits of two phosphonium-based ionic liquids (ILs) as additives to a synthetic lubricant without additives and to a fully formulated and commercially available wind turbine oil. In this work, AISI 52100 steel disks were tested in a ball-on-flat reciprocating tribometer against AISI 440C steel balls. Surface finish also affects the tribological properties of gear surfaces. In order to understand the combined effect of using the ILs with surface finish, two surface finishes were also used in this study. Adding ILs to the commercial available or synthetic lubricant reduced the wear scar diameter for both surface finishes. This decrease was particularly important for trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) amide, where a wear reduction of the steel disk around 20% and 23% is reached when 5 wt % of this IL is added to the commercially available lubricant and to the synthetic lubricant without additives, respectively. Keywords: ionic liquids; wear; lubrication; steel-steel contact

1. Introduction Wind is a source of clean and sustainable energy. Wind energy does not produce harmful pollution gases such as carbon dioxide, sulfur dioxide, and other gases that have contributed to global warming. In addition, wind is a renewable source of energy, and a good alternative to current limited fossil fuel-generated electricity. While there is a big push in the use of wind energy, there is a downside regarding the cost of maintaining the wind turbines, more specifically the gearboxes [1,2]. Gears and bearing components often fail due to contact fatigue, causing costly repairs and downtime of the turbines [2]. The high cost of maintaining both land-based and off-shore wind turbines is a critical aspect of lowering the cost of wind energy and achieving the US Department of Energy (DOE) goal of generating 20% of the nation’s electricity from wind energy in 2030 [3]. Several lines of action can be followed to achieve this goal. Improving the overall gearbox design [4–6] and using surface treatments [2] or coatings [7] to increase the wear resistance of the materials used for the contacting components are two of the most common ways investigated to reduce the maintenance costs of the turbines and increase the overall reliability and efficiency of wind technology. In the last decade, the use of ionic liquids (ILs) has attracted attention in the tribology community as potential lubricants and lubricants additives for challenging contacts [8–11]. ILs, also known as fused salts and molten salts, are ionic compounds with meting points lower than 100 ˝ C. They are typically

Lubricants 2016, 4, 14; doi:10.3390/lubricants4020014

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composed of bulky asymmetric organic cations and a weakly coordinating anion. Their unique properties, including high thermal stability, non-volatility, non-flammability, high ionic conductivity, Lubricants 2016, 4, 14 2 of 13 wide electrochemical window, and miscibility with organic compounds make them ideal candidates typically composed applications. of bulky asymmetric organic cations weakly Their for many engineering In addition, ILs haveand the a ability tocoordinating form stableanion. ordered layers unique properties, including thermal stability, the non‐volatility, non‐flammability, high ionic and protective tribo-films [12–14]high in the area between two materials in contact, reducing friction conductivity, wide electrochemical window, and miscibility with organic compounds make them and wear. ideal candidates for many engineering applications. In addition, ILs have the ability to form stable Some researchers [15,16] have shown that the addition of small amounts of halogenated ILs to ordered layers and protective tribo‐films [12–14] in the area between the two materials in contact, mineral and synthetic wind turbine oils has a positive effect on the wear behavior of materials in reducing friction and wear. contact. However, halogen-containing anions are very sensitive to moisture and, in the presence Some researchers [15,16] have shown that the addition of small amounts of halogenated ILs to of water, will react, causing: (1)turbine the corrosion metallic surfaces formation halides mineral and synthetic wind oils has a of positive effect on the by wear behavior of of metallic materials in and (2) the evolution of toxic species (HF) from hydrolysis of the anions [17,18]. Recent literature contact. However, halogen‐containing anions are very sensitive to moisture and, in the presence of water, will react, causing: (1) the corrosion of metallic surfaces by formation of metallic halides and has suggested the potential of using ILs with halogen-free anions [11,19,20]; however, studies on the (2) the evolution of toxic species from hydrolysis of the anions [17,18]. Recent literature has tribological performance of these ILs(HF) as additives of lubricants for wind turbine applications are still suggested the potential of using ILs with halogen‐free anions [11,19,20]; however, studies on the very scarce. tribological performance of these ILs as additives of lubricants for wind turbine applications are still The objective of this work was to investigate the potential tribological benefits of the use of two very scarce. phosphonium-based ILs (one of which is halogen-free) as additives of lubricants, in combination with The objective of this work was to investigate the potential tribological benefits of the use of two two surface finishes, on gearboxes of both land-based and off-shore wind turbines. The ILs were added phosphonium‐based ILs (one of which is halogen‐free) as additives of lubricants, in combination with to a synthetic oil finishes, without on additives andof toboth a fully formulated commercially available wind turbine two surface gearboxes land‐based and and off‐shore wind turbines. The ILs were lubricant. The effect of the IL concentration was also studied and compared to the currently available added to a synthetic oil without additives and to a fully formulated and commercially available wind gearbox oil. turbine lubricant. The effect of the IL concentration was also studied and compared to the currently available gearbox oil.

2. Experimental 2. Experimental

AISI 52100 steel disks were tested in a ball-on-flat reciprocating (Figure 1) tribometer against AISI 440CAISI 52100 steel disks were tested in a ball‐on‐flat reciprocating (Figure 1) tribometer against steel. Two different surface finishes (super-finish, Ra « 0.02 µm and normal ground, AISI 440C steel. Two different surface finishes (super‐finish, Ra ≈ 0.02 μm and normal ground, Ra ≈ Ra « 0.1 µm) were used for the disks. Table 1 summarizes the properties and dimensions of the 0.1 μm) were used for the disks. Table 1 summarizes the properties and dimensions of the materials materials used in this study. The following experimental parameters were kept constant for all tests: used in this study. The following experimental parameters were kept constant for all tests: normal normal load = 14.7 N (corresponding to mean Hertzian contact pressure = 1.03 GPa and maximum load = 14.7 N (corresponding to mean Hertzian contact pressure = 1.03 GPa and maximum Hertzian Hertzian contact pressure = GPa), 1.55 GPa), frequency = 50amplitude Hz, amplitude = 0.8 sliding mm, sliding distance 288 m, contact pressure = 1.55 frequency = 50 Hz, = 0.8 mm, distance = 288 =m, ˝ C, and relative humidity = 30%–35%. temperature = 23 temperature = 23 °C, and relative humidity = 30%–35%.

Figure 1. Ball‐on‐flat tester with reciprocating motion. Figure 1. Ball-on-flat tester with reciprocating motion.

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Table 1. Properties and dimensions of the materials used in this study. Table 1. Properties and dimensions of the materials used in this study. Table 1. Properties and dimensions of the materials used in this study. Table 1. Properties and dimensions Disk of the materials used in this study. Ball

Material Material Material Hardness (HV) Hardness (HV) Hardness (HV) Material Hardness (HV) Dimension (mm) Dimension (mm) Dimension (mm) Dimension (mm)

Disk Disk BallBall AISI 52100 steel AISI 440C Disk Ball AISI 52100 steel AISI 440C AISI 52100 steel AISI 440C 500 690 500 500 steel 690 690 AISIDiameter = 19 52100 AISI 440C 500 690 Spherical radius = 3 Diameter = 19 Diameter = 19 Thickness = 7 Spherical radius = 3 Spherical radius = 3 Diameter = 19 Thickness = 7 Spherical radius = 3 Thickness = 7 Thickness = 7

ILs used in this study were commercially available from Sigma‐Aldrich (USA). Their molecular ILs used in this study were commercially available from Sigma‐Aldrich (USA). Their molecular ILs used in this study were commercially available from Sigma‐Aldrich (USA). Their molecular structure, name, and abbreviation (code) are shown in Table 2. Both ILs have identical cations with ILs used in this study were commercially available from Sigma-Aldrich (USA). Their molecular structure, name, and abbreviation (code) are shown in Table 2. Both ILs have identical cations with structure, name, and abbreviation (code) are shown in Table 2. Both ILs have identical cations with varying anion structures. Lubricating mixtures were prepared by adding 2.5 or 5 wt % ratio of ILs to structure, name, and abbreviation (code) are shown in Table 2. Both ILs have identical cations with varying anion structures. Lubricating mixtures were prepared by adding 2.5 or 5 wt % ratio of ILs to varying anion structures. Lubricating mixtures were prepared by adding 2.5 or 5 wt % ratio of ILs to a polyalphaolefin—Synton PAO‐40 (PAO)—base stock and to a commercially available, fully varying anion structures. Lubricating mixtures were prepared adding 2.5 or 5available, wtavailable, % ratio offully a polyalphaolefin—Synton PAO‐40 (PAO)—base stock to a commercially a polyalphaolefin—Synton PAO‐40 (PAO)—base stock and and to bya commercially fully formulated, wind turbine gearbox lubricant—Mobilgear SHC XMP 320 (MG). Before each test, steel ILs to a polyalphaolefin—Synton PAO-40 (PAO)—base stock and to a commercially available, fully formulated, wind turbine gearbox lubricant—Mobilgear SHC XMP 320 (MG). Before each test, steel formulated, wind turbine gearbox lubricant—Mobilgear SHC XMP 320 (MG). Before each test, steel disks were covered with 2 mL of the lubricant, and no additional lubricant was added during the test. formulated, wind turbine gearbox lubricant—Mobilgear SHC XMP 320 (MG). Before each test, steel disks were covered with 2 mL of the lubricant, and no additional lubricant was added during the test. disks were covered with 2 mL of the lubricant, and no additional lubricant was added during the test. Optical micrographs of wear track were obtained using a Zeiss 3‐D stereoscope, and SEM images disks were covered with 2 mL of the lubricant, and no additional lubricant was added during the Optical micrographs of wear track were obtained using a Zeiss 3‐D stereoscope, and SEM images Optical micrographs of wear track were obtained using a Zeiss 3‐D stereoscope, and SEM images were obtained using an AMRAY 1830 scanning electron microscope with energy‐dispersive X‐ray test. Optical micrographs of wear track were obtained using a Zeiss 3-D stereoscope, and SEM images were obtained using an AMRAY 1830 scanning electron microscope with energy‐dispersive X‐ray were obtained using an AMRAY 1830 scanning electron microscope with energy‐dispersive X‐ray spectroscopy (EDS) capability. were obtained using an AMRAY 1830 scanning electron microscope with energy-dispersive X-ray spectroscopy (EDS) capability. spectroscopy (EDS) capability. spectroscopy (EDS) capability. Table 2. Molecular structure, name, and abbreviation of ionic liquids (ILs) used in this study. Table 2. Molecular structure, name, and abbreviation of ionic liquids (ILs) used in this study. Table 2. Molecular structure, name, and abbreviation of ionic liquids (ILs) used in this study. Table 2. Molecular structure, name, and abbreviation of ionic liquids (ILs) used in this study. Structure

Code Code Code Code

[THTDP][Deca] [THTDP][Deca] [THTDP][Deca] [THTDP][Deca]

[THTDP][NTf2] [THTDP][NTf2]

[THTDP][NTf2] [THTDP][NTf2]

Structure Structure Cation Anion Cation Structure Anion Anion Cation Cation Anion

IUPAC name IUPAC name IUPAC name IUPAC name

Trihexyltetradecylphosphonium Trihexyltetradecylphosphonium Trihexyltetradecylphosphonium Trihexyltetradecylphosphonium decanoate decanoate decanoate decanoate Trihexyltetradecylphosphonium Trihexyltetradecylphosphonium Trihexyltetradecylphosphonium Trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) bis(trifluoromethylsulfonyl) amide bis(trifluoromethylsulfonyl) bis(trifluoromethylsulfonyl) amide amide amide

Upon completion of a test, the wear ball and disk were removed from the tribometer and cleaned Upon completion of a test, the wear ball and disk were removed from the tribometer and cleaned Upon completion of a test, the wear ball and disk were removed from the tribometer and cleaned Upon completion of a test, the wear ball and disk were removed from the tribometer and cleaned with isopropyl alcohol. TheThe major andand minor axes of the wear scarscar were measured according to to with isopropyl alcohol. major minor axes of the wear were measured according with isopropyl alcohol. The major and minor axes of the wear scar were measured according with isopropyl alcohol. The major and minor axes of the wear scar were measured according to to ASTM-D6079. Three tests were run for each lubricant mixture, and the reported value is the average ASTM‐D6079. Three tests were run for each lubricant mixture, and the reported value is the average ASTM‐D6079. Three tests were run for each lubricant mixture, and the reported value is the average ASTM‐D6079. Three tests were run for each lubricant mixture, and the reported value is the average value of the three tests. value of the three tests. value of the three tests. value of the three tests. Densities at room temperature were determined by weighing a known volume. Viscosities at Densities at room temperature were determined by weighing a known volume. Viscosities at Densities at room temperature were determined by weighing a known volume. Viscosities at 40 ˝40 °C and 100 °C were obtained using a LVDV2T Brookfield viscometer with a Thermosel System CDensities at room temperature were determined by weighing a known volume. Viscosities at and 100 ˝ C were obtained using a LVDV2T Brookfield viscometer with a Thermosel System 40 °C and 100 °C were obtained using a LVDV2T Brookfield viscometer with a Thermosel System 40 °C and 100 °C were obtained using a LVDV2T Brookfield viscometer with a Thermosel System (Model 106) for elevated temperature testing. The thermal stabilities of the lubricants were studied (Model 106) for elevated temperature testing. The thermal stabilities of the lubricants were studied (Model 106) for elevated temperature testing. The thermal stabilities of the lubricants were studied (Model 106) for elevated temperature testing. The thermal stabilities of the lubricants were studied using a TA Instruments TGA-2950 at a 10 ˝ C/min heating rate in a nitrogen atmosphere (flow rate of using a TA Instruments TGA‐2950 at a 10 °C/min heating rate in a nitrogen atmosphere (flow rate of using a TA Instruments TGA‐2950 at a 10 °C/min heating rate in a nitrogen atmosphere (flow rate of using a TA Instruments TGA‐2950 at a 10 °C/min heating rate in a nitrogen atmosphere (flow rate of 20 mL/min). 20 mL/min). 20 mL/min). 20 mL/min). 3. Results 3. Results 3. Results 3. Results 3.1. Viscosity, Density, and Thermal Stability of the Lubricants 3.1. Viscosity, Density, and Thermal Stability of the Lubricants 3.1. Viscosity, Density, and Thermal Stability of the Lubricants 3.1. Viscosity, Density, and Thermal Stability of the Lubricants The density at 23 ˝ C and dynamic viscosity at 40 ˝ C and 100 ˝ C of the lubricants used in this study The density at 23 °C and dynamic viscosity at 40 °C and 100 °C of the lubricants used in this are listed in Table 3. PAO and MG have similar densities at 23 ˝ C. While adding [THTDP][NTf2] did The density at 23 °C and dynamic viscosity at 40 °C and 100 °C of the lubricants used in this The density at 23 °C and dynamic viscosity at 40 °C and 100 °C of the lubricants used in this study are listed in Table 3. PAO and MG have similar densities at 23 °C. While adding [THTDP][NTf2] not have any effect on the density of the base oils, the addition of [THTDP][Deca] slightly increased study are listed in Table 3. PAO and MG have similar densities at 23 °C. While adding [THTDP][NTf2] study are listed in Table 3. PAO and MG have similar densities at 23 °C. While adding [THTDP][NTf2] did not have any effect on the density of the base oils, the addition of [THTDP][Deca] slightly the density ofhave the mixtures. as seen inoils, Table 3, PAO isof a [THTDP][Deca] little more viscous than did any effect on the density of be the base addition of [THTDP][Deca] slightly did increased the density of the mixtures. Additionally, as can be seen in Table 3, PAO is a little more not not have any effect on Additionally, the density of can the base oils, the the addition slightly MG. The addition of ILs at such low concentrations caused little change in both oil viscosities. increased the density of the mixtures. Additionally, as can be seen in Table 3, PAO is a little more increased the density of the mixtures. Additionally, as can be seen in Table 3, PAO is a little more viscous than MG. The addition of ILs at such low concentrations caused little change in both oil viscous than MG. addition of ILs at such concentrations caused little change in both viscous than MG. The The addition of ILs at such low low concentrations caused little change in both oil oil viscosities. viscosities. viscosities. The thermogravimetric analysis (TGA) curves of the lubricants are shown in Figure 2. The onset The thermogravimetric analysis (TGA) curves of the lubricants are shown in Figure 2. The onset The thermogravimetric analysis (TGA) curves of the lubricants are shown in Figure 2. The onset decomposition temperatures for MG and PAO are 291 and 295 °C, respectively. The addition of the decomposition temperatures for MG and PAO are 291 and 295 °C, respectively. The addition of the decomposition temperatures for MG and PAO are 291 and 295 °C, respectively. The addition of the ILs, in most cases, slightly decreased the thermal stability of the lubricants. In either case, degradation ILs, in most cases, slightly decreased the thermal stability of the lubricants. In either case, degradation ILs, in most cases, slightly decreased the thermal stability of the lubricants. In either case, degradation for the mixtures does not reach 1% weight loss until 270 °C. for the mixtures does not reach 1% weight loss until 270 °C. for the mixtures does not reach 1% weight loss until 270 °C.


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Table 3. Density and viscosity values of the lubricants (* Viscosity at 70 ˝ C). Lubricants 2016, 4, 14

Lubricant

Density at 20 ˝ C (g/cm3 )

Dynamic Viscosity (cP)4 of 13 40 ˝ C

100 ˝ C

284

31

Table 3. Density and viscosity values of the lubricants (* Viscosity at 70 °C).

MG PAO Lubricant [THTDP][Deca] [THTDP][NTf2] MG MG + 2.5%[THTDP][Deca] PAO MG + 5%[THTDP][Deca] MG + 2.5%[THTDP][NTf2] [THTDP][Deca] MG + 5%[THTDP][NTf2] [THTDP][NTf2] PAO + 2.5%[THTDP][Deca] PAO MG + 2.5%[THTDP][Deca] + 5%[THTDP][Deca] PAO + MG + 5%[THTDP][Deca] 2.5%[THTDP][NTf2] PAO MG + 2.5%[THTDP][NTf2] + 5%[THTDP][NTf2] MG + 5%[THTDP][NTf2]

0.86 0.84 Density at 20 °C (g/cm3) 0.88 1.07 0.86 0.86 0.860.84 0.870.88 0.87 1.07 0.84 0.840.86 0.850.86 0.850.87 0.87

Dynamic Viscosity (cP) 331 32

1600 * 40 °C 141 284 285 331 297 288 1600 * 290 141 343 285 359 332 297 332 288

100 °C 17

290

32

31 32 17 17 31 32 32

17 31 32 32 32 32 32 32 32

The thermogravimetric analysis (TGA) curves 0.84 of the lubricants are shown in Figure 2. The onset PAO + 2.5%[THTDP][Deca] 343 32 ˝ C, respectively. The addition of the decomposition temperatures for MG and PAO are 291 and 295 PAO + 5%[THTDP][Deca] 0.84 359 32 ILs, in most cases, slightly decreased the thermal stability of the lubricants. In either case, degradation PAO + 2.5%[THTDP][NTf2] 0.85 332 32 ˝ C. for the mixtures does not reach 1% weight loss until 270 PAO + 5%[THTDP][NTf2] 0.85 332 32

(a)

(b) Figure 2. Weight loss vs. temperature thermogravimetric analysis (TGA) curves for the lubricants: (a) Mobilgear SHC XMP 320 (MG) and MG mixtures with ILs; (b) Synton PAO-40 (PAO) and PAO mixtures with ILs.


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Figure 2. Weight loss vs. temperature thermogravimetric analysis (TGA) curves for the lubricants: (a) Mobilgear SHC XMP 320 (MG) and MG mixtures with ILs; (b) Synton PAO‐40 (PAO) and PAO Lubricants 2016, 4, 14 5 of 12 mixtures with ILs.

3.2. Effect of Ionic Liquid 3.2. Effect of Ionic Liquid Figure 3 shows the wear scar diameters on AISI 52100 steel flat disks with normal ground finish Figure 3 shows the wear scar diameters on AISI 52100 steel flat disks with normal ground finish (Ra ≈ 0.1 μm) after ball‐on‐flat reciprocating tests, using different lubricant mixtures. As expected, (Ra « 0.1 µm) after ball-on-flat reciprocating tests, using different lubricant mixtures. As expected, when PAO is used as lubricant, the steel disks presented higher wear than when commercially when PAO is used as lubricant, the steel disks presented higher wear than when commercially available available MG is used. The reason for the larger diameters is the PAO does not contain any additive, MG is used. The reason for the larger diameters is the PAO does not contain any additive, while the while the additive package contained in the MG reduces friction and wear. Figure 3 also shows that additive package contained in the MG reduces friction and wear. Figure 3 also shows that adding ILs adding ILs to MG and PAO reduced the wear scar diameter. Comparing the two ILs, the decrease in to MG and PAO reduced the wear scar diameter. Comparing the two ILs, the decrease in the wear the wear scar diameter is greater for the [THTDP][NTf2], where a wear reduction of 20% and 23% is scar diameter is greater for the [THTDP][NTf2], where a wear reduction of 20% and 23% is reached reached when 5 wt % of this IL is added to MG and PAO, respectively. In general, when when 5 wt % of this IL is added to MG and PAO, respectively. In general, when [THTDP][NTf2] [THTDP][NTf2] is used as an additive to both oils, increasing the ratio of the IL, wear of the steel is used as an additive to both oils, increasing the ratio of the IL, wear of the steel disks decreased. disks decreased. However, if [THTDP][Deca] is added, the effect of increasing the IL ratio is opposite However, if [THTDP][Deca] is added, the effect of increasing the IL ratio is opposite in both oils. A in both oils. A slight improvement of disk wear is observed when the ratio of [THTDP][Deca] is slight improvement of disk wear is observed when the ratio of [THTDP][Deca] is increased from increased from 2.5 to 5 wt % in MG. However, when the same ratio of this IL is increased in PAO, the 2.5 to 5 wt % in MG. However, when the same ratio of this IL is increased in PAO, the wear of the steel wear of the steel disk increased. disk increased.

Figure 3. Wear scar scar diameters diameters on on AISI AISI 52100 52100 steel steel flat flat disks disks (Ra 0.1 μm) µm) after after ball-on-flat Figure 3. Wear (Ra « ≈ 0.1 ball‐on‐flat reciprocating tests. reciprocating tests.

Figure shows wear wear scars scars on on AISI AISI 52100 52100 steel steel flat flat disks disks (Ra (Ra « ≈ 0.1 ball‐on‐flat Figure 4 4 shows 0.1 μm) µm) after after a a ball-on-flat reciprocating test using MG, MG + 5% [THTDP][NTf2], PAO, and PAO + 5% [THTDP][NTf2] as reciprocating test using MG, MG + 5% [THTDP][NTf2], PAO, and PAO + 5% [THTDP][NTf2] as lubricants. From the figure, the addition of the IL to the oils not only reduced the diameter of the lubricants. From the figure, the addition of the IL to the oils not only reduced the diameter of the wear scar, but also its depth. When the IL is not present, the worn surface presented severe grooving wear scar, but also its depth. When the IL is not present, the worn surface presented severe grooving and plastic deformation, particularly for PAO (Figure 4c). The presence of cracks (Figure 5a) also and plastic deformation, particularly for PAO 4c). The oils presence of ILs cracks 5a) also confirms a fatigue component of the wear of the(Figure surfaces when without are (Figure used. However, confirms a fatigue component of the wear of the surfaces when oils without ILs are used. However, when [THTDP][NTf2] is added, a more superficial and smoother wear track is obtained, and the occurrence of fatigue cracks is reduced 5b). and It has been reported [12–21] that ILs have the when [THTDP][NTf2] is added, a more (Figure superficial smoother wear track is obtained, and the ability to form highly ordered absorbed layers on metal surfaces. This layer prevents direct contact occurrence of fatigue cracks is reduced (Figure 5b). It has been reported [12–21] that ILs have the between mating surfaces, reducing friction and wear. In this study, the presence of F and S on the worn ability to form highly ordered absorbed layers on metal surfaces. This layer prevents direct contact surface (Figure 5b) after a test using [THTDP][NTf2] as an additive of PAO suggests the formation of a between mating surfaces, reducing friction and wear. In this study, the presence of F and S on the tribo-film on the steel disk surface that reduces the wear. worn surface (Figure 5b) after a test using [THTDP][NTf2] as an additive of PAO suggests the formation of a tribo‐film on the steel disk surface that reduces the wear.

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(a)

(b)

(c)

(d)

Figure 4. Wear scars on AISI 52100 steel flat disks (Ra ≈ 0.1 μm) after a ball‐on‐flat reciprocating test Figure 4. Wear scars on AISI 52100 steel flat disks (Ra « 0.1 µm) after a ball-on-flat reciprocating test using the following lubricants: (a) MG; (b) MG + 5% [THTDP][NTf2]; (c) PAO; and (d) PAO + 5% using the following lubricants: (a) MG; (b) MG + 5% [THTDP][NTf2]; (c) PAO; and (d) PAO + 5% Lubricants 2016, 4, 14 7 of 13 [THTDP][NTf2]. [THTDP][NTf2].

(a)

(b) Figure 5. Cont. F


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(b)

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(a) F

S

(b)

Figure SEMmicrographs micrographsand and energy‐dispersive energy-dispersive X-ray (EDS) analysis of of thethe wear scars Figure 5. 5.SEM X‐ray spectroscopy spectroscopy (EDS) analysis wear F on AISI 52100 steelsteel flat disks (Ra «(Ra 0.1 µm) ball-on-flat reciprocating test using the following scars on AISI 52100 flat disks ≈ 0.1 after μm) aafter a ball‐on‐flat reciprocating test using the lubricants: (a) PAO; (b) PAO + 5% [THTDP][NTf2]. following lubricants: (a) PAO; (b) PAO + 5% [THTDP][NTf2].

3.3. Effect of Ionic Liquid 3.3. Effect of Ionic Liquid Figures 6 and 7 show the wear scar diameters on AISI steel flat disks for both surface Figures 6 and 7 show the wear scar diameters on AISI 52100 steel flat disks for both surface S 52100 finishes and both additives after a ball‐on‐flat reciprocating test using MG and PAO as base lubricants, finishes and both additives after a ball-on-flat reciprocating test using MG and PAO as base lubricants, respectively. addition of of ILs to to both oils not only reduced the wear diameter on onsteel respectively.The The addition ILs both oils not only reduced the wear diameter steeldisks disks (Figures 6 and 7) but also smoothened the worn surface (Figure 8), resulting in a much milder wear (Figures 6 and 7) but also smoothened the worn surface (Figure 8), resulting in a much milder Figure 5. SEM micrographs and energy‐dispersive spectroscopy (EDS) analysis of the wear both IL mechanism. When MG was used as used the base of X‐ray the mixtures (Figure 6), increasing wear mechanism. When MG was as theoil base oil of the mixtures (Figure 6), increasing both scars on AISI 52100 steel flat disks (Ra ≈ 0.1 μm) after a ball‐on‐flat reciprocating test using the concentrations resulted in a decrease in wear. However, as mentioned in Section 3.1, increasing the IL concentrations resulted in a decrease in wear. However, as mentioned in Section 3.1, increasing following lubricants: (a) PAO; (b) PAO + 5% [THTDP][NTf2]. [THTDP][Deca] ratio in PAO (Figure 7) had a negative effect on the wear of the disk for both surface the [THTDP][Deca] ratio in PAO (Figure 7) had a negative effect on the wear of the disk for both finishes. surface finishes. 3.3. Effect of Ionic Liquid In addition, studies have shown that super-finishing the working surfaces of mechanical Figures 6 and 7 show the wear scar diameters on AISI 52100 steel flat disks for both surface components can reduce friction, pitting fatigue and wear [22,23]. As can be seen in Figures 6 and 7 finishes and both additives after a ball‐on‐flat reciprocating test using MG and PAO as base lubricants, (a) a baseThe (b) only respectively. when oil addition is used with super-finishing thewear surface of theon steel had a of ILs no to additives, both oils not only reduced the diameter steel disk disks positive effect on wear. When the lubricant contains additives, i.e., either ILs Super-finish or additives present in (Figures 6 and 7) but also smoothened the worn surface (Figure 8), resulting in a much milder wear Super-finish mechanism. When improving MG was used as the base oil (Ra of the mixtures (Figure 6), increasing IL wear the MG formulation, the surface finish « 0.02 µm) of the disk produces both larger tracksconcentrations resulted in a decrease in wear. However, as mentioned in Section 3.1, increasing the (Figure 8) than those obtained with a normal finish (Ra « 0.1 µm) on the surface. This suggests [THTDP][Deca] ratio in PAO (Figure 7) had a negative effect on the wear of the disk for both surface that additives need a certain level of asperities to form a tribo-layer that improves the wear properties finishes. Normal ground of the metals. Normal ground

(a)

(b) Super-finish

Super-finish

Normal ground

Normal ground

Figure 6. Wear scar diameters on AISI 52100 steel flat disks for both surface finishes after a ball-on-flat reciprocating test using MG as base lubricant and (a) [THTDP][NTf2], and (b) [THTDP][Deca] as an additive.


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Figure 6. Wear scar diameters on AISI 52100 steel flat disks for both surface finishes after a ball‐on‐ flat reciprocating test using MG as base lubricant and (a) [THTDP][NTf2], and (b) [THTDP][Deca] as Lubricants 2016, 4, 14 8 of 13 Lubricants 2016, 4, 14 8 of 12 an additive. Figure 6. Wear scar diameters on AISI 52100 steel flat disks for both surface finishes after a ball‐on‐ flat reciprocating test using MG as base lubricant and (a) [THTDP][NTf2], and (b) [THTDP][Deca] as (a) (b) an additive.

Super-finish

(a)

Super-finish

(b)

Super-finish

Super-finish

Normal ground

Normal ground

Normal ground

Normal ground

Figure 7. Wear scar diameters on AISI 52100 steel flat disks for both surface finishes after a ball‐on‐ Figure 7. Wear scar diameters on AISI 52100 steel flat disks for both surface finishes after a ball-on-flat flat reciprocating test using PAO as base lubricant and (a) [THTDP][NTf2], and (b) [THTDP][Deca] reciprocating test using PAO as base lubricant and (a) [THTDP][NTf2], and (b) [THTDP][Deca] as Figure 7. Wear scar diameters on AISI 52100 steel flat disks for both surface finishes after a ball‐on‐ as an additive. flat reciprocating test using PAO as base lubricant and (a) [THTDP][NTf2], and (b) [THTDP][Deca] an additive. as an additive.

(a

(b a

b

Normal ground

Super-finish

Normal ground

Super-finish (c

c

d

(d

Figure 8. Wear scars on AISI 52100 steel flat disks after a ball-on-flat reciprocating test using: (a) and (b) MG + 5% [THTDP][Deca]; (c) and (d) PAO + 5% [THTDP][NTf2].

As can be seen in Figures 9 and 10 when MG and PAO are used without ILs, abrasive wear and plastic deformation, particularly for PAO (Figure 10), occurred. In addition, higher magnification

positive effect on wear. When the lubricant contains additives, i.e., either ILs or additives present in the MG formulation, improving the surface finish (Ra ≈ 0.02 μm) of the disk produces larger wear tracks (Figure 8) than those obtained with a normal finish (Ra ≈ 0.1 μm) on the surface. This suggests that additives need a certain level of asperities to form a tribo‐layer that improves the wear properties of the metals. Lubricants 2016, 4, 14 9 of 12 As can be seen in Figures 9 and 10, when MG and PAO are used without ILs, abrasive wear and plastic deformation, particularly for PAO (Figure 10), occurred. In addition, higher magnification imaging of these worn surfaces showed that there was surface fatigue in these regions. Figure 9 imaging of these worn surfaces showed that there was surface fatigue in these regions. Figure 9 also also shows EDS analysis of the wear scars AISI 52100 steel disks « 0.02 after shows the the EDS analysis of the wear scars on on AISI 52100 steel flat flat disks (Ra (Ra ≈ 0.02 μm) µm) after a a reciprocating test using MG and MG + 5% [THTDP][NTf2]. TheThe presence of sulfur on the surface reciprocating test using MG and MG + 5% [THTDP][NTf2]. presence of sulfur on worn the worn suggests that this element, present in the MG formulation, reacted with the steel to form a sulfide surface suggests that this element, present in the MG formulation, reacted with the steel to form a layer [24] on the surface of the disk, reducing the worn area and the presence of cracks inside the sulfide layer [24] on the surface of the disk, reducing the worn area and the presence of cracks inside the track. wear track. The combination of [THTDP][NTf2] with the additives had a wear The combination of [THTDP][NTf2] with the additives presentpresent in MG in hadMG a synergistic synergistic effect [15,25], producing a smoother and smaller wear track (Figure 9). effect [15,25], producing a smoother and smaller wear track (Figure 9).

(a)

S

(b) F

S

Figure 9. SEM micrographs and EDS analysis of the wear scars on AISI 52100 steel flat disks (Ra ≈ 0.02 Figure 9. SEM micrographs and EDS analysis of the wear scars on AISI 52100 steel flat disks μm) after µm) a ball‐on‐flat reciprocating test using the following lubricants: (a) MG; (b) MG + (Ra « 0.02 after a ball-on-flat reciprocating test using the following lubricants: (a)5% MG; Lubricants 2016, 4, 14 10 of 13 [THTDP][NTf2]. (b) MG + 5% [THTDP][NTf2].

It should also be noted that, under the experimental conditions studied, the AISI 440C steel balls (a) presented no apparent wear loss, but showed an adhered layer of steel particles from the disk (Figure 11) when IL was not present in the lubricant.

(b)

Figure 10. Cont.

(a) Lubricants 2016, 4, 14

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(b)

(b)

Figure 10. SEM micrographs of the wear scars on AISI 52100 steel flat disks (Ra ≈ 0.02 μm) after a ball‐ Figure 10. SEM micrographs of the wear scars on AISI 52100 steel flat disks (Ra « 0.02 µm) after a on‐flat reciprocating test using the following lubricants: (a) PAO; (b) PAO + 5% [THTDP][NTf2]. ball-on-flat reciprocating test using the following lubricants: (a) PAO; (b) PAO + 5% [THTDP][NTf2].

It should aalso be noted that, under the experimental 440C steel b conditions studied, the AISI balls presented no apparent wear loss, but showed an adhered layer of steel particles from the disk Figure 10. SEM micrographs of the wear scars on AISI 52100 steel flat disks (Ra ≈ 0.02 μm) after a ball‐ (Figure 11) when IL was not present in the lubricant. on‐flat reciprocating test using the following lubricants: (a) PAO; (b) PAO + 5% [THTDP][NTf2].

a

b

Super-finish c

Normal ground d

Super-finish c

Normal ground d

Figure 11. Optical micrographs of steel balls after a test lubricated with: (a) and (b) PAO; (c) and (d) PAO + 5% [THTDP][NTf2].

4. Conclusions This work investigates the potential tribological benefits of the use of two phosphonium-based ILs as additives of lubricants, in combination with two surface finishes, on gearboxes of both land-based and off-shore wind turbines. Adding ILs to MG and PAO reduced the wear scar diameter for both surface finishes. This decrease is particularly important for [THTDP][NTf2], where a wear reduction of the steel disk


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(Ra « 0.1 µm) around 20% and 23% is reached when 5 wt % of this IL is added to MG and PAO, respectively. When MG is used as the base oil of the mixtures, increasing both IL concentrations results in a decrease in wear. However, increasing the [THTDP][Deca] ratio in PAO has a negative effect on the wear of the disk for both surface finishes. When the lubricant contains additives, i.e., either ILs or additives present in the MG formulation, improving the surface finish of the disk produces larger wear tracks than those obtained with a normal finish on the surface. This suggests that additives need a certain level of asperities to form a tribo-layer that improves the wear properties of the metals. When the IL is not present, the worn surface presented severe grooving and plastic deformation, particularly for PAO. In addition, the presence of cracks confirms a fatigue component of the wear of the surfaces when oils without ILs are used. The presence of F and S on the worn surface after a test using [THTDP][NTf2] as an additive of PAO suggests the formation of a tribo-film on the steel disk surface that reduces the wear. The combination of [THTDP][NTf2] with the additives present in MG had a synergistic effect producing a smoother and smaller wear track. Acknowledgments: Funding provided by the NYS Pollution Prevention Institute through a grant from the NYS Department of Environmental Conservation. Any opinions, findings, conclusions, or recommendations expressed are those of the author(s) and do not necessarily reflect the views of the Department of Environmental Conservation. Author Contributions: Miguel A. Gutierrez performed the experiments under the supervision of Patricia Iglesias and Michael Haselkorn. All authors contributed equally to the design of the experiments and analysis of the data. Patricia Iglesias wrote the paper with cooperation of Michael Haselkorn. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript: IL [THTDP][NTf2] [THTDP][Deca] SEM EDS PAO TGA MG

Ionic Liquid Trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) amide Trihexyltetradecylphosphonium decanoate Scanning electron microscope Energy-dispersive X-ray spectroscopy Polyalphaolefin Thermogravimetric analysis Mobilgear SHC XMP 320

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