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May 5, 2016 - Miguel A. Gutierrez 1, Michael Haselkorn 2 and Patricia Iglesias 1,* ..... Greco, A.; Sheng, S.; Keller, J.; Erdemir, A. Material wear and fatigue in ...
lubricants Article

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

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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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Greco, A.; Sheng, S.; Keller, J.; Erdemir, A. Material wear and fatigue in wind turbine Systems. Wear 2013, 302, 1583–1591. [CrossRef] Greco, A.; Mistry, K.; Sista, V.; Eryilmaz, O.; Erdemir, A. Friction and wear behaviour of boron based surface treatment and nano-particle lubricant additives for wind turbine gearbox applications. Wear 2011, 271, 1754–1760. [CrossRef] U.S. Department of Energy. 20% Wind Energy by 2030. Increasing Wind Energy’s Contribution to U.S. Electricity Supply; US Department of Energy, National Renewable Energy Laboratory, Co.: Oak Ridge, TN, USA, 2008. Giger, D.U. Redesign of a Gearbox for 5 MW Wind Turbines. In Proceedings of ASME 2011 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference, Washington, DC, USA, 28–31 August 2011; pp. 635–641. Hall, J.F.; Mecklenborg, C.A.; Chen, D.; Pratap, S.B. Wind energy conversion with a variable-ratio gearbox: Design and analysis. Renew. Energy 2011, 36, 1075–1080. [CrossRef]

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