Effects of protic ionic liquid crystal additives on the water-lubricated sliding wear and friction of sapphire against stainless steel

Effects of protic ionic liquid crystal additives on the water-lubricated sliding wear and friction of sapphire against stainless steel

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Author’s Accepted Manuscript Effects of protic ionic liquid crystal additives on the water-lubricated sliding wear and friction of sapphire against stainless steel M.D. Avilés, F.J. Carrión, J. Sanes, M.D. Bermúdez www.elsevier.com/locate/wear

PII: DOI: Reference:

S0043-1648(18)30394-6 https://doi.org/10.1016/j.wear.2018.04.015 WEA102407

To appear in: Wear Received date: 26 March 2018 Revised date: 23 April 2018 Accepted date: 24 April 2018 Cite this article as: M.D. Avilés, F.J. Carrión, J. Sanes and M.D. Bermúdez, Effects of protic ionic liquid crystal additives on the water-lubricated sliding wear and friction of sapphire against stainless steel, Wear, https://doi.org/10.1016/j.wear.2018.04.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effects of protic ionic liquid crystal additives on the water-lubricated sliding wear and friction of sapphire against stainless steel M.D. Avilés, F.J. Carrión, J. Sanes, M.D. Bermúdez Grupo de Ciencia de Materiales e Ingeniería Metalúrgica, Departamento de Ingeniería de Materiales y Fabricación, Universidad Politécnica de Cartagena Campus de la Muralla del Mar., 30202-Cartagena (Spain)

Abstract Additives for water lubrication of a sapphire-stainless steel contact have been developed by adding 1 wt.% protic ammonium carboxylate ionic liquid crystals (ILCs) derived from stearic and palmitic fatty acids. Triprotic (2-hydroxyethyl)ammonium stearate [HOCH2CH2NH3]+ [CH3(CH2)16COO]- (MES), diprotic bis(2-hydroxyethyl)ammonium stearate

[(HOCH2CH2)2NH2]+

[CH3(CH2)16COO]-

(DES)

and

bis(2-

hydroxyethyl)ammonium palmitate [(HOCH2CH2)2NH2]+ [CH3(CH2)14COO]- (DPA) have been studied. In unidirectional, pin-on-disk tests of sapphire against type 316L stainless steel, the additives reduce the coefficient of friction up to 80%, from the start of the sliding, and reduce or prevent the increase of the friction at the lubricated-dry contact transition after water evaporation. The wear rate of the stainless steel is reduced by one order of magnitude. The main mechanism for prevention of surface damage is the reduction of iron oxidation inside the wear track due to adsorption of the ILCs additives. SEM/EDX, TEM, XPS surface analysis, Raman spectroscopy and profilometry results are discussed.

Keywords: Ionic liquid crystals; fatty acids; water; bio-based lubricants.

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1. Introduction There is an increasing need for green, environmentally friendly or even biocompatible lubricant formulations [1-3], with sustainable applications in energy saving and in extension of the useful life of components. Water-based lubricants could be biodegradable and biocompatible, but there is a need for green friction-reducing, wearpreventing additives [4-8]. Ionic liquids (ILs) are considered as excellent candidates for the development of green lubricants [9-17]. The last generation IL lubricants are developed for their biodegradability and biocompatibility [18-20]. The combination of a renewable source biocompatible base lubricant such as water with biodegradable non-toxic IL additives could find applications as lubricants in the manufacturing processes of engineering materials and even in such fields as food processing or the pharmaceutical industry [21]. From the first studies of ionic liquids as lubricants in water [22-24] to the most recent research [25-28] there has been a continuous search for new water+IL lubricants. A growing number of studies consider the use of, not only halogen-free [29-35], which could be toxic and corrosive, but fully organic ionic liquids as lubricants or lubricant additives. The ionic liquids used in this work belong to a group of protic ionic liquids (PILs) [3641] which have been shown to be biodegradable and can be even biocompatible [42-44]. These characteristics have triggered the research on PILs in lubrication and tribology [45-46]. As PILs contain carboxylate anions derived from fatty acids [47] and di- or triprotic 2-hydroxyethylammonium cations, their chemical synthesis [37-41] from Brønsted acids and hydroxyethyl amines is very simple and clean, as the only byproduct is water.

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PILs containing mono-, di- or tricarboxylated short alkyl chain anions such as salycilate, adipate, succinate or citrate, have previously shown outstanding performance as neat lubricants, both in full fluid film and in thin film lubrication conditions [48-50]. When used as 1 weight percent additives in water, their friction and wear reducing performance is observed once the high contact temperature under the sliding conditions produces the evaporation of water. However, a high friction coefficient during the running-in period still remains [48]. To obtain low friction from the start of the sliding, neat PILs should be used, either in full-fluid or thin film lubrication, after controlled water evaporation under static conditions, before the sliding begins [50], with the corresponding increase of the costs. The use of a long chain fatty acid protic ionic liquid [38] such as bis(2hydroxyethyl)ammonium oleate [49] as neat lubricant in copper-copper contacts produced very low friction and no wear during the break-in period, but failed to prevent friction and wear after a short sliding distance. Liquid crystals have attracted great interest in lubrication and tribology [51-54] due to their long-range molecular order and their ability to adsorb on sliding surfaces. The main disadvantages of neutral ionic liquids are their low thermal stability and their weak interactions with surfaces. The first ionic liquid crystal (ILC) used as lubricant additive was studied steel-aluminium lubrication [51] was an ammonium chloride salt which showed better lubrication performance than neutral LCs under high contact pressure or temperature. However, the presence of the chloride anion was not desirable for practical applications, as it could cause metal corrosion. The synthesis and characterization of the stearate and palmitate protic ionic liquid crystals used in the present study have been very recently described [56, 57]. They belong to the family of bio-based [58] ionic liquid crystals (ILCs), which are not room-

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temperature ionic liquids but present moderate melting points, thermotropic mesophases and amphiphilic structures in water. Although it was anticipated that they would be promising candidates as lubricants or lubricant additives, their tribological performance has not been previously described. The main purpose of the present study is to develop new environmentally friendly lubricants based on water modified by a small proportion of a bio-based [58] ionic liquid additive synthesized following a clean process, without contaminant byproducts. As water-based lubricants can fail under the high contact temperatures reached at steelceramic interfaces, the stainless-steel-sapphire sliding contact ahs been chosen. 2. Experimental Ionic

liquid

crystals

[CH3(CH2)16COO][(HOCH2CH2)2NH2]+

(2-hydroxyethyl)ammonium

(MES),

diprotic

[CH3(CH2)16COO]-

stearate

[HOCH2CH2NH3]+

bis(2-hydroxyethyl)ammonium (DES)

[56]

stearate

and

bis(2-

hydroxyethyl)ammonium palmitate [(HOCH2CH2)2NH2]+ [CH3(CH2)14COO]- (DPA) and DPA [57] were kindly provided by Dr. M. Iglesias [37-41] and used as received (Figure 1). Water+MES, Water+DES and Water+DPA were prepared adding 1 wt.% of the corresponding ionic liquid crystal to deionized water. To obtain the corresponding emulsions used as lubricants, the mixtures were sonicated for 30 minutes at 30ºC. Viscosity measurements were carried out using an ARG2 rheometer (TA Instruments, Germany) in air with a maximum rotor speed of 300 s-1. The order of viscosity values at room temperature is: 6.2×10-3 Pa·s for (Water+DES) > 4.2×10-3 Pa·s for (Water+DPA) > 1.8×10-3 Pa·s for (Water+MES). Contact angles on AISI 316L steel surface were measured using a DSA 30B (Krüss, Germany) equipment and are average values after 5 measurements, once the contact angles stabilize after 5 minutes.

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Tribological tests (ASTM G99-05 standard) were carried out at room temperature (1926 ºC; 51-64 % HR) in a pin-on-disk ISC-200 equipment [49]. Sapphire balls [(Goodfellow Cambridge Ltd. UK) (Al2O3; 99.9 %; HV 2,750; Young’s modulus 445 GPa; Poisson’s ratio 0.24) of 0.75 mm sphere radius]-AISI 316L stainless steel disks [(HV 200; Young modulus 197 GPa; Poisson’s ratio 0.27; surface roughness Ra < 0.1 m) of 25 mm diameter and 5 mm thickness] contacts were studied under the following conditions: normal load 1 N (mean hertzian contact pressure 1.30 GPa; maximum hertzian contact pressure 1.95 GPa), sliding velocity 0.10 ms-1, sliding radius 9 mm and sliding distance

1,500 m. At least three tests were carried out for each set of

experimental conditions. AISI 316L disks were cleaned with distilled water and dried with hot air before and after the tests and the lubricants were added before each test, to cover completely the surface of the steel disk. Sapphire balls showed no wear or surface damage under the experimental conditions. 3D surface topography images, cross section profiles, and wear volumes measurements for stainless steel were determined using a Talysurf CLI optical profiler. Scanning electron microscopy (SEM) micrographs and energy dispersive (EDX) analyses were obtained using a Hitachi S3500 N. Surface analysis by X-ray photoelectron spectroscopy (XPS) [59] and binding energy values (precision ± 0.1 eV) were obtained with a VG-Microtech Multilab 3,000. Binding energies were referenced to C 1 s peak (285.0 eV) as internal standard. 3. Results and discussion 3.1. Contact angles of the lubricants on stainless steel Stearate derivatives MES and DES increase the contact angle (Table 1) of water on stainless steel surface. The palmitate salt DPA is the only additive which increases the wettability of water on stainless steel, reducing the contact angle in a 30% with respect to water. This result could be due to a strong interaction between DPA molecules and

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the steel surface. Although no direct relationship is observed between viscosity values (see Materials and methods) and contact angles, the more viscous lubricant (Water+DES) also shows the highest contact angle (Table 1). 3.2. Friction coefficients and wear rates Deionized water as neat lubricant (Figure 2) shows a very high friction coefficient of 0.39 up to 1387 m (Table 2), when the evaporation at the sliding contact produces the transition to dry wear [46], and an increase of the friction coefficient to 0.81. All additives are able to reduce the friction coefficient of water up to 80% (Figure 2; Table 2). Moreover, in the case of Water+MES and Water+DPA, no friction increase is observed along the total sliding distance. The more viscous Water+DES is the only lubricant that shows a friction increase after the evaporation of water for a sliding distance of 1340 m. However, the maximum friction coefficient for Water+DES is 0.203, 75% lower than that observed for neat water. Wear rates (Table 2) and surface roughness (Table 3) inside the wear tracks are reduced with respect to those obtained for water. The lowest wear rate is obtained for Water+DPA and the highest one for Water+DES, probably due to the final high friction period observed for this additive, as a consequence of water evaporation at the interface. 3.3. Surface analysis In agreement with wear rate values, Water+DES also shows the highest surface roughness inside the wear track (Table 3) after the tribological tests. 3D surface topography (Figure 3a) and cross section profiles (Figure 3b) show the ability of the ILCs additives to reduce surface damage on stainless steel with respect to water. SEM-EDX study (Figure 4) shows the severe surface damage (Figure 4a) and oxidation on the stainless steel wear track after lubrication with water. The increase of the oxygen

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concentration inside the wear track can be observed by EDX element map (Figure 4b) and by the oxygen and iron line analysis across the wear track (Figure 4c), which shows an increase of the oxygen concentration inside the wear track, with the corresponding decrease of the iron content. The large wear debris particles are mainly composed of iron oxide (Figure 4d). Oxidation of the wear track is also observed for Water+MES (Figures 5b and 5c), although the surface damage (Figure 5a) is milder than that produced on the steel surface after lubrication with Water+DES (Figure 6), where severe abrasion and fracture takes place (Figures 6a and 6b) with extensive oxidation (Figures 6c and 6d). In contrast, the surface lubricated with Water+DPA shows a smooth (figure 7a) almost oxygen-free (Figures 7b and 7c) wear track, in agreement with the lower wear rate and roughness values (Tables 2 and 3) obtained for this lubricant. Moreover, in this case, the EDX line analysis (Figure 7d) shows no significant changes in the carbon, oxygen and iron concentrations inside and outside the wear path. The stainless steel surface after lubrication with Water+MES was selected for XPS analysis inside and outside the wear track, as the surface damage after lubrication with Water+DES was too severe, and too mild in the case of Water+DPA, for accurate comparison. XPS analysis was carried out for neat MES (Table 4) as a reference for the results obtained for the stainless steel surface after lubrication with Water+MES (Table 5). Aliphatic adventitious carbon (285.0 eV) concentration decreases inside the wear track. C1s binding energies at 286.5; 288.0 and 288.7 eV could be assigned to C-N; C-O and C=O, respectively, in agreement with the presence of the ammonium cation and the stearate anion. Nitrogen concentration from the ammonium cation is higher inside than outside the wear track. This result is in agreement with the adsorption of ammonium

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cation at the wear track surface. The total oxygen concentration is higher inside the wear track. O1s binding energies at 529.9; 531.5 and 532.8 eV are assigned to oxygen in iron oxide (Fe2O3); C-O and C=O, respectively. Fe2p3/2 peaks at 707.0 eV and 710.4 eV are assignable to Fe0 and Fe2O3, respectively, while the binding energy at 712.3 eV, can be tentatively assigned to Fe-OC- in iron (III) coordination compounds [59]. These results show the interaction between the ionic liquid crystals and the steel surface.

4. Conclusions Di- and triprotic ammonium ionic liquid crystals derived from stearic and palmitic fatty acids have been used as additives in water for the lubrication of the sapphire-stainless steel contact. All additives reduce the friction coefficient up to 80% with respect to water. The main advantage of these additives is their ability to reduce friction coefficient from the start of the sliding and to maintain this reduction after the evaporation of water at the interface has taken place. Wear rates of stainless steel are reduced in one order of magnitude with respect to water. The best performance is observed for the palmitate derivative, which prevents the extension of iron oxidation in contact with water. The ionic liquid crystal additives described here could be useful in new water-based lubricant formulations with no detrimental effects on the environment.

Acknowledgements The authors acknowledge the Ministerio de Economía, Industria y Competitividad (MINECO, Spain), the EU FEDER Program (Grant # MAT2017-85130-P), “Este trabajo es resultado de la actividad desarrollada en el marco del Programa de Ayudas a Grupos de Excelencia de la Región de Murcia, de la Fundación Séneca, Agencia de Ciencia y Tecnología de la Región de Murcia (Grant # 19877/GERM/15)”. M.D. Avilés is grateful to MINECO for a research grant (BES-2015-074836). 8

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Table 1. Contact angles on AISI 316L. Lubricant

Contact Angle on AISI 316L (after 5 minutes)

Average Contact Angle

Water

42.43 (±1.80)

Water+MES

47.93 (±1.46)

Water+DES

59.73 (±0.90)

Water+DPA

29.90 (±1.06)

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Table 2. Coefficients of friction and wear rates of AISI 316L. Lubricant

Coefficient of Friction

Water

Water+MEs

0.378 (±0.010) (average value from 0 to 1,348 m) 0.129 (±0.003)

Water+DEs

0.117 (±0.005)

Water+DPa

0.107 (±0.006)

Wear rate (mm3/N·m) 1.86×10-5 (±1.7×10-6) 2.63×10-6 (±9.4×10-8) 3.67×10-6 (±1.8×10-7) 2.27×10-6 (±4.9×10-8)

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Table 3. Roughness (Ra) values after the tribological tests.

Lubricant

Roughness (Ra) Roughness (Ra) outside the wear track (µm) inside the wear track (µm) 0.042 1.59 Water (±8.3×10-3) (±6.7×10-2) 0.034 0.51 Water+MEs (±2.1×10-3) (±4.6×10-2) 0.034 0.55 Water+DEs (±3.5×10-3) (±2.3×10-2) 0.033 0.43 Water+DPa (±2.8×10-3) (±5.3×10-2)

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Table 4. XPS binding energies, atomic percentages and assignations for MES. MES Binding Energy (eV) Atomic % 285.0 79.9 286.3 4.9 C1s 287.7 2.2 289.2 3.5 399.7 1.0 N1s 532.2 5.2 O1s 533.4 3.2

Assignation (-C-H) -C-N -C-O -C(O)O-N-H -OH -O-C

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Table 5. XPS binding energies, atomic percentages inside and outside the wear track after lubrication with Water+MES. Element C1s

N1s O1s

Cr2p

Fe2p

Outside the wear track Inside the wear track Binding Energy (eV) Atomic % Binding Energy (eV) Atomic % 285.0 66.6 285.0 62.1 286.1 3.7 286.5 3.5 287.0 1.0 288.0 1.4 288.4 2.7 288.7 2.2 399.7 0.2 398.2 0.4 401.5 0.1 399.9 0.7 530.2 11.6 529.9 13.3 531.7 9.0 531.5 10.3 533.2 0.9 532.8 1.3 574.2 0.1 575.8 0.5 576.4 0.6 577.0 0.5 578.0 0.3 578.1 0.2 707.4 0.4 707.0 0.4 710.6 1.5 710.4 2.4 712.4 0.9 712.3 0.6

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Figure 1. Protic ionic liquid crystals. Figure 2. Coefficient of friction ()-sliding distance records. Figure 3. a) Surface topography; b) Cross sections of the wear tracks. Figure 4. a) SEM micrograph of the wear track on AISI 316L after lubrication with water; b) Fe and O element maps; c) Fe and O concentration along the arrow line crossing the wear track; SEM micrograph and EDX spectrum of wear debris. Figure 5. a) SEM micrograph of the wear track on AISI 316L after lubrication with Water+MES; b) Fe and O element maps; c) EDX spectrum inside the wear track. Figure 6. a) SEM micrograph of the wear track on AISI 316L after lubrication with Water+DES; b) Detail of the wear track; c) Fe and O element maps; c) EDX spectrum inside the wear track. Figure 7. a) SEM micrograph of the wear track on AISI 316L after lubrication with Water+DPA; b) Fe and O element maps; c) EDX spectrum inside the wear track; d) Fe, O and C concentration along the arrow line crossing the wear track.

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(2-hydroxyethyl)ammonium stearate + [CH3(CH2)16COO ] [(HOCH2CH2)NH3] (MES)

Bis[(2-hydroxyethyl)ammonium] stearate + [CH3(CH2)16COO ] [(HOCH2CH2)2NH2] (DES)

Bis[(2-hydroxyethyl)ammonium]) palmitate + [CH3(CH2)14COO ] [(HOCH2CH2)2NH2] (DPA)

Figure 1. Protic ionic liquid crystals.

22

2D Graph 1 2D Graph 1

2D Graph 1

2D Graph 2D Graph 1 1 Water+DES Water+MES Water+DES Water+DPA Water+MES Water WDES Water+DPA WDES Water+DES WMES WaterWater+MES WMES WDPA WDPA Water+DPA AGUA AGUA Water

0.8 0,8 0,8 0.8 0.8

0.6 0.6 0,6



0,6

COF COF



0.6

0.4

0.4 0,4



0,4

0.4

0.2 0.2 0,2

0,2 0.2 0.0 0.0

0,0

00

200 200

0

200

400 400

400

600 600

0.0

0,0

800 800 1000 1000 1200 1200 1400 1400

800

1000

1200

1400

Distance, Distancia,mm 0

0

600

200

200

400

400

Distance, 600 800 m 1000

600

800

1000

1200

1200

1400

1400

Distance, Figure 2. Coefficient of friction ()-slidingm distance records.

Distancia, m

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a)

Water+DES

Water+MES

Water+DPA

Water

b)

Water+DES

Water+MES

Water+DPa

Water

Figure 3. a) Surface topography; b) Cross sections of the wear tracks.

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a)

b)

c)

Fe e

O

Fe

O

d) O

Fe

C

Cr Fe Ni

Figure 4. a) SEM micrograph of the wear track on AISI 316L after lubrication with water; b) Fe and O element maps; c) Fe and O concentration along the arrow line crossing the wear track; SEM micrograph and EDX spectrum of wear debris.

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a)

b)

Fe

O

c) Fe Fe O

C

Cr

Mo

Ni

Figure 5. a) SEM micrograph of the wear track on AISI 316L after lubrication with Water+MES; b) Fe and O element maps; c) EDX spectrum inside the wear track.

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a)

b)

c)

Fe

O

d)

Fe

Fe

Cr O C

Ni

Ni

Figure 6. a) SEM micrograph of the wear track on AISI 316L after lubrication with Water+DES; b) Detail of the wear track; c) Fe and O element maps; c) EDX spectrum inside the wear track.

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a)

b)

Fe

O

c) Fe

Fe

Cr

Ni Ni

d) Fe e

O C

Figure 7. a) SEM micrograph of the wear track on AISI 316L after lubrication with Water+DPA; b) Fe and O element maps; c) EDX spectrum inside the wear track; d) Fe, O and C concentration along the arrow line crossing the wear track.

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Highlights:



Wear of stainless steel is reduced after friction tests with sapphire-steel contacts

compared to pure water. 

The palmitate derivative improves wettability of water on stainless steel.



The new additives eliminate the high friction running-in period with respect to

other protic ionic liquid additives with short chain carboxylate anions.

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