Oleate-based protic ionic liquids as lubricants for aluminum 1100

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Oleate-based protic ionic liquids as lubricants for aluminum 1100 Maria Rita Ortega Vega, Juliano Ercolani, Silvana Mattedi, César Aguzzoli, Carlos Arthur Ferreira, Alexandre Silva Rocha, and CELIA MALFATTI Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02426 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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Industrial & Engineering Chemistry Research

Oleate-based protic ionic liquids as lubricants for aluminum 1100. Maria-Rita Ortega-Vega1*, Juliano Ercolani2, Silvana Mattedi3, César Aguzzoli4, Carlos Arthur. Ferreira5, Alexandre S. Rocha2, Célia F. Malfatti1. 1 – Laboratório de Pesquisa em Corrosão – LAPEC – Universidade Federal do Rio Grande do Sul – UFRGS. Av. Bento Gonçalves 9500, Block 4, BLDG 43427, 2nd FL. CEP: 91501-970. Porto Alegre, RS, Brazil. 2 – Laboratório de Transformação Mecânica – LdTM – Universidade Federal do Rio Grande do Sul – UFRGS. Av. Bento Gonçalves 9500. CEP: 91501-970. Porto Alegre, RS, Brazil. 3 – Laboratório de Termodinâmica Aplicada, Universidade Federal da Bahia - UFBA, Aristides Novis St. 2, 2nd FL, CEP 40210-630, Salvador, BA, Brazil. 4 – Programa de Pós-Graduação em Engenharia e Ciência dos Materiais - PGMAT, Universidade de Caxias do Sul - UCS, Francisco Getúlio Vargas St., 1130 - Caxias do Sul, RS, Brazil. 5 – Laboratório de Polímeros – LAPOL – Universidade Federal do Rio Grande do Sul – UFRGS. Av. Bento Gonçalves 9500, Block 4, BLDG 43426, 1st FL. CEP: 91501-970. Porto Alegre, RS, Brazil.

* Corresponding author. E-mail: [email protected]

Abstract

Due to their structures and tunable properties, viscosity among them, oleate-based protic ionic liquids (PILs) were used as lubricants for aluminum, aiming to apply them in metal forming processes. Three oleate-based PILs with three ammonium-based cations were tested as lubricants, in order to evaluate the influence of the cation structure on the tribological performance. Coefficient of friction (COF) determination and subsequent wear computations were conducted. Erichsen tests were also conducted to evaluate their performance in metal forming processes. Oleate-based PILs

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maintained a low COF value, with the formation of uniform tribofilms that promoted the wear reduction down to 98% compared to the dry condition. Considering wear, their performance surpassed the one of a studied commercial lubricant. In spite of this, Erichsen test revealed that the PILs yielded a performance comparable to that of the commercial lubricant. The advantages of the use of these PILs are their simple synthetic route, low cost, low toxicity and chemical stability.

1. INTRODUCTION

The initial studies of lubrication using ionic liquids (ILs) pointed out the use of alkylimidazolium in the cation moiety hexafluorophosphate in the anion

4–6

1–3

as well as tetrafluoroborate or

. However, these ILs tended to hydrolyze and their

viscosity values are low compared to other ones. As the researches advanced, the leading specifications for the ionic liquids were higher viscosity to increase the lubricity and structures harmless for the tribopair integrity. Lubricity could be increased with the presence of long, linear alkyl chains, as stated by other authors

4,6,7

. However and according to Jiménez and Bermúdez 8, care must be

taken with high molecular polarity induced by longer alkyl branches, as they found out for Ti6Al4V-steel employing alkylimidazolium-based ILs. Thus, it is necessary to conduct specific studies for different types of combinations of ionic liquid structures and tribopairs. Aiming to keep the tribopair integrity, different halogen-free and ammonium-based structures have attracted attention 9–11; the authors 12,13 reported that the use of halogenfree PILs for metallic contacts allowed attaining low and ultra-low coefficients of friction (COF) without any corrosion evidences. Some authors

4

stated the importance

of more hydrophobic molecules to improve the thermal and chemical stability; however, they also reported that with the presence of longer branches thermo-oxidative stability decreases. The use of halogen-free, ammonium-based ILs with long alkyl branches, gives away more advantages than exclusively the high viscosity value and the chemical stability of the tribopair materials. As a point in common, the lubrication mechanism promoted by the ILs is mainly attributed to mixed lubrication regime 14,15. This mechanism refers to a film with molecular thickness and lack of fluidity but capable of cushioning load coupled to the asperities of the surfaces to support part of the load


17

16

. In order to

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promote the formation of this film, the molecular structure of the film must contain functional groups capable of different types of interactions. π-π interaction is relevant when a film for thin film lubrication on a metallic substrate is wanted

18,19

. Some

functional groups capable of promoting π-π interactions with metallic surfaces are the carboxylic and amino functions, because of the presence of more electronegative elements

18

. The diversity of functional groups also enhance the adsorption with other

active compounds, like oxides or hydroxides

14

. In addition, the adsorbed film remains

for long-term due to the low-volatility proper of the ILs, a remarkable property for application under vacuum 20. In this work, three ammonium-based protic ionic liquids (PILs) were tested as lubricants. These ILs possess different structures in the cation moiety while the anion structure was fixed. The anion precursor corresponded to oleic acid that was neutralized with an amine. Literature reports that these type of PILs can be adsorbed on metallic substrates, due to the amino and carboxylic functions

21

and their performance as

lubricants can be comparable to that of a commercial lubricant used for metal forming processes, depending on the anion chain length

22

. The novelty in this work lies in the

influence of the cation structure, keeping a very long anion chain (18 carbon atoms), on the coefficient of friction, wear and metal forming performance, being the cation structure a mono- or di-substituted ammonium. Their advantages are the easiness of synthesis and low cost, as well as the absence of halogen atoms 23.

2. EXPERIMENTAL

2.1.PILs and commercial lubricant characterization

The studied protic ionic liquids are present in Table 1. For the evaluation of their performance as lubricants for aluminum 1100, a commercial lubricant was also tested for comparison purposes.

PIL

Structure OH

2HEAOl O-

2-hydroxyethylammonium oleate

NH 3

+

CH3 O



m-2HEAOl

H3C

N-methyl-2hydroxyethylammonium

NH 2

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OH

+

OCH3 O

oleate BHEAOl

HO NH 2

Bis-2-

+

OH

O-

hydroxyethylammonium oleate

CH3 O

Table 1. Structures of the studied protic ionic liquids.

Physical-chemical characterization of the lubricant fluids included thermogravimetric analysis and rheology. TGA measurements were conducted using TA Instruments Q50 equipment, under N2 atmosphere (10 mL.min-1) with heating rate of 20 °C.min-1, starting from the room temperature up to 640 °C. Rheology studies were conducted by measuring the fluid viscosity through a plate and cone Brookfield HB DV - II viscometer (highest torque: 0.05745 N.m). The cone radius was 1.2 cm and its angle, 3.0 °; the plate radius was 5.3 cm. Rheological behavior data were obtained by using an Anton Paar Physica MCR 102 rheometer, in parallel plate configuration; the plates diameter was of 25 mm. The shear rates were varied from the lowest to the highest values and then, backwards in order to determine if there is hysteresis or not in the viscosity as a function of the applied shear.

2.2.Plate and ball morphology

The composition of the studied aluminum 1100 was already reported by the authors in another paper 22. The obtained value of the material hardness was 48 HV0.01, determined with Insize ISH-TDV1000 equipment, a load of 10 g and 10 seconds of application, and its yield stress, as reported by other author 24, corresponded to 124 MPa. Both ball and plate were used as received from the manufacturers. Surface topography of the used aluminum plate appears in Figure 1-a). It shows the presence of lines product of the rolling process. The surface presented more valleys than peaks, (surface skewness = -0.161), and they were not gradually distributed (surface kurtosis = 2.625) 25. The surface of the alumina ball was dull, and it presented pores and imperfections, as seen in Figure 1-b); its average surface roughness (Sa) was of 0.5 µm. Both ball and plate were used as received from the manufacturers. Aluminum plate


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roughness is product of the rolling process and ball finish is full of imperfections due to the manufacturing process of the alumina.

a)

b) Figure 1. a) Topography of the aluminum plate used for the tribological tests. b) Micrograph of the alumina ball surface.

2.3.Wear tests

Wear tests were conducted using a CETR UMT (Universal Micro Tribometer) tribometer with a ball-on-plate modulus. An alumina ball (4.76 mm) with a load of 0.5 N, frequency of 1 Hz and 2 mm as track length for 120 minutes (total distance = 28.24 m). Tests were conducted in a room with 50 % - 55 % HR and the temperature was 23 °C. In order to guarantee the reproducibility of the tests, at least five tests were conducted for each lubricant fluid and dry condition. For the COF values, track profile and ball and plate micrographs and EDS results, only the sample that reached the worst result was presented. Balls and tracks were evaluated before and after by optical microscopy, using an Olympus CX31 microscope. In addition, plate surface topography, and track profiles



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were obtained with a Bruker Contour GT-K optical interferometer. Worn volume was calculated as stated in ASTM G133 – 05 (2016) standard

26

, section 9.3.1; plastic

deformation contribution was also considered in the computing as proposed by

12

. The

obtained worn volume was divided by the total distance in order to obtain the wear rate (mm3.m-1). For the worn volume computations, all the results of the five repetitions were considered, in order to calculate the mean and standard deviation values. Lubricant fluids were applied in amount of 4.7 mg ± 0.2 mg. SEM images of the balls were obtained with a Phenom ProX equipment, meanwhile Shimadzu Superscan SSX-550 coupled with EDS was employed for capture the track images and confirm the presence of the adsorbed layer of the PIL.

2.4.Erichsen test

Erichsen test was conducted using a Universal Testing Machine EMIC DL 6.000 with maximum capacity for 600 kN and a module for Erichsen test following the standard EN ISO 20482:2003. Aluminum plates of 1 mm thickness were cut to obtain 90 mm x 90 mm dimensions. Punch velocity was set to 5 mm.min-1. The tests were conducted with and without lubrication.

3. RESULTS AND DISCUSSION

3.1.Lubricant fluids characterization Structure of the PILs was confirmed by 1H and 13C NMR and FTIR spectroscopy; the obtained spectra were similar to those found by other authors

23

. Concerning the

commercial lubricant, manufacturers relate that it is water-based and it contains siloxanes. The manufacturer describes this lubricant as suitable for deep drawing and stamping processes for metallic, ceramic and polymeric materials. The siloxanes presence was confirmed by FTIR spectroscopy. The PILs and commercial lubricant structural characterization can be found in the S1 section of the Supporting Information. Thermograms for the oleate-based PILs can be found in Figure 2. Up to 100 °C, water evaporation took place. This water was product of absorption due to the hydrophilic character of the PILs23,27. Given the high environmental relative humidity, absorbed water amount is hard to control. The PILs were not dried in order to represent


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the real condition of the PILs employment. Hence, with the difference of the mass loss values, it is possible to determine the PILs water amount that was less than 7 % (Table 2). These PILs were highly stable: mass loss augmented over 200 °C (Figure 2-a). This behavior was formerly reported for fatty-acid-based-ionic liquids

28

. At 250 °C was

found the most important mass loss for 2HEAOl and m-2HEAOl, which was also found for BHEAOl at 358 °C, as the highest peaks in Figure 2-b); these temperature values coincide with the degradation temperature for oleic acid 29. However, the existence of more than two peaks in Figure 2-b) revealed that the PIL decomposition followed a two-step reaction mechanism and possibly the formation of intermediate products in each: initially the cation moiety was shorter and presented more functional groups, which can be more reactive and at higher temperature the oleic acid aliphatic backbone broke, since these structures need more energy to break the strong C-C bonds 30. 2HEAOl BHEAOl m-2HEAOl

100

80

Weight %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 0

a)

100

200

300

400

Temperature (°C)


500

600


2HEAOl BHEAOl m-2HEAOl

Degradation first step

1.4 1.2 1.0

dW/dT (%/°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Degradation second step

0.8 0.6 Water evaporation

0.4 0.2 0.0 -0.2 0

100

200

300

400

500

600

Temperature (°C)

b)

Figure 2. a) Thermograms and b) mass loss derivative vs. temperature obtained for oleate-based PILs. Atmosphere: N2 (10 mL.min-1). Heating rate: 20 °C.min-1.

According to the thermogram for the commercial lubricant (Figure 3), there was an important mass loss up to 100 °C (42%), associated to water evaporation as aforementioned; thus, the commercial lubricant was water-based. However, at 87 °C a smaller peak was found (Figure 3) that can correspond to light, volatile organic substances in the lubricant formulation. Other components of this lubricant were to degrade at 250 °C and at more than 400 °C. At the end of the thermogravimetric experiments, all the studied lubricant fluids left a residual char with 3% to 6% of the initial mass; this behavior was also reported by other authors 22,31. Lubricant fluid

Water content* (%)

Viscosity** (mPa.s)

2HEAOl

2.69

3800

BHEAOl

3.04

4000

m-2HEAOl

7.16

3600

Commercial lubricant

42.0

2281***

Table 2. Water content and viscosity of the studied lubricant fluids. *Obtained by TGA. **50 rpm, 25% torque, 23 °C. ***50 rpm, 14.5% torque, 23 °C.


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Commercial Lubricant Derivative function

1.25

100 1.00 80

60 0.50 40 0.25

dW/dT (%/°C)

0.75

Weight %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 0.00 0 -0.25 0

100

200

300

400

500

600

Temperature (°C)

Figura 3. Thermogram and mass loss derivative vs. Temperature plots for the commercial lubricant.

Regarding viscosity, both the PILs and the commercial lubricant presented high viscosity values and in the same order of magnitude (Table 2). This is a property whose high value is desirable for lubricity enhancement, since higher viscosity in general promotes the formation of thicker and more stable lubricant films, and in consequence, the separation between surfaces in contact and relative motion

32

. In order to obtain a

stable viscosity measure, it was necessary to set a torque value in the equipment; the presented torque values correspond to those at which the viscosity measure was stable. Among the studied lubricant fluids, BHEAOl had the highest viscosity value, followed by 2HEAOl and m-2HEAOl. The lowest viscosity belonged to the commercial lubricant. Aiming to complement these results, rheological behavior was also measured (Figure 4). All the lubricant fluids had shear-thinning behavior: their viscosity (µ) value, in Pa.s, decreased with the increase of the shear rate (γ), in 1/s, and is described by a power law as stated in Eq. (1) 33,34. 𝜇 = 𝛾 (𝑛−1) + 𝑐

Eq. (1)

Values for the exponent n and for the constant c appear in Table 3, as well as the goodness of the fitting R2. These results are in agreement with those reported elsewhere; the authors

23

concluded that oleate-based PILs present non-Newtonian behavior at

temperatures below 65 °C, which also applies for the results obtained in this work at 23 °C.



This behavior is adequate to maintain the fluid in liquid state and to ensure full-film lubricity 35,36. However, care must be taken when applying higher shear rates, since the fluid viscosity can diminish as low as to break the lubricant film and do not provide any lubrication.

Forward

Lubricant fluid

Backwards

n

c (Pa.s)

R2

n

c (Pa.s)

R2

2HEAOl

0.33

94.2

0.97

0.47

40.0

0.94

BHEAOl

0.49

48.2

0.97

0.59

25.6

0.95

m-2HEAOl

0.46

38.5

0.96

0.56

20.6

0.94


0.35

90.1

0.97

0.49

34.9

0.97

Table 3. Values obtained for the rheological behavior power-law fitting parameters n and c of the studied lubricant fluids.

Forward Backward

Rheometry - 2HEAOl 12000

10000

Viscosity [mPa·s]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8000

6000

4000

2000

0 0

125

250

375

500

625

750

875

1000

Shear Rate [1/s]

a)


1125

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Forward Backward

Rheometry - BHEAOl 12000

Viscosity [mPa·s]

10000

8000

6000

4000

2000

0 0

125

250

375

500

625

750

875

1000

1125

Shear Rate [1/s]

b) Viscosity Viscosity

Rheometry - m-2HEAOl 12000

10000

Viscosity [mPa·s]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8000

6000

4000

2000

0 0

125

250

375

500

625

750

875

1000

Shear Rate [1/s]

c)


1125


Forward Backward

Rheometry - D35S 12000

10000

Viscosity [mPa·s]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8000

6000

4000

2000

0 0

125

250

375

500

625

750

875

1000

1125

Shear Rate [1/s]

d) Figure 4. Rheological behavior of the studied lubricant fluids: a) 2HEAOl, b) BHEAOl, c) m2HEAOl and d) commercial lubricant.

3.2.Wear test Coefficient of friction behavior with the sliding distance appears in Figure 5. The contacts without lubrication showed the highest COF, natural between both the metal – ceramic surface, considered as the worst condition, where oscillations along the whole experiment were observed. With the employment of all the lubricant fluids different behaviors considering COF were observed. In general, the oleate-based PILs yielded low COF values vs. dry condition (Figure 5). For similar molecules in other works

37,38

and at low sliding speed, this behavior was

associated to mixed lubrication as lubrication regime. COF value for BHEAOl was the lowest and stable along the whole experiment. Meanwhile, two behaviors were observed for the other oleate-based PILs: first, COF values remained approximately constant up to 8 m and then they increased with an oscillating behavior. The COF reduction with the use of PILs demonstrated their lubricity: they kept the surfaces separated and the tribofilm properties (thickness, chemical interaction with the surfaces) were adequate to cushion the load. On the other hand, the tested commercial lubricant maintained a low, stable COF up to 5 m. From this distance on and till the end of the experiment, COF oscillated with an increasing trend and at 16 m COF reached the value for the naked aluminum alloy.


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Thus, three steps of the tribofilm life span were observed: a stable tribofilm up to 5 m of sliding distance, then the formation of debris that increased the COF in an unstable way 39

up to 20 m; finally, there was the complete rupture of the tribofilm and the contact

was then between the aluminum and the alumina as observed by the COF values and behavior (Figure 5). Initial and final values of the COF for all the studied conditions are presented in Table 4. Despite COF behavior did not follow any linear trend vs. the sliding distance, it was fitted following the least squares algorithm as a gross approach to its variation; intercept, slope and correlation coefficient (R2) obtained for the fitting are also reported in Table 4. For the variation in the COF behavior along the experiment, the intercept parameter will not be considered, but only initial and final COF and slope values. Since the initial and final COF values were very close between each other for the cases of lubrication using oleate-based PILs, and each slope value was close to zero (order of magnitude of 10-3 and less); then negligible variation of COF also suggests the capability of the PILs to attain some stable lubrication conditions 40. The initial and final COF values for the dry condition and the test with the commercial lubricant were very separated and the slope were higher than those found with PIL; the important variation in the COF behavior with the sliding distance may be attributed to the surface modifications along the wear tests. Al-Al2O3 0.5 N; 1 Hz; 2 mm; 120 min

1.0 0.9 0.8 0.7 0.6

COF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dry 2HEAOl BHEAOl m-2HEAOl Commercial lubricant

0.5 0.4 0.3 0.2 0.1 0.0 0

4

8

12

16

20

24

28

32

Distance (m)

Figure 5. Coefficient of friction vs. sliding distance obtained for the dry condition and for the studied lubricant fluids.



Lubricant fluid

Initial COF

Final COF

Dry

0.25

2HEAOl

0.11

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Fitting parameters Intercept

Slope

R2

0.86

0.61

0.0095

0.58

0.15

0.067

0.0027

0.89 -4

BHEAOl

0.12

0.076

0.0647

1.52 x 10

0.051

m-2HEAOl

0.12

0.15

0.056

0.0038

0.92


0.12

0.83

-0.084

0.03372

0.87

Table 4. Initial and final COF values and parameters for least square fitting.

Optical microscopy images of the tracks and the balls after the wear tests appear in Figures 6 and 7 respectively. Samples in dry condition (Figure 6-a) and with the use of the commercial lubricant (Figure 6-e) yielded the biggest wear scars on the aluminum surface. This is in agreement with the results obtained in the aforementioned COF determination (Figure 5): the higher the COF the more the wear on the surface. The obtained tracks were dark, perhaps related to oxide formation

41,42

. Observing the

corresponding ball morphology, both for dry condition (Figure 7-a) and with the use of the commercial lubricant (Figure 7-e) there was material transfer with the sliding direction designed. The material transfer suggested the development of adhesion

43,44

between the bodies and with the displacement of the ball, the material removal from the plate took place and formed the third body (debris). As abrasion grooves were present, the wear mechanism also had a contribution coming from abrasion, promoted by the debris produced during adhesion. In the case of the commercial lubricant, the lubricant film was not stable, as suggested by Figure 5 and Table 4, broke and left exposed the metallic surface to contact with the ceramic ball. For this latter case, a slurry formed by removed material and lubricant for the system corresponded to the third body. On the opposite, with the use of the oleate-based PILs, the wear tracks diminished their dimensions (Figures 6-b, 6-c and 6-d) and presented a light color. However, among them, PIL BHEAOl, the one with the lowest COF along the whole sliding test, yielded a track bigger than the ones using the other PILs (Figure 6-c). Regarding the counter-bodies (Figures 7-b, 7-c and 7-d), balls appeared clean with a similar aspect to that before the tests (Figure 4); thus, material transfer was not observed. In this case, the presence of the PIL film successfully separated the two contacting surfaces and eliminated adhesion; however, the predominant mechanism was abrasion promoted by the third body formed (debris) at the start of the experiment and that left abrasion grooves in the tracks (Figure 6-b, 6-c, 6d)

39,44

. The formation of this debris promoted


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the oscillations and the increase of the COF observed for the PILs 2HEAOl and m2HEAOl (Figure 5). Those results were in agreement with the ones of COF vs. sliding (Figure 5), because with the low COF values obtained with the PILs as lubricants the wear reduction was expected.

a) Dry




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b) 2HEAOl



c) BHEAOl


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



d) m-2HEAOl


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e) Commercial lubricant



Figure 6. Track optical and SEM images after wear tests for: a) Dry condition, b) 2HEAOl, c) BHEAOl, d) m-2HEAOl and e) commercial lubricant with the respective EDS chemical composition.


Page 22 of 43

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a) Dry

Transferred material




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b) 2HEAOl



c) BHEAOl


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



d) m-2HEAOl


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e) Commercial lubricant


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Figure 7. Ball optical and SEM images after wear tests for: a) Dry condition, b) 2HEAOl, c) BHEAOl, d) m-2HEAOl and e) commercial lubricant with the respective EDS chemical composition.



SEM images (Figure 7) for the tracks showed the heterogeneous skinning of the aluminum plate for the dry condition and with the employment of commercial lubricant; as well as the smooth aspect for the tracks obtained with the studied PILs. EDS results showed the presence of aluminum and oxygen for all the samples, both track and ball. On the tracks and balls images obtained using PILs as lubricants, traces of carbon were detected; the presence of these carbon atoms confirm the adsorption of the PILs on the aluminum plate as part of the wear mechanism. On the track and ball images obtained for the commercial lubricant, there was the presence of silicon; it suggested the presence of this element in the lubricant, which is in agreement with the manufacturer information. Track profiles obtained by optical interferometry (Figure 8) revealed the presence of plastic deformation in the wear mechanism in absence of lubricant and with the commercial lubricant, observed with the highest peaks above the zero line and at each side of the tracks. For the dry condition, the plastic deformation was also distributed at the track contour (Figure 9). Meanwhile, the commercial lubricant promoted the concentration of the plastic deformation at every end of the track (Figure 9); as the track itself became a lubricant reservoir, at every displacement of the ball, some lubricant was displaced too in the same sliding direction, and its pressure promoted the movement upwards of the aluminum. Comparing the tracks (Figure 8 and 9), the tracks obtained with the PILs presented a semicircle-shaped profile well defined, corresponding to the indentation of the ball, opposite to the heterogeneous, irregular semicircle obtained for the dry condition and with the commercial lubricant. These latter ones are in agreement with the micrographs, which show marks of abrasion from very small particles product of the third body formed during the first moments of the contact

44

, revealed as the deepest valleys

observed in the track profile (Figure 8). The width and depth of the tracks were the highest for the dry condition followed by the system using commercial lubricant, followed by the track obtained with BHEAOl and the smallest ones were the ones obtained with 2HEAOl and m-2HEAOl.


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Page 33 of 43

10

Dry 2HEAOl BHEAOl m-2HEAOl Commercial Lubricant

Track profile Al - Al2O3 0.5 N; 2 mm; 1 Hz; 120 min

5 0 -5 -10

z (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-15 -20 -25 -30 -35 -40 0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

y (mm)

Figure 8. Track profiles obtained by optical interferometry for all the dry and lubricated conditions. Alumina ball diameter = 4.76 mm. Normal load = 0.5 N. Frequency = 1 Hz. Stroke length = 2 mm. Sliding distance = 28.24 m.

Dry

2HEAOl



BHEAOl

m-2HEAOl


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Figure 9. 3D optical interferometry images for the tracks obtained at all the studied conditions.

The small dimensions of the tracks obtained with the PILs are explained because the suppression of the contact between the materials when the load was applied finally reduced the fractional contact area. It was favored by the PILs high viscosity (Table 2) and the low speed (1 Hz) during the wear test 17. Worn volume computation followed the procedure reported in the ASTM G133 standard 26. In addition, the discount of the plastic deformation followed the procedure already reported by other authors

12

. Worn volume was divided by the total sliding

distance and the wear rate was calculated. These values are displayed in Table 4. These results agree with the other morphological characterization images: The dry condition and the commercial lubricant promoted the highest wear rates, two and one orders of magnitude higher than the wear rates achieved using PILs, respectively. It was translated into wear reduction down to 96 % with BHEAOl and even lower percentages with 2HEAOl (98.5 %) and m-2HEAOl (98.4 %), even when the latter ones presented higher coefficients of friction. It can be due to the efficient adsorption eased by the absence of steric hindrance at the amino function that allowed the interaction with the metal 46. Lubricant fluid

Wear rate (mm3.m-1) (SD)

Dry

1.67 x 10-3 (1.56 x 10-4)

2HEAOl

Wear reduction (%)* 0

-5

-5

98.5

-5

-5

2.51 x 10 (1.10 x10 )

BHEAOl

7.17 x 10 (2.00 x10 )

95.7

m-2HEAOl

2.60 x 10-5 (1.09 x 10-5)

98.4


-4

-4

7.48 x 10 (1.52 x 10 ) *vs. Dry condition.


55.3


Page 36 of 43

Table 4. Wear rates and wear reduction percentages obtained after wear tests for the dry and lubricated conditions.

3.3.Erichsen test

Following the successful results obtained in the wear tests, the same conditions (dry and lubricated) were applied to Erichsen test, aiming to evaluate the performance of the studied PILs as future lubricants in stamping or deep drawing processes. These processes are characterized by high friction and high adhesion between the contacting materials

48

, which could be diminished with the use of PILs. Erichsen tests results

appear in Table 5. In this test, punch applies a force to deform the sample down to a depth. The maximum force is applied at the rupture and the depth is measured; these values are reported. As observed in these results, all the lubricant fluids had comparable performance for the stamping application both in force and depth of indentation. Opposite to the wear test, the commercial lubricant showed a good performance for short-term load application on the metal; however, the use of this lubricant fluid was not promising for long-term, cyclic load application, which opens an important field for industrial application of the PILs as alternative lubricants, whose lubricant effects are long-lasting.

Maximum force (kN)

Depth in rupture (mm)

Mean

SD

Mean

SD

Dry

5.74

0.054

9.16

0.15

2HEAOl

5.96

0.065

9.58

0.15

BHEAOl

5.96

0.065

9.64

0.088

m-2HEAOl

6.07

0.11

9.56

0.16


6.13

0.17

9.60

0.20

Lubricant fluid

Table 5. Results obtained from the Erichsen tests.

Despite the PILs yielded a result similar to that of the commercial lubricant, these fluids are non-toxic, more environmentally friendly, an advantage in terms of disposal, and they are less prone to oxidation, which ensures the chemical integrity of the tools and components, and can be reused. In addition, the suppression of the adhesion in the wear mechanism due to the PIL film is beneficial, since it promotes the good finishing of the components and the preservation of the tool surface for a longer service life 42.


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4. CONCLUSION

The studied PILs presented high lubricity, because of their big sizes and the presence of amino and carboxylic functional groups; however, the lubricity was negatively affected by the presence of a longer branch at the nitrogen atom of the amino function in BHEAOl. The addition of a methyl substituent at the amino function (m-2HEAOl) did not yield an important difference in the coefficient of friction or in the wear rate. The high viscosity of the PILs played an important role of the organic film in the elimination of the adhesion between the contacts. Opposite to the dry condition and the commercial lubricant whose mechanism was mixed with both abrasion, adhesion and with third body presence, as well as plastic deformation. The use of PILs allowed wear reduction of 96 % - 98.5 %. The presence of the PIL molecule on the track confirmed the adsorption of the PIL onto the aluminum plate, which promoted the formation of a tribofilm during the sliding test. Deeper studies involving Raman spectroscopy and XPS surface analyses are necessary to characterize this tribofilm. PILs were adequate to support long-term loads; meanwhile the commercial lubricant film broke after some sliding cycles. The performance in metal forming process simulation yielded the same for all the studied conditions; in consequence, all the studied lubricants worked out well for short-term load application. Among the PILs, the ones with the best performance were 2HEAOl and m-2HEAOl regarding wear reduction. In general, PILs performance was better than that of the commercial lubricant. ACKNOWLEDGEMENTS Authors thank the financial support of the Brazilian Government Agencies CAPES (Conselho de Aperfeiçoamento do Pessoal de Nível Superior) and National Council for Scientific

and

Technological

Development

CNPq

(Conselho

Nacional

de

Desenvolvimento Científico e Tecnológico). The authors also thank the Laboratório Central de Microscopia (LCMIC) at the Universidade de Caxias do Sul (UCS).

Supporting Information Structural characterization of the PILs (NMR and FTIR spectrum) and the commercial lubricant (FTIR spectrum).



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ABSTRACT GRAPHIC


Oleate-based protic ionic liquids as lubricants for aluminum 1100

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