steel contacts

Tribology International 73 (2014) 83–87

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The study of TEMPOs as additives in different lubrication oils for steel/steel contacts Dongmei Li, Ping Gao, Xiaojun Sun, Songwei Zhang, Feng Zhou n, Weimin Liu n State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

art ic l e i nf o

a b s t r a c t

Article history: Received 18 July 2013 Received in revised form 12 December 2013 Accepted 6 January 2014 Available online 11 January 2014

The anti-wear and antioxidant behaviors of 2,2,6,6-tetramethyl-1-piperidinooxy (TEMPO) and its derivatives were studied. The tribological properties of lubrication oils with or without the addition of TEMPOs were investigated on an Optimol SRV IV oscillating friction wear tester and the oxidative stability was evaluated by RBOT test using SH/T-0193 and ASTM D2272 procedures. The results showed that the addition of TEMPOs can improve the antiwear and antioxidant behaviors of lubrication oils remarkably. Moreover, the addition of TEMPOs can improve the performance of lubrication oil under atomic oxygen radiation. & 2014 Elsevier Ltd. All rights reserved.

Keywords: TEMPOs Anti-wear Anti-oxidant Anti-atomic oxygen

1. Introduction

2. Materials and experimental procedures

The development of antiwear and antioxidant additives with superior performance properties is one of the major tasks in lubricant chemistry [1–3]. Today, the most widely used antiwear additives are the dihydrocarbyl dithiophosphates [4–9], which form protective polymeric films on the sliding surfaces of the metal parts [10]. Conversely, ionic liquids interact with the metal surface via an electrostatic interaction to construct a protective film [11–14]. Typically, hindered phenols and alkylated amines are used as antioxidants in lubricating oils, but they are not very effective antiwear additives. It would be ideal if some of the antiwear additives would also improve the antioxidancy of the lubricant [3,15]. Additionally, apart from the lubricant performance, environmental factors should also be considered, and additives that are only made from ashless biodegradable components (C,H, O, N) should be preferred [16,17]. Herein we want to present a novel class of antioxidation additives, i.e. TEMPOs, which have never been tested as additives in lubricating oils. The typical structures are shown in Scheme 1. The presence of free radical group in the molecules can inhibit the oxidation during lubrication and the results show it really can improve the antiwear and antioxidant behaviors of different lubrication oils. More importantly, these additives do not contain more elements than carbon, hydrogen, oxygen and nitrogen and thus are biodegradable. The TEMPOs studied in this manuscript are shown in Scheme 1.

Three typical lubrication oils such as poly(ethylene glycol)-400 (PEG), eicosyl naphthenate (E20, average molecular of naphthenate¼180350) and eicosyl oleate (U20) were tested in this work. After dissolving the TEMPO derivatives into the base oils, all the samples were stored in the sealed tubes for 3 days before testing. The TEMPOs were fully soluble in the base oil and maintained stable during the measurement under different conditions. The friction and wear tests were carried out on an Optimal SRV-IV oscillating reciprocating friction and a wear tester at 25 1C. These tests were conducted under conditions similar to previous investigations [3,8,9]. The contact between the frictional pair was achieved by pressing the upper running ball (diameter 10 mm, AISI 52100 steel) against the lower stationary disc (24  7.9 mm2, AISI 52100 steel) which was driven to reciprocate at a given frequency and displacement. The experiments were conducted at the frequency of 25 Hz, at an amplitude of 1 mm, and relative humidity of 35–45%. Prior to the friction and wear test, 0.12 mL lubricant was dropped to the ball-disc contact area. The wear volume loss of the lower disc was measured by a MicroXAM 3D noncontact surface mapping profiler. EHL minimum film thickness (hmin) could be calculated for the various lubricants based on testing conditions with the following equations [18]:

n

Corresponding authors. E-mail addresses: [email protected] (F. Zhou), [email protected] (W. Liu).

0301-679X/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.triboint.2014.01.005

!1=21
 0 h0 U η0 α 5=7 E R2 ¼ 1:73 R R W

hmin ¼ 0:75h0

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D. Li et al. / Tribology International 73 (2014) 83–87

w e a r s c a r d ia m e te r/m m

Scheme 1. TEMPOs used in this work. From left to right: TEMPO, TEMPO–OH, TEMPO–CO and TEMPO–CONH2.

0.5

3. Results and discussions 0.4 PEG 0.5wt% TEMPO-OH/PEG 1.0wt% TEMPO-OH/PEG 2.0wt% TEMPO-OH/PEG 4.0wt% TEMPO-OH/PEG

0.3

50N

100N

150N

200N

250N

300N

Load/N Fig. 1. Wear scar diameter of steel balls lubricated by PEG with or without the addition of TEMPO–OH at 25 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min).

Wear volume /10 -4 mm3

The oxidative stability of the TEMPOs additives in PEG was evaluated by RBOT test using SH/T-0193 and ASTM D 2272 procedures. A total of 50 g of sample, 5 cm3 of water, and catalytic copper wire were placed in an oxygen bomb. Oxygen was introduced at room temperature and 620 kPa initial pressure. The oxygen bomb was rotated at a rate of 100 rpm at 100 1C, and the oxygen pressure inside the reactor was monitored. The test was continued until the pressure reached a predetermined value. The test duration was reported as OIT, which characterized the oxidative stability of specimen. The operations of RBOT test were repeated 3 times.

4

2 PEG 0.5wt% TEMPO-OH/PEG 1.0wt% TEMPO-OH/PEG 2.0wt% TEMPO-OH/PEG 4.0wt% TEMPO-OH/PEG

0 50N

100N

150N

200N

250N

300N

Load/N Fig. 2. Wear volumes of steel disks lubricated by PEG with or without the addition of TEMPO–OH at 25 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min).

The pressure–viscosity coefficients (α) of the three lubricants were 6 GPa  1 (αPEG) [19], 14.5 GPa  1 (αU20 and αE20) [20]. The hmin of PEG yields the following results: 0.015 μm (50 N) and 0.014 μm (150 N). U20, having a higher α value, yields higher hmin: 0.021 μm (50 N) and 0.020 μm (300 N). E20 values are intermediate: 0.018 μm (50 N) and 0.017 μm (250 N). The lubrication regime of these lubricants under the testing conditions could be judged by the parameter λ ratios (film thickness to composite surface roughness). Based on a surface roughness of about 0.025 μm, all the tribological tests under these testing conditions were in the boundary lubrication regime (λ o1). The atomic oxygen radiation resistance measurement was performed at 2.5  10  4 Pa oxygen, beam energy 5 eV, flux density 5  1015 atom/cm2 s, and 4 h. All the operations were repeated 3 times and the average value was used in Figs. 1–8. The X-ray photoelectron spectrometer (XPS) analysis was carried out on a PHI-5702 multifunctional XPS, using Al Kα radiation as the exciting source. The binding energies of the target elements were determined at a pass energy of 29.35 eV, with a resolution of about 70.3 eV, using the binding energy of organic carbon (C1S: 284.8 eV) as the reference.

3.1. Physical properties of PEG, E20 and U20 before and after the addition of TEMPO–OH The physical properties of PEG, E20 and U20 before and after the addition of TEMPO–OH are given in Table 1. Clearly, the viscosity and density of PEG, E20 and U20 were not significantly changed after the addition of TEMPO–OH, which is typical for the addition of a low amount of additive. 3.2. Tribological properties at ambient conditions The tribological properties of PEG, E20 and U20 (before and after the addition of TEMPOs) were tested. First, TEMPO/PEG was used as a model system to explore the suitable addition amount. As the results in Figs. 1 and 2 about the wear scar diameter and wear volume have shown, the addition of TEMPO into PEG improves the antiwear ability significantly. The antiwear ability of PEG itself was only  150 N and it reached 250–300 N when 1.0–4.0 wt% TEMPO was added. Meanwhile, the wear scar diameter after lubrication with PEG at 150 N was 0.49 mm and the wear volume was 4.20  10  4 mm3. After the addition of 2.0 wt% TEMPO–OH, the wear scar diameter and wear volume decreased to 0.40 mm and 1.27  10  4 mm3 respectively. The corresponding morphological feature pictures of the steel surface after lubrication are given in Fig. 3. Moreover, the addition of TEMPO–OH results in smooth lubrication and low friction co-efficiency, Fig. 4. If lubricating by pure PEG, a shaky lubrication curve was observed with  0.13 friction coefficient. After the addition of 2.0 wt% TEMPO–OH, a smooth lubrication curve was obtained with o0.10 friction coefficient. By using 2.0 wt% concentration, the antiwear ability of TEMPOs with different functional groups was checked. The results about the wear scar diameter and wear volumes are given in Figs. 5 and 6. Clearly, the addition of TEMPOs with different functional groups could improve the antiwear performance of PEG too, and the presence of functional groups such as hydroxyl, carbonyl and amide does not influence the antiwear behavior remarkably. All the TEMPO derivatives exhibited similar performance as the pure TEMPO. Due to the slightly better performance, the TEMPO–OH was chosen for the next set of experiments in the study. E20 and U20 oils are extensively used in industry and the TEMPO–OH was added into E20 and U20 for measurement. The wear scar diameters are given in Figs. 7 and 8. Clearly, the addition of TEMPO–OH does not improve the antiwear performance of E20 obviously. Similar extreme pressure and wear scar diameter were observed with or without the addition of TEMPO–OH. 3.3. Stability under atomic oxygen radiation Although similar performance exhibited when TEMPO–OH was added into E20 and U20, the presence of free radical in TEMPOs

D. Li et al. / Tribology International 73 (2014) 83–87

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Fig. 3. Morphological feature pictures of the steel surface lubricated for 30 min by PEG (left) and 2.0 wt% TEMPO–OH/PEG (right) at 25 1C (stroke: 1 mm, frequency: 25 Hz, load: 150 N, duration: 30 min).

w e a r s c a r d i a m e te r/m m

Friction coefficient

0.20 0.15 PEG

0.10 2.0wt%TEMPO-OH/PEG

0.05 0.00 0

500

1000

1500

0.6

0.5

0.4 E20 2.wt% TEMPO-OH/E20

0.3

2000

50N

Time/s

150N

250N

300N

Fig. 7. Wear scar diameter of steel balls lubricated by E20 or TEMPO–OH/E20 at 25 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min).

w e a r s c a r d i a m e te r /m m

0.7

0.5

0.4 PEG 2.0wt% TEMPO /PEG 2.0wt% TEMPO-OH/PEG 2.0wt% TEMPO-CO/PEG 2.0wt% TEMPO-CONH2/PEG

0.3

50N

100N

150N

200N

250N

0.6 0.5 0.4 U20 2.0wt% TEMPO-OH/U20

0.3 0.2

300N

50N

100N

Load/N

150N

200N

250N

300N

Load/N

Fig. 5. Wear scar diameter of steel balls lubricated by PEG or TEMPOs/PEG at 25 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min).

Wear volume /10-4mm3

200N

Load/N

Fig. 4. The friction coefficient with time at 25 1C (stroke: 1 mm, frequency: 25 Hz, load: 150 N, time: 30 min).

w e a r s c a r d i a m e te r/m m

100N

Fig. 8. Wear scar diameter of steel discs lubricated by U20 or TEMPO–OH/U20 at 25 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min).

Table 1 PEG, E20 and U20 before and after the addition of TEMPO–OH.

4

2 PEG 2.0wt% TEMPO /PEG 2.0wt% TEMPO-OH/PEG 2.0wt% TEMPO-CO/PEG 2.0wt% TEMPO-CONH2/PEG

0 50N

100N

150N

200N

250N

300N

Load/N Fig. 6. Wear volumes of steel discs lubricated by PEG or TEMPOs/PEG at 25 1C (stroke: 1 mm, frequency: 25 Hz, duration: 30 min).

U20 2.0 wt% TEMPO– OH/U20 E20 2.0 wt% TEMPO– OH/E20 PEG 2.0 wt% TEMPO– OH/PEG a b

Kviscosity (mm2/s) 40 1Ca

Kviscosity (mm2/s) 100 1C

Vindexb d/kg/m3 25 1C

35.04 38.54

7.15 7.40

173.1 161.6

875.2 879.5

26.99 27.39

5.05 5.18

115.0 120.7

889.1 890.8

39.68 40.45

7.15 7.01

144.0 134.0

1129.4 1127.2

Kinematic viscosity. Viscosity index.

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D. Li et al. / Tribology International 73 (2014) 83–87

Fig. 9. Pictures of U20 (left) and 2.0 wt% TEMPO–OH/U20 (right) on steel discs after been radiated under atomic oxygen radiation for 4 h.

10

10

O1s

Fe2p 8

In te n s ity (a .u .)

Intensity (a.u.)

8

6

4

2

6

4

2

0

0 740

730

720

710

545

700

540

535

530

525

Binding Energy/eV

Binding Energy/eV 10

N1s In te n s ity (a .u .)

8

6

4

2

0 415

410

405

400

395

Binding Energy/eV Fig. 10. XPS spectra of Fe, O and N of worn surface lubricated by PEG, U20 and E20 with or without addition of TEMPO–OH at room temperature (stroke: 1 mm, frequency: 25 Hz, PEG: 150 N, U20 and E20: 200 N, duration: 30 min).

suggests that it might improve the atomic oxygen resistance of oils under atomic oxygen radiation. U20 and 2.0 wt% TEMPO–OH/U20 was used as the model system for the test. Both U20 and 2.0 wt% TEMPO–OH/U20 on discs were radiated under atomic oxygen radiation for 4 h at room temperature. As the pictures shown in Fig. 9, it is clear that TEMPO–OH as additive can improve the stability of U20 under atomic oxygen radiation. The U20 oil turned to be a solid film on the steel plate but fine liquid film maintained after the addition of TEMPO–OH. Clearly, the TEMPO additive behaves as a radical scavenger and it inhibits the oxidation reaction induced by atomic

oxygen radiation. These results indicate that TEMPOs might be potential additive in space lubricating oils. 3.4. Oxidative stability measurement The presence of free radical group in TEMPOs could also improve the oxidative stability of PEG because it can quench the radicals generated during oxidation reaction. The oxidative stability of PEG and 2.0 wt% TEMPO–OH/PEG was evaluated with the RBOT test. Addition of 2.0 wt% of TEMPO–OH into PEG resulted in 299 min of prolonged

D. Li et al. / Tribology International 73 (2014) 83–87

protection time. Meanwhile, the reference oil, pure PEG only sustained induction time of 87 min. The results reveal that the compounds containing free radical group can enhance the oxidative stability significantly. 3.5. Surface analysis Fig. 10 shows the XPS spectra of the worn surfaces of the steels lubricated with PEG, U20 and E20 with or without addition of TEMPO–OH. From the XPS spectra of Fe2p and O1s, it can be seen that similar spectra were obtained after the addition of TEMPO–OH. Meanwhile, there was no obvious N1s peak after lubricating with U20 and E20 containing TEMPO–OH. It suggests that the TEMPO–OH molecules did not interact strongly with the steel surface when it was dissolved in U20 or E20. This is reasonable because similar extreme pressure results were obtained before and after the addition of TEMPO–OH into U20 and E20. Clear peak for N1s was observable on the worn surfaces of the steels lubricated with PEG with TEMPO– OH additive. That means that relatively stronger interaction between TEMPO–OH and the steel surface might be the reason for the better lubrication performance of TEMPO–OH/PEG.

4. Conclusions In conclusion, TEMPOs as lubricating oil additives in PEG, U20 and E20 were studied. The results showed that TEMPOs can improve the lubrication behavior of PEG and can enhance the oxidative stability significantly with RBOT test. Moreover, the addition of TEMPOs into U20 resulted in atomic radiation resistance and thus it might be a potential additive for space lubrication oils.

Acknowledgments The authors would like to thank the financial support of this work by NSFC (51105354) and National Key Basic Research Program of China (973) (2013CB632301). The authors also thank Bo Wang for his help with the XPS analysis.

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