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210

Tribology OnlineJapanese Society of Tribologists

http://www.tribology.jp/trol/

Vol. 16, No. 4 (2021) 210-215.ISSN 1881-2198

DOI 10.2474/trol.16.210

Article

Effect of Degree of Unsaturation in Vegetable Oils on Friction Properties of DLC Coatings

Kentaro Yoshida1)*, Yasuhiro Naganuma1) and Makoto Kano2)

1) Material and Machanical Technology Division, Kanagawa Institute of Industrial Science and Technology,705-1 Shimo-imaizumi, Ebina, Kanagawa 243-0435, Japan

2) Institute of Innovation Research, Tokyo Institute of Technology,2-12-1 Oookayama, Meguro-ku, Tokyo 152-0033, Japan

*Corresponding author: Kentaro Yoshida ([email protected])

Manuscript received 27 May 2021; accepted 06 September 2021; published 15 October 2021

Abstract

Recently, it has been reported that combinations of various types of vegetable oils containing organic acids and DLC coatings are effective for reducing friction, but there are few reports of detailed investigations into the relationship between differences in the chemical structures of vegetable oil lubricants and friction reduction. Therefore, the authors investigated the influence on friction properties of two types of DLC coatings, a-C:H and ta-C under lubrication with vegetable oils which have different degrees of unsaturation. The ta-C coating displayed markedly lower friction coefficients than the a-C:H coating and the uncoated steel with all of the vegetable oil lubricants used. It was found that the low friction properties of the ta-C coating showed even lower friction coefficients with vegetable oils containing a higher content of monounsaturated fatty acids. Additionally, the sliding surfaces of ta-C coated discs and cylinders were analyzed by XPS and ToF-SIMS analysis. The results confirmed the formation of a surface layer consisting of C-OH bonds, and a lower friction coefficient was seen as the detected intensity of these bonds increased.

Keywords

DLC, vegetable oil, friction, lubrication, unsaturation, fatty acid, XPS, ToF-SIMS

Copyright © 2021 Japanese Society of TribologistsThis article is distributed under the terms of the Creative Commons BY-NC-ND 4.0 License.

1 Introduction

Abnormal weather conditions have been occurring more frequently and with greater intensity around the world in recent years accompanying global warming on a worldwide scale. One causal factor cited in this regard is the increase in atmospheric emissions of carbon dioxide (CO2). As measures for reducing CO2 emissions from automotive engines, technologies are steadily being adopted to improve fuel economy by lowering the friction levels of engine sliding parts. Another serious issue for the global environment is the low rate of biodegradability of polymer materials. In this regard, engine and powertrain lubricants still make use of mineral oils and various types of additives containing toxic, low biodegradable materials [1].

Against this backdrop, diamond-like coatings (DLCs) have begun to be applied for reducing the friction of engine sliding parts. It has been found that the tetrahedral amorphous carbon (ta-C) coating in particular, which is essentially hydrogen free, displays ultra-low friction properties with a friction coefficient of 0.01 or lower when lubricated with environmentally friendly alcohols such as glycerin or organic acid lubricants like oleic acid [2-4]. It has been reported that combinations of various types of vegetable oils containing organic acids and DLC

coatings are effective for reducing friction [5, 6]. Such friction properties are influenced by various factors, but there are few reports of detailed investigations into the relationship between differences in the chemical structures of vegetable oil lubricants and friction reduction. Therefore, the authors investigated the influence on friction properties of vegetable oils have different degrees of unsaturation and which are known to have a low environmental impact and a pronounced effect on reducing friction. The aim of this work is to obtain a guideline for combining materials such as environmentally friendly DLC coatings and lubricants for future application. Especially for the sliding components in the power train system of the future vehicle, the smooth ta-C coating will be required to display the super-low friction property as well. This paper presents the results of this investigation.

2 Experimental procedure

2.1 Test specimensTest discs (33 mm in diameter and 3 mm thick) and

cylinders (9 mm in diameter and 9 mm long) were made of quenched AISI52100 bearing steel having a hardness of 60 HRC. All the discs and cylinders were lapped to a mirror-like

Tribology Online, Vol. 16, No. 4 (2021) /211Japanese Society of Tribologists (http://www.tribology.jp/)


finish, with the discs having a root mean square roughness (Rq) of approximately 3 nm and that of the cylinders being approximately 12 nm. The surface roughness was measured five times for different areas for each sample by using the surface roughness instrument, ACCRETECH Surfcom 2000 SD. For cylinder, the surface of the cylinder along parallel to the line direction was measured. Following lapping, a hydrogenated DLC coating (a-C:H: hydrogen content of approximately 20 at%, coating hardness 20 GPa measured by nano-indentation method and coating thickness t = 1.0 μm) was deposited on the surface by plasma chemical vapor deposition (PCVD) and a hydrogen-free DLC coating (ta-C: coating hardness 60 GPa and coating thickness t = 0.3 μm) was deposited on the surface by T-shaped filtered arc deposition (T-FAD) [7].

2.2 LubricantsSix types of vegetable oils (Table 1) were used as lubricants.

The table shows the respective contents (%) of the saturated fatty acids, monounsaturated fatty acids, diunsaturated fatty acids and triunsaturated fatty acids composing the ester of each oil. The oils were selected on the basis of the relative quantities of these fatty acids.

2.3 Friction testsReciprocating cylinder-on-disc friction tests were

conducted for 600 s using three types of cylinder/disc material combinations, consisting of AISI52100/AISI52100 (as-is base material), a-C:H/a-C:H and ta-C/ta-C. The friction test procedure is shown schematically in Fig. 1. Each material combination of the friction tests was conducted for 10 minutes in air at room temperature(23°C) and repeated three times. The symmetrical line contact was obtained by using the flexible cylinder holder which bended along the line direction at loading. A 5-N load was applied to the stationary cylinder secured on the disc that underwent reciprocating movement with a stroke of 10 mm and a maximum sliding speed of 50 mm/s. The calculated contact

pressure using with the physical properties of AISI52100 is 67 MPa. Friction torque was measured at the maximum sliding speed during reciprocating movement and then divided by the 5-N load to obtain a time history graph of the friction coefficient. The uncoated AISI52100 cylinders and discs and the specimens coated with the DLC coatings were immersed in 2-propanol before the sliding friction tests for ultrasonic cleaning for 5 min. and were then dried in hot air.

The six types of vegetable oils listed in Table 1 with different degrees of unsaturation were used as lubricants. Ten μl of a lubricant were dripped on the sliding friction surface before the test. In total, 18 levels of friction tests were conducted using the three types of materials and six types of lubricants. Table 2 shows the kinematic viscosity and oil film parameter of vegetable oils in sliding tests. The lubrication state was

LoadFixed cylinder

Reciprocating disc

Added lubricant

before sliding

Mono Di TriCoconut Oil Coc 87 6 2 - 5

Olive Oil Oli 14 73 11 - 2Rapeseed Oil Rap 8 63 19 9 1Soybean Oil Soy 16 23 51 7 3

Sunflower Seed Oil Sun 10 20 70 - -Linseed Oil Lin 9 19 14 53 5

Oil Saturated, % Unsaturated, % Others, %Abbreviation

Table 1 Properties of vegetable oils

Fig. 1　Schematic illustration of cylinder-on-disc friction test

Kinematic Viscosity Oil Film Thickness λratio37.8℃, cSt m -

Coconut Oil 29.79 0.022 1.79Olive Oil 46.68 0.033 2.68

Rapeseed Oil 50.64 0.036 2.88Soybean Oil 28.49 0.021 1.72

Sunflower Seed Oil 33.31 0.024 1.97Linseed Oil 29.60 0.022 1.78

Oil

Table 2 Viscosity and oil film parameter


Kentaro Yoshida, Yasuhiro Naganuma and Makoto Kano

estimated as the mixed lubrication judging from the lambda value ranging from 1 to 3.

3 Test results and discussion

3.1 Average friction coefficients of DLC coating/lubricant combinationsFigure 2 shows the average friction coefficients obtained

for each material combination with the six lubricants in the friction tests. Average friction coefficients were calculated by averaging friction coefficients at the max speed in oscillating motion for 10 minutes. The friction coefficients of the ta-C coating were markedly lower at less than 0.03 compared with the values of the other materials for all six types of vegetable oils used. In contrast, although the a-C:H coating displayed lower friction coefficients of around 0.05 compared with values of around 0.08 for the uncoated AISI52100, it did not have such a pronounced effect on reducing friction as the ta-C coating. The AISI52100 and a-C:H specimens showed the lowest friction coefficients with rapeseed oil, which has a large content of monounsaturated fatty acids. For the ta-C coating that showed low friction coefficients with all of the oil types, the results of enlarging the vertical axis are presented in Fig. 3 so as to make the differences in the friction coefficients more distinct. The ta-C

coating also showed the lowest friction coefficients of 0.017 and 0.018 with rapeseed oil and olive oil, respectively. These two oils contain large amounts, 60-70%, of monounsaturated fatty acids. These values were followed by friction coefficients of 0.02 and 0.022 with soybean oil and sunflower seed oil, which contain around 20% of monounsaturated fatty acids, and then 0.026 with linseed oil containing 19% of monounsaturated fatty acids. The highest friction coefficient displayed by the ta-C coating was 0.027 with coconut oil having the smallest content of monounsaturated fatty acids at 6%. The results showed a relationship between low friction properties and the quantity of double bond compounds present in the lubricants for the ta-C coating lubricated with the vegetable oils.

3.2 Appearance and surface roughness of friction surfaceThe appearance and surface roughness of the sliding

surfaces of the ta-C and a-C:H coated discs and cylinders were examined when lubricated with rapeseed oil and linseed oil. These two oils showed distinctly different effects on the friction coefficients of the ta-C coating in the friction tests. The photographs in Fig. 4 show the appearance of the sliding surfaces of the discs and cylinders.

With both lubricants, no pronounced wear scars were observed on the sliding surfaces of the a-C:H and ta-C coatings on the discs. In contrast, the sliding surfaces of the ta-C coating on the mating cylinders showed the formation of distinct sliding marks. The band-like part was judged as the shallow wear scar which displayed the different color of ta-C coating by the optical interference. Therefore, the wear depth of ta-C coating was seemed to be very shallow to have substantial thickness of ta-C coating. In addition, with the rapeseed oil lubrication that exhibited low friction coefficients, wear marks with more distinct sliding marks over a wider area were observed compared with the sliding surface lubricated with linseed oil. The results suggest that ultra-low friction attributable to tribochemical reactions occurred on the ta-C coating under oleic acid lubrication, which is similar to the phenomenon reported by Dr.Stefan Makowski in Scientific Reports, Physical Review [8].

The surface roughness results for the above-mentioned DLC-coated discs and cylinders in their initial state before sliding and after the sliding friction tests under linseed oil lubrication and rapeseed oil lubrication are shown in Figs. 5 (a) and (b), respectively. The surface roughness was measured five times for different areas for each sample by using the

Fig. 3 Average friction coefficients for ta-C / ta-C lubricated with vegetable oils

Fig. 2 Average friction coefficients for each material pair lubricated with vegetable oils



surface roughness instrument, ACCRETECH Surfcom 2000 SD. Observation of the disc side reveals that no clear decline in surface roughness is seen for the a-C:H coating compared with its initial state before sliding, whereas the ta-C coating clearly

shows a reduction of surface roughness. It is notable that a low level of surface roughness created by tribo-chemical polishing wear is observed for rapeseed oil, for which the lowest friction coefficients were seen in the friction tests. In contrast, for the cylinders, surface roughness increased for both the a-C:H and ta-C coatings. That can probably be attributed to a more severe wear condition for the cylinder owing to its state of constant contact compared with the disc. Consequently, as noted here, surface roughness of the cylinders increased accompanying the deterioration of their surface due to the occurrence of tribochemical reactions, whereas polishing action on the mating discs presumably produced a mirror-like surface.

3.3 Friction surface analysisAs explained above, it was assumed that the ta-C coating

displayed a marked friction reduction under vegetable oil lubrication owing to tribochemical reactions at the sliding surfaces. Accordingly, both the a-C:H and ta-C coatings were subjected to a surface analysis following the sliding tests to examine differences in the chemisorption states and to consider the connection with friction characteristics.

As the first surface analysis, X-ray photoelectron spectroscopy (XPS) was used to investigate the types of chemisorbed bonds on the surface and to evaluate their strength. The main conditions of the analysis are shown in the following. XPS instrument:PHI 5000 VersaProbe II of ULVAC-PHI inc., The analyzed area :100μmΦ, Pass energy: 58.7 eV., Peak fitting software: MultiPak. Just before the analysis, the coating specimens underwent ultrasonic cleaning for five minutes in n-hexane to remove contaminants adhering to the surface. Figure 6 presents an example of the four component peaks deconvoluted from the C1s peak obtained by XPS analysis from the surface an a-C:H coating lubricated with rapeseed oil. The state of carbon bonds can be found from the C1s profile obtained by this analysis. It was estimated from the obtained peaks that the following components were co-present on the specimen surface: hydrocarbons (C-C or C-H: 284.3 eV), carbon-oxygen single bonds (C-O: 285.5 eV), carbonyl groups (C=O: 287.9 eV) and carboxyl groups (O=C-O: 289.0 eV).

The peak area proportions were obtained by deconvoluting the constituent peaks of the C1s spectra obtained by XPS analysis of the disc friction surface of the a-C:H coating and the ta-C coating for four types of combinations: a-C:H/Rap, a-C:H/Lin, ta-C/Rap and ta-C/Lin. The results are compared in Fig. 7. It was noted earlier that relatively low friction coefficients

(b)

(a)

Fig. 4 (a) Sliding surfaces on discs, (b) Sliding surfaces on cylinders

(b)

(a)

Fig. 5 (a) Surface roughness of discs, (b) Surface roughness of cylinders

Fig. 6 C1s peak fitting of a-C:H coating lubricated with rapeseed oil


Kentaro Yoshida, Yasuhiro Naganuma and Makoto Kano

were obtained with rapeseed oil lubrication and high friction coefficients with linseed oil lubrication. It was found that bonds containing oxygen, such as C-O, C=O, O-C=O, etc. were present on the a-C:H coating surface. In contrast, oxygen containing bonds on the ta:C coating surface were limited to C-O alone. These results suggest that fatty acid components originating from the vegetable oils remained on the a-C:H coating surface without undergoing chemical reactions. In contrast to that, the fatty acids originally in the vegetable oils formed a surface layer containing only Cn-O(H) single bonds on the ta-C coating surface accompanying the chemical reactions [9].

The four types of DLC coating surface/lubrication combinations analyzed by XPS were then subjected to a time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis. The main conditions of the ToF-SIMS analysis are shown in the following. The instrument: TRIFT III of ULVAC-PHI, Analyzed area: 10 μm × 10 μm, Primary ion source: Ga+, Primary ion output voltage: 15 kV, Analyzed mode: Mass decomposition priority mode. As one example of the results, Fig. 8 presents the intensities of the mass numbers of the a-C:H/Rap specimen in a range of 0-100. The mass numbers of Cn-O(H) in this range were 29, 41, 53, 67, 77 and 89. Therefore, the intensities of the mass numbers of each specimen are plotted for comparison in Fig. 9. The results confirm that for mass numbers 29, 41 and 53, the combinations with rapeseed oil displayed higher detected intensities than the combinations with linseed oil. Accordingly, this suggests the presence of many Cn-O(H) bonds. It is inferred that the development of this surface state led to the appearance of low friction coefficients.

The reason for the friction coefficient reduction and detection of many Cn-O(H) bonds observed with oils containing a large quantity of monounsaturated fatty acids is attributed to the formation of a strong tribochemical reaction film due to a cross-linking effect [10] stemming from functional groups with high chemical reactivity and the presence of double bonds in two places. It is assumed that the presence of a double bond in only one place would make it more difficult to form links with functional groups and that the presence of a double bond in three places would hinder the cross-linking effect. For the possible reason of different chemical reactive performance between a-C:H and ta-C, the a-C:H coating top surface of which carbon elements were terminated with hydrogen originally without undergoing chemical reactions was not active. On the contrary, the ta-C coating surface of which carbon elements had the dangling bonds created the chemical reactions easily. The fatty acid has the two chemically active portions which were the double bonding and the carboxyl functional group. Those portions were easily chemically adsorbed with the dangling bond initially on the sliding surface of the ta-C coating.

4 Conclusions

This study evaluated the effect of different degrees of unsaturation of vegetable oils on the friction properties obtained with two types of DLC coatings. The results obtained are summarized below.(1) The ta-C coating displayed markedly lower friction

coefficients than the a-C:H coating and the uncoated steel with all of the vegetable oil lubricants used.

(2) It was found that the low friction properties of the ta-C coating showed even lower friction coefficients with vegetable oils containing a higher content of monounsaturated fatty acids.

(3) The sliding surfaces of ta-C coated discs and cylinders were analyzed by XPS and ToF-SIMS analysis. The results confirmed the formation of a surface layer consisting of C-OH bonds, and a lower friction coefficient was seen as the detected intensity of these bonds increased.

References

[1] Kalin, M. and Simic, R., “Atomic Force Microscopy and Tribology Study of the Adsorption of Alcohols on Diamond-Like Carbon Coatings and Steel,” Applied Surface Science, 271, 2013, 317-328.

[2] Kano, M., Yasuda, Y., Okamoto, Y., Mabuchi, Y., Hamada, T., Ueno,

Fig. 7 Peak area proportions for C1s peak fitting

Fig. 8 Intensity of mass numbers detected from sliding surface of a-C:H coating lubricated with rapeseed oil

Fig. 9 Comparison of intensity of Cn-OH mass numbers detected from sliding surface



T., Ye, J., Konishi, S., Takeshima, S., Martin, J. M., De Barros Bouchet, M. I. and Le Mognee, T., “Ultralow Friction of DLC in Presence of Glycerol Mono-Oleate (GMO),” Tribology Letters, 18, 2, 2005, 245-251.

[3] Kano, M., Martin, J. M., Yoshida, K. and De Barros Bouchet, M. I., “Super-Low Friction of Ta-C Coating in Presence of Oleic Acid,” Friction, 2, 2, 2014, 156-163.

[4] De Barros Bouchet, M. I., Martin, J. M., Avila, J., Kano, M., Yoshida, K., Tsuruda, T., Bai, S., Higuchi, Y., Ozawa, N., Kubo, M. and Asensio, M. C., “Diamond-Like Carbon Coating under Oleic Acid Lubrication: Evidence for Graphene Oxide Formation in Superlow Friction,” Scientific Reports, 7, 2017, 46394.

[5] Carrell, J., Slatter, T., Little, U. and Lewis, R., “Combining DLC, Shot Blasting, Chemical Dip and Nano Fullerene Surface Treatments to Reduce Wear and Friction When Used with Bio-Lubricants in Automotive Contacts,” SAE Technical Papers, 2017-01-0878, 2017.

[6] Al Mahmud, K. A. H., Kalam, M. A., Masjuki, H. H. and Abdollah, M. F. B., “Tribological Study of a Tetrahedral Diamond-Like

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[7] Takikawa, H., Izumi, K., Miyano, R. and Sakakibara, T., “DLC Thin Film Preparation by Cathodic Arc Deposition with a Super Droplet-Free System,” Surface and Coatings Technology, 163-164, 2003, 368-373.

[8] Makowski, S., Schaller, F., Weihnacht, V., Englberger, G. and Becker, M., “Tribochemical Induced Wear and Ultra-Low Friction of Superhard Ta-C Coatings,” Wear, 392-393, 2017, 139-151.

[9] Yoshida, K., Kano, M., Masuko, M., Kawaguchi, M. and Martin, J. M., “Relation between Frictional Reducing Property and Tribo-Chemical Reaction about DLC Coating with Oleic Acid Lubrication during Sliding,” Journal of Japan Society of Tribologists, 58, 10, 2013, 773-782 (in Japanese).

[10] Kuwahara, T., Romero, P. A., Makowski, S., Weihnacht, V., Moras, G. and Moseler, M., “Mechano-Chemical Decomposition of Organic Friction Modifiers with Multiple Reactive Centres Induces Superlubricity of Ta-C,” Nature Communications, 10, 2019, 151.

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