Friction and Wear Performance of Boundary-lubricated DLC/DLC Contacts in Synthetic base Oil

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Procedia Engineering 68 ( 2013 ) 518 – 524

Available online at www.sciencedirect.com

Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia

doi: 10.1016/j.proeng.2013.12.215

ScienceDirect

The Malaysian International Tribology Conference 2013, MITC2013

Friction and wear performance of boundary-lubricated DLC/DLC

contacts in synthetic base oil

Haci Abdullah TASDEMIR

a,

*

,

Takayuki TOKOROYAMA

a

, Hiroyuki KOUSAKA

a

,

Noritsugu UMEHARA

a

and Yutaka MABUCHI

b

a_{Department of Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya} 464-8603, Japan

b

Nissan Motor Co., Japan

Abstract

The use of diamond-like carbon (DLC) coatings in engine components can be one of the potential solutions for automotive industry to improve fuel efficiency by controlling the friction and wear. The tribological behavior of various types of DLC coatings in liquid lubricated systems and their interaction with lubricants are needed for proper usage of these coatings. In this study, the friction and wear performance of four different DLC coatings in synthetic base oil polyalpha-olefin (PAO) were investigated using pin-on-disc tribometer under boundary lubricated condition. One hydrogen-free ta-C DLC, one hydrogenated a-C:H, one silicon doped a-C:H:Si and one chromium doped a-C:H:Cr were deposited on SUJ2 bearing steel pins and discs for this study. The structural, mechanical and topographical characterizations of these films were analyzed using Raman spectroscopy, Nano indenter and Atomic Force Microscopy (AFM).

The results indicate that structure and doping elements greatly affect the tribological performance of DLC coatings. Ta-C coatings gave the lowest boundary friction, while Si-doped a-C:H:Si showed the highest boundary friction. Hydrogenated a-C:H coating exhibited the highest wear rate, but silicon and chromium incorporation in a-C:H coatings greatly increased the wear resistance. Cr-doped a-C:H:Cr showed the lowest wear rate, followed by Si-doped a-C:H:Si coating.

Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia.

Keywords: DLC; Friction; Wear; Boundary Lubrication

1. Introduction

Diamond-like coatings (DLC), namely amorphous carbon films, can be deposited by various advanced chemical vapor deposition (CVD) and physical vapor deposition (PVD) techniques [1]. These coatings consist of mixture of sp3_{and sp}2_{hybridized carbon and can be alloyed with certain elements such as hydrogen, nitrogen, silicon, titanium,}

* Corresponding author. Tel.: +81-080 -4542-1907 E-mail address: tasdemirhaci@hotmail.com

Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia

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boron, fluorine to improve their properties [2]. DLC coatings have been used widely in many areas as surface coating due to their promising mechanical and tribological properties such as high hardness, chemical inertness, low coefficient of friction and high wear resistance [3]. The various types of DLC coatings have unique tribological properties depending on structural and chemical nature, humidity, temperature, working conditions, surrounding environment, substrate materials and counterpart materials [4, 5, 6]. Hydrogen-free ta-C and a-C DLC coatings exhibit low friction coefficient in humid environments, but wear rate of a-C DLC is higher than ta-C DLC [5, 7, 8]. Hydrogenated DLC coatings show super-low friction in dry and inert environments, but the presence of dopants in hydrogenated DLC coatings greatly affect the tribological performance [9, 10, 11, 12].

In recent years, use of DLC coatings has increased in many automotive components to improve the friction and durability of mechanical components [13, 14, 15]. Some tribological components in engine operate under lubricated conditions. Therefore, tribological behavior of various DLC coatings under lubricated conditions and their interaction with lubricant are needed to achieve superior performance [16, 17, 18]. This paper compares the tribological behavior of four different types of DLC coatings in synthetic base oil under boundary lubricated condition.

2. Experimental details

In this study, four different types of DLC coatings were tested; one hydrogen-free tetrahedral amorphous coating (ta-C) by filtered cathodic vacuum arc (FCVA), one hydrogenated a-C:H by plasma-enhanced CVD, one Si-doped a-C:H:Si by plasma-enhanced CVD and one Cr-doped a-C:H:Cr by plasma-enhanced CVD. The coatings were deposited onto SUJ2 bearing steel discs and pins with the thickness about 1 and 2 ȝm. A thin metal interlayer was used to increase the adhesion between the coating and the substrate. Average hardness of substrate was 62 HRC for SUJ2 steel disc and pin. Hydrogen contents of the DLC films were measured with Elastic Recoil Detection analysis (ERDA) system. Details of important characteristics of DLC coatings tested in this study are described in table 1. Table 1. Important characteristics of DLC coatings tested in this study

Materials Deposition Method Thickness (ȝm) Hydrogen (at. %) Hardness (GPa) Elastic Modulus (GPa) Roughness Disc (nm) a-C:H PECVD 1Ͳ2 25 18±3 122±10 11±3 a-C:H:Si PECVD 1Ͳ2 27 24±3 189±12 14±4 a-C:H:Cr PECVD 1Ͳ2 22 20±2 248±12 4±2 ta-C FCVA 1 <1 75±5 900±50 17±5

Tribological tests were performed using home-build pin on disc type unidirectional tribotester (Fig.1). Tests were carried out for self-mated DLC/DLC contacts for clear evaluation of tribological properties of coatings. In all experiments, Ø 5 mm x 5 mm flat ended circular cylindrical pin and Ø 22.5 mm x 4 mm flat disc were used. The lower, flat disc was mounted on a steel holder fixed to a rotary turnable, while the upper, pin was located 6 mm in diameter from the center of the disc. The pin was fixed to prevent from rotating to ensure pure sliding condition. The DLC-coated pin was loaded and rubbed against DLC-coated disc with 5 N applied load which correspond to a maximum initial Hertzian contact pressure of around 150 MPa. The temperature, duration and total sliding cycles of the tribological tests were 80 °C, 1 h and 10700 cycles, respectively. Both pin and disc were totally immersed in lubricant during tests. All samples were washed with acetone in an ultrasonic bath to remove oil species and contaminants before and after the tests. The tests were repeated at least 3 times for verification and reproducibility of results.

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Fig. 1. Schematic representation of pin-on-disc configuration

Synthetic polyalpha-olefin (PAO4) having viscosity of 19 mm2_{/s and pressure-viscosity coefficient of 17.09 GPa} -1_{at 40 °C was used in this study. The viscosity and pressure-viscosity coefficient of oil at 80 °C were 6.11 mm}2_/s

and 12.85 GPa-1_{, respectively. Under these conditions, the lambda ratio was calculated using operating parameters,}

lubricant properties and surface roughness to determine the lubrication regime [19]. The calculated lambda ratio was less then unity which indicates that operating regime was boundary lubrication. The wear rate calculation was performed only on pin specimens by measuring the width of the wear scar. The wear rate calculated using Archard wear equation (Eq. (1))

kFs

V

(1) where k is the wear rate (m3_{/N.m), F the normal load (N), s the sliding distance (m), V the wear volume (m}3_).

Optical microscopy and scanning electron microscopy were used to measure the width of the wear scar. 3. Results and discussion

The friction coefficients as a function of sliding cycles for self-mated DLC/DLC contacts using four different DLC coatings in synthetic base oil are shown in Fig. 2. The ta-C coating showed the lowest friction coefficient, while Si-doped a-C:H:Si showed the highest friction coefficient. The running-in periods of a-C:H and a-C:H:Si was very short. The friction coefficient of a-C:H coating decreased from 0.1 to 0.06 after around 800 cycles and remained almost constant throughout the test. The friction coefficient of a-C:H:Si coatings decreased slightly from 0.11 to around 0.1 and reached the steady state value. Cr-doped a-C:H:Cr coating exhibited a gradual reduction of friction with sliding cycles that showed initial friction coefficients of 0.09 and finally reached to less than 0.06 after 10700 cycles. The running-in period of ta-C coating was longer than those a-C:H and a-C:H:Si which reached the steady state value after around 7000 cycles. The friction coefficient of ta-C coating gradually decreased from 0.08 to 0.025.

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Fig. 2. Friction coefficients of self-mated DLC/DLC contacts as a function of sliding cycles for four different DLC coatings in PAO oil. The generated wear tracks on the DLC coated discs were barely distinguishable. Therefore, accurate wear measurement on disc specimens was found to be impracticable. The wear rate calculations are done only for pin specimens by using Eq. (1). The wear rates of different DLC pins with standard error are displayed in Fig. 3 when rubbed against self-mated DLC coated discs in PAO oil. Hydrogenated a-C:H coating gave the highest wear rate, followed by hydrogen-free ta-C coating and the lowest wear rate was provided by a-C:H-Cr as can be seen in Fig. 3. Although silicon incorporation in a-C:H coating resulted in higher friction coefficient as it is seen in Fig.2, it greatly increased the wear resistance of DLC coating. Compared to a:C:H coating, Si-doped a-C:H:Si film showed better wear resistance. In terms of chromium incorporation in a-C:H coating, it does not changed the friction behavior so much compared to a-C:H coating, but significantly increased the wear resistance.

Fig. 3. Wear rates of different DLC pins when rubbed against self-mated DLC coated discs in base oil.

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Raman spectroscopy analysis was performed on the wear scar of DLC coatings for better understanding and interpretation of the experimental results. It is suggested that structural transformation or graphitization of DLC coatings can be responsible for the low friction performance. Raman spectroscopy is a powerful, non-destructive method for identification of friction-induced graphitization of DLC coatings. An increase in ID/IG ratio where ID and

IG are maximum intensity of D-peak and G-peak, respectively, can be attributed to the graphitization of DLC coating.

In Fig. 4, we compared the Raman spectra of as deposited DLC coating with those obtained after friction tests. Raman spectra obtained inside of the wear scar of the hydrogenated a-C:H coating showed a significant increase in the ID/IG ratio compared to those as-deposited spectra (Fig. 4a). On the other hand, Raman spectra obtained inside of

the wear scar of hydrogen-free ta-C, Si-doped DLC and Cr-doped DLC indicate that no significant structural changes of the coating occurred during the friction tests (Fig. 4b, 4c, and 4d).

Results revealed that tribological properties of DLC coatings in oil lubricated condition greatly affected by dopant elements. a-C:H, a-C:H:Si and a-C:H:Cr were deposited by same PECVD method. Si-doped coating have similar hydrogen content and surface roughness, but have higher mechanical properties compared to non-doped a-C:H coating. Therefore, higher friction coefficient and improved wear resistance of Si-doped DLC could be attributed to silicon incorporation into the coating undoubtedly. Cr-doped DLC also have better mechanical properties than non-doped a-C:H coating. Furthermore, Cr-doped DLC is provided superior wear resistance and slightly less friction coefficient than non-doped a-C:H coating. Overall, it is noted that doped DLC coatings increased the wear resistance compared to non-doped a-C:H coating and this behavior attributed to the better mechanical properties of doped coatings like high hardness and elastic modulus. However, in term of friction, such generalization cannot be done, since different dopant element resulted in totally different friction coefficient.

Non-hydrogenated ta-C DLC exhibited the lowest friction coefficient. Raman analysis revealed that only a-C:H showed the clear increase in the ID/IG ratio which is indicator of graphitization. The friction reduction of a-C:H

coating could be linked the graphitization. However, comparing to low friction behavior and Raman spectra analysis of various DLC coatings, it could be noted that graphitization is not the crucial factor for low friction. It is believed that sp3 _{hybridization, hydrogen content or deposition methods of DLC coatings could be more important than}

graphitization for low friction behavior of DLC coatings under oil lubricated conditions.

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Fig. 4. Raman Spectra of (a) a-C:H. (b) a-C:H:Si, (c) a-C:H:Cr and (d) ta-C DLC coatings before and after sliding in base oil. 4. Conclusions

In the study, tribological behavior of four different DLC coatings under boundary lubricated condition has been tested in pin-on-disc configuration. The following conclusions can be drawn from this study:

The ta-C DLC coatings exhibited the lowest friction coefficient than other types of DLC coatings. Hydrogenated a-C:H DLC showed the highest wear rate, but doping silicon or chromium into a-C:H

greatly increased the wear resistance.

Silicon incorporation into a-C:H DLC resulted in higher friction coefficient, but Si-doped DLC showed less wear than a-C:H coating.

Cr-doped DLC exhibited the lowest wear rate and showed slightly lower friction coefficient than a-C:H coating.

Dopant elements enhance the durability of DLC coatings. However, friction coefficients of doped DLC coatings vary depending on dopant elements.

Raman spectra analysis reveals that only non-doped a-C:H coating showed clear graphitization. The friction reduction of a-C:H coating can be linked to the surface graphitization. However, overall low friction behavior of various DLC coatings cannot be explained by surface graphitization. Deposition methods, sp3 hybridization or hydrogen content could be more crucial for low friction of DLC coating under lubricated conditions.

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