Achieving a high adhesion and excellent wear resistance diamond-like carbon film coated on NBR rubber by Ar plasma pretreatment

Accepted Manuscript Achieving a high adhesion and excellent wear resistance diamond-like carbon film coated on NBR rubber by Ar plasma pretreatment

Changning Bai, Aimin Liang, Zhongyue Cao, Li Qiang, Junyan Zhang PII: DOI: Reference:

S0925-9635(18)30353-4 doi:10.1016/j.diamond.2018.08.013 DIAMAT 7186

To appear in:

Diamond & Related Materials

Received date: Revised date: Accepted date:

23 May 2018 26 July 2018 22 August 2018

Please cite this article as: Changning Bai, Aimin Liang, Zhongyue Cao, Li Qiang, Junyan Zhang , Achieving a high adhesion and excellent wear resistance diamond-like carbon film coated on NBR rubber by Ar plasma pretreatment. Diamat (2018), doi:10.1016/ j.diamond.2018.08.013

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ACCEPTED MANUSCRIPT Achieving a high adhesion and excellent wear resistance diamond-like carbon film coated on NBR rubber by Ar plasma pretreatment Changning Baia,b, Aimin Lianga, Zhongyue Caoa,b, Li Qiang a*, Junyan Zhang a* State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China b University of Chinese Academy of Sciences, Beijing 100049, P. R. China

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Abstract:

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Nitrile-butadiene rubber (NBR) exhibits the high friction and wear as a dynamic seal sliding against the engineering materials. To improve its friction and wear resistance,

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diamond-like carbon (DLC) films are devised to deposit on NBR rubber via plasma-enhanced chemical vapor deposition (PECVD). Prior to the deposition, Ar plasma pretreatment of NBR

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substrate is carried out, and the influence of Ar plasma pretreatment time on the adhesion and tribological behavior of DLC coated NBR is studied systematically. The typical micrometer-

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scale patches divided by random cracks are always observed on the DLC film coated NBR

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rubber, and the size of the patches increases with increasing pretreatment time. Moreover, the hardness of the films decreases monotonously as the increase of the pretreatment time, which means that Ar plasma pretreatment has an effect on the properties of the films. It is worth to

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mention that the adhesion strength between DLC films and rubber first increases and then decreases along with the change of Ar plasma pretreatment time. The tribotests results show

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that the DLC film with 30 min pretreatment exhibits the lowest friction coefficient of 0.2 (compared to ~0.8 of uncoated NBR) during 12000 laps. The wear life of the superior DLC/NBR is studied further, and it is functional after 120000 laps tribotests under the normal load of 10 N. Therefore, the DLC film can be considered as a promising candidate coating for the enhancement of wear resistance of rubbers. Keywords: Nitrile-butadiene rubber; DLC films; Surface morphology; Adhesion; Tribological performance

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Corresponding author. Tel.: +86 0931 4968005; 4968295. E-mail address: [email protected] (L. Qiang); [email protected] (J.Y. Zhang).

ACCEPTED MANUSCRIPT 1. Introduction Rubber is commonly used in many engineering sealing systems to avoid the entrance of dirt and the leakage of sealing medium owning to its excellent elasticity and flexibility. Nitrile butadiene rubber (NBR), which has good oil resistance (especially resistant to mineral oil, animal and vegetable oils, liquid fuels and solvents) due to the presence of strong polar groups

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(-CN) in the molecular chain, is one of the most widely used and the lowest cost rubber sealing material in automotive, aerospace industries [1, 2]. However, rubber exhibits the high friction

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energy dissipation and failure of the sealing system [3].

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and severe wear in sliding contact mode at a relatively high speed and high load, which cause

So far, two common methods can be used to reduce the friction and wear of rubber:

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lubricating oil and advance lubricating coating. Lubricating oil or grease is often used to improve the friction of rubbers, but it has deadly drawbacks such as the swelling behavior, the

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degradation of lubricants and environmental pollution problems. Therefore, designing and developing an advanced lubricating coating may be a positive strategy for enhancing the

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performance of rubber.

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The following aspects should be considered in the preparation of coatings on rubbers: (1) low deposition temperature (below the maximum working temperatures of rubber); (2)

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sufficient flexibility (to adapt large deformation of rubber under loading); (3) strong adhesion; (4) low friction and wear rate. Fortunately, diamond-like carbon (DLC) films have been widely

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recognized as an optimal protective coating to meet the above requirements [4-9]. Besides the excellent performance of the DLC films itself, such as relatively high hardness, chemical inertness, low friction coefficient and wear rates, the deposition temperature and flexibility of DLC films can be tailored by adjusting the deposition conditions [10]. On the other hand, the chemical composition of DLC films (mainly composed by C and H) exhibits a good compatibility with rubber. Therefore, DLC films will effectively protect the rubber body itself and reduce the adhesive interaction between the rubber and its counterpart. One of the most important concerns is how to achieve the excellent adhesion between the hard DLC film and flexible rubber. Because it does not sound logical to apply a hard film on a

ACCEPTED MANUSCRIPT soft substrate with elastic deformation without the occurrence of interfacial delamination. Therefore, tailoring a rubber with modified properties on the surface is one of the most intense areas of research. Argon plasma treatment [11-13] becomes a commonly used way to modify NBR or HNBR surfaces with desired surface properties in a controlled manner. Argon is an inert gas, and thus there is no chemical reaction between Ar and the NBR surface. The main

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effect of argon plasma treatment of NBR is the transfer of energy from the plasma species towards the rubber surface (bombardment effect), which are effective at creating free radicals

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but not at adding directly new functionalities on the rubber surface [14]. As a result, it provides a possibility for the binding of carbon atoms to rubber, thereby enhancing the adhesion

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between DLC films and NBR rubber. J. H. Kim et al. [11] found that form-like nanostructures had formed on the NBR surface after argon plasma treatment, resulting in chain scission

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responsible for better bonding with a grease lubricant to have a low friction coefficient. However, D. J. Wolthuizen et al. [12] studied that NBR and HNBR by Ar plasma treatment

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possessed low friction coefficient under the lack of lubricating oil. It can be interpreted in terms of enhancement of cross-linking and reduction of rubber tackiness. D. Z. Segu [13] also

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suggested that a low friction coefficient and wear could ascribe to the enhancement of the

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resistance to mircro-crack, delamination and plastic deformation of NBR after Ar plasma treatment. In general, the Ar plasma treatments resultes in an enhancement of the crosslinking of rubber surface and/or modification of the surface bonding. The rubber surface cross-linking

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involves three main process steps: obstruction of hydrogen atoms from molecular chains by energetic species (such as plasma, uncharged particles), formation of radicals at hydrogen

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obstruction sites and reaction between produced radicals leading to the formation of the cross-linking. The Ar plasma treatment of rubber can break molecular chains and those detached molecular chains may link together to form the crosslinking [15]. However, the surface modified layer of the rubber is relatively thin after the plasma pretreatment, and the problem of poor durability still exists under the high friction load. Although there are some inadequacies in the application of Ar plasma modifying the rubber, the method of the plasma pretreatment provides a new route for improving the adhesion and tribological performance of DLC films on rubber. More recently, a few exploratory works

ACCEPTED MANUSCRIPT on the deposition of DLC films onto NBR [16-19] or HNBR [20-24] substrates confirm this view. Firstly, a clean substrate surface could be provided for thin film deposition by plasma pretreatment. Secondly, the chain scission caused by the plasma pretreatment can provide the possibility of the bonding of carbon ions to rubber. Thirdly, the surface morphology of the rubber can be modified as well as the cleavage of much C-H bonds produce active sites by Ar

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plasma pretreatment [25]. Therefore, Ar plasma pretreatment is undoubtedly an excellent choice to improve the adhesion between rubbers and coating. Although some works [26, 27]

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have been carried out on the related research, further studies are still needed because the

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relevant work is still not expected.

In this study, hydrogenated DLC films were successfully deposited on the NBR rubber by

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plasma enhanced chemical vapor deposition (PECVD) in Ar/CH4 plasma. The rubber substrate was pretreated by Ar plasma before the DLC film deposition in order to improve the adhesion.

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The adhesion of DLC films and NBR was evaluated by X-cut method and the scratch tester. The tribological performance of the coated rubbers was investigated via the rotating ball-on-disc configuration under normal load of 10N. The selection of the normal load of 10 N

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was an accelerated wear test. The purpose was to get a better understanding of the high

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adhesion strength and excellent tribological performance of DLC films on NBR, and to apply in a wider range of fields or harsh working conditions.

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2. Experimental details

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2.1. Sample preparation

Black nitrile butadiene rubber (NBR) sheet of 2 mm thickness is made by northwest rubber plastics research and design institute Co., Ltd. (Xianyang, China), the detail properties are shown in Table 1. Before plasma pretreatment, the rubber substrates of 20×20 mm2 size were first cleaned by soap in an ultrasonic cleaner at 60 °C deionized water for 15 min to remove oil contamination, this procedure was repeated 3 times. Subsequently, boiling water was then used under ultrasonic to eliminate residual soap and wax, and the sample was dried in a centrifugal machine to evaporate all absorbed water at 100 °C for 20 min. Finally, the substrates were cooled down to room temperature for further use.

ACCEPTED MANUSCRIPT Table 1. Properties of NBR rubber. Rubber

Color

Max. stable working temperature (°C)

NBR

Black

120

Roughness (nm)

Shore’s hardness A

Modulus (MPa)

Coefficient of thermal expansion (×10-6K-1)

80

11

175

200

2.2. Plasma pretreatment and film deposition Argon plasma pretreatment of rubber substrates and deposition of DLC films were carried

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out in the direct current plasma-enhanced chemical vapor deposition system. The NBR rubber

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were put onto the substrate holder in the chamber, and then the chamber was evacuated to <1.0×10-3 Pa. Prior to deposition, the NBR substrates were treated by argon plasma with 300

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sccm gas flow at 5.8 Pa working pressure and the bias voltage of -900V for 0, 15, 30 and 75 min, respectively. The film deposition process followed immediately after the Ar plasma

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pretreatment. The power supply was a constant current high frequency monopolar-pulse power system. The applied dc-negative bias power voltage was −800 V, f was 60 KHz, duty cycle was

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60% and duration time was 120 min. The gas flow rate was set at Ar/CH4=15/10 sccm and the deposition pressure was kept constant at 20 Pa via throttle control. The deposition temperature

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was strictly guaranteed to be lower than the upper limit of the NBR rubber by water cooling

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deposition chamber in whole process. The deposition temperature was measured with a surface thermometer fixed at the same position as the NBR substrates. The films are thus named as DLC1 film (DLC films/non-pretreatment), DLC2 film (DLC films/Ar plasma pretreated 15

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min), DLC3 film (DLC films/Ar plasma pretreated 30 min), DLC4 film (DLC films/Ar plasma

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pretreated 75 min).

2.3. Sample characterization The surface morphology and the cross section of coated rubber were characterized with scanning electron microscope (LYRA3 TESCAN, SEM). Cross sections of the coated rubbers were made by fracturing after cooling in liquid nitrogen for 10 min. Raman spectrum was acquired to investigate the chemical bonding of these films by using a HORIBA Jobin Yvon S.A.S. Raman Spectroscope at the excitation wavelength of the 532 nm Ar laser line and with a spot size of 5 μm. The spectral resolution was less than 1 cm−1. The hardness measurement was carried out on the DLC coated Si wafer, which was prepared under the similar deposition

ACCEPTED MANUSCRIPT conditions of DLC/rubber to remove the influence of substrate deformation. The hardness and elastic modulus of the film were obtained by using a Nano-indenter DCM system (Hysitron TI-950 tribo-indenter, America) with a three-sided pyramidal Berkovich diamond tip having a radius of 50 nm, and the maximum indentation depth was set to 80 nm (about 10% of the film thickness). Noteworthy, the structure of coatings deposited on rubber is rather different from

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that on Si wafer. The hardness of the coating on rubber might be lower than that on Si due to release of residual stresses, but the trend of hardness results is expected to be the same.

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In order to determine the adhesion of DLC films, on the one hand, the X-cut method was

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used qualitatively to evaluate the adhesion. The rubber was cut into the ‘X’ type with the blade, and the angle of the two cross lines was about 30-45 degrees. A cellophane adhesive tape

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(Scotch® with adhesion to steel of 47 N/100 mm width) was carefully adhered to the surface of the dissected film under 10 N pressure conditions, then waiting for 2 min, the tape was pulled

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off rapidly (not jerked). The peeling off or delamination of the film at the X-cut area was observed by SEM. On the other hand, the adhesion levels for the coated NBR were also evaluated quantitatively using the scratch tester. The type of the scratch tester is MFT-4000

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Multi-functional Tester for Material Surface Properties, which manufactured by Lanzhou

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huahui Instrument Technology Co., Ltd, (Lanzhou, China). A GCr15 steel ball (ø6 mm) was used as counterparts. The test method was suitable for hard coating/soft substrate arrangements

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and was confirmed by W. Kaczorowski [28]. In the examination, the applied force increased lineally from 5 N up to 100 N on a distance of 10 mm and sliding velocity of 0.07 mm/s. We

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determined the magnitude of the adhesion by the loading force corresponding to the mutation in the friction coefficient and the acoustic signal. The adhesion strength can be more reliably determined by two methods. The tribological performance was conducted at room temperature (23°C) on selfdeveloped friction and wear tester with the rotating ball-on-disc configuration selecting a commercial ø6 mm GCr15 steel balls as counterparts. The coated rubber sheets were glued with double-sided tap onto 30×30 mm2 polished stainless steel sheet, and a steel ring was fixed on the sample surface at four corners with screws to prevent samples from moving under the large friction load. The parameters of tribological tests were as followings: the frictional load

ACCEPTED MANUSCRIPT was 10 N, the linear speed was 83.73 mm/s, the radius of rotation was 4 mm, the testing time was 60 min, the sliding distance was 301.44 m, the number of rotations was 12,000 and a constant humidity=25±1% kept with a humidity regulator. In addition, wear life of the DLC3 film was studied further. The testing time, sliding distance and the number of rotations were respectively 10 times that of the above, and the other parameters remained unchanged. All the

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tribotests were carried out at least three times to ensure the repeatability. After testing, the surface morphology of wear tracks of DLC film coated rubber was characterized with SEM,

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OLYMPUS optical microscope and Canon 500D camera.

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3. Results and discussion

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3.1. Surface and cross-sectional morphology

An overview of the surface of DLC films coated NBR under the different Ar plasma

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pretreated time is shown in Fig. 1, with inset of high magnification view. It can be seen that random crack is always observed on all the DLC films (as shown in Fig. 1a-d), these crack networks divided the films into micrometer-scale patches, which is a typical feature of hard

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coatings deposited on soft substrates such as rubber [23]. The reason is directly related to the

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huge difference in the coefficient of the thermal expansion (CTE) of NBR and DLC films, and such mismatch thermal strain determines the density of the crack network and the size of DLC

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film segments. Meanwhile, it can be found that the size of the patches (the patch size can be estimated by averaging the length of straight lines connecting opposite sides of the patch)

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increases from 80 µm to 170 µm with increasing pretreatment time from 0 min to 75 min. Moreover, all the films exhibit a cauliflower-like morphology (Fig. 1a1-d1), which reflects a columnar growth of DLC films. The structure is consistent with the investigated in work of J. Th. M. De Hosson group [26, 29]. The size of these globular particles formed on the film non-pretreated are around 350 nm (Fig. 1a1) and is up to around 1300 nm (Fig. 1d1) for the DLC film deposited at 75 min pretreatment. The result may be related to the Ar plasma treated rubber surface morphology [29]. In addition, some delamination bands are observed in the insert of Fig. 1a and d, implying a poor adhesion.

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Fig. 1. The surface morphologies of the films with the different pretreatment time of 0 min (a, a1), 15 min (b, b1), 30 min (c, c1) and 75 min (d, d1).

Fig. 2 reveals the fracture cross sections of the DLC films prepared on NBR under the

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different Ar plasma pretreated time. There is no essential difference in the fracture morphology of the DLC films on NBR substrates. All films exhibit a dense columnar microstructure which can be related to interface shadowing initiated from a rough surface. Furthermore, the cross section reveals that the thickness of samples measured at different positions is not very homogeneous. Such a minor difference may be interrelated to the rough interface of rubber substrates and Ar plasma etching. In addition, interfacial delamination could be observed for the untreated sample due to a sharp step along the interface (Fig. 2a), implying indirectly a weak adhesion between the non-pretreated NBR substrate and films.

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Fig. 2. The fracture cross sections of (a) DLC1 film, (b) DLC2 film, (c) DLC3 film, (d) DLC4 film.

3.2. Raman analysis

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Raman spectroscopy is generally used to obtain the detailed bonding structure of DLC films. Fig. 3 represents the typical Raman spectra of the films deposited on rubber at different pretreatment time in the wavelength range of 800-2000 cm-1, showing the prominent G peak

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(centred at approximately 1500 cm-1) and a smaller shoulder D peak (centred at approximately

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1350 cm-1), which represents the typical characteristic of hydrogenated amorphous DLC films. Normally, the G peak position and ID/IG ratio increase with the increasing of the sp2/sp3 ratio in

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hydrogenated amorphous DLC films [30]. The G peak position and ID/IG ratio of the films as a function of the Ar pretreatment time are shown in Table 2, it should be noted that the intensity

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of both G and D peaks significantly increase along with a shift in the G peak position from 1523 cm-1 to 1538 cm-1 when the pretreatment time was increased from 0 min to 75 min. Moreover, the ID/IG ratio is also observed to increase monotonically from 0.41 to 0.61 with increasing the pretreatment time from 0 min to 75 min, which is an indication of major increase in sp2 content. The increase of substrate pretreatment time means the increase of substrate temperature because of the continued bombardment of high-energy ions, which also implies an increase in initial deposition temperature of the films. When the deposition is completed, there are minor differences in the temperature of these films. For example, the deposition temperature of DLC1 rose from 23 °C to 88 °C, while the deposition temperature of DLC4

ACCEPTED MANUSCRIPT reduced from 112.5 °C to 95 °C. The longer pretreatment time reflects the higher the temperature of the film deposition at the early stage, therefore, the film may contain higher the concentration of sp2 [31]. The hardness and elastic modulus of films are given in Table 2. It´s clear that there is a continuously decrease in the hardness with the increase of pretreatment time from 0 to 75 min. It is well known that there is a close relationship between the hardness

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and the sp2/sp3 ratio. According to the Raman analysis, the increase of sp2 content could be obtained with the increase of pretreatment time, which finally also contributes to the decrease

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of hardness.

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Fig. 3. Raman spectra of the DLC films deposited at the different pretreatment time. Table 2. G peak position, ID/IG ratio, hardness and elastic modulus of the films.

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DLC1 film

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ID/IG ratio G peak position(cm-1) Hardness(GPa) Elastic modulus(GPa)

0.41 1523 28.84±1 164.42±3

DLC2 film

DLC3 film

DLC4 film

0.44 1526 27.59±3 142.35±10

0.48 1527 23.37±1 155.94±3

0.61 1538 21.57±1 134.21±3

3.3. Adhesion

The adhesion levels of the samples were firstly determined following the X-cut method. Fig. 4 shows SEM images of X-cut locations after peel tests for these films. It is observed that the DLC3 film had higher adhesion levels than the others. For the DLC1 film, jagged removal along incisions is observed (Fig. 4a). At the same time, the brittle fracture occurred during the cutting and a lot of debris along the cutting line is sticky away from the adhesive tape (such as

ACCEPTED MANUSCRIPT the area of the dashed line), while a severe warping can be observed for the DLC2 film. Those phenomena also qualitatively indicate that the DLC1, DLC2 and DLC4 film shows a poor adhesion. A high adhesion level is determined for the DLC3 films due to trace peelings

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occurring along incisions with little peeling observed (Fig. 4c).

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Fig. 4. SEM images of X-cut locations after peel tests for (a) DLC1 film, (b) DLC2 film, (c) DLC3 film and (d) DLC4 film.

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The above qualitative judgments were still not enough to assess the subtle differences in the adhesion of them, but the deficiency was remedied by subsequent scratch tests. Table 3

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presented the results of the adhesion measurement in detail. In addition, the friction coefficient and acoustic signal curve of the example manufactured at argon plasma pretreatment for 30

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min was shown in Fig. 5. Based on the change of friction coefficient and acoustic signal, the LC2 (lower critical load) caused the film to break and LC3 (upper critical load) required to remove the film from the NBR surface were determined. For example, it was obtained that the films peel off when the friction coefficient suddenly increases and the acoustic signal fluctuates violently and is at a higher value as showed in Fig. 5. Compared to DLC deposited on the NBR without pretreatment (the LC2 and LC3 are found to be 9.26 N and 30.83 N, respectively), the adhesion of DLC deposited on the NBR with pretreatment is improved. Especially, the critical force of DLC deposited on the NBR with pretreatment for 30 min is almost twice that of the non-pretreatment comparatively. Meanwhile,

ACCEPTED MANUSCRIPT it can be found that the adhesion between film and NBR increased firstly and then decreased as the pretreatment time of Ar plasma. Finally, these data confirm the optimum time of plasma pretreatment for adhesion is 30 min, and the results are in agreement with that of X-cut

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method.

Fig. 5. Friction coefficient and acoustic signal as a function of the loading force for the DLC3 film.

Film

Pretreatment time 0 min 15 min 30 min 75 min

Critical load Lc2[N]

Critical load Lc3[N]

9.26 15.24 21.26 11.28

30.83 38.59 63.68 29.35

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DLC1 DLC2 DLC3 DLC4

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Table 3. Comparison of adhesion of carbon coatings to the NBR substrates

The excellent adhesion for the DLC3 film may be attributed to the two following reasons:

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(1) the physical removal of surface contaminants may occur through the ion bombardment. In its strictest sense, a surface remains contaminated after any cleaning process that finishes with a liquid rinse. Ar plasmas are capable of removing organic contamination from rubber, but it is important to plasma clean rubber for a sufficiently long time to remove all of the surface contamination. In addition, X-ray photoeletron spectroscopy (XPS) or other analytical techniques are difficult to detect contaminant material because they have a very similar chemistry to that of NBR. Therefore, we can qualitatively determine the residual amount of contaminant by the SEM photograph. For the DLC1 film, there may be a large number of pollutants on the rubber surface, which leads to the weak adhesion strength of the film (as

ACCEPTED MANUSCRIPT shown in Fig. 6a). For the DLC2 film, the pretreatment time is so short that the NBR rubber may not be completely cleaned though plasma bombardment is applied to rubber (as marked in Fig. 6b), resulting in no obvious improvement in adhesion. For the DLC3 and DLC4 film, the surface morphology of NBR rubber is distinctly changed by the plasma pretreatment, and the surface is bright and clean. In other words, the rubber surface could be cleaned completely

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because of the longer pretreatment time (Fig. 6c and d).

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Fig. 6. Surface morphology of NBR rubber pretreated by Ar plasma at the different time of (a) 0 min, (b) 15min, (c) 30 min and (d) 75 min.

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(2) The breakage of C–H and C=C bonds and some free radicals are produced due to the plasma bombardment, which may result in a chemistry binding between the film and NBR

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rubber to improve adhesion. However, the problem is how to measure the number of activated groups on the surface because the in-situ detection cannot be performed during the plasma bombardment period. Generally, when the plasma-treated NBR are taken out of the plasma reactor chamber, the radicals may capture molecular O2 and H2O from air atmosphere, producing oxygen-based groups. Therefore, we choose to measure the oxygen atomic content to indirectly account for changes in free radicals as Ar plasma treatment time. Fig.7 shows the the carbon and oxygen element mapping images of the native and argon plasma treated NBR. It can be observed that there is the growth trend of the percentage of oxygen content with increasing time of Ar plasma treatment. The increased oxygen content indirectly means that

ACCEPTED MANUSCRIPT free radicals increase with plasma treating time. However, for the DLC4 film, although the number of free radicals reaches a maximum, many vacancies or voids are created as the bombardment time increased further (as shown in Fig.6d), and a weak boundary layer is formed because of the excessive plasma pretreatment [11]. As a result, the mechanical strength of rubber surfaces decreases although the chemical binding can also form between the film and

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NBR rubber, leading to the plastic fracture of rubber and peeling of films during the cutting. Furthermore, the increased temperature should be considered. The surface temperature of

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rubber increases with the increase of pretreatment time. When Ar plasma pretreatment time is 15 min, 30 min and 75 min，the temperature of the substrate can respectively reach to 43 °C,

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80 °C and 112.5 °C. The elevated temperature accelerates the expansion of the rubber, and cause the plasma to gather deeper location of the NBR surface for enhancing bombardment

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[32]. These may limit the improvement in adhesion and makes difficult to understand the real

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interface formed between the rubber and the adhesive surfaces.

Fig. 7. The carbon and oxygen element mapping images of NBR rubber pretreated by Ar plasma at the different time of (a) 0 min, (b) 15min, (c) 30 min and (d) 75 min.

3.4. Tribological behavior The friction coefficient curves of uncoated and coated NBR rubber with the different pretreatment time sliding against GCr15 steel ball (diameter of 6 mm) are shown in Fig.8a, under dry condition at the sliding velocity of 84 mm/s and the applied load of 10 N. For the virgin NBR rubber, the friction coefficient is relatively low (~0.7) at the beginning of the sliding. After about 400 laps, the friction coefficient increases rapidly and reaches the stable

ACCEPTED MANUSCRIPT values of ~0.8 with a slight fluctuation. Such an increase in friction coefficient is just the opposite of the references [33], their results show that the friction coefficient of the virgin rubber decreases rapidly at the beginning of the tribotests because of the effect of flash temperature rising on the contact area. At the higher load, however, the surface temperature of the virgin rubber rises sharply at the beginning of the friction, which leads to a serious adhesive

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wear and makes the friction coefficient increase significantly. For the coated rubber, it is clear that the friction coefficient of all rubbers is drastically reduced (4 times lower), it can be

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mainly attributed that DLC films can weaken the interfacial adhesive wear effects between the film and the counterpart. But the average friction coefficient of the film with the different

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pretreatment time shows different characteristics. The DLC1 film (non-pretreatment) exhibits the highest friction coefficient (~0.28) and its friction coefficient increases monotonously from

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the initial friction coefficient of about 0.2 at the beginning of sliding to the value of 0.35 at the end of the test. As the pretreatment time increases, The DLC2 film presents a lower friction

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coefficient of 0.22 than that of the DLC1 film and its friction coefficient is relatively stable except for a slight increase, no obvious fluctuation could be observed. As the pretreatment time

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increases from 15 to 30 min, the DLC3 film displays the lowest friction coefficient (～0.2) and

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it is stable within 60 min in Fig. 8a. As the pretreatment time increases further, however, the friction coefficient of the DLC4 film shows the same trend as the DLC1 film, that is, it increases gradually during the entire period of the sliding and eventually reaches 0.28. The

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gradual increase in the friction coefficient indicates that a serious wear of the DLC1 and DLC4 film occurred and a large amount of wear debris has been generated (which can be verified in

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Fig. 9), implying a gradual damage of the films. The main reason for the difference of the friction coefficient between the different films with different pretreatment time is discussed in the following.

Finally, wear life of the DLC3 film was studied further. Fig. 8b shows the friction life of the DLC3 film. Clearly, the friction coefficient of the film shows a significant increase after the 40000 laps tribotests, implying a gradual damage of the films. However, it should be noted that the friction coefficient is less than half of that of uncoated NBR rubber (0.31 compared to 0.8) although the film was partially damaged, but is still functional after 120000 laps tribotests,

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for the industrial applications.

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Fig. 8. (a) Friction coefficient of uncoated and coated NBR rubbers with the different pretreatment time and (b) friction life of DLC3 film.

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SEM micrographs of the wear tracks of uncoated and coated NBR rubbers with the different Ar plasma pretreatment time are shown in Fig. 9. For the virgin NBR, the width of the wear tracks is quite wide (~2.89 mm), and a ‘‘liquid-like’’ third body on the wear tracks could be found due to the increase of flash temperature during the friction. From this point, the friction coefficient of virgin NBR increases sharply at the beginning of sliding can be related with the effects of viscous friction. As a consequence, a strong wearing is occurred for virgin NBR rubber. The coated NBR without pretreatment is heavily worn (Fig. 9a), the width of the wear tracks is up to 2.56 mm and a large number of block wear debris can be observed on the wear tracks. Its friction coefficient increases monotonously from the initial ~0.2 to the value of

ACCEPTED MANUSCRIPT 0.35 at the end of the test, partly due to the poor adhesion, evidenced by the SEM observation (Fig. 4a), which causes the film failure and the formed hard debris acts as the abrasive particle that results in severe wear of the film. After 15 min Ar plasma pretreatment, the friction coefficient of the DLC2 film is lower (0.22) and relatively stable than that of the DLC1 film, the width of wear tracks is also reduced to 1.88 mm and the film located at the hill of the

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rubber stripe is gradually smoothed out (Fig. 9c), which means a lower wear. As the pretreatment time increases from 15 to 30 min, the DLC3 film displays the lowest friction

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coefficient (～0.2) and wear, because the width of the wear tracks is also the smallest (1.83 mm), the worn areas were seen only on the top of the patches and a small amount of wear

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debris can be found on the contact spots.

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Fig. 9. SEM images of wear track of (a) the virgin NBR rubber, (b) DLC1 film, (c) DLC2 film, (d) DLC3 film and (e) DLC4 film.

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Fig. 10 presents the Raman spectra of the DLC films and the corresponding wear tracks with the different pretreatment time, and G peak position of the DLC films and the corresponding wear tracks are shown in Table 4. It can be found that ID/IG ratio is increased from 0.44 for the DLC2 film to 0.98 for the corresponding wear tracks, the G peak position shifts from 1526 cm-1 to 1563 cm-1. Similarly, the ID/IG ratio and G peak position of DLC3 film and the corresponding wear tracks shows a same trend, but its graphitization is relatively mild, which indicates that all films are graphitized at different degrees during the friction. This result is quite different from the result of Bui et al [34], because the load applied is so small in their studies that the contact stress is not high enough to induce the graphitization. Fortunately, it can

ACCEPTED MANUSCRIPT be seen that an appropriate pretreatment time gives the coatings good adhesion and a higher load promotes the mild graphitization, which guarantees a good tribological performance of the DLC3 film. As the pretreatment time increases further, however, the friction coefficient of the DLC4 film increases gradually, the larger wear area can be observed, this may be attributed to

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the lower hardness and poorer adhesion.

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Fig. 10. Raman spectra of the DLC films and the corresponding wear tracks with the different pretreatment time of (a) 0 min; (b) 15 min; (c) 30 min and (d) 75 min. Table 4. G peak position and ID/IG ratio of the films and the corresponding wear tracks. DLC1 film

Friction state

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Samples

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DLC4 film

Before

After

before

After

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0.41 1523

1.0 1583

0.44 1526

0.98 1563

0.48 1527

0.65 1536

0.61 1538

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ID/IG ratio G position (cm-1)

DLC2 film

After 0.95 1557

4. Conclusions

In summary, we demonstrate that optimal adhesion strength and tribological performance of DLC/NBR are drastically improved by appropriate Ar plasma pretreatment time. The micrometer-scale patches separated by random crack are always observed on all films as a result of the large difference in thermal expansion coefficient between the DLC film and NBR. Moreover, the hardness of the films decreases monotonously as the increase of the pretreatment

ACCEPTED MANUSCRIPT time due to the increase of sp2 content by the Raman analysis, which may be related to initial deposition temperature of the films caused by Ar pretreatment. Finally, the enhancement of adhesion can be attributed to the Ar plasma cleaning and the production of surface active sites. There is a weak adhesion because of the existence of surface pollutants of NBR rubber without pretreatment and short pretreatment time, while the mechanical strength of rubber surfaces

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decreases because of the formation of a weak boundary layer caused by the excessive plasma pretreatment, leading to the plastic fracture of rubber and peeling of films. The appropriate

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pretreatment times result in the perfect adhesion of DLC/NBR and the mild occurrence of graphitization, which guarantees a positive tribological performance of the DLC/NBR. The

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investigations performed within the scope of this work will be helpful to engineering

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applications.

Acknowledgements

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The authors are grateful to the National Natural Science Foundation of China (Grant no. U1737213), Gansu Provincial Youth Science and Technology Fund Program (Grant no.

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1606RJYA223) and Youth Innovation Promotion Association CAS (Grant no. 2017459) for the

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financial support. They also wish to thank especially Binji Gao and Sancheng Yu for testing the samples and providing the NBR substrates, respectively. The help rendered by the other

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Graphical Abstract

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1. Ar plasma pretreatment not only influenced on the surface morphology of NBR, but also had a significant effect on the properties of the films. 2. The adhesion between DLC films and rubber has a direct correlation with the pretreatment time, and a high adhesion could be obtained by a suitable Ar plasma pretreatment. 3. The DLC film coated NBR with suitable Ar plasma pretreatment exhibits the lowest friction coefficient of 0.2 and slightly wear, and is still functional after 120000 laps under high normal load of 10 N.