Vacuum tribological performance of DLC-based solid–liquid lubricating coatings: Influence of sliding mating materials

Wear 292–293 (2012) 124–134

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Vacuum tribological performance of DLC-based solid–liquid lubricating coatings: Influence of sliding mating materials Xiufang Liu a,b, Liping Wang a,n, Zhibin Lu a, Qunji Xue a a b

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 October 2011 Received in revised form 29 May 2012 Accepted 30 May 2012 Available online 19 June 2012

The friction and wear behaviors of DLC-based solid–liquid lubricating coatings for three liquid lubricants sliding against different counterface materials were examined under high vacuum conditions. Seven kinds of balls with the diameter of 3 mm were chosen as counterparts, which were GCr15, bronze, ZrO2, Al2O3, SiC, WC, and Si3N4. Under high vacuum condition, the friction coefficient (COF) and wear rate of carbon-based solid–liquid lubricating coatings sliding against different counterparts were diverse, due to different liquid lubrications and counterface materials. In analyzing the friction and wear mechanism, the contact radius and the contact pressure were introduced. The COF showed the inverse varied trend with Hertzian contact radius for the three liquid lubricants. COF decreased with the increase of contact radius, which was different from the dry sliding of DLC film. The possible reason was that the synergetic lubrication between the DLC films and the liquid lubricants formed a new solid–liquid synergistic system (DLC-oil-DLC), which improved the ability of plasticity-resistant deformation, reduced the real contact area and friction coefficient. The contact pressure was consistent with the wear rate varied trend for the three liquid lubricants, and this varied trend was in good agreement with each other for MACs lubricant. & 2012 Elsevier B.V. All rights reserved.

Keywords: Diamond-like carbon Sliding friction Sliding wear Hardness High vacuum

1. Introduction Currently, diamond-like carbon films (DLC) is referenced as a potential space tribological material due to their high hardness, low friction, and low wear [1,2]. However, under high vacuum condition the adhesion and cold-welding between the DLC film and the counterpart often occurred, resulting in a large friction coefficient, especially for the non/low hydrogenated DLC film [3,4]. A high hydrogenated DLC film increases lifetime through hydrogen termination of active carbon bonds, but not for long durations due to hydrogen depletion after about 104 cycles [5,6]. A ‘‘chameleon’’ composite coating can hold low friction and long lifetime under high vacuum condition, but its fabrication process is complicated and it has unstable performance [7,8]. Alternatively, recent studies on DLC-based solid–liquid synergetic lubricating coatings can effectively improve tribological behavior of DLC films under high vacuum condition [9,10]. Previous studies showed that excellent tribological behaviors in high vacuum of DLC-based solid–liquid synergetic lubricating coatings could be attributed to the synergy lubrication mechanism by the combination of the solid lubrication effect of DLC film

n

Corresponding author. Tel.: þ86 931 4968080; fax: þ 86 931 8277088. E-mail address: [email protected] (L. Wang).

0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.05.023

and the boundary lubrication of the liquid lubricants [9]. The different liquid lubricants would also lead to different tribological performance of DLC-based solid–liquid synergetic lubricating coatings, which were possible due to the differences in viscosity and components of the liquid lubricants [10]. However, the different tribological behavior of DLC-based solid–liquid synergetic lubricating coatings sliding against different counterface materials under high vacuum condition is still unintelligible, even though there have been some studies on the effect of different counterparts on friction and wear behaviors of DLC films. The influence of pairs on wear behavior of DLC films is complicated because different chemical environments, different mechanical natures, and even different tribological mechanisms can be caused by the different pair materials [11–16]. Konca et al. studied dry sliding friction and wear behavior of non-hydrogenated DLC coatings against Al, Cu and Ti in ambient air and argon using a vacuum pin-on-disc tribometer [14]. It indicated that the tribological behaviors of non-hydrogenated DLC coatings against different counterparts were distinct in different testing conditions. Waesche et al. analyzed the high temperature tribological performance of tetrahedral amorphous carbon coatings against different counterbody materials [15]. The results showed that the counterbody materials influenced the friction and wear behaviors of DLC coatings and therefore coatings’ life time was different when sliding against different counterbodies. Liu et al. also

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Table 1 Physical properties of TiC/a-C:H film.

TiC/a-C:H

125

Table 2 Typical physical properties of the liquid lubricants.

Hardness (GPa)

Elastic modulus (GPa)

Thickness of layer (mm)

18.4

150.7

2

investigated the influence of sliding mating materials on tribological behavior of diamond-like carbon films [16]. The results showed that in sliding against hard materials, DLC films exhibited a low, stable, friction coefficient; in sliding with easy-transfer soft metals, their friction coefficient is also low; in sliding with the difficult-transfer soft metals, DLC films exhibit a high friction coefficient. Unfortunately, the friction and wear behaviors and mechanisms of DLC-based solid–liquid synergetic lubricating coatings sliding against different counterparts under high vacuum condition have not been studies as yet. Thus, understanding the influence mechanism of different pairs against the DLC-based solid–liquid synergetic lubricating coatings would make great practical significance for space application in the future. In the literature, we investigated the friction and wear behaviors of DLC-based solid–liquid synergetic lubricating coatings sliding against different counterface materials (GCr15, bronze, ZrO2, Al2O3, SiC, WC, Si3N4) under high vacuum condition. Additionally, effect of the combined mechanical properties of the mated solid–liquid synergetic lubricating friction system on the friction and wear behaviors of DLC-based solid–liquid synergetic lubricating coatings is also discussed.

2. Experimental section The TiC/a-C:H films used in this investigation were deposited on stainless steel substrates in an argon/methane atmosphere with unbalanced magnetron sputtering technique. Prior to deposition, the stainless steel substrates were first ultrasonically cleaned in acetone for 20 min and then ultrasonically cleaned in alcohol for 10 min. After drying with dried gas, the samples were put into the depositing chamber. A Ti interlayer with good bonding with substrates was deposited at radio frequency (RF) power of 400 W and bias voltage of 300 V. The distance between the Ti target and sample was  15 cm and the duty cycle was 50%. The thickness of the Ti interlayer was about 150 nm. Subsequently, TiC/a-C:H coatings were deposited on the Ti interlayer at bias voltage of 1000 V and duty cycle of 20% in the flowing Arþ CH4 atmosphere. The flow rate of Ar gas was 120 sccm, and the CH4 was introduced into the system with an interval increment of 2 sccm per 2 min from 8 to 40 sccm. Then, the coating was deposited for 120 min. The Nanoindentation tester (Nano IndenterXP, Nano Instruments) equipped with a MTS three-sided pyramidal diamond indenter was employed to measure the hardness of the TiC/a-C:H coating. The liquid lubricants used in this study were ionic liquid (3-hexyl-1-methyl-imidazolium hexafluorophosphate, short as IL), Multiply-alkylated cyclopentane (MACs), Poly(tetrafluoroethylene oxide-co-difluoromethylene oxide) a,o-diol (Zdol), respectively. The MACs and IL were synthesized by ourselves followed by the similar procedures as proposed in previous references [17,18], respectively. Zdol was obtained from Aldrich Chemical Company and used as received. The density, viscosity, and viscosity–temperature index of the liquid lubricants were measured by a SVM3000 Stabinger Viscometer. The liquid lubricants were coated on the DLC films spinning method with a small brush. Then the DLC-based solid–liquid duplex coating was successfully formed on steel for friction and wear test. The thickness of the lubricants layers were in a wide range of 0.5–2.0 mm. The interaction between the DLC film and the

Kinematic viscosity (mm2/s), 40 oC IL 129 MACs 109 Zdol 146

Kinematic viscosity (mm2/s), 100 oC

Viscosity index

Density (g/ cm3), 15 oC

14 15 46

106 143 351

1.298 0.846 1.82

Table 3 Mechanical properties of tribo-pair balls in the friction tests. Hardness (GPa) GCr15 6.9 Bronze  0.8 ZrO2 11.8 Al2O3 16.2 SiC 27.4 WC 14.6 Si3N4 14.7

Elastic modulus (GPa)

Poisson ratio

Thermal conductivity (W/m K)

208 127 340 210 440 635 300

0.3 0.34 0.22 0.3 0.17 0.22 0.26

40 407 2 26 120 79.6 25

liquid lubricants layers were mainly physical adsorption. Tribological behaviors of the DLC-based solid–liquid synergistic lubricating coatings were investigated by a rotational high-vacuum tribometer. The rotational radius was set as 6 mm, and the sliding speed was 500 rev/min, and the normal load was 3 N. Two-hour’s (corresponding to a total sliding distance of 2262 m) tests were performed at room temperature and under high vacuum (10  5 Pa) condition. 3 mm diameter balls of GCr15, bronze, ZrO2, Al2O3, SiC, WC and Si3N4 were used as tribo-pairs in this paper. Physical properties of the balls are displayed in Table 3. The wear track profiles after the friction test were obtained by a noncontact 3D surface profiler (model MicroMAXTM, made by ADE Phase Shift, Tucson, AZ, USA), and the wear scars of mating surfaces were investigated by an optical microscope.

3. Results and discussion 3.1. Properties of diamond-like carbon films and liquid lubricants Table 1 shows the properties of TiC/a-C:H film required in this paper. The whole thickness of TiC/a-C:H film is approximately 2 mm. The hardness and the Young’s modulus of the films are determined to be about 18.4 GPa and 150.7 GPa, respectively. Table 2 presents the physical properties of three liquid lubricants. The viscosities were in the range of 109–146 cP at 40 1C, and they decreased significantly to between 14 and 46 cP at 100 1C. At 40 1C, the magnitude of kinematic viscosities increased in the following order: MACsoIL oZdol. 3.2. Tribological behavior with different counterface materials The friction coefficient curves and average COF of three liquid lubricants’ DLC-based solid–liquid synergistic lubricating coatings under high vacuum condition with different counterface materials are presented in Fig. 1. Under high vacuum condition, all friction coefficients of three liquid lubricants’ DLC-based solid– liquid synergistic lubricating coatings are less than 0.1. For the three liquid lubricants, the average COF of different tribo-pairs increased in the following order: ZrO2 oGCr15obronzeoSi3N4 oSiCoAl2O3 oWC for IL; ZrO2 obronzeoSi3N4 oGCr15o Al2O3 oWCoSiC for MACs; bronzeoZrO2 o Si3N4 oGCr15o Al2O3 oWCoSiC for Zdol, respectively. When running against

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Fig. 1. Friction coefficient and average COF for DLC-based solid–liquid synergistic lubricating coatings under high vacuum condition with different counterface materials: (a) (b) IL, (c) (d) MACs and (e) (f) Zdol.

the metallic counterbody, the COF curves of bronze for the three liquid lubricants exhibit large fluctuation during the whole rotational sliding process. Whereas, switching the tribo-pairs from metal balls to ceramic balls, COF curves are smooth during the friction process for most of ceramic balls. In test against the five ceramic materials, much bigger differences are observed. For

against ZrO2, no running-in process occurs, and the friction is the smallest for the three liquid lubricants. The SiC and WC ceramic ball show high friction values. For the three liquid lubricants, the COF curves of MACs are relatively stable and smooth, except for bronze; the other two liquid lubricants show relatively unstable and fluctuant COF curves.

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127

Fig. 2. Disc wear rate for DLC-based solid–liquid synergistic lubricating coatings under high vacuum condition with different counterface materials: (a) IL, (b) MACs and (c) Zdol.

Fig. 2 shows the disc wear rates for three liquid lubricants’ DLCbased solid–liquid synergistic lubricating coatings under high vacuum condition with different counterface materials. Obviously, disc wear rate of DLC-based solid–liquid synergistic lubricating coatings for three liquid lubricants against bronze displays the lowest value, which could be attributed to the low hardness of bronze. The wear rates of GCr15 counterbody are the largest for IL and Zdol, which is not coincided with the hardness. The possible reason is that in the presence of oxygen and/or water in the liquid lubricant, some active macro-radicals can be generated by the cracking of C–C and C–H. These active macro-radicals will react with the activated Fe atoms to form Fe–C bonds under high driving reaction force. Strong adhesive friction at the interface would be promoted by these reactions between the a-C:H film and the steel ball, causing severe adhesion wear along the wear track [19]. Non-contact 3D surface profiler images of the wear tracks for DLC-based solid–liquid synergistic lubricating coatings under high vacuum condition with different counterface materials for three liquid lubricants are presented in Figs. 3–5, respectively. The insets were the corresponding cross-section profiles of the wear tracks. The width and depth of the wear tracks for different friction tests are different. The smallest and narrowest wear tracks that against bronze balls for the three liquid lubricants were about 140 nm, 50 nm and 270 nm for IL, MACs and Zdol, respectively. No significant plastic deformation was observed on DLC films for all wear tracks, which could be attributed to the synergy lubrication

mechanism by the combination of the solid lubrication effect of the DLC-based film and the boundary lubrication of the liquid lubricants. The solid–liquid synergy lubrication not only kept the yield strength, which was determined by hard film of DLC, but improved the shear strength, which depended on the soft film of liquid lubricants [20]. Moreover, the liquid lubricants could improve the ability of plasticity-resistant deformation, so that the deformation and the real contact area were reduced when counterpart was indented. Thus the adhesive deformation and plowing deformation were also decreased, and the ability of friction-reducing and wear resistance were significantly improved [21]. The wear scar optical micrographs of counterpart surfaces for MACs lubricant are showed in Fig. 6. These optical micrographs of counterparts were magnified 100 times. It is clear that large wear scars are observed on the softer counterpart surfaces, example for bronze, GCr15. During the initial stages of sliding, the sharp and hard protruding surface asperities of the rough coating could cause large contact pressure on the contact tip, leading to considerable wear of the mated softer balls and only a little wear of the hard one, consistent with the optical micrographs of the wear scars on the mated balls as shown in Fig. 6. 3.3. Relationship between COF and properties of counterpart In the contact of two rough surfaces, a large number of asperities of different shapes and sizes are pressed against each

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Fig. 3. Non-contact 3D surface profiler images of the wear tracks for DLC-based solid–liquid synergistic lubricating coatings of IL under high vacuum condition with different counterface materials: (a) GCr15, (b) bronze, (c) ZrO2, (d) Al2O3, (e) SiC, (f) WC and (g) Si3N4.

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129

Fig. 4. Non-contact 3D surface profiler images of the wear tracks for DLC-based solid–liquid synergistic lubricating coatings of MACs under high vacuum condition with different counterface materials: (a) GCr15, (b) bronze, (c) ZrO2, (d) Al2O3, (e) SiC, (f) WC and (g) Si3N4.

other [12]. Tips of surface asperities on solid bodies are sometimes considered spherically shaped so that the contact of two macroscopically flat bodies can be reduced to the study of an array of spherical contacts deforming at their tips [22,23]. From

Hertz analysis, we have the contact radius: a¼

  3WR 1=3 4E*

ð1Þ

130

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Fig. 5. Non-contact 3D surface profiler images of the wear tracks for DLC-based solid–liquid synergistic lubricating coatings of Zdol under high vacuum condition with different counterface materials: (a) GCr15, (b) bronze, (c) ZrO2, (d) Al2O3, (e) SiC, (f) WC and (g) Si3N4.

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131

Fig. 6. Optical micrographs of wear scar on pair surface for MACs lubricant under high vacuum condition: (a) GCr15, (b) bronze, (c) ZrO2, (d) Al2O3, (e) SiC, (f) WC and (g) Si3N4. Table 4 Effective elastic modulus of the counterparts, contact radius and contact pressure on the film surface against different counterparts in Hertzian contact theory.

Effective elastic modulus (GPa) Contact radius (  10  4 m) Contact pressure (GPa)

GCr15

Bronze

ZrO2

Al2O3

SiC

WC

Si3N4

87.39 0.68 0.31

68.92 0.73 0.27

87.74 0.68 0.31

104.42 0.64 0.35

112.25 0.62 0.37

121.80 0.61 0.38

100.31 0.65 0.34

where W is an applied normal load, R is the radii of the ball, and E* is the effective elastic modulus. 1 1n1 2 1n2 2 ¼ þ E* E1 E2

ð2Þ

n1 is Poisson ratio of the DLC film, n2 is Poisson ratio of the counterface materials, E1 is the elastic modulus of DLC film, and E2 is the elastic modulus of the counterface materials. Physical properties of the balls are displayed in Table 3. The effective elastic modulus and contact radius of counterparts are displayed in Table 4.

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Fig. 7. Relationship between contact radius and COF for DLC-based solid–liquid synergistic lubricating coatings under high vacuum condition with different counterface materials: (a) IL, (b) MACs and (c) Zdol.

Dry sliding friction behavior of DLC films is always explained by the build-up of a friction-induced transfer film on the counterpart and easy shear material formed in the interface between the DLC and the counterpart, attributed to the phase transformation (sp3-sp2) of the DLC film during the sliding process [24]. The relationship of Hertzian contact radius with COF is usually used to evaluate the friction behavior of DLC films [11,13]. For the dry sliding of DLC films, the COF shows the same varied trend with contact radius [13]. However, for the DLC-based solid–liquid synergetic lubricating coatings, the friction and wear behaviors may be different from the dry sliding. The relationship between Hertzian contact radius and the COF is presented in Fig. 7. Clearly, it can be seen that the COF shows the inverse varied trend with contact radius for the three liquid lubricants. This inspires us to further explore the possible reason. The DLC-based solid–liquid synergetic lubrication is different from the dry sliding of DLC film and the single oil lubrication due to the synergetic lubrication between the DLC films and the liquid lubricants. Because the solid–liquid synergy lubrication not only kept the yield strength, which was determined by hard film of DLC, but improved the shear strength, which depended on the soft film of liquid lubricants [20]. Moreover, the liquid lubricants could improve the ability of plasticity-resistant deformation, so that the deformation and the real contact area were reduced when counterpart was indented. Thus the adhesive deformation and plowing

deformation were also decreased, and the ability of frictionreducing was significantly improved [21]. Additionally, repetitious sliding in liquid lubricants resulted in the formation of oil-containing carbon-rich tribofilm on the surface of counterpart. When carbon material transformed from DLC films to the counterpart, a new solid–liquid synergistic system (DLC-oil-DLC) was formed, and the friction behavior was governed by this new system [9]. So the relationship of Hertzian contact radius with COF of the DLC-based solid–liquid synergetic lubricating coatings against different counterparts is opposite to that of the dry sliding of DLC films against different counterparts. 3.4. Relationship between wear rate and properties of counterpart To quantify the wear evolution, many researches about wear behavior of amorphous carbon films against different counterparts have been reported by other authors. Li et al. indicated that the formation of oxide on the counterpart could influence the formation of transfer film and affect the wear behavior of DLC films [19,25]. Shaha et al. pointed out that the hardness ratio of the tribo-pair and the coating material makes markedly sense on the wear behavior of coating [26]. When the hardness ratio is above rc1 (0.5 to 0.8), the wear rate of the counterpart decreases; as the increased hardness ratio is close to rc2 (1 to 1.4), almost no wear is observed. Lu et al. studied the influence of the elastic

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133

Fig. 8. Relationship between wear rate and contact pressure for DLC-based solid–liquid synergistic lubricating coatings under high vacuum condition with different counterface materials: (a) IL, (b) MACs and (c) Zdol.

energy (Epp) of the counterparts on the wear volume of DLC coatings, finding out that the elastic energy is more appropriate to predict the wear resistance of the DLC coatings compared with the hardness of the tribo-pairs [11]. However, these theories were not suitable for explaining the wear behavior of our solid–liquid synergistic lubricating system under high vacuum condition. Hardness has been regarded as a primary material property that defines wear resistance for a long time, recognized by the classical theories of wear [27]. Furthermore, elastic modulus plays an important role in determining the wear behavior of material, attributed to that the elastic strain to failure is related to the ratio of hardness (H) and elastic modulus (E). As discussed earlier, we have understood the relationship between effective elastic modulus and contact radius by the formula (1). When the counterparts loaded on the solid–liquid lubricating coating, contact pressure will occur between the two surfaces. The contact pressure is related to the load and the contact radius, given as: P0 ¼

3W ¼ 2pa2

6WE*

p3 R2

2

!1=3 ð3Þ

where W is an applied normal load, a is the contact radius, R is the radii of the ball, and E* is the effective elastic modulus. The contact pressures on the film surface against different counterparts were listed in Table 4. In this study, we investigated the relationship between contact pressure and wear rate for different counterparts, as shown in Fig. 8. It reveals that the contact pressure is generally consistent with the wear rate varied trend for the three liquid lubricants. The DLC film has a rough surface, and the comparatively harder pairs can quickly

remove the top of the surface asperities and reduce the roughness in the wear track in the initial stage of sliding. But for the softer pairs, it is difficult to remove the asperities on the DLC film surface. Therefore, hard pair causes high contact pressure on the contact surface compared with the softer one, due to its high elastic modulus, and with small contact area. Thus hard pair can cause high contact pressure, resulting in a severe wear. As analyzed above, COF is inversely proportional to contact radius (mp1/R), and wear rate is proportional to contact pressure (KpP0). Furthermore, contact pressure is also inversely proportional to contact radius (P0p1/R), so the wear rate is proportional to the COF (Kpm). With the increasing of friction coefficient, the tensile stress increased [28], and the plastically deforming area also increased, leading to increasing abrasion [29]. In the three liquid lubricants, these two varied trend is in good agreement with each other for MACs lubricants. But for the different counterparts, it is clear that the wear rate of SiC is not coincided with the effective elastic modulus trend, which might suggest that besides the influence of mechanical character, other factors had important effect to the wear rate for the contact of SiC and DLC, especially liquid lubricants. But for the other counterparts, the wear mechanism of the solid–liquid synergy lubrication of DLC was consistent with that of the dry sliding of DLC. Because the key factor of wear was the mechanical character in spite of different chemical and atmosphere environments.

4. Conclusion In summary, we have studied the effects of the different counterface materials on the friction and wear behaviors of

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DLC-based solid–liquid synergetic lubricating coatings under high vacuum condition, and discussed the relationship of friction and wear with contact radius and contact pressure. It is shown that the friction and wear of the DLC-based solid–liquid synergetic lubricating coatings sliding against different counterface materials were different, due to the difference physical properties of liquid lubrications and counterparts. In the paper, the contact radius and the contact pressure were introduced to explain the friction and wear mechanism. Different from dry sliding of DLC, the COF shows the inverse varied trend with Hertzian contact radius for the three liquid lubricants. The formation of a new solid–liquid synergistic system (DLC-oil-DLC) in the DLC-based solid–liquid synergistic system seemed to explain this result. Additionally, the contact pressure is generally consistent with the wear rate varied trend for the three liquid lubricants, and this is in good agreement with the dry sliding of DLC films. Because the key factor of wear was the mechanical character in spite of different chemical and atmosphere environments. Hard pairs cause high contact pressure on the contact surface compared with the softer ones, due to its high elastic modulus, and with small contact area.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 11172300) and Youth Science Fund of the National Nature Science Foundation (No. 11004203). The authors gratefully acknowledge Mr. Lichun Bai for useful discussions.

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