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Friction and wear performance of dual lubrication systems combining WS2–MoS2 composite film and low volatility oils under vacuum condition
Tribology International 99 (2016) 57–66
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Friction and wear performance of dual lubrication systems combining WS2–MoS2 composite film and low volatility oils under vacuum condition Xin Quan a,b, Ming Hu a, Xiaoming Gao a, Yanlong Fu a, Lijun Weng a, Desheng Wang a, Dong Jiang a, Jiayi Sun a,n a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 January 2016 Received in revised form 5 March 2016 Accepted 10 March 2016 Available online 19 March 2016
In the present work, WS2–MoS2 nanocomposite films were sputtered, based on which solid/liquidlubrication systems were realized by combining the WS2–MoS2 films with space oils including MACs (multialkylated cyclopentanes) and CPSO (chlorinated-phenyl with methyl-terminated silicone oil). The tribological behaviors of the solid/liquid systems were investigated in vacuum. It was revealed that in comparison to pure WS2 film, the WS2–MoS2 composite film exhibited an enhanced tribological performance. Moreover, our results identified that the WS2–MoS2/MACs system integrated better the advantages of the solid and liquid lubrications and thereby a synergistic lubricating effect was generated. A tribological model was established to explain the synergistic lubricating mechanism. & 2016 Elsevier Ltd. All rights reserved.
Keywords: WS2–MoS2 film Liquid lubricant Solid/liquid lubrication Friction and wear
1. Introduction It is well known that transition metal disulfides (denoted as MS2), such as molybdenum disulfide and tungsten disulfide, are widely used as solid lubricant owing to the inter-lamellar hexagonal structure and inert basal planes existing in the individual crystallites where shear occurs easily [1–3]. During the last few decades, the MS2 films deposited by sputtering technique have been intensively studied and widely applied for moving parts in vacuum [4–6]. However, the easy reaction of unsaturated and dangling bonds of crystal MS2 with the moisture is inclined to result in degradation of tribological performance of these films. For this reason, many proposals had been made in the literature to improve the tribological properties of MoS2 or WS2 films, mainly including nanocomposites and multilayer constructions [7–9]. The results from related works revealed that the tribological performances of MoS2 or WS2 films in dry and even in humid atmosphere can be obviously improved by multilayer and nanocomposite architectures [9–13]. Despite all this, the performance of the present materials still cannot satisfy the requirements absolutely for future long-term space exploration, e.g. higher reliability and durability, extreme environments and harsh n
Corresponding author. Tel.: þ 86 931 4968092; fax: þ 86 931 8277088. E-mail address: [email protected] (J. Sun).
http://dx.doi.org/10.1016/j.triboint.2016.03.009 0301-679X/& 2016 Elsevier Ltd. All rights reserved.
working conditions [4,14]. NASA report also pointed out that there is no solid lubricants and films currently exist could completely meet those demands [15]. Recently, a novel lubrication technology, combining solid and liquid lubricants on friction interfaces, or namely duplex lubrication systems, shows excellent applying potential for some applications with increasingly demands. It is found that proper combination of solid and liquid lubricants may trigger a significant improvement of the tribological performances in comparison with the single lubricant [16–20]. Some results given in the papers [17,21] by Erdemir et al. indicated that Ag/polyester synthetic oil solid/liquid system exhibits good capability in controlling friction and wear for both ceramic and metallic friction pairs. The tribological properties of DLC-based solid/liquid duplex lubrication systems had been systematically studied in vacuum circumstance in works by Liu et al. [14–16,22,23]. The results revealed that some of DLC-based solid–liquid systems, such as DLC/IL, DLC/IL/Graphene and DLC/MACs systems, exhibit promising application prospect in space which mainly attribute to the substantial loadbearing capability of DLC film and good lubricity from liquid lubricant. In our recent work, we have investigated the pure WS2 film based solid/liquid lubricating systems [24]. The results demonstrated that proper combination of pure WS2 film with special oil, such as WS2/SiCH (silahydrocarbons) system, can achieve significant improvement in tribological performances than
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pure WS2 film both under vacuum and air conditions. Well selflubricating property of WS2 film as well as surface property of WS2 crystallographic planes together in response to the tribological behavior of WS2/SiCH system. Despite of some studies on solid/liquid duplex lubrication systems, to address specific applications however, it should be studied in more details mainly involving the following respects:
Compatibility and wettability between a variety of liquid lubricants and all kinds of solid lubricants.
The mechanisms of the interaction of physical and chemical
and n-type Si (100) substrates were selected for the tribological performance tests and the analyses of the film composition and structure, respectively. The steel substrates were surface-polished and then ultrasonically cleaned in acetone before the deposition. Then, they were mounted under the target for the vertical distance of 80 mm. The substrate surfaces were etched by an Ar plasma for 15 min at a DC bias of 500 V to eliminate possible contaminants before the deposition. Afterwards, the WS2–MoS2 composite films were sputtered under a target power density of 0.068 W mm2 in an Ar pressure of 5.0 Pa and a substrate bias voltage of 70 V for the deposition time of 12 min.
properties between liquid and solid lubricants.
The effects of functional group, polarity and additives with
respect to liquid lubricants on the tribological properties of duplex lubrication system. The effects of modified microstructures, especially nanocomposites and multilayers, of solid lubricants on the tribological properties of duplex lubrication system.
In this study, we firstly fabricated WS2–MoS2 nanocomposite film to optimize the microstructure and tribological properties. And then, the solid/liquid lubricating systems were designed by WS2–MoS2 nanocomposite film with two specific liquid lubricants. The selected oils, including CPSO (chlorinated-phenyl with methyl-terminated silicone oil) and MACs (multialkylated cyclopentanes), have been successfully used in space moving components for their excellent lubrication property and well physicochemical properties, e.g. extremely low volatility, high viscosity index, wide running temperature, and low pour point. The main aim of this present work is to investigate tribological performance of MS2 nanocomposite film based solid/liquid system under vacuum condition and further to demonstrate the friction and wear mechanism.
2. Experimental details 2.1. Materials The MACs and CPSO oils were provided by the State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics [25]. Their typical properties are listed in Table 1. 2.2. WS2–MoS2 composite film preparation A radio frequency sputtering equipment and WS2–MoS2 composite target (80 mm in diameter, and mass ratio of is 1/2) were used to deposit WS2/MoS2 nanocomposite films in this work. The composite target was composed of pure WS2 and MoS2 (99.9% purity) in mass ratio of 1–2. The AISI 440C (Ø25 mm 4 mm) steel
2.3. Preparation of the solid/liquid lubricating system In this work, the solid/liquid composite systems were prepared by combining the sputtered WS2–MoS2 film with oils. The MACs and CPSO oil were respectively coated on the pre-deposited WS2– MoS2 film surfaces to form a homogeneous oil layer by a spin coating method. The calculated oil layer thickness was about 20 mm for both MACs and CPSO. Then the WS2-based solid/liquid lubricating systems were prepared as references. The detailed calculation method has been reported in detail in a previous work [24]. 2.4. Friction and wear test The tribological behaviors of the WS2–MoS2 composite film and the solid/liquid lubricating systems were evaluated using a ballon-disk tribometer in vacuum condition at room temperature (10 4 Pa, 30% RH, 20 75 °C). The disks were the solid/liquid lubricating systems or WS2–MoS2 films. The counterpart was cleaned AISI 440C steel ball (HRC 60, Ra 0.1 mm and 8 mm in diameter). The normal load was 3 N, corresponding to a theoretical Hertzian contact pressure of 0.25 GPa. The rotational speed was 1000 rev/min for all tests, equaling to a linear speed of 0.52 m/s. Each friction test was repeated three times under the same conditions to check the repeatability of the results. The wear rates (K) after the sliding tests were calculated via equation of K ¼ V/(FS), where V is the wear volume loss (mm3), F is normal load (N) and S is total sliding distance (m). 2.5. Characterization Field emission scanning electron microscopy (FESEM, JSM6701F) was used to examine the morphology of WS2–MoS2 composite films. The crystal structure of the films was investigated using grazing incidence X-ray diffraction(GIXRD, Philips, X'Pert Pro) with Cu Κɑ radiation in the scanning range of 2θ from 5° to 80°. The contact angle of the oils on the surface of films was measured by using a contact angle measurement instrument
Table 1 Typical properties of MACs and CPSO oils. MACs
CPSO
56 9.3 148 858.3 5.6 10 6
120.4 48.6 415 1014 8.4 10 8
Molecular structure
Viscosity at 40 °C/mm2 s1 Viscosity at 100 °C/mm2 s1 Viscosity index Density at 20 °C/kg m3 Vapor pressure at 20 °C/Pa
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3. Results and discussion
disk was about 0.5 mm which was measured by 3D surface profiler. It can be seen that the oil layer was much thicker than WS2–MoS2 film and its surface roughness. When MoS2 was co-deposited, the surface of the composite film shows a feather-like morphology (cf. Fig. 2a); whereas the pure WS2 film exhibits an acicular-like structure as revealed in our former work [24]. This demonstrates that the microstructure of WS2–MoS2 composite film was much denser than the pure WS2 film. The cross-sectional FESEM micrograph manifests that WS2–MoS2 film possesses a compact double-layer microstructure, containing an upper columnar platelet and a lower dense layer near the substrate with the thickness about 100 nm.
3.1. Microstructure and composition of WS2–MoS2 composite film
3.2. Tribological properties
Fig. 1a shows the GIXRD spectra for the WS2–MoS2 composite film. Since the lattice constants of WS2 and MoS2 are parallel, the diffraction peak positions of them are almost identical. It could be found that the film exhibits both hcp-WS2 and hcp-MoS2 (002), (101) and (112) diffraction peaks besides the substrate peak. The strongest peak was in the vicinity of 35° which demonstrated that the film preferred was grown along (101) crystal plane. In addition, the growth of (002) plane was thought to be helpful to improve the tribological properties for the composite film. The result of film element composition determined by EDS is manifested in Fig. 1b. The contents of S, W, and Mo elements in the composite coating were 41.7 at%, 8.1 at% and 22.3 at%, respectively. Fig. 2 gives the FESEM micrographs of the WS2–MoS2 composite film. The mean thickness of the film was in the range of 3.2– 3.6 mm. The surface roughness of WS2–MoS2 film deposited on
The tribological properties of WS2–MoS2 composite film in vacuum were firstly investigated. Fig. 3a shows that the WS2– MoS2 film exhibited well lubricating property with a mean friction coefficient of 0.05 and a wear life about 8.0 105 cycles, which was much longer than that of pure WS2 film [24]. It is well known that the mainly deterioration mechanism for WS2 or MoS2 film is own to the oxidation reaction with the moisture and O2 [8]. It is deemed that the excellent tribological behavior of WS2–MoS2 film is related to its dense microstructure, which could effectively prevent the film contacting with ambient atmosphere and therefore efficiently decrease the friction. Then, we evaluated the friction and wear behaviors of the two composite systems in vacuum condition. For comparison, the friction coefficients of oil lubricated steel/steel were also studied. It can be seen (in Fig. 3b) that the mean friction coefficients were 0.11 for
(DSA100). After friction tests, the morphology and composition of the wear track and counterpart surface were analyzed by a scanning electron microscopy (SEM, JMS-5600L JEOL) equipped with an Energy Dispersive Spectrometry (EDS) and MicroXAM 3D noncontact surface profiler. The wear debris of the solid/liquid lubricating systems after the friction tests were characterized by transmission electron microscopy (TEM JEM-1200EX/S). The Fourier transformation infrared spectroscopy (FTIR, IFS 66v/s) was used to analyze the structure of oils after the tribological tests.
Fig. 1. GIXRD and EDS analysis spectrum of WS2–MoS2 composite film.
Fig. 2. Typical surface and corresponding cross-sectional FESEM micrographs of the sputtered WS2–MoS2 composite film.
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Fig. 3. Typical friction curves in vacuum condition for: (a) WS2–MoS2 film, (b) oil lubricated steel/steel, (c) WS2–MoS2/MACs system, and (d) WS2–MoS2/CPSO system.
MACs and 0.14 for FCPSO during the sliding cycles of 1.2 105 r, respectively. As mixed together with WS2–MoS2 films, the friction coefficients for both combined solid/liquid systems were apparently decreased, as clearly seen from Fig. 3c and d. Moreover, it is demonstrated that the WS2–MoS2/MACs system exhibited a much better friction-reducing performance than the WS2–MoS2/CPSO system. During the sliding cycles of 20 105 r, the WS2–MoS2/MACs system exhibited a notably stable and low friction coefficient (the mean value of 0.06). However, an increasing trend of the friction coefficient (the mean value of 0.08) occurred for the WS2–MoS2/ CPSO system, even if the lifetime of the WS2–MoS2 film was limited within the sliding cycles of 1.0 105 r. Fig. 4 gives the wear rates of the two combined systems as well as WS2–MoS2 film after the friction tests. Clearly, as CPSO and MACs oils are combined respectively with the WS2–MoS2 films, the wear rate shows opposite tendencies. In comparison with the WS2–MoS2 film (about 57 10 8 mm3/Nm), the wear rate for the WS2–MoS2/ CPSO system (about 1187 10 8 mm3/Nm) is increased by two order magnitude, while decreased by one order magnitude for the WS2–MoS2/MACs system (about 1.7 10 8 mm3/Nm). 3.3. Friction and wear mechanism From the tribological results described above, a synergistic lubricating effect was achieved for WS2–MoS2/MACs system, however, which was absent to WS2–MoS2/CPSO system. The wettability of oils with the WS2–MoS2 film was assessed by measuring the contact angles. Fig. 5 shows the typical contact angle images of the two oils on the surface of WS2–MoS2 film. The measurement was repeated more than eight times for each oil
Fig. 4. Wear rates of the composite systems and WS2–MoS2 film.
sample. The mean contact angle is 19.7° for CPSO and 9.5° for MACs. The results indicated that MACs have better wettability and thereby more easy to spread out on the film surface. The wellwettability was favorable to the contacting of solid and liquid lubricants. After the friction tests, the liquid lubricants were collected from the wear track area and then analyzed by FTIR. It can be seen (shown in Fig. 6a and b) that the selected oils have good stability in the friction process because no evident structure changes were founded from FTIR results. Thus, it could be inferred that the opposite tribological performances for the two systems may be not caused by the degradation of oils in vacuum condition. Fig. 7 gives the SEM and 3D profile images of the wear tracks after friction testing. It can be seen from Fig. 7c and d that the wear track of WS2–MoS2/CPSO system was significantly wide and
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Fig. 5. Contact angle images of liquid lubricants on the surfaces of WS2–MoS2 film: (a) CPSO; (b) MACs.
Fig. 6. FTIR spectra of the selected oils before and after tribotests: (a) MACs; (b) CPSO.
irregular, demonstrating a severe wear happened in the sliding process. Nevertheless, a smooth and narrow wear track was noticed for WS2–MoS2/MACs system (Fig. 7e and f), in good accordance with the stable lubricating effect and high anti-wear capacity. By comparing with the EDS results shown in Table 2, it can be found that the highest sum of W, Mo and S elements was detected on the surface of wear track of WS2–MoS2/MACs system. However, for the WS2–MoS2/CPSO system, few contents of the film elements were detected. Besides, there were many wear debris inside and at the edge of the wear track for WS2–MoS2 film (Fig. 7a), which mostly generated from the fracture of the upper coarse columnar structure, indicating that the remaining thin lower lubricating layer mainly played the lubricating effect [10–11]. It was well known that the columnar platelets will be cracked under the effect of shear force during the sliding process for sputtered MoS2 or WS2 films [8]. So, the liquid lubricant would be firstly mixed with the upper numerous cracked WS2–MoS2 fragmentations under the shearing effect for composite systems. Then, a duplex lubricating layers were formed on the substrate including upper film/oil mixed layer and lower denser film. The wear debris was contained in the oil on the sliding surface for the durable lubricating effect. This characteristics of solid/liquid systems could make the wear debris repeated contact with the sliding surface to decrease the friction. The wear scars on the counterpart balls for WS2–MoS2 film and the composite systems after the tests were also examined. For WS2–MoS2 film, a transfer layer surrounded by a great deal of wear debris is clearly observed from Fig. 8a and b. EDS results shown in Fig. 9 reveal the high contents of W, Mo and S elements in the area of transfer layer. It is well known that transfer film plays an important role on the lubricating performance for the lamellar solid films. The suitable composite of WS2 and MoS2 not only aroused the microstructure evolution, but also assisted on the formation of transfer film on the counterpart. However, for WS2– MoS2/CPSO system, a wide and rough wear scar with numerous grooves was observed on surface of counterpart ball (Fig. 8b and c). Meanwhile, there was no obvious transfer film formed since
few S, W and Mo elements are detected by EDS. In comparison, the surface of the steel ball for WS2–MoS2/MACs was covered by a smooth layer (Fig. 8e and f), which has high film elements from Fig. 9. Therefore, it seems that an effective transfer film has been developed for WS2–MoS2/MACs system which was indispensable for the durable lubricating life. The film transferring action was distinct between WS2–MoS2 film and solid/liquid systems due to the existence of liquid lubricant. On the other hand, different oils had a considerable influence to the generating of transfer film on the counterpart ball surface. In order to further illustrate the mechanism, it is of importance to investigate wear debris in the region of wear track after friction tests. As seen from Fig. 10, the wear debris of the two composite systems show distinct structures analyzed by TEM. For WS2–MoS2/ CPSO system, the wear debris was clavate-like with a small aspect ratio from (cf. Fig. 10a and b). However, thin plate-like film debris was observed for WS2–MoS2/MACs system, as shown in Fig. 10c and d, which have a high aspect ratio. As is well known, the surfaces of WS2 or MoS2 crystals possess basal and edge plane areas. The basal plane surfaces are composed of S atoms in a hexagonal arrangement and have a relatively low surface energy as well as non-polarity [26]. Therefore, the basal planes are thought to be responsible for easy cleavage of the crystals along their surfaces and play a key role on lubrication action. However, the edges of the crystal planes are thought to be active sites with high surface energy and polarity. Besides, an interesting surface property, for lamellar solids including MoS2, WS2, graphite as well as BN, is that the basal planes of these lamellar solids have strong affinities for normal hydrocarbons compounds such as liquid paraffins or mineral oils, whereas the edge sites absorbing, independently, polar compounds [27–31]. Additionally, another important fact is that the basal plane crystals of graphite and MoS2 always present a relatively higher aspect ratio than edge site crystals, as claimed by Giltrow et al [26,32]. As shown in Table 1, there are many long hydrocarbon chains in the structure of MACs molecules, such as octyl, dodecyl and decyl groups, which are absent for CPSO.
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Fig. 7. Typical SEM and corresponding 3D profile images of the wear tracks in vacuum after tribotest: (a, b) for WS2–MoS2 composite film; (c, d) for WS2–MoS2/CPSO system; (e, f) for WS2–MoS2/MACs system. Table 2 EDS analysis results of the wear tracks for WS2–MoS2 film and composite systems.
WS2–MoS2 film WS2–MoS2/MACs WS2–MoS2/CPSO
S (at%)
W (at%)
Mo (at%)
Fe (at%)
Cr (at%)
O (at%)
6.63 8.09 –
1.6 2.44 –
3.75 4.45 1.07
64.21 61.59 72.88
17.59 17.50 19.18
6.22 5.93 6.87
Therefore, the basal planes of WS2–MoS2 film would have more preferential and powerful combination with MACs than CPSO. It has also reported that strong affinity between lamellar solids and normal hydrocarbon could convert the polar surface into a basal surface to some extent, relies on the presences of distinct fluid
medium to influence the cleavage of the crystal during the grinding process [27,32,33]. That means lamellar solids combine with normal hydrocarbon or mineral oil may improve the proportion of basal plane surfaces. It could be concluded that, therefore, the debris of WS2–MoS2/MACs system seems to have higher of basal-oriented planes. Apparently, the plate-like debris were more immediate and easy to lie flat and deposit on the contact surface to prevent the direct rubbing between the sliding pairs and even can repair the worn surface for WS2–MoS2/MACs system. In opposite, edge plane debris with relatively lower aspect ratio has smaller contacting area with the rubbing surfaces. Besides, the hardness of edge planes is higher than basal planes as the crystallographic anisotropy for MS2 lamellar solids [34]. Thus, wear
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Fig. 8. Typical SEM micrographs of the wear scars in vacuum after tribotests: (a) for WS2–MoS2 film; (c) for WS2–MoS2/MACs system; (e) for WS2–MoS2/CPSO system; (b, d, f) is the magnification for (a, c and e) respectively.
Fig. 9. Comparisons of atomic percent (at%) by EDS analysis of S, W and Mo elements on the wear scars after the friction test for WS2–MoS2 film and the composite systems.
debris with high proportion of edge-oriented crystal were more likely to embed in the sliding surfaces and even to scratch them. As aforementioned, a synergistic lubricating model was proposed to explain the excellent tribological behavior of WS2– MoS2/MACs system. The schematic graph of the synergistic lubricating mechanism is given in Fig. 11. For WS2–MoS2/MACs system, the synergistic lubricating effect mainly attributed to the following aspects: the film fragmentations were more expedient to contact and combine with the rubbing surface to generate strong adhering and thereafter decrease the direct rubbing of the friction pairs. In addition, a enhanced load-carrying capacity and stable lubricating effect were achieved due to the strong combination of solid and liquid lubricants, especially the upper film/ oil mixed layer. Besides, the well film-transferring capability on counterpart may also improve the anti-wear performance of the lubrication system. For WS2–MoS2/CPSO system, however, the film fragmentations were arduous to contact with rubbing
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Fig. 10. Typical TEM picture of worn debris after the vacuum friction test: (a, b) for WS2–MoS2/CPSO system; (c, d) for WS2–MoS2/MACs system.
Fig. 11. Schematic graph for demonstrating the synergistic lubricating mechanism.
surface. Increasingly film debris would be accumulated in the sliding area to make the friction track outspread quickly. Afterwards, the probability of the direct contact for metal to metal was vastly increased. These results were well corresponding to the observed surface morphology differences of wear tracks for the two composite systems. The synergistic lubricating effect between WS2 and MACs oil is evidenced further via an additional experiment. In the experiment, pure WS2 powders were directly mixed with MACs and CPSO oils
on the polished AISI 440 C substrate. Then the tribological behavior was also investigated in vacuum condition. The mass of WS2 powders was equal to which of the WS2–MoS2 film deposited on the substrate. The quality of oils was consistent with previous tests. It can be clearly seen from Fig. 12 that the tribological property of WS2 powders mixed with MACs was much better than that of mixing with CPSO, exhibiting the same tendency towards WS2–MoS2 film compositing with the corresponding oil. Fig. 13 gives the SEM images of the wear track after the tribotests for the added experiment. As pure WS2 powders mixed with MACs, the surface of wear track (shown in Fig. 13a and b) was smooth and complete. Besides, there were white strips distributing on the rubbing surface which were identified to contain W and S elements (shown Fig. 13c) by EDS analysis. Obviously, WS2 powders have been successfully transferred on the sliding surface. Whereas, the mixture of CPSO and WS2 particles, leads to the generation of a great number of plowing grooves on the surface of wear track (shown Fig. 13d and e). In addition, few signals of S and W elements were detected (cf. Fig. 9f) demonstrating the occurrence of severe wear and high friction. Based on the above results one can conclude that the WS2– MoS2/MACs system can more effectively integrate the advantages of solid and liquid lubricants to generate the synergistic lubricating effect than that of WS2–MoS2/CPSO system. Moreover, it is identified that liquid lubricant having more hydrocarbon chains should be preferentially selected while designing the MS2-based (M is molybdenum or tungsten) solid/liquid lubricating system.
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Fig. 12. Typical friction curves in vacuum condition for: (a) WS2 powders with MACs; (b) WS2 powders with CPSO.
Fig. 13. Typical SEM micrographs of the wear tracks in vacuum after tribotests for: (a) WS2 powders with MACs; (b) magnified picture for a; (c) EDS spectrum of the wear track surface for WS2 powders mixed with MACs; (d) WS2 powders with CPSO; (e) magnified picture for d; (f) EDS spectrum of the wear track surface for WS2 powders mixed with CPSO.
4. Conclusions
(1) WS2–MoS2 nanocomposite film was deposited by RF-sputter method, which exhibited an excellent tribological behavior than pure WS2 film for the improvement of the microstructure. (2) The solid/liquid dual lubrications systems were realized by combining WS2–MoS2 films with two specific oils including MACs and CPSO. The tribological performances of WS2-MoS2/ MACs system were much better than those of WS2–MoS2/ CPSO system. (3) For WS2–MoS2/MACs system, the good properties of lubrication and anti-wear were mainly attributed to the synergistic lubricating effect by solid and liquid lubricants. The preferential affinity between basal planes of WS2–MoS2 film and MACs was effective to form a boundary lubricating film and thereby reduce the direct rubbing between the friction pairs. However, these effects were absent for WS2–MoS2/CPSO system.
(4) Our results recommend that while designing MS2-based solid/ liquid system, liquid lubricants those contain long hydrocarbon chains in molecular structure should be preferentially selected.
Acknowledgments This work was supported by National Key Basic Research Program of China (973) (Grant no. 2013CB632302) and National Natural Science Foundation of China (Grant no. 51227804). The authors gratefully acknowledge Prof. Litian Hu at State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics for offering the lubricating oils.
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[18] Hao JY, Weng LJ, Sun JY, Liu WM. Solid lubricating film/oil or grease complex system I: tribological properties of MoS2 film at dry sliding and coated with trifluorinated-butyl with methylterminated silcon oil or the related grease formulated with powder of PTFE. Tribology 2010;30:105–10 [in Chinese]. [19] Hao JY, Wang P, Liu XQ, Liu WM. Solid lubricating film/oil or grease complex system II: tribological properties of DLC film coated with space oils and the related greases. Tribology 2010;30:217–22 [in Chinese]. [20] Erdemir A. Review of engineered tribological interfaces for improved boundary lubrication. Tribol Int 2005;38:249–56. [21] Erdemir A, Erck R, Fenske G, Hong H. Solid/liquid lubrication of ceramics at elevated temperatures. Wear 1997;203:588–95. [22] Liu XF, Wang LP, Xue QJ. High vacuum tribological performance of DLC-based solid–liquid lubricating coatings: influence of atomic oxygen and ultraviolet irradiation. Tribol Int 2013;60:36–44. [23] Liu XF, Wang LP, Lu ZB, Xue QJ. Vacuum tribological performance of DLCbased solid–liquid lubricating coatings: influence of sliding mating materials. Wear 2012;292:124–34. [24] Quan X, Gao XM, Weng LJ, Hu M, Jiang D, Wang DS, et al. Tribological behavior of WS2-based solid/liquid lubricating systems dominated by the surface properties of WS2 crystallographic planes. RSC Adv 2015;5:64892–901. [25] Weng LJ, Wang HZ, Feng DP, Pan GM, Duan YR, Liu WM, et al. Synthesis and tribological behavior of a chlorinated-phenyl methyl-terminated silicon oil as aerospace lubricant. Tribology 2005;25:254–7 [in Chinese]. [26] Wyckoff RWG. Crystal structures. 2nd ed.. New York: Interscience Publishers; 1963. [27] Andrews G, Groszek A, Hairs N. Measurement of surface areas of basal plane and polar sites in graphite and MoS2 powders. Tribol Trans 1972;15:184–91. [28] Groszek A. Preferential adsorption of compounds with long methylene chains on cast iron, graphite, boron nitride, and molybdenum disulfide. Tribol Trans 1966;9:67–76. [29] Groszek A. Preferential adsorption of long-chain normal paraffins on MoS2, WS2 and graphite from n-Heptane. Nature 1964;204:680. [30] Groszek A. Preferential adsorption of normal hydrocarbons on cast iron. Nature 1962;196:531–3. [31] Groszek A. Selective adsorption at graphite/hydrocarbon interfaces. Proc R Soc Lond A – Math Phys 1970;314:473–98. [32] Giltrow J, Groszek A. The effect of particle shape on the abrasiveness of lamellar solids. Wear 1969;13:317–29. [33] Groszek A, Witheridge R. Surface properties and lubricating action of graphite and MoS2. Tribol Trans 1971;14:254–66. [34] Lancaster J. Anisotropy in the mechanical properties of lamellar solids and its effect on wear and transfer. Wear 1966;9:169–88.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Friction and wear performance of dual lubrication systems combining WS2–MoS2 composite film and low volatility oils under vacuum condition Xin Quan a,b, Ming Hu a, Xiaoming Gao a, Yanlong Fu a, Lijun Weng a, Desheng Wang a, Dong Jiang a, Jiayi Sun a,n a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 January 2016 Received in revised form 5 March 2016 Accepted 10 March 2016 Available online 19 March 2016
In the present work, WS2–MoS2 nanocomposite films were sputtered, based on which solid/liquidlubrication systems were realized by combining the WS2–MoS2 films with space oils including MACs (multialkylated cyclopentanes) and CPSO (chlorinated-phenyl with methyl-terminated silicone oil). The tribological behaviors of the solid/liquid systems were investigated in vacuum. It was revealed that in comparison to pure WS2 film, the WS2–MoS2 composite film exhibited an enhanced tribological performance. Moreover, our results identified that the WS2–MoS2/MACs system integrated better the advantages of the solid and liquid lubrications and thereby a synergistic lubricating effect was generated. A tribological model was established to explain the synergistic lubricating mechanism. & 2016 Elsevier Ltd. All rights reserved.
Keywords: WS2–MoS2 film Liquid lubricant Solid/liquid lubrication Friction and wear
1. Introduction It is well known that transition metal disulfides (denoted as MS2), such as molybdenum disulfide and tungsten disulfide, are widely used as solid lubricant owing to the inter-lamellar hexagonal structure and inert basal planes existing in the individual crystallites where shear occurs easily [1–3]. During the last few decades, the MS2 films deposited by sputtering technique have been intensively studied and widely applied for moving parts in vacuum [4–6]. However, the easy reaction of unsaturated and dangling bonds of crystal MS2 with the moisture is inclined to result in degradation of tribological performance of these films. For this reason, many proposals had been made in the literature to improve the tribological properties of MoS2 or WS2 films, mainly including nanocomposites and multilayer constructions [7–9]. The results from related works revealed that the tribological performances of MoS2 or WS2 films in dry and even in humid atmosphere can be obviously improved by multilayer and nanocomposite architectures [9–13]. Despite all this, the performance of the present materials still cannot satisfy the requirements absolutely for future long-term space exploration, e.g. higher reliability and durability, extreme environments and harsh n
Corresponding author. Tel.: þ 86 931 4968092; fax: þ 86 931 8277088. E-mail address: [email protected] (J. Sun).
http://dx.doi.org/10.1016/j.triboint.2016.03.009 0301-679X/& 2016 Elsevier Ltd. All rights reserved.
working conditions [4,14]. NASA report also pointed out that there is no solid lubricants and films currently exist could completely meet those demands [15]. Recently, a novel lubrication technology, combining solid and liquid lubricants on friction interfaces, or namely duplex lubrication systems, shows excellent applying potential for some applications with increasingly demands. It is found that proper combination of solid and liquid lubricants may trigger a significant improvement of the tribological performances in comparison with the single lubricant [16–20]. Some results given in the papers [17,21] by Erdemir et al. indicated that Ag/polyester synthetic oil solid/liquid system exhibits good capability in controlling friction and wear for both ceramic and metallic friction pairs. The tribological properties of DLC-based solid/liquid duplex lubrication systems had been systematically studied in vacuum circumstance in works by Liu et al. [14–16,22,23]. The results revealed that some of DLC-based solid–liquid systems, such as DLC/IL, DLC/IL/Graphene and DLC/MACs systems, exhibit promising application prospect in space which mainly attribute to the substantial loadbearing capability of DLC film and good lubricity from liquid lubricant. In our recent work, we have investigated the pure WS2 film based solid/liquid lubricating systems [24]. The results demonstrated that proper combination of pure WS2 film with special oil, such as WS2/SiCH (silahydrocarbons) system, can achieve significant improvement in tribological performances than
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pure WS2 film both under vacuum and air conditions. Well selflubricating property of WS2 film as well as surface property of WS2 crystallographic planes together in response to the tribological behavior of WS2/SiCH system. Despite of some studies on solid/liquid duplex lubrication systems, to address specific applications however, it should be studied in more details mainly involving the following respects:
Compatibility and wettability between a variety of liquid lubricants and all kinds of solid lubricants.
The mechanisms of the interaction of physical and chemical
and n-type Si (100) substrates were selected for the tribological performance tests and the analyses of the film composition and structure, respectively. The steel substrates were surface-polished and then ultrasonically cleaned in acetone before the deposition. Then, they were mounted under the target for the vertical distance of 80 mm. The substrate surfaces were etched by an Ar plasma for 15 min at a DC bias of 500 V to eliminate possible contaminants before the deposition. Afterwards, the WS2–MoS2 composite films were sputtered under a target power density of 0.068 W mm2 in an Ar pressure of 5.0 Pa and a substrate bias voltage of 70 V for the deposition time of 12 min.
properties between liquid and solid lubricants.
The effects of functional group, polarity and additives with
respect to liquid lubricants on the tribological properties of duplex lubrication system. The effects of modified microstructures, especially nanocomposites and multilayers, of solid lubricants on the tribological properties of duplex lubrication system.
In this study, we firstly fabricated WS2–MoS2 nanocomposite film to optimize the microstructure and tribological properties. And then, the solid/liquid lubricating systems were designed by WS2–MoS2 nanocomposite film with two specific liquid lubricants. The selected oils, including CPSO (chlorinated-phenyl with methyl-terminated silicone oil) and MACs (multialkylated cyclopentanes), have been successfully used in space moving components for their excellent lubrication property and well physicochemical properties, e.g. extremely low volatility, high viscosity index, wide running temperature, and low pour point. The main aim of this present work is to investigate tribological performance of MS2 nanocomposite film based solid/liquid system under vacuum condition and further to demonstrate the friction and wear mechanism.
2. Experimental details 2.1. Materials The MACs and CPSO oils were provided by the State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics [25]. Their typical properties are listed in Table 1. 2.2. WS2–MoS2 composite film preparation A radio frequency sputtering equipment and WS2–MoS2 composite target (80 mm in diameter, and mass ratio of is 1/2) were used to deposit WS2/MoS2 nanocomposite films in this work. The composite target was composed of pure WS2 and MoS2 (99.9% purity) in mass ratio of 1–2. The AISI 440C (Ø25 mm 4 mm) steel
2.3. Preparation of the solid/liquid lubricating system In this work, the solid/liquid composite systems were prepared by combining the sputtered WS2–MoS2 film with oils. The MACs and CPSO oil were respectively coated on the pre-deposited WS2– MoS2 film surfaces to form a homogeneous oil layer by a spin coating method. The calculated oil layer thickness was about 20 mm for both MACs and CPSO. Then the WS2-based solid/liquid lubricating systems were prepared as references. The detailed calculation method has been reported in detail in a previous work [24]. 2.4. Friction and wear test The tribological behaviors of the WS2–MoS2 composite film and the solid/liquid lubricating systems were evaluated using a ballon-disk tribometer in vacuum condition at room temperature (10 4 Pa, 30% RH, 20 75 °C). The disks were the solid/liquid lubricating systems or WS2–MoS2 films. The counterpart was cleaned AISI 440C steel ball (HRC 60, Ra 0.1 mm and 8 mm in diameter). The normal load was 3 N, corresponding to a theoretical Hertzian contact pressure of 0.25 GPa. The rotational speed was 1000 rev/min for all tests, equaling to a linear speed of 0.52 m/s. Each friction test was repeated three times under the same conditions to check the repeatability of the results. The wear rates (K) after the sliding tests were calculated via equation of K ¼ V/(FS), where V is the wear volume loss (mm3), F is normal load (N) and S is total sliding distance (m). 2.5. Characterization Field emission scanning electron microscopy (FESEM, JSM6701F) was used to examine the morphology of WS2–MoS2 composite films. The crystal structure of the films was investigated using grazing incidence X-ray diffraction(GIXRD, Philips, X'Pert Pro) with Cu Κɑ radiation in the scanning range of 2θ from 5° to 80°. The contact angle of the oils on the surface of films was measured by using a contact angle measurement instrument
Table 1 Typical properties of MACs and CPSO oils. MACs
CPSO
56 9.3 148 858.3 5.6 10 6
120.4 48.6 415 1014 8.4 10 8
Molecular structure
Viscosity at 40 °C/mm2 s1 Viscosity at 100 °C/mm2 s1 Viscosity index Density at 20 °C/kg m3 Vapor pressure at 20 °C/Pa
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3. Results and discussion
disk was about 0.5 mm which was measured by 3D surface profiler. It can be seen that the oil layer was much thicker than WS2–MoS2 film and its surface roughness. When MoS2 was co-deposited, the surface of the composite film shows a feather-like morphology (cf. Fig. 2a); whereas the pure WS2 film exhibits an acicular-like structure as revealed in our former work [24]. This demonstrates that the microstructure of WS2–MoS2 composite film was much denser than the pure WS2 film. The cross-sectional FESEM micrograph manifests that WS2–MoS2 film possesses a compact double-layer microstructure, containing an upper columnar platelet and a lower dense layer near the substrate with the thickness about 100 nm.
3.1. Microstructure and composition of WS2–MoS2 composite film
3.2. Tribological properties
Fig. 1a shows the GIXRD spectra for the WS2–MoS2 composite film. Since the lattice constants of WS2 and MoS2 are parallel, the diffraction peak positions of them are almost identical. It could be found that the film exhibits both hcp-WS2 and hcp-MoS2 (002), (101) and (112) diffraction peaks besides the substrate peak. The strongest peak was in the vicinity of 35° which demonstrated that the film preferred was grown along (101) crystal plane. In addition, the growth of (002) plane was thought to be helpful to improve the tribological properties for the composite film. The result of film element composition determined by EDS is manifested in Fig. 1b. The contents of S, W, and Mo elements in the composite coating were 41.7 at%, 8.1 at% and 22.3 at%, respectively. Fig. 2 gives the FESEM micrographs of the WS2–MoS2 composite film. The mean thickness of the film was in the range of 3.2– 3.6 mm. The surface roughness of WS2–MoS2 film deposited on
The tribological properties of WS2–MoS2 composite film in vacuum were firstly investigated. Fig. 3a shows that the WS2– MoS2 film exhibited well lubricating property with a mean friction coefficient of 0.05 and a wear life about 8.0 105 cycles, which was much longer than that of pure WS2 film [24]. It is well known that the mainly deterioration mechanism for WS2 or MoS2 film is own to the oxidation reaction with the moisture and O2 [8]. It is deemed that the excellent tribological behavior of WS2–MoS2 film is related to its dense microstructure, which could effectively prevent the film contacting with ambient atmosphere and therefore efficiently decrease the friction. Then, we evaluated the friction and wear behaviors of the two composite systems in vacuum condition. For comparison, the friction coefficients of oil lubricated steel/steel were also studied. It can be seen (in Fig. 3b) that the mean friction coefficients were 0.11 for
(DSA100). After friction tests, the morphology and composition of the wear track and counterpart surface were analyzed by a scanning electron microscopy (SEM, JMS-5600L JEOL) equipped with an Energy Dispersive Spectrometry (EDS) and MicroXAM 3D noncontact surface profiler. The wear debris of the solid/liquid lubricating systems after the friction tests were characterized by transmission electron microscopy (TEM JEM-1200EX/S). The Fourier transformation infrared spectroscopy (FTIR, IFS 66v/s) was used to analyze the structure of oils after the tribological tests.
Fig. 1. GIXRD and EDS analysis spectrum of WS2–MoS2 composite film.
Fig. 2. Typical surface and corresponding cross-sectional FESEM micrographs of the sputtered WS2–MoS2 composite film.
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Fig. 3. Typical friction curves in vacuum condition for: (a) WS2–MoS2 film, (b) oil lubricated steel/steel, (c) WS2–MoS2/MACs system, and (d) WS2–MoS2/CPSO system.
MACs and 0.14 for FCPSO during the sliding cycles of 1.2 105 r, respectively. As mixed together with WS2–MoS2 films, the friction coefficients for both combined solid/liquid systems were apparently decreased, as clearly seen from Fig. 3c and d. Moreover, it is demonstrated that the WS2–MoS2/MACs system exhibited a much better friction-reducing performance than the WS2–MoS2/CPSO system. During the sliding cycles of 20 105 r, the WS2–MoS2/MACs system exhibited a notably stable and low friction coefficient (the mean value of 0.06). However, an increasing trend of the friction coefficient (the mean value of 0.08) occurred for the WS2–MoS2/ CPSO system, even if the lifetime of the WS2–MoS2 film was limited within the sliding cycles of 1.0 105 r. Fig. 4 gives the wear rates of the two combined systems as well as WS2–MoS2 film after the friction tests. Clearly, as CPSO and MACs oils are combined respectively with the WS2–MoS2 films, the wear rate shows opposite tendencies. In comparison with the WS2–MoS2 film (about 57 10 8 mm3/Nm), the wear rate for the WS2–MoS2/ CPSO system (about 1187 10 8 mm3/Nm) is increased by two order magnitude, while decreased by one order magnitude for the WS2–MoS2/MACs system (about 1.7 10 8 mm3/Nm). 3.3. Friction and wear mechanism From the tribological results described above, a synergistic lubricating effect was achieved for WS2–MoS2/MACs system, however, which was absent to WS2–MoS2/CPSO system. The wettability of oils with the WS2–MoS2 film was assessed by measuring the contact angles. Fig. 5 shows the typical contact angle images of the two oils on the surface of WS2–MoS2 film. The measurement was repeated more than eight times for each oil
Fig. 4. Wear rates of the composite systems and WS2–MoS2 film.
sample. The mean contact angle is 19.7° for CPSO and 9.5° for MACs. The results indicated that MACs have better wettability and thereby more easy to spread out on the film surface. The wellwettability was favorable to the contacting of solid and liquid lubricants. After the friction tests, the liquid lubricants were collected from the wear track area and then analyzed by FTIR. It can be seen (shown in Fig. 6a and b) that the selected oils have good stability in the friction process because no evident structure changes were founded from FTIR results. Thus, it could be inferred that the opposite tribological performances for the two systems may be not caused by the degradation of oils in vacuum condition. Fig. 7 gives the SEM and 3D profile images of the wear tracks after friction testing. It can be seen from Fig. 7c and d that the wear track of WS2–MoS2/CPSO system was significantly wide and
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Fig. 5. Contact angle images of liquid lubricants on the surfaces of WS2–MoS2 film: (a) CPSO; (b) MACs.
Fig. 6. FTIR spectra of the selected oils before and after tribotests: (a) MACs; (b) CPSO.
irregular, demonstrating a severe wear happened in the sliding process. Nevertheless, a smooth and narrow wear track was noticed for WS2–MoS2/MACs system (Fig. 7e and f), in good accordance with the stable lubricating effect and high anti-wear capacity. By comparing with the EDS results shown in Table 2, it can be found that the highest sum of W, Mo and S elements was detected on the surface of wear track of WS2–MoS2/MACs system. However, for the WS2–MoS2/CPSO system, few contents of the film elements were detected. Besides, there were many wear debris inside and at the edge of the wear track for WS2–MoS2 film (Fig. 7a), which mostly generated from the fracture of the upper coarse columnar structure, indicating that the remaining thin lower lubricating layer mainly played the lubricating effect [10–11]. It was well known that the columnar platelets will be cracked under the effect of shear force during the sliding process for sputtered MoS2 or WS2 films [8]. So, the liquid lubricant would be firstly mixed with the upper numerous cracked WS2–MoS2 fragmentations under the shearing effect for composite systems. Then, a duplex lubricating layers were formed on the substrate including upper film/oil mixed layer and lower denser film. The wear debris was contained in the oil on the sliding surface for the durable lubricating effect. This characteristics of solid/liquid systems could make the wear debris repeated contact with the sliding surface to decrease the friction. The wear scars on the counterpart balls for WS2–MoS2 film and the composite systems after the tests were also examined. For WS2–MoS2 film, a transfer layer surrounded by a great deal of wear debris is clearly observed from Fig. 8a and b. EDS results shown in Fig. 9 reveal the high contents of W, Mo and S elements in the area of transfer layer. It is well known that transfer film plays an important role on the lubricating performance for the lamellar solid films. The suitable composite of WS2 and MoS2 not only aroused the microstructure evolution, but also assisted on the formation of transfer film on the counterpart. However, for WS2– MoS2/CPSO system, a wide and rough wear scar with numerous grooves was observed on surface of counterpart ball (Fig. 8b and c). Meanwhile, there was no obvious transfer film formed since
few S, W and Mo elements are detected by EDS. In comparison, the surface of the steel ball for WS2–MoS2/MACs was covered by a smooth layer (Fig. 8e and f), which has high film elements from Fig. 9. Therefore, it seems that an effective transfer film has been developed for WS2–MoS2/MACs system which was indispensable for the durable lubricating life. The film transferring action was distinct between WS2–MoS2 film and solid/liquid systems due to the existence of liquid lubricant. On the other hand, different oils had a considerable influence to the generating of transfer film on the counterpart ball surface. In order to further illustrate the mechanism, it is of importance to investigate wear debris in the region of wear track after friction tests. As seen from Fig. 10, the wear debris of the two composite systems show distinct structures analyzed by TEM. For WS2–MoS2/ CPSO system, the wear debris was clavate-like with a small aspect ratio from (cf. Fig. 10a and b). However, thin plate-like film debris was observed for WS2–MoS2/MACs system, as shown in Fig. 10c and d, which have a high aspect ratio. As is well known, the surfaces of WS2 or MoS2 crystals possess basal and edge plane areas. The basal plane surfaces are composed of S atoms in a hexagonal arrangement and have a relatively low surface energy as well as non-polarity [26]. Therefore, the basal planes are thought to be responsible for easy cleavage of the crystals along their surfaces and play a key role on lubrication action. However, the edges of the crystal planes are thought to be active sites with high surface energy and polarity. Besides, an interesting surface property, for lamellar solids including MoS2, WS2, graphite as well as BN, is that the basal planes of these lamellar solids have strong affinities for normal hydrocarbons compounds such as liquid paraffins or mineral oils, whereas the edge sites absorbing, independently, polar compounds [27–31]. Additionally, another important fact is that the basal plane crystals of graphite and MoS2 always present a relatively higher aspect ratio than edge site crystals, as claimed by Giltrow et al [26,32]. As shown in Table 1, there are many long hydrocarbon chains in the structure of MACs molecules, such as octyl, dodecyl and decyl groups, which are absent for CPSO.
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Fig. 7. Typical SEM and corresponding 3D profile images of the wear tracks in vacuum after tribotest: (a, b) for WS2–MoS2 composite film; (c, d) for WS2–MoS2/CPSO system; (e, f) for WS2–MoS2/MACs system. Table 2 EDS analysis results of the wear tracks for WS2–MoS2 film and composite systems.
WS2–MoS2 film WS2–MoS2/MACs WS2–MoS2/CPSO
S (at%)
W (at%)
Mo (at%)
Fe (at%)
Cr (at%)
O (at%)
6.63 8.09 –
1.6 2.44 –
3.75 4.45 1.07
64.21 61.59 72.88
17.59 17.50 19.18
6.22 5.93 6.87
Therefore, the basal planes of WS2–MoS2 film would have more preferential and powerful combination with MACs than CPSO. It has also reported that strong affinity between lamellar solids and normal hydrocarbon could convert the polar surface into a basal surface to some extent, relies on the presences of distinct fluid
medium to influence the cleavage of the crystal during the grinding process [27,32,33]. That means lamellar solids combine with normal hydrocarbon or mineral oil may improve the proportion of basal plane surfaces. It could be concluded that, therefore, the debris of WS2–MoS2/MACs system seems to have higher of basal-oriented planes. Apparently, the plate-like debris were more immediate and easy to lie flat and deposit on the contact surface to prevent the direct rubbing between the sliding pairs and even can repair the worn surface for WS2–MoS2/MACs system. In opposite, edge plane debris with relatively lower aspect ratio has smaller contacting area with the rubbing surfaces. Besides, the hardness of edge planes is higher than basal planes as the crystallographic anisotropy for MS2 lamellar solids [34]. Thus, wear
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Fig. 8. Typical SEM micrographs of the wear scars in vacuum after tribotests: (a) for WS2–MoS2 film; (c) for WS2–MoS2/MACs system; (e) for WS2–MoS2/CPSO system; (b, d, f) is the magnification for (a, c and e) respectively.
Fig. 9. Comparisons of atomic percent (at%) by EDS analysis of S, W and Mo elements on the wear scars after the friction test for WS2–MoS2 film and the composite systems.
debris with high proportion of edge-oriented crystal were more likely to embed in the sliding surfaces and even to scratch them. As aforementioned, a synergistic lubricating model was proposed to explain the excellent tribological behavior of WS2– MoS2/MACs system. The schematic graph of the synergistic lubricating mechanism is given in Fig. 11. For WS2–MoS2/MACs system, the synergistic lubricating effect mainly attributed to the following aspects: the film fragmentations were more expedient to contact and combine with the rubbing surface to generate strong adhering and thereafter decrease the direct rubbing of the friction pairs. In addition, a enhanced load-carrying capacity and stable lubricating effect were achieved due to the strong combination of solid and liquid lubricants, especially the upper film/ oil mixed layer. Besides, the well film-transferring capability on counterpart may also improve the anti-wear performance of the lubrication system. For WS2–MoS2/CPSO system, however, the film fragmentations were arduous to contact with rubbing
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Fig. 10. Typical TEM picture of worn debris after the vacuum friction test: (a, b) for WS2–MoS2/CPSO system; (c, d) for WS2–MoS2/MACs system.
Fig. 11. Schematic graph for demonstrating the synergistic lubricating mechanism.
surface. Increasingly film debris would be accumulated in the sliding area to make the friction track outspread quickly. Afterwards, the probability of the direct contact for metal to metal was vastly increased. These results were well corresponding to the observed surface morphology differences of wear tracks for the two composite systems. The synergistic lubricating effect between WS2 and MACs oil is evidenced further via an additional experiment. In the experiment, pure WS2 powders were directly mixed with MACs and CPSO oils
on the polished AISI 440 C substrate. Then the tribological behavior was also investigated in vacuum condition. The mass of WS2 powders was equal to which of the WS2–MoS2 film deposited on the substrate. The quality of oils was consistent with previous tests. It can be clearly seen from Fig. 12 that the tribological property of WS2 powders mixed with MACs was much better than that of mixing with CPSO, exhibiting the same tendency towards WS2–MoS2 film compositing with the corresponding oil. Fig. 13 gives the SEM images of the wear track after the tribotests for the added experiment. As pure WS2 powders mixed with MACs, the surface of wear track (shown in Fig. 13a and b) was smooth and complete. Besides, there were white strips distributing on the rubbing surface which were identified to contain W and S elements (shown Fig. 13c) by EDS analysis. Obviously, WS2 powders have been successfully transferred on the sliding surface. Whereas, the mixture of CPSO and WS2 particles, leads to the generation of a great number of plowing grooves on the surface of wear track (shown Fig. 13d and e). In addition, few signals of S and W elements were detected (cf. Fig. 9f) demonstrating the occurrence of severe wear and high friction. Based on the above results one can conclude that the WS2– MoS2/MACs system can more effectively integrate the advantages of solid and liquid lubricants to generate the synergistic lubricating effect than that of WS2–MoS2/CPSO system. Moreover, it is identified that liquid lubricant having more hydrocarbon chains should be preferentially selected while designing the MS2-based (M is molybdenum or tungsten) solid/liquid lubricating system.
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Fig. 12. Typical friction curves in vacuum condition for: (a) WS2 powders with MACs; (b) WS2 powders with CPSO.
Fig. 13. Typical SEM micrographs of the wear tracks in vacuum after tribotests for: (a) WS2 powders with MACs; (b) magnified picture for a; (c) EDS spectrum of the wear track surface for WS2 powders mixed with MACs; (d) WS2 powders with CPSO; (e) magnified picture for d; (f) EDS spectrum of the wear track surface for WS2 powders mixed with CPSO.
4. Conclusions
(1) WS2–MoS2 nanocomposite film was deposited by RF-sputter method, which exhibited an excellent tribological behavior than pure WS2 film for the improvement of the microstructure. (2) The solid/liquid dual lubrications systems were realized by combining WS2–MoS2 films with two specific oils including MACs and CPSO. The tribological performances of WS2-MoS2/ MACs system were much better than those of WS2–MoS2/ CPSO system. (3) For WS2–MoS2/MACs system, the good properties of lubrication and anti-wear were mainly attributed to the synergistic lubricating effect by solid and liquid lubricants. The preferential affinity between basal planes of WS2–MoS2 film and MACs was effective to form a boundary lubricating film and thereby reduce the direct rubbing between the friction pairs. However, these effects were absent for WS2–MoS2/CPSO system.
(4) Our results recommend that while designing MS2-based solid/ liquid system, liquid lubricants those contain long hydrocarbon chains in molecular structure should be preferentially selected.
Acknowledgments This work was supported by National Key Basic Research Program of China (973) (Grant no. 2013CB632302) and National Natural Science Foundation of China (Grant no. 51227804). The authors gratefully acknowledge Prof. Litian Hu at State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics for offering the lubricating oils.
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