Next Article in Journal
Identification and Regulation of Cold Rolling Interface State Based on a Novel Modified Forward Slip Model
Next Article in Special Issue
Friction and Wear Properties of AgCuNi Alloy/Au-Electroplated Layer Sliding Electrical Contact Material
Previous Article in Journal
Impact of Soot on Internal Combustion Engine Lubrication—Oil Condition Monitoring, Tribological Properties, and Surface Chemistry
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Lubricants
Volume 12
Issue 12
10.3390/lubricants12120403
lubricants-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Investigation of Tribological Performance of Ti:WS2/PFPE Composite Lubricating System Under Proton Radiation

by
Jian Liu
1,2,3,
Zhen Yan
1,2,3,*,
Junying Hao
1,2,3,* and
Weimin Liu
1
1
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
2
Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
3
Qingdao Center of Resource Chemistry and New Materials, Qingdao 266000, China
*
Authors to whom correspondence should be addressed.
Lubricants 2024, 12(12), 403; https://doi.org/10.3390/lubricants12120403
Submission received: 15 October 2024 / Revised: 7 November 2024 / Accepted: 18 November 2024 / Published: 21 November 2024
(This article belongs to the Special Issue Space Tribology)
Browse Figures
Versions Notes

Abstract

:
The tribological performance of PFPE oil and the Ti:WS2/PFPE composite lubricating system with different oil amounts was investigated under a proton radiation (PR) irradiation environment. After PR irradiation, PFPE molecules occurred during cross-linking and a polymerization reaction and formed a volatile small molecular compound, which deteriorates the tribological performance of the Ti:WS2/PFPE system. The tribological properties of the Ti:WS2/PFPE system rely strongly on oil amount. For an unirradiated Ti:WS2/PFPE system, the amorphous layer of transfer film near the sliding contact area was converted into a well-defined crystalline WS2 layer with a (002) plane induced by the friction process. After PR irradiation, the transfer film became thicker and showed a wholly amorphous structure due to the difficulty in preventing the entrance of O and showed no reorientation with induced friction.

1. Introduction

As one of the classic members of the group of transitional metal dichalcogenides (TMDs), WS2 film possesses outstanding tribological properties under a vacuum environment due to its extremely anisotropic layered structure in which two adjacent layers of WS2 are held together through weak van der Waals forces [1,2,3,4]. Nonetheless, space lubrication presents problems including not only vacuum but also earth’s atmosphere. And WS2 film is easily degraded in a high-humidity environment, which makes H2O and O2 absorb at defective dangling bonds and edge sites, resulting in the failure of WS2 film [3,5]. In recent decades, co-sputtering with metal (Ti, Cr, Au, etc.) to improve the mechanical properties and oxidation resistance of WS2 film has been explored [5,6,7,8,9]. Unfortunately, the high friction noise and limited wear life of these films makes them shrivel, making them unsuitable for use in future deep space exploration [10,11].
Liquid lubricants with ultra-low vapor pressure such as polyalphaolefin (PAO) and perfluoropolyether (PFPE) have been widely applied to reduce friction and mechanical noise in space mechanical components, such as in bearings, gyroscopes, oxidizer pumps, sensors, and so on [12,13]. The development of a new type of solid–liquid composite lubricating system through the combination of solid film and liquid lubricant will be of great significance in improving the state of lubrication in space motion mechanisms, as this is regarded as an ideal lubrication pattern for space applications [13,14,15,16]. As is well known, instantaneous temperature is very high during dry friction, which perhaps degrades the physicochemical properties of film, but the local temperature can be reduced significantly after adding some oil [13]. The ability of liquid oil to replenish the wear area during fiction, especially under harsh conditions, therefore extends wear life [17,18]. Although there is a small amount of lubricating oil in the wear area, it can play a good role in lubrication. Furthermore, the wear debris produced by solid films during friction is physically or chemically adsorbed with the liquid lubricant, and friction-induced recrystallization occurs, which changes tribological properties greatly [19]. Therefore, the solid–liquid composite lubricating system can minimize its own shortcomings and exhibit higher reliability and a longer lubricating life than a singular lubricating material, which exhibits adaptive lubrication [20].
However, the harsh space environment includes factors such as atomic oxygen, ultraviolet light and charged particles (like protons and electrons), beyond vacuum and microgravity [21,22]. In low earth orbits (LEOs) with altitudes ranging from 200 km to 800 km, the primary influencing factor is atomic oxygen (AO). AO can cause space lubricants as well as the solid–liquid composite lubricating system to be oxidized, decomposed and polymerized, therefore greatly degrading tribological performance [23,24,25]. However, due to the rapid development of the aerospace industry toward deep space exploration, proton radiation (PR) exposure has become the main factor [22]. The damage to the structure and tribological properties of MoS2 under low-energy PR irradiation was studied by Nicholson et al., and the results indicate that the loss of tribological performance is a combination of irradiation damage effects including the loss of stoichiometry, altered structure and increased reactivity [22]. The tribological behavior of MoS2-based solid–liquid systems established by Zhuang et al. was improved greatly via the synergistic lubricating effects of solid and liquid lubricants, and synergistic effects are still maintained despite the damage caused by PR irradiation [20]. The composition, structure and tribological behaviors of PFPE utilized with WS2 nanoparticles during PR irradiation were investigated by Ren et al., and the results indicated that severe deterioration and degradation of PFPE took place and that surface oxidation of WS2 occurred under PR irradiation [26].
Ti:WS2 film prepared by physical vapor deposition technology displays excellent tribological behaviors after Ti incorporation. PFPE oil benefits from its chemical inertness, thermal stability and low friction coefficient, which have made it one of the optimal lubricants for space applications [12,27]. The Ti:WS2/PFPE composite lubricating systems are created by combining Ti:WS2 films and PFPE oil. As far as the authors are aware, for the Ti:WS2/PFPE composite lubricating system, detailed studies into the effect of the PFPE oil amount and PR irradiation on tribological properties have not been undertaken. In this work, the effect of simulated PR irradiation on the microstructure and the physicochemical properties of PFPE oil was investigated. The tribological properties of PFPE oil and the Ti:WS2/PFPE systems with different oil amounts were evaluated on a ball-on-disk tribometer under a high-vacuum and PR environment using space tribology experimental equipment. The chemical composition and morphologies of wear areas were systematically analyzed, and the tribological mechanism was also elucidated based on these results. Our research provides guidance that supports the development of a high-performance solid–liquid composite lubricating system toward application in deep space mechanical devices.

2. Experimental Details

2.1. Film Deposition and Construction of Ti:WS2/PFPE Composite Lubricating System

The Ti:WS2 films were prepared on n-type Si substrates and a stainless steel disk (9Cr18, Φ 25 × 8 mm, Ra ~ 0.03 μm) by a magnetron sputtering deposition system (UDP700 produced by Teer Coating Ltd., Droitwich, UK) with two WS2 targets and one Ti target (345 × 145 × 8 mm3, 99.99% purity). Prior to deposition, the vacuum chamber was pumped to a base pressure of 1.0 × 10−3 Pa for Ar+ ion bombardment in order to remove possible contaminants on the substrate surface. Afterward, a film with a thickness of 2 μm was produced with a bias voltage of −20 V, Ar flow of 70 sccm, WS2 target power of 600 W, Ti target current of 0.15 A and deposition time of 120 min.
Fomblin Y25 perfluoropolyether (PFPE) oil in amounts of 10 μL, 5 μL and 2 μL was dropped on the Ti:WS2 film surface in order to construct the Ti:WS2/PFPE composite lubricating system. From Figure 1, it can be seen that PFPE oil can spread on the Ti:WS2 film surface faster than on 9Cr18 steel at room temperature, indicating that the oil can repair the wear area of the film surface in a timely manner.

2.2. PR Irradiation and Tribological Test

PR irradiation was conducted using a self-made ground-based simulation facility, a schematic diagram and photo of which are shown in Figure 2. Before PR irradiation, the chamber was pumped down to 1.0 × 10−4 Pa with an Edward mechanical pump and an Agilent molecular pump (Agilent, Santa Clara, CA, USA). PR irradiation was performed at an accelerated voltage of 25 kV and a flux of 3.0 × 1017 protons·m−2·s−1. The exposure time was set to 10 min, and total fluence was 1.8 × 1020 protons·m−2. The simulated deep space PR irradiation experiment could explore irradiation-induced damage to lubricants in situ and promote the development of lubricating materials toward special space applications. Then, the upper sample was moved onto the lubricating system using a mechanical arm to obtain the normal load under a vacuum state. Finally, the tribological tests were carried out without exposure to the atmosphere. And the tribological performance of the Ti:WS2/PFPE system was evaluated using a ball-on-disk tribometer with a line speed of 0.2 m/s (500 r/min) under a vacuum environment (20 °C, 1.0 × 10−4 Pa). In these tests, a 9Cr18 steel ball with a 6 mm diameter was held on a rotating disk. Hertz contact pressure was 1 GPa at the normal load of 5 N. After the friction test, the three-dimensional morphologies of the wear surface were obtained by a noncontact surface profiler (MicroXAM-800, KLA-Tencor, Milpitas, CA, USA).

2.3. Characterization

The weight of PFPE oil before and after PR irradiation was obtained by an analytical balance (SHIMADZU, AUW220D, Kyoto, Japan). The NATZSCH thermo-gravimetric analyzer (NATZSCH, Selb, Germany) was used to evaluate the thermal stability of PFPE oil with and without PR irradiation. The molecular weight (Mw) and polydispersion index (PDI) of PFPE oil before and after PR irradiation were measured by gel permeation chromatography (GPC, Agilent 1260, Agilent, Santa Clara, CA, USA). The wear areas were analyzed by the DXR 2 Raman spectrometer (ThermoFisher Scientific, Waltham, MA, USA). BRUKER V70 Fourier transform infrared spectroscopy (FT-IR) (BRUKER, Billerica, MA, USA) was used to examine the molecular structures of PFPE oil. The morphologies of the wear area were characterized by field emission scanning electron microscopy (FE-SEM, JSM-7610F, JEOL, Akishima, Japan). Chemical composition was analyzed using an X-ray photoelectron spectroscope (XPS) with a monochromatic Al Kα X-ray radiation source (200 μm spot size) of 1486.6 eV, which was calibrated by an adventitious carbon C 1s peak at 284.8 eV. A focused ion beam (FIB, Gatan 95) technique was used to prepare the cross-section of transfer film on the wear scar, which was then examined by a high-resolution transmission electron microscope (HRTEM, Talos F200, ThermoFisher Scientific).

3. Results and Discussion

3.1. The Influence of PR Irradiation on the PFPE Oil

Figure 3 exhibits photographs of the PFPE oil before and after PR irradiation. It is clear that the PFPE base oil exists in a liquid state, but the irradiated oil has a solid state and a black aggregate surface, indicating that PR irradiation caused cross-linking and polymerization between molecules and the formation of polymers. Furthermore, it can be calculated that the weight loss is −14.4 mg (weight loss rate of 11.8%) after 10 min of PR irradiation. Compared with PR-irradiated P201 oil (15 min, weight loss rate of 3%), the weight loss of PFPE is significantly larger. The results showed that the molecular structure of PFPE oil is more sensitive to PR irradiation. Similarly, it can be inferred that the weight decrease of oil is due to the molecular chain being broken under high-energy particle bombardment, resulting in the formation and volatilization of small molecular compounds, leading to weight loss.
By measuring the weight-average molecular weight (Mw) and polydispersion index (PDI) of PFPE oil before and after PR irradiation, the effects of irradiation on oil can be seen more clearly. As shown in Figure 4, the oils were dissolved in C2Cl3F3 (99.5% purity) to form a uniform solution. In contrast, it can be seen that the Mw and PDI increase after PR irradiation. It is illustrated that cross-linking and a polymerization reaction took place after irradiation as well as the formation and volatilization of a small molecular compound, leading to an increase in molecular weight and widening the distribution of molecular weight.
The thermo-gravimetric analysis of PFPE and PR-irradiated PFPE oil was carried out and is shown in Figure 5. The decomposition of PFPE oil occurs at 305 °C, but after irradiation, its initial decomposition temperature reduces by 24 °C, which suggests that PR irradiation induces the breakage of some chemical bonds and the formation of small molecular compounds. Obviously, there is some undecomposed material at 600 °C, which can be ascribed to the formation of high-molecular-weight polymers by PR irradiation.
Figure 6 gives the FTIR spectra of PFPE and PR-irradiated PFPE oil. The characteristic peaks at 1425–1065 cm−1 and 2400–1425 cm−1 are due to C-F and C=O vibration, respectively. And the peak that appears at 980 cm−1 is attributed to C-O vibration [28]. Although the positions of characteristic peaks before and after PR irradiation are almost identical, their intensities are obviously different because of irradiation-induced decomposition as well as cross-linking and a polymerization reaction. More specifically, compared with the PFPE base oil, the irradiated PFPE oil shows lower C-F vibration intensity and higher C-O and C=O vibration intensity, which further confirms that PR irradiation damages PFPE oil very obviously.
The tribological performance of PFPE oil with and without PR irradiation was investigated, and the friction curves are shown in Figure 7. Compared with the PFPE base oil, the 10 min irradiated oil increases the friction coefficient by ~17%, and the 15 min irradiated oil increases the friction coefficient to more than 0.5 within 50 s, implying that the damaging of physicochemical properties leads to the deterioration of tribological performance.

3.2. Tribological Properties of Ti:WS2/PFPE System

The tribological behaviors of the Ti:WS2/PFPE system with good synergistic lubrication under a vacuum environment before and after PR irradiation were investigated in detail and are shown in Figure 8. As previously reported, the Ti:WS2/PFPE system with 10 μL of PFPE oil (Ti:WS2/PFPE10) has a low friction coefficient (0.055). Furthermore, the influence of PFPE oil amount on the tribological performance of the Ti:WS2/PFPE system was studied. The Ti:WS2/PFPE system with 5 μL of PFPE oil (Ti:WS2/PFPE5) shows a large fluctuating signal in the running-in stage, followed by a relatively stable friction coefficient (0.065). The Ti:WS2/PFPE system with 2 μL of PFPE oil (Ti:WS2/PFPE2) displays an ever-increasing friction coefficient in the running-in stage, and then shows an incredibly volatile friction curve (0.08~0.11), probably because of the rough wear surface, and lubrication failure finally occurs after 1.5 × 105 r. According to the reports of researchers, PR irradiation has an important effect on the properties of lubricating materials [20,26]. Here, the tribological properties of the Ti:WS2/PFPE system under harsh PR irradiation were systematically evaluated. Although Ti:WS2/PFPE10 displays a high initial friction coefficient (0.13) under PR irradiation, it has a stable friction curve (0.07) after that. However, Ti:WS2/PFPE5, which performs well under vacuum, exhibits significant fluctuation and a limited wear life (4.5 × 104 r) with PR irradiation, suggesting that irradiation has an obvious influence on the tribological properties of this system. The friction curve of Ti:WS2/PFPE2 after PR irradiation presents a very stable and low friction coefficient (0.025) within 7.8 × 104 r, except during the initial friction process, and it shows a wear life of about 1.1 × 105 r.
The 3D topographies of wear tracks on the substrates with and without PR irradiation are shown in Figure 9. The worn surface of Ti:WS2/PFPE2 is observed to have some wide and deep wear tracks formed by severe wear, while those of Ti:WS2/PFPE5 and Ti:WS2/PFPE10 show shallow wear tracks, which is in accordance with the tribological results. Notably, the wear area of Ti:WS2/PFPE10 is covered with a smooth and intact tribolayer. Interestingly, 3D morphology of Ti:WS2/PFPE2 with PR irradiation shows a shallow wear track, suggesting that irradiated oil does not participate in the friction process due to the loss of a small amount of PFPE oil exposed to irradiation. In contrast, Ti:WS2/PFPE5 with good tribological properties in vacuum shows wide and deep wear tracks after PR irradiation, as well as some pits which formed on it, indicating that this system experienced serious abrasive wear and adhesive wear under irradiation. Similarly, Ti:WS2/PFPE10 with PR irradiation shows a discontinuous tribolayer due to a portion of PFPE oil being corrupted by irradiation.
In summary, oil amount and PR irradiation have obvious influences on the tribological properties of the Ti:WS2/PFPE system. In-depth and detailed studies were then carried out to reveal the mechanism.

3.3. The Tribological Mechanisms

Figure 10 shows SEM images of the wear track surface of the Ti:WS2/PFPE system with and without PR irradiation. The wear track of Ti:WS2/PFPE2 has some deep grooves along the sliding direction, which is consistent with the results shown in Figure 9. As the amount of PFPE oil increases, the wear track becomes smooth, especially for the Ti:WS2/PFPE10 system, and high-resolution morphology shows characteristic loose WS2 clusters. After PR irradiation, the 2 μL of PFPE oil on the surface of the film is almost fully consumed, and Ti:WS2/PFPE2-PR displays some characteristics of onefold dry friction [29,30]. When the quantity of PFPE oil is 5 μL, the oil is completely irradiated, and some polymer compounds are generated, which gather the wear debris together. When friction occurs, the aggregates are rapidly consumed, and lubrication failure occurs. As the quantity of oil increases to 10 μL, the loose WS2 clusters generated during synergistic lubrication disappear and a dense tribolayer forms instead.
Micrographs of wear scars formed on the counter balls before and after PR irradiation were also measured and are shown in Figure 11. For the unirradiated Ti:WS2/PFPE system, with an increase in PFPE oil amount, the size of the wear scar decreases and the dispersion degree of the transfer film becomes more uniform. After PR irradiation, a large continuous film-like surface with dry friction was formed with respect to the Ti:WS2/PFPE2-PR system. For the Ti:WS2/PFPE5-PR system, the size of the wear scar was large and the transfer film shows some distinct aggregation. It can be inferred that the underlying unirradiated oil plays a key role in the Ti:WS2/PFPE10-PR system, and some wear debris still exists in a dispersed form.
Figure 12 presents the Raman spectra at the center of the wear surface of the Ti:WS2/PFPE system before and after PR irradiation. Almost all spectra display a WS2 Raman region (50~600 cm−1), a WO3 peak (850 cm−1) and a C-C Raman region (1060~2000 cm−1) [31,32,33]. At the wear track surface, the WS2 Raman signal appears in the Ti:WS2/PFPE5, Ti:WS2/PFPE10 and Ti:WS2/PFPE10-PR systems, and all of these showed good tribological performance. It is clear that the intensity of WO3 vibration increases after PR irradiation, which may be due to the weakened fluidity of irradiated oil and the inability of irradiated oil to protect the friction region. Meanwhile, a large number of PFPE oil signals can be detected only on the oil-rich Ti:WS2/PFPE10 surface. With respect to the counter ball, strong WS2 vibration modes were detected in all systems due to the transfer of WS2 debris from the film to the counter ball. Like the wear track, the intensity of WO3 signals on the counter ball are also enhanced after irradiation. On the other hand, a large number of oil signals appear in the aggregation of the Ti:WS2/PFPE2 and Ti:WS2/PFPE5-PR systems rather than on the Ti:WS2/PFPE10 surface. The results indicate that the uniformity of the transfer film will be reduced when a large quantity of wear debris is absorbed in a limited amount of PFPE oil.
To illustrate the influence of oil amount and irradiation-induced changes on the composition and structure of the Ti:WS2/PFPE system, XPS measurement was carried out. The high-resolution XPS spectra of W 4f and C 1s on the wear track along with Gaussian–Lorentzian fitting are shown in Figure 13. For all samples, the deconvolution of W 4f shows two doublets, which could be attributed to WS2 (32.4 and 34.4 eV) and WO3 (35.4 and 37.5 eV), and the fitted results are listed in Table 1 [34,35]. WO3 appears on the wear track of all systems, indicating that friction-induced oxidation occurs during friction, even under high-vacuum conditions. Compared with the unirradiated Ti:WS2/PFPE2 system, the irradiation-induced volatilization of PFPE oil makes the oil decrease, and a large continuous film formed at the Ti:WS2/PFPE2-PR wear area acts as a barrier to the atmosphere. Therefore, the content of WO3 on the wear track surface of Ti:WS2/PFPE2-PR decreases very obviously. As the amount of PFPE oil increases, the oil can protect the film from oxidation, and the WO3 content decreases, especially in the case of Ti:WS2/PFPE10, which shows the highest content of WS2. Unfortunately, oil volatilization and cross-linking as well as polymerization after PR irradiation result in an increase in WO3. For C 1s XPS spectra, the peaks are located at a binding energy of 284.8 and 290.4 ~ 296 eV and can be assigned to the C-C/C-H and C-F bonds, respectively [35,36]. It is observed that the intensity of the C-F peak increases after PR irradiation on the wear track surface within several nm, suggesting that irradiated oil is more likely to adsorb on the wear track surface or that irradiated oil is not completely dissolved in the solvent.
Deep insights into the lubrication failure mechanism of the Ti:WS2/PFPE system after PR irradiation were obtained using HRTEM observations, and the results are shown in Figure 14. Figure 14a–c show cross-sectional TEM images of the unirradiated Ti:WS2/PFPE system, and it can be clearly seen that a transfer film with a thickness of 63 nm is formed between the 9Cr18 substrate and the W/C protective layer. Furthermore, the transfer film can be divided into two layers: the upper layer is a crystalline WS2 layer with an interplanar spacing value of 0.62 nm, and the lower layer is an amorphous structure [37,38]. As shown in Figure 15, the results of mapping confirm that the upper layer of the transfer film is enriched with the W and S elements and is accompanied by a small amount of the Ti element, which acts as filling. It can be inferred that the well-defined WS2 (002) crystal planes play a role in reducing friction during sliding, and some crystalline WS2 exists in the amorphous layer, which will be converted into a crystalline WS2 layer during a friction-induced process.
As shown in Figure 14d–f, after PR irradiation, the thickness of the transfer film increased to 1200 nm, which is 19 times that of the unirradiated system. Except for a small amount of crystalline WS2 on the top of the transfer film, the whole transfer film has an amorphous structure, as shown in the inset of Figure 14f. Figure 16 shows the HRTEM element mapping for the irradiated Ti:WS2/PFPE system, and the results indicated that O content was very high, though the quantities of the W and S elements were not. It is confirmed that the irradiated system could not prevent the entrance of O, and recrystallization cannot occur in the presence of a high O content with induced friction. It was found that the high number of molecular compounds formed by irradiation weakened the fluidity of lubricating oil and covered the wear debris through an adhesive effect. The transfer film without a crystallization rearrangement was poor in crystallization and contained a large number of pores. Consequently, the abrasive wear caused by frictional oxidation gave rise to an increase in the friction and wear of composite lubricating systems. Therefore, the Ti:WS2/PFPE system with PR irradiation showed poor tribological performance.

4. Conclusions

The Ti:WS2/PFPE composite lubricating system was constructed by a combination of sputtered Ti:WS2 film and different amounts of PFPE oil. PR irradiation results in cross-linking and a polymerization reaction within PFPE molecules as well as the formation and volatilization of small molecular compounds, therefore increasing molecular weight and widening the distribution of molecular weight. Also, PR irradiation induces an increase in the friction coefficient and a decrease in the wear life of PFPE oil. Meanwhile, the tribological properties of the Ti:WS2/PFPE system rely strongly on oil amount. The Ti:WS2/PFPE system with 5 μL and 10 μL of PFPE oil displays better tribological properties than that with 2 μL of PFPE oil. This is mainly attributed to the well-defined WS2 (002) crystal planes in the upper layer of the transfer film, and some WS2 that exist in the amorphous layer will be converted into a crystalline WS2 layer during a friction-induced process. After PR irradiation, the Ti:WS2/PFPE system with more oil exhibits good tribological properties, because the underlying unirradiated oil still plays a lubricating role. However, the entirety of the transfer film of Ti:WS2/PFPE is found to display an amorphous structure due to the difficulty in preventing the entrance of O and showed no reorientation with induced friction. This study indicates that PR irradiation has a significant influence on the friction and wear of the Ti:WS2/PFPE system.

Author Contributions

J.L.: Conceptualization, Data curation, Investigation, Writing—original draft, Formal analysis. Z.Y.: Methodology, Investigation, Validation. J.H.: Writing—review and editing, Project administration. W.L.: Funding acquisition, Supervision, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Basic Research Project (No.2020-JCJQ-ZD-155-12), Major Science and Technology Project of Gansu Province (No. 23ZDGA011), Gansu Natural Science Foundation under grant No. 22JR5RA109, and the Young Elite Scientists Sponsorship Program by CAST (2021QNRC001).

Data Availability Statement

All the data will be available on request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

  1. Uzoma, P.C.; Hu, H.; Khadem, M.; Penkov, O.V. Tribology of 2D nanomaterials: A review. Coatings 2020, 10, 897. [Google Scholar] [CrossRef]
  2. Yang, J.F.; Parakash, B.; Hardell, J.; Fang, Q.F. Tribological properties of transition metal di-chalcogenide based lubricant coatings. Front Mater. Sci. 2012, 6, 116–127. [Google Scholar] [CrossRef]
  3. Marian, M.; Berman, D.; Rota, A.; Jackson, R.L.; Rosenkranz, A. Layered 2D nanomaterials to tailor friction and wear in machine elements—A review. Adv. Mater. Interfaces 2021, 9, 2101622. [Google Scholar] [CrossRef]
  4. Joseph, A.; Vijayan, A.S.; Shebeeb, C.M.; Akshay, K.S.; John Mathew, K.P.; Sajith, V. A review on tailoring the corrosion and oxidation properties of MoS2-based coatings. J. Mater. Chem. A 2023, 11, 3172–3209. [Google Scholar] [CrossRef]
  5. Vazirisereshk, M.R.; Martini, A.; Strubbe, D.A.; Baykara, M.Z. Solid lubrication with MoS2: A review. Lubricants 2019, 7, 57. [Google Scholar] [CrossRef]
  6. Tedstone, A.A.; Lewis, D.J.; O’Brien, P. Synthesis, properties, and applications of transition metal-doped layered transition metal dichalcogenides. Chem. Mater. 2016, 28, 1965–1974. [Google Scholar] [CrossRef]
  7. Ren, S.; Li, H.; Cui, M.; Wang, L.; Pu, J. Functional regulation of Pb-Ti/MoS2 composite coatings for environmentally adaptive solid lubrication. Appl. Surf. Sci. 2017, 401, 362–372. [Google Scholar] [CrossRef]
  8. Scharf, T.W.; Goeke, R.S.; Kotula, P.G.; Prasad, S.V. Synthesis of Au-MoS2 nanocomposites: Thermal and friction-induced changes to the structure. ACS Appl. Mater. Interfaces 2013, 5, 11762–11767. [Google Scholar] [CrossRef]
  9. Serles, P.; Nicholson, E.; Tam, J.; Barri, N.; Chemin, J.B.; Wang, G.; Michel, Y.; Veer Sigh, C.; Choquet, P.; Saulot, A.; et al. High performance space lubrication of MoS2 with tantalum. Adv. Funct. Mater. 2022, 32, 2110429. [Google Scholar] [CrossRef]
  10. Zhang, J.; Jiang, D.; Wang, D.; Yu, Q.; Bai, Y.; Cai, M.; Weng, L.; Zhou, F.; Liu, W. MoS2 lubricating film meets supramolecular gel: A novel composite lubricating system for space applications. ACS Appl. Mater. Interfaces 2021, 13, 58036–58047. [Google Scholar] [CrossRef]
  11. Quan, X.; Hu, M.; Gao, X.; Fu, Y.; Weng, L.; Wang, D.; Jiang, D.; Sun, J. Friction and wear performance of dual lubrication systems combining WS2–MoS2 composite film and low volatility oils under vacuum condition. Tribol. Inter. 2016, 99, 57–66. [Google Scholar] [CrossRef]
  12. Zaretsky, E.V. Liquid lubrication in space. Tribol. Inter. 1990, 23, 75–93. [Google Scholar] [CrossRef]
  13. Fan, X.; Xue, Q.; Wang, L. Carbon-based solid-liquid lubricating coatings for space applications-a review. Friction 2015, 3, 191–207. [Google Scholar] [CrossRef]
  14. Liu, X.; Wang, L.; Xue, Q. DlC-based solid–liquid synergetic lubricating coatings for improving tribological behavior of boundary lubricated surfaces under high vacuum condition. Wear 2011, 271, 889–898. [Google Scholar] [CrossRef]
  15. Lv, M.; Yang, L.; Wang, Q.; Wang, T.; Liang, Y. Tribological performance and lubrication mechanism of solid–liquid lubricating materials in high-vacuum and irradiation environments. Tribol. Lett. 2015, 59, 20. [Google Scholar] [CrossRef]
  16. Quan, X.; Gao, X.; Weng, L.; Hu, M.; Jiang, D.; Wang, D.; Sun, J.; Liu, W. Tribological behavior of WS2-based solid/liquid lubricating systems dominated by the surface properties of WS2 crystallographic planes. RSC Adv. 2015, 5, 64892. [Google Scholar] [CrossRef]
  17. Lu, Z.; Lin, Q.; Cao, Z.; Li, W.; Gong, J.; Wang, Y.; Hu, K.; Hu, X. MoS2 nanomaterials as lubricant additives: A review. Lubricants 2023, 11, 527. [Google Scholar] [CrossRef]
  18. Cai, M.; Yu, Q.; Liu, W.; Zhou, F. Ionic liquid lubricants: When chemistry meets tribology. Chem. Soc. Rev. 2020, 49, 7753. [Google Scholar] [CrossRef]
  19. Quan, X.; Zhang, S.; Hu, M.; Gao, X.; Jiang, D.; Sun, J. Tribological properties of WS2 /MoS2-Ag composite films lubricated with ionic liquids under vacuum conditions. Tribol. Inter. 2017, 115, 389–396. [Google Scholar] [CrossRef]
  20. Zhuang, W.; Fan, X.; Li, W.; Li, H.; Zhang, L.; Peng, J.; Cai, Z.; Mo, J.; Zhang, G.; Zhu, M. Comparing space adaptability of diamond-like carbon and molybdenum disulfide films toward synergistic lubrication. Carbon 2018, 134, 163–173. [Google Scholar] [CrossRef]
  21. Nautiyal, H.; Singh, S.; Gautam, R.; Goswami, R.; Khatri, O.; Verma, P.; Tyagi, R. The state of art on lubrication methods in space environment. Phys. Scr. 2024, 99, 022003. [Google Scholar] [CrossRef]
  22. Nicholson, E.; Serles, P.; Wang, G.; Filleter, T.; Davis, J.W.; Singh, C.V. Low energy proton irradiation tolerance of molybdenum disulfide lubricants. Appl. Surf. Sci. 2021, 567, 150677. [Google Scholar] [CrossRef]
  23. Wei, R.; Wilbur, P.J.; Buchholz, B.W.; Kustas, S.M. In situ tribological evaluation of greases and solid lubricants in a simulated atomic oxygen environment. Tribol. Trans. 1995, 38, 950–958. [Google Scholar] [CrossRef]
  24. Fan, X.; Shi, Y.; Cui, M.; Ren, S.; Wang, H.; Pu, J. MoS2/WS2 nanosheet-based composite films irradiated by atomic oxygen: Implications for lubrication in space. ACS Appl. Nano Mater. 2021, 4, 10307–10320. [Google Scholar] [CrossRef]
  25. Wu, Y.; Liu, Y.; Li, H.; Chen, J.; Yu, S.; Zhou, B.; Tang, B. The effect of vacuum atomic oxygen and ultraviolet radiations on Ag/a-C:H nanocomposite film. Tribol. Inter. 2016, 101, 395–401. [Google Scholar] [CrossRef]
  26. Ren, J.; Gong, K.; Zhao, G.; Lou, W.; Wu, X.; Wang, X. Investigation of the tribological performances of graphene and WS2 nanosheets as additives for perfluoroalkylpolyethers under simulated space environment. Tribol. Lett. 2021, 69, 45. [Google Scholar] [CrossRef]
  27. Minn, M.; Sinha, S.K.; Lee, S.K.; Kondo, H. High-speed tribology of PFPEs with different functional groups and molecular weights coated on DLC. Tribol. Lett. 2006, 24, 67–76. [Google Scholar] [CrossRef]
  28. Cui, H.C.; Li, D.C.; Wang, C. Infrared spectrum analysis of perfluoro polyethers (PFPE) of fluorocarbon-based magnetic fluid’s base liquid. Adv. Mater. Res. 2011, 391–392, 1311–1314. [Google Scholar] [CrossRef]
  29. Yang, J.; Wang, D.; Fu, Y.; Wang, Q.; Hu, M.; Jiang, D.; Gao, X.; Sun, J.; Weng, J. Improving the tribological and anti-corrosion property of the WS2 film through Ta doping. Vacuum 2021, 192, 110485. [Google Scholar] [CrossRef]
  30. Takahashi, A.; Takeichi, Y.; Kimura, M.; Hashimoto, K. Low friction mechanism survey of tungsten disulfide by using XRD, XPS, and XAFS. Tribol. Lett. 2021, 69, 84. [Google Scholar] [CrossRef]
  31. Mignuzzi, S.; Pollard, A.J.; Bonini, N.; Brennan, B.; Gilmore, I.S.; Pimenta, M.A.; Pichards, D.; Roy, D. Effect of disorder on Raman scattering of single-layer MoS2. Phys. Rev. B 2015, 91, 195411. [Google Scholar] [CrossRef]
  32. Windom, B.C.; Sawyer, W.G.; Hahn, D.W. A Raman Spectroscopic study of MoS2 and MoO3: Applications to tribological systems. Tribol. Lett. 2011, 42, 301–310. [Google Scholar] [CrossRef]
  33. He, M.; Yeo, C.D. Micro-wear and friction of carbon film with molecularly thin PFPE lubricants. J. Mech. Sci. Technol. 2018, 32, 3291–3297. [Google Scholar] [CrossRef]
  34. Alov, N.V. XPS study of MoO3 and WO3 oxide surface modification by low-energy Ar+ ion bombardment. Phys. Status Solidi. C 2015, 12, 263–266. [Google Scholar] [CrossRef]
  35. Liu, J.; Yan, Z.; Hao, J.; Liu, W. Tribological performance and failure mechanism of Ti:WS2/P201 hybrid lubrication system under atomic oxygen irradiation. Vacuum 2023, 211, 111976. [Google Scholar] [CrossRef]
  36. Pan, F.M.; Lin, Y.L.; Horng, S.S. Decomposition of perfluoropolyethers during XPS measurements. Appl. Surf. Sci. 1991, 47, 9–16. [Google Scholar] [CrossRef]
  37. Pu, J.; Ren, S.; Lu, Z.; Wang, L. A feasible multilayer structure design for solid lubricant coatings in a lunar environment. RSC Adv. 2016, 6, 65504–65517. [Google Scholar] [CrossRef]
  38. Gustavsson, F.; Svahn, F.; Bexell, U.; Jacobson, S. Nanoparticle based and sputtered WS2 low-friction coatings—differences and similarities with respect to friction mechanisms and tribofilm formation. Surf. Coat. Technol. 2013, 232, 616–626. [Google Scholar] [CrossRef]
Lubricants 12 00403 g001
Figure 1. Photographs of 10 μL of PFPE oil dropped on the surface of 9Cr18 steel and film at different times.
Figure 1. Photographs of 10 μL of PFPE oil dropped on the surface of 9Cr18 steel and film at different times.
Lubricants 12 00403 g001
Lubricants 12 00403 g002
Figure 2. Schematic diagram and photo of self-made vacuum tribometer with PR irradiation.
Figure 2. Schematic diagram and photo of self-made vacuum tribometer with PR irradiation.
Lubricants 12 00403 g002
Lubricants 12 00403 g003
Figure 3. Photographs of PFPE oil before and after PR irradiation.
Figure 3. Photographs of PFPE oil before and after PR irradiation.
Lubricants 12 00403 g003
Lubricants 12 00403 g004
Figure 4. PFPE oil with and without PR irradiation dissolved in solvent.
Figure 4. PFPE oil with and without PR irradiation dissolved in solvent.
Lubricants 12 00403 g004
Lubricants 12 00403 g005
Figure 5. The thermo-gravimetric analysis curves (decomposition temperature) of PFPE and PR-irradiated PFPE oil.
Figure 5. The thermo-gravimetric analysis curves (decomposition temperature) of PFPE and PR-irradiated PFPE oil.
Lubricants 12 00403 g005
Lubricants 12 00403 g006
Figure 6. FTIR spectra of PFPE and PR-irradiated PFPE oil.
Figure 6. FTIR spectra of PFPE and PR-irradiated PFPE oil.
Lubricants 12 00403 g006
Lubricants 12 00403 g007
Figure 7. The friction curves of PFPE oil with and without PR irradiation.
Figure 7. The friction curves of PFPE oil with and without PR irradiation.
Lubricants 12 00403 g007
Lubricants 12 00403 g008
Figure 8. The friction curves of the Ti:WS2/PFPE system with 2 μL, 5 μL and 10 μL of PFPE oil before and after PR irradiation.
Figure 8. The friction curves of the Ti:WS2/PFPE system with 2 μL, 5 μL and 10 μL of PFPE oil before and after PR irradiation.
Lubricants 12 00403 g008
Lubricants 12 00403 g009
Figure 9. The 3D morphologies of wear tracks of the Ti:WS2/PFPE system (2μL, 5 μL and 10 μL) before and after PR irradiation.
Figure 9. The 3D morphologies of wear tracks of the Ti:WS2/PFPE system (2μL, 5 μL and 10 μL) before and after PR irradiation.
Lubricants 12 00403 g009
Lubricants 12 00403 g010
Figure 10. SEM images and clearer observation of wear track surface of Ti:WS2/PFPE system with and without PR irradiation.
Figure 10. SEM images and clearer observation of wear track surface of Ti:WS2/PFPE system with and without PR irradiation.
Lubricants 12 00403 g010
Lubricants 12 00403 g011
Figure 11. The micrographs of wear scars (size) formed on the counter balls before and after AO irradiation.
Figure 11. The micrographs of wear scars (size) formed on the counter balls before and after AO irradiation.
Lubricants 12 00403 g011
Lubricants 12 00403 g012
Figure 12. Raman spectra at the center of the wear track and the counter ball surface of the Ti:WS2/PFPE system.
Figure 12. Raman spectra at the center of the wear track and the counter ball surface of the Ti:WS2/PFPE system.
Lubricants 12 00403 g012
Lubricants 12 00403 g013
Figure 13. The high-resolution XPS spectra of W 4f (WO3 and WS2) and C 1s (C-F and C-C/C-H) on the wear track.
Figure 13. The high-resolution XPS spectra of W 4f (WO3 and WS2) and C 1s (C-F and C-C/C-H) on the wear track.
Lubricants 12 00403 g013
Lubricants 12 00403 g014
Figure 14. HRTEM images of the wear scar of (a–c) Ti:WS2/PFPE and (d–f) Ti:WS2/PFPE-PR.
Figure 14. HRTEM images of the wear scar of (a–c) Ti:WS2/PFPE and (d–f) Ti:WS2/PFPE-PR.
Lubricants 12 00403 g014
Lubricants 12 00403 g015
Figure 15. The HRTEM elemental mapping scan (scale 50 nm) of the W, S, Ti, C, O and Fe elements in the Ti:WS2/PFPE system.
Figure 15. The HRTEM elemental mapping scan (scale 50 nm) of the W, S, Ti, C, O and Fe elements in the Ti:WS2/PFPE system.
Lubricants 12 00403 g015
Lubricants 12 00403 g016
Figure 16. The HRTEM elemental mapping scan (scale 500 nm) of the W, S, Ti, C, O and Fe elements in the Ti:WS2/PFPE-PR system.
Figure 16. The HRTEM elemental mapping scan (scale 500 nm) of the W, S, Ti, C, O and Fe elements in the Ti:WS2/PFPE-PR system.
Lubricants 12 00403 g016
Table 1. The fitted XPS data for the Ti:WS2/PFPE system on the wear tracks.
Table 1. The fitted XPS data for the Ti:WS2/PFPE system on the wear tracks.
Before IrradiationAfter Irradiation
PFPE oil content (μL)25102510
WS2 (atom%)55.755.865.862.349.655.7
WO3 (atom%)44.344.234.237.750.444.3
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Liu, J.; Yan, Z.; Hao, J.; Liu, W. Investigation of Tribological Performance of Ti:WS2/PFPE Composite Lubricating System Under Proton Radiation. Lubricants 2024, 12, 403. https://doi.org/10.3390/lubricants12120403

AMA Style

Liu J, Yan Z, Hao J, Liu W. Investigation of Tribological Performance of Ti:WS2/PFPE Composite Lubricating System Under Proton Radiation. Lubricants. 2024; 12(12):403. https://doi.org/10.3390/lubricants12120403

Chicago/Turabian Style

Liu, Jian, Zhen Yan, Junying Hao, and Weimin Liu. 2024. "Investigation of Tribological Performance of Ti:WS2/PFPE Composite Lubricating System Under Proton Radiation" Lubricants 12, no. 12: 403. https://doi.org/10.3390/lubricants12120403

APA Style

Liu, J., Yan, Z., Hao, J., & Liu, W. (2024). Investigation of Tribological Performance of Ti:WS2/PFPE Composite Lubricating System Under Proton Radiation. Lubricants, 12(12), 403. https://doi.org/10.3390/lubricants12120403

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop