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Article

A Promising Conductive Lubricant for Space Sliding Electrical Contact: NbSe2-Ti Film

by
Yang Yang
1,2,
Guan Wang
1,*,
Xingchen Zhou
1,
Xingkang Su
1 and
Long Gu
1
1
School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China
2
Key Laboratory of Science and Technology on Wear and Protection of Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
*
Author to whom correspondence should be addressed.
Lubricants 2024, 12(2), 44; https://doi.org/10.3390/lubricants12020044
Submission received: 26 December 2023 / Revised: 29 January 2024 / Accepted: 31 January 2024 / Published: 4 February 2024
Browse Figures
Versions Notes

Abstract

:
Vacuum-sliding electrical contacts find extensive application in aerospace components, yet they face limitations related to inadequate lubrication performance. In this study, we analyzed the design of an emerging conductive lubricant material, NbSe2. A series of NbSe2-Ti films with varying doped Ti contents were prepared through magnetron sputtering technology. We investigated the correlation between the sputtering current and composition, microstructure, mechanical properties, and current-carrying tribological properties of the films. The results indicate that under vacuum and current-carrying conditions, the NbSe2-Ti films demonstrate significant advantages over existing electrical-contact lubrication materials. Compared with electroplated gold films, the NbSe2-Ti films reduced the coefficient of friction from 0.25 to 0.015, thereby improving the wear life by more than six times. This result demonstrates that magnetron-sputtered NbSe2 film can be used as a lubricant for space current-carrying sliding contacts.

1. Introduction

With advances in space exploration, aerospace technology has encountered significant challenges. Conductive lubrication materials play a pivotal role in aerospace systems, and an increasing number of these materials are extensively used in the space environment [1,2,3]. For example, the lubricity and wear resistance of a slip-ring assembly are crucial factors in determining the ability of a spacecraft to operate stably over an extended period. The primary engineering challenges associated with these applications, particularly in the case of long-service-life slip rings, involve minimizing wear and ensuring stable wear performance, maintaining acceptable friction levels to minimize mechanical energy losses, achieving low and stable electrical contact resistance (ECR) to minimize Joule heating, and ensuring the thermal stability of sliding materials. Unlike general mechanical wear, conductive lubricated parts are influenced by a combination of factors, including loading current and sliding friction during service. Excessive heat from Joule heating and frictional heat generated by strong currents can lead to high temperatures near the electrical contact interface. This can degrade the film properties, alter the coefficient of friction, and cause severe wear [4,5]. Conventional friction-testing machines face difficulties in realizing vacuum current-carrying environment testing, and there is insufficient guidance for material design and performance improvement [6].
Currently, precious metals remain the primary materials for vacuum electric contact lubrication. The excellent electrical conductivity and chemical stability of precious metals are guaranteed for practical applications [7]. Typically, precious metals such as gold are prepared using electroplating methods. There is growing concern regarding the environmental hazards associated with the use and disposal of cyanides and arsenic in electroplating chemistry [8]. However, under vacuum current-carrying conditions, friction pairs involving precious metals can exhibit severe adhesion wear and liquid spatter, with a friction coefficient typically exceeding 0.3 [9]. This issue significantly affects lubrication life.
However, the unique characteristics and high standards of in-service sliding electrical contacts limit the use of conventional lubricants. Coatings incorporating transition metal dichalcogenides (TMDs) as solid lubricants garnered significant interest among researchers due to their remarkable combination of lubricious properties and conductivity [10]. The layered structure and semiconducting properties inherent in TMDs contribute significantly to their favorable tribological behaviors and exceptional transport characteristics [11]. Their distinct properties offer promising avenues for advancing the understanding of solid lubricant materials and their potential applications. Graphite serves as a solid lubricant renowned for its commendable electrical conductivity, but its wear resistance deteriorates considerably in a vacuum and dry environment [12,13]. The higher resistivity of MoS2 leads to instability in electrical signal transmission during the current-carrying friction process. NbSe2, characterized by its distinctive layered structure, shares lubricating properties akin to MoS2. Additionally, it exhibits metal conductivity and, intriguingly, even demonstrates superconductivity properties [14,15]. For example, NbSe2 displays a resistivity of 3.5 × 10−4 Ω·cm, in contrast to MoS2 with a resistivity of 8.5 Ω·cm. NbSe2 materials were used in precise electric brushes for motors and special bearings in atmospheric environments. However, the applications of NbSe2 in specific spaces remain unexplored [16,17]. Exploring the optimization of its microstructure composition and investigating the tribology and electric contact properties in the actual service conditions of space vacuum current-carrying holds significant merit.
In this study, a series of NbSe2-Ti films were produced via magnetron sputtering under low-pressure and low-sputtering energy deposition conditions. The composition, microstructure, and properties of these films were systematically adjusted by modulating the sputtering current of the Ti target, preventing the generation of NbSe3 and Nb2O5. This approach resulted in excellent tribological and electrical contact properties under vacuum current-carrying conditions, showcasing significant advantages over presently employed electroplated films.

2. Experimental Details

2.1. Film Deposition

The NbSe2-Ti films were fabricated using a Teer Plasma CF-800 closed unbalanced field magnetron sputtering system(Teer Coatings), maintaining a substrate-target distance of 150 mm, with the substrate rotating in front of each target at a rate of 5.0 rpm. Pulsed DC power supply at a 40 V bias voltage during deposition, coupled with a tailored deposition process involving low-pressure conditions (0.11 Pa) and minimal sputtering energy, aimed to preserve NbSe2 purity while suppressing interference phases. After the base pressure dropped below 1 × 10−5 mbar, Ar gas introduction and a 20 min etching process using Ar+ ions facilitated the removal of native oxides from the substrate surface. A 250 nm Ti interlayer was deposited to enhance adhesion. Substrate materials included an n-type Si (100) substrate and polished stainless steel (24 mm × 8 mm 1Cr18Ni9Ti). The sputtering current for the NbSe2 target was maintained at 1.2 A, while Ti target currents were set to 0 A, 0.2 A, 0.4 A, and 0.6 A, respectively, designating the corresponding samples as Ti-0, Ti-0.2, Ti-0.4, and Ti-0.6. Table 1 provides a comprehensive overview of the parameters employed during the film deposition process.

2.2. General Characterization

The chemical compositions of the NbSe2-Ti films were analyzed using both an energy dispersive spectrometer (SEM, JSM-5600LV, Japan) and an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi). X-ray diffractometry (GIXRD, EMPYREAN, PANalytical) was conducted in the swept mode (2°) at a 2θ angle, covering a range of 10° to 80° for X-ray diffraction (XRD) patterns. Additionally, the morphologies of the NbSe2-Ti composite films were observed using FE-SEM (JSM-6701F, JEOL). Cross-sectional images of the films were obtained through the destructive analysis of the silicon substrate post-film deposition. The determination of nanohardness and elastic modulus values for NbSe2-Ti composite films on stainless steel substrates was carried out using a nanoindenter (Nano Hardness Tester, CSM) equipped with a Berkovich diamond probe tip. To mitigate the impact of the substrate, the indentation depth was maintained below 10% of the film’s thickness. The evaluation of bond strength for films on the steel substrate was performed utilizing a CSM Revetest scratch tester, employing a load range from 1 to 10 N and a loading rate of 4 N·min−1. The square resistance of the films was determined using a four-probe measuring instrument (McP-y610; Loresta GP, Japan). The resistivity was calculated using the formula R = ρ/W, where W represents the film thickness, and ρ denotes the square resistance. The resistivity (ρ) of the film was determined using the formula ρ = R_s × W, where W represents the film thickness, and R_s is the sheet resistance.

2.3. Tribological Test

Conventional friction testing machines face challenges in simulating real vacuum current-carrying test conditions and providing data closer to actual working conditions. Therefore, we enhanced the assembly of the CSM vacuum friction testing machine and configured the current loading system, current, and voltage testing device (as shown in Figure 1) for real-time monitoring of electrical signals. This setup facilitated a genuine evaluation of the tribological performance of vacuum current-carrying. A DC-stabilized power supply (KXN-10050D) was connected to the positive pole and the conductive contact, with the negative pole connected to the conductive sample. The conductive contact and sample were linked to a friction-testing machine through ceramic insulation. When the conductive contact made contact with the sample surface, the test loop switched from off to on. The contact voltage between the contact and sample coating was measured using a high-precision real-time voltage and current-measuring instrument (FLUKE8846A). A 6Cr18 steel ball with a 6.0 mm diameter was selected as the counterpart, with a normal load of 1.0 N, reciprocating amplitude of 5 mm, frequency of 5 Hz, and current-carrying of 1 A. All tests were carried out in a vacuum (pressure lower than 2 × 10−5 mbar). The experiment was terminated when the samples became worn. Each friction experiment was repeated at least four times.

3. Results and Discussion

3.1. Composition and Structure of Films

XPS was utilized to analyze the chemical composition variations of the NbSe2-Ti films with different Ti contents. The XPS results for the NbSe2-Ti films are depicted in Figure 2. Figure 2a displays a typical Nb 3d peak for Ti-0.4, which, through fitting, can be decomposed into four peaks at 203.2, 205.9, 207.3, and 210.0 eV. These correspond to the Nb 3d5/2 and Nb 3d3/2 peaks of Nb4+ in NbSe2 and Nb 3d3/2 peaks of Nb5+ in Nb2O5, respectively [18]. The red line in Figure 2a,b represents the comparison between the XPS fit and the original curve. The majority of the unetched Ti-0.4 film surface comprises Nb2O5. Previous studies demonstrated oxidation pollution on sputtered film surfaces, attributed to the adsorption of surface oxygen or retention of residual oxygen during the deposition process [19]. Furthermore, during transportation from preparation to characterization and testing, exposure to the atmosphere exacerbates surface oxidation levels. To ascertain the actual chemical composition of the NbSe2-Ti film, argon ion etching was employed to treat the film surface for 1 min, eliminating external influences on the chemical composition. In Figure 2b, XPS outcomes for the Ti-0.4 film following the etching procedure are illustrated. Following etching, there is a notable reduction in the Nb2O5 content of the Ti-0.4 film, with NbSe2 emerging as the predominant component. This indicates that oxidation predominantly occurred at the film surface, with minimal oxidation product formation within the film.
Figure 2c illustrates the XPS results for the NbSe2-Ti films with varying Ti concentrations under identical etching conditions. As Ti content increased, there was a proportional increase in NbSe2 content and a concurrent reduction in oxidation. This phenomenon is ascribed to increased Ti incorporation into the solid solution within the NbSe2 matrix, effectively diminishing the internal grain size of the NbSe2 coating. The Ti dopants were localized at both the grain boundaries of the columnar structures and the interlayer between the NbSe2 layers, reducing the number of active dangling bonds formed during preparation [20]. Figure 2d shows typical Ti 2p peaks of the films. With increasing Ti content, the Ti 2p peak at 455.5 intensifies and exhibits significant broadening.
The XRD patterns of the NbSe2-Ti films are presented in Figure 3. Peaks of the Ti and NbSe2 phases were identified for all films, with the absence of Nb2O5 crystal phases indicating an amorphous structure. As Ti content increased, the intensity of the NbSe2 peaks gradually decreased. The strongest NbSe2 (002) diffraction peaks in the Ti-0 films correspond to the HCP-NbSe2 (002) crystal surface, suggesting that NbSe2 films without Ti doping exhibited the highest crystallinity. These findings indicate that the basal plane of pure NbSe2 is primarily aligned parallel to the substrate orientation, resulting in coatings with low friction coefficients and prolonged frictional lifetimes [21]. In comparison with the standard XRD pattern of NbSe2, the (002) diffraction peak shows a certain degree of low-angle deviation, indicating lattice distortion in the NbSe2 film. This distortion is mainly due to the oxidation of the NbSe2 film after exposure to air, consistent with results obtained by sputtering MoS2 and WS2 films [22]. The presence of Ti during deposition induces the reorganization of deposited NbSe2 and the establishment of nucleation sites, promoting preferential basal crystalline growth [23]. Despite XPS findings revealing an augmentation in NbSe2 content with increasing Ti concentration in the film, XRD analysis demonstrates a concomitant decrease in the intensity of the NbSe2 diffraction peaks. This discrepancy suggests that Ti doping induces the amorphization of the entire film and constrains the growth of the NbSe2 crystals.
Figure 4 illustrates the cross-sectional and surface morphologies of the NbSe2-Ti films. All films displayed smooth and flat surfaces without noticeable defects. In Figure 4a,e, the surface of the single-component NbSe2 film exhibited a loose morphology with an amorphous structure in the cross-section. As the sputtering current increases, a clear columnar crystal structure appears in the film, and the thickness of the film decreases to become denser, typical of radio frequency sputter-deposited thin film structures of transition metal dichalcogenides (TMDs) [24]. The incorporation of Ti atoms refined the nanosized grains in the NbSe2 base, enhancing the internal density of the film, though fine pores were still observed between the aggregated grains. In the Ti-0.6 film, columnar crystals were uniformly observed across the cross-section, accompanied by the appearance of longitudinal cracks displaying denser columnar crystals. During the sliding process, the sputtered film exhibits a propensity to undergo fragmentation within the columnar zone, resulting in the deposition of a thin film onto the substrate. This is the actual film thickness, which is responsible for effective lubrication [25].

3.2. Physical Properties

The film’s structure was altered by varying Ti concentrations, and nanoindentation tests were performed to explore the impact of these distinct structures on the mechanical properties of the films. Figure 5 presents the hardness and elastic moduli of the films. The incorporation of Ti resulted in an enhancement of both hardness and elastic modulus. Specifically, the microhardness of Ti-0 measured 3.1 GPa, with an associated elastic modulus of 82 GPa. Following Ti doping, both hardness and elastic modulus increased, with Ti-0.4 composite coatings exhibiting peak values of 6.2 GPa for hardness and 106 GPa for elastic modulus. This behavior aligns with findings reported by Ding et al. in metal-doped thin films of transition metal dichalcogenides (TMDs) [26]. This behavior is ascribed to the deformation of the soft metal grains. Both XRD and SEM results align with the Hall–Petch relationship, indicating an increase in the hardness of NbSe2-Ti composite coatings with a decrease in grain size. However, when the grain size diminishes below a certain threshold or the coating adopts an amorphous structure, the strength and hardness of the coating experience a rapid decline due to grain boundary sliding, where the fraction of grain boundaries increases [27]. The observed minimum hardness and elastic modulus of Ti-0.6 substantiate this assertion. The observed trend revealed an initial increase followed by a subsequent decrease with an increase in the Ti contents. These results underscore the significant impact of Ti doping on the mechanical properties of the films, with the density of the films playing a critical role in determining their hardness.
The scratch test, commonly employed to assess adhesion between substrates and films, is depicted in Figure 6. With an increase in the sputtering current from 0 to 0.4 A, a notable increase in adhesive strength, denoted by the critical load for film peeling, was observed from 2 to 4 N. However, upon further a increase to 0.6 A, the adhesive strength subsequently decreased to 2 N. Notably, the Ti-0.4 film exhibited the highest adhesive strength among the tested conditions. Results from hardness and modulus of elasticity assessments, along with bonding experiments, demonstrated that Ti doping contributed to a measurable enhancement in the mechanical properties of the films, holding promise for potential improvements in tribological properties, particularly under current-carrying conditions.
Resistivity, a fundamental parameter characterizing electrical resistance, was investigated to further understand the electrical conductivity of the sample films using four probes. Figure 7 displays the electrical resistivity results for the NbSe2-Ti films. Previous studies show that the electrical resistivity of well-crystallized Au coating increases significantly upon doping with small amounts of impurities [28]. Similarly, the resistivity results for the NbSe2-Ti films confirmed this trend. The Ti-0.6 film exhibited the highest resistivity, while the Ti-0 film demonstrated the lowest. Film conductivity is influenced by both carrier concentration and Hall mobility [29]. Excessive Ti doping impeded electron transport, leading to increased resistance. Conversely, compared with Ti-0, the crystallinity of NbSe2 within the film decreased in the Ti-0.4 and Ti-0.6 films due to the introduction of Ti, significantly reducing the carrier concentration and resulting in the high resistivities observed in these films.

3.3. Tribological Property under the Combined Current-Carrying and Vacuum Conditions

Figure 8a,b illustrate the typical tribological properties of NbSe2-Ti films under combined current-carrying and vacuum conditions. The friction coefficient and wear rate of NbSe2-Ti films exhibit an increasing trend with the rise in Ti content. Among them, Ti-0 and Ti-0.2 films have the best tribological properties. The friction coefficient was approximately 0.028, while the wear rate was 0.92 × 10−5 mm3/(N·m). Transition metal dichalcogenides (TMDs) typically demonstrate favorable lubricity owing to their lamellar structure. This structure is characterized by weak interlayer interactions that facilitate sliding between neighboring atomic layers [30]. Furthermore, the pristine (002) crystal orientation promotes the formation of ordered laminar structures on the sliding surfaces, contributing to the observed low friction. Based on the XRD analysis of the NbSe2-Ti films, it is evident that the intensity of the NbSe2 (002) diffraction peaks progressively decreases with increasing Ti content. The poor crystallinity and absence of (002) crystallographic orientation are reasons for the inferior tribological properties of the Ti-0.4 and Ti-0.6 films. Another significant contributing factor to this outcome is that excessive Ti inclusions, when the counterpart ball slides on the NbSe2-Ti film, led to the formation of hard particles (TiO2) on the transfer film, causing severe damage during sliding [31].
Figure 9a depicts the typical tribological properties of NbSe2-Ti films under combined current-carrying and vacuum conditions. During the reciprocating sliding experiments at a 3 mm oscillation amplitude, the Ti-0 sample exhibited noticeable fluctuations in the coefficient of friction throughout the entire testing process, ranging from 0.01 to 0.03, until coating failure occurred at approximately 28,000 cycles. With an increase in Ti content, the Ti-0.2 sample demonstrated optimal tribological performance, maintaining a coefficient of friction between 0.01 and 0.02 until coating failure at around 33,000 cycles. Notably, the Ti-0.4 film exhibited the highest coefficient of friction of approximately 0.035 with a frictional lifespan of 25,000 cycles. The Ti-0.6 film, which exhibited the poorest mechanical performance, experienced the shortest failure lifespan, lasting only 12,000 cycles.
Among the series of prepared NbSe2-Ti films, the Ti-0.2 film exhibits excellent tribological performance in both current-carrying and vacuum conditions. To evaluate its practical application, comparisons were made with an electroplated Au coating and traditional MoS2 space-lubricating film materials alongside Ti-0.2. Figure 9b illustrates a comparison of the Ti-0.2 film with in-service electrical contact materials. The friction coefficient of the electroplated gold coating was around 0.22, and it exhibited a wear life of 6000 cycles (tests ceased if the friction coefficient exceeded 0.4). The MoS2 space-lubricating film showed a lower friction coefficient of about 0.05, along with a wear life of 7000 cycles (tests ceased if the friction coefficient exceeded 0.2). In contrast, the Ti-0.2 film demonstrated a remarkably low friction coefficient of only about 0.015, which is 10 times lower than that of the electroplated Au coating. The NbSe2 film experienced wear after 33,000 cycles, with a wear life that was at least 5 and 4.5 times longer than that of the electroplated Au coating and MoS2 film, respectively.
Figure 9c illustrates the real-time contact voltage of Ti-0.2 samples in comparison with the electroplated gold coating and MoS2 film. It is evident that the contact voltage of MoS2 remains stable at around 0.6 V, attributable to its high resistance. In contrast, the contact voltage of the Ti-0.2 film closely resembles that of electroplated gold, highlighting its excellent conductivity. Figure 9d illustrates the current fluctuations exhibited by the three distinct electrical contact materials throughout the frictional process. Notably, the electroplated gold manifests the most pronounced current fluctuations, attributed to its elevated friction coefficient. Conversely, both MoS2 and Ti-0.2 exhibit a sustained current fluctuation, maintaining values above and below a stable baseline. This behavior is attributed to their comparatively lower friction coefficients, indicating a more stable and controlled electrical contact state. Concerning the stability of electrical contacts, variations in contact voltage and current suggest that the NbSe2 film displayed reduced and optimal electrical noise. This highlights the unique benefits and potential applications of NbSe2 films as a novel lubricant for sliding contacts in space applications involving current transmission.
Figure 10a–c represent the wear tracks of electroplated gold, MoS2, and NbSe2 materials under electrical current conditions. As depicted in Figure 10a, the electroplated gold film suffered severe damage, with Joule heat generated by the current softening the film, leading to adhesion wear and liquid spattering under vacuum and current-carrying conditions. In Figure 10b, discernible erosion marks (depicted by dashed lines) are evident on the wear track of the MoS2 film. Poor electrical conductivity induces significant Joule heating when a current passes through, generating an arc between the friction pairs that ablates and destroys the film structure. Previous studies [32,33] indicate that the application of electrical current significantly affects the friction coefficient of graphite films, increasing it to approximately 0.1 and triggering severe abrasion phenomena. Fouvry’s [34] investigation revealed that the durability of coatings is intricately tied not only to their inherent wear resistance characteristics but also to the robustness of their interface with the substrate. Importantly, the application of loading current significantly diminishes the interfacial strength of gold (Au) films. A weakened interface correlates with a reduction in effective worn thickness and a consequent decrease in the overall lifespan of the coating. In contrast, the wear track of the NbSe2-Ti film appears flat and smooth, with no traces of arc ablation. This observation elucidates the longest friction life and the lowest friction coefficient. The amalgamation of superior electrical conductivity and lubricating properties in the NbSe2-Ti film facilitates the establishment of a stable electrical contact state, reducing current erosion and mitigating Joule heating.
The emergence of NbSe2 as a novel material with exceptional conductivity and lubrication properties is attributable to its unique structural characteristics. As shown in Figure 11, the lubricating properties of NbSe2 stem from the presence of weak van der Waals forces between its layers. This inherent property allows the material to exhibit superior lubrication, thereby reducing friction and enhancing wear resistance. Weak van der Waals forces contribute to the ease of sliding between layers, making NbSe2 an ideal candidate for applications where efficient lubrication is paramount [35]. However, the remarkable conductivity of NbSe2 arises from the free ionic movement within its structure. The crystal lattice of the material facilitates ion mobility, leading to enhanced electrical conductivity. This feature positions NbSe2 as a promising material for applications in electrical and electronic devices where high conductivity is a critical requirement. NbSe2’s multifaceted properties, encompassing both exceptional lubrication and conductivity, make it a compelling material for diverse applications ranging from advanced lubricants to high-performance electronic components.

4. Conclusions

A set of NbSe2-Ti films was produced through the DC closed-field magnetron sputtering technique, with a primary focus on investigating the feasibility of these films as novel types of space electrical contact lubricants. This study revealed that an optimal level of Ti doping enhanced the stability of the contact voltage while maintaining the original tribological properties. Notably, the Ti-0.2 film demonstrated the best overall performance in terms of vacuum current-carrying tribological properties.
Compared with the currently employed electroplated Au coating, the Ti-0.2 film exhibited a substantial advantage under real-service conditions involving vacuum and current carrying. The friction coefficient decreased notably, from 0.25 to 0.015, and the wear life increased significantly, advancing from 6000 to over 33,000 cycles. These results signify a substantial enhancement in performance.

Author Contributions

Investigation, X.Z.; Writing—original draft, Y.Y.; Writing—review & editing, Y.Y. and G.W.; Supervision, G.W.; Funding acquisition, X.S. and L.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the China Postdoctoral Science Foundation (2023M731458), the Natural Science Foundation of Gansu Province (23JRRA1099), and the National Outstanding Youth Science Fund Project of the National Natural Science Foundation of China (12122512).

Data Availability Statement

The authors declare that the work described is original research.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. CSM ball-on-flat tribometer in a vacuum chamber equipped with a current control system.
Figure 1. CSM ball-on-flat tribometer in a vacuum chamber equipped with a current control system.
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Figure 2. XPS spectra of Ti-0.4 film (a) before etching and (b) after etching and (c) Nb 3d peaks and (d) Ti2p of the NbSe2-Ti films.
Figure 2. XPS spectra of Ti-0.4 film (a) before etching and (b) after etching and (c) Nb 3d peaks and (d) Ti2p of the NbSe2-Ti films.
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Figure 3. XRD patterns of NbSe2 films with different Ti contents.
Figure 3. XRD patterns of NbSe2 films with different Ti contents.
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Figure 4. FESEM images of (a–d) cross-section and (e–h) surface of NbSe2-Ti films.
Figure 4. FESEM images of (a–d) cross-section and (e–h) surface of NbSe2-Ti films.
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Figure 5. Hardness and elastic modulus of NbSe2 films with different Ti contents.
Figure 5. Hardness and elastic modulus of NbSe2 films with different Ti contents.
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Figure 6. The scratch morphologies of the NbSe2-Ti films.
Figure 6. The scratch morphologies of the NbSe2-Ti films.
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Figure 7. Electrical resistivity of the NbSe2-Ti films.
Figure 7. Electrical resistivity of the NbSe2-Ti films.
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Figure 8. (a) Typical friction curve, (b) wear rate and average friction coefficient of the NbSe2-Ti films under the combined current-carrying and vacuum conditions.
Figure 8. (a) Typical friction curve, (b) wear rate and average friction coefficient of the NbSe2-Ti films under the combined current-carrying and vacuum conditions.
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Figure 9. (a) Wear life of NbSe2 films with different Ti contents, (b) friction curves, (c) contact voltage, and (d) current fluctuation of the three materials under the combined current-carrying and vacuum conditions.
Figure 9. (a) Wear life of NbSe2 films with different Ti contents, (b) friction curves, (c) contact voltage, and (d) current fluctuation of the three materials under the combined current-carrying and vacuum conditions.
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Figure 10. SEM images of the worn surfaces.
Figure 10. SEM images of the worn surfaces.
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Figure 11. Schematic structure of NbSe2.
Figure 11. Schematic structure of NbSe2.
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Table 1. Process parameters for NbSe2 composite films with different Ti contents.
Table 1. Process parameters for NbSe2 composite films with different Ti contents.
NbSe2 Target Current (A)Ti Target Current (A)Thickness (μm)Ti (at. %)
Ti-01.202.40
Ti-0.21.20.22.256.6
Ti-0.41.20.42.19.3
Ti-0.61.20.62.112.2
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Yang, Y.; Wang, G.; Zhou, X.; Su, X.; Gu, L. A Promising Conductive Lubricant for Space Sliding Electrical Contact: NbSe2-Ti Film. Lubricants 2024, 12, 44. https://doi.org/10.3390/lubricants12020044

AMA Style

Yang Y, Wang G, Zhou X, Su X, Gu L. A Promising Conductive Lubricant for Space Sliding Electrical Contact: NbSe2-Ti Film. Lubricants. 2024; 12(2):44. https://doi.org/10.3390/lubricants12020044

Chicago/Turabian Style

Yang, Yang, Guan Wang, Xingchen Zhou, Xingkang Su, and Long Gu. 2024. "A Promising Conductive Lubricant for Space Sliding Electrical Contact: NbSe2-Ti Film" Lubricants 12, no. 2: 44. https://doi.org/10.3390/lubricants12020044

APA Style

Yang, Y., Wang, G., Zhou, X., Su, X., & Gu, L. (2024). A Promising Conductive Lubricant for Space Sliding Electrical Contact: NbSe2-Ti Film. Lubricants, 12(2), 44. https://doi.org/10.3390/lubricants12020044

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