Magnetron sputtering NbSe2 film as lubricant for space current-carrying sliding contact

This study demonstrates that magnetron-sputtered NbSe2 film can be used as a lubricant for space current-carrying sliding contact, which accommodates both metal-like conductivity and MoS2-like lubricity. Deposition at low pressure and low energy is performed to avoid the generation of the interference phase of NbSe3. The composition, microstructure, and properties of the NbSe2 films are further tailored by controlling the sputtering current. At an appropriate current, the film changed from amorphous to crystalline, maintained a dense structure, and exhibited excellent comprehensive properties. Compared to the currently available electrical contact lubricating materials, the NbSe2 film exhibits a significant advantage under the combined vacuum and current-carrying conditions. The friction coefficient decreases from 0.25 to 0.02, the wear life increases more than seven times, and the electric noise reduces approximately 50%.


Introduction
Sliding electrical contact is a mechanical design used to transmit electric energy and signals in the sliding contact state [1][2][3]. It is widely used in aerospace applications. For example, electrically conductive slip rings are used to transfer energy collected by a solar panel to a spacecraft system. The lubrication and wear resistance of the electrical contact materials directly determine the reliability of the entire system. The extreme service conditions, such as high vacuum and carrying-current, bring huge challenges for electrical contact materials. Compared with traditional friction, the one with current-carrying is affected also by the electric field accompanied by arc discharge and other phenomena [4]. During electrical contact sliding, frictional heating and Joule heat generated by the current can lead to high temperature near the electrical contact interface and thus the severe adhesive wear [5]. Therefore, electrical contact materials must have good electrical conductivity and lubricity [6][7][8]. At present, the research in this field is still not sufficient. The major reason is that vacuum environment and current-carrying evaluation conditions are difficult to achieve simultaneously, and the guidance for material design is insufficient.
Gold, platinum, and other precious metals are widely used as lubricating materials for space current-carrying sliding contact owing to the excellent electrical conductivity and chemical stability [9,10]. However, precious metal materials exhibit high friction coefficient (usually above 0.3) and serious adhesion wear, which seriously limits the wear life. The specific characteristic of space sliding electrical contact limits the applicability of a lot of traditional solid lubricants. For example, although graphite is a solid lubricant with good electrical conductivity, it has high friction and wear in vacuum [11,12]. MoS 2 exhibit low friction in vacuum, but has high resistivity and poor transport characteristic. Therefore, it is meaningful to develop new space lubrication material systems for sliding electrical contact.
NbSe 2 is a transition metal compound with a unique layered structure, which accommodates both metal-like conductivity (even superconductivity at low temperature) and MoS 2 -like lubricity [13][14][15]. For example, the electrical resistivity of NbSe 2 is 3.5 × 10 -4 Ω·cm, whereas that of MoS 2 is 8.5 × 10 2 Ω·cm. NbSe 2 has been used in motor brushes and specific bearings in atmospheric environments. However, the application of NbSe 2 in a specific space environment is yet to be investigated [16]. Liu et al. [17] used the radio-frequency (RF) magnetron sputtering method to prepare NbSe 2 films. The as-prepared films exhibited the excellent dual-functional characteristics of lubrication and electrical conductivity in air. But the existence of the interfering substances such as NbSe 3 and Nb 2 O 5 in the films could adversely affect the lubrication and conductivity performances. Thus it is of great significance to further optimize the magnetron sputtering NbSe 2 film and explore its application potential for space current-carrying system.
In this study, NbSe 2 films were prepared using the DC closed-field magnetron sputtering method. Deposition at low pressure and low energy was performed to keep the purity of NbSe 2 sputtering products and avoid the generation of interference phase such as NbSe 3 . The composition, microstructure, and properties of the NbSe 2 films were further tailored by controlling the sputtering current. The tribological properties under real service conditions with currentcarrying in vacuum were systematically investigated and compared with those of traditional electrical contact materials, such as the electroplated Au coating and the MoS 2 film.

Film deposition
NbSe 2 films were deposited using a Teer PlasMag CF-800 closed unbalanced field magnetron sputtering system (a single NbSe 2 sputtering target and two Ti sputtering targets). The bias voltage was supplied by a pulsed DC power source, while the target current was supplied by a DC supply. Deposition at low pressure of 0.11 Pa (benefit from high ionization closed magnetic field tech.) and low sputtering energy (DC power) was specially designed to keep the purity of NbSe 2 sputtering products and avoid the generation of interference phase. The substrates were stainless steel disk (1Cr18Ni9Ti, with surface roughness below 30 nm), Si (100) wafer (with surface roughness below 10 nm), and copper disk (with surface roughness below 30 nm). The stainless steel samples were used for the mechanical properties test, the Si (100) wafer samples were used for the composition, structure, and morphology analyses, and the copper samples were used for X-ray diffraction (XRD) analysis and tribological properties test. Before being placed into the deposition system, the substrates were washed by ultrasonic waves sequentially in deionised water and acetone for 20 min respectively and then dried in nitrogen. The distance between the substrates and targets was maintained at 150 mm. There was no additional heat source to heat the substrate during the deposition. The substrates were rotated in front of each target at a speed of 5 rpm. The Ar gas was introduced when the based pressure was lower than 1×10 -5 mbar, the native oxides on the surface of the substrates were removed by Ar + ions etching for 20 min. A thin interlayer of Ti (250 nm) was deposited to improve the adhesion. The sputtering current of the NbSe 2 target was 0.4, 0.5, 0.6, 0.7, and 0.8 A, and the corresponding samples were denoted as C0.4, C0.5, C0.6, C0.7, and C0.8, respectively. The process parameters for film deposition were listed in Table 1. Traditional materials, such as electroplated Au coating [18] and MoS 2 film [19], were selected to compare the current-carrying tribological properties to those of the NbSe 2 film.

General characterization
The chemical compositions of the NbSe 2 films were  characterized using energy-dispersive spectroscopy (EDS)-scanning electron microscopy (SEM, JSM-5600LV, Japan) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). XRD patterns were obtained using an X-ray diffractometer (GIXRD, EMPYREAN, PANalytical) in the grazing mode (2°). The morphologies of the NbSe 2 films were observed using field-emission scanning electron microscopy (FESEM, JSM-6701F, JEOL). Cross-sectional images of the films were obtained by breaking the silicon substrate after the film deposition. Focused ion beam (FIB, FEI scios) was used to fabricated the nanoscale-thick lamellar specimens of NbSe 2 film for HRTEM (FEI Talos-F200S) observation. The nanohardnesses and elastic modulus of the NbSe 2 films were measured using a nanoindenter (Nano Hardness Tester, CSM) equipped with a Berkovich diamond probe tip. The indentation depth was controlled to below 10% of the film thickness to minimise the effect of the substrate. The adhesive strength of the films was evaluated using a CSM Revetest Scratch tester. The normal load range was 1 to 10 N (loading rate: 4.5 N/min). The square resistance of the film was assessed using a four-probe measuring instrument (McP-y610, Loresta GP, Japan). The resistivity was calculated by R = ρ/W, where W is the film thickness and ρ is the square resistance.

Tribological test
The tribological properties were evaluated under the combined vacuum and current-carrying conditions. The ball-on-disk tester (CSM) was configured with the current loading system and voltage testing device (Fig. 1). The counterpart was a GCr15 bearing steel ball (φ = 6 mm, Ra = 20 nm). The current applied during the friction test (1 A) was supplied by the power supply (KXN-10050D), and the current density was similar to the actual working value. All tests were carried out in vacuum (pressure lower than 2 × 10 -5 mbar) with a load of 1 N, sliding frequency of 3 Hz, reciprocating amplitude of 5 mm, and duration of 6,000 cycles. The tribological tests were repeated four times.

Composition and structure
XPS was used to characterize the chemical composition of the NbSe 2 films with different sputtering currents. Figure 2(a) showed the typical Nb 3d peak of the film at a sputtering current of 0.6 A (C0.6). By fitting, it could be decomposed into four peaks at 203.2, 205.9, 207.3, and 210.0 eV, corresponding to Nb 3d 5/2 and Nb 3d 3/2 peaks of Nb 4+ in NbSe 2 and Nb 3d 3/2 and Nb 3d 3/2 peaks of Nb 5+ in Nb 2 O 5 , respectively [20,21]. The surface of the NbSe 2 film was consisted of the majority of Nb 2 O 5 . Previous studies had demonstrated oxidation pollution on the surface of sulphur (selenide) sputtered films due to surface oxygen adsorption or residual oxygen during deposition [17,22]. To characterize actual chemical composition of films, The Ar ion was employed to etch the surfaces of the NbSe 2 films for 2 min to eliminate the external influence on chemical composition. Figure 2(b) showed the typical XPS results for the C0.6 film after the etching process. The Nb 2 O 5 content of the C0.6 film was significantly reduced after the etching process. NbSe 2 became the major component of the film. It can be attributed to the existence of the Nb 2 O 5 oxidized surface, partly due to surface oxidation during the transportation process and inevitable residual oxygen-containing substances in the membrane chamber during the preparation process. Figure 2(c) showed the XPS results With the increase of the sputtering current, the content proportion of NbSe 2 increased and the oxidation decreased. This is because that the low sputtering current results an inadequate sputtering deposition, and the residual O in the membrane chamber can be more easily combined with Nb. The detection depth of XPS was only tens of nanometres, while EDS can analyze the composition in the range of micrometres, and can more comprehensively characterize the composition of the films. The EDS elemental analysis results for the five different samples were shown in Table 2. With the increase of the sputtering current from 0.4 to 0.8 A, the Se/Nb ratio increased from 1.78 to 2.03. The stoichiometric ratio of the films prepared at high sputtering current was very close to that of NbSe 2 . The EDS and XPS results showed that the film prepared under high sputtering current has a lower oxidation content [23]. Compared to the interior of the film, there was obvious oxidation on its surface. In this study, deposition at low pressure and low sputtering energy was specially designed to maintain the purity of NbSe 2 sputtering products and avoided the generation of interference phase such as NbSe 3 , which provided favourable conditions for obtaining excellent conductivity and lubrication properties.
The XRD results of the NbSe 2 films with different sputtering currents were shown in Fig. 3. To eliminate the interference of the Cu substrate, the XRD pattern of the Cu substrate was referenced. In addition to the diffraction peak of the Cu substrate, the C0.6, C0.7, and C0.8 films exhibited hcp-NbSe 2 (002), (101), (103), (112) diffraction peaks with a preferred orientation of (002) plane. The absence of Nb 2 O 5 crystal phases indicated that it should be amorphous structure. The C0.4 and C0.5 films did not exhibit distinct NbSe 2 diffraction peaks, which indicated that they are mainly  amorphous structure. When the sputtering current reached 0.6 A, there exhibited a distinct NbSe 2 (002) diffraction peak. With the further increase of the sputtering current, the diffraction peak became stronger. It showed that a high sputtering current was conducive to the crystallization of the NbSe 2 films. When the sputtering current was less than 0.6 A, the film existed in an amorphous structure. When the sputtering current reached 0.6 A, the films existed in the form of hcp-NbSe 2 (002) preferentially oriented crystals. The low sputtering current can significantly reduce the diffusion ability of the free particles, inhibit the formation and development of crystal nuclei, and eventually lead to a decrease of crystallization. Figure 4 showed the cross-sectional and surface morphologies of NbSe 2 films with different sputtering currents. The structure of the C0.4 film was very dense, which corresponded to the amorphous structure detected by XRD. With the increasing sputtering current, a columnar crystal structure was evident observed from the cross-sectional morphology of the C0.7 and C0.8 films, and the films became loose. Particularly, when the sputtering current was 0.8 A, columnar crystal with distinct gaps penetrated the cross-section of the film. For surface morphologies, with the increasing sputtering current, the cracks at the surface of the film gradually increased, corresponding to the columnar crystal structure. The sputtering current of 0.6 A was the change tipping point, that it exhibited crystalline characteristic while avoiding a several columnar crystals and maintaining a relatively dense structure. According to the XPS results, the Nb 2 O 5 content in the C0.6 film was significantly reduced, maintaining the excellent NbSe 2 chemical composition and crystallization. These observations were consistent with the XRD results. The apparent structural difference was related to the migration ability of the deposited particles at the substrate surface. The low sputtering current inhibited the nucleation and growth of base-plane-oriented grains, and thus the film structure tended to be amorphous [24][25][26][27]. The formation of the crystal structure ( Fig. 4(i)) was mainly caused by sufficient surface diffusion and  [13,28,29], which benefited from the high sputtering current. Figure 5 showed the HRTEM cross-section images and selected-area diffraction result of the C0.6 film. The films exhibited a special nanocrystalline/amorphous composite superlattice structure. The spacings of the nanocrystal grains were approximately 0.63 and 0.23 nm, corresponding to the (002) and (102) crystal planes of NbSe2, respectively. The continuous rings of the NbSe2 (002) and (102) planes can be observed in the selected area electron diffraction pattern ( Fig. 5(b)). Many grain boundaries existed in the nanocrystalline/amorphous composite superlattice, which can effectively impede the movement of dislocations. The internal stress can be released by grain boundary sliding. Such a special nanocrystalline/ amorphous composite superlattice structure can effectively impede the movement of dislocations by grain boundaries, and release internal stress by the sliding of the grain boundary [30].

Mechanical properties
The hardness and elastic modulus of the NbSe 2 films with different sputtering currents were shown in Fig. 6. As the increased sputtering current, the hardness decreased gradually. However, the film has the highest elastic modulus when the sputtering current is 0.6A. The C0.6 film had excellent mechanical properties with the highest elastic modulus and relatively high hardness. It can be understood from two aspects. On one hand, the film denseness was one of the key factors affecting the hardness. According to the structural characterization results, with the increased sputtering current, the films changed from amorphous structure to nanocrystalline/amorphous composite structure. The films prepared under the low sputtering current had a denser microstructure and thus the higher hardness. On the other hand, for the nanocrystalline/ amorphous composite structure, the amorphous boundary can limit the crack diffusion in the amorphous matrix, which yields the high hardness and elastic modulus of the C0.6 film [31,32]. Figure 7 showed scratch morphologies of the NbSe 2 films with different sputtering currents. As the sputtering current increased from 0.4 to 0.6 A, the adhesive strength (corresponds to the critical load for the film peeling) increased significantly from 4 to 8 N. While as it further increased to 0.6 A, the adhesive strength decreased to 2 N. The C0.6 film had the best adhesive strength. It was worth noted that brittle cracks appeared at the edge of the scratch track for C0.4, C0.7, and C0.8 films, while none for C0.5 and C0.6 films, which also indicated the better mechanical strength and elasticity for the nanocrystalline/ amorphous composite structure [33]. Figure 8 showed the electrical resistivity results of the NbSe 2 films with different sputtering currents. The as-prepared NbSe 2 films had excellent conductivity with resistivity in the magnitude of 10 -5 Ω·m. The resistivity of the C0.8 sample was highest, whereas that of the C0.6 sample was lowest. The conductivity of the film was determined by the carrier concentration and Hall mobility. The periodic potential field and scattering process of free electrons inside the film hinder electron transmission. In contrast, the NbSe 2 films with high crystallinities can effectively weaken   this effect and promote electron transmission. Thus, the resistivity of the C0.6 film was lower than those of the C0.4 and C0.5 films. C0.7 and C0.8 films had a loose structure, which was considered to lead to structural defects and dislocations with an increased scattering of a large number of columnar platelet boundaries, and weakening the carrier concentration of the conductive material, resulting in high resistivities [34,35]. Compared with the NbSe 2 film prepared by the RF magnetron sputtering method, the conductivity was reduced from 1 × 10 -3 to 6 × 10 -5 Ω·m for that by DC magnetron sputtering, owing to the low-energy-sputtering induce denser structure and purer composition [17].

Tribological property under the combined current-carrying and vacuum conditions
Figures 9(a) and 9(b) showed typical tribological properties of the NbSe 2 films under the combined current-carrying and vacuum conditions. The friction coefficient and wear rate of the NbSe 2 films initially decreased, and then decreased with the increase of the sputtering current. The C0.6 film had the best tribological properties. The friction coefficient was approximately 0.025, while the wear rate was 3.8 × 10 -6 mm 3 /(N·m). Figure 9(c) showed the online contact voltage curves of the NbSe 2 films. The C0.5 and C0.7 samples can maintain a low contact voltage of 0.3 V before 1,000 cycles, and then slowly increased during friction process. The contact voltage of the C0.8 film reached approximately 0.5 V and fluctuated violently. The contact voltage of the C0.6 film was lowest and stable, owing to the excellent electrical conductivity and mechanical and tribological properties. The stable stability of electrical signal transmission capability was due to its excellent tribological properties. In contrast, C0.4 and C0.8 exhibited high contact voltages and fluctuation. As known, transition metal dichalcogenides (TMDs) materials usually have good lubricity owing to the lamellar structure, which had weak interlayer interaction and easy sliding nature between neighboring atomic layers [36,37]. As for the sputtering TMDs films, the original (002) crystal orientation was also conducive to the formation of the ordered lamellar structure at the sliding surface, thus leading to low friction. This can explain the high friction coefficient and wear rate for the films prepared by the low sputtering current. The films prepared by the high sputtering current had poor mechanical strength, and were easily damaged during the friction process [38][39][40].

Comparison with the available electrical contact lubricating materials
In a series of NbSe 2 films prepared by us, the C0.6 film exhibited the most excellent tribological properties under the combined current-carrying and vacuum conditions. To evaluate its practical application value, electroplated Au coating and traditional MoS 2 space lubricating film materials were selected to compare with C0.6. Their tribological performances and failure mechanisms under real service conditions were systematically studied. Figure 10 Regarding the electrical contact performance (Figs. 10(a) and 10(b)), the contact voltages of the electroplated Au coating and NbSe 2 film were at the same level of magnitude, which indicated that the NbSe 2 films had excellent electrical conductivity similar to that of the electroplated Au coating. The MoS 2 film exhibited a very high contact voltage (0.6 V) owing to its poor conductivity. Regarding the electrical contact stability, the contact voltage and current fluctuations showed that the NbSe 2 film exhibited a lower and optimal electrical noise. It showed the distinct advantages and potential applications of the NbSe 2 films as a new lubricant for space current-carrying sliding contact. as lubricant for space current-carrying sliding contact.
To explore the effect mechanism of the currentcarrying on tribological properties, we also carried out the comparative tests without current-carrying in vacuum. Figure 11 showed SEM images of the wear surfaces of the tribo-pairs (NbSe 2 film, MoS 2 film, www.Springer.com/journal/40544 | Friction and electroplated Au coating VS steel ball) without current-carrying in vacuum. The wear tracks were flat and smooth, and a dense transfer film was observed on the counterpart ball surface for all three materials. As shown in Fig. 12(c), obvious traces of erosion were observed (dashed line) on the wear track of the MoS 2 film. The Joule heat generated by the high contact voltage damaged tribo-pairs severely. This was inconsistent with the tribological comparison results, which the wear life of the MoS 2 film in vacuum is reduced from 1.3 × 10 5 cycles (see the Electronic Supplementary Material (ESM)) without currentcarrying to 2 × 10 4 cycles with current-carrying. Upon sliding against the electroplated Au coating, the contact areas of the wear scars on the counterpart ball become largest. The film was severely damaged and a large amount of gold was transferred to the counterpart ball surface. This also showed that the NbSe 2 film exhibits excellent tribological properties under the vacuum and current-carrying conditions. The current and voltage applied to the friction pair have arc erosion effects. When the film had poor electrical conductivity, an arc is generated between the friction pairs, ablating and destroying the film structure (e.g., the MoS 2 film). On the other hand, the Joule heat generated by the current softened the film and caused    very large wear (e.g., electroplated Au and MoS 2 film). The combination of the excellent electrical conductivity and lubricity of the NbSe 2 film enables it to form a stable electrical contact state, weakening the current erosion and Joule heating [41,42].

Conclusions
A series of NbSe 2 films were prepared using the DC closed-field magnetron sputtering method. The relationship between the sputtering current and composition, microstructure, and mechanical and tribological properties under the combined vacuum and current-carrying conditions were studied. The feasibility of the NbSe 2 film as a new type of space electrical contact lubricant was investigated. The main conclusions are as follows.
1) Deposition at low pressure and low energy was performed to keep the purity of NbSe 2 sputtering products and avoid the generation of interference phase such as NbSe 3 , which endowed it excellent electrical conductivity and lubricity integrated performance.
2) The low sputtering current resulted in the amorphization and densification trend of NbSe 2 film, and the high sputtering current leaded to the loose structure of NbSe 2 film with significant columnar crystals. While at an appropriate sputtering current, the NbSe 2 film not only changed from amorphous to crystalline, but also maintained a dense structure with excellent comprehensive properties.
3) As compared with electroplated Au coating now in service, NbSe 2 film showed significant advantage at real service conditions with vacuum and currentcarrying, where the friction coefficient decreased from 0.25 to 0.02, the wear life increased from 14,000 cycles to more than 100,000 cycles, and the electric noise fluctuation was reduced by about 50%. It developed a new material system, and provided a new idea for solving the lubrication problem of space sliding electrical contact.