Macro-superlubricity in sputtered MoS2-based films by decreasing edge pinning effect

To date, MoS2 can only be achieved at microscale. Edge pinning effect caused by structure defects is the most obvious barrier to expand the size of structural superlubricity to macroscale. Herein, we plan to pin edge planes of MoS2 with nanospheres, and then the incommensurate structure can be formed between adjacent rolling nanoparticles to reduce friction. The sputtered MoS2 film was prepared by the physical vapor deposition (PVD) in advance. Then enough Cu2O nanospheres (∼40 nm) were generated in situ at the edge plane of MoS2 layers by liquid phase synthesis. An incommensurate structure (mismatch angle (θ) = 8°) caused by MoS2 layers was formed before friction. The friction coefficient of the films (5 N, 1,000 r/min) was ∼6.0×10−3 at the most. During friction, MoS2 layers pinned on numerous of Cu2O nanoparticles reduced its edge pinning effect and decreased friction. Moreover, much more incommensurate was formed, developing macro-superlubricity.


Introduction
Friction and wear take part in the dominant role in mechanical energy consumption [1].In that case, an increasing research focuses on structural superlubricity, in which friction nearly vanishes between two incommensurate solid surfaces [2].To date, superlubricity is only primarily viable in some nearly ideal condition [3][4][5][6][7][8][9], such as atomically smooth substrate, perfectly crystalline materials, low velocity and load, and strict structural incommensurate structure.Those factors restrict it to apply in actual engineering.Taking the most common solid lubrication MoS 2 -based films as an example, MoS 2 has a special layer-crystal structure, in which a sheet of Mo atoms is sandwiched between two hexagonally packed S layers [10].The binding within S-Mo-S layer is the strong covalent bond, and between the layers are the weak van der Waals force [11].It provides an ideally rigid layer-by-layer sliding interface.Hence, in theory, if its structure is adjusted to the incommensurate structure, superlubricity can be easily achieved in MoS 2 films.However, superlubricity has only been demonstrated for incommensurate MoS 2 monolayers [3,4] by mechanical exfoliation and simple combination, as well as heterojunctions between MoS 2 and WS 2 /hexagonal-BN (h-BN)/graphite [7] with poor adhesion on steel substrate.Therefore, superlubricity has been primarily realized in a limited experimental stage, but not widely be applied in engineering.In order to quickly introduce the application of superlubricity to engineering, current mature technology of preparing MoS 2 films in engineering is the first choice.To date, the physical vapor deposition (PVD) technology is one of the most common technology for preparing MoS 2 -based films, and the sputtered MoS 2 -based films are also widely applied in industrial applications.Hence, shifting sputtered MoS 2 films may be a desirable choice for accelerating superlubricity to apply in engineering.Nevertheless, during deposition, impact with highenergy makes S atoms' loss, forming dangling bonds, unpaired bonds, and disorder [12].When the films slid in high loads and high sliding velocities, it was inevitably influenced by absorption, chemical reaction, and structual damage, resulting in increasing edge pinning effect [2], and then losing their incommensurate and superlubricity property in practical applications.Clearly, edge pinning effect is the primary obstacle for the as-deposited MoS 2 -based films to achieving superlubricity.As far as we know, except for the fullerene-like nanospheres [13], it also does not have a proper research on how to effectively deal with the edge planes in the sputtered MoS 2 films.Meanwhile, this method of preparing fullerene-like nanoparticles is complicated and has a low yield, so it is not appropriate to reduce the exposed edge planes.On that account, a novel and effective method about dealing with the exposed edge planes should be explored.
In recent years, multi-contact interfacial geometries get more and more attention.It is a special incommensurate structure caused by a contact of two-dimensional (2D) materials in different dimensions.For example, it was demonstrated that ultra-low friction can be achieved in centimeter-long double-walled carbon nanotubes [6].Ultra-low friction coefficients have been observed in mesoscale contacts between diamond-like carbon (DLC) spheres and graphene on a silicon dioxide surface [14].The multiple-contact interfacial superlubricity is less susceptible to effects of external loads and velocities, and thereby those kinds of structure can be considered to construct in the sputtered MoS 2 films.While these kinds of superlubricity is also hard to realize in macroscale.For one reason, the perfect and huge 2D curved surfaces are difficult to be prepared.In other words, edge pinning effect is still a factor that hinders superlubricity from applying at engineering.Besides, the incommensurate structure is arduous to form and maintain during friction [14].The 2D material can stay on rigid spheres by physical adsorption, and then an incommensurate curved surface is formed.
Thereby, the bonding between the 2D material and rigid spheres is poor, and it takes a certain amount of running-in time to stick and form the incommensurate structure.For this reason, chemical bonds were considered to replace the physical adsorption, thereby shortening the time to reach superlubricity.Since the exposed edge planes have high-activity, here we plan to fix them into different nanoparticles, and hope that the particles can provide a micro-contact incommensurate position for MoS 2 layers.Subsequently, the multi-contact interfacial geometries can be constructed rapidly and stably in the sputtered MoS 2 film.While, for the traditional co-depositing technology, when the dopants (such as Au [11,15], Cu [16], Ni [17], Ag [18,19], and Sb 2 O 3 [20]) were introduced to films, dangling bonds were not eliminated [21,22] in nature.Moreover, the dopants can induce the grain of MoS 2 to fineness, resulting in enhanced edge pinning effect [2] and thereby increased residual friction.Although some previous studies have dispersed some particles (such as DLC [1] and SiO 2 [14]) into graphene or h-BN by the mechanical method to realize structural superlubricity, it is yet limited by specific external conditions, such as normal load, contact size, sliding velocity, and distance.In that case, nanoparticles are considered to be generated in situ at the most of the position of the exposed edge planes by liquid phase synthesis.

Results and discussion
Considering that sodium citrate (C 6 H 6 Na 2 O 7 •2H 2 O) is a strong Mo complex agent [23][24][25][26], here we plan to select C 6 H 6 Na 2 O 7 •2H 2 O as a "connector" to assemble nanoparticles with a regular shape to the edge plane of MoS 2 layers in advance.Furthermore, nanoparticlecombined MoS 2 layers are expected to construct a multi-contact geometry.The macroscale of multi-contact interfacial geometries can be expected to be achieved by increasing the number of nanoparticles.Nanoparticles (Au or Cu | https://mc03.manuscriptcentral.com/frictionedge crystal plane of MoS 2 .Reference [2] infers that edge pinning effect is often the dominant reason for remaining high friction coefficients of the films.To reduce the negative impact of this effect, nanoparticles were considered to form in situ on the position of dangling bonds by liquid phase synthesis.Considering that MoS 2 has poor resistance to moisture, boiling water, which equipped with excellent properties (such as friendly oxygen-free environment, flowing medium, and uniform heating conditions), was chosen as a key condition for the preparation of the MoS 2 -Cu 2 O composite films (Fig. 1(II)).Then we use C 6 H 6 Na 2 O 7 •2H 2 O to connect nanoparticles to the position of dangling bonds in the films.In the sputtered MoS 2 film, 2H-MoS 2 area can be observed, whose layer-to-layer spacing is 0.62 nm.In this area, lots of fractures can be detected, exposing many edge planes (Fig. 1(c 111) plane (PDF#34-1354).MoS 2 layers were marked with number 3, and its typical d-spacing was ~0.60 nm corresponding to the MoS 2 (002) plane (PDF#37-1492).Meanwhile, between areas 1 and 3, tidy edge planes were detected, and it was marked with the number 2. In area 1, friction has no obvious damage to Cu 2 O nanoparticles, and even some particles provide smooth curved surface supports for MoS 2 layers.During friction, those curved surface-induced MoS 2 layers connected on the nanoparticles to increase their contact dimension between layer to layer.From area 2, we can see that even the film was slid for 1.0×10 4 r, the edge planes can also be wholly occupied by Cu 2 O     In MoS 2 -Au films, the TEM image (Fig. S5 in the ESM) reveals that Au nanoparticles can also induce MoS 2 layers to develop an incommensurate structure, and its θ is 17°.However, when the MoS 2 -Au film slid under the contact pressure (0.9 GPa), the pressure can drive Au disperse into MoS 2 , which can also be verified by the EDS mapping images (Fig. S5(c) in the ESM).Only a microcrystalline phase can be observed, and its θ of MoS 2 layers had vanished (Fig. S5(b) in the ESM).Consequently, Au nanoparticles not only cannot provide a satisfactory multi-contact interfacial structure for MoS 2 layers, but also few incommensurate structure formed before friction can be maintained.This may be the main reason that MoS 2 -Au film cannot reach structural superlubricity.Furthermore, when the content of Cu 2 O was reduced to less than 1 at% in the sputtered MoS 2 film (Fig. S6  Figure 5 gives the typical friction coefficient curves of the MoS 2 -Cu 2 O composite films.The composite film loses its macroscale superlubricity at 4.6×10 5 r, which is more than 10 times of the wear life for the pure MoS 2 film.Subsequently, the friction and wear behaviors of the composite films were investigated.Figure 6 shows the three-dimensional (3D) non-contact surface mappings of the wear track and optical photos of the wear scar and SEM images of debris on the surface of counterpart ball.Compared with the pure MoS 2 film, the composite film shows narrower wear track.Meanwhile, the optical photos and SEM images of the surface of counterpart ball showed that the composite films have smaller wear spots as well as more finely debris.The results indicated that Cu 2 O nanoparticles can effectively reduce the wear rate of the composite film.
The transfer-layers of the MoS 2 and MoS 2 -Cu 2 O composite films on the surface of counterpart ball are further researched, and then displayed in Fig. 7.The transfer-film of the pure MoS 2 film has a typical flat state after layer-by-layer sliding.The EDS results showed that the main components of the transfer-film are O, S, and Mo.Meanwhile, for the MoS 2 -Cu 2 O composite films, dispersed particles with different sizes can be observed on the surface of the transfer-film, and particle sizes range from nanometers to micrometers.The EDS results showed that the main components of the transfer-film are O, S, Mo, and Cu.Compared with that of pure MoS 2 films, the S/Mo ratio was significantly increased.This may be related to the growth process of Cu 2 O nanoparticles.Reference [30] has shown that trisodium citrate acts as a bridge between the edge plane of MoS 2 and Cu 2 O nanoparticles.When parts of C 6 H 6 Na 2 O 7 •2H 2 O were combined with Mo vacancies, super-hydrophilicity trisodium citrate brings parts of Mo into the solution, leading to an increased S/Mo ratio.Besides, the O content is greatly increased, which is mainly caused by the introduction of Cu 2 O (and CuO) on the dangling bonds.
The XPS spectra of MoS 2 -Cu 2 O composite film before and after friction are subsequently compared in Fig. 8. Consistent with those reported in Ref. [30], the Mo 3d and S 2p peaks with low intensities for the composite film before friction can mainly attribute to the low content of MoS 2 phase on the surface of the film.After sliding, the Mo 3d and S 2p peaks showed strong intensities for the sample.In the Cu 2p spectra (Fig. 8(c)), whether before or after friction, two sets of satellite peaks exist at 941.0 and 961.0 eV and 944.bend, and then form an incommensurate structure.A high load (10 N) may damage nanoparticles, and thereby an incommensurate structure can be quickly dispread.
On the basis of the mentioned above, two vital factors need to meet for the MoS 2 films to achieve macroscale superlubricity.The foremost factor is the reduction of exposed edge planes.References [31][32][33] mainly focused on how to achieve the mismatched structure of MoS 2 layers to expand scale of superlubricity, but not paid attention to edge pinning effect [2,34], which was the chief cause of increasing residual friction in macroscale.Here we first adopted a method of occupying edge planes with nanoparticles (Fig. 10(a)).This method can reliably decrease the quantity of exposed edge planes, and it is also a critical step for MoS 2 -based films to accomplish macroscale superlubricity.As friction can induce incommensurate MoS 2 layers to that of commensurate structure [35], only decreasing the damage of edge pinning effect cannot realize superlubricity.Another factor is the incommensurate structure at the friction interface.Although maintaining the incommensurate structure is always a serve task for the MoS 2 layer during friction, we found that when the MoS 2 layers were pinned to nanoparticles (Fig. 10 2 O) were occupied to the edge plane of the sputtered MoS 2 films by liquid phase synthesis (Figs.1(I)-1(III)).The MoS 2 layers with a commensurate structure were damaged, obviously by atoms impacting during deposition, forming numerous dangling bonds (Figs.1(a)-1(c)) on the Friction 12(1): 52-63 (2024) )).The corresponding high-resolution transmission electron microscope (HRTEM) images of the MoS 2 -Cu 2 O film can clearly observe the Cu 2 O nanosphere-like particle (40 nm in size).Its lattice spacing is 0.29 nm, corresponding to the Cu 2 O(110) plane (PDF#05-0667).Moreover, the Cu 2 O nanospherelike particle was successfully formed at the edge plane of the MoS 2 (Figs.1(e) and 1(f)).Hence, it can effectively reduce the quantity of edge planes in the MoS 2 -Cu 2 O film by introducing Cu 2 O nanoparticles.Sputtered MoS 2 -based films displayed a typical coarse columnar platelet (Figs.2(a)-2(e)) as different growth rates in the edge and the basal direction.Different from previous nano-and micro-macroscale superlubricity [1, 14], Au (10 nm in size) and Cu 2 O (40 nm in size) sphere-like particles were embedded in platelets rather than simply being dispersed or concentrated on surface of the films (Figs.2(c) and 2(e), respectively).Those particles have great crystalline observed from the TEM images, and its typical d-spacing was ~0.236 and 0.302 nm corresponding to the Au(111) plane (PDF#04-0784) and Cu 2 O(110) plane (PDF#05-0667), respectively (Figs. 2(d) and 2(f), respectively).The X-ray diffraction (XRD) pattern and Raman spectra of those films are displayed in Fig. S1 in the Electronic Supplementary Material (ESM), and all the films mainly exhibited 2H-MoS 2 diffraction peaks of (100) and (110) planes.Despite that MoS 2 has very poor humidity resistance [27, 28], it is not oxidized after the post-treatment in boiling water.The Au(111), Cu 2 O(111), and Cu 2 O(200) planes were observed at 38.2 (PDF#04-0784), 36.4, and 42.3 (JCPDS No. 05-0667), respectively, and no diffraction peak corresponding to MoO 3 had yet been observed from the XRD pattern in the composite films, which was the most common oxidation product of MoS 2 .Furthermore, the Raman spectra (Fig. S1(b) in the ESM) mainly exhibit two characteristic peaks of MoS 2 , in which the one at 408 cm −1 is the 12gE mode due to an in-plane motion of Mo and S atoms, and the other at 370 cm −1 is the A 1g mode due to an out-of-plane

Fig. 1 (
Fig. 1 (I-III) Schematic illustration of the preparation of MoS 2 -Cu 2 O film.(a, b) TEM image and (c) sandwich-like structure schematic of MoS 2 film.(d, e) TEM image and (f) structure schematic of MoS 2 -Cu 2 O film.

Fig. 3
Fig. 3 Incommensurate structures of MoS 2 layers.(a) TEM image of interface at adjacent Cu 2 O nanoparticles; the inset is the FFT image of Cu 2 O nanoparticles marked with the yellow frame.(b) FFT image of area marked with the navy frame, which is an incommensurate structure (θ = 8°) caused by MoS 2 layers.(c) Contact structure schematic of two adjacent Cu 2 O nanoparticles.(d) Moiré pattern of incommensurate structure of MoS 2 layers at the interface between two adjacent Cu 2 O nanoparticles.

Friction 12 ( 1 )
: 52-63 (2024) 57 www.Springer.com/journal/40544| Friction the TEM image of wear debris for the composite film.The results show that the MoS 2 layer can bend around the Cu 2 O nanoparticles, which is consistent with the generation of the incommensurate structure.On the balance, the multi-contact interfacial structure can promote further reduction in friction coefficient for the MoS 2 -Cu 2 O composite films.
in the ESM), the size of particles was reduced to 10 nm or less.No obvious θ was formed before friction in the FFT image (Fig. S6(b) in the ESM).The friction coefficient of MoS 2 -1 at% Cu 2 O film was 0.02 (Fig. S7 in the ESM).As a consequence, small particles hardly induced mismatch in MoS 2 layers and constructed structural superlubricity with MoS 2 layers.

2 Friction 12 ( 1 )
2 and 964.2 eV, corresponding to the Cu 2p 3/2 and 2p 1/: 52-63 (2024) | https://mc03.manuscriptcentral.com/frictionpeaks, respectively (Chinese Academy of Sciences (CAS) Registry No. 1317-38-0).Hence, the XPS results indicated that the surface of the composite film is mainly consisted of Cu 2 O and parts of CuO.Besides, no obvious changes of the valence states and binding energy for the Mo 3d, S 2p, and Cu 2p peaks can be seen, indicating that during friction, no obvious chemical reaction occurs.The effect of different loads on the lubrication performance for the MoS 2 -Cu 2 O composite films was then investigated, and the friction curves are shown in Fig. 9.No matter what the load is loaded in the range from 1 to 10 N, the composite films show a low friction coefficient.When the load is in the range from 1 to 5 N, the increased load induced the friction coefficient drop in a quicker way.When the load is 10 N, the friction coefficient of the film decreases rapidly in the early stage, but it increases to 0.15 after 6×10 3 r.The reduction rate of the friction coefficient may be related to the speed of the formation of the incommensurate structure.At low loads, composite films require more time to induce the MoS 2 layers to

Fig. 6
Fig. 6 Wear characteristics of pure MoS 2 and MoS 2 -Cu 2 O composite films after the films were worn.(a, b) 3D non-contact surface mappings of wear track and (c, d) optical photos of wear scar and SEM images of wear debris on the surface of counterpart ball.

Fig. 7 Fig. 9
Fig. 7 SEM images and EDS results of transfer-layer for (a, b) MoS 2 and (c, d) MoS 2 -Cu 2 O composite films on the surface of counterpart ball.

Fig. 10
Fig. 10 Principle for preparing macroscale multi-contact superlubricity.(a) Plenty of edge planes pinned in Cu 2 O nanospheres, developing macroscale multi-contact interfacial geometries.(b) Macroscale multi-contact interfacial geometries consisted of microscale incommensurate contact.(c) State for MoS 2 film equipped with small Cu 2 O nanospheres.

Fig. 8
Fig. 8 XPS spectra of MoS 2 -Cu 2 O composite film before and after friction.