A study of the tribological properties of Mo-S-Ti composite films prepared by magnetron sputtering

Pure MoS2 films exhibit disadvantages such as low densities, poor mechanical properties, and weak wear resistance under atmospheric environments. For this reason, Ti was chosen as the doping element in this work. Mo-S-Ti composite films with different Ti contents were deposited using the magnetron sputtering system, and the effects of Ti content on the chemical composition, mechanical and tribological properties of these films were analyzed by numerous characterization methods to determine the doping content of Ti elements. The results show that the tribological performance of Mo-S-Ti composite film under atmospheric environments reaches the best when the Ti content is 13.48 at%. In addition, the present work also found that longitudinal load and reciprocating frequency have a significant effect on the tribological properties of this film. It is easier to form high quality transfer film on GCr15 balls under higher longitudinal load and lower reciprocating frequency, thus transforming the friction between film and GCr15 ball into the friction between film and transfer film, resulting in excellent tribological performance.


Introduction
Solid lubrication films have significant advantages over the conventional lubricant and grease in reducing friction, lowering wear, and extending service life of friction pair under severe service conditions where conventional lubricant and grease are not qualified [1].Transition metal disulfides (TMDs), represented by MoS 2 , play an important role in the field of solid lubrication and are commonly used in inert gas and vacuum environments.The excellent tribological properties of MoS 2 originate from its graphene-like layer structure, and the weak Van der Waals force between the Mo-S-Mo unit layers is easily broken by the tangential force, thus making the unit layers susceptible to slip [2].
Magnetron sputtering technology has become an important method in the field of film preparation due to its advantages of simple equipment, easy control, large coating area and strong adhesion [3].In their series of research by Zheng et al, TiN/TiCN multilayer films prepared by DC magnetron sputtering with appropriate surface treatments (Ar+ ion bombardment or Plasma-nitriding treatment) showed good tribological and mechanical properties [4][5][6].In their another research work [7], the TiN/TiCN multilayer film (prepared by DC magnetron sputtering, thickness: 23.5 μm) has a bonding force of up to 29 N with the substrate and a hardness of 21.4 GPa, which results in good wear resistance and a long service life.The MoS 2 films prepared by magnetron sputtering can be classified into three types, namely, amorphous (randomly oriented), Type I ((002) crystal plane perpendicular to the substrate) and Type II ((002) crystal plane parallel to the substrate) [8].The amorphous MoS 2 film has basically no lubrication capability.Type I films exhibit disadvantages such as low densities, easy oxidation, low hardness, and short service life.Unfortunately, the majority of MoS 2 films prepared by magnetron sputtering are Type I films.In comparison, the tribological performance of Type II films is significantly better than that of the first two films.Numerous research efforts have been used to improve the tribological properties of MoS 2 films in multi-service environments.It has been proven that doping with a certain content of metallic (Ti, Cr, Ni, Au) [9][10][11][12] or non-metallic (C, N, Si) [13][14][15] elements is a simple and effective method.With the addition of these elements, the friction coefficient and wear rate of MoS 2 films were significantly reduced.In the past decades, Ti was often used as a doping element for MoS 2 to prepare Mo-S-Ti composite films.The doping with a certain amount of Ti not only promotes the growth of MoS 2 crystals with (002) crystalline planes in a preferential orientation, but also significantly improves the mechanical properties of MoS 2 films, which results in a better performance of these films in friction and wear tests [16][17][18].Obviously, Ti is a promising dopant element.The application of Mo-S-Ti composite films is mostly limited to atmospheric environments, such as the Mo-S-Ti composite coated tools (named MoST TM ) produced by Teer [19].It is well known that the test parameters (load, sliding speed and temperature) during the friction and wear tests have a significant influence on the tribological performance of the films [20].However, there are few studies on the effects of load and sliding speed on the tribological properties of Mo-S-Ti composite films [21,22].
In this work, a series of Mo-S-Ti composite films with different Ti contents were prepared by magnetron sputtering system.At first, the effects of Ti content on the elemental composition, microscopic morphology, surface roughness, phase composition, elemental valence and mechanical properties of Mo-S-Ti composite films were investigated.Then, the effect of Ti content on the friction coefficient, wear rate and wear mechanism of Mo-S-Ti composite films was investigated.Finally, based on the first two steps, the Mo-S-Ti composite film with optimal Ti doping content was studied.The influence of longitudinal load and reciprocating frequency on the tribological properties of this film was investigated with the variables of longitudinal load and reciprocating frequency during the ball-disk reciprocating friction wear experiments.

Experimental details 2.1. Preparation of films
All films were deposited on GCr15 bearing steel and monocrystalline Si substrates using a laboratory selfassembled magnetron sputtering system that included one Ti target and one MoS 2 target.Direct current (DC) and Radio Frequency (RF) power supplies were applied to the Ti target and MoS 2 target, respectively.Prior to deposition, the substrates were ultrasonically cleaned in acetone and alcohol for 20 min to remove contaminants from the substrate surface, respectively, and then dried with hot air.These cleaned substrates were mounted on a sample holder in the center of the chamber.This sample holder was kept rotating at a uniform speed (2 r min −1 ) during the deposition process to ensure the uniformity of the films.After the vacuum reached 1.5 × 10 −3 Pa, 40 sccm Ar was injected into the chamber, and then the two targets were pre-sputtered for 5 min to ensure the cleanliness of the targets.After that, the Ti transition layer with a thickness of about 250 nm was firstly deposited on the substrates to enhance the adhesion between the films and the substrates.Finally, Mo-S-Ti composite films with different Ti contents were prepared by varying the Ti target currents (0, 0.1 A, 0.2 A, 0.3 A), named as Ti-0, Ti-0.1, Ti-0.2 and Ti-0.3 in order.Among them, the Ti-0 film is a pure MoS 2 film without Ti elements.Throughout the formal deposition process, the RF power applied to the MoS 2 target was kept at 200 V, and the working pressure and substrate temperature were kept at fixed values of 0.6 Pa and 300 °C, respectively.In addition, the deposition time was 90 min for all films.

General characterization
The microscopic morphology of the films was observed by field emission scanning electron microscopy (FESEM, FEI Inspect F50).The elemental composition and content of the films were analyzed by x-ray energy dispersive spectrometry (EDS, EDAX super octane).The three-dimensional morphology and roughness of the film surface were obtained by Atomic force microscopy (AFM, Bruker Dimension Icon).The phase composition of the films was analyzed by grazing incidence x-ray diffractometer (GIXRD, Smartlab 9KW), the x-ray source, grazing incidence angle, scanning speed and scanning range were Gu Kα (λ = 0.154 nm), 1°, 4°min −1 and 5°to 90°, respectively.The Raman spectra of the films were tested by a laser confocal Raman spectrometer (Raman, Horiba LabRAM HR Evolution) with a laser wavelength of 532 nm.The chemical bonding of the constituent elements of the films was obtained by x-ray photoelectron spectrometry (XPS, Thermo Fisher ESCALAB 250Xi) with Al Kα irradiation at the pass energy of 160 eV.Prior to testing, the films were bombarded with a highenergy A + beam to remove contaminants.The hardness and elastic modulus of the films were obtained with a nanoindenter (Bruker Hysitron TI980) with an indentation depth no more than 10% of the film thickness.

Friction and wear test
The coefficient of friction of the films was obtained by a ball and disc reciprocating tribometer (Rtec MFT-5000 tribometer).GCr15 balls are selected as sliding match material with a diameter of 6 mm.The reciprocating stroke was 10 mm and the sliding time was 20 min.In addition, the friction and wear tests were conducted at 25 °C with humidity of approximately 40%-50%.After that, the morphology and volume of the abrasion marks were obtained by an optical profilometer (Mahr MarSurf LD130).The wear rate of the films was calculated by the equation K = V/(S × F), where K represents the wear rate of the films, V represents the wear volume, S represents the sliding distance, and F represents the longitudinal load applied.The wear mechanism of the films was determined by analysing the morphology and phase composition of the wear tracks, where the phase composition was obtained by analysing its Raman spectrum.
Firstly, the effect of Ti content on the tribological property of Mo-S-Ti composite films was investigated to determine an optimal doping content, in which the longitudinal load and reciprocating frequency were 5 N and 3 Hz, respectively.After that, the influence of longitudinal load (5 N, 10 N) and reciprocating frequency (2 Hz, 4 Hz) on the tribological property of Mo-S-Ti composite film with optimal Ti content was investigated.Finally, the morphology and element distribution of the transfer film on GC15 balls were observed by FESEM and EDS, and the effects of longitudinal load and reciprocating frequency on the quality of the transfer film were analysed.

Chemical composition
The surface (a ∼ d) and cross-sectional (e ∼ h) morphologies of Mo-S-Ti composite films with different Ti contents were observed by FESEM, as shown in figure 1.Similar to related research, the surface morphology of pure MoS 2 film (figure 1(a)) shows a typical 'worm-like structure' with a large number of voids [17].The crosssection of pure MoS 2 film is composed of numerous columnar crystals with numerous pores between them, which become channels for O 2 and H 2 O to enter the interior of the film.The surface morphology of Mo-S-Ti composite films with different Ti contents (figures 1(b) ∼ (d)) shows a 'cauliflower-like structure' with fewer voids between grains, and the grain size gradually decreases with the increase of Ti content.As shown in figures 1(g) ∼ (h), the cross section of Mo-S-Ti composite film has no obvious characteristics, and the densities of these films are significantly enhanced.Overall, the columnar growth is inhibited by Ti, which significantly enhances the densities of the films.
The contents of the constituent elements of the films were obtained by EDS, as shown in table 1.The content of Ti element in Ti-0 ∼ Ti-0.3 films gradually increases from 0 to 20.92 at%, among which Ti-0 film does not contain Ti element and is a pure MoS 2 film.The S/Mo ratio of pure MoS 2 film is less than 2, and that of Mo-S-Ti composite films has improved but is still less than 2. This phenomenon can be explained by the following two reasons [9].First, the relative atomic mass of element S is much smaller than that of element Mo.As a result, S elements are more likely to be sputtered out of the MoS 2 target, leading to S element deficiency in the MoS 2  target.Second, the S element reacts with the residual hydrogen and oxygen in the chamber, resulting in the absence of S element.The absence of S elements is prevented by the Ti doping to some extent.The three-dimensional morphology and roughness of the film surface were obtained by AFM, as shown in figure 2. Many large-sized mound-like bumps are distributed on the surface of pure MoS 2 films, and the voids between these bumps are large.The roughness of the pure MoS 2 film is 12.69 nm, and its surface is distributed with many large-sized mound-like bumps, and the voids between these mound-like bumps are wide.The Mo-S-Ti composite films with different Ti contents still have a large number of mound-like bumps distributed on the surface, but the size of these mound-like bumps is significantly reduced.With the increase of Ti content, the distribution of grains becomes more compact, the surface becomes smoother, and the roughness decreases from 9.13 nm to 5.83 nm.This is consistent with the results of the analysis of the surface morphology of the films.In short, the surface of Mo-S-Ti composite films is smoother than that of pure MoS 2 film, and the higher the Ti content, the smoother the film surface.
The phase composition of the films was obtained by GIXRD, as shown in figure 3. It is well known that the XRD pattern of MoS 2 contains mainly three diffraction peaks near 14.4°, 33.7°and 58.5°, corresponding to the (002), (100) and (110) crystal planes, respectively [16].In this work, the diffraction peaks corresponding to the (002), (100) and (110) crystal planes are significantly present for the pure MoS 2 film and the intensity of the diffraction peak corresponding to the (100) crystal plane is significantly stronger than that of the other two diffraction peaks, which indicates that this film is a polycrystalline film and is grown with the (100) crystal plane in a preferential orientation.Besides, the positions of the three diffraction peaks show some degree of shift, which is related to the internal stress of the films.For the Mo-S-Ti composite films, the diffraction peaks corresponding to the (100) and (110) crystal planes basically disappear, and only the diffraction peak corresponding to the (002) crystal plane exists.As the Ti content increases, the intensity of the diffraction peak of the (002) crystal plane gradually decreases and the width of the diffraction peak of the (002) crystal plane gradually increases.It is reported that the peak width of the diffraction peak is related to the crystallinity of the  material, the larger the peak width, the lower the crystallinity [23].Therefore, the doping of Ti element makes the (002) crystal plane which is parallel to the substrate grow in a selective orientation, and the crystallinity of the film gradually decreases with the increase of Ti content.
To better confirm the presence of MoS 2 , Raman spectrum investigation was also carried out, as shown in figure 4.There are two first-order Raman activity modes, E 1 2g (383 cm −1 ) and A 1g (408 cm −1 ), in most of the reported MoS 2 Raman spectroscopy studies [24].The full width at half maximum (FWHM) of the Raman peak is related to the crystallinity of materials, the larger the FWHM, the lower the crystallinity.So, the doping of Ti reduces the crystallinity of the film, and the higher the Ti content, the lower the crystallinity of the Mo-S-Ti composite films.Besides, the two Raman peaks (E 1 2g and A 1g ) of the Mo-S-Ti composite films are shifted toward the low wavenumber direction, which indicates the presence of compressive stresses within the films.A certain amount of compressive stress is beneficial to the film, owing to the fact that the presence of compressive stress makes the grains more compact with each other, which avoids crack extension and improves the fatigue strength of the film [25].The above analysis is consistent with that of XRD pattern.
To analyse the chemical valence states of the constituent elements of the films, XPS was used to characterize the Ti-0.2 film, as shown in figure 5.The Mo3d spectrum (figure 5(a)) consists of three parts.The double peaks located at 228.7 and 231.9 eV are associated with MoS 2 ; the double peaks located at 229.5 and 232.8 eV are related to MoS 2-x (x < 2;); the peak located at 226.8 eV belongs to S2s (the overlap peak between Mo and S).The S2p spectrum (figure 5(b)) contains two parts.The double peaks at 164.2 and 165.3 eV are associated with S 2− (MoS 2 ); while the double peaks at 162.6 and 163.9 eV are associated with S 2−x (x < 2; MoS 2-x ), which is in agreement with the analysis of the Mo3d spectrum and explains the S/Mo ratio less than 2. The Ti2p spectrum (figure 5(c)) consists of two parts.The double peaks located at 456.0 and 462.1 eV are associated with Ti 2+ (TiO); while the double peaks located at 458.9 and 465.2 eV are associated with Ti 4+ (TiS 2 ) [16].

Mechanical property
The tribological property of film is closely related to its mechanical property [26], therefore, nanoindentation was used to obtain the mechanical property (hardness and elastic modulus) of the films, as shown in figure 6.It is difficult for pure MoS 2 films to meet the requirements of practical applications due to its extremely poor mechanical property.In this paper, the hardness and elastic modulus (figure 6(a)) of Mo-S-Ti composite films show an increasing trend as the increase of Ti content, with the hardness increasing from 6.07 GPa to 12.68 GPa and the elastic modulus increasing from 78.77 GPa to 160.45 GPa.It is reported that the ratio of hardness to elastic modulus (H/E) of the films is positively correlated with its wear resistance [27].The wear resistance of the  Mo-S-Ti composite film is shown in figure 6(b).As the Ti content increases, the H/E tends to increase first and then decrease.The H/E of the Ti-0.3 film reaches the maximum, which indicates that it has the best wear resistance.When Ti atoms dissolved in the MoS 2 lattice, they replaced some of the Mo atoms to form a replacement solid solution (TiS 2 ), resulting in a solid solution strengthening effect, which led to a significant increase in the mechanical properties (hardness and elastic modulus) of the Mo-S-Ti composite films.However, it does not mean that the higher the content of Ti the better the mechanical property of Mo-S-Ti composite films.

Tribological property
The friction coefficient versus sliding time of the Mo-S-Ti composite films with different Ti contents under atmospheric environment are shown in figure 7(a).It can be seen that the Mo-S-Ti composite films with Ti content of 7.11 at% and 13.48 at% exhibit low and stable friction coefficient and long sliding time.When the Ti content is 20.92 at%, the friction coefficient of Mo-S-Ti composite film shows a large fluctuation.However, the friction coefficient of pure MoS 2 film increases abruptly after a very short sliding time (about 100 s), indicating that the film is worn out. Figure 7(b) shows the average friction coefficient and wear rate of these films.Apparently, the average friction coefficient and wear rate of these films show a decreasing and then increasing trend as the Ti content increases from 0 to 20.92 at%.The average friction coefficient and wear rate of Mo-S-Ti composite film with Ti content of 13.48 at% both reach the minimum value (0.027 and 5.25 × 10 −5 mm 3 N −1 m −1 ).The results show that Mo-S-Ti composite film with Ti content of 13.48 at% has the best tribological properties under atmospheric environment.
To understand the effect of Ti content on the wear mechanism of Mo-S-Ti composite films, the wear tracks of the films were observed, as shown in figure 8.For the pure MoS 2 film, many flaking and grooves were observed in the wear tracks, indicating that its wear mechanism is abrasive wear caused by hard particles.In addition, substrates were observed, indicating that the wear resistance of pure MoS 2 film was extremely poor.When the Ti content was 7.11 at%, only a few grooves were observed in the wear tracks of the Mo-S-Ti composite film.As the Ti content increased to 13.48 at%, there were a few slight scratches in the wear tracks of Mo-S-Ti composite film.However, when the Ti content increased to 20.92 at%, many wear debris were observed at the edge of the wear track of the Mo-S-Ti composite film, indicating that excessive Ti reduces the wear resistance of the Mo-S-Ti composite film.It is reported that the wear resistance of MoS 2 films is closely related to their densities, and the densities of MoS 2 films are significantly improved after doping with Ti, thus the wear resistance of Mo-S-Ti composite films is significantly better than that of pure MoS 2 films.However, the  excessive Ti will generate TiO 2 under the action of frictional heat generation, aggravating the wear of the film [28].
The results show that the Mo-S-Ti composite film with Ti content of 13.48 at% has the best tribological properties.It is well known that the tribological performance of a friction pair is closely related to the load and sliding speed [20].Therefore, it is necessary to study the effect of longitudinal load and reciprocating frequency (also known as sliding speed) on the tribological performance of Mo-S-Ti composite films (Ti content: 13.48 at%), and the results are shown in figure 9.The friction coefficient versus sliding time of the film under different longitudinal loads (5 N and 10 N) and reciprocating frequencies (2 Hz and 4 Hz) are shown in figure 9(a).The friction coefficient of the Mo-S-Ti composite film (Ti-0.2film) is unstable at the condition of 5N-2Hz, fluctuating up and down around 0.03.At the conditions of 5N-4Hz and 10N-2Hz, the Ti-0.2 film exhibits a low and stable friction coefficient.However, when the test parameter shifts to 10N-4Hz, the friction coefficient of the Ti-0.2 film is unstable and increases abruptly after 920 s, indicating that the film is worn out.When the longitudinal load is 5 N, the actual contact area is small and the unevenness of the film surface has a significant effect on the friction, causing the fluctuation of friction.As the reciprocating frequency increases from 2 Hz to 4 Hz, the film surface is smoothed rapidly, making the actual contact area increase and thus the friction coefficient becomes stable and decreasing.The actual contact area between the friction interface is larger when the longitudinal load is 10 N. The smaller reciprocating frequency (2 Hz) is favourable to the formation of the transfer film, which transforms the friction between the film and GC15 ball into the friction between the film and the transfer film, thus making the friction coefficient exhibit low and stable.However, after the reciprocating frequency increased from 2 Hz to 4 Hz, the consumption of the film and the transfer film was aggravated by the friction chemical reaction, resulting in the fluctuation of the friction coefficient.reciprocating frequency are 5 N and 2 Hz, respectively, the Raman spectra of the wear tracks exhibit MoS 2 signals, and the Raman signal of FeMoO 4 is also observed near 920 cm −1 .High-quality MoS 2 Raman signals were observed in the spectra of the wear tracks at both 5N-4Hz and 10N-2Hz.When the test conditions were changed to 10N-4Hz, the Raman signal of MoS 2 was still observed in the spectrum of the wear tracks, but its intensity was very weak, and the Raman signal of MoO 3 was also observed near 666 cm −1 [29].
FESEM and EDS were applied to observe the morphology and elemental composition of the transfer film on GCr15 balls, as shown in figure 11.It is well known that the quality of the transfer film has an important influence on the tribological properties of the film, and the higher the quality of the transfer film, the better the tribological properties of the film [17,26].The transfer film on the GCr15 ball was most complete when the longitudinal load and reciprocating frequency were 10N-2Hz, respectively.In contrast, only a few transfer films were observed on the GCr15 spheres at 5N-2Hz and 10N-4Hz, which explains to some extent the fluctuation of the friction coefficient of the films under these conditions.It is obvious that the film is more likely to form a transfer film on GCr15 balls at 10N-2Hz, thus exhibiting excellent tribological properties.

Figure 1 .
Figure 1.(a) and (e) are the surface and cross-section morphology of Ti-0, respectively; (b) and (f) are the surface and cross-section morphology of Ti-0.1, respectively; (c) and (g) are the surface and cross-section morphology of Ti-0.2, respectively; (d) and (h) are the surface and cross-section morphology of Ti-0.3, respectively.

Figure 3 .
Figure 3.The XRD patterns of Mo-S-Ti composite films with different Ti contents.

Figure 4 .
Figure 4.The Raman spectra of Mo-S-Ti composite films with different Ti contents.

Figure 6 .
Figure 6.The mechanical properties of Mo-S-Ti composite films ((a): hardness and elastic modulus, (b): ratio of hardness to elastic modulus H/E).

Figure 7 .
Figure 7.The tribological properties of Mo-S-Ti composite films ((a): friction coefficient versus sliding time, (b): average friction coefficient and wear rate).
Figure 9(b) gives the average friction coefficient and wear rate of the Ti-0.2 film at different longitudinal loads and reciprocating frequencies.It can be seen that the average friction coefficient first decreases and then increases from 5N-2Hz to 10N-4Hz, reaching a minimum value of 0.019 at 10N-2Hz, and the wear rate also shows the same trend, reaching a minimum value of 3.81 × 10 −5 mm 3 N −1 m −1 at 10N-2Hz.The composition of the wear tracks of the film under different longitudinal loads and reciprocating frequencies was analysed using Raman spectroscopy, as shown in figure 10.When the longitudinal load and

Figure 9 .
Figure 9.The tribological properties of the Ti-0.2 film under different longitudinal loads and reciprocating frequencies ((a): friction coefficient versus sliding time, (b): average friction coefficient and wear rate).

Figure 10 .
Figure 10.The Raman spectra of wear tracks of the Ti-0.2 film under different longitudinal loads and reciprocating frequencies.

Figure 11 .
Figure 11.The morphology and elemental composition of transfer films on GCr15 ball corresponding to sample Ti-0.2 under different loading and reciprocating frequency conditions.

Table 1 .
The elemental composition and thickness of Mo-S-Ti composite films with different Ti contents.