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Crystal structure and tribological properties of molybdenum disulfide films prepared by magnetron sputtering technology
Current Applied Physics 19 (2019) 1318–1324
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Crystal structure and tribological properties of molybdenum disulfide films prepared by magnetron sputtering technology
T
Chenyang Gonga, Jianrong Xiaoa,∗, Liwen Zhua, Meng Qia, Songshan Mab a b
College of Science, Guilin University of Technology, Guilin, 541004, PR China School of Physics and Electronics, Central South University, Changsha, 410083, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molybdenum disulfide film Magnetron sputtering Friction coefficient Radio-frequency power Surface morphology
Molybdenum disulfide (MoS2) is widely used in practice due to its excellent lubricating properties. However, research on the tribological properties of magnetron sputtering for depositing MoS2 films remains limited. Herein, the tribological properties of MoS2 films were investigated in detail through a series of characterization and friction coefficient tests. MoS2 films were deposited onto silicon substrates by magnetron sputtering under different radio-frequency powers (Prf). With increased Prf, the crystallinity of the films gradually increases, whereas the friction coefficient initially decreases and then increases. Prf also affects the chemical composition, surface morphology, and grain size of MoS2 films. At Prf = 300 W, the film surface is dense and smooth, the grain distribution is uniform. Moreover, the films have superior tribological properties and low friction coefficient, which can be attributed to the weak van der Waals force among MoS2 layers and the microscopic morphology of the films. All these results indicate that by reasonably controlling the preparation parameters, MoS2 films with excellent tribological properties can be prepared by magnetron sputtering.
1. Introduction Applications of graphene two-dimensional nanomaterials in the fields of material science, solid-state physics, and engineering are rapidly developing and becoming a research hotspot [1–4]. However, many defects of such nanomaterials limit their use [1,5,6]. Accordingly, other two-dimensional nanomaterials such as transition-metal disulfides [7,8], transition-metal oxides, and hexagonal boron nitride are attracting considerable research interest [9–11]. Among these nanomaterials, two-dimensional transition-metal disulfide has attracted considerable attention due to its excellent electrical, optical, and mechanical properties [12–16]. Molybdenum disulfide (MoS2) is a typical transition-metal disulfide. Its thickness can be reduced from large to single layer because surface dangling bond do not exist, thereby becoming a two-dimensional material [17]. MoS2 has excellent physical and chemical properties and is abundant in nature [14,18]. Moreover, it is widely used in the fields of electrochemistry [19–21], energy storage [22–24], catalysis [25–28], lubrication, and so on [29–32]. MoS2 follows a hexagonal-layered structure in which S and Mo are bonded by covalent bonds, whereas the covalently bonded layers of S–Mo–S are connected by weak van der Waals forces (Fig. 1). These weak van der Waals forces lead to low frictional behavior, thereby making it an effective solid lubricant [33–35]. MoS2 has been widely ∗
used in automotive components, sensors, optoelectronic devices, and biomedical fields [12,36–39]. However, the MoS2 coatings prepared by different techniques have remarkable differences in friction properties. Thus, its application fields are also different. For example, Zhou et al. obtained MoS2 for fire proofing materials by chemical stripping [7,9,40]. Xue et al. used chemical deposited MoS2 films for lightemitting devices [41,42]. Pak et al. obtained magnetron sputtering for MoS2 films for detectors [43,44]. Although MoS2 films have been prepared by magnetron sputtering, their structure and properties that are formed by various sputtering conditions are remarkably different during sputtering. Thus, the deposition process of magnetron sputtering MoS2 films is worth exploring. In this study, other conditions were constant, and a batch of MoS2 films were prepared by controlling the sputtering power. The films were characterized by structural characterization and friction properties. The composition and microstructure of the films were analyzed. The surface morphology and friction properties of the films were discussed. The best sputtering power of the magnetron sputtering MoS2 films was obtained.
Corresponding author. E-mail address: [email protected] (J. Xiao).
https://doi.org/10.1016/j.cap.2019.08.017 Received 15 March 2019; Received in revised form 13 August 2019; Accepted 20 August 2019 Available online 20 August 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
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Fig. 1. Schematic of the MoS2 films deposited by magnetron sputtering on silicon wafer substrates.
2. Experimental details
diffraction peaks at 2θ is 69.4° and 69.6° correspond to the (202) crystal planes of MoS2 (JCPDS card number: 89–2905) and the (400) crystal planes of Si (JCPDS card number: 01–0791), sharp diffraction peaks indicate that all four silicon substrate surfaces have remarkable MoS2 crystals [45,46]. Additionally, when Prf is less than 300 W, its diffraction peak sharpers with gradually increased Prf. When Prf exceeds 300 W, the diffraction intensity is relatively weak. When Prf is 300 W, the intensity of the diffraction peak is strong, indicating that the crystallization of MoS2 on the Si substrate is better when Prf is 300 W. The Raman test was performed on 300 W power deposited films, and the result is shown in Fig. 2(b). The two Raman characteristic peaks E2g and A1g of MoS2 can be clearly seen from the figure, wherein the E2g peak of 384.6 cm−1 is the in-plane vibration of two S atoms and Mo atoms in opposite directions, and the A1g peak of 404.3 cm−1 is the vibration of the S atom perpendicular to the in-plane direction, which is consistent with the literature report [15,47,48]. The sharp Raman peak and the narrow half-width indicate that MoS2 forms better crystallization on the silicon substrate, which is consistent with the XRD test results. The crystallinity of MoS2 films is the key to affecting the friction coefficient. EDS was used to detect the distribution of elements in films prepared by different Prf's. Fig. 3 shows the EDS spectrum. Table 1 shows the atomic percentage of each element. The EDS data show that the ratio of Mo to S atoms in the films is approximately 1:2, and with increased Prf, the content of Mo and S atoms in the films also increases. This phenomenon indicates that a large amount of MoS2 is present in the films (and a small number of defects is also present) [49]. In addition, EDS detects the presence of O and Si atoms, which are mainly derived from residual oxygen in the atmosphere and vacuum chamber, and high content in the films. The Si element is attributed to the single crystal silicon substrates. The oxygen element in the material indicates the formation of oxides, which have an effect on the friction properties of the films. XPS was used to further investigate the samples prepared by different Prf's to clarify the chemical components of Mo and S elements in different MoS2 films [50]. Fig. 4(a) shows the XPS measurement spectrum, thereby verifying the presence of Mo and S elements, and 540 eV corresponds to the XPS peak of O1s due to O atoms in the atmospheric and residual gases in the vacuum chamber. Fig. 4(b) shows the highresolution XPS spectrum of Mo3d, where the two peaks at 228.5 and 232.3 eV correspond to Mo 3d5/2, respectively. Mo 3d3/2 is the characteristic of Mo4+ in MoS2 [51]. Moreover, the peak at 234.2 eV corresponds to the Mo–O bond, which indicates that the film surface is partially oxidized [49], and the S2s peak is also observed at 226.1 eV
2.1. Preparation of MoS2 films MoS2 films were deposited onto silicon (100) substrates by magnetron sputtering using argon (Ar, purity 99.999%) as a sputtering gas. The magnetron was operated with a radio-frequency power (Prf) supply. The silicon substrates were ultrasonically cleaned in acetone and ethanol solution for 15 min, and then rinsed with deionized water. Subsequently, the substrate was dried using a heater and placed in a substrate holder for film deposition. The sputtering chamber was evacuated to 9.9 × 10−4 Pa. The Prf of the MoS2 target varied at 100, 200, 300, and 400 W using a high-purity MoS2 target (MoS2, diameter 2.362 in, purity 99.99%). Prior to sputtering, impurities on the surface of the target were etched by plasma Ar+ ions. During the sputtering process, the films were deposited at room temperature, the total gas flow was fixed at 30 sccm, the vacuum chamber pressure was controlled at 1.2 Pa during deposition, and all samples were deposited for 60 min. 2.2. Characterization of MoS2 films The structure of the deposited films was examined using Cu kα radiation X-ray diffraction (XRD) (X'Pert PRO, Holland) with a wavelength of 0.154 nm. The elemental composition of the films was measured using an X-ray energy spectrometer (EDS) (S-001223, USA). X-ray photoelectron spectroscopy (XPS) (Escalab, USA) was used to characterize the chemical composition and bonding of the films. The surface and cross-sectional morphology of the films were characterized using a scanning electron microscopy (SEM) (S-4800, Japan). Raman spectroscopy was characterized by Thermo Fisher Scientific DXR system (Raman, Thermo Fisher Scientific DXR, USA) at an excitation light of 532 nm. The surface morphology and roughness of the films were characterized using atomic force microscopy (AFM) (Lyrique MA, USA). The friction coefficient of the films was tested by a friction material testing machine (Brook UMT-1, USA) at room temperature under dry sliding conditions, which is the fixed load of 5 N and rubbing time of 3 min. 3. Results and discussion XRD can be used to identify the crystallization of MoS2 films. Fig. 2 (a) shows that four samples with different Prf's present two diffraction peaks in the spectrum. The search results of ICSD indicates that the 1319
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Fig. 2. (a) XRD pattern of MoS2 films deposited at different Prf's; (b) Raman spectrum of MoS2 films deposited at 300 W Prf's.
[50], thereby indicating the formation of sulfides. Fig. 4(c) shows the XPS spectrum of S2p. The XPS results show that the two peaks, namely, 161.8 and 162.5 eV, correspond to the S 2p3/2 and S 2p1/2 of MoS2 [49], respectively. Fig. 4(d) shows the S/Mo atomic ratio of MoS2 films deposited by different sputtering powers. It can be clearly seen that as the RF power increases, the S/Mo atomic ratio reaches a minimum at 200 W, but when the power is 300 W, S/MO = 2.14, which is closest to 2. The existence of a large amount of MoS2 in the films was further verified, which is in good agreement with the XRD and EDS results. The presence of Mo–O bonds indicates that the oxide in the material is MoO3 [52–55], which has certain influence on the friction properties of the films. Fig. 5 shows the SEM image of the MoS2 films deposited onto the Si substrate. The surface of the Si substrate is uniformly distributed with MoS2 particles [56]. The surface morphology of the MoS2 films samples prepared at different Prf's show that due to the increase in Prf, the thickness of the MoS2 films increases and then decreases, and the thickness of the films is the largest at 300 W. The dense surface and uniform particle distribution enable the films to possess lubricating properties. Fig. 5 shows that the 100 and 200 W films have holes and defects, thereby resulting in their poor friction performance and high friction coefficient. At 300 W, the films possess the smoothest dense surface and uniform particle distribution [57], and the weak van der Waals interaction of MoS2 promotes the formation of interlayer sliding and lubricating films. The lubricating films made of MoS2 can provide relatively low friction to the contact surface [58]. In order to further study the surface morphology of MoS2 films,
Table 1 EDS spectral data of MoS2 films deposited by different Prf's.
Si Mo S O
100 W
200 W
300 W
400 W
4.67 22.45 36.53 36.35
2.09 24.86 39.07 33.99
1.23 24.87 41.40 32.51
1.51 26.90 45.32 26.28
MoS2 films with different Prf's were characterized by AFM analysis. The results are shown in Fig. 6. From the two-dimensional image of the MoS2 films AFM with different Prf's, it can be clearly seen that the surface morphology of the films is significantly different. When the Prf's is 100 W, the surface of the films is undulating, and the average roughness is 12.6 nm. When the Prf's is 300 W, the surface of the films is relatively flat, the grain distribution is uniform, and the average roughness is relatively low at 2.95 nm. It can be seen from Fig. 6. (e) that the surface roughness of the MoS2 films decreases first and then gradually increases with the increase of the Prf's. When the Prf's is 300 W, the average roughness is relatively low, and the films surface is relatively smooth. The smaller roughness is the direct cause of the relatively low friction coefficient at the 300 W films. The tribological properties of magnetron sputtering MoS2 films at different Prf's were investigated using a reciprocating friction tester [45]. Fig. 7(a) shows the scratch pattern after sample testing in the atmospheric environment. The wear surface appears on the film surface
Fig. 3. (a) EDS spectrum of MoS2 films deposited by different Prf's; (b) Atomic percentage of MoS2 films deposited by different Prf's. 1320
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Fig. 4. XPS spectrum of MoS2 films: (a) Measurement spectrum; (b) Mo 3d core energy level spectrum; (c) S 2p core energy level spectrum; (d) Atomic percentage.
the center of the friction, the film surface is still relatively smooth and intact. As a whole, the scratch diagrams of four different Prf's show that the films with Prf of 300 W has shallow scratch and smooth and complete, except for partial shedding. Fig. 7(b) shows the test results of the sample friction tester prepared at different Prf's. The friction coefficient
after the friction test, and peeling can be observed at the edge of the wear track. This phenomenon occurred because the high internal stress and the weak bond strength in the films make the films brittle and result in the cracking and falling off of the local films during the rubbing process [59]. Although visible scratches or even shedding can be seen in
Fig. 5. SEM surface and cross-sectional micrographs of the MoS2 films at different Prf's: (a) 100 W; (b) 200 W; (c) 300 W; (d) 400 W. 1321
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condition is relatively large, the friction coefficient is approximately 0.2, the friction coefficients of the sample prepared under other Prf are low, the friction coefficient is approximately 0.1. The film surface prepared at 300 W is dense and smooth. The crystallinity is high, and the friction coefficient is the smallest at approximately 0.0863. The combination of the results in SEM and XRD is not difficult to explain. The friction coefficient of the films prepared at 100 W is large because of the existence of porous holes and defects. Moreover, and the crystal particles are uneven; hence, the friction coefficient is large. The friction coefficients of the films prepared by 300 and 400 W slightly increase first and then gradually decrease. This phenomenon is mainly caused by the large particles that break away from the material surface under the tangential force of the friction pair, thereby forming a transfer layer, which plays a lubricating role with the friction test. Consequently, the friction coefficient begins to increase again because MoO3 in the material acts as a friction. Combined with the results of the previous EDS and XPS analyses, a small amount of MoO3 was present in the films. Because friction generates frictional heat, these MoO3 particles expand with increased temperature, which enlarges the friction coefficient of the material [60]. When a certain frictional heat is reached, the surface layer of MoS2 films produces sulfur vapor, and “vapor lubrication” occurs among MoS2 so that the friction coefficient is gradually reduced.
4. Conclusions MoS2 films of different Prf's were prepared and deposited onto silicon wafer substrates by magnetron sputtering. The effect of Prf on the crystal structure, chemical composition, surface morphology, and friction properties of the films were discussed. Microstructure studies show that with increased Prf, the MoS2 (202) diffraction peak gradually increases. Thus, the crystallinity of MoS2 gradually increases. Furthermore, Prf has great effect on the surface morphology of MoS2 films. When Prf is 300 W, the film surface is smooth and dense, and the crystal grains are evenly distributed. The weak van der Waals force between the MoS2 films layer and the layer is easily slipped under the action of external tangential force, which mainly causes its lubricating property. Furthermore, it is affected by the large particle transfer layer, oxide expansion, and gas adsorption during the friction process so that the friction coefficient of the MoS2 films increases with increased Prf and then decreases. The friction coefficient at 300 W is relatively low at approximately 0.0863. In general, the Prf of 300 W is the best Prf to improve the tribological properties of MoS2 films deposited onto silicon wafer substrates.
Fig. 6. (a–d) 2D image of MoS2 films atomic force microscope (3 μm × 3 μm); (e) Surface roughness of MoS2 films at different Prf's.
of all samples is reduced, indicating that the MoS2 films prepared by magnetron sputtering has remarkable tribological properties. The test results in Fig. 7(b) shows that the sample prepared under 100 W
Fig. 7. (a) Microscopic images of the wear surface of MoS2 films with different Prf's; (b) Friction coefficient diagram of MoS2 films deposited by different Prf's. 1322
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Author contributions
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Crystal structure and tribological properties of molybdenum disulfide films prepared by magnetron sputtering technology
T
Chenyang Gonga, Jianrong Xiaoa,∗, Liwen Zhua, Meng Qia, Songshan Mab a b
College of Science, Guilin University of Technology, Guilin, 541004, PR China School of Physics and Electronics, Central South University, Changsha, 410083, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molybdenum disulfide film Magnetron sputtering Friction coefficient Radio-frequency power Surface morphology
Molybdenum disulfide (MoS2) is widely used in practice due to its excellent lubricating properties. However, research on the tribological properties of magnetron sputtering for depositing MoS2 films remains limited. Herein, the tribological properties of MoS2 films were investigated in detail through a series of characterization and friction coefficient tests. MoS2 films were deposited onto silicon substrates by magnetron sputtering under different radio-frequency powers (Prf). With increased Prf, the crystallinity of the films gradually increases, whereas the friction coefficient initially decreases and then increases. Prf also affects the chemical composition, surface morphology, and grain size of MoS2 films. At Prf = 300 W, the film surface is dense and smooth, the grain distribution is uniform. Moreover, the films have superior tribological properties and low friction coefficient, which can be attributed to the weak van der Waals force among MoS2 layers and the microscopic morphology of the films. All these results indicate that by reasonably controlling the preparation parameters, MoS2 films with excellent tribological properties can be prepared by magnetron sputtering.
1. Introduction Applications of graphene two-dimensional nanomaterials in the fields of material science, solid-state physics, and engineering are rapidly developing and becoming a research hotspot [1–4]. However, many defects of such nanomaterials limit their use [1,5,6]. Accordingly, other two-dimensional nanomaterials such as transition-metal disulfides [7,8], transition-metal oxides, and hexagonal boron nitride are attracting considerable research interest [9–11]. Among these nanomaterials, two-dimensional transition-metal disulfide has attracted considerable attention due to its excellent electrical, optical, and mechanical properties [12–16]. Molybdenum disulfide (MoS2) is a typical transition-metal disulfide. Its thickness can be reduced from large to single layer because surface dangling bond do not exist, thereby becoming a two-dimensional material [17]. MoS2 has excellent physical and chemical properties and is abundant in nature [14,18]. Moreover, it is widely used in the fields of electrochemistry [19–21], energy storage [22–24], catalysis [25–28], lubrication, and so on [29–32]. MoS2 follows a hexagonal-layered structure in which S and Mo are bonded by covalent bonds, whereas the covalently bonded layers of S–Mo–S are connected by weak van der Waals forces (Fig. 1). These weak van der Waals forces lead to low frictional behavior, thereby making it an effective solid lubricant [33–35]. MoS2 has been widely ∗
used in automotive components, sensors, optoelectronic devices, and biomedical fields [12,36–39]. However, the MoS2 coatings prepared by different techniques have remarkable differences in friction properties. Thus, its application fields are also different. For example, Zhou et al. obtained MoS2 for fire proofing materials by chemical stripping [7,9,40]. Xue et al. used chemical deposited MoS2 films for lightemitting devices [41,42]. Pak et al. obtained magnetron sputtering for MoS2 films for detectors [43,44]. Although MoS2 films have been prepared by magnetron sputtering, their structure and properties that are formed by various sputtering conditions are remarkably different during sputtering. Thus, the deposition process of magnetron sputtering MoS2 films is worth exploring. In this study, other conditions were constant, and a batch of MoS2 films were prepared by controlling the sputtering power. The films were characterized by structural characterization and friction properties. The composition and microstructure of the films were analyzed. The surface morphology and friction properties of the films were discussed. The best sputtering power of the magnetron sputtering MoS2 films was obtained.
Corresponding author. E-mail address: [email protected] (J. Xiao).
https://doi.org/10.1016/j.cap.2019.08.017 Received 15 March 2019; Received in revised form 13 August 2019; Accepted 20 August 2019 Available online 20 August 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
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Fig. 1. Schematic of the MoS2 films deposited by magnetron sputtering on silicon wafer substrates.
2. Experimental details
diffraction peaks at 2θ is 69.4° and 69.6° correspond to the (202) crystal planes of MoS2 (JCPDS card number: 89–2905) and the (400) crystal planes of Si (JCPDS card number: 01–0791), sharp diffraction peaks indicate that all four silicon substrate surfaces have remarkable MoS2 crystals [45,46]. Additionally, when Prf is less than 300 W, its diffraction peak sharpers with gradually increased Prf. When Prf exceeds 300 W, the diffraction intensity is relatively weak. When Prf is 300 W, the intensity of the diffraction peak is strong, indicating that the crystallization of MoS2 on the Si substrate is better when Prf is 300 W. The Raman test was performed on 300 W power deposited films, and the result is shown in Fig. 2(b). The two Raman characteristic peaks E2g and A1g of MoS2 can be clearly seen from the figure, wherein the E2g peak of 384.6 cm−1 is the in-plane vibration of two S atoms and Mo atoms in opposite directions, and the A1g peak of 404.3 cm−1 is the vibration of the S atom perpendicular to the in-plane direction, which is consistent with the literature report [15,47,48]. The sharp Raman peak and the narrow half-width indicate that MoS2 forms better crystallization on the silicon substrate, which is consistent with the XRD test results. The crystallinity of MoS2 films is the key to affecting the friction coefficient. EDS was used to detect the distribution of elements in films prepared by different Prf's. Fig. 3 shows the EDS spectrum. Table 1 shows the atomic percentage of each element. The EDS data show that the ratio of Mo to S atoms in the films is approximately 1:2, and with increased Prf, the content of Mo and S atoms in the films also increases. This phenomenon indicates that a large amount of MoS2 is present in the films (and a small number of defects is also present) [49]. In addition, EDS detects the presence of O and Si atoms, which are mainly derived from residual oxygen in the atmosphere and vacuum chamber, and high content in the films. The Si element is attributed to the single crystal silicon substrates. The oxygen element in the material indicates the formation of oxides, which have an effect on the friction properties of the films. XPS was used to further investigate the samples prepared by different Prf's to clarify the chemical components of Mo and S elements in different MoS2 films [50]. Fig. 4(a) shows the XPS measurement spectrum, thereby verifying the presence of Mo and S elements, and 540 eV corresponds to the XPS peak of O1s due to O atoms in the atmospheric and residual gases in the vacuum chamber. Fig. 4(b) shows the highresolution XPS spectrum of Mo3d, where the two peaks at 228.5 and 232.3 eV correspond to Mo 3d5/2, respectively. Mo 3d3/2 is the characteristic of Mo4+ in MoS2 [51]. Moreover, the peak at 234.2 eV corresponds to the Mo–O bond, which indicates that the film surface is partially oxidized [49], and the S2s peak is also observed at 226.1 eV
2.1. Preparation of MoS2 films MoS2 films were deposited onto silicon (100) substrates by magnetron sputtering using argon (Ar, purity 99.999%) as a sputtering gas. The magnetron was operated with a radio-frequency power (Prf) supply. The silicon substrates were ultrasonically cleaned in acetone and ethanol solution for 15 min, and then rinsed with deionized water. Subsequently, the substrate was dried using a heater and placed in a substrate holder for film deposition. The sputtering chamber was evacuated to 9.9 × 10−4 Pa. The Prf of the MoS2 target varied at 100, 200, 300, and 400 W using a high-purity MoS2 target (MoS2, diameter 2.362 in, purity 99.99%). Prior to sputtering, impurities on the surface of the target were etched by plasma Ar+ ions. During the sputtering process, the films were deposited at room temperature, the total gas flow was fixed at 30 sccm, the vacuum chamber pressure was controlled at 1.2 Pa during deposition, and all samples were deposited for 60 min. 2.2. Characterization of MoS2 films The structure of the deposited films was examined using Cu kα radiation X-ray diffraction (XRD) (X'Pert PRO, Holland) with a wavelength of 0.154 nm. The elemental composition of the films was measured using an X-ray energy spectrometer (EDS) (S-001223, USA). X-ray photoelectron spectroscopy (XPS) (Escalab, USA) was used to characterize the chemical composition and bonding of the films. The surface and cross-sectional morphology of the films were characterized using a scanning electron microscopy (SEM) (S-4800, Japan). Raman spectroscopy was characterized by Thermo Fisher Scientific DXR system (Raman, Thermo Fisher Scientific DXR, USA) at an excitation light of 532 nm. The surface morphology and roughness of the films were characterized using atomic force microscopy (AFM) (Lyrique MA, USA). The friction coefficient of the films was tested by a friction material testing machine (Brook UMT-1, USA) at room temperature under dry sliding conditions, which is the fixed load of 5 N and rubbing time of 3 min. 3. Results and discussion XRD can be used to identify the crystallization of MoS2 films. Fig. 2 (a) shows that four samples with different Prf's present two diffraction peaks in the spectrum. The search results of ICSD indicates that the 1319
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Fig. 2. (a) XRD pattern of MoS2 films deposited at different Prf's; (b) Raman spectrum of MoS2 films deposited at 300 W Prf's.
[50], thereby indicating the formation of sulfides. Fig. 4(c) shows the XPS spectrum of S2p. The XPS results show that the two peaks, namely, 161.8 and 162.5 eV, correspond to the S 2p3/2 and S 2p1/2 of MoS2 [49], respectively. Fig. 4(d) shows the S/Mo atomic ratio of MoS2 films deposited by different sputtering powers. It can be clearly seen that as the RF power increases, the S/Mo atomic ratio reaches a minimum at 200 W, but when the power is 300 W, S/MO = 2.14, which is closest to 2. The existence of a large amount of MoS2 in the films was further verified, which is in good agreement with the XRD and EDS results. The presence of Mo–O bonds indicates that the oxide in the material is MoO3 [52–55], which has certain influence on the friction properties of the films. Fig. 5 shows the SEM image of the MoS2 films deposited onto the Si substrate. The surface of the Si substrate is uniformly distributed with MoS2 particles [56]. The surface morphology of the MoS2 films samples prepared at different Prf's show that due to the increase in Prf, the thickness of the MoS2 films increases and then decreases, and the thickness of the films is the largest at 300 W. The dense surface and uniform particle distribution enable the films to possess lubricating properties. Fig. 5 shows that the 100 and 200 W films have holes and defects, thereby resulting in their poor friction performance and high friction coefficient. At 300 W, the films possess the smoothest dense surface and uniform particle distribution [57], and the weak van der Waals interaction of MoS2 promotes the formation of interlayer sliding and lubricating films. The lubricating films made of MoS2 can provide relatively low friction to the contact surface [58]. In order to further study the surface morphology of MoS2 films,
Table 1 EDS spectral data of MoS2 films deposited by different Prf's.
Si Mo S O
100 W
200 W
300 W
400 W
4.67 22.45 36.53 36.35
2.09 24.86 39.07 33.99
1.23 24.87 41.40 32.51
1.51 26.90 45.32 26.28
MoS2 films with different Prf's were characterized by AFM analysis. The results are shown in Fig. 6. From the two-dimensional image of the MoS2 films AFM with different Prf's, it can be clearly seen that the surface morphology of the films is significantly different. When the Prf's is 100 W, the surface of the films is undulating, and the average roughness is 12.6 nm. When the Prf's is 300 W, the surface of the films is relatively flat, the grain distribution is uniform, and the average roughness is relatively low at 2.95 nm. It can be seen from Fig. 6. (e) that the surface roughness of the MoS2 films decreases first and then gradually increases with the increase of the Prf's. When the Prf's is 300 W, the average roughness is relatively low, and the films surface is relatively smooth. The smaller roughness is the direct cause of the relatively low friction coefficient at the 300 W films. The tribological properties of magnetron sputtering MoS2 films at different Prf's were investigated using a reciprocating friction tester [45]. Fig. 7(a) shows the scratch pattern after sample testing in the atmospheric environment. The wear surface appears on the film surface
Fig. 3. (a) EDS spectrum of MoS2 films deposited by different Prf's; (b) Atomic percentage of MoS2 films deposited by different Prf's. 1320
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Fig. 4. XPS spectrum of MoS2 films: (a) Measurement spectrum; (b) Mo 3d core energy level spectrum; (c) S 2p core energy level spectrum; (d) Atomic percentage.
the center of the friction, the film surface is still relatively smooth and intact. As a whole, the scratch diagrams of four different Prf's show that the films with Prf of 300 W has shallow scratch and smooth and complete, except for partial shedding. Fig. 7(b) shows the test results of the sample friction tester prepared at different Prf's. The friction coefficient
after the friction test, and peeling can be observed at the edge of the wear track. This phenomenon occurred because the high internal stress and the weak bond strength in the films make the films brittle and result in the cracking and falling off of the local films during the rubbing process [59]. Although visible scratches or even shedding can be seen in
Fig. 5. SEM surface and cross-sectional micrographs of the MoS2 films at different Prf's: (a) 100 W; (b) 200 W; (c) 300 W; (d) 400 W. 1321
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condition is relatively large, the friction coefficient is approximately 0.2, the friction coefficients of the sample prepared under other Prf are low, the friction coefficient is approximately 0.1. The film surface prepared at 300 W is dense and smooth. The crystallinity is high, and the friction coefficient is the smallest at approximately 0.0863. The combination of the results in SEM and XRD is not difficult to explain. The friction coefficient of the films prepared at 100 W is large because of the existence of porous holes and defects. Moreover, and the crystal particles are uneven; hence, the friction coefficient is large. The friction coefficients of the films prepared by 300 and 400 W slightly increase first and then gradually decrease. This phenomenon is mainly caused by the large particles that break away from the material surface under the tangential force of the friction pair, thereby forming a transfer layer, which plays a lubricating role with the friction test. Consequently, the friction coefficient begins to increase again because MoO3 in the material acts as a friction. Combined with the results of the previous EDS and XPS analyses, a small amount of MoO3 was present in the films. Because friction generates frictional heat, these MoO3 particles expand with increased temperature, which enlarges the friction coefficient of the material [60]. When a certain frictional heat is reached, the surface layer of MoS2 films produces sulfur vapor, and “vapor lubrication” occurs among MoS2 so that the friction coefficient is gradually reduced.
4. Conclusions MoS2 films of different Prf's were prepared and deposited onto silicon wafer substrates by magnetron sputtering. The effect of Prf on the crystal structure, chemical composition, surface morphology, and friction properties of the films were discussed. Microstructure studies show that with increased Prf, the MoS2 (202) diffraction peak gradually increases. Thus, the crystallinity of MoS2 gradually increases. Furthermore, Prf has great effect on the surface morphology of MoS2 films. When Prf is 300 W, the film surface is smooth and dense, and the crystal grains are evenly distributed. The weak van der Waals force between the MoS2 films layer and the layer is easily slipped under the action of external tangential force, which mainly causes its lubricating property. Furthermore, it is affected by the large particle transfer layer, oxide expansion, and gas adsorption during the friction process so that the friction coefficient of the MoS2 films increases with increased Prf and then decreases. The friction coefficient at 300 W is relatively low at approximately 0.0863. In general, the Prf of 300 W is the best Prf to improve the tribological properties of MoS2 films deposited onto silicon wafer substrates.
Fig. 6. (a–d) 2D image of MoS2 films atomic force microscope (3 μm × 3 μm); (e) Surface roughness of MoS2 films at different Prf's.
of all samples is reduced, indicating that the MoS2 films prepared by magnetron sputtering has remarkable tribological properties. The test results in Fig. 7(b) shows that the sample prepared under 100 W
Fig. 7. (a) Microscopic images of the wear surface of MoS2 films with different Prf's; (b) Friction coefficient diagram of MoS2 films deposited by different Prf's. 1322
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Author contributions
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