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MoS2 coatings at 25–700 °C
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Influence of Mo contents on the tribological properties of CrMoN/MoS2 coatings at 25–700 °C Cheng Lua,b, Junhong Jiaa,c,∗, Yingying Fua,∗∗, Gewen Yia, Xiaochun Fenga,b, Jingjing Yanga,b, Qi Zhoua, Erqing Xied, Yuan Sune a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, PR China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, PR China c College of Mechanical and Electrical Engineering, Shanxi University of Science & Technology, Xi'an, 710021, PR China d School of Physical Science and Technology, Lanzhou University, Lanzhou, 730000, PR China e Department of Superalloys, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: CrMoN/MoS2 composite coating Mechanical property Tribological property High-temperature
CrMoN/MoS2 coatings with different Mo contents were fabricated by magnetron sputtering technique. The microstructure, mechanical and wear properties at elevated temperatures of coatings were investigated. The results show that the coating (Mo: 18.5 at.%) exhibited moderate hardness and high adhesion strength, which was primarily due to the effect of solid solution enhancement. Almost all of the Mo added composite coatings demonstrated good wear resistance performance with low wear rates of 10−6 mm3 (N m)−1 magnitude although they had high friction coefficients at RT and 500 °C. Particularly, the coating (Mo: 18.5 at.%) exhibited excellent wear resistance property at 700 °C, which was attributed to the outstanding adhesive strength of coating and lubrication of Cr2O3, MoO2 and MoO3.
1. Introduction CrN hard coating with metastable face-centered cubic structure have been extensively used for protective coatings on cutting, forming tools, piston rings, aerospace rolling bearings, etc. as a result of high hardness and excellent wear resistance property over the past two decades [1–5]. Nevertheless, its failure, includes mainly hot-wearing, oxidizing and hot-fatigue, further limited the application of CrN. The addition of aluminum [6,7], silicon [8,9] or titanium [10,11] etc, to CrN coating is an effective approach for improving their properties in many aspects. CrTiN composite coatings present an improvement in wear resistance property due to the solid solution structure and the formation of hard nitride phase [12]. Likewise, CrAlN coatings exhibit high hardness and strong oxidation resistance because of dense Al2O3 films [13]. However, the main drawbacks of those coatings are poor wear resistance and high friction coefficient at rising temperatures. As the oxides of Si, Ti and Al are unfavorable to lubricating at high temperature. Thus, improving wear resistance property of CrN coating at high temperature for long-term application is still a problem that needs to be solved.
Ternary Cr–Mo–N coatings, combining the advantages of CrN and MoN, could have superior mechanical and thermal properties [14]. Mo (N) is suggested due to the fact that it is easy to form the lubricating oxide of MoO3 at about 500 °C, which is beneficial to improve the wear resistance property of coating [15]. Meanwhile, MoS2 was mostly used for reduction of wear rate of coating at room temperature as a result of its low shear strength [16,17]. Choi et al. [18] found that the hardness of CrN coating was largely improved from 18 GPa to around 34 GPa due to the optimal doping of Mo content (21 at.%), and the friction coefficient was obviously reduced from 0.59 to 0.37 of Cr–Mo (30.4 at.%)-N coating. The enhanced hardness and lubrication performance of above composite coatings are ascribed to the solid solution hardening and MoO3 tribo-layer formed in the wear track, respectively. Wang et al. [19] reported CrMoN/MoS2 multilayer coatings possess excellent wear resistance under the heavy load conditions. For CrMoN-based coatings, however, most of these studies have concentrated on the characterization and improvement of their microstructure, mechanical and tribological properties under room temperature (RT), rarely concentrated on researching the wear resistance property of CrMoN/MoS2 coating from RT to 700 °C.
∗
Corresponding author. State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (J. Jia), [email protected] (Y. Fu). https://doi.org/10.1016/j.surfcoat.2019.125072 Received 10 July 2019; Received in revised form 11 October 2019; Accepted 14 October 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Cheng Lu, et al., Surface & Coatings Technology, https://doi.org/10.1016/j.surfcoat.2019.125072
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Therefore, in the present study, the CrMoN/MoS2 coatings containing small amount of MoS2 and different Mo contents were deposited by dc magnetron sputtering technique at different currents of Mo target. The effects of Mo contents on the microstructure and wear resistance properties of CrMoN/MoS2 coatings were investigated systematically at elevated temperatures. And the wear mechanisms at different temperatures were analyzed.
Table 1 Chemical compositions of CrMoN/MoS2 coatings. Samples
S1 S2 S3 S4
2. Experimental details 2.1. Deposition
0 2 4 5
Composition (at.%) Cr
Mo
N
S
49.2 36.1 29.0 26.9
2.3 18.5 29.7 37.2
47.1 44.2 39.8 34.5
1.4 1.2 1.5 1.3
3. Results and discussion
CrMoN/MoS2 coatings with different Mo contents were fabricated on the (100) silicon wafers and polished Inconel 718 sheets (51.1% Ni, 20.3% Fe, 18.6% Cr, 5.1% Nb, 2.9% Mo) by a magnetron sputtering system in Ar + N2 atmosphere. A rotating substrate holder was surrounded by three targets (Cr, Mo, MoS2). Prior to the deposition, the substrates were ultrasonically cleaned for 20 min in acetone and ethanol baths, and then sputter-cleaned with high energy Ar+ for 15 min to achieve surface activation. A thin adhesive layer (Cr layer) was deposited between the substrates and CrMoN/MoS2 coatings to increase the adhesion strength. The current on Cr and MoS2 targets was fixed at 4 A and 0.4 A, and that on Mo target was 0 A, 2 A, 4 A and 5 A respectively. The deposition chamber was evacuated to approximately 2 × 10−3 Pa and the rotation speed of substrate was fixed at 1.1 rpm. Hereafter, the CrMoN/MoS2 composite coatings with varied Mo contents were denoted as S1, S2, S3 and S4. During deposition, the high purity Ar and N2 fluxes were fixed at 16 sccm and 24 sccm respectively. the substrate temperature and chamber pressure were respectively controlled at 150 °C–200 °C and 0.6 Pa.
The chemical compositions of CrMoN/MoS2 composite coatings with various Mo contents are shown in Table 1. It was observed that the Mo content in the coating increased from 2.3 at.% to 37.2 at.% as the Mo target current changed from 0 A to 5 A. Meanwhile, the S content in the coatings was almost constant at 1.3 at.% and the actomic ratio of S/ Mo in S1 coating was less than 2:1. The low S content in the coating was attributed to the fact that it's easier to sputter off S out of target than Mo. Ar+ erosion would cause content deviation from the stoichiometry [21]. The XRD patterns of CrMoN/MoS2 coatings are presented in Fig. 1. The CrN in coatings presented a face-centered cubic (fcc) crystal structure (JCPDS No.77–0047) with (111), (200), (220) and (311) multiple orientations. With the increasing of Mo contents in the composite coating, the peak intensity of preferred orientation of (111) increased along with the intensity of (200) peak of Mo2N (JCPDS No.25–1366) increased. As the Mo contents in CrMoN/MoS2 coatings increased to 18.5 at%, the diffraction peaks of S2 coating were shifted slightly toward lower diffraction angle. The unremarkable shift phenomena of diffraction peaks in S2 coating demonstrated that the Mo atoms was partly melted into CrN lattice in the coating by replacing the Cr atoms and reflected that the prepared CrMoN in composite coatings were substitutional solid solutions. Similar studies were also be reported by other researchers [19,22]. However, the diffraction peaks of CrMoN/MoS2 coatings were shifted toward higher diffraction angle when the Mo contents increased to 29.7 at.% and 37.2 at%. Especially the peak (311) was dislocated for S4 coating. It could be attributed to the lattice distortion caused by residual stress [23]. The XPS spectra of Mo 3d and S 2s obtained from sample S2 coating are presented in Fig. 2. For S2 coating, the Mo 3d spectra (Fig. 2a) consisted of characteristic peaks of 229.1 eV, 232.2 eV and 235.5 eV, which could be identified as Mo–S, Mo–N and Mo–O from that of MoS2, Mo2N and MoO3, respectively [24,25]. The peak at 225.4 eV was the
2.2. Characterization The chemical compositions of CrMoN/MoS2 composite coatings were characterized by an energy dispersive spectroscopy (EDS). The morphology and microstructure of coatings were measured by Tescan Mira 3 field emission scanning electron microscope (FESEM) and TECNAI G2 high resolution transmission electron microscope (HRTEM) operating at accelerating voltage of 200 kV, respectively. The crystal structures of coatings were revealed by X'Pert X-ray diffractometer (XRD) at 45 kV and 40 mA with scan speed of 9°/min and scan range of 15°–90°. X-ray photo-electron spectroscopy (XPS) was conducted on a VG Scientific ESCALAB 250Xi-XPS photoelectron spectrometer equipped with a monochromatic source of Al Kα (1486.68 eV) radiation to analyze the elemental compositions and binding states of molybdenum and sulfur in the coatings. The hardness and elastic modulus were determined by Anton Paar Nanoindenter (TTX-NHT3) with a diamond Berkovich (three-sided pyramid) indenter tip and calculated by the method of Oliver Pharr [20]. The average data was obtained from ten test points. Adhesive strength measurement was performed by micro scratch tester (J&L Tech, JLCST022) equipped with acoustic sensor with scratch speed of 50 N/min and test distance of 5 mm. The friction and wear properties of coatings at different temperatures (RT, 500 °C and 700 °C) were evaluated through ball-on-disc tribometer (UMT-3) for a total time of 1800 s. The applied load was 10 N and the sliding speed was set at 0.15 m/s. The Al2O3 ball (10 mm) was used as counterpart. Each sliding test at a particular temperature was repeated at least three times. The morphology of wear track after each sliding test was analyzed by a Raman spectrometer (JY-HR800) at 532 nm. The wear rates (W) were obtained by the followed equation:
W=
Current of Mo target (A)
V FS
(1) 3
where V is wear volume (mm ), determined by a three dimensional profilometer (MicroXAM-800), F is the applied test load (N) and S is the entire sliding length (m).
Fig. 1. XRD patterns of as-deposited coatings with various Mo contents. 2
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Fig. 2. High-resolution XPS spectra for (a) Mo and (b) S of sample S2.
As Mo content increased from 2.3 at.% to 37.2 at.%, the fibrous columns and columnar grain boundary became increasingly apparent. The microstructure transition of CrMoN/MoS2 coatings was closely connected with the phase structure changes from CrN to Mo2N with a number of preferential nucleation sites. For further verification of the structure of composite coatings, HRTEM investigations are conducted. Fig. 5 shows a bright field images and SAED patterns for the CrMoN/MoS2 coatings. In Fig. 5a, the deposited CrN phase crystallized in a typical cubic structure with strong (111) and (200) crystallographic orientations were observed in S1 coating. The deposited S2 coating showed typical columnar structure with clear columnar boundaries (Fig. 5b), which was consistent with Fig. 4b. The inserted SAED pattern in Fig. 5b demonstrated the existence of a polycrystalline grain structure in the S2 coating. The representative nanoindentation curves and nanohardness and H3/E*2 ratio of CrMoN/MoS2 coatings are shown in Fig. 6. The hardness of S2–S4 composite coatings declined from 12.44 GPa to 11.40 GPa, compared to that of S1 coating for 16.60 GPa. As the Mo content continually increased to 37.2 at.%, the hardness of S4 coating changed slightly to 12.40 GPa. The hardness of composite coating is lower than other CrMoN coating (34 GPa) [18]. The decline of hardness of CrMoN/MoS2 coatings indicated that solid solution hardening [29,30] were not obvious in the composite coatings. This phenomenon might be ascribed to two reason: First, the Mo atom, which radius is larger than that of Cr atom, can't completely replace the Cr atom in the lattice to form solid solution. Second, the redundant Mo atoms formed Mo2N and its hardness is lower than CrN. The H3/E*2 ratio is a major
peak of S 2s and reveals Mo–S [26]. In Fig. 2b, the peak centered at 161.1 eV was detected and that could be attributed to Cr–S bonds (Cr2S3) [14]. The spectrum peak centered at 162.1 eV was the signal of S2− in MoS2 [24]. In addition, the peak at approximately 168.1 eV was observed, corresponding to the oxidation state S and thus supporting that some MoS2 was oxidized due to the exposure to the air, which was consistent with the results in Ref. [27]. Therefore, the XRD and XPS analysis demonstrated that the MoS2 exist in the coating, but in very small amounts. The surface morphologies of CrMoN/MoS2 coatings are shown in Fig. 3. On the whole (Fig. 3 a-d), no huge difference was observed in the morphology amidst the CrMoN/MoS2 composite coatings with various Mo contents. All the samples displayed dense and compact surface structures consisting of a large number of particles with spherical shape. The particles sizes of CrMoN/MoS2 coatings increased slightly and the surfaces exhibited unnoticeable defects like pinholes, particularly the S3 coating (Mo: 29.72 at.%). Additionally, the surfaces became rough which can be attributed to fierce bombardment at higher current. From the higher magnification images (Fig. 3 a'-d´), the grain size seemed to be decreased, but the change was small. The change in grain size might be caused by higher deposition rate and doping [23]. The cross-sectional micrographs of CrMoN/MoS2 coatings with various Mo contents are indicated in Fig. 4. It is obvious that all the prepared coatings exhibited distinct columnar crystal structures owing to the development of cumulative interface waves in magnetron sputtered process. It was accorded well with other report [28]. In addition, all the crystal structures of S1–S4 coatings were continuous and dense.
Fig. 3. Surface morphologies of CrMoN/MoS2 coatings: (a), a' S1; (b), b' S2; (c), c' S3; (d), d' S4. 3
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Fig. 4. Cross-section morphologies of CrMoN/MoS2 coatings: (a) S1; (b) S2; (c) S3; (d) S4.
different temperatures against Al2O3 balls. Through the representative curves of coefficient of friction versus time for S1–S4 coatings at different temperatures, the running-in periods were observed at RT (Fig. 8a), resulting from the hard asperities on the coating surfaces. However, no obvious running-in periods were observed at 500 °C and 700 °C for composite coatings, which could be ascribed to the decrease of hardness caused by the oxidation of coating surface at high temperature. Therefore, the oxidation film produced on the coating surface at high temperature is beneficial to shorten the running-in period. At 700 °C, the evident increase of friction coefficient of S1 coating implied that the coating has been gradually destroyed. While the friction coefficient of S2–S4 coatings tested at 700 °C gradually decreased, which could be lying in the continuous formation of tribo-layer containing molybdenum oxide in friction process. At room temperature, the average friction coefficient of the coatings was not as low as expected due to the low content of MoS2 (Table 1). Meanwhile, the friction coefficient at room temperature slightly declined from 0.55 to 0.52 with increase of Mo content to 18.5 at.%. The highest value reached up to about 0.67 when the Mo content was 29.3 at.%, resulting from the combined effect of low hardness and H3/E*2 ratio of composite coating. With the further increase of Mo content, the friction coefficient reduced to 0.43 might due to the lubricating property of Mo2N compared to CrN,
factor of coating's wear resistance property rather than hardness only [31,32]. That's to say, the coatings with higher H3/E*2 ratio generally exhibit better wear resistance properties. The tendency of H3/E*2 ratio of the prepared coatings was in agreement with that of hardness. With the increase of Mo content, the H3/E*2 ratio as a whole decreased obviously. The S2–S4 coatings showed a declined hardness compared with S1 coating due to the essential soft feature of Mo2N in the coatings. The scratch test was used to obtain the coating's critical load and quantitatively evaluate the adhesion strength of coating to the substrate. The critical load (Lc1) is defined as the force of crack initiation [33], which is presented in Fig. 7. It can be seen that the critical load increased from 13.4 N of S1 coating to 32.7 N when the Mo content reached up to 18.5 at.%, then decreased to 15.8 N with the Mo content further increased to 37.2 at.%. According to the definition of Lc1, the S2 coating, contained 18.5 at.% Mo element, exhibited the strongest crack resistance. Thus, it is obvious that moderate amounts of Mo atoms can improve the adhesion strength of composite coating. Through previous analysis, it was found that the prepared CrMoN/ MoS2 composite coatings had reasonable hardness and adhesion strength. Therefore, the tribological properties of CrMoN/MoS2 composite coatings at elevated temperatures were systematically studied. Fig. 8 shows the friction coefficients of composite coatings tested at
Fig. 5. HR-TEM images with the inserted SAED pattern showing a columnar grain morphology (a) CrN/MoS2 (S1) and (b) CrMoN/MoS2 (S2). 4
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Fig. 6. (a) Representative nanoindentation curves and (b) Hardness and H3/E*2of CrMoN/MoS2 coatings.
with 18.5 at.% Mo content showed the lowest wear rate about 3.9 × 10−7 mm3 (N m)−1, meaning that the coating has excellent wear resistance property at RT due to the appropriate doping of Mo atoms. At 500 °C, the wear rates of S1–S4 coatings reduced sharply from 1.5 × 10−5 to 3.2 × 10−6 mm3 (N m)−1 with the increase of Mo content from 2.3 at.% to 37.2 at.%. The reason may be that the CrN and Mo2N soften at 500 °C, leading to the adhesive wear. At 700 °C, the wear rate of S1 coating (Mo: 2.26 at.%) reached up to 4.7 × 10−5 mm3 (N m)−1, demonstrating that the coating was badly damaged. While, the wear rate of the S2 coating, with 18.5 at.% Mo content, reduced rapidly to 6.87 × 10−6 mm3 (N m)−1. With the continuous increase of Mo content, the wear rates increased within a narrow range. Thereout, the CrMoN/MoS2 coating with Mo content of 18.5 at.% shows excellent wear resistance property at different test temperatures. Meanwhile, combining the results of wear rate and the H3/E*2 ratios mentioned before, it can be concluded that the H3/E*2 ratio of CrMoN/MoS2 coating is not the only parameter to affect the wear resistance property in the sliding process at elevated temperatures. The new lubrication phases formed in tribological reaction process at high temperature should also be considered. In order to better discuss the tribological mechanism at 700 °C, the local surface morphologies of the CrMoN/MoS2 coatings and corresponding friction pairs after wear at 700 °C were characterized and the results are represented in Fig. 11. For S1 coating, the worn surface was
but still higher than other CrMoN coating (0.37) [18]. In contrast to S3 and S4 coatings, S1 and S2 coatings performed lower friction coefficients of 0.45 and 0.25 when test respectively at 500 °C and 700 °C, meaning that the CrMoN/MoS2 coatings with low Mo content possess moderate wear resistance performance at elevated temperatures. The profiles of the wear track tested at 700 °C are valuable and worthy to be explored and discussed. Fig. 9 presents the 2D profile curves of the wear tracks tested at 700 °C. The wear track of the S1 coating (Mo: 2.3 at.%) was broad and deep, approximately 0.6 mm and 8 μm, indicating that the coating was totally worn out by the friction pair in tribological test. However, the wear track of the S2 coating (Mo: 18.5 at.%) became shallow and narrow evidently, around 0.25 mm and 0.7 μm. This result demonstrated that the moderate Mo content could evidently improve the wear resistance property of composite CrMoN/ MoS2 coating at 700 °C, though it exhibited high friction coefficient in sliding test. The improvement of the wear resistance at 700 °C was due to the excellent adhesion strength and compact microstructure of the composite coating [18]. The wear tracks of S3 and S4 coatings became wider and deeper than S2 coating, meaning that the abrasion became severe. The results are consistent with adhesion strength (Fig. 7). Fig. 10 indicates the wear rates of the composite coatings with various Mo contents at different temperatures. The wear rates of all of the coatings kept in order of magnitude of 10−7 mm3 (N m)−1 at RT due to the high hardness of coatings. It was worth noting that the S2 coating
Fig. 7. Images of the scratch groove and acoustical signal curves on CrMoN/MoS2 coatings: (a) S1; (b) S2; (c) S3; (d) S4. 5
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Fig. 8. Friction property of S1–S4 coatings: (a) friction coefficient versus time at RT, (b) friction coefficient versus time at 500 °C, (c) friction coefficient versus time at 700 °C, (d) friction coefficient as a function of the temperature.
Fig. 10. Coatings wear rates vs. temperatures.
Fig. 9. Corresponding profiles of CrMoN/MoS2 coating wear tracks, tested at 700 °C.
resistance property of coating. The low friction coefficient and wear rate might be ascribed to the formation of new oxides from Mo and Cr elements in the coatings at above 500 °C. Those oxides are excellent solid lubricating material at high temperature [34,35], which contributed to the reduction of the friction coefficient and wear rate. XRD studies (Fig. 12) were carried out to understand the changes in the phase composition of these composite coatings after friction tests at 700 °C, though it was not real time investigation at 700 °C and the whole sample was being probed not just the contact area. For S1
characterized with brittle fracture. Meanwhile, the wear track on friction pair (Fig. 11a') was large and there was lots of wear debris in the wear track. Smooth and continuous lamellar transfer layers were obviously generated on the worn surfaces of S2–S4 coatings and transferred to the corresponding friction pairs (Fig. 11b'-d´), indicating that the wear mechanism was adhesion wear. Obviously, the lubrication film generated in the friction process is beneficial to improve the wear 6
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Fig. 11. SEM micrographs of wear track on CrMoN/MoS2 coating and corresponding friction pair at 700 °C (a) S1; (b) S2; (c) S3; (d) S4.
Fig. 12. XRD patterns of coatings after friction tests at 700 °C.
Fig. 13. Raman patterns of coatings after friction tests at 700 °C.
coating, the detected phases after test at 700 °C were CrN, Cr2N and Cr2O3, indicating that the S1 coating has good resistance to oxidation at 700 °C. As the increase of Mo contents, the diffraction peaks of Cr2N and Cr2O3 became weak and the intensities of the diffraction peaks of MoO2 and MoO3 gradually increased. This result demonstrated that the doping of Mo atoms tended to reduce the anti-oxidant ability of composite coating. The differences in the phase change on the worn surface of CrMoN/ MoS2 coatings after friction tests at 700 °C have been further investigated by Raman analysis, as shown in Fig. 13. For S1 coating, Fe3O4 was the only Raman active phase detected by the instrument. This demonstrated that the coating was worn out and the Fe element from substrate was oxidized. As the Mo content increased to 18.5 at.%, the peaks associated with MoO2, MoO3 and Cr2O3, generated by high temperature and tribochemical reaction, were observed on the wear track of the composite coating. Peaks corresponding to NiMoO4 appeared in conjunction with MoO2 peaks as the Mo contents of composite coatings increased to 29.7 at.% and 37.2 at.%. The presence of NiMoO4 in S3 and S4 coatings at 700 °C was due to that the Ni atoms in the substrate diffused to the damaged locations of coating and reacted with Mo and O atoms. The migration mechanism of substrate atoms was similar with other reports [36,37]. The HRTEM images of the products on S2 coating surface tested after 700 °C are presented in Fig. 14. The interval of 0.24 nm and 0.47 nm between the lattice fringes in the
HRTEM images can be indexed to the (110) plane of Cr2O3 and the (001) plane of MoO2 respectively, which were further proved by SAED pattern (Fig. 14b) and well matched with the Raman analysis.
4. Conclusions The CrMoN/MoS2 composite coatings containing various Mo contents deposited by dc magnetron sputtering deposition technique showed dense and compact structure and high adhesion to the substrate. As the Mo content increased from 2.3 at.% to 18.5 at.%, the hardness of coating slightly decreased from 16.6 GPa to 12.44 GPa, but the critical load of coating was largely increased from 13.4 N to 32.7 N. With the doping of Mo atoms, the obvious microstructure transformation occurred in the composite coating, which contributed to the improvement of mechanical strength of composite coating. Superior tribological properties of CrMoN/MoS2 composite coating at high temperature by controlling molybdenum content were highlighted. The tribological property of CrMoN/MoS2 coatings showed high dependence of Mo content at high temperatures. The CrMoN/MoS2 composite coating, with 18.5 at.% of Mo addition, showed the lowest wear rate about 6.87 × 10−6 mm3 (N m)−1 at 700 °C. The tribo-layers contained MoO2, MoO3 and Cr2O3 formed in the wear track of CrMoN/MoS2 composite coating, acted a pivotal part in lubrication and wear reduction. 7
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Fig. 14. HRTEM image of products on the surfaces of S2 film after friction test at 700 °C.
Declaration of competing interest [15]
We would like to submit the revised manuscript entitled “Influence of Mo Contents on the Tribological Properties of CrMoN/MoS2 Coatings at 25–700°C″, which we wish to be considered for publication in “Surface and Coatings Technology”. All authors approve its publication and no conflict of interest exists in the submission of this manuscript. I would like to state on behalf of my co-authors that the work described is original research. Neither the entire paper nor any part of its content has been published or has been accepted elsewhere. It is not being submitted to any other journal.
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Acknowledgements The authors acknowledge the financial supports by the National Natural Science Foundation of China (Grant No. 51575505, 51675508) and the CAS “Light of West China” Program as well as the fund of State Key Laboratory of Solid Lubrication (LSL-1610).
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structural and mechanical properties of CrMoN/MoS2 composite coatings, rare metal, Mater. Eng. 43 (2014) 264–268. A.D. Pogrebnjak, V.M. Beresnev, O.V. Bondar, B.O. Postolnyi, K. Zaleski, E. Coy, S. Jurga, M.O. Lisovenko, P. Konarski, L. Rebouta, J.P. Araujo, Superhard CrN/Mon coatings with multilayer architecture, Mater. Des. 153 (2018) 47–59. N.M. Renevier, J. Hamphire, V.C. Fox, J. Witts, T. Allen, D.G. Teer, Advantages of using self-lubricating, hard, wear-resistant MoS2-based coatings, Surf. Coat. Technol. 142–144 (2001) 67–77. S. Watanabe, J. Noshiro, S. Miyake, Tribological characteristics of WS2/MoS2 solid lubricating multilayer films, Surf. Coat. Technol. 183 (2004) 347–351. E.Y. Choi, M.C. Kang, D.H. Kwon, D.W. Shin, K.H. Kim, Comparative studies on microstructure and mechanical properties of CrN, Cr-C-N and Cr-Mo-N coatings, J. Mater. Process. Technol. 187 (2007) 566–570. X.C. Wang, Q. Li, R.T. Li, Y.L. Di, Syntheses and tribological property of CrMoN/ MoS2 multilayer films on piston rings of heavy vehicle engine, Journal of Wuhan University of Technology-Mater. 31 (2016) 429–434. W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. Y. Wu, H. Li, L. Ji, Y. Ye, J. Chen, H. Zhou, Preparation and properties of MoS2/a-C films for space tribology, J. Phys. D Appl. Phys. 46 (2013). K.H. Kim, E.Y. Choi, S.G. Hong, B.G. Park, J.H. Yoon, J.H. Yong, Syntheses and mechanical properties of Cr-Mo-N coatings by a hybrid coating system, Surf. Coat. Technol. 201 (2006) 4068–4072. J. Jin, H. Liu, D. Zhang, Z. Zhu, Effects of Mo content on the interfacial contact resistance and corrosion properties of CrN coatings on SS316L as bipolar plates in simulated PEMFCs environment, Int. J. Hydrogen Energy 43 (2018) 10048–10060. G. Eda, H. Yamaguchi, D. Voiry, T. Fugita, M. Chen, M. Chhowalla, Photoluminescence from chemically exfoliated MoS2, Nano Lett. 11 (2011) 5111–5116. Y. Jang, J.B. Kim, T.E. Hong, S.J. Yeo, S. Lee, E.A. Jung, B.K. Park, T.-M. Chung, C.G. Kim, D.-J. Lee, H.-B.-R. Lee, S.-H. Kim, Highly-conformal nanocrystalline molybdenum nitride thin films by atomic layer deposition as a diffusion barrier against Cu, J. Alloy. Comp. 663 (2016) 651–658. J. Q. Wang, Z. Liu, C. H. Zhan, K. X. Zhang, X. Y. Lai, J. C. Tu, Y. Cao, 3D hierarchical NiS2/MoS2 nanostructures on CFP with enhanced electrocatalytic activity for hydrogen evolution reaction, J. Mater. Sci. 10.1016/j.jmst.2019.05.037. S.H. Song, B.H. Kim, D.H. Choe, J. Kim, D.C. Kim, D.J. Lee, J.M. Kim, K.J. Chang, S. Jeon, Bandgap widening of phase quilted, 2D MoS2 by oxidative intercalation, Adv. Mater. 27 (2015) 3152–3158. D. He, J. Pu, Z. Lu, L. Wang, G. Zhang, Q. Xue, Simultaneously achieving superior mechanical and tribological properties in WC/a-C nanomultilayers via structural design and interfacial optimization, J. Alloy. Comp. 698 (2017) 420–432. M.C. Kang, S.K. Je, K.H. Kim, B.S. Shin, D.H. Kwon, J.S. Kim, Cutting performance of CrN-based coatings tool deposited by hybrid coating method for micro drilling applications, Surf. Coat. Technol. 202 (2008) 5629–5632. Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, Z.P. Lu, Microstructures and properties of high-entropy alloys, Prog. Mater. Sci. 61 (2014) 1–93. H. Tao, M.T. Tsai, H.-W. Chen, J.C. Huang, J.-G. Duh, Improving high-temperature tribological characteristics on nanocomposite CrAlSiN coating by Mo doping, Surf. Coat. Technol. 349 (2018) 752–756. A. Leyland, A. Matthews, On the significance of the H/E ratio in wear control: a nanocomposite coating approach to optimised tribological behavior, Wear 246 (2000) 1–11. S. Zhang, D. Sun, Y.Q. Fu, H.J. Du, Toughness measurement of thin films: a critical review, Surf. Coat. Technol. 198 (2005) 74–84. P. Hones, N. Martin, M. Regula, F. Levy, Structural and mechanical properties of chromium nitride, molybdenum nitride, and tungsten nitride thin films, J. Phys. D Appl. Phys. 36 (2003) 1023–1029. J.H. Ouyang, S. Sasaki, K. Umeda, Effects of different additives on microstructure and high-temperature tribological properties of plasma-sprayed Cr2O3ceramic coatings, Wear 249 (2001) 440–451. J.L. Li, D.S. Xiong, H.Y. Wu, J.H. Dai, T. Cui, Tribological properties of Mon layer on silver-containing nickel-base alloy at high temperatures, Wear 271 (2011) 987–993. N. He, H. Li, L. Ji, X. Liu, H. Zhou, J. Chen, Reusable chromium oxide coating with lubricating behavior from 25 to 1000° C due to a self-assembled mesh-like surface structure, Surf. Coat. Technol. 321 (2017) 300–308.
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Influence of Mo contents on the tribological properties of CrMoN/MoS2 coatings at 25–700 °C Cheng Lua,b, Junhong Jiaa,c,∗, Yingying Fua,∗∗, Gewen Yia, Xiaochun Fenga,b, Jingjing Yanga,b, Qi Zhoua, Erqing Xied, Yuan Sune a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, PR China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, PR China c College of Mechanical and Electrical Engineering, Shanxi University of Science & Technology, Xi'an, 710021, PR China d School of Physical Science and Technology, Lanzhou University, Lanzhou, 730000, PR China e Department of Superalloys, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: CrMoN/MoS2 composite coating Mechanical property Tribological property High-temperature
CrMoN/MoS2 coatings with different Mo contents were fabricated by magnetron sputtering technique. The microstructure, mechanical and wear properties at elevated temperatures of coatings were investigated. The results show that the coating (Mo: 18.5 at.%) exhibited moderate hardness and high adhesion strength, which was primarily due to the effect of solid solution enhancement. Almost all of the Mo added composite coatings demonstrated good wear resistance performance with low wear rates of 10−6 mm3 (N m)−1 magnitude although they had high friction coefficients at RT and 500 °C. Particularly, the coating (Mo: 18.5 at.%) exhibited excellent wear resistance property at 700 °C, which was attributed to the outstanding adhesive strength of coating and lubrication of Cr2O3, MoO2 and MoO3.
1. Introduction CrN hard coating with metastable face-centered cubic structure have been extensively used for protective coatings on cutting, forming tools, piston rings, aerospace rolling bearings, etc. as a result of high hardness and excellent wear resistance property over the past two decades [1–5]. Nevertheless, its failure, includes mainly hot-wearing, oxidizing and hot-fatigue, further limited the application of CrN. The addition of aluminum [6,7], silicon [8,9] or titanium [10,11] etc, to CrN coating is an effective approach for improving their properties in many aspects. CrTiN composite coatings present an improvement in wear resistance property due to the solid solution structure and the formation of hard nitride phase [12]. Likewise, CrAlN coatings exhibit high hardness and strong oxidation resistance because of dense Al2O3 films [13]. However, the main drawbacks of those coatings are poor wear resistance and high friction coefficient at rising temperatures. As the oxides of Si, Ti and Al are unfavorable to lubricating at high temperature. Thus, improving wear resistance property of CrN coating at high temperature for long-term application is still a problem that needs to be solved.
Ternary Cr–Mo–N coatings, combining the advantages of CrN and MoN, could have superior mechanical and thermal properties [14]. Mo (N) is suggested due to the fact that it is easy to form the lubricating oxide of MoO3 at about 500 °C, which is beneficial to improve the wear resistance property of coating [15]. Meanwhile, MoS2 was mostly used for reduction of wear rate of coating at room temperature as a result of its low shear strength [16,17]. Choi et al. [18] found that the hardness of CrN coating was largely improved from 18 GPa to around 34 GPa due to the optimal doping of Mo content (21 at.%), and the friction coefficient was obviously reduced from 0.59 to 0.37 of Cr–Mo (30.4 at.%)-N coating. The enhanced hardness and lubrication performance of above composite coatings are ascribed to the solid solution hardening and MoO3 tribo-layer formed in the wear track, respectively. Wang et al. [19] reported CrMoN/MoS2 multilayer coatings possess excellent wear resistance under the heavy load conditions. For CrMoN-based coatings, however, most of these studies have concentrated on the characterization and improvement of their microstructure, mechanical and tribological properties under room temperature (RT), rarely concentrated on researching the wear resistance property of CrMoN/MoS2 coating from RT to 700 °C.
∗
Corresponding author. State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (J. Jia), [email protected] (Y. Fu). https://doi.org/10.1016/j.surfcoat.2019.125072 Received 10 July 2019; Received in revised form 11 October 2019; Accepted 14 October 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Cheng Lu, et al., Surface & Coatings Technology, https://doi.org/10.1016/j.surfcoat.2019.125072
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Therefore, in the present study, the CrMoN/MoS2 coatings containing small amount of MoS2 and different Mo contents were deposited by dc magnetron sputtering technique at different currents of Mo target. The effects of Mo contents on the microstructure and wear resistance properties of CrMoN/MoS2 coatings were investigated systematically at elevated temperatures. And the wear mechanisms at different temperatures were analyzed.
Table 1 Chemical compositions of CrMoN/MoS2 coatings. Samples
S1 S2 S3 S4
2. Experimental details 2.1. Deposition
0 2 4 5
Composition (at.%) Cr
Mo
N
S
49.2 36.1 29.0 26.9
2.3 18.5 29.7 37.2
47.1 44.2 39.8 34.5
1.4 1.2 1.5 1.3
3. Results and discussion
CrMoN/MoS2 coatings with different Mo contents were fabricated on the (100) silicon wafers and polished Inconel 718 sheets (51.1% Ni, 20.3% Fe, 18.6% Cr, 5.1% Nb, 2.9% Mo) by a magnetron sputtering system in Ar + N2 atmosphere. A rotating substrate holder was surrounded by three targets (Cr, Mo, MoS2). Prior to the deposition, the substrates were ultrasonically cleaned for 20 min in acetone and ethanol baths, and then sputter-cleaned with high energy Ar+ for 15 min to achieve surface activation. A thin adhesive layer (Cr layer) was deposited between the substrates and CrMoN/MoS2 coatings to increase the adhesion strength. The current on Cr and MoS2 targets was fixed at 4 A and 0.4 A, and that on Mo target was 0 A, 2 A, 4 A and 5 A respectively. The deposition chamber was evacuated to approximately 2 × 10−3 Pa and the rotation speed of substrate was fixed at 1.1 rpm. Hereafter, the CrMoN/MoS2 composite coatings with varied Mo contents were denoted as S1, S2, S3 and S4. During deposition, the high purity Ar and N2 fluxes were fixed at 16 sccm and 24 sccm respectively. the substrate temperature and chamber pressure were respectively controlled at 150 °C–200 °C and 0.6 Pa.
The chemical compositions of CrMoN/MoS2 composite coatings with various Mo contents are shown in Table 1. It was observed that the Mo content in the coating increased from 2.3 at.% to 37.2 at.% as the Mo target current changed from 0 A to 5 A. Meanwhile, the S content in the coatings was almost constant at 1.3 at.% and the actomic ratio of S/ Mo in S1 coating was less than 2:1. The low S content in the coating was attributed to the fact that it's easier to sputter off S out of target than Mo. Ar+ erosion would cause content deviation from the stoichiometry [21]. The XRD patterns of CrMoN/MoS2 coatings are presented in Fig. 1. The CrN in coatings presented a face-centered cubic (fcc) crystal structure (JCPDS No.77–0047) with (111), (200), (220) and (311) multiple orientations. With the increasing of Mo contents in the composite coating, the peak intensity of preferred orientation of (111) increased along with the intensity of (200) peak of Mo2N (JCPDS No.25–1366) increased. As the Mo contents in CrMoN/MoS2 coatings increased to 18.5 at%, the diffraction peaks of S2 coating were shifted slightly toward lower diffraction angle. The unremarkable shift phenomena of diffraction peaks in S2 coating demonstrated that the Mo atoms was partly melted into CrN lattice in the coating by replacing the Cr atoms and reflected that the prepared CrMoN in composite coatings were substitutional solid solutions. Similar studies were also be reported by other researchers [19,22]. However, the diffraction peaks of CrMoN/MoS2 coatings were shifted toward higher diffraction angle when the Mo contents increased to 29.7 at.% and 37.2 at%. Especially the peak (311) was dislocated for S4 coating. It could be attributed to the lattice distortion caused by residual stress [23]. The XPS spectra of Mo 3d and S 2s obtained from sample S2 coating are presented in Fig. 2. For S2 coating, the Mo 3d spectra (Fig. 2a) consisted of characteristic peaks of 229.1 eV, 232.2 eV and 235.5 eV, which could be identified as Mo–S, Mo–N and Mo–O from that of MoS2, Mo2N and MoO3, respectively [24,25]. The peak at 225.4 eV was the
2.2. Characterization The chemical compositions of CrMoN/MoS2 composite coatings were characterized by an energy dispersive spectroscopy (EDS). The morphology and microstructure of coatings were measured by Tescan Mira 3 field emission scanning electron microscope (FESEM) and TECNAI G2 high resolution transmission electron microscope (HRTEM) operating at accelerating voltage of 200 kV, respectively. The crystal structures of coatings were revealed by X'Pert X-ray diffractometer (XRD) at 45 kV and 40 mA with scan speed of 9°/min and scan range of 15°–90°. X-ray photo-electron spectroscopy (XPS) was conducted on a VG Scientific ESCALAB 250Xi-XPS photoelectron spectrometer equipped with a monochromatic source of Al Kα (1486.68 eV) radiation to analyze the elemental compositions and binding states of molybdenum and sulfur in the coatings. The hardness and elastic modulus were determined by Anton Paar Nanoindenter (TTX-NHT3) with a diamond Berkovich (three-sided pyramid) indenter tip and calculated by the method of Oliver Pharr [20]. The average data was obtained from ten test points. Adhesive strength measurement was performed by micro scratch tester (J&L Tech, JLCST022) equipped with acoustic sensor with scratch speed of 50 N/min and test distance of 5 mm. The friction and wear properties of coatings at different temperatures (RT, 500 °C and 700 °C) were evaluated through ball-on-disc tribometer (UMT-3) for a total time of 1800 s. The applied load was 10 N and the sliding speed was set at 0.15 m/s. The Al2O3 ball (10 mm) was used as counterpart. Each sliding test at a particular temperature was repeated at least three times. The morphology of wear track after each sliding test was analyzed by a Raman spectrometer (JY-HR800) at 532 nm. The wear rates (W) were obtained by the followed equation:
W=
Current of Mo target (A)
V FS
(1) 3
where V is wear volume (mm ), determined by a three dimensional profilometer (MicroXAM-800), F is the applied test load (N) and S is the entire sliding length (m).
Fig. 1. XRD patterns of as-deposited coatings with various Mo contents. 2
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Fig. 2. High-resolution XPS spectra for (a) Mo and (b) S of sample S2.
As Mo content increased from 2.3 at.% to 37.2 at.%, the fibrous columns and columnar grain boundary became increasingly apparent. The microstructure transition of CrMoN/MoS2 coatings was closely connected with the phase structure changes from CrN to Mo2N with a number of preferential nucleation sites. For further verification of the structure of composite coatings, HRTEM investigations are conducted. Fig. 5 shows a bright field images and SAED patterns for the CrMoN/MoS2 coatings. In Fig. 5a, the deposited CrN phase crystallized in a typical cubic structure with strong (111) and (200) crystallographic orientations were observed in S1 coating. The deposited S2 coating showed typical columnar structure with clear columnar boundaries (Fig. 5b), which was consistent with Fig. 4b. The inserted SAED pattern in Fig. 5b demonstrated the existence of a polycrystalline grain structure in the S2 coating. The representative nanoindentation curves and nanohardness and H3/E*2 ratio of CrMoN/MoS2 coatings are shown in Fig. 6. The hardness of S2–S4 composite coatings declined from 12.44 GPa to 11.40 GPa, compared to that of S1 coating for 16.60 GPa. As the Mo content continually increased to 37.2 at.%, the hardness of S4 coating changed slightly to 12.40 GPa. The hardness of composite coating is lower than other CrMoN coating (34 GPa) [18]. The decline of hardness of CrMoN/MoS2 coatings indicated that solid solution hardening [29,30] were not obvious in the composite coatings. This phenomenon might be ascribed to two reason: First, the Mo atom, which radius is larger than that of Cr atom, can't completely replace the Cr atom in the lattice to form solid solution. Second, the redundant Mo atoms formed Mo2N and its hardness is lower than CrN. The H3/E*2 ratio is a major
peak of S 2s and reveals Mo–S [26]. In Fig. 2b, the peak centered at 161.1 eV was detected and that could be attributed to Cr–S bonds (Cr2S3) [14]. The spectrum peak centered at 162.1 eV was the signal of S2− in MoS2 [24]. In addition, the peak at approximately 168.1 eV was observed, corresponding to the oxidation state S and thus supporting that some MoS2 was oxidized due to the exposure to the air, which was consistent with the results in Ref. [27]. Therefore, the XRD and XPS analysis demonstrated that the MoS2 exist in the coating, but in very small amounts. The surface morphologies of CrMoN/MoS2 coatings are shown in Fig. 3. On the whole (Fig. 3 a-d), no huge difference was observed in the morphology amidst the CrMoN/MoS2 composite coatings with various Mo contents. All the samples displayed dense and compact surface structures consisting of a large number of particles with spherical shape. The particles sizes of CrMoN/MoS2 coatings increased slightly and the surfaces exhibited unnoticeable defects like pinholes, particularly the S3 coating (Mo: 29.72 at.%). Additionally, the surfaces became rough which can be attributed to fierce bombardment at higher current. From the higher magnification images (Fig. 3 a'-d´), the grain size seemed to be decreased, but the change was small. The change in grain size might be caused by higher deposition rate and doping [23]. The cross-sectional micrographs of CrMoN/MoS2 coatings with various Mo contents are indicated in Fig. 4. It is obvious that all the prepared coatings exhibited distinct columnar crystal structures owing to the development of cumulative interface waves in magnetron sputtered process. It was accorded well with other report [28]. In addition, all the crystal structures of S1–S4 coatings were continuous and dense.
Fig. 3. Surface morphologies of CrMoN/MoS2 coatings: (a), a' S1; (b), b' S2; (c), c' S3; (d), d' S4. 3
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Fig. 4. Cross-section morphologies of CrMoN/MoS2 coatings: (a) S1; (b) S2; (c) S3; (d) S4.
different temperatures against Al2O3 balls. Through the representative curves of coefficient of friction versus time for S1–S4 coatings at different temperatures, the running-in periods were observed at RT (Fig. 8a), resulting from the hard asperities on the coating surfaces. However, no obvious running-in periods were observed at 500 °C and 700 °C for composite coatings, which could be ascribed to the decrease of hardness caused by the oxidation of coating surface at high temperature. Therefore, the oxidation film produced on the coating surface at high temperature is beneficial to shorten the running-in period. At 700 °C, the evident increase of friction coefficient of S1 coating implied that the coating has been gradually destroyed. While the friction coefficient of S2–S4 coatings tested at 700 °C gradually decreased, which could be lying in the continuous formation of tribo-layer containing molybdenum oxide in friction process. At room temperature, the average friction coefficient of the coatings was not as low as expected due to the low content of MoS2 (Table 1). Meanwhile, the friction coefficient at room temperature slightly declined from 0.55 to 0.52 with increase of Mo content to 18.5 at.%. The highest value reached up to about 0.67 when the Mo content was 29.3 at.%, resulting from the combined effect of low hardness and H3/E*2 ratio of composite coating. With the further increase of Mo content, the friction coefficient reduced to 0.43 might due to the lubricating property of Mo2N compared to CrN,
factor of coating's wear resistance property rather than hardness only [31,32]. That's to say, the coatings with higher H3/E*2 ratio generally exhibit better wear resistance properties. The tendency of H3/E*2 ratio of the prepared coatings was in agreement with that of hardness. With the increase of Mo content, the H3/E*2 ratio as a whole decreased obviously. The S2–S4 coatings showed a declined hardness compared with S1 coating due to the essential soft feature of Mo2N in the coatings. The scratch test was used to obtain the coating's critical load and quantitatively evaluate the adhesion strength of coating to the substrate. The critical load (Lc1) is defined as the force of crack initiation [33], which is presented in Fig. 7. It can be seen that the critical load increased from 13.4 N of S1 coating to 32.7 N when the Mo content reached up to 18.5 at.%, then decreased to 15.8 N with the Mo content further increased to 37.2 at.%. According to the definition of Lc1, the S2 coating, contained 18.5 at.% Mo element, exhibited the strongest crack resistance. Thus, it is obvious that moderate amounts of Mo atoms can improve the adhesion strength of composite coating. Through previous analysis, it was found that the prepared CrMoN/ MoS2 composite coatings had reasonable hardness and adhesion strength. Therefore, the tribological properties of CrMoN/MoS2 composite coatings at elevated temperatures were systematically studied. Fig. 8 shows the friction coefficients of composite coatings tested at
Fig. 5. HR-TEM images with the inserted SAED pattern showing a columnar grain morphology (a) CrN/MoS2 (S1) and (b) CrMoN/MoS2 (S2). 4
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Fig. 6. (a) Representative nanoindentation curves and (b) Hardness and H3/E*2of CrMoN/MoS2 coatings.
with 18.5 at.% Mo content showed the lowest wear rate about 3.9 × 10−7 mm3 (N m)−1, meaning that the coating has excellent wear resistance property at RT due to the appropriate doping of Mo atoms. At 500 °C, the wear rates of S1–S4 coatings reduced sharply from 1.5 × 10−5 to 3.2 × 10−6 mm3 (N m)−1 with the increase of Mo content from 2.3 at.% to 37.2 at.%. The reason may be that the CrN and Mo2N soften at 500 °C, leading to the adhesive wear. At 700 °C, the wear rate of S1 coating (Mo: 2.26 at.%) reached up to 4.7 × 10−5 mm3 (N m)−1, demonstrating that the coating was badly damaged. While, the wear rate of the S2 coating, with 18.5 at.% Mo content, reduced rapidly to 6.87 × 10−6 mm3 (N m)−1. With the continuous increase of Mo content, the wear rates increased within a narrow range. Thereout, the CrMoN/MoS2 coating with Mo content of 18.5 at.% shows excellent wear resistance property at different test temperatures. Meanwhile, combining the results of wear rate and the H3/E*2 ratios mentioned before, it can be concluded that the H3/E*2 ratio of CrMoN/MoS2 coating is not the only parameter to affect the wear resistance property in the sliding process at elevated temperatures. The new lubrication phases formed in tribological reaction process at high temperature should also be considered. In order to better discuss the tribological mechanism at 700 °C, the local surface morphologies of the CrMoN/MoS2 coatings and corresponding friction pairs after wear at 700 °C were characterized and the results are represented in Fig. 11. For S1 coating, the worn surface was
but still higher than other CrMoN coating (0.37) [18]. In contrast to S3 and S4 coatings, S1 and S2 coatings performed lower friction coefficients of 0.45 and 0.25 when test respectively at 500 °C and 700 °C, meaning that the CrMoN/MoS2 coatings with low Mo content possess moderate wear resistance performance at elevated temperatures. The profiles of the wear track tested at 700 °C are valuable and worthy to be explored and discussed. Fig. 9 presents the 2D profile curves of the wear tracks tested at 700 °C. The wear track of the S1 coating (Mo: 2.3 at.%) was broad and deep, approximately 0.6 mm and 8 μm, indicating that the coating was totally worn out by the friction pair in tribological test. However, the wear track of the S2 coating (Mo: 18.5 at.%) became shallow and narrow evidently, around 0.25 mm and 0.7 μm. This result demonstrated that the moderate Mo content could evidently improve the wear resistance property of composite CrMoN/ MoS2 coating at 700 °C, though it exhibited high friction coefficient in sliding test. The improvement of the wear resistance at 700 °C was due to the excellent adhesion strength and compact microstructure of the composite coating [18]. The wear tracks of S3 and S4 coatings became wider and deeper than S2 coating, meaning that the abrasion became severe. The results are consistent with adhesion strength (Fig. 7). Fig. 10 indicates the wear rates of the composite coatings with various Mo contents at different temperatures. The wear rates of all of the coatings kept in order of magnitude of 10−7 mm3 (N m)−1 at RT due to the high hardness of coatings. It was worth noting that the S2 coating
Fig. 7. Images of the scratch groove and acoustical signal curves on CrMoN/MoS2 coatings: (a) S1; (b) S2; (c) S3; (d) S4. 5
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Fig. 8. Friction property of S1–S4 coatings: (a) friction coefficient versus time at RT, (b) friction coefficient versus time at 500 °C, (c) friction coefficient versus time at 700 °C, (d) friction coefficient as a function of the temperature.
Fig. 10. Coatings wear rates vs. temperatures.
Fig. 9. Corresponding profiles of CrMoN/MoS2 coating wear tracks, tested at 700 °C.
resistance property of coating. The low friction coefficient and wear rate might be ascribed to the formation of new oxides from Mo and Cr elements in the coatings at above 500 °C. Those oxides are excellent solid lubricating material at high temperature [34,35], which contributed to the reduction of the friction coefficient and wear rate. XRD studies (Fig. 12) were carried out to understand the changes in the phase composition of these composite coatings after friction tests at 700 °C, though it was not real time investigation at 700 °C and the whole sample was being probed not just the contact area. For S1
characterized with brittle fracture. Meanwhile, the wear track on friction pair (Fig. 11a') was large and there was lots of wear debris in the wear track. Smooth and continuous lamellar transfer layers were obviously generated on the worn surfaces of S2–S4 coatings and transferred to the corresponding friction pairs (Fig. 11b'-d´), indicating that the wear mechanism was adhesion wear. Obviously, the lubrication film generated in the friction process is beneficial to improve the wear 6
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Fig. 11. SEM micrographs of wear track on CrMoN/MoS2 coating and corresponding friction pair at 700 °C (a) S1; (b) S2; (c) S3; (d) S4.
Fig. 12. XRD patterns of coatings after friction tests at 700 °C.
Fig. 13. Raman patterns of coatings after friction tests at 700 °C.
coating, the detected phases after test at 700 °C were CrN, Cr2N and Cr2O3, indicating that the S1 coating has good resistance to oxidation at 700 °C. As the increase of Mo contents, the diffraction peaks of Cr2N and Cr2O3 became weak and the intensities of the diffraction peaks of MoO2 and MoO3 gradually increased. This result demonstrated that the doping of Mo atoms tended to reduce the anti-oxidant ability of composite coating. The differences in the phase change on the worn surface of CrMoN/ MoS2 coatings after friction tests at 700 °C have been further investigated by Raman analysis, as shown in Fig. 13. For S1 coating, Fe3O4 was the only Raman active phase detected by the instrument. This demonstrated that the coating was worn out and the Fe element from substrate was oxidized. As the Mo content increased to 18.5 at.%, the peaks associated with MoO2, MoO3 and Cr2O3, generated by high temperature and tribochemical reaction, were observed on the wear track of the composite coating. Peaks corresponding to NiMoO4 appeared in conjunction with MoO2 peaks as the Mo contents of composite coatings increased to 29.7 at.% and 37.2 at.%. The presence of NiMoO4 in S3 and S4 coatings at 700 °C was due to that the Ni atoms in the substrate diffused to the damaged locations of coating and reacted with Mo and O atoms. The migration mechanism of substrate atoms was similar with other reports [36,37]. The HRTEM images of the products on S2 coating surface tested after 700 °C are presented in Fig. 14. The interval of 0.24 nm and 0.47 nm between the lattice fringes in the
HRTEM images can be indexed to the (110) plane of Cr2O3 and the (001) plane of MoO2 respectively, which were further proved by SAED pattern (Fig. 14b) and well matched with the Raman analysis.
4. Conclusions The CrMoN/MoS2 composite coatings containing various Mo contents deposited by dc magnetron sputtering deposition technique showed dense and compact structure and high adhesion to the substrate. As the Mo content increased from 2.3 at.% to 18.5 at.%, the hardness of coating slightly decreased from 16.6 GPa to 12.44 GPa, but the critical load of coating was largely increased from 13.4 N to 32.7 N. With the doping of Mo atoms, the obvious microstructure transformation occurred in the composite coating, which contributed to the improvement of mechanical strength of composite coating. Superior tribological properties of CrMoN/MoS2 composite coating at high temperature by controlling molybdenum content were highlighted. The tribological property of CrMoN/MoS2 coatings showed high dependence of Mo content at high temperatures. The CrMoN/MoS2 composite coating, with 18.5 at.% of Mo addition, showed the lowest wear rate about 6.87 × 10−6 mm3 (N m)−1 at 700 °C. The tribo-layers contained MoO2, MoO3 and Cr2O3 formed in the wear track of CrMoN/MoS2 composite coating, acted a pivotal part in lubrication and wear reduction. 7
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Fig. 14. HRTEM image of products on the surfaces of S2 film after friction test at 700 °C.
Declaration of competing interest [15]
We would like to submit the revised manuscript entitled “Influence of Mo Contents on the Tribological Properties of CrMoN/MoS2 Coatings at 25–700°C″, which we wish to be considered for publication in “Surface and Coatings Technology”. All authors approve its publication and no conflict of interest exists in the submission of this manuscript. I would like to state on behalf of my co-authors that the work described is original research. Neither the entire paper nor any part of its content has been published or has been accepted elsewhere. It is not being submitted to any other journal.
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Acknowledgements The authors acknowledge the financial supports by the National Natural Science Foundation of China (Grant No. 51575505, 51675508) and the CAS “Light of West China” Program as well as the fund of State Key Laboratory of Solid Lubrication (LSL-1610).
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