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Wear resistance and solid lubricity of molybdenum-containing nitride coatings deposited by cathodic arc evaporation
Wear resistance and solid lubricity of molybdenum-containing nitride coatings deposited by cathodic arc evaporation Qi Yang PII: DOI: Reference:
S0257-8972(17)31059-9 doi:10.1016/j.surfcoat.2017.10.026 SCT 22791
To appear in:
Surface & Coatings Technology
Please cite this article as: Qi Yang, Wear resistance and solid lubricity of molybdenumcontaining nitride coatings deposited by cathodic arc evaporation, Surface & Coatings Technology (2017), doi:10.1016/j.surfcoat.2017.10.026
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ACCEPTED MANUSCRIPT Wear Resistance and Solid Lubricity of Molybdenum-Containing Nitride Coatings Deposited by Cathodic Arc Evaporation
Aerospace Portfolio
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Qi Yang
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National Research Council of Canada
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1200 Montreal Road, Ottawa, Ontario, K1A 0R6, Canada
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Abstract
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Molybdenum-containing MoTiN, MoAlTiN, MoCrN and MoZrN coatings were deposited on the 17-4 PH stainless steel substrate by the cathodic arc evaporation technique. Pin-on-disc dry
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sliding tests were performed to investigate their wear resistance and the solid lubricity of the
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coatings. All of these coatings, with MoTiN and MoAlTiN in particular, demonstrated superior
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wear resistance and a significantly lower coefficient of friction against WC-6Co ball material when compared to their corresponding Mo-free TiN, AlTiN, CrN and ZrN monolithic coatings. For example, the specific wear rate of the MoAlTiN coating is only 0.14% of the wear rate of the AlTiN coating, and its coefficient of friction is only 0.28 compared to 0.60 of AlTiN. The excellent tribological performance of the Mo-containing coatings is attributed to the formation of a MoO3 surface layer on the wear tracks through the tribo-oxidation process. Further wear tests of the MoTiN and MoAlTiN coatings against Al2O3 ball material revealed less improvement in wear resistance and reduction of the coefficient of friction. When tested against Si3N4 ball material, both coatings, though still showing noticeably better wear resistance than their corresponding Mo-free coatings, did not demonstrate any beneficial effect of Mo on lowing the coefficient of friction. The scanning electron microscopy and energy dispersive X-ray spectroscopy analyses of the wear track surfaces illustrated the importance of retaining a stable 1
ACCEPTED MANUSCRIPT and sufficiently thick MoO3 surface layer in order to maintain the beneficial effect of Mo on the
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tribological performance of the coatings.
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Keywords: Solid Lubricity; Wear; Molybdenum; Nitride Coatings; Cathodic Arc
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Introduction
Due to the environmental concerns of using lubrication fluids to reduce friction, solid lubricant
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powders, such as graphite and molybdenum (Mo) disulfides, have been used in various
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applications where low friction between two sliding surfaces is needed. However, there are
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several disadvantages limiting the usages of solid lubricant powders, such as low thermal conductivity of the powders that makes heat dissipation from the sliding surface difficult,
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potential significant fluctuations in the coefficient of friction (CoF), performance degradation due to aging and oxidation, and difficulty in replenishment [1]. To address these issues, there are increasing research interests in developing novel coatings with a solid lubricious capability [2]. Several approaches have been adapted to design and optimize solid lubricious coatings through producing coatings with the same chemistry of known solid lubricants such as lamellar-layered transition metal dichalcogenides, MoS2 and WS2 for example, or including a solid lubricant in a composite coating. Responding to the sliding process, the easy-to-shear planes of the lamellar structure orientate parallel to the sliding direction, leading to extremely low friction [ 3 ]. Researches have revealed that MoS2 [4], WS2 [5], metal doped MoS2 [6, 7] or CrN-WS2 [8] coatings exhibited low coefficient of friction in dry and vacuum environments, but were sensitive to attack in a moisture presenting environment due to the tribo-oxidation process. Utilizing 2
ACCEPTED MANUSCRIPT carbon based coatings, such as graphite and diamond like carbon (DLC) coatings, is another approach to producing solid lubricious coatings [9]. DLC is a generic term for amorphous carbon coatings, which can display high hardness, low coefficient of friction and excellent wear
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resistance [10]. The low friction of DLC coatings was attributed to either the formation of a
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carbon-rich transfer layer or the surface graphitization process [11]. However, these coatings are
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very sensitive to environments with oxygen or moisture as their coefficient of friction and wear rate upsurge due to the surface tribo-oxidation process. Also, the high internal stress makes
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adherence of DLC coatings to substrate difficult and limits the coating thickness [12]. Solid
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lubricous coatings can also be designed by incorporating soft easy-to-shear metal element(s) such as Au, Ag and Cu in an oxide, carbide or nitride coating [13,14]. Finally, in-situ formation of
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lubricous oxides, such as MoO3, WO3 and V2O5 in Mo [15], W [16, 17] or V [18,19] containing
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coatings, is effective in reducing friction through the tribo-oxidation process. As these oxides
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have high ionic potentials, the cations are well separated and completely screened by the surrounding oxygen anions, and hence have a reduced ability to chemically interact with other cations. In general, these oxides are soft and easy to shear, resulting in low friction [2]. In the past two decades, Mo had been incorporated in transition metal nitride coatings or AlN coating to improve their tribological performance. Low coefficient of friction and excellent wear resistance were observed in various Mo-alloyed nitride coatings, such as TiMoN [ 20 - 23 ], TiMoCN [24], TiAlMoN [25], TiMoSiN [26], CrMoN [17, 27, 28] and MoAlN [15].
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instance, it was reported that with the increase in Mo content in TiMoN, the coefficient of friction decreased and the wear resistance was significantly improved [20]. It is well accepted that the low friction of Mo-containing coatings is attributed to the formation of a lubricious MoO3 surface layer through tribo-oxidation [20, 26, 29]. However, the majority of the research has been performed on transition metal nitride coatings with a Mo atomic percentage usually less or 3
ACCEPTED MANUSCRIPT significantly less than other metallic elements [25, 26, 29 , 30 ].
Also, the tribological
characteristics of the coatings were investigated through tests frequently with a short sliding distance of 100 meters [22, 29] or less [25, 26, 31]. It would be impossible to reveal the friction
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stability of Mo-containing nitride coatings from such short sliding tests.
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In this study, four coatings, namely MoTiN, MoAlTiN, MoCrN and MoZrN with Mo as the
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dominant metallic element, were fabricated by using the cathodic arc evaporation technique. The
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tribological characteristics and friction stability of the coatings were evaluated through drysliding tests with a long sliding distance of 5000 m. To reveal the effect of Mo on the solid
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lubricity and wear resistance of these coatings against WC-6Co ball material, monolithic TiN, AlTiN, CrN and ZrN were also tested for comparison. Considering that no single lubricious
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coating can provide both low friction and excellent wear resistance over a broad range of
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applications or testing conditions [1], the MoTiN and MoAlTiN coatings, along with the TiN and
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AlTiN coatings, were further dry-sliding tested against different ball materials, i.e. Al2O3 and Si3N4, to reveal whether the two Mo-containing nitride coatings could perform similar to or different from, the performance tested using the WC-6Co ball material. In addition to the measurements of the coefficient of friction and the specific wear rate of each coating, the chemical compositions and morphologies of the wear debris as well as the worn surfaces were investigated to provide comprehensive information about the dry sliding wear process of the Mocontaining coatings.
Experimental
Four Mo-containing nitride coatings, MoTiN, MoAlTiN, MoCrN and MoZrN were deposited on flat discs of a 17-4 PH precipitation-hardening stainless steel substrate using a Metaplas Ionon 4
ACCEPTED MANUSCRIPT MZR-304 cathodic arc coater. Two cathodes were vertically installed on two flanges 180 apart, with pure Mo as one cathode and Ti, Al67Ti33, Cr or Zr as another. Flat substrate discs of 50
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mm in diameter and 3 mm in thickness were mechanically grounded and polished to a mirror finish, followed by ultrasonic cleaning in alcohol and drying with warm air. The discs were held
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by fixtures and then placed on a single-axial rotation carousal, which was rotated at a speed of 2
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rpm during the whole coating process. Prior to coating deposition, the discs were further cleaned by argon ions with a bias of -300 V using an arc enhanced glow discharge (AEGD) technique.
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During the deposition stage, a mixture of Ar and N2 gases was introduced into the chamber with a
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fixed Ar flow rate of 185 sccm and a regulated N2 flow to maintain the chamber pressure at 5 Pa (5E-2 mbar). A current of 160 A was applied on the Mo evaporator, while 80 A was employed
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on another evaporator. A bias voltage of -40 V was applied on the carousal and the deposition
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temperature was maintained at ~ 400C.
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The surface morphology and chemical composition of the coatings were characterized using a Philips XL-30 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). To identify the phase(s) in the coatings, X-ray diffraction (XRD) analysis was performed using a Bruker AXS D8 Discover diffractometer with the Bragg-Brentano configuration and Co K radiation with a wavelength of 1.789Å. Mechanical properties were measured using a CSM Instruments nano-indentation tester with a Berkovich diamond indenter. To reduce the influence from the substrate on the measurements, a maximum load of 50 mN was employed to ensure that the maximum penetration depth of the indenter was less than 1/10 of the coating thickness. Both loading and unloading speeds were set at 100 mN/min. Using the Oliver-Pharr method [32] and assuming a Poisson’s ratio of 0.30, the coating hardness (H) and Young’s modulus (E) were derived from the loading and unloading 5
ACCEPTED MANUSCRIPT curves. The average H and E values from seven measurements were used to represent the coating mechanical properties in this study.
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The tribological properties of the coatings were evaluated using a Teer pin-on-disc tester under
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dry sliding conditions. The tests were performed at room temperature (22 2C) under a
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humidity of 30 5% RH. The MoTiN, MoAlTiN, MoCrN and MoZrN coated discs were tested against a 5 mm diameter WC-6Co ball under a normal load of 10 N and at a sliding speed of 20
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cm/s. The frictional force as a function of sliding distance was recorded. To study the stability of
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the coatings’ coefficient of friction, the total sliding distance was set at 5000 m. In addition, Mofree CrN, ZrN, TiN and AlTiN monolithic coatings with a thickness of 11 µm were deposited
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under similar processing conditions. The Mo-free coatings were tested for comparison with the
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Mo-containing coatings but at different sliding distances, i.e. 200 m for ZrN, 1000 m for AlTiN,
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2000 m for TiN and 50000 m for CrN, to avoid coating perforation. To further study the coating tribological behavior against the different ball materials, MoTiN and MoAlTiN, along with TiN and AlTiN, were tested against the Al2O3 and Si3N4 balls. The morphologies and chemical compositions of the wear scars and debris were analyzed using SEM/EDS to develop a better understanding of the wear mechanisms. The depth profiles along the radial directions across the circular wear tracks were measured using a DekTak 150 surface profilometer. For each wear track, six measurements were performed at evenly spaced locations in order to calculate the average wear track cross-sectional area. The wear volume was calculated based on the wear track diameter of 8 mm and the average cross-sectional area. The specific wear rate was obtained by normalizing the wear volume using the total sliding distance and the normal load applied.
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ACCEPTED MANUSCRIPT Results
Microstructure, Surface roughness and Mechanical properties
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SEM images showing the surface morphologies of the four Mo-containing nitride coatings are
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presented in Figure 1. Circular shaped droplets or macro-particles, a typical defect on cathodic
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arc deposited coatings, were observed with a size ranging from less than one micron to several microns. Some of these poorly adhered droplets, due to high intrinsic stress, could easily detach
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from the coating, leaving craters on the surface [33]. Apparently, these droplets roughened the
dense in areas without droplets.
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coating surface, which had a Ra value in the range of 0.05-0.12 m. Moreover, the coatings were Table 1 lists the chemical compositions of the four Mo-
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containing nitride coatings and shows an atomic percentage of Mo remarkably higher than other
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metallic element(s).
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Figure 2 presents the cross-sectional SEM back scattering electron images of the MoTiN, MoAlTiN, MoCrN and MoZrN coatings, revealing their nanolayered coating structures of alternating light Mo2N layers and dark TiN, AlTiN, CrN or ZrN layers with a modulation period of approximately 100 nm or larger. Consistent with the high Mo contents in the coatings, Mo2N layers are markedly thicker than other constituent layers. All the coatings had an overall dense structure with embedded droplets. The coating thickness values, measured from the crosssections, are also presented in Table 1. The XRD spectra in Figure 3 show that there are two sets of peaks for the Mo-containing coatings, one set from body-centered tetragonal -Mo2N and the other set from rock salt facecentered cubic B1 structured TiN, AlTiN, CrN or ZrN.
It should be noted that both face-
centered cubic γ-Mo2N and body-centered tetragonal -Mo2N can be formed in nanolayered 7
ACCEPTED MANUSCRIPT coatings deposited using the cathodic arc or magnetron sputtering technique [34]. For the coatings with a small modulation period, γ-Mo2N due to the template effect could be stabilized in nanolayered coatings such as TiN/Mo2N and CrN/Mo2N [31]. In this paper, the Mo2N in the
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nanolayered coatings had the tetragonal -Mo2N structure, the same as in the monolithic layered
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Mo2N coating also deposited in this study. The large modulation periods prevented Mo2N from
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forming the same face-centered cubic structure as the TiN, AlTiN, CrN or ZrN layer constituent.
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The coating hardness and Young’s modulus values are presented in Table 2. All the Mocontaining coatings possessed hardness values higher than 26 GPa. Both MoTiN (27.7 GPa) and
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MoZrN (28.2GPa) had hardness values similar to TiN (28.0 GPa) and ZrN (27.2 GPa), respectively, but showed a lower Young’s modulus than their corresponding counterparts.
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However, MoCrN had Young’s modulus value similar to CrN, but with higher hardness.
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Different from other coating systems, MoAlTiN had a hardness value comparable to AlTiN, but
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with a slightly higher Young’s modulus. Tribological Performance 1. Against WC-6Co ball
The CoFs of the coatings as a function of sliding distance are presented in Figure 4, which evidently demonstrates that the Mo-containing coatings, after the initial running-in stage, had much lower CoFs than their corresponding Mo-free coatings. The test results indicate that Mo in the coatings can effectively lead to a reduction in friction against WC-6Co. Also, it needs to be noted that the CoFs of all four Mo-incorporated coatings were relatively stable during the sliding tests for 5000 m. The average CoF values of the coatings over the whole sliding distance are summarized in Figure 5. Among the Mo-free coatings, ZrN had the highest CoF (0.68) while CrN had the lowest one (0.53). Regarding the Mo-containing coatings, however, the highest CoF 8
ACCEPTED MANUSCRIPT was only 0.40 for MoZrN and the lowest was 0.28 for MoAlTiN, with MoCrN having the second lowest valueof 0.295. Furthermore, the CoF dropped significantly from 0.59 for TiN to 0.32 for
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MoTiN, and decreased substantially from ~ 0.60 for AlTiN to 0.28 for MoAlTiN. The images in Figure 6 show the wear debris accumulated around the wear tracks on the disc
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surface after wear testing against WC-6Co. An extensive amount of wear debris was generated
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for all four Mo-free coatings, among which the one tested for a shorter sliding distance produced
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more debris than the ones tested for a longer distance. For instance, the ZrN coating that was tested only for 200 m created the most amount of debris, while the CrN coating, though being
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tested for the longest distance (5000 m), formed markedly less debris than others. All four Mocontaining coatings, particularly MoTiN, after being tested for 5000 m, generated a substantially
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less amount of wear debris than their Mo-free counterparts, demonstrating that the Mo-containing
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coatings have significantly improved wear resistance. Further examination under a SEM revealed
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that the wear debris adjacent to the wear tracks were agglomerates of fine particles, as exemplified in Figure 7. Their chemical compositions measured by the EDS technique are listed in Table 3 and show a high oxygen content in the range of ~ 57-76 at.%. No nitrogen was detected from the wear debris formed on all the coating systems, except that on TiN. Generally, the wear debris are heavily oxidized coating materials mixed with a trace of the ball material, except for the CrN and MoCrN coatings that contained oxidized coating materials with a considerable amount of the ball material. The Mo-free coatings had deep groves on the worn surfaces that were partially covered with wear debris patches, as shown in Figure 8 (a); ZrN had more track areas covered by the debris patches (Figure 8 (b)) than the others.
In contrast, the
worn surfaces of the Mo-containing coatings were smooth and clean (Figures 8 (c) and (d)), with wear debris being smeared over only a small percentage of the wear track surfaces.
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ACCEPTED MANUSCRIPT The wear track profiles were measured after the loose wear debris were removed from the sample surfaces using compressed air, and the results are presented in Figure 9. The Mo-containing coatings, as shown in Figure 9, all exhibited much shallower and narrower wear tracks, when
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compared to their corresponding Mo-free counterparts. ZrN, which was tested only for 200
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meters, had an average maximum wear track depth of 8.71 µm. TiN, AlTiN and CrN, after being
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tested for 2000 m, 1000 m and 5000 m, respectively, also demonstrated deep wear tracks with a corresponding track depth of 8.45, 10.65 and 1.99 µm, respectively. On the contrary, MoTiN,
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MoAlTiN, MoCrN and MoZrN, after being tested for a long sliding distance of 5000 m, only had
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an average maximum wear track depth of 0.29, 0.30, 0.39 and 1.27 µm, respectively. It should be noted that the rough as-deposited coating surfaces, mainly due to the presence of droplets, had
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surface profile peak amplitude normally in the range of 0.2-0.5 µm, which was close to the
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maximum depth of the shallow wear tracks on the Mo-containing coatings. Consequently, it
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would be impossible to accurately measure the wear track profiles on the as-deposited coating surfaces. Therefore, in this study, wear tests were performed on the Mo-containing coated discs that had been tumbled in a tumbler to achieve an improved surface finish of Ra 0.02 - 0.03 µm. The specific wear rates of the coatings, shown Figure 10, reveal that Mo incorporation results in a substantial improvement in wear resistance. ZrN (3.4x10-5 mm3/(N.m)) had the poorest wear resistance among all coatings; CrN, though demonstrating the highest wear resistance among Mofree nitride coatings, had a wear rate (3.0*10-7 mm3/(N.m)) much higher than any of the Mocontaining nitride coatings. For example, the wear rate of MoZrN (7.5 x10-8 mm3/(N.m)) ˗ the worst performing coating amongst the Mo-containing coatings ˗ is nearly one quarter of the wear rate of CrN and only 0.22% of the rate of ZrN; MoTiN and MoAlTiN, the two highest performing coatings, had respective wear rates of 7.5 x10-9 mm3/(N.m) and 1.2x10-8 mm3/(N.m),
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ACCEPTED MANUSCRIPT which are only 0.2% of the wear rate of TiN (3.9 x10-6 mm3/(N.m)) and 0.14% of the rate of AlTiN (8.8 x10-6 mm3/(N.m)), respectively.
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As all the four Mo-containing nitride coatings demonstrated lower friction and much superior wear resistance, when compared to their corresponding Mo-free counterparts, the test results
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imply that Mo can play an important role in improving tribological performance. For further
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comparison, a MoN coating mainly with the tetragonal -Mo2N phase was produced and dry-
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sliding tested against WC-6Co ball material as well. The MoN coating survived a short test duration with a total sliding distance of 500 m (Figure 11 (a)), showing a low CoF of 0.44 and a
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wear rate of 4.46 x10-8 mm3/(N.m), which was lower than those for MoZrN and other Mo-free nitride coatings. These results are in a good agreement with the dry sliding test results of MoN
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and Mo2N coatings showing markedly lower coefficients of friction and smaller wear depth than
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TiN coatings [35]. However, the coating encountered localized coating failures, such as cracking
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and chipping (Figures 11 (b)-(d)) after being tested at sliding distances of 960 m for one test and 1240 m for a repeated test, indicating the brittle nature of the coating. This cracking phenomenon has been also reported on the wear track of MoAlN coatings with various Al contents [36]. Cracking or chipping in these coatings will undoubtedly limit their applications as lubricious coatings. Importantly, it should be pointed out that all the Mo-containing nitride coatings in this study didn’t show any cracking or chipping on their wear tracks during the tests for a sliding distance of 5000 m against WC-6Co. 2. Against Al2O3 ball The CoF curves of the MoTiN and MoAlTiN coatings as a function of sliding distance are presented along with those of TiN and AlTiN in Figure 12; the average CoF values of the coatings over the whole test duration are presented in Figure 13. Evidently, the two Mo11
ACCEPTED MANUSCRIPT containing coatings maintained lower CoFs than their corresponding Mo-free coatings when tested against Al2O3, but the friction reduction was not as significant as against WC-6Co. The CoF dropped from 0.75 for TiN to 0.63 for MoTiN, while it decreased from 0.72 for AlTiN to
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0.50 for MoAlTiN. Additionally, the CoFs of the MoTiN and MoAlTiN coatings against Al2O3
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are higher than those against WC-6Co.
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Them images in Figure 14 show that there is a large amount of wear debris on the TiN and AlTiN
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coatings; MoTiN and MoAlTiN, however, exhibited much lower amount of wear debris around the wear tracks even after being tested for a 5000 m sliding distance. EDS analysis results show
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that the debris consisted of mainly heavily oxidized coating materials mixed with a small amount of Al2O3 ball materials. The wear tracks on MoTiN and MoAlTiN were narrower than those on
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their TiN and AlTiN counterparts. All the worn surfaces were nearly free of wear debris patches. The wear track profiles of TiN, AlTiN, MoTiN and MoAlTiN, presented in Figure 15, show that
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the two Mo-containing nitride coatings had shallower and narrower tracks than the Mo-free coatings. MoTiN and MoAlTiN, as shown in Figure 16, had wear resistances superior to their TiN and AlTiN counterparts, though at a lower extent than compared to the tests against WC-6Co. The wear rate of MoTiN (3.6 x10-8 mm3/(N.m)) is only 4.6% of that for TiN (7.8 x10-7 mm3/(N.m)), and MoAlTiN (3.5 x10-8 mm3/(N.m)) has a wear rate that is only 0.9% of that for AlTiN (3.8 x10-6 mm3/(N.m)). 3. Against Si3N4 ball Different from being tested against WC-6Co and Al2O3, the MoTiN and MoAlTiN coatings, when tested against Si3N4, did not show any reduction in CoF, as illustrated in Figure 17. With an increase in sliding distance, the coatings first exhibited a stable CoF stage with a value of ~ 0.9 for MoTiN and ~ 0.7 for MoAlTiN after the running-in stage. Subsequently, the CoF became 12
ACCEPTED MANUSCRIPT unstable and increased to a markedly higher value (~1.3 or higher). Upon increasing the sliding distance further, the CoF of the MoTiN coating recovered to a relatively stable and lower CoF (~
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0.8), but the MoAlTiN coating remained with a high CoF. The images in Figure 18 show that all the four coatings generated a considerable amount of wear
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debris. The debris, again, were heavily oxidized coating materials mixed with a small amount of
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the Si3N4 ball material. All the worn surfaces were smooth and almost free of wear debris
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patches. As shown Figure 19, while MoTiN had a shallower and narrower wear track than TiN tested for 2000 m, the MoAlTiN coating tested for 5000 m had comparable track width and depth
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as the AlTiN coating tested for 1000 m. The specific wear rates, presented in Figure 20, still demonstrated noticeable improvement in wear resistance of the MoTiN (5.4 x10-7 mm3/(N.m))
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and MoAlTiN (4.5 x10-7 mm3/(N.m)) coatings over the corresponding TiN (6.2 x10-6 mm3/(N.m))
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and AlTiN (1.5 x10-6 mm3/(N.m) coatings, but with a dwindling margin. MoTiN had a wear rate
Discussion
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of only 8.8% of that for TiN; however, MoAlTiN had a wear rate of 30.5% of that for AlTiN.
EDS analysis of the wear debris indicated that the heat generated due to sliding friction could raise the contact surface temperature sufficiently high for oxidation to occur. In this study, the Mo-free nitride coatings, i.e. TiN, ZrN, AlTiN and CrN, when tested against WC-6Co, Al2O3 or Si3N4, all exhibited large CoFs. The heat generated due to sliding contact between the coated surface and the ball led to the tribo-oxidation reaction of the coating materials, forming oxides, such as TiO2 on TiN and ZrO2 on ZrN. As both TiO2 and ZrO2 have low ionic potentials [2], their cations can strongly interact with each other to form strong ionic or covalent bonds, which make them difficult to shear. Consequently, forming a thin oxide layer or wear debris patches of such 13
ACCEPTED MANUSCRIPT oxides on the surface does not provide any lubricious effect, but leads to high CoFs. The complex Al2O3-TiO2 oxide system formed on the AlTiN, also provided high CoFs due to the
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small ionic potential difference between the two oxides [2]. It is well recognized that the tribo-oxidatively formed oxide tribo-layer with low shear strength
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can act as a lubricious medium, resulting in an effective reduction in friction [37]. The research
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studies performed on the tribological characteristics of Mo-containing coatings show evidences
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of forming MoO3 on the dry sliding wear track surface [20, 26, 29]. MoO3 has a low melting point and orthorhombic crystallographic structure that has double layers of distorted edge-sharing
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MoO6 octahedra parallel to (010) planes. Successive layers are bonded together by a weak van der Waals force, therefore leading to low shear strength along (010) planes [38]. MoO3 with this
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structural feature can effectively accommodate the velocity between the friction pair, making the
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soft and non-abrasive MoO3 that is known as a solid lubricant [39, 40].
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In this study, the MoTiN, MoAlTiN, MoCrN and MoZrN coatings did show low and stable CoFs during the long duration of wear testing against WC-6Co. Detailed SEM/EDS examinations were performed on the wear tracks of MoTiN and MoAlTiN over the clean area without any debris patch or loose debris. Since no oxygen peak could be identified in the EDS spectra when using an acceleration voltage of 20 kV, the voltage was reduced to 5 kV in order to increase the sensitivity of the surface analysis. It is evident in Figure 21 that a thin tribo-oxidized surface layer did form on the wear track surface against WC-6Co, as exemplified by the strong oxygen peaks, which led to the observed low friction. In contrast, the clean track surface areas of MoTiN and MoAlTiN that were tested against the Al2O3 sliding counterpart, showed much weaker oxygen peaks, implying that there could be a much thinner tribo-oxidized layer formed on the coating surface; this explains why the effect of Mo on the CoF reduction of the MoTiN and MoAlTiN coatings tested vs. Al2O3 was not as effective compared to the coatings tested against WC-6Co. 14
ACCEPTED MANUSCRIPT Furthermore, the oxygen peaks were barely discernible if the sliding counterpart was changed to Si3N4, indicating the tribo-oxidative layer was extremely thin; correspondingly, the Mocontaining coatings simply were not able to exhibit the beneficial effect of Mo in providing
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lubricity through forming lubricous MoO3 oxide layer. It might be beneficial to the solid lubricity
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of the Mo-containing nitride coatings if they are partially oxidized during the coating deposition
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process through introduction of oxygen gas, but further investigations are needed.
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In fact, there are two concurrent competing processes that occur during wear tests: (1) tribooxidation process to produce a surface oxide layer consisting of MoO3 and (2) sliding wear action
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to remove materials from the worn surface. If the oxide formation rate exceeds the wear rate, a lubricous oxide layer of sufficient thickness is able to remain on the coating surface for solid
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lubrication. Otherwise, reducing friction through the tribo-oxidation process becomes less
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effective or cannot be realized at all. The test results in Figure 10 show that all the four MoFor
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containing nitride coatings had exceptional wear resistance against the WC-6Co ball.
example, both the MoTiN and MoAlTiN coatings had very low wear rates of 7.5 x10-9 and 1.2 x10-8 mm3/(N.m), respectively. These low wear rates helped to retain the tribo-oxidation surface layer so that the low friction between the coatings and the ball could be maintained; the low friction, in return, contributed to the overall low wear rate. In the wear tests against Al2O3, the increased wear rates of ~ 3.5 x10-8 mm3/(N.m) for both MoTiN and MoAlTiN most likely led to the formation of a thinner lubricious oxide layer and hence less effective reduction in friction, when compared to the tests against WC-6Co. The much higher wear rates of MoTiN and MoAlTiN tested against Si3N4 made it even more difficult to form an effective lubricious layer on the surface, which was clearly reflected by the high and nonstable CoFs shown in Figure 20. The different tribological characteristics of the MoTiN and MoAlTiN coatings against different sliding counterparts, as revealed in this study, prove that it is difficult to find a Mo-containing 15
ACCEPTED MANUSCRIPT lubricous coating that can achieve low friction coefficient and excellent wear resistance over a broad range of conditions. Special precautions and more investigations need to be undertaken before the coatings are applied for specific applications. Also, it is important to note that MoO3
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becomes volatile at temperatures higher than 550°C [41], and therefore the sublimation of the
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tribo-chemically formed MoO3 at an elevated temperature could result in deteriorated tribological
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properties of Mo-containing nitride coatings, such as increased CoF and wear damage as being
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observed at a temperature of 650°C for the TiAlMoN coating [29].
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Conclusions
Four Mo-containing nitride coatings, namely MoTiN, MoAlTiN, MoCrN and MoZrN, were
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produced by cathodic arc evaporation. When tested against WC-6Co ball material, the coatings
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demonstrated significantly lower and stable coefficients of friction and exceptionally higher wear
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resistance compared to their corresponding Mo-free TiN, AlTiN, CrN and ZrN monolithic coatings. Their outstanding tribological performance was attributed to the formation of a sufficiently thick MoO3 surface layer through the in-situ tribo-oxidation process. However, the MoTiN and MoAlTiN coatings, when tested against Al2O3, showed less improvements in reducing friction and increasing wear resistance. Furthermore, when tested against Si3N4, though MoTiN and MoAlTiN still exhibited noticeable improvements in wear resistance compared to TiN and AlTiN, they didn’t demonstrate any friction reduction and their coefficients of friction were not stable during the dry sliding tests. These observations suggest that more thorough investigations are needed before these Mo-containing coatings can be fully utilized as dry lubricious coatings for specific applications.
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preparing the metallographic samples.
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10.7
Cr
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Ti 18.0 6.1
17.9
16.1
N 49.1 40.2 50.9 55.1
Thickness, m 5.4 5.6 4.9 7.6
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MoTiN MoAlTiN MoCrN MoZrN
Mo 32.9 43.1 31.2 28.8
atomic % Al Zr
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Table 1. Chemical compositions and thickness values of MoTiN, MoAlTiN, MoCrN and MoZrN coatings
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Table 2. Mechanical properties of the coatings
Coating
H, GPa
E, GPa
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MoCrN
26.3
304
MoZrN
28.2 0.9
MoTiN MoAlTiN
27.7 1.5 31 1.0
E, GPa
CrN
19.9 1.8
318 36
312 13
ZrN
27.2 1.1
475 37
378 23 369 14
TiN AlTiN
28.0 1.1 31.6 1.0
437 17 348 6
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H, GPa
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Table 3. Chemical compositions of wear debris in atomic % Al
Cr
Zr
21.7 23.6
5.0 14.0
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8.1
1.4
N 8.3
O 57.2 67.0 62.1 75.0 70.8 70.1 62.0 75.7
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16.9 21.1 17.4 15.5
W 0.6 0.1 11.4 0.6 1.0 0.8 5.2 0.8
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TiN AlTiN CrN ZrN MoTiN MoAlTiN MoCrN MoZrN
Ti 34.0 11.0
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Mo
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Figure 1. SEM images showing the as-deposited (a) MoTiN, (b) MoAlTiN, (c) MoCrN and (d) MoZrN coatings.
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Figure 2. SEM cross-sectional images showing the nano-layered structure of (a) MoTiN, (b) MoAlTiN, (c) MoCrN and (d) MoZrN coatings.
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Figure 3. XRD spectra of (a) MoTiN, (b) MoAlTiN, (c) MoCrN and (d) MoZrN coatings.
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Figure 4. Plots of the CoF as a function of sliding distance: (a) TiN and MoTiN, (b) AlTiN and MoAlTiN, (c) CrN and MoCrN, and (d) ZrN and MoZrN coatings. Figure 5. Average CoFs of the coatings tested against WC-6Co.
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Figure 6. Images showing wear debris around the wear tracks of the coatings tested against WC6Co . The numbers in the blanks are the sliding distances. All the wear tracks have a diameter of 8 mm.
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Figure 7. Low magnification SEM images showing the wear debris being accumulated adjacent to the wear tracks of (a) AlTiN and (b) MoAlTiN, and high magnification images showing morphologies of the debris at on (c) AlTiN and (d) MoAlTiN after being tested against WC-6Co.
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Figure 8. SEM images of the wear track surfaces of (a) AlTiN, (b) ZrN, (c) MoCrN and (d) MoAlTiN coatings after being tested against WC-6Co. Figure 9. Wear track profiles of (a) TiN and MoTiN, (b) AlTiN and MoAlTiN, (c) CrN and MoCrN, and (d) ZrN and MoZrN coatings after being tested against WC-6Co. Figure 10. Specific wear rates of the coatings tested against WC-6Co. Figure 11. SEM images of the MoN coatings showing (a) a smooth wear track after a test of 500 m, (b) local coating damage after a test of 960 m, (c) coating chipping and (d) cracking after a test of 1240 m. Figure 12. CoF as a function of sliding distance for (a) TiN and MoTiN, and (b) AlTiN and MoAlTiN that were tested against Al2O3. Figure 13. Average CoFs of the coatings tested against Al2O3. Figure 14. Images showing wear debris around the wear tracks of TiN, AlTiN, MoTiN and MoAlTiN coatings tested against Al2O3. The numbers in the parentheses are the sliding distances. All the wear tracks have a diameter of 8 mm. Figure 15. Wear track profiles of (a) TiN and MoTiN, (b) AlTiN and MoAlTiN coatings tested against Al2O3. 21
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Figure 18. Images showing wear debris around the wear tracks of the TiN, AlTiN, MoTiN and MoAlTiN coatings tested against Si3N4. The numbers in the parentheses are the sliding distances. All the wear tracks have a diameter of 8 mm.
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Figure 19. Wear track profiles of (a) TiN and MoTiN, (b) AlTiN and MoAlTiN coatings tested against Si3N4.
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Figure 20. Specific wear rates of the TiN, AlTiN, MoTiN and MoAlTiN coatings tested against Si3N4.
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Figure 21. EDS spectra of (a) MoTiN and (b) MoAlTiN coatings, which were collected at clean wear track locations without wear debris patch or loose debris.
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Highlights
Mo-containing nitride coatings demonstrate improved wear resistance and reduced
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Their tribological characteristics change for different sliding counterpart materials.
Solid lubricity is attributed to the formation of MoO3 surface layer.
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S0257-8972(17)31059-9 doi:10.1016/j.surfcoat.2017.10.026 SCT 22791
To appear in:
Surface & Coatings Technology
Please cite this article as: Qi Yang, Wear resistance and solid lubricity of molybdenumcontaining nitride coatings deposited by cathodic arc evaporation, Surface & Coatings Technology (2017), doi:10.1016/j.surfcoat.2017.10.026
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ACCEPTED MANUSCRIPT Wear Resistance and Solid Lubricity of Molybdenum-Containing Nitride Coatings Deposited by Cathodic Arc Evaporation
Aerospace Portfolio
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Qi Yang
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National Research Council of Canada
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1200 Montreal Road, Ottawa, Ontario, K1A 0R6, Canada
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Abstract
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Molybdenum-containing MoTiN, MoAlTiN, MoCrN and MoZrN coatings were deposited on the 17-4 PH stainless steel substrate by the cathodic arc evaporation technique. Pin-on-disc dry
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sliding tests were performed to investigate their wear resistance and the solid lubricity of the
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coatings. All of these coatings, with MoTiN and MoAlTiN in particular, demonstrated superior
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wear resistance and a significantly lower coefficient of friction against WC-6Co ball material when compared to their corresponding Mo-free TiN, AlTiN, CrN and ZrN monolithic coatings. For example, the specific wear rate of the MoAlTiN coating is only 0.14% of the wear rate of the AlTiN coating, and its coefficient of friction is only 0.28 compared to 0.60 of AlTiN. The excellent tribological performance of the Mo-containing coatings is attributed to the formation of a MoO3 surface layer on the wear tracks through the tribo-oxidation process. Further wear tests of the MoTiN and MoAlTiN coatings against Al2O3 ball material revealed less improvement in wear resistance and reduction of the coefficient of friction. When tested against Si3N4 ball material, both coatings, though still showing noticeably better wear resistance than their corresponding Mo-free coatings, did not demonstrate any beneficial effect of Mo on lowing the coefficient of friction. The scanning electron microscopy and energy dispersive X-ray spectroscopy analyses of the wear track surfaces illustrated the importance of retaining a stable 1
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tribological performance of the coatings.
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Keywords: Solid Lubricity; Wear; Molybdenum; Nitride Coatings; Cathodic Arc
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Introduction
Due to the environmental concerns of using lubrication fluids to reduce friction, solid lubricant
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powders, such as graphite and molybdenum (Mo) disulfides, have been used in various
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applications where low friction between two sliding surfaces is needed. However, there are
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several disadvantages limiting the usages of solid lubricant powders, such as low thermal conductivity of the powders that makes heat dissipation from the sliding surface difficult,
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potential significant fluctuations in the coefficient of friction (CoF), performance degradation due to aging and oxidation, and difficulty in replenishment [1]. To address these issues, there are increasing research interests in developing novel coatings with a solid lubricious capability [2]. Several approaches have been adapted to design and optimize solid lubricious coatings through producing coatings with the same chemistry of known solid lubricants such as lamellar-layered transition metal dichalcogenides, MoS2 and WS2 for example, or including a solid lubricant in a composite coating. Responding to the sliding process, the easy-to-shear planes of the lamellar structure orientate parallel to the sliding direction, leading to extremely low friction [ 3 ]. Researches have revealed that MoS2 [4], WS2 [5], metal doped MoS2 [6, 7] or CrN-WS2 [8] coatings exhibited low coefficient of friction in dry and vacuum environments, but were sensitive to attack in a moisture presenting environment due to the tribo-oxidation process. Utilizing 2
ACCEPTED MANUSCRIPT carbon based coatings, such as graphite and diamond like carbon (DLC) coatings, is another approach to producing solid lubricious coatings [9]. DLC is a generic term for amorphous carbon coatings, which can display high hardness, low coefficient of friction and excellent wear
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resistance [10]. The low friction of DLC coatings was attributed to either the formation of a
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carbon-rich transfer layer or the surface graphitization process [11]. However, these coatings are
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very sensitive to environments with oxygen or moisture as their coefficient of friction and wear rate upsurge due to the surface tribo-oxidation process. Also, the high internal stress makes
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adherence of DLC coatings to substrate difficult and limits the coating thickness [12]. Solid
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lubricous coatings can also be designed by incorporating soft easy-to-shear metal element(s) such as Au, Ag and Cu in an oxide, carbide or nitride coating [13,14]. Finally, in-situ formation of
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lubricous oxides, such as MoO3, WO3 and V2O5 in Mo [15], W [16, 17] or V [18,19] containing
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coatings, is effective in reducing friction through the tribo-oxidation process. As these oxides
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have high ionic potentials, the cations are well separated and completely screened by the surrounding oxygen anions, and hence have a reduced ability to chemically interact with other cations. In general, these oxides are soft and easy to shear, resulting in low friction [2]. In the past two decades, Mo had been incorporated in transition metal nitride coatings or AlN coating to improve their tribological performance. Low coefficient of friction and excellent wear resistance were observed in various Mo-alloyed nitride coatings, such as TiMoN [ 20 - 23 ], TiMoCN [24], TiAlMoN [25], TiMoSiN [26], CrMoN [17, 27, 28] and MoAlN [15].
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instance, it was reported that with the increase in Mo content in TiMoN, the coefficient of friction decreased and the wear resistance was significantly improved [20]. It is well accepted that the low friction of Mo-containing coatings is attributed to the formation of a lubricious MoO3 surface layer through tribo-oxidation [20, 26, 29]. However, the majority of the research has been performed on transition metal nitride coatings with a Mo atomic percentage usually less or 3
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Also, the tribological
characteristics of the coatings were investigated through tests frequently with a short sliding distance of 100 meters [22, 29] or less [25, 26, 31]. It would be impossible to reveal the friction
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stability of Mo-containing nitride coatings from such short sliding tests.
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In this study, four coatings, namely MoTiN, MoAlTiN, MoCrN and MoZrN with Mo as the
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dominant metallic element, were fabricated by using the cathodic arc evaporation technique. The
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tribological characteristics and friction stability of the coatings were evaluated through drysliding tests with a long sliding distance of 5000 m. To reveal the effect of Mo on the solid
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lubricity and wear resistance of these coatings against WC-6Co ball material, monolithic TiN, AlTiN, CrN and ZrN were also tested for comparison. Considering that no single lubricious
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coating can provide both low friction and excellent wear resistance over a broad range of
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applications or testing conditions [1], the MoTiN and MoAlTiN coatings, along with the TiN and
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AlTiN coatings, were further dry-sliding tested against different ball materials, i.e. Al2O3 and Si3N4, to reveal whether the two Mo-containing nitride coatings could perform similar to or different from, the performance tested using the WC-6Co ball material. In addition to the measurements of the coefficient of friction and the specific wear rate of each coating, the chemical compositions and morphologies of the wear debris as well as the worn surfaces were investigated to provide comprehensive information about the dry sliding wear process of the Mocontaining coatings.
Experimental
Four Mo-containing nitride coatings, MoTiN, MoAlTiN, MoCrN and MoZrN were deposited on flat discs of a 17-4 PH precipitation-hardening stainless steel substrate using a Metaplas Ionon 4
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mm in diameter and 3 mm in thickness were mechanically grounded and polished to a mirror finish, followed by ultrasonic cleaning in alcohol and drying with warm air. The discs were held
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by fixtures and then placed on a single-axial rotation carousal, which was rotated at a speed of 2
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During the deposition stage, a mixture of Ar and N2 gases was introduced into the chamber with a
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fixed Ar flow rate of 185 sccm and a regulated N2 flow to maintain the chamber pressure at 5 Pa (5E-2 mbar). A current of 160 A was applied on the Mo evaporator, while 80 A was employed
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on another evaporator. A bias voltage of -40 V was applied on the carousal and the deposition
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temperature was maintained at ~ 400C.
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The surface morphology and chemical composition of the coatings were characterized using a Philips XL-30 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). To identify the phase(s) in the coatings, X-ray diffraction (XRD) analysis was performed using a Bruker AXS D8 Discover diffractometer with the Bragg-Brentano configuration and Co K radiation with a wavelength of 1.789Å. Mechanical properties were measured using a CSM Instruments nano-indentation tester with a Berkovich diamond indenter. To reduce the influence from the substrate on the measurements, a maximum load of 50 mN was employed to ensure that the maximum penetration depth of the indenter was less than 1/10 of the coating thickness. Both loading and unloading speeds were set at 100 mN/min. Using the Oliver-Pharr method [32] and assuming a Poisson’s ratio of 0.30, the coating hardness (H) and Young’s modulus (E) were derived from the loading and unloading 5
ACCEPTED MANUSCRIPT curves. The average H and E values from seven measurements were used to represent the coating mechanical properties in this study.
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The tribological properties of the coatings were evaluated using a Teer pin-on-disc tester under
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dry sliding conditions. The tests were performed at room temperature (22 2C) under a
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humidity of 30 5% RH. The MoTiN, MoAlTiN, MoCrN and MoZrN coated discs were tested against a 5 mm diameter WC-6Co ball under a normal load of 10 N and at a sliding speed of 20
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cm/s. The frictional force as a function of sliding distance was recorded. To study the stability of
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the coatings’ coefficient of friction, the total sliding distance was set at 5000 m. In addition, Mofree CrN, ZrN, TiN and AlTiN monolithic coatings with a thickness of 11 µm were deposited
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under similar processing conditions. The Mo-free coatings were tested for comparison with the
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Mo-containing coatings but at different sliding distances, i.e. 200 m for ZrN, 1000 m for AlTiN,
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2000 m for TiN and 50000 m for CrN, to avoid coating perforation. To further study the coating tribological behavior against the different ball materials, MoTiN and MoAlTiN, along with TiN and AlTiN, were tested against the Al2O3 and Si3N4 balls. The morphologies and chemical compositions of the wear scars and debris were analyzed using SEM/EDS to develop a better understanding of the wear mechanisms. The depth profiles along the radial directions across the circular wear tracks were measured using a DekTak 150 surface profilometer. For each wear track, six measurements were performed at evenly spaced locations in order to calculate the average wear track cross-sectional area. The wear volume was calculated based on the wear track diameter of 8 mm and the average cross-sectional area. The specific wear rate was obtained by normalizing the wear volume using the total sliding distance and the normal load applied.
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Microstructure, Surface roughness and Mechanical properties
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SEM images showing the surface morphologies of the four Mo-containing nitride coatings are
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presented in Figure 1. Circular shaped droplets or macro-particles, a typical defect on cathodic
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arc deposited coatings, were observed with a size ranging from less than one micron to several microns. Some of these poorly adhered droplets, due to high intrinsic stress, could easily detach
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from the coating, leaving craters on the surface [33]. Apparently, these droplets roughened the
dense in areas without droplets.
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coating surface, which had a Ra value in the range of 0.05-0.12 m. Moreover, the coatings were Table 1 lists the chemical compositions of the four Mo-
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containing nitride coatings and shows an atomic percentage of Mo remarkably higher than other
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metallic element(s).
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Figure 2 presents the cross-sectional SEM back scattering electron images of the MoTiN, MoAlTiN, MoCrN and MoZrN coatings, revealing their nanolayered coating structures of alternating light Mo2N layers and dark TiN, AlTiN, CrN or ZrN layers with a modulation period of approximately 100 nm or larger. Consistent with the high Mo contents in the coatings, Mo2N layers are markedly thicker than other constituent layers. All the coatings had an overall dense structure with embedded droplets. The coating thickness values, measured from the crosssections, are also presented in Table 1. The XRD spectra in Figure 3 show that there are two sets of peaks for the Mo-containing coatings, one set from body-centered tetragonal -Mo2N and the other set from rock salt facecentered cubic B1 structured TiN, AlTiN, CrN or ZrN.
It should be noted that both face-
centered cubic γ-Mo2N and body-centered tetragonal -Mo2N can be formed in nanolayered 7
ACCEPTED MANUSCRIPT coatings deposited using the cathodic arc or magnetron sputtering technique [34]. For the coatings with a small modulation period, γ-Mo2N due to the template effect could be stabilized in nanolayered coatings such as TiN/Mo2N and CrN/Mo2N [31]. In this paper, the Mo2N in the
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nanolayered coatings had the tetragonal -Mo2N structure, the same as in the monolithic layered
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Mo2N coating also deposited in this study. The large modulation periods prevented Mo2N from
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forming the same face-centered cubic structure as the TiN, AlTiN, CrN or ZrN layer constituent.
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The coating hardness and Young’s modulus values are presented in Table 2. All the Mocontaining coatings possessed hardness values higher than 26 GPa. Both MoTiN (27.7 GPa) and
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MoZrN (28.2GPa) had hardness values similar to TiN (28.0 GPa) and ZrN (27.2 GPa), respectively, but showed a lower Young’s modulus than their corresponding counterparts.
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However, MoCrN had Young’s modulus value similar to CrN, but with higher hardness.
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Different from other coating systems, MoAlTiN had a hardness value comparable to AlTiN, but
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with a slightly higher Young’s modulus. Tribological Performance 1. Against WC-6Co ball
The CoFs of the coatings as a function of sliding distance are presented in Figure 4, which evidently demonstrates that the Mo-containing coatings, after the initial running-in stage, had much lower CoFs than their corresponding Mo-free coatings. The test results indicate that Mo in the coatings can effectively lead to a reduction in friction against WC-6Co. Also, it needs to be noted that the CoFs of all four Mo-incorporated coatings were relatively stable during the sliding tests for 5000 m. The average CoF values of the coatings over the whole sliding distance are summarized in Figure 5. Among the Mo-free coatings, ZrN had the highest CoF (0.68) while CrN had the lowest one (0.53). Regarding the Mo-containing coatings, however, the highest CoF 8
ACCEPTED MANUSCRIPT was only 0.40 for MoZrN and the lowest was 0.28 for MoAlTiN, with MoCrN having the second lowest valueof 0.295. Furthermore, the CoF dropped significantly from 0.59 for TiN to 0.32 for
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MoTiN, and decreased substantially from ~ 0.60 for AlTiN to 0.28 for MoAlTiN. The images in Figure 6 show the wear debris accumulated around the wear tracks on the disc
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surface after wear testing against WC-6Co. An extensive amount of wear debris was generated
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for all four Mo-free coatings, among which the one tested for a shorter sliding distance produced
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more debris than the ones tested for a longer distance. For instance, the ZrN coating that was tested only for 200 m created the most amount of debris, while the CrN coating, though being
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tested for the longest distance (5000 m), formed markedly less debris than others. All four Mocontaining coatings, particularly MoTiN, after being tested for 5000 m, generated a substantially
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less amount of wear debris than their Mo-free counterparts, demonstrating that the Mo-containing
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coatings have significantly improved wear resistance. Further examination under a SEM revealed
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that the wear debris adjacent to the wear tracks were agglomerates of fine particles, as exemplified in Figure 7. Their chemical compositions measured by the EDS technique are listed in Table 3 and show a high oxygen content in the range of ~ 57-76 at.%. No nitrogen was detected from the wear debris formed on all the coating systems, except that on TiN. Generally, the wear debris are heavily oxidized coating materials mixed with a trace of the ball material, except for the CrN and MoCrN coatings that contained oxidized coating materials with a considerable amount of the ball material. The Mo-free coatings had deep groves on the worn surfaces that were partially covered with wear debris patches, as shown in Figure 8 (a); ZrN had more track areas covered by the debris patches (Figure 8 (b)) than the others.
In contrast, the
worn surfaces of the Mo-containing coatings were smooth and clean (Figures 8 (c) and (d)), with wear debris being smeared over only a small percentage of the wear track surfaces.
9
ACCEPTED MANUSCRIPT The wear track profiles were measured after the loose wear debris were removed from the sample surfaces using compressed air, and the results are presented in Figure 9. The Mo-containing coatings, as shown in Figure 9, all exhibited much shallower and narrower wear tracks, when
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compared to their corresponding Mo-free counterparts. ZrN, which was tested only for 200
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meters, had an average maximum wear track depth of 8.71 µm. TiN, AlTiN and CrN, after being
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tested for 2000 m, 1000 m and 5000 m, respectively, also demonstrated deep wear tracks with a corresponding track depth of 8.45, 10.65 and 1.99 µm, respectively. On the contrary, MoTiN,
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MoAlTiN, MoCrN and MoZrN, after being tested for a long sliding distance of 5000 m, only had
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an average maximum wear track depth of 0.29, 0.30, 0.39 and 1.27 µm, respectively. It should be noted that the rough as-deposited coating surfaces, mainly due to the presence of droplets, had
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surface profile peak amplitude normally in the range of 0.2-0.5 µm, which was close to the
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maximum depth of the shallow wear tracks on the Mo-containing coatings. Consequently, it
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would be impossible to accurately measure the wear track profiles on the as-deposited coating surfaces. Therefore, in this study, wear tests were performed on the Mo-containing coated discs that had been tumbled in a tumbler to achieve an improved surface finish of Ra 0.02 - 0.03 µm. The specific wear rates of the coatings, shown Figure 10, reveal that Mo incorporation results in a substantial improvement in wear resistance. ZrN (3.4x10-5 mm3/(N.m)) had the poorest wear resistance among all coatings; CrN, though demonstrating the highest wear resistance among Mofree nitride coatings, had a wear rate (3.0*10-7 mm3/(N.m)) much higher than any of the Mocontaining nitride coatings. For example, the wear rate of MoZrN (7.5 x10-8 mm3/(N.m)) ˗ the worst performing coating amongst the Mo-containing coatings ˗ is nearly one quarter of the wear rate of CrN and only 0.22% of the rate of ZrN; MoTiN and MoAlTiN, the two highest performing coatings, had respective wear rates of 7.5 x10-9 mm3/(N.m) and 1.2x10-8 mm3/(N.m),
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ACCEPTED MANUSCRIPT which are only 0.2% of the wear rate of TiN (3.9 x10-6 mm3/(N.m)) and 0.14% of the rate of AlTiN (8.8 x10-6 mm3/(N.m)), respectively.
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As all the four Mo-containing nitride coatings demonstrated lower friction and much superior wear resistance, when compared to their corresponding Mo-free counterparts, the test results
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imply that Mo can play an important role in improving tribological performance. For further
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comparison, a MoN coating mainly with the tetragonal -Mo2N phase was produced and dry-
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sliding tested against WC-6Co ball material as well. The MoN coating survived a short test duration with a total sliding distance of 500 m (Figure 11 (a)), showing a low CoF of 0.44 and a
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wear rate of 4.46 x10-8 mm3/(N.m), which was lower than those for MoZrN and other Mo-free nitride coatings. These results are in a good agreement with the dry sliding test results of MoN
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and Mo2N coatings showing markedly lower coefficients of friction and smaller wear depth than
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TiN coatings [35]. However, the coating encountered localized coating failures, such as cracking
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and chipping (Figures 11 (b)-(d)) after being tested at sliding distances of 960 m for one test and 1240 m for a repeated test, indicating the brittle nature of the coating. This cracking phenomenon has been also reported on the wear track of MoAlN coatings with various Al contents [36]. Cracking or chipping in these coatings will undoubtedly limit their applications as lubricious coatings. Importantly, it should be pointed out that all the Mo-containing nitride coatings in this study didn’t show any cracking or chipping on their wear tracks during the tests for a sliding distance of 5000 m against WC-6Co. 2. Against Al2O3 ball The CoF curves of the MoTiN and MoAlTiN coatings as a function of sliding distance are presented along with those of TiN and AlTiN in Figure 12; the average CoF values of the coatings over the whole test duration are presented in Figure 13. Evidently, the two Mo11
ACCEPTED MANUSCRIPT containing coatings maintained lower CoFs than their corresponding Mo-free coatings when tested against Al2O3, but the friction reduction was not as significant as against WC-6Co. The CoF dropped from 0.75 for TiN to 0.63 for MoTiN, while it decreased from 0.72 for AlTiN to
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0.50 for MoAlTiN. Additionally, the CoFs of the MoTiN and MoAlTiN coatings against Al2O3
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are higher than those against WC-6Co.
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Them images in Figure 14 show that there is a large amount of wear debris on the TiN and AlTiN
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coatings; MoTiN and MoAlTiN, however, exhibited much lower amount of wear debris around the wear tracks even after being tested for a 5000 m sliding distance. EDS analysis results show
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that the debris consisted of mainly heavily oxidized coating materials mixed with a small amount of Al2O3 ball materials. The wear tracks on MoTiN and MoAlTiN were narrower than those on
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their TiN and AlTiN counterparts. All the worn surfaces were nearly free of wear debris patches. The wear track profiles of TiN, AlTiN, MoTiN and MoAlTiN, presented in Figure 15, show that
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the two Mo-containing nitride coatings had shallower and narrower tracks than the Mo-free coatings. MoTiN and MoAlTiN, as shown in Figure 16, had wear resistances superior to their TiN and AlTiN counterparts, though at a lower extent than compared to the tests against WC-6Co. The wear rate of MoTiN (3.6 x10-8 mm3/(N.m)) is only 4.6% of that for TiN (7.8 x10-7 mm3/(N.m)), and MoAlTiN (3.5 x10-8 mm3/(N.m)) has a wear rate that is only 0.9% of that for AlTiN (3.8 x10-6 mm3/(N.m)). 3. Against Si3N4 ball Different from being tested against WC-6Co and Al2O3, the MoTiN and MoAlTiN coatings, when tested against Si3N4, did not show any reduction in CoF, as illustrated in Figure 17. With an increase in sliding distance, the coatings first exhibited a stable CoF stage with a value of ~ 0.9 for MoTiN and ~ 0.7 for MoAlTiN after the running-in stage. Subsequently, the CoF became 12
ACCEPTED MANUSCRIPT unstable and increased to a markedly higher value (~1.3 or higher). Upon increasing the sliding distance further, the CoF of the MoTiN coating recovered to a relatively stable and lower CoF (~
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0.8), but the MoAlTiN coating remained with a high CoF. The images in Figure 18 show that all the four coatings generated a considerable amount of wear
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debris. The debris, again, were heavily oxidized coating materials mixed with a small amount of
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the Si3N4 ball material. All the worn surfaces were smooth and almost free of wear debris
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patches. As shown Figure 19, while MoTiN had a shallower and narrower wear track than TiN tested for 2000 m, the MoAlTiN coating tested for 5000 m had comparable track width and depth
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as the AlTiN coating tested for 1000 m. The specific wear rates, presented in Figure 20, still demonstrated noticeable improvement in wear resistance of the MoTiN (5.4 x10-7 mm3/(N.m))
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and MoAlTiN (4.5 x10-7 mm3/(N.m)) coatings over the corresponding TiN (6.2 x10-6 mm3/(N.m))
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and AlTiN (1.5 x10-6 mm3/(N.m) coatings, but with a dwindling margin. MoTiN had a wear rate
Discussion
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of only 8.8% of that for TiN; however, MoAlTiN had a wear rate of 30.5% of that for AlTiN.
EDS analysis of the wear debris indicated that the heat generated due to sliding friction could raise the contact surface temperature sufficiently high for oxidation to occur. In this study, the Mo-free nitride coatings, i.e. TiN, ZrN, AlTiN and CrN, when tested against WC-6Co, Al2O3 or Si3N4, all exhibited large CoFs. The heat generated due to sliding contact between the coated surface and the ball led to the tribo-oxidation reaction of the coating materials, forming oxides, such as TiO2 on TiN and ZrO2 on ZrN. As both TiO2 and ZrO2 have low ionic potentials [2], their cations can strongly interact with each other to form strong ionic or covalent bonds, which make them difficult to shear. Consequently, forming a thin oxide layer or wear debris patches of such 13
ACCEPTED MANUSCRIPT oxides on the surface does not provide any lubricious effect, but leads to high CoFs. The complex Al2O3-TiO2 oxide system formed on the AlTiN, also provided high CoFs due to the
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small ionic potential difference between the two oxides [2]. It is well recognized that the tribo-oxidatively formed oxide tribo-layer with low shear strength
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can act as a lubricious medium, resulting in an effective reduction in friction [37]. The research
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studies performed on the tribological characteristics of Mo-containing coatings show evidences
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of forming MoO3 on the dry sliding wear track surface [20, 26, 29]. MoO3 has a low melting point and orthorhombic crystallographic structure that has double layers of distorted edge-sharing
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MoO6 octahedra parallel to (010) planes. Successive layers are bonded together by a weak van der Waals force, therefore leading to low shear strength along (010) planes [38]. MoO3 with this
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structural feature can effectively accommodate the velocity between the friction pair, making the
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soft and non-abrasive MoO3 that is known as a solid lubricant [39, 40].
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In this study, the MoTiN, MoAlTiN, MoCrN and MoZrN coatings did show low and stable CoFs during the long duration of wear testing against WC-6Co. Detailed SEM/EDS examinations were performed on the wear tracks of MoTiN and MoAlTiN over the clean area without any debris patch or loose debris. Since no oxygen peak could be identified in the EDS spectra when using an acceleration voltage of 20 kV, the voltage was reduced to 5 kV in order to increase the sensitivity of the surface analysis. It is evident in Figure 21 that a thin tribo-oxidized surface layer did form on the wear track surface against WC-6Co, as exemplified by the strong oxygen peaks, which led to the observed low friction. In contrast, the clean track surface areas of MoTiN and MoAlTiN that were tested against the Al2O3 sliding counterpart, showed much weaker oxygen peaks, implying that there could be a much thinner tribo-oxidized layer formed on the coating surface; this explains why the effect of Mo on the CoF reduction of the MoTiN and MoAlTiN coatings tested vs. Al2O3 was not as effective compared to the coatings tested against WC-6Co. 14
ACCEPTED MANUSCRIPT Furthermore, the oxygen peaks were barely discernible if the sliding counterpart was changed to Si3N4, indicating the tribo-oxidative layer was extremely thin; correspondingly, the Mocontaining coatings simply were not able to exhibit the beneficial effect of Mo in providing
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lubricity through forming lubricous MoO3 oxide layer. It might be beneficial to the solid lubricity
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of the Mo-containing nitride coatings if they are partially oxidized during the coating deposition
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process through introduction of oxygen gas, but further investigations are needed.
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In fact, there are two concurrent competing processes that occur during wear tests: (1) tribooxidation process to produce a surface oxide layer consisting of MoO3 and (2) sliding wear action
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to remove materials from the worn surface. If the oxide formation rate exceeds the wear rate, a lubricous oxide layer of sufficient thickness is able to remain on the coating surface for solid
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lubrication. Otherwise, reducing friction through the tribo-oxidation process becomes less
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effective or cannot be realized at all. The test results in Figure 10 show that all the four MoFor
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containing nitride coatings had exceptional wear resistance against the WC-6Co ball.
example, both the MoTiN and MoAlTiN coatings had very low wear rates of 7.5 x10-9 and 1.2 x10-8 mm3/(N.m), respectively. These low wear rates helped to retain the tribo-oxidation surface layer so that the low friction between the coatings and the ball could be maintained; the low friction, in return, contributed to the overall low wear rate. In the wear tests against Al2O3, the increased wear rates of ~ 3.5 x10-8 mm3/(N.m) for both MoTiN and MoAlTiN most likely led to the formation of a thinner lubricious oxide layer and hence less effective reduction in friction, when compared to the tests against WC-6Co. The much higher wear rates of MoTiN and MoAlTiN tested against Si3N4 made it even more difficult to form an effective lubricious layer on the surface, which was clearly reflected by the high and nonstable CoFs shown in Figure 20. The different tribological characteristics of the MoTiN and MoAlTiN coatings against different sliding counterparts, as revealed in this study, prove that it is difficult to find a Mo-containing 15
ACCEPTED MANUSCRIPT lubricous coating that can achieve low friction coefficient and excellent wear resistance over a broad range of conditions. Special precautions and more investigations need to be undertaken before the coatings are applied for specific applications. Also, it is important to note that MoO3
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becomes volatile at temperatures higher than 550°C [41], and therefore the sublimation of the
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tribo-chemically formed MoO3 at an elevated temperature could result in deteriorated tribological
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properties of Mo-containing nitride coatings, such as increased CoF and wear damage as being
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observed at a temperature of 650°C for the TiAlMoN coating [29].
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Conclusions
Four Mo-containing nitride coatings, namely MoTiN, MoAlTiN, MoCrN and MoZrN, were
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produced by cathodic arc evaporation. When tested against WC-6Co ball material, the coatings
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demonstrated significantly lower and stable coefficients of friction and exceptionally higher wear
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resistance compared to their corresponding Mo-free TiN, AlTiN, CrN and ZrN monolithic coatings. Their outstanding tribological performance was attributed to the formation of a sufficiently thick MoO3 surface layer through the in-situ tribo-oxidation process. However, the MoTiN and MoAlTiN coatings, when tested against Al2O3, showed less improvements in reducing friction and increasing wear resistance. Furthermore, when tested against Si3N4, though MoTiN and MoAlTiN still exhibited noticeable improvements in wear resistance compared to TiN and AlTiN, they didn’t demonstrate any friction reduction and their coefficients of friction were not stable during the dry sliding tests. These observations suggest that more thorough investigations are needed before these Mo-containing coatings can be fully utilized as dry lubricious coatings for specific applications.
Acknowledgements 16
ACCEPTED MANUSCRIPT This work was supported by the Factory of the Future Program of the National Research Council of Canada under [Project # A1-006885]. The author would like to thank Ms. Olga Lupandina for
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preparing the metallographic samples.
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10.7
Cr
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Ti 18.0 6.1
17.9
16.1
N 49.1 40.2 50.9 55.1
Thickness, m 5.4 5.6 4.9 7.6
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MoTiN MoAlTiN MoCrN MoZrN
Mo 32.9 43.1 31.2 28.8
atomic % Al Zr
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Coating
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Table 1. Chemical compositions and thickness values of MoTiN, MoAlTiN, MoCrN and MoZrN coatings
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Table 2. Mechanical properties of the coatings
Coating
H, GPa
E, GPa
Coating
MoCrN
26.3
304
MoZrN
28.2 0.9
MoTiN MoAlTiN
27.7 1.5 31 1.0
E, GPa
CrN
19.9 1.8
318 36
312 13
ZrN
27.2 1.1
475 37
378 23 369 14
TiN AlTiN
28.0 1.1 31.6 1.0
437 17 348 6
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H, GPa
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Table 3. Chemical compositions of wear debris in atomic % Al
Cr
Zr
21.7 23.6
5.0 14.0
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11.3 3.1
Co
2.9
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24.4
8.1
1.4
N 8.3
O 57.2 67.0 62.1 75.0 70.8 70.1 62.0 75.7
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16.9 21.1 17.4 15.5
W 0.6 0.1 11.4 0.6 1.0 0.8 5.2 0.8
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TiN AlTiN CrN ZrN MoTiN MoAlTiN MoCrN MoZrN
Ti 34.0 11.0
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Mo
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Figure 1. SEM images showing the as-deposited (a) MoTiN, (b) MoAlTiN, (c) MoCrN and (d) MoZrN coatings.
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Figure 2. SEM cross-sectional images showing the nano-layered structure of (a) MoTiN, (b) MoAlTiN, (c) MoCrN and (d) MoZrN coatings.
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Figure 3. XRD spectra of (a) MoTiN, (b) MoAlTiN, (c) MoCrN and (d) MoZrN coatings.
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Figure 4. Plots of the CoF as a function of sliding distance: (a) TiN and MoTiN, (b) AlTiN and MoAlTiN, (c) CrN and MoCrN, and (d) ZrN and MoZrN coatings. Figure 5. Average CoFs of the coatings tested against WC-6Co.
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Figure 6. Images showing wear debris around the wear tracks of the coatings tested against WC6Co . The numbers in the blanks are the sliding distances. All the wear tracks have a diameter of 8 mm.
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Figure 7. Low magnification SEM images showing the wear debris being accumulated adjacent to the wear tracks of (a) AlTiN and (b) MoAlTiN, and high magnification images showing morphologies of the debris at on (c) AlTiN and (d) MoAlTiN after being tested against WC-6Co.
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Figure 8. SEM images of the wear track surfaces of (a) AlTiN, (b) ZrN, (c) MoCrN and (d) MoAlTiN coatings after being tested against WC-6Co. Figure 9. Wear track profiles of (a) TiN and MoTiN, (b) AlTiN and MoAlTiN, (c) CrN and MoCrN, and (d) ZrN and MoZrN coatings after being tested against WC-6Co. Figure 10. Specific wear rates of the coatings tested against WC-6Co. Figure 11. SEM images of the MoN coatings showing (a) a smooth wear track after a test of 500 m, (b) local coating damage after a test of 960 m, (c) coating chipping and (d) cracking after a test of 1240 m. Figure 12. CoF as a function of sliding distance for (a) TiN and MoTiN, and (b) AlTiN and MoAlTiN that were tested against Al2O3. Figure 13. Average CoFs of the coatings tested against Al2O3. Figure 14. Images showing wear debris around the wear tracks of TiN, AlTiN, MoTiN and MoAlTiN coatings tested against Al2O3. The numbers in the parentheses are the sliding distances. All the wear tracks have a diameter of 8 mm. Figure 15. Wear track profiles of (a) TiN and MoTiN, (b) AlTiN and MoAlTiN coatings tested against Al2O3. 21
ACCEPTED MANUSCRIPT Figure 16. Specific wear rates of the TiN, AlTiN, MoTiN and MoAlTiN coatings tested against Al2O3. Figure 17. CoF as a function of sliding distance for (a) TiN and MoTiN, and (b) AlTiN and MoAlTiN that were tested against Si3N4.
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Figure 18. Images showing wear debris around the wear tracks of the TiN, AlTiN, MoTiN and MoAlTiN coatings tested against Si3N4. The numbers in the parentheses are the sliding distances. All the wear tracks have a diameter of 8 mm.
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Figure 19. Wear track profiles of (a) TiN and MoTiN, (b) AlTiN and MoAlTiN coatings tested against Si3N4.
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Figure 20. Specific wear rates of the TiN, AlTiN, MoTiN and MoAlTiN coatings tested against Si3N4.
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Figure 21. EDS spectra of (a) MoTiN and (b) MoAlTiN coatings, which were collected at clean wear track locations without wear debris patch or loose debris.
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Highlights
Mo-containing nitride coatings demonstrate improved wear resistance and reduced
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Their tribological characteristics change for different sliding counterpart materials.
Solid lubricity is attributed to the formation of MoO3 surface layer.
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