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Structure and tribological performances of CrAlSiN coatings with different Si percentages in seawater
Tribology International 115 (2017) 591–599
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Structure and tribological performances of CrAlSiN coatings with different Si percentages in seawater S.Q. Sun a, b, Y.W. Ye a, c, **, Y.X. Wang a, *, M.Q. Liu a, X. Liu b, J.L. Li a, L.P. Wang a a Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China b School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China c Corrosion & Protection Center, Key Laboratory of Corrosion and Protection of Ministry of Education, University of Science and Technology Beijing, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: CrAlSiN coating Friction Wear Seawater
Nanocomposite CrAlSiN coatings with different Si percentages were deposited using a multi-arc ion plating technique. The coating microstructures were characterized by SEM, XRD, XPS and TEM. The mechanical properties and tribological performances were measured. The results demonstrated that the microstructure of the CrAlSiN coatings was composed of (Cr, Al) N crystallites and an amorphous Si3N4 phase. With increasing Si content, the thickness of the as-deposited coating increased gradually, and the hardness, toughness and adhesion force first increased and then decreased. The CrAlSiN coating containing 5.5 at.% Si exhibited the optimal tribological properties in seawater. The significant improvement in the tribological performances is closely related to the formation of self-lubricating SiO2 and the enhancement of comprehensive properties.
1. Introduction As ocean exploitation has developed into the blue revolution, a growing number of studies have concentrated on high-performance materials for marine environments, which are necessary for the reliable operation of marine equipment [1–3]. Certain moving components, such as valves and gears in marine equipment, including pumps, hydraulic systems, and floating cranes, operate directly in the seawater environment, which creates severe damage from wear and corrosion [4–6]. The life, safety and reliability of the mechanical components in seawater all depend strongly on their tribological and corrosion properties [7]. In most cases, this type of damage in seawater is much more severe than either corrosion or wear alone due to the interaction between wear and corrosion. Thus, the modern marine industry requires satisfactory antiwear materials for use in seawater environments. Coating materials have been widely used in many fields, including the marine, manufacturing, automotive, aerospace, and electronics industries, because the designed coatings impart new functions or better performance than substrates. Due to their favorable mechanical properties and satisfactory anti-wear performance, CrN-based coatings have become one of the most attractive working surfaces for many moving
parts [2,8,9]. Monticelli et al. [4] studied the corrosion and tribocorrosion behaviors of thermally sprayed coatings and thermally sprayed/ nanoscale multilayer CrN/NbN coatings that were deposited on steel specimens in 3.5% NaCl solutions, and the results revealed that the cermet coatings suffered severe corrosion under tribocorrosion conditions, whereas the duplex nanoscale multilayer CrN/NbN/WC-12Co coating afforded superior corrosion resistance both in the absence and in the presence of wear. Wang et al. [5] reported that a CrN coating exhibited better corrosion resistance than TiN and TiAlN coatings in a 1 M H2SO4 solution. Frutos et al. [6] noted that a duplex treatment consisting of plasma nitriding and CrN coating by physical vapor deposition (PVD) could significantly enhance both the mechanical properties and electrochemical behavior of AISI 304 steel. However, the application of CrN coating is limited due to the low hardness and high friction resistance [10–12], especially in tribology field. In order to improve the mechanical and tribological properties, some scholars had attempted to increase the hardness and reduce the friction resistance of CrN coatings by doping a third-party element of Ti, Al, V, Si, C [11–17]. For instance, Beliardouh et al. [15] studied the mechanical and tribological properties of CrAlN coating after Al was doped into CrN coating, and found that the formation of mix aluminum and chromium oxides could effectively
* Corresponding author. ** Corresponding author. Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail addresses: [email protected] (Y.W. Ye), [email protected] (Y.X. Wang). http://dx.doi.org/10.1016/j.triboint.2017.06.038 Received 12 May 2017; Received in revised form 10 June 2017; Accepted 24 June 2017 Available online 26 June 2017 0301-679X/© 2017 Elsevier Ltd. All rights reserved.
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coatings. A loading rate of 200 N/min, loading scale of 0–120 N and trace length of 3 mm were used as the test parameters. The corrosion behaviors of the deposited coatings were measured using a 273A electrochemical workstation (Solartron Analytical) in a seawater solution. Before measurement of the open circuit potential, the specimens were immersed in the electrolyte for 30 min to establish the steady state potential.
improve the mechanical and tribological properties. Liu et al. [18] investigated the wear performance of (nc-AlTiN)/(a-Si3N4) coating and (nc-AlCrN)/(a-Si3N4) coating in dry and minimum quantity lubrication (MQL) conditions, and pointed out that high oxidation resistance and hardness of coatings were beneficial to improve the wear performance. Polcar et al. [19] comparatively discussed the oxidation resistance of CrAlN and AlCrSiN coatings, and indicated that the AlCrSiN showed a better high oxidation resistance than that of CrAlN coating. Chen et al. [20] systemically reported the thermal conductivity of TiN, TiAlCN, TiAlN, AlTiN, TiAlSiN and CrAlSiN coatings, and showed that the CrAlSiN coating presented the lowest thermal conductivity, which attributed to the solid solution of Si in CrAlN and the reducation of grain size or the formation of amorphous phases. Meanwhile, the CrAlSiN coating demonstrated better corrosion resistance and wear resistance than most other CrN-based coatings due to the incorporation of Al and Si elements, particularly Si [21]. Though the research that Si content influenced the structure and properties of CrAlSiN coating was common, while the investigations about tribological performances of CrAlSiN coatings in seawater were lack. Therefore, this paper presented a detailed study on the tribological performance of CrAlSiN coatings containing various Si contents in an artificial seawater environment. The main objective was to explore the tribological mechanism and optimize the tribological performance of CrAlSiN coatings in a seawater environment, which would be significant for the potential protecting application of advanced CrAlSiN coatings in marine equipment.
2.3. Tribological tests Tribological tests of the CrAlSiN coatings were conducted on planar surfaces using the reciprocating ball-on-disc module Rtec tribometer (Rtec Instruments, USA). Measurements were performed in an artificial seawater environment at room temperature (25 ± 5 C) and a relative humidity (RH) of 45 ± 5%. The artificial seawater was prepared according to standard ASTMD 1141–98, and its chemical composition is listed in Table 1. WC balls with a diameter of 6 mm were used as a counterpart material for the planar test. The measurement parameters were as follows: the speed, load, sliding length, frequency, and sliding time were 1.2 m/min, 10 N, 5 mm, 2 Hz, and 90 min, respectively. After each test, the morphologies of the wear tracks were observed using a Zeiss large-chamber scanning electron microscope (EV018, Germany) equipped with an INCA x-sight module (EDS). The wear surfaces of the coatings were first analyzed using a stylus profilometer. Six traces were collected on each worn surface using an Alpha-Step IQ profilometer (Alpha-Step IQ, USA) to achieve wear depths and cross-sectional areas by taking average measurements along the wear track. The volume V of the coating loss during friction was evaluated. The wear rate of the coating was calculated based on the classical equation:
2. Experimental details 2.1. Coating preparation
w¼
CrAlSiN coatings were deposited on 316L stainless steel (30 mm 20 mm 2 mm) and single-crystal silicon substrates using a multi-arc ion plating system (Hauzer Flexicoat 850). Prior to coating deposition, all of the substrates were cleaned continuously for half an hour using acetone and 98% ethanol ultrasonic, respectively. The chamber was pumped down to a pressure of approximately 4 103 Pa. The substrates were then cleaned by Arþ bombardment for 2 min with negative bias voltages of 900 V, 1100 V and 1200 V to remove any oxides and contaminants on the surface. The CrAlSiN coatings were prepared using Cr and AlSi targets. To deposit CrAlSixN coatings with varying Si percentages, the Al and Si contents of the AlSi targets were chosen to be 9:1 8:2, 7:3, 6:4, and 5:5, which are labeled M1, M2, M3, M4, and M5, respectively. Deposition of the Cr interlayer was conducted for 20 min under a voltage of 40 V, a target current of 60 A and an Ar flow rate of 350 sccm. A deposition pressure of 0.04 mbar with an N2 flow rate of 600 sccm was employed for 160 min to prepare the CrAlSiN coating.
V FS
(1)
where F is the load applied, S is the distance of the sliding, and V is the volume of the coating loss. 3. Results and discussion 3.1. Microstructures of the CrAlSiN coatings Table 2 listed the chemical components of the CrAlSiN coatings with varying Si contents in the AlSi target, which were measured by XPS. The CrAlSiN coatings contained predominantly Cr, Al, Si, N, C and O. The presence of O indicated that partial oxidation occurred on the surface, and the existence of C implied some carbon pollution in the contact area. As a result of varying the Al and Si contents of the AlSi targets, the Si content increased significantly from 2.87 to 15.61 at.%, and the Al content decreased from 28.15 to 15.87 at.%. However, the Cr and N contents remained nearly unchanged at 29 and 33 at.%, respectively. This finding indicated that the conclusion showed a good agreement with experimental design. The surface morphologies of the as-deposited coatings were depicted in Fig. 1. Many microparticles were observed on the surface of each coating, indicating the presence of holes and droplets that were inherent to deposition by cathodic arc [22,23] and originated from excess ions from the targets [24]. Through EDS analysis, the surface droplets were determined to contain Cr, Al, Si, N and C, as illustrated in Fig. 1(f). The number of these defects decreased with increasing Si content. The crosssectional morphologies of the as-deposited coatings were revealed in Fig. 2. The thickness of the coatings clearly increased from 3.49 to
2.2. Coating characterization The crystal structure was investigated using grazing incidence X-ray diffraction (XRD, Bruker D8 X-ray facility) with Cu Kα radiation. The cross-sectional microstructure of the coatings was investigated using a field emission scanning electron microscope (FE-SEM, FEI Quanta FEG 250, Japan). The chemical compositions of the coatings were measured by X-ray photoelectron spectroscopy (XPS) with a Kratos spectrometer (AXIS UTLRADLD, Japan). The microstructure was studied in detail by TEM, high-resolution TEM (HR-TEM) (Tecnai F20, USA). The hardness (H) and elastic modulus (E) were measured using an MTS Nano Indenter G200 system equipped with a Berkovich indenter by the load–depth-sensing nanoindentation method and the continuous stiffness measurement (CSM) mode. To obtain a reliable mean value and standard deviation, at least 6 points were tested for each sample. A CSM Revetest tester equipped with a diamond cone (radius of 200 μm, cone angle of 120 ) was employed to determine the scratch adhesion of the
Table 1 Composition of seawater (g/L).
592
Solution Concentration
NaCl 24.53
Na2SO4 4.09
MgCl2 5.20
CaCl2 1.16
SrCl2 0.025
Solution Concentration
KCl 0.695
NaHCO3 0.201
KBr 0.101
H3BO3 0.027
NaF 0.003
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CrAlSiN coatings [25]. Meanwhile, three main peaks at 37.7 , 43.6 , and 63.3 corresponded to reflections (111), (200) and (220) of cubic-CrN, respectively. In addition, three low-intensity signal at 37.1 , 44.8 and 51.2 attributed to (002), (200) and (102) of the hexagonal-AlN, respectively. ICCD cards revealed that the peaks of hexagonal AlN and cubic CrN were also close to this position, which could create difficulties in interpreting the experimental data. With the increase of Si content, the peaks exhibited a broadening and weakening trend as a whole, which indicated the formation of a fine-grained structure and the decrease of crystallize size, this was due to the incorporation of amorphous Si3N4 in the coatings. At the same time, the development of the crystal phase was disturbed by the amorphous Si3N4, causing the nitride grains to grow discontinuously and forming a general nitride mix of aluminum and chromium, which could effectively affect the property of coating [26]. An investigation of the chemical states of the CrAlSiN coatings was performed by XPS. Fig. 4 depicted the Cr 2p, N 1s, Si 2p and Al 2p XPS spectra of the CrAlSiN coatings with varying Si contents. As shown in Fig. 4(a), the Cr 2p3/2 and Cr 2p1/2 peaks at 575.3 eV/586.3 eV and 576.5 eV/584.5 eV corresponded to Cr2O3 and CrN, respectively [27]. In Fig. 4(b), the Al 2p peaks at 73.9 eV corresponded to an AlN phase. The Si 2p binding energies of the CrAlSiN coatings were listed in Fig. 4(c), and
Table 2 The chemical components of CrAlSiN coatings at different Si contents. Coatings
M1 M2 M3 M4 M5
Si (at.%)
2.87 5.52 8.40 11.71 15.61
(at.%) N
Si3N4
33.59 32.13 33.07 33.47 32.41
16.81 18.20 28.37 34.22 44.85
Cr (at.%)
Al (at.%)
C (at.%)
O (at.%)
27.94 28.58 29.46 29.48 30.43
30.15 24.49 20.93 16.76 15.87
5.02 3.88 2.80 3.06 2.51
5.44 4.40 5.34 5.51 4.17
6.03 μm with increasing Si content, which could be explained by the high deposition rate of Si. Additionally, the microstructure of the coatings changed from a columnar feature to a glass feature as the Si content increased to the highest level. This structural change was closely related to the formation of new phases and indicated that formation of an amorphous phase was favored and dominated inside the composite coatings with increasing Si content. Fig. 3 displayed the XRD analysis of the as-deposited coatings. Obviously, regardless of the Si content, the same sequence of phases was observed, and a peak corresponding to crystalline silicon nitride was not found, which indicated amorphous phase of Si3N4 was formed on
Fig. 1. Surface SEM images of CrAlSiN coatings at different Si contents.
Fig. 2. The cross-sectional SEM micrographs of CrAlSiN coatings at different Si contents: (a) M1, (b) M2, (c) M3, (d) M4, (e) M5. 593
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results of the XRD and XPS analyses. In addition, the SAED pattern in Fig. 5(c) revealed the presence of polycrystalline phases of CrN and AlN in which the (111), (200), and (220) reflections of CrN and the (002), (200), and (102) reflections of AlN could be identified. The perfect composite structure that included amorphous and nanocrystalline phases played a positive role in the mechanical properties, corrosion resistance and tribological performance. 3.2. Mechanical properties of the CrAlSiN coatings The nanohardness and elastic modulus of the CrAlSiN coatings were determined by nanoindention and the results were provided in Fig. 6 and Table 3. With Si content increased from 2.87 to 5.52 at. %, the hardness of coating showed a slightly increase trend until 5.52 at. %, where the hardness reached a maximum value of approximately 37.9 GPa. This enhancement of hardness was attributed to the solid solution hardening effect created by the replacement of Cr atoms with Al atoms and the grain refinement. The resultant lattice distortion and stress area could increase the difficulty of dislocation propagation. The formation of the nanocomposite structure may also contributed to the enhancement of hardness. The structure consists of crystalline nanograins embedded in an amorphous matrix. Thus, a continuous rise in the amorphous fraction with increasing silicon concentration was evident in the data provided in Table 2. When the Si content exceeded 5.52 at. %, the content of the amorphous phase increased sharply, which leaded directly to a downtrend in the hardness of coating, this attributed to the low diffusion rate of Si3N4 in coating [25], which was well agreement with Park et al. [29]. In addition, the toughness was a physical quantity that measures the ability of a material to resist crack initiation and propagation, which was proportional to H/E and H3/E2, where H was the hardness and E was the modulus [30]. A nanoindentation tester was used to measure the H and E values, and the H/E and H3/E2 ratios were calculated and are listed in Table 3. The H/E and H3/E2 ratios of M2 were approximately 0.078 and 0.229 GPa, respectively, which indicated that the M2 coating exhibited the best resistance against elastic strain to failure and the more favorable plastic deformation performance. In the other words, the M2 coating owned a better toughness compared to other coatings. The scratch morphologies of these coatings were displayed in Fig. 7. The position of the first crack was detected and defined as the critical load Lc of the coatings. For the M1 coating, the first crack appeared at 42 N. When the silicon content first increased, the Lc of the coating exhibited an ascending trend up to that of coatings M2 and M3, in which
Fig. 3. XRD spectra of CrAlSiN coatings at different Si contents.
two peaks are found at 100.5 eV and 102.1 eV, which were known to be the binding energies of SiO2 and Si3N4, respectively. Therefore, the formation of a Si3N4 phase was confirmed. The content of Si3N4 in the CrAlSiN coatings was evaluated by fitting the Si 2p spectra; the results were provided in Table 2. With increasing Si content, the Si3N4 content increased from 16.81 to 44.85 at.%. In Fig. 4(d), the N 1s peaks were decomposed to two peaks at 397.8 eV and 396.4 eV, which corresponded to Si3N4 and AlN (CrN), respectively. From the XPS results, it was important to note that the coatings were contaminated with oxygen, which was frequently reported for various nitride coatings [28] and might be correlated to oxidation during sample handling and storage in air. To investigate the structure in more detail, HR-TEM was conducted, and the resulting images were depicted together with the corresponding selected area electron diffraction (SAED) pattern of the M3 coating in Fig. 5. As illustrated in Fig. 5(a) and (b), the coating exhibited a typical nanocomposite structure in which nanocrystalline compounds were embedded in the amorphous Si3N4 matrix, and the amorphous Si3N4 around the nanocrystalline CrN and AlN boundaries exhibited a finegrained crystalline structure. These features were consistent with the
Fig. 4. The XPS spectra of CrAlSiN coatings at different Si contents: (a) Cr 2p; (b) Al 2p; (c) Si 2p; (d) N 1s. 594
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Fig. 5. TEM, HRTEM images and corresponding SAED pattern of the M3 coating: (a) (b) HRTEM images, (c) SAED pattern.
corrosion current density for each coating was listed in Table 4. The corrosion current density value (icorr) of the CrAlSiN coatings clearly presented a descending trend with increasing silicon content, and the M5 coating exhibited the lowest value of 5.69 107 A/cm2. The reason was that the higher nucleation site density of a dense structure in a passive film creates higher passive layer fraction, which formed a lower passive current density. Thus, permeation of seawater could be prevented efficiently by using a dense structure, which could enhance the anticorrosion property of the coating. Moreover, defects accelerated the speed of corrosion. Certain distinct defects (e.g., pinholes and penetrating holes) visible in Fig. 2 were presented in the as-fabricated coatings. Thus, microcracks might appear on the surface due to a combination of the cyclic stress and water erosion, leading to spallation or delamination in areas with weak points. Additionally, the M2 coating exhibited the highest polarization potential value of 0.3 V that was accompanied by a passivation phenomenon, indicating the formation of a passivation layer on the coating surface that protected the substrate from exposure to the ambient environment for a short period.
40
Hardness (GPa)
35 30 25 20 15
M1
M2
M3
M4
M5
10 5 0
0
100 200 300 400 500 600 700 800 900 1000
Displacement Into Surface (nm) Fig. 6. The nanohardness of CrAlSiN coatings at different Si contents.
3.4. Tribological performances of the CrAlSiN coatings Table 3 The H, E, H/E and H3/E2 of the coatings. Coating
M1
M2
M3
M4
M5
H (GPa) E (GPa) H/E H3/E2 (GPa)
36.2 492.4 0.073 0.192
37.9 488.9 0.078 0.229
35.1 472.4 0.074 0.0192
30.9 425.5 0.073 0.165
26.6 388.0 0.069 0.125
Friction curves of the CrAlSiN coatings in seawater were provided in Fig. 9. According to the friction curves, all of the as-deposited coatings exhibited running-in periods, although the lengths of the running-in periods were different. The coatings with proper silicon contents exhibited a shorter running-in period. As for the more important stable periods, the COF of the M1 coating was approximately 0.20, which was the highest COF value among the coatings. When the Si content increased from 2.87 to 8.40 at.%, the COF of coating showed a descending trend. Subsequently, the COF of the CrAlSiN coatings increased gradually with increasing Si content from 8.40 to 15.61 at.%. In addition, the wear rates of the CrAlSiN coatings with varying Si contents upon sliding against WC balls in seawater were illustrated in Fig. 10. Similar to the COF, the wear rate first decreased and then sharply increased. When the silicon content was approximately 5.52 at.%, the wear rate decreased to 3 107 mm3 N1 m1, which was the lowest value. However, the wear rate increased by an order of magnitude when the silicon content was further increased. To understand the wear process in detail, Fig. 11 depicted the crosssectional profiles of wear tracks on the CrAlSiN coatings containing different Si contents in seawater. The max wear depth of the M1 coating was nearly 0.85 μm, and the depth decreased to 0.7 μm in sample M2. However, when the content of Si exceeded 5.52 at.% (M2 coating), the
Lc reached its highest value of approximately 55 N. When the silicon content was further increased, the Lc of the coatings demonstrated a descending trend. The adhesion strength between the coating and substrate was an important property in wear-resistant protective coatings. If stripping of the coating occurs during operation, the protection capabilities of the coating would be greatly reduced, and the cracked debris could also cause severe abrasive wear in the friction system. Therefore, coatings with a high level of adhesion force were essential for improving the service life and reliability. 3.3. Corrosion performances of the CrAlSiN coatings The investigation of the potentiodynamic polarization curves of the CrAlSiN coatings in a seawater solution was depicted in Fig. 8, and the 595
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1.5
0.40
1.0
0.35
0.5
Friction coefficient
E (V/SCE)
Fig. 7. The scratch morphologies of CrAlSiN coatings at different Si contents.
M1 M2 M3 M4 M5
0.0
-0.5
M2 M5
M3
0.25 0.20 0.15 0.10
-1.0 -1.5
M1 M4
0.30
0.05
-9
-8
-7
-6
2
-5
-4
-3
logi (A/cm )
0
1000
2000
3000
Time(s)
4000
5000
6000
Fig. 9. Friction curves of the CrAlSiN coatings in seawater. Fig. 8. the potentiodynamic polarization curves of the CrAlSiN coatings in a seawater solution.
Table 4 The corrosion current densities of CrAlSiN coatings at different Si contents. Coating Icorr (10
7
2
A/cm )
M1
M2
M3
M4
M5
9.20
7.35
6.71
6.10
5.69
max depth value exhibited an increasing trend, implying that the proper Si content in the coatings had a positive effect on the wear resistance in seawater. In addition, noted that the max wear depth of the M5 coating reached 6.9 μm, which was larger than the thickness of this coating in Fig. 2 and indicated that the surface of the coating partially failed. SEM and EDS analyses were conducted to investigate the wear surface of these samples, and the corresponding results were depicted in the insert of Fig. 12. Using the M2 and M5 coatings as examples, the wear surface of the M2 coating was relatively smooth with little debris, as shown clearly in Fig. 12(a). EDS analysis revealed that the debris did not contain tungsten, indicating no detectable adhesion wear. Several micropits formed at the center of the wear track due to corrosion by seawater. Fig. 12(b) revealed that the main elements in the M2 coating were Cr, Al, Si, O and N, and the debris did not contain Fe, indicating that the M2 coating was not worn away. Na, Mg and W were observed by EDS, implying that elements transferred from the seawater and counterpart to the coating surface. In addition, Shan et al. [3] determined that calcium ions and magnesium ions could react with carbonate and hydroxide to
Fig. 10. Wear rates of the CrAlSiN coatings in seawater.
form calcium carbonate and magnesium hydroxide, which could act as a lubricant. For the M5 coating, as Fig. 12(c) depicted, a significant amount of abrasive dust was observed on the edge of the wear track due to debris being caught in the dimple, and this dust then filled the pits in the friction process. White particles were observed on the wear track, showing that salt precipitated from seawater and effectively filled the pits of the M5 coating. The EDS results indicated significant wear of the coating, which was further supported by the depth profiles in which the depth of the
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molecule to pass through, and the adhesion strength between the asprepared coatings and the substrates could decrease as a result, which would contribute to delamination or spallation of the areas that contain weak points. Microcracks primarily intersected with the surface, leading to wear-loss [31]. Seawater contained a high concentration of Cl ions, which was highly destructive to coatings because it caused erosion. In the friction process, this erosion enhanced coating delamination, resulting in the formation of a clean surface track that was exposed to the corrosive environment. Wear-loss increased in a corrosive medium because it promoted additional defects and accelerated the corrosion speed [32]. The wear surface of the coatings was demonstrated to contain oxygen in Fig. 12(b) and (d), which indicated that a tribochemical reaction occurred. Fig. 13 presented the XPS analysis of oxygen on the M2 coating surface before and after the tribochemical reaction. The spectra were fitted by resolution into their component Gaussian line shapes to identify the various bonding schemes of oxygen. As shown in Fig. 13(a), before friction occurred, the peaks at 320.2 eV and 532.1 eV corresponded to Cr2O3 and SiO2. However, in Fig. 13(b), after friction occurred, Cr2O3 at 529.4 eV, Al2O3 at 531.6 eV, and SiO2 at 533 eV were observed after tribochemical reactions occurred on the coating surface. Oxygen reacts with Al/Cr/Si to form Al2O3/Cr2O3/SiO2 oxidation films during the wear process. Generally, these films could effectively minimize contact between the slider and the coating, leading to improved wear resistance. According to a previous study, the tribochemical reaction occurred when the CrAlSiN coating was tested at room temperature [33]. In addition, the sliding surface temperature increased with increasing time in a seawater environment, which promoted the following reactions: 2CrN þ 3H2O ¼ Cr2O3 þ 2NH3
(2)
Si3N4 þ 6H2O ¼ 3SiO2 þ 4NH3
(3)
Fig. 11. The cross-sectional profiles of wear tracks on the CrAlSiN coatings containing different Si contents in seawater.
2AlN þ 3H2O ¼ Al2O3 þ 2NH3
(4)
wear track was deeper than any other coating. The EDS analysis also revealed the presence of Cr, Al, Si, N, Fe, Mo, Ni and O in the wear track in Fig. 12(d), which indicated that the coatings were fatigue flaked. On the sliding surface of the M5 coating, the uplifts were deformed by the influence of a recurring sliding action and microcracks were created by the plastic deformation. Formation of the penetrating microcracks would result in further deformation upon their extension and propagation parallel to the surface at any depth, which might then create throughcoating channels for water molecules. The channels allowed water
Upon increasing the percentage of silicon, the COF decreased because SiO2 self-lubricating films formed [34]. SiO2 could react with water to form silica gel, whose mud structure effectively reduced wear and friction. Nevertheless, the experimental results were different. For example, the M5 coating with the highest content of Si3N4 was worn out, whereas the M2 coating with a relatively lower Si3N4 content exhibited perfect tribological properties. With increasing silicon content, the structure gradually became denser and thicker, and the synergistic actions of both mechanical stress and corrosion also played an important role in the friction and wear processes. Under mechanical stress, a coating with low
Fig. 12. SEM and EDS images of wear tracks on the CrAlSiN coatings in seawater: (a) (b) M2, (c) (d) M5. 597
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synergistic action of both mechanical stress and corrosion in seawater.
Intensity (arb. units)
a-O 1s
Acknowledgments This study was supported by the National Natural Science Foundation of China (grant no. 51475449), the National Basic Research Program of China (973 Program, grant no. 2014CB643302), the Gansu Science and Technology Minister (1604GKCA005) and the Key Research and Development Program of Jiangsu Province (grant no. BE2016115). We gratefully acknowledge these organizations for their financial support.
532.1 eV 530.2 eV
References 520
524
528
532
536
Binding Energy (eV)
[1] Wang YX, Zhang S, Lee J-W, Lew WS, Li B. Influence of bias voltage on the hardness and toughness of CrAlN coatings via magnetron sputtering. Surf Coat Technol 2012; 206:5103–7. [2] Kong JZ, Hou TJ, Wang QZ, Yin L, Zhou F, Zhou ZF, et al. Influence of titanium or aluminum doping on the electrochemical properties of CrN coatings in artificial seawater. Surf Coat Technol 2016;307:118–24. [3] Chen H, Ye Y, Wang Y, Li J, Zhou S, Lu X. Tribological properties of uniform and gradient CrCN coatings in seawater. Rare Metal Mater Eng 2016;45:2036–40. [4] Zum Gahr KH, Mathieu M, Brylka B. Friction control by surface engineering of ceramic sliding pairs in water. Wear 2007;263:920–9. [5] Jeong N, Jwa E, Kim C, Choi JY, Nam J-y, Hwang KS, et al. One-pot large-area synthesis of graphitic filamentous nanocarbon-aligned carbon thin layer/carbon nanotube forest hybrid thin films and their corrosion behaviors in simulated seawater condition. Chem Eng J 2017;314:69–79. [6] Shan L, Wang Y, Li J, Jiang X, Chen J. Architecture of multilayer Cr/CrN coatings for wear protection in seawater: Cr/CrN ratio and total thickness. Tribol Trans 2015;58:914–23. [7] Shan L, Wang Y, Li J, Li H, Wu X, Chen J. Tribological behaviours of PVD TiN and TiCN coatings in artificial seawater. Surf Coat Technol 2013;226:40–50. [8] Guan X, Wang Y, Zhang G, Jiang X, Wang L, Xue Q. Microstructures and properties of Zr/CrN multilayer coatings fabricated by multi-arc ion plating. Tribol Int 2017; 106:78–87. [9] Shan L, Wang Y, Zhang Y, Zhang Q, Xue Q. Tribocorrosion behaviors of PVD CrN coated stainless steel in seawater. Wear 2016;362:97–104. [10] Shah HN, Jayaganthan R, Kaur D. Influence of silicon content on the microstructure and hardness of CrN coatings deposited by reactive magnetron sputtering. Mater Chem Phys 2010;121:567–71. [11] Martinez E, Sanjines R, Karimi A. Mechanical properties of nanocomposite and multilayered Cr–Si–N sputtered thin films. Surf Coat Technol 2004;180:570–4. [12] Ye Y, Wang Y, Chen H, Li J, Yao Y, Wang CT. Doping carbon to improve the tribological performance of CrN coatings in seawater. Tribol Int 2015;90:362–71. [13] Wang Q, Zhou F, Yan J. Evaluating mechanical properties and crack resistance of CrN, CrTiN, CrAlN and CrTiAlN coatings by nanoindentation and scratch tests. Surf Coat Technol 2016;285:203–13. [14] Panjan P, Drnovsek A, Kovac J, Gselman P, Boncina T, Paskvale S, et al. Oxidation resistance of CrN/(Cr,V)N hard coatings deposited by DC magnetron sputtering. Thin Solid Films 2015;591:323–9. [15] Beliardouh NE, Bouzid K, Nouveau C, Tlili B, Walock MJ. Tribological and electrochemical performances of Cr/CrN and Cr/CrN/CrAlN multilayer coatings deposited by RF magnetron sputtering. Tribol Int 2015;82:443–52. [16] Gilewicz A, Warcholinski B. Tribological properties of CrCN/CrN multilayer coatings. Tribol Int 2014;80:34–40. [17] Ye Y, Wang Y, Wang C, Li J, Yao Y. An analysis on tribological performance of CrCN coatings with different carbon contents in seawater. Tribol Int 2015;91:131–9. [18] Liu Z, An Q, Xu J, Chen M, Han S. Wear performance of (nc-AlTiN)/(a-Si3N4) coating and (nc-AlCrN)/(a-Si3N4) coating in high-speed machining of titanium alloys under dry and minimum quantity lubrication (MQL) conditions. Wear 2013; 305:249–59. [19] Polcar T, Cavaleiro A. High-temperature tribological properties of CrAlN, CrAlSiN and AlCrSiN coatings. Surf Coat Technol 2011;206:1244–51. [20] Chen HW, Chan YC, Lee JW, Duh JG. Oxidation behavior of Si-doped nanocomposite CrAlSiN coatings. Surf Coat Technol 2010;205:1189–94. [21] Lin CH, Duh JG. Electrochemical impedance spectroscopy (EIS) study on corrosion performance of CrAlSiN coated steels in 3.5 wt.% NaCl solution. Surf Coat Technol 2009;204:784–7. [22] Gåhlin R, Bromark M, Hedenqvist P, Hogmark S, Håkansson G. Properties of TiN and CrN coatings deposited at low temperature using reactive arc-evaporation. Surf Coat Technol 1995;76–77:174–80. [23] Boxman RL. SG. Macroparticle contamination in cathodic arc coatings: generation, transport and control. Surf Coat Technol 1992;52:39–50. [24] Polcar T, Cavaleiro A. High temperature properties of CrAlN, CrAlSiN and AlCrSiN coatings-Structure and oxidation. Mater Chem Phys 2011;129:195–201. [25] Barshilia HC, Deepthi B, Rajam KS. Deposition and characterization of CrN/Si3N4 and CrAlN/Si3N4 nanocomposite coatings prepared using reactive DC unbalanced magnetron sputtering. Surf Coat Technol 2007;201:9468–75. [26] Polcar T, Vitu T, Sondor J, Cavaleiro A. Tribological performance of CrAlSiN coatings at high temperatures. Plasma Process Polym 2009;6:935–40. [27] Ho WY, Shen CH, Chang CL, Wang DY. Corrosion behaviors of Cr(N,O)/CrN multilayered coatings by cathodic arc deposition. Surf Coat Technol 2007;202:745–9.
540
Intensity (arb. units)
b-O 1s 531.6 eV
533 eV 529.4 eV
520
524
528
532
536
540
Binding Energy (eV) Fig. 13. The XPS analysis of oxygen on the M2 coating surface before (a) and after (b) the tribochemical reaction.
hardness and toughness was liable to deform or even crack, and the chloride ions in seawater could then easily entered the crack to induce the corrosion; in turn, this mechanism promoted expansion of the crack and further accelerated wear. The increased wear triggers more defects and attracts further corrosion, thereby initiating a self-enhancing cycle. In addition, even though silica gel functions as a lubricant, the tribochemical products Cr2O3 and Al2O3 had high hardness and were therefore difficult to deform and could scratch the surface, thereby forming microcracks and further advancing wear. From the above discussion, it could be concluded that the proper silicon content could significantly enhance the comprehensive performance of CrAlSiN coatings. 4. Conclusions In this paper, CrAlSiN coatings containing different silicon contents were successfully prepared using a multi-arc ion plating technique. The relation between the silicon content and tribological performance of the CrAlSiN coatings in seawater was systematically studied. The results demonstrate than the CrAlSiN coatings were characterized by a mixture of nano-crystallites and amorphous composite microstructures of Cr(Al)N embedded in a Si3N4 amorphous matrix. During friction and wear tests, the COF and wear rate first decreased and then increased with increasing silicon content. The CrAlSiN coating containing 5.52 at.% Si possessed the best comprehensive properties: the hardness of 37.9 GPa, the adhesion force of 55 N, the COF of 0.13 and the wear rate of 3 107 mm3/ Nm in seawater. The high hardness and toughness had strong inhibitory effects on the initiation and propagation of cracks when the coating was subjected to an external force, which could effectively resist the 598
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Tribology International 115 (2017) 591–599 [32] Shan L, Wang Y, Li J, Chen J. Effect of N2 flow rate on microstructure and mechanical properties of PVD CrNx coatings for tribological application in seawater. Surf Coat Technol 2014;242:74–82. [33] Xu J. KK. Formation of tribochemical layer of ceramics sliding in water and its role for low friction. Wear 2000;245:61–75. [34] Zhang S, Wang L, Wang Q, Li M. A superhard CrAlSiN superlattice coating deposited by multi-arc ion plating: I. Microstructure and mechanical properties. Surf Coat Technol 2013;214:160–7.
[28] Vladescu A, Braic V, Braic M, Balaceanu M. Arc plasma deposition of TiSiN/Ni nanoscale multilayered coatings. Mater Chem Phys 2013;138:500–6. [29] Park I-W, Kang DS, Moore JJ, Kwon SC, Rha JJ, Kim KH. Microstructures, mechanical properties, and tribological behaviors of Cr–Al–N, Cr–Si–N, and Cr–Al–Si–N coatings by a hybrid coating system. Surf Coat Technol 2007;201: 5223–7. [30] Musil J, Jirout M. Toughness of hard nanostructured ceramic thin films. Surf Coat Technol 2007;201:5148–52. [31] Wang J, Chen J, Chen B, Yan F, Xue Q. Wear behaviors and wear mechanisms of several alloys under simulated deep-sea environment covering seawater hydrostatic pressure. Tribol Int 2012;56:38–46.
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Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Structure and tribological performances of CrAlSiN coatings with different Si percentages in seawater S.Q. Sun a, b, Y.W. Ye a, c, **, Y.X. Wang a, *, M.Q. Liu a, X. Liu b, J.L. Li a, L.P. Wang a a Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China b School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China c Corrosion & Protection Center, Key Laboratory of Corrosion and Protection of Ministry of Education, University of Science and Technology Beijing, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: CrAlSiN coating Friction Wear Seawater
Nanocomposite CrAlSiN coatings with different Si percentages were deposited using a multi-arc ion plating technique. The coating microstructures were characterized by SEM, XRD, XPS and TEM. The mechanical properties and tribological performances were measured. The results demonstrated that the microstructure of the CrAlSiN coatings was composed of (Cr, Al) N crystallites and an amorphous Si3N4 phase. With increasing Si content, the thickness of the as-deposited coating increased gradually, and the hardness, toughness and adhesion force first increased and then decreased. The CrAlSiN coating containing 5.5 at.% Si exhibited the optimal tribological properties in seawater. The significant improvement in the tribological performances is closely related to the formation of self-lubricating SiO2 and the enhancement of comprehensive properties.
1. Introduction As ocean exploitation has developed into the blue revolution, a growing number of studies have concentrated on high-performance materials for marine environments, which are necessary for the reliable operation of marine equipment [1–3]. Certain moving components, such as valves and gears in marine equipment, including pumps, hydraulic systems, and floating cranes, operate directly in the seawater environment, which creates severe damage from wear and corrosion [4–6]. The life, safety and reliability of the mechanical components in seawater all depend strongly on their tribological and corrosion properties [7]. In most cases, this type of damage in seawater is much more severe than either corrosion or wear alone due to the interaction between wear and corrosion. Thus, the modern marine industry requires satisfactory antiwear materials for use in seawater environments. Coating materials have been widely used in many fields, including the marine, manufacturing, automotive, aerospace, and electronics industries, because the designed coatings impart new functions or better performance than substrates. Due to their favorable mechanical properties and satisfactory anti-wear performance, CrN-based coatings have become one of the most attractive working surfaces for many moving
parts [2,8,9]. Monticelli et al. [4] studied the corrosion and tribocorrosion behaviors of thermally sprayed coatings and thermally sprayed/ nanoscale multilayer CrN/NbN coatings that were deposited on steel specimens in 3.5% NaCl solutions, and the results revealed that the cermet coatings suffered severe corrosion under tribocorrosion conditions, whereas the duplex nanoscale multilayer CrN/NbN/WC-12Co coating afforded superior corrosion resistance both in the absence and in the presence of wear. Wang et al. [5] reported that a CrN coating exhibited better corrosion resistance than TiN and TiAlN coatings in a 1 M H2SO4 solution. Frutos et al. [6] noted that a duplex treatment consisting of plasma nitriding and CrN coating by physical vapor deposition (PVD) could significantly enhance both the mechanical properties and electrochemical behavior of AISI 304 steel. However, the application of CrN coating is limited due to the low hardness and high friction resistance [10–12], especially in tribology field. In order to improve the mechanical and tribological properties, some scholars had attempted to increase the hardness and reduce the friction resistance of CrN coatings by doping a third-party element of Ti, Al, V, Si, C [11–17]. For instance, Beliardouh et al. [15] studied the mechanical and tribological properties of CrAlN coating after Al was doped into CrN coating, and found that the formation of mix aluminum and chromium oxides could effectively
* Corresponding author. ** Corresponding author. Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail addresses: [email protected] (Y.W. Ye), [email protected] (Y.X. Wang). http://dx.doi.org/10.1016/j.triboint.2017.06.038 Received 12 May 2017; Received in revised form 10 June 2017; Accepted 24 June 2017 Available online 26 June 2017 0301-679X/© 2017 Elsevier Ltd. All rights reserved.
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coatings. A loading rate of 200 N/min, loading scale of 0–120 N and trace length of 3 mm were used as the test parameters. The corrosion behaviors of the deposited coatings were measured using a 273A electrochemical workstation (Solartron Analytical) in a seawater solution. Before measurement of the open circuit potential, the specimens were immersed in the electrolyte for 30 min to establish the steady state potential.
improve the mechanical and tribological properties. Liu et al. [18] investigated the wear performance of (nc-AlTiN)/(a-Si3N4) coating and (nc-AlCrN)/(a-Si3N4) coating in dry and minimum quantity lubrication (MQL) conditions, and pointed out that high oxidation resistance and hardness of coatings were beneficial to improve the wear performance. Polcar et al. [19] comparatively discussed the oxidation resistance of CrAlN and AlCrSiN coatings, and indicated that the AlCrSiN showed a better high oxidation resistance than that of CrAlN coating. Chen et al. [20] systemically reported the thermal conductivity of TiN, TiAlCN, TiAlN, AlTiN, TiAlSiN and CrAlSiN coatings, and showed that the CrAlSiN coating presented the lowest thermal conductivity, which attributed to the solid solution of Si in CrAlN and the reducation of grain size or the formation of amorphous phases. Meanwhile, the CrAlSiN coating demonstrated better corrosion resistance and wear resistance than most other CrN-based coatings due to the incorporation of Al and Si elements, particularly Si [21]. Though the research that Si content influenced the structure and properties of CrAlSiN coating was common, while the investigations about tribological performances of CrAlSiN coatings in seawater were lack. Therefore, this paper presented a detailed study on the tribological performance of CrAlSiN coatings containing various Si contents in an artificial seawater environment. The main objective was to explore the tribological mechanism and optimize the tribological performance of CrAlSiN coatings in a seawater environment, which would be significant for the potential protecting application of advanced CrAlSiN coatings in marine equipment.
2.3. Tribological tests Tribological tests of the CrAlSiN coatings were conducted on planar surfaces using the reciprocating ball-on-disc module Rtec tribometer (Rtec Instruments, USA). Measurements were performed in an artificial seawater environment at room temperature (25 ± 5 C) and a relative humidity (RH) of 45 ± 5%. The artificial seawater was prepared according to standard ASTMD 1141–98, and its chemical composition is listed in Table 1. WC balls with a diameter of 6 mm were used as a counterpart material for the planar test. The measurement parameters were as follows: the speed, load, sliding length, frequency, and sliding time were 1.2 m/min, 10 N, 5 mm, 2 Hz, and 90 min, respectively. After each test, the morphologies of the wear tracks were observed using a Zeiss large-chamber scanning electron microscope (EV018, Germany) equipped with an INCA x-sight module (EDS). The wear surfaces of the coatings were first analyzed using a stylus profilometer. Six traces were collected on each worn surface using an Alpha-Step IQ profilometer (Alpha-Step IQ, USA) to achieve wear depths and cross-sectional areas by taking average measurements along the wear track. The volume V of the coating loss during friction was evaluated. The wear rate of the coating was calculated based on the classical equation:
2. Experimental details 2.1. Coating preparation
w¼
CrAlSiN coatings were deposited on 316L stainless steel (30 mm 20 mm 2 mm) and single-crystal silicon substrates using a multi-arc ion plating system (Hauzer Flexicoat 850). Prior to coating deposition, all of the substrates were cleaned continuously for half an hour using acetone and 98% ethanol ultrasonic, respectively. The chamber was pumped down to a pressure of approximately 4 103 Pa. The substrates were then cleaned by Arþ bombardment for 2 min with negative bias voltages of 900 V, 1100 V and 1200 V to remove any oxides and contaminants on the surface. The CrAlSiN coatings were prepared using Cr and AlSi targets. To deposit CrAlSixN coatings with varying Si percentages, the Al and Si contents of the AlSi targets were chosen to be 9:1 8:2, 7:3, 6:4, and 5:5, which are labeled M1, M2, M3, M4, and M5, respectively. Deposition of the Cr interlayer was conducted for 20 min under a voltage of 40 V, a target current of 60 A and an Ar flow rate of 350 sccm. A deposition pressure of 0.04 mbar with an N2 flow rate of 600 sccm was employed for 160 min to prepare the CrAlSiN coating.
V FS
(1)
where F is the load applied, S is the distance of the sliding, and V is the volume of the coating loss. 3. Results and discussion 3.1. Microstructures of the CrAlSiN coatings Table 2 listed the chemical components of the CrAlSiN coatings with varying Si contents in the AlSi target, which were measured by XPS. The CrAlSiN coatings contained predominantly Cr, Al, Si, N, C and O. The presence of O indicated that partial oxidation occurred on the surface, and the existence of C implied some carbon pollution in the contact area. As a result of varying the Al and Si contents of the AlSi targets, the Si content increased significantly from 2.87 to 15.61 at.%, and the Al content decreased from 28.15 to 15.87 at.%. However, the Cr and N contents remained nearly unchanged at 29 and 33 at.%, respectively. This finding indicated that the conclusion showed a good agreement with experimental design. The surface morphologies of the as-deposited coatings were depicted in Fig. 1. Many microparticles were observed on the surface of each coating, indicating the presence of holes and droplets that were inherent to deposition by cathodic arc [22,23] and originated from excess ions from the targets [24]. Through EDS analysis, the surface droplets were determined to contain Cr, Al, Si, N and C, as illustrated in Fig. 1(f). The number of these defects decreased with increasing Si content. The crosssectional morphologies of the as-deposited coatings were revealed in Fig. 2. The thickness of the coatings clearly increased from 3.49 to
2.2. Coating characterization The crystal structure was investigated using grazing incidence X-ray diffraction (XRD, Bruker D8 X-ray facility) with Cu Kα radiation. The cross-sectional microstructure of the coatings was investigated using a field emission scanning electron microscope (FE-SEM, FEI Quanta FEG 250, Japan). The chemical compositions of the coatings were measured by X-ray photoelectron spectroscopy (XPS) with a Kratos spectrometer (AXIS UTLRADLD, Japan). The microstructure was studied in detail by TEM, high-resolution TEM (HR-TEM) (Tecnai F20, USA). The hardness (H) and elastic modulus (E) were measured using an MTS Nano Indenter G200 system equipped with a Berkovich indenter by the load–depth-sensing nanoindentation method and the continuous stiffness measurement (CSM) mode. To obtain a reliable mean value and standard deviation, at least 6 points were tested for each sample. A CSM Revetest tester equipped with a diamond cone (radius of 200 μm, cone angle of 120 ) was employed to determine the scratch adhesion of the
Table 1 Composition of seawater (g/L).
592
Solution Concentration
NaCl 24.53
Na2SO4 4.09
MgCl2 5.20
CaCl2 1.16
SrCl2 0.025
Solution Concentration
KCl 0.695
NaHCO3 0.201
KBr 0.101
H3BO3 0.027
NaF 0.003
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CrAlSiN coatings [25]. Meanwhile, three main peaks at 37.7 , 43.6 , and 63.3 corresponded to reflections (111), (200) and (220) of cubic-CrN, respectively. In addition, three low-intensity signal at 37.1 , 44.8 and 51.2 attributed to (002), (200) and (102) of the hexagonal-AlN, respectively. ICCD cards revealed that the peaks of hexagonal AlN and cubic CrN were also close to this position, which could create difficulties in interpreting the experimental data. With the increase of Si content, the peaks exhibited a broadening and weakening trend as a whole, which indicated the formation of a fine-grained structure and the decrease of crystallize size, this was due to the incorporation of amorphous Si3N4 in the coatings. At the same time, the development of the crystal phase was disturbed by the amorphous Si3N4, causing the nitride grains to grow discontinuously and forming a general nitride mix of aluminum and chromium, which could effectively affect the property of coating [26]. An investigation of the chemical states of the CrAlSiN coatings was performed by XPS. Fig. 4 depicted the Cr 2p, N 1s, Si 2p and Al 2p XPS spectra of the CrAlSiN coatings with varying Si contents. As shown in Fig. 4(a), the Cr 2p3/2 and Cr 2p1/2 peaks at 575.3 eV/586.3 eV and 576.5 eV/584.5 eV corresponded to Cr2O3 and CrN, respectively [27]. In Fig. 4(b), the Al 2p peaks at 73.9 eV corresponded to an AlN phase. The Si 2p binding energies of the CrAlSiN coatings were listed in Fig. 4(c), and
Table 2 The chemical components of CrAlSiN coatings at different Si contents. Coatings
M1 M2 M3 M4 M5
Si (at.%)
2.87 5.52 8.40 11.71 15.61
(at.%) N
Si3N4
33.59 32.13 33.07 33.47 32.41
16.81 18.20 28.37 34.22 44.85
Cr (at.%)
Al (at.%)
C (at.%)
O (at.%)
27.94 28.58 29.46 29.48 30.43
30.15 24.49 20.93 16.76 15.87
5.02 3.88 2.80 3.06 2.51
5.44 4.40 5.34 5.51 4.17
6.03 μm with increasing Si content, which could be explained by the high deposition rate of Si. Additionally, the microstructure of the coatings changed from a columnar feature to a glass feature as the Si content increased to the highest level. This structural change was closely related to the formation of new phases and indicated that formation of an amorphous phase was favored and dominated inside the composite coatings with increasing Si content. Fig. 3 displayed the XRD analysis of the as-deposited coatings. Obviously, regardless of the Si content, the same sequence of phases was observed, and a peak corresponding to crystalline silicon nitride was not found, which indicated amorphous phase of Si3N4 was formed on
Fig. 1. Surface SEM images of CrAlSiN coatings at different Si contents.
Fig. 2. The cross-sectional SEM micrographs of CrAlSiN coatings at different Si contents: (a) M1, (b) M2, (c) M3, (d) M4, (e) M5. 593
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results of the XRD and XPS analyses. In addition, the SAED pattern in Fig. 5(c) revealed the presence of polycrystalline phases of CrN and AlN in which the (111), (200), and (220) reflections of CrN and the (002), (200), and (102) reflections of AlN could be identified. The perfect composite structure that included amorphous and nanocrystalline phases played a positive role in the mechanical properties, corrosion resistance and tribological performance. 3.2. Mechanical properties of the CrAlSiN coatings The nanohardness and elastic modulus of the CrAlSiN coatings were determined by nanoindention and the results were provided in Fig. 6 and Table 3. With Si content increased from 2.87 to 5.52 at. %, the hardness of coating showed a slightly increase trend until 5.52 at. %, where the hardness reached a maximum value of approximately 37.9 GPa. This enhancement of hardness was attributed to the solid solution hardening effect created by the replacement of Cr atoms with Al atoms and the grain refinement. The resultant lattice distortion and stress area could increase the difficulty of dislocation propagation. The formation of the nanocomposite structure may also contributed to the enhancement of hardness. The structure consists of crystalline nanograins embedded in an amorphous matrix. Thus, a continuous rise in the amorphous fraction with increasing silicon concentration was evident in the data provided in Table 2. When the Si content exceeded 5.52 at. %, the content of the amorphous phase increased sharply, which leaded directly to a downtrend in the hardness of coating, this attributed to the low diffusion rate of Si3N4 in coating [25], which was well agreement with Park et al. [29]. In addition, the toughness was a physical quantity that measures the ability of a material to resist crack initiation and propagation, which was proportional to H/E and H3/E2, where H was the hardness and E was the modulus [30]. A nanoindentation tester was used to measure the H and E values, and the H/E and H3/E2 ratios were calculated and are listed in Table 3. The H/E and H3/E2 ratios of M2 were approximately 0.078 and 0.229 GPa, respectively, which indicated that the M2 coating exhibited the best resistance against elastic strain to failure and the more favorable plastic deformation performance. In the other words, the M2 coating owned a better toughness compared to other coatings. The scratch morphologies of these coatings were displayed in Fig. 7. The position of the first crack was detected and defined as the critical load Lc of the coatings. For the M1 coating, the first crack appeared at 42 N. When the silicon content first increased, the Lc of the coating exhibited an ascending trend up to that of coatings M2 and M3, in which
Fig. 3. XRD spectra of CrAlSiN coatings at different Si contents.
two peaks are found at 100.5 eV and 102.1 eV, which were known to be the binding energies of SiO2 and Si3N4, respectively. Therefore, the formation of a Si3N4 phase was confirmed. The content of Si3N4 in the CrAlSiN coatings was evaluated by fitting the Si 2p spectra; the results were provided in Table 2. With increasing Si content, the Si3N4 content increased from 16.81 to 44.85 at.%. In Fig. 4(d), the N 1s peaks were decomposed to two peaks at 397.8 eV and 396.4 eV, which corresponded to Si3N4 and AlN (CrN), respectively. From the XPS results, it was important to note that the coatings were contaminated with oxygen, which was frequently reported for various nitride coatings [28] and might be correlated to oxidation during sample handling and storage in air. To investigate the structure in more detail, HR-TEM was conducted, and the resulting images were depicted together with the corresponding selected area electron diffraction (SAED) pattern of the M3 coating in Fig. 5. As illustrated in Fig. 5(a) and (b), the coating exhibited a typical nanocomposite structure in which nanocrystalline compounds were embedded in the amorphous Si3N4 matrix, and the amorphous Si3N4 around the nanocrystalline CrN and AlN boundaries exhibited a finegrained crystalline structure. These features were consistent with the
Fig. 4. The XPS spectra of CrAlSiN coatings at different Si contents: (a) Cr 2p; (b) Al 2p; (c) Si 2p; (d) N 1s. 594
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Fig. 5. TEM, HRTEM images and corresponding SAED pattern of the M3 coating: (a) (b) HRTEM images, (c) SAED pattern.
corrosion current density for each coating was listed in Table 4. The corrosion current density value (icorr) of the CrAlSiN coatings clearly presented a descending trend with increasing silicon content, and the M5 coating exhibited the lowest value of 5.69 107 A/cm2. The reason was that the higher nucleation site density of a dense structure in a passive film creates higher passive layer fraction, which formed a lower passive current density. Thus, permeation of seawater could be prevented efficiently by using a dense structure, which could enhance the anticorrosion property of the coating. Moreover, defects accelerated the speed of corrosion. Certain distinct defects (e.g., pinholes and penetrating holes) visible in Fig. 2 were presented in the as-fabricated coatings. Thus, microcracks might appear on the surface due to a combination of the cyclic stress and water erosion, leading to spallation or delamination in areas with weak points. Additionally, the M2 coating exhibited the highest polarization potential value of 0.3 V that was accompanied by a passivation phenomenon, indicating the formation of a passivation layer on the coating surface that protected the substrate from exposure to the ambient environment for a short period.
40
Hardness (GPa)
35 30 25 20 15
M1
M2
M3
M4
M5
10 5 0
0
100 200 300 400 500 600 700 800 900 1000
Displacement Into Surface (nm) Fig. 6. The nanohardness of CrAlSiN coatings at different Si contents.
3.4. Tribological performances of the CrAlSiN coatings Table 3 The H, E, H/E and H3/E2 of the coatings. Coating
M1
M2
M3
M4
M5
H (GPa) E (GPa) H/E H3/E2 (GPa)
36.2 492.4 0.073 0.192
37.9 488.9 0.078 0.229
35.1 472.4 0.074 0.0192
30.9 425.5 0.073 0.165
26.6 388.0 0.069 0.125
Friction curves of the CrAlSiN coatings in seawater were provided in Fig. 9. According to the friction curves, all of the as-deposited coatings exhibited running-in periods, although the lengths of the running-in periods were different. The coatings with proper silicon contents exhibited a shorter running-in period. As for the more important stable periods, the COF of the M1 coating was approximately 0.20, which was the highest COF value among the coatings. When the Si content increased from 2.87 to 8.40 at.%, the COF of coating showed a descending trend. Subsequently, the COF of the CrAlSiN coatings increased gradually with increasing Si content from 8.40 to 15.61 at.%. In addition, the wear rates of the CrAlSiN coatings with varying Si contents upon sliding against WC balls in seawater were illustrated in Fig. 10. Similar to the COF, the wear rate first decreased and then sharply increased. When the silicon content was approximately 5.52 at.%, the wear rate decreased to 3 107 mm3 N1 m1, which was the lowest value. However, the wear rate increased by an order of magnitude when the silicon content was further increased. To understand the wear process in detail, Fig. 11 depicted the crosssectional profiles of wear tracks on the CrAlSiN coatings containing different Si contents in seawater. The max wear depth of the M1 coating was nearly 0.85 μm, and the depth decreased to 0.7 μm in sample M2. However, when the content of Si exceeded 5.52 at.% (M2 coating), the
Lc reached its highest value of approximately 55 N. When the silicon content was further increased, the Lc of the coatings demonstrated a descending trend. The adhesion strength between the coating and substrate was an important property in wear-resistant protective coatings. If stripping of the coating occurs during operation, the protection capabilities of the coating would be greatly reduced, and the cracked debris could also cause severe abrasive wear in the friction system. Therefore, coatings with a high level of adhesion force were essential for improving the service life and reliability. 3.3. Corrosion performances of the CrAlSiN coatings The investigation of the potentiodynamic polarization curves of the CrAlSiN coatings in a seawater solution was depicted in Fig. 8, and the 595
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1.5
0.40
1.0
0.35
0.5
Friction coefficient
E (V/SCE)
Fig. 7. The scratch morphologies of CrAlSiN coatings at different Si contents.
M1 M2 M3 M4 M5
0.0
-0.5
M2 M5
M3
0.25 0.20 0.15 0.10
-1.0 -1.5
M1 M4
0.30
0.05
-9
-8
-7
-6
2
-5
-4
-3
logi (A/cm )
0
1000
2000
3000
Time(s)
4000
5000
6000
Fig. 9. Friction curves of the CrAlSiN coatings in seawater. Fig. 8. the potentiodynamic polarization curves of the CrAlSiN coatings in a seawater solution.
Table 4 The corrosion current densities of CrAlSiN coatings at different Si contents. Coating Icorr (10
7
2
A/cm )
M1
M2
M3
M4
M5
9.20
7.35
6.71
6.10
5.69
max depth value exhibited an increasing trend, implying that the proper Si content in the coatings had a positive effect on the wear resistance in seawater. In addition, noted that the max wear depth of the M5 coating reached 6.9 μm, which was larger than the thickness of this coating in Fig. 2 and indicated that the surface of the coating partially failed. SEM and EDS analyses were conducted to investigate the wear surface of these samples, and the corresponding results were depicted in the insert of Fig. 12. Using the M2 and M5 coatings as examples, the wear surface of the M2 coating was relatively smooth with little debris, as shown clearly in Fig. 12(a). EDS analysis revealed that the debris did not contain tungsten, indicating no detectable adhesion wear. Several micropits formed at the center of the wear track due to corrosion by seawater. Fig. 12(b) revealed that the main elements in the M2 coating were Cr, Al, Si, O and N, and the debris did not contain Fe, indicating that the M2 coating was not worn away. Na, Mg and W were observed by EDS, implying that elements transferred from the seawater and counterpart to the coating surface. In addition, Shan et al. [3] determined that calcium ions and magnesium ions could react with carbonate and hydroxide to
Fig. 10. Wear rates of the CrAlSiN coatings in seawater.
form calcium carbonate and magnesium hydroxide, which could act as a lubricant. For the M5 coating, as Fig. 12(c) depicted, a significant amount of abrasive dust was observed on the edge of the wear track due to debris being caught in the dimple, and this dust then filled the pits in the friction process. White particles were observed on the wear track, showing that salt precipitated from seawater and effectively filled the pits of the M5 coating. The EDS results indicated significant wear of the coating, which was further supported by the depth profiles in which the depth of the
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molecule to pass through, and the adhesion strength between the asprepared coatings and the substrates could decrease as a result, which would contribute to delamination or spallation of the areas that contain weak points. Microcracks primarily intersected with the surface, leading to wear-loss [31]. Seawater contained a high concentration of Cl ions, which was highly destructive to coatings because it caused erosion. In the friction process, this erosion enhanced coating delamination, resulting in the formation of a clean surface track that was exposed to the corrosive environment. Wear-loss increased in a corrosive medium because it promoted additional defects and accelerated the corrosion speed [32]. The wear surface of the coatings was demonstrated to contain oxygen in Fig. 12(b) and (d), which indicated that a tribochemical reaction occurred. Fig. 13 presented the XPS analysis of oxygen on the M2 coating surface before and after the tribochemical reaction. The spectra were fitted by resolution into their component Gaussian line shapes to identify the various bonding schemes of oxygen. As shown in Fig. 13(a), before friction occurred, the peaks at 320.2 eV and 532.1 eV corresponded to Cr2O3 and SiO2. However, in Fig. 13(b), after friction occurred, Cr2O3 at 529.4 eV, Al2O3 at 531.6 eV, and SiO2 at 533 eV were observed after tribochemical reactions occurred on the coating surface. Oxygen reacts with Al/Cr/Si to form Al2O3/Cr2O3/SiO2 oxidation films during the wear process. Generally, these films could effectively minimize contact between the slider and the coating, leading to improved wear resistance. According to a previous study, the tribochemical reaction occurred when the CrAlSiN coating was tested at room temperature [33]. In addition, the sliding surface temperature increased with increasing time in a seawater environment, which promoted the following reactions: 2CrN þ 3H2O ¼ Cr2O3 þ 2NH3
(2)
Si3N4 þ 6H2O ¼ 3SiO2 þ 4NH3
(3)
Fig. 11. The cross-sectional profiles of wear tracks on the CrAlSiN coatings containing different Si contents in seawater.
2AlN þ 3H2O ¼ Al2O3 þ 2NH3
(4)
wear track was deeper than any other coating. The EDS analysis also revealed the presence of Cr, Al, Si, N, Fe, Mo, Ni and O in the wear track in Fig. 12(d), which indicated that the coatings were fatigue flaked. On the sliding surface of the M5 coating, the uplifts were deformed by the influence of a recurring sliding action and microcracks were created by the plastic deformation. Formation of the penetrating microcracks would result in further deformation upon their extension and propagation parallel to the surface at any depth, which might then create throughcoating channels for water molecules. The channels allowed water
Upon increasing the percentage of silicon, the COF decreased because SiO2 self-lubricating films formed [34]. SiO2 could react with water to form silica gel, whose mud structure effectively reduced wear and friction. Nevertheless, the experimental results were different. For example, the M5 coating with the highest content of Si3N4 was worn out, whereas the M2 coating with a relatively lower Si3N4 content exhibited perfect tribological properties. With increasing silicon content, the structure gradually became denser and thicker, and the synergistic actions of both mechanical stress and corrosion also played an important role in the friction and wear processes. Under mechanical stress, a coating with low
Fig. 12. SEM and EDS images of wear tracks on the CrAlSiN coatings in seawater: (a) (b) M2, (c) (d) M5. 597
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synergistic action of both mechanical stress and corrosion in seawater.
Intensity (arb. units)
a-O 1s
Acknowledgments This study was supported by the National Natural Science Foundation of China (grant no. 51475449), the National Basic Research Program of China (973 Program, grant no. 2014CB643302), the Gansu Science and Technology Minister (1604GKCA005) and the Key Research and Development Program of Jiangsu Province (grant no. BE2016115). We gratefully acknowledge these organizations for their financial support.
532.1 eV 530.2 eV
References 520
524
528
532
536
Binding Energy (eV)
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540
Intensity (arb. units)
b-O 1s 531.6 eV
533 eV 529.4 eV
520
524
528
532
536
540
Binding Energy (eV) Fig. 13. The XPS analysis of oxygen on the M2 coating surface before (a) and after (b) the tribochemical reaction.
hardness and toughness was liable to deform or even crack, and the chloride ions in seawater could then easily entered the crack to induce the corrosion; in turn, this mechanism promoted expansion of the crack and further accelerated wear. The increased wear triggers more defects and attracts further corrosion, thereby initiating a self-enhancing cycle. In addition, even though silica gel functions as a lubricant, the tribochemical products Cr2O3 and Al2O3 had high hardness and were therefore difficult to deform and could scratch the surface, thereby forming microcracks and further advancing wear. From the above discussion, it could be concluded that the proper silicon content could significantly enhance the comprehensive performance of CrAlSiN coatings. 4. Conclusions In this paper, CrAlSiN coatings containing different silicon contents were successfully prepared using a multi-arc ion plating technique. The relation between the silicon content and tribological performance of the CrAlSiN coatings in seawater was systematically studied. The results demonstrate than the CrAlSiN coatings were characterized by a mixture of nano-crystallites and amorphous composite microstructures of Cr(Al)N embedded in a Si3N4 amorphous matrix. During friction and wear tests, the COF and wear rate first decreased and then increased with increasing silicon content. The CrAlSiN coating containing 5.52 at.% Si possessed the best comprehensive properties: the hardness of 37.9 GPa, the adhesion force of 55 N, the COF of 0.13 and the wear rate of 3 107 mm3/ Nm in seawater. The high hardness and toughness had strong inhibitory effects on the initiation and propagation of cracks when the coating was subjected to an external force, which could effectively resist the 598
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