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The influence of Ni concentration on the structure, mechanical and tribological properties of Ni–CrSiN coatings in seawater
Journal of Alloys and Compounds xxx (xxxx) xxx
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The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater Qianzhi Wang a, b, c, d, e, *, Yungen Lin c, Fei Zhou a, b, c, **, Jizhou Kong c a
State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China National Key Laboratory of Science and Technology on Helicopter Transmission, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China c College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China d State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China e Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 August 2019 Received in revised form 7 November 2019 Accepted 11 November 2019 Available online xxx
The impact of Ni content on the microstructure, hardness, fracture toughness and tribological properties of NieCrSiN coatings in seawater was investigated by using X-ray diffraction, dynamic ultra micro hardness tester and ball-on-disk tribometer. The structural analyses indicated that the Ni existed in the form of amorphous phase regardless of concentrations. The hardness of NieCrSiN coating maintained around 28.1 GPa at low Ni incorporation but decreased to 20.1 GPa at 10.8 at% Ni concentration. Then, owing to grain refinement, the hardness increased to 24.5 GPa at 13.9 at% Ni concentration. All coatings presented compressive stress that led to the formation of circular cracks around indentation impression. The Ni incorporation especially at high concentration was able to improve coatings toughness against circular cracks. Although the Ni incorporation had no effect in reducing the friction coefficient of CrSiN coatings in seawater, but the moderate Ni incorporation (5.2 at%) could enhance the wear resistance of CrSiN coatings in seawater due to the favorable combination of hardness and toughness. © 2019 Elsevier B.V. All rights reserved.
Keywords: CrSiN Nickel Toughness Tribology Seawater
1. Introduction As compared to CrN coatings, non-metal element doped CrNbased coatings, such as CrCN, CrBN and CrSiN generally present high hardness, which is a critical parameter of wear resistant materials. For instance, the CrCN coating deposited at the C2H2 flow of 10 sccm presented a higher hardness of 32 GPa as well as a lower wear rate of 7.7 107 mm3/Nm than those of CrN coating [1]. Similarly, Ma et al. [2] reported the high hardness and low wear rate of CrBN coating as compared with CrN coating. Moreover, in comparison to CrN coating, a high hardness of 22 GPa and a low wear
* Corresponding author. State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China. ** Corresponding author. State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China. E-mail addresses: [email protected] (Q. Wang), [email protected] (F. Zhou).
rate of 0.7 107 mm3/Nm were obtained for CrSiN coating containing 12.65 at% Si [3]. These results demonstrate that high hardness is beneficial to improve the wear resistance of coatings. However, some researchers found that the wear resistance of the above-mentioned composite coatings was inversely proportional to hardness [4e8]. As listed in Table 1, Wang et al. [4] found that the CrCN coating deposited at a bias voltage of 160 V presented the highest hardness of 29 GPa but suffered the highest wear rate of 3.6 106 mm3/Nm in air. Ding et al. [5] reported that the wear rate of CrBN coating with the high hardness (24 GPa) was almost 10 times than that of CrBN coating with a relatively low hardness (23 GPa). Moreover, the CrSiN coating containing 12.8 at% Si presented the highest hardness (15.3 GPa) but suffered the highest wear rate (6.2 104 mm3/Nm) [6]. The same phenomena were also reported in studies [7,8]. The inverse relationship between hardness and wear resistance implied that there was another reason simultaneously impacting the wear resistance of coatings. As reported, fracture toughness closely related to the ability of resisting crack initiation and propagation is believed to be this reason [9,10]. Thus, hard yet tough coatings became the desired
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Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Table 1 Mechanical and tribological properties of some composite coatings. Coatings
Deposition variable
Hardness (GPa)
Wear rate (mm3/Nm)
CrCN [4]
40 bias voltage 130 bias voltage 160 bias voltage
18 27 29
1.0 106 2.9 106 3.6 106
CrBN [5]
BN target power 200 W BN target power 600 W BN target power 1000 W
22 23 24
7.0 108 4.0 108 3.1 107
CrSiN [6]
9.0 at% Si 12.8 at% Si
11.5 15.3
3.3 104 6.2 104
objective of researchers. Among the ways towards toughening coatings, toughening agents introducing is the effective one [11]. Thus, nickel (Ni) with good ductility has been introduced into coatings as a toughening agent. For binary CrN coating, the Ni incorporation changed the deformation mechanism from intercolumnar shear sliding to the plastic deformation, and therefore CrNiN coating presented a higher toughness as compared to CrN coating [12]. Similarly, the plastic deformation instead of cracks along scratch proved a better toughness of CreNieN coating as compared to CrN coating [13]. Moreover, less circular cracks were found around Rockwell-C indentation impression of CrNiN coating [14,15]. For ternary coating, the toughness of TiSiN coating was enhanced from pffiffiffiffiffi pffiffiffiffiffi 1.1 MPa$ m to 1.24 MPa$ m by Ni incorporation [16]. Thus, CrSiN coating with similar structure as TiSiN coating is expected to be also toughened by Ni incorporation, which may affect its tribological properties as well. In this study, NieCrSiN coatings with different Ni concentrations were deposited using unbalanced magnetron sputtering technology. The microstructure, mechanical and tribological properties of NieCrSiN coatings subject to Ni concentration were systematically investigated and analyzed.
2. Experimental details
atmosphere for 15 min. On the top of the CrN layer, a CrSiN layer was deposited by sputtering two Cr and one Si targets in Ar (50 sccm) and N2 atmosphere for 15 min. 3) Top layer deposition: NieCrSiN top layer was deposited by sputtering two Cr, one Si and one NiCr targets in Ar (50 sccm) and N2 atmosphere. The Ni concentration and the thickness of top layer were controlled by adjusting the current of NiCr target (0 A, 0.4A, 0.8 A, 1.2 A, 1.6 A and 2.0 A) and the corresponding sputtering time (1.50 h, 1.42 h, 1.33 h, 1.25 h, 1.16 h and 1.08 h). During the depositions of the gradient binding layer and the top layer, the flow of N2 was controlled automatically by an optical emission monitor (OEM) (Preset at 50%) whilst a bias voltage of 80 V was applied. The NieCrSiN coatings deposited at each current (0 A, 0.4 A, 0.8 A, 1.2 A, 1.6 A and 2.0 A) would be denoted as CrSiN, NieCrSiN-0.4, NieCrSiN-0.8, NieCrSiN-1.2, NieCrSiN-1.6 and NieCrSiN-2.0.
2.2. Structural analyses The chemical composition and bonding condition of coatings were measured using X-ray photoelectronic spectroscopy (ESCALAB 250, Thermo Scientific). The XPS measurement was conducted using an Al-Ka X-ray source at 150 W with a 500 mm spot size. The crystal structure of coatings was characterized using an X-ray diffractometer (XRD, Ultima IV, Japan). The XRD measurement was conducted with a Cu Ka radiation (l ¼ 0.15404 nm) and operated at 40 kV and 40 mA. The phase pattern was acquired over a range of 2q from 20 to 80 under a q-2q symmetrical mode. Afterwards, the size of crystal grain was obtained according to Scherrer’s formula [17] as below: D ¼ Kl/Bcosq
(1)
Where K is shape factor equal to 0.89, l ¼ 1.5406 Å is the wavelength of the Cu Ka radiation. B is the full width at half maximum (FWHM) of diffraction peak whilst q is the corresponding diffraction angle.
2.1. Coatings deposition A closed-field unbalanced magnetron sputtering system (UDP650, Teer Coatings Limited, UK) equipped with four rectangular targets (Two Cr, one Si and one NiCr targets) on the sidewalls was used to deposit NieCrSiN as well as CrSiN reference coatings. Two kinds of substrates including Ti6Al4V alloy disks (F30 mm 4 mm) and Si(100) wafers were used. The coatings deposited on Si(100) wafers were only used to measure the residual stress while the coatings deposited on Ti6Al4V disks were used for the rest tests. After an ultrasonic clean in ethanol for 10 min, Ti6Al4V disks and Si(100) wafers were fastened on the rotational sample holder, which rotated at 10 rpm during coatings deposition. Afterwards, the chamber was vacuumed using molecular pump until the background pressure reached 2.0 106 Torr. The coatings deposition process contained three successive steps: 1) Ion beam cleaning: Ar gas (50 sccm) was introduced into chamber to maintain the chamber pressure at 1.3 103 Torr. An ion beam gun was used to ionize Ar gas to generate Arþ, and the generated Arþ was accelerated at a bias voltage of 450 V to clean substrates for 30 min. 2) Gradient binding layer deposition: a Cr layer was first deposited on substrate by sputtering two Cr targets in Ar (50 sccm) atmosphere for 10 min. Then, a CrN layer was deposited on the Cr layer by sputtering two Cr targets in Ar (50 sccm) and N2
2.3. Mechanical properties The hardness (H) and the elastic modulus (E) of coatings were measured using a dynamic ultra micro hardness tester (DUH211s, SHIMADZU, Japan). Ten spots on each coating were tested with a constant depth of 200 nm, and the obtained results were averaged to get the mean values of H and E. Moreover, the indentation with a big depth of 2.0 mm was carried out on coatings to initiate the cracks. The cracks distribution around indentation impression was observed using a field-emission scanning electron microscope (SEM) (SIGMA 500, Zeiss). In addition, the residual stress of coatings (sc) was evaluated according to Stoney’s formula [18e20] as below:
sc ¼
1 Es t 2s 1 1 6 ð1 ns Þtc R2 R1
(2)
Where Es, ns and ts are the elastic modulus, Poisson ratio (0.27) and thickness of silicon wafer, while tc is the thickness of coating. R1 and R2 are curvature radiuses of silicon wafer before and after coating deposition. The curvature radius R ¼ (l2 þ c2)/2c. Where l and c are the measured length and height of samples as shown in Fig. 1. The curvature radius R used in Eq. (2) is the average value based on three measurements.
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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morphologies and compositions of wear tracks on coatings were observed and analyzed using a field-emission scanning electron microscope (FEI Quanta 200 FEG). The morphologies and compositions of wear scars on SiC balls were also observed and analyzed using a field-emission scanning electron microscope (SEM) (SIGMA 500, Zeiss) equipped with an energy dispersive spectrum (EDS) (xmax 80, Oxford).
3. Experimental results 3.1. Microstructure of NieCrSiN coatings
Fig. 1. Contours of Si wafers before and after coating deposition.
2.4. Tribological properties The tribological properties of coatings in artificial seawater were studied by using a ball-on-disk tribometer, which can be found in elsewhere [21]. The artificial seawater was formulated according to ASTM D1141-98 (Table 2) whilst SiC ceramic was chosen as counterpart. The load (F), velocity (V) and sliding distance (L) were set as 9 N, 0.1 m/s and 1000 m, respectively. After tribotest, the wear rates of coatings (wc) and SiC ball (wb) were calculated according to the Eqs. (3) and (4) as below:
wc ¼
2prA FL
(3)
wb ¼
pd4 256FL
(4)
r: the radius of wear track on coatings, unit in mm. A: the average cross-sectional area of wear track on coatings, unit in mm2 d: the diameter of wear scar on SiC ball, unit in mm. F: the load, unit in N. L: the sliding distance, unit in m. Among them, d was measured using an optical microscope (XJZ6, China) whilst A was the average cross-sectional area based on four measurements using a white light interferometer (Contour GTK, Bruker). In order to reveal the corresponding wear mechanisms, the
Table 2 Chemical composition of artificial seawater. Components
Concentration (g/L)
NaCl MgCl2 Na2SO4 CaCl2 NaHCO3 KCl KBr H3BO3 SrCl2 NaF
24.53 5.20 4.09 1.16 0.201 0.695 0.101 0.027 0.025 0.003
The chemical compositions of CrSiN and NieCrSiN coatings are listed in Table 3. As the current of NiCr target increases from 0.4 A to 2.0 A, the Ni concentration increases gradually from 2.4 at% to 13.9 at%. In contrast, the concentrations of Cr, Si and N decrease accordingly. The around 4.0 at% O is attributed to the residual oxygen in vacuum chamber. As seen in Fig. 2, both of CrSiN and NieCrSiN coatings present a fcc B1 type crystalline structure that can be confirmed by a set of typical diffraction peaks at 37.60 , 43.69 , 63.51 and 76.21 related to CrN (111), CrN (200), CrN (220) and CrN (311) (JCPDS 11e0065). The intensity of the CrN (200) major peak increases gradually as the Ni concentration increases from 2.4 to 8.3 at% but decreases as the Ni concentration further increases to 13.9 at%. It demonstrates that the low Ni incorporation could promote the formation of CrN (200). According to Scherrer’s formula, the size of CrN grain perpendicular to (200) crystal plane is listed in Table 4. The grain size of CrSiN coating is 8.7 nm and increases to 9.6 nm at 5.2 at% Ni, but then decreases to 7.3 nm at 13.9 at% Ni. The variation of grain size is almost consistent with the intensity variation of CrN (200) peak. In order to confirm the existence form of Ni and Si elements, which are absence in XRD patterns, the Ni 2p and Si 2p core level XPS spectra of NieCrSiN-0.4, NieCrSiN-1.2 and NieCrSiN-2.0 coatings are illustrated in Fig. 3. The major peak in Ni 2p core level XPS spectra is NieNi bond at 853.0 eV. The weak NieO bonds at 854.7 eV and 856.5 eV are believed to be attributed to the residual oxygen on the coatings surface. It is indicated that the Ni concentration has no effect in forming new chemical bonds. Tan et al. [22] investigated the structure of CrNiN coatings and reported that no diffraction peaks related to nickel compounds were found due to ultra-low content of nickel (2.92 at.%) or amorphous phase (8.79 at.%). Thus, based on the NieNi bond in XPS but no Ni crystal peak in XRD pattern, Ni in NieCrSiN coatings is believed to exist as amorphous phase as well regardless of concentration. In addition, the major peak in Si 2p core level XPS spectra is SieN bond at 101.3 eV. However, it is worth noting that a SieSi bond starts to form in NieCrSiN-1.2 coating and becomes strong in NieCrSiN-2.0 coating. It is indicated that Si exists in the form of amorphous SiNx in NieCrSiN coating with low Ni concentration (5.2 at%) but in the form of both amorphous SiNx and amorphous Si in NieCrSiN coating with high Ni concentration (8.3 at%).
Table 3 Chemical compositions of the as-deposited coatings. Coatings
Ni (at.%)
Cr (at.%)
Si (at.%)
N (at.%)
O (at.%)
CrSiN NieCrSiN-0.4 NieCrSiN-0.8 NieCrSiN-1.2 NieCrSiN-1.6 NieCrSiN-2.0
0 2.4 5.2 8.3 10.8 13.9
39.5 38.0 37.5 36.8 36.4 34.8
9.4 10.0 8.3 7.9 7.2 6.9
46.7 45.4 44.8 43.0 41.7 40.7
4.4 4.2 4.2 4.0 3.9 3.8
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Fig. 2. XRD patterns of NieCrSiN coatings.
3.2. Mechanical properties of NieCrSiN coatings According to Oliver and Pharr approach [23], the average hardness H and elastic modulus E with standard deviation are listed in Table 4. As compared to CrSiN coating (28.9 GPa), the hardness of NieCrSiN-0.4 coating decreases to 26.9 GPa, but then the hardness of NieCrSiN-0.8 and NieCrSiN-1.2 coatings increases to 28.1 GPa and 28.6 GPa. As the current of NiCr target increases to 1.6 A, the hardness of NieCrSiN-1.6 coating decreases sharply to 20.1 GPa. In comparison, NieCrSiN-2.0 coating presents a relatively high hardness of 24.5 GPa. Taking elastic modulus into account, the ratio of H3/E2 as an indicator proportional to plastic deformation resistance is also listed in Table 4 [24]. It is obvious that the values of H3/E2 for NieCrSiN coatings are lower than that of CrSiN coating. It indicates
that Ni incorporation would enhance the plastic deformation ability of CrSiN coating due to the nature ductility of Ni. At the meantime, the ratio of hr/hmax could be calculated based on the loading and unloading curves. Where hr is the residual depth after removing the load whilst hmax is the maximum depth. As is known, the ratio of hr/hmax can be deemed as the plastic deformation ability of coatings as well [25,26]. As seen in Table 4, the most values of hr/ hmax for NieCrSiN coatings are higher than that of CrSiN coating. It proves that the Ni incorporation is able to enhance the plastic deformation ability of CrSiN coating from another point of view. The morphologies of indentation impressions under a penetration depth of 2.0 mm as well as the corresponding loading and unloading curves are illustrated in Fig. 4. No radial crack can be observed at impressions corner, but several circular cracks are observed around the impressions edge for all coatings. Moreover, the impressions insides of CrSiN, NieCrSiN-0.4, NieCrSiN-0.8 and NieCrSiN-1.2 coatings show some picture frame cracks, which are absence inside the impressions of NieCrSiN-1.6 and NieCrSiN-2.0 coatings. Additionally, it is worth noting that an obvious pop-in is only found on the loading curve of CrSiN coating while the loading curves of NieCrSiN coatings are smooth.
3.3. Tribological properties The friction behavior of coatings in artificial seawater is illustrated in Fig. 5a, which can be divided into two stages. The first stage is known as running-in period, in which the friction coefficient generally increases gradually with the sliding distance. It could be seen that the running-in period of CrSiN coating lasts for 250 m. After 2.4 at% and 5.2 at% Ni incorporations, the running-in periods of NieCrSiN-0.4 and NieCrSiN-0.8 coatings extend to
Table 4 Mechanical properties of NieCrSiN coatings. Coatings
H (GPa)
CrSiN NieCrSiN-0.4 NieCrSiN-0.8 NieCrSiN-1.2 NieCrSiN-1.6 NieCrSiN-2.0
28.9 26.9 28.1 28.6 20.1 24.5
± ± ± ± ± ±
3.5 2.5 2.7 2.3 2.7 2.4
E (GPa) 297 295 292 293 251 257
± ± ± ± ± ±
14 22 22 17 21 14
H3/E2 (GPa)
hr/hmax
sc (MPa)
D (nm)
0.274 0.224 0.262 0.273 0.129 0.224
0.512 0.527 0.515 0.501 0.580 0.523
450.4 364.2 1065.4 1522.6 28.5 1120.3
8.7 8.5 9.6 8.8 8.7 7.3
Fig. 3. (a) Ni 2p and (b) Si 2p core level XPS spectra of NieCrSiN-0.4, NieCrSiN-1.2 and NieCrSiN-2.0 coatings.
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Fig. 4. Morphologies of indentation under 600 mN and the corresponding loading and unloading curves of (a1) (a2) CrSiN (b1) (b2) NieCrSiN-0.4 (c1) (c2) NieCrSiN-0.8 (d1) (d2) NieCrSiN-1.2 (e1) (e2) NieCrSiN-1.6 (f1) (f2) NieCrSiN-2.0 coatings.
Fig. 5. (a) Friction behavior and (b) Mean-steady friction coefficient of NieCrSiN coatings.
around 450 m. When the Ni incorporation increases to 8.3 at%, the running-in period of NieCrSiN-1.2 coating decreases to 400 m. Furthermore, as the Ni concentration increases to 10.8 at% and 13.9 at%, the running-in periods of NieCrSiN-1.6 and NieCrSiN-2.0 coatings decrease to 130 m and 50 m, respectively. It indicates that the low Ni incorporation would extend the running-in period of CrSiN coating, but the high Ni incorporation would shorten it. The second stage is the steady period, in which friction coefficient fluctuates slightly around a certain value. It is obvious that except CrSiN coating, all NieCrSiN coatings present a typical steady period. In contrast, the friction coefficient of CrSiN coating keeps steady from 250 to 500 m, but then decreases gradually until the end of tribotest. By averaging the friction coefficient over steady period, the mean-steady friction coefficients of coatings are illustrated in Fig. 5b. The mean steady friction coefficients of all NieCrSiN
coatings (0.145e0.211) are higher than that of CrSiN coating (0.122), indicating that the Ni incorporation has no effect of reducing the friction coefficient of CrSiN coating. Generally, the friction coefficient is closely related to the interface condition especially roughness. The 3D morphologies as well as the surface roughness Sa of wear tracks are presented in Fig. 6. As seen in Fig. 6e and f, the morphologies of wear tracks on NieCrSiN-1.6 and NieCrSiN-2.0 coatings present many deep grooves, and the peaks of some grooves are even higher than the original coatings surface. On the contrary, the grooves on the wear tracks of CrSiN, NieCrSiN0.4, NieCrSiN-0.8 and NieCrSiN-1.2 coatings become shallow and are lower than the original coatings surface. The wear rates of coatings and SiC balls are illustrated in Fig. 7. The wear rates of SiC ball vary in the range of 4.0 107 mm3/Nm to 8.0 107 mm3/Nm. The SiC balls sliding against NieCrSiN coatings present lower wear rate than that of SiC ball sliding
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Fig. 6. 3D profiles of wear tracks on (a) CrSiN (b) NieCrSiN-0.4 (c) NieCrSiN-0.8 (d) NieCrSiN-1.2 (e) NieCrSiN-1.6 (f) NieCrSiN-2.0 coatings.
Fig. 7. (a) Coatings wear rate and (b) SiC balls wear rate.
against CrSiN coating. In contrast, the wear rates of coatings vary in the range of 8.5 108 mm3/Nm to 2.7 107 mm3/Nm. As compared to CrSiN coating, the wear rate of NieCrSiN-0.4 coating increases from 1.2 107 mm3/Nm to 1.5 107 mm3/Nm. As the current of NiCr target increases to 0.8 A, the wear rate of NieCrSiN0.8 coating decreases to 8.5 108 mm3/Nm. Then, the wear rate increases to the highest value of 2.7 107 mm3/Nm for NieCrSiN1.6 coating. At last, the wear rate of NieCrSiN-2.0 coating decreases to 1.7 107 mm3/Nm. 4. Discussions 4.1. Microstructure According to the standard reference sample (JCPDS 11e0065), the 2q value of CrN (200) should be 43.69 , but the CrN (200) diffraction peaks of CrSiN, NieCrSiN-0.4, NieCrSiN-0.8, NieCrSiN1.2, NieCrSiN-1.6 and NieCrSiN-2.0 coatings shift to low angles of 43.46 , 43.48 , 43.50 , 43.38 , 43.54 and 43.40 , respectively. It demonstrates that the distance between two (200) crystal planes is enlarged at different degrees. Since the radii of Si atom (Ra ¼ 0.118 nm) and Ni atom (Ra ¼ 0.124 nm) are both smaller than that of Cr atom (Ra ¼ 0.128 nm), thus the replacement of Cr atom in CrN lattice by Si or Ni atoms should shrink the distance between two (200) crystal planes. It implies that there is another reason
contributing to the distance enlargement of crystal planes. As described in Section 2.2, the XRD measurement was conducted under a q-2q symmetrical mode, and therefore the detected crystal planes of CrN (200) should be paralleled to coatings surface. As listed in Table 4, all coatings possess compressive stress that squeezes the coatings at the horizontal direction. Under such a circumstance, the distance of (200) crystal plane paralleled to coatings surface is enlarged [27]. As a result, the CrN (200) diffraction peaks of all coatings shift to low angle. The similar phenomenon has been reported in CrN coatings deposited by different methods [28]. As shown in Fig. 2, as compared to the CrN (200) peak of CrSiN coating (43.46 ), the CrN (200) peaks of NieCrSiN-0.4, NieCrSiN0.8 and NieCrSiN-1.6 coatings shift to high values (43.48 , 43.50 and 43.54 ) while those of NieCrSiN-1.2 and NieCrSiN-2.0 coatings shift to low values (43.38 and 43.40 ). This difference is attributed to the combination of two major aspects. The first aspect is the compressive stress of coatings, which has mentioned above. The second aspect is the effect of substitution solid solution. Because the radius of Ni atom (Ra ¼ 0.124 nm) is less than that of Cr atom (Ra ¼ 0.128 nm), the replacement of Cr atom by Ni atom in CrN lattice would cause the shrink of plane distance, which leads to a right shift of diffraction peak based on Bragg formula [29]. Jayaganthan et al. [30] reported the right shift of (200) orientation for CrSiN coatings due to the substitution of Cr atom (Ra ¼ 0.128 nm) by
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Si atom (Ra ¼ 0.118 nm). Thus, as compared to CrSiN coating (43.46 ), the relatively low angles of CrN (200) for NieCrSiN-1.2 and NieCrSiN-2.0 coatings (43.38 and 43.40 ) demonstrate that the effect of compressive stress is dominant, which is well consistent with their high compressive stresses (1522.6 MPa and 1120.3 MPa vs 450.4 MPa). With regards to NieCrSiN-0.4 and NieCrSiN-1.6 coatings, the enlarging effect of compressive stress (364.2 MPa and 28.5 MPa vs 450.4 MPa) becomes weak. Namely, the lattice shrinkage from substitution solid solution counteracts the enlarging effect of compressive stress at a certain degree. Therefore, the diffraction peaks of NieCrSiN-0.4 and NieCrSiN-1.6 coatings shift to relatively high angles (43.48 and 43.54 ). 4.2. Mechanical properties As listed in Table 4, the hardness of all NieCrSiN coatings (20.1e28.6 GPa) is lower than that of CrSiN coating (28.9 GPa). It is believed that the soft feature of Ni contributes to this result. In addition, the hardness of NieCrSiN coatings is dependent on the Ni concentration. According to Hall-Petch law [31e33], small grain would contribute to high hardness. Meanwhile, high compressive stress played a significant role in contributing to high hardness as well [34,35]. Thus, the combination of grain size and compressive stress determines the hardness variation of NieCrSiN coatings. As compared to CrSiN coating (8.7 nm and 450.4 MPa), NieCrSiN-0.4 coating has a small crystal grain size (8.5 nm) and a low compressive stress (364.2 MPa). Thus, NieCrSiN-0.4 coating presents a lower hardness (26.9 GPa) than that of CrSiN coating (28.9 GPa). As the current of NiCr target increases to 0.8 A and 1.2 A, the crystal grain size of NieCrSiN-0.8 and NieCrSiN-1.2 coatings increases slightly to 9.6 nm and 8.8 nm, but their compressive stress increases sharply to 1065.4 MPa and 1522.6 MPa. It implies that the enhancement effect of high compressive stress is over the weaken effect of big grain size. As a consequence, the hardness of NieCrSiN-0.8 and NieCrSiN-1.2 coatings increases to 28.1 and 28.6 GPa. In contrast, a big drop of compressive stress (28.5 MPa) and more Ni incorporation lead to the lowest hardness of NieCrSiN-1.6 coating (20.1 GPa). With regards to NieCrSiN-2.0 coating, the grain size decreases to 7.3 nm and the compressive stress increases to 1120.3 MPa. Under the enhancement effects of both grain size and compressive stress, the hardness of NieCrSiN-2.0 coating increases to 24.5 GPa eventually. Wang et al. [36,37] and Jungk et al. [38] pointed out that no radial crack was found on the coatings possessing high compressive stress. It indicates that high compressive stress is beneficial to inhibit the formation of radial cracks. Thus, except NieCrSiN-1.6 coating, the high compressive stress of the rest coatings (from 364.2 to 1522.6 MPa) in this study prevents the formation of radial cracks in Fig. 4. For NieCrSiN-1.6 coating, its good plastic deformation ability proved by the lowest ratio of H3/E2 and the highest ratio of hr/hmax retards the formation of radial cracks. On the contrary, the compressive stress is able to promote the formation of circular crack especially under the “sink-in” condition as shown in Fig. 8. The morphologies of impression in Fig. 4 show that the impressions of all coatings are under sink-in condition and display the circular cracks. As seen in Fig. 8, the end point of sink-in was prone to the formation of circular crack, and the continuous circular cracks as the indentation proceeding would form the picture frame cracks as shown in Fig. 4, which have been pointed out in studies [39,40]. As seen in Fig. 4, the CrSiN, NieCrSiN-0.4, NieCrSiN-0.8 and NieCrSiN-1.2 coatings all present picture frame cracks inside the impression. In comparison, no obvious picture frame cracks are observed on NieCrSiN-1.6 and NieCrSiN-2.0 coatings. It demonstrates that the critical load of generating circular cracks for NieCrSiN-1.6 and NieCrSiN-2.0 coatings is high, thus indicating
Fig. 8. Schematic diagram of impression under sink-in condition.
that the high Ni content incorporation (10.8 at% and 13.9 at%) could enhance the toughness of CrSiN coating. In addition, as mentioned in the Section 3.2, a pop-in is only found on the loading curve of CrSiN coating. The pop-in on the loading curve meant a sudden depth penetration of indenter tip, which was attributed to the formation of crack as reported in studies [41,42]. According to the relationship of penetration depth h and edge length a as below [43]:
pffiffiffi a ¼ 2 3h tan65:27
(5)
The corresponding edge length h of indentation impression at pop-in depth (1.35 mm) should be 10.15 mm, which has been marked in Fig. 4a1. It can be seen that the position of the triangle with 10.15 mm edge corresponds to an obvious picture frame crack, which results from the formation of circular crack at 300 mN during loading process. In contrast, no obvious pop-in is observed on the loading curves of all NieCrSiN coatings. It is indicated from another point of view that Ni incorporation can improve the toughness of CrSiN coating. To compare the toughness of CrSiN and NieCrSiN coatings with low Ni incorporation (2.4e8.3 at%), the corresponding toughness KIC was calculated according to Eq. (6) based on the picture frame cracks as reported in studies [39,40].
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ufra E KIC ¼ 1 n2 Afra
(6)
Where Ufra is the fracture dissipated energy, E is the elastic modulus of coatings, Poisson ratio n ¼ 0.25 is used in the absence of better data for the coatings of interest. Afra is the total fracture area. Among them, the Ufra could be obtained using Eq. (7) as below: Ufra ¼ W T-We-Wp
(7)
Where WT and We are the total work and elastic deformation work, which could be derived from loading and unloading curves. Wp is plastic deformation work that could be estimated by Eq. (8):
hr Wp ¼ 1:27 0:27 WT hmax
(8)
In addition, the total fracture area Afra can be calculated using Eq. (9) as below:
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Q. Wang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
Afra ¼
3b2 t 2s
(9)
Where b and s are the radial dimension of impression and the space between cracks as shown in Fig. 8 t is the thickness of coatings. As listed in Table 5, the toughness of NieCrSiN-0.4, NieCrSiNpffiffiffiffiffi 0.8 and NieCrSiN-1.2 coatings (5.95e8.05 MPa$ m) is all higher pffiffiffiffiffi than that of CrSiN coating (5.91 MPa$ m), which is well consistent with the pop-in phenomena on loading curves in Fig. 4. NieCrSiNpffiffiffiffiffi 0.8 coating presents the highest toughness of 8.05 MPa$ m. Since no obvious picture frame cracks are observed on NieCrSiN-1.6 and NieCrSiN-2.0 coatings, thus we assume their fracture toughness pffiffiffiffiffi pffiffiffiffiffi higher than 8.05 MPa$ m(>8.05 MPa$ m). The relationship of fracture toughness, Ni concentration and mechanical properties is illustrated in Fig. 9. Apparently, the fracture toughness is dependent on the combination of compressive stress and hr/hmax. To be specific, without Ni incorporation (0 at%), the compressive stress and hr/hmax of CrSiN coating are both low, thus leading to the lowest pffiffiffiffiffi fracture toughness of 5.91 MPa$ m. As Ni concentration increases to 2.4 at%, 5.2 at% and 8.3 at%, either compressive stress or hr/hmax increases, which contributes to the improved fracture toughness of NieCrSiN-0.4, NieCrSiN-0.8 and NieCrSiN-1.2 coatings pffiffiffiffiffi (5.95e8.05 MPa$ m). When Ni concentration reaches 10.8 at% and 13.9 at%, the combination effect of compressive stress and hr/hmax becomes more beneficial to enhance fracture toughness. As a result, pffiffiffiffiffi the fracture toughness higher than 8.05 MPa$ m is obtained for NieCrSiN-1.6 and NieCrSiN-2.0 coatings.
4.3. Tribological properties As shown in Fig. 5b, the mean-steady friction coefficients of NieCrSiN coatings (0.145e0.211) are higher than that of CrSiN coating (0.122) especially NieCrSiN-1.6 coating presenting the highest value of 0.211. The surface roughness Sa of wear track on each coating is illustrated in Fig. 6. The roughness Sa of NieCrSiN0.4 and NieCrSiN-1.6 coatings are 76 nm and 99 nm that are higher than that of CrSiN coating (65 nm). Thus, the mean-steady friction coefficients of NieCrSiN-0.4 (0.175) and NieCrSiN-1.6 coatings (0.211) are much higher than that of CrSiN coating (0.122). The surface roughness Sa of wear track on the rest coatings (54e59 nm) is similar as that of CrSiN (65 nm), thus their mean-steady friction coefficient is close to that of CrSiN coating. The reason to the different surface roughness of wear track is believed to be the plastic deformation ability of coatings. As mentioned above, H3/E2 is an indicator proportional to plastic deformation resistance while hr/hmax reflects plastic deformation ability. According to the values of H3/E2 and hr/hmax in Table 4, NieCrSiN-1.6 coating should have the best plastic deformation ability. As a consequence, the NieCrSiN-1.6 coating material is prone to plastic deformation and forms deep grooves with the highest roughness Sa (99 nm) that leads to its highest friction coefficient (0.211). The morphologies and chemical compositions of wear scars on SiC balls are illustrated in Fig. 10. Apparently, some obvious grooves
Table 5 Toughness of coatings based on picture frame cracks. Coatings
a (mm)
s (nm)
CrSiN NieCrSiN-0.4 NieCrSiN-0.8 NieCrSiN-1.2 NieCrSiN-1.6 NieCrSiN-2.0
7.647 7.428 7.426 7.226 e e
525 521 456 508 e e
± ± ± ±
30 38 11 17
t (mm)
Ufra ( 109 N m)
pffiffiffiffiffi KIC (MPa$ m)
2.73 2.36 2.82 2.77 2.36 2.50
50.4 55.9 106.5 48.3 e e
5.91 ± 6.85 ± 8.05 ± 5.95 ± >8.05 >8.05
1.42 1.86 1.26 1.09
Fig. 9. The relationship of fracture toughness, Ni concentration and mechanical properties of NieCrSiN coatings.
are observed on the wear scars of SiC balls, which are well consistent with the grooves on the wear track of coatings as shown in Fig. 6. The light and dark areas can be observed on the wear scar of SiC balls, but the chemical compositions on the different areas are quite similar (See inset in Fig. 10). Thus, the reason contributing to the light and dark areas is light reflex rather than the different materials. Owing to quite low concentration of Cr (0.1e0.2 at%) detected, almost no transfer layer from coatings adheres on SiC balls. A small amount of O (0.9e4.6 at%) was detected on SiC balls, which indicated the hydration reaction of SiC in aqueous environment that have been widely reported [44e46]. On the contrary, C at different concentrations was detected on the wear track of all coatings as shown in Fig. 11. It demonstrates that the wear debris of SiC balls transfers onto coatings surface. Moreover, the detected Na, Mg and Cl elements are the residual of seawater. It is worth noting that the ratio of N/Cr on the wear track (0.18e0.22) (See in Fig. 11) is much lower as compared to the original ratio (1.18e1.19) (See in Table 3). It implies that N in coating was lost during friction. As analyzed in microstructure, CrSiN and NieCrSiN coatings consist of CrN crystal dispersing into a-SiNx matrix. According to the Gibbs free energy of equations as below, the chemical reactions of CrN and a-SiNx might occur as similar as Eqs. (10) and (11) [21,47]: 2CrNþ3H2O]Cr2O3þ2NH3
(10)
DGf298 ¼ 250:10 kJ,mol1 Si3N4þ6H2O ¼ 3SiO2þ4NH3
(11)
DGf298 ¼ 570:06 kJ,mol1 Thus, N was removed in the form of NH3 gas, and the formed Cr2O3 and SiO2 contributed to the O element detection on the wear track. In contrast, the Ni in coatings was hard to react with water as the Gibbs free energy of reaction is larger than zero as below: Ni þ H2O]NiO þ H2
(12)
DGf298 ¼ 26:14 kJ,mol1 Thus, based on the above analyses, both coating and SiC ball could have reactions with water and form oxides layer, which is called tribochemical wear. Zhou et al. [48,49] have pointed out that
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Fig. 10. SEM images of wear scars on SiC balls sliding against (a1-a3) CrSiN (b1-b3) NieCrSiN-0.4 (c1-c3) NieCrSiN-0.8 (d1-d3) NieCrSiN-1.2 (e1-e3) NieCrSiN-1.6 (f1-f3) NieCrSiN2.0 coatings.
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Q. Wang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
Fig. 11. SEM images of wear scars on SiC balls sliding against (a1-a3) CrSiN (b1-b3) NieCrSiN-0.4 (c1-c3) NieCrSiN-0.8 (d1-d3) NieCrSiN-1.2 (e1-e3) NieCrSiN-1.6 (f1-f3) NieCrSiN2.0 coatings.
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Acknowledgement This work has been supported by National Natural Science Foundation of China (Grant No. 51705245 and 51775271); Natural Science Foundation of Jiangsu Province (Grant No. BK20170794). The Tribology Science Fund of State Key Laboratory of Tribology (Grant No. SKLTKF17B05). The Foundation of Jiangsu Provincial Key Laboratory of Bionic Functional Materials. The Foundation of Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (Grant No.2018K03). We would like to acknowledge them for their financial support. References
Fig. 12. Relationship between coatings hardness and wear rate.
the wear tracks on both coating and ball should be smooth when tribochemical wear was dominant during friction process. However, as seen in Figs. 6 and 10, obvious grooves are found on both of coating and ball. It is indicated that the abrasive wear rather than tribochemical wear dominates the wear process in this study. As seen in Fig. 12, the hardness and coatings wear rate presents a quasi-linear relationship. Namely, the coatings hardness dominates the wear resistance of coatings in this study. On this basis, the pffiffiffiffiffi improved toughness (8.05 MPa$ m) and the similar hardness (28.1 GPa) of NieCrSiN-0.8 coating make it present a slightly lower wear rate (8.5 108 mm3/Nm) than that of CrSiN coating (1.2 107 mm3/Nm).
5. Conclusions To improve the fracture toughness and tribological properties of CrSiN coatings in seawater, Ni at various concentrations has been incorporated into CrSiN coatings. Through indentation measurements and tribotests, it was found that: 1) The incorporated Ni in NieCrSiN coatings existed in the form of amorphous phase. 2) The Ni incorporation induced the increment of compressive stress first but then the decrement of compressive stress, which dominated the variation of hardness. 3) The Ni incorporation especially at high concentration was able to enhance the toughness of CrSiN coating but sacrificing the hardness. 4) 5.2 at% Ni incorporation made NieCrSiN-0.8 present a better wear resistance than that of CrSiN coating due to the improved pffiffiffiffiffi toughness (8.05 MPa$ m) and similar hardness (28.1 GPa).
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater Qianzhi Wang a, b, c, d, e, *, Yungen Lin c, Fei Zhou a, b, c, **, Jizhou Kong c a
State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China National Key Laboratory of Science and Technology on Helicopter Transmission, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China c College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China d State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China e Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 August 2019 Received in revised form 7 November 2019 Accepted 11 November 2019 Available online xxx
The impact of Ni content on the microstructure, hardness, fracture toughness and tribological properties of NieCrSiN coatings in seawater was investigated by using X-ray diffraction, dynamic ultra micro hardness tester and ball-on-disk tribometer. The structural analyses indicated that the Ni existed in the form of amorphous phase regardless of concentrations. The hardness of NieCrSiN coating maintained around 28.1 GPa at low Ni incorporation but decreased to 20.1 GPa at 10.8 at% Ni concentration. Then, owing to grain refinement, the hardness increased to 24.5 GPa at 13.9 at% Ni concentration. All coatings presented compressive stress that led to the formation of circular cracks around indentation impression. The Ni incorporation especially at high concentration was able to improve coatings toughness against circular cracks. Although the Ni incorporation had no effect in reducing the friction coefficient of CrSiN coatings in seawater, but the moderate Ni incorporation (5.2 at%) could enhance the wear resistance of CrSiN coatings in seawater due to the favorable combination of hardness and toughness. © 2019 Elsevier B.V. All rights reserved.
Keywords: CrSiN Nickel Toughness Tribology Seawater
1. Introduction As compared to CrN coatings, non-metal element doped CrNbased coatings, such as CrCN, CrBN and CrSiN generally present high hardness, which is a critical parameter of wear resistant materials. For instance, the CrCN coating deposited at the C2H2 flow of 10 sccm presented a higher hardness of 32 GPa as well as a lower wear rate of 7.7 107 mm3/Nm than those of CrN coating [1]. Similarly, Ma et al. [2] reported the high hardness and low wear rate of CrBN coating as compared with CrN coating. Moreover, in comparison to CrN coating, a high hardness of 22 GPa and a low wear
* Corresponding author. State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China. ** Corresponding author. State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China. E-mail addresses: [email protected] (Q. Wang), [email protected] (F. Zhou).
rate of 0.7 107 mm3/Nm were obtained for CrSiN coating containing 12.65 at% Si [3]. These results demonstrate that high hardness is beneficial to improve the wear resistance of coatings. However, some researchers found that the wear resistance of the above-mentioned composite coatings was inversely proportional to hardness [4e8]. As listed in Table 1, Wang et al. [4] found that the CrCN coating deposited at a bias voltage of 160 V presented the highest hardness of 29 GPa but suffered the highest wear rate of 3.6 106 mm3/Nm in air. Ding et al. [5] reported that the wear rate of CrBN coating with the high hardness (24 GPa) was almost 10 times than that of CrBN coating with a relatively low hardness (23 GPa). Moreover, the CrSiN coating containing 12.8 at% Si presented the highest hardness (15.3 GPa) but suffered the highest wear rate (6.2 104 mm3/Nm) [6]. The same phenomena were also reported in studies [7,8]. The inverse relationship between hardness and wear resistance implied that there was another reason simultaneously impacting the wear resistance of coatings. As reported, fracture toughness closely related to the ability of resisting crack initiation and propagation is believed to be this reason [9,10]. Thus, hard yet tough coatings became the desired
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Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Table 1 Mechanical and tribological properties of some composite coatings. Coatings
Deposition variable
Hardness (GPa)
Wear rate (mm3/Nm)
CrCN [4]
40 bias voltage 130 bias voltage 160 bias voltage
18 27 29
1.0 106 2.9 106 3.6 106
CrBN [5]
BN target power 200 W BN target power 600 W BN target power 1000 W
22 23 24
7.0 108 4.0 108 3.1 107
CrSiN [6]
9.0 at% Si 12.8 at% Si
11.5 15.3
3.3 104 6.2 104
objective of researchers. Among the ways towards toughening coatings, toughening agents introducing is the effective one [11]. Thus, nickel (Ni) with good ductility has been introduced into coatings as a toughening agent. For binary CrN coating, the Ni incorporation changed the deformation mechanism from intercolumnar shear sliding to the plastic deformation, and therefore CrNiN coating presented a higher toughness as compared to CrN coating [12]. Similarly, the plastic deformation instead of cracks along scratch proved a better toughness of CreNieN coating as compared to CrN coating [13]. Moreover, less circular cracks were found around Rockwell-C indentation impression of CrNiN coating [14,15]. For ternary coating, the toughness of TiSiN coating was enhanced from pffiffiffiffiffi pffiffiffiffiffi 1.1 MPa$ m to 1.24 MPa$ m by Ni incorporation [16]. Thus, CrSiN coating with similar structure as TiSiN coating is expected to be also toughened by Ni incorporation, which may affect its tribological properties as well. In this study, NieCrSiN coatings with different Ni concentrations were deposited using unbalanced magnetron sputtering technology. The microstructure, mechanical and tribological properties of NieCrSiN coatings subject to Ni concentration were systematically investigated and analyzed.
2. Experimental details
atmosphere for 15 min. On the top of the CrN layer, a CrSiN layer was deposited by sputtering two Cr and one Si targets in Ar (50 sccm) and N2 atmosphere for 15 min. 3) Top layer deposition: NieCrSiN top layer was deposited by sputtering two Cr, one Si and one NiCr targets in Ar (50 sccm) and N2 atmosphere. The Ni concentration and the thickness of top layer were controlled by adjusting the current of NiCr target (0 A, 0.4A, 0.8 A, 1.2 A, 1.6 A and 2.0 A) and the corresponding sputtering time (1.50 h, 1.42 h, 1.33 h, 1.25 h, 1.16 h and 1.08 h). During the depositions of the gradient binding layer and the top layer, the flow of N2 was controlled automatically by an optical emission monitor (OEM) (Preset at 50%) whilst a bias voltage of 80 V was applied. The NieCrSiN coatings deposited at each current (0 A, 0.4 A, 0.8 A, 1.2 A, 1.6 A and 2.0 A) would be denoted as CrSiN, NieCrSiN-0.4, NieCrSiN-0.8, NieCrSiN-1.2, NieCrSiN-1.6 and NieCrSiN-2.0.
2.2. Structural analyses The chemical composition and bonding condition of coatings were measured using X-ray photoelectronic spectroscopy (ESCALAB 250, Thermo Scientific). The XPS measurement was conducted using an Al-Ka X-ray source at 150 W with a 500 mm spot size. The crystal structure of coatings was characterized using an X-ray diffractometer (XRD, Ultima IV, Japan). The XRD measurement was conducted with a Cu Ka radiation (l ¼ 0.15404 nm) and operated at 40 kV and 40 mA. The phase pattern was acquired over a range of 2q from 20 to 80 under a q-2q symmetrical mode. Afterwards, the size of crystal grain was obtained according to Scherrer’s formula [17] as below: D ¼ Kl/Bcosq
(1)
Where K is shape factor equal to 0.89, l ¼ 1.5406 Å is the wavelength of the Cu Ka radiation. B is the full width at half maximum (FWHM) of diffraction peak whilst q is the corresponding diffraction angle.
2.1. Coatings deposition A closed-field unbalanced magnetron sputtering system (UDP650, Teer Coatings Limited, UK) equipped with four rectangular targets (Two Cr, one Si and one NiCr targets) on the sidewalls was used to deposit NieCrSiN as well as CrSiN reference coatings. Two kinds of substrates including Ti6Al4V alloy disks (F30 mm 4 mm) and Si(100) wafers were used. The coatings deposited on Si(100) wafers were only used to measure the residual stress while the coatings deposited on Ti6Al4V disks were used for the rest tests. After an ultrasonic clean in ethanol for 10 min, Ti6Al4V disks and Si(100) wafers were fastened on the rotational sample holder, which rotated at 10 rpm during coatings deposition. Afterwards, the chamber was vacuumed using molecular pump until the background pressure reached 2.0 106 Torr. The coatings deposition process contained three successive steps: 1) Ion beam cleaning: Ar gas (50 sccm) was introduced into chamber to maintain the chamber pressure at 1.3 103 Torr. An ion beam gun was used to ionize Ar gas to generate Arþ, and the generated Arþ was accelerated at a bias voltage of 450 V to clean substrates for 30 min. 2) Gradient binding layer deposition: a Cr layer was first deposited on substrate by sputtering two Cr targets in Ar (50 sccm) atmosphere for 10 min. Then, a CrN layer was deposited on the Cr layer by sputtering two Cr targets in Ar (50 sccm) and N2
2.3. Mechanical properties The hardness (H) and the elastic modulus (E) of coatings were measured using a dynamic ultra micro hardness tester (DUH211s, SHIMADZU, Japan). Ten spots on each coating were tested with a constant depth of 200 nm, and the obtained results were averaged to get the mean values of H and E. Moreover, the indentation with a big depth of 2.0 mm was carried out on coatings to initiate the cracks. The cracks distribution around indentation impression was observed using a field-emission scanning electron microscope (SEM) (SIGMA 500, Zeiss). In addition, the residual stress of coatings (sc) was evaluated according to Stoney’s formula [18e20] as below:
sc ¼
1 Es t 2s 1 1 6 ð1 ns Þtc R2 R1
(2)
Where Es, ns and ts are the elastic modulus, Poisson ratio (0.27) and thickness of silicon wafer, while tc is the thickness of coating. R1 and R2 are curvature radiuses of silicon wafer before and after coating deposition. The curvature radius R ¼ (l2 þ c2)/2c. Where l and c are the measured length and height of samples as shown in Fig. 1. The curvature radius R used in Eq. (2) is the average value based on three measurements.
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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morphologies and compositions of wear tracks on coatings were observed and analyzed using a field-emission scanning electron microscope (FEI Quanta 200 FEG). The morphologies and compositions of wear scars on SiC balls were also observed and analyzed using a field-emission scanning electron microscope (SEM) (SIGMA 500, Zeiss) equipped with an energy dispersive spectrum (EDS) (xmax 80, Oxford).
3. Experimental results 3.1. Microstructure of NieCrSiN coatings
Fig. 1. Contours of Si wafers before and after coating deposition.
2.4. Tribological properties The tribological properties of coatings in artificial seawater were studied by using a ball-on-disk tribometer, which can be found in elsewhere [21]. The artificial seawater was formulated according to ASTM D1141-98 (Table 2) whilst SiC ceramic was chosen as counterpart. The load (F), velocity (V) and sliding distance (L) were set as 9 N, 0.1 m/s and 1000 m, respectively. After tribotest, the wear rates of coatings (wc) and SiC ball (wb) were calculated according to the Eqs. (3) and (4) as below:
wc ¼
2prA FL
(3)
wb ¼
pd4 256FL
(4)
r: the radius of wear track on coatings, unit in mm. A: the average cross-sectional area of wear track on coatings, unit in mm2 d: the diameter of wear scar on SiC ball, unit in mm. F: the load, unit in N. L: the sliding distance, unit in m. Among them, d was measured using an optical microscope (XJZ6, China) whilst A was the average cross-sectional area based on four measurements using a white light interferometer (Contour GTK, Bruker). In order to reveal the corresponding wear mechanisms, the
Table 2 Chemical composition of artificial seawater. Components
Concentration (g/L)
NaCl MgCl2 Na2SO4 CaCl2 NaHCO3 KCl KBr H3BO3 SrCl2 NaF
24.53 5.20 4.09 1.16 0.201 0.695 0.101 0.027 0.025 0.003
The chemical compositions of CrSiN and NieCrSiN coatings are listed in Table 3. As the current of NiCr target increases from 0.4 A to 2.0 A, the Ni concentration increases gradually from 2.4 at% to 13.9 at%. In contrast, the concentrations of Cr, Si and N decrease accordingly. The around 4.0 at% O is attributed to the residual oxygen in vacuum chamber. As seen in Fig. 2, both of CrSiN and NieCrSiN coatings present a fcc B1 type crystalline structure that can be confirmed by a set of typical diffraction peaks at 37.60 , 43.69 , 63.51 and 76.21 related to CrN (111), CrN (200), CrN (220) and CrN (311) (JCPDS 11e0065). The intensity of the CrN (200) major peak increases gradually as the Ni concentration increases from 2.4 to 8.3 at% but decreases as the Ni concentration further increases to 13.9 at%. It demonstrates that the low Ni incorporation could promote the formation of CrN (200). According to Scherrer’s formula, the size of CrN grain perpendicular to (200) crystal plane is listed in Table 4. The grain size of CrSiN coating is 8.7 nm and increases to 9.6 nm at 5.2 at% Ni, but then decreases to 7.3 nm at 13.9 at% Ni. The variation of grain size is almost consistent with the intensity variation of CrN (200) peak. In order to confirm the existence form of Ni and Si elements, which are absence in XRD patterns, the Ni 2p and Si 2p core level XPS spectra of NieCrSiN-0.4, NieCrSiN-1.2 and NieCrSiN-2.0 coatings are illustrated in Fig. 3. The major peak in Ni 2p core level XPS spectra is NieNi bond at 853.0 eV. The weak NieO bonds at 854.7 eV and 856.5 eV are believed to be attributed to the residual oxygen on the coatings surface. It is indicated that the Ni concentration has no effect in forming new chemical bonds. Tan et al. [22] investigated the structure of CrNiN coatings and reported that no diffraction peaks related to nickel compounds were found due to ultra-low content of nickel (2.92 at.%) or amorphous phase (8.79 at.%). Thus, based on the NieNi bond in XPS but no Ni crystal peak in XRD pattern, Ni in NieCrSiN coatings is believed to exist as amorphous phase as well regardless of concentration. In addition, the major peak in Si 2p core level XPS spectra is SieN bond at 101.3 eV. However, it is worth noting that a SieSi bond starts to form in NieCrSiN-1.2 coating and becomes strong in NieCrSiN-2.0 coating. It is indicated that Si exists in the form of amorphous SiNx in NieCrSiN coating with low Ni concentration (5.2 at%) but in the form of both amorphous SiNx and amorphous Si in NieCrSiN coating with high Ni concentration (8.3 at%).
Table 3 Chemical compositions of the as-deposited coatings. Coatings
Ni (at.%)
Cr (at.%)
Si (at.%)
N (at.%)
O (at.%)
CrSiN NieCrSiN-0.4 NieCrSiN-0.8 NieCrSiN-1.2 NieCrSiN-1.6 NieCrSiN-2.0
0 2.4 5.2 8.3 10.8 13.9
39.5 38.0 37.5 36.8 36.4 34.8
9.4 10.0 8.3 7.9 7.2 6.9
46.7 45.4 44.8 43.0 41.7 40.7
4.4 4.2 4.2 4.0 3.9 3.8
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Fig. 2. XRD patterns of NieCrSiN coatings.
3.2. Mechanical properties of NieCrSiN coatings According to Oliver and Pharr approach [23], the average hardness H and elastic modulus E with standard deviation are listed in Table 4. As compared to CrSiN coating (28.9 GPa), the hardness of NieCrSiN-0.4 coating decreases to 26.9 GPa, but then the hardness of NieCrSiN-0.8 and NieCrSiN-1.2 coatings increases to 28.1 GPa and 28.6 GPa. As the current of NiCr target increases to 1.6 A, the hardness of NieCrSiN-1.6 coating decreases sharply to 20.1 GPa. In comparison, NieCrSiN-2.0 coating presents a relatively high hardness of 24.5 GPa. Taking elastic modulus into account, the ratio of H3/E2 as an indicator proportional to plastic deformation resistance is also listed in Table 4 [24]. It is obvious that the values of H3/E2 for NieCrSiN coatings are lower than that of CrSiN coating. It indicates
that Ni incorporation would enhance the plastic deformation ability of CrSiN coating due to the nature ductility of Ni. At the meantime, the ratio of hr/hmax could be calculated based on the loading and unloading curves. Where hr is the residual depth after removing the load whilst hmax is the maximum depth. As is known, the ratio of hr/hmax can be deemed as the plastic deformation ability of coatings as well [25,26]. As seen in Table 4, the most values of hr/ hmax for NieCrSiN coatings are higher than that of CrSiN coating. It proves that the Ni incorporation is able to enhance the plastic deformation ability of CrSiN coating from another point of view. The morphologies of indentation impressions under a penetration depth of 2.0 mm as well as the corresponding loading and unloading curves are illustrated in Fig. 4. No radial crack can be observed at impressions corner, but several circular cracks are observed around the impressions edge for all coatings. Moreover, the impressions insides of CrSiN, NieCrSiN-0.4, NieCrSiN-0.8 and NieCrSiN-1.2 coatings show some picture frame cracks, which are absence inside the impressions of NieCrSiN-1.6 and NieCrSiN-2.0 coatings. Additionally, it is worth noting that an obvious pop-in is only found on the loading curve of CrSiN coating while the loading curves of NieCrSiN coatings are smooth.
3.3. Tribological properties The friction behavior of coatings in artificial seawater is illustrated in Fig. 5a, which can be divided into two stages. The first stage is known as running-in period, in which the friction coefficient generally increases gradually with the sliding distance. It could be seen that the running-in period of CrSiN coating lasts for 250 m. After 2.4 at% and 5.2 at% Ni incorporations, the running-in periods of NieCrSiN-0.4 and NieCrSiN-0.8 coatings extend to
Table 4 Mechanical properties of NieCrSiN coatings. Coatings
H (GPa)
CrSiN NieCrSiN-0.4 NieCrSiN-0.8 NieCrSiN-1.2 NieCrSiN-1.6 NieCrSiN-2.0
28.9 26.9 28.1 28.6 20.1 24.5
± ± ± ± ± ±
3.5 2.5 2.7 2.3 2.7 2.4
E (GPa) 297 295 292 293 251 257
± ± ± ± ± ±
14 22 22 17 21 14
H3/E2 (GPa)
hr/hmax
sc (MPa)
D (nm)
0.274 0.224 0.262 0.273 0.129 0.224
0.512 0.527 0.515 0.501 0.580 0.523
450.4 364.2 1065.4 1522.6 28.5 1120.3
8.7 8.5 9.6 8.8 8.7 7.3
Fig. 3. (a) Ni 2p and (b) Si 2p core level XPS spectra of NieCrSiN-0.4, NieCrSiN-1.2 and NieCrSiN-2.0 coatings.
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Fig. 4. Morphologies of indentation under 600 mN and the corresponding loading and unloading curves of (a1) (a2) CrSiN (b1) (b2) NieCrSiN-0.4 (c1) (c2) NieCrSiN-0.8 (d1) (d2) NieCrSiN-1.2 (e1) (e2) NieCrSiN-1.6 (f1) (f2) NieCrSiN-2.0 coatings.
Fig. 5. (a) Friction behavior and (b) Mean-steady friction coefficient of NieCrSiN coatings.
around 450 m. When the Ni incorporation increases to 8.3 at%, the running-in period of NieCrSiN-1.2 coating decreases to 400 m. Furthermore, as the Ni concentration increases to 10.8 at% and 13.9 at%, the running-in periods of NieCrSiN-1.6 and NieCrSiN-2.0 coatings decrease to 130 m and 50 m, respectively. It indicates that the low Ni incorporation would extend the running-in period of CrSiN coating, but the high Ni incorporation would shorten it. The second stage is the steady period, in which friction coefficient fluctuates slightly around a certain value. It is obvious that except CrSiN coating, all NieCrSiN coatings present a typical steady period. In contrast, the friction coefficient of CrSiN coating keeps steady from 250 to 500 m, but then decreases gradually until the end of tribotest. By averaging the friction coefficient over steady period, the mean-steady friction coefficients of coatings are illustrated in Fig. 5b. The mean steady friction coefficients of all NieCrSiN
coatings (0.145e0.211) are higher than that of CrSiN coating (0.122), indicating that the Ni incorporation has no effect of reducing the friction coefficient of CrSiN coating. Generally, the friction coefficient is closely related to the interface condition especially roughness. The 3D morphologies as well as the surface roughness Sa of wear tracks are presented in Fig. 6. As seen in Fig. 6e and f, the morphologies of wear tracks on NieCrSiN-1.6 and NieCrSiN-2.0 coatings present many deep grooves, and the peaks of some grooves are even higher than the original coatings surface. On the contrary, the grooves on the wear tracks of CrSiN, NieCrSiN0.4, NieCrSiN-0.8 and NieCrSiN-1.2 coatings become shallow and are lower than the original coatings surface. The wear rates of coatings and SiC balls are illustrated in Fig. 7. The wear rates of SiC ball vary in the range of 4.0 107 mm3/Nm to 8.0 107 mm3/Nm. The SiC balls sliding against NieCrSiN coatings present lower wear rate than that of SiC ball sliding
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Fig. 6. 3D profiles of wear tracks on (a) CrSiN (b) NieCrSiN-0.4 (c) NieCrSiN-0.8 (d) NieCrSiN-1.2 (e) NieCrSiN-1.6 (f) NieCrSiN-2.0 coatings.
Fig. 7. (a) Coatings wear rate and (b) SiC balls wear rate.
against CrSiN coating. In contrast, the wear rates of coatings vary in the range of 8.5 108 mm3/Nm to 2.7 107 mm3/Nm. As compared to CrSiN coating, the wear rate of NieCrSiN-0.4 coating increases from 1.2 107 mm3/Nm to 1.5 107 mm3/Nm. As the current of NiCr target increases to 0.8 A, the wear rate of NieCrSiN0.8 coating decreases to 8.5 108 mm3/Nm. Then, the wear rate increases to the highest value of 2.7 107 mm3/Nm for NieCrSiN1.6 coating. At last, the wear rate of NieCrSiN-2.0 coating decreases to 1.7 107 mm3/Nm. 4. Discussions 4.1. Microstructure According to the standard reference sample (JCPDS 11e0065), the 2q value of CrN (200) should be 43.69 , but the CrN (200) diffraction peaks of CrSiN, NieCrSiN-0.4, NieCrSiN-0.8, NieCrSiN1.2, NieCrSiN-1.6 and NieCrSiN-2.0 coatings shift to low angles of 43.46 , 43.48 , 43.50 , 43.38 , 43.54 and 43.40 , respectively. It demonstrates that the distance between two (200) crystal planes is enlarged at different degrees. Since the radii of Si atom (Ra ¼ 0.118 nm) and Ni atom (Ra ¼ 0.124 nm) are both smaller than that of Cr atom (Ra ¼ 0.128 nm), thus the replacement of Cr atom in CrN lattice by Si or Ni atoms should shrink the distance between two (200) crystal planes. It implies that there is another reason
contributing to the distance enlargement of crystal planes. As described in Section 2.2, the XRD measurement was conducted under a q-2q symmetrical mode, and therefore the detected crystal planes of CrN (200) should be paralleled to coatings surface. As listed in Table 4, all coatings possess compressive stress that squeezes the coatings at the horizontal direction. Under such a circumstance, the distance of (200) crystal plane paralleled to coatings surface is enlarged [27]. As a result, the CrN (200) diffraction peaks of all coatings shift to low angle. The similar phenomenon has been reported in CrN coatings deposited by different methods [28]. As shown in Fig. 2, as compared to the CrN (200) peak of CrSiN coating (43.46 ), the CrN (200) peaks of NieCrSiN-0.4, NieCrSiN0.8 and NieCrSiN-1.6 coatings shift to high values (43.48 , 43.50 and 43.54 ) while those of NieCrSiN-1.2 and NieCrSiN-2.0 coatings shift to low values (43.38 and 43.40 ). This difference is attributed to the combination of two major aspects. The first aspect is the compressive stress of coatings, which has mentioned above. The second aspect is the effect of substitution solid solution. Because the radius of Ni atom (Ra ¼ 0.124 nm) is less than that of Cr atom (Ra ¼ 0.128 nm), the replacement of Cr atom by Ni atom in CrN lattice would cause the shrink of plane distance, which leads to a right shift of diffraction peak based on Bragg formula [29]. Jayaganthan et al. [30] reported the right shift of (200) orientation for CrSiN coatings due to the substitution of Cr atom (Ra ¼ 0.128 nm) by
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Si atom (Ra ¼ 0.118 nm). Thus, as compared to CrSiN coating (43.46 ), the relatively low angles of CrN (200) for NieCrSiN-1.2 and NieCrSiN-2.0 coatings (43.38 and 43.40 ) demonstrate that the effect of compressive stress is dominant, which is well consistent with their high compressive stresses (1522.6 MPa and 1120.3 MPa vs 450.4 MPa). With regards to NieCrSiN-0.4 and NieCrSiN-1.6 coatings, the enlarging effect of compressive stress (364.2 MPa and 28.5 MPa vs 450.4 MPa) becomes weak. Namely, the lattice shrinkage from substitution solid solution counteracts the enlarging effect of compressive stress at a certain degree. Therefore, the diffraction peaks of NieCrSiN-0.4 and NieCrSiN-1.6 coatings shift to relatively high angles (43.48 and 43.54 ). 4.2. Mechanical properties As listed in Table 4, the hardness of all NieCrSiN coatings (20.1e28.6 GPa) is lower than that of CrSiN coating (28.9 GPa). It is believed that the soft feature of Ni contributes to this result. In addition, the hardness of NieCrSiN coatings is dependent on the Ni concentration. According to Hall-Petch law [31e33], small grain would contribute to high hardness. Meanwhile, high compressive stress played a significant role in contributing to high hardness as well [34,35]. Thus, the combination of grain size and compressive stress determines the hardness variation of NieCrSiN coatings. As compared to CrSiN coating (8.7 nm and 450.4 MPa), NieCrSiN-0.4 coating has a small crystal grain size (8.5 nm) and a low compressive stress (364.2 MPa). Thus, NieCrSiN-0.4 coating presents a lower hardness (26.9 GPa) than that of CrSiN coating (28.9 GPa). As the current of NiCr target increases to 0.8 A and 1.2 A, the crystal grain size of NieCrSiN-0.8 and NieCrSiN-1.2 coatings increases slightly to 9.6 nm and 8.8 nm, but their compressive stress increases sharply to 1065.4 MPa and 1522.6 MPa. It implies that the enhancement effect of high compressive stress is over the weaken effect of big grain size. As a consequence, the hardness of NieCrSiN-0.8 and NieCrSiN-1.2 coatings increases to 28.1 and 28.6 GPa. In contrast, a big drop of compressive stress (28.5 MPa) and more Ni incorporation lead to the lowest hardness of NieCrSiN-1.6 coating (20.1 GPa). With regards to NieCrSiN-2.0 coating, the grain size decreases to 7.3 nm and the compressive stress increases to 1120.3 MPa. Under the enhancement effects of both grain size and compressive stress, the hardness of NieCrSiN-2.0 coating increases to 24.5 GPa eventually. Wang et al. [36,37] and Jungk et al. [38] pointed out that no radial crack was found on the coatings possessing high compressive stress. It indicates that high compressive stress is beneficial to inhibit the formation of radial cracks. Thus, except NieCrSiN-1.6 coating, the high compressive stress of the rest coatings (from 364.2 to 1522.6 MPa) in this study prevents the formation of radial cracks in Fig. 4. For NieCrSiN-1.6 coating, its good plastic deformation ability proved by the lowest ratio of H3/E2 and the highest ratio of hr/hmax retards the formation of radial cracks. On the contrary, the compressive stress is able to promote the formation of circular crack especially under the “sink-in” condition as shown in Fig. 8. The morphologies of impression in Fig. 4 show that the impressions of all coatings are under sink-in condition and display the circular cracks. As seen in Fig. 8, the end point of sink-in was prone to the formation of circular crack, and the continuous circular cracks as the indentation proceeding would form the picture frame cracks as shown in Fig. 4, which have been pointed out in studies [39,40]. As seen in Fig. 4, the CrSiN, NieCrSiN-0.4, NieCrSiN-0.8 and NieCrSiN-1.2 coatings all present picture frame cracks inside the impression. In comparison, no obvious picture frame cracks are observed on NieCrSiN-1.6 and NieCrSiN-2.0 coatings. It demonstrates that the critical load of generating circular cracks for NieCrSiN-1.6 and NieCrSiN-2.0 coatings is high, thus indicating
Fig. 8. Schematic diagram of impression under sink-in condition.
that the high Ni content incorporation (10.8 at% and 13.9 at%) could enhance the toughness of CrSiN coating. In addition, as mentioned in the Section 3.2, a pop-in is only found on the loading curve of CrSiN coating. The pop-in on the loading curve meant a sudden depth penetration of indenter tip, which was attributed to the formation of crack as reported in studies [41,42]. According to the relationship of penetration depth h and edge length a as below [43]:
pffiffiffi a ¼ 2 3h tan65:27
(5)
The corresponding edge length h of indentation impression at pop-in depth (1.35 mm) should be 10.15 mm, which has been marked in Fig. 4a1. It can be seen that the position of the triangle with 10.15 mm edge corresponds to an obvious picture frame crack, which results from the formation of circular crack at 300 mN during loading process. In contrast, no obvious pop-in is observed on the loading curves of all NieCrSiN coatings. It is indicated from another point of view that Ni incorporation can improve the toughness of CrSiN coating. To compare the toughness of CrSiN and NieCrSiN coatings with low Ni incorporation (2.4e8.3 at%), the corresponding toughness KIC was calculated according to Eq. (6) based on the picture frame cracks as reported in studies [39,40].
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ufra E KIC ¼ 1 n2 Afra
(6)
Where Ufra is the fracture dissipated energy, E is the elastic modulus of coatings, Poisson ratio n ¼ 0.25 is used in the absence of better data for the coatings of interest. Afra is the total fracture area. Among them, the Ufra could be obtained using Eq. (7) as below: Ufra ¼ W T-We-Wp
(7)
Where WT and We are the total work and elastic deformation work, which could be derived from loading and unloading curves. Wp is plastic deformation work that could be estimated by Eq. (8):
hr Wp ¼ 1:27 0:27 WT hmax
(8)
In addition, the total fracture area Afra can be calculated using Eq. (9) as below:
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Q. Wang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
Afra ¼
3b2 t 2s
(9)
Where b and s are the radial dimension of impression and the space between cracks as shown in Fig. 8 t is the thickness of coatings. As listed in Table 5, the toughness of NieCrSiN-0.4, NieCrSiNpffiffiffiffiffi 0.8 and NieCrSiN-1.2 coatings (5.95e8.05 MPa$ m) is all higher pffiffiffiffiffi than that of CrSiN coating (5.91 MPa$ m), which is well consistent with the pop-in phenomena on loading curves in Fig. 4. NieCrSiNpffiffiffiffiffi 0.8 coating presents the highest toughness of 8.05 MPa$ m. Since no obvious picture frame cracks are observed on NieCrSiN-1.6 and NieCrSiN-2.0 coatings, thus we assume their fracture toughness pffiffiffiffiffi pffiffiffiffiffi higher than 8.05 MPa$ m(>8.05 MPa$ m). The relationship of fracture toughness, Ni concentration and mechanical properties is illustrated in Fig. 9. Apparently, the fracture toughness is dependent on the combination of compressive stress and hr/hmax. To be specific, without Ni incorporation (0 at%), the compressive stress and hr/hmax of CrSiN coating are both low, thus leading to the lowest pffiffiffiffiffi fracture toughness of 5.91 MPa$ m. As Ni concentration increases to 2.4 at%, 5.2 at% and 8.3 at%, either compressive stress or hr/hmax increases, which contributes to the improved fracture toughness of NieCrSiN-0.4, NieCrSiN-0.8 and NieCrSiN-1.2 coatings pffiffiffiffiffi (5.95e8.05 MPa$ m). When Ni concentration reaches 10.8 at% and 13.9 at%, the combination effect of compressive stress and hr/hmax becomes more beneficial to enhance fracture toughness. As a result, pffiffiffiffiffi the fracture toughness higher than 8.05 MPa$ m is obtained for NieCrSiN-1.6 and NieCrSiN-2.0 coatings.
4.3. Tribological properties As shown in Fig. 5b, the mean-steady friction coefficients of NieCrSiN coatings (0.145e0.211) are higher than that of CrSiN coating (0.122) especially NieCrSiN-1.6 coating presenting the highest value of 0.211. The surface roughness Sa of wear track on each coating is illustrated in Fig. 6. The roughness Sa of NieCrSiN0.4 and NieCrSiN-1.6 coatings are 76 nm and 99 nm that are higher than that of CrSiN coating (65 nm). Thus, the mean-steady friction coefficients of NieCrSiN-0.4 (0.175) and NieCrSiN-1.6 coatings (0.211) are much higher than that of CrSiN coating (0.122). The surface roughness Sa of wear track on the rest coatings (54e59 nm) is similar as that of CrSiN (65 nm), thus their mean-steady friction coefficient is close to that of CrSiN coating. The reason to the different surface roughness of wear track is believed to be the plastic deformation ability of coatings. As mentioned above, H3/E2 is an indicator proportional to plastic deformation resistance while hr/hmax reflects plastic deformation ability. According to the values of H3/E2 and hr/hmax in Table 4, NieCrSiN-1.6 coating should have the best plastic deformation ability. As a consequence, the NieCrSiN-1.6 coating material is prone to plastic deformation and forms deep grooves with the highest roughness Sa (99 nm) that leads to its highest friction coefficient (0.211). The morphologies and chemical compositions of wear scars on SiC balls are illustrated in Fig. 10. Apparently, some obvious grooves
Table 5 Toughness of coatings based on picture frame cracks. Coatings
a (mm)
s (nm)
CrSiN NieCrSiN-0.4 NieCrSiN-0.8 NieCrSiN-1.2 NieCrSiN-1.6 NieCrSiN-2.0
7.647 7.428 7.426 7.226 e e
525 521 456 508 e e
± ± ± ±
30 38 11 17
t (mm)
Ufra ( 109 N m)
pffiffiffiffiffi KIC (MPa$ m)
2.73 2.36 2.82 2.77 2.36 2.50
50.4 55.9 106.5 48.3 e e
5.91 ± 6.85 ± 8.05 ± 5.95 ± >8.05 >8.05
1.42 1.86 1.26 1.09
Fig. 9. The relationship of fracture toughness, Ni concentration and mechanical properties of NieCrSiN coatings.
are observed on the wear scars of SiC balls, which are well consistent with the grooves on the wear track of coatings as shown in Fig. 6. The light and dark areas can be observed on the wear scar of SiC balls, but the chemical compositions on the different areas are quite similar (See inset in Fig. 10). Thus, the reason contributing to the light and dark areas is light reflex rather than the different materials. Owing to quite low concentration of Cr (0.1e0.2 at%) detected, almost no transfer layer from coatings adheres on SiC balls. A small amount of O (0.9e4.6 at%) was detected on SiC balls, which indicated the hydration reaction of SiC in aqueous environment that have been widely reported [44e46]. On the contrary, C at different concentrations was detected on the wear track of all coatings as shown in Fig. 11. It demonstrates that the wear debris of SiC balls transfers onto coatings surface. Moreover, the detected Na, Mg and Cl elements are the residual of seawater. It is worth noting that the ratio of N/Cr on the wear track (0.18e0.22) (See in Fig. 11) is much lower as compared to the original ratio (1.18e1.19) (See in Table 3). It implies that N in coating was lost during friction. As analyzed in microstructure, CrSiN and NieCrSiN coatings consist of CrN crystal dispersing into a-SiNx matrix. According to the Gibbs free energy of equations as below, the chemical reactions of CrN and a-SiNx might occur as similar as Eqs. (10) and (11) [21,47]: 2CrNþ3H2O]Cr2O3þ2NH3
(10)
DGf298 ¼ 250:10 kJ,mol1 Si3N4þ6H2O ¼ 3SiO2þ4NH3
(11)
DGf298 ¼ 570:06 kJ,mol1 Thus, N was removed in the form of NH3 gas, and the formed Cr2O3 and SiO2 contributed to the O element detection on the wear track. In contrast, the Ni in coatings was hard to react with water as the Gibbs free energy of reaction is larger than zero as below: Ni þ H2O]NiO þ H2
(12)
DGf298 ¼ 26:14 kJ,mol1 Thus, based on the above analyses, both coating and SiC ball could have reactions with water and form oxides layer, which is called tribochemical wear. Zhou et al. [48,49] have pointed out that
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
Q. Wang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
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Fig. 10. SEM images of wear scars on SiC balls sliding against (a1-a3) CrSiN (b1-b3) NieCrSiN-0.4 (c1-c3) NieCrSiN-0.8 (d1-d3) NieCrSiN-1.2 (e1-e3) NieCrSiN-1.6 (f1-f3) NieCrSiN2.0 coatings.
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
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Q. Wang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
Fig. 11. SEM images of wear scars on SiC balls sliding against (a1-a3) CrSiN (b1-b3) NieCrSiN-0.4 (c1-c3) NieCrSiN-0.8 (d1-d3) NieCrSiN-1.2 (e1-e3) NieCrSiN-1.6 (f1-f3) NieCrSiN2.0 coatings.
Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998
Q. Wang et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
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Acknowledgement This work has been supported by National Natural Science Foundation of China (Grant No. 51705245 and 51775271); Natural Science Foundation of Jiangsu Province (Grant No. BK20170794). The Tribology Science Fund of State Key Laboratory of Tribology (Grant No. SKLTKF17B05). The Foundation of Jiangsu Provincial Key Laboratory of Bionic Functional Materials. The Foundation of Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences (Grant No.2018K03). We would like to acknowledge them for their financial support. References
Fig. 12. Relationship between coatings hardness and wear rate.
the wear tracks on both coating and ball should be smooth when tribochemical wear was dominant during friction process. However, as seen in Figs. 6 and 10, obvious grooves are found on both of coating and ball. It is indicated that the abrasive wear rather than tribochemical wear dominates the wear process in this study. As seen in Fig. 12, the hardness and coatings wear rate presents a quasi-linear relationship. Namely, the coatings hardness dominates the wear resistance of coatings in this study. On this basis, the pffiffiffiffiffi improved toughness (8.05 MPa$ m) and the similar hardness (28.1 GPa) of NieCrSiN-0.8 coating make it present a slightly lower wear rate (8.5 108 mm3/Nm) than that of CrSiN coating (1.2 107 mm3/Nm).
5. Conclusions To improve the fracture toughness and tribological properties of CrSiN coatings in seawater, Ni at various concentrations has been incorporated into CrSiN coatings. Through indentation measurements and tribotests, it was found that: 1) The incorporated Ni in NieCrSiN coatings existed in the form of amorphous phase. 2) The Ni incorporation induced the increment of compressive stress first but then the decrement of compressive stress, which dominated the variation of hardness. 3) The Ni incorporation especially at high concentration was able to enhance the toughness of CrSiN coating but sacrificing the hardness. 4) 5.2 at% Ni incorporation made NieCrSiN-0.8 present a better wear resistance than that of CrSiN coating due to the improved pffiffiffiffiffi toughness (8.05 MPa$ m) and similar hardness (28.1 GPa).
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Please cite this article as: Q. Wang et al., The influence of Ni concentration on the structure, mechanical and tribological properties of NieCrSiN coatings in seawater, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152998