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Influence of the negative bias in ion plating on the microstructural and tribological performances of Ti–Si–N coatings in seawater
Surface & Coatings Technology 280 (2015) 154–162
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of the negative bias in ion plating on the microstructural and tribological performances of Ti–Si–N coatings in seawater Yirong Yao a,b, Jinlong Li a,⁎, Yongxin Wang a, Yuwei Ye a, Lihui Zhu b a b
Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China School of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China
a r t i c l e
i n f o
Article history: Received 9 April 2015 Revised 3 September 2015 Accepted in revised form 4 September 2015 Available online 10 September 2015 Keywords: TiSiN coatings Multi-arc ion plating Tribological performance Seawater
a b s t r a c t The TiSiN coatings were deposited on Ti6Al4V alloy by multi-arc ion plating with different substrate negative bias to improve the tribological properties of titanium alloy in seawater. The TiSiN coatings have a coupled structure of amorphous Si3N4 and nanocrystalline TiN. The grain size decreases and the coatings become dense with the increase of the negative bias. The hardness of TiSiN coatings increases from 39 GPa to 47 GPa with the increase of substrate negative bias from 10 V to 40 V. The tribological tests reveal that the TiSiN coatings have a lower friction coefficient and higher wear rate in seawater compared with in atmosphere. Moreover, the wear rates increase with the increase of bias voltage. The TiSiN coating deposited at bias voltage of 10 V shows the best tribological properties in seawater. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Titanium alloys have been widely used in manufacturing ships, submersibles, ocean exploration and marine equipment as the key components due to its lightweight, high strength, particularly excellent corrosion resistant in marine environment. However, the poor wear resistance has restricted the application of titanium alloy. Especially, some crucial frictional components (such as pump, hydraulic system, valve, gear, shaft and propeller) which are surrounded by seawater directly have to face the complex work condition containing both wear and corrosion [1–2]. Thus, excellent tribological resistance is crucial for the service life and safety of these components. It is a promising method to improve the tribological resistance by depositing coatings with excellent performance on substrates using PVD technology. TiN coating is extensively used to improve the wear resistance due to its low friction coefficient, good chemical stability and high hardness [2–7]. The element Si was incorporated into TiN coating to form the TiSiN coating, which has higher hardness and lower wear rate due to its special nanocomposite structure compared with the TiN [8,9]. S.Z. Li et al. had firstly reported the TiSiN coating deposited by PCVD using chlorides as source of Si and Ti [10]. Veprek et al. revealed a TiSiN coating with super hardness about 50–60 GPa [11], and the authors argue that the hardness enhancement attributed to the binary nc-TiN/aSi3N4 nature of these coatings (nc- and a- means nanocrystalline and ⁎ Corresponding author. E-mail address: [email protected] (J. Li).
http://dx.doi.org/10.1016/j.surfcoat.2015.09.005 0257-8972/© 2015 Elsevier B.V. All rights reserved.
amorphous, respectively). The TiSiN coatings often were prepared by CVD [12], PCVD [10] and PVD [13–15] methods. However, for CVD processes, the requirement of poisonous gases (SiCl4) and high deposition temperature (N 600 °C) restricts its application. PVD techniques are considered more suitable to synthesis than the TiSiN coatings [13], such as unbalance magnetron sputtering [14] and electron beam ion plating [15]. Many researchers have reported the hardness enhancement mechanism, thermal stability and tribological behavior in atmosphere of the TiSiN coatings [16–18]. However, there is a lack of information on tribological behavior of the TiSiN coating in marine environment. In this paper, the TiSiN coatings were deposited by multi-arc ion plating with different negative bias. The effects of substrate negative bias on the structure and tribological behaviors in seawater of the TiSiN coating were investigated. 2. Experimental details The commercial Ti6Al4V alloy was used as the substrate. The samples with a size of 15 mm × 15 mm × 4 mm were polished to mirror and ultrasonically cleaned in acetone for 20 min before deposition. The chamber was pumped down to base pressure of 4 × 10−5 Pa. The TiSi (Ti 90 wt.% and Si 10 wt.%) target was used to prepare coatings by multi-arc ion plating system (Hauzer Flexicoat 850), The substrates were etched for 2 min to clean the impurities with negative bias of 900 V, 1100 V and 1200 V, respectively, in the gas of Ar (99.99%). Firstly, the thin TiSiN interlayer was deposited with a N2 (99.99%) pressure of 3 × 10−2 Pa and bias negative voltage of 40 V, at temperature of 450 °C
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Fig. 1. Surface morphology of TiSiN coatings deposited with different negative bias: (a) 10 V (b) 20 V (c) 30 V (d) 40 V.
Table 1 Roughness and thickness of TiSiN coatings at different bias voltages. Bias (−V)
10
20
30
40
Roughness (nm) Thickness (μm)
134.9 ± 1.89 1.8
110.2 ± 1.24 2.0
122.5 ± 1.37 1.7
138.4 ± 1.48 1.2
for 10 min. Secondly, the TiSiN coatings were deposited with a target current of 60 A and a distance from the substrate to the target of 100 mm, the negative bias was 10 V, 20 V, 30 V and 40 V, respectively, for 120 min. During deposition, the temperature is fixed at 450 °C and nitrogen pressure is 3 × 10−2 Pa.
The phase structure was investigated by X-ray diffraction (Bruker D8X-ray facility) with Cu Kα radiation (λ = 0.154 nm). The scanning angle was ranged from 20 to 90°, every sample was scanned for 8 min. Field emission scanning electron-microscope (Hitachi S4800) and Zeiss large chamber scanning electron microscope (EV018) were used to observe the cross-section and wear track morphologies of the coatings. The element chemical states were determined by X-ray photoelectron spectroscopy (AXIS-ULTRA, Kratos) with Al (mono) Kα X-ray under 12 kV and 10 mA, the surface of these samples were etched about 2 min before XPS experiment. The structure was characterized by transmission electron microscope (FEI Tecnai F20). Hardness tests were measured in a MTS Nano G200 system with a Berkovich indenter using the continuous stiffness measurement (CSM).
Fig. 2. SEM images of cross-section fractured TiSiN coatings with different negative bias: (a) 10 V (b) 20 V (c) 30 V (d) 40 V.
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Fig. 3. XPS spectra for TiSiN coatings deposited at bias voltage of 30 V:(a) Ti 2p (b) Si 2p (c) N 1s.
UMT-3MT tribometer (CETR, USA) was used to investigate the tribological performance of the coatings. The ball-on-disk reciprocating module was selected to carry out the test in seawater environment at room temperature. The artificial seawater was prepared according to standard ASTMD 1141–98. SiC with a diameter of 3 mm were used as the mating balls. The parameters of the measurement were as follows: sliding speed is 300 rpm, load is 5 N, sliding length is 5 mm, the frequency is 5 Hz, and sliding time is 1800 s. The depth of wear track was measured by Alpha-Step IQ profilometer. The volume V of the coating loss during friction could be evaluated. The wear rate of the coating was calculated based on the classical equation K¼
V FS
where F is the load applied and S is the distance of the sliding. Table 2 The chemical components of TiSiN coatings at different bias voltage. Bias (−V)
10 20 30 40
(at.%) Si
Si3N4
6.39 6.99 7.41 7.1
75.38 77.33 80.73 85.65
O (at.%)
Ti (at.%)
N (at.%)
C (at.%)
15.59 17.86 17.03 18.01
13.58 13.9 12.57 15.73
22.19 23.27 20.01 23.67
42.26 37.98 42.97 35.49
Fig. 4. XRD patterns of TiSiN coatings.
Y. Yao et al. / Surface & Coatings Technology 280 (2015) 154–162 Table 3 Grain size of TiSiN coatings at different bias voltages. Bias (V)
TiN (111)
TiN (220)
Average grain size
−10 V −20 V −30 V −40 V
25.3 20.4 14.5 –
23.9 11.0 9.4 10.8
37.9 ± 0.329 19.9 ± 0.117 11.5 ± 0.826 11.7 ± 0.659
3. Results 3.1. Morphologies and structure Fig. 1 shows SEM images of the TiSiN coatings. Many big particles can be observed on the surface of all samples. The roughness of the samples was measured by Alpha-Step IQ profilometer to comment the number and size of these particles. As shown in Table 1, with the increase of the bias voltage from 10 to 40 V, the roughness value of the TiSiN coatings is fluctuant in a small range, and the roughness decreases from 10 to 20 V, and then increases from 20 to 40 V. The particles were originated from the liquid drops emitted from the targets during deposition. Fig. 2 shows cross-section images of the TiSiN coatings. The thicknesses of the coatings measured by SEM are displayed in Table 1. The change of coating thickness can be explained by the change of ion bombarding energy. By increasing bias voltage, the energy of the ions arriving to the substrates increases, which can deliver high energy to deposited coating and then results in the thickness increase of the coating; this is closely related to the diffusion of adatom, however the further increase of bias voltage will lead to the occurrence of resputtering which can cause a decrease of thickness [19–21]. When the bias voltage is 10 V, the TiSiN coating displays a distinct columnar structure, and further increase of bias voltage induces a decrease of columnar width and the formation of a more dense structure. The X-ray photoelectron spectroscopy (XPS) was used to investigate the element chemical states of the TiSiN coatings. Fig. 3 show The XPS wide-scan spectra and high-resolution spectra of Ti 2p, Si 2p and N 1 s for TiSiN coatings deposited at negative bias voltage of 30 V. The survey
157
general spectra indicate that the coatings are mainly composed of Ti, Si, N, C and O, the result was summarized in Table 2. With the increase of bias voltage from 10 to 40 V, the concentrations of Ti, Si, N, C and O are fluctuant in a small range. This can be explained by the change of ion sputtering energy. The increase of bias voltage leads to the increase of ion energy, consequently, the content of these elements changes slightly [22]. The content of Si3N4 in the TiSiN coatings was evaluated from the fitting of the Si 2p spectra. It revealed that the Si3N4 had a rising tendency as bias voltage is increased. The Ti 2p peaks at 455.1, 456.3, 458.3 eV are corresponding to TiN, Ti2O3, TiO2 respectively [23–25], the Si 2p peaks at 101.7 eV, 102.2 eV are assigned to Si3N4 and SiO2 [26–27], the N 1s peaks are decomposed to three peaks at 396.3, 397.0, 398.5 eV, and they are Si3N4, TiON, TiN, respectively [28–30]. According to the results of the XPS, no free silicon or silicide (TiSix) was formed, which is consistent with other TiSiN reported [9]. From the result of XPS, it is important to note that the coatings contamination with oxygen, this was frequently reported for various nitrides coatings [31], this may correlate with the oxidation during samples handling and storage in free atmosphere [18]. Fig. 4 shows X-ray diffraction patterns of the TiSiN coatings deposited on titanium alloy. The results reveal that the coatings mainly consist with TiN (111) and TiN (200) at 2 theta = 36.7° and 2 theta = 43°, this is in agreement with TiSiN films deposited by cathodic arc technique [32–33]. At the same time, other littery peaks were the signal from substrate. The XRD patterns at around 2 theta = 43° have a movement towards small angle as bias voltage increase. The reason is that the change of negative bias voltage may have a significant effect on residual stress of the coatings and further induce the peak shift of diffraction peaks. TiN which can be assigned to the cubic B1 NaCl structure with a lattice parameter of approximately 0.430 nm exhibited (111), (200) [34]. When the substrate negative bias was 10 V, TiN exhibited strong (111) and (200) preferred orientation. Along with the negative bias voltage increased the (111) peak gradually changed to (200), when the substrate negative bias reached to 40 V, TiN presented a strong (200) orientation. Which was consistent with the reported results of Alina Vladescu [18]. When the negative bias further increased, the preferred coating orientation of TiN changed from (111) to (200), and the
Fig. 5. High-resolution electron microscopy images for TiSiN coatings with different negative bias: (a) 10 V (b) 20 V (c) 30 V (d) 40 V.
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Fig. 8. Wear rates of TiSiN coatings in air and seawater.
Fig. 6. Hardness of TiSiN coatings deposited at different negative bias: (a) 10 V (b) 20 V (c) 30 V (d) 40 V.
peaks broadened. Which is related to grain sizes and/or micro-strains due to defects [35]. The preferred orientation is dominated by minimization of the overall coating energy, namely the sum of strain and surface energies. The negative bias influenced the bombardment parameters (such as momentum and kinetic energy, etc.) [8]. The TiN
(111), TiN (220) and average grain size were also measured using XRD by Scherrer formula: size ¼
kλ FWðSÞ cosðθÞ
[36], which are listed in Table 3. As the (111) and (200) planes own the lowest surface and strain energy, respectively [35], thus the texture change from (111) to (200) occurred when the residual stress significantly decreases [18]. In other words, the change of negative bias voltage may have a significant effect on residual stress of the coatings. The high resolution TEM images of the TiSiN coatings are shown in Fig. 5. For the coatings deposited at negative bias voltage of 10 V, the grains are large, while the increase of negative bias leads to the finite crystallite size [37]. The TiSiN coatings consist of two phases: nanocrystalline TiN and amorphous Si3N4 around the TiN grain boundaries (nc-TiN/a-Si3N4) [38]. As shown in Table 3, the grain size from XRD and TEM were at same order of magnitude, the two first rows were obtained from the XRD analysis and the last row from the TEM analysis, and with the increase of substrate negative bias, the grain sizes decreased from 37.9 nm to 11.5 nm, indicating that the grain sizes were refined much more. 3.2. Mechanical and tribological behaviors
Fig. 7. Coefficient of friction of TiSiN coatings under different conditions: (a) air (b) seawater.
Fig. 6 shows the hardness of the TiSiN coatings deposited at different negative bias. With the increase of substrate negative bias from 10 to 40 V, the hardness increases from about 39 to 47 GPa due to the grain refining and structure densification. Fig. 7 shows the variation of coefficient of friction (COF) with sliding time for different samples. In atmosphere, the coefficients gradually increase from 0.2 to 0.4 in the initial stage (sliding time 0–800 s) for the TiSiN coatings deposited at 10 V and 20 V. However, for the coatings deposited at bias voltages of 30 V and 40 V, the coefficients are 0.4 for all sliding time. While in artificial seawater, the coefficients for coatings deposited at bias voltages of 10, 20, 30 V are approximately kept at 0.30, which was obviously lower than that sliding in the air. For the coatings deposited at negative bias of 40 V, the coefficient is evidently higher than other samples, and this attribute to its high hardness and roughness. Fig. 8 shows the wear rate for different TiSiN coatings in atmosphere and artificial seawater. The wear rate increases with the increase of the negative bias, especially with negative bias of 10 V the coatings have the best wear resistant. Fig. 9 shows the morphologies of the wear track on the TiSiN coatings deposited with negative bias of 10 V and the corresponding EDS analysis in seawater. EDS and wear track morphology reveal that the coating was peeled off partly. The images of the wear tracks in other
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Fig. 9. Wear track of TiSiN coatings with negative bias for 10 V in seawater: (a) surface morphology, (b) flake pits, (c) depth profile, (d) EDS analysis.
TiSiN coating sliding in seawater environment are shown in Fig. 10– Fig. 12, respectively. Fig. 10 indicates that some pits formed and some microparticles were pulled out. Some deformed particles and cracks can be observed around the particles from Fig. 11. These cracks provide a favorable path for corrosion liquid. Fig. 12 shows the wear track of the coatings deposited at 40 V, the morphology of wear track presented as plowing grooves, and the coatings are completely destroyed. The results indicate that the negative bias voltage has a clear effect on tribological properties of the coatings.
Under lower negative bias voltage, the wear tracks are shallow and the wear rates are lower than the coatings deposited at higher substrate negative bias. The EDS results reveal that the coatings are peeled off more seriously as the negative bias voltage further increase, and the profiles show that the depth of the wear track are deeper at a condition of higher negative bias, and the detachment of the coatings is more serious. EDS analysis on the wear tracks also indicates that Ti, Al, V, Si, N, C and O elements are found on the wear track; these elements are mainly the composition of the
Fig. 10. Wear track of TiSiN coatings with negative bias for 20 V: (a) surface morphology, (b) flake pits, (c) depth profile, (d) EDS analysis.
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Fig. 11. Wear track of TiSiN coatings with negative bias for 30 V: (a) surface morphology, (b) flake pits, (c) depth profile, (d) EDS analysis.
Ti6Al4V substrate and the coatings, and indicate that the coatings are fatigue flaked. On the other hand, while sliding in seawater, the debris are also observed, and the friction coefficients are lower than that in ambient air. This attributes to the lubrication of seawater. Fig. 13 shows the micrograph of Vickers indentations with a load of 10 N for the TiSiN coatings. Severe edge cracks are observed in the coating deposited at negative bias voltage of 10 V, much more cracks are found compared with the higher bias voltage. With increase of the
bias, the coatings become brittle and are easy to wear out. On the other hand, the hardness of SiC matting ball is about 28 GPa [39], while coating deposited at negative bias voltage of 10 V has the hardness of 39 GPa, the wear mechanism between balls and coatings is the adhesive wear. However when the bias voltage increased to 40 V, the plough grooves were found, which attribute to the high hardness of the TiSiN coating about 47 GPa, and the wear mechanism is mainly the abrasive wear. In summary, the wear rate of coatings in artificial
Fig. 12. Wear track of TiSiN coatings with negative bias for 40 V: (a) surface morphology, (b) flake pits, (c) depth profile, (d) EDS analysis.
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Fig. 13. The micrograph of indentations for TiSiN coatings deposited at different negative bias: (a) 10 V (b) 20 V (c) 30 V (d) 40 V.
seawater was higher than that in air, and this is due to the corrosion of aqueous medium. However, the high wear rate of the coatings deposited at high bias voltage mainly attributes to the coating's brittleness and the change of wear mechanism during friction process. Fig. 14 shows the wear model of the TiSiN coatings sliding against SiC ball in seawater, Fig. 14 (a) is the contact model between the coatings and mating ball, Fig. 14 (b) is the wear mechanism of the coating. There are two kinds of micro particles in the coatings. One can be pulled out during cycle sliding and the other one can be named deformed microparticle, which is stubborn and does not fall off from the coatings after cycle friction. The deformed
microparticles have a distinct effect on the coefficient of friction during run-in period and lead to a high value of COF. The wear rates of the coatings are mainly from chipping and debris, and the wear mechanism is adhesive wear. The images in Figs. 9–12 indicate that the depth of the wear track become deeper when the bias voltage increases, at the same time the wear image of Fig. 11 and Fig. 12 presented plough grooves, the coatings are worn out and the number of micro cracks increased, which was consistent with the result of indentations. Thus the corrosion effect of seawater becomes more serious, which accelerates local delamination of the coatings, and the abrasive wear mechanism could dominate.
Fig. 14. Wear model of the TiSiN coatings sliding against SiC ball in seawater.
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3.3. Discussion The formation energy of TiN is about ΔG0298 = − 309.2 kJ/mol, and Si3N4 is ΔG0298 = − 665.40 kJ/mol [40]. Thus during deposition, amorphous Si3N4 with lower formation energy and atomic mobility was more stable, which grow along the crystal boundary of TiN. At the same time, even though with higher formation energy, TiN should easily form under pure N2 atmosphere as well. When the coatings deposited at low negative bias, the morphologies of cross-section presented as columnar structure. While the substrate negative bias increases, the columnar structure of the TiSiN coatings becomes dense. The XPS results show that the amount of amorphous Si3N4 increased. This change has a significant influence on the mechanical properties of the coatings. The columnar structure has an open inter-grain boundary which provided an access for corrosive seawater to degrade the bonding of the coating [38]. The nanocomposite of nc-TiN/a-Si3N4 structure which is proposed by Veprek is spinodal phase segregation, which is thermodynamically driven [41]. The increase of negative bias leads to the enhancement of the atomic diffusion and adatom surface mobility, which can provide necessary thermodynamic driving force for spinodal decomposition during deposition and result in the formation of nc-TiN/ a-Si3N4 nanocomposite [37]. The structure with smaller grains and greater grain boundaries can limit the sliding of dislocation and the extension of flaw. On the other hand, the thermodynamically driven phase segregation generates a strong interface between the nanocrystals TiN and the Si3N4, which lead to higher hardness of the coatings. However, the amount of amorphous Si3N4 should be controlled properly, otherwise, the mechanical properties will be declined [41]. The increase of the negative bias leads to a dense structure and high hardness of the coatings. But the residual stress increases as substrate negative bias increases, which may induce brittlement of the coatings; this is another important factor that affected the mechanical properties of the TiSiN coatings. 4. Conclusions The TiSiN coatings were deposited by multi-arc ion plating with different substrate negative bias. The effect of substrate negative bias on structures and tribocorrosion properties in artificial seawater was studied. The results reveal that the deposited TiSiN coatings have columnar structure, and the columnar structure becomes dense with the increase of the substrate negative bias. TEM images reveal that the TiSiN coatings have a coupled structure of amorphous Si3N4 and nanocrystalline TiN, and the grain size decreases with the increase of the voltage. The hardness of the TiSiN coatings increased from 39 to 47 GPa with the increase of voltage bias from 10 V to 40 V. Compared to atmosphere, the TiSiN coatings have lower friction coefficient and higher wear rate in seawater. Moreover, with the increase of the negative bias voltage, the wear mechanism changes from adhesion wear to abrasive wear, and the wear rate increases. At the same time the corrosion media accelerates the delamination of the coatings and induces the increase of wear
rate. The TiSiN coatings deposited at negative bias of 10 V show the best tribological properties in seawater. Acknowledgments This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LY14E010005), the Ningbo International Cooperation Project (2013D10005), and the National Natural Science Foundation of China (Grant No. 51575510). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]
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Influence of the negative bias in ion plating on the microstructural and tribological performances of Ti–Si–N coatings in seawater Yirong Yao a,b, Jinlong Li a,⁎, Yongxin Wang a, Yuwei Ye a, Lihui Zhu b a b
Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China School of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China
a r t i c l e
i n f o
Article history: Received 9 April 2015 Revised 3 September 2015 Accepted in revised form 4 September 2015 Available online 10 September 2015 Keywords: TiSiN coatings Multi-arc ion plating Tribological performance Seawater
a b s t r a c t The TiSiN coatings were deposited on Ti6Al4V alloy by multi-arc ion plating with different substrate negative bias to improve the tribological properties of titanium alloy in seawater. The TiSiN coatings have a coupled structure of amorphous Si3N4 and nanocrystalline TiN. The grain size decreases and the coatings become dense with the increase of the negative bias. The hardness of TiSiN coatings increases from 39 GPa to 47 GPa with the increase of substrate negative bias from 10 V to 40 V. The tribological tests reveal that the TiSiN coatings have a lower friction coefficient and higher wear rate in seawater compared with in atmosphere. Moreover, the wear rates increase with the increase of bias voltage. The TiSiN coating deposited at bias voltage of 10 V shows the best tribological properties in seawater. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Titanium alloys have been widely used in manufacturing ships, submersibles, ocean exploration and marine equipment as the key components due to its lightweight, high strength, particularly excellent corrosion resistant in marine environment. However, the poor wear resistance has restricted the application of titanium alloy. Especially, some crucial frictional components (such as pump, hydraulic system, valve, gear, shaft and propeller) which are surrounded by seawater directly have to face the complex work condition containing both wear and corrosion [1–2]. Thus, excellent tribological resistance is crucial for the service life and safety of these components. It is a promising method to improve the tribological resistance by depositing coatings with excellent performance on substrates using PVD technology. TiN coating is extensively used to improve the wear resistance due to its low friction coefficient, good chemical stability and high hardness [2–7]. The element Si was incorporated into TiN coating to form the TiSiN coating, which has higher hardness and lower wear rate due to its special nanocomposite structure compared with the TiN [8,9]. S.Z. Li et al. had firstly reported the TiSiN coating deposited by PCVD using chlorides as source of Si and Ti [10]. Veprek et al. revealed a TiSiN coating with super hardness about 50–60 GPa [11], and the authors argue that the hardness enhancement attributed to the binary nc-TiN/aSi3N4 nature of these coatings (nc- and a- means nanocrystalline and ⁎ Corresponding author. E-mail address: [email protected] (J. Li).
http://dx.doi.org/10.1016/j.surfcoat.2015.09.005 0257-8972/© 2015 Elsevier B.V. All rights reserved.
amorphous, respectively). The TiSiN coatings often were prepared by CVD [12], PCVD [10] and PVD [13–15] methods. However, for CVD processes, the requirement of poisonous gases (SiCl4) and high deposition temperature (N 600 °C) restricts its application. PVD techniques are considered more suitable to synthesis than the TiSiN coatings [13], such as unbalance magnetron sputtering [14] and electron beam ion plating [15]. Many researchers have reported the hardness enhancement mechanism, thermal stability and tribological behavior in atmosphere of the TiSiN coatings [16–18]. However, there is a lack of information on tribological behavior of the TiSiN coating in marine environment. In this paper, the TiSiN coatings were deposited by multi-arc ion plating with different negative bias. The effects of substrate negative bias on the structure and tribological behaviors in seawater of the TiSiN coating were investigated. 2. Experimental details The commercial Ti6Al4V alloy was used as the substrate. The samples with a size of 15 mm × 15 mm × 4 mm were polished to mirror and ultrasonically cleaned in acetone for 20 min before deposition. The chamber was pumped down to base pressure of 4 × 10−5 Pa. The TiSi (Ti 90 wt.% and Si 10 wt.%) target was used to prepare coatings by multi-arc ion plating system (Hauzer Flexicoat 850), The substrates were etched for 2 min to clean the impurities with negative bias of 900 V, 1100 V and 1200 V, respectively, in the gas of Ar (99.99%). Firstly, the thin TiSiN interlayer was deposited with a N2 (99.99%) pressure of 3 × 10−2 Pa and bias negative voltage of 40 V, at temperature of 450 °C
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Fig. 1. Surface morphology of TiSiN coatings deposited with different negative bias: (a) 10 V (b) 20 V (c) 30 V (d) 40 V.
Table 1 Roughness and thickness of TiSiN coatings at different bias voltages. Bias (−V)
10
20
30
40
Roughness (nm) Thickness (μm)
134.9 ± 1.89 1.8
110.2 ± 1.24 2.0
122.5 ± 1.37 1.7
138.4 ± 1.48 1.2
for 10 min. Secondly, the TiSiN coatings were deposited with a target current of 60 A and a distance from the substrate to the target of 100 mm, the negative bias was 10 V, 20 V, 30 V and 40 V, respectively, for 120 min. During deposition, the temperature is fixed at 450 °C and nitrogen pressure is 3 × 10−2 Pa.
The phase structure was investigated by X-ray diffraction (Bruker D8X-ray facility) with Cu Kα radiation (λ = 0.154 nm). The scanning angle was ranged from 20 to 90°, every sample was scanned for 8 min. Field emission scanning electron-microscope (Hitachi S4800) and Zeiss large chamber scanning electron microscope (EV018) were used to observe the cross-section and wear track morphologies of the coatings. The element chemical states were determined by X-ray photoelectron spectroscopy (AXIS-ULTRA, Kratos) with Al (mono) Kα X-ray under 12 kV and 10 mA, the surface of these samples were etched about 2 min before XPS experiment. The structure was characterized by transmission electron microscope (FEI Tecnai F20). Hardness tests were measured in a MTS Nano G200 system with a Berkovich indenter using the continuous stiffness measurement (CSM).
Fig. 2. SEM images of cross-section fractured TiSiN coatings with different negative bias: (a) 10 V (b) 20 V (c) 30 V (d) 40 V.
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Fig. 3. XPS spectra for TiSiN coatings deposited at bias voltage of 30 V:(a) Ti 2p (b) Si 2p (c) N 1s.
UMT-3MT tribometer (CETR, USA) was used to investigate the tribological performance of the coatings. The ball-on-disk reciprocating module was selected to carry out the test in seawater environment at room temperature. The artificial seawater was prepared according to standard ASTMD 1141–98. SiC with a diameter of 3 mm were used as the mating balls. The parameters of the measurement were as follows: sliding speed is 300 rpm, load is 5 N, sliding length is 5 mm, the frequency is 5 Hz, and sliding time is 1800 s. The depth of wear track was measured by Alpha-Step IQ profilometer. The volume V of the coating loss during friction could be evaluated. The wear rate of the coating was calculated based on the classical equation K¼
V FS
where F is the load applied and S is the distance of the sliding. Table 2 The chemical components of TiSiN coatings at different bias voltage. Bias (−V)
10 20 30 40
(at.%) Si
Si3N4
6.39 6.99 7.41 7.1
75.38 77.33 80.73 85.65
O (at.%)
Ti (at.%)
N (at.%)
C (at.%)
15.59 17.86 17.03 18.01
13.58 13.9 12.57 15.73
22.19 23.27 20.01 23.67
42.26 37.98 42.97 35.49
Fig. 4. XRD patterns of TiSiN coatings.
Y. Yao et al. / Surface & Coatings Technology 280 (2015) 154–162 Table 3 Grain size of TiSiN coatings at different bias voltages. Bias (V)
TiN (111)
TiN (220)
Average grain size
−10 V −20 V −30 V −40 V
25.3 20.4 14.5 –
23.9 11.0 9.4 10.8
37.9 ± 0.329 19.9 ± 0.117 11.5 ± 0.826 11.7 ± 0.659
3. Results 3.1. Morphologies and structure Fig. 1 shows SEM images of the TiSiN coatings. Many big particles can be observed on the surface of all samples. The roughness of the samples was measured by Alpha-Step IQ profilometer to comment the number and size of these particles. As shown in Table 1, with the increase of the bias voltage from 10 to 40 V, the roughness value of the TiSiN coatings is fluctuant in a small range, and the roughness decreases from 10 to 20 V, and then increases from 20 to 40 V. The particles were originated from the liquid drops emitted from the targets during deposition. Fig. 2 shows cross-section images of the TiSiN coatings. The thicknesses of the coatings measured by SEM are displayed in Table 1. The change of coating thickness can be explained by the change of ion bombarding energy. By increasing bias voltage, the energy of the ions arriving to the substrates increases, which can deliver high energy to deposited coating and then results in the thickness increase of the coating; this is closely related to the diffusion of adatom, however the further increase of bias voltage will lead to the occurrence of resputtering which can cause a decrease of thickness [19–21]. When the bias voltage is 10 V, the TiSiN coating displays a distinct columnar structure, and further increase of bias voltage induces a decrease of columnar width and the formation of a more dense structure. The X-ray photoelectron spectroscopy (XPS) was used to investigate the element chemical states of the TiSiN coatings. Fig. 3 show The XPS wide-scan spectra and high-resolution spectra of Ti 2p, Si 2p and N 1 s for TiSiN coatings deposited at negative bias voltage of 30 V. The survey
157
general spectra indicate that the coatings are mainly composed of Ti, Si, N, C and O, the result was summarized in Table 2. With the increase of bias voltage from 10 to 40 V, the concentrations of Ti, Si, N, C and O are fluctuant in a small range. This can be explained by the change of ion sputtering energy. The increase of bias voltage leads to the increase of ion energy, consequently, the content of these elements changes slightly [22]. The content of Si3N4 in the TiSiN coatings was evaluated from the fitting of the Si 2p spectra. It revealed that the Si3N4 had a rising tendency as bias voltage is increased. The Ti 2p peaks at 455.1, 456.3, 458.3 eV are corresponding to TiN, Ti2O3, TiO2 respectively [23–25], the Si 2p peaks at 101.7 eV, 102.2 eV are assigned to Si3N4 and SiO2 [26–27], the N 1s peaks are decomposed to three peaks at 396.3, 397.0, 398.5 eV, and they are Si3N4, TiON, TiN, respectively [28–30]. According to the results of the XPS, no free silicon or silicide (TiSix) was formed, which is consistent with other TiSiN reported [9]. From the result of XPS, it is important to note that the coatings contamination with oxygen, this was frequently reported for various nitrides coatings [31], this may correlate with the oxidation during samples handling and storage in free atmosphere [18]. Fig. 4 shows X-ray diffraction patterns of the TiSiN coatings deposited on titanium alloy. The results reveal that the coatings mainly consist with TiN (111) and TiN (200) at 2 theta = 36.7° and 2 theta = 43°, this is in agreement with TiSiN films deposited by cathodic arc technique [32–33]. At the same time, other littery peaks were the signal from substrate. The XRD patterns at around 2 theta = 43° have a movement towards small angle as bias voltage increase. The reason is that the change of negative bias voltage may have a significant effect on residual stress of the coatings and further induce the peak shift of diffraction peaks. TiN which can be assigned to the cubic B1 NaCl structure with a lattice parameter of approximately 0.430 nm exhibited (111), (200) [34]. When the substrate negative bias was 10 V, TiN exhibited strong (111) and (200) preferred orientation. Along with the negative bias voltage increased the (111) peak gradually changed to (200), when the substrate negative bias reached to 40 V, TiN presented a strong (200) orientation. Which was consistent with the reported results of Alina Vladescu [18]. When the negative bias further increased, the preferred coating orientation of TiN changed from (111) to (200), and the
Fig. 5. High-resolution electron microscopy images for TiSiN coatings with different negative bias: (a) 10 V (b) 20 V (c) 30 V (d) 40 V.
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Fig. 8. Wear rates of TiSiN coatings in air and seawater.
Fig. 6. Hardness of TiSiN coatings deposited at different negative bias: (a) 10 V (b) 20 V (c) 30 V (d) 40 V.
peaks broadened. Which is related to grain sizes and/or micro-strains due to defects [35]. The preferred orientation is dominated by minimization of the overall coating energy, namely the sum of strain and surface energies. The negative bias influenced the bombardment parameters (such as momentum and kinetic energy, etc.) [8]. The TiN
(111), TiN (220) and average grain size were also measured using XRD by Scherrer formula: size ¼
kλ FWðSÞ cosðθÞ
[36], which are listed in Table 3. As the (111) and (200) planes own the lowest surface and strain energy, respectively [35], thus the texture change from (111) to (200) occurred when the residual stress significantly decreases [18]. In other words, the change of negative bias voltage may have a significant effect on residual stress of the coatings. The high resolution TEM images of the TiSiN coatings are shown in Fig. 5. For the coatings deposited at negative bias voltage of 10 V, the grains are large, while the increase of negative bias leads to the finite crystallite size [37]. The TiSiN coatings consist of two phases: nanocrystalline TiN and amorphous Si3N4 around the TiN grain boundaries (nc-TiN/a-Si3N4) [38]. As shown in Table 3, the grain size from XRD and TEM were at same order of magnitude, the two first rows were obtained from the XRD analysis and the last row from the TEM analysis, and with the increase of substrate negative bias, the grain sizes decreased from 37.9 nm to 11.5 nm, indicating that the grain sizes were refined much more. 3.2. Mechanical and tribological behaviors
Fig. 7. Coefficient of friction of TiSiN coatings under different conditions: (a) air (b) seawater.
Fig. 6 shows the hardness of the TiSiN coatings deposited at different negative bias. With the increase of substrate negative bias from 10 to 40 V, the hardness increases from about 39 to 47 GPa due to the grain refining and structure densification. Fig. 7 shows the variation of coefficient of friction (COF) with sliding time for different samples. In atmosphere, the coefficients gradually increase from 0.2 to 0.4 in the initial stage (sliding time 0–800 s) for the TiSiN coatings deposited at 10 V and 20 V. However, for the coatings deposited at bias voltages of 30 V and 40 V, the coefficients are 0.4 for all sliding time. While in artificial seawater, the coefficients for coatings deposited at bias voltages of 10, 20, 30 V are approximately kept at 0.30, which was obviously lower than that sliding in the air. For the coatings deposited at negative bias of 40 V, the coefficient is evidently higher than other samples, and this attribute to its high hardness and roughness. Fig. 8 shows the wear rate for different TiSiN coatings in atmosphere and artificial seawater. The wear rate increases with the increase of the negative bias, especially with negative bias of 10 V the coatings have the best wear resistant. Fig. 9 shows the morphologies of the wear track on the TiSiN coatings deposited with negative bias of 10 V and the corresponding EDS analysis in seawater. EDS and wear track morphology reveal that the coating was peeled off partly. The images of the wear tracks in other
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Fig. 9. Wear track of TiSiN coatings with negative bias for 10 V in seawater: (a) surface morphology, (b) flake pits, (c) depth profile, (d) EDS analysis.
TiSiN coating sliding in seawater environment are shown in Fig. 10– Fig. 12, respectively. Fig. 10 indicates that some pits formed and some microparticles were pulled out. Some deformed particles and cracks can be observed around the particles from Fig. 11. These cracks provide a favorable path for corrosion liquid. Fig. 12 shows the wear track of the coatings deposited at 40 V, the morphology of wear track presented as plowing grooves, and the coatings are completely destroyed. The results indicate that the negative bias voltage has a clear effect on tribological properties of the coatings.
Under lower negative bias voltage, the wear tracks are shallow and the wear rates are lower than the coatings deposited at higher substrate negative bias. The EDS results reveal that the coatings are peeled off more seriously as the negative bias voltage further increase, and the profiles show that the depth of the wear track are deeper at a condition of higher negative bias, and the detachment of the coatings is more serious. EDS analysis on the wear tracks also indicates that Ti, Al, V, Si, N, C and O elements are found on the wear track; these elements are mainly the composition of the
Fig. 10. Wear track of TiSiN coatings with negative bias for 20 V: (a) surface morphology, (b) flake pits, (c) depth profile, (d) EDS analysis.
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Fig. 11. Wear track of TiSiN coatings with negative bias for 30 V: (a) surface morphology, (b) flake pits, (c) depth profile, (d) EDS analysis.
Ti6Al4V substrate and the coatings, and indicate that the coatings are fatigue flaked. On the other hand, while sliding in seawater, the debris are also observed, and the friction coefficients are lower than that in ambient air. This attributes to the lubrication of seawater. Fig. 13 shows the micrograph of Vickers indentations with a load of 10 N for the TiSiN coatings. Severe edge cracks are observed in the coating deposited at negative bias voltage of 10 V, much more cracks are found compared with the higher bias voltage. With increase of the
bias, the coatings become brittle and are easy to wear out. On the other hand, the hardness of SiC matting ball is about 28 GPa [39], while coating deposited at negative bias voltage of 10 V has the hardness of 39 GPa, the wear mechanism between balls and coatings is the adhesive wear. However when the bias voltage increased to 40 V, the plough grooves were found, which attribute to the high hardness of the TiSiN coating about 47 GPa, and the wear mechanism is mainly the abrasive wear. In summary, the wear rate of coatings in artificial
Fig. 12. Wear track of TiSiN coatings with negative bias for 40 V: (a) surface morphology, (b) flake pits, (c) depth profile, (d) EDS analysis.
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Fig. 13. The micrograph of indentations for TiSiN coatings deposited at different negative bias: (a) 10 V (b) 20 V (c) 30 V (d) 40 V.
seawater was higher than that in air, and this is due to the corrosion of aqueous medium. However, the high wear rate of the coatings deposited at high bias voltage mainly attributes to the coating's brittleness and the change of wear mechanism during friction process. Fig. 14 shows the wear model of the TiSiN coatings sliding against SiC ball in seawater, Fig. 14 (a) is the contact model between the coatings and mating ball, Fig. 14 (b) is the wear mechanism of the coating. There are two kinds of micro particles in the coatings. One can be pulled out during cycle sliding and the other one can be named deformed microparticle, which is stubborn and does not fall off from the coatings after cycle friction. The deformed
microparticles have a distinct effect on the coefficient of friction during run-in period and lead to a high value of COF. The wear rates of the coatings are mainly from chipping and debris, and the wear mechanism is adhesive wear. The images in Figs. 9–12 indicate that the depth of the wear track become deeper when the bias voltage increases, at the same time the wear image of Fig. 11 and Fig. 12 presented plough grooves, the coatings are worn out and the number of micro cracks increased, which was consistent with the result of indentations. Thus the corrosion effect of seawater becomes more serious, which accelerates local delamination of the coatings, and the abrasive wear mechanism could dominate.
Fig. 14. Wear model of the TiSiN coatings sliding against SiC ball in seawater.
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3.3. Discussion The formation energy of TiN is about ΔG0298 = − 309.2 kJ/mol, and Si3N4 is ΔG0298 = − 665.40 kJ/mol [40]. Thus during deposition, amorphous Si3N4 with lower formation energy and atomic mobility was more stable, which grow along the crystal boundary of TiN. At the same time, even though with higher formation energy, TiN should easily form under pure N2 atmosphere as well. When the coatings deposited at low negative bias, the morphologies of cross-section presented as columnar structure. While the substrate negative bias increases, the columnar structure of the TiSiN coatings becomes dense. The XPS results show that the amount of amorphous Si3N4 increased. This change has a significant influence on the mechanical properties of the coatings. The columnar structure has an open inter-grain boundary which provided an access for corrosive seawater to degrade the bonding of the coating [38]. The nanocomposite of nc-TiN/a-Si3N4 structure which is proposed by Veprek is spinodal phase segregation, which is thermodynamically driven [41]. The increase of negative bias leads to the enhancement of the atomic diffusion and adatom surface mobility, which can provide necessary thermodynamic driving force for spinodal decomposition during deposition and result in the formation of nc-TiN/ a-Si3N4 nanocomposite [37]. The structure with smaller grains and greater grain boundaries can limit the sliding of dislocation and the extension of flaw. On the other hand, the thermodynamically driven phase segregation generates a strong interface between the nanocrystals TiN and the Si3N4, which lead to higher hardness of the coatings. However, the amount of amorphous Si3N4 should be controlled properly, otherwise, the mechanical properties will be declined [41]. The increase of the negative bias leads to a dense structure and high hardness of the coatings. But the residual stress increases as substrate negative bias increases, which may induce brittlement of the coatings; this is another important factor that affected the mechanical properties of the TiSiN coatings. 4. Conclusions The TiSiN coatings were deposited by multi-arc ion plating with different substrate negative bias. The effect of substrate negative bias on structures and tribocorrosion properties in artificial seawater was studied. The results reveal that the deposited TiSiN coatings have columnar structure, and the columnar structure becomes dense with the increase of the substrate negative bias. TEM images reveal that the TiSiN coatings have a coupled structure of amorphous Si3N4 and nanocrystalline TiN, and the grain size decreases with the increase of the voltage. The hardness of the TiSiN coatings increased from 39 to 47 GPa with the increase of voltage bias from 10 V to 40 V. Compared to atmosphere, the TiSiN coatings have lower friction coefficient and higher wear rate in seawater. Moreover, with the increase of the negative bias voltage, the wear mechanism changes from adhesion wear to abrasive wear, and the wear rate increases. At the same time the corrosion media accelerates the delamination of the coatings and induces the increase of wear
rate. The TiSiN coatings deposited at negative bias of 10 V show the best tribological properties in seawater. Acknowledgments This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LY14E010005), the Ningbo International Cooperation Project (2013D10005), and the National Natural Science Foundation of China (Grant No. 51575510). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]
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