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Preparation and corrosion behavior of electrodeposited Ni–TiN composite coatings
Int. Journal of Refractory Metals and Hard Materials 35 (2012) 295–299
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Preparation and corrosion behavior of electrodeposited Ni–TiN composite coatings Fafeng Xia a,⁎, Chao Liu b, Chunhua Ma c, Dianqing Chu c, Liang Miao a a b c
School of Mechanical Science and Engineering, Northeast Petroleum University, Daqing 163318, PR China School of Electronics Science, Northeast Petroleum University, Daqing 163318, PR China No.4 oil production plant, Petrochina Daqing Oilfield, Daqing, 163000, PR China
a r t i c l e
i n f o
Article history: Received 9 March 2012 Accepted 12 July 2012 Keywords: Corrosion Deposition Composite coating
a b s t r a c t Ni–TiN composite coatings were successfully prepared by direct current (DC), pulse current (PC) and ultrasonic pulse current (UPC) deposition methods. The morphology, mechanical properties and the corrosion behavior of Ni–TiN composite coatings were investigated using atomic force microscope (AFM), scanning electronic microscope (SEM), X-ray diffraction (XRD) and gravimetric analysis. The results show that the Ni–TiN composite coatings synthesized by UPC deposition method possess a compact and exiguous surface morphology. The XRD results demonstrate that the average grain diameter of Ni and TiN in composite coating prepared by UPC deposition is 52.6 and 35.7 nm, respectively. In the corrosion tests, the coating prepared by UPC deposition exhibits the best corrosion resistance, whereas the coating fabricated by DC deposition suffers the most serious damage. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction TiN, which is well known as a metal nitride ceramics, has been utilized in many applications for its fine physical and chemical properties [1–3]. TiN nano-size particles are usually introduced into nickel based composite coatings for enhancing properties such as corrosion resistance, wear resistance and micro-hardness [4–7]. Generally, the electrodeposition technique may be a simple and inexpensive method for obtaining composite coatings in the metal matrix. At present, there are two ways to electrodeposit composite coatings. One is direct current (DC) deposition, and the other is pulse current (PC) deposition [8–12]. Recent literatures on the electro-deposition of metallic composite coatings are studied. K. H. Hou [13] reported that Ni–W/Al2O3 composite coatings can be produced by PC deposition. L. Chen [14] demonstrated that Ni–Al2O3 composite coatings were electrodeposited from a Watts-type bath. It was found that the addition of HPB (hexadecylpyridinium bromide) could improve the amount of co-deposited Al2O3 particles, reduce the agglomeration of particles and achieve a more uniform distribution of Al2O3 particles in the nickel matrix. Study has shown that the properties of composite coatings mainly depend on the matrix phases, the amount and distribution of co-deposited particles, which are related to many process parameters, including particle characteristics (particle shape, size and concentration in solution), electrolyte composition (electrolyte concentration, additives, wetting agent, surfactant and concentration) and applied current (direct current, pulsed current and current density) [15]. ⁎ Corresponding author. Tel./fax.: +86 459 6507757. E-mail address: [email protected] (F. Xia). 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2012.07.002
Despite many investigations on direct current (DC) deposition or pulse current (PC) deposition of metals, however, few reports concerning the application of ultrasonic pulse current (UPC) deposition for producing Ni–TiN composite coatings appear in literatures. Our recent work indicated that Ni–TiN composite coatings with superior properties could be prepared by ultrasonic pulse current (UPC) deposition. In this paper, three types of Ni–TiN composite coatings were produced by DC, PC and UPC deposition methods, respectively. The morphology and mechanical properties of Ni–TiN composite coatings were discussed. The corrosion behavior and corrosion mechanism of the coatings were investigated. 2. Experimental Ni–TiN composite coatings with thickness of ~ 60 μm were deposited on the 20 mm × 30 mm mild steel substrates by DC, PC and UPC deposition methods, respectively. The mild steel substrates were used as the cathodes. Prior to deposition, the substrates were mechanically polished to a 0.10–0.15 μm surface finish, sequentially cleaned to remove surface contamination, activated for 10 s in a mixed acidic bath, and rinsed with distilled water and ethylic alcohol. A similar dimension of pure nickel (99.9%) plate was used as the anode. In order to obtain electrodeposited Ni–TiN composite coatings, the composition of the electrolyte was as follows: 250 g/l nickel sulfate, 40 g/l nickel chloride, 30 g/l boric acid and 2–10 g/l TiN particles. The temperature was kept at 50 °C at pH 4–5, adjusted using ammonium hydroxide or dilute sulfuric acid. During electrodepositing, the TiN particles in the range of ~ 30 nm were introduced into the electrolyte in various ratios. The plating parameters for electrodepositing Ni– TiN composite coatings are shown in Table 1.
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F. Xia et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 295–299
Table 1 The plating parameters. Direct current deposition Current density (A/dm2) Electroplating time (min) Pulse current deposition Current density (A/dm2) Pulsed frequency (Hz) Duty cycle Electroplating time (min) Ultrasonic pulse current deposition Ultrasonic power (W) Current density (A/dm2) Pulsed frequency (Hz) Duty cycle Electroplating time (min)
4 60 4 100 0.5 60 200 4 100 0.5 60
Surface morphology of the Ni–TiN composite coating was determined by SEM (JEOL, JSM-6460LV) with energy dispersive X-ray analysis (EDS) and AFM (DI, Nanoscope IIIa). To determine the phase structure of Ni–TiN composite coatings, X-ray diffraction (XRD) analysis was performed on a Rigaku D/Max-2400 instrument using Cu Kα radiation (λ = 0.15418 nm). The operating target voltage was 40 kV and the tube current was 100 mA. Using the Scherrer equation, the average grain diameter could be calculated as follows: 180Kλ D ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi π β2 −ω cosθ
ð1Þ
where K is the figure factor of the grains (K = 0.89), λ the wavelength, β the breadth of the diffraction peak at half height, ω the standard Full Width at Half Maximum (FWHM) and θ is the Bragg angle.
Vickers hardness was measured by means of 401 MVT microhardness tester at loads of 100 gf for 10 s. The contents of TiN particles in composite coatings were surveyed by gravimetric analysis (HIDEN, IGA-003). Corrosion tests were carried out on the Ni–TiN composite coatings by immersing samples in 5 wt.% NaCl solution for 150 h at 30 °C, then rinsed with distilled water, and finally dried in a drying oven (CIXI, FY-DR-1). The weight loss was measured on an electronic analytical balance (SARTORIUS, BS210S).
3. Results and discussion 3.1. Surface morphology and microstructure Fig. 1 shows the AFM surface morphology of the three types of Ni– TiN composite coatings. Based on the experiment results, the Ni–TiN composite coating synthesized by PC deposition displays a uniform and fine structure among micro-regions, whereas the coating prepared by DC deposition appears relatively coarse and shows irregular crystalline grain structure due to the presence of pulse current, which can lead to an increase in the nuclei number for nucleation of nickel grains and inhibition of grain growth. However, the Ni–TiN composite coating produced by UPC deposition exhibits a compact and exiguous surface morphology. The grain size in this coating is much smaller than that in the other coatings due to the ultrasonication and the pulse current breaking the normal growth of nickel crystals and disrupting larger crystals from producing smaller nuclei. Moreover, the moderate ultrasonication leads to homogeneous dispersion of TiN particles in the coatings. Therefore, the introduction of ultrasonication results in the formation of smaller grains. All these results indicate that UPC deposition method tends to
Fig. 1. AFM images of Ni–TiN composite coatings deposited by DC (a), PC (b) and UPC (c) methods.
F. Xia et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 295–299
297
1000
Micro-Hardness (HV)
950 900 850 800
(a) (b) (c)
750 700 650 600
Fig. 2. XRD patterns of Ni–TiN composite coatings deposited by DC (a), PC (b) and UPC (c) methods.
550
0
2
4
6
8
10
V (%)
3.2. Mechanical properties of Ni–TiN composite coatings The micro-hardness of the three types of Ni–TiN composite coatings as a function of TiN particle contents is shown in Fig. 3. The micro-hardness of the composite coatings increases a little when the contents of TiN particles increase from 0% to 2%. However, micro-hardness increases greatly with the TiN particle contents increasing from 2% to 9%. The maximal micro-hardness for Ni–TiN composite coatings prepared by DC, PC, and UPC deposition methods is 861 HV, 907 HV, and 950 HV, respectively. The micro-hardness improvement in composite coating is related to the dispersion hardening effect caused by TiN particles, which has higher micro-hardness and enhances the properties of Ni–TiN composite coatings.
Fig. 3. The effect of the TiN particle contents on micro-hardness of Ni–TiN composite coatings deposited by DC (a), PC (b) and UPC (c) methods.
methods is basically the same: they rise very quickly at the beginning and then change slowly. The three coatings by UPC deposition experience the least weight loss, whereas those by DC deposition have the biggest under similar content of TiN. This finding establishes that Ni–TiN composite coatings prepared by UPC deposition have
(a) 3.5 3.02% TiN (DC) 2.98% TiN (PC) 2.95% TiN (UPC)
3.0
Weight change (mg mm-2)
grow Ni–TiN composite coatings with compact and fine grain size compared with DC and PC deposition methods. The XRD patterns of the Ni–TiN composite coatings were detected by X-ray diffraction to further confirm the existence of TiN particles. Scans were recorded for the range 2θ = 20° to 80° with a scan step of 0.02°. The XRD patterns of the three types of Ni–TiN composite coatings are presented in Fig. 2. The figure shows that the composite coatings consist of Ni phase and TiN phase. For Ni, the diffraction peaks at 44.82°, 52.21°, and 76.77° correspond to (1 1 1), (2 0 0), and (2 2 0). For TiN, the diffraction peaks at 36.66°, 42.60°, and 61.81° correspond to (1 1 1), (2 0 0), and (2 2 0), respectively. According to the XRD data, the average grain size of Ni and TiN can be calculated using Eq. (1). The results are shown in Table 2. The XRD results demonstrate that the average grain diameters of Ni and TiN in the composite coating prepared by UPC deposition are 52.6 and 35.7 nm, respectively. The average grain diameters of Ni and TiN by DC deposition are 215.5 and 131.2 nm, respectively. These results are consistent with the AFM results.
2.5 2.0 1.5 1.0 0.5 0.0
0
60
90
120
150
180
120
150
180
Time (h)
(b) 2.0 Weight change (mg mm-2)
3.3. Corrosion dynamics of Ni–TiN composite coatings Corrosion dynamic curves of different Ni–TiN composite coatings are shown in Fig. 4. Fig. 4a and b displays the weight loss curves of Ni–TiN composite coatings after corrosion. Their V in composite coatings is about 3% and 9%, respectively. Evidently, corrosion weight loss decreases as TiN content increases. The trend of corrosion curves for Ni–TiN composite coatings produced by DC, PC, and UPC deposition
30
8.97% TiN (DC) 9.05% TiN (PC) 9.01% TiN (UPC)
1.5
1.0
0.5
Table 2 The average grain size of Ni and TiN in Ni–TiN composite coatings. Types of coatings
DNi (nm)
DTiN (nm)
(a) (b) (c)
215.5 168.3 52.6
131.2 106.8 35.7
0.0
0
30
60
90
Time (h) Fig. 4. Weight loss curves of Ni–TiN composite coatings after corrosion.
298
F. Xia et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 295–299
better corrosion resistance than those by DC or PC deposition under the same content of TiN due to moderate ultrasonication, which is conducive to homogeneous dispersion of TiN particles in Ni–TiN composite coatings. In addition, the TiN particles embedded in the coatings improve the compactness of the coatings and enhance their corrosion resistance. 3.4. Corrosion morphology of Ni–TiN composite coatings Fig. 5 shows the SEM surface morphology of samples after corrosion tests for 150 h. The Ni–TiN composite coating produced by UPC deposition consists of regular and compact oxides with uniform and fine grains. The coating by DC deposition consists of coarse and
Fig. 6. EDS spectra of the Ni–TiN composite coating deposited by UPC method.
incompact oxides. It is also found from Fig. 5(c) that after immersion in 5 wt.% NaCl solution for 150 h, only a few small pits appear on the surface of Ni–TiN composite coating, indicating that the coating prepared by UPC deposition exhibits the best corrosion resistance in this test. However, the coating prepared by DC deposition suffers the most serious damage, as shown in Fig. 5(a). 3.5. Corrosion mechanism of Ni–TiN composite coatings Fig. 6 illustrates the EDS spectra of the Ni–TiN composite coating deposited by UPC method after corrosion tests. The result indicates that the major corrosion products are NiO and NiCl2. From the corrosion results, the corrosion mechanism of Ni–TiN composite coatings can be explained as follows. First, Ni atoms of composite coatings are oxidized to NiO (2Ni + O2 = 2NiO), and then the oxide layer is produced. Second, NaCl on the coatings reacts with NiO and induces the formation of NaCl2 (NiO + 2Cl − = NaCl2 + O 2−). The formed NiCl2 can be eventually decomposed to form Cl2 (NaCl2 = Ni ++ 2Cl −, 2Cl − + 2e = Cl2). Consequently, the Ni–TiN composite coatings are destroyed and more point defects are formed. The loose oxide layer with poor adhesion to the matrix fails to provide effective protection to the matrix, which is one of the reasons why the corrosion rate of coatings is accelerated. As the corrosion test continues, the oxide layer gradually thickens, which inhibits the increase of the corrosion reaction. This phenomenon is one of the reasons why corrosion dynamic curves tend to level off when corrosion lasts for a certain period of time. The TiN particles embedded in the coatings cannot change the corrosion mechanism. However, they can improve the coating structure and make the surface smoother and more compact, which can impede solution contact with the coatings. Consequently, the corrosion resistance of the Ni–TiN composite coatings is enhanced. 4. Conclusions
Fig. 5. SEM images of samples after the corrosion test: (a) DC, (b) PC and (c) UPC.
Ni–TiN composite coatings were produced by DC, PC and UPC deposition methods, respectively. AFM results demonstrate that UPC deposition method tends to grow Ni–TiN composite coatings with compact and fine grain size compared with DC and PC deposition methods. The XRD results illuminate that the average grain diameters of Ni and TiN in composite coating prepared by UPC deposition are 52.6 and 35.7 nm, respectively. In corrosion tests, the coating prepared by UPC deposition exhibits the best corrosion resistance. However, that prepared by DC deposition suffers the most serious damage.
F. Xia et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 295–299
Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (Grant 51101027) and National Key Technology Support Program (2012BAH28F03). References [1] [2] [3] [4]
Blackwood DJ, Seah KHW. Mater Sci Eng C 2009;29:1233. Miguel M, Carmen JH, Carmen M. Biomaterials 2000;21:1755. Ming JJ, Wang XX. Mater Lett 2009;63:2286. Kwok CT, Wonga PK, Cheng FT, Man HC. Appl Surf Sci 2009;255:6736.
[5] [6] [7] [8] [9] [10] [11] [12]
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Ng KW, Man HC, Yue TM. Appl Surf Sci 2008;254:6725. Vohra A, Goswami DY, Deshpande DA, Block SS. Appl Catal B 2006;64:57. Mahmoodi NM, Arami M, Limaee NY. J Colloid Interface Sci 2006;295:159. Ao YH, Xu JJ, Fu DG, Yuan CW. Carbon 2008;46:596. Wang WD, Silva CG, Faria JL. Appl Catal B 2007;70:470. Cheng YP, Sun HQ, Jin WQ, Xu NP. Chem Eng J 2007;128:127. Satuf ML, Brandi RJ, Cassano AE, Alfano OM. Appl Catal B Environ 2008;82:37. Cordero T, Duchamp C, Chovelon JM, Ferronato C, Matos J. J Photochem Photobiol A 2007;191:122. [13] Hou KH, Chen YC. Appl Surf Sci 2011;257:6340. [14] Chen L, Wang LP, Zeng ZX, Zhang JY. Mater Sci Eng A 2006;434:319. [15] Wu G, Li N, Zhou DR. Surf Coat Technol 2004;176:157.
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Preparation and corrosion behavior of electrodeposited Ni–TiN composite coatings Fafeng Xia a,⁎, Chao Liu b, Chunhua Ma c, Dianqing Chu c, Liang Miao a a b c
School of Mechanical Science and Engineering, Northeast Petroleum University, Daqing 163318, PR China School of Electronics Science, Northeast Petroleum University, Daqing 163318, PR China No.4 oil production plant, Petrochina Daqing Oilfield, Daqing, 163000, PR China
a r t i c l e
i n f o
Article history: Received 9 March 2012 Accepted 12 July 2012 Keywords: Corrosion Deposition Composite coating
a b s t r a c t Ni–TiN composite coatings were successfully prepared by direct current (DC), pulse current (PC) and ultrasonic pulse current (UPC) deposition methods. The morphology, mechanical properties and the corrosion behavior of Ni–TiN composite coatings were investigated using atomic force microscope (AFM), scanning electronic microscope (SEM), X-ray diffraction (XRD) and gravimetric analysis. The results show that the Ni–TiN composite coatings synthesized by UPC deposition method possess a compact and exiguous surface morphology. The XRD results demonstrate that the average grain diameter of Ni and TiN in composite coating prepared by UPC deposition is 52.6 and 35.7 nm, respectively. In the corrosion tests, the coating prepared by UPC deposition exhibits the best corrosion resistance, whereas the coating fabricated by DC deposition suffers the most serious damage. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction TiN, which is well known as a metal nitride ceramics, has been utilized in many applications for its fine physical and chemical properties [1–3]. TiN nano-size particles are usually introduced into nickel based composite coatings for enhancing properties such as corrosion resistance, wear resistance and micro-hardness [4–7]. Generally, the electrodeposition technique may be a simple and inexpensive method for obtaining composite coatings in the metal matrix. At present, there are two ways to electrodeposit composite coatings. One is direct current (DC) deposition, and the other is pulse current (PC) deposition [8–12]. Recent literatures on the electro-deposition of metallic composite coatings are studied. K. H. Hou [13] reported that Ni–W/Al2O3 composite coatings can be produced by PC deposition. L. Chen [14] demonstrated that Ni–Al2O3 composite coatings were electrodeposited from a Watts-type bath. It was found that the addition of HPB (hexadecylpyridinium bromide) could improve the amount of co-deposited Al2O3 particles, reduce the agglomeration of particles and achieve a more uniform distribution of Al2O3 particles in the nickel matrix. Study has shown that the properties of composite coatings mainly depend on the matrix phases, the amount and distribution of co-deposited particles, which are related to many process parameters, including particle characteristics (particle shape, size and concentration in solution), electrolyte composition (electrolyte concentration, additives, wetting agent, surfactant and concentration) and applied current (direct current, pulsed current and current density) [15]. ⁎ Corresponding author. Tel./fax.: +86 459 6507757. E-mail address: [email protected] (F. Xia). 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2012.07.002
Despite many investigations on direct current (DC) deposition or pulse current (PC) deposition of metals, however, few reports concerning the application of ultrasonic pulse current (UPC) deposition for producing Ni–TiN composite coatings appear in literatures. Our recent work indicated that Ni–TiN composite coatings with superior properties could be prepared by ultrasonic pulse current (UPC) deposition. In this paper, three types of Ni–TiN composite coatings were produced by DC, PC and UPC deposition methods, respectively. The morphology and mechanical properties of Ni–TiN composite coatings were discussed. The corrosion behavior and corrosion mechanism of the coatings were investigated. 2. Experimental Ni–TiN composite coatings with thickness of ~ 60 μm were deposited on the 20 mm × 30 mm mild steel substrates by DC, PC and UPC deposition methods, respectively. The mild steel substrates were used as the cathodes. Prior to deposition, the substrates were mechanically polished to a 0.10–0.15 μm surface finish, sequentially cleaned to remove surface contamination, activated for 10 s in a mixed acidic bath, and rinsed with distilled water and ethylic alcohol. A similar dimension of pure nickel (99.9%) plate was used as the anode. In order to obtain electrodeposited Ni–TiN composite coatings, the composition of the electrolyte was as follows: 250 g/l nickel sulfate, 40 g/l nickel chloride, 30 g/l boric acid and 2–10 g/l TiN particles. The temperature was kept at 50 °C at pH 4–5, adjusted using ammonium hydroxide or dilute sulfuric acid. During electrodepositing, the TiN particles in the range of ~ 30 nm were introduced into the electrolyte in various ratios. The plating parameters for electrodepositing Ni– TiN composite coatings are shown in Table 1.
296
F. Xia et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 295–299
Table 1 The plating parameters. Direct current deposition Current density (A/dm2) Electroplating time (min) Pulse current deposition Current density (A/dm2) Pulsed frequency (Hz) Duty cycle Electroplating time (min) Ultrasonic pulse current deposition Ultrasonic power (W) Current density (A/dm2) Pulsed frequency (Hz) Duty cycle Electroplating time (min)
4 60 4 100 0.5 60 200 4 100 0.5 60
Surface morphology of the Ni–TiN composite coating was determined by SEM (JEOL, JSM-6460LV) with energy dispersive X-ray analysis (EDS) and AFM (DI, Nanoscope IIIa). To determine the phase structure of Ni–TiN composite coatings, X-ray diffraction (XRD) analysis was performed on a Rigaku D/Max-2400 instrument using Cu Kα radiation (λ = 0.15418 nm). The operating target voltage was 40 kV and the tube current was 100 mA. Using the Scherrer equation, the average grain diameter could be calculated as follows: 180Kλ D ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi π β2 −ω cosθ
ð1Þ
where K is the figure factor of the grains (K = 0.89), λ the wavelength, β the breadth of the diffraction peak at half height, ω the standard Full Width at Half Maximum (FWHM) and θ is the Bragg angle.
Vickers hardness was measured by means of 401 MVT microhardness tester at loads of 100 gf for 10 s. The contents of TiN particles in composite coatings were surveyed by gravimetric analysis (HIDEN, IGA-003). Corrosion tests were carried out on the Ni–TiN composite coatings by immersing samples in 5 wt.% NaCl solution for 150 h at 30 °C, then rinsed with distilled water, and finally dried in a drying oven (CIXI, FY-DR-1). The weight loss was measured on an electronic analytical balance (SARTORIUS, BS210S).
3. Results and discussion 3.1. Surface morphology and microstructure Fig. 1 shows the AFM surface morphology of the three types of Ni– TiN composite coatings. Based on the experiment results, the Ni–TiN composite coating synthesized by PC deposition displays a uniform and fine structure among micro-regions, whereas the coating prepared by DC deposition appears relatively coarse and shows irregular crystalline grain structure due to the presence of pulse current, which can lead to an increase in the nuclei number for nucleation of nickel grains and inhibition of grain growth. However, the Ni–TiN composite coating produced by UPC deposition exhibits a compact and exiguous surface morphology. The grain size in this coating is much smaller than that in the other coatings due to the ultrasonication and the pulse current breaking the normal growth of nickel crystals and disrupting larger crystals from producing smaller nuclei. Moreover, the moderate ultrasonication leads to homogeneous dispersion of TiN particles in the coatings. Therefore, the introduction of ultrasonication results in the formation of smaller grains. All these results indicate that UPC deposition method tends to
Fig. 1. AFM images of Ni–TiN composite coatings deposited by DC (a), PC (b) and UPC (c) methods.
F. Xia et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 295–299
297
1000
Micro-Hardness (HV)
950 900 850 800
(a) (b) (c)
750 700 650 600
Fig. 2. XRD patterns of Ni–TiN composite coatings deposited by DC (a), PC (b) and UPC (c) methods.
550
0
2
4
6
8
10
V (%)
3.2. Mechanical properties of Ni–TiN composite coatings The micro-hardness of the three types of Ni–TiN composite coatings as a function of TiN particle contents is shown in Fig. 3. The micro-hardness of the composite coatings increases a little when the contents of TiN particles increase from 0% to 2%. However, micro-hardness increases greatly with the TiN particle contents increasing from 2% to 9%. The maximal micro-hardness for Ni–TiN composite coatings prepared by DC, PC, and UPC deposition methods is 861 HV, 907 HV, and 950 HV, respectively. The micro-hardness improvement in composite coating is related to the dispersion hardening effect caused by TiN particles, which has higher micro-hardness and enhances the properties of Ni–TiN composite coatings.
Fig. 3. The effect of the TiN particle contents on micro-hardness of Ni–TiN composite coatings deposited by DC (a), PC (b) and UPC (c) methods.
methods is basically the same: they rise very quickly at the beginning and then change slowly. The three coatings by UPC deposition experience the least weight loss, whereas those by DC deposition have the biggest under similar content of TiN. This finding establishes that Ni–TiN composite coatings prepared by UPC deposition have
(a) 3.5 3.02% TiN (DC) 2.98% TiN (PC) 2.95% TiN (UPC)
3.0
Weight change (mg mm-2)
grow Ni–TiN composite coatings with compact and fine grain size compared with DC and PC deposition methods. The XRD patterns of the Ni–TiN composite coatings were detected by X-ray diffraction to further confirm the existence of TiN particles. Scans were recorded for the range 2θ = 20° to 80° with a scan step of 0.02°. The XRD patterns of the three types of Ni–TiN composite coatings are presented in Fig. 2. The figure shows that the composite coatings consist of Ni phase and TiN phase. For Ni, the diffraction peaks at 44.82°, 52.21°, and 76.77° correspond to (1 1 1), (2 0 0), and (2 2 0). For TiN, the diffraction peaks at 36.66°, 42.60°, and 61.81° correspond to (1 1 1), (2 0 0), and (2 2 0), respectively. According to the XRD data, the average grain size of Ni and TiN can be calculated using Eq. (1). The results are shown in Table 2. The XRD results demonstrate that the average grain diameters of Ni and TiN in the composite coating prepared by UPC deposition are 52.6 and 35.7 nm, respectively. The average grain diameters of Ni and TiN by DC deposition are 215.5 and 131.2 nm, respectively. These results are consistent with the AFM results.
2.5 2.0 1.5 1.0 0.5 0.0
0
60
90
120
150
180
120
150
180
Time (h)
(b) 2.0 Weight change (mg mm-2)
3.3. Corrosion dynamics of Ni–TiN composite coatings Corrosion dynamic curves of different Ni–TiN composite coatings are shown in Fig. 4. Fig. 4a and b displays the weight loss curves of Ni–TiN composite coatings after corrosion. Their V in composite coatings is about 3% and 9%, respectively. Evidently, corrosion weight loss decreases as TiN content increases. The trend of corrosion curves for Ni–TiN composite coatings produced by DC, PC, and UPC deposition
30
8.97% TiN (DC) 9.05% TiN (PC) 9.01% TiN (UPC)
1.5
1.0
0.5
Table 2 The average grain size of Ni and TiN in Ni–TiN composite coatings. Types of coatings
DNi (nm)
DTiN (nm)
(a) (b) (c)
215.5 168.3 52.6
131.2 106.8 35.7
0.0
0
30
60
90
Time (h) Fig. 4. Weight loss curves of Ni–TiN composite coatings after corrosion.
298
F. Xia et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 295–299
better corrosion resistance than those by DC or PC deposition under the same content of TiN due to moderate ultrasonication, which is conducive to homogeneous dispersion of TiN particles in Ni–TiN composite coatings. In addition, the TiN particles embedded in the coatings improve the compactness of the coatings and enhance their corrosion resistance. 3.4. Corrosion morphology of Ni–TiN composite coatings Fig. 5 shows the SEM surface morphology of samples after corrosion tests for 150 h. The Ni–TiN composite coating produced by UPC deposition consists of regular and compact oxides with uniform and fine grains. The coating by DC deposition consists of coarse and
Fig. 6. EDS spectra of the Ni–TiN composite coating deposited by UPC method.
incompact oxides. It is also found from Fig. 5(c) that after immersion in 5 wt.% NaCl solution for 150 h, only a few small pits appear on the surface of Ni–TiN composite coating, indicating that the coating prepared by UPC deposition exhibits the best corrosion resistance in this test. However, the coating prepared by DC deposition suffers the most serious damage, as shown in Fig. 5(a). 3.5. Corrosion mechanism of Ni–TiN composite coatings Fig. 6 illustrates the EDS spectra of the Ni–TiN composite coating deposited by UPC method after corrosion tests. The result indicates that the major corrosion products are NiO and NiCl2. From the corrosion results, the corrosion mechanism of Ni–TiN composite coatings can be explained as follows. First, Ni atoms of composite coatings are oxidized to NiO (2Ni + O2 = 2NiO), and then the oxide layer is produced. Second, NaCl on the coatings reacts with NiO and induces the formation of NaCl2 (NiO + 2Cl − = NaCl2 + O 2−). The formed NiCl2 can be eventually decomposed to form Cl2 (NaCl2 = Ni ++ 2Cl −, 2Cl − + 2e = Cl2). Consequently, the Ni–TiN composite coatings are destroyed and more point defects are formed. The loose oxide layer with poor adhesion to the matrix fails to provide effective protection to the matrix, which is one of the reasons why the corrosion rate of coatings is accelerated. As the corrosion test continues, the oxide layer gradually thickens, which inhibits the increase of the corrosion reaction. This phenomenon is one of the reasons why corrosion dynamic curves tend to level off when corrosion lasts for a certain period of time. The TiN particles embedded in the coatings cannot change the corrosion mechanism. However, they can improve the coating structure and make the surface smoother and more compact, which can impede solution contact with the coatings. Consequently, the corrosion resistance of the Ni–TiN composite coatings is enhanced. 4. Conclusions
Fig. 5. SEM images of samples after the corrosion test: (a) DC, (b) PC and (c) UPC.
Ni–TiN composite coatings were produced by DC, PC and UPC deposition methods, respectively. AFM results demonstrate that UPC deposition method tends to grow Ni–TiN composite coatings with compact and fine grain size compared with DC and PC deposition methods. The XRD results illuminate that the average grain diameters of Ni and TiN in composite coating prepared by UPC deposition are 52.6 and 35.7 nm, respectively. In corrosion tests, the coating prepared by UPC deposition exhibits the best corrosion resistance. However, that prepared by DC deposition suffers the most serious damage.
F. Xia et al. / Int. Journal of Refractory Metals and Hard Materials 35 (2012) 295–299
Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (Grant 51101027) and National Key Technology Support Program (2012BAH28F03). References [1] [2] [3] [4]
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