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Effects of annealing temperature on the crystal structure and properties of electroless deposited Ni–W–Cr–P alloy coatings
Applied Surface Science 255 (2008) 1686–1691
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of annealing temperature on the crystal structure and properties of electroless deposited Ni–W–Cr–P alloy coatings Ling Zhang a, Yong Jin b,*, Bo Peng b, Yifan Zhang b, Xuejuan Wang b, Qingsong Yang b, Jian Yu b a b
Department of Chemistry and Environmental Engineering, Changsha University of Science and Technology, Changsha 410076, China Department of Materials Science, Sichuan University, Wangjiang Road, Chengdu 610064, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 October 2007 Received in revised form 4 June 2008 Accepted 6 June 2008 Available online 12 June 2008
Ni–W–Cr–P alloy coatings were prepared by electroless deposition. The effects of the annealing temperature on the crystal structure, microhardness, and corrosion resistance of these Ni–W–Cr–P alloy coatings were studied. The experimental results show that the crystal structure of these alloy coatings changes with the increase of annealing temperature. That is, as-deposited Ni–W–Cr–P alloy coatings possess amorphous structure, gradually begin to crystallize at 300 8C; Ni3P phase is completely formed at 500 8C, and Ni17–W3 and Cr–Fe phases were formed at 600 8C; Ni3P is completely decomposed into Ni, W, Cr, Fe alloy films at 700 8C; the alloy coatings mainly consisted of Ni17W3 and Ni–Cr–Fe phases at 800 8C. Annealing temperature dependence of the microhardness and corrosion resistance of the alloy coatings was also measured. These results show that the Ni–W–Cr–P alloy coatings annealed at 800 8C exhibit better microhardness and corrosion resistance than those of the alloy coatings with other annealing temperature. The related strengthening mechanisms in the electroless deposited Ni–W–Cr–P alloy coatings were also discussed. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Electroless deposition Ni–W–Cr–P alloy coatings Annealing temperature Crystal structure Microhardness Corrosion resistance
1. Introduction Ni–P plating is well known for its good performance as a surface coating and as a low cost process for a wide range or industrial applications [1–6]. Electroless deposition is considered as the most effective method to alter the chemical and physical properties of Ni–P-based alloy deposits [7–9]. Many factors may affect the performance and crystallization behavior of Ni–P-based coatings. Among these, the doping element plays an important role on Ni–P-based alloy coatings. Codeposition of Cr or W in binary electroless Ni–P deposit improves the deposit characteristics such as wear resistance, corrosion resistance, thermal stability, and electrical resistance [10–13]. Even small amounts of tungsten in the Ni–P matrix greatly increased hardness compared to plain Ni–P deposits [14]. But there are almost no reports on the preparation of the Ni–W–Cr–P alloy coatings deposited by electroless deposition. In this work, Cr and W elements are used to modify Ni–P alloy coatings because of their high melting point and superior hardness. The ternary Ni–P alloy coatings doped with Cr and W were prepared by electroless deposition. The effects of the annealing
* Corresponding author. E-mail address: [email protected] (Y. Jin). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.06.012
temperature on the crystal structure, microhardness, and corrosion resistance of Ni–W–Cr–P alloy coatings were mainly studied. The related mechanisms for enhanced microhardness and corrosion resistance of Ni–W–Cr–P alloy coatings with proper annealing temperature were also discussed. 2. Experimental details The substrates for the plating were carbon steel with a size of 30 mm 15 mm 3 mm. They were firstly ground with emery paper of 600 grit, degreased in acetone, cathodically cleaned in sodium hydroxide solution, and rinsed in running water and deionized water. Thus, degreased sample was deoxidized in sulphuric acid solution, rinsed in running water and deionized water. Finally, the sample was put into the electroless solution for plating. Ni–W–Cr–P alloy coatings were prepared by altering the chemistry solution with proper ratio of A:B solution. That is, Ni– W–P system was formed in A solution, Cr system was formed in B solution, and then the A and B solutions were mixed together. pH value of the bath used in the electroless plating was adjusted to 8.8–9.2 by ammonia and 30% acetic acid, and the reaction temperature of the coatings keeps 90–95 8C. To investigate the effects of the annealing temperature on the crystallization behavior of Ni–W–Cr–P alloy coatings, the as-deposited coatings were annealed from 300 to 800 8C for 1 h in a nitrogen-purged
L. Zhang et al. / Applied Surface Science 255 (2008) 1686–1691 Table 1 Composition and operating conditions of the plating baths Chemical compounds A
Concentration
Chemical compounds B
Concentration
NiSO46H2O Na2WO42H2O Na3C6H5O72H2O NH4Cl NaH2PO2H2O NH3H2O Stability PH
30–40 g/L 25–30 g/L 80–100 g/L 40–60 g/L 10–20 g/L 10–20 ml/L 10–30 mg/L 8.8–9.2
CrCl36H2O NaH2PO4H2O K2C2O4H2O NaC2H3O2
10–15 g/L 10–20 g/L 4–6 g/L 10–20 g/L
Stability 4–6
10–20 mg/L
atmosphere and then furnace—cooled to room temperature. The compositions of the bath and the deposition conditions for Ni–W– Cr–P alloy coatings are shown in Table 1. Chemical composition of the as-deposited layers with different mixed solutions A + B was shown in Table 2. X-ray diffraction (XRD) characterization of the alloy coatings was performed by using Cu Ka radiation (l = 1.54178 A˚) in the
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Table 2 Chemical composition of as-deposited Ni–W–Cr–P alloy coatings with different mixed solutions A + B Concentration (%)
A4 + B6
A5 + B5
A6 + B4
A7 + B3
A8 + B2
W Cr P Ni
4.1 3.2 5.6 87.1
5.2 3.0 5.2 86.6
5.7 2.7 5.0 86.6
6.1 2.5 4.8 86.6
6.2 2.3 4.6 86.9
u-2u scan mode (DX1000, Dandong, China). The cross-section of the as-deposited layers was measured by a scanning emission microscope (SEM) (JSM-5900, Japan). The chemical composition of the layers was measured by Rigaku X-ray Fluorescence ZSX100e. The microhardness of the alloy coatings was measured using a HV2100 microhardness tester, microhardness values of the coatings were calculated according to the average values of a sample measured five times. The polarization curves of the alloy coatings were measured using Autolab PG302 Potentiostat Electrochemistry Workstation.
Fig. 1. XRD patterns of (a) substrate (carbon steel) and (b) as-deposited Ni–W–Cr–P alloy coatings without heat treatment.
Fig. 2. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 300 8C for 1 h.
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Fig. 3. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 400 8C for 1 h.
3. Results and discussion 3.1. Crystallization behavior of electroless deposited Ni–W–Cr–P alloy coatings Fig. 1(a) and (b) show the XRD patterns of substrate (carbon steel) and as-deposited Ni–W–Cr–P alloy coatings without annealing temperature, respectively. As shown in Fig. 1(b), there is a noncrystal peak at 2u = 458. So it was thought that the asdeposited alloy coatings belong to amorphous structure. In Ni– W–Cr–P system, the original amorphous structure was transformed into a mixture of amorphous, Ni and Ni3P crystallites at 300 8C for 1 h, as shown in Fig. 2. With the increase of annealing temperature, it was found from Fig. 3 that the stable phase of Ni3P was completely formed at 400 8C for 1 h, the peak of Ni becomes narrow, and the peak of Fe–Cr was also formed. After annealed at 500 8C, the diffraction peak of alloy coatings
becomes narrower, which shows the increase of coatings’ grains; the diffraction peaks of Ni disappear, and the diffraction peaks of Ni2.9–Fe0.36–Cr0.7 and Fe–Cr phases were formed, as shown in Fig. 4. As compared with Fig. 4, the coatings and substrate interdiffuses and penetrates after annealed at 600 8C, which results in a formation of grads Ni17–W3 alloy coatings, and thus strengthens the coalescent force between coatings and substrate (Fig. 5). After heating to 700 8C, Ni3P phase was completely decomposed, and formed Ni2.9Fe0.36Cr0.7, Fe–Cr, and Fe4W2N phases (Fig. 6). The alloy coatings annealed at 800 8C consisted of Ni17W3 and Ni–Cr–Fe phases (Fig. 7). The corresponding phase compositions of the layers with different annealing temperature were also shown in Table 3. As mentioned above, a strong annealing temperature dependence of the crystal structure of alloy coatings was observed, that is, the annealing temperature plays an important role in the crystal structure of Ni–W–Cr–P alloy coatings.
Fig. 4. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 500 8C for 1 h.
L. Zhang et al. / Applied Surface Science 255 (2008) 1686–1691
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Fig. 5. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 600 8C for 1 h.
3.2. Cross-section of electroless deposited Ni–W–Cr–P alloy coating Fig. 8 shows the cross-section of as-deposited Ni–W–Cr–P alloy coatings. As shown in Fig. 8, the thickness of amorphous Ni–W–Cr– P alloy coatings is about 13 mm. These images show that asdeposited Ni–W–Cr–P alloy coatings are dense and well adhered on the carbon steel. 3.3. Microhardness of electroless deposited Ni–W–Cr–P alloy coating Fig. 9 shows the microhardness of the Ni–W–Cr–P alloy coatings as function of annealing temperatures. As shown in Fig. 8, the microhardness of amorphous Ni–W–Cr–P alloy coatings is HV580, increases with the increase of annealing temperature, slightly decreases at 600 8C, and reaches maximum (HV1140) at 800 8C.
When the annealing temperature is between room temperature and 500 8C, the crystal structure of Ni–W–Cr–P alloy coatings changes from the amorphous to crystal, and forms highly dispersed supersaturated solid solution [Ni(P) and Ni3P compound]. With the further increase of annealing temperature, a number of dispersed Ni3P were formed, and therefore result in enhancement of the coatings’ microhardness. However, the decrease in microhardness after annealing at higher temperature (>500 8C) may be due to the increase of grain sizes, i.e. the socalled Hall–Petch effect for increasing diffusion of the substrate [15,16]. When the annealing temperature is above 600 8C, the microhardness gradually increases because of the formation of new hard compound (Fe3Ni2, Ni2.9Fe0.36Cr0.7, Fe–Cr, and Fe4W2N). When the annealing temperature reaches 800 8C, the microhardness reaches maximum (HV1140). In this work,
Fig. 6. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 700 8C for 1 h.
L. Zhang et al. / Applied Surface Science 255 (2008) 1686–1691
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Fig. 7. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 800 8C for 1 h.
enhanced microhardness of the alloy coatings should be attributed to completely crystallized hard compound (Ni17W3 and Ni–Cr–Fe phases). 3.4. Corrosion resistance of electroless deposited Ni–W–Cr–P alloy coating In order to evaluate the corrosion resistance of Ni–W–Cr–P alloy coatings, as-deposited (i.e., without annealing) Ni–P, Ni–W– P, and Ni–W–Cr–P alloy coatings with the same thicknesses were put into the solution of 10% NaOH, 5% NaCl, 10% HCl and 10% H2SO4
for 12 days, respectively. Corrosion rate of these alloy coatings is calculated by weight method. Detailed results were shown in Table 4. It was found from Table 4 that the corrosion resistance of as-deposited Ni–W–Cr–P alloy coatings shows better than those of Ni–P and Ni–W–P coatings in different mediums, especially for 10% H2SO4. The Ni–W–Cr–P alloy coatings is still bright except for 10% NaOH solution. As a result, the as-deposited Ni–W–Cr–P alloy coatings show good corrosion resistance.
Table 3 Phase composition of Ni–W–Cr–P alloy coatings with different annealing temperature Annealing temperature (8C)
Phase composition
300 400 500 600 700 800
Ni3P, Ni Ni, Ni3P, Fe–Cr Ni3P, Ni2.9–Fe0.36–Cr0.7, Fe–Cr Ni3P, Ni17W3, Cr17Ni3, Fe–Cr, Ni2.9Cr0.7Fe0.36 Fe3Ni2, Fe–Cr, Fe4W2N, Ni2.9Fe0.36Cr0.7 Ni17W3, Ni–Cr–Fe
Fig. 9. Microhardness of the Ni–W–Cr–P alloy coatings as a function of annealing temperature.
Table 4 Corrosion rate of as-deposited coatings in different mediums (mg/(day cm2))
Fig. 8. Cross-section of the as-deposited Ni–W–Cr–P alloy coatings.
As-deposited coatings
10% NaOH
5% NaCl
10% HCl
10% H2SO4
Ni–P Ni–W–P Ni–W–Cr–P
1.27 1.20 1.17
1.04 0.09 0.06
335 310 280
60 32 27
L. Zhang et al. / Applied Surface Science 255 (2008) 1686–1691
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Table 5 Corrosion potential, corrosion current density, and polarization resistance of the Ni–W–Cr–P alloy coatings with different annealing temperature
Corrosion potential (V) Polarization resistance (V cm 2) Corrosion current density (mA cm
2
)
As-deposited (A)
300 8C (B)
500 8C (C)
600 8C (D)
800 8C (E)
0.306 1587 7.709
0.332 1251 13.87
0.389 858.2 23.68
0.403 613.3 36.8
0.352 1023 18.67
Fig. 10. Polarization curves of Ni–W–Cr–P alloy coatings as a function of annealing temperature. (A) As-deposited; (B) 300 8C; (C) 500 8C; (D) 600 8C; (E) 800 8C.
Fig. 10 shows polarization curves of Ni–W–Cr–P alloy coatings as a function of annealing temperature. Table 5 shows the corrosion potential, corrosion current density, and polarization resistance of the Ni–W–Cr–P alloy coatings annealed at different temperature which were calculated according to the polarization curves. As shown in Table 5 and Fig. 10, as-deposited Ni–W–Cr–P alloy coatings deposited by this method exhibit relatively high corrosion resistance, low corrosion potential, and low corrosion current density. Moreover, the Ni–W–Cr–P alloy coatings annealed at proper temperature also show enhanced corrosion resistance. It was also found that the corrosion resistance slightly decreased and corrosion current density increased because the increase of grain sizes breaks the uniformity of structure after annealed at 500 and 600 8C, and the corrosion resistance was improved and corrosion current density was decreased for the formation of Ni17W3 and Ni–Cr–Fe phases at the annealing temperature of 800 8C. 4. Conclusion In this work, the Ni–W–Cr–P alloy coatings were prepared by electroless deposition. Some results were obtained, as listed below: Annealing temperature strongly affects the crystal structure of the Ni–W–Cr–P alloy coatings. As-deposited Ni–W–Cr–P alloy
coatings possess amorphous structure; the crystal structure of the coatings gradually changes with the increase of annealing temperature, the Ni and Ni3P phases were formed at the annealing temperature of 300 8C; Ni3P was completely crystallized along with the formation of NiFeCr and FeCr phases at 500 8C; after annealed at 600 8C, Ni3P begins to decompose, and Ni17–W3 phase gradually appears; Ni3P completely decomposes, and results in the formation of Fe3Ni2, Ni2.9Fe0.36Cr0.7, Fe–Cr, and Fe4W2N phases after annealed at 700 8C; Ni17W3 and Ni–Cr–Fe phase with good alloy were obtained at 800 8C. Microhardness of the Ni–W–Cr–P alloy coatings strongly depends on the annealing temperature. When the annealing temperature is below 500 8C, the microhardness of the Ni–W–Cr–P alloy coatings increases with the increase of annealing temperature. However, the microhardness of the Ni–W–Cr–P alloy coatings decreases due to the increase of grain sizes and uncompletely crystalline at 500 8C; after annealed at 600 8C, the microhardness of the Ni–W–Cr–P alloy coatings continuously increases due to the formation of NiFeCr and Ni17–W3 phases, and reaches maximum (HV1140) at 800 8C. Corrosion resistance of the Ni–W–Cr–P alloy coatings strongly depends on the annealing temperature. The corrosion resistance of as-deposited Ni–W–Cr–P alloy coatings is better than those of asdeposited Ni–P and Ni–W–P alloy coatings in different mediums. Moreover, the corrosion resistance of the Ni–W–Cr–P alloy coatings gradually increases with the increase of annealing temperature, and the corrosion resistance of the alloy coating annealed at 800 8C reaches best. References [1] J.N. Balaraju, S.M. Jahan, K.S. Rajam, Surf. Coat. Technol. 201 (2006) 507–512. [2] Y. Gao, Z.J. Zheng, M. Zhu, C.P. Luo, Mater. Sci. Eng. A 381 (2004) 98–103. [3] P. de Lima-Neto, G.P. da Silva, A.N. Correia, Electrochim. Acta 51 (2006) 4928– 4933. [4] W.Y. Chen, S.K. Tien, F.B. Wu, J.G. Duh, Surf. Coat. Technol. 182 (2004) 85–91. [5] D.Y. Ding, J.N. Wang, A. Dozier, J. Appl. Phys. 95 (2004) 5006–5009. [6] T.X. Wang, J.L. Meng, Y.J. Hu, Surf. Technol. 34 (3) (2005) 27–29. [7] J.E. Williams, C. Davidson, J. Electrochem. Soc. 137 (1990) 3260. [8] D. Osmola, P. Nolan, U. Erb, G. Palumbo, K.T. Aust, Phys. Status Solidi, A 131 (1992) 569. [9] A. Robertson, U. Erb, G. Palumbo, Nanostruct. Mater. 12 (1999) 1035. [10] J. Li, X.G. Hu, D.L. Wang, Plat. Surf. Finish. 83 (1996) 62. [11] G. Lu, G. Zangari, Electrochem. Acta 47 (2002) 2969. [12] Y.Y. Tsai, F.B. Wu, Y.I. Chen, P.J. Peng, J.G. Duh, S.Y. Tsai, Surf. Coat. Technol. 146/ 147 (2001) 502. [13] K. Aoki, O. Takano, Plat. Surf. Finish. 77 (3) (1990) 48. [14] B.W. Zhang, W.Y. Hu, Q.L. Zhang, X.Y. Qu, Mater. Charact. 37 (1996) 119. [15] E.O. Hall, Proc. Phys. Soc. B 64 (1951) 747. [16] N.J. Petch, J. Iron Steel Inst. 173 (1953) 25.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of annealing temperature on the crystal structure and properties of electroless deposited Ni–W–Cr–P alloy coatings Ling Zhang a, Yong Jin b,*, Bo Peng b, Yifan Zhang b, Xuejuan Wang b, Qingsong Yang b, Jian Yu b a b
Department of Chemistry and Environmental Engineering, Changsha University of Science and Technology, Changsha 410076, China Department of Materials Science, Sichuan University, Wangjiang Road, Chengdu 610064, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 October 2007 Received in revised form 4 June 2008 Accepted 6 June 2008 Available online 12 June 2008
Ni–W–Cr–P alloy coatings were prepared by electroless deposition. The effects of the annealing temperature on the crystal structure, microhardness, and corrosion resistance of these Ni–W–Cr–P alloy coatings were studied. The experimental results show that the crystal structure of these alloy coatings changes with the increase of annealing temperature. That is, as-deposited Ni–W–Cr–P alloy coatings possess amorphous structure, gradually begin to crystallize at 300 8C; Ni3P phase is completely formed at 500 8C, and Ni17–W3 and Cr–Fe phases were formed at 600 8C; Ni3P is completely decomposed into Ni, W, Cr, Fe alloy films at 700 8C; the alloy coatings mainly consisted of Ni17W3 and Ni–Cr–Fe phases at 800 8C. Annealing temperature dependence of the microhardness and corrosion resistance of the alloy coatings was also measured. These results show that the Ni–W–Cr–P alloy coatings annealed at 800 8C exhibit better microhardness and corrosion resistance than those of the alloy coatings with other annealing temperature. The related strengthening mechanisms in the electroless deposited Ni–W–Cr–P alloy coatings were also discussed. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Electroless deposition Ni–W–Cr–P alloy coatings Annealing temperature Crystal structure Microhardness Corrosion resistance
1. Introduction Ni–P plating is well known for its good performance as a surface coating and as a low cost process for a wide range or industrial applications [1–6]. Electroless deposition is considered as the most effective method to alter the chemical and physical properties of Ni–P-based alloy deposits [7–9]. Many factors may affect the performance and crystallization behavior of Ni–P-based coatings. Among these, the doping element plays an important role on Ni–P-based alloy coatings. Codeposition of Cr or W in binary electroless Ni–P deposit improves the deposit characteristics such as wear resistance, corrosion resistance, thermal stability, and electrical resistance [10–13]. Even small amounts of tungsten in the Ni–P matrix greatly increased hardness compared to plain Ni–P deposits [14]. But there are almost no reports on the preparation of the Ni–W–Cr–P alloy coatings deposited by electroless deposition. In this work, Cr and W elements are used to modify Ni–P alloy coatings because of their high melting point and superior hardness. The ternary Ni–P alloy coatings doped with Cr and W were prepared by electroless deposition. The effects of the annealing
* Corresponding author. E-mail address: [email protected] (Y. Jin). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.06.012
temperature on the crystal structure, microhardness, and corrosion resistance of Ni–W–Cr–P alloy coatings were mainly studied. The related mechanisms for enhanced microhardness and corrosion resistance of Ni–W–Cr–P alloy coatings with proper annealing temperature were also discussed. 2. Experimental details The substrates for the plating were carbon steel with a size of 30 mm 15 mm 3 mm. They were firstly ground with emery paper of 600 grit, degreased in acetone, cathodically cleaned in sodium hydroxide solution, and rinsed in running water and deionized water. Thus, degreased sample was deoxidized in sulphuric acid solution, rinsed in running water and deionized water. Finally, the sample was put into the electroless solution for plating. Ni–W–Cr–P alloy coatings were prepared by altering the chemistry solution with proper ratio of A:B solution. That is, Ni– W–P system was formed in A solution, Cr system was formed in B solution, and then the A and B solutions were mixed together. pH value of the bath used in the electroless plating was adjusted to 8.8–9.2 by ammonia and 30% acetic acid, and the reaction temperature of the coatings keeps 90–95 8C. To investigate the effects of the annealing temperature on the crystallization behavior of Ni–W–Cr–P alloy coatings, the as-deposited coatings were annealed from 300 to 800 8C for 1 h in a nitrogen-purged
L. Zhang et al. / Applied Surface Science 255 (2008) 1686–1691 Table 1 Composition and operating conditions of the plating baths Chemical compounds A
Concentration
Chemical compounds B
Concentration
NiSO46H2O Na2WO42H2O Na3C6H5O72H2O NH4Cl NaH2PO2H2O NH3H2O Stability PH
30–40 g/L 25–30 g/L 80–100 g/L 40–60 g/L 10–20 g/L 10–20 ml/L 10–30 mg/L 8.8–9.2
CrCl36H2O NaH2PO4H2O K2C2O4H2O NaC2H3O2
10–15 g/L 10–20 g/L 4–6 g/L 10–20 g/L
Stability 4–6
10–20 mg/L
atmosphere and then furnace—cooled to room temperature. The compositions of the bath and the deposition conditions for Ni–W– Cr–P alloy coatings are shown in Table 1. Chemical composition of the as-deposited layers with different mixed solutions A + B was shown in Table 2. X-ray diffraction (XRD) characterization of the alloy coatings was performed by using Cu Ka radiation (l = 1.54178 A˚) in the
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Table 2 Chemical composition of as-deposited Ni–W–Cr–P alloy coatings with different mixed solutions A + B Concentration (%)
A4 + B6
A5 + B5
A6 + B4
A7 + B3
A8 + B2
W Cr P Ni
4.1 3.2 5.6 87.1
5.2 3.0 5.2 86.6
5.7 2.7 5.0 86.6
6.1 2.5 4.8 86.6
6.2 2.3 4.6 86.9
u-2u scan mode (DX1000, Dandong, China). The cross-section of the as-deposited layers was measured by a scanning emission microscope (SEM) (JSM-5900, Japan). The chemical composition of the layers was measured by Rigaku X-ray Fluorescence ZSX100e. The microhardness of the alloy coatings was measured using a HV2100 microhardness tester, microhardness values of the coatings were calculated according to the average values of a sample measured five times. The polarization curves of the alloy coatings were measured using Autolab PG302 Potentiostat Electrochemistry Workstation.
Fig. 1. XRD patterns of (a) substrate (carbon steel) and (b) as-deposited Ni–W–Cr–P alloy coatings without heat treatment.
Fig. 2. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 300 8C for 1 h.
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Fig. 3. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 400 8C for 1 h.
3. Results and discussion 3.1. Crystallization behavior of electroless deposited Ni–W–Cr–P alloy coatings Fig. 1(a) and (b) show the XRD patterns of substrate (carbon steel) and as-deposited Ni–W–Cr–P alloy coatings without annealing temperature, respectively. As shown in Fig. 1(b), there is a noncrystal peak at 2u = 458. So it was thought that the asdeposited alloy coatings belong to amorphous structure. In Ni– W–Cr–P system, the original amorphous structure was transformed into a mixture of amorphous, Ni and Ni3P crystallites at 300 8C for 1 h, as shown in Fig. 2. With the increase of annealing temperature, it was found from Fig. 3 that the stable phase of Ni3P was completely formed at 400 8C for 1 h, the peak of Ni becomes narrow, and the peak of Fe–Cr was also formed. After annealed at 500 8C, the diffraction peak of alloy coatings
becomes narrower, which shows the increase of coatings’ grains; the diffraction peaks of Ni disappear, and the diffraction peaks of Ni2.9–Fe0.36–Cr0.7 and Fe–Cr phases were formed, as shown in Fig. 4. As compared with Fig. 4, the coatings and substrate interdiffuses and penetrates after annealed at 600 8C, which results in a formation of grads Ni17–W3 alloy coatings, and thus strengthens the coalescent force between coatings and substrate (Fig. 5). After heating to 700 8C, Ni3P phase was completely decomposed, and formed Ni2.9Fe0.36Cr0.7, Fe–Cr, and Fe4W2N phases (Fig. 6). The alloy coatings annealed at 800 8C consisted of Ni17W3 and Ni–Cr–Fe phases (Fig. 7). The corresponding phase compositions of the layers with different annealing temperature were also shown in Table 3. As mentioned above, a strong annealing temperature dependence of the crystal structure of alloy coatings was observed, that is, the annealing temperature plays an important role in the crystal structure of Ni–W–Cr–P alloy coatings.
Fig. 4. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 500 8C for 1 h.
L. Zhang et al. / Applied Surface Science 255 (2008) 1686–1691
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Fig. 5. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 600 8C for 1 h.
3.2. Cross-section of electroless deposited Ni–W–Cr–P alloy coating Fig. 8 shows the cross-section of as-deposited Ni–W–Cr–P alloy coatings. As shown in Fig. 8, the thickness of amorphous Ni–W–Cr– P alloy coatings is about 13 mm. These images show that asdeposited Ni–W–Cr–P alloy coatings are dense and well adhered on the carbon steel. 3.3. Microhardness of electroless deposited Ni–W–Cr–P alloy coating Fig. 9 shows the microhardness of the Ni–W–Cr–P alloy coatings as function of annealing temperatures. As shown in Fig. 8, the microhardness of amorphous Ni–W–Cr–P alloy coatings is HV580, increases with the increase of annealing temperature, slightly decreases at 600 8C, and reaches maximum (HV1140) at 800 8C.
When the annealing temperature is between room temperature and 500 8C, the crystal structure of Ni–W–Cr–P alloy coatings changes from the amorphous to crystal, and forms highly dispersed supersaturated solid solution [Ni(P) and Ni3P compound]. With the further increase of annealing temperature, a number of dispersed Ni3P were formed, and therefore result in enhancement of the coatings’ microhardness. However, the decrease in microhardness after annealing at higher temperature (>500 8C) may be due to the increase of grain sizes, i.e. the socalled Hall–Petch effect for increasing diffusion of the substrate [15,16]. When the annealing temperature is above 600 8C, the microhardness gradually increases because of the formation of new hard compound (Fe3Ni2, Ni2.9Fe0.36Cr0.7, Fe–Cr, and Fe4W2N). When the annealing temperature reaches 800 8C, the microhardness reaches maximum (HV1140). In this work,
Fig. 6. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 700 8C for 1 h.
L. Zhang et al. / Applied Surface Science 255 (2008) 1686–1691
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Fig. 7. XRD pattern of the Ni–W–Cr–P alloy coatings annealed at 800 8C for 1 h.
enhanced microhardness of the alloy coatings should be attributed to completely crystallized hard compound (Ni17W3 and Ni–Cr–Fe phases). 3.4. Corrosion resistance of electroless deposited Ni–W–Cr–P alloy coating In order to evaluate the corrosion resistance of Ni–W–Cr–P alloy coatings, as-deposited (i.e., without annealing) Ni–P, Ni–W– P, and Ni–W–Cr–P alloy coatings with the same thicknesses were put into the solution of 10% NaOH, 5% NaCl, 10% HCl and 10% H2SO4
for 12 days, respectively. Corrosion rate of these alloy coatings is calculated by weight method. Detailed results were shown in Table 4. It was found from Table 4 that the corrosion resistance of as-deposited Ni–W–Cr–P alloy coatings shows better than those of Ni–P and Ni–W–P coatings in different mediums, especially for 10% H2SO4. The Ni–W–Cr–P alloy coatings is still bright except for 10% NaOH solution. As a result, the as-deposited Ni–W–Cr–P alloy coatings show good corrosion resistance.
Table 3 Phase composition of Ni–W–Cr–P alloy coatings with different annealing temperature Annealing temperature (8C)
Phase composition
300 400 500 600 700 800
Ni3P, Ni Ni, Ni3P, Fe–Cr Ni3P, Ni2.9–Fe0.36–Cr0.7, Fe–Cr Ni3P, Ni17W3, Cr17Ni3, Fe–Cr, Ni2.9Cr0.7Fe0.36 Fe3Ni2, Fe–Cr, Fe4W2N, Ni2.9Fe0.36Cr0.7 Ni17W3, Ni–Cr–Fe
Fig. 9. Microhardness of the Ni–W–Cr–P alloy coatings as a function of annealing temperature.
Table 4 Corrosion rate of as-deposited coatings in different mediums (mg/(day cm2))
Fig. 8. Cross-section of the as-deposited Ni–W–Cr–P alloy coatings.
As-deposited coatings
10% NaOH
5% NaCl
10% HCl
10% H2SO4
Ni–P Ni–W–P Ni–W–Cr–P
1.27 1.20 1.17
1.04 0.09 0.06
335 310 280
60 32 27
L. Zhang et al. / Applied Surface Science 255 (2008) 1686–1691
1691
Table 5 Corrosion potential, corrosion current density, and polarization resistance of the Ni–W–Cr–P alloy coatings with different annealing temperature
Corrosion potential (V) Polarization resistance (V cm 2) Corrosion current density (mA cm
2
)
As-deposited (A)
300 8C (B)
500 8C (C)
600 8C (D)
800 8C (E)
0.306 1587 7.709
0.332 1251 13.87
0.389 858.2 23.68
0.403 613.3 36.8
0.352 1023 18.67
Fig. 10. Polarization curves of Ni–W–Cr–P alloy coatings as a function of annealing temperature. (A) As-deposited; (B) 300 8C; (C) 500 8C; (D) 600 8C; (E) 800 8C.
Fig. 10 shows polarization curves of Ni–W–Cr–P alloy coatings as a function of annealing temperature. Table 5 shows the corrosion potential, corrosion current density, and polarization resistance of the Ni–W–Cr–P alloy coatings annealed at different temperature which were calculated according to the polarization curves. As shown in Table 5 and Fig. 10, as-deposited Ni–W–Cr–P alloy coatings deposited by this method exhibit relatively high corrosion resistance, low corrosion potential, and low corrosion current density. Moreover, the Ni–W–Cr–P alloy coatings annealed at proper temperature also show enhanced corrosion resistance. It was also found that the corrosion resistance slightly decreased and corrosion current density increased because the increase of grain sizes breaks the uniformity of structure after annealed at 500 and 600 8C, and the corrosion resistance was improved and corrosion current density was decreased for the formation of Ni17W3 and Ni–Cr–Fe phases at the annealing temperature of 800 8C. 4. Conclusion In this work, the Ni–W–Cr–P alloy coatings were prepared by electroless deposition. Some results were obtained, as listed below: Annealing temperature strongly affects the crystal structure of the Ni–W–Cr–P alloy coatings. As-deposited Ni–W–Cr–P alloy
coatings possess amorphous structure; the crystal structure of the coatings gradually changes with the increase of annealing temperature, the Ni and Ni3P phases were formed at the annealing temperature of 300 8C; Ni3P was completely crystallized along with the formation of NiFeCr and FeCr phases at 500 8C; after annealed at 600 8C, Ni3P begins to decompose, and Ni17–W3 phase gradually appears; Ni3P completely decomposes, and results in the formation of Fe3Ni2, Ni2.9Fe0.36Cr0.7, Fe–Cr, and Fe4W2N phases after annealed at 700 8C; Ni17W3 and Ni–Cr–Fe phase with good alloy were obtained at 800 8C. Microhardness of the Ni–W–Cr–P alloy coatings strongly depends on the annealing temperature. When the annealing temperature is below 500 8C, the microhardness of the Ni–W–Cr–P alloy coatings increases with the increase of annealing temperature. However, the microhardness of the Ni–W–Cr–P alloy coatings decreases due to the increase of grain sizes and uncompletely crystalline at 500 8C; after annealed at 600 8C, the microhardness of the Ni–W–Cr–P alloy coatings continuously increases due to the formation of NiFeCr and Ni17–W3 phases, and reaches maximum (HV1140) at 800 8C. Corrosion resistance of the Ni–W–Cr–P alloy coatings strongly depends on the annealing temperature. The corrosion resistance of as-deposited Ni–W–Cr–P alloy coatings is better than those of asdeposited Ni–P and Ni–W–P alloy coatings in different mediums. Moreover, the corrosion resistance of the Ni–W–Cr–P alloy coatings gradually increases with the increase of annealing temperature, and the corrosion resistance of the alloy coating annealed at 800 8C reaches best. References [1] J.N. Balaraju, S.M. Jahan, K.S. Rajam, Surf. Coat. Technol. 201 (2006) 507–512. [2] Y. Gao, Z.J. Zheng, M. Zhu, C.P. Luo, Mater. Sci. Eng. A 381 (2004) 98–103. [3] P. de Lima-Neto, G.P. da Silva, A.N. Correia, Electrochim. Acta 51 (2006) 4928– 4933. [4] W.Y. Chen, S.K. Tien, F.B. Wu, J.G. Duh, Surf. Coat. Technol. 182 (2004) 85–91. [5] D.Y. Ding, J.N. Wang, A. Dozier, J. Appl. Phys. 95 (2004) 5006–5009. [6] T.X. Wang, J.L. Meng, Y.J. Hu, Surf. Technol. 34 (3) (2005) 27–29. [7] J.E. Williams, C. Davidson, J. Electrochem. Soc. 137 (1990) 3260. [8] D. Osmola, P. Nolan, U. Erb, G. Palumbo, K.T. Aust, Phys. Status Solidi, A 131 (1992) 569. [9] A. Robertson, U. Erb, G. Palumbo, Nanostruct. Mater. 12 (1999) 1035. [10] J. Li, X.G. Hu, D.L. Wang, Plat. Surf. Finish. 83 (1996) 62. [11] G. Lu, G. Zangari, Electrochem. Acta 47 (2002) 2969. [12] Y.Y. Tsai, F.B. Wu, Y.I. Chen, P.J. Peng, J.G. Duh, S.Y. Tsai, Surf. Coat. Technol. 146/ 147 (2001) 502. [13] K. Aoki, O. Takano, Plat. Surf. Finish. 77 (3) (1990) 48. [14] B.W. Zhang, W.Y. Hu, Q.L. Zhang, X.Y. Qu, Mater. Charact. 37 (1996) 119. [15] E.O. Hall, Proc. Phys. Soc. B 64 (1951) 747. [16] N.J. Petch, J. Iron Steel Inst. 173 (1953) 25.