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The heat treatment effect on the structure and mechanical properties of electrodeposited nano grain size Ni–W alloy coatings
Thin Solid Films 518 (2010) 7535–7540
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
The heat treatment effect on the structure and mechanical properties of electrodeposited nano grain size Ni–W alloy coatings Kung-Hsu Hou a,⁎, Yun-Feng Chang b, Sha-Ming Chang c, Chia-Hua Chang d a
Department of Power Vehicle and System Engineering, Chung Cheng Institute of Technology, National Defense University, Taoyuan 335, Taiwan Graduate School of Defense Science, Chung Cheng Institute of Technology, National Defense University, Taoyuan 335, Taiwan Post-graduate School of Vehicle and Transport Engineering, Chung Cheng Institute of Technology, National Defense University, Taoyuan 335, Taiwan d Chemical Systems Research Division, Chung-Shan Institute of Science & Technology, Taoyuan 335, Taiwan b c
a r t i c l e
i n f o
Available online 10 May 2010 Keywords: Ni–W alloy Electrodeposition Amorphous-like Heat treatment Wear resistance
a b s t r a c t The Ni–W alloy coatings with tungsten content from 32.5 to 61.2 wt.% were prepared in this study by electrodeposition. Experimental results show that the grain size of Ni–W coatings evaluated by XRD decreased with increasing tungsten content in coatings, however, the micro-hardness increased with increasing tungsten content. As-deposited Ni–61.2 wt.%W coating has amorphous-like structure and the grain size is around 1.5 nm, after annealing at 500 °C, the hardness of the coating is promoted to 1293 Hv owing to formation of Ni4W and NW precipitates. In addition, the heat-treated Ni–W coatings show a better wear resistance than the as-plated Ni–W coatings. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the development of optical micro-molding tooling plays an important role in progress of micro-injection techniques for fabrication of optical components. In particular, LIGA (an acronym from the German words for lithography, electroplating, and molding) is one of the main process techniques for fabrication of high precision optical devices. Generally, LIGA fabrication has relied mainly on electrodeposited pure nickel [1] or copper [2] as the structural materials. However, the use of pure metal is limited when properties like mechanical strength and thermal stability are crucial. This may be the main reason why the electroforming structural tooling with high toughness, high hardness, good wear resistance and excellent thermal stability has received significant interests. Alloys, so far, provide obvious alternatives, as their properties can be tailored with different combinations of metals and compounds. Significant research has gone into electroforming alloys from a wide variety of electrolytes, including Ni–Fe [3], Ni–P [4], Ni–Mo [5] and Ni–W [6], etc. Owing to its highest melting point of all metals and excellent thermal stability, tungsten can render excellent properties as an alloying element for Ni. It will result in longer service life and better precision when used as the tooling materials. Haj-Taieb et al. [7] reported the micro-hardness value of pure Ni drops strongly from 300 to 100 Hv after annealed at 700 °C while Ni–W alloys have much higher micro-hardness
⁎ Corresponding author. Tel.: + 886 3 3809257; fax: + 886 3 3906385. E-mail address: [email protected] (K.-H. Hou). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.041
than pure Ni at both as-deposited and heat-treated states. Their study suggested that Ni–W alloy is a possible alternative for Ni in microelectromechanical systems (MEMS) device applications [7]. It is believed that heat treatment will cause changes in microstructures of metals, such as phase, grain size, and residual stress. However, systematic researches of heat treatment effect on the mechanical properties of Ni– W alloy coatings have seldom been published in literatures. Only in Deter and Schuh's study, grain growth of Ni–W alloy during the heat treatment was reported [8]. It has been demonstrated that thermal stability, mechanical strength, anti-corrosion and anti-wear properties of Ni–W alloys would increase with tungsten content [9–12]. However, the research on the mechanical properties and wear characteristics of electroforming Ni–W coating is limited. Therefore, the objectives in this study are to discuss how the microstructures, mechanical strengths and wear characteristics of Ni–W alloys with various tungsten contents are influenced by heat treatment temperature. The results would be useful for developing an advanced material process for fabrication of optical micro-molding tools with electroforming Ni–W alloys. 2. Experimental details Direct current electroforming was conducted to prepare the Ni–W alloy coating in electrolytes containing 0.5 M Na3Cit, 0.5 M NH4Cl, varying concentrations of NiSO4 in the range of 0.03–0.1 M and Na2WO4 in the range of 0.05–0.3 M. The pH was adjusted to a value of 8.5 and the current density was constant at 15 mA/cm2. The used analytical Ni–W coatings were deposited on a 33 mm× 50 mm stainless steel sheet. After the samples were cut into
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Table 1 Electroforming conditions and its effect on deposition rate, current efficiency and composition of Ni–W coatings. Sample case
#A #B #C
Concentration [Ni2+]/[Ni2+] + [W6+]
Deposition rate (μm/h)
Current efficiency (%)
Composition of coatings (wt.%) W
Ni
0.18 0.3 0.4
18.5 24.9 31.2
17 25 40
61.2 49.0 32.5
38.8 51.0 67.5
Grain size (nm) 1.5 2.1 8.7
Fig. 1. Variation of deposition rate and current efficiency with W content of Ni–W alloy coatings.
small area of 1 cm2 using the electrical wire machine. Heat treatment was conducted with a heating rate of 10 °C/min to a pre-set temperature from 300 to 700 °C in a vacuum oven for 1 h, and then the samples were cooled to room temperature. Structural analysis was performed by means of X-ray diffraction (XRD) using Cu target, Ka radiation in a MAC Science, MXP 18 X-ray diffractometer operated at a scanning rate of 4°/min. The scanning angle range was from 25° to 80°. The grain size (d) was estimated using the Scherrer equation: d = 0.89λ / β cosθ, where λ is the wavelength of Cu Ka, β is the half-maximum width and θ is the diffraction angle. On the other hand, Vickers micro-hardness test was used to measure the hardness and the mean value for the hardness is calculated after 10 tests, loading with 100 g and 15 s. The morphology of the deposits was observed with a scanning electron microscope (SEM, Jeol model JSM-6500). The attached energy dispersive spectroscopy (EDS, Oxford Isis system) was used to determine the approximate composition of the alloy. The friction and wear tests were carried out on a rotational wear test machine [13] using a ring on disk pair (same as thrust washer adapter). The steel ring was made by JIS SKD-11 tool steel and thermal treated to achieve an average hardness of HRC 62 ± 1. The stationary iron disk was coated with Ni–W deposit with thickness about 200 μm on the counter surface. Before wear test, machined contact surfaces of friction pairs were ground with #600, #800, and #1200 SiC abrasive sheets, and then polished with 0.25 μm alumina powder using a low speed polish machine. For the highest tungsten content coating deposition conditions, 5 as-plated and 5 heat-treated (heat treatment at 700 °C, 1 h) samples were prepared for wear test. The weight loss of the Ni–W coatings was obtained by measuring the disk weight before and after each wear test using an electrical balance with 0.01 mg weight scale accuracy.
3. Results and discussion 3.1. W content and properties of Ni–W alloy coatings The tungsten content, deposition rate of the Ni–W coating and current efficiency for various nickel concentration in the electroforming baths are presented in Table 1. In the formation of the Ni–W coating, the tungsten content of the as-deposited Ni–W coating decreases with the increase of ([Ni2+]/[Ni2+] + [W6+]) ratio in the bath, while the current
Fig. 2. SEM images of Ni–W alloy coatings with different tungsten contents (a) 32.5 wt.%W, and (b) 49.0 wt%W.
K.-H. Hou et al. / Thin Solid Films 518 (2010) 7535–7540
efficiency and deposition rate increase with ([Ni2+]/[Ni2+] + [W6+]) ratio. Fig. 1 also shows the variation of deposition rate and current efficiency with tungsten content of Ni–W alloy coatings. It should be noted that both deposition rate and current efficiency decreased as tungsten content increased. It mainly arises from the influence of tungsten content on reduction rate of Ni2+ ions on the diffusion layer of cathode. With a lower Ni2+ concentration, the deposition rate of W increases and W, leading to a reduction in nickel crystallite size, would occupy the nucleation sites. A similar variation trend of the tungsten content and the current efficiency for Ni–W coatings have been reported by Eliaza et al. [14], and Younes et al. [15], respectively. According to the mechanism proposed by Younes et al. [15], deposition of tungsten occurs through the reduction reactions, which proceed through the production of some complexes (either with citrate or with NH3) in the
7537
bulk of solution. As the concentration of the complexes in the bath solution is very low, it can cause mass transport limitation for deposition process. Therefore, high nickel content alloy coatings would be obtained together with high current efficiency. Fig. 2 presents the SEM images of Ni–32.5 wt.%W and Ni–49 wt.% W, respectively. SEM images reveal that the surfaces of the Ni– 32.5 wt.%W alloy coating had a rough granular structure; nevertheless, the surfaces of the Ni–49 wt.%W alloy coating had a more homogeneous and smoother surface appearance. It indicates that increase in the tungsten content in the alloy would lead to the decrease of the surface granular size and the morphology transform to form fine spherical nodules and smoothing the surface texture. This may cause reduced grain size as the tungsten increased in coatings (see the XRD analysis results).
Fig. 3. XRD patterns of Ni–W alloy coating with different tungsten contents (a) 32.5 wt.%W, and (b) 49.0 wt.%W alloy.
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Fig. 4. The effect of heat treatment temperature on the crystallite size of Ni–W alloys.
3.2. The heat treatment effect on the structure of Ni–W alloy coating XRD measurements were carried out to investigate the structural evolution of Ni–W coatings before and after heat treatment. Fig. 3(a) shows the XRD patterns for Ni–W alloys with tungsten content of 32.5 wt.%. The microstructure of Ni–W alloy coating is dependent on the heat treatment temperature. For the as-deposited alloy, three diffraction peaks located at 44°, 51° and 76° were detected. These peaks are assigned as the (111), (200) and (220) reflection planes of FCC phase structure of metallic nickel. After annealing, the XRD pattern of Ni– 32.5 wt.%W coating shows that the relative strength of the diffraction peak at (111) increased with increase in the heat-treated temperature. It reveals that the growth and coarsening of grain will be promoted thermally. After heat-treated at a temperature up to 500 °C, slim precipitate of Ni4W phase was observed, as shown in XRD pattern.
The XRD diffraction patterns for Ni–W alloys with tungsten content of 49 wt.% are shown in Fig. 3(b), in which the as-deposited coating shows that the diffraction halo is around 44° broadening with peak shifting towards lower scanning angle (2θ). A similar shift has been reported by Yamasaki et al. [16], in Ni–20.7 at.%W coating structure. This may be attributed to a reduction in the crystallite size of the alloys with an increase in the amount of alloying tungsten element. The average nearest neighbor distance was expanded by the W atoms incorporated into the lattice of nickel and leading to form an amorphous-like structure [15]. It is also observed from Fig. 3(b) that upon heating on 300 °C no change is apparent in the XRD pattern as compared to the as-deposited one. After heat treating at 500 °C, the Ni(111) peak became narrower, however, no other peaks appeared. At 700 °C, additional peaks arise. A thorough phase analysis of the diffraction peaks identifies the formation
Fig. 5. The heat treatment temperature effect on the micro-hardness of Ni–W alloys.
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Fig. 6. The friction coefficients of Ni–61.2 wt.%W alloys as-deposited and after annealing at 700 °C.
of NiW and Ni4W inter-metallic phases. In addition, the pattern peak also revealed that the unremarkable oxide peak exists on the surface which may be influenced by the chamber environment. The XRD profiles for Ni–61.2 wt.%W alloy before and after heat treatment are quite similar to those of Ni–49 wt.%W alloy. Fig. 4 displays the variation of grain size correlated with heat treatment temperature for Ni–W alloy coating with different W contents, in which reported data were obtained by applying the Scherrer formula to calculate the grain size from the diffraction peak of Ni (111) and the broad maximum halo of amorphous phase on XRD patterns. It shows that the as-deposited Ni–61.2 wt.%W coating exhibits the finest grain size at approximately 1.5 nm and the grain size increases with decreasing W content of the Ni–W coating. The grain sizes were nearly unchanged for all samples annealed at 300 °C for 1 h. However, the grain sizes of Ni–W alloys increase to 6.3–9.9 nm after samples annealed at 500 °C. As the heat treatment temperature is increased to 700 °C, the crystallites grow continuously and their sizes are coarsened to 16.8 nm–25.7 nm, respectively. It is obvious that the grain size in the alloys became larger with increasing annealing temperature. The enlargement of the grain size might be caused by the crystallization of the amorphous region in the deposit during the annealing process.
Typical friction coefficient curves for Ni–61.2 wt.%W alloys are shown in Fig. 6. Significant fluctuation (0.35–0.45) occurs for the friction coefficient of as-deposited sample (curve (a)), indicating the domination of adhesive sliding friction. It is clearly seen from (curve (b)) that
3.3. The heat treatment effect on the hardness and wear characteristics Fig. 5 presents the heat treatment temperature effect on the hardness of deposited Ni–W alloy coatings. The hardness for all asdeposited samples with different tungsten content is ranged from 593.7 to 823 Hv. The hardness of Ni–W alloy deposits increased with increasing W content. Fig. 5 also indicate that the hardness increases with a heat treatment temperature up to 500 °C. The hardness increased to reach the maximum of 1293.3 Hv at 500 °C for Ni– 61.2 wt.%W. As the temperature is increased to 700 °C, the hardness of the deposits drops slightly. Over 300 °C, the precipitation of intermetallic compounds such as Ni4W and NiW seemed to contribute to the increase of hardness. However, excessive annealing at 700 °C tends to decrease the hardness of Ni–W alloy due to grain coarsening. It is worth to mention that the hardness of Ni–32.5 wt.%W coating treated at 500 °C was as high as that of as-deposited Ni–61.2 wt.%W coating, indicating that the evolution temperature is not high enough to drive the great amount of precipitation of Ni4W inter-metallic compound for low tungsten coatings.
Fig. 7. Worn surface morphology of Ni–61.2 wt%W alloy (a) as-deposited and (b) annealed at 700 °C.
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after heat treatment at 700 °C, the friction coefficient curve shows less fluctuation and all friction coefficients are below 0.3. The morphology of worn surface of as-deposited Ni–61.2 wt.%W alloy is shown in Fig. 7(a). The worn surface shows the presence of plastic deformation. Meanwhile, the red debris, which was identified as ferrous oxide by EDX, was distributed uniformly on the worn surface of the coating. It is reasonable to consider that the iron is fragmented and transmitted to the rubbing coatings surface from the SKD steel counterpart during the sliding period. However, the micrograph shown in Fig. 7(b) indicates that the worn area is smooth for the coating under heat treatment at 700 °C. It means that there is no plastic deformation appearing on the surface. Based on the above results, it is suggested that heat-treated Ni–W alloy has better wear characteristic than that of as-deposited one. Our results showed that the improvement of wear resistance of Ni–W alloys with hardness increase fits with Archard's law. From those results of tungsten contents, X-ray grain size, microhardness and wear resistance for the Ni–W coatings, we can correlate the heat treatment effect on the Ni–W coatings. The strengthened mechanism of the Ni–W alloy in this research can be divided into two cases. The first case is the as-deposited coating strengthened with increasing the tungsten (32.5 wt.%–61.2 wt.%W) in coating, which was caused by solid solution and grain size refinement (8.7 nm– 1.5 nm) [17]. Another case is the heat-treated coating that was strengthened by the precipitation of inter-metallic compounds such as Ni4W and NiW [18], as heat-treated temperature promoted upon 500 °C. In this study, the Ni–61.2 wt.%W coating treated at 500 °C exhibited the maximum hardness of 1293.3 Hv, however, on further increasing the heat-treated temperature to 700 °C, the hardness drops slightly which can be ascribed to the grain coarsening effect. 4. Conclusions In this study, the Ni–W coatings with W content ranged from 32.5 to 61.2 wt.% were prepared by a detailed control of the bath composition. With comparison of their microstructures, strength as well as wear behaviours between as-plated and heat-treated coatings, the conclusions can be summarized as follows. The grain size of the Ni–W alloys decreases with increasing of tungsten content. XRD results indicated that the as-deposited Ni–W alloys with tungsten content of 49 wt.% or higher have an amorphouslike microstructure. With heat treatment at 700 °C, the amorphous-like
Ni–W would transform into NiW and Ni4W inter-metallic crystalline phases. On the other hand, both as-deposited and heat-treated Ni– 32.5 wt.%W alloys have a Ni FCC lattice structure. The hardness of Ni–W alloy deposits increased with increasing W content. As the heat treatment temperature was increased, the hardness increased and reached the maximum of 1293 Hv at 500 °C for Ni–61.2 wt.%W coating. The hardness of Ni–61.2 wt.%W alloy with annealing at 700 °C is 8–9% lower than that of 500 °C heated one. After heat treatment at 700 °C, the friction behaviour of Ni–61.2 wt.%W alloy transferred from adhesive sliding type to non-adhesive friction type, leading to lowering the friction coefficient (less than 0.3). The wear behaviour of heat-treated coating is much better than that of as-deposited coating. Acknowledgements The supports of this work from N.S.C. No. NSC96-2622-E-606-003CC3 and C.S.I.S.T. No. SIST-800-V113(98) are appreciated. References [1] Y. Guo, G. Liu, Y. Xiong, J. Wang, X. Huang, Y. Tian, J. Microelectromech. Syst. 16 (2007) 589. [2] K.K. Chakravorty, C.P. Chien, J.M. Cech, L.B. Branson, J.M. Atencio, T.M. White, L.S. Lathrop, B.W. Aker, M.H. Tanielian, P.L. Young, IEEE Trans. Components Hybrids. Manuf. Technol. 13 (1990) 135. [3] C.W. Su, F.J. He, H. Ju, Y.B. Zhang, E.L. Wang, Electrochim. Acta 54 (2009) 6257. [4] A.P. Ordine, S.L. D´ıaz, I.C.P. Margarit, O.E. Barcia, O.R. Mattos, Electrochim. Acta 51 (2006) 1480. [5] N.V. Krstajic, V.D. Jovic, Lj. Gajic´-Krstajic, B.M. Jovic, A.L. Antozzi, G.N. Martelli, Int. J. Hydrogen Energy 33 (2008) 3676. [6] A.S.M.A. Haseeb, K. Bade, Microsyst. Technol. 14 (2008) 379. [7] M. Haj-Taieb, A.S.M.A. Haseeb, J. Caulfield, K. Bade, J. Aktaa, K.J. Hemker, Microsyst. Technol. 14 (2008) 1531. [8] A.J. Detor, C.A. Schuh, J. Mater. Res. 22 (2007) 3233. [9] P. Schloûmacher, T. Yamasaki, Mikrochim. Acta 132 (2000) 309. [10] C.A. Schuh, T.G. Nieh, H. Iwasaki, Acta Mater. 51 (2003) 431. [11] K.R. Sriraman, S. Ganesh-Sundara-Raman, S.K. Seshadri, Mater. Sci. Eng. A 460/461 (2007) 39. [12] K.R. Sriraman, S. Ganesh-Sundara-Raman, S.K. Seshadri, Mater. Sci. Eng. A 418 (2006) 303. [13] K.H. Hou, M.D. Ger, L.M. Wang, S.T. Ke, Wear 253 (2002) 994. [14] N. Eliaza, T.M. Sridhara, E. Gileadi, Electrochim. Acta 50 (2005) 2893. [15] O. Younes, L. Zhu, Y. Rosenberg, Y. Shacham-Diamand, E. Gileadi, Langmuir 17 (2001) 8270. [16] T. Yamasaki, Scr. Mater. 44 (2001) 1497. [17] F.B. Wu, S.K. Tien, W.Y. Chen, J.G. Duh, Surf. Coat. Technol. 177–178 (2004) 312. [18] W.S. Hwang, J.J. Lee, Mater. Sci. Forum 510–511 (2006) 1126.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
The heat treatment effect on the structure and mechanical properties of electrodeposited nano grain size Ni–W alloy coatings Kung-Hsu Hou a,⁎, Yun-Feng Chang b, Sha-Ming Chang c, Chia-Hua Chang d a
Department of Power Vehicle and System Engineering, Chung Cheng Institute of Technology, National Defense University, Taoyuan 335, Taiwan Graduate School of Defense Science, Chung Cheng Institute of Technology, National Defense University, Taoyuan 335, Taiwan Post-graduate School of Vehicle and Transport Engineering, Chung Cheng Institute of Technology, National Defense University, Taoyuan 335, Taiwan d Chemical Systems Research Division, Chung-Shan Institute of Science & Technology, Taoyuan 335, Taiwan b c
a r t i c l e
i n f o
Available online 10 May 2010 Keywords: Ni–W alloy Electrodeposition Amorphous-like Heat treatment Wear resistance
a b s t r a c t The Ni–W alloy coatings with tungsten content from 32.5 to 61.2 wt.% were prepared in this study by electrodeposition. Experimental results show that the grain size of Ni–W coatings evaluated by XRD decreased with increasing tungsten content in coatings, however, the micro-hardness increased with increasing tungsten content. As-deposited Ni–61.2 wt.%W coating has amorphous-like structure and the grain size is around 1.5 nm, after annealing at 500 °C, the hardness of the coating is promoted to 1293 Hv owing to formation of Ni4W and NW precipitates. In addition, the heat-treated Ni–W coatings show a better wear resistance than the as-plated Ni–W coatings. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the development of optical micro-molding tooling plays an important role in progress of micro-injection techniques for fabrication of optical components. In particular, LIGA (an acronym from the German words for lithography, electroplating, and molding) is one of the main process techniques for fabrication of high precision optical devices. Generally, LIGA fabrication has relied mainly on electrodeposited pure nickel [1] or copper [2] as the structural materials. However, the use of pure metal is limited when properties like mechanical strength and thermal stability are crucial. This may be the main reason why the electroforming structural tooling with high toughness, high hardness, good wear resistance and excellent thermal stability has received significant interests. Alloys, so far, provide obvious alternatives, as their properties can be tailored with different combinations of metals and compounds. Significant research has gone into electroforming alloys from a wide variety of electrolytes, including Ni–Fe [3], Ni–P [4], Ni–Mo [5] and Ni–W [6], etc. Owing to its highest melting point of all metals and excellent thermal stability, tungsten can render excellent properties as an alloying element for Ni. It will result in longer service life and better precision when used as the tooling materials. Haj-Taieb et al. [7] reported the micro-hardness value of pure Ni drops strongly from 300 to 100 Hv after annealed at 700 °C while Ni–W alloys have much higher micro-hardness
⁎ Corresponding author. Tel.: + 886 3 3809257; fax: + 886 3 3906385. E-mail address: [email protected] (K.-H. Hou). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.041
than pure Ni at both as-deposited and heat-treated states. Their study suggested that Ni–W alloy is a possible alternative for Ni in microelectromechanical systems (MEMS) device applications [7]. It is believed that heat treatment will cause changes in microstructures of metals, such as phase, grain size, and residual stress. However, systematic researches of heat treatment effect on the mechanical properties of Ni– W alloy coatings have seldom been published in literatures. Only in Deter and Schuh's study, grain growth of Ni–W alloy during the heat treatment was reported [8]. It has been demonstrated that thermal stability, mechanical strength, anti-corrosion and anti-wear properties of Ni–W alloys would increase with tungsten content [9–12]. However, the research on the mechanical properties and wear characteristics of electroforming Ni–W coating is limited. Therefore, the objectives in this study are to discuss how the microstructures, mechanical strengths and wear characteristics of Ni–W alloys with various tungsten contents are influenced by heat treatment temperature. The results would be useful for developing an advanced material process for fabrication of optical micro-molding tools with electroforming Ni–W alloys. 2. Experimental details Direct current electroforming was conducted to prepare the Ni–W alloy coating in electrolytes containing 0.5 M Na3Cit, 0.5 M NH4Cl, varying concentrations of NiSO4 in the range of 0.03–0.1 M and Na2WO4 in the range of 0.05–0.3 M. The pH was adjusted to a value of 8.5 and the current density was constant at 15 mA/cm2. The used analytical Ni–W coatings were deposited on a 33 mm× 50 mm stainless steel sheet. After the samples were cut into
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Table 1 Electroforming conditions and its effect on deposition rate, current efficiency and composition of Ni–W coatings. Sample case
#A #B #C
Concentration [Ni2+]/[Ni2+] + [W6+]
Deposition rate (μm/h)
Current efficiency (%)
Composition of coatings (wt.%) W
Ni
0.18 0.3 0.4
18.5 24.9 31.2
17 25 40
61.2 49.0 32.5
38.8 51.0 67.5
Grain size (nm) 1.5 2.1 8.7
Fig. 1. Variation of deposition rate and current efficiency with W content of Ni–W alloy coatings.
small area of 1 cm2 using the electrical wire machine. Heat treatment was conducted with a heating rate of 10 °C/min to a pre-set temperature from 300 to 700 °C in a vacuum oven for 1 h, and then the samples were cooled to room temperature. Structural analysis was performed by means of X-ray diffraction (XRD) using Cu target, Ka radiation in a MAC Science, MXP 18 X-ray diffractometer operated at a scanning rate of 4°/min. The scanning angle range was from 25° to 80°. The grain size (d) was estimated using the Scherrer equation: d = 0.89λ / β cosθ, where λ is the wavelength of Cu Ka, β is the half-maximum width and θ is the diffraction angle. On the other hand, Vickers micro-hardness test was used to measure the hardness and the mean value for the hardness is calculated after 10 tests, loading with 100 g and 15 s. The morphology of the deposits was observed with a scanning electron microscope (SEM, Jeol model JSM-6500). The attached energy dispersive spectroscopy (EDS, Oxford Isis system) was used to determine the approximate composition of the alloy. The friction and wear tests were carried out on a rotational wear test machine [13] using a ring on disk pair (same as thrust washer adapter). The steel ring was made by JIS SKD-11 tool steel and thermal treated to achieve an average hardness of HRC 62 ± 1. The stationary iron disk was coated with Ni–W deposit with thickness about 200 μm on the counter surface. Before wear test, machined contact surfaces of friction pairs were ground with #600, #800, and #1200 SiC abrasive sheets, and then polished with 0.25 μm alumina powder using a low speed polish machine. For the highest tungsten content coating deposition conditions, 5 as-plated and 5 heat-treated (heat treatment at 700 °C, 1 h) samples were prepared for wear test. The weight loss of the Ni–W coatings was obtained by measuring the disk weight before and after each wear test using an electrical balance with 0.01 mg weight scale accuracy.
3. Results and discussion 3.1. W content and properties of Ni–W alloy coatings The tungsten content, deposition rate of the Ni–W coating and current efficiency for various nickel concentration in the electroforming baths are presented in Table 1. In the formation of the Ni–W coating, the tungsten content of the as-deposited Ni–W coating decreases with the increase of ([Ni2+]/[Ni2+] + [W6+]) ratio in the bath, while the current
Fig. 2. SEM images of Ni–W alloy coatings with different tungsten contents (a) 32.5 wt.%W, and (b) 49.0 wt%W.
K.-H. Hou et al. / Thin Solid Films 518 (2010) 7535–7540
efficiency and deposition rate increase with ([Ni2+]/[Ni2+] + [W6+]) ratio. Fig. 1 also shows the variation of deposition rate and current efficiency with tungsten content of Ni–W alloy coatings. It should be noted that both deposition rate and current efficiency decreased as tungsten content increased. It mainly arises from the influence of tungsten content on reduction rate of Ni2+ ions on the diffusion layer of cathode. With a lower Ni2+ concentration, the deposition rate of W increases and W, leading to a reduction in nickel crystallite size, would occupy the nucleation sites. A similar variation trend of the tungsten content and the current efficiency for Ni–W coatings have been reported by Eliaza et al. [14], and Younes et al. [15], respectively. According to the mechanism proposed by Younes et al. [15], deposition of tungsten occurs through the reduction reactions, which proceed through the production of some complexes (either with citrate or with NH3) in the
7537
bulk of solution. As the concentration of the complexes in the bath solution is very low, it can cause mass transport limitation for deposition process. Therefore, high nickel content alloy coatings would be obtained together with high current efficiency. Fig. 2 presents the SEM images of Ni–32.5 wt.%W and Ni–49 wt.% W, respectively. SEM images reveal that the surfaces of the Ni– 32.5 wt.%W alloy coating had a rough granular structure; nevertheless, the surfaces of the Ni–49 wt.%W alloy coating had a more homogeneous and smoother surface appearance. It indicates that increase in the tungsten content in the alloy would lead to the decrease of the surface granular size and the morphology transform to form fine spherical nodules and smoothing the surface texture. This may cause reduced grain size as the tungsten increased in coatings (see the XRD analysis results).
Fig. 3. XRD patterns of Ni–W alloy coating with different tungsten contents (a) 32.5 wt.%W, and (b) 49.0 wt.%W alloy.
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Fig. 4. The effect of heat treatment temperature on the crystallite size of Ni–W alloys.
3.2. The heat treatment effect on the structure of Ni–W alloy coating XRD measurements were carried out to investigate the structural evolution of Ni–W coatings before and after heat treatment. Fig. 3(a) shows the XRD patterns for Ni–W alloys with tungsten content of 32.5 wt.%. The microstructure of Ni–W alloy coating is dependent on the heat treatment temperature. For the as-deposited alloy, three diffraction peaks located at 44°, 51° and 76° were detected. These peaks are assigned as the (111), (200) and (220) reflection planes of FCC phase structure of metallic nickel. After annealing, the XRD pattern of Ni– 32.5 wt.%W coating shows that the relative strength of the diffraction peak at (111) increased with increase in the heat-treated temperature. It reveals that the growth and coarsening of grain will be promoted thermally. After heat-treated at a temperature up to 500 °C, slim precipitate of Ni4W phase was observed, as shown in XRD pattern.
The XRD diffraction patterns for Ni–W alloys with tungsten content of 49 wt.% are shown in Fig. 3(b), in which the as-deposited coating shows that the diffraction halo is around 44° broadening with peak shifting towards lower scanning angle (2θ). A similar shift has been reported by Yamasaki et al. [16], in Ni–20.7 at.%W coating structure. This may be attributed to a reduction in the crystallite size of the alloys with an increase in the amount of alloying tungsten element. The average nearest neighbor distance was expanded by the W atoms incorporated into the lattice of nickel and leading to form an amorphous-like structure [15]. It is also observed from Fig. 3(b) that upon heating on 300 °C no change is apparent in the XRD pattern as compared to the as-deposited one. After heat treating at 500 °C, the Ni(111) peak became narrower, however, no other peaks appeared. At 700 °C, additional peaks arise. A thorough phase analysis of the diffraction peaks identifies the formation
Fig. 5. The heat treatment temperature effect on the micro-hardness of Ni–W alloys.
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Fig. 6. The friction coefficients of Ni–61.2 wt.%W alloys as-deposited and after annealing at 700 °C.
of NiW and Ni4W inter-metallic phases. In addition, the pattern peak also revealed that the unremarkable oxide peak exists on the surface which may be influenced by the chamber environment. The XRD profiles for Ni–61.2 wt.%W alloy before and after heat treatment are quite similar to those of Ni–49 wt.%W alloy. Fig. 4 displays the variation of grain size correlated with heat treatment temperature for Ni–W alloy coating with different W contents, in which reported data were obtained by applying the Scherrer formula to calculate the grain size from the diffraction peak of Ni (111) and the broad maximum halo of amorphous phase on XRD patterns. It shows that the as-deposited Ni–61.2 wt.%W coating exhibits the finest grain size at approximately 1.5 nm and the grain size increases with decreasing W content of the Ni–W coating. The grain sizes were nearly unchanged for all samples annealed at 300 °C for 1 h. However, the grain sizes of Ni–W alloys increase to 6.3–9.9 nm after samples annealed at 500 °C. As the heat treatment temperature is increased to 700 °C, the crystallites grow continuously and their sizes are coarsened to 16.8 nm–25.7 nm, respectively. It is obvious that the grain size in the alloys became larger with increasing annealing temperature. The enlargement of the grain size might be caused by the crystallization of the amorphous region in the deposit during the annealing process.
Typical friction coefficient curves for Ni–61.2 wt.%W alloys are shown in Fig. 6. Significant fluctuation (0.35–0.45) occurs for the friction coefficient of as-deposited sample (curve (a)), indicating the domination of adhesive sliding friction. It is clearly seen from (curve (b)) that
3.3. The heat treatment effect on the hardness and wear characteristics Fig. 5 presents the heat treatment temperature effect on the hardness of deposited Ni–W alloy coatings. The hardness for all asdeposited samples with different tungsten content is ranged from 593.7 to 823 Hv. The hardness of Ni–W alloy deposits increased with increasing W content. Fig. 5 also indicate that the hardness increases with a heat treatment temperature up to 500 °C. The hardness increased to reach the maximum of 1293.3 Hv at 500 °C for Ni– 61.2 wt.%W. As the temperature is increased to 700 °C, the hardness of the deposits drops slightly. Over 300 °C, the precipitation of intermetallic compounds such as Ni4W and NiW seemed to contribute to the increase of hardness. However, excessive annealing at 700 °C tends to decrease the hardness of Ni–W alloy due to grain coarsening. It is worth to mention that the hardness of Ni–32.5 wt.%W coating treated at 500 °C was as high as that of as-deposited Ni–61.2 wt.%W coating, indicating that the evolution temperature is not high enough to drive the great amount of precipitation of Ni4W inter-metallic compound for low tungsten coatings.
Fig. 7. Worn surface morphology of Ni–61.2 wt%W alloy (a) as-deposited and (b) annealed at 700 °C.
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after heat treatment at 700 °C, the friction coefficient curve shows less fluctuation and all friction coefficients are below 0.3. The morphology of worn surface of as-deposited Ni–61.2 wt.%W alloy is shown in Fig. 7(a). The worn surface shows the presence of plastic deformation. Meanwhile, the red debris, which was identified as ferrous oxide by EDX, was distributed uniformly on the worn surface of the coating. It is reasonable to consider that the iron is fragmented and transmitted to the rubbing coatings surface from the SKD steel counterpart during the sliding period. However, the micrograph shown in Fig. 7(b) indicates that the worn area is smooth for the coating under heat treatment at 700 °C. It means that there is no plastic deformation appearing on the surface. Based on the above results, it is suggested that heat-treated Ni–W alloy has better wear characteristic than that of as-deposited one. Our results showed that the improvement of wear resistance of Ni–W alloys with hardness increase fits with Archard's law. From those results of tungsten contents, X-ray grain size, microhardness and wear resistance for the Ni–W coatings, we can correlate the heat treatment effect on the Ni–W coatings. The strengthened mechanism of the Ni–W alloy in this research can be divided into two cases. The first case is the as-deposited coating strengthened with increasing the tungsten (32.5 wt.%–61.2 wt.%W) in coating, which was caused by solid solution and grain size refinement (8.7 nm– 1.5 nm) [17]. Another case is the heat-treated coating that was strengthened by the precipitation of inter-metallic compounds such as Ni4W and NiW [18], as heat-treated temperature promoted upon 500 °C. In this study, the Ni–61.2 wt.%W coating treated at 500 °C exhibited the maximum hardness of 1293.3 Hv, however, on further increasing the heat-treated temperature to 700 °C, the hardness drops slightly which can be ascribed to the grain coarsening effect. 4. Conclusions In this study, the Ni–W coatings with W content ranged from 32.5 to 61.2 wt.% were prepared by a detailed control of the bath composition. With comparison of their microstructures, strength as well as wear behaviours between as-plated and heat-treated coatings, the conclusions can be summarized as follows. The grain size of the Ni–W alloys decreases with increasing of tungsten content. XRD results indicated that the as-deposited Ni–W alloys with tungsten content of 49 wt.% or higher have an amorphouslike microstructure. With heat treatment at 700 °C, the amorphous-like
Ni–W would transform into NiW and Ni4W inter-metallic crystalline phases. On the other hand, both as-deposited and heat-treated Ni– 32.5 wt.%W alloys have a Ni FCC lattice structure. The hardness of Ni–W alloy deposits increased with increasing W content. As the heat treatment temperature was increased, the hardness increased and reached the maximum of 1293 Hv at 500 °C for Ni–61.2 wt.%W coating. The hardness of Ni–61.2 wt.%W alloy with annealing at 700 °C is 8–9% lower than that of 500 °C heated one. After heat treatment at 700 °C, the friction behaviour of Ni–61.2 wt.%W alloy transferred from adhesive sliding type to non-adhesive friction type, leading to lowering the friction coefficient (less than 0.3). The wear behaviour of heat-treated coating is much better than that of as-deposited coating. Acknowledgements The supports of this work from N.S.C. No. NSC96-2622-E-606-003CC3 and C.S.I.S.T. No. SIST-800-V113(98) are appreciated. References [1] Y. Guo, G. Liu, Y. Xiong, J. Wang, X. Huang, Y. Tian, J. Microelectromech. Syst. 16 (2007) 589. [2] K.K. Chakravorty, C.P. Chien, J.M. Cech, L.B. Branson, J.M. Atencio, T.M. White, L.S. Lathrop, B.W. Aker, M.H. Tanielian, P.L. Young, IEEE Trans. Components Hybrids. Manuf. Technol. 13 (1990) 135. [3] C.W. Su, F.J. He, H. Ju, Y.B. Zhang, E.L. Wang, Electrochim. Acta 54 (2009) 6257. [4] A.P. Ordine, S.L. D´ıaz, I.C.P. Margarit, O.E. Barcia, O.R. Mattos, Electrochim. Acta 51 (2006) 1480. [5] N.V. Krstajic, V.D. Jovic, Lj. Gajic´-Krstajic, B.M. Jovic, A.L. Antozzi, G.N. Martelli, Int. J. Hydrogen Energy 33 (2008) 3676. [6] A.S.M.A. Haseeb, K. Bade, Microsyst. Technol. 14 (2008) 379. [7] M. Haj-Taieb, A.S.M.A. Haseeb, J. Caulfield, K. Bade, J. Aktaa, K.J. Hemker, Microsyst. Technol. 14 (2008) 1531. [8] A.J. Detor, C.A. Schuh, J. Mater. Res. 22 (2007) 3233. [9] P. Schloûmacher, T. Yamasaki, Mikrochim. Acta 132 (2000) 309. [10] C.A. Schuh, T.G. Nieh, H. Iwasaki, Acta Mater. 51 (2003) 431. [11] K.R. Sriraman, S. Ganesh-Sundara-Raman, S.K. Seshadri, Mater. Sci. Eng. A 460/461 (2007) 39. [12] K.R. Sriraman, S. Ganesh-Sundara-Raman, S.K. Seshadri, Mater. Sci. Eng. A 418 (2006) 303. [13] K.H. Hou, M.D. Ger, L.M. Wang, S.T. Ke, Wear 253 (2002) 994. [14] N. Eliaza, T.M. Sridhara, E. Gileadi, Electrochim. Acta 50 (2005) 2893. [15] O. Younes, L. Zhu, Y. Rosenberg, Y. Shacham-Diamand, E. Gileadi, Langmuir 17 (2001) 8270. [16] T. Yamasaki, Scr. Mater. 44 (2001) 1497. [17] F.B. Wu, S.K. Tien, W.Y. Chen, J.G. Duh, Surf. Coat. Technol. 177–178 (2004) 312. [18] W.S. Hwang, J.J. Lee, Mater. Sci. Forum 510–511 (2006) 1126.