UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

1012 IMMC 2018 | 19th International Metallurgy & Materials Congress

Eff ect of Heat Treatment on the Microstructure and Surface Properties of Ni-Based Platings

Mertcan Başkan¹, Metehan Erdoğan², İshak Karakaya¹

¹Middle East Technical University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Ankara, Turkey²Ankara Yıldırım Beyazıt University, Faculty of Engineering and Natural Sciences,

Department of Metallurgical and Materials Engineering, Ankara, Turkey

Abstract

Pure nickel and nickel-cobalt alloy platings are one of the most prominent materials due to their wear and corrosion resistances, thermal conductivities, magnetic properties and electrocatalytic properties. Although Ni based coatings have been plated with Watts solution for a long time, today, especially for the engineering applications, Nickel sulfamate based solutions are commonly preferred due to the advantages of low internal stress, high deposition rate and enhanced mechanical properties. In this study, the effects of heat treatment (HT) regimes on microstructure and surface properties of Ni based platings obtained under sulfamate electrolytes were investigated. Different heat treatments were applied to both pure Ni and Ni-Co alloy platings in order to reveal the altered properties of the coating with heat treatment temperature and time. After all, the platings were characterized in terms of grain size and shape, composition of the plating and hardness.

1. Introduction

Pure Ni and Ni-Co alloy platings are important electrplating materials due to their alterable magnetic properties [1-2], wear resistances [3-4], electrocatalytic properties [5] and applicability to electroforming [6]. Nickel based platings can be done for aesthetic purposes in consumer electronics, telecommunications, whereas the enginnering applications include communications, wear resistant erosion shields, even in aerospace and rocket systems. Ni-based deposits were plated in Watts Solution for a long time, today for engineering purposes, their deposition were conducted in sulfamate electrolytes due to various advantages such as low internal stress even deposition at high current densities, high current efficiencies and higher hardness as compared to the conventional Watts electrolyte [7].

The effects of processing parameters on Ni-based platings have been deeply investigated by many research groups so far. For example, Bai et al. characterized the electrolytes by electrochemical methods like cyclic voltammetry to see the reaction mechanism of the Ni-Co electrodeposition [8]. Bakhit et al. studied the corrosion behavior of Ni-Co alloy and Ni-Co/SiC(p) composite platings and concluded that cobalt additions improved the corrosion resistance but the composite

structure deteriorated the corrosion resistance [9]. Vazquez-Arenas et al. studied the application of different current regimes (i.e. direct, pulse and pulse reverse) during electroplating of Ni-Co alloys to see the structural properties like grain size, cobalt content and surface roughness [10]. Recently, . et al. [11] revealed the effects of plating parameters on the microstructure, hardness and cobalt content.

The effects of heat treatment on the properties of pure Ni and Ni-Co alloy platings which has not been studiedthoroughly, were investigated in this study. The heat treated platings were characterised in terms of microhardness, microstructure and composition.

2. Experimental Procedure

Both Ni and Ni-Co alloys were plated in sulfamate based electrolyte with a composition of 350 g/l nickel sulfamate, 15 g/l nickel chloride and 30 g/l boric acid. No additives were used during the experiments, only the pH of the electrolyte was set by the addition of sulfamic acid. High purity nickel anodes were used as nickel source, whereas Co2+ ions from cobalt sulfate was the

temperature controller and the electrolyte was stirred with at a speed of 600 rpm by magnetic stirrer.

Cu plates, which had been mechanically polished before, were used as cathode materials. All cathodes were chemically cleansed by firstly dipping into 1M NaOH then rinsing with water, then dipping into 25 vol% HNO3. The surface to be plated was defined by masking and kept nearly 5 cm2. Fresh electrolytes were used in each experiment. The electroplating tests were conducted under galvanostatic conditions. All deposits were aimed to have 40

perfect current efficiency.

Heat treatment (HT) operations were conducted under open atmosphere muffle furnaces. Different heat treatments were applied with the samples plated using the same electroplating parameters. The experimental parameters and design of experiments are summarised in Table 1.

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

101319. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

Table 1. Experimental parameters and DOE. Electrolyte Constituents Ni(SO3NH2)2 350 g/lt NiCl2.6H2O 15 g/lt H3BO3 30 g/lt pH 4.0 [Co2+]/[Ni2+] 0 ; 0.05 Electroplating Parameters Temperature Current density 4 A/dm2

Design of Experiments All platings Control samples (No HT)

591 for 1, 2 and 6 hrs. Nomenclature T(Temperature in celcius)t(time in hrs) Ex: T245t2 meant HT at 245°C for 2 hours.

3. Results and Discussion

3.1. Heat Treatment of Pure Ni Platings

Figure 1 showed the results of hardness measurements of pure Ni platings. Black and red bars represented the hardness values of coatings before and after heat treatment. Approximate values shown by black bars indicated reproducible and precise as-plated hardness values of about 260 HV0.2. The hardness values were seemed to be affected negatively from the heat treatments. Especially at high temperature cycles, the hardness was decreased to about 120 HV0.2.

Figure 1 Hardness values of pure Ni platings

The microstructure of pure Ni deposits before and after HT were illustrated in Figure 2(a) and 2(b), respectively. Similar microstructures Ni before and after HT at 245 Ccan be seen. However, at high temperature heat treatments, the platings exhibited very low hardness values and changes in surface properties. The microstructure of nickel coating was completely changed and even copper substrate was visible at some regions. Cross-sectional analyses were conducted to examine the metallurgical phenomena of the system. .

Figure 2 Microstructure of T245t6 Ni coating (a) Before and (b) After HT

Figure 3 shows the composition profile of the cross-section from sample T591t6. It can be seen that there was a gradual change in the composition at the Cu-Ni interface after the heat treatment, which was not the case for as-plated samples. Since the Cu-Ni phase diagram had complete miscibility at heat treatment temperature, the structure behaved as diffusion couple and the Cu substrate and Ni plating layer diffused each other by the aid of temperature.

Figure 3 Line analysis from the sample T591t6.

Boltzmann-Matano Analysis:

Boltzmann-Matano Analysis derived from Second Law gave the composition dependent diffusion coefficients of miscible systems according to the following formula:

(1)

Where x represents the distance, cx was composition at position x and c* was defined as the composition at Matano interface. In order to calculate the diffusion coefficients the following procedure was applied:

A sigmoidal curve was fitted to the composition profile (see Figure 4).

Analytical integration was used rather than numerical integration to minimize the numerical errors.

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

1014 IMMC 2018 | 19th International Metallurgy & Materials Congress

The position of matano interface was determined and substituted to the formula above.

Figure 4 Atom fraction of Ni at the Cu-Ni interface

Figure 5 shows the diffusion coefficient of Ni changing with the atomic fraction of Ni. It was observed that there was a minimum in the diffusivity of Ni at arround 25 at% Ni and started to increase again until pure Cu composition.

Figure 5 Diffusivitiy of Ni changing with compositions

3.2. Heat Treatment of Ni-Co Alloy Platings

The results of microhardness measurements were illustrated in Figure 6. Again the black and red bars represented the hardness values before and after heat treatments, respectively. All hardness values were nearly 400 HV0.2 before the heat treatments, which were quite reproducible like pure Ni platings. Also, Ni-Co alloy deposits yielded better hardness values than pure Ni platings for both as-plated and heat-treated conditions due to the effect of solid solution hardening, which was illustrated in detail from the previous studies of the authors [11].

Figure 6 Hardness values of Ni-Co alloy platings

When the hardness values were compared before and after HT according to the Figure 6, all samples lost hardness to some extent. This loss was not so prominent for samples T245t2 and T245t6. However, the loss was such severe for heat-treated samples at 591 C that the resultant hardness values were even less than half of as-plated ones. The microstructures of T245t2 and T591t6 before and after HT are shown in Figure 7. According to Figure 7(a), no drastic change in the microstructure was observed, although grain sizes of the T245t2 sample were slightly increased, which could be attributed to the minor hardness decrease in Figure 6. On the other hand, the microstructure was completely changed before and after HT of T591t6 sample as shown in Figure 7(b). The microstructures of Ni-Co alloys were consisted of globular grains. However, these grains transformed into the acicular grain structure after the application of heat treatment. According to the literature, acicular grains were observed at Co-rich region of the Ni-Co phase diagram, which was far behind of the composition range expected in the samples. This observation indicated the change in the chemistry of the samples during heat treatment.

Figure 7 Microstructures of (a) T245t2 and (b) T591t6.

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

101519. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

Figure 8(a) showed the cobalt concentrations on the surface. Cobalt concentrations of the samples T245t2 and T245t6 were close to each other and similar grade with the values before the heat treatment operations. However, at high temperature heat treatments, the cobalt concentration was significantly increased by the heat treatment duration. This could be correlated with the microstructures in Figure 7. However, this contradicted the line EDS analysis conducted through the cross section of the plating according to the Figure 8(b). The cobalt content exhibited subtle decrease as the plating thickness increase due to the consumption of Co ions in the electrolyte, however it was assumed to be equal and very close to non-heat treated samples. Therefore, nickel atoms diffused into the copper substrate during heat treatment.

Figure 8 Co concentration (a) on the surface and (b) through the cross-section

4. Conclusion

The effects of heat treatment on the hardness, microstructure and composition of the pure Ni and Ni-Co alloy platings were investigated in this study. Hardness was slightly decreased at low temperature heat treatments and the effect was much more prominent at high temperature heat treatments. A diffusional analysis was conducted for determination of effective diffusion coefficients of the Cu-Ni system. The microstructural change from globular to acicular, was attributed to

surface composition becaming Co rich after high temperature heat treatments in the Ni-Co alloy platings.

5. References

[1] A. Karpuz, H. Kockar, and M. Alper, J. Mater. Sci. Mater. Electron., 24 (2013) 3376 3381. [2] G. Chatzipirpiridis, E. Avilla, O. Ergeneman,

IEEE Transactions on Magnetics, 50 (2014) 10 12. [3] L. Xuewu, X. Yunhua, Q. Yi, and S. Lilong, Integr. Ferroelectr., 152 (2014) 144 151 2014. [4] L. Wang, Y. Gao, Q. Xue, H. Liu, and T. Xu, Appl. Surf. Sci., 242 (2005) 326 332. [5] C. Hu, C. Tsay, and A. Bai, Electrochim. Acta, 48 (2003) 565 572. [6] fabricatio Nuove Finiture (Italy), 1999. [7] M. Saitou, S. Oshiro, and S. M. Asadul Hossain, J. Appl. Electrochem., 38 (2008) 309 313. [8] A. Bai and C. Hu, Electrochim. Acta, 47 (2002) 3447 3456. [9] B. Bakhit, A. Akbari, F. Nasirpouri, and M. G. Hosseini, Appl. Surf. Sci, 307 (2014) 351 359. [10] J. Vazquez-Arenas, T. Treeratanaphitak, and M. Pritzker, Electrochim. Acta, 62 (2012) 63 72. [11] Effects of pH and Cobalt Concentration on the Properties of Nickel Cobalt Alloy Plating Proceedings

Book, 740-743.