1 Introduction

Fe–Ni alloy with Fe 64–36 wt% Ni, commercially known as invar, has a low thermal expansion coefficient (CTE) than any other metals [1]. Based on its dimensional stability, Fe–Ni invar has been used as the materials of fine metal mask(FMM), a key component in RGB patterning of organic light-emitting diode (OLED) display, to avoid distortion of patterns during evaporation process at high temperature [2,3,4].

Nowadays, commercialized FMM has been produced by wet etching process of invar film with over 20 μm in thickness made by rolling process. However, it is impossible to get higher resolution than QHD with extruded invar thickness [5, 6], therefore, several studies about alternative technology for higher resolution have been conducted. Electrodeposition is considered as one of the methods to make thinner FMM of 15 μm or less, whose resolution is higher than QHD by relatively simple equipment and process, compared to other technologies [7].

However, the properties of electrodeposited Fe–Ni film, such as composition, microstructure, surface morphology, are significantly influenced by various process conditions (bath composition, current density, temperature, pH, additive and etc.) [8, 9, 11]. For this reason, it is important to understand the effects of each variable on the deposits in order to apply electrodeposition to mass production of high resolution FMM. In this study, the electrodeposition of Fe–Ni alloy was conducted at various bath composition of ferrous ion (Fe2+) and current density. Also, ferric ion (Fe3+) was added to bath to observe the effect of ferric ion on Fe–Ni electrodeposition behavior. Finally, Fe–Ni films close to invar composition were obtained and the uniformity of deposited was evaluated.

2 Experimental Details

In this study, Fe–Ni alloy electrodeposition was conducted in sulfate–chloride bath using dimensionally stabilized anode (DSA) as anode, Ti plate with exposed area of 2 cm × 2 cm as cathode, which cleaned with 10 wt% NaOH solution and 10 wt% H2SO4 solution in ultrasonic cleaner before electrodeposition. The electrolyte consisted of 0.95 M nickel(II) sulfate, 0.17 M nickel(II) chloride, 0.25–0.4 M iron(II) sulfate, 0.5 M boric acid, 0.05 M malonic acid and 2 g/L sodium saccharin. Iron(III) sulfate was added for investigation of Fe3+ effects. The pH of bath was adjusted to 2.3 with 20 wt% NaOH solution and 10 wt% H2SO4 solution. The temperature of bath was maintained at 50 ℃ in water jacket and bath was stirred with 300 rpm using 28 mm magnetic bar. The schematic diagram of the electrodeposition cell is shown in Fig. 1. The current density was varied from 10 to 200 mA/cm2, and deposition time was adjusted with each current density to maintain total charge of 192 C. The Fe–Ni deposits was cleaned with distilled water and dried after electrodeposition.

Fig. 1
figure 1

Schematic image of the Fe–Ni electrodeposition cell

The compositions of the Fe–Ni deposits were analyzed using energy dispersive X-ray spectroscopy (EDS). Field emission scanning electron microscope (FESEM) was used for analysis of surface morphology and thickness for calculation of current efficiency. Potentiodynamic polarization was conducted at three electrode system, consisting of Ag/AgCl electrode as reference electrode, Ti plate as working electrode, and DSA as counter electrode. The scan rate was 5 mV/s and operating voltage range was from 0.05 V (vs open circuit voltage, OCV) to − 1.8 V (vs Ag/AgCl). Residual stress of deposits were analyzed using deposit stress analyzer model 683, and the condition of temperature and total charge for the electrodeposition of strip were room temperature (RT) and 45 C, respectively.

3 Results and Discussion

Figure 2 shows the composition of the deposits from bath with 0.25, 0.30, 0.35, 0.40 M iron(II) sulfate (FeSO4). For all bath conditions, the Fe content of deposits were higher than Fe content of the baths. It means that Fe–Ni alloy was electrodeposited anomalously, as active Fe preferentially deposited than Ni [10, 11]. The deposits became Fe-richer as the concentration of Fe2+ increased. In 0.35 M Fe2+ bath, the Fe–Ni film with 35.4 wt% Ni, similar to invar composition, was obtained. The surface morphologies of the deposits are shown in Fig. 3. The morphology of deposits at low Fe2+ concentration showed the bright surface, however the surface morphology was changed to rougher and lost the brightness as the Fe2+ concentration increased. The current efficiency of each bath was calculated from the thickness of deposits. As shown in Fig. 4, the current efficiency was decreased as the concentration of Fe2+ became higher, from 57.9% at 0.25 M to 44.7% at 0.35 M Fe2+. It means that competing reactions were occurred during Fe–Ni alloy electrodeposition, such as hydrogen evolution reaction and Fe2+/Fe3+ redox reactions. Standard reduction potential(Eo) of possible redox reactions in the Fe–Ni electrodeposition cell are listed in Table 1 [12].

Fig. 2
figure 2

Composition of the deposits vs Concentration of iron(II) sulfate (current density (i) = 50 mA/cm2)

Fig. 3
figure 3

Surface morphology of Fe–Ni electrodeposits at various concentration of Fe2+ (a 0.25 M, b 0.30 M, c 0.35 M, d 0.40 M)

Fig. 4
figure 4

Current efficiency at various concentration of Fe2+

Table 1 Standard reduction potential(Eo) of possible redox reactions in the Fe–Ni electrodeposition cell [12]

To investigate the effect of Fe3+ in bath, Fe–Ni electrodeposition was conducted in the modified bath containing 0, 0.05, 0.10 M iron(III) sulfate, which the sum of Fe2+ and Fe3+ was kept at 0.30 M. As shown in Fig. 5, the Fe content of deposits from the bath with higher Fe3+ content was lower. In addition, the current efficiency was reduced from 56.5 to 51.8% as the concentration of Fe3+ increased (Fig. 6). It means that Fe3+ produced on the DSA suppress Fe reduction on the cathode due to consumption of charge for Fe2+/Fe3+ redox reactions.

Fig. 5
figure 5

Composition of deposits from different Fe3+/Fe2+ bath

Fig. 6
figure 6

Current efficiency at various Fe3+/Fe2+ conditions

Figure 7 shows the composition of deposit from 0.35 M Fe2+ bath depending on current density. The composition change in the high current density region over 50 mA/cm2 was negligible compared to that in the low current density region from 10 to 50 mA/cm2. It is due to the current density reached the diffusion controlled region as shown in the cathodic polarization plot (Fig. 8), where charge transfer rate was so fast that total reaction rate is determined by diffusion of reactants [13, 14]. Figures 9 and 10 show the residual stress and current efficiency at various current density respectively. All deposits showed the tensile stress and it was increased with current density. The current efficiency was decreased from 73% at 10 mA/cm2 to 44% at 100 mA/cm2. As the current density became higher, the rate of charge transfer for reduction of proton in bath get faster whereas the rate of reduction of metal ions became diffusion-controlled. As the results, more hydrogen gases were evolved on deposit, leading to low efficiency and high tensile stress [15, 16].

Fig. 7
figure 7

Composition of deposits at various current density ([Fe2+] = 0.35 M)

Fig. 8
figure 8

Polarization plots in 0.35 M iron(II) sulfate bath

Fig. 9
figure 9

Residual stress at various current efficiency ([Fe2+] = 0.35 M, T = RT)

Fig. 10
figure 10

Current efficiency of deposits with varying current density

The composition range of electrodeposited Fe–Ni, which is showing minimum CTE, was 36–41 wt%, on the other hand the pyrometallurgically produced one was about 36 wt% Ni [17]. For this reason, the Fe–Ni alloy was electrodeposited at 50 mA/cm2 in 0.30 M Fe2+ bath for 5, 10, 15, 30 min. Figure 11 shows thickness of deposits(δ) and current efficiency depending on deposition duration. For all conditions, the thickness of deposits were thinner than theoretical thickness(δexp) and the difference between them became larger, leading to decrease in current efficiency. The results of composition analysis are shown in Fig. 12. The Ni contents of the front side of deposits were 36–40 wt% Ni regardless of deposition time, however, the back side of deposits contain more Fe than front side at all conditions. The Fe–Ni alloy was electrodeposited anomalously, and the commonly reported mechanism of anomalous codeposition was shown below [18,19,20,21].

$${\text{2H}}_{{2}} {\text{O }} + {\text{ 2e}}^{ - } \to {\text{H}}_{{2}} + {\text{ 2OH}}^{ - }$$
$${\text{M}}^{{{2} + }} + {\text{ OH}}^{ - } \to {\text{MOH}}^{ + }$$
$${\text{MOH}}^{ + } \to \left( {{\text{MOH}}^{ + } } \right)_{{{\text{ads}}}}$$
$$\left( {{\text{MOH}}^{ + } } \right)_{{{\text{ads}}}} + {\text{ 2e}}^{ - } \to {\text{M }} + {\text{ OH}}^{ - }$$
Fig. 11
figure 11

Current efficiency and thickness of deposits vs deposition time (i = 50 mA/cm2, [Fe2+] = 0.30 M)

Fig. 12
figure 12

Composition of front and back side of deposits vs. deposition time

In these equation, M is iron group metal (Fe, Ni, Co) and the adsorption ability of each metal monohydroxide ion (MOH+) is strong in order of Fe, Co, Ni [22]. Thus Fe(OH)+, preferentially adsorbed on the cathode, inhibited adsorption of Ni(OH) + and caused higher Fe content of the back side of deposits compared to that of the front side. 

4 Conclusion

The effects of bath composition and current density on Fe–Ni electrodeposition behavior and properties of deposits were investigated. The Fe–Ni alloy films with 36–40 wt% Ni were deposited at optimized conditions. The anomalous codeposition of Fe–Ni, the preferential deposition of more active Fe, was shown in all conditions and it was profound at higher Fe2+ concentration and higher current density. However, the decrease in current efficiency was observed at the same time, due to Fe2+/Fe3+ redox reaction and hydrogen evolution. For efficient and stable electrodeposition, Fe3+ in the bath should be controlled as low as possible and the excessively high current density should be avoided. In addition there was the difference in composition between the front and back of deposits in all deposition time. Therefore, for the application of electrodeposition to fabrication of FMM, the uniformity of deposits’ properties should be secured by additional process.