Relationship between Current Density, Crystal Grain Size, Composition and Mechanical Properties in Electrodeposited Ni-Co Alloys
Authors: Jiang Yiming , Chien Yu-An , Chen Chun-Yi , Chang Tso-Fu Mark , Sone Masato 
Published: December 2020

Nickel [1], cobalt [2], and their alloys have been investigated as important engineering materials because of their unique properties, such as magnetic, heat-conductive and high hardness. Much interest is focused on the application of Ni-Co alloy films in micro-electrical-mechanical systems (MEMS) devices especially their considerable potential in manufacturing of magnetic actuator due to excellent electrical and mechanical properties [3].
Reliability of electronic components is highly dependent on the mechanical property, which is closely related to its average grain size according to grain boundary strengthening mechanism [4]. Nanocrystalline materials can offer enhanced mechanical strength compared with coarse-grained counterparts when device components are scaled to the micro-scale regime with synergistic effect of the sample size effect [5]. Meanwhile, alloying allows utilization of the solid solution strengthening mechanism to further enhance the mechanical strength [6], and Ni-Co forms a solid solution over the whole concentration range, making it easy to control the mechanical and magnetic properties.
Electroplating is a promising technique in controlling crystalline properties of Ni-Co alloy films because the morphology, composition, grain size, and deposition rate of the deposited materials can be facilely controlled by varying the electroplating parameters, such as the current density, bath composition, and temperature [7]. Therefore, in this study, the effects of applied current density on average grain size, composition, and micro-mechanical properties of electrodeposited Ni-Co films are evaluated for fabrication of micro-components in electronic devices.
The electroplating was carried out at 55 ℃, and the current density was varied from 5 to 20 mA/cm2. A piece of Pt plate was used as the anode. Crystalline structure of the Ni-Co films was characterized by X-ray diffraction (XRD), average grain size was determined using XRD in conjunction with the Scherrer method. Microhardness tests were conducted on a Vicker's hardness tester using a load of 0.025 kg, applied for 15 s. Composition was analysed by energy dispersive X-ray spectroscopy (EDX) system. Furthermore, alloy films are processed into micro-pillars in a size of 10×10×20 μm3with focused ion beam (FIB, FB2100, Hitachi) milling. The optical microscope photograph of Ni-Co alloy micro-pillar fabricated with current density of 18 mA/cm2 is shown in Fig. 1. A high yield stress of 1.65 GPa is determined after the micro-compression test.
Vickers hardness as a function of
d

-1/2 for the alloy specimens is shown in the scatter plot of Fig. 2, where
d
is average grain size. Microhardness value has a positive linear relationship with
d

-1/2 approximately, which corresponds well to Hall-Petch relationship. Maximum Vicker's hardness value of 526 Hv was obtained with a current density of 10 mA/cm2, which is much higher than those of pure Ni (306 Hv) and Co (403 Hv) films prepared in this study, indicating the effect of solid solution strengthening.
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Figure 1