Nanocrystalline structure and microhardness of cobalt-chromium alloys electrochemically synthesized using a metal hydroxide coprecipitation technique

The effect of glycine as a complexing agent on metal hydroxide formation, such as Co(OH)2 and Cr(OH)3, was investigated based on potential-pH diagrams and titration curves for Co2+-H2O and Cr3+−H2O systems. Using a potentiostatic electrodeposition technique, Co–Cr alloy-based composite films containing Cr2O3 were synthesized from a non-suspended aqueous solution within an optimized pH range. Chromium content in the composite films was controlled up to 38.9% by adjusting the cathode potential during the alloy electrodeposition. Based on the XRD profiles and electron diffraction patterns, an amorphous-like nanocrystalline structure was observed in the composite films with high chromium content. The average crystal grain size declined due to Cr2O3 particles and hydrogen evolution during the electrodeposition process. Saturation magnetization of the composite films decreased with an increase in the chromium content. Synergistic contribution of increasing dislocation density and refining crystal grain size improved the microhardness of the composite films. The microhardness reached 624.2 kgf mm−2 and greatly exceeded that of pure cobalt (ca. 250–300 kgf mm−2).


Introduction
Metal-based composite (MBC) films containing a certain amount of metal oxide fine particles (Cr 2 O 3 , Al 2 O 3 , SiO 2 , TiO 2 , etc) have attracted much attention due to their characteristic performance, such as mechanical, electromagnetic, and electrochemical properties. So far, various types of MBC films (e.g., Fe-Ni alloy with TiO 2 [1], Co metal with Cr 2 O 3 , Ni metal with Cr 2 O 3 [2], Co-Ni alloy with Al 2 O 3 [3]) have been synthesized using an electrodeposition technique. Some researchers have reported that the MBC films exhibited excellent mechanical properties and corrosion resistance. Xiong et al investigated the crystal structure and microhardness of electrodeposited CoNiP alloy-based composite films with Cr 2 O 3 [4]. Their report revealed that the microhardness was improved by increasing the content of Cr 2 O 3 particles in the electrodeposited alloy films. They concluded that the improvement of microhardness was caused by the crystal grain size refinement, which was induced by increasing the crystal nucleation density during the electrodeposition. Usually, MBC films can be synthesized by a co-electrodeposition of metals and particles using an electrolytic solution containing suspended fine particles [5]. However, there are some technical difficulties in the co-electrodeposition of metals and particles using a suspension type bath. In the co-electrodeposition of metals and particles, a vigorous stirring technique and controlling the concentration of the dispersing agent are required to prevent the agglomeration of fine particles during the electrodeposition. Furthermore, it is difficult to create a homogeneous dispersion of fine particles in the electrodeposited MBC films because fine particle agglomeration easily occurs on the cathode. However, recently, an electrodeposition method has been proposed to use a non-suspended bath to synthesize MBC films that include metal oxide or hydroxide nanoparticles [6]. During metal deposition from an Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. electrolytic solution, the pH near the cathode surface increases with increasing current density due to the reduction of hydrogen ions. Hayashi et al explored the fabrication process of MBC films via the hydrolysis reaction of metal ions [7]. Molybdenum oxide ions (MoO 4 2− ) or tungsten oxide ions (WO 4 2− ) can be reduced to the metallic state in the presence of iron-group metal ions (Ni 2+ , Co 2+ , Fe 2+ , etc) [8,9]. However, chromium ions (Cr 3+ , etc) will transform to hydroxide or oxide through dehydration reaction near the cathode surface with increasing current density [10]. After the transformation, the chromium hydroxide is reduced to the metallic state.
Co-Cr alloys are widely used as biomaterials, such as artificial knee joints and dental prostheses, due to their excellent corrosion resistance and biocompatibility [11,12]. Also, ferromagnetic Co-Cr alloys can be applied to high-density magnetic recording media [13,14]. So far, an aqueous solution containing hexavalent chromium ions (Cr 6+ ) was used for electrodeposition of metallic chromium and its alloys. However, Cr 6+ ions can harm the human body and induce environmental pollution [15,16]. As an alternative to Cr 6+ ions, trivalent chromium ions (Cr 3+ ) have been considered the most promising candidate to realize an environmental-friendly electrodeposition method of chromium alloys. Cr 3+ ions can form complexes with six water molecules 6 ] 3+ ions inhibit the electrochemical reduction process to the metallic state because the [Cr(H 2 O) 6 ] 3+ ions can form a hydroxo-bridged colloidal polymer through the hydrolysis reaction in an acidic pH region (pH>4.5) [17,18]. Hence, an acidic bath (pH<2) has been used so far for the electrodeposition of metallic chromium. However, pH in the vicinity of cathode will increase with a decrease in the concentration of proton (H + ) due to the hydrogen evolution during an electrolysis with a high current density. To avoid the hydrolysis reaction at the vicinity of cathode, complexing agents such as glycine, dimethylformamide (DMF), and formic acid should be added to the electrolysis bath because Cr 3+ ions can form the stable complex ions with these complexing agents even in a neutral pH range. For example, Li et al investigated the Cr 3+ -glycine complexes in an acidic aqueous solution by UV-visible spectrophotometry [19].
Based on their report, the formation of complex species such as [Cr(gly)(H 2 O) 4 ] 2+ ions was promoted with an increase in the gly:Cr ratio (1<gly/Cr<2.5). Some researchers have reported that metallic chromium films were able to be electrodeposited from an aqueous solution containing the above complexing agents [20][21][22]. Among of them, glycine is an attractive complexing agent because it is harmless to the human body and global environment. The formation process of Cr 3+ -glycine complexes and the electrochemical reduction process to the metallic state can be expressed by the following equations (1) 4 ] + ions will be reduced to metallic chromium as shown in equation (3). According to the above complexing mechanism due to glycine, the olation reaction of [Cr(H 2 O) 6 ] 3+ ions in the vicinity of cathode will be inhibited. Furthermore, the pH buffer activity of glycine will also be effective to inhibit the olation reaction [24]. Structure and corrosion resistance of the electrodeposited Co-Cr alloy films have been explored [25,26]. In contrast, there are few reports on the mechanical properties of Co-Cr alloys that were electrochemically synthesized from an electrolytic solution containing Cr 3+ ions. Mechanical properties such as tensile strength, ductility, and microhardness are important indexes to assess whether the materials can be applied to a practical application. Microstructures strongly affect the mechanical properties of electrodeposited films. Surface morphology, crystal grain size, and alloy composition of electrodeposited metallic films can be controlled by adjusting some electrolysis parameters, such as solution composition, temperature, pH, current density, cathode potential, etc. It is also possible to synthesize metal/ceramics composite films by utilizing an electrodeposition method with a large over potential.
Hence, in this study, the effects of the complexing agent (glycine) on the pH buffer activity of an electrolytic bath were examined to reveal the hydrolysis behavior of Co 2+ and Cr 3+ ions based on the potential-pH diagrams and titration curves. Then, Co-Cr alloy-based composite films containing Cr 2 O 3 were synthesized by a potentiostatic electrodeposition technique from an aqueous chloride bath containing glycine. Furthermore, the effects of microstructure and chemical state of electrodeposited composite films on saturation magnetization and microhardness were also investigated.

Method
Co-Cr alloy-based composite films were synthesized by a potentiostatic electrodeposition technique using an electrolytic solution containing 0.25 M cobalt chloride hexahydrate, 0.5 M chromium chloride hexahydrate, 0.4 M boric acid, 1.0 M glycine, and 1.0 M ammonium chloride. The solution temperature and pH were adjusted to 40°C and ca. 1.6, respectively, while the electrolytic bath was stirred using a magnetic stirrer (300 rpm) during the electrodeposition. A copper foil and a carbon rod were utilized as cathodes and anodes, respectively, while a silver chloride electrode (Ag/AgCl/Sat. KCl) was used as a reference electrode. Cathode potentials during the electrodeposition were fixed to −1.0 V, −1.5 V, −2.0 V, −2.5 V, −3.0 V, and −3.5 V versus Ag/AgCl, which were determined based on the cathodic polarization curve.
The alloy composition of electrodeposited Co-Cr alloy-based composite films was evaluated using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, ULTIMA2, Horiba Ltd, Kyoto, Japan). Surface texture of the electrodeposited alloy samples was investigated using Scanning Electron Microscopy (SEM, JCM-5700, JEOL Ltd, Tokyo, Japan). Constituent phases were identified using Transmission Electron Microscopy (TEM, JEM-2010-HT, JEOL Ltd, Tokyo, Japan) and an x-ray Diffractometer (XRD, Miniflex 600-DX, Rigaku Corp., Tokyo, Japan). Chemical state of cobalt and chromium in the electrodeposited films was analyzed using x-ray Photoelectron Spectroscopy (XPS, AXIS-ULTRA, Shimadzu Corp., Kyoto, Japan). Saturation magnetization of the electrodeposited films was investigated using a Vibrating Sample Magnetometer (VSM, TM-VSM1014-CRO, Tamakawa Co., Sendai, Japan) at room temperature. Microhardness of the electrodeposited films was measured using a micro-Vickers hardness testing machine (HM-211, Mitutoyo, Kanagawa, Japan). The microhardness test was performed on a polished cross-section of the electrodeposited films by applying a load of 0.05 kgf. For comparison, the saturation magnetization and microhardness of commercially available cobalt foil (NILACO, Co-103228, purity: 99.9%) were also investigated.

Results and discussion
3.1. Potential-pH diagrams for Co 2+ -H 2 O and Cr 3+ -H 2 O systems Figure 1 shows the potential-pH diagrams for Co 2+ -H 2 O (a) and Cr 3+ -H 2 O (b) systems at 25°C. Based on the potential-pH diagrams, the thermodynamically stable state of chemical species can be estimated. These diagrams were plotted using the standard electrode potentials of Co/Co 2+ and Cr/Cr 3+ (E Co° =−0.474 V, E Cr° =−0.937 V versus Ag/AgCl), the stability constants of Co(OH) 2 and Cr(OH) 3 (pK sp = 14.8, 30.2 [27]), and Nernst's equation. According to figure 1(a), it is predicted that the electrodeposition of metallic cobalt will be possible at the cathode potential region less-noble than the equilibrium potential of Co/Co 2+ (E Co eq =−0.492 V versus Ag/AgCl). In addition, Co 2+ ions can form hydroxide when the solution pH increases up to 6.9. Conversely, the electrodeposition of metallic chromium will be possible at the cathode potential region lessnoble than the equilibrium potential of Cr/Cr 3+ (E Cr eq =−0.943 V versus Ag/AgCl) according to the potential-pH diagram for the Cr 3+ -H 2 O system ( figure 1(b)). Furthermore, it is also predicted that Cr 3+ ions can form hydroxide when the solution pH rises to 4.0. During the electrodeposition process from an aqueous solution, pH in the vicinity of cathode increases with decreasing H + ions concentration due to the hydrogen evolution at a high current density. Therefore, it seems to be quite difficult to make an electrochemical reduction of Cr 3+ ions from an aqueous solution that does not contain complexing agents. In solutions A and B, each pH was stabilized at 7 and 3, respectively. These pH buffer activities seem to be induced by the hydroxide formation process of Co 2+ and Cr 3+ ions because the pH7 and pH3 correspond well with the pH values that are estimated from the pHpotential diagrams for Co 2+ -H 2 O and Cr 3+ -H 2 O systems (figure 1). However, in solution C, the pH stabilized only at 3. Hence, Co 2+ and Cr 3+ ions seem to form hydroxides simultaneously at pH3. Such a phenomenon is known as 'coprecipitation'. The following several theories (I-III) have been proposed as the mechanism of coprecipitation [28,29].

Titration curves of electrolytic solutions containing glycine
(I) Substitution: If the two kinds of heterologous metal ions have a similar size, the substitution of metal ions will occur at a lattice position of a metal hydroxide precipitate.
(II) Occlusion: Metal ions and solvent molecules will be trapped in the metal hydroxide precipitate.
(III) Adsorption: Metal and hydroxide ions will be adsorbed on the surface of the metal hydroxide precipitate.
Cr 3+ ions seem to exist as Cr 2 O 3 ·nH 2 O or Cr(OH) 3 in the hydroxide precipitate. Cr 2 O 3 and Co(OH) 2 have the same hexagonal structure. Furthermore, the ionic radii of Cr 3+ (61.5 pm [30]) and Co 2+ (65 pm [30]) are quite similar in value. Hence, the mixed crystal of Cr 2 O 3 and Co(OH) 2 will be formed due to the abovementioned coprecipitation process. These solutions showed pH buffer activities around pH 9. In solutions D, E, and F with glycine, no clear stagnation of pH value was observed, unlike the pH buffer activities of solutions A, B, and C without glycine. Hence, the effect of glycine addition seems to suppress the formation of chromium hydroxide in the low pH region (∼pH 4) and the production of cobalt hydroxide associated with the coprecipitation phenomenon.

Electrodeposition of Co-Cr alloy-based composite films
To investigate the electrodeposition behavior of Co-Cr alloy-based composite films, a cathodic polarization curve was plotted ( figure 3(a)). During the measurement, the bath temperature was kept at 40°C, and the potential was scanned at a rate of 50 mVs −1 . The bath composition and electrodes were the same as those used for electrodepositing the composite films. From figure 3(a), by scanning the electrode potential to a less-noble region, the cathode current density began to increase sharply at the electrode potential of ca. −0.5 V versus Ag/ AgCl. According to Nernst's equation, the equilibrium potential of Co/Co 2+ (E Co eq ) can be calculated to ca. −0.49 V versus Ag/AgCl based on the bath temperature and metal ions concentration (40°C, [Co 2+ ]=0.25 M). Hence, the increase in the cathode current density should be caused by the deposition current of Co 2+ ions. Subsequently, by scanning the electrode potential to a less-noble region, the cathode current density significantly increased again at the electrode potential region of ca. −0.85 V versus Ag/AgCl. Likewise, the equilibrium potential of Cr/Cr 3+ (E Cr eq ) can be determined to ca. −0.94 V versus Ag/AgCl ([Cr 3+ ]=0.5 M). Hence, it is considered that the deposition of metallic chromium began to proceed at a potential region which is nobler than the theorical equilibrium potential due to an under potential deposition (UPD) phenomenon. Although the mechanism of UPD remains unsolved, some researchers have proposed that some kinds of metal ions (Cd 2+ , Zn 2+ , etc) can be deposited by a way of the hydroxide intermediates, even in a potential region that is nobler than the equilibrium potential [31]. Furthermore, by shifting the potential to a region that is less noble than −1.0 V versus Ag/AgCl, the slope of the polarization curve decreased due to the decrease in H + , Co 2+ , and Cr 3+ ion concentrations in the vicinity of cathode. Consequently, for electrodepositing Co-Cr alloy-based composite films, the optimum cathode potential condition was determined, ranging from −1.0 V to −3.5 V versus Ag/AgCl. Figure 3(b) shows the effect of cathode potential on the chromium content (X Cr ) in Co-Cr alloy-based composite films. By scanning the electrode potential to a less-noble region, X Cr in the alloy films increased. According to figures 3(a) and (b), the co-deposition behavior of Co-Cr alloys in this study can be considered a 'normal co-deposition type' based on Brenner's classification [32]. In the normal co-deposition type, the metal ions, which have an electrochemically nobler equilibrium potential (i.e., cobalt), were preferentially deposited rather than the less-noble metal ions (i.e., chromium). In this study, X Cr in Co-Cr alloy-based composite films increased up to 38.9% by scanning the electrode potential down to −3.5 V versus Ag/AgCl. Figure 4 shows the effect of chromium content on the SEM images of Co-Cr alloy-based composite films ((a) X Cr =2.4%, (b) X Cr =15.6%, (c) X Cr =21.5%, (d) X Cr =29.9%, (e) X Cr =34.0%, (f) X Cr =38.9%). From figure 4(a) in the sample with X Cr of 2.4%, a needle-shaped microstructure was observed. In contrast, from figures 4(b)-(f), by increasing X Cr from 15.6% to 38.9%, a nodule-like morphology was confirmed. Especially, from figures 4(e) and (f), the samples with high chromium content (X Cr =34.0% and 38.9%) had some microcracks, which seem to be caused by hydrogen embrittlement. As shown in figure 3(b), with increasing the cathode potential, the chromium content also increased. It is well known that the crystal nucleation density of electrodeposited metals increases with an increase in the cathodic overpotential if the rate-determining process is the charge transfer on the electrode [33]. According to figure 3(a), at the cathode potential region nobler than −1.1 V, the charge transfer process seems to be the rate-determining. Hence, the sample ( figure 4(a)) which was obtained at −1.0 V seems to be composed of fine grains with relatively smooth surface due to the uniform nucleation. On the other hand, the reduction rate seems to be controlled by the mass transfer process of metal ions in the potential region less noble than −1.1 V. In this potential region, the sub-reaction such as hydrogen evolution will proceed on the cathode and the non-uniform nucleation process will be occurred during the electrodeposition process. Consequently, the nodule-like deposits seem to be caused on the surface of electrodeposited films with high chromium content (figures 4(b)-(f)). The increase in X Cr will cause the disturbance of the atomic arrangement in the cobalt crystal lattice. Figure 5 shows the effect of chromium content on the XRD profiles of Co-Cr alloy-based composite films that were separated from copper substrates. For comparison, the XRD profile of pure Co film (X Cr =0%) was added in figure 5. The Co film was electrochemically synthesized from an aqueous solution (40°C) containing 1.0 M cobalt sulfate and 0.4 M boric acid. In the profile of electrodeposited Co film, the sharp peak associated with hcp-Co (0 0 2) was observed. In addition, the small peaks associated with hcp-Co (1 0 0) and (1 0 1) were also confirmed. Based on the XRD profile, the electrodeposited Co film seems to have a crystalline structure. On the contrary, in the composite films with X Cr of 2.4% and 15.6%, broad peaks associated with hcp- Co (1 0 0), (0 0 2), and (1 0 1) were observed. However, in the composite films with X Cr exceeding 20%, no diffraction peaks were  confirmed. Figure 6 shows the effect of chromium content on the TEM bright field images ((a) and (b)) and electron diffraction patterns ((a′) and (b′)) of Co-Cr alloy-based composites ((a) X Cr =15.6% and (b) X Cr =34.0%). From figures 6(a)′ and (b)′, concentric and vague diffraction patterns (halo-patterns) were observed. Hence, figures 5 and 6 show that the structure of Co-Cr alloy-based composite films was transformed from a nanocrystalline phase to an amorphous phase with an increase in X Cr .

Structure of electrodeposited Co-Cr alloy-based composite films
The chemical binding energy of cobalt (figure 7) and chromium (figure 8) in Co-Cr alloy-based composite films was investigated according to the XPS spectra. The surface of each sample was etched by argon ions for a duration of 0, 12, 24, 36, 48 and 60 min. Based on a reference [34], the binding energy derived from metallic cobalt corresponds to 792.95 eV (Co°−2p 1/2 ) and 777.9 eV (Co°−2p 3/2 ). Notably, iron-group metal ions (M 2+ =Fe 2+ , Co 2+ , Ni 2+ ) can form hydroxide ions (M(OH) + ), and the ions can adsorb on the surface of cathode [35]. Subsequently, the M(OH) + ions will be reduced to the metallic state as electrodeposited crystals via a multi-step reaction process. Therefore, cobalt seems to exist as a metallic state in Co-Cr alloy-based composite films. However, the binding energy derived from metallic chromium corresponds to 583.4 eV (Cr°−2p 1/2 ) and 574.1eV (Cr°−2p 3/2 ), while that of trivalent chromium corresponds to 586.3 eV (Cr 3+ −2p 1/2 ) and 576.6eV (Cr 3+ −2p 3/2 ) [36]. Hence, according to the XPS spectra (figure 8), chromium seems to exist as a mixture of metallic and oxide states. Based on the above experimental results, chromium ions should form hydroxide in the vicinity of the cathode through a hydrolysis reaction during the electrodeposition process with a large over potential. Furthermore, the hydroxide could form metallic oxide and be included into the electrodeposited alloy film via dehydration reaction. Figure 9 shows the electrodeposition mechanism of Co-Cr alloy-based composite films by way of the glycine complex precursors. As mensioned above, an amorphous-like nanocrystalline structure was confilmed in the composite films. The crystal grain size seems to decline due to solute Cr atoms, Cr 2 O 3 particles and hydrogen evolution during the electrodeposition process. Figure 10 shows the effect of chromium content on the magnetic hysteresis loops of Co-Cr alloy-based composite films ((a) X Cr =2.4%, (b) X Cr =15.6%, (c) X Cr =21.5%, (d) X Cr =29.9%, (e) X Cr =34.0%, (f) X Cr =38.9%). An external magnetic field was applied to the in-plane direction of the electrodeposited alloy films during the magnetization measurement. Figure 11(a) shows the effect of chromium content on the saturation magnetization of Co-Cr alloy-based composite films. In the figure, the saturation magnetization value (146.3 emu g −1 ) of commercially available pure cobalt foil (purity: 99.9%) was also plotted as X Cr =0%. The saturation magnetization of Co-Cr alloy-based composite films decreased to zero with an increase in X Cr up to ca. 40%. Ishida et al reported that the Curie temperature of Co-Cr alloys decreased to room temperature with an increase in X Cr up to ca. 20% [37]. Hence, in this study, almost half of the Cr atoms seem to exist outside of the Co-Cr alloy phase. Figure 11(b) shows the effect of chromium content on the microhardness of Co-Cr alloy-   based composite films. It has been reported that the hardness of pure cobalt is approximately 250 to 300 kgf mm −2 (HV 0.05 ) [38,39]. In this study, the microhardness of commercially available cobalt foil was determined to be 291.1 kgf mm −2 (HV 0.05 ). From figure 11(b), the microhardness increased up to 624.2 kgf mm −2 (HV 0.05 ) with an increase in X Cr up to ca. 40%. According to the mechanism of solid solution strengthening of metallic materials, the density of lattice defects, such as dislocations and grain boundaries, increases with an increase in the concentration of solute atoms. Furthermore, the average crystal grain size will decrease with an increase in the density of lattice defects. Hence, in this study, the synergistic effect of solid solution strengthening and crystal grain refinement strengthening seems to have contributed to improving the microhardness of Co-Cr alloybased composite films.

Conclusion
Based on the potential-pH diagrams and titration curves for Co 2+ -H 2 O and Cr 3+ -H 2 O systems, it was revealed that glycine as a complexing agent suppressed the formation of chromium hydroxide in the low pH region (∼ pH 4) and the production of cobalt hydroxide associated with the coprecipitation phenomenon. Co-Cr alloybased composite films containing Cr 2 O 3 were synthesized from a non-suspended aqueous solution using a potentiostatic electrodeposition technique. Chromium content in the composite films was controlled up to Figure 9. Electrodeposition mechanism of Co-Cr alloy-based composite films by way of the glycine complex precursors. 38.9% by scanning the electrode potential down to −3.5 V versus Ag/AgCl. Amorphous-like nanocrystalline structure was observed in Co-Cr alloy-based composite films with high chromium content. Saturation magnetization of Co-Cr alloy-based composite films decreased to zero with an increase in X Cr up to ca. 40%. Microhardness increased up to 624.2 kgf mm −2 with an increase in X Cr up to ca. 40%. It was concluded that the synergistic effect of solid solution strengthening and crystal grain refinement strengthening contributed to improving the microhardness of Co-Cr alloy-based composite films. Figure 10. Effect of chromium content on the magnetic hysteresis loops of Co-Cr alloy-based composite films ((a) X Cr =2.4%, (b) X Cr =15.6%, (c) X Cr =21.5%, (d) X Cr =29.9%, (e) X Cr =34.0%, (f) X Cr =38.9%). An external magnetic field was applied to inplane direction to the electrodeposited alloy films. Figure 11. Effect of chromium content on the saturation magnetization (a) and microhardness (b) of Co-Cr alloy-based composite films.