The Structure and Micro-Mechanical Properties of Electrodeposited Cobalt Films by Micro-Compression Test

Cobalt films were electrodeposited on pure copper (99.99%) substrates having a dominant {111} crystal orientation, Pure platinum (99.99%) was used as the anode electrode. Both copper and platinum substrates have dimensions of 10 mm × 20 mm × 0.3 mm. Composition of the electrodeposition bath was 350 g l⁻¹ of CoSO₄ · 6H₂O, 45 g l⁻¹ of CoCl₂ · 6H₂O and 40 g l⁻¹ of H₃BO₃. The electrodeposition temperature was 303 K. Different current densities at 5, 10, 12, and 15 A dm⁻² (ASD) were used, and the deposition time was adjusted to produce cobalt films having a thickness of about 100 μm when assuming 100% of the current efficiency.

Surface morphology and crystal characteristics of the electrodeposited cobalt films were examined by a scanning electron microscope (SEM, Nova NanoSEM 450, US) using 15 kV. The crystal characteristics were evaluated by X-ray diffraction (XRD, Ultima IV, Rigaku, Japan) with Cukα radiation at 40 kV and 40 mA, step size of 0.01° (2θ) and scan rate of 0.2° min⁻¹. The XRD patterns were analyzed using a MDI Jade software for the value of the full width at half maximum (β, FWHM). Average grain size of the film was calculated using the Scherrer equation shown in the following: ²⁷

$$\begin{eqnarray}&&{\rm{Average}}\,{\rm{grain}}\,{\rm{size}}\,\left({\rm{nm}}\right)={\rm{K}}\lambda /\beta \,\cos \,\theta \end{eqnarray} \tag{ 1 }$$

Where K is the Scherrer constant, generally about 1.0 for spherical particles; λ is the X-ray wavelength, Cukα = 0.15418 nm; β is the FWHM in radians; θ is the peak position (2θ/2), in radians. Transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan) and selected area electron diffraction (SAED) equipped in the TEM were also used to characterize the crystal structure. The Vickers hardness was tested on a micro hardness tester (MHT, HMV-G20S, Shimadzu, Japan) at a load of 0.01 kg and a loading time of 15 s. The Vickers hardness value reported in this study were averaged using 5 points obtained from each cobalt film.

Micro-pillars were fabricated from the cobalt films using focused ion beam (FIB, FB-2100, Hitachi, Japan). Details of the micro-pillars fabrication method are reported in a previous work. ²⁸ The micro-pillars had a square cross-section and dimensions of 10 μm × 10 μm × 20 μm, and the square cross-section was perpendicular to the cobalt/copper interface. The micro-compression was conducted at a constant strain rate of 5.0 × 10⁻³ s⁻¹ using a piezo-electric actuator.

Compression of bulk-size cobalt was also conducted as comparisons with the micro-compression test. The test samples had dimensions of ϕ 3 mm × 3 mm were fabricated from a hot-rolled cobalt plate (99.98%) by wire cutting. The average grain size was about 1 μm. The compression was performed on a universal test machine (UTM, UTM5305H, Suns, China) at the same strain rate with micro-compression test.

The surface morphology of the electroplated cobalt films is shown in Fig. 1. The surfaces were composed of particle-like structures. The cobalt particles grew spirally along one or more screw directions on the surface to form particles as polygonal cones by the spiral growth mechanism. ²⁹ When these particles grow up and connect to each other, they merged into a new particle, and this could lead to formation of defects as pores between them. It was clear that the average particle size became smaller and the pore-like features between the inter particles became shallower following an increase in the current density. The refined particle size was suggested to be a result of the promoted overpotential and the nucleation rate as the current density increased. The particles growth changed from the layer-by-layer growth advantage to the spiral growth advantage as the current density increased from 5 to 12 ASD. ²⁹ At higher current density, the surface morphology was more uniform and smoother, and amount of the surface defects was reduced. At 15 ASD, due to the excessive current density, cobalt particles began to form at edges of the copper plate with abnormally large sizes, which reveal an uneven distribution of the current density from the edge to middle area of the copper substrate.

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Both HCP and FCC crystal structures of cobalt were observed in the electrodeposited cobalt prepared in this study as shown in in X-ray diffraction (XRD) patterns in Fig. 2. FCC and HCP crystal structures of cobalt had two same diffraction peak positions, {002}_HCP//{111}_FCC and {110)_HCP//{220}_FCC. In Fig. 2, {002}_HCP//{111}_FCC diffraction angle was close to {111}_FCC of the copper substrate. In Fig. 2, the XRD patterns of electrodeposited cobalt films at 5, 10, 12, 15 ASD were also compared. A decrease in the peak intensity of the {100}_HCP was observed following an increase in the current density. At 12 ASD, the peak intensity of the {200}_FCC increased, which indicated an increase in ratio of the FCC crystal structure in cobalt film at high current density.

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Average grain sizes of the cobalt films were calculated separately from the {100}_HCP, {002}_HCP//{1$\bar{1}\,$1}_FCC, {101}_HCP, {200}_FCC and {110}_HCP//{220}_FCC peaks by the Scherrer equation and presented in Fig. 3, and the values obtained from the five peaks were average by the least squares method. ³⁰ The values estimated from the {101}_HCP showed a faster decreasing trend following an increase in the current density than those of the {002}_HCP//(1$\bar{1}$1}_FCC. On the other hand, the values estimated from the {100}_HCP and {200}_FCC gradually increased with the current density. The {101}_HCP and {002}_HCP//{1$\bar{1}$1}_FCC are close-packed planes. This implies that electrodeposition at a high current density promoted the nucleation and limited the grain growth, which resulted the grain refinement and formation of small approximately equiaxed grains transformed from the elongated grains.

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SAED and TEM observation were conducted to further understand the crystal structure of the electrodeposited cobalt. The results are shown in Fig. 4. The Laue diffraction patterns are shown in Fig. 4b. According to the distance and angles of the Laue diffraction spots, the crystal planes corresponding to the spots can be calculated as (100), (001) and (101) of HCP cobalt, intersecting at the [010] zone axis. The TEM observation revealed the electrodeposited cobalt film was composed of HCP crystal structure without FCC crystal structure. From [010] zone axis, parallel to the electron beam, the TEM image surface in Fig. 4a is (010). Several lamellas on (010) facet were observed, and the angles between the lamellas in two directions were about 120 degrees. The lamellas in the two directions are (10$\bar{1}$) & (001) facet and (101) & (001) facet by calculation, respectively. The positional relationship of the facet was showed in Fig. 4c. The lamellas are suggested to be nanotwins in Fig. 4a, which is reported in cobalt with low stacking fault energies under proper conditions. ^31,32 Cobalt grains grew spirally around screw directions and along crystal planes such as (10$\bar{1}$), (001) and (101) to form regular hexagonal cone-shaped particles. ²⁹ When the crystal grows, nanotwin boundaries may be formed at the interface.

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Relationship between the average grain size and Vickers hardness of the cobalt films are plotted in Fig. 5. Averaged values of the five XRD peaks were used in the plotting. The average grain size was ranged from 13.8 to 15.4 nm as the current density varied. 0.2% proof strength of single crystal of cobalt is reported to be 432 MPa, and the Hall-Petch slope is 1900 MPa*nm^1/2 for pure cobalt. ³³ The yield strength estimated by Hall-Petch equation using the average grain size obtained in this study was 916, 933, 942 and 943 MPa. The change in the yield strength was not significant as the grain size varied from 13.8 to 15.4 nm. For the cobalt films evaluated in this study, the Vickers hardness increased significantly as the current density increased, which the Vickers hardness was found to be ranged from 3138 to 4288 MPa. The Vickers hardness can be converted to the yield strength using the Tabor factor, and the Tabor factor is approximately three. ³⁴ After doing the conversion, values of the Vickers hardness obtained in this study were all higher than the yield strengths estimated from the Hall-Petch equation. The Hall-Petch relationship came from Karimpoor's ³⁵ research data for bulk tensile samples with about 2.5 × 10⁻³ of strain rate by tensile test for polycrystalline and nanocrystalline cobalt. Cobalt is HCP structure, {10$\bar{1}$2} twins causes metals with the HCP structure to exhibit significantly different yield strength by tensile and compression test. This is tension-compression yield asymmetry (TCYS). ³⁶ The yield strength from the Hall-Petch relationship is an approximate value, lower than the yield strength by micro-compression test. The yield strength obtained by the tabor factor is also an approximate value. The Vickers hardness approximates to three times of the ultimate tensile strength, ³⁴ and reflects the elastic deformation and plastic deformation of the materials. This method uses the yield strength to be approximately equal to ultimate tensile strength. For insufficient work-hardened materials, the difference between the yield strength and the ultimate tensile strength causes the tabor factor to deviate from three. Internal compressive stress exists on the electrodeposited cobalt films. The internal residual stress enhanced hardness and ultimate tensile strength more than the yield strength. So the yield strength obtained by the tabor factor is higher than the yield strength by micro-compression test for internal compressive stress.

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Images of the micro-pillars fabricated from the electrodeposited cobalt films are shown in Fig. 6. The micro-pillars were fixed at the bottom ends only, which resulted severe deformations near the top ends of the micro-pillars. At middle part of the micro-pillars, slips were observed, and the slips were at roughly 45 degree angle with respect to the compressive direction. The deformed micro-pillars showed a drum-like shape with no fractures but some cracks on side edge, and bended slightly in the direction perpendicular to the cobalt/copper interface. Engineering stress-engineering strain (SS) curves generated from the micro-compression test and the compression test of bulk cobalt at a constant strain rate of 5.0 × 10⁻³ s⁻¹ are plotted in Fig. 7a. The yield strength (0.2% proof strength) was determined from the SS curves, and the values for cobalt micro-pillars were 948, 1075, and 996 MPa for the cobalt film prepared at 10, 12 and 15 ASD, respectively. For the bulk cobalt, the yield strength was 565 MPa. The highest yield strength was obtained from a micro-pillar fabricated from the cobalt film electrodeposited at 12 ASD, which was about twice the value of bulk cobalt. The promoted strength was suggested to be mostly caused by the fine grain size at 13.8 nm according to the Hall-Petch relationship and the sample size effect. All the micro-pillars had a beyond 20% maximum strain without fracture in Fig. 7b. They were lower than the ∼40% maximum strain of the bulk cobalt, because finer grains have poorer maximum strain. However, the maximum strain values obtained from the micro-compression tests conducted in this study were all higher than the elongation (∼10%) of bulk ultrafine-grained (200 nm grain size) cobalt at a constant strain rate of 1.0 × 10⁻³ s⁻¹ by a combination of high-energy ball milling and subsequent spark plasma sintering. ²⁶ The micro-pillars and bulk cobalt exhibited typical strain hardening across the entire region of plastic deformation, and the micro-pillars exhibited weaker strain hardening when compared to the bulk cobalt.

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The yield strengths of cobalt by tensile or compression test from Karimpoor, ³⁵ Marek, ²⁶ Hug, ³⁷ Betteridge, ³⁸ Knapek ³⁹ and this work were compared with the average grain size as shown in Fig. 8. The trends of the yield strength and the average grain size is in good agreement with the Hall-Petch relationship with a pronounced different stages. ³⁷ A Hall-Petch relationship equation was obtained by linear least-squares fitting of the yield strength data obtained from bulk cobalt and micro-pillars in this study as listed in following:

$$\begin{eqnarray}&&{\sigma }_{y}=396\,{\rm{MPa}}+72.3\,{\rm{MPa}}/{{\rm{nm}}}^{-1/2}\,\times \,{{\rm{D}}}^{-1/2}\end{eqnarray} \tag{ 2 }$$

Where σ_y is the yield strength. D is the average grain size. The equation is similar to the equations with bulk cobalt from Karimpoor and of Marek. This indicates that the micro-compression yield strength and average grain size of cobalt micro-pillars are also applicable to the Hall-Petch relationship of bulk crystalline cobalt. When compared with the yield strength estimated from the Hall-Petch equation, the yield strengths obtained from the micro-compression test were all higher. The electrodeposited cobalt films evaluated in this study were composed of both FCC and HCP crystal structures. This might be the main reason causing the difference.

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In this study, cobalt films were prepared by electrodeposition at current densities ranged from 5 to 15 ASD. The cobalt films were found to be composed of both HCP and FCC crystal structures from XRD. The electrodeposited cobalt films grew spirally along the (10$\bar{1}$), (001) and (101) planes, forming regular hexagonal particles, and nanotwins were observed in the HCP lamellas. The average grain sizes were evaluated from the XRD information and the Scherer equation, and refinement of the average grain size was observed following an increase in the current density. The Vickers hardness reached 4288 MPa, and the relationship with the average grain size followed the Hall-Petch relationship well. Micro-pillars were fabricated from the electrodeposited cobalt films by FIB and underwent micro-compression testing at a constant strain rate of 5 × 10⁻³ s⁻¹. The micro-pillars showed the yield strength from 948 to 1075 MPa, which were about twice of the yield strength of bulk cobalt at 565 MPa. The high yield strength was mainly caused by the grain boundary strengthening and the sample size effect.

This study was supported by the Research Center for Biomedical Engineering, the National Natural Science Foundation of China (No. 51764005, 52064010) and the project from Youth Science and Technology Personnel Training Program of Education Department of Guizhou Province - Guizhou Institute of Technology (QJH-KY-2017222).