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Nanocrystalline cobalt–iron alloy: Synthesis and characterization
Materials Science and Engineering A 550 (2012) 388–394
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Nanocrystalline cobalt–iron alloy: Synthesis and characterization N.M. Nik Rozlin a,b,∗ , Akram M. Alfantazi a a b
Department of Materials Engineering, The University of British Columbia, 6350 Stores Road, Vancouver, BC V6T1Z4, Canada Faculty of Mechanical and Materials Engineering, University Teknologi Mara, 40450 Shah Alam, Selangor, Malaysia
a r t i c l e
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Article history: Received 14 February 2012 Received in revised form 16 March 2012 Accepted 24 April 2012 Available online 2 May 2012 Keywords: Electrodeposition CoFe Nanostructured materials Microhardness Grain refinement
a b s t r a c t Nanocrystalline cobalt–iron (CoFe) alloys were electrodeposited from a sulfate bath by direct current. The bath temperature and pH was maintained at 40 ◦ C and 2.5 respectively. The iron content was varied to study the effect on the surface morphology, grain size, current efficiency and microhardness. The experimental results showed that grain size of the CoFe alloy coating decreased with an increase in the iron content from about 6–25 wt%. Nanocrystalline CoFe coatings with an average grain size of 15 nm were achieved in coatings with 25 wt% Fe. Furthermore, the nanocrystalline CoFe coating was roughly three times harder than pure polycrystalline cobalt. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It has been almost 23 years since Gleiter first introduced what was then referred to as ‘microcrystalline materials’ [1]. Nowadays, these are better known as nanocrystalline materials or sometimes also referred to as nanocrystals, nanostructures or nanophase materials. In recent years, there has been tremendous interest in the development of nanocrystalline materials with ultra fine grain sizes lower than 100 nm. These materials often benefit from enhanced and sometimes novel, chemical and physical properties when compared to their conventional polycrystalline counterparts. An enhancement in hardness, fatigue behavior, coercivity, wear and corrosion resistance is often observed from the reduction in the grain size toward the sub-100 nm scale. Several synthesis techniques have been used to produce these unique materials such as inert gas condensation [2,3], mechanical alloying [4,5], sol gel [6,7] and rapid solidification [8,9]. Severe plastic deformation has also been extensively used to generate nanocrystalline materials [10]. However, electrodeposition is found to be a much simpler and economical method in producing nanocrystalline material. Several advantages include that 100% dense nanomaterials can be obtained without secondary processing steps, various materials can be deposited as nanomaterials and the very low set up cost and easy handling of alloy production as compared to other methods.
∗ Corresponding author at: Department of Materials Engineering, The University of British Columbia, Frank Forward Building, 309-6350 Stores Road, Vancouver, BC V6T 1Z4, Canada. Tel.: +1 604 822 2676; fax: +1 604 822 3619. E-mail addresses: [email protected] (N.M. Nik Rozlin), [email protected] (A.M. Alfantazi). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.04.092
Various pure metals, alloys, composites and ceramics have been synthesized by the electrodeposition process [11–17]. Electrodeposited alloys have been the subject of various studies in terms of its corrosion behavior [18–21], thermal stability [22], structural and magnetic properties [23,24]. Nowadays, electrodeposited cobalt–iron (CoFe) alloys are also gaining much attention for applications in microelectrochemical systems (MEMS) and protective coatings due to their high saturation magnetic flux density and high Curie temperature [25]. Several researchers have reported on the electrodeposited CoFe alloys. Kim et al. [26] investigated the magnetic properties of iron group thin films (Co, Ni, Fe, CoFe, NiCo and CoNiFe) that were electrodeposited from both chloride and sulfate baths. The results show that magnetic saturation of CoFe alloy was increased with an increase in Fe content in the deposit. An increase in coercivity with Co content increase in both baths was also reported from this study. Furthermore, higher current efficiency was obtained from a chloride bath due to low H2 limiting current compared to those from sulfate baths. Koza et al. [27] studied the influence of homogeneous magnetic fields on the properties of the electrodeposited CoFe. They reported that the magnetic field did significantly affect the properties of the deposits in terms of their microstructure, roughness, internal stress state, and chemical composition. Khan and Petrikowski [28] also studied the magnetic and structural properties of CoFe nanowires. The effect of Fe3+ on the magnetic moment of electrodeposited CoFe was studied by Brankovic et al. [29]. They concluded that the Fe(OH)3 nucleation and precipitation decreases the saturation magnetic flux density of electrodeposited CoFe. Electrodeposited CoFe was also studied in terms of its tensile stress by Shao et al. [30]. They reported that grain size plays an important role on the tensile stress of CoFe coatings. The smaller
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the grain size leads to higher tensile stress of the electrodeposits. However, higher deposition temperature was found to significantly reduce the film stress. Result concerning the stress of electrodeposited CoFe was also presented by Tabakovic et al. [31]. They investigated the possible causes of tensile stress which included interfacial stress between CoFe films and Cu-substrate, crystal texture and grain size, coalescence and stress evolution during film growth, and hydrogen adsorption/desorption. It was concluded that the most dominant stress factor was due to the adsorption/desorption of hydrogen. In addition an increase in Fe content in the alloy also resulted in an increase of stress in the alloy deposit. Sahari et al. [32] produced Co and CoFe electrodeposited from a mixed sulfate chloride bath to study the electrochemical nucleation and growth by voltammetric, chronoamperometric and AFM measurements. They obtained a compact and granular structure for both Co and CoFe alloys and concluded that the type of nucleation and growth was mainly controlled by the co-deposition phenomena as well as the bath composition. Several papers by Lallemand et al. reported on the production of electrodeposited CoFe containing different organic additives and its effect on the properties of the alloy coating [33–35]. In their study three different organic additives (saccharin (SAC), phthalimide (PHTA) and (OAS) were employed. They demonstrated that deposits produced from bath containing PHTA and OAS were inhomogeneous and dark while a bright and homogeneous coating was obtained from the use of SAC. Corrosion resistance was also improved in deposits obtained from additives containing the sulfonamide group. Ricq et al. [36] also investigated the influence of sodium saccharin on the electrochemical process as well as the corrosion of CoFe deposits. The results showed that corrosion resistance decreases in deposits obtained from solutions containing high concentration of sodium saccharin. The authors concluded that the poor corrosion behavior was due to the high sulfur content in the alloy coating. Although much research has been done on the thin films of CoFe alloys in terms of their magnetic properties [26,28,29] and stress [30,31] few studies have focused on the development of nanocrystalline CoFe alloys. Therefore, in the present work, nanocrystalline cobalt–iron alloy will be synthesized by direct current electrodeposition method in a sulfate based solution on a titanium substrate. The effects of varying the iron content of the nanocrystalline alloy deposits on the microstructure, grain size, surface morphology and microhardness will be investigated.
2. Experimental procedure 2.1. Electrodeposition of CoFe alloy A standard three-electrode cell configuration was used to synthesize nanocrystalline CoFe films. Titanium was used as the cathode substrate with an exposed area of 10 cm2 and the anode material was a graphite rod placed 20 cm from the cathode. The substrate was bonded to a copper wire using a conductive silver epoxy to establish an electrical connection. Prior to electrodeposition, the titanium substrate was ground with silicon carbide papers of 240, 320, 600, 800 and 1200 grit, polished with 1 m alumina powder and then rinsed with distilled water. The substrate was then emerged in sulfuric acid for a few seconds to eliminate all contaminants. Specimens were weighed before and after the electrodeposition process. The plating bath for CoFe deposits consisted of cobalt sulfate (CoSO4 ), iron sulfate (FeSO4 ), sodium chloride (NaCl), boric acid (H3 BO3 ) and saccharin (C7 H4 NO3 S). Boric acid and saccharin were added as a pH buffer and grain refiner respectively. Iron sulfate concentration was adjusted to produce CoFe deposits with
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Table 1 Bath composition and electrodepositing parameters for CoFe alloy deposits. Variable
Range
CoSO4 , g/L FeSO4 , g/L H3 BO3 , g/L NaCl, g/L Saccharin, g/L pH Temperature, ◦ C Peak current density, A/cm2
250 10–80 30 25 1.5 2.5 ± 0.1 40 0.5–3
various iron contents. The chemical composition of the solution is shown in Table 1. All chemicals were reagent grade and immersed in distilled water. The temperature of the bath was maintained at 40 ◦ C by employing a water bath and the solution was continuously stirred using a magnetic stirrer. Sulfuric acid (H2 SO4 ) and sodium hydroxide (NaOH) was used to adjust the pH of the bath solution to 2.5. During the electrodeposition process, direct current between 0.5 A/cm2 and 3 A/cm2 was applied. The electrodeposition process was carried out for 30 min to 1 h depending on the current density used to obtain deposits with an average thickness of about 80–100 m. Agitation was applied to maintain the homogeneity of the solution and in order to eliminate pitting due to the accumulation of hydrogen at the surface of the cathode [37]. After plating, the deposits were immediately taken out from the solution and washed under running water for several minutes to make sure that all unwanted salts on the surface were removed. All deposits were then dried with hot air and mechanically stripped from the substrate for further analysis. 2.2. Characterization The microstructure of nanocrystalline CoFe alloy coatings was characterized by scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDS) attached to the SEM and electron-probe micro-analyses (EPMA) were employed to determine the approximate composition of the deposited alloy. An average of 6 points was taken for the final composition value. X-ray diffraction (XRD) was performed on Rigaku x-ray diffractometer using Cu K␣ radiation (40 kV, 20 mA). The XRD data were recorded in a range from 10◦ to 70◦ with a step width of 0.04◦ . The average grain size of crystallites was calculated using the Scherrer formula according to the XRD peak broadening. The grain size of the CoFe deposits was also determined using a transmission electron microscopy (TEM). Thin films were prepared from 3 mm disks using the electropolishing method in an electrolyte comprising 10% perchloric acid and 90% acetic acid at a temperature of 15 ◦ C and a voltage of 20 V. Microhardness of the CoFe alloy deposits was measured by using a Vickers microhardness tester under a load of 50 g and a dwelling time of 15 s. Ten measurements at different locations throughout the alloy coating surface were performed and the average value was taken as the final hardness value. The deposits were polished and then rinsed with water prior to the hardness testing. In order to confirm that the substrate had no effect on the hardness value, the depth of indentation for all hardness measurement was less than 1 m. 3. Results and discussion 3.1. Effect of FeSO4 concentration The iron content of the CoFe alloys as a function of the iron sulfate concentration in the bath is shown in Fig. 1 while the EDS mapping analysis corresponding to the iron content in the
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of iron to 20–60 g/L in the plating bath lead to a rapid increase in the iron content of the alloy coatings to approximately 8–18 wt% Fe. Meanwhile the content of iron in the deposit was seen to be almost constant at about 25 wt% between 60 and 80 g/L of iron sulfate in solution. We were not able to analyze coatings deposited from solutions with more than 80 g/L of iron sulfate as they had multiple cracks on the coating surface and were very brittle to handle. 3.2. Current efficiency The current efficiency (j ) was calculated after determining the composition of cobalt and iron in the deposits using the expression given by Lowenheim [38]: Fig. 1. Iron content (wt%) of the CoFe alloy coatings as a function of the FeSO4 in the solution.
electrodeposits are shown in Fig. 2(a) and (b). As expected an increase in the iron content in the deposits was observed with an increase in the concentration of iron sulfate. A linear relationship was obtained from concentrations of iron sulfate between 10 g/L and 60 g/L. The electrodeposits contain about 6 wt% Fe from the lowest concentration of 10 g/L FeSO4 in solution. Further addition
j =
m m (Co εFe + Fe εCo ) = m mt ItεCo εFe
where m—mass of deposited alloy, mt —theoretical mass, Co —wt% of Co in the deposit, Fe —wt% of Fe in the deposit, εFe and εCo —electrochemical equivalents of Fe and Co, respectively; I—current; t—duration of the deposition process. Fig. 3 shows the current efficiency (CE) as a function of the current density. The current efficiency was seen to increase with an increase in the current density. The current efficiency was calculated to be about 70% with the highest current density of 3 A/cm2 employed in this study. The
Fig. 2. EDS spectrum of CoFe (a) 8 wt% Fe and (b) 25 wt% Fe.
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lowest amount of current efficiency at lower current density may be due to the large amount of hydrogen evolution in the electroplating process [39]. 3.3. SEM and TEM
Fig. 3. Current efficiency (%) as a function of current density (A/cm2 ).
Fig. 4 shows the SEM images of CoFe alloy coating with different iron content. A smooth and uniform surface morphology was obtained for all deposits. An acicular morphology was obtained from the deposit with 0 wt% Fe (Fig. 4(a)) while equiaxed grains were observed with increase iron content in the alloy coatings. The deposit with an iron content of about 6 wt% Fe (Fig. 4(b)) showed a relatively large grain size. However, increasing the iron contents from 11 to 25 wt% Fe resulted in a decrease in the grain size as shown in Fig. 4(c)–(e). From Fig. 4(f), it can be observed that
Fig. 4. SEM images of CoFe alloy electrodeposited with different iron contents. (a) 0 wt% Fe, (b) 6 wt% Fe, (c) 11 wt% Fe, (d) 17 wt% Fe, (e) 25 wt% Fe. (f) Deposit consisting of multiple cracks.
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Fig. 5. TEM images of CoFe alloy electrodeposited with different iron contents. (a) 17 wt% Fe, (b) 25 wt% Fe.
multiple cracks were present in the CoFe deposits containing iron content above 25 wt%. This could be associated with high film stresses obtained in the alloy coatings which has been previously found to be related to the increase in iron content [30]. Furthermore, Fig. 5(a) and (b) shows the TEM micrographs which further confirm the existence of the fine grains in the CoFe deposit with an average grain size of about 18 nm. 3.4. XRD analysis All CoFe deposit samples were characterized by means of XRD using Cu K␣ radiation on a standard Siemens D5000 diffractometer at a scan rate of 0.04◦ /s. Fig. 6 shows the XRD patterns for CoFe deposits with different iron contents. Furthermore, the diffraction pattern of the pure Co is also shown. CoFe alloys can exist in three distinct phases namely the (HCP), ␥ (FCC) and ␣ (BCC) phases at room temperature. It can be observed that the diffraction patterns change considerably in peak locations and intensities with the addition of Fe in the deposits. Significant broadening of the peaks could also be observed with an increase in Fe content indicating the presence of finer grains. All deposits were seen to have similar crystallographic structure at the same peak positions of about 43◦ corresponding to CoFe (1 1 0) of cubic structure. However, the XRD pattern for pure Co deposit exhibited the HCP crystal structure with a strong (0 0 2) texture. The diffraction pattern also indicates the presence of ␥ (FCC) reflections together with the (HCP) peaks
Fig. 6. X-ray diffraction patterns for Co and CoFe alloy coatings.
in deposits containing low iron content (6 wt% Fe). Deposits with 11–17 wt% Fe exhibit both phases (FCC + BCC). Further analysis of the diffraction peaks obtained from deposits with increased iron content (18–25 wt% Fe) revealed the deposits exhibited a BCC crystal structure. This result was in agreement with a previous study done [40] where considerable crystal structure change was also observed from pure Co to deposits with different range of Fe ions present in the thin films. From the phase diagram of CoFe alloys [41], a complete substitutional solid solution between Co and Fe is formed at room temperature. In another study it was also demonstrated that as the Fe amount in the deposit increases the peak position will also shift to higher angles [26]. Hence, from the result presented it can be observed that crystallographic structure or preferred orientation of phases strongly depends on the composition of the electrodeposit alloy coatings. 3.5. Grain size decreases with iron content The grain size of the CoFe alloy deposits was observed to decrease with an increase in the iron content of the deposit as shown in Fig. 7. The XRD using a diffractometer with Cu K␣ radiation ( = 0.15405) was employed and the grain sizes obtained were calculated from the line broadening of the X-ray peaks from XRD according to Scherrer’s formula as follows [42]: D=
ˇ cos
Fig. 7. Variation of grain sizes of CoFe alloy coatings as a function of iron content (wt%).
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Fig. 8. Variation of microhardness of CoFe alloy coatings as a function of Fe content (wt%).
where D is the average crystallite size, is the wave length of the radiation, is the Bragg angle and ˇ is the full width at the half maximum (FWHM). The average grain size value of the CoFe deposits decreased from 56 nm to 15 nm with an increase in the iron content of the alloy coatings. From the figure it is shown that coatings with iron content of up to 5 wt% Fe had grain sizes of about 56 nm. A further increase of 8–11 wt% Fe in the deposit resulted in grain sizes ranging from 40 to 31 nm respectively. Smaller grain sizes around 15 nm were achieved through CoFe electrodeposits with the highest iron composition of 25 wt% Fe. The effect of grain refining due to the increase of Fe in the alloy deposit has also been reported in studies concerning the nanocrystalline Ni Fe [40,43]. 3.6. Microhardness The microhardness of the electrodeposited CoFe alloy coatings was measured using a microhardness tester with a pyramidal indenter. A load of 50 g was used for an indentation time of 15 s. Fig. 8 shows the change in hardness as a function of the Fe content in the deposits. Alloy coatings between 0 and 26 wt% Fe were employed for the microhardness test. From the figure it can be seen that the hardness increased linearly from about 181 HV for pure Co to 323 HV (0–8 wt% Fe). A significant increase in hardness for deposits with a composition above 11 wt% Fe was obtained where the hardness was measured to be about 450 HV. The highest hardness was measured at about 620 HV for alloy coatings with 15 wt% Fe and was relatively constant thereafter. It is interesting to note that this hardness value is about three times higher than that for pure Co. The maximum in hardness for the deposits with 15 wt% Fe is most probably the result of the presence of dual phases (FCC + BCC) as determined by XRD. However a slight decrease in hardness was later seen for deposits above 25 wt% Fe. A similar hardness trend has been reported in several studies [43–47]. To the best of the authors’ knowledge, there is no systematic data that has been reported on the strength of either polycrystalline or nanocrystalline CoFe as a function of iron content. Therefore, comparisons are basically done with other electrodeposited alloy coatings reported in the literature. Although several theories have been proposed and investigated in order to further clarify and understand the reason of this deviation from the normal Hall–Petch behavior, this unique phenomenon is still not fully understood and progressive studies are still being carried out until today. Among the factors that have been considered to be affecting this particular behavior, diffusional creep [48], decrease in interfacial excess volume [49], annealing effects [50] and change in grain size [51]. The gradual increase obtained initially at lower iron content and the drop at higher iron content maybe attributed to the change in the phase structure as iron is
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further increased in the deposits. As can be seen the hardness increases and reaches the maximum value of about 620 HV at around 18 wt% Fe content which coincides with the BCC phase from deposits with an average grain size of 20 nm. Another important factor affecting the behavior of the microhardness of the nanocrystalline CoFe in this study may be attributed to the grain size effect. Initially, the increase at low Fe content in the deposits could be the result from the refinement in grain size as the Fe increases in the alloy coatings. A study by Li and Ebrahimi [52] also experienced an increase in the hardness of Ni Fe. They suggested that the increase in strength was due to the solid solution hardening with an increase in Fe. In another study [53] the formation of an ordered Ni3 Fe in nanocrystalline Ni Fe due to faster grain boundary diffusion was proposed as the main factor for the increase in strength. The above findings may be associated with an increase in hardness for CoFe electrodeposits in this study as the amount of Fe content in the deposits is within the same range. On the other hand, the decrease in hardness at higher Fe content maybe due to different reasons such as that the solid solution hardening is no longer effective for deposits with higher than 25 wt% Fe. The softening effect resulted from the refinement of the grain size may be due the significant increase of the intercrystalline volume fraction associated with the fraction of triple junction [54]. Thus, it could be assumed in this study that below a particular fine grain size this softening effect significantly contributes in the deviation of hardness from the normal Hall–Petch behavior. 4. Conclusions Several conclusions were drawn from this study: (a) Nanocrystalline CoFe alloy coatings have been successfully synthesized using direct current electrodeposition from a sulfate-based solution. Different iron compositions of CoFe alloys were obtained by varying the FeSO4 concentration in the plating bath. (b) An increase in FeSO4 concentration (10–80 g/L) in the plating bath increased the iron content of the alloy coatings (6–25 wt% Fe). (c) Grain size can be significantly reduced (60 nm–15 nm) with an increase of iron content in the deposits. (d) Different phases (HCP, FCC and BCC) were present according to the iron composition of the alloy coatings. (e) This study is the first to report on the microhardness of electrodeposited nanocrystalline CoFe where microhardness was increased to about three times with an increase in Fe (0–15 wt% Fe). (f) The deviation from the normal Hall–Petch behavior for deposits with higher Fe content (25 wt% Fe) could be attributed to the change in crystal structure as well as the softening effect below certain grain sizes. (g) Variation in the content of Fe has a very strong influence on the morphology, preferred orientation, grain size and microhardness of the CoFe electrodeposits. Acknowledgments Authors are grateful to the Natural Sciences Engineering Research Council of Canada (NSERC). A scholarship granted by Ministry of High Education, Malaysia and Universiti Teknologi Mara to Nik Rozlin Nik Masdek is also gratefully acknowledged. References [1] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427–556. [2] G.W. Nieman, J.R. Weertman, R.W. Siegel, Scr. Metall. 23 (1989) 2013–2018.
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Nanocrystalline cobalt–iron alloy: Synthesis and characterization N.M. Nik Rozlin a,b,∗ , Akram M. Alfantazi a a b
Department of Materials Engineering, The University of British Columbia, 6350 Stores Road, Vancouver, BC V6T1Z4, Canada Faculty of Mechanical and Materials Engineering, University Teknologi Mara, 40450 Shah Alam, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 14 February 2012 Received in revised form 16 March 2012 Accepted 24 April 2012 Available online 2 May 2012 Keywords: Electrodeposition CoFe Nanostructured materials Microhardness Grain refinement
a b s t r a c t Nanocrystalline cobalt–iron (CoFe) alloys were electrodeposited from a sulfate bath by direct current. The bath temperature and pH was maintained at 40 ◦ C and 2.5 respectively. The iron content was varied to study the effect on the surface morphology, grain size, current efficiency and microhardness. The experimental results showed that grain size of the CoFe alloy coating decreased with an increase in the iron content from about 6–25 wt%. Nanocrystalline CoFe coatings with an average grain size of 15 nm were achieved in coatings with 25 wt% Fe. Furthermore, the nanocrystalline CoFe coating was roughly three times harder than pure polycrystalline cobalt. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It has been almost 23 years since Gleiter first introduced what was then referred to as ‘microcrystalline materials’ [1]. Nowadays, these are better known as nanocrystalline materials or sometimes also referred to as nanocrystals, nanostructures or nanophase materials. In recent years, there has been tremendous interest in the development of nanocrystalline materials with ultra fine grain sizes lower than 100 nm. These materials often benefit from enhanced and sometimes novel, chemical and physical properties when compared to their conventional polycrystalline counterparts. An enhancement in hardness, fatigue behavior, coercivity, wear and corrosion resistance is often observed from the reduction in the grain size toward the sub-100 nm scale. Several synthesis techniques have been used to produce these unique materials such as inert gas condensation [2,3], mechanical alloying [4,5], sol gel [6,7] and rapid solidification [8,9]. Severe plastic deformation has also been extensively used to generate nanocrystalline materials [10]. However, electrodeposition is found to be a much simpler and economical method in producing nanocrystalline material. Several advantages include that 100% dense nanomaterials can be obtained without secondary processing steps, various materials can be deposited as nanomaterials and the very low set up cost and easy handling of alloy production as compared to other methods.
∗ Corresponding author at: Department of Materials Engineering, The University of British Columbia, Frank Forward Building, 309-6350 Stores Road, Vancouver, BC V6T 1Z4, Canada. Tel.: +1 604 822 2676; fax: +1 604 822 3619. E-mail addresses: [email protected] (N.M. Nik Rozlin), [email protected] (A.M. Alfantazi). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.04.092
Various pure metals, alloys, composites and ceramics have been synthesized by the electrodeposition process [11–17]. Electrodeposited alloys have been the subject of various studies in terms of its corrosion behavior [18–21], thermal stability [22], structural and magnetic properties [23,24]. Nowadays, electrodeposited cobalt–iron (CoFe) alloys are also gaining much attention for applications in microelectrochemical systems (MEMS) and protective coatings due to their high saturation magnetic flux density and high Curie temperature [25]. Several researchers have reported on the electrodeposited CoFe alloys. Kim et al. [26] investigated the magnetic properties of iron group thin films (Co, Ni, Fe, CoFe, NiCo and CoNiFe) that were electrodeposited from both chloride and sulfate baths. The results show that magnetic saturation of CoFe alloy was increased with an increase in Fe content in the deposit. An increase in coercivity with Co content increase in both baths was also reported from this study. Furthermore, higher current efficiency was obtained from a chloride bath due to low H2 limiting current compared to those from sulfate baths. Koza et al. [27] studied the influence of homogeneous magnetic fields on the properties of the electrodeposited CoFe. They reported that the magnetic field did significantly affect the properties of the deposits in terms of their microstructure, roughness, internal stress state, and chemical composition. Khan and Petrikowski [28] also studied the magnetic and structural properties of CoFe nanowires. The effect of Fe3+ on the magnetic moment of electrodeposited CoFe was studied by Brankovic et al. [29]. They concluded that the Fe(OH)3 nucleation and precipitation decreases the saturation magnetic flux density of electrodeposited CoFe. Electrodeposited CoFe was also studied in terms of its tensile stress by Shao et al. [30]. They reported that grain size plays an important role on the tensile stress of CoFe coatings. The smaller
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the grain size leads to higher tensile stress of the electrodeposits. However, higher deposition temperature was found to significantly reduce the film stress. Result concerning the stress of electrodeposited CoFe was also presented by Tabakovic et al. [31]. They investigated the possible causes of tensile stress which included interfacial stress between CoFe films and Cu-substrate, crystal texture and grain size, coalescence and stress evolution during film growth, and hydrogen adsorption/desorption. It was concluded that the most dominant stress factor was due to the adsorption/desorption of hydrogen. In addition an increase in Fe content in the alloy also resulted in an increase of stress in the alloy deposit. Sahari et al. [32] produced Co and CoFe electrodeposited from a mixed sulfate chloride bath to study the electrochemical nucleation and growth by voltammetric, chronoamperometric and AFM measurements. They obtained a compact and granular structure for both Co and CoFe alloys and concluded that the type of nucleation and growth was mainly controlled by the co-deposition phenomena as well as the bath composition. Several papers by Lallemand et al. reported on the production of electrodeposited CoFe containing different organic additives and its effect on the properties of the alloy coating [33–35]. In their study three different organic additives (saccharin (SAC), phthalimide (PHTA) and (OAS) were employed. They demonstrated that deposits produced from bath containing PHTA and OAS were inhomogeneous and dark while a bright and homogeneous coating was obtained from the use of SAC. Corrosion resistance was also improved in deposits obtained from additives containing the sulfonamide group. Ricq et al. [36] also investigated the influence of sodium saccharin on the electrochemical process as well as the corrosion of CoFe deposits. The results showed that corrosion resistance decreases in deposits obtained from solutions containing high concentration of sodium saccharin. The authors concluded that the poor corrosion behavior was due to the high sulfur content in the alloy coating. Although much research has been done on the thin films of CoFe alloys in terms of their magnetic properties [26,28,29] and stress [30,31] few studies have focused on the development of nanocrystalline CoFe alloys. Therefore, in the present work, nanocrystalline cobalt–iron alloy will be synthesized by direct current electrodeposition method in a sulfate based solution on a titanium substrate. The effects of varying the iron content of the nanocrystalline alloy deposits on the microstructure, grain size, surface morphology and microhardness will be investigated.
2. Experimental procedure 2.1. Electrodeposition of CoFe alloy A standard three-electrode cell configuration was used to synthesize nanocrystalline CoFe films. Titanium was used as the cathode substrate with an exposed area of 10 cm2 and the anode material was a graphite rod placed 20 cm from the cathode. The substrate was bonded to a copper wire using a conductive silver epoxy to establish an electrical connection. Prior to electrodeposition, the titanium substrate was ground with silicon carbide papers of 240, 320, 600, 800 and 1200 grit, polished with 1 m alumina powder and then rinsed with distilled water. The substrate was then emerged in sulfuric acid for a few seconds to eliminate all contaminants. Specimens were weighed before and after the electrodeposition process. The plating bath for CoFe deposits consisted of cobalt sulfate (CoSO4 ), iron sulfate (FeSO4 ), sodium chloride (NaCl), boric acid (H3 BO3 ) and saccharin (C7 H4 NO3 S). Boric acid and saccharin were added as a pH buffer and grain refiner respectively. Iron sulfate concentration was adjusted to produce CoFe deposits with
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Table 1 Bath composition and electrodepositing parameters for CoFe alloy deposits. Variable
Range
CoSO4 , g/L FeSO4 , g/L H3 BO3 , g/L NaCl, g/L Saccharin, g/L pH Temperature, ◦ C Peak current density, A/cm2
250 10–80 30 25 1.5 2.5 ± 0.1 40 0.5–3
various iron contents. The chemical composition of the solution is shown in Table 1. All chemicals were reagent grade and immersed in distilled water. The temperature of the bath was maintained at 40 ◦ C by employing a water bath and the solution was continuously stirred using a magnetic stirrer. Sulfuric acid (H2 SO4 ) and sodium hydroxide (NaOH) was used to adjust the pH of the bath solution to 2.5. During the electrodeposition process, direct current between 0.5 A/cm2 and 3 A/cm2 was applied. The electrodeposition process was carried out for 30 min to 1 h depending on the current density used to obtain deposits with an average thickness of about 80–100 m. Agitation was applied to maintain the homogeneity of the solution and in order to eliminate pitting due to the accumulation of hydrogen at the surface of the cathode [37]. After plating, the deposits were immediately taken out from the solution and washed under running water for several minutes to make sure that all unwanted salts on the surface were removed. All deposits were then dried with hot air and mechanically stripped from the substrate for further analysis. 2.2. Characterization The microstructure of nanocrystalline CoFe alloy coatings was characterized by scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDS) attached to the SEM and electron-probe micro-analyses (EPMA) were employed to determine the approximate composition of the deposited alloy. An average of 6 points was taken for the final composition value. X-ray diffraction (XRD) was performed on Rigaku x-ray diffractometer using Cu K␣ radiation (40 kV, 20 mA). The XRD data were recorded in a range from 10◦ to 70◦ with a step width of 0.04◦ . The average grain size of crystallites was calculated using the Scherrer formula according to the XRD peak broadening. The grain size of the CoFe deposits was also determined using a transmission electron microscopy (TEM). Thin films were prepared from 3 mm disks using the electropolishing method in an electrolyte comprising 10% perchloric acid and 90% acetic acid at a temperature of 15 ◦ C and a voltage of 20 V. Microhardness of the CoFe alloy deposits was measured by using a Vickers microhardness tester under a load of 50 g and a dwelling time of 15 s. Ten measurements at different locations throughout the alloy coating surface were performed and the average value was taken as the final hardness value. The deposits were polished and then rinsed with water prior to the hardness testing. In order to confirm that the substrate had no effect on the hardness value, the depth of indentation for all hardness measurement was less than 1 m. 3. Results and discussion 3.1. Effect of FeSO4 concentration The iron content of the CoFe alloys as a function of the iron sulfate concentration in the bath is shown in Fig. 1 while the EDS mapping analysis corresponding to the iron content in the
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of iron to 20–60 g/L in the plating bath lead to a rapid increase in the iron content of the alloy coatings to approximately 8–18 wt% Fe. Meanwhile the content of iron in the deposit was seen to be almost constant at about 25 wt% between 60 and 80 g/L of iron sulfate in solution. We were not able to analyze coatings deposited from solutions with more than 80 g/L of iron sulfate as they had multiple cracks on the coating surface and were very brittle to handle. 3.2. Current efficiency The current efficiency (j ) was calculated after determining the composition of cobalt and iron in the deposits using the expression given by Lowenheim [38]: Fig. 1. Iron content (wt%) of the CoFe alloy coatings as a function of the FeSO4 in the solution.
electrodeposits are shown in Fig. 2(a) and (b). As expected an increase in the iron content in the deposits was observed with an increase in the concentration of iron sulfate. A linear relationship was obtained from concentrations of iron sulfate between 10 g/L and 60 g/L. The electrodeposits contain about 6 wt% Fe from the lowest concentration of 10 g/L FeSO4 in solution. Further addition
j =
m m (Co εFe + Fe εCo ) = m mt ItεCo εFe
where m—mass of deposited alloy, mt —theoretical mass, Co —wt% of Co in the deposit, Fe —wt% of Fe in the deposit, εFe and εCo —electrochemical equivalents of Fe and Co, respectively; I—current; t—duration of the deposition process. Fig. 3 shows the current efficiency (CE) as a function of the current density. The current efficiency was seen to increase with an increase in the current density. The current efficiency was calculated to be about 70% with the highest current density of 3 A/cm2 employed in this study. The
Fig. 2. EDS spectrum of CoFe (a) 8 wt% Fe and (b) 25 wt% Fe.
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lowest amount of current efficiency at lower current density may be due to the large amount of hydrogen evolution in the electroplating process [39]. 3.3. SEM and TEM
Fig. 3. Current efficiency (%) as a function of current density (A/cm2 ).
Fig. 4 shows the SEM images of CoFe alloy coating with different iron content. A smooth and uniform surface morphology was obtained for all deposits. An acicular morphology was obtained from the deposit with 0 wt% Fe (Fig. 4(a)) while equiaxed grains were observed with increase iron content in the alloy coatings. The deposit with an iron content of about 6 wt% Fe (Fig. 4(b)) showed a relatively large grain size. However, increasing the iron contents from 11 to 25 wt% Fe resulted in a decrease in the grain size as shown in Fig. 4(c)–(e). From Fig. 4(f), it can be observed that
Fig. 4. SEM images of CoFe alloy electrodeposited with different iron contents. (a) 0 wt% Fe, (b) 6 wt% Fe, (c) 11 wt% Fe, (d) 17 wt% Fe, (e) 25 wt% Fe. (f) Deposit consisting of multiple cracks.
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Fig. 5. TEM images of CoFe alloy electrodeposited with different iron contents. (a) 17 wt% Fe, (b) 25 wt% Fe.
multiple cracks were present in the CoFe deposits containing iron content above 25 wt%. This could be associated with high film stresses obtained in the alloy coatings which has been previously found to be related to the increase in iron content [30]. Furthermore, Fig. 5(a) and (b) shows the TEM micrographs which further confirm the existence of the fine grains in the CoFe deposit with an average grain size of about 18 nm. 3.4. XRD analysis All CoFe deposit samples were characterized by means of XRD using Cu K␣ radiation on a standard Siemens D5000 diffractometer at a scan rate of 0.04◦ /s. Fig. 6 shows the XRD patterns for CoFe deposits with different iron contents. Furthermore, the diffraction pattern of the pure Co is also shown. CoFe alloys can exist in three distinct phases namely the (HCP), ␥ (FCC) and ␣ (BCC) phases at room temperature. It can be observed that the diffraction patterns change considerably in peak locations and intensities with the addition of Fe in the deposits. Significant broadening of the peaks could also be observed with an increase in Fe content indicating the presence of finer grains. All deposits were seen to have similar crystallographic structure at the same peak positions of about 43◦ corresponding to CoFe (1 1 0) of cubic structure. However, the XRD pattern for pure Co deposit exhibited the HCP crystal structure with a strong (0 0 2) texture. The diffraction pattern also indicates the presence of ␥ (FCC) reflections together with the (HCP) peaks
Fig. 6. X-ray diffraction patterns for Co and CoFe alloy coatings.
in deposits containing low iron content (6 wt% Fe). Deposits with 11–17 wt% Fe exhibit both phases (FCC + BCC). Further analysis of the diffraction peaks obtained from deposits with increased iron content (18–25 wt% Fe) revealed the deposits exhibited a BCC crystal structure. This result was in agreement with a previous study done [40] where considerable crystal structure change was also observed from pure Co to deposits with different range of Fe ions present in the thin films. From the phase diagram of CoFe alloys [41], a complete substitutional solid solution between Co and Fe is formed at room temperature. In another study it was also demonstrated that as the Fe amount in the deposit increases the peak position will also shift to higher angles [26]. Hence, from the result presented it can be observed that crystallographic structure or preferred orientation of phases strongly depends on the composition of the electrodeposit alloy coatings. 3.5. Grain size decreases with iron content The grain size of the CoFe alloy deposits was observed to decrease with an increase in the iron content of the deposit as shown in Fig. 7. The XRD using a diffractometer with Cu K␣ radiation ( = 0.15405) was employed and the grain sizes obtained were calculated from the line broadening of the X-ray peaks from XRD according to Scherrer’s formula as follows [42]: D=
ˇ cos
Fig. 7. Variation of grain sizes of CoFe alloy coatings as a function of iron content (wt%).
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Fig. 8. Variation of microhardness of CoFe alloy coatings as a function of Fe content (wt%).
where D is the average crystallite size, is the wave length of the radiation, is the Bragg angle and ˇ is the full width at the half maximum (FWHM). The average grain size value of the CoFe deposits decreased from 56 nm to 15 nm with an increase in the iron content of the alloy coatings. From the figure it is shown that coatings with iron content of up to 5 wt% Fe had grain sizes of about 56 nm. A further increase of 8–11 wt% Fe in the deposit resulted in grain sizes ranging from 40 to 31 nm respectively. Smaller grain sizes around 15 nm were achieved through CoFe electrodeposits with the highest iron composition of 25 wt% Fe. The effect of grain refining due to the increase of Fe in the alloy deposit has also been reported in studies concerning the nanocrystalline Ni Fe [40,43]. 3.6. Microhardness The microhardness of the electrodeposited CoFe alloy coatings was measured using a microhardness tester with a pyramidal indenter. A load of 50 g was used for an indentation time of 15 s. Fig. 8 shows the change in hardness as a function of the Fe content in the deposits. Alloy coatings between 0 and 26 wt% Fe were employed for the microhardness test. From the figure it can be seen that the hardness increased linearly from about 181 HV for pure Co to 323 HV (0–8 wt% Fe). A significant increase in hardness for deposits with a composition above 11 wt% Fe was obtained where the hardness was measured to be about 450 HV. The highest hardness was measured at about 620 HV for alloy coatings with 15 wt% Fe and was relatively constant thereafter. It is interesting to note that this hardness value is about three times higher than that for pure Co. The maximum in hardness for the deposits with 15 wt% Fe is most probably the result of the presence of dual phases (FCC + BCC) as determined by XRD. However a slight decrease in hardness was later seen for deposits above 25 wt% Fe. A similar hardness trend has been reported in several studies [43–47]. To the best of the authors’ knowledge, there is no systematic data that has been reported on the strength of either polycrystalline or nanocrystalline CoFe as a function of iron content. Therefore, comparisons are basically done with other electrodeposited alloy coatings reported in the literature. Although several theories have been proposed and investigated in order to further clarify and understand the reason of this deviation from the normal Hall–Petch behavior, this unique phenomenon is still not fully understood and progressive studies are still being carried out until today. Among the factors that have been considered to be affecting this particular behavior, diffusional creep [48], decrease in interfacial excess volume [49], annealing effects [50] and change in grain size [51]. The gradual increase obtained initially at lower iron content and the drop at higher iron content maybe attributed to the change in the phase structure as iron is
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further increased in the deposits. As can be seen the hardness increases and reaches the maximum value of about 620 HV at around 18 wt% Fe content which coincides with the BCC phase from deposits with an average grain size of 20 nm. Another important factor affecting the behavior of the microhardness of the nanocrystalline CoFe in this study may be attributed to the grain size effect. Initially, the increase at low Fe content in the deposits could be the result from the refinement in grain size as the Fe increases in the alloy coatings. A study by Li and Ebrahimi [52] also experienced an increase in the hardness of Ni Fe. They suggested that the increase in strength was due to the solid solution hardening with an increase in Fe. In another study [53] the formation of an ordered Ni3 Fe in nanocrystalline Ni Fe due to faster grain boundary diffusion was proposed as the main factor for the increase in strength. The above findings may be associated with an increase in hardness for CoFe electrodeposits in this study as the amount of Fe content in the deposits is within the same range. On the other hand, the decrease in hardness at higher Fe content maybe due to different reasons such as that the solid solution hardening is no longer effective for deposits with higher than 25 wt% Fe. The softening effect resulted from the refinement of the grain size may be due the significant increase of the intercrystalline volume fraction associated with the fraction of triple junction [54]. Thus, it could be assumed in this study that below a particular fine grain size this softening effect significantly contributes in the deviation of hardness from the normal Hall–Petch behavior. 4. Conclusions Several conclusions were drawn from this study: (a) Nanocrystalline CoFe alloy coatings have been successfully synthesized using direct current electrodeposition from a sulfate-based solution. Different iron compositions of CoFe alloys were obtained by varying the FeSO4 concentration in the plating bath. (b) An increase in FeSO4 concentration (10–80 g/L) in the plating bath increased the iron content of the alloy coatings (6–25 wt% Fe). (c) Grain size can be significantly reduced (60 nm–15 nm) with an increase of iron content in the deposits. (d) Different phases (HCP, FCC and BCC) were present according to the iron composition of the alloy coatings. (e) This study is the first to report on the microhardness of electrodeposited nanocrystalline CoFe where microhardness was increased to about three times with an increase in Fe (0–15 wt% Fe). (f) The deviation from the normal Hall–Petch behavior for deposits with higher Fe content (25 wt% Fe) could be attributed to the change in crystal structure as well as the softening effect below certain grain sizes. (g) Variation in the content of Fe has a very strong influence on the morphology, preferred orientation, grain size and microhardness of the CoFe electrodeposits. Acknowledgments Authors are grateful to the Natural Sciences Engineering Research Council of Canada (NSERC). A scholarship granted by Ministry of High Education, Malaysia and Universiti Teknologi Mara to Nik Rozlin Nik Masdek is also gratefully acknowledged. References [1] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427–556. [2] G.W. Nieman, J.R. Weertman, R.W. Siegel, Scr. Metall. 23 (1989) 2013–2018.
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