Effect of carbon content on structural and mechanical properties of Fe-Co-C alloy electrodeposited from aqueous solutions

Article abstract of DOI:10.1149/1.1931468

Fe-Co-C alloy coatings were electrodeposited from a sulfate solution containing citric acid and L-ascorbic acid as additives. Crystal structure and mechanical property of the coatings were examined by using X-ray diffraction (XRD) and Vickers hardness tester. The atomic ratio of Fe/Co in Fe-Co-C alloy coatings is essentially the same as that in carbon-free Fe-Co alloy coatings. However, the crystal structure of Fe-Co-C alloy is quite different from that of Fe-Co alloy. In the case of Fe-Co-C alloys, coatings with 30.0 to 83.8 mass % Co, an intermediate phase was detected. XRD analysis indicated that the phase had the same crystal structure as the cubic cobalt phase that was produced by the evaporation of Co on cold substrate under He atmosphere. However, the cubic structure changes to a tetragonal system by the incorporation of carbon. This intermediate phase is quite unstable and then transforms to bcc-Fe and hep-Co phases during pulverization process. The hardness of Fe-Co alloy coatings was improved by the codeposition of carbon. The maximum hardness of ca. Hv 800 was obtained for the Fe-10.9 mass % Co-1.1 mass % C alloy. The improvement in hardness of Fe-Co-C alloy coatings is discussed in detail from the viewpoint of alloy composition and crystal structure.

Full text of DOI:10.1149/1.1931468

C488
Journal of The Electrochemical Society, 152 ͑7͒ C488-C492 ͑2005͒
0
013-4651/2005/152͑7͒/C488/5/$7.00 © The Electrochemical Society, Inc.
Effect of Carbon Content on Structural and Mechanical
Properties of Fe-Co-C Alloy Electrodeposited from Aqueous
Solutions
Norikazu Miyamoto, Shinsuke Sakamoto, Hiroto Tamura, Masao Matsuoka,*,z
and Jun Tamaki*
Department of Applied Chemistry, Faculty of Science and Engineering, Ritsumeikan University,
Nojihigashi, Kusatsu, Shiga 535-8577, Japan
Fe-Co-C alloy coatings were electrodeposited from a sulfate solution containing citric acid and L-ascorbic acid as additives.
Crystal structure and mechanical property of the coatings were examined by using X-ray diffraction ͑XRD͒ and Vickers hardness
tester. The atomic ratio of Fe/Co in Fe-Co-C alloy coatings is essentially the same as that in carbon-free Fe-Co alloy coatings.
However, the crystal structure of Fe-Co-C alloy is quite different from that of Fe-Co alloy. In the case of Fe-Co-C alloys, coatings
with 30.0 to 83.8 mass % Co, an intermediate phase was detected. XRD analysis indicated that the phase had the same crystal
structure as the cubic cobalt phase that was produced by the evaporation of Co on cold substrate under He atmosphere. However,
the cubic structure changes to a tetragonal system by the incorporation of carbon. This intermediate phase is quite unstable and
then transforms to bcc-Fe and hcp-Co phases during pulverization process. The hardness of Fe-Co alloy coatings was improved by
the codeposition of carbon. The maximum hardness of ca. Hv 800 was obtained for the Fe-10.9 mass % Co-1.1 mass % C alloy.
The improvement in hardness of Fe-Co-C alloy coatings is discussed in detail from the viewpoint of alloy composition and crystal
structure.
©
2005 The Electrochemical Society. ͓DOI: 10.1149/1.1931468͔ All rights reserved.
Manuscript submitted September 20, 2004; revised manuscript received February 7, 2005. Available electronically June 7, 2005.
+ CoSO ·6H O͒, 6.2 ϫ 10−3 mol dm−3 citric acid, and 17
4 2
As-deposited Fe-C alloy coatings prepared by cathodic deposi-
tion from iron͑II͒ salts solution containing citric acid at 323 K
ϫ 10− mol dm L-ascorbic acid. Carbon-free Fe-Co alloy coat-
ings were prepared from the corresponding electrolyte free from
organic acid. All the solutions were prepared with deionized water
and special grade chemicals. The solution was adjusted to pH 2.7 by
using sulfuric acid. Stainless steel ͑SUS304͒ or brass sheets used as
cathodes for electrodeposition were degreased in an electrolyte con-
3
−3
showed the Vickers hardness of Hv 800, and it increased to the
1
maximum value of Hv 1150 by annealing at 623 K. This value is
larger than twice the industrial iron electrodeposits and comparable
to those of the carbonized steels or nitriding steels. Therefore, Fe-C
alloy is a promising material as an alternative to hard chromium
electrodeposits or heat-treated nickel-phosphorus coatings. For the
sisting of 1.1 mol dm− KOH and 0.10 mol dm K P O , and
3
−3
4
2
7
2
practical use of Fe-C alloy, it is necessary to improve the mechani-
rinsed with deionized water prior to each run. Two rectangular steel
cal properties such as hardness and wear resistance, etc.
sheets ͑7.0 ϫ 15.0 cm, JIS SPCC͒ were used as active anode. Elec-
−
2
A few attempts have been made to improve the mechanical prop-
erties of Fe-C alloy coatings by introducing a third element. For
example, ternary Fe-C-B alloy coatings had excellent wear
trodeposition was carried out galvanostatically at 30 mA cm under
the unstirred condition. The thickness of the alloy coatings was con-
trolled to be ca. 20 ␮m. The carbon and oxygen contents of the
coating were determined using a carbon–sulfur analyzer ͑Horiba
EMIA-920V͒ and oxygen analyzer ͑LECO TC-436AR͒, respec-
tively. The iron and cobalt contents of the coating were determined
by atomic absorption spectrophotometry ͑Hitachi Z-8200͒. Scanning
electron microscope ͑SEM, Hitachi S-2460͒ was used for observa-
tion of surface morphology. X-ray diffraction was carried out using
Cu K␣ radiation ͑RIGAKU RINT2000; 40 kV, 20 mA͒. The diffrac-
tion angles were calibrated with reference to pure Si diffraction
lines. The deconvolution of diffraction peaks was done with the
least-square fitting technique based on a pseudo-Voight formula by
3
resistance. In addition, Fe-C-P alloy coatings had excellent hard-
ness and wear resistance, due to the formation of solid solution in
4
which carbon and phosphorous were supersaturated. In both cases,
the third elements ͑B and P͒ effectively reduced the oxygen content
in the alloys, which significantly improved wear resistance of the
coatings.
An addition of cobalt as the third element to Fe-C alloy coatings
is of interest to improve mechanical properties of Fe-C alloy coat-
ings. In addition, the electrodeposited Fe-Co alloy ͑90 atom % Co͒
is an attractive material because of the high saturation magnetization
5
7
͑
1.9 T͒ as reported by Liao. Although the mechanical properties are
using the computer programs supplied by Toraya. X-ray photoelec-
an important factor in engineering the soft magnetic thin coatings,
there are few reports on the mechanical properties of Fe-Co alloy
coatings. Klingenmaier pointed out that the Fe-6 wt % Co alloy
electrodeposited from a chloride solution yielded low stress and the
tron spectroscopy ͑Ulvac-Phi ESCA-5700MC͒ was performed by
−9
using Mg K␣ radiation under vacuum of ca. 10 Torr. Binding
energy was calibrated with reference to the O 1s peak of iron oxide.
Hardness measurements were carried out using a Vickers hardness
tester ͑Akashi MS-55͒ under the load of 0.098 N.
6
Vickers hardness number of Hv 640.
In this study, the ternary Fe-Co-C alloys were systematically
investigated to improve the mechanical properties as a function of
Co content. The Fe-Co-C alloy coatings were electrodeposited gal-
vanostatically, and the effect of alloy composition on the crystal
structure and mechanical properties of Fe-C alloy coatings were
discussed based on the fundamental data derived from X-ray diffrac-
tion ͑XRD͒ and the measurements of Vickers hardness.
Results and Discussion
Chemical composition of Fe-Co-C coatings.—Cobalt, iron,
carbon, and oxygen contents of Fe-Co and Fe-Co-C alloy coatings
are shown in Fig. 1a and b, as a function of Co2 concentration.
Cobalt content of Fe-Co alloy coatings linearly increased and
iron content linearly decreased with an increase in Co concen-
The behavior of so-called anomalous codeposition, which
was frequently observed for electrodeposition of the iron-group bi-
+
2
+
Experimental
8-10
tration.
Fe-Co-C alloy coatings were prepared at 323 K from an
aqueous solution consisting of 0.14 mol dm− ͑FeSO ·7H O
3
11
4
2
nary alloy system, was not detected in this study. The Co/Fe ratio
in Fe-Co-C coatings was essentially the same as in carbon-free
Fe-Co alloy coatings. Carbon content of Fe-Co-C alloy coatings was
ca. 1.1 mass % and this value was kept constant in the range from 0
*
Electrochemical Society Active Member.
E-mail: matsuoka@se.ritsumei.ac.jp
z
2+
to 60 mol % Co ; however, it decreased abruptly with a further
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Journal of The Electrochemical Society, 152 ͑7͒ C488-C492 ͑2005͒
C489
Figure 3. Powder XRD patterns of Fe-Co alloy coatings as a function of Co
content. ͑a͒ Fe; ͑b͒ Fe-18.8% Co; ͑c͒ Fe-38.5% Co; ͑d͒ Fe-57.8% Co; ͑e͒
Fe-79.4% Co; ͑f͒ Fe-91.6% Co. Filled circle: bcc-Fe; filled square: hcp-Co.
cance of the oxide layer formed on the coating surface has been
3
12
pointed out by Izaki and Fujiwara. Judging from the low oxygen
content for the coatings deposited from organic acid-free electrolyte,
it is concluded that oxygen uptake through oxide layer is quite
small.
Crystal structure of Fe-Co and Fe-Co-C coatings.—The pow-
der XRD spectra of carbon-free Fe-Co alloy with different compo-
sitions are shown in Fig. 3. The diffraction profiles from ͑a͒ to ͑d͒
indicate that the carbon-free Fe-Co alloys take a single phase with a
body-centered cubic ͑bcc͒ structure when the Co content is less than
60 mass %. However, the other phases assigned to hexagonal close-
packed ͑hcp͒-Co and bcc-Fe are detected for the Fe-Co alloys with
further increase in Co content ͓profiles ͑e͒ and ͑f͔͒. The results are in
fairly good agreement with the previous report.
Figure 1. ͑a͒ Composition of Fe-Co alloy coatings as a function of Co2+
concentration. Open triangle: Fe; open square: Co; open diamond: O. ͑b͒
8-10
Composition of Fe-Co-C alloy coatings as a function of Co2+ concentration.
The powder X-ray diffraction spectra of Fe-Co-C alloy ͑0 to
Filled triangle: Fe; filled square: Co; filled diamond: O; filled circle: C.
18.6 mass % Co͒ are shown in Fig. 4. All diffraction lines are as-
signed to bcc structure with the lattice constant of
a
a
increase in Co2+ concentration. The oxygen content of Fe-Co-C
= 0.286 55 nm. The diffraction peaks were shifted to the higher
angle direction with an increase in Co content. Such changes are
attributable to solid solution formation of cobalt and iron atoms.
However, a discernible shoulder was detected at the higher angle to
the side of the ͕211͖ line, which could be divided into ͑112͒ and
coatings was remarkably higher than that of Fe-Co coatings as a
whole. The relationship between carbon and oxygen contents of
Fe-Co and Fe-Co-C alloy coatings is shown in Fig. 2. It was dis-
closed that the oxygen content increased in proportion to the carbon
content. The same tendency was also detected for the martensitic
͑211͒ lines. The deconvolution results of ͕211͖ diffraction lines are
4,12
shown in Fig. 4 as the insertion. In the case of ͕200͖ diffraction
Fe-C and Fe-P-C coatings.
Consequently, an increase in oxygen
content with carbon content is exclusively attributable to inclusion
of a reaction intermediate derived from organic acid. The signifi-
Figure 4. Powder XRD patterns of Fe-Co-C alloy coatings and an insertion
showing deconvolution profiles of ͕211͖ diffraction line. ͑a͒ Fe-1.1% C; ͑b͒
Fe-10.9% Co-1.2% C; ͑c͒ Fe-18.6% Co-1.1% C.
Figure 2. Relationship between C and O contents in Fe-Co and Fe-Co-C
alloy coatings. Filled circle: Fe-Co-C; open circle: Fe-Co.
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C490
Journal of The Electrochemical Society, 152 ͑7͒ C488-C492 ͑2005͒
Table I. Crystallographic parameters of intermediate phase with
a composition of Fe-57.3% Co-1.2% C alloy. The observed inter-
mediate phase had a = 0.4567 nm as cubic system and the re-
1
3
ported intermediate phase of cubic system had a = 0.460 nm.
No.
dobs/nm
dcal/nm
Difference/nm
h, k, l
1
2
3
4
0.2292
0.1867
0.1260
0.1144
0.2284
0.1865
0.1267
0.1142
0.0008
0.0003
−0.0007
0.0002
2, 0, 0
2, 1, 1
3, 2, 0
4, 0, 0
bcc-Fe by the pulverization. The intermediate phase would be in-
duced by solute carbon, because such a phase could not be detected
in carbon-free Fe-Co alloy. Further details of the intermediate phase
will be discussed in the following section.
Although SEM images are not shown here, the surface mor-
phologies of Fe-Co-C alloy coatings were smoother than those of
Fe-Co alloy, regardless of cobalt content. However, few cracks were
detected for the Fe-Co-C alloy with 64.9-83.8 mass % Co. In the
case of the Fe-Co-C coating ͑Ͼ89.3 mass % Co͒, the needle-shaped
crystal growth characterizing hexagonal structure was detected.
Figure 5. XRD patterns of as-prepared Fe-Co-C alloy coatings with different
Co contents. ͑d͒ Fe-30.0% Co-1.1% C; ͑e͒ Fe-39.1% Co-1.2% C; ͑f͒ Fe-
48.1% Co-1.2% C; ͑g͒ Fe-57.3% Co-1.2% C; ͑h͒ Fe-64.9% Co-1.0% C; ͑i͒
Fe-74.2% Co-0.8% C; ͑j͒ Fe-83.8% Co-0.5% C; ͑k͒ Fe-88.5% Co-0.05% C.
Filled circle: bcc-Fe; filled square: hcp-Co; filled star: intermediate; filled
triangle: unknown phase.
Identification of the intermediate phase.—An attempt was
made to identify the intermediate phase from the XRD spectra of
as-prepared Fe-Co-C coatings. The calculated and observed crystal-
lographic parameters of typical intermediate phase ͑Fe-57.3 mass %
Co-1.2 mass % C alloy͒ are shown in Table I. All the peaks are
assigned to a cubic cobalt phase with a lattice constant of 0.460 nm,
which has the same structure as detected for the cobalt evaporated
lines, the shoulder was detected at the lower angle to the side of the
͕
200͖ line, which could be deconvoluted into ͑002͒ and ͑200͒ lines.
The shoulders detected in ͕200͖ and ͕211͖ lines are caused by the
solute carbon atoms occupying the interstitial site. As reported by
1
Izaki et al. the interstitial carbon atoms in Fe-C alloy occupied ͑0,
1
3
0, 1/2͒ and ͑1/2, 1/2, 0͒ sites and the c-axial and the axial ratio c/a
on a cold substrate under He atmosphere. All the peaks observed at
9, 49, 75, and 85° are assigned to the index of ͕200͖, ͕211͖, ͕320͖,
increased in proportion to the carbon content. The observed peak
splits indicate that all the Fe-Co-C alloys take a martensitic single
phase with a body-centered tetragonal structure.
3
and ͕400͖, respectively. This intermediate phase was exclusively
detected for Fe-Co-C coatings with the cobalt content of 30.0-
The XRD spectra of as-prepared and pulverized Fe-Co-C alloy
83.8 mass %.
͑
30.0 to 83.8 mass % Co͒, are shown in Fig. 5 and 6, respectively. In
As shown in Fig. 5, a discernible shoulder can be seen at a lower
the case of as-prepared Fe-Co-C alloy coatings ͑30.0 to 83.8 mass %
Co͒, the diffraction peaks located at 39°, 49°, 76°, and 85° could not
be assigned to any phase of hcp-Co, face-centered cubic ͑fcc͒-Co, or
bcc-Fe. In this paper, this unknown phase is referred to as the inter-
mediate phase. The powder XRD spectra indicate that main peaks
are assigned to the hcp-Co or bcc-Fe phase. The intensity of the
hcp-Co peak increased and that of the bcc-Fe decreased with an
increase in cobalt content. Consequently, the crystal structure of an
intermediate phase is transformed to the mixture of hcp-Co and
angle to the side of the ͕320͖ line and at higher angle to the side of
the ͕400͖ line, respectively. The ͕320͖ and ͕400͖ lines were deconvo-
luted by the same manner as described in the previous section. The
deconvolution result is shown in Fig. 5 as the insertion. Such a
deconvolution result indicates that the crystal structure of interme-
diate phase belongs to a tetragonal system. This situation is quite
similar to that of a martensitic single phase being detected for the
Fe-Co-C alloys ͑Ͻ18.6 mass %͒. In this case, the interstitial carbon
atom also plays an important role in the formation of the tetragonal
phase.
For the alloys with the composition of 30.0-57.3 mass % Co, the
intensity of ͕200͖ and ͕400͖ lines was larger than that of ͕211͖ and
Figure 6. Powder XRD patterns of Fe-Co-C alloy coatings with different Co
contents. ͑d͒ Fe-30.0% Co-1.1% C; ͑e͒ Fe-39.1% Co-1.2% C; ͑f͒ Fe-48.1%
Co-1.2% C; ͑g͒ Fe-57.3% Co-1.2% C; ͑h͒ Fe-64.9% Co-1.0% C; ͑i͒ Fe-
7
4.2% Co-0.8% C; ͑j͒ Fe-83.8% Co-0.5% C. Filled circle: bcc-Fe; filled
Figure 7. Lattice constants as a function of Co content in Fe-Co-C alloy
coatings. Filled circle: a-axial length; filled square: c-axial length.
square: hcp-Co; filled star: intermediate; filled triangle: unknown phase.
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Journal of The Electrochemical Society, 152 ͑7͒ C488-C492 ͑2005͒
C491
Figure 8. Co 2p, Fe 2p, O1s, and C1s
photoelectron spectra of Fe-Co-C al-
loy coatings after Ar etching for 600 s.
͑
a͒ Fe-10.9% Co-1.2% C; ͑b͒ Fe-
39.1% Co-1.2% C; ͑c͒ Fe-64.9% Co-
1.0% C.
͕
320͖, but the reverse was true for the alloys containing 64.9-83.8
bonds. The chemical states of corresponding elements for the mar-
tensitic and intermediate phases resemble each other, independent of
cobalt content. In addition, the chemical states of corresponding
elements for the Fe-Co-C alloy coatings are essentially identical to
mass % Co. This fact suggests that the orientation of the intermedi-
ate phase changes as a function of cobalt content.
The small peaks originated from an unknown phase unassigned
to the intermediate, and the other phases were detected for the alloys
with higher Co content as shown in Fig. 5 and 6. However, it was
impossible to identify the unknown phase because of its weak dif-
fraction intensity.
1
those for Co-free Fe-C alloy coatings except for cobalt.
Effect of cobalt and carbon contents on mechanical properties of
Fe-Co-C alloy coatings.—The hardness of Fe-Co and Fe-Co-C al-
loy coatings as a function of Co content is shown in Fig 9. The
hardness of Co-free Fe-C alloy coating was found to be ca. Hv 700.
The hardness of martensitic Fe-Co-C alloy coatings was remarkably
greater than that of Fe-Co alloys. Such a hardening was dependent
on the amount of interstitial carbon atoms as reported for Fe-C
Lattice parameters of Fe-Co-C alloy coatings.—The effect of
cobalt content on lattice parameters of Fe-Co-C alloy coatings is
shown in Fig. 7. The lattice constants a and c increased abruptly at
ca. 20-30 mass % Co accompanying the changes in crystal structure.
The lattice parameters a and c decreased with increasing Co content
both in the martensitic and intermediate regions. The decrease in
lattice constants in the martensitic and intermediate region is ex-
plained by the formation of Fe-Co solid solution as observed for the
1
alloys. Maximum hardness of ca. Hv 800 was obtained for Fe-10.9
mass % Co-1.1mass % C alloy coating. The higher hardness would
be ascribed to the lattice strain produced during the formation of
carbon-supersaturated solid solution.
8
Fe-Co alloy with bcc structure.
Although carbon content was kept constant at ca. 1.1 mass % for
Chemical state of elements in the alloys.—Figure 8 shows the
typical photoelecton spectra being measured to figure out the chemi-
cal states of each element in Fe-Co-C coatings with different cobalt
contents. The O 1s peak at 530 eV is ascribed to Fe-O and Co-O
1,3,4
14
bond as detected for Fe O3 and Fe O ,
Co O , and Co O .
2
3
4
2 3 3 4
Although the alloys contained ca. 5 mass % O, and O 1s peak of
iron or cobalt oxides was detected, no diffraction lines correspond-
ing to oxides were detected in the XRD profile shown in Fig. 4.
Therefore, iron oxides are supposed to be in an amorphous state.
3
/2
The Co 2p peaks located at 778 and 794 eV are assigned to Co 2p
1
/2
15
and Co 2p in the metallic state, respectively. The Fe 2p peaks
located at 707 and 720 eV are assigned to Fe 2p and Fe 2p in
3
/2
1/2
1,3,4
the metallic state, respectively.
The intensity of Co 2p and Fe 2p
peaks was proportional to the iron and cobalt contents. In the C 1s
spectra, the main peak at 283 eV and shoulder peak at 286 eV was
detected. The peak around 286 eV is assigned to the oil contamina-
1
tion from the apparatus. The binding energy of Co-C bond is un-
known; however, it is estimated to be 283 eV, judging from the
16
16
1,3,4
binding energy reported for Cr-C, Ni-C, and Fe-C.
Conse-
Figure 9. Vickers hardness number of Fe-Co and Fe-Co-C alloy coatings as
quently, the C 1s peak around 283 eV is ascribed to Fe-C and Co-C
a function of Co content. Filled circle : Fe-Co-C alloy; filled square : Fe-Co.
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C492
Journal of The Electrochemical Society, 152 ͑7͒ C488-C492 ͑2005͒
the alloys with a composition of 0 to 64.9 mass % Co, the hardness
of intermediate phase ͑30.0-83.8 mass % Co͒ was smaller than that
of Fe-Co-C alloys with a composition of 0-18.6 mass % Co. This
result indicates that the lattice strain produced in the intermediate
phase is not as excessive as that in a martensitic phase, despite the
constancy of carbon content. Fe-Co-C alloy coatings gave the higher
hardness, compared to binary Fe-Co alloy coatings. Supersaturated
carbon is found to be effective to improve the mechanical property
of Fe-Co alloy coating.
rated interstitial carbon and substitutional cobalt synergistically play
an important role to give rise to the maximum hardness.
Acknowledgments
The authors thank Dr. Masanobu Izaki of Osaka Municipal Tech-
nical Research Institute for his kind assistance and helpful discus-
sion on XPS measurements.
Ritsumeikan University assisted in meeting the publication costs of this
article.
Conclusions
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