Effect of H3PO3 concentration on the electrodeposition of nanocrystalline Ni-P deposited in an emulsified supercritical CO2 bath

Article abstract of DOI:10.1016/j.electacta.2011.10.018

The electrodeposition of Ni-P alloy in an emulsified supercritical carbon dioxide (sc-CO₂) bath was investigated to explore the effect of H₃PO₃ concentration (ranging from 0 to 1.6 M) on the material characteristics of the deposits. The experimental results showed that the P content in the deposit increased with increasing H₃PO ₃ concentration in the emulsified sc-CO₂ bath, while the weight gain of the Ni-P film decreased. At the same concentration of H ₃PO₃, the significant decreases in weight gain and P content of the Ni-P films deposited in the emulsified sc-CO₂ bath were found, as compared with that plated in the conventional aqueous electrolyte at ambient pressure. X-ray photoelectron analyses revealed that the carbon-containing Ni-P deposit could be obtained in the bath with the presence of sc-CO₂ fluid. Transmission electron microscopy was employed for microstructure analysis. The average grain size of the Ni-P films deposited in the conventional aqueous electrolyte at ambient pressure or in the emulsified sc-CO₂ bath varied, depending on the H₃PO₃ concentration and P content. The Ni-P films deposited from the emulsified sc-CO₂ bath had a higher hardness as compared with that formed in conventional electrolyte. The hardness of Ni-P alloy deposited in the emulsified sc-CO₂ bath was enhanced by the additional solid solution strengthening mechanism.

Full text of DOI:10.1016/j.electacta.2011.10.018

Electrochimica Acta 58 (2011) 571–577
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Effect of H3PO3 concentration on the electrodeposition of nanocrystalline Ni–P
deposited in an emulsified supercritical CO2 bath
Sung-Ting Chung, Yan-Chi Chuang, Shih-Yi Chiu, Wen-Ta Tsai∗
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 August 2011
Received in revised form 1 October 2011
Accepted 1 October 2011
Available online 14 October 2011
The electrodeposition of Ni–P alloy in an emulsified supercritical carbon dioxide (sc-CO2) bath was
investigated to explore the effect of H3PO3 concentration (ranging from 0 to 1.6 M) on the material char-
acteristics of the deposits. The experimental results showed that the P content in the deposit increased
with increasing H3PO3 concentration in the emulsified sc-CO2 bath, while the weight gain of the Ni–P film
decreased. At the same concentration of H3PO3, the significant decreases in weight gain and P content of
the Ni–P films deposited in the emulsified sc-CO2 bath were found, as compared with that plated in the
conventional aqueous electrolyte at ambient pressure. X-ray photoelectron analyses revealed that the
carbon-containing Ni–P deposit could be obtained in the bath with the presence of sc-CO2 fluid. Transmis-
sion electron microscopy was employed for microstructure analysis. The average grain size of the Ni–P
films deposited in the conventional aqueous electrolyte at ambient pressure or in the emulsified sc-CO2
bath varied, depending on the H3PO3 concentration and P content. The Ni–P films deposited from the
emulsified sc-CO2 bath had a higher hardness as compared with that formed in conventional electrolyte.
The hardness of Ni–P alloy deposited in the emulsified sc-CO2 bath was enhanced by the additional solid
solution strengthening mechanism.
Keywords:
Electrodeposition
Supercritical CO2
Ni–P deposits
Nanocrystalline
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
overshadow their advantages [14,15], and low P alloys (P < 7 wt.%)
with nanocrystalline grains have thus attracted research attention.
Ni–P films have been widely used as hard coatings owing to
their satisfactory mechanical properties, reasonable cost and ease
of manufacturing [1–3]. Numerous studies have been devoted to
the fabrication of Ni–P alloy films using electrodeposition and elec-
troless deposition methods [4–6]. Electrodeposition processes are
less expensive, have faster deposition rates, and more stable baths
than electroless deposition [6]. Despite their different deposition
mechanisms, the electrodeposition and electroless deposition of
Ni–P alloys all lead to higher micro-hardness values than the pure
Ni. It has been shown that most of the properties of electrode-
posited nickel are structure-dependent, and that the structure is
closely related to the P content of the deposits [7]. The explanations
proposed for the increase in micro-hardness include solid solution
hardening, precipitation hardening, and grain size strengthening
[8–10]. The effect of deposition parameters, such as solution pH,
current density and the concentration of H3PO3 on composition,
structure and mechanical properties of Ni–P electrodeposits has
been studied extensively [1,10–16]. Deposits with high P content
(P > 7 wt.%) exhibit an amorphous structure. However, the extreme
brittleness and high sensitivity to notch with the high P alloys
Furthermore, the substitution of toxic electrolytes with envi-
ronmentally friendly ones is vitally important to minimize
environmental pollution. Recently, several researchers [17–19]
have begun to develop a new method for electrodeposition in a
bath involving supercritical CO2 (sc-CO2), and our previous stud-
ies [20–22] also showed that the Ni deposited in such a bath had
low surface roughness, small grain size, high micro-hardness, and
high corrosion resistance. We also found that the fine grain size and
solid solution strengthening due to carbon were responsible for the
higher hardness observed.
In order to further improve the mechanical performance of Ni
film, P addition in the Ni matrix was attempted in this investigation
by an electrodeposition process in an emulsified sc-CO2 bath. The
corresponding material and mechanical properties were examined
systematically. The Ni/P content ratio was controlled by adjust-
ing the H3PO3 concentration in the modified deposition solution,
and an improvement in the hardness of the Ni coating due to the
introduction of P element was found.
2. Experimental
A brass sheet with a surface area of 2 cm × 2.4 cm was used as
the cathode for electrodeposition. Each substrate was successively
ground with SiC paper to a grit of #2000, followed by polishing
∗
Corresponding author. Tel.: +886 6 275 7575x62927; fax: +886 6 275 4395.
E-mail address: wttsai@mail.ncku.edu.tw (W.-T. Tsai).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.10.018

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Table 1
Bath composition and conditions used for Ni–P electrodeposition.
Electrolyte
Concentration
Deposition conditions
Parameters
Value
NiSO4·6H2O
NiCl2·6H2O
H3BO3
Critic acid
Saccharin
C7H9NO2S
H3PO3
120 g/L
20 g/L
15 g/L
15 g/L
1 g/L
0.5 g/L
0–1.6 M
0.1 vol.%
Current density
Bath temperature
Deposition time
Solution pH
2 A/dm2
50 ◦
C
90 min
3.0
Conventional electrolyte (exposed to air)
0.1 MPa
Electrolyte in Ar gas
Electrolyte in sc-CO2
10 MPa
10 MPa
C12EO8
with Al2O3 slurry to 0.3 ␮m, degreased in 0.1 M NaOH solution and
followed by water rinsing, etched in 0.1 M H2SO4 solution, rinsed
with de-ionized water, and finally dried using a stream of hot air.
A high pressure cell made of stainless steel, with a volume of
650 mL, was fabricated for the electroplating in a bath contain-
ing sc-CO2 fluid. The electroplating system has been described
elsewhere [20]. Two types of electrodeposition baths, namely
conventional and emulsified sc-CO2, were used in this investiga-
tion. The conventional one consisted of 120 g/L NiSO4·6H2O, 20 g/L
NiCl2·6H2O, 15 g/L H3BO3, 15 g/L citric acid, 1 g/L saccharin, 0.5 g/L
C7H9NO2S and various concentrations of H3PO3 (ranging from 0
to 1.6 M). When the conventional bath was used, electrodeposition
was conducted either at ambient pressure or at 10 MPa under Ar
atmosphere. The emulsified sc-CO2 bath was prepared by purging
pressurized CO2 at 10 MPa into the conventional bath with proper
addition of C12EO8 surfactant to ensure an emulsified fluid was
attained. The bath consisted of 350 mL of the above aqueous solu-
tion and 300 mL of sc-CO2 fluid. The composition of the bath and
the operating conditions employed in this investigation are listed
in Table 1. Electroplating was performed under constant current
density conditions at 2 A/dm2 for 90 min.
Fig. 1. Weight gain of Ni–P films electrodeposited in the conventional electrolyte
containing different H3PO3 concentrations at ambient pressure, in Ar atmosphere
at 10 MPa, and in the emulsified sc-CO2 bath at 10 MPa.
After electroplating, material characterization was performed.
The chemical composition of the Ni–P alloy was measured by
energy dispersive spectrometry (EDS). The crystal structure of the
as-plated film was analyzed by X-ray diffraction (XRD, Rigaku Mini-
FlexII) using monochromatic Cu K␣ (ꢀ = 1.540562 nm) radiation.
X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe) was
carried out using Al K␣ excitation with a fixed analyzer transmis-
sion of 117.4 and 23.5 eV pass energy, 1 and 0.2 eV step size for
survey and high-resolution spectra, respectively. The microstruc-
ture of the nickel films was observed with a transmission electron
microscope (TEM, FEI Tecnai G2 20 S-Twin) with an emission volt-
age of 200 kV. The hardness measurements were carried out using
a nano-indenter equipped with a diamond tip. The loading was
applied at a strain rate of 0.05/s till a penetration depth of 1000 nm.
The corresponding hardness was then determined. The average of
five readings was taken as the hardness value of each Ni–P film.
In the emulsified sc-CO2 bath at 10 MPa, the weight gain of the
electrodeposited Ni–P film was lower than that formed in aqueous
electrolyte at ambient pressure, as shown in Fig. 1. The character-
istics of emulsion between water and supercritical carbon dioxide,
as addressed by many researchers [23,24], could play important
role in electroplating. In this emulsified bath, based on the work of
Sone and his co-workers [25], the formation of micelle (either CO2
in H2O or H2O in CO2) caused a reduction in the number of active
sites at the substrate/electrolyte interface for electrochemical reac-
tions. As a result, the deposition rate was hindered and the weight
gain mainly associated with Ni and/or P reductions was lower as
compared with that performed in the conventional bath. The lower
conductivity of the emulsified sc-CO2 bath, as pointed out by Kim
et al. [26], was also responsible for the lower weight gain observed.
In addition, the lower pH value of the aqueous electrolyte could be
maintained through the formation of carbonic acid by dissolving
carbon dioxide in water following the chemical reaction [10,27]:
3. Results and discussion
−
CO2(g) + H2O(aq) ⇔ H2CO3(aq) ⇔ H+(aq) + HCO3
(1)
(aq)
3.1. Weight gain
Previous studies [21,28,29] had also demonstrated the effect of
solution pH via the above reaction in affecting the electrodepo-
sition of Ni and the−electroless Ni–B deposition, respectively. The
adsorption of HCO3 on the deposit surface might also retard the
reduction of Ni2+, leading to a lower current efficiency observed in
the emulsified sc-CO2 bath.
The cross-section micrographs of the Ni–P alloy films electrode-
posited in the conventional and the emulsified sc-CO2 baths are
shown in Fig. 2. With a deposition time of 90 min at 2 A/dm2,
the thickness of the deposit formed in the emulsified sc-CO2 bath
was 17 ␮m, thinner than that electrodeposited in the conventional
aqueous electrolyte (25 ␮m). Obviously, the deposition rate in the
The weight gains of pure Ni and Ni–P alloy films electrode-
posited both in conventional aqueous electrolyte at ambient
pressure and in the emulsified sc-CO2 bath with various concen-
trations of H3PO3 were measured. As shown in Fig. 1, the weight
gain of the deposits decreased with increasing H3PO3 concentra-
tion in both baths. The weight gain of the Ni–P deposit formed in
the conventional bath with 0.35 M of H3PO3 under an Ar atmo-
sphere of 10 MPa was also determined. The result was the same as
that formed at ambient pressure, suggesting the insignificant effect
of pressure on the electrodeposition behavior in the conventional
bath.

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573
Fig. 3. P content in the Ni–P films electrodeposited in the conventional electrolyte
containing different H3PO3 concentrations at ambient pressure, in Ar atmosphere
at 10 MPa, and in the emulsified sc-CO2 bath at 10 MPa.
Similar results were found for the Ni–P films deposited in the
emulsified sc-CO2 bath. The P content in the deposit increased
from 2.0 to 7.7 at.% with increasing H3PO3 concentration from 0.05
to 1.6 M. The results shown in Fig. 3 clearly reveal that the Ni–P
films deposited in the emulsified sc-CO2 bath had a lower P con-
tent than those formed in the conventional bath with the same
H3PO3 concentration. The P content in the Ni–P alloy layers formed
in a conventional bath at ambient pressure and in that containing
sc-CO2 is shown in Table 2. The retardation of P co-deposition with
the presence of sc-CO2 fluid was significant, which was attributed
to the co-deposition of carbon, as will be discussed latter.
Fig. 2. Cross-section micrographs of the Ni–P films electrodeposited in the con-
ventional electrolyte containing 0.35 M H3PO3 at 2 A/dm2 for 90 min (a) at ambient
pressure (0.1 MPa) and (b) in the emulsified sc-CO2 bath at 10 MPa.
emulsified sc-CO2 bath was lower than that found in the conven-
tional bath, and these results are consistent with the weight gain
measurements.
3.2. Chemical composition analysis
3.2.2. XPS analysis
3.2.1. P content in the as deposited Ni–P alloy films
XPS was employed for chemical composition analysis of the
Ni–P deposits. Before the analysis, each Ni or Ni–P film was cleaned
by Ar+ ion etching for 1 min. The C1s spectra for the Ni–P film
deposited in the bath with or without sc-CO2 fluid are shown in
Fig. 4. The absence of C1s in the Ni–P film deposited in conventional
bath at ambient pressure indicates that Ar+ ion etching was ade-
quate for surface cleaning to eliminate any C contamination. For
the Ni–P film deposited in the conventional aqueous electrolyte
under an Ar atmosphere of 10 MPa, a similar result (without dis-
cernable C1s peak) was obtained. For the Ni–P film electrodeposited
in the emulsified sc-CO2 bath, a distinct C1s peak was observed,
EDS was employed to determine the P content in the deposit.
The variation of P content with H3PO3 concentration is demon-
strated in Fig. 3. For deposits formed in the conventional bath at
ambient pressure, P content increased from 4.6 to 24.9 at.%, with
increasing H3PO3 concentration from 0.05 to 1.6 M. By applying an
Ar atmosphere of 10 MPa to the bath containing 0.35 M H3PO3, the
P content in the deposit (about 9.7 at.%) was almost the same as that
formed at ambient pressure. The results indicate that the P content
in the Ni–P deposit formed in the conventional bath is independent
of the applied pressure.
Table 2
P content, grain size, and hardness of Ni–P films electrodeposited in conventional electrolyte containing different H3PO3 concentrations at ambient pressure and in the sc-CO2
bath at 10 MPa.
Condition of coating
[H3PO3] (M)
P content (at.%)
Grain size (nm)
Hardness (GPa)
Conventional electrolyte 0.1 MPa
Ni deposit
0
0
4.6
8.1
9.7
14.4
20.9
25.0
43
37
22
9
7
4
5.2
8.2
7.8
9.0
10.3
9.3
0.05
0.15
0.35
0.6
0.8
1.6
Ni–P deposit
3
8.8
Sc-CO2 bath 10 MPa
Ni deposit
0
0
17
18
16
14
12
13
11
9.5
12.1
12.0
12.4
12.5
12.0
11.9
0.05
0.15
0.35
0.6
0.8
1.6
2.0
3.7
4.9
5.7
6.5
7.7
Ni–P deposit

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Fig. 4. X-ray photoelectron spectra of the C1s region of Ni–P films electrodeposited
in the conventional electrolyte containing 0.35 M H3PO3 at ambient pressure, in
Ar atmosphere at 10 MPa, and in the emulsified sc-CO2 bath at 10 MPa, after Ar+
sputtering to remove atmospheric contaminants.
Fig. 5. X-ray diffraction patterns of the Ni deposit and Ni–P films electrodeposited
in the conventional electrolyte with varying H3PO3 concentrations: 0.35 M, 0.8 M,
and 1.6 M.
as shown in Fig. 4. The C1s binding energy obtained in this case
was 283.6 eV, close to that for graphitic carbon (284.6 eV) [30]. The
small deviation from the graphitic carbon C1s binding energy was
due to the formation of metal–carbon bonds, as has been reported
elsewhere [21,31,32]. In the electrolyte containing sc-CO2 fluid, as
described in this investigation, the formation of Ni–P deposit could
be accompanied by the following reaction:
H2CO3 + 4H+ + 4e− → C(s) + 3H2O
(2)
Electro-carburization of Ni in the system incorporating sc-CO2
fluid has been reported in our previous studies [21,22]. The XPS
results obtained in this investigation further confirmed the alloying
of carbon into Ni–P film electrodeposited in the emulsified sc-CO2
bath. Since the electro-carburization described in reaction (2) is
a reduction reaction which competes for electrons, therefore, the
amounts of Ni and P reduced from the electrolyte under constant
charges in the bath containing sc-CO2 are less than those in the
conventional bath, as revealed in Figs. 1 and 3.
3.3. Crystal structure
The XRD patterns of the Ni–P coatings prepared in the con-
ventional bath at ambient pressure and in the emulsified sc-CO2
bath, both with different H3PO3 concentrations, are illustrated in
Figs. 5 and 6, respectively. As shown in Fig. 5, pure Ni (P free) elec-
troplated in the conventional bath was crystalline and exhibited a
face-centered cubic structure with a preferred crystallographical
plane of (2 0 0). In the bath containing 0.35 M H3PO3, an obvi-
ous change of the preferred crystallographical plane from (2 0 0)
to (1 1 1) was observed. As the H3PO3 concentration was further
increased to 0.8 M, the diffraction peak intensity decreased sig-
nificantly, indicating the loss of crystallinity. At 1.6 M H3PO3, the
diffraction peak almost disappeared, implying the formation of
an amorphous phase in the electrodeposit. The effect of P on the
change of crystal structure has been addressed elsewhere [33–35].
Owing to the different atomic radii, the alloying of P causes a dis-
tortion of the base Ni lattices. As the P content increases, the lattice
Fig. 6. X-ray diffraction patterns of the Ni deposit and Ni–P films electrodeposited
in the emulsified sc-CO2 bath at 10 MPa with varying H3PO3 concentrations: 0.35 M,
0.8 M, and 1.6 M.
distortion becomes large enough such that the crystalline phase
cannot be maintained, and this leads to the formation of amorphous
phase, as demonstrated in Fig. 5.
When electrodeposition was conducted in the emulsified sc-CO2
containing electrolyte, a relatively weak dependence of crys-
tallinity on H3PO3 concentration was observed, as depicted in
Fig. 6. Although the change in preferred crystallographical plane
from (2 0 0) to (1 1 1) was still observed with P alloying, the
deposit still had a crystalline structure, even it was formed in the

S.-T. Chung et al. / Electrochimica Acta 58 (2011) 571–577
575
electrolyte containing 1.6 M H3PO3. The lower P content in the Ni–P
film deposited in the emulsified sc-CO2 bath, as compared with
that formed in the conventional electrolyte with the same H3PO3
concentration, was the main reason for the crystalline structure.
However, a gradual decrease in diffraction peak intensity accom-
panied by a slight shift and widening of the Ni (1 1 1) peak was found
when the concentration of H3PO3 in the emulsified sc-CO2 bath was
increased. A gradual decrease in grain size with increasing P con-
tent was expected, as confirmed by TEM analysis and discussed in
the following section.
3.4. Microstructure
The TEM bright field images and the SAD patterns of the Ni–P
coatings electrodeposited in conventional aqueous electrolyte at
ambient pressure and in the emulsified sc-CO2 bath are shown in
Figs. 7 and 8, respectively. In both cases, the TEM images reveal
that the deposits exhibited a nanocrystalline microstructure and
the grain size became finer as the concentration of H3PO3 in both
baths was increased. For the Ni–P film formed in conventional
aqueous electrolyte, the grain size determined from the dark-field
image decreased from 37 to 3 nm as the H3PO3 concentration was
increased from 0.05 to 1.6 M. The decrease in average grain size of
the Ni–P films was attributed to the increasing P content, as men-
tioned above. For the Ni–P films fabricated in the emulsified sc-CO2
bath, the average grain size varied from 18 to 11 nm with increas-
ing H3PO3 concentration from 0.05 to 1.6 M. The effect of sc-CO2
in reducing the grain size of the deposit has been addressed by
Yoshida et al. [18] and Chang et al. [25]. They reported that the
“periodic” deposition characteristics in the emulsified sc-CO2 bath,
which affects the nucleation and grain growth behaviors, is the
main factor leading to the grain size refinement. More specifically,
the micelles formed in the emulsified bath contact the cathode
surface in dynamic mode, which is similar to pulse plating. The
on-and-off situation as appeared in pulse plating [36] retarding
the crystal grown and leading to the formation of a fine crystalline
nickel film. The TEM results were basically in agreement with those
of the X-ray diffraction analyses. The grain sizes of the Ni–P alloys
deposited either in the conventional bath and or in emulsified
sc-CO2 bath with different H3PO3 concentration are also listed in
Table 2.
3.5. Hardness
A nanoindentation test was employed to determine the hard-
ness of the electrodeposited Ni–P films. Fig. 9 shows the hardness
of the Ni–P alloys with different P contents. The values of the P
content and hardness of the various Ni–P films are summarized in
Table 2. The hardness of pure Ni film formed at ambient pressure
was measured as 5.2 GPa. P alloying caused a significant increase in
the hardness of the Ni–P deposits. For the Ni–P films, the hardness
increased with increasing P content to a peak of 10 GPa at about
9.7 at.% P, and then decreased afterwards.
Notably, a substantial increase in hardness to about 9.5 GPa was
observed if pure Ni film was fabricated in the sc-CO2 fluid con-
taining bath. The significant increase in hardness was attributed to
the fine grain size and solid solution strengthening from carbon,
as reported previously [21]. For the Ni–P film electrodeposited in
the sc-CO2 bath, a higher hardness was observed as compared with
that formed in the conventional bath with the same concentration
of H3PO3. The changes in crystallographical orientation and grain
size associated with the alloying of P and C are responsible for the
substantial increase in hardness.
According to Hall–Petch equation [37,38], the strength of a solid
decreases linearly with the square root of grain size. Fig. 10 shows
the Hall–Petch form of hardness vs. grain size relationship for Ni
Fig. 7.

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S.-T. Chung et al. / Electrochimica Acta 58 (2011) 571–577
Fig. 9. Hardness of the Ni–P films electrodeposited at different P contents from
conventional electrolyte at ambient pressure and in the emulsified sc-CO2 bath at
10 MPa.
Fig. 10. Variation of hardness with grain size of various Ni and Ni–P films electrode-
posited in the conventional and in the emulsified sc-CO2 baths.
and Ni–P deposits with a P content less than 9.7 at.%. Obviously,
the improvement in hardness of the Ni–P coatings is attributed to
grain size strengthening. However, further increasing the P con-
tent beyond 9.7 at.% caused the hardness of the Ni–P deposits to
decrease with decreasing grain size (see Table 2). For the Ni–P
deposits with a higher P content (average grain size <10 nm), relax-
ation of atomic spacing and re-arrangement of atomic sites in the
amorphous phase could render a low hardness, obeying the inverse
Hall–Petch equation [39,40], namely
ꢁy = ꢁ0 + (−ky)d−1/2
(3)
where ꢁy is the tensile yield strength, ꢁ0 is the intrinsic stress
resisting dislocation motion, and ky is the proportional constant.
Because the hardness of a material is generally proportional to its
yield strength, the negative slope observed in Fig. 10 for the films
with much finer grain size implies that the inverse Hall–Petch rela-
tionship is obeyed.
4. Conclusions
Co-deposition of Ni with P via electrodeposition could be per-
formed in an emulsified sc-CO2 bath. The P content in the Ni–P
film, either deposited in a conventional or emulsified sc-CO2 bath,
Fig. 8.

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577
increased with increasing H3PO3 concentration, but with a sacrifice
in current efficiency due to electron competition from H+ and phos-
phorus oxyacid reductions. The weight gain of Ni and/or Ni–P film
formed in the emulsified sc-CO2 bath was lower than that found in
the conventional bath, which was mainly attributed to the reduced
number of active sites at the substrate/electrolyte interface for elec-
trochemical reaction and the lower conductivity of the bath with
the introduction of sc-CO2 into the aqueous electrolyte. The Ni–P
films deposited in the emulsified sc-CO2 bath had a lower P content
than that formed in the conventional bath with the same H3PO3
concentration. Electro-carburization of Ni and Ni–P film was found
when electrodeposition was performed in the sc-CO2 containing
bath. Nanocrystalline microstructure was observed and the grain
size of the deposit became finer as the P content in the deposit
increased. Furthermore, the presence of sc-CO2 in the electrode-
position bath enhanced the grain size refinement. A substantial
increase in hardness of the Ni–P film was observed when it was
fabricated in the emulsified sc-CO2 bath. The changes in crystallo-
graphical orientation, grain size refinement and the alloying of P
and C were responsible for the substantial increase in hardness of
the Ni–P electrodeposits obtained.
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