Boron Particle Composite Plating with Ni–B Alloy Matrix
Susumu Arai,
a,
*
,z
Shuji Kasai,
a
and Ikuo Shohji
b
a
Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan
b
Graduate School of Engineering, Gunma University, Gunma 376-8515, Japan
Ni–B alloy films containing amorphous boron particles 共referred to as “Ni–B alloy composite films”兲 were fabricated by elec-
trodeposition and were subsequently subjected to heat-treatment. Their compositions and microstructures were characterized, and
their hardness was evaluated. The content of boron particles in the alloy composite films increased with boron particle concen-
tration in the plating baths. In addition, the total boron content in the films increased with decreasing current density, reaching a
maximum value of 34.3 atom %. The boron particles were homogeneously distributed in these alloy composite films and exhibited
no cohesion. Heat-treatment of the alloy composite films consisting of a Ni–B alloy matrix and the boron particles led to a phase
conversion from an inhomogeneous amorphous phase to stable homogeneous crystalline phases, which were similar to those in the
Ni–B binary alloy phase diagram. The hardness of the Ni–B alloy 34.3 atom % B composite film was higher than that of a Ni–B
alloy film both before and after heat-treatment.
© 2009 The Electrochemical Society. 关DOI: 10.1149/1.3271099兴 All rights reserved.
Manuscript submitted July 6, 2009; revised manuscript received November 11, 2009. Published December 29, 2009.
Ni–B alloy films are attractive for industrial applications because
they possess several desirable properties, including high hardness
and wear resistance. To date, such films have mainly been deposited
by electroless plating methods,
1-11
and their applications have also
been studied.
12-18
However, very few studies have been undertaken
on Ni–B alloy films produced by electroplating methods.
19-21
Most
reports discuss alloy films with a boron content of under 25 atom %,
which have the stable phases of Ni and Ni
3
B according to the Ni–B
binary phase diagram. However, the phase diagram also indicates
that Ni
2
B and other crystalline phases are stable when the boron
content of Ni–B alloys exceeds 25 atom %. A comparison of the
microstructure and characteristics of Ni–B alloy films having boron
contents above and below 25 atom % is interesting from both an
academic and a practical viewpoint. We initially attempted to fabri-
cate Ni–B alloy films using a conventional electroplating method.
However, the maximum boron content that could be achieved by
this method was less than 25 atom %. We have already reported that
Ni–P alloy films containing over 25 atom % phosphorus could be
obtained using a unique technique, which involves the formation of
a composite film of a Ni–P alloy and phosphorus particles and a
subsequent heat-treatment.
22
In the present study, to fabricate Ni–B alloy films with boron
contents greater than 25 atom %, we used a similar technique, pro-
ducing composite films of a Ni–B alloy and boron particles and then
subjecting them to heat-treatments. Furthermore, we analyzed the
microstructure and hardness of the alloy composite films both before
and after heat-treatment.
Experimental
The compositions of the plating baths used in the present study
are shown in Table I. The baths were based on the Watts bath, and a
Ni–B alloy and a Ni–B composite bath were prepared for compari-
son. Trimethylamine borane was used as a boron source for the
Ni–B alloy plating bath,
19
and boron particles were used as a boron
source for the Ni–B composite plating bath. Both sources were used
for the Ni–B alloy composite plating bath. Amorphous boron par-
ticles 共Rare Metallic Co., Ltd.兲 with an 800 nm average diameter
were used in this study. An electrolytic cell 共model I, Yamamoto-Ms
Co., Ltd.兲 with internal dimensions of 65 ⫻ 65 ⫻ 95 mm was em-
ployed for electrodeposition. The volume of the plating bath was
250 cm
3
. Pure copper plates 共C1020P兲 and stainless steel plates
共SUS304兲, both having exposed surface areas of 10 cm
2
共3
⫻ 3.3 cm
2
兲, were used as substrates. A pure nickel plate was used
as the anode. Plating was performed under galvanostatic conditions.
The Ni–B alloy plating and the Ni–B alloy composite plating were
carried out at 45°C with aeration. The Ni–B composite plating was
conducted at 25°C with aeration. The compositions of the deposited
films were measured using an inductively coupled plasma emission
spectrometer 共ICPS-7500, Shimadzu Seisakusho Co.兲. Before the
measurement, the deposited films were dissolved in nitric acid, re-
sulting in nickel ions 共Ni
2+
兲 and boric acid ions 共BO
3
3−
兲. Boron
concentration was determined as the boric acid ion concentration.
The deposited films were heat-treated in vacuum using an IR heating
furnace. Their phase structures were analyzed using an X-ray dif-
fractometer 共XRD, XRD-6000, Shimadzu Seisakusho Co.兲. Surface
morphologies and cross-sectional textures were observed using a
field-emission scanning electron microscope 共SEM, JSM-7000F,
JEOL兲. A mapping analysis of the cross sections of the deposits was
performed using an electron probe X-ray microanalyzer 共EPMA,
EPMA-1610, Shimadzu Seisakusho Co.兲. A cross-section polisher
共SM-09010, JEOL兲 was used to prepare cross-sectional samples for
observation. Hardness testing of the deposited films was performed
using a micro-Vickers hardness tester 共DUH-201, Shimadzu Sei-
sakusho Co.兲.
Results and Discussion
Figure 1 shows the relationship between the concentration of
boron particles in the plating bath and the boron content in the
electrodeposited films. The applied current density was 1 A dm
−2
.
The boron content in the films includes the boron component of the
Ni–B alloy matrix and the boron particles in the films. The Ni–B
alloy film electrodeposited from the bath without boron particles
contained 6.4 atom % boron. The boron content of the deposited
films increased as the concentration of boron particles in the plating
bath increased, reaching a maximum value of 22.5 atom %. Homo-
geneous composite films could not be fabricated when the boron
particle concentration in the plating bath exceeded 100 g dm
−3
.
Figure 2 shows surface and cross-sectional SEM images of films
with various boron contents. Figure 2a and d shows the surface and
the cross section of a Ni–6.4 atom % B alloy film. The film has a
smooth surface morphology and a homogeneous microstructure.
Figure 2b, e, c, and f shows the surfaces and cross sections of the
Ni–17.1 atom % B alloy composite film and the Ni–22.5 atom % B
alloy composite film, respectively. Boron particles are homoge-
neously distributed in the films without any cohesion. The quantity
of boron particles in the films evidently increased with the overall
boron content of the films, which is related to the boron particle
concentration in the plating baths. Therefore, the increase in the
boron content in the films in Fig. 1 is considered to be caused
mainly by the increase in the number of boron particles in the films.
Figure 3 shows the effect of current density on the boron content
in the electrodeposited films. The boron particle concentration in the
alloy composite bath was 100 g dm
−3
. The boron content in the
Ni–B alloy films increased with decreasing current density. This
*
Electrochemical Society Active Member.
z
E-mail: araisun@shinshu-u.ac.jp
Journal of The Electrochemical Society, 157 共2兲 D119-D125 共2010兲
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tendency is similar to that reported by Krishnaveni et al. using the
dimethylamine borane as a boron source.
20
The boron content in the
Ni–B composite films slightly increased with decreasing current
density. For the Ni–B alloy composite films, the boron content also
increased with decreasing current density, reaching a maximum
value of 34.3 atom %, which is obviously higher than 25 atom %.
The boron content of the Ni–B alloy composite films at any given
current density seems to be the sum of that of the Ni–B alloy film
and the Ni–B composite film at the same current density. This sug-
gests that the boron component of the Ni–B alloy matrix and the
boron in the particles are independent of each other.
Figure 4 shows surface and cross-sectional SEM images of the
Ni–B alloy composite films electrodeposited under various current
densities. A uniform distribution of boron particles was found for
every alloy composition, with no evidence of cohesion. Whereas the
boron content of the film electrodeposited at 0.5 A dm
−2
共Ni–34.3
atom % B兲 is clearly higher than that of the film electrodeposited at
1Adm
−2
共Ni–22.5 atom % B兲, the difference in the amount of
boron particles in those films is not so obvious. Therefore, the dif-
ference in the overall boron content is thought to be due to differing
amounts of boron in the Ni–B alloy matrix. Figure 5 shows XRD
patterns of the Ni–B alloy composite films. For each film, a broad
peak assigned to Ni appears at around 44°, indicating that the alloy
composite films have low crystallinity or an amorphous structure. In
particular, the peak for the Ni–34.3 atom % B alloy composite film
is extremely broad. This is thought to be largely due to the increase
in the boron content in the Ni–B alloy matrix and supports the idea
that the difference in the overall boron content between the Ni–34.3
atom % and the Ni–22.5 atom % B alloy composite film is mainly
due to the difference in the boron content in the Ni–B alloy matrix.
Figure 6 shows surface SEM images of the Ni–34.3 atom % B
alloy composite film following heat-treatment at various tempera-
tures. No significant defects such as cracks could be observed in any
of the films, and morphological changes did not occur up to 400°C.
However, the size of the boron particles became smaller above
500°C possibly due to the accelerated interdiffusion between the
boron particles and the Ni–B alloy matrix. Figure 7 shows cross-
sectional SEM images of the Ni–34.3 atom % B alloy composite
film following heat-treatments at various temperatures. No cracks
are observed in any of the films, and no changes in the microstruc-
ture are seen up to 400°C. Above 500°C, the number of boron
particles appears to decrease. Figure 8 shows magnified images of
Table I. Compositions of various plating baths.
Chemicals
Ni–B alloy bath
共M兲 Ni–B composite bath Ni–B alloy composite bath
NiSO
4
·6H
2
O 1 1M 1M
NiCl
2
·6H
2
O 0.2 0.2 M 0.2 M
H
3
BO
3
0.5 0.5 M 0.5 M
Trimethylamine borane 0.1 0.1 M
Saccharin sodium dehydrate 0.1 M
2-Butyne-1,4-diol 2.5 ⫻ 10
−3
M
Boron particle 100 g dm
−3
30–100 g dm
−3
Figure 1. Relationship between the boron particle concentration in the plat-
ing bath and the boron content in the electrodeposited film. The current
density is 1 A dm
−2
.
Figure 2. Surface and cross-sectional
SEM images of electrodeposited films
with various boron contents. 共a兲–共c兲 show
surface SEM images of the Ni–6.4 atom
% B alloy film, the Ni–17.1 atom % B
alloy composite film, and the Ni–22.5
atom % B alloy composite film, respec-
tively. 共d兲–共f兲 show the cross-sectional
SEM images of 共a兲–共c兲, respectively.
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the regions shown in Fig. 7. Again, no change in the microstructure
is seen up to 400°C. However, above 500°C, the density of boron
particles begins to decrease, and many voids start to appear. These
voids are most likely the result of the Kirkendall effect due to the
enhanced interdiffusion between the boron particles and the Ni–B
alloy matrix. The diffusion velocity of boron from the boron par-
ticles to the Ni–B alloy matrix may be higher than that of nickel
from the Ni–B alloy matrix to the boron particles, leading to void
formation.
To clarify the interdiffusion behavior of the boron particles and
the Ni–B alloy matrix, EPMA analysis was carried out. Figure 9
shows EPMA elemental mappings of cross sections of the Ni–34.3
atom % B alloy composite films following heat-treatment at differ-
ent temperatures. Before heat-treatment, it is evident from the boron
mapping that boron particles undoubtedly exist in the film. No ob-
vious difference in the boron distribution is seen following heat-
treatment at 400°C. However, the amount of boron particles obvi-
ously decreases at 500°C, and a relatively homogeneous boron
distribution can be seen at 600°C. These results strongly suggest
that the interdiffusion of boron and nickel between the boron par-
ticles and the Ni–B alloy matrix is highly accelerated over 500°C. It
is also evident that a Ni–B alloy film with a homogeneous micro-
structure can be formed from a Ni–B alloy composite film by heat-
treatment.
Figure 10 shows XRD patterns of the Ni–34.3 atom % B alloy
composite film following heat-treatment at various temperatures.
The broad peak previously assigned to nickel becomes sharper at
200°C, and the sharp peaks assigned to nickel and Ni
3
B appear at
300 and 400°C. Using differential scanning calorimetry and XRD,
Lee et al. determined that the phase transition in Ni–B alloy films
from a metastable amorphous phase to a stable crystalline phase,
i.e., the precipitation of the Ni
3
B phase, occurred at different tem-
peratures depending on the boron content in the films, and precipi-
Figure 3. Effect of current density on the boron content in films electrode-
posited in different plating baths. White circles with short dashed lines: Ni–B
alloy plating bath; double circles with dashed lines: Ni–B composite plating
bath; and black circles with solid lines: Ni–B alloy composite plating bath
共the boron particle concentration is 100 g dm
−3
兲.
Figure 4. Surface and cross-sectional
SEM images of films electrodeposited un-
der various current densities. 共a兲–共c兲 show
the surface SEM images of the Ni–16.6
atom % B alloy composite film, the Ni–
22.5 atom % B alloy composite film, and
the Ni–34.3 atom % B alloy composite
film, respectively. 共d兲–共f兲 show the cross-
sectional SEM images of 共a兲–共c兲, respec-
tively.
Figure 5. XRD patterns of 共a兲 the Ni–16.6 atom % B alloy composite film,
共b兲 the Ni–22.5 atom % B alloy composite film, and 共c兲 the Ni–34.3 atom %
B alloy composite film.
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tation of the Ni
3
B phase was observed at 300°C for boron contents
above 9 atom %.
19
Because the boron content in the Ni–B alloy
matrix of the Ni–34.3 atom % B alloy composite film is considered
to be at least more than 9 atom % in Fig. 3-5, the appearance of the
peaks assigned to the Ni
3
B phase at 300 and 400°C is almost cer-
tainly due to the phase transition of the Ni–B alloy matrix. On the
contrary, above 500°C, the only sharp peaks observed are those
assigned to Ni
3
B and Ni
2
B. Although these peaks often overlap and
make the assignment difficult, especially for Ni
2
B, clear peaks due
to Ni
2
B do appear at around 25 and 36°. According to the Ni–B
Figure 6. Surface SEM images of the Ni–
34.3 atom % B alloy composite film heat-
treated at various temperatures: 共a兲 Before
heat-treatment, 共b兲 200, 共c兲 300, 共d兲 400,
共e兲 500, and 共f兲 600°C.
Figure 7. Cross-sectional SEM images of
the Ni–34.3 atom % B alloy composite
film heat-treated under various tempera-
tures: 共a兲 Before heat-treatment, 共b兲 200,
共c兲 300, 共d兲 400, 共e兲 500, and 共f兲 600°C.
Figure 8. Enlarged images of the regions
represented in Fig. 7: 共a兲 Before heat-
treatment, 共b兲 200, 共c兲 300, 共d兲 400, 共e兲
500, and 共f兲 600°C.
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binary alloy phase diagram 共Fig. 11兲,
23
at temperatures above
700°C, the Ni
2
B and o-Ni
4
B
3
phases are the stable phases for boron
content from 33.3 to 41.4 atom %, whereas the Ni
3
B and Ni
2
B
phases are the stable phases for boron content from 25.0 to 33.3
atom %. If the stable phases at room temperature are assumed to be
the same as those above 700°C, the Ni–34.3 atom % B alloy com-
posite film heat-treated at 600°C would be expected to consist of the
Ni
2
B and o-Ni
4
B
3
phases. However, as shown in Fig. 10, it is actu-
ally composed of the Ni
3
B and Ni
2
B phases. One possible explana-
tion may be that the interdiffusion between the boron particles and
the Ni–B alloy matrix does not reach completion, and the composi-
tion of the Ni–B alloy matrix does not exceed 33.3 atom %. A few
boron particles remain in the film, as seen in Fig. 9. Furthermore,
some of the boron may have diffused into the iron substrate, causing
a decrease in the boron content, resulting in the Ni
3
B and Ni
2
B
phases.
Figure 12 shows the effects of the heat-treatment temperature on
the hardness of the Ni–34.3 atom % B alloy composite film. The
result for the Ni–6.4 atom % B alloy film is also shown for com-
parison. The hardness increased with increasing heat-treatment tem-
perature, reaching a maximum value of 1250 HV at 300°C before
decreasing again, and was higher than that of the Ni–6.4 atom % B
alloy film over the full temperature range studied. For temperatures
up to 200°C, the main contribution to the increased hardness of the
Ni–B alloy composite film may be the dispersion hardening of boron
particles. Between 300 and 400°C, both the dispersion hardening of
boron particles and the precipitation hardening of the Ni
3
B phase are
likely to be the main causes of the higher hardness. The hardness of
the Ni–6.4 atom % B alloy film rapidly decreases with increasing
heat-treatment temperature over 300°C, which is almost the same
result reported by Lee et al.
19
The strength 共or hardness兲 of poly-
crystalline materials is expected to increase with decreasing grain
Figure 9. EPMA mapping analysis of
cross sections of the Ni–34.3 atom % B
alloy composite film heat-treated at vari-
ous temperatures.
Figure 10. XRD patterns of the Ni–34.3 atom % B alloy composite film
heat-treated at various temperatures: 共a兲 Before heat-treatment, 共b兲 200, 共c兲
300, 共d兲 400, 共e兲 500, and 共f兲 600°C.
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