Electrocodeposition of nickel nanocomposites using an impinging jet electrode

Article abstract of DOI:10.1149/1.2767415

The electrocodeposition of nickel alumina nanocomposites was investigated using an impinging jet electrode. The effects of jet flow rate, particle loading, and current density on the particle incorporation were studied. The amount of codeposited particles was determined using both electrogravimetric measurements and energy dispersive X-ray analysis. A maximum particle incorporation of about 5 wt % was found for a flow rate of 2.5 L min-1 and a current density of 10 A dm-2. The microstructure of the coatings was investigated via X-ray diffraction. As a result of increasing current density and particle incorporation, a loss of (100) texture and a relative enhancement of the (111), (220), and (311) reflections appeared. The microhardness of the nickel films increased significantly with the inclusion of alumina nanoparticles.

Full text of DOI:10.1149/1.2767415

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Journal of The Electrochemical Society, 154 ͑10͒ D510-D515 ͑2007͒
0013-4651/2007/154͑10͒/D510/6/$20.00 © The Electrochemical Society
Electrocodeposition of Nickel Nanocomposites Using an
Impinging Jet Electrode
*
a,**,z
Denny Thiemig,a, Andreas Bund,
b,***
and Jan B. Talbot
aDepartment of Physical Chemistry, Dresden University of Technology, D-01062 Dresden, Germany
bChemical Engineering Program, University of California, San Diego, La Jolla, California 92093-0411, USA
The electrocodeposition of nickel alumina nanocomposites was investigated using an impinging jet electrode. The effects of jet
flow rate, particle loading, and current density on the particle incorporation were studied. The amount of codeposited particles was
determined using both electrogravimetric measurements and energy dispersive X-ray analysis. A maximum particle incorporation
of about 5 wt % was found for a flow rate of 2.5 L min−1 and a current density of 10 A dm−2. The microstructure of the coatings
was investigated via X-ray diffraction. As a result of increasing current density and particle incorporation, a loss of ͑100͒ texture
and a relative enhancement of the ͑111͒, ͑220͒, and ͑311͒ reflections appeared. The microhardness of the nickel films increased
significantly with the inclusion of alumina nanoparticles.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2767415͔ All rights reserved.
Manuscript submitted February 6, 2007; revised manuscript received June 6, 2007. Available electronically August 9, 2007.
Metal matrix nanocomposite films can be cost-effectively pre-
Experimental
pared by means of electrocodeposition.1 The inert, nanosized par-
ticles, ͑e.g., ceramics or organic materials͒ are suspended in a con-
ventional plating bath and are subsequently embedded into the
growing metal film. The quantity and distribution of incorporated
particles depends on a variety of interactive variables, including
particle characteristics ͑type, size, shape, and concentration͒, elec-
trolyte composition ͑concentrations, additives, surfactants͒, operat-
ing conditions ͑hydrodynamics, ac or dc current, current density,
pH, temperature͒ and particle-bath and particle-electrode inter-
actions.2-4
Unsubmerged impinging jet cell.— Figure 1 shows a schematic
diagram of the jet electroplating system, which was developed by
Osborne.15 The electroplating cell was directly placed over the 8 L
rectangular solution tank. An electric diaphragm pump ͑Jabsco
model 31801-0115͒ with an 8 L min−1 capacity was used to circu-
late the electrolyte. The flow rate was accurately adjusted with two
ball valves that divided the flow into a main and a bypass stream.
Flow rate was measured with a digital paddle flowmeter ͑Blue-
White Industries, F-1000-RB͒. The particles were kept in suspension
using a laboratory stirrer ͑Fisher Scientific, model 47͒ with a paddle
impeller. A 1.1 cm inner diameter stainless steel tube served as an-
ode and nozzle. The chemical analyses of the plated films did not
show any traces of iron. Therefore, we conclude that the stainless-
steel anode is passivated and does not dissolve under our experimen-
tal conditions. The nozzle-substrate gap distance was 0.65 cm and
the angle at which the jet impinges onto the surface was 90°. The
electrolyte was continuously recirculated to the stirring bath. Further
details concerning the geometry of the IJE system can be found
elsewhere.15,18
Bath agitation is usually necessary to maintain a dispersed sus-
pension and to transport the particles to the cathode surface. Most
studies on electrocodeposition have focused on a parallel plate elec-
trode configuration because of its simplicity. However, due to the
various ways of agitating the suspension, an analysis and compari-
son of hydrodynamics is almost impossible with this configuration.
The control of hydrodynamics can be achieved by using a rotating
5-7
cylinder ͑RCE͒ or rotating disk electrode ͑RDE͒.6,8,9 However,
both the RCE and the RDE configurations are not typically a viable
industrial method, because of their limitation of specific cathode
shapes. Another promising way to control hydrodynamics is the im-
pinging jet electrode ͑IJE͒ configuration.10 The IJE provides the ad-
vantages of selective11 and high-speed plating.12 Furthermore, it is
an attractive method to electrodeposit gradient coatings while con-
trolling the volume fraction of particles by changing the jet
velocity.13 As long as the nozzle is placed close to the substrate, the
electrodeposition is hydrodynamically limited to an area propor-
tional to the width of the nozzle.14 It is important to distinguish
between submerged and unsubmerged jet systems. The main benefit
of the unsubmerged system is to avoid the entrainment of the bulk
fluid found in the submerged configuration.15
Deposition experiments.— Electrocodeposition experiments
were conducted at room temperature with an acidic sulfamate elec-
trolyte and the working conditions as listed in Table I. Suspensions
were prepared by adding a specified mass of 50 nm ␥-Al2O3 par-
ticles ͑Buehler Micropolish II͒ to 4 L of the electrolyte. As dis-
cussed in a previous paper,5 the particles are spherically shaped with
a size in the range of 0.03–0.06 ␮m. The mixture was covered and
stirred using a magnetic stirrer for 12–24 h. The pH of the suspen-
sions was measured before and after each experiment and if neces-
sary adjusted to pH 4.3 with either NaHCO3 or H2SO4.
A piece of copper shim stock ͑2 cm ϫ 2 cm ϫ 0.15 cm͒ was
employed as a substrate. Prior to electroplating, a substrate was
placed in an isopropanol bath and ultrasonicated for 20 min. The
There is little research on the electrocodeposition of nickel-based
composites plated with an IJE.13,16,17 Also, no systematic investiga-
tion of the effects of the plating parameters on the codeposition of
nanosized alumina particles and nickel has been reported to our
knowledge.
In the present study, the electrolytic codeposition of alumina
nanoparticles and nickel from a sulfamate bath was investigated
using an unsubmerged impinging jet cell. The amount and distribu-
tion of embedded particles in the nickel matrix were examined as
well as the microstructure of the nanocomposites. The hardness was
investigated with regard to the particle content of the layer.
*
**
Electrochemical Society Student Member.
Electrochemical Society Active Member.
Electrochemical Society Fellow.
***
z E-mail: andreas.bund@chemie.tu-dresden.de
Figure 1. Unsubmerged impinging jet electrode system.15
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Journal of The Electrochemical Society, 154 ͑10͒ D510-D515 ͑2007͒
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Table I. Bath composition and working conditions.
Table II. Percent cathode current efficiency of pure nickel elec-
trodeposition from a sulfamate bath for various IJE flow rates
Composition and working conditions
c.d./A dm−2
1 L min−1
2.5 L min−1
6.5 L min−1
Ni͑NH2SO3͒
350 g L−1
10 g L−1
40 g L−1
0–120 g L−1
4.3
2
1
5
90
98
99
98
97
–
89
97
98
98
99
–
87
96
97
98
99
97
96
NiCl2·6H2O
H3BO3
10
15
20
25
30
Particles in suspension
pH
Temperature
Flow rate
Curent density
Charge
25 °C
1–7 L min−1
1–30 A dm−2
100 C
–
–
to about 97–99%. Within this current density range, the activation
overvoltage of hydrogen is much higher than that of nickel. At high
current densities, the diffusion of nickel ions to the cathode becomes
the controlling step and the cathode efficiency decreases with in-
creasing current density.21 It has been shown that the presence of
alumina particles in the bath does not affect the deposition rate of
nickel, because the particle loading is low.22 The current efficiency
is lower at higher flow rates at current densities up to 20 A dm−2
͑Table II͒. The amount of nickel ions supplied at the electrode can
be adjusted by varying the flow rate. The current density at which
the reaction reaches mass transfer limitations increases with the flow
rate.17 However, in the case of nickel sulfamate the efficiency of
nickel deposition is lower at a low percentage of the limiting current
density.21
plating area was 1 cm diameter circle cut into electroplaters mask-
ing tape ͑3M͒ which covered the rest of the substrate area.
The codeposition was carried out galvanostatically with a gal-
vanostat ͑model 273, EG&G Princeton Applied Research͒ from
electrolytes containing 0–120 g L−1 Al2O3 with a current density
between 5 and 30 A dm−2 for a total charge of about 100 C. The
corresponding film thickness was approximately 40 ␮m. After elec-
trolysis, the samples were ultrasonically cleaned in water for 10 min
to remove loosely adherent particles from the surface.
Determination of particle incorporation.— The amount of code-
posited alumina was determined by the electrogravimetric method.5
The efficiency of pure nickel deposition using the sulfamate bath
was determined within the investigated current density range. The
theoretical mass of deposited nickel was calculated from Faraday’s
law and subsequently compared to the actual plated nickel mass.
The current efficiency was in the range of 87–99%. Hence, the
amount of nickel electrodeposited during codeposition was calcu-
lated from the total charge passed, taking into account the current
efficiency of pure nickel plating. The typical weight of the codepos-
ited films was 0.1 g, resulting in an accuracy in the particle incor-
poration of 0.3 wt %. The amount of codeposited alumina was
determined by subtracting the amount of nickel determined from
coulometry from the actual weight gain.
The amount of codeposited alumina calculated by the electro-
gravimetric method was validated by energy dispersive X-ray analy-
sis ͑EDX͒ performed with an FEI Quanta 600. Three randomly cho-
sen points, each covering an area of 655 ␮m2, were analyzed and an
average value was calculated. The accuracy of EDX analysis was
approximately 0.2 wt %. The alumina concentration was evaluated
from the aluminum signal, taking into consideration the stoichio-
metric ratio of oxygen to aluminum in Al2O3.
Particle loading.— In Fig. 2 the effect of the amount of sus-
pended alumina particles from 10 to 120 g L−1 on the codeposition
with nickel is given at a current density of 10 A dm−2 for three
different flow rates. The maximum possible loading in these studies
with this IJE system was 120 g L−1. Beyond this concentration,
sedimentation of particles occurred quickly. As expected, an in-
crease in incorporation of particles with an increase in the loading in
suspension was found.2 A particle incorporation of ϳ1 to 3.5 wt %
was measured even with a particle concentration in suspension as
low as 10 g L−1. Maximum particle incorporation was about 5 wt %
at a flow rate of 2.5 L min−1. In comparison, in the case of a parallel
plate electrode system the maximum incorporation of 13 nm Al2O3
within a nickel matrix deposited from an acidic sulfamate electrolyte
at a current density of 1 A dm−2 was only 1.6 wt %.23,24
Flow rate.— The effect of the flow rate on the amount of code-
posited alumina nanoparticles is shown in Fig. 3 for high loadings of
90 and 120 g L−1 at 10 A dm−2. The flow rate was varied from
1 to 6.5 L min−1. A maximum incorporation of 5.2 wt % Al2O3 was
Deposit analysis.— Scanning electron micrographs ͑SEM͒ were
taken with an FEI Quanta 600 to investigate the surface morphology
and the distribution of particles within the metallic matrix. The hard-
ness of samples was measured with a Vickers microhardness mea-
suring device ͑FischerScope HM2000 S͒. Ten hardness values were
measured for each sample and an average is reported with an accu-
racy of approximately 10%. X-ray diffraction ͑XRD, Siemens
D5000͒ technique was used to study the preferred orientation of the
pure nickel and nickel alumina composite films. The scan rate was
0.12° min−1 over the two-theta range from 10 to 100°. The Debye-
Scherrer formula method using ͑200͒ and ͑111͒ reflections was em-
ployed for estimating the crystallite size of nickel films.19
Results and Discussion
Current efficiency.— The electrochemical reactions occurring
during nickel deposition include hydrogen evolution as a side
reaction.20 However, the efficiency of nickel deposition should be
high in pH 4.3 sulfamate bath.20,21 The cathodic current efficiencies
for the electrodeposition of pure nickel from the nickel sulfamate
bath used are listed in Table II for various current densities and flow
rates for the IJE system with an accuracy of about 0.8%. For all
flow rates, the cathode efficiency increases with the current density
Figure 2. Effect of particle loading of the electrolyte on the particle incor-
poration in nickel at a current density of 10 A dm−2. ͑᭿͒ 1 L min−1, ͑b͒
2.5 L min−1, ͑᭡͒ 6.5 L min−1
.
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Journal of The Electrochemical Society, 154 ͑10͒ D510-D515 ͑2007͒
Figure 3. Correlation between the Al2O3 content in the layer and the flow
Figure 4. Influence of current density on the incorporation of alumina par-
ticles in nickel at a particle loading of 90 g L−1 Al2O3. ͑᭿͒ 1 L min−1, ͑b͒
rate for different particle loadings of the electrolyte at a current density of
10 A dm−2. ͑᭿͒ 90 g L−1, ͑b͒ 120 g L−1
.
2.5 L min−1, ͑᭡͒ 6.5 L min−1
.
observed at a flow rate of 2.5 L min−1 as shown in Fig. 3. Regard-
less of the alumina concentration in the electrolyte, a maximum
particle incorporation was found in the range of 2–3 L min−1. As
mentioned above, the amount of incorporated alumina within a
nickel film is about three times smaller when using a parallel plate
electrode system.24 These results are quite similar to those reported
by Osborne et al. for the codeposition of 50 nm ␥-alumina particles
in copper.18 Increasing the flow rate increases the quantity of par-
ticles arriving at the electrode, but reduces the residence time of the
particles at the electrode surface. The interaction between these ef-
fects influences the process of particle incorporation. As a result, a
decrease of the residence time of the particles at the electrode leads
to an inhibition of particle codeposition. Interestingly, the particle
incorporation increases at a flow rate of 7.5 L min−1. Higher levels
of shear force acting on the electrode surface lead to a decrease in
particle incorporation.5,6,25 However, within the investigated flow
region shear force might not be the most important factor in deter-
mining particle codeposition. Instead, the increase in particle incor-
poration might be a result of the interaction between flow rate and
limiting current density.12,26 Further investigations are needed with a
higher capacity pump in order to determine the fluid flow range
where hydrodynamic shear force effectively starts to affect the par-
ticle incorporation.
density was increased from 5 to 10 A dm−2 the particle incorpora-
tion increased significantly to maximum value between
a
5–15 A dm−2 and then decreased for all flow rates. This behavior
has been observed previously with both RDE and RCE systems,5,6,27
and is related to the balance of particle adsorption or entrapment vs
metal deposition rate. A similar dependence of the particle incorpo-
ration on the plating current density appeared at all other particle
contents of the electrolyte.
Interactions between current density and hydrodynamics can be
accounted for by normalizing the current density with the limiting
current density, the maximal possible current density under mass
transfer conditions.5 The relationship between the limiting current
density and impinging jet velocity was discussed in Ref. 12 and 26
as
−0.09
nFCϱD
d
H
ͩ ͪ
d
iL = 0.9
Re1/2Sc1/3
͓1͔
where n, F, Cϱ, D, H, and d are, respectively, the number of elec-
trons transferred in the electrochemical reaction, Faraday’s constant,
the bulk concentration, and the diffusivity of the metal ion in solu-
tion, as well as the nozzle height and diameter. Sc is the Schmidt
number ͑Sc = ␷/D͒ and Re ͓Re = ͑d/␷͔͒ is the nozzle Reynolds
v0
number, where
is the jet velocity and ␷ the viscosity of the elec-
vo
Current density.— The current density is known to affect the
particle content in composite films regardless of the codeposition
system; however, the effects can be quite contradictory depending
on the system.2 As current density is one of the most influential
process variables, it is desirable that the current distribution is as
uniform as possible. Otherwise, variations in particle codeposition
across the width and depth of the film may occur. Jet electroplating
allows varying of the current density over a wide range and to very
high values as compared to conventional plating systems.16 How-
ever, the current distribution varies as a function of the radial posi-
tion on the electrode. According to Alkire and Chen11 there is a
region of “uniform accessibility” in the impingement zone of the
unsubmerged impinging jet electrode system, where the concentra-
tion of the depositing metal ions, as well as the current distribution,
are uniform. Hence, masking off the region outside of the impinge-
ment zone ensures that plating occurs at a fairly constant rate over
the substrate area.
trolyte. Because the Reynolds number is directly proportional to the
velocity at the nozzle exit, the limiting current density is related to
the square root of the fluid velocity ͑Eq. 1͒. The velocity was cal-
culated from the volumetric flow rate divided by the cross-sectional
area of the nozzle and ranged from ϳ18 to ϳ 116 cm s−1. The par-
ticle incorporation vs the current density, normalized by the square
root of the jet velocity, is shown in Fig. 5 at a particle loading of
90 g L−1. Particle incorporation was found to initially increase with
the normalized current density from approximately 1000 to
2000 A s m−5/2. A further increase of the normalized current density
leads to a decrease in codeposition, and finally a relatively constant
particle incorporation is reached. This behavior is similar to those
observed for the codeposition of Cu–Al2O3 composites using an
RCE.5
Particle incorporation validation.— In order to validate the in-
corporation results of the electrogravimetric measurements, EDX
was performed on the surface of the coatings. The comparison of
both methods used to determine the alumina content in the nano-
composites is shown in Fig. 6. Although the EDX analysis shows a
significantly higher weight content of alumina in the composites, the
overall trend with flow rate is similar. This difference can be ex-
plained by the size differences of the analyzed area. In the case of
To determine the effect of the current density on the particle
incorporation, depositions were performed at three different flow
rates with an alumina concentration of 10, 30, 60, 90, and 120 g L−1
in the electrolyte as the current density was varied from
5 to 30 A dm−2. The particle contents of the coatings plated with
suspension loading of 90 g L−1 are shown in Fig. 4. As the current
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Journal of The Electrochemical Society, 154 ͑10͒ D510-D515 ͑2007͒
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Figure 7. SEM surface morphology of
a pure Ni film and a
Ni–Al2O3-composite coating plated at 10 A dm−2 and 6.5 L min−1. ͑a͒
0 g L−1, ͑b͒ 120 g L−1 Al2O3.
tribution within the composite film. Others have observed that par-
ticularly in the first micrometer of the coating exclusively metal is
deposited.28,29
Figure 5. Particle incorporation vs normalized current density at a particle
loading of 90 g L−1 Al2O3. ͑᭿͒ 10 A dm2, ͑b͒ 15 A dm2, ͑᭡͒ 20 A dm2.
Microscopy.— The effects of particle incorporation, current den-
sity, and flow rate on the surface morphology of the coatings were
studied using SEM. The surface of a nickel deposit from the sulfa-
mate bath is made up of truncated pyramidal crystals.30 Due to the
particle incorporation and the decrease of the current density, a re-
finement of the surface morphology appeared as shown in Fig. 7 for
a composite film deposited at a current density of 10 A dm−2, a flow
rate of 6.5 L min−1, and a particle loading of 120 g L−1. The surface
morphology of the composite coating is more refined and uniform
than that of a pure nickel coating. A change in the flow rate had
almost no influence on the surface morphology of the nickel coat-
ings ͑micrographs not shown͒.
EDX the area of analysis is limited to approximately 625 ␮m2 on
the surface of the coating. In comparison, the electrogravimetric
method accounted for the entire composite film. Another possible
explanation for the observed discrepancy is an uneven particle dis-
In order to study the distribution of the embedded nanoparticles
within the metallic matrix, backscattered electron images ͑BSE͒
were taken on the coatings. Figure 8 shows the BSE image of a
composite film deposited at 10 A dm−2 and 6.5 l min−1 with
120 g L−1 Al2O3 particles in suspension. The alumina particles ap-
pear as dark spots in the lighter nickel matrix, as indicated by EDX
spectroscopy. The particles appear to be incorporated individually as
well as agglomerates. By varying the working parameters, such as
current density, flow rate, or particle content of the electrolyte, no
characteristic change of the deposit quality and dispersion of larger
agglomerates was observed.
Microstructural characterization.— The XRD patterns of Ni
films and Ni–Al2O3 composites are shown in Fig. 9a and b, respec-
tively. The nickel matrix exhibited a face-centered cubic ͑fcc͒ lattice
with different preferred orientations which were mainly influenced
by the plating current density and the incorporation of alumina
nanoparticles. The XRD patterns of pure nickel deposits from the
sulfamate bath revealed
a strong ͑100͒ preferred crystalline
orientation31 detected by the intensive reflection ͑200͒, as presented
in Fig. 9a. However, the quality of ͑100͒ texture alters with variation
of the working parameters. Increasing current density leads to a loss
of texture that could be observed in the reduction of the line ͑200͒
Figure 6. Comparison of the particle incorporation determined by different
methods at a current density of 10 A dm−2 and a particle loading of ͑a͒
90 g L−1, ͑b͒ 120 g L−1. ͑᭿͒ Electrogravimetric measurement, ͑b͒ EDX
analysis.
Figure 8. Backscatter electron image of a composite film deposited at
10 A dm−2 and 6.5 L min−1 with 120 g L−1 Al2O3 particles in suspension.
͑a͒ 10,000-times magnification, ͑b͒ 40,000-times magnification.
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Journal of The Electrochemical Society, 154 ͑10͒ D510-D515 ͑2007͒
Figure 10. Vickers microhardness vs concentration of Al2O3 in the electro-
lyte at a current density of 10 A dm−2, ͑᭿͒ 1 L min−1, ͑b͒ 2.5 L min−1, ͑᭡͒
6.5 L min−1
.
hardness, depending on the flow rate of the plating bath. However,
there is no clear correlation between the microhardness and the ap-
plied current density and flow rate.
Figure 10 illustrates the dependency of deposit hardness on the
content of alumina nanoparticles in the plating bath. An increase of
the hardness of nickel composite coatings is due to the high hard-
ness and strength of the particles. Thus, the nanoparticles will inhibit
the plastic flow of the relatively soft nickel matrix.35 Pure nickel
films plated from an acidic sulfamate bath exhibited a well-defined
columnar structure belonging to the field-oriented texture type.24,36
Due to the particle incorporation, the microstructure of the nickel
film changed to the unoriented dispersion type and a grain refine-
ment occurred.24,37 Thus, the increased hardness of the composites
can be explained by means of the particle incorporation as well as
by the change of the grain size and structure of the nickel
coating.24,37
Figure 9. X-ray diffraction pattern of deposits obtained at a current density
of 10 A dm−2 and a flow rate of 2.5 L min−1. ͑a͒ Pure nickel film, ͑b͒ com-
posite film plated from electrolyte containing 120 g L−1 Al2O3.
Conclusion
and in the relative enhancement of the ͑111͒, ͑220͒, and ͑311͒ re-
flections. A relatively strong ͑111͒ texture has been found to be
associated with larger grains.31 The increase of grain size with the
current density also has been observed in the SEM micrographs of
the surface morphology of pure nickel films.32
In the case of the Ni–Al2O3 nanocomposites a mixed ͑100͒ and
͑111͒ texture is apparent in Fig. 9b. Furthermore, the incorporation
of alumina nanoparticles leads to a relative enhancement of the
͑111͒, ͑220͒, and ͑311͒ lines.
The crystallite size of the nickel deposits can be estimated by the
Scherrer line broadening.19 As a result of the particle codeposition
the width of the reflections increased, which means a decrease in the
crystallite size of the nickel matrix appeared ͑Fig. 9a and b͒. The
crystallite size of a pure nickel film deposited at a flow rate of
2.5 L min−1 and a current density of 10 A dm−2 was about 115 nm.
In comparison, a nickel alumina composite plated under the same
working conditions from an electrolyte containing 120 g L−1
showed a nickel crystallite size of about 30 nm.
An unsubmerged IJE system was used to investigate the electro-
lytic codeposition of Ni–Al2O3 nanocomposites. It has been shown
that 50 nm ␥-alumina particles codeposit with nickel in amounts as
large as 5.2 wt %. The effect of current density, flow rate, and par-
ticle content in the bath on the structure and properties of these
coatings was studied. The amount of codeposited alumina particles
was determined by means of the electrogravimetric method and vali-
dated by EDX analysis. Due to the incorporation of alumina nano-
particles the microhardness of the composites increased. The pure
nickel deposits showed a strong ͑100͒ texture. Due to the particle
incorporation, a decrease in the crystallite size and a loss of texture
was found.
Acknowledgments
This work was financially supported by the “Deutsche For-
schungsgemeinschaft” ͑grant no. BU 1200/10-1͒. D.T. thanks Pro-
fessor J. B. Talbot for the opportunity to perform this study at Uni-
versity of California San Diego, and “Gesellschaft von Freunden
und Förderern der TU Dresden e.V.” for a travel grant.
Microhardness.— Microhardness measurements were performed
for pure nickel and particle reinforced films. The microhardness of
the coatings varied from 210 HV for pure nickel to 580 HV for
composite films, which is in good agreement with the literature.33
The hardness of metal films and composite coatings is related to the
microstructure and the quantity and distribution of the reinforcement
phase. Furthermore, it has been shown that nickel deposits with a
͑100͒ texture show a maximum ductility and a minimum of hardness
and internal stress.34 In the case of the pure nickel films, an increase
in current density leads to a decrease or increase of the coating
Dresden University of Technology assisted in meeting the publication
costs of this article.
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