Stress measurements in nanocrystalline Ni electrodeposits

Article abstract of DOI:10.1016/j.jallcom.2004.08.010

X-ray diffraction method was used to assess the internal stresses in nanocrystalline nickel coatings electrodeposited onto various substrates. In addition, internal stresses in nanocrystalline nickel deposits without substrates were also investigated using the same technique. The preferred orientation of nanocrystalline nickel deposits was found unaffected by substrate type and bath addition. All deposits showed that compressive internal stresses are present. The substrate type was found to affect the magnitude of internal stresses in these deposits. Furthermore, compared with microcrystalline coatings, the nanoprocessed deposits contained higher compressive stresses. The addition of small amounts of sodium lauryl sulphonate to a saccharin-containing Watts' bath was found to decrease the internal stresses in deposits. It is concluded that nanoprocessed Ni-coatings are strong contenders for applications requiring high fatique strength of the underlying substrate metal.

Full text of DOI:10.1016/j.jallcom.2004.08.010

Journal of Alloys and Compounds 389 (2005) 140–143
Stress measurements in nanocrystalline Ni electrodeposits
A.M. El-Sherika, J. Shirokoffb,∗, U. Erbc
a
Dofasco Inc., P.O. Box 2460, Hamilton, Ontario, Canada L8N 3J5
b
Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X5
c
Department of Metallurgy and Materials Science, University of Toronto, Toronto, Ontario, Canada M5S 3E4
Received 29 July 2004; accepted 6 August 2004
Abstract
X-ray diffraction method was used to assess the internal stresses in nanocrystalline nickel coatings electrodeposited onto various substrates.
In addition, internal stresses in nanocrystalline nickel deposits without substrates were also investigated using the same technique. The
preferred orientation of nanocrystalline nickel deposits was found unaffected by substrate type and bath addition. All deposits showed
that compressive internal stresses are present. The substrate type was found to affect the magnitude of internal stresses in these deposits.
Furthermore, compared with microcrystalline coatings, the nanoprocessed deposits contained higher compressive stresses. The addition of
small amounts of sodium lauryl sulphonate to a saccharin-containing Watts’ bath was found to decrease the internal stresses in deposits. It is
concluded that nanoprocessed Ni-coatings are strong contenders for applications requiring high fatique strength of the underlying substrate
metal.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline coatings; Electrodeposition; X-ray diffraction
1. Introduction
differences in thermal coefficient of expansion at the inter-
face between the substrate and coating, and (3) residual or
intrinsic stress that results from particular plating conditions
and bath composition. It is far more difficult to account for
intrinsic stress, and no single universally accepted theory has
been put forward. Many theories have been proposed such
as the co-deposited hydrogen theory, crystallite-joining the-
ories, dislocation theory, excess energy theory, Kushner the-
ory, and Gabe and West theory [3].
Residual stress has been measured in polycrystalline
nickel [4], nanocrystalline nickel [5], nanocrystalline metals
[6,7], nanocrystalline metal alloys [8,9] and nanocompos-
ites [10] using various methods. For electrodeposited poly-
crystalline nickel the residual stress has been measured to
be −130 MPa (compression) by X-ray diffraction (sin2Ψ
method) [4]. Nanocrystalline nickel (9–25 nm) produced by
DC magnetron sputtering under varied conditions (up to
−100 V bias) was measured at 500–1000 MPa residual stress
(tension) determined by a method involving the curvature
change due to deposition in a Si-wafer cantilever beam [5].
DC magnetron sputtered nanocrystalline iron (7.7–16 nm)
The study of internal stress in electrodeposits is of great
technological and fundamental importance. Internal stresses
in coatings can cause adverse affects on some of the most
important protective, mechanical and physical properties of
electrodeposits (e.g., corrosion resistance, a wear, hardness,
adhesion, fatigue strength, and toughness etc.) [1]. It is well
established that tensile stresses in electrodeposits are delete-
rious, and the higher the stress the lower the fatigue strength
of the substrate [2]. However, stress may serve a useful pur-
pose. For example, in the deposition of magnetic films for
use in high speed computers, stress in iron, nickel and cobalt
electrodeposits will bring about preferred directions of easy
magnetization [2]. Three kinds of stresses exist in plated coat-
ings: (1) lattice misfit stresses resulting from distortion due
to differences in lattice parameters at the interface between
the coating and substrate (2) thermal stresses arising due to
∗
Corresponding author. Tel.: +1-709-737-2601; fax: +1-709-737-4042.
E-mail address: shirokof@engr.mun.ca (J. Shirokoff).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.08.010

A.M. El-Sherik et al. / Journal of Alloys and Compounds 389 (2005) 140–143
141
had a residual stress (tension) reported as 600 MPa which be-
came increasing compressive for increasing alloy additions
(Ta, Si, Al, and N) up to −900 MPa for composition Fe-
10%TaN using a Flexus 2320 thin-film stress measurement
by way of cantilever beam wafer curvature change [6]. In an-
other study, nanocrystalline Pd (6–17 nm) and Cu (6–34 nm)
deposited by inert gas condensation was found to have a
residual stress (compression) of −20 to −105 MPa for in-
creasing depth below the surface using the sin2Ψ method
[7]. The residual stress in electrodeposited (at high over po-
tential conditions) nanocrystalline (5.5–7 nm) alloys of Fe
(12–15%Ni) was 1400 MPa (tension) measured by a flexible
beam curvature method similar to the others [8]. Electrode-
posited nanocrystalline (3 nm) alloy Ni20%W produced a
stress of up to 2300 MPa using an Instron-type tensile testing
machine on 35 ␮m thick samples [9]. In a study using neu-
tron diffraction to measure residual stress in nanocomposites
of Al2O3/SiC (12–115 nm mean particle size) a mean matrix
stress of 150–200 MPa (tension) and reinforcement stresses
of −1600 to −1750 MPa (compression) was found [10].
Generally speaking, any method capable of producing ma-
terials with ultra fine grains can be employed in the synthesis
of nanocrystalline solids. However, a number of processes
have proven to be more feasible than others in terms of over-
coming engineering barriers to mass production; these meth-
ods include inert gas condensation, ball milling and elec-
troplating [2]. Both direct current and pulsed current plating
havebeensuccessfulinproducingavarietyofnanocrystalline
materials [11–15]. For example, a number of nanocrystalline
metals (Ni, Co), binary alloys (Ni–Fe, Co–W and Zn–Ni)
ternary alloys as well as metal matrix composites have been
produced. For applications as coatings and electrode position
has many advantages over other nanoprocessing techniques
including(1)thepotentiallyverylargenumberofpuremetals,
alloys and composite systems which can be deposited with
grain size less than 100 nm (2) the comparatively low initial
capital investment required to synthesize these materials (3)
high production rates (4) fewer size and shape limitations
and (5) the relatively minor ‘technological barriers’ to be
overcome in transferring this technology from the research
laboratory to existing electroplating and electroforming in-
dustries.
resulted in a decrease in hardness (inverse Hall–Petch) down
to 560kg/mm2 [22]. Microhardness testing of nanoprocessed
nickel coatings having a grain size of 10 nm deposited onto
various substrates showed that substrate type had no effect on
the hardness values on these coatings [23]. Similarly, coat-
ing/substrate adhesion of nanocrystalline nickel coatings was
found unaffected by substrate type, surface finish and coat-
ing thickness [23]. The wear resistance of nanocrystalline
nickel electrodeposits with an average grain size of 10 nm
was greatly enhanced as compared with conventional poly-
crystalline nickel [24]. Potentiodynamic testing of nanocrys-
talline nickel electrodeposits in 2N H2SO4 showed the reg-
ular active–passive-transpassive behaviour that is common
for normal crystalline nickel [25]. Although the overall dis-
solution rates in the passive range were somewhat enhanced
in nanoprocessed material, it was found that nanocrystalline
nickel exhibits superior resistance to localized corrosion [25].
The corrosion resistance of nanocrystalline nickel coatings
in 3.5% NaCl neutral salt spray environment was similar to
conventional polycrystalline nickel coatings with grain size
of 10 ␮m [23].
Given its excellent wear, adhesion and corrosion proper-
ties, nanocrystalline nickel can be a more suitable coating
candidate than conventional nickel. However, for this new
class of coatings to gain industry acceptance, knowledge of
the type and magnitude of residual stresses in these coatings
is of great importance.
The purpose of the present paper is to assess internal
stresses in these novel coatings using X-ray diffraction meth-
ods. Theeffectsofsubstratetype, coatingthickness, grainsize
and bath additions on the internal stress of nanocrystalline
nickel deposits will be discussed. The effects of substrate
type, grain size and bath additions on preferred orientation
of nickel will be presented.
2. Experimental
Nanocrystalline nickel coatings of 99.95% purity were
pulse-plated from a modified Watts’ bath A and B containing
organic additives [23]. The electroplating parameters were
adjusted to produce Ni electrodeposits with a grain size of
10 nm as determined by transmission electron microscopy
(TEM) and X-ray line broadening techniques. In addition,
microcrystalline nickel deposits with a grain size of about
3–5 ␮m were plated from an additive-free Watt’s bath C using
similar pulse plating parameters. Table 1 shows the composi-
tion and plating conditions for baths A, B and C used for the
deposition of nanocrystalline and microcrystalline nickel, re-
spectively. Nanocrystalline coatings with thickness of 25 ␮m
were electrodeposited onto copper, brass and mild steel sub-
strates. A microcrystalline coating with a thickness of 75 ␮m
was initially deposited onto Ti substrates prior to mechani-
cally peeling them off. Table 2 shows the parameters involved
for each coating/substrate system. In all cases, surface prepa-
rationofthesubstratesinvolvedthefollowingsteps:(1)grind-
Over the past years, we have particularly studied the syn-
thesis, structure and properties of nanocrystalline nickel [16].
We have already shown [17–20], that grain refinement of
electroplated nickel into the nanometer range (<100 nm) re-
sults in unique and, in many cases, improved properties as
compared to conventional polycrystalline nickel. For exam-
ple, the hardness of electrodeposited nickel initially increases
linearly far into the nanocrystalline range from the hard-
ness of about 150kg/mm2 for deposits with 100 ␮m grain
size to about 650kg/mm2 at 10 nm [17]. However, starting at
grain sizes less than 30 nm, a clear deviation from the regu-
lar Hall–Petch relationship [21,22] is observed leading to a
plateau in the hardness curve at about 650kg/mm2 for grain
size of 10 nm. Further decrease in grain size down to 6 nm

142
A.M. El-Sherik et al. / Journal of Alloys and Compounds 389 (2005) 140–143
Table 1
done using the slopes m, from these graphs and Eq. (2).
Chemical composition of the plating baths and plating conditions for pro-
duction of nanocrystalline (baths A and B) and microcrystalline (bath C) Ni
electrodeposits
mE
σ =
(2)
d0(1 + υ)
Chemical composition
Bath A
Bath B
Bath C
Values for Young’s modulus E = 28,900 psi were used for
both microcrystalline and nanocrystalline nickel and Pois-
son’s ratio ν = 0.31.
Ni2SO4.7H2O (g/L)
NiCl2.6H2O (g/L)
Boric acid (g/L)
Saccharin (g/L)
Sodium lauryl sulphonate (g/L)
pH
Temperature (◦C)
300
45
45
5.0
0.25
≈2.0
65
300
45
45
5.0
0.25
≈2.0
65
300
45
45
5.0
0.25
≈2.0
65
3. Results and discussion
3.1. Structural analysis
ing of substrates down to 800 grit using SiC emery paper (2)
mechanical polishing down to 0.05 ␮m Al2O3 (3) washing
with soap and cleaning with a jet of distilled water and (4)
degreasing ultrasonically in acetone prior to plating.
XRD patterns of microcrystalline nickel deposited onto
mild steel used in this study previously reported elsewhere
were found to have a strong (2 0 0) fibre texture [15]. This
result is commonly consistent with the texture previously re-
ported for nickel produced from organic-free Watts’ baths
operated under similar conditions [28,29]. XRD patterns of
nanocrystalline nickel coatings of 25 ␮m thicknesses on me-
chanically polished mild steel, copper and brass substrates
all indicated line intensities similar to those found in samples
with random grain orientation distribution with the excep-
tion of the (2 2 0) line which is somewhat reduced in inten-
sity. Also, XRD patterns of nanocrystalline nickel coatings
of 75 ␮m thickness prepared from baths A and B and initially
deposited onto Ti substrates all showed line intensities similar
to those found in samples with random grain orientation dis-
tribution with the exception of the (2 2 0) line which is some-
what reduced in intensity. In all samples, the crystal orienta-
tion of nickel was observed to be unaffected by substrate type.
TEM brightfield, darkfield and selected area (electron)
diffraction patterns of the nickel deposits indicated a uniform
structure with an average grain size of about 10 nm electrode-
posited to a coating thickness of 25 ␮m on mechanically pol-
ished copper substrates [15]. It is also known from density
measurements that these materials have negligible porosity
[31]. Grain size measurements of the same deposit using the
X-ray line broadening technique showed similar results as
those obtained from the TEM analysis.
Brightfield and darkfield transmission electron micro-
graphs were taken for the examination of the structure and
grain size of nanocrystalline deposits. The grain size of the
nanocrystalline electrodeposits was determined directly from
darkfield transmission electron micrographs by measuring
approximately 250 grains. X-ray diffraction patterns for the
investigation of residual stress of the nickel deposits were
˚
obtained using CuKα radiation (λ=1.54184 A) on a diffrac-
tometer with Ψ–tilt and θ–2θ scanning.
The sin2Ψ analysis method was used to measure internal
stress in the nickel electrodeposits [26,27]. The diffraction
data and patterns were generated using a Cu X-ray tube oper-
ating at 45 KV and 30 mA and a monochromator. This anal-
ysis was computer assisted so that the inter-planar spacing
values can be corrected for the instrument error function by
analyzing a silicon standard and subsequent analysis method.
The internal stress in the deposit coatings is related to the
slope of the plot of strain ε = ꢀd/d0 versus sin2Ψ using the
following Eq. (1):
(1 − υ)σsin2ψ
ε =
(1)
E
Where E is Young’s modulus, ν is the Poisson’s ratio and
σ is the stress coefficient.
The d-spacing measurements were conducted on the Ni
(3 1 1) plane at Ψ = 0◦, 10◦, 20◦, and 30◦ (higher angles)
known to improve the accuracy of the stress measurement.
The ꢀd-spacing plotted against sin2Ψ for each of the six
samples Ni-1–Ni-6. Calculation of the residual stress was
3.2. Internal stress analysis
Table 3 presents results of internal stresses in nickel elec-
trodeposits. It is clear that all these nickel deposits have
Table 2
Sample substrates, thickness, lattice mismatch, bath additives and grain size parameters
Sample
Substrate
Coating thickness (␮m)
Lattice mismatch (%)
Bath additives
Grain size (nm)
Ni-1
Ni-2
Ni-3
Ni-4
Ni-5
Ni-6
Copper
Brass
Steel
Steel
N/A
25
25
25
10
75
75
2.51
SA + SLS
SA + SLS
SA + SLS
N/A
SA + SLS
SA
10
10
10
17.20
18.66
18.66
N/A
≈5000
10
N/A
N/A
10
(SA: Saccharin, SLS: Sodium lauryl sulphonate).

A.M. El-Sherik et al. / Journal of Alloys and Compounds 389 (2005) 140–143
143
Table 3
ronment coupled with its enhancement of fatigue strength
of the underlying substrate, manufacturers and end-users of
protective coatings may benefit from nanoprocessed nickel
electrodeposits.
Calculated internal stresses for nanocrystalline and microcrystalline nickel
electrodeposits with and without various substrates as measured by XRD
using Cu K-diffraction from (3 1 1) planes
Sample
Lattice mismatch
(%)
Internal stress
(kg/mm2)
Internal stress
(MPa)
Ni-1
Ni-2
Ni-3
Ni-4
Ni-5
Ni-6
2.51
−46.12
−80.05
−85.85
−14.60
−68.48
−76.17
−452.3
−785.1
−841.9
−143.2
−671.6
−747.0
Acknowledgement
17.20
18.66
18.66
N/A
The financial support of the Natural Sciences and Engi-
neering Research Council of Canada is gratefully acknowl-
edged.
N/A
Table 4
Room temperature coefficient of thermal expansion of the materials studied
in the present work
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literature for nickel deposits prepared from Watts’ baths con-
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saccharin. However, compressive stresses in microcrystalline
nickel deposit (Ni-4) prepared from additive-free Watts’ bath
isincontradictionwithpublishedliteraturefornickeldeposits
produced from similar baths using conventional DC plating.
This may be explained in terms of the effect of pulse plating
on decreasing the hydrogenation of the coatings. It can also be
seen from this table that the internal compressive stresses in
nanoprocessed nickel coatings increase with increasing coat-
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from distortion due to differences in lattice parameters at the
interface between the coating and substrate (Tables 3 and 4).
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A and B, respectively, shows that Ni-5 has lower internal
stresses. This may be attributed to the presence of the surfac-
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4. Conclusions
For the conditions studied in this work, all nickel deposits
contained compressive internal stresses. The internal stresses
in nanoprocessed (10 nm) nickel coatings are affected by sub-
strate type. Furthermore, the nanoprocessed coatings showed
about six times (−841.9 MPa) the internal stress of micro-
crystalline (5000 nm) nickel (−143.2 MPa) with a similar
substrate type. Given the excellent wear resistance, undimin-
ished adhesion and corrosion resistance to salt spray envi-