S. CHERNEVA et al.: INVESTIGATION OF THE MECHANICAL PROPERTIES OF ELECTROCHEMICALLY DEPOSITED ...
689–693
INVESTIGATION OF THE MECHANICAL PROPERTIES OF
ELECTROCHEMICALLY DEPOSITED Au-In ALLOY FILMS
USING NANO-INDENTATION
PREISKAVA MEHANSKIH LASTNOSTI ELEKTROKEMIJSKO
NANE[ENEGA FILMA ZLITINE Au-In Z NANOVTISKOVANJEM
Sabina Cherneva1, Roumen Iankov1, Martin Georgiev2, Tsvetina Dobrovolska2,
Dimitar Stoychev2
1Bulgarian Academy of Sciences, Institute of Mechanics, Acad. G. Bonchev str., Bl. 4, 1113 Sofia, Bulgaria
2Bulgarian Academy of Sciences, Institute of Physical Chemistry, Acad. G. Bonchev str., Bl. 11, 1113 Sofia, Bulgaria
sabina_cherneva@yahoo.com
Prejem rokopisa – received: 2015-06-26; sprejem za objavo – accepted for publication: 2015-09-09
doi:10.17222/mit.2015.129
Thin Au-In alloy films containing different amounts of In were electrochemically deposited on a CuZn substrate with a 500-μm
thickness. The thicknesses of the obtained films varied from 0.4 μm to 2.7 μm. The chemical and phase compositions, as well as
the structures of the films, were investigated by XRF, XRD and SEM analysis. The mechanical properties of the films and
substrates were investigated using nano-indentation experiments. As a result, load–displacement curves were obtained and two
mechanical characteristics of the substrate and investigated films – indentation hardness and indentation modulus – were
calculated using the Oliver & Pharr approximation method. The dependence of the indentation modulus and the indentation
hardness on the depth of the indentation and the content of In, the structure and the phase compositions of the films were
investigated and discussed as well.
Keywords: gold-indium alloy, electrochemical deposition, mechanical properties, nano-indentation
Tanke plasti zlitine Au-In, z razli~no vsebnostjo In, so bile elektrokemijsko nane{ene na podlago iz CuZn, debeline 500 μm.
Debeline dobljene plasti so bile od 0,4 μm to 2,7 μm. Kemijska sestava in sestava faz, kot tudi mikrostruktura plasti, so bile
preiskovane z XRF, XRD in s SEM analizami. Mehanske lastnosti preiskovanih plasti so bile preiskane s pomo~jo preizkusa z
nanovtiskovanjem. Kot rezultat so bile dobljene krivulje obremenitev-raztezek. Dve mehanski lastnosti podlage in preiskovanih
plasti – trdota vtiskovanja in modul vtiskovanja – sta bili izra~unani s pomo~jo Oliver & Pharr metode pribli`ka. Preiskovana in
prediskutirana je bila odvisnost modula vtiskovanja in trdota vtiskovanja na globino vtiska od vsebnosti In.
Klju~ne besede: zlitina zlato-indij, elektrokemijsko nana{anje, mehanske lastnosti, nanovtiskovanje
1 INTRODUCTION
The phase diagram of the gold–indium1 system
shows the presence of several intermetallic compounds
existing at a temperature lower than the melting point of
Indium (~ 156 °C), including stable AuIn and AuIn2 pha-
ses with a cubic lattice, similar to the -phase of gold.
The indium phase starting from a 53 % mass fraction is
tetragonal. The average microhardness obtained for the
metallurgical alloy system Au–In is as follows: Au
(99.999 %) = 0.660,  AuIn (8 % of amount fractions of
In) = 1.700, 1-phase = 3.68, AuIn = 2.73, AuIn2 = 0.77
GPa and the alloy with 80 % of amount fractions of
In = 2.07 GPa.2 The microhardness in the hardened
condition of the compound Au3In2 ( phase) is 1.83 GPa,
and increases in the uniformity of the phase deviations in
the stoichiometric composition.3 In contrast to metallur-
gically obtained, the electrochemically deposited Au-In
alloys are not well studied. At the same time, electroche-
mically deposited thin layers of Au–In alloys will find
wide application (instead of pure gold coatings) in
electrical engineering, micro-electronics, the manufact-
uring of various sensors, the jewelery industry, etc. The
interest in studying the impact of the content of In in the
Au–In alloy on a number of colors, decorative, optical,
corrosion, mechanical and other properties, regardless of
the method of their production, has also increased.4–6 The
aim of the present work is to investigate the indentation
hardness and the indentation modulus of electrochemi-
cally deposited thin layers of Au–In alloys as a function
of the indentation depth and considering the effect on
them of the nature of the substrate, the content of In, the
structure and phase composition of the alloy films as
well as the surface roughness of the films.
2 EXPERIMENTAL PART
The Au-In alloy films with thicknesses between
0.4 μm and 2.7 μm were deposited onto brass sheet sub-
strates (2 cm × 1 cm × 0.03 cm) in a standard electro-
chemical glass cell equipped with two Pt anodes as the
counter electrodes. The standard preliminary treatment
of the brass cathode-substrates includes a procedure for
electrochemical decreasing, followed by pickling in a 20 %
water solution of sulphuric acid at room temperature.
The investigated Au–In alloy films were electrodeposited
Materiali in tehnologije / Materials and technology 50 (2016) 5, 689–693 689
UDK 620.1:67.017:669.055:669.21:669.872 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(5)689(2016)
in galvanostatic mode (in the range of cathodic current
densities from 0.2 to 1.8 A dm–2) of an acetate-citrate
electrolyte (containing 1 g/L Au as a metal (KAu (CN)2);
3 g/L In as a metal (InCl3); 90 g/L CH3COONa; 14 g/L
citric acid). The electrolysis process was performed
without any stirring of the electrolyte and at room tem-
perature. The thickness, content of In and percentage
composition (Au:In) of the thin alloy films were deter-
mined by X-ray fluorescence analysis (Fischerscope
XDAL). The structure and morphology of the layer sur-
face were investigated by scanning electron microscopy
(SEM) using a JSM 6390 microscope. The phase com-
position was characterized by X-ray diffraction (XRD)
using a PANalytical Empyrean Equipped with a multi-
channel detector (Pixel 3D) using Cu–K (45 kV-40 mA)
radiation in the 20–115° 2 range with a scan step of
0.01° per 20 s. The mechanical properties of the Au–In
alloy films containing different amounts of In onto the
CuZn substrate were investigated by means of nano-in-
dentation experiments, using a Nano Indenter G200
(Keysight Technologies, USA), equipped with a Berko-
vich three-sided diamond pyramid with a centerline-
to-face angle of 65.3° and a 20-nm radius at the tip of the
indenter. We realized a series of 25 indentations on each
sample probe. We used an indentation method that was
proposed in 7. The indentation hardness and indentation
modulus are determined using the stiffness calculated
from the slope of the load–displacement curve during
each unloading cycle. As a result load–displacement
curves were obtained and two mechanical characteristics
of the substrate and the investigated films – indentation
hardness (HIT) and indentation modulus (EIT) – were
calculated using the Oliver & Pharr approximation
method.8
3 RESULTS AND DISCUSSION
Table 1 shows the results of the XRF analyses on the
chemical composition, thickness () and the micro
roughness (Rz and Ra) of the tested alloy samples and the
brass substrate on which they were deposited. From the
results it can be seen that the interval of changes in
content 49–63 % for the mass fractions of indium in the
resulting thin alloy layers and changes of their micro
roughness. Information about the surface morphology
and the structure of the electrodeposited pure Au and In
coatings of the working electrolyte for the preparation of
the alloy coatings of which in the first case the presence
of In ions is excluded, and in the second case, the
presence of Au ions is excluded, give the microphoto-
graphs presented in Figure 1a and 1f. While the Au
coating is dense and uniform, formed by spherical
crystallites having a size ~ 0.5–1.2 μm (Figure 1a), the
coatings of In have an uneven thickness – over the fine
crystal thin indium layer which covers the entire surface
690 Materiali in tehnologije / Materials and technology 50 (2016) 5, 689–693
S. CHERNEVA et al.: INVESTIGATION OF THE MECHANICAL PROPERTIES OF ELECTROCHEMICALLY DEPOSITED ...
Figure 1: SEM microphotographs of deposited a) Au 100 %: f) In 100
% and Au–In alloy layers in which the content of indium (in mass
fractions, (w/%) is: b) 49.4 % In, c) 54.2 % In, d) 56.0 % In and e) 63
% In (samples No. 2, 7, 3, 4, 5, 6 described in Table 1)
Slika 1: SEM-posnetek nanosa: a) Au 100 %, f) In 100 % in nanosi
AuIn z razli~no vsebnostjo In (v masnih odstotkih, (w/%): b) 49,4 %
In, c) 54,2 % In, d) 56,0 % In, e) 63 % In (vzorci {t. 2, 7, 3, 4, 5, 6
opisani v Tabeli 1)
Table 1: Chemical composition, thickness, Rz and Ra of the investigated Au–In alloy layers and the substrate on which they are deposited
Tabela 1: Kemijska sestava, debelina, Rz in Ra preiskovanih AuIn plasti in podlage, na katero so bile nane{ene
No Sample Content in mass fractions,(w/%) , ìm Ra, ìm Rz, ìm J, A dm
2 deposition time, min
1. Brass substrate (pickled) Cu – 65.80; Zn – 34.20 300 1.61 9.13
2. Àu/Brass Au – 100 0.64 1.13 4.77 1.0; 20
3. AuIn/Brass Au – 50.6; In – 49.4 0.56 1.14 4.90 1.8; 7
4. AuIn/Brass Au – 45.8; In – 54.2 0.75 1.40 5.37 1.2; 15
5. AuIn/Brass Au – 44.1; In – 56.0 1.42 1.15 5.00 0.6; 20
6. AuIn/Brass Au – 37.0; In – 63.0 2.76 1.50 8.93 0.2; 30
7. In/Brass In – 100 0.49 1.18 3.83 1.0; 20
of the brass substrate, they grew, not fully coalesced,
spheroidal agglomerates with size ~ 1–10 μm (Figure
1f). The influence of changes in the content of indium on
the surface morphology and structure of the Au–In alloy
layers is presented in Figures 1b to 1e. From the
microphotographs it is clear that at the lowest content of
indium (49.4 %) (Figure 1b) the film has a morphology
and structure that is different from that of the pure gold
film. The alloy coating is formed by homogeneously dis-
tributed agglomerates of a size of the base several times
larger than that of the spherical grains constituting the
gold coating. Moreover, there was no phase hetero-
geneity in the regime of back scattering electrons. The
reasons for this conclusion give the images on the left-
hand side of the SPI electron microscopic image (Figure
1b), obtained in the regime of back scattering electrons
(BEI), while the right-hand part of the photograph shows
an image that was obtained in the regime of a secon-
dary-electron image (SEI). With the same purpose (the
recording of possible phase heterogeneity) are the SPI
electron microscopic images for a higher content of In
(Figures 1c to 1e). Increasing the content of indium in
the alloy layer to ~ 54 % (Figure 1c), leads to a levelling
of the morphology, respectively, to a finer structure
compared with those at a content of ~ 49 % (Figure 1b),
in which an even greater degree was observed in the next
amount (~ 56 %) of indium (Figure 1d). Obviously, the
observed differences in morphology are not related to the
phase, but are related to the topographic heterogeneity.
When the content of co-deposited In, however, reached
63 %, the morphology and the structure drastically change;
they are characterized by large aggregates (3–15 μm),
composed of crystallites with dimensions of 0.5–1 μm.
Moreover, the BEI image (left-hand side) of the electron
microscopic micrograph, at this content (Figure 1e)
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Materiali in tehnologije / Materials and technology 50 (2016) 5, 689–693 691
Figure 5: Dependence of the indentation hardness on the content of In
Slika 5: Odvisnost trdote vtiskovanja od vsebnosti In
Figure 3: Dependence of the indentation hardness on the depth of the
indentation
Slika 3: Odvisnost trdote vtiskovanja od globine vtiskovanja
Figure 4: Dependence of indentation modulus on the depth of the
indentation
Slika 4: Odvisnost modula vtiskovanja od globine vtiskovanja
Figure 2: XRD patterns of the: a) Au 100 %, f) In 100 % and Au–In
alloy layers, containing: b) 49.4 % In, c) 54.2 % In, d) 56.0 % In,
e) 63 % In; (o) – reflections of CuZn substrates, () – reflections of
AuIn2
Slika 2: Rentgenogram: a) Au 100 %, f) In 100 % in AuIn nanosa z:
b) 49,4 % In, c) 54,2 % In, d) 56,0 % In, e) 63 % In; (o) odboji CuZn
podlage, () odboji AuIn2
indicates the occurrence of phase heterogeneity. The
X-ray phase analysis of the same alloy samples (Figure
2) showed that for electrodeposited samples of electro-
lyte containing only gold ions (in the absence of indium
ions) the diffractogram showed the presence of reflec-
tions of the cubic lattice (a = b = c = 4.08) of the phase
of gold (pdf 98-004-4362), and reflections of the brass
substrate (pdf 98-062-9457) (Figure 2a). In the case of
the electrodeposition of an indium film (in the absence of
gold ions in the electrolyte), the diffractogram of the
obtained sample indicates the presence of reflections of
an In tetragonal phase (pdf 98 005-3091) with the lattice
parameters a = 0.3253 nm, b = 0.49455 nm and reflec-
tions of the substrate made of brass (pdf 98-062-9457)
(Figure 2f). Since the content of indium in the electro-
deposited alloy film is in the range 49–63 % of mass
fractions of In, then, according to the phase diagram for
the system Au–In, they fall into the area of the phase
AuIn2. This is confirmed by the recorded reflections in
the diffractograms presented in Figures 2b to 2e. The
AuIn2 phase has a cubic lattice with highly expanded
parameters regarding the -phase of the gold (a = b = c =
6.502). Only in the alloy composition containing over 63
% of mass fractions of In (Figure 2e) is there a presence
of both the phase AuIn2 and the tetragonal phase of In
(pdf 98005-3091), which is the most likely cause for
registered, strongly emphasized, morphological hetero-
geneity of the alloy coating (Figure 1). The dependence
of the indentation hardness and the indentation modulus
of the investigated alloy films on the depth of the inden-
tation are shown in Figures 3 and 4. With an increasing
depth of indentation, the indentation hardness and the
modulus change a great deal. There are two possible
reasons for this: the influence of the substrate and the
effect of the difference in the structure with depth. The
dependence of the indentation hardness and the inden-
tation modulus of the investigated alloy films (at load =
1.15 mN, in order to be far enough from the influence of
the substrate) on the content of In is shown in Figures 5
and 6. With an increase of the content of In from 0 to
49.4 %, the indentation hardness increases too, and after
this (from 54.2 % to 63 % content of In) it starts to
decrease. It is obvious from Figure 6 that with an
increase in the In content up to 56.0 % and 63 % the
indentation modulus of the investigated Au–In films
decreases. This could be explained by the influence of
the simultaneously existing two phases on the surface
electrode: In and AuIn2. The effect of the non-regularity
is very strong due to the different type of crystal lattice –
tetragonal in case of the In phase and cubic in the case of
the AuIn2 phase. Most probably, the inhomogeneity of
these two phases, as well as their roughness limit the
accuracy, due to the randomly of both phases during the
performed measurements.
4 CONCLUSIONS
In the present work the mechanical properties of
electrochemically deposited thin layers of Au–In alloys
as a function of the indentation depth and considering
the effect on them of the nature of the substrate, the
content of In, the structure and the phase composition of
the alloy films as well as the surface roughness of the
films were investigated. The results showed that with an
increasing content of In from 0 % to 49.4 %, the indenta-
tion hardness increased too and after this (from 54.2 %
to 63 % content of In) it starts to decrease. Moreover,
with an increase in the In content up to 56.0 % and 63 %
the indentation modulus of the investigated Au-In films
decreases. This could be explained by the influence of
the simultaneously existing two phases on the surface
electrode: In and AuIn2. The effect of the non-regularity
is very strong due to the different types of crystal lattice:
tetragonal in the case of the In phase and cubic in the
case of the AuIn2 phase.
Acknowledgments
Authors gratefully acknowledge the financial support
of Bulgarian National Science Fund under Grant No.
T02-22/12.12.2014.
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