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Cu/Co-W Nanolayers Electrodeposited from Single Bath and Investigations of their Nanohardness
N. Tsyntsarua b, S. Belevskyb, H. Cesiulisc, A. Dikusarb, J.-P. Celisa
aKatholieke Universiteit Leuven, Dept. MTM,
Kasteelpark Arenberg 44, B-3001 Leuven, Belgium, e-mail: ashra_nt@yahoo.com bInstitute of Applied Physics, Academy of Sciences of Moldova,
5, Academiei str., Chisinau, MD-2028, Republic of Moldova, cVilnius University, Dept. Phys.Chem., Naugarduko 24, Vilnius, LT-03225, Lithuania
For the first time thick (~ 8 pm) Cu/Co-W multilayered coatings with individual layers ranging from 5 to 200 nm were electrodeposited from a single bath. The content of tungsten in rich-in Co-W layers was controlled by varying current densities in a citrate-borate bath. Continuous Multi-Cycle (CMC) nanoindentation technique was used to analyze mechanical properties of those deposits. Optical examination of the indented zone revealed the absence of cracks inside and outside the indentation area in the interval of the normal loads used. The hardness of Cu/Co-W multilayers varied with the bi-layer period and the electrodeposition parameters. The Cu/Co-W multilayers showed an increased hardness compared to that of Co-W coatings electrodeposited under the same conditions.
УДК 620.3
INTRODUCTION
Multilayers are attractive materials because a high density of interfaces induces physical and mechanical properties that differ from those of their bulk constituents. An electrodeposited multilayer thin film consists of alternating layers of at least two different materials with submicrometer layer thicknesses [1-4] or even in the nanometer range [5-9]. In most cases the co-depositing metals have quite different electrodeposition potentials. Frequently copper or gold are chosen as the noble metal, and an iron-group metal or its alloy - as an active metal [10]. Since both Cu(II) and Au(I) reduce easily, the concentration of these metals in the electrolyte is relatively low that enables electrodeposition of Cu or Au under mass-transport control. On the contrary, metals of the iron group are electrodeposited under kinetic control.
The electrodeposition of multilayers can be performed from a single bath under pulse current or pulse potential mode and the periodicity of the current or potential pulses triggers the alternated growth of the noble (active) metal layer at low (high) values of current or potential [10]. The electrodeposition of a number of multilayered coatings was reported, namely, of Cu/Ni, NiCo/Cu, Cu/Co, Cu/NiFe, Au/Co, Au/FeAu, and Cu/CoB/CoW(B)P [11-17]. Multilayered materials, tailored with a repetitive sequence of magnetic segments (e.g. iron group metals) interspaced by noble metal layers, display unique magnetic properties [18-20].
Functionally graded and periodic multilayer films often are investigated with intentions to use for applications as hard coating because of their superior properties which result from layered structure. Functionally graded multilayer coatings have been explored to improve toughness, hardness and the adhesion to the substrate [21]. The CMC nanoindentation test can be used to study delamination and the interfacial strength between layers [22].
The Co-W coatings electrodeposited from similar citrate baths potentially might be used as anticorrosive and wear resistance coatings [23, 24].
The aim of this study is to electrodeposit Cu/Co-W multilayers from a single bath and to investigate their mechanical properties.
EXPERIMENTAL
The electrodeposition of coatings was carried out in a standard 3-electrode cell. A graphite rod was used as an anode, a saturated Ag/AgCl electrode as the reference electrode. All potentials are reported versus the saturated Ag/AgCl reference electrode. Mild steel flat plates (working area 8 cm2, composition 98.5 at.% Fe, 0.5 at.% C, 0.7 at.% Si, others 0.3 at.%) were used as substrates. Prior to electrodeposition, the substrates were mechanically polished. In order to improve the adhesion of the Cu/Co-W multilayers onto the substrates, the electrodeposition of a nickel seed layer was done from an electrolyte containing 1 M NiCl2 and 2.2 M HCl at 3 А/dm2 for 1 min. The electrodeposition conditions were controlled and monitored using a PARSTAT 2273 system which was also used to record potentiodynamic polarization curves at a scan rate of 2 mV/s. In such experiments, a Cu wire was used as the working electrode.
© Tsyntsaru N., Belevsky S., Cesiulis H., Dikusar A., Celis J.-P., Электронная обработка материалов, 2012, 48(5), 49-57.
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Cu/Co-W multilayred coatings were electrodeposited from a single electrolyte (SOLUTION 1) at pH
6.7 and 60°C. The contents of all solutions used in this study are presented in table 1.
The hardness of multilayers was determined by continuous multicycling nano-hardness tests (CSM Nano tester). The applied load was varied between 10 mN up to 200 mN without removing completely the indenter from the surface.
The surface roughness of as-deposited coatings was quantified by non-contact white light interferometry (WYKO NT 3300). The coating morphology was investigated by scanning electron microscopy (SEM) with Philips XL 30 FEG equipped with the energy dispersive x-ray spectroscope and a super-ultrathin window. The composition of electrodeposited alloys reported in this study was calculated as the content of metallic phases. FIB (focus ion beam) cross-sections were made to reveal the multilayered structure.
Table 1. The compositions of electrolytes
No. CCoSO4> M CNa2WO4> M CCuSO4> M CC6H8O7> M CNa3C6H5O7, M ch3bo3> M
Solution 1 0.2 0.2 0.002 0.04 0.25 0.65
Solution 2 - - 0.002 0.04 0.25 0.65
Solution 3 0.2 0.2 - 0.04 0.25 0.65
Mechanical properties of the electrodeposited Cu/Co-W multilayers were compared to those of Co-W coatings electrodeposited at 3 A/dm2 from a similar electrolyte as described elsewhere [24].
RESULTS AND DISCUSSIONS
Electrodeposition of multilayered coatings
The electrodeposition conditions were selected based on the potentiodynamic polarization curves recorded in three electrolytes whose compositions are presented in table 1. The obtained potentiodynamic polarization curves are shown in fig. 1. As it is seen, the electrodeposition of pure Cu starts at a more positive potential (approx -0.1 V) than the electrodeposition of Co-W (approx. -0.5 V), and electrodeposition at reasonable rate is observed at potentials more negative than -0.7 V. In all multilayer deposition experiments described hereafter the current density used to electrodeposite Cu layer was kept as constant so that the electrodeposition potential would remain more positive than that for Co-W electrodeposition. The current densities for Co-W electrodeposition were varied. The pulse duration was chosen in such a way that the thickness of both the Cu-layers and the Co-W would be almost the same, namely 5-200 nm. The electrodeposition cycle contained also a pause needed to remove hydrogen bubbles from the cathode surface and to settle the values of pH in the pre-electrode space increased during the electrodeposition of Co-W layer due to hydroxyl ion production in the pre-cathode layer. To keep the constant pH values in course of the electrodeposition is very important when the electrodeposition of tungsten or molybdenum alloys is performed in deep recesses [25-27].
Fig. 1. Cathodic branches of potentiodynamic polarization curves recorded in Solutions 1-3.
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Table 2. Conditions used for electrodeposition of Cu/Co-W multilayers and some coating characteristics obtained
No. of the plating conditions А ш s iCu, A/dm2 tCoW, s iCoW A/dm2 tpause, s Sublayer thickness, nm Coating thickness, pm Content of metallic elements, Co/W/Cu at. % Mean surface roughness Ra, nm
1 37.5 0.05 1.25 3 2.5 ~ 5 10 76,2/19,0/4,8 110
2 1500 50 3 100 ~ 200 10 61,6/18,7/19,7 853
3 45 2.4 2 12 ~ 7 8 79,2/14,3/6,5 150
4 90 4.8 2 24 ~ 12 7 71,9/16,8/11,3 201
5 45 3.2 1 16 ~ 4.5 7 82,5/14,2/3,3 480
The following electrodeposition sequence was finalized to obtain multilayered coatings: (step 1) current pulse for the electrodeposition of Cu layer (duration tCu and amplitude iCu); (step 2) current pulse for the electrodeposition of the rich-in-Co-W layer (duration tCoW and amplitude iCoW); and (step 3) pause (duration tpause and amplitude ipause = 0). The parameters used for ectrodeposition of multilayers are summarized in table 2. The potential response on the applied current path shown in fig. 2 illustrates that the potential recorded during the galvanostatic growth of Cu sublayers remains sufficiently more positive than the one recorded during the growth of Co-W sublayers.
Fig. 2. Cathode potential changes during electrodeposition under conditions No 2 and 4 (see table 2). Potentials are reported without ohmic drop compensation.
Structure of the multilayered coatings
Fig. 3 presents the FIB-analysis of multilayers electrodeposited under the plating condition No. 2 and indicated in table 2. The dark layers are copper sublayers and the grey ones are Co-W sublayers.
The FIB-analysis allows to estimate the thickness of the sublayers and to derive the current efficiency. As is shown in fig. 3, the thickness of each layer was determined as ~200 nm and this values is close to the calculated one using Faraday’s law (equation). Multilayers with thick sublayers of 200 nm grow uniformly till a total thickness does not exceed 1-2 pm (fig. 3,c and d). When the coatings become thicker, the uniformity of the thickness of individual layers is lost probably due to the following reasons: (1) the interfacial roughness and/or substitutional disorder (i.e. mixing of the adjacent metal atoms at the interface) that is always present in experiments [28]; (2) the influence of hydrogen evolution leading to the intensification of roughness [29]; (3) Cu(II) electroreduction from citrate solutions occurs under diffusion control [30]. All these peculiarities might cause the sequential decrease in the thickness of layers.
The last hypothesis can be qualitatively explained in the following way. Cu sublayers are electrode-posited under diffusion that limits current control (fig. 1), therefore the concentration of Cu(II) complexes at the cathode surface is [Cu(II)]x=0=0. During electrodeposition of the rich-in-Co-W layer, Cu electroreduction occurs also at [Cu(II)]x=0=0, but with the gradually decreasing value of the concentration fluxion
. The partial current density for Cu electrodeposition (/Cu) is proportional to this term, as be-
x=0
low:
d[Cu(II)] ^
dx J
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Jci
= nFD,
Cu(II)
d[Cu(II)]
dx
' x=0
(1)
where DCu(II) is the diffusion coefficient of Cu(II) species.
a
b
c d
Fig. 3. SEM images taken on a FIB cross-section of a Cu/Co-W multilayer coating electrodeposited under the set of parameters 2 listed in table 2. The a, b, c and d are the images taken in different spots of cross-section.
The decrease of the concentration fluxion with time under condition [Cu(II)]x=0=0 can be describe based on the Cottrell equation, and is expressed in a following way:
Г d[Cu(II)] "l = [Cu(II)]0 (2)
V dx J x=0 'y/nDCu(II)t
where [Cu(II)]0 is concentration in the bulk of electrolyte.
The decrease of the concentration fluxion results in the growing thickness of the diffusion layer during the pulse duration, because of the distance where concentration flux of [Cu(II)] = [Cu(II)]0 becomes longer. Probably, during the pause time the concentration of Cu(II) at the cathode surface does not restore fully to its initial value but remains less than in the bulk of the electrolyte. Therefore, each subsequent deposition cycle starts at a gradually lower [Cu(II)]x=0, which results in a lower partial current for Cu and thus in a sublayer thickness deviation.
Another reason is dealing with Cu electrodeposits obtained at limiting current density. In this case a relatively rough surface is formed, which results in widening the real surface area, thus causing a decrease in current density at constant current applied. Lowering of the Cu sublayer thickness appears more pronounced at a thicker initial sublayer thickness.
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1
3
2
4
5
Fig. 4. SEM images of as-plated surfaces of Cu/Co-W multilayers. Numbers under images correspond to numbers of electrodeposition parameters listed in table 2.
SEM images of multilayers electrodeposited under different conditions are shown in fig. 4. The coatings differ in as-plated roughness (table 2) and in surface inhomogeneities. The coatings deposited under condition No. 2 (table 2), resulting in the highest sublayer thickness, have the highest roughness, whereas coatings deposited under other conditions indicated in table 2 exhibit only “islands” of inhomogeneity. It is worth noting here that an intensified roughness is also obtained when the low current densities are applied for the electrodeposition of Co-W layers (condition No. 5).
Mechanical behaviour of Cu/Co-W multilayers
The nanohardness of Cu/Co-W multiyares was investigated by CMC nanoindentation. In CMC, the coatings are loaded up to a specific value, then unloaded and immediately re-loaded to a higher load at the same place; and a cyclic nanoindentation curve is recorded. This method allows to check stability and crack initiation during continuous and high-speed loading. This test can be used to study buckling, delamination of the coating and the interfacial strength between the layers and the substrate. To understand the influence of the layered structure, 19 cycles were performed in one CMC test with the progressive loading from 10 mN up to 200 mN (fig. 5). This allowed to keep the penetration depth not exceeding 10% of the coating thickness.
The topographies of the indented zone acquired by optical microscopy of the investigated coatings after CMC indentations are shown in fig. 6, numbered respectively on the electrodeposition mode used. Several factors influence the hardness: the formation of cracks in the coating, properties of the material, indentation size, influence of the substrate effect and others [31]. As can be noticed from fig. 6, the indentation
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imprints did not show any cracks, but such factors as tungsten content, hydrogen evolution, thickness of sublayers do have effect on the resulting topography and hardness of the multilayered coating (figs. 6 and 7).
Fig. 5. Typical CMC loading nanoindentation test on Cu/Co-W multilayers.
1
3
4
2
5
Fig. 6. Images of nanoindentation imprints of Cu/Co-W coatings electrodeposited under electrochemical conditions as shown in table 2. Condition No. 4 can have different degrees of roughness and is presented by 2 images.
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Thus, a higher current density (3 A/dm2, condition No. 1) applied for Co-W electrodeposition leads to a higher cathodic polarization and tungsten content in the multilayered coatings (table 2). The presence of more tungsten in the coating results in higher hardness values (fig. 7 graph 7), but simultaneously it promotes a more intensive hydrogen bubbles formation (fig. 6, image 7). Increase of surface heterogeneities under condition No. 1 in comparison with electrodepostion under conditions No. 3 and 5 leads to higher scattering values of hardness, which can be bigger or smaller than those for the Co-W ones (~ 9 GPa). Even, under the electodeposition condition No. 2, with the same current density as in No. 1, but with ~ 200 nm sublayers, the indentation cannot be carried out correctly, because of the extremely high roughness of the as-deposited Cu/Co-W coating. Increase of the thickness of sublayers requires longer deposition time, that leads to intensification of hydrogen evolution and roughness.
The negative influence of the increased sublayer thickness on the hardness of the investigated multilayers can be also noticed for the coatings electrodeposited under condition No. 4: coatings with ~ 12 nm sublayers have increased copper content and hence the hardness values can be very low (fig. 7, graph 4).
Penetration i
5
0.2 0.4 0.6 0.8 1.0 1.2
Penetration depth, pm PeneLracion depth, pm
Fig. 7. Nanohardness measured by CMC method for the deposition conditions No 1, 3-5 (see table 2). Different curves correspond to measurements at different spots on the coating.
1
3
4
The remaining coatings electrodeposited under conditions No. 3 and 5 are more uniform and their hardness varies within a narrower range. The scattering of the hardness values at small penetration depths (fig. 7) can be due to the mixing of adjacent atoms at the interface of the upper layers of the coating. Nevertheless, for the majority of cases, the Cu/Co-W multilayers have better elastic recovery, which resulted in lower values of the penetration depth in comparison to those of Co-W coatings (fig. 7). The hardness of Cu/Co-W coatings is close to or even exceeds the values obtained for the electrodeposited Co-W coatings.
CONCLUSIONS
The electrodeposition of Cu/Co-W multilayers was performed for the first time.The challenge of this study was to produce Cu/Co-W multilayers with a high total thickness of coatings and nanosized sublayers.
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The mechanical properties of these Cu/Co-W multilayered coatings were compared with those of Co-W coatings electrodeposited from a similar bath. The hardness of Cu/Co-W multilayers varies with the electrodeposition parameters. The Cu/Co-W multilayers show, in most cases, an increase in hardness compared to that of Co-W coatings electrodeposited under the same conditions.
ACKNOWLEDGEMENT
This research was funded by the FP7 grants “NANOALLOY” (n° 252407), and “TEMADEP” (n° 247659) and also from the Research Council of Lithuania (grant n° MIP-134/2010-2011).
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Реферат
Received 24.01.12
Впервые были получены толстые (~ 8 мкм) Cu/Co-W многослойные покрытия из одной ванны с индивидуальными слоями от 5 до 200 нм. Содержание W в обогащенных Co-W слоях контролировалось изменением плотности тока при электроосаждении из цитрат-боратной ванны. Для анализа свойств таких покрытий использовано наноиндентирование непрерывным многоцикличным методом. Оптическая оценка зон индентирования показала отсутствие трещин как на внешней, так и внутренней областях индентирования в интервале использованных нагрузок на индентор. Показано, что твердость исследованных многослойных покрытий варьировала в зависимости от толщины индивидуальных слоев и параметров электроосаждения. Cu/Co-W покрытия обладали более высокой твердостью в сравнении с кобальт-вольфрамовыми, полученными в тех же условиях.
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