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97
Electrodeposition of Fe and Composite Fe/ZrO2 Coatings from a Methanesulfonate Bath
V. S. Protsenkoa, E. A. Vasil'evaa, I. V. Smenovaa, A. S. Baskevicha, I. A. Danilenkob, T. E. Konstantinovab, and F. I. Danilova
a Ukrainian State University of Chemical Technology, Gagarin, Av 8, Dnepropetrovsk, 49005, Ukraine, e-mail: [email protected] b Donetsk Institute for Physics and Engineering named after O.O. Galkin of the NAS of Ukraine, R. Luxembourg, str. 72, Donetsk, 83114, Ukraine
The electrodeposition of iron and composite iron-zirconia coatings from a methanesulfonate electrolyte was investigated. The current efficiency of iron deposition reaction was stated to be sufficiently higher in methanesulfonate electrolytes than in usual sulfate baths. Iron coatings electrodeposited from a methanesulfonate bath have a nano-crystalline structure. The Fe coatings obtained from methanesulfonate baths are harder than those deposited from sulfate baths because of the strengthening effect by the Hall-Petch mechanism. The composite Fe/ZrO2 coatings can be obtained from the iron electroplating baths containing the particles of zirconia stabilized by 3 mol % yttria. The kinetics of ZrO2 particles co-deposition with iron in methanesulfonate electrolytes obeys Guglielmi's model. The insertion of zirconia particles into the iron matrix results in an appreciable increase of the coatings microhardness via the dispersion strengthening mechanism.
Keywords: electroplating, iron, zirconia, composite coating, methanesulfonate electrolyte.
УДК 544.65:621.357
INTRODUCTION
The electrodeposition of iron, its alloys and composites has been widely used for various engineering applications such as electrotypes, electroforming, repairing worn and corroded machine parts, magnetics components in computer and electronic industries, etc. [1-7]. Iron electrodeposits are usually obtained from acid sulfate, chloride, fluoroborate, and sulfamate Fe(II) electrolytes. Acid iron electroplating baths are well studied. They are highly productive and their compositions are relatively simple. Nevertheless, acid electrolytes for iron electrodepo-sition are rather corrosive and toxic; therefore, the development of novel acid Fe(II) baths is an important problem of modern electroplating.
Aqueous Fe(II) baths on the base of methanesul-fonic acid CH3SO3H (MSA) seem to be an attractive and perspective alternative to common iron electroplating baths since MSA is considered to be a "green acid" due to its environmental advantages [8]. MSA is known to be far less corrosive and toxic than the usual mineral acids used in different branches of industry [9]. Methanesulfonates of various metals are highly soluble in water; the conductivity of the corresponding aqueous solutions is high. In addition, MSA is easily biodegradable. Because of these advantages, electrochemical systems on the base of MSA and its salts have been shown to be very promising for different electrochemical technologies, and especially for electroplating of some metals and alloys [10-15].
However, only several papers have been published, in which the electrodeposition of iron from methanesulfonate baths was investigated [16-18]. Thus, it has been revealed that methanesulfonate iron plating baths ensure a wide range of operating current densities and reduced hydrogen saturation of coatings [16]. The application of MSA-containing iron plating baths was shown to provide satisfactory adhesion of iron coatings with steel and cast iron [17]. Nevertheless, the data presented in those studies are very limited. The effects of deposition conditions on Fe electroplating from methanesul-fonate electrolytes remain practically unexplored.
The performance characteristics of iron-based electrodeposits may be sufficiently improved by the incorporation of inert particles into metallic coatings (i.e. when depositing composite coatings) [6, 7, 19]. We report here the results of our attempts to prepare composite Fe/ZrO2 coatings. Zirconia is of a persistent interest as an engineering material because of its outstanding combination of properties such as: strength, toughness, high melting point, chemical inertness, hardness, etc. The electrodeposition of zirconia-containing composite coatings with Ni and other matrices has been described in a number of papers [20-22]. Composite Ni/ZrO2 coatings exhibit improved physicochemical and service properties compared with those of the pure nickel under the same electrodeposition conditions.
In our recent work [23], we have demonstrated that the ZrO2-containing hard composite coatings on the base of the iron matrix may be electrodeposited
© Protsenko V.S., Vasil'eva E.A., Smenova I.V., Baskevich A.S., Danilenko I.A., Konstantinova T.E., and Danilov F.I., Электронная обработка материалов, 2015, 51(1), 66-75.
using methanesulfonate electrolytes. However, the kinetics of iron-based composite coatings from MSA-containing electrochemical systems and the hardening mechanism remain still undetermined. Thus, the purpose of this work is to study the kinetics and mechanism of Fe and composite Fe/ZrO2 electrodeposition processes from MSA-containing plating baths. The effects of different factors on the microhardness of deposits obtained and probable hardening mechanisms are also revealed and discussed.
EXPERIMENTAL
All solutions were prepared using twice-distilled water and reagent grade chemicals. Iron (II) methanesulfonate was synthesized by the procedure described in detail in [8]. The pH value of the plating bath was controlled through a common potentiometric method. If needed, the pH of the bath was adjusted to the required value by adding either Na2CO3 or MSA solutions. The concentration of Fe(II) ions in the plating bath was determined by the titrometric analysis. To this end, K2Cr2O7 solution was used as a titrant in the presence of difenilamin-sulfonat as a redox indicator.
Electrodeposition was performed at a steady current density in a usual thermostated glass cell. Iron and iron-zirconia films were deposited both on the mild steel plates and on the disc electrode of the copper foil (S = 1 cm2) fixed in a plastic holder. Prior to each experiment, the cathode surface was treated with magnesium oxide, etched for several minutes in 1:1 (vol.) hydrochloric acid solution and then thoroughly rinsed with twice-distilled water. The electrolysis was carried out with anodes made from mild steel. The current efficiency of iron deposition was calculated from the cathode gain in weight.
Voltammetric measurements were performed using Potentiostat/Galvanostat Reference 3000 (Gamry) in a conventional glass three-electrode cell deaerated by blowing with electrolytic hydrogen. The electrochemical cell was thermostated by a thermostat Flussigkeitsthermostate Baureihe U/UH8. The cathode and anodic compartments were separated by a porous glass diaphragm. A freshly electro-deposited Fe layer with a thickness of 15 ^m on the platinum substrate was used as the working electrode (S = 0.28 cm2). The counter electrode was made of the platinum wire. All potentials were measured with respect to the saturated Ag/AgCl-electrode and recalculated to a standard hydrogen electrode.
For obtaining composite coatings, the doped zir-conia nanopowder ZrO2+3 mol% Y2O3 was incorporated into the electrodeposition bath. Zirconia nanopowder was synthesized via co-precipitation
technique from ZrOCl2nH2O and Y(NO3)3 salts. The details of this procedure were published elsewhere [24-26]. The procedure allowed obtaining mono-dispersed nanopowders of stabilized zirconia with a prescribed particle size of about 18 nm. It should be noted that doping by 3 mol % Y2O3 is used in order to stabilize the tetragonal phase of zir-conia [26].
The sedimentation analysis of the size of ZrO2 particles in the suspensions was performed with a digital analytical balance Vibra HT-120 (Shinko denshi).
The surface morphology of deposits was investigated by the scanning electron microscopy (SEM) utilizing scanning electron microscope JSM-6490. In order to choose a typical picture for each coating, four to six locations (in the central zone) were taken.
The chemical composition of Fe/ZrO2 composite layers was determined both by the energy dispersive X-ray (EDX) microanalyser coupled to the SEM instrument and by the chemical analysis. In the latter case, a coating obtained on the copper substrate was solved in sulfuric acid (1:1 vol.) and then the solution was titrated according to the procedure described above.
The X-ray diffraction (XRD) analysis was performed by an X-ray diffractometer DRON-3.0 in the monochromatized Cu-Ka radiation. The crystalline size was estimated by the Scherrer equation.
Vickers microhardness (HV) was determined using a PMT-3 apparatus at a 100 g load, the coating thickness being not less than 20 ^.m. The average value of the deposits microhardness was calculated from more than 10 separate measurements.
RESULTS AND DISCUSSION
Electrodeposition of iron from methanesulfonate electrolyte
Iron electrodeposition from methanesulfonate baths has not been sufficiently described in literature so far. Therefore, it is reasonable to characterize the main features of electroplating process in this system in comparison with a "common" sulfate bath.
The current efficiency (CE) increases with the current density both in a methanesulfonate electrolyte and in the sulfate one (Fig. 1). However, the growth of the CE values ceases in MSA-containing bath at i > 20 A/dm2. We have stated that an increase in the bath temperature leads to a decrease in the CE values [23].
All other conditions being equal, the current efficiency in MSA-containing electrolyte is considerably higher than that in case of the sulfate bath. This is the most evident advantage of the methanesul-fonate system over the sulfate one.
i, A/dm2
Fig. 1. Effect of current density on current efficiency of Fe electrodeposition from (1) methanesulfonate electrolyte containing 1.25M Fe(CH3SO3)2 and (2) sulfate electrolyte containing 1.25M FeSO4. Bath temperature 298K, pH 1.3.
The nature of anion affects the surface appearance of deposits. Dull coatings are obtained from sulfate electrolytes and bright iron coatings deposits from MSA-containing plating baths.
The CE of iron electrodeposition increases with an increase in the bath pH (Fig. 2), this dependence is more sharply pronounced for the sulfate electrolyte. At a high acidity (pH 1.0), the CE in the methanesulfonate system is about 90%, while it approaches only about 8% in the sulfate bath. It should be stressed that an increase in the electrolyte pH is not desirable in consideration of the increase of the rate of Fe(II) ions oxidation by atmospheric oxygen. At pH > 1.5, a rapid oxidation of Fe(II) is especially noticeable and iron deposits become dark and rough. Taking into account all these facts, one may assume that the bath pH should be equal to about 1.3.
E, V
Fig. 3. Steady-state polarization curves obtained in (1)—(3) sulfate electrolyte containing 1.25M FeSO4 and (1 ')—(3') methanesulfonate electrolyte containing 1.25M Fe(CH3SO3)2. (1) and (1') - total current density; (2) and (2') - partial current density of iron deposition; (3) and (3') - partial current density of hydrogen evolution reaction. Bath temperature 298K, pH 1.3.
The iron deposition reaction in acid aqueous solutions is always accompanied by the hydrogen evolution reaction (HER):
pH
Fig. 2. Effect of pH on current efficiency of Fe electrodeposition from (1) methanesulfonate electrolyte containing 1.25M Fe(CH3SO3)2 and (2) sulfate electrolyte containing 1.25M FeSO4. Bath temperature 298K. Current density 25 A/dm2.
Fe+2 + Fe, (1)
2H++ 2eH2. (2)
Hence, detected differences in the CE values of iron deposition for the two electroplating baths under consideration may be associated with some differences in the kinetics of the parallel electrochemical reactions (1) and (2). Figure 3 gives partial-polarization curves of iron electrodeposition and of the HER. As can be seen, the kinetics of Fe(II) ions discharge in the methanesulfonate and sulfate systems is similar since the respective partial-polarization curves almost coincide. Meanwhile, the hydrogen evolution reaction in MSA containing electrolyte occurs with a significantly higher cathode polarization than in the sulfate bath. This is due to some differences in the chemical properties of MSA and sulfuric acid. Indeed, MSA is a monobasic strong acid (pKa = 21.9) [8] while, H2SO4 is a dibasic acid in whose aqueous solutions both sulfate and bisulfate anions are present. As shown in [27], the current density of the proton reduction in acid Fe(II) sulfate electrolytes was higher than that measured in chloride or perchlorate solutions at the same pH value. The difference was explained by the presence of bisulfate ions since the following chemical equilibrium exists in the sulfate electrolytes:
HSO;» H++ SO4~ pK = 1.99. (3)
The bisulfate ion acts as a proton source. Thus, both "free" proton ions and those generated from the dissociation of HSO4- ions may take part in the hydrogen evolution reaction. The appearance of an additional proton source in a solution leads to an increase in the current density of the HER. Evidently, such phenomenon cannot occur in methanesulfonate electrolytes. A higher partial current density of the HER in sulfate baths is the reason of the low CE of the Fe deposition reaction when compared with the MSA-containing baths.
Sedimentation analysis of the size of dispersed phase particles
In accordance with [19], the composition and properties of composite electrodeposits are determined, to a large extent, by the dimensions of dispersed phase particles. As stated above, the mono-dispersed nanopowder of yttria-stabilized zirconia with a prescribed particle size of 18 nm was used to produce composite coatings. However, it is well known that the addition of electrolytes to a colloidal system leads to particle aggregation. The size of dispersed phase particles in iron plating baths was determined sedimentometrically. As follows from the data presented in Fig. 4, the colloidal systems both in a methanesulfonate electrolyte and in a sulfate one become polydispersed. The main fraction of the suspension contains particles with the aggregate size of about 3 ^m.
20 г
r, jj.m
Fig. 4. Differential curves of particle size distribution: (1) in methanesulfonate electrolyte containing 1.25M Fe(CH3SO3)2, 20 g/dm3 ZrO2; (2) in sulfate electrolyte containing 1.25M FeSO4, 20 g/dm3 ZrO2. Bath temperature 298K, pH 1.3.
Such behavior may be explained by the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory whereby the compression of the double layer, when increasing ionic strength, has dramatic effects on the stability of colloidal dispersions. At high electrolyte concentrations, the diffuse layer is compressed, and the particles approach more closely. The coarsening of the particles results in their enhanced sedimentation and subsequent phase separation. This is why the electrochemical synthesis of the composite Fe/ZrO2 was performed under the conditions of a continuous bath agitation with a magnetic stirrer (~ 60 rev/min-1). It should be noted that the reproducibility of the experimental results was quite satisfactory under the circumstances.
Electrodeposition of composite Fe/ZrO2 coatings
As follows from the data given in Fig. 5, the content of ZrO2 particles in the composite coatings ob-
tained from the methanesulfonate electrolyte is appreciably higher than that in composite coatings deposited from the sulfate bath. A relatively low zirconia content in the latter case can be explained by the fact that the current efficiency of metal deposition in the sulfate electrolytes is less than 100% (in contrast to the methanesulfonate baths). Hence, the electro-deposition process is accompanied by an intensive hydrogen evolution which provides additional agitation in the near-electrode layer. Too much agitation should decrease the particle content, because the particles are ejected by a turbulent flow from the electrode surface before being incorporated [28].
12 г
«
N
Current density, A/dm2
Fig. 5. Effect of current density on ZrO2 content in composite coatings (1) in methanesulfonate electrolyte containing 1.25M Fe(CH3SO3)2, 20 g/dm3 ZrO2; (2) in sulfate electrolyte containing 1.25M FeSO4, 20 g/dm3 ZrO2. Bath temperature 298K, pH 1.3.
It is worth noting here that the composition of deposits and the current efficiency of the iron deposition do not practically depend on the electrolysis duration (Table 1).
Table 1. Content of ZrO2 in composite coatings and current efficiency of Fe electrodeposition at different electrolysis duration
Electrolysis duration, min ZrO2 content, wt.% Current efficiency, %
10 9.3 96
15 9.4 95
20 9.2 96
30 9.5 95
Bath composition: 1.25M FeCCHsSOsK 20 g/dm3 ZrO2; pH 1.3; temperature 298K; current density 15 A/dm2.
The content of ZrO2 particles in the composite coatings increases with an increase in the suspension concentration and decreases somewhat with an increase in the total current density (Fig. 6).
Many theories have been suggested in order to describe the mechanism of the composite coatings formation [19]. In this work, we applied a model proposed by Guglielmi [29]. According to this mo-
Fig. 6. Effect of current density and zirconia concentration in the bath on ZrO2 content in composite coatings deposited from methanesulfonate electrolyte containing 1.25M Fe(CH3SO3)2. Bath temperature 298K, pH 1.3.
del, the particles inclusion in the metallic matrix occurs in two consecutive steps of adsorption. In the first step, called a "loose adsorption", the ion-coated particles are reversibly adsorbed on the electrode surface and yield a high degree of coverage. The second step is a "strong adsorption" of particles. The reduction of metal ions adsorbed on the particles creates the circumstances of an irreversible strong adsorption. Further, the particles are engulfed by the growing of the metallic matrix. Guglielmi's model may be expressed by the following main equation
C a
Mi,
nFPmV0
0 e(A-B)n
1+C к
(4)
where a is the volume fraction of the particles in the coating, C is the volume fraction of the particle in the plating bath, M is the atomic weight of the elec-trodeposited metal, n is the valence of the deposited metal, F is the Faraday constant, pm is the density of the electrodeposited metal, ^ is the cathodic overpotential, A is the constant in the kinetic equation of the electrochemical reaction i = i0eAл, i0 is the exchange current density, к is the Langmuir isotherm constant, B and Vo are the constants related to the particle co-deposition.
In accordance with Eq. (4), a plot of C/a vs. C at a constant current density should give a straight line, from whose intercept at C/a = 0 the value of (-1/к) can be calculated. Figure 7 presents the lines obtained for the Fe/ZrO2 composite under study in coordinates C/a vs. C.
The intercept of all the lines at C/a = 0 gives the value of 0.035; then the constant к is equal to 28.6 for yttria stabilized zirconia particles co-deposited with iron in methanesulfonate electrolytes.
Fig. 7. Ratio C/a vs zirconia concentration in plating baths at various current densities, A/dm2: (1) 20; (2) 15; (3) 10; (4) 5. Baths contain 1.25M Fe(CH3SO3)2. Temperature 298K, pH 1.3.
The value of к indicates the net reaction constant of the particle adsorbed on the electrode surface and can be expressed as
к
к = f (5)
kd
where ка is the adsorption coefficient and кл is the desorption coefficient of particles on the electrode. If к > 1, it indicates that the adsorption rate of particles is faster than the desorption rate [30].
Guglielmi's model implies the Langmuir adsorption isotherm modified as follows [29]:
a = JC_ (1 - 0) (6)
1+C '
where с is the loose adsorption coverage and 0 is the strong adsorption coverage.
The loose adsorption coverage may be estimated by Eq. (6) if we assume that the surface coverage of the strong adsorption is close to the volume fraction of the particles (0 « a) [29].
As can be seen from the data presented in Table 2, at all current densities and ZrO2 concentrations, the loose adsorption of the particles is much higher than their concentration in the bath. Furthermore, just a little fraction of the loosely adsorbed particles can be co-deposited by the reduction of the metallic ions surrounding them. Consequently, the rate determining step in the co-deposition of zirconia in the iron methanesulfonate plating bath is controlled by the transfer from the loose adsorption to the strong adsorption [29-31].
Table 3 gives the slopes (tg9) of the lines which were plotted in coordinates C/a vs. C (see Fig. 7) at different current densities. For these dependences, Guglielmi deduced the following expression [29]:
lg 0w) = lg M0^+
nFPmV0
1 - a I i.
(7)
From the experimental results, the curve lg (tg^) vs. lg i was plotted (Fig. 8). It is clear that the slope of such dependences enables the calculation
Table 2. Loose and strong adsorption coverage at various current densities and suspension concentrations
Current density, A/dm2 ZrO2 particles concentrations in bath, vol. %
0.02 0.05 0.09 0.18 0.35
CT % 0 % CT % 0 % CT % 0 % CT % 0 % CT % 0 %
5 31.7 6.7 54.6 9.4 63.9 10.8 72.3 13.3 77.6 14.8
10 32.1 5.5 55.0 8.8 64.1 10.5 73.6 11.8 78.5 13.7
15 32.2 5.3 55.4 8.0 64.7 9.6 74.1 11.1 79.5 12.6
20 32.3 4.9 55.8 7.5 65.1 9.1 74.5 10.8 80.5 11.5
Bath contains 1.25M Fe(CH3SO3)2; pH 1.3; temperature 298K.
of the value B/A. As can be seen, the dependence lg (tg^) vs. lg i consists of two linear sections with various slopes: section I (at relatively low current densities) and section II (at relatively high current densities). It should be mentioned that we have used the values of the partial current densities of iron deposition, calculated on the basis of the above mentioned current efficiencies (see Fig. 1), but not the total current densities when plotting the dependence lg (tg^) vs. lg i.
Table 3. Dependence of slope of lines plotted in coordinates C/a vs C on current density
Current density, A/dm2 tg 9
5 6.21
10 6.67
15 7.29
20 7.94
Bath contains 1.25M Fe(CH3SO3)2; pH 1.3; temperature 298K.
Fig. 8. Plot of lg (tg^) versus lg i according to Eq. (7).
The fact that the dependence shown in Fig. 8 is not entirely linear suggests that the values A and (or) B in Eq. (4) change somewhat with the current density. It is important to underline that the value lg (tg^) increases with an increase of lg i (the slopes are equal to 0.1 and 0.24 in sections I and II, respectively). Then, according to Eq. (7), the following inequality will be valid: (1 - B/A) > 0, i.e. B < A. This means that the Fe cations adsorbed on the particles are electroreduced more slowly than the solvated Fe cations [28]. In addition, the fact that B
is lower than A involves a decrease of a with an increase of the current density; this conclusion is in good agreement with the experimental data obtained.
SEM investigation, EDX analysis, and XRD characterization
Figure 9 shows the typical SEM images of iron and composite iron-zirconia coatings deposited from methanesulfonate baths. The surface morphology of "pure" iron is smooth but there is a superficial network of cracks. The ZrO2 agglomerates incorporated into the deposits structure are irregular and the surface morphology becomes heterogeneous. The ZrO2 dispersed particles are easy to distinguish in SEM micrographs, their average size (about several ^m) correlates accurately with the data of the sedimentation analysis (see above).
The EDX analysis of composite iron-zirconia coatings was performed in various areas of the surface: (i) in the region that is free from the particles entrapped, and (ii) directly in the area of the incorporated agglomerate. In agreement with these results (Table 4), the EDX analysis provided evidence that the surface of the regions without visible dispersed particles consists essentially of iron and oxygen, a small amount of zirconium being also detected.
The detection of oxygen in the region that is free from the incorporated dispersed particles can be explained by the fact that even a brief contact of the freshly deposited iron with electrolyte (after current interruption) or with atmospheric oxygen and moist air was sufficient to create a thin oxide layer on the iron surface.
The surface of the dispersed particles embedded in the coating is predictably enriched with Zr, O and Y. The EDX analysis also revealed that the surface of the dispersed particles, entrapped into the deposit, contains small amounts of such elements as sulfur, carbon, sodium and iron (together with zirconium, oxygen and yttrium). This is presumably due to the engulfment of electrolyte traces by the spongy aggregates of ZrO2 particles. Then the source of sulfur and carbon may be MSA; iron is included as adsorbed Fe(II) ions; sodium is present in electrolyte as a result of pH adjustment by the adding of Na2CO3 solution.
"M чХ
SEUW» affijsim M©
(a) (b)
Fig. 9. SEM micrographs of (a) iron and (b) iron-zirconia composite deposited at cathodic current density of 20 A/dm2. Bath composition: (a) 1.25M Fe(CH3SO3)2; (b) 1.25M Fe(CH3SO3)2, 20 g/dm3 ZrO2. Temperature 298K, pH 1.3.
Table 4. Results of EDX analysis of the Fe/ZrO2 composite surface
Element Content, wt.%
Region free from the particles entrapped Region of incorporated agglomerate
Fe 96.5 40.4
O 2.7 18.6
Zr 0.3 33.3
Y - 1.7
Other elements 0.5 6.0
(a)
30 40
50 60 70 20, degree
80 90 100
(b)
30 40
50 60 70 20, degree
80 90 100
(c)
<N w m w A
30 40
50
60 70 20, degree
80 90 100
Fig. 10. XRD patterns of (a) iron coating deposited from sulfate plating bath; (b) iron coating deposited from methanesulfonate plating bath; (c) iron-zirconia composite coating deposited from methanesulfonate plating bath.
The XRD patterns of pure iron and composite iron-zirconia coatings are shown in Fig. 10. All coatings exhibit a single phase of the a-Fe matrix with the body centered cubic (bcc) lattice. ZrO2 is present as tetragonal (t) phase. The reflections of the Cu foil substrate were also detectable (they are not presented in Fig. 10).
In order to estimate the Fe crystalline size, the Scherrer equation was used:
D =
0.9X P cos 0
(8)
where D is the crystalline size, A is the wavelength of X-rays, P is the corrected peak width at a halfmaximum intensity and 0 is the angular position.
The calculated average crystallite sizes are given in Table 5. As can be seen, the Fe coatings under study are nanocrystalline, the D value being higher for the sulfate electrolyte than that in case of a methanesulfonate bath. When iron is co-deposited with ZrO2 particles, the crystallite size increases, although insignificantly.
Table 5. Crystallite size of Fe and Fe/ZrO2 composite coatings
Type of coating D, nm
Fe from sulfate bath 60
Fe from methanesulfonate bath 42
Fe/ZrO2 from methanesulfonate bath 48
H = H 0 + kd
-1/2
(9)
Fig. 11. Effect of current density on the microhardness of Fe deposits obtained from (1) methanesulfonate electrolyte containing 1.25M Fe(CH3SO3)2 and (2) sulfate electrolyte containing 1.25M FeSO4. Bath temperature 298K, pH 1.3.
Figure 12 shows the dependence of Vickers microhardness upon the grain size for Fe coatings in linear coordinates according to the Hall-Petch equation. For comparison, the corresponding values of nano-crystalline iron borrowed from [35] are also presented. It can be easily seen that the data obtained in this work satisfactorily coincide with the independent data given in literature.
Microhardness of deposits
Figure 11 demonstrates the dependence of Fe electrodeposits microhardness on current density both for a sulfate bath and a methanesulfonate one. The value of microhardness increases with the current density in the range from 5 to 20 A/dm2. At higher current densities, the microhardness remains practically unvarying. The iron coatings obtained from a methanesulfonate electrolyte are appreciably harder than those from the sulfate system. This may be caused by lowering the nano-crystallite size. Indeed, the microhardness of electrodeposits belongs to the properties which exhibit a strong grain size dependence [32]. The relationship between the microhardness and crystallite size can be expressed by the Hall-Petch equation [33, 34]:
where H0 is the microhardness value at a large grain size, d in the grain size and k is a certain constant for each material.
With reference to this equation, smaller grain sizes introduce higher grain boundary densities and, therefore, higher concentrations of obstacles for the dislocation slip. As a result, the hardness of material grows.
Fig. 12. Vickers microhardness versus grain size d-0.5 for iron powder [35] and for iron nano-crystalline coatings obtained in the present work.
The introduction of yttria-stabilized ZrO2 particles into the iron matrix leads to an appreciable increase in the microhardness of coatings (Fig. 13). A growth in the suspension concentration results in a continuous increase in the deposits microhardness.
The enhanced microhardness of composite Fe/ZrO2 coatings in comparison with pure iron might be due to two possible hardening mechanisms, namely, the grain refinement (i.e. a strengthening effect from the Hall-Petch relationship) and the dispersion strengthening (i.e. a strengthening effect by the Orowan mechanism) [20, 36, 37]. Since embedding zirconia particles in the metal matrix does not lead to the grain refinement (see Table 5), we may
conclude that the strengthening effect from the Hall-Petch relationship is not responsible for the observed growth of the coatings hardness. A dispersion-strengthened composite is characterized by a dispersion of fine particles with an average diameter ranging from about 0.5 to 5 ^m. In this case, the fine dispersed particles impede the motion of dislocations in the metallic matrix resulting in an increase in the material hardness.
Fig. 13. Effect of current density and zirconia concentration in the bath on deposits microhardness. Electrolytes contain 1.25M Fe(CH3SO3)2. Bath temperature 298K, pH 1.3.
To sum up, in the present study the improvement of composite coatings microhardness was achieved by the dispersion hardening effect (i.e. by the Orowan mechanism). As was reported in a number of previous publications [20, 37-39], the effect of the Orowan strengthening may be much stronger than the Hall-Petch strengthening for various composite coatings.
CONCLUSIONS
1. The CE of the iron electrodeposition reaction in a methanesulfonate electrolyte is appreciably higher than that in a typical sulfate bath. The differences in the values of the CE in the electrochemical systems under consideration are related to the features of the HER rather than to the kinetics of the metal deposition.
2. The introduction of yttria-stabilized ZrO2 particles into the iron plating bath allows obtaining composite coatings with the zirconia content up to 10-12 wt.%. It has been shown that the aggregation of ZrO2 nano-particles occurs in the iron plating baths and the aggregates with the average size of several micrometers are embedded in the metal matrix. The content of the zirconia phase in composite coatings increases with an increase in the suspension concentration and a decrease in the current density.
3. The mechanism of co-deposition proposed by Guglielmi proved to be valid for the Fe/ZrO2 composite plating in a methanesulfonate electrolyte. The rate determining the step is controlled by the parti-
cles transference from the loose adsorption to the strong one.
4. Iron coatings obtained from a methanesulfonate bath are harder than those deposited from a sulfate bath due to the strengthening effect by the Hall-Petch mechanism. The inclusion of ZrO2 particles in the iron matrix leads to the enhancement of the coatings microhardness by the dispersion strengthening mechanism.
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Received 22.04.14 Accepted 05.05.14
Реферат
Исследовано электроосаждение железных и композиционных железо-диоксидно-циркониевых покрытий из метансульфонатного электролита. Установлено, что выход по току реакции осаждения железа в метансульфонатном электролите существенно выше, чем в обычном сульфатном электролите. Железные покрытия, электроосажденные из метансульфонатно-го электролита, обладают нанокристаллической структурой. Fe осадки, полученные из метансульфо-натных электролитов, отличаются большей твердостью в сравнении с покрытиями, осажденными из сульфатных электролитов, в результате проявления эффекта упрочнения по механизму Хэлла-Петча. Композиционные Fe/ZrO2 покрытия могут быть получены из электролитов железнения, содержащих частицы диоксида циркония, стабилизированного 3 мол. % оксида иттрия. Кинетика соосаждения частиц ZrO2 с железом из метансульфонатных электролитов описывается моделью Гуглиельми. Включение частиц диоксида циркония в железную матрицу приводит к существенному увеличению микротвердости вследствие механизма дисперсионного упрочнения.
Ключевые слова: электроосаждение, железо, диоксид циркония, композиционное покрытие, метан-сульфонатный электролит.
