ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2016, Vol. 52, No. 2, pp. 254–266. © Pleiades Publishing, Ltd., 2016. Original Russian Text © V.N. Tseluikin, 2016, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2016, Vol. 52, No. 2, pp. 171–184.

NEW SUBSTANCES, MATERIALS, AND COATINGS

On the Structure and Properties of Composite Electrochemical Coatings. A Review V. N. Tseluikin Engels Technological Institute (subdivision) Yuri Gagarin Technical University of Saratov, pl. Svobody 17, Engel’s, Saratov oblast, 413100 Russia e-mail: [email protected] Received March 5, 2015

Abstract—The results of studies of composite electrochemical coatings modified by nano- and microparticles of various natures are presented. Functional properties (hardness, wear resistance, friction coefficient, corrosion resistance, etc.) and structural features of main types of composite coatings are considered. DOI: 10.1134/S2070205116010251

INTRODUCTION Classic galvanic coatings are deposited from homophase electrolyte. Composite electrochemical coatings (CECs) form upon electrocrystallization from suspension electrolytes containing disperse particles of various sizes and types. Being incorporated into a metallic matrix, particles enhance the operational properties of surfaces (hardness, wear resistance, corrosion resistance, etc.) and impart new qualities (antifriction, magnetic, catalytic, etc.). In a number of cases, substitution of classic galvanic coatings by CECs allows one to cut expenses on expensive nonferrous metals and cheapens the process of electrodeposition. Thus, modification of metallic surfaces via CEC deposition finds applications in various branches of industry (mechanical engineering, instrument engineering, production of medical instruments, chemical apparatuses, etc.). Development of new composite coatings and searching for new ways of control of their properties are important scientific and technical problems. CEC matrices are mainly metals and alloys deposited by the galvanic method (nickel, chrome, copper, zinc, etc.). Moreover, composite coatings can be formed basing on anodic oxide films and alloys obtained by chemical (no-current) reduction (Ni–P, Ni–B, Cu–P, etc.) Solid particles with sizes of microand nanometer scales (oxides, carbides, nitrides, metal and nonmetal powders, polymers, etc.) are introduced as disperse phase. Formation of composite coatings is due to interaction forces on three interfaces: disperse phase–electrolyte, electrode–electrolyte, and disperse phase– electrode. The following stages of CEC formation process are distinguished [1]: transport of the disperse particles to the cathode, retention of them near the

cathode surface, and silting of the particles by the deposited metal. Disperse particles can be transported to the cathode surface by stirring, Brownian motion, or an electrophoretic process; under the effect of gravitational forces; or by adsorption of deposited metal cations or other ions (for example, H+, NH 4+ , etc.) on them. Particles retained on the electrode initiate nucleation in the areas of contact with its surface, which stimulates silting of these particles by metal. Various methods are used for intensification of deposition of composite coatings: nonstationary electrolysis modes, ultrasound action, application of magnetic field, preliminary chemical treatment of disperse phase, etc. The first work to have reported the formation of a CEC was published in the United States in 1929 [2]. In our country, systematic works directed to obtaining and studying CECs began in the 1960s. The works of R.S. Saifiullin [3–6], L.I. Antropov [7], G.V. Gur’yanov [8], Yu.M. Polukarov [9–11], et al. made a significant contribution to the development of this scientific direction. Review [12] considered the results of studies of CEC obtained in 1993–2008. The present review presents the results on the studies of structure and functional properties of CEC published by Russian and foreign authors in 2009–2014, as well as a number of earlier works not reflected in [12]. NICKEL-BASED COMPOSITE COATINGS Coatings with nickel matrix are the CECs to have found widest application [1, 3–7]. Composite coatings based on nickel are characterized by high hardness and wear resistance, resistance to corrosive

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5 μm

(а)

255

5 μm

(b)

Fig. 1. Microstructure of nickel coatings obtained from (a) Watts electrolyte and (b) electrolyte modified by CNT (0.06 g/L).

media, and good visual appearance. They are used for machinery and machine parts operating under hard and, especially, severe conditions [7]. Nickel possesses affinity to disperse particles of various natures and easily forms composite coatings with them. Practically all the known electrolytes can be used for production of nickel-based CECs, but sulfamate and sulfate-chloride (Watts electrolyte) electrolytes are used most often [3–7]. CEC Nickel–Carbon Nanotubes Carbon nanotubes (CNTs) are cylindrical structures formed as a result of convolution of flat atomic graphite layers (graphenes). They can be one- and multilayered (consist of several coaxial cylinders). The internal diameter of nanotubes lies in the range from 0.4 to several nanometers, and their length, as a rule, does not exceed tens of micrometers. CNTs can be open and confined by hemispheres consisting of penta- or hexagons [13, 14]. A significant number of publications [15–25] have been devoted to studies of the structure and properties of Nickel-CNT CEC. The introduction of nanotubes into Watts electrolyte leads to a significant increase of microhardness and allows production of non-porous nickel deposits (Table 3). It should be noted that enhancement of the operational properties of composite coatings is achieved upon low CNT content in the Table 1. Influence of CNT content in the electrolyte on the properties of nickel coatings CNT content, g/L 0 0.02 0.04 0.06 0.08 0.10

Microhardness, kg/mm2

Porosity, pores/cm2

390 610 560 770 620 520

7–12 3–8 0.1–1.1 0 0 0

electrolyte (0.05–0.08 g/L). At higher concentrations of disperse phase, microhardness decreases, and coatings of black color with dendrites at CNT concentration in the electrolyte higher than 0.2 g/L [21]. It is assumed that the properties of nickel-CNT CEC enhance due to perfection of the coating structure under influence of nanotubes [21, 22]. The addition of disperse CNT particles into electrolyte leads to refinement of the grain, significant change of morphology and texture of the forming deposits [23]. Microphotographs (Fig. 1) show that the size of nickel-CNT CEC crystals is significantly smaller than the size of crystals of nickel coating without disperse phase. Fine crystalline structure, according to the authors of [21], provides increase of microhardness of nickel CEC modified by CNT. Nickel-CNT CECs deposited from ammonium electrolyte in the pulse mode possess better properties than composite coatings obtained with direct current [16]. The microhardness of these CECs increase to 627 kg/mm2, while the porosity decreases to 0.9% (Table 1). The wear resistance of coatings modified by CNTs increases by three to four times as compared to nickel deposits without disperse phase. In its turn, sliding friction coefficient of the coatings containing 0.5 wt % of multilayer CNT decreases to 0.13 [18]. Density of cathodic current and the presence of surfactant additives affect the structure and properties of nickel-CNT CEC deposited from Watts electrolyte [19, 20, 23]. Introduction of 0.6 g/L of sodium dodecylsulfate (SDS) leads to refinement of the grain in composite materials, and their adhesion to the substrate enhances. Microhardness of the nickel CEC reaches 920 kg/mm2 at SDS concentration of 2–6 g/L and CNT content of 0.4 g/L in the Watts electrolyte [23]. CNT content in the coatings initially increases with increase of current density, reaching the maximum at 8 A/dm2, and then decreases [19, 20]. NickelCNT CEC deposited at 8 A/dm2 possess the best physico-mechanical properties (adhesion, microhardness, corrosion resistance).

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(а)

1 2 3 4

500 nm

(а)

500 nm

(b)

Fig. 2. Surface morphology of (a) pure nickel and (b) composite nickel–nano-SiC coating.

Composite nickel-CNT coatings obtained from Watts electrolyte with additives of saccharine, phthalimide, and butinediol-1,4 exhibit high activity in the hydrogen and oxygen evolution reactions [25]. This effect, conditioned by the developed surface of CEC, allows decrease of the overvoltage of hydrogen evolution by 0.20–0.30 V, and oxygen evolution by 0.30–0.36 V as compared to nickel coatings without disperse phase. CEC Nickel-SiC Hardness and wear resistance of nickel coatings can be significantly increased upon inclusion of nanoparticles of various carbides into their composition, specifically SiC [26]. Application of pulse current upon deposition of nickel-nanosized SiC CEC allows one to increase the rate of disperse particles inclusion into the coatings and to decrease internal stresses [27]. In the case of CECs, the use of pulse current leads to the formation of coatings with a higher content of disperse particles, smaller grain size, and more homogenous distribution of the particles introduced into nickel matrix than in the case of the direct current mode. Figure 2 shows the morphology of a surface of pure nickel deposit and nickel-nano-SiC CEC. It was established using scanning electron microscopy method [28] that SiC particles, generally, are distributed homogeneously in the nickel matrix. The proper-

Vickers microhardness, kgf/mm2

400

200

(b)

5 6 3 4

500

400

300

200 0

30 60 Duty cycle, %

90

Fig. 3. Vickers microhardness values of (a, b) pure nickel and (a) Ni–micro-SiC and (b) Ni–nano-SiC composite deposits obtained in the mode of (2, 4, 6) direct and (1, 3, 5) pulse currents at constant value of pulse sequence frequency v = 0.1 Hz as a function of the duty cycle: (1, 2) Ni–micro-SiC; (3, 4) Ni; (5, 6) Ni–nano-Sic.

ties of nickel-nanosized SiC CEC depend on the type of applied current, size of the introduced particles, content of disperse phase in the deposit, and modification of the microstructure, introduced by both codeposited SiC particles and specific conditions of the current mode [29]. Codeposition of silicon carbide nanoparticles leads to significant reinforcement of the composite nickel coatings obtained in both direct current and pulse modes, as compared to the pure nickel [30]. Application of the pulse current upon electrodeposition facilitates significant increase of hardness of the composite nickel-SiC deposits, specifically, if the pause time is higher than pulse duration, which leads to production of finer grain and increase of fraction of SiC particles in the volume of composite nickel coatings [31]. Nickel-nanosized SiC deposits obtained from Watts electrolyte possess increased hardness as compared to nickel-micro SiC CEC (Fig. 3), which can be explained by increase of silicon carbide particles in the nickel matrix. Moreover, change of the hardness due to codeposition of SiC particles of various size can be associated with the differences in the mechanism of introduction of disperse phase into metallic matrix and, namely, with the intercrystallite

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Table 2. Microhardness of nickel coatings in dependence on disperse phase content La2O3 concentration in the electrolyte, g/L

Coating

Microhardness, kg/mm2

La2O3 content in the coating, wt %

Nickel Nickel–micro-La2O3

0 20

0 10.6

295 378

Nickel–nano-La2O3

20

2.4

510

type of introduction in the case of silicon carbide of micron size and, partially, intracrystallite type for SiC nanoparticles [30]. The “introducibility” of these particles into nickel coating deposited from sulfamate electrolyte can be significantly increased via preliminary treatment of SiC in HF, and SiO2 in HNO3 [32]. The mechanism of inclusion of SiC into the nickel matrix and, therefore, the rate of inclusion, at low current densities is determined by adsorption, while at high current densities it is determined by transport of the particles of the second phase [33]. Yield by nickel current increases in the presence of silicon carbide in the electrolyte; however, the degree of texturing of the nickel-SiC CEC amounts to 15–90% of the one characteristic to pure nickel coatings. This is probably due to the fact that not only nickel cations and silicon carbide particles adsorb on the cathode, but hydrogen also [30]. Other Nickel-Based CECs Functional properties of composite coatings, such as hardness, wear resistance, and corrosion resistance, depend on the state of intercrystallite boundaries between grains of the disperse phase and metallic matrix. Addition of TiO2 particles into the nickelation electrolyte enhances mechanical and corrosion properties of the electrolytical nickel [34–39]. The authors of [39] have noted the increase of microhardness by 1.3–1.6 times for nickel–TiO2 CEC as compared to electrolytical nickel without disperse phase. And inclusion of titanium dioxide into the nickel matrix leads to increase of corrosion resistance of the coatings by 1.5–2 times [25, 39]. Nickel-based CECs deposited from Watts electrolyte with TiO2 nanoparticles in pulse current contain more inclusions of disperse phase than do the deposits obtained in the direct current mode [36]. The frequency of the pulses affects the orientation of the crystals and grain size. In addition, the microhardness of nickel–TiO2 CEC is higher than that of pure nickel independently of the deposition mode, which is due to not only compacting of the deposit, but also increase of the perfection degree of the texture [34, 36]. Structural changes have been noted also for nickel–TiO2 deposits obtained in the direct current mode [39]. Nickel-fullerene C60 CECs are studied in [40, 41]. C60 molecule has a confined shell and abundance of

multiple bonds and can reversibly receive electrons without destruction of its structure [42]. Fullerenes are hydrophobic and are soluble only in apolar or weakly polar solvents; however, there are various methods for preparation of aqueous dispersion of C60 [43–47]. Nickel–C60 CECs are deposited from sulfate-chloride electrolyte at less negative potential values than nickel coatings without disperse phase [50]. Fullerene C60, being an electron acceptor, tends to assume a negative charge in solution. Due to this, it adsorbs nickel cations on its surface, which facilitate transport of disperse particles to the cathode and inclusion of them into crystalline lattice of the deposit. Analysis of nickel–C60 CECs by method of secondary-ion mass-spectroscopy has shown the presence of carbon and C–H bonds in them [40]. Carbon content in the coatings amounts to about 1.5wt %. Obviously, fullerene particles are hydrated by cathodically co-discharging hydrogen in the process of electrodeposition. Surface layers of CEC deposits contain the highest amount of disperse phase. Friction coefficients of nickel–C60 CEC obtained from sulfate-chloride electrolyte decrease by 2.5– 3 times as compared to pure nickel deposits [40]. Increase of hardness by 13–17% and wear resistance by 1.5 times is observed in forming deposits upon joint deposition of nickel with fullerene from sulfamate electrolyte [41]. A number of works [48, 49] have been devoted to studies of CECs modified by micro- and nanoparticles. The wear resistance of nickel composite coatings containing nanosized La2O3 particles exceeds the values of this parameter of the nickel deposits with inclusions of lanthanum oxide microparticles [48]. The size of the crystals of the forming deposit decreases upon join deposition of nickel with La2O3 nanoparticles from sulfamate electrolyte, which leads to a significant increase in microhardness (Table 2). From the results of studies [49] of nickel coatings containing chrome carbonitride, it follows that wear resistance of nickel–nano Cr3(C0.8N0.2)2 CEC is 1.5 times higher than the wear resistance of nickel–micro Cr3(C0.8N0.2)2 and by 1.7 times greater than that of nickel deposits without disperse phase (Fig. 4). In addition microhardness, corrosion resistance, and heat resistance of CEC-modified chrome nanocarbo-

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–E, V (vs. Ag/AgCl) 1.6

Δm × 102, kg/m2

1

10 3 1.5 8

2 1.4 2 1

6

1.3

Δ = ± 0.25 –2.4

4 250

500

750

–2.0

1000 τ, s

Fig. 4. Wear resistance of CEC: (1) Ni–nano Cr3(C0.8N0.2)2 CEC, (2) Ni–micro Cr3(C0.8N0.2)2 CEC, and (3) Ni.

nitride also increase as compared to the coatings, containing microparticles of Cr3(C0.8N0.2)2. The authors of [50–52] studied the deposition and properties of nickel–ZrO2 CEC. The Vickers microhardness of nickel deposits increases by three to five times upon inclusion of zirconium oxide particles. Microhardness of composite coatings increases with increase of ZrO2 + disperse phase content in the electrolyte. This effect is due to refinement of the grain and decrease of plastic deformation in the nickel matrix upon inclusion of zirconium oxide particles. Nickel coatings can be modified by powders of various metals. The friction coefficient and rate of wear of the forming coatings decrease upon introduction of titanium nanoparticles into Watts electrolyte [53]. Nickel-niobium CECs possess more rough morphology as compared to the coatings without disperse phase obtained under the same conditions [54]. Moreover, nickel coatings modified by niobium have increased microhardness and corrosion resistance, which is due to refinement of the grain and change of the deposit structure. In [55], it was established that cathodic evolution of hydrogen from KOH solution on electrodes coated by nickel–V2O5 CEC occurs at less negative potentials than at electrodes with pure nickel coating. Figures 5 and 6 show polarization curves of hydrogen evolution in Tafel coordinates. It can be seen that an increase of electrolyte temperature leads to increase of cathodic hydrogen evolution rate. Slopes of the linear parts increase from 118 to 134 mV upon an increase in temperature from 25 to 70°C. From the data of angular coefficients it is assumed [55] that hydrogen is released in accordance with the Volmer–Tafel, Volmer–Hey-

–1.6

–1.2

log ik, (ik, А/сm2) Fig. 5. Polarization curves of hydrogen evolution on steel electrodes with surface modification of (1) nickel and (2) nickel + V2O5 in Tafel coordinates.

–E, V (vs. Ag/AgCl) 1 2 3 4

1.40 1.35 1.30 1.25 1.20 1.15 –2.4

–2.0

–1.6

–1.2

log ik, (ik, А/сm2) Fig. 6. Polarization curves of hydrogen evolution on steel electrode coated by nickel with V2O5 inclusions at temperatures of 30% KOH solution (°C): (1) 25, (2) 40, (3) 55, and (4) 70 in Tafel coordinates.

rovsky, or Tafel–Goriuchi mechanisms. Application of nickel–V2O5 CEC allows one to decrease power consumption in hydrogen production by electrolysis of alkaline solutions. CHROME-BASED COMPOSITE COATINGS Electrolytic chrome coatings are used to increase the hardness and wear resistance of metallic surfaces, and also for recovery of worn parts. In the latter two cases, as a rule, thick layers of chrome have to be

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ND CECs from electrolyte of a composition, g/L, of CrO3 250, H2SO4 2.5, and ND 2–20, the microdensity of the coatings significantly increases, while the friction coefficients and wear decrease. The ND content in the coating amounts to 0.3–1.0 wt %.

(а)

10 μm

(b)

10 μm

(c)

1 μm

Fig. 7. Microphotographs of the electrodeposited coatings surfaces: (a) Cr (1000×), (b) Cr–ND (1000×), and (b) Cr–ND (10000×).

applied. However, this is associated with difficulties, both economic (high cost) and technological (low yield by chrome current). One methods for decreasing the thickness of chrome coatings upon enhancement of their physico-mechanical properties is production of CECs. A significant number of publications have been devoted to joint deposition of chrome and nanodiamonds (NDs) [56–60]. ND particles have sizes around 4–6 nm, an oval or spherical form, and a developed surface (up to 450 m2/g) and high surface energy. Structurally, ND is a diamond core, confined into a shell of X-ray-amorphous carbon structures. In [56] it was shown that, upon production of chrome–

The authors of [58, 59] studied the structure and physico-mechanical properties of chrome–ND CECs deposited from electrolytes based on Cr(III). Microhardness of coatings increases from 1000 kg/mm2 (pure chrome) to 1365 kg/mm2 (CEC) upon introduction of ND particles into chrome-plating electrolyte in concentration of 20 g/L. Increase of ND content in the solution up to 30 g/L leads to decrease of microhardness to 940 kg/mm2, which, is, probably, associated with increase of brittleness of the coatings upon inclusion of higher number of disperse particles. Optimal content of disperse phase in chrome–ND CEC, at which a maximum of microhardness and minimum of microbrittleness of the deposits is observed [59] amounts to 10.5 v % (5.6 wt %). The optimal concentration of ND in electrolytes based on Cr(III) is 17 g/L. The structure of chrome coatings and a chrome– ND CEC is shown in Fig. 7. Wide open cracks are seen on the chrome deposit (Fig. 7a), while narrow blindend cracks are observed in CECs (Fig. 7b) [58]. Studies of the composite coatings using scanning electron microscopy at various magnifications have shown that a chrome-ND CEC is characterized by inclusion of a large number of disperse particles of various size, which are homogenously distributed in the metallic matrix (Figs. 11b, 11c). Disperse ND phase, being incorporated into the coatings, changes the nature of the formation and growth of crystals, decreasing their size and internal stresses. Such changes, in turn, increase the microhardness of a CEC. ND particles tend to form aggregates. According to the data of [58], the average radius of diamond aggregates in the chrome-plating electrolytes amounts to 4530 nm, while in composite coatings it is only 204 nm. Disaggregation of ND particles and inclusion of smaller formations into a CEC probably occurs in the near-electrode layer. The authors of [61, 62] studied joint deposition of chrome with Al2O3, SiC, Nb2N, and Ta2N from sulfate-oxalate solutions. Inclusion mechanisms of the aforementioned particles into the coatings differ. In the case of Al2O3 (dielectric) and SiC (semiconductor), inclusion is conditioned by kinetic factors (adhesion of particles does not occur in the absence of current). Nb2N and Ta2N particles (conductors) have strong adhesion in the absence of current. Distribution of disperse phase of Al2O3 and SiC is inhomogeneous in its coating thickness, while Nb2N and Ta2N are characterized by the accumulation of the particles near the substrate in concentrations insignificantly differing from their content in surface layers. Increase

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(а)

10 μm

5 μm

(b)

Fig. 8. Surface of chrome deposits obtained at current density ik = 60 A dm–2 from solutions with additives of (a) Nb2N and (b) Ta2N.

of the solution temperature facilitates codeposition of Al2O3, SiC, Nb2N, and Ta2N with chrome. The surface of the deposit of pure chrome is covered with spherical elements characteristic for amorphous structure growth. Smoother coatings are formed upon the introduction of disperse Al2O3 phase into electrolyte. Aluminum oxide particles are included only into the surface layer of the deposit and affect catalytic properties of the surface in regards of hydrogen evolution reaction. Al2O3 is not incorporated into the CEC bulk volume. Silicon carbide particles incorporated into the coatings significantly increase the roughness of a CEC. Inclusion of SiC into the volume of the deposit leads to the appearance of pitting [62]. The surface of the coatings obtained from electrolyte containing Nb2N and Ta2N is developed with rodlike inclusions (Fig. 8). Codeposition of particles of all types does not affect the structure of the matrix. The coatings have x-ray-amorphous structure, which indicates the mechanism of electrocatalytic reduction of oxalates on the newly-formed chrome surface with subsequent capsulation of the chrome clusters by the products of oxalate reduction [61]. A technique for production of chrome coatings modified by CNT is proposed in [63]. It is established that introduction of CNT particles into standard chrome-plating electrolyte in a concentration of 0.06 g/L leads to an increase in the microhardness of the forming deposits by 28%. Inhomogeneity of the chrome coating decreases by 2.5 times. Figure 8 shows the morphology of the surface of the electrolytic chrome deposits. The size of spheroids in the case of a chrome-CNT CEC is smaller than that of chrome coating without disperse phase. This conditions the increase of microhardness of a CEC modified by carbon nanotubes. OTHER TYPES OF COMPOSITE COATINGS Copper-Based Composite Coatings The main purpose of copper-based CEC is to impart hardness, wear resistance, heat resistance, and antifriction properties to metallic surfaces [4–6, 8].

(а)

(b) Fig. 9. Images of surface morphology of chrome coatings obtained in (a) standard chrome-plating electrolyte and (b) electrolyte with CNT (0.04 g/L).

Ethylenediamine, pyrophosphate, and sulfate electrolytes are usually used for production of copper-based CEC. The authors of [56] studied the process of copper–ND CEC deposition from the solution with composition, g/L: CuSO4 80; H2SO4 100. The introduction of ND into the electrolyte in concentrations from 0.1 to 5.0 g/L does not change the nature and mechanism of the electrode process. Scattering power of the electrolyte with addition of ND increases by 3 times as compared to initial solution. Inclusion of ND into copper matrix leads to decrease of the number of pores from 10 per 1 cm2 (0.1 g/L ND) to total absence of them (5.0 g/L ND). As a result, the coatings are more dense and fine-grained. No decrease of mass was observed in the process of corrosion tests. Microhardness of the coatings deposited from the electrolyte with

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(а)

50 μm

(b)

50 μm

10 μm

(c)

50 μm

Fig. 10. Microphotographs (scanning electron microscopy) of the surface morphology of (a) pure copper, (b) composite coatings copper–0.4 wt % Si3N4, and (c) copper–3 wt % Si3N4.

ND concentration of 5.0 g/L increases by almost 1.5 times as compared to the deposits obtained from the base electrolyte. The wear of copper–ND CEC is nine to ten times lower than that of pure copper. The structure and physico-mechanical properties of copper-based coatings modified by silicon nitride

261

were studied in [64, 65]. According to the obtained results, friction coefficients decrease from 0.52 for pure copper to 0.16–0.24 for copper–Si3N4 CEC. Microhardness of the composite coatings increases upon increase of the Si3N4 content in the copper matrix. Morphology and structure of the deposits is determined by the inclusion of the silicon nitride particles (Fig. 10). Fine-grain coatings form in the presence of Si3N4, and the dominant orientation of the crystals changes. Crystallites of smaller size than in deposits without disperse phase form in copper-based composite coatings modified by carbon nanotubes [66]. Due to structural changes, copper-CNT CECs possess higher electroconductivity, microhardness, and wear resistance than do pure copper coatings. The authors of [67] studied the deposition of copper–SnO2 CECs from pyrophosphate electrolyte onto aluminum substrate without application of intermediate layers. The addition of nanocrystalline SnO2 (average particle size of 13 nm) into the electrolyte leads to decrease of the slope of the cathodic branch of the polarization curve in the potential range below –0.8 V, and to decrease of the anodic current density. The corrosion of aluminum samples with copper–SnO2 coatings in acidic and alkaline media slows down by two to four times. A decrease in corrosion currents is also observed for copper–SiO2 coatings obtained from sulfate electrolyte with the addition of silicon oxide nanoparticles [68]. Corrosion process slows down due to the fact that inclusion of disperse SiO2 phase into copper matrix leads to a decrease of the active surface of the coating in contact with corrosion medium. Zinc-Based Composite Coatings Zinc-based composite coatings are used for protection of steel surfaces from corrosion with enhancement of their physico-mechanical properties [8]. Zinc coatings are deposited from various electrolytes, which conditionally can be divided into acidic, neutral and weakly alkaline, and alkaline [69]. The corrosion properties of Zinc–ND CEC deposited from alkaline (zincate) and weakly acidic chloride electrolyte are studied in [14, 70]. It has been established experimentally [14] that the optimal content of nanodiamonds (ND) in the electrolyte amounts to 10 g/L, and 0.7 of ND is incorporated into the coating. The introduction of disperse ND particles into zincate electrolyte leads to shift of the potential into positive direction by 20– 25 mV (at a concentration of ND 10 g/L), which indicates partial depolarization of the double electric layer [92]. The amount of disperse phase in the coatings and grains of deposits becomes finer with an increase of ND concentration [14]. Corrosion resistance of zincND CEC significantly increases upon passivation of

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(а)

10 μm

(b)

10 μm

(c)

10 μm

(d)

10 μm

Fig. 11. Images obtained by the scanning electron microscopy method: (a) zinc coating at high resolution, (b) zinc coating with low resolution, (c) zinc–CNT CEC at high resolution, and (d) zinc–CNT CEC at low resolution.

the coatings by standard chromate treatment and phosphating. Phosphating is preferable, since it is less ecologically hazardous. The authors of [73–76] studied the deposition and functional properties of zinc-based CECs modified by carbon nanotubes. Friction coefficient decreased by 1.5 times and corrosion resistance increases by 1.6– 1.7 times for zinc-CNT CECs obtained from alkaline (zincate) electrolyte as compared to zinc deposits without disperse phase [73, 74]. Shift of the zinc deposition potentials to the region of more positive values occurs upon introduction of CNT into acidic sulfate zinc-plating electrolyte [76]. Non-porous composite coatings form. Microphotographs (Fig. 11) noticeably show the differences in the morphology of the pure zinc and zinc–CNT CEC surfaces. Performed studies revealed that increase of corrosion resistance of these CEC is more than two times better than that of zinc deposits without disperse phase. This is due to the filling of micropores on the zinc surface, which are active centers of metal dissolution, by CNT particles. Nanocomposite zinc–TiO2 coatings deposited from acidic chloride electrolyte [77] also have higher corrosion resistance than pure zinc deposits. Iron-Based Composite Coatings Electrolytic iron coatings are close to steel in hardness, and so they are used for recovery of parts of machines [8]. Such coatings are reinforced even more upon use of ND as disperse phase [13, 78]. However, upon deposition of an iron-based CEC, it is more

effective to use not pure nanodiamonds, but less expensive diamond charge. The microhardness of a CEC increases twofold as compared to iron–ND coatings upon introduction of the charge. The corrosion behavior of iron–Al2O3 CEC in 0.05 M Na2SO4 solution and 5% NaCl was studied in [79]. Introduction of aluminum oxide disperse phase into the iron matrix leads to corrosion potential shift into positive direction and decrease of anodic dissolution currents. CEC corrosion rate decreases as compared to pure iron deposits. Researchers of [80, 81] obtained iron–TiO2 and iron–ZrO2 CEC from methanesulfonate electrolyte. Methanesulfonic acid is relatively ecologically safe as compared to other mineral acids. Introduction of nanodisperse particles of both titanium dioxide and zirconium dioxide leads to increase of microhardness of the coating due to the effect of dispersion reinforcement. Iron–TiO2 CEC exhibits photochemical activity in regards to the reaction of methyl-orange dye decomposition reaction. Composite Coatings Based on Electrolytic Alloys Process of electrodeposition of alloys is more technically difficult than is the deposition of individual metals. Nevertheless, electrolytical alloys have found wide application as protective–decorative, wear resistant, corrosion resistant, and other functional coatings. The inclusion of disperse particles into the alloy matrix allows one to significantly enhance their operational properties.

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Deposition of nickel–cobalt–nanodiamond composite coatings from Watts electrolyte with CoSO4 addition was studied in [82, 83]. For CEC, microhardness increases twofold, and wear rate decreases by 3– 3.5 times as compared to nickel-cobalt alloy without disperse phase. The roughness of the nickel–cobalt– ND coatings is lower than nickel–diamond deposits [82]. The wear resistance of these CECs in the dry friction mode exceeds wear resistance of chrome coatings by 1.5 times [83]. The presence of cobalt cation in the Watts electrolyte leads to an increase of percent content of disperse phase in the coating. This is due to the fact that Co2+ ions adsorb on the diamond surface more easily than do Ni2+ ions. As a result, diamond particles take on a positive charge, and their electrostatic attraction to the cathode surface increases. Reduction potentials shift to the direction of more negative values upon introduction of carbon nanotube dispersion into sulfate electrolyte of nickel–cobalt alloy deposition [84]. CNTs do not significantly affect the alloy electrodeposition process; however, they determine the structure and properties of formed CECs. CNT particles homogenously distribute through the Ni–Co matrix. Composite coatings possess higher values of microhardness and elasticity modulus than the alloys without disperse phase. Nickel–cobalt–Al2O3 CECs deposited from sulfate-chloride electrolyte have been proposed [85] as an alternative to chrome coatings, since their wear resistance at load upon a frictional contact of 2 MPa exceeds the wear resistance of electrolytic chrome. Fullerene C60 particles in chloride electrolyte of iron–nickel alloy deposition facilitate an increase of the cathodic process rate and are incorporated into the composition of formed coatings [86]. Sliding friction coefficients of iron–nickel–C60 decrease by 1.5–2 times as compared to pure iron–nickel deposits. Anodic Oxide Films Anodic oxide films (AOFs) are obtained on metals that can move into a passive state (Al, Ta, Nb, Zr, Ti, etc.) [87]. Deposition of AOFs is the main method for protection from corrosion and increasing the durability and wear resistance of these metals and alloys based on them. This process is used in mechanical engineering, aviation, and the chemical industry. An AOF on aluminum consists of densely packed Al2O3 particles and has a polymer-colloid structure [7]. Growth of the film occurs in the layer directly adjacent to the metallic surface. This thin layer that is constantly being replenished is impermeable to ions that participate in the formation of AOF. Anodic oxidation of aluminum is usually performed in acidic medium, but sometimes in alkaline medium. Nanodiamond particles in oxidation electrolytes take on a negative charge and move toward the anode. There, they fill the pores of the forming AOF and are

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retained in them by mechanical and Van der Waals forces. In addition, the mass of the filled oxide film increases by two to four times [14]. The presence of ND in the solution does not decrease the AOF growth rate and increases the scattering power of the electrolyte. The value of filling of oxide films by ND particles depends on the electrolysis mode and concentration of nanodiamonds in the solution. Optimal ND content in the oxidation electrolytes amounts to 2–15 g/L. Wear resistance of AOF with ND inclusions increases by 10–13 times, their corrosion resistance, electric insulator properties, and visual appearance are improved [14]. One of the methods for production of a composite AOF is plasma-electrolytic oxidation (PEO)—oxidation of the metallic surface by electric discharges [88]. According to the data of [89–92], particles of disperse phase, slightly fusing, attach to the AOF surface, mainly unchanged. The authors of [91] obtained oxide films on aluminum from phosphate and silicate electrolytes containing ZrO2 nanoparticles of monoclinic modification by the PEO method. The disperse zirconium phase is located, mainly, in the surface layer of the coating, as well as inside the AOF cavities. In addition, the particles in the coating are analogous to those introduced into the electrolyte in form, size, and phase composition. The average zirconium concentration in near-surface layer of AOF obtained on aluminum from silicate electrolyte amounts to about 1 at %. Oxide films of magnesium obtained from silicate electrolyte with ZrO2 nanoparticles consist of two main layers: external and internal [90]. Zirconium nanoparticles are mainly located on the deposit surface and on the interface between the internal and external layers of oxide films. Upon local heating from microdischarges, zirconium reacts with magnesium, thus forming oxide with Mg2Zr5O12 composition in the interval layer of the coating. The authors of [92] obtained oxide films on titanium from borate and silicate electrolytes with MnO2, Mn2O3, NiO particles by PEO method. Coatings that have a globular surface structure form in sodium tetraborate-based electrolytes. Inclusion of disperse phase into the composite coatings is significantly higher, probably due to physico-chemical properties of the aqueous sodium silicate solutions. AOF on titanium containing manganese and nickel compounds are of interest for catalysis. Manganese-containing oxide-ceramic coatings obtained on aluminum and its alloys by microarc oxidation method can be used as catalytically active material [93]. The possibility of their application in this capacity is confirmed in the process of electrosynthesis of oxidizers used in oxidation of phenols.

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Resin Epoxyy Resin Epoxyy

Substrate

Substrate 50 μm

(а)

50 μm

(b)

Fig. 12. Microphotographs of sections of no-current coatings (a) Ni and (b) Ni–PTFE–MoS2.

Chemical Composite Coatings Chemical reduction of metals is an autocatalytic process of deposition of metal coatings via interaction of metal ions in the solution and reducing agent [94]. This method has some advantages over electrochemical deposition. First, chemical coatings can be deposited by a uniform layer onto parts of complex configuration. Second, dielectrics and semiconductors can be metalized by the chemical method in order to modify their surface. Thus, along with electrochemical ones, chemical composite coatings (CCCs) are studied. Inclusion of solid particles into chemical nickel coating induces shifts in its structure, which enhance the functional properties of the CCC [95]. For example, the hardness of chemical nickel coatings increases in the presence of diamond nanoparticles in them. Nanosized silicon oxide particles also provide high hardness of a CCC based on nickel and copper [41]. Inclusion of disperse SiC phase into the matrix of a copper–P CCC increases the corrosion resistance of these coatings in NaCl and HCl solutions [96]. Molybdenum disulfide increases the hardness and wear resistance of chemical coatings [42]. The author of [97] established that friction coefficients of Ni–P– nano-MoS2 CCC decrease by two to three times as compared to analogous coatings without disperse phase (Fig. 12). The microhardness of a nickel CCC modified by molybdenum disulfide reaches a maximum value (about 750 kg/mm2) after thermal treatment at 400°C [98]. Nanocomposite Ni–P–carbon soot coatings have low radiative ability [99]. Diamond particles increase the thermal conductivity of nickel-based CCCs [99]. Moreover, nanodiamonds significantly increase [100] the corrosion resistance of no-current Ni–P coatings.

researchers is being devoted to CECs with inclusions of nanodisperse materials can be noted as a general tendency. The selection of the disperse phase for inclusion into metallic matrix is conditioned by functional purpose of the coatings. CECs based on nickel and chrome, as well as copper, are finding application as hard wear- and corrosion-resistant coatings. Correspondingly, particles incorporated into their composition should enhance these properties. While NDs or SiC increase the microhardness and wear resistance of CECs due to their intrinsic high hardness, CNTs facilitate the formation of a dense fine crystal matrix. Zinc-based CECs are used mainly as corrosionresistant coatings. Disperse particles increase their corrosion resistance both due to filling of micropores in the coatings and uniform distribution of corrosion currents, as well as the formation of centers that hinder their propagation. The surfaces of metals that can turn into a passive state are modified by AOF deposition. Particles that are part of the AOF composition facilitate increase their durability and corrosion resistance. Chemical composite coatings are deposited for modification of dielectrics and semiconductor surfaces. The inclusion of disperse particles into CCCs leads to enhancement of their functional properties. Based on the considered material, a conclusion can be drawn that new coatings with increased hardness, wear resistance, corrosion resistance, and other valuable properties are being developed presently. Many composite coatings are finding industrial applications. CECs modified by ND, CNT, SiC, and other particles are used in industrial production both in Russia and abroad. Thus, one can boldly assume that interest in this field of galvanic technology will only increase. REFERENCES

CONCLUSIONS Studies of the structure and functional properties of CECs are continuing to successfully develop [101, 102]. The fact that significant attention on the part of

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