ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2017, Vol. 53, No. 3, pp. 433–436. © Pleiades Publishing, Ltd., 2017. Original Russian Text © V.N. Tseluikin, 2017, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2017, Vol. 53, No. 3, pp. 278–281.
NANOSCALE AND NANOSTRUCTURED MATERIALS AND COATINGS
Electrodeposition and Properties of Composite Coatings Modified by Fullerene C60 V. N. Tseluikin Engels Technological Institute (Branch), Gagarin Saratov State Technical University, Saratov, Saratov oblast, 413100 Russia e-mail:
[email protected] Received May 19, 2016
Abstract⎯Composite electrochemical coatings (CECs) based on nickel and fullerene C60 as the dispersed phase are obtained. The process of electrodeposition of these coatings is studied. The method of secondary ion-mass spectrometry was used to determine the composition of nickel–fullerene C60 CECs. It is found that coefficients of sliding friction of the obtained CECs decrease by from 1.8 to 2.5 times as compared to similar nickel coatings with no dispersed phase. Corrosion behavior of nickel–fullerene C60 CECs is studied in 0.5 M H2SO4 solution. DOI: 10.1134/S2070205117030248
INTRODUCTION Composite electrochemical coatings (CECs) are obtained by simultaneous electrodeposition of metals with different dispersed particles from suspension electrolytes [1–3]. Particles incorporated into the metal matrix improve performance characteristics of galvanic deposits and confer new properties on them. In a number of cases, replacement of monophase coatings on CECs allows economizing on base metals and cheapens the electrodeposition process. Therefore, CECs find application in different branches of industry (mechanical engineering, instrumentation, manufacturing of medical appliances, chemical apparatus, etc.) and development of new composite coatings is an important research and technology problems. The greatest acceptance among CECs was gained by coatings with a nickel matrix [1, 2]. They are characterized by high hardness and wear resistance, as well as stability in corrosive media. CEC characteristics are largely determined by the properties of the dispersed phase. Nickel manifests an affinity to dispersed particles of different natures and easily forms composite coatings with them. In particular, nickel-based CECs modified by SiC [4, 5], TiO2 [6–10], La2O3 [11], ZrO2 [12, 13], etc., have been obtained and studied in recent years. Analysis of publications of Russian and foreign authors [14] shows that researchers pay significant attention to composite coatings modified by different nanoparticles, as intercalation of nanosize particles into the metal matrix allows obtaining structural materials exceeding their coarse-grained counterparts
by their functional properties. One promising nanomaterial is fullerene C60. Fullerene molecules have a closed shell and an abundance of multiple bonds. They can easily and reversibly accept one to six electrons without structure degradation [15, 16]. The aim of the work is to obtain nickel-based CECs modified by fullerene C60 and to study the electrodeposition, tribological properties, and corrosion behavior of these coatings. EXPERIMENTAL A colloid aqueous dispersion of fullerene C60 free of organic solvents was prepared according to the method of [17]. Adding this dispersion to nickel-plating electrolyte yielded a solution with the following concentration of components, g/L: NiSO4 · 7H2O: 220; NiCl2 · 6H2O: 40; CH3COOK: 30; C60: 0.05. Nickel-fullerene C60 CECs were electrodeposited onto a steel support (steel 45) at a temperature of 25°C with constant stirring of electrolyte. Nickel coatings with no dispersed phase were deposited from C60-free electrolyte of the above composition. Analyses of deposits of nickel-fullerene C60 CECs were carried out using the method of secondary ion mass spectrometry (SIMS) on an MI-1305 magnetic mass spectrometer equipped by a universal add-on for studying solids. Coatings for analysis were deposited onto a copper support. Tribological tests of the coatings were carried out in the case of friction with no lubricants. The counter-
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–E, V (SHE)
–log ik, [i, А/сm2]
1
1.2 1.0
5
2
0.8 0.6 4
0.4 0.2
3
0 0.5
2
1 0.4
1.0
1.5
2.0
2.5
3.0
3.5
1 2
0.5
0.6
0.7 0.8 –E, V (SHE)
Fig. 1. Cathodic polarization deposition curves of (1) nickel coatings and (2) nickel–fullerene C60 CEC.
body used was a steel sample (carbon steel). The counterbody mass was 1 g in all tests. The coefficient of sliding friction of electrolytic coatings was determined according to the following formula:
Ffr = tan α, Р where Ffr is the sliding frictional force and P is the normal pressure force. Electrochemical measurements were carried out on a P-5848 potentiostat coupled with a KSP-4 automatic potentiometer. Potentials were set versus the saturated silver/silver chloride reference electrode and recalculated to the hydrogen scale. f =
RESULTS AND DISCUSSION The initial deposition currents of nickel coatings and nickel–C60 CECs were used to construct the log ik–E-dependences (Fig. 1). Curves of cathodic polarization show that introduction of fullerene particles into electrolyte facilitates the cathodic process: in the studied potential range, CECs are deposited at lower negative potentials E than the nickel coating containing no dispersed phase. Transport of dispersed-phase particles to the cathode can occur through the stage of adsorption of cations of the deposited metal on their surface [1, 3]. When the particles are charged, they are transported to the cathode, where a deposit grows over them. Ions
4.0 t, s
Fig. 2. Galvanostatic curves of nickel deposition (1) with no additive and (2) together with fullerene C60 at current density ik = 8 A/dm2.
adsorbed on particles participate in the bridge bonding of the dispersed phase to the cathode surface [3]. This bonding attenuates the disjoining pressure of the liquid streak between the particle and the cathode, i.e., enhances adhesion. Fullerene C60, being an electron acceptor, tends to acquire a negative charge in the solution of electrolyte under electric current. This, in its turn, promotes adsorption of nickel cations on it. Aggregated dispersed particles then move toward the cathode and are embedded into the crystal lattice of the electrolytic deposit. In the case of deposition in the galvanic mode, potentials are also shifted in the positive direction when fullerene is added to the sulfate chloride nickelplating electrolyte (Fig. 2). At the initial time moment, galvanostatic curves contain a potential jump; then, the process passes into the steady-state mode. This jump can be explained by the fact that, when power is switched on, a large amount of energy is consumed at first in formation of metal nuclei. The results of galvanostatic measurements were used to calculate the polarization capacitance of the electrodeposition process using the following expression:
C=
i , ∂E ∂t
where i is the current density, A/cm2; E is the potential, V; and t is the time, s. The values of polarization capacitance decrease at a transition from nickel deposits to nickel–C60 CEC deposits (Table 1). This can be explained by the fact that introduction of dispersed particles into the electric double layer results in an increase in its size. Analysis of the composition of composite nickel– fullerene C60 coatings using the SIMS technique showed the presence of carbon and C–H bonds in the
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It needs to be noted that the greatest amount of the dispersed phase is present in the surface layers of the studied composite coatings. This is indicated by a decrease in the intensity of secondary ions in the case of advancement inward the deposit (Fig. 3, curve 2). Nickel content, on the contrary, increases in the above direction (Fig. 3, curve 1). Addition of dispersed particles into the coating results in structural changes in the metal matrix [3]. In turn, the change in the structure of the electrolytic deposit must affect its performance characteristics. Tribological characteristics of metallic surfaces and particularly sliding friction coefficient f are of considerable practical interest. According to Amontons’ law, the friction coefficient depends on the nature of the contacting materials. Under the usual conditions, in the case of dry friction in air f is generally in the range of 0.1–1. In the case of nickel–fullerene C60 CECs, as dependent on the cathodic current, f varies from 0.20 to 0.12 (Table 2), i.e., the sliding friction coefficients of the composite coatings decrease by from 1.8 to 2.5 times. This is related to the fact that fullerenes incorporated into the coating in the course of electrodeposition function as a dry lubricant (the experiment was carried out under the conditions of dry friction). It is shown in [19, 20] that addition of fullerene C60 (up to 5 wt %) into industrial oil results in a three- to fourfold decrease in the friction coefficient. Besides, abnormally small wear of counterbodies is observed. These phenomena are explained by the fact that mineral oil is polymerized on rubbing metal surfaces in the course of friction. Here, a thin plastic layer (tribopolymer film) with a low shearing resistance is formed, which facilitates the breaking of adhesion compounds (Rehbinder effect). Fullerene C60 being a chemically active compound interacts with molecules of lubrication oil and promotes formation of friction polymers bound to the metal surface. It should be expected that the presence of fullerene C60 particles directly in the galvanic coating must facilitate the process of formation of a tribopolymer film as a result of addition of lubrication oil, increase the strength of this film, and improve its adhesion to the metal surface.
Table 1. Values of polarization capacitance C × 103, F/cm2 in the case of deposition of nickel and nickel– fullerene C60 CEC C × 103, F/cm2
ik, A dm–2
Nickel
nickel–fullerene C60 CEC
2 3 4 5 6 7 8 9 10
22.3 40.1 52.9 67.4 79.7 92.8 107.3 119.9 133.2
11.6 14.9 23.2 24.1 29.9 34.9 46.5 52.3 66.4
Table 2. Coefficients of sliding friction f for nickel coatings at different cathodic current densities ik, A dm–2
6
Nickel CEC nickel–fullerene C60
7
8
9
10
0.38 0.34 0.34 0.33 0.30 0.20 0.19 0.17 0.15 0.12
One of the most important physicochemical properties of galvanic coatings is corrosion stability. Figure 4 shows anodic potentiodynamic curves (PDCs) of nickel and nickel–fullerene C60 CECs. Dispersed fullerene particles cause an increase in the potential and accordingly a decrease in the current of active anodic dissolution of the studied coatings. Electro-
Intensity. au
deposits. The carbon content in composite coatings is about 1.5 wt %. The presence of C–H bonds in the deposit structure is due to the fact that fullerene particles are hydrated before their inclusion into the coating by cathodic hydrogen discharged simultaneously. This likely occurs at the stage of formation of Hads. The possibility of electrochemical hydration of fullerene C60 in the course of cathodic reduction in aqueous solution was proven by the authors in [18]. Intercalation of hydrogen into the deposit together with dispersed particles must affect the values of internal stress. However, nickel-based CECs are characterized by a decrease in internal stress as compared to coatings with no dispersed phase [1].
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95 90 85 80 75 70
1
1.5 1.4 1.3 1.2 1.1 1.0 0.9
2 0
100
200
300 400 500 Spray time, s
600
700
Fig. 3. Profiles of concentrations of (1) nickel and (2) carbon in nickel–fullerene C60 composite coatings.
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REFERENCES
–E, V (SHE) –0.5 2
0
1
0.5 1.0 1.5 2.0
2.5
2.0
1.5
1.0 0.5 0 –log ik, [i, А/cm2]
Fig. 4. Potentiodynamic polarization curves of (1) nickel and (2) nickel–fullerene C60 CEC in 0.5 M H2SO4 (vsw = 8 mV/s).
chemical properties of composite coatings are largely due to the properties of the metal matrix, so that the potentials of the beginning of activation of the nickel coating and nickel–C60 CECs are close. A characteristic feature of anodic PDC of nickel–C60 CEC is the significant widening of the passive region. In the far anodic range of potentials, dispersed fullerene particles in the coatings produce the most significant effect on the course of PDC (potentials of repassivation of the above coatings differ significantly). Studies performed in 0.5 M H2SO4 show that corrosion resistance of nickel–C60 CECs is much higher than that of nickel deposits free of dispersed phase. Thus, addition of dispersion of fullerene C60 into nickel-plating electrolyte facilitates the electrodeposition process and promotes formation of CECs. The greatest amount of the dispersed phase is present in the surface deposit layers. When incorporated into coatings, C60 particles produce determining influence on their tribological and corrosion–electrochemical properties.
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PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES
Translated by M. Ehrenburg
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