Table 1.

Microhardness (GPa) by Vickers galvanic coatings of metals and metal alloys.


As an example, one can analyze the contribution of individual groups of process parameters (Figure 1) to the formation of synergistic alloys [12]. Thus, varying the composition of the electrolyte, the modes and parameters of the electrolysis, we electroplated iron alloys with refractory metals of different compositions and morphologies [13–15], which determine the level of functional properties (Figure 2).

Cobalt-based alloys are widely used as construction materials for different technic applications, for example, super alloys for aircraft turbine vanes and blades, alloys for powerful, high-coercive force magnets, hard metal alloys for cutting tool materials, and protective hard coatings. Cobalt is utilized as a matrix of special materials, including alloys for dental and surgical implants or bone fracture fixation, thermal resistant materials (Fe▬Ni▬Co, Co▬Fe▬Cr), magnetic recording thin films, catalysts [16], etc.

Electrodeposition peculiarities of binary and ternary cobalt alloys with molybdenum as well as their properties are reflected in promising publications. Cobaltmolybdenum (ω<sup>W</sup> is 10%) films with low coercivity and high saturation magnetization were formed from a sulfate-citrate bath [17]. It was shown that the dependence of electrolytic alloy structure on the current density, namely a close-packed hexagonal structure, was formed at low cathodic polarization, and both crystalline and amorphous structures were appeared at higher polarization [18]. High adherent, compact, and uniform cobalt and cobalt-molybdenum coatings with 1–8 wt. % Mo were deposited onto copper substrates from ionic liquids based on choline

molybdenum-boron amorphous electrolytic alloys of composition, wt. %: Co—51, Mo—47, and B—2, were deposited from citrate-phosphate-ammonia bath at a cathode current efficiency of 29–65%. Above materials exhibit high hardness, corrosion resistance, wear resistance, and also sufficient ductility [20]. Cobaltmolybdenum-phosphorus (Co▬Mo▬P) coatings containing 8% Mo, 20% P, and

recommended as barrier layer to replace nickel [21]. The amorphous Co▬Mo▬C coatings were deposited with additional exposure of working electrode in the magnetic field. The content of Mo in the coatings deposited in a magnetic field increases up to 34.2 at. % as compared with traditionally deposited alloys [22]. A growth of overvoltage during hydrogen evolution at above materials was also observed.

Thus, in particular, the light lustrous uniform coatings made of iron binary and ternary alloys with refractory metals were deposited from complex electrolytes both at direct and pulse current. It was noted fairly high deposition rate up to 20 μm/h and the current efficiency of 60–85%, which is much higher as compared with the results of other scientists [23–25]. The morphology and topography of the coatings were shown to be depended on the nature of alloying refractory metals and the electrolysis modes on the course of the electrochemical alloy forming reactions. The reason was the change in the nature of the discharging particles and the limiting

Co balance were deposited from citrate-phosphate electrolyte and were

, at 90–100°C [19]. Cobalt-

chloride (ChCl) at a current density of 7–25 mA cm<sup>2</sup>

Internal factors forming the functional properties of the alloys.

Composition Electrolytic Coatings with Given Functional Properties

DOI: http://dx.doi.org/10.5772/intechopen.84519

Figure 2.

stage of the net electrode process.

97

The addition of refractory metals (molybdenum, tungsten, zirconium, etc.) even in small amounts to cobalt significantly extends the functional properties of materials. In some cases, of particular interest is the implementing of these properties in thin surface layers. High adhesion and a wide range of coating composition are provided by electrodeposition from aqueous and nonaqueous solutions.

Figure 1. The main factors of electrolytic alloying.

Composition Electrolytic Coatings with Given Functional Properties DOI: http://dx.doi.org/10.5772/intechopen.84519

• an impulse of movement, including the ultrasound field (mixing, transfer of

• a radiation (radiation field), which, depending on the nature, can lead, for example, to radiolysis, change of electrolysis conditions under the action of laser irradiation or structure and surface properties of the electrode material, as well as other changes in the state and properties of individual phases and even

• a gravitational field, which changes the conditions for the electrochemical processing (e.g., the stages of transportation), as well as other, less significant

As an example, one can analyze the contribution of individual groups of process parameters (Figure 1) to the formation of synergistic alloys [12]. Thus, varying the composition of the electrolyte, the modes and parameters of the electrolysis, we electroplated iron alloys with refractory metals of different compositions and morphologies [13–15], which determine the level of functional properties (Figure 2). Cobalt-based alloys are widely used as construction materials for different technic applications, for example, super alloys for aircraft turbine vanes and blades, alloys for powerful, high-coercive force magnets, hard metal alloys for cutting tool materials, and protective hard coatings. Cobalt is utilized as a matrix of special materials, including alloys for dental and surgical implants or bone fracture fixation, thermal resistant materials (Fe▬Ni▬Co, Co▬Fe▬Cr), magnetic recording

The addition of refractory metals (molybdenum, tungsten, zirconium, etc.) even in small amounts to cobalt significantly extends the functional properties of materials. In some cases, of particular interest is the implementing of these properties in thin surface layers. High adhesion and a wide range of coating composition are provided by electrodeposition from aqueous and nonaqueous solutions.

reactants, or electrolysis products);

transformation routes;

thin films, catalysts [16], etc.

Figure 1.

96

The main factors of electrolytic alloying.

effects.

Applied Surface Science

Figure 2. Internal factors forming the functional properties of the alloys.

Electrodeposition peculiarities of binary and ternary cobalt alloys with molybdenum as well as their properties are reflected in promising publications. Cobaltmolybdenum (ω<sup>W</sup> is 10%) films with low coercivity and high saturation magnetization were formed from a sulfate-citrate bath [17]. It was shown that the dependence of electrolytic alloy structure on the current density, namely a close-packed hexagonal structure, was formed at low cathodic polarization, and both crystalline and amorphous structures were appeared at higher polarization [18]. High adherent, compact, and uniform cobalt and cobalt-molybdenum coatings with 1–8 wt. % Mo were deposited onto copper substrates from ionic liquids based on choline chloride (ChCl) at a current density of 7–25 mA cm<sup>2</sup> , at 90–100°C [19]. Cobaltmolybdenum-boron amorphous electrolytic alloys of composition, wt. %: Co—51, Mo—47, and B—2, were deposited from citrate-phosphate-ammonia bath at a cathode current efficiency of 29–65%. Above materials exhibit high hardness, corrosion resistance, wear resistance, and also sufficient ductility [20]. Cobaltmolybdenum-phosphorus (Co▬Mo▬P) coatings containing 8% Mo, 20% P, and Co balance were deposited from citrate-phosphate electrolyte and were recommended as barrier layer to replace nickel [21]. The amorphous Co▬Mo▬C coatings were deposited with additional exposure of working electrode in the magnetic field. The content of Mo in the coatings deposited in a magnetic field increases up to 34.2 at. % as compared with traditionally deposited alloys [22]. A growth of overvoltage during hydrogen evolution at above materials was also observed.

Thus, in particular, the light lustrous uniform coatings made of iron binary and ternary alloys with refractory metals were deposited from complex electrolytes both at direct and pulse current. It was noted fairly high deposition rate up to 20 μm/h and the current efficiency of 60–85%, which is much higher as compared with the results of other scientists [23–25]. The morphology and topography of the coatings were shown to be depended on the nature of alloying refractory metals and the electrolysis modes on the course of the electrochemical alloy forming reactions. The reason was the change in the nature of the discharging particles and the limiting stage of the net electrode process.

The biligand citrate-pyrophosphate electrolyte [26, 27] was used for codeposition cobalt with molybdenum and tungsten to overcome significant potential difference of alloying metals which also are reduced multistage [12, 13]. The binary Co▬Mo [26] and Co▬W [5] deposits were obtained, and their composition was controlled by the variation of the pH and the concentration ratio of alloying metals' and ligands in a bath. The formation of hetero-nuclear complexes cobalt, molybdate, and tungstate with citrate and pyrophosphate, and complexes subsequently reducing into an alloy, may be competing with each other as it was shown for iron ternary [28, 29], Co▬Mo▬Zr [30] and Co▬Mo▬W deposits.

Ternary Co▬Mo▬Zr alloys were also deposited from citrate-pyrophosphate electrolyte with optimal concentration ratio of molybdate and tungstate for ligands [30, 31]. It was shown the advantage to use electrolyte with tungstate excess compared with molybdate. In addition, the use of pulse current was contributing to the deposition of ternary alloys with a high content of refractory metals.

intermediate molybdenum and tungsten oxides instead of metals as suggested by the appearance of the deposited coatings. Those plated using current densities 8 A dm<sup>2</sup> are smooth, compact, and microglobular. Those obtained using higher temperature of 50°C (Figure 4b) have developed globular surface with larger sizes of agglomerates and microspheroids. With an increase in the temperature, the relative content of molybdenum also increases, and tungsten one changes slightly.

Morphology and refractory metal content (at. % in terms of metal) in Co▬Mo▬W coatings, deposited at: (a) T = 30°C, Co—86.6, Mo—7.0, W—6.4; (b) T = 50°C, Co—83.2, Mo—10.7, W—6.1. Magnification

Composition Electrolytic Coatings with Given Functional Properties

DOI: http://dx.doi.org/10.5772/intechopen.84519

It is obviously molybdenum and tungsten compete with each other during deposition into an alloy, and therefore the atomic ratio of metals in the coating differs from the ratio of oxoanion concentrations in the electrolyte [30]. In turn, the atomic ratio of metals in the alloy determines the structure of the morphology of the surface layers. So, it is the increase in ω(W) that causes deep microcracks in the coating to a greater extent than the growth of the total refractory metal

Time parameters of pulsed electrolysis namely on/off time impact the current efficiency and composition of ternary alloys (Figure 5). Increasing the polarization time within the range of 2–5 ms at the current density of 10 A dm<sup>2</sup> and pause time 20 ms promotes content, both refractory components, in the coating (Figure 5a) but more significantly for tungsten. No any significant change in refractory metal content observed at longer polarization. As it follows from experiment (Figure 5b), the optimal off-time for alloying metal content is in the range of 15–20 ms when ontime is 5 ms. Thus, we can conclude that the pulse/pause ratio of 1/ (3–4) provides the maximum content of molybdenum and tungsten in the alloy. Considering the molybdenum competition with tungsten when forming hetero-nuclear complexes discharged at the cathode, the energy and time parameters of electrolysis despite

Dependence of refractory metal content and current efficiency for Co▬Mo▬W alloys deposited from bi-ligand

bath on parameters of pulse electrolysis: on-time (a) at toff = 20 ms and off-time (b) at ton = 5 ms.

<sup>4</sup> ratio in the electrolyte of 1:2, the coatings are enriched

Despite WO<sup>2</sup>

Figure 4.

2000/5000.

content [33].

Figure 5.

99

with molybdenum.

<sup>4</sup> : MoO2

### 2. Co▬Mo▬W electrolytic alloys

Figure 3 shows the current efficiency and composition of deposits Co▬Mo▬W plated from bi-ligand (citrate-pyrophosphate) bath in pulse regime at ratio ton/ toff = 5/20 ms. The total deposition time is 30 min. As we can see from the plots, the total alloying metal content in the deposits Co▬Mo▬W rises from 19 at. % up to 30 at. % with increasing cathodic polarization from 4 to 10 A dm<sup>2</sup> . This phenomenon is quite natural and is due to the shift of the electrode potential in the site of oxometalates reducing with increasing current density. The decrease in the alloying metal content with an increase in the current density of more than 10 A dm<sup>2</sup> is associated with the intensification of the side reaction of hydrogen evolution. The current efficiency decreases with current density also due to the impact of the side reaction of hydrogen evolution. Indirectly, a decrease in the current efficiency with an increase in the refractory metal content may be due to the alloy catalytic activity in the electrolytic hydrogen evolution.

Both the morphology and composition of ternary alloys deposited in pulse mode at current density 8 A dm<sup>2</sup> , on/off time 5/20 ms depend on the electrolyte temperature (Figure 4). Since it was established earlier that lower current densities 5–7 A dm<sup>2</sup> favor higher current efficiency, this growth is due to the formation of

#### Figure 3.

Dependence of current efficiency Ce, total refractory metal content (a), and Mo or W content (b) in Co▬Mo▬W alloy on applied current density.

Composition Electrolytic Coatings with Given Functional Properties DOI: http://dx.doi.org/10.5772/intechopen.84519

#### Figure 4.

The biligand citrate-pyrophosphate electrolyte [26, 27] was used for codeposition cobalt with molybdenum and tungsten to overcome significant potential difference of alloying metals which also are reduced multistage [12, 13]. The binary Co▬Mo [26] and Co▬W [5] deposits were obtained, and their composition was controlled by the variation of the pH and the concentration ratio of alloying metals' and ligands in a bath. The formation of hetero-nuclear complexes cobalt, molybdate, and tungstate with citrate and pyrophosphate, and complexes subsequently reducing into an alloy, may be competing with each other as it was shown for iron ternary [28, 29], Co▬Mo▬Zr [30] and Co▬Mo▬W

Ternary Co▬Mo▬Zr alloys were also deposited from citrate-pyrophosphate electrolyte with optimal concentration ratio of molybdate and tungstate for ligands [30, 31]. It was shown the advantage to use electrolyte with tungstate excess compared with molybdate. In addition, the use of pulse current was contributing to the

Figure 3 shows the current efficiency and composition of deposits Co▬Mo▬W

. This phenom-

plated from bi-ligand (citrate-pyrophosphate) bath in pulse regime at ratio ton/ toff = 5/20 ms. The total deposition time is 30 min. As we can see from the plots, the total alloying metal content in the deposits Co▬Mo▬W rises from 19 at. % up to

enon is quite natural and is due to the shift of the electrode potential in the site of oxometalates reducing with increasing current density. The decrease in the alloying metal content with an increase in the current density of more than 10 A dm<sup>2</sup> is associated with the intensification of the side reaction of hydrogen evolution. The current efficiency decreases with current density also due to the impact of the side reaction of hydrogen evolution. Indirectly, a decrease in the current efficiency with an increase in the refractory metal content may be due to the alloy catalytic activity

Both the morphology and composition of ternary alloys deposited in pulse mode

perature (Figure 4). Since it was established earlier that lower current densities 5–7 A dm<sup>2</sup> favor higher current efficiency, this growth is due to the formation of

Dependence of current efficiency Ce, total refractory metal content (a), and Mo or W content (b) in

, on/off time 5/20 ms depend on the electrolyte tem-

deposition of ternary alloys with a high content of refractory metals.

30 at. % with increasing cathodic polarization from 4 to 10 A dm<sup>2</sup>

2. Co▬Mo▬W electrolytic alloys

in the electrolytic hydrogen evolution.

Co▬Mo▬W alloy on applied current density.

at current density 8 A dm<sup>2</sup>

Figure 3.

98

deposits.

Applied Surface Science

Morphology and refractory metal content (at. % in terms of metal) in Co▬Mo▬W coatings, deposited at: (a) T = 30°C, Co—86.6, Mo—7.0, W—6.4; (b) T = 50°C, Co—83.2, Mo—10.7, W—6.1. Magnification 2000/5000.

intermediate molybdenum and tungsten oxides instead of metals as suggested by the appearance of the deposited coatings. Those plated using current densities 8 A dm<sup>2</sup> are smooth, compact, and microglobular. Those obtained using higher temperature of 50°C (Figure 4b) have developed globular surface with larger sizes of agglomerates and microspheroids. With an increase in the temperature, the relative content of molybdenum also increases, and tungsten one changes slightly. Despite WO<sup>2</sup> <sup>4</sup> : MoO2 <sup>4</sup> ratio in the electrolyte of 1:2, the coatings are enriched with molybdenum.

It is obviously molybdenum and tungsten compete with each other during deposition into an alloy, and therefore the atomic ratio of metals in the coating differs from the ratio of oxoanion concentrations in the electrolyte [30]. In turn, the atomic ratio of metals in the alloy determines the structure of the morphology of the surface layers. So, it is the increase in ω(W) that causes deep microcracks in the coating to a greater extent than the growth of the total refractory metal content [33].

Time parameters of pulsed electrolysis namely on/off time impact the current efficiency and composition of ternary alloys (Figure 5). Increasing the polarization time within the range of 2–5 ms at the current density of 10 A dm<sup>2</sup> and pause time 20 ms promotes content, both refractory components, in the coating (Figure 5a) but more significantly for tungsten. No any significant change in refractory metal content observed at longer polarization. As it follows from experiment (Figure 5b), the optimal off-time for alloying metal content is in the range of 15–20 ms when ontime is 5 ms. Thus, we can conclude that the pulse/pause ratio of 1/ (3–4) provides the maximum content of molybdenum and tungsten in the alloy. Considering the molybdenum competition with tungsten when forming hetero-nuclear complexes discharged at the cathode, the energy and time parameters of electrolysis despite

#### Figure 5.

Dependence of refractory metal content and current efficiency for Co▬Mo▬W alloys deposited from bi-ligand bath on parameters of pulse electrolysis: on-time (a) at toff = 20 ms and off-time (b) at ton = 5 ms.

the ratio of the molybdate and tungstate concentrations in a solution are the tools for ternary alloy composition control.

3. Co▬Mo▬Zr electrolytic alloys

DOI: http://dx.doi.org/10.5772/intechopen.84519

Composition Electrolytic Coatings with Given Functional Properties

However, at current densities above 8 A dm<sup>2</sup>

ability in the composition of the surface layers [31].

site hydrogen evolution reaction at more negative potentials.

rising current density from 5 to 8 A dm<sup>2</sup>

ton/toff 2/10 ms; T 20–25°C; рН 8; plated time 30 min.

Figure 7.

101

current density than of 4 A dm<sup>2</sup>

The molybdenum content in Co▬Mo▬Zr alloys changes with current density increasing in the range of 2–4 A dm<sup>2</sup> (Figure 7a) similar to the Co▬Mo▬W films. At the same time, we observe a wider range of current densities of 4–8 A dm<sup>2</sup> providing coatings with molybdenum content of 24–25 at. %. Coating enrichment by molybdenum with increasing current density is entirely predictable since the molybdate electrochemical behavior is associated with multi-stage process following by chemical reducing of intermediate molybdenum oxides with hydrogen adatoms Had [27, 29, 35]. The cathode potential at Co▬Mo▬Zr electrodeposition is rather negative (2.0–2.8) V (Figure 7b) and it becomes more negative with current density, that resulting in acceleration of side reaction producing Had which are involved in a chemical step of reducing intermediate molybdenum oxides. Exactly due to above reasons, the molybdenum content in the deposits is increased.

becomes the dominant as evidenced by the decreasing current efficiency (Figure 7b), whereby the molybdenum content in the alloy decreases.

The zirconium content in the ternary coatings reaches 3.6–3.7 at. % with increasing current density up to 4 A dm<sup>2</sup> (Figure 7a). As it follows from the experimental data as well as previous investigations [26], molybdenum and zirconium competition is observed when codeposited to get the ternary alloy at higher

mechanisms of alloying metals reducing from citrate-pyrophosphate electrolyte. Really, molybdate reducing to metallic state proceeds through the six electrons transfer accompanied by the removal of four oxygen atoms. Therefore, deposits may also contain incompletely reduced intermediate molybdenum oxides. Zirconium is likely included in the deposit in the form of oxygen compounds ZrOx due to higher binding energy Zr–O [31]. The EDS analysis data confirm the oxygen avail-

Nonlinear dependences of current efficiency Ce on the deposition current density (Figure 7b) were obtained, and Ce increases by 20% and reaches 63% with a

current efficiency up to 47%. Such behavior may be attributed with acceleration of

Pulsed electrolysis allows the use of higher current densities, and not only energy parameter but also polarization time ton and current interruption time toff as well as its ratio are effectively used to control deposit composition and current efficiency. The shortest pulse duration should ensure the achievement of the alloy

Pulse current density influence on the composition (a) and current efficiency (b) for Co▬Mo▬Zr coatings;

, hydrogen evolution reaction

; however, further increase in i reduces the

. Such behavior is associated with the different

Current efficiency decreases in the range of 87–52% when increasing on-time is caused by the hydrogen evolution enhancement (Figure 5a). Prolong the pause as compared with pulse time positively influences the current efficiency as subsequent chemical reactions accompanying the alloying metals discharge are more fully. A larger current interruption than for 20 ms reduces the efficiency of the process (Figure 5b) as it was also observed in [34]. Thus, high refractory metal content in the Co▬Mo▬W alloy while maintaining a reasonable value of current efficiency Ce 70–75% at a current density of 10 A dm<sup>2</sup> is achieved with ton/toff = 5/20 ms.

Figure 6 shows X-ray diffraction patterns for Co▬Mo▬W alloys deposited on a copper substrate at a pulse current amplitude of 8 A dm<sup>2</sup> ; ton/toff = 2/10 ms (Figure 6, black line) and a direct current density of 4 A dm<sup>2</sup> (Figure 6, red line); T = 30°C; coating thickness is 20 μm.

The X-ray diffraction patterns indicate an amorphous-crystalline structure of the alloys. The high intensity peaks at 60 and 90° are copper substrate lines. We can see some peaks corresponding to α-Co phase, intermetallic phase Co7W6 as previously for binary Co▬W alloy [32], additional reflections of the intermetallic compound Co7Mo6, and rather wide halo with width about 15° is detected at angles 2θ of 43–58° (Figure 6) that reflects amorphous structure for both coatings deposited at pulse and direct current. The most important fact is the appearance of reflexes of metallic molybdenum and tungsten on XRD patterns for Co▬Mo▬W alloys deposited at pulse current. Such a character of X-ray diffraction patterns for coatings obtained by pulse current confirms our proposed mechanism of alloy formation [14, 28]. Metallic tungsten and molybdenum are formed in the chemical stage of reduction of refractory metal intermediate oxides during the pause of polarization. Such a pattern, along with decreasing tungsten content in the ternary alloy Co▬Mo▬W compared with a binary Co▬W [32], also indicates molybdenum and tungsten competition during deposition in the alloy. The coherent-scattering region size of the amorphous part is 2–8 nm.

#### Figure 6.

X-ray diffraction patterns for electrolytic alloys Co▬Mo▬W of composition, at. %: (1) Co—75, Mo—16, W—9; (2) Co—85, Mo—9, W—5.
