Improving reliability and service life of industrial products via composite electrochemical coatings

The paper considers a method for obtaining hard, wear-resistant composite electrochemical tungsten carbide-cobalt (WC-Co) coatings with high adhesive strength. Such coatings are used to enhance surface hardness and wear resistance of parts and tooling. It is challenging to obtain hard, wear-resistant coatings on the surface of industrial parts and tooling. It is proposed to use an eco-friendly cobalt electrolyte with tungsten carbide additive for the electrodeposition of composite electrochemical coatings. The electrodeposition technique provides high output and quality of the applied coating with the ability to control its thickness.


Introduction
The industrial sector focuses on increasing efficiency and quality of product processing via the development of alloys and coatings to offer both corrosion and heat resistances, strength and other service properties, and to increase profitability.Materials with such characteristics are widely used in mechanical and aircraft engineering, and even in medicine.
A detailed study and practical research on coatings with enhanced wear resistance, uniform wear, high adhesion and heat resistance, and strength is vital for the development of various industrial sectors.High-performance cutting can be achieved due to the improved wear resistance, since nondeformed coating will significantly enhance productivity to increase the efficiency.
Recent developments in coating technologies are based on producing materials with certain qualitative characteristics.Many materials, such as steel, can be modified based on novel technologies to improve output and product quality.
A large share of metalworking machinery manufacturing market belongs to foreign producers.Given current conditions, it is urgent to substitute imported parts and tooling with domestic competitive ones that meet modern requirements.
Protective coatings rank special place among structural materials in the industrial complex.The advanced material should be lighter and more resistant.Composite materials can exceed steel or aluminum alloys by 50-100% in terms of specific strength, while providing saving the weight of the structure by 20-50%.It is feasible to obtain modern materials and products with the required combination by selecting the composition and properties of the matrix, filler, and the ratio thereof.Chromium coatings are often used to increase surface hardness and wear resistance.These coatings have high adhesive strength and hardness, wear resistance and reliable corrosion protection of steel products [1].However, high aggressiveness against most structural materials, environmental hazard, and toxicity of hexavalent chromium compounds to living organisms have initiated research on alternatives to using chromium coatings [2,3].
Composite coatings are the most promising for replacing electroplated coatings with chromium.Wherein, ultradispersed diamonds and metal carbides included in a metal matrix are the dispersed phase.The matrix wets particles of the dispersed phase and has a high adhesive strength to the surface of the backing material.
Composite coatings are obtained by deposition of metals.Substance particles of such coatings have high wear resistance, and prevent corrosion and erosion.Currently, the technique for the deposition of composite coatings is being actively developed.Tungsten carbide-cobalt (WC-Co) coatings are widely used in many industrial sectors.Tungsten carbide (WC) has an increased hardness, its density is two times greater that of lead, and it preserves its properties both at room and high temperature (up to 400 °C).
WC-based coatings have excellent corrosion performance.Due to carbon, the compound becomes very hard and durable, and resistant to oxidation.Cobalt (Co) is a binder with improved plasticity in this composite coating.It is due to Co that the high elasticity of WC-Co is achieved.Varying Co content or its partial replacing with another component can significantly change the properties of composite coatings, enhance or reduce hardness and corrosion potential of coating.
The carbide particle size greatly affects both the strength and structure of coating, and the abrasion resistance.If WC grain size is diminished, the abrasive wear resistance will increase.Corrosion resistance enhances with increasing the grain size.
Low impact toughness leads to impact failure.When composite coating is applied, the reduction in porosity enhances its protective function against corrosion.The coating strength directly depends on Co content, since the pore size grows with a decrease therein.
Co content in the composite coating affects the growth of grains, which size rapidly increases with an increase in the binder content of the composition.Cobalt forms a thick film on the coating surface to protect the part from rapid wear.Therefore, an increase in Co content leads to a decrease in the wear resistance of coating.If we partially replace Co with Al2O3 in the structure of a composite electrochemical coating (CEC), the latter will acquire high impact toughness, strength, and hardness.Such coatings are mostly applied onto drilling parts.
Decarburization occurs at high temperatures, which leads to a higher content of a binding plastic agent in the composition.It causes the coating to become brittle and lose its properties.
These CECs are used as protection against cavitation of metal structures.Due to their high hardness, impact toughness and adhesion, WC-Co coatings are able to cope with the wear process in an aqueous environment.
Various compositions and deposition modes are proposed to form a composite electrochemical Ni-WC coating on steel [4].One of this CEC disadvantages is the lower ability of nickel to wet WC particles as compared to cobalt.High nickel toxicity and its ability to cause allergies in contact with the skin are also limiting factors for the widespread use of this coating.
Ni-WC coating is also fabricated by a laser deposition to implement the laser cladding technology.The laser power was 1700 W, the spot diameter of laser was 3 mm, and the scanning speed was 100 mm/min [5].This technology is more expensive than galvanic deposition and is not suitable for processing large parts with complex geometric shape.
Given better ability of cobalt to wet WC particles, its lower toxicity compared to nickel, the European restrictions on the use of nickel as coatings [6], higher hardness of WC-Co coatings compared to Ni-WC coatings, cobalt is intended to be used as a metal matrix for composite electrochemical coatings to provide high surface hardness and wear resistance.
The main criteria for this approach are to reduce environmental hazard, to increase productivity, to preserve the quality of such structural electrochemical coating, and to fabricate coatings with desired properties.Increasing environmental protection is achieved due to the evidence-based composition of the electrolyte to reduce the treating loads.It is also feasible to change the technology for workpiece washing to help save water resources via recycling.The use of dissolved metal ions in electrolytes increases the depth of diffusion limitations during electrocrystallization leading to coating quality deterioration and a significant decrease in the deposition rate.This problem can be solved by choosing a proper non-stationary electrolysis mode.For example, the use of pulsed current for electrode polarization has a favorable effect on the concentration-diffusion mode in the cathode region and promotes an increase in the reaction rate and improvement in the coating morphology.
The wear resistance of WC depends on hardness, part size and impact toughness of the substance particles.The impact toughness does not directly depend on residual micropores due to large WC grains [3].It was proved that this indicator depends not only on the size, but also on the location of the defect [4].Low impact toughness leads to impact failure.When composite coating is applied, the reduction in porosity enhances its protective function against corrosion [5].The coating strength directly depends on Co content, since the pore size grows with a decrease therein [7].These coatings are superior to chrome ones in terms of heat resistance.

Materials and methods
The studies were carried out in a thermostatically controlled rectangular glass cell of 0.15 L [8].The IPS-1 power supply was used as a constant current source.We used К0 cobalt-based anode material, and М00 copper plates of 2×10 -4 m 2 as a cathode.To study the effect of electrolyte composition and electrolysis modes on the rate of the electrodeposition process, and to assess the quality of coatings and cobalt current efficiency, we prepared solution of 35 mL/L cobalt chloride hexahydrate, 100 mL/L ammonium chloride, and 45 mL/L urotropin.

Results and discussion
An increase in the cathode current density from 5 to 15 A/dm 2 gains cobalt current efficiency up to 95-97%.A further increase in the current density to 50 A/dm 2 reduces the cathode current efficiency of the metal and degrades the quality of coatings.High-quality coatings (fine-crystalline, light, semigloss and homogeneous) are formed in the range of current densities from 15 to 30 A/dm 2 .
To form WC-Co CEC, an additive of tungsten carbide was introduced into the indicated electrolyte (particle size of 10-100 μm with the concentration of 10 g/L obtained by the electric arc dispersion method).The deposition was carried out at a current density of 3.5-4.5A/dm 2 , a temperature of 18-22°C using a magnetic stirrer (300-600 rpm) to ensure a uniform distribution of dispersed phase particles in the electrolyte volume [9,10].
The proposed solution is easy to prepare, it lacks toxic additives and allows working with both soluble and insoluble anodes.The inserted urotropin improves surface wettability of the part with coating and suppresses acid corrosion of metals.Homogeneous semi-gloss coatings with high adhesive strength are formed from this electrolyte under the indicated modes.Hardness and wear resistance of the obtained coatings are higher than those of developed analogues [11].
The experiments have demonstrated some data on WC-Co CEC.The scratch hardness test and the bend testing procedure were used as adhesive strength tests for the coatings.The microhardness of coating was 6.452±0.23GPa.The wear resistance of coating was 4500±150 dual reciprocating motions of the indenter per 1 μm of the coating thickness at an indentation load of 2 N.

Conclusion
The advantages of the industrial use of the considered method are as follows: • the proposed electrolyte is low-component, it does not contain toxic organic additives, and provides for obtaining coatings with a high current efficiency value; • the electrolyte can operate both with inert and soluble anodes; • light and semi-gloss WC-Co composite coatings with high adhesive strength are fabricated from this electrolyte at a temperature of 18-25 °C.The development of technology to fabricate WC-Co CEC for tooling machine-building production will increase the competitiveness of Russian production in the world market.
with a size of 10-100 µm are obtained by the method of electric arc dispersion.The codeposition of WC particles into the cobalt-based coating will lead to the formation of WC-Co CEC.The choice of electrolyte composition is based on the principles of reducing environmental hazards.Photomicrographs of the coating samples are shown in figure 1 and figure 2.

Figure 1 .
Figure 1.A micrograph of a coating sample at a current density of 4 A/dm 2 and stirring speed of 300 rpm.

Figure 2 .
Figure 2. A micrograph of a coating sample at a current density 4 A/dm 2 and stirring speed of 600 rpm.

Table 1
and table 2 illustrate the dependences of Co current efficiency on the electrolysis mode.

Table 1 .
Dependence of Co current efficiency on the current density at a stirring speed of 300 rpm and 10 g/L WC concentration in the electrolyte.

Table 2 .
Dependence of Co current efficiency on the current density at a stirring speed of 600 rpm and 10 g/L WC concentration in the electrolyte.