Comprehensive interpretation of the development of brush electroplating technology
01 Introduction to brush electroplating
Brush electroplating is a process in which a coating is deposited on the surface of a workpiece by electrolysis. Its purpose is to strengthen and improve the surface properties of the workpiece, such as obtaining a decorative appearance, corrosion resistance, wear resistance, and special optical, electrical, magnetic, and thermal properties; it can also change workpiece dimensions, improve mechanical fitting, and repair workpieces that have been scrapped due to out-of-tolerance errors or wear. It is therefore widely used in industry.
Brush electroplating technology (referred to as brush plating) is an important branch of electroplating technology. In addition to the common functions mentioned above, it focuses more on repair applications for workpieces and functional surface strengthening of small- and medium-batch workpieces.
In practice, therefore, on-site or in-line plating is more often required, and on the basis of ensuring coating quality, greater emphasis is placed on rapid and efficient deposition of the coating.
The basic brush plating process is as follows: a plating pen (anode) wrapped with a cover and soaked in a special plating solution is brought into contact with the area of the workpiece (cathode) to be plated, and relative motion is maintained to form the coating. The brush plating power supply is connected in series between the two electrodes.
In order to stably supply sufficient metal ions to the liquid layer on the workpiece surface, the high-concentration brush plating solution is either pumped directly or allowed to return naturally between the anode and cathode.
02 Brush electroplating service content
(1) Repair and strengthening of worn parts. Shafts, housings, end covers and other parts made of different materials in various types of machinery can be repaired by brush electroplating after wear. The repaired hardness range is HRC 20-60, which can meet the requirements of various working conditions and greatly extend service life.
(2) Repair of engine crankshafts with wear beyond tolerance. Brush electroplating can restore the original dimensions of worn, out-of-tolerance crankshafts, greatly extending their service life.
(3) Restoration of machining out-of-tolerance. For valuable mechanical parts that are out of tolerance after machining, brush electroplating can be conveniently used to correct their geometric shape and dimensional accuracy.
(4) Non-disassembly repair of large mechanical parts. For large, high-precision, and structurally complex mechanical parts, local repair can be carried out on site without disassembly, eliminating the steps of dismantling, hoisting, and transportation. This is efficient, economical, and time-saving.
(5) Surface strengthening of new products and new workpieces. It can be applied in the production processes of new products and new workpieces to perform strengthening treatments so that they have specific mechanical and physicochemical properties.
(6) Preparation of surface protective layers for workpieces, equipment, and metal structural components. A protective layer can be coated or plated on the surface of workpieces, equipment, or steel structural parts to give the surface special properties such as high corrosion resistance, oxidation resistance, and high-temperature resistance, which cannot be matched by ordinary paint or coating protection. Brush electroplating is also used to repair printed circuits, contacts of electrical devices, and pins and sockets of electronic components.
03 New developments in the technology
With continued research and the promotion and application of brush electroplating technology, brush plating has developed further. Like other surface engineering technologies, it is no longer limited to the repair of damaged parts and has made considerable progress.
It is now applied in both materials engineering and manufacturing engineering, and its processes will become even more complete and mature.
1 Research on brush-plated composite coatings
Composite coatings are an important development direction in coating research and application.
By structure, composite coatings can be divided into layered coatings and dispersion coatings. Layered coatings are multi-layer coatings formed by the sequential deposition of two or more metal elements.
A rationally designed multi-layer structure based on service performance can effectively improve the bonding strength between the coating and the substrate, optimize the distribution of internal stress in the coating and the crack propagation direction, and achieve some special and excellent properties.
Through coating structure design and process adjustment, coatings with different properties can also be prepared according to the structural characteristics and working conditions of parts to meet different service requirements (e.g., friction reduction, wear resistance, corrosion resistance, high-temperature resistance, etc.).
Dispersion coatings refer to the addition of high-performance solid particles to a conventional plating solution. These particles are uniformly suspended in the solution and co-deposit with metal ions during brush plating, thereby producing composite coatings with excellent performance.
Studies have shown that adding micron-sized dispersed second-phase particles (such as SiC, Al2O3, SiO2, ZrO2, diamond, etc.) to the brush plating solution provides a certain dispersion-strengthening effect on the composite coating formed after co-deposition.
However, when the dispersed particles are relatively large in size, it is difficult to ensure uniformity of the coating structure and stability of performance, which limits industrial application. Further improvement of the coating structure and microstructure is still needed.
2 New progress of brush electroplating technology in nano surface engineering
(1) Composite materials. Related research shows that dispersion strengthening with micron-sized second-phase particles does not produce a very significant toughening effect.
Composite with nanoparticles, by contrast, can achieve remarkable toughening and strengthening effects. In 1988, Izaki et al. used nano-SiC to reinforce Si3N4 ceramics, significantly changing the mechanical properties of SiC/Si3N4 ceramics compared with single-phase Si3N4 ceramics. Since Niihara Koichi of Japan synthesized high-strength nano-composite ceramics by adding SiC nanopowder to micron-sized Al2O3, nano-composite ceramics have become one of the research hotspots in the ceramic materials field in recent years because they have superior properties to micron-composite ceramics, especially strength and high-temperature performance, and have good prospects for development into a new type of structural material.
Similarly, for coating materials, composite coatings with nano-nano or nano-micron particles as the second dispersed phase or multiple dispersed phases may lead to significant improvements in mechanical properties.
(2) Due to the special surface and structural characteristics of nanomaterials, they possess excellent properties that conventional materials are difficult to achieve. This provides favorable conditions for improving the composition and performance of coating materials and makes it highly possible to functionalize materials.
Combining nanopowders with brush electroplating technology to prepare nano or nanopowder-containing surface composite coatings can improve the mechanical, physical, and chemical properties of the substrate surface, achieving the goal of surface modification and functionalization of materials.
Compared with traditional coatings, nano-coatings should have high strength, high toughness, better corrosion resistance, and superior wear resistance and thermal fatigue resistance.
In particular, the characteristics of nanomaterials have opened up a new field for preparing nanostructured coatings and functional nano-coatings that serve under severe conditions or special environments (such as ultra-high/low temperature, ultra-high pressure, vacuum, corrosive environments, radiation, sound absorption, signal shielding, point loads, etc.).
Preparing coatings with nanomaterials not only gives full play to the performance advantages of nanostructured materials, but also minimizes the high cost caused by using nanostructured materials as the base material.
(3) Ma Yajun, Xu Longtang, et al. used brush electroplating technology to prepare a nickel-based composite coating containing Ni-coated nanopowder.
The use of nickel-coated nanoparticles solved the problem of uniform distribution of particles during co-deposition in the coating. The surface morphology, microstructure characteristics, and microhardness of the composite coating were tested, and a mechanism for the co-deposition of Ni-coated nano-Al2O3 powder, Ni-coated nano-SiC powder, and nickel was proposed.
Zhang Yufeng et al. proposed a brush electroplating process for nano Ni-ZrO composite coatings. Tests showed that the amount of ZrO particles incorporated in the coating increased with the ZrO content in the plating solution. Aging heat treatment could significantly improve the hardness of the coating; after aging at 400°C, the hardness reached a maximum. The high-temperature wear resistance of the nano Ni-ZrO composite coating was 5–7 times that of the substrate (40Cr steel).
This is because the presence of nano ZrO in the composite brush coating inhibited grain growth, thereby improving the hardness and wear resistance of the coating. The composite brush nano Ni-ZrO coating has excellent high-temperature wear resistance and can be used for repairing equipment worn at high temperatures (e.g., cylinder bodies of high-speed diesel engines operating above 600°C, engine bearing liners, and main shafts of automotive gas turbine engines).
Zhang Wei et al. used a nickel plating solution containing nano-diamond powder to produce brush-plated composite coatings, and systematically investigated the effects of nano-diamond in the composite coating on its microstructure, mechanical properties, and wear resistance.
The results showed that the dispersion strengthening effect of nano-diamond could effectively improve coating growth, reduce internal stress, and increase the microhardness of the coating.
The composite coating containing nano-diamond powder exhibited excellent fatigue resistance and wear resistance at room temperature under high load, with wear resistance 4 times that of pure nickel coating.
3 Progress in composite brush electroplating processes. The combination of brush electroplating with other surface engineering technologies (especially high-energy beam processing technologies such as laser beams and ion beams) is an important development direction for composite brush electroplating processes.
It can be divided into composites with brush electroplating as the preceding process (brush electroplating + other surface engineering technologies) and composites with brush electroplating as the subsequent process (other surface engineering technologies + brush electroplating).
Among them, when brush electroplating is the preceding process, the main function of the subsequent process is to re-treat the coating. At this time, the existing brush coating can be strengthened, the interfacial bonding strength can be increased, special properties can be imparted to the coating, and a new material layer can also be prepared on the basis of the existing coating. For example, when brush electroplating is combined with laser micro-finishing technology, the laser is used to produce regular micro-dents on the coating surface of important friction pairs, which can strengthen the coating, improve oil storage capacity, and enhance the wear resistance of friction pairs. When brush electroplating is combined with laser remelting and laser strengthening technologies, the bonding strength of the brush coating and the surface properties of the material are improved after laser remelting or strengthening of the metal or alloy coating.
Combining brush electroplating with ion implantation technology, nitrogen ions are implanted into nickel coatings, nickel-tungsten coatings, and copper coatings, thereby strengthening the coating and further improving the wear resistance of the brush electroplated coating.
When brush electroplating is combined with brazing technology, transitional coatings of copper, tin, silver, etc. are plated on some difficult-to-braze materials, followed by brazing. This can resolve performance differences between the base material and the brazing filler material and enhance interfacial bonding. The "sandwich brazing plating" used to repair scratches on cast iron guideways of machine tools is a typical application example.
When brush electroplating is used as the subsequent process in composite technology, if brush electroplating is the final machining operation, the surface quality after brushing and the errors of the brush coating determine the final quality of the part; if additional machining is required after brushing, machining allowance should be reserved.
For example, when brush electroplating is combined with thermal spraying, the thermal sprayed coating is used to quickly restore dimensions, and then brush electroplating is performed on the coating to seal the pores of the sprayed coating, reduce surface roughness, and obtain the required coating properties.
When brush electroplating is combined with adhesive coating technology, for deep scratches, grooves, and dents on large parts where build-up welding, brazing, and thermal spraying are inconvenient, a conductive adhesive can first be used to fill the grooves; after the adhesive cures, a metal coating is brush-plated onto the adhesive.
It should be said that there are many surface engineering technologies that can be combined with brush electroplating. Appropriate composite technologies should be selected according to actual applications to achieve the best synergistic effect and meet the requirements for preparing high-performance surface working layers at low cost.
04 Applications of brush electroplating
1 Applications of brush electroplating technology in mechanical engineering
Brush electroplating technology has been widely used in aerospace, rolling stock, ships and vessels, petrochemical engineering, textile printing and dyeing, construction machinery, electronics and electric power, cultural relic restoration, handicraft decoration, localized gold plating, silver plating, and many other fields.
In general, it includes the following aspects: repair of worn journal surfaces; repair of hole-type parts; repair of rolling bearings.
(1) Brush electroplating can quickly repair machining out-of-tolerance, wear, pits, and scratches on mechanical parts, restore the dimensions of worn and out-of-tolerance parts, and meet tolerance requirements;
(2) It has good adhesion on various metal materials such as carbon steel, stainless steel, cast iron (steel), copper (alloys), and aluminum (alloys). The coating has high hardness and good wear resistance, and repair thickness can reach more than 1.0 mm, meeting the performance requirements of various repairs.
Brush-plated protective layers for new products, used to improve wear resistance, surface corrosion resistance, and high-temperature oxidation resistance of parts;
(3) Repair and protection of molds. For example, brush plating a mirror finish coating on the surface satisfies the requirements for corrosion resistance and surface gloss, and improves the service performance and life of molds;
(4) Repair of local wear, scratches, pits, and corrosion spots on large and precision parts such as crankshafts, hydraulic cylinders, plungers, machine bodies, and guide rods;
(5) Improving the metallurgical properties of part surfaces, such as improving the brazability of materials and local anti-carburizing and anti-nitriding treatment of parts;
(6) Improving the interference and fitting performance of bearings and mating surfaces, such as increasing interference and enhancing the wear resistance and corrosion resistance of mating surfaces;
(7) Repair and protection of printed circuit boards, such as gold plating and silver plating on pins;
(8) Repair and protection of electrical contacts, connectors, and high-voltage switches;
(9) Operations that are difficult to complete with conventional tank plating. These include repair of defective plated parts, workpieces that cannot be immersed in a plating tank, workpieces already installed on equipment, workpieces requiring only localized plating, and certain deep holes and blind holes;
(10) Construction at normal temperature ensures that the base material does not undergo thermal deformation or metallographic structure changes, thereby extending the service life of parts. For example, repairs of sand holes in castings and quenching cracks are almost invisible;
2 Application and prospects of brush electroplating technology in mechanical engineering
(1) Repair of worn and damaged surfaces of moving parts is the main application of brush electroplating in mechanical engineering. It can be divided into external surface repair and internal surface repair. External surfaces include: piston rods, engine crankshafts, transmission shafts and pin shafts, gear shafts and other various cylindrical surfaces, as well as guide rail planes used as motion references for machine tools. Internal surfaces include: hydraulic cylinders, bearing housing bores, shaft sleeves, box bore systems, etc. For assemblies with fitting performance or clearance requirements, brush electroplating can be used as a method to adjust clearances of machine tools, fixtures, or assemblies. For example, when the bearing clearance of a diesel engine becomes too large, a copper coating can be brushed onto the bearing shell back of the connecting rod bearing to obtain the specified bearing clearance, conveniently meeting engineering requirements.
For imported mechanical equipment such as CNC machine tools and machining centers, where spare parts are expensive and procurement cycles are long, brush electroplating repair can reduce maintenance costs and shorten repair cycles. Moreover, the repaired surface performance can exceed that of the original component, delivering remarkable economic benefits.
Similarly, repairing damaged mold parts used in mechanical engineering is another important application of brush electroplating repair.
In addition, brush electroplating can also be used as a surface strengthening method for various molds. Brushing wear-resistant, impact-resistant, or high-temperature-resistant materials onto mold surfaces can extend the service life of molds used in mechanical engineering.
This is a new application field developed in recent years.
(2) Brush electroplating will receive increasing attention due to its outstanding advantages such as simple equipment, mature process, low temperature operation, and controllable parameters.
Brushing thin metal layers (below 100 μm in thickness) onto a substrate to improve surface properties and restore dimensions is the most widespread application of brush electroplating technology.
Meanwhile, nanostructured thin layers produced by brush electroplating offer high wear resistance and corrosion resistance, along with high hardness and excellent adhesion to the substrate, making them ideal protective coatings. With a low wear rate and low friction coefficient, they are suitable for applications requiring both high wear resistance and a low friction coefficient, such as automobile engines and hydraulic pistons.
Brush-electroplated nano-alloys can be used for aircraft landing gear components, automotive shock absorbers, and hydraulic pistons.
Brush electroplating of nanocrystalline structures on the inner surfaces of nuclear generator piping, enabling on-site repair of pipes that have cracked or failed due to intergranular corrosion and stress corrosion, is another important industrial application of nanocrystalline brush electroplating materials.