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Electrolytic co-deposition

Dr. Dmitri Kopeliovich

Electrolytic co-deposition is an Electroplating method of incorporation of non-metallic particles into metallic coatings obtained from electrolytes containing the particles in a suspended state.

The second phase particles dispersed within the metal base form a metal matrix composite, properties of which may be significantly different from those of the pure metallic deposit.

The metals used as matrices for electrolytic co-deposition:

Nickel (Ni)
Copper (Cu)
Silver (Ag)
Chromium (Cr)
Cobalt (Co)
Iron (Fe)
Lead (Pb) alloys
Aluminum (Al)
Gold (Au)
Zinc (Zn)

The following substances are used as the second phase particles:

Aluminum oxide (Al₂O₃)
Carbide ceramics|Silicon carbide (SiC)
Tungsten carbide (WC)
Diamond (C)
Silicon oxide (SiO₂)
Graphite (C)
Thermoplastic Polytetrafluoroethylene (PTFE)
Molybdenum disulphide
Boron nitride (BN)

Content of the dispersed phase is commonly 2-27 oz/gal (appr. 15-200 g/l).

Mechanism of electrolytic co-deposition

The second phase particles suspended in the electrolyte adsorb the positively charged metal ions. The particles gain positive electric charge.
The particles surrounded by the positive metal ions reach the cathode surface driven by the electrostatic attraction and the electrolyte convection.
The particles stick to the cathode surface and discharge. The particles may either stay on the cathode surface or disconnect from it. Bonding force retaining the particles on the cathode surface is determined by the relationship between the interfacial energies particle-electrolyte, particle-cathode and cathode electrolyte.
The metal ions are deposited on the cathode surface around the particles. The particles are incorporated into the metallic deposit.

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Effect of electroplating process parameters on co-deposition

Metal ion concentration. Higher ion concentration leads to denser adsorption of the ions on the particles surface resulting in increase of the driving force of the co-deposition. Increased concentration of the positively charged metal ions enhances the deposition of the particles suspended in the electrolyte.
Additives promote deposition of the second phase particles from the electrolytic suspensions. Additives promote adsorption of the metal ions on the particle surface and stabilize suspension preventing particles agglomeration.
Current density. Increased current density enhances incorporation of the particles into the deposit.
Electrolyte agitation increases convection and therefore enhances the flux of the particles reaching the cathode surface however too intensive agitation may cause adverse effect caused by disconnection and removal of the particles by turbulent streams of the electrolyte.
Electrolyte temperature. Elevated temperature increases the electrolyte flow and the ions mobility due to lower viscosity and density. Higher temperature also causes stronger bonding between the particles and the cathode surface. Thus increased temperature enhances incorporation of the particles into the deposit.

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Wear resistant coatings with incorporated hard particles

Metal Matrix Composites with dispersed hard particles as the second phase possess enhanced wear resistance and abrasion resistance.
The composite coating reinforced by hard particles have also improved mechanical properties (hardness, strength) than plain metallic coatings.

The most popular wear resistant composite coatings are nickel based. Nickel matrix composites with various dispersed phases (Al₂O₃, SiC, WC, diamond, SiO₂) are fabricated by electrolytic co-deposition from Nickel Sulfamate and Watts electrolytes. Nickel based wear resistant coatings are used in abrasive tools, measuring tools and gauges, moving details of machines and engines.

Anti-friction coating of Engine bearings consisting of lead-tin-copper alloy and reinforced by alumina (Al₂O₃) is fabricated by electrolytic co-deposition from electrolyte solution of lead, tin and copper with alumina particles.

Aluminum matrix material reinforced by silica (SiO₂) is prepared from AlCl₃-dimethylsulfone electrolyte containing fine silica particles.

Incorporation of dispersed hard particles by electrolytic co-deposition is also used for obtaining wear resistant coatings of copper, cobalt, silver, gold and zinc.

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Co-deposition of solid lubricant particles

Incorporation of solid lubricant particles into a metallic coating improves its antifriction properties: decreases the coefficient of friction and increases Seizure resistance (compatibility) and wear resistance.

Particles of Thermoplastic Polytetrafluoroethylene (PTFE), molybdenum disulfide, graphite and boron nitride (BN) are used as the second phase of solid lubricant in the composite antifriction coatings.

The most popular composite antifriction coatings are nickel based with co-deposited PTFE.
Copper-graphite and cobalt-BN are other examples of depositions with incorporated particles of solid lubricants.

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Co-deposition of corrosion protection coatings

Composite coating containing incorporated dispersed non-metallic particles demonstrate enhanced corrosion and oxidation resistance.

Strong compounds (oxides, nitrides, carbides, borides) are used as the second phase in corrosion resistant coatings: Al₂O₃, Cr₂O₃, TiO₂, SiO₂, SiC, TiC, Cr₃C₂, Si₃N₄, ZrB.
Nickel, cobalt and chromium are commonly used as the matrix materials in corrosion resistant composite coatings.

Examples of corrosion and oxidation resistant composite coatings: Ni-Al₂O₃, Ni-TiO₂, Ni-Cr₂O₃, Ni-TiC, Co-Cr₃C₂, Cr-ZrB.

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