Question: Due to customer inquiries, we are discussing in our electroplating shop whether we should expand our program in the medium term to include dispersion deposition. Unfortunately, none of us have any practical experience in this area and the technical literature is not particularly comprehensive.
Answer: In fact, dispersion deposition is pretty much a blank spot in the German-language literature. If at all, it is only mentioned that it exists. Only a few authors discuss the details. We also have no practical experience in this area and can unfortunately only reproduce what we can find in our archives.
The term dispersion layers is used when mechanical particles added to the electrolyte are also deposited. Diamond powder was used together with nickel for the first time in the 1930s. It was used to manufacture dental instruments and nail files, among other things. In general, nickel (galvanic and chemical) is the main application for dispersion layers.
Essentially, the process works by keeping the mechanical additive in a corresponding suspension and simultaneously nickel-plating the parts.
Around the beginning of the 1960s, nickel dispersion coatings became even more important. All conceivable solid materials are incorporated, such as silicon carbide, corundum, quartz, titanium dioxide, titanium carbide, boron nitride, oxides of thorium, uranium or zirconium, as well as graphite and plastics such as Teflon powder or molybdenum sulphide. The latter produce nickel layers with a high self-lubricating effect. The most prominent properties of nickel dispersion coatings are
- Abrasion or wear resistance
- temperature resistance
- Sliding properties
- ductility
- strength
- Corrosion resistance
Typical areas of application are weapons technology and the nickel plating of engine parts, for example the electroplating of the running surface of rotary engines or cylinders or cylinder liners and piston rings. However, hard chrome plating is still superior for piston rings. Other areas of application include dental tools or bone cutters, in components of textile or plastic injection molding machines, on cylinder head surfaces or saw wires.
In addition to nickel, chromium, iron, copper, silver, gold, zinc and tin electrolytes are also used.
Electrolytes
Normal Watts or sulphamate electrolytes are used as nickel electrolytes [1], to which the particles to be dispersed are added and kept in solution as a suspension by agitation and appropriate surfactants. This limits the particle quantity, the particle mass and thus the particle size. It is particularly important to homogenize the particle incorporation, which is achieved using various convection strategies and modified particles (see practical information).
Hard material
The most common hard material is silicon carbide SiC, which is used in grain sizes of 0.1 to 3 μm and average contents of 150 g/L. The achievable incorporation rate is approx. 8 to 10 mass %, which is usually sufficient. Increasing the solids content in the electrolyte only slightly increases the incorporation rate.
Aluminum oxide (Al2O3) and titanium oxide (TiO2) are also sometimes used. The aluminum oxide is added as a powder with a particle size of 13 nm, titanium oxide with a particle size of 21 nm. The particle concentration can be between 1 and 10 % by weight. These layers have a more favorable material structure compared to pure nickel layers and can therefore offer more favorable material and wear properties. Particularly noteworthy are the refinement of the microstructure, the increase in hardness, the reduction in stresses and the increase in wear resistance.
Practical tips
With regard to the adhesive strength of the coating, it has proven to be favorable to first deposit approx. 10 μm nickel without solid content.
Additional organic compounds can be used to further increase the hardness of the coating.
Uniform distribution of the particles in the area close to the cathode is also essential for uniform incorporation, which is why a corresponding electrolyte movement using mechanical agitators or air injection is required. More complex pump systems are also used.
Agglomeration is a problem with small particles. The particles are held together in the agglomerates by so-called van der Waals forces. The inclusion of agglomerates in the dispersion layer is undesirable because they have a negative effect on the layer properties. Strategies for deagglomeration have therefore been developed. [2] To prevent this, dispersant molecules are introduced into the solution, which form a hydrate shell of approx. 20 µm around the particles so that the particles "no longer see" each other and thus do not form agglomerates. During deposition, the stabilizer must be stripped off again.
Logically, the electrolyte can only be filtered after the dispersed phase has settled. It is also recommended to wash and activate the dispersant at certain intervals.
In the production of dispersion layers with hard materials as abrasives, the nickel layer only acts as a binder. The finished coated component represents an abrasive body in which larger particles (diamond, cubic boron nitride) with diameters of 5 to > 100 μm are grown around by the nickel layer. First, the part to be coated is briefly pre-nickel-plated without hard material particles. The hard material particles are then usually applied to the part as a monolayer and enclosed by the growing nickel layer in such a way that they still protrude from the nickel layer but do not break out under stress.
Ni/P dispersion layers
Of course, dispersion layers in electroless nickel are also possible and are applied accordingly in practice. Ni/P+diamond and Ni/P+PTFE are mainly used. These have 20 % PTFE by volume and are used for lightweight hydraulic and pneumatic components, door closers, nozzle sleeves, pump parts, choke guides for carburetors that can withstand temperatures of up to 300 °C, ball plugs, bearing and gear parts, seals, printing machine parts, casting and press molds for plastics, rubber and similar. Ni/P+SiC (automotive lamellas, carburetor parts, brake discs, clutch parts, gear wheels, cylinders, Al pistons, etc.) and Ni/P+graphite are also used.
Ni/P+diamond coatings with phosphorus contents of around 6 to 7 % offer optimum wear resistance. For highly abrasive tools, Ni/P dispersion coatings with diamond inclusions with a grain size of 2 to 6 µm have proven to be useful.
Applications for Ni/P+SiC coatings in the aerospace sector focus on rails, running surfaces, bushings, bolts and couplings. By extending the service life of these parts, maintenance cycles on aircraft can be reduced. This not only lowers the cost of new coatings, but also leads to a significant reduction in expensive aircraft downtimes. Ni/P+SiC coatings are not attacked by water, acids, alkalis and oil. They are largely temperature-resistant and inexpensive.
Ni/P+PTFE coatings are used wherever low coefficients of friction and good abrasion resistance are required. They are used in particular for molds, connecting elements and pump parts where sliding wear and anti-adhesive properties are required. Due to the special properties of these coatings, even parts that are difficult to access, such as gear, bearing or piston assemblies with extremely tight tolerances, can be provided with a durable, self-lubricating coating.
Further information
[1] Online course: Galvanic nickel plating https://www.galvanotechnik-for-you.de/uebersicht-kurse/die-galvanische-vernickelung/
[2] Dispersion deposition - ZOG seminar in Aalen Galvanotechnik 02/2022 Eugen G. Leuze Verlag
