Electroless nickel layers

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Electroless nickel layers are chemical, i. H. Coatings made of nickel and i. produced by means of a redox reaction . d. Usually another alloy partner such as phosphorus or boron on a substrate with a catalytically active surface.


The chemical nickel deposition takes place autocatalytically , i. H. the deposited nickel alloy itself catalyzes the further deposition, so that the layer thickness that can be deposited is not limited from a process point of view. Typically, hypophosphite PH 2 O 2 - or boranate BH 4 - or organic boranes such as dimethylaminoborane (H 3 C) 2 HNBH 3 are used as reducing agents .

The deposited layers have a content of phosphorus or boron that can be adjusted over a wide range and, depending on the P or B content, are crystalline to amorphous. The Ni / P layers are categorized as low (low-phos, 1–3% P, crystalline in the deposited state), medium (mid-phos, 4–9% P, partially crystalline in the deposited state) and high-phosphorus (high- phos, at least 10% P, X-ray amorphous in the deposited state). They show a very high corrosion resistance, which increases with increasing phosphorus content. In the area of ​​Ni / B layers, a distinction is made between low-boron (1-2% B) and high-boron layers (5-6% B). Both Ni / P and Ni / B layers are also very hard in the deposited state. By heat treatment in the range of 350–400 ° C for 1–2 hours (depending on the component mass), the hardness can be increased again significantly, so that for (medium and high phosphorus) Ni / P or (high boron) Ni / B Coating values ​​around 1000 HV or 1400 HV can be achieved, which is almost or significantly more than the hardness of hard chrome layers (approx. 1100 HV). The increase in hardness is due to phase precipitation of Ni 3 P or Ni 3 B and Ni, which leads to a dispersion hardening effect . In the deposited state, the low-phosphorus layers of the Ni / P layers have the highest hardness, which can be up to 700 HV. Among the Ni / B layers, layers with a high B content have the highest deposition hardness, in the range 800–850 HV.

Electroless nickel dispersion layers

Electroless nickel layers are suitable for many dispersoids as the ideal matrix for the deposition of dispersion layers . The composite layers obtained in this way synergistically combine the properties of the matrix (high layer hardness, corrosion resistance and dimensional accuracy) and the respective embedded dispersoids, e.g. B. hard materials such as diamond or silicon carbide to produce abrasive, wear-resistant or microstructured surfaces or dry lubricants such as hexagonal boron nitride h-BN or perfluorinated polymers such as polytetrafluoroethylene (PTFE) or perfluoroalkoxy copolymer (PFA).

In order to achieve a sufficient deposition rate, the electrolytes are heated well above room temperature during deposition, e.g. B. up to 90 ° C. According to the RGT rule, the separation rate increases exponentially with the temperature. Depending on the process, the deposition rate can be, for. B. with medium-phosphorus chemical nickel up to around 25 microns / h. Too high a deposition rate leads to uneven, cauliflower-like layer growth, which can have a negative effect on the layer gloss and other layer properties. In addition, if the electrolyte temperatures are too high, the decomposition from homogeneous solution takes place too quickly. If too many Ni / P nuclei are formed in the colloidal state in this way, the electrolyte spontaneously self-decomposes, which means that the coating process must be stopped and repeated. Since the deposition takes place independently of an electrical field, the deposition rates are locally very homogeneous even on complex geometric components, so that, in contrast to galvanic processes that work with external current, there are only the slightest deviations in the layer thickness over the entire component. In addition, a specified target layer thickness can be achieved very precisely over the duration of the deposition, whereby excessive coatings can be avoided. This leads to a further advantage on components with high demands on dimensional accuracy.

Even rotationally symmetrical, geometrically complex components such as opening rollers of spinning machines (open-end spinning process) or aluminum compressor wheels of turbochargers can be coated in this way after the balancing process, without being re-imbalanced by the coating process.

Mission requirements and shift selection

Layers containing medium phosphorus are most frequently used. These cover approx. 60%, high-phosphorus layers 30–35% and low-phosphorus layers 5–10% of the market. If the electroless nickel layer is to have the highest possible hardness and if no heat treatment in the range of 350 ° C required for precipitation hardening can be carried out, as is the case, for example, with many aluminum alloys or hardened steels, i. d. Usually low-phosphorus electroless nickel layers are used. If the substrate material allows an appropriate heat treatment, layers with a medium or high phosphorus content can be used. However, the latter are i. d. Usually used primarily for corrosion protection applications due to their amorphous structure in the deposited state, which is why no heat treatment to increase hardness is carried out here to maintain the amorphous state.

A heat treatment of layers with a high phosphorus content is carried out when the high hardness levels that can be achieved in combination with the higher elongation at break of layers with a high phosphorus content are to be used. If both high hardness and good corrosion resistance are required, the most common medium-phosphorus layers are used. Due to the almost double deposition rate, these are usually cheaper than the high-phosphorus layers with the same layer thickness. In order to achieve the highest possible level of protection against corrosion, the layers must not have any pores reaching through to the base material , which is the case from layer thicknesses of 20-25 µm. Pores can be formed, since elemental hydrogen is always formed during the deposition , which can form the origin of a pore through the formation of bubbles if no suitable measures such as the use of surfactants or sufficient convection are taken.

Electroless nickel layers are therefore always used when one or more of the following process or layer properties are required:

  1. High dimensional accuracy , especially on geometrically demanding substrates (no "dog bone" effect)
  2. Very high corrosion resistance , etc. a. also against alkaline media
  3. Very high hardness and therefore high resistance to abrasive wear

The high dimensional accuracy in connection with the very high corrosion resistance and the very high hardness is unique for electroless nickel. As with all layers, the protection that can be achieved in practice depends on the specific load collective and the applied layer thickness. The corrosion protection of electroless nickel layers is on ferrous substrates i. d. R. anodic nature, e.g. B. also tin / nickel layers. Alternatives from the area of ​​cathodic corrosion protection are, for example, zinc and zinc alloy layers. With regard to point 3, only galvanically produced hard chrome layers have a higher layer hardness. In summary, however, it can be stated that an electroless nickel layer, like any other layer system, is unique in terms of its collective properties, so that depending on the load collective, requirements for dimensional accuracy and layer thickness as well as the incurring layer costs, electroless nickel can meet the requirements in a unique way .

Ni / P layers on light metal components

Light metals such as aluminum and magnesium alloys can be used as materials under atmospheric conditions because a dense oxide layer shields the reactive base material from aggressive environmental influences. For numerous applications, however, the chemical and mechanical load profile of a component exceeds the resistance of these oxide layers. Surface finishing is essential for such applications. Since complex geometrical shapes are the rule for light metal components, electroless metal deposition, in particular chemical nickel plating, is preferred. Optimized pretreatment sequences for the base material have the task of suppressing the formation of partial oxide layers in the initial phase of the coating. Oxidic islands limit the adhesion between the base metal and the coating and can therefore lead to component failure. If this key procedural step is under control, the entire range of properties of Ni / P alloy layers is available for optimizing the component.

Applications of Ni / P layers

Chemical nickel layers are often used in the following areas due to their excellent dimensional stability, their very high hardness and their excellent corrosion protection:

  • Automotive and aviation industry, mechanical engineering: components for injection systems, air conditioning compressors, coolant pumps, airbag systems, synchronizer rings, servo valves, chassis components, bearing journals, flanges, compressor wheels, textile machine components, etc.
  • Chemical industry, petrochemical: heat exchangers, turbine blades, spray heads, pressure vessels, reactors etc.
  • Electronics: PC drive components, plug connections, base layer for soldered connections, electromagnetic shielding of sensitive components, etc.
  • Human medicine: base layers for metal-ceramic soldered joints (joint prostheses) etc.
  • Plastic metallization: decorative and functional layers