No external current
As external current (English electroless derived from " electrodeless ") are in the electroplating process referred to in which no external circuit , d. H. without a current or voltage source ( rectifier ). Either the substrate metal (dipping process, brewing process), which then has to be less noble than the metal to be deposited from the electrolyte, or a correspondingly less noble metal that is brought into electrical contact with it (contact process) serves as the reducing agent . In both cases the redox process taking place is called cementation . The layer thicknesses that can be achieved with this method are limited, however, since the redox process comes to a standstill as soon as there are no more pores reaching as far as the substrate material. Since the deposition rate also depends on the electron exchange taking place via the pores, due to the local dissolution of the substrate metal, the layer thickness asymptotically approaches a limit value.
An electroless chemical coating process, on the other hand, is a process in which the reducing agent used to reduce the metal ions in solution is also in solution. The redox reaction from homogeneous solution must be prevented, which is done by adding so-called stabilizers. The substrate to be coated must therefore act as a heterogeneous catalyst and offer the reaction an energetically favorable reaction path. Depending on the type of substrate, a distinction is made between the following groups:
- Substrate itself acts as a catalyst, e.g. B. Ni and z. T. Ni alloys (depending on the Ni content and type of alloy partner), Ir , Co , Os , Pd , Pt , Ru , Rh
- Must be germinated with self-catalytic metal (especially with plastic substrates)
- Ni nucleation by cementation on z. B. Al , Be , Fe , Ti (however, possibly no adhesive layers)
- Ni nucleation through contact deposition, current impulse (or Ni strike) or activation (Pd nucleation etc.) on z. B. Au , Ag , Cu
- Ionic or colloidal germination with Pd or Ag or the use of highly filled pastes with a subsequent baking process - is also used on non-conductors such as plastics, glass and ceramics, the latter only on glass and ceramics
Since the requirement for a catalytically active surface also relates to the deposited layer material, the electroless methods with dissolved reducing agent are also called autocatalytic methods. The most important representatives here are chemical nickel and chemical copper . However, a number of other metals can also be autocatalytically deposited, e.g. B. Fe, Co, Ag, Au, Pd, Rh, Ru , Pt, Sn, Pb and some of their alloys with an expanded range of metals and non-metals. Hypophosphite PH 2 O 2 - (usually combined with a co-deposition of phosphorus ), formaldehyde H 2 CO and boranate BH 4 - or organic boranes, e.g. B. Dimethylaminoborane (H 3 C) 2 HNBH 3 (usually combined with a cob deposition of boron ) is used. Hydrazine H 2 N-NH 2 is of minor importance because of its high toxicity . Furthermore, depending on the potential position, Sn 2+ can also be used as a reducing agent, e.g. B. in the germination of non-conductive plastic substrates with Pd germs. The potential positions of the reducing agents used are usually pH- dependent. Depending on the type of metal to be deposited and the other process conditions such as free metal ion concentration (depending, among other things, on the type and concentration of complexing agents present ), temperature and stabilizer content, the pH value and the type of reducing agent or its concentration are adjusted to create adhesive layers with the desired functional and maintain decorative properties.
- G. O. Mallory, JB Hajdu (eds.): Electroless Plating - Fundamentals and applications , American Electroplaters and Surface Finishers Society, Orlando, 1990.
- G. G. Gawrilov: Chemical (currentless) nickel plating , Leuze-Verlag, Bad Saulgau, 1974.
- N. Kanani: chemical nickel plating , Leuze-Verlag, Bad Saulgau, 2007.
- N. Kanani: Kupferschichten , Leuze-Verlag, Bad Saulgau, 2000.