Heat treatment in plasma
Oxidation following plasma nitriding or plasma nitrocarburizing increases the corrosion resistance of the material. The treatment creates a 1-2 µm thin oxide layer of Fe3O4 (magnetite), which gives the surface an anthracite to black appearance and makes it significantly less susceptible to corrosion.
Procedure and physical basis
Post-oxidation is a treatment stage following nitriding or nitrocarburizing, in which an oxide layer is created on the material surface with the aid of an oxygen donor. The aim of this layer is to additionally increase the corrosion resistance compared to the nitriding treatment. Prior nitriding or nitrocarburizing of the materials is absolutely necessary for the post-oxidation process to ensure sufficient formation of the oxide layer.
During nitriding, nitrogen is diffused into the surface layer. If carbon is introduced in addition to nitrogen, this is called nitrocarburizing. The structure of the resulting layers is similar in many respects. On the surface there is the compound layer, which is a few μm thin. Directly underneath is the diffusion layer, which in contrast to the upper layer is several hundred μm thick. The formation of the layers is determined by the alloying elements of the base material and the particles of the treatment medium introduced. Typical nitride formers include the alloying elements chromium (Cr), aluminum (Al) and molybdenum (Mo). The elements manganese (Mn), vanadium (V) and titanium (Ti) tend to form carboxyl groups.
Furthermore, the oxide layer is mainly formed by a transformation of the surface areas of a previously created compound layer. The free iron molecules and the iron nitrides of the compound layer, which are formed during oxidation, form a stable iron oxide in combination with the oxygen donor. This resulting 1-2 μm thick oxide layer is chemically resistant. In order to achieve high corrosion resistance, a pure magnetite (Fe3O4) is used.
A post-oxidation is carried out as the last process step directly after plasma nitriding or plasma nitrocarburizing. An additional treatment step and subsequent cooling in an oxidizing atmosphere transforms the top layer of the compound layer produced into a protective oxide layer. Afterwards, no further processing of the part may be carried out.
Effect and advantage of the process
The structure of the layers described above leads to an increase in resistance to tribological and dynamic mechanical stresses. This results in an improvement in corrosion resistance and wear resistance. Subsequent oxidation further increases the corrosion resistance in particular. The previously formed bonding layer is necessary for further treatment, since the oxide layer to be formed finds better hold on iron nitride than on ferrite.
Through oxidation, a resistance of up to 300 h (depending on the shape of the treated parts) can be demonstrated in the salt spray test. The application of the process is particularly useful as an additional treatment for plasma-nitrided workpieces made of lower alloyed materials, which must be both wear and corrosion resistant (e.g. transmission spindles, hydraulic cylinders or various vehicle pieces).