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Thermochemistry - Nitriding
A layer of iron nitrides (basically Ɛ-Epsilon type but also ϒ'-Gamma prime type) and nitrides of alloy elements called "combination layer", which has the particular feature of being surface porous,
A diffusion zone below the combination layer, in which nitrogen is present in solid form between the iron (or alloy element) atoms.
Indeed, one obtains from the surface a hardness gradient and a compressive stress gradient which make it possible to improve in particular wear and fatigue resistance.
Thicknesses of combination layers and diffusion depths depend on the nature of the substrate, as well as on the CLIN process carried out. However, these are of the order of 0 to 30 µm for the combination layer and can be as much as several tenths of a millimetre for the diffusion zone.
With the aim of offering in addition excellent corrosion resistance (in an atmospheric, alkaline or acid environment), a passivation stage in an oxidizing bath creating Fe304 oxides (magnetite) embedded in the porous layer and a final organic type anti-corrosive impregnation which will fill this porosity while at the same time offering a dry-to-the-touch surface, are carried out subsequently.
Corrosion resistance can exceed 700 hours in salt spray on simple parts and can commonly reach 400 hours in salt spray on more complex parts.
In the case of special roughness requirements (for example friction with polymers and elastomers), a post-treatment polishing stage is necessary. The corresponding processes are of the QP or QPQ type.