Surface engineering of Fe-C coatings

This doctoral thesis presents research work on the Fe-C coatings, which were electrochemically deposited using an iron(II)sulfate based electrolyte with citric acid as the additive and obtained a high microhardness around 800 HV that increased to about 1300 HV by dedicated annealing. Electrodeposited iron-carbon coatings are a promising wear protective alternative to hard chrome coatings, because they allow environmental friendly deposition of a coating with promising mechanical properties.

The organic additives in the electrolyte are adsorbed in the coating during deposition and improve the mechanical properties by refining the microstructure. Fe-C coatings with the same internal structure have been reproducibly deposited independently of the coatings thickness and the applied metallic substrate.

A simple method is proposed specially adapted to estimate the adhesion between the Fe-C coating and a substrate after different pre-treatments. The influence of the operation conditions on the resulting material properties of the Fe-C coating has been systematically investigated and suggestions for the optimal operation conditions for long-term large-scale operation are provided in detail, which are required for the next step towards testing and implementation of the Fe-C coatings in industry.

The internal structure of the as-deposited Fe-C coating has been investigated, both at room temperature and during annealing, by advanced methods of materials characterization applying thermal analysis, microscopic and diffraction-based in-situ and ex-situ methods. The results essentially contribute to the understanding of the growth characteristics, co-deposition of light elements and the phase formation during electrodeposition of the Fe-C coatings.

The main cause of the high hardness of the as-deposited Fe-C coating is interplay between the nanocrystalline microstructure and the coherent or semi-coherent precipitates inside the ferrite grains. The characterization of an “organic compound” based on thermal analysis improved the understanding of the nature of the high amount of co-deposited carbon, oxygen and hydrogen within the as-deposited microstructure.

The temperature induced evolution of the co-deposited elements has been studied ex-situ and in-situ and is discussed in relation to dedicated post deposition surface engineering to improve the properties of the coating even further. The results essentially contribute to understanding of the internal structure of the  electrodeposited Fe-C coatings and described the recommended operation conditions for a long-term large-scale production, which are needed to be considered as a sustainable alternative to hard coatings.