Adhesion optimization of DLC thin films through conditioning of steel surfaces by plasma nitriding

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Diamond-like carbon films (DLC) are essential due to their excellent properties in many tribological applications. In many cases, unsatisfactory adhesion and mechanical load capacity limits their application. One solution, rarely used until now with DLC coatings is pretreatment plasma nitriding. In addition to an improved supportive effect, an improved coating adhesion was also observed. Reasons for the previously seldom use of this approach is the poor reproducibility of results and the low level of knowledge about its adhesion.The aim of this project is the identification, description and assessment of the adhesion criteria of DLC films on plasma-nitrided steel surfaces. Preliminary work has shown that plasma nitriding of steel surfaces allows for a very good adhesion of DLC coatings, without going into further detail. However, it was determined that the modifications of the surface by suitable selection of parameters can be specifically adjusted during the nitriding process. These results form the basis for the following working hypotheses:• The improved adhesion of DLC coatings is caused by changes to the surface structure, which is a result of the plasma nitriding process. These include changes of the grid structure, the solubility and diffusion rate of carbon, the reaction conditions and also phase transformations.• The correlations between these changes and the adhesion of the coatings are still largely unknown, but can be experimentally determined.• Special attention will be paid to the carbon solubility, which is responsible for an increase of diffusion processes during the coating process and therefore has a positive influence on the adhesion of the coating. From these working hypotheses, sub-objectives are derived and listed in the following.1. To reach a deeper understanding of the link between the nitriding parameters and the resulting microstructure and material properties.2. The modeling of diffusion processes during plasma nitriding and DLC coating, taking the micro-structure into account.3. Explanation of the connections between the resulting surface modifications through plasma nitriding and the adhesion of the DLC coatings.4. The identification of suitable modifications of DLC coatings, that allow a good adhesion on plasma-nitrided surfaces.To achieve these objectives, suitable steels are selected, under variation of process parameters, plasma nitrided and their edge layer composition is analyzed. Then the nitrided samples are DLC coated in a PACVD process under defined conditions and the coatings adhesion is examined after-wards. The findings from the examinations are incorporated in the modeling and simulation of the processes taking place during nitriding in the material. Also a model concept is developed that com-bines the results of the various investigations. This allows the identification and evaluation of the influencing parameters of the nitriding and coating processes that have a significant influen