Investigations regarding a novel combination treatment of electron beam alloying and diamond-like carbon coating for highly stressed Al alloys

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Al alloys are attractive lightweight materials, but their low hardness limits their usability for components under tribological stress. Thin hard coatings, especially diamond-like carbon coatings (DLC), show excellent friction and wear behavior. But due to the lack of supporting effect on soft materials they can only be used to a limited extent. This lack can be eliminated by electron beam (EB) liquid phase surface treatment of the Al substrate and the resulting increase in hardness. The aim of the project is to investigate the material-specific cause-and-effect relationships and the sequence of elastic-plastic deformations and crack formation in the material composite Al-substrate - EB surface layer - DLC coating. This enables a knowledge-based design of the material composite for high loads. For a comprehensive understanding, a scientifically based chain of knowledge of the relationships between process parameters - microstructure - properties - stress behaviour is to be established stepwise. In phase I, a fundamental material and mechanistic understanding of the interface between Al-based model alloy/DLC-coating as a basis for the functionality of a layer composite on soft Al-substrate will be generated. Thereby, the focus is on the analysis of the microstructure of the model alloys and their relevant properties for the desired support and adhesion function for the DLC coating. The stepwise investigation of Al, Al-Si and Al-Si-Ni(x) model alloys allows an easier extraction of the effects of the microstructural constituents on e.g. oxidation and selective sputtering processes, and thus, on the adhesion and load capacity of the DLC coating. The comprehensive analysis of the individual components of the material composite with regard to their mechanical characteristics and the residual stress state is incorporated into the contact modelling. The first step is to generate a realistic elastic load/contact model of the layered composite in order to calculate the complete stress and strain states. Attention is paid to the exceeding of critical von Mises stresses and to crack-critical stress parameters in the interface Al-based model-alloy/DLC-coating as well as within the DLC. The decision whether a stress or strain parameter is ultimately critical, i.e. whether it leads to the initiation or propagation of cracks, is made in comparison with the crack phenomena actually observed in the crack test and in the pop-in analysis by means of nanoindentation. Based on these new basic findings, the transition to application-relevant technical alloys is to be made in phase II in order to be able to interpret the mechanisms of the heterogeneous, multiphase multi-material alloy.