ICAMS / Interdisciplinary Centre for Advanced Materials Simulation
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CVD-diamond coatings on heat-treatable steels – experimental and computational Investigation of the adhesive layer system Cr/CrxCy/diamond

Date: 04.09.2008
Place: Materials Science and Engineering 2008, Nürnberg, Germany

Andreas Böhner, Department of materials science and engineering, Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Germany
Rebecca Janisch
Alexander Hartmaier
Karsten Kellermann, Institute of Science and Technology of Metals, University Erlangen-Nürnberg, Erlangen, Germany
Stefan Rosiwal, Werkstoffkunde und Technologie der Metalle, University Erlangen-Nürnberg, Erlangen, Germany
Robert Singer, University Erlangen-Nürnberg, Erlangen, Germany
Karsten Durst, Werkstoffwissenschaften, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
Jens Schaufler, General Materials Properties, University Erlangen-Nürnberg, Erlangen, Germany
Mathias Göken, Department of Materials Science and Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany

Superhard carbon coatings, e.g. CVD-diamond on ductile heat-treatable steel, promise to feature exceptional resistance to frictional wear and corrosion. For wear protective coatings the adhesion between the coating and the substrate is of great importance. In this study tool steel was coated with diamond in a CVD process, using Cr as an adhesive layer between diamond and the substrate. To this end we diffusion-chromized samples of 1.0501 heat-treatable steel which were then coated with CVD-diamond. The emerging chromium carbide interlayer prevents the graphitization effect of iron during CVD-diamond deposition and enables metallic bonding to the iron substrate and covalent bonding to the growing CVD-diamond layer. Tensile tests were performed at the coated samples to determine the critical stress until delamination of the coating. The failure mechanisms during tensile deformation were afterwards characterized by light and scanning electron microscopy and focused ion beam cross sections. It is found that under tensile stress cracks at equal spacing evolved perpendicular to the loading direction, in the diamond coating as well as the underlying carbide. This brittle failure mechanism in principle enables a determination of the interfacial strength after the model of Agrawal and Raj. However, cross sections produced by the focused ion beam raise ambiguities concerning the weakest interface in the adhesive layer system (Cr/CrCx/diamond). To clarify this point we performed computational case studies of model interfaces between chromium and chromium carbides of different stoichiometry as well as chromium carbides and diamond. For this we used ab-initio density functional calculations employing plane waves and pseudopotentials. After relaxing macroscopic and microscopic degrees of freedom, the interfaces were characterized by calculations of the work of separation and investigation of the electronic structure at the boundary. The influence of superimposed stresses and strains on the results can be made visible. The results show an increasing work of separation with decreasing C concentration in the carbide for both types of interfaces. The absolute values suggest that failure occurs inside the carbide layer.

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