Constitution and mechanical properties of nanocrystalline reactive magnetron sputtered V-Al-C-N hard coatings as a function of the carbon content

C. Ziebert, U. Albers, M. Stüber, S. Ulrich, H. Holleck

Forschungszentrum Karlsruhe, Institut für Materialforschung I, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

Nanocrystalline V-AI-C-N hard coatings were deposited by reactive r.f.-magnetron sputtering in an Ar/CH₄ mixture from a segmented target composed of one half each of pure VC and AlN ceramics by using a combinatorial materials science approach. For each experiment, six hard metal inserts were placed on the substrate table in fixed, equidistant positions under the target. Consequently, six coatings of different chemical composition and/or microstructure could be grown in a single experiment. The Ar gas flow was kept constant at 660 sccm, while the CH₄ gas flow was systematically varied between 0 and 60 sccm to study the formation of metastable and nanocomposites phases and to investigate the influence of the carbon content on the constitution and the properties of the coatings. The chemical composition of the coatings was determined by electron microprobe analysis and the crystal structure of the films was characterized by X-ray diffraction. Concerning the mechanical properties hardness and reduced elastic modulus have been investigated by nanoindentation and the critical load of failure by scratch test. The topographical changes in dependence of the carboncontent have been studied by AFM. Significant changes in the coatings topography, in the related surface roughness and in the mechanical properties were observed both as a function of the sample position and of the carbon content. Especially it was possible to achieve a large variation of the hardness in the range between 15 and 35 GPa and in the reduced elastic modulus in the range between 120 GPa and 600 GPa. Finally, the conditions for the formation of metastable (V,AI)(C,N thin films and nanocomposites are described in terms of surface processes during film growth and thermodynamic considerations.