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F. Klimashin, H. Euchner, P.H. Mayrhofer:
"Phase evolution, microstructure, and mechanical properties of Mo-Cr-N hard coatings";
Talk: Vienna Young Scientists Symposium (VSS) 2015, Wien; 2015-06-25 - 2015-06-26.

The constantly growing demands for improved high-performance coatings to protect tools used in machining and forming applications or components in automotive and aerospace applications triggers the development of new material combinations. For these applications, where thermal stability, wear and mechanical resistivity are important, ceramic-like coatings are ideal candidates. Whereas there are extensive research activities for TiN- and CrN-based coatings, only little is known about MoN-based materials. Especially the cubic phase of molybdenum nitride (γ-Mo2N) exhibits excellent mechanical properties, but the phase is extremely sensitive to the nitrogen content. Consequently, the phase evolution of Mo-N coatings is extremely sensitive to the nitrogen partial pressure used during physical vapor deposition. Furthermore, MoNx coatings are sensitive to exposure in oxygen containing atmosphere, as the nitride easily transforms to molybdenum oxides exhibiting a rather high vapor pressure. Therefore, we started to investigate the phase evolution of Mo-N coatings as a function of the N2-to-total pressure ratio (pN2/pT) used during PVD (magnetron sputtering), see Fig. 1. Based on these results we used pN2/pT ratios of 0.32, 0.44, and 0.69 to improve the oxidation resistance by developing ternary Mo-Cr-N coatings. Our development was targeted towards single-phase Mo-N based coatings with a high Cr-content, as multiphased coatings typically exhibit a lower thermal stability due to their increased phase boundary fractions. The development of ternary Mo-Cr-N coatings is guided by detailed ab initio calculations within the binary Mo-N (Fig. 2) and Cr-N, and ternary Mo-Cr-N systems. Whereas all available literature reports deal with the Cr-rich side - Cr/(Mo+Cr) > 0.5 - our experimental studies concentrate on the Mo-rich side with Cr/(Mo+Cr) < 0.5. All ternary Mo-Cr-N coatings have a fine crystalline (column diameter of 10-30 nm) cubic structure (see the cross sectional TEM image, Fig. 3) and high hardnesses of around 30 GPa. The Mo1-xCrxNy coating with a Cr/(Mo+Cr) ratio x of ~0.15 exhibits the highest hardness of about 34 GPa as well as the highest resistance against plastic deformation with (H3/E2) of ~0.2 GPa, when prepared at pN2/pT = 0.32 (for comparison, H3/E2 values for TiN and CrN are about 0.1 GPa). We furthermore show that by decreasing pN2/pT from 0.69 to 0.32 during deposition, the structural and elemental evolution within our Mo1-xCrxNy coatings follows the quasi-binary tie lines MoN-CrN, Mo2N-CrN, or Mo2N-Cr2N. The coatings prepared with pN2/pT = 0.44 exhibit lattice parameters, which excellently agree with ab initio obtained values along the Mo2N-CrN tie line (Fig. 4) verifying their structural description. The coatings prepared with pN2/pT = 0.69 exhibit lattice parameters, which follow the MoN-CrN tie line only for high Cr contents, Cr/(Cr+Mo) > 0.5. Our results show, that depending on the nitrogen partial pressure used and Cr/(Cr+Mo) ratio Mo1-xCrxNy coatings with distinct structure and properties can be developed.

Mo-Cr-N; hard coatings; reactive sputtering; nitrogen partial pressure; vacancies.