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F. Klimashin, H. Euchner, P.H. Mayrhofer:
"Composition driven phase evolution in Mo-Cr-N and Mo-Al-N hard coatings";
Talk: Oorgandagarna, Visby, Schweden; 2015-06-15 - 2015-06-17.

Mo-N-based coatings provide a variety of phases depending on their chemical composition and N2-partial pressure used during deposition. Thereby, they can be design for a variety of applications requesting for tailor-made properties. For example, hexagonal δ-MoN has a superconducting transition temperature of 29 K, whereas cubic γ-Mo2N exhibits excellent mechanical properties and is therefore an ideal candidate for wear-resistant coatings. The major limitation of Mo-N coatings is their limited resistance against oxidation to form oxides with a high vapour pressure. Therefore, we have developed Mo-Cr-N and Mo-Al-N coatings, as the additional Cr and Al content not just increases their oxidation resistance by forming dense oxides but improves their mechanical properties by solid solution and precipitation strengthening.
The excellent correlation between experimental and ab initio results verifies our structural model for the Mo-Cr-N and Mo-Al-N coatings developed. Mo-Cr-N forms a solid solution fcc-Mo1-xCrxN over the whole compositional range 0 < x < 1, whereas within the Mo-Al-N system, the maximum Al solubility within fcc-Mo1-yAlyN was found to be at y ≈ 0.6. In both Mo-rich ternary systems, the evolution of the chemical composition (especially with respect to the N content) is in excellent agreement with the Mo2N-Cr2N or Mo2N-Al2N tie lines. These data suggest the formation of solid solutions where the metal species are substituted within the metal sublattice without a full occupation of the octahedral interstices within the nitrogen sublattice. Our ab initio studies highlight, however, that compositions along the quasi-binary MoN-CrN or MoN-AlN tie lines are energetically favourable, where all octahedral interstices are occupied. This can especially be observed during our experiments when preparing Al-rich MoN-AlN coatings (Mo1-xAlxN), which are hexagonal wurtzite structured w-Mo1-yAlyN for y values above ~0.6.
The formation of a solid solution along the Mo2N-Cr2N or Mo2N-Al2N results in highly (200)-growth oriented coatings. For both systems a pronounced solid solution strengthening effect, especially at the Mo-rich side up to x or y of ~0.2, is obtained exhibiting hardnesses of ~34 GPa for Mo1-xCrxN0.5 and 38 GPa for Mo1-yAlyN0.5. Further increasing the Cr content leads to a reduction in hardness to about 30 GPa, whereas a further increase in Al content has no measureable effect on the hardness of fcc-Mo1-yAlyN. Finally, fcc-Mo1-xCrxN and
fcc-Mo1-yAlyN show also a high resistance against plastic deformation with H3/E2 of 0.20 and 0.24 GPa, respectively.

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