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F. Klimashin:
"Molybdenum-based hard nitrides";
Supervisor, Reviewer: P.H. Mayrhofer, G. Ramanath; E308, 2016; oral examination: 2016-03-15.

Molybdenum-based cubic-structured nitrides are fascinating materials, predicted by empirical criteria (e.g. valence electron concentration, elastic constants G/B and C12-C44, ionic potential) and proved experimentally as self-lubricating hard coatings with improved toughness. The application area is restricted, however, to lower temperatures, since the weak chemical non-metal-metal bonds tend to break at 300-500 &deg;C and, if exposed to an oxygen-containing atmosphere, the volatile MoO3 can form. Cubic-structured molybdenum nitride is also difficult to synthesise due to its high sensitivity to the nitrogen content present. Hence, to the nitrogen partial pressure used during physical vapour deposition. The cubic-structured of the molybdenum nitride is the high-temperature allotropy of the tetragonal-structured ß-phase. The use of non-equilibrium synthesising techniques such as magnetron sputtering allows, however, the deposition of metastable phases. The distinctive feature of the cubic c-MoNx, as compared to the majority of B1-structured transition metal nitrides, is the highly defected nitrogen sublattice, being 50% vacant. The experimental results, supported by ab initio calculations, suggest that increasing vacancy content at the nitrogen sublattice leads to the formation of coherent domains with high Mo-content (where the interstitial sites are vacant), while decreasing vacancy content at the nitrogen sublattice leads to the formation of coherent domains with partially ordered pseudo-cubic c'-Mo3N2 (actually c'-MoN0.67, where 1/3 of the nitrogen sublattice is vacant). The slightly overstoichiometric c-MoN0.53 (as revealed by elastic recoil detection analysis) was found to exhibit the highest indentation hardness 33 GPa, which decreases to 28 GPa, when the vacancy content is reduced and partially ordered pseudo-cubic c'-MoN0.67 developed. To extend the temperature range and further improve the properties of c-MoNx-based materials, ternary Mo-Cr-N and Mo-Al-N alloys were developed. Both elements, chromium and aluminium, are well known to significantly improve the oxidation resistance through the formation of dense oxide scales Al2O3 or Cr2O3, preventing or significantly reduce further metal (e.g., Mo) outward and oxygen inward diffusion. For the ternary Mo-Cr-N coatings, the nitrogen partial pressure used during magnetron sputtering is crucial to allow the synthesis of single-phase cubic-structured solid solutions along the entire Mo-Cr composition. For a narrow nitrogen partial pressure window, the solid solution follows the MoN0.5-CrN quasi-binary tie line and, thus, forms a continuous cubic-structured solid solution. The chemical formula along this tie line can be described with Mo1-xCrxN0.5(1+x), indicating that for every vacancy at the nitrogen sublattice that is populated with nitrogen, we need to substitute two Mo-ions with Cr-ions at the metal sublattice. However, high nitrogen partial pressures during deposition favour the formation of fully occupied for combining the cubic structure and high Al content. As soon as the hexagonal phase forms, for too high nitrogen partial pressures or Al contents, the hardness drastically decreases to 22 GPa. The knowledge gained during the investigation and development of the binary MoNy and the ternaries Mo1-xCrxNy and Mo1-xAlxNy, allowed the development of quaternary Al-rich (x = 0.5 or even 0.6 depending on the Cr-content) single-phase cubic-structured fcc-Mo1-x-yAlxCryNz coatings with hardnesses above 40 GPa. Particularly, fcc-Mo0.39Al0.52Cr0.09N0.98 exhibits the highest hardness, H, of 41 GPa among all coatings studied. The combination with a relatively low indentation modulus, E, allows for low H/E- and H3/E2-ratios (0.1 and 0.35, respectively). These empirical criteria, related with the elastic strain to failure and resistance to the plastic deformation, respectively, suggest for excellent wear protection and a high potential for severe applications. Summarizing, the combining of experimental and computational materials science provides deeper insights into the complex nature of the substoichiometric nitrides (nitrogen/metal-ratio < 1). Especially the development of Mo-based nitride materials requires a knowledge-driven tuning of the deposition process, due to the determining role of the nitrogen vacancies (and vacancies in general). The study clearly shows that the understanding of atomic scale processes is needed for the development of new high-performance materials.