[Back]

F. Klimashin, H. Euchner, P.H. Mayrhofer:
"Experimental and computational studies on Mo-N";
Talk: TACT 2015, Tainan; 2015-11-15 - 2015-11-18.

Cubic B1 molybdenum nitride (γ-Mo2N) is a promising candidate as a wear-protective coating because of its excellent mechanical and tribological characteristics [1]. In contrast to the materials with the typical B1 structure, formation of the nitrogen deficient NaCl structure with the half-populated N-sublattice is energetically favorable within the Mo-N system. Although in the past 40 years numerous research activities have been devoted to the Mo-N system, to the best knowledge of the authors no computation-supported studies aimed at studying the influence of N-vacancies on phase composition and mechanical properties were carried out.
In this work we reactively sputtered Mo-N coatings at varying nitrogen partial pressures. Evolution of the phase composition with increasing N2-to-total pressure ratio was investigated by means of XRD and SAED (TEM) and can be described as following: Mo → Mo + c-Mo2N → c-Mo2N → c-Mo2N + h-MoN. The employed first-principles calculations demonstrate the role of the nitrogen vacancies and the entropy in the energy of formation of c-MoNx coatings. Considering the entropy factor at 700 K (equivalent to the substrate temperature used during deposition), c-MoNx (γ-Mo2N-type) can be synthesised in the wide range of N/Mo ratio, x, 0.31 - 0.67. This excellently correlates with our experimental results. The indentation hardnesses of c-MoNx range between 30 and 33 GPa, where the maximum of ~ 33 GPa show the nearly 2:1-stoichiometric MoNx - c-MoN0.53.

Mo-N, magnetron sputtering, nitrogen vacancies, DFT, hardness