Coating of overstoichiometric transition metal nitrides (TMNx (x > 1)) by magnetron sputtering

This article reports on the formation of strongly overstoichiometric ZrNx>1 and Ti(Al, V)Nx>1 coatings by reactive magnetron sputtering. Problems in the formation of overstoichiometric coatings and possible ways to form strongly overstoichiometric TMNx>1 nitride coatings up to TMNx=2 dinitride coatings are discussed; here, TM are transition metals such as Ti, Zr, Mo, Ta, Nb, W, etc. The coating stoichiometry x = N/TM strongly influences its electrical and mechanical properties. The creation and properties of reactively sputtered ZrNx coatings were investigated. It was found that (1) the electrical resistivity of the ZrNx coating varies with increasing x from well electrically conducting films with x ≤ 1 through semi-conducting films with x ranging from 1 to ≤ 1.26 to non-conductive with x ≥ 1.3, showing that the stoichiometry x is a strong parameter which enables to control an electric conductivity of the coating in a wide range, (2) electrically conductive coatings with x ≤ 1 are harder than the semiconducting and electrically insulating coatings, and (3) the ZrN2 dinitride film cannot be created due to the formation of a Zr3N4 phase whose formation enthalpy is greater than that of a ZrN2 phase. Further, it is shown that the main problem in the formation of strongly overstoichiometric TMNx>1 and dinitride TMN2 coatings is a strong increase of ionization of the nitrogen sputtering gas to achieve a necessary high ratio N/TM > 1. Trends enabling the mastery of formation of the TMN2 dinitride coatings are briefly outlined.