Reduction of the coefficient of friction of niobium nitride coatings by the addition of bismuth

Composite coatings of niobium nitride containing bismuth inclusions were deposited using a dual magnetron sputtering system with independent power suppliers for each targets; Bi and Nb. The deposition was performed under and Ar/N2 atmosphere at a 2.3 flow rate ratio, which allows the stoichiometric NbN phase to be obtained. The aim of this work was to evaluate the effect of increasing concentrations of Bi, proposed as a lubricous second phase, in the structural, mechanical and tribological properties of niobium nitride (NbN) coatings, searching for a reduction in the coefficient of friction but trying to keep hardness values larger than the tool steels (>10 GPa). The power applied to the Nb target was fixed, while the power on the Bi target was increased; obtaining Bi atomic percentage values of 1.7, 4.9 and 8.4 at%. The Bi inclusions showed different sizes and distribution in the volume and surface of the coatings. The results indicated a reduction in the hardness from 30 GPa of the NbN coating to 13.9 GPa for the highest Bi concentration. However, a more significant reduction was observed for the coefficient of friction (CoF) from 0.8 of the NbN to 0.3 for Bi concentrations between 4.9 and 8.4 at%.