Sputter deposited NbCxNy films: Effect of nitrogen content on structure and mechanical and tribological properties

NbCxNy thin films were deposited on Si(100) substrates by direct current magnetron sputtering from a Nb-C compound target in a mixture of N2 and Ar. The mechanical and tribological properties show a significant dependency on composition, chemical bonding and structure, as characterized by X-ray diffraction, and photoelectron and Raman spectroscopies. Increasing the nitrogen flow rate from 0 to 5 to 30 sccm causes a continuous increase of the nitrogen content y from 0 to 0.44 to 1.01. The films exhibit a Nb(C,N) solid solution at low N content, while an additional hexagonal NbN phase develops for y ≥ 0.69. A maximum hardness H = 42.1 GPa and elastic modulus E = 362 GPa is reached for the NbC1.44 N0.44 film, which also exhibits the highest compressive stress of 6.1 GPa and the highest H3/E2 value, yielding the best wear resistance. Increasing the N content leads to an increasing amorphous phase, which ultimately causes a reduction of stress, hardness, H3/E2, and wear resistance. The nitrogen also reduces the root mean square surface roughness from 1.27 to 0.46 nm, and causes a reduction of the friction due to an increase in the sp2-graphitelike carbon phase, with the lowest friction coefficient of 0.18 for the NbC1.99 N0.69 film.