The properties of molybdenum nitride coatings obtained by cathodic arc evaporation

Among the many techniques for the deposition of Mo-N coatings, AC and DC magnetron sputtering, ion implantation and cathodic arc evaporation, the latter is the most comprehensive, owing to the high degree of particle ionisation in the plasma, high density and coating quality. The phase structure of Mo-N coatings strongly depends on the deposition technology, mainly on the pressure of nitrogen in the working chamber, and the substrate bias voltage. Coatings deposited at four nitrogen pressures: 0.6, 1.0, 1.8 and 3.0 Pa, and four negative substrate bias voltages U-B: 10. 70, 150 and 250 V, were widely tested. EDS and WDS were used to evaluate the chemical composition, XRD to determine the phase composition, scanning and an optical microscopy to assess the morphology and surface quality, the scratch test and Daimler-Benz test to estimate the coatings adhesion, and ball-on-disc test to define the specific wear rate. The increase in the nitrogen pressure changes the phase structure of the body-centred cubic lattice of molybdenum Mo, through cubic gamma-Mo2N to the hexagonal delta-MoN. Due to the different structures of the crystal lattice, the given phases exhibit different physical properties. The substrate bias voltage rise initiates the resputtering of the coating, causing a reduction in nitrogen atoms in the coating. This affects the changes in the chemical composition and the coating morphology. It can be the cause of the cubic gamma-Mo2N phase existing in the deposition conditions of the hexagonal delta-MoN phase. The relatively large amount of macroparticles on the coating surface depends on the nitrogen pressure and the substrate bias voltage. The surface roughness Ra of the coating deposited at substrate bias voltages of more than -70 V is relatively small and almost independent of the nitrogen pressure. The coatings deposited at low substrate bias voltages (U-B = -10 V) are characterised by more than twice the surface roughness Ra, decreasing as the partial pressure of nitrogen increases. The mechanical parameters of the deposited coatings depend on their phase composition due to the different properties of Mo, Mo2N and MoN. (C) 2013 Published by Elsevier B.V.