Nanostructured molybdenum nitride-based coatings: effect of nitrogen concentration on microstructure and mechanical properties

Abstract

Molybdenum nitride (MoNₓ) coatings were deposited onto AISI M2 tool steel substrates (hardened to HRC 60) by magnetron sputtering. The nitrogen content was controlled by means of a closed-loop optical emission monitor. The coating microstructure was examined by X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. This showed that the increased nitrogen content led to a transformation from body-centered cubic α-Mo phase to γ-Mo₂N phase (NaCl-B1 type) and then to a face-centered cubic MoN phase. The mechanical and tribological properties of these coatings were evaluated by nano-indentation, wear and scratch testing. The γ-Mo₂N coating showed the highest hardness and the highest values of H (hardness)/E* (effective elastic modulus) and H³/E* ratio. During scratch and wear tests, the γ-Mo₂N coating exhibited the best resistance to deformation and the lowest wear rate.