Effect of nitrogen flow rate on structure and properties of MoNx coatings deposited by facing target sputtering

Molybdenum nitride (MoNx) coatings with different nitrogen content were deposited by reactively sputtering molybdenum metal target with various N2 flow rate. The structure of the coatings were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectrometry (XPS). The mechanical and tribological properties of the coating were evaluated by nano-indentation and ball-on-disc wear tests, respectively. The structural analysis showed that these coatings changed from bcc α-Mo phase to fcc γ-Mo2N phase, and then to fcc B1-MoN with the increase in N2 flow rate. As the phase structure changed from α-Mo to γ-Mo2N, the MoNx coatings exhibit finer grain sizes and denser crystal structure, which have dramatic contribution on hardness enhancement. The γ-Mo2N coating exhibits the highest hardness with the value of 32.5 GPa. As phase structure further changed to B1-MoN coating, a slight decline of hardness can be observed. The γ-Mo2N structured coatings exhibited lower COF than that of mainly Mo structured MoNx coating or B1-MoN coating.