Comparative study of chromium nitride coatings deposited by unbalanced and balanced magnetron sputtering

Chromium nitride (CrN) coatings are increasingly being used as an alternative to titanium nitride coatings and electroplated hard chromium in various applications. This work reports the results of a study of chromium nitride coatings deposited using unbalanced magnetron sputtering, with and without substrate bias, in comparison to films deposited using conventional balanced magnetron sputtering. Samples of CrNx were deposited in the two systems under identical deposition conditions, that is the same substrate temperature, plasma power, pressure, substrate to target distance and Ar/N2 ratio flow-rate. The film microstructure and composition were analysed by X-ray diffraction (XRD), cross-section scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The sample porosity and corrosion resistance were studied by electrochemical methods. Prior to deposition, the magnetic field geometry was measured by means of Hall probe, and plasma properties were determined using planar and cylindrical Langmuir probes. The rf bias voltage primarily influenced the crystal orientation and hardness of the deposits. Films deposited without bias presented about 80% of (200) orientation and 20% (111), whereas at rf biasing the orientation was preferentially (200). The hardness values on stainless steel substrates varied from 1200 to 1600 Hv0.01, and cross-section SEM images showed that the CrN films had a dense microstructure where columnar growth was inhibited. The relationship between growth conditions, microstructure and plasma parameters is presented and discussed