Growth of CrNx films by DC reactive magnetron sputtering at constant N2/Ar gas flow

Chromium nitride coatings have been deposited on silicon by magnetron sputtering in an Ar+N2 atmosphere at different bias substrate voltages. The poisoning effect has been studied monitoring the discharge voltage and total pressure. The chemical composition and crystalline structure have been analysed by Glow Discharge Optical Emission Spectroscopy (GDOES) and X-ray Diffraction (XRD), respectively. Changing simultaneously the relative N2 (f(N2)) and Ar (f(Ar)) flow rates, while keeping constant the total gas flow (at 11 sccm), different crystalline phases are observed. At low N2 concentrations, f(N2) < 2%, pure Cr is detected. At increasing N2 flow rates (5% < f(N2) < 40%) the following sequence of phases is found: [Cr+N], [Cr+CrNx] and [Cr+CrN]. Finally, in the range 50% < f(N2) < 100% only the hexagonal phase Cr2N is obtained. Scanning electron microscopy (SEM) has been used to obtain the microstructure, revealing that it changes from columnar to granular when the substrate bias varies in the range from +20 V to -150 V. Nanoindentation and pin-on-disk experiments have been also carried out to correlate the tribological behaviour of the coatings with the observed crystalline phases.