A comparative study of CrNx coatings Synthesized by dc and pulsed dc magnetron sputtering

Chromium nitride (CrNx) coatings were prepared by reactively sputtering chromium metal target with various nitrogen flow rate percentages (fN2) using a closed field unbalanced magnetron sputtering system operated in dc and middle frequency pulsed condition (100 kHz and 50% duty cycle). In this study, plasma examination proved that a large amount of ions with a wide range of ion energies (up to 65 eV and mainly from 10–30 eV region) was identified in the pulsed plasma compared to the low ion flux and energy (0–10 eV) in a dc discharged plasma. The results showed that the phase structure of CrNx coatings was changed from nitrogen doped Cr(N) to pure β-Cr2N, and to a mixture of β-Cr2N and c-CrN and then to pure c-CrN phases with an increase in the fN2 in both dc and pulsed conditions. However, the pulsed CrNx coatings exhibit lower N concentrations than dc CrNx coatings prepared under the same fN2, which leads to the existing of β-Cr2N phase within a wide range of fN2 (30–50%). In comparison with the typical large columnar structure in the dc sputtered coatings, the pulsed CrNx coatings exhibit dramatic microstructure improvements which benefited from the improved plasma density and ion bombardment from the pulsed plasma, where the super dense and nearly equi-axial structures were observed in a wide range of fN2. The microstructure improvements contributed to the enhancements in the hardness and wear resistance of pulsed CrNx coatings. In the pulsed CrNx coatings, the hardness values were above 30 GPa when the fN2 is in the range of 30–40%, which is related to the formation of the β-Cr2N phase. With the formation of a mixture of β-Cr2N and c-CrN phases in the coatings deposited with 40–50% fN2, a low COF of 0.36 and wear rate of 1.66 × 10− 6 mm3 N− 1 m− 1 can be achieved.