Zr-Nb-N hard coatings deposited by high power pulsed sputtering using different pulse modes

High power pulse magnetron sputtering processes enable protective hard layer deposition at a high rate. Their capacity and strength rely to a large extent on pulse power supplies with high flexibility for adaptation to different discharge conditions to achieve specific film structures. In the present work, this is demonstrated for the deposition of Zr-Nb-N hard protective coatings by reactive co-sputtering of zirconium and niobium using a new versatile pulse power supply. The choice of pulse mode, namely single bipolar or pulse package mode, and pulse times in adjustment to gas flow rates allows tailoring the composition of the coatings. In a wide range of elemental compositions (i.e. for a Zr:Nb ratio between 0.6 and 1.1, and for a nitrogen to metal ratio between 0.6 and 1.1) the coatings exhibit a two-phase microstructure, containing a cubic (Zr,Nb)N phase and a hexagonal β-Nb2N phase, as evidenced by x-ray diffraction. The appearance of a two-phase microstructure even for a nominal coating composition Zr0.5Nb0.5N is attributed to the sequential passing of the rotating substrates through the niobium and zirconium target sputtering zones. Substrate movement also leads to a nanolayer structure of the coatings that has been detected by x-ray reflectometry. The coatings deposited on floating substrates exhibit hardness values in the range of 20 GPa to 30 GPa, which can be extended to 40 GPa by application of a substrate bias voltage of -60 V. In scratch testing, the hardest coatings of 5 µm thickness exhibit cohesive failure at Lc = 20 N, but no adhesive failure is observed up to a critical load of 30 N.