Effect of Deposition Conditions on the Structure and Properties of CrAlN Films Prepared by Pulsed DC Reactive Sputtering in FTS Mode at High Al Content

Abstract

CrAlN films were prepared by a pulsed DC magnetron sputtering in FTS mode with Cr-Al alloy targets (Cr/Al = 30 at%/70 at%) in a mixed atmosphere of Ar and N2. Effects of different deposition conditions (frequency, duty cycle, nitrogen flow rate and pulsed DC power) on the films structure and phase formation have been investigated. XRD analyses were carried out to determine the phases of the films. The surface morphology was observed using FE-SEM. Transmission electron microscopy studies were carried out for selected films. In order to investigate the relationship between the mechanical properties and microstructure of the films, the hardness was measured by a nanoindentation system. All films exhibited mixture of fcc-CrN and hcp-AlN structures with changing the sputtering conditions. With increasing pulse frequency, the relative percent of fcc-CrN phase decreased from ∼100% to ∼20%. In the current study it was found that there is an optimal duty cycle (around 82%) where the fcc-CrN relative% reaches to its highest value (about 98%). On the other hand, grain size of the films kept in the range of 20–25 nm below 82% of duty cycle, then increased up to ∼50 nm with increasing duty cycle. Changing the nitrogen flow rate affected the fcc-CrN relative % and films morphology. Under optimal nitrogen flow rate, fcc-CrN relative % reached to about 100% in addition to the formation of micro columnar morphology, which resulted in the highest plastic hardness of the films. It seems that the mechanical properties of the CrAlN films are directly influenced not only by fcc/hcp phase ratio, but also by morphology. These results suggest that the dominant fcc-CrN phase and high hardness of CrAlN films containing high Al content (Cr:Al=3:7) can be obtained under the restricted condition by the pulsed DC magnetron sputtering in FTS mode.