Biased magnetron sputter deposition of corrosion resistant Al–Zn alloy thin films

The reason for the high corrosion resistance of Al–Zn films is unclear. To relate this behavior to the structure and fabrication we have studied films deposited by dc biased magnetron sputtering onto Fe–Cr substrates. Changes of composition, structure, and electrochemical polarization behavior of resultant films were measured as a function of experimental parameters, including on‐axis and off‐axis substrate positions. The films, 1.6–4 μm thick, were deposited with a chamber Ar pressure of 10 mTorr (1.33 Pa), substrate temperature of 22–90 °C, and substrate bias of −60 to −380 V. Transmission electron photomicrographs of thin alloy films exhibited polycrystalline structure comprising one or two phases depending on film composition. Lower bias (−60 V) tends to produce two phases. Reflection high‐energy electron diffraction photomicrographs also presented polycrystal ring patterns. Energy‐dispersive x‐ray spectroscopy analysis of as‐deposited films revealed that Al vapor flux was more likely than Zn to go axially. The enrichment of Al in the on‐axis films increased dramatically with increasing substrate bias. The bias effect was small in off‐axis films. The enrichment of Al in films also varied with target composition. Film surface morphology and cross sections were examined with scanning electron microscopy: zinc‐rich targets and lower biases led to porous structures. Higher biases (−380 V) and Al‐rich targets led to dense columnar structures. Transmission electron microscopy images showed a reduction of average grain size with increasing substrate bias. The deposited thin films presented active–passive transitions in potentiodynamic tests. Step passivity was observed on porous films but not on dense films. Alloy films had higher corrosion resistance than corresponding bulk material as a result of reduced grain size and better chemical uniformity, which enhances the films’ passive behavior.