Corrosion protection of magnetron sputtered TiN coatings deposited on high strength aluminium alloys

TiNx coatings were deposited on aluminium AA7075-T6 substrates by reactive DC magnetron sputtering. The main objective was to improve the impermeability to corrosive media by controlling the morphology of the coatings. It was found that by increasing the bias voltage during deposition to −150 V the normally arising columnar structure is reduced due to enhanced defect and nucleation site formation. In consequence these coatings exhibit a reduced porosity and therefore fewer possible paths for corrosive media like aqueous sodium chloride solution to reach the interface substrate/coating. A further effective improvement of the corrosion behaviour compared to stoichiometric TiN coatings was achieved by decreasing the nitrogen content in the TiNx layers below 20 at.% N. This has been confirmed by potentiodynamic measurements as well as salt spray tests. It is proposed that this further improvement is due to the formation of a thicker and denser surface oxide layer as soon as the coating is exposed to air.