Corrosion Properties of Physically Deposited Thin Coatings (PVD coatings)

Abstract

Sputtering, the first invented physical vapour deposition (PVD) coating technology has been known for more than 150 years. Although the PVD and related technologies have been commercialized for over 70 years, their expansion in the last 20 years has become enormous - from decorative and cutting tool to special optical applications and microelectronics. This is due to their excellent properties such as chemical inertness, hardness, tribological, tribochemical and optical properties. In many applications the PVD coatings are exposed to corrosive environments. Moreover, PVD coatings of environment-friendly nature are often considered to be potential successors of cadmium-based anticorrosion coatings. This fact promotes numerous studies of corrosion properties of thin coatings. Hard coatings (including nitrides, carbides and oxides) are usually more corrosion-resistant than the base material. Some of them (e.g. titanium nitride,− the most common PVD coating) have columnar microstructure which brings about the possibility of high micropore density. In local defects and/or pores, local corrosion can be promoted by galvanic effects. As a result, this type of hard coatings is useless for anticorrosion purposes. As perspective anticorrosion coatings, PVD-deposited aluminium-based coatings are frequent subjects of studies. Similarly to cadmium coatings, they usually provide sacrificial properties, which can be enhanced by alloying with magnesium. Tribological properties and corrosion resistance can be enhanced by alloying with molybdenum. Given the excellent flexibility, adaptability of PVD technology and its ability to deposit multilayers or graded coatings, new coatings with functional properties comparable with traditional anticorrosion coating systems could be developed.