Incorporation of oxygen at column boundaries in (Cr,Al)ON hard coatings

Abstract

Chromium-based hard coatings deposited using physical vapor deposition are commonly applied protective coatings in several technical applications such as cutting or plastics processing. Particularly enhanced oxidation resistance can be achieved using the oxy-nitride coating system (Cr,Al)ON. Although several studies on (Cr,Al)ON have been conducted, details on the incorporation of oxygen into the coating system are still subject of research. This will be discussed in the present paper through three mechanisms. Considering the (Cr,Al)N with a cubic structure, oxygen can be interstitially stored in the lattice. Here, the nitrogen anions are replaced by the oxygen anions leading to the formation of metal vacancies. In addition to the cubic (Cr,Al)ON, other phases can be formed in the coating, e.g. cubic phases of the type (Cr,Al)2+θO3 with θ ≤ 1 or corundum phase α-(Cr,Al)2O3. In both latter phases, nitrogen can be additionally dissolved in the phases. Moreover, oxygen might segregate preferably at column boundaries. This phenomenon has hardly been explored. Therefore, the column boundaries of a (Cr,Al)ON coating are investigated in this work using transmission electron microscopy. The coating deposition was conducted in an industrial scale coating unit using a hybrid technology consisting of direct current and high power pulse magnetron sputtering. The results suggest that higher oxygen concentrations can be detected at column boundaries. Oxygen-rich column boundaries would contribute to explaining most recently studied phenomena in thin hard chromium-based coatings. Among such phenomena are the column separation manifested by a reduced cohesion between microstructural columns or the grain boundary sliding.