Influence of Ag additions on the structure, mechanical properties and oxidation behaviour of Cr-O coatings deposited by HiPIMS

Abstract

Here, we investigate the influence of Ag additions on the properties of Cr-O coatings deposited by magnetron sputtering. The deposition rate of Cr-O coating is 1.6 times higher than a single Cr coating, taken as reference, deposited with identical conditions. Incorporation of Ag in the Cr-O coating increases the deposition rate. Transition from a dominant columnar to a very compact morphology, as well as from crystalline to amorphous structure, was observed with the O addition. However, no morphological changes were observed with Ag additions. Annealing at temperature ≥ 500 °C promotes the crystallization of Cr-O and Cr-O-Ag coatings, i.e. the formation of a crystalline composite structure, formed by: Cr, Cr2O3 and Ag phases, the later in the case of Cr-O-Ag coatings. The increase of coatings crystallinity improved the mechanical properties. Thermogravimetric analysis showed that O and Ag additions to Cr have no influence on the onset point of oxidation. The oxidation kinetics of the Cr-based coatings is controlled by the outward diffusion of Cr ions through a top Cr2O3 layer. A dual oxide layer was formed for Cr-O and Cr-O-Ag coatings, when exposed to a continuous increase of the temperature up to 1200 °C. A compact layer of Cr2O3 grows on top of the coating, due to the Cr ions outwards diffusion, leaving behind a porous layer formed by Cr2O3 grains resulting from the recrystallization process, together with Ag agglomerates inside the empty porous zones.