Mechanism of high corrosion resistance for cemented carbide materials coated with CrN via interface analysis

Abstract

Cemented carbide materials contain expensive rare metals and are used in diverse processes as substrates for moulds, cutting tools, and metal products due to their hardness and strength. Applying a hard coating to these substrates extends the service life and enhances functionality such as corrosion and wear resistance. Hard coatings are comprised of different elements (e.g., Ti, Cr, or multiple elements) and take different forms (mono and multilayers). Although research on hard coatings is becoming more sophisticated, the interaction between the substrate and hard coating is rarely examined. Cr-based hard coatings are often applied on cemented carbide materials because it generates a high adhesion and corrosion resistance. However, the origin of these features is unclear. Here, we focus on the interaction that occurs at the interface between substrates (cemented carbide and high-speed steel) and hard coatings (CrN and TiN). Specifically, the elemental composition and crystal structure of the interface are analysed to elucidate the factors that lead to high adhesion and corrosion resistance. Cr ions more readily penetrate the surface of cemented carbide and react with W than Ti ions. Coating CrN on cemented carbide spontaneously forms an approximately 10-nm-thick CrW bonding layer at the interface. Also, the formation of the bonding layer does not occur when the substrate is high-speed steel. Additionally, electrochemical characterisation shows that the CrW bonding layer protects cemented carbide materials from corrosion under both acidic and basic conditions. Thus, using a thin film as a bonding layer contributes not only to high adhesion between the substrate and hard coating but also corrosion resistance of cemented carbide materials coated with CrN. These results should help effectively select the appropriate elements, substrates, and hard coatings according to the intended purpose.