Microstructure, Mechanical Properties, Tribological and Scratch Adhesion Behaviour of Graded Ternary Chromium Nitride-Based Coatings

Graded Cr-CrN-Cr(1 - x)AlxN and Cr-CrN-Cr(1 - x)TixN ternary coatings were deposited on M42 high-speed tool steels by closed-field unbalanced magnetron sputtering (CFUBMS). These coatings consisted of a coarse columnar grained Cr adhesive layer followed by a fine columnar grained CrN buffer layer and a compositionally graded columnar grained CrAlN/CrTiN outer coating layer. The structure, composition, mechanical properties, scratch adhesion and tribological behaviour of these coatings were investigated to study the durability of the graded coatings under severe dynamic loading conditions and methods to improve coating architecture in order to enhance damage tolerance. XPS and XRD investigations both showed the formation of a solid solution nitride as the outer coating layer, with a fcc B1 (NaCl) crystal structure. This was further confirmed from diffraction analysis from TEM investigations. Moreover, the concentration of Al and Ti in their respective coating systems increased with increasing target current. The mechanical properties of the coatings, such as hardness, modulus, elastic strain to failure (H/E ratio) and plastic deformation resistance (H3/E2 ratio) determined from nanoindentation tests, increased with increasing Al and Ti concentrations. This improvement in mechanical properties also led to increases in axial deformation resistance under high load indentation conditions. However, the scratch adhesion and tribological behaviour of the coatings revealed a complex trend that contrasts with the conventional understanding that coatings with increasing hardness and modulus, higher H/E and H3/E2 ratios exhibit better crack and wear resistance. This decrease in scratch toughness and wear resistance was ascribed to increasing coating-substrate modulus mismatch, as a result of increase in coating modulus. By exploring the influence of target current on the structure and properties of graded coatings, it was found that increasing hardness and modulus does not necessarily lead to increases in scratch toughness and wear resistance. The damage tolerance of the coatings can be improved by reducing the modulus of the coatings, which leads to a wider distribution of load. As a result, the modulus mismatch between coating and substrate decreases, resulting in enhanced resistance to plastic deformation, thus leading to greater resistance of the coating to cracking and enhanced durability in severe dynamic loading conditions