Contact damage under monotonic and cyclic loads of coated tool materials

Cemented carbides have a wide application in cutting and forming tools due to their excellent combination of hardness, toughness and wear resistance. In industrial field, hard coatings are always applied on these tool materials in order to improve tribological properties and extend their lifetime. Complicated service conditions involved wear and cyclic contact are normally implied on these materials. Accordingly, surface integrity and mechanical behavior under contact loads for coated hardmetals emerge as critical requirements, from both design and material selection perspectives, if they want to be used effectively in forming tools or structural components. In this study, the contact mechanical behavior of tool materials (a tool steel and a cemented carbide) coated with distinct ceramic films: AlCrN and WC/C, either as monolayer or bilayer, is studied by means of indentation techniques. Experimental procedure is based on hertzian indentation to analyze the contact response. Both monotonic and cyclic spherical indentation loading conditions are performed in order to observe emergence and evolution of distinct damage modes at the coating surface: circumferential cracking or cohesive failures. These tests are accompanied by an extensive microstructural and mechanical characterization of the coating such as hardness, adhesion resistance or wear resistance. The results indicate that all the coating/substrate systems evaluated are susceptible to fatigue degradation associated with repetitive contact loading. This is clearly indicated by the presence of more consequent damage under cyclic loading. Compared with AlCrN coated on tool steel system, AlCrN-WC/Co system revealed a higher critical monotonic load and showed less damage under fatigue test. Moreover a higher adhesive strength is observed for the system in which WC/Co as the substrate. It was ascribed to the harder and stiffer hardmetal as a more supportive substrate for the upward coating. Concerning the coating effects, the bilayers containing both of DLC and AlCrN have a better damage resistance than the monolayer AlCrN under monotonic indentation. However this advantage was not conserved during the fatigue test. Nevertheless, the bilayer system has a satifactory resistant to adhesive failure, which was considered to be an advantage in sliding or rolling contact pressure application.Incomin