Influence of localized cyclic substrate plastification on residual stress, load stress and cracking near the interface between hard coating and WC-Co hard metal substrate

Abstract WC-Co hard metal covered with thin ceramic coatings represents a composite structure with a supreme performance in many metalworking tool applications. The failure behaviour of this substrate-coating composite is significantly influenced by the plastic deformation of the hard metal, especially if the plastification is localized and associated with changes in the residual stress state. Today, there is no complete understanding of the effect of localized cyclic substrate plastification on the residual stress, and its interaction with the superimposed stress induced by external load. The current work sheds light on this interplay via an approach combining experimental observations and finite element simulation. To induce different levels of localized plastification, uncoated and AlCrN coated notched specimens were subjected to different levels of uniaxial cyclic compressive loads at a stress ratio R = σmin/σmax of minus infinity. The associated shift of residual stress towards tension at the notch root was determined in the hard metal for the same load situations experimentally via X-ray diffraction and in simulations. The applied finite element simulations included an experimentally parameterized material model to describe the cyclic deformation behaviour of the hard metal. A type of fatigue crack oriented parallel to the substrate-coating interface was observed close to a surface asperity via scanning electron microscopy in a cross section prepared by focused ion beam milling. The observed type of crack can be interpreted as an early stage of coating delamination. Simulation results indicate that the maximum applied compressive load induces tensile stresses acting perpendicular to the substrate-coating interface, when considering the surface roughness features present in the specimen. The magnitude of the observed tensile stresses, that drive the growth of the observed crack type, decreases rapidly due to the cyclic plastification of the hard metal substrate.