Near-interface-cracking in coated WC-Co hard metals triggered by local plastic deformation

WC-Co hard metals coated with micrometre-thin ceramic layers represent a well-established solution to cope with elevated load levels in e.g. industrial metalworking. Improving the fracture resistance of hard metal-coating composites requires a thorough understanding of the acting failure mechanisms. The degradation of these composite structures is significantly influenced by residual stresses that are superimposed with external stresses induced by in-service loading. Both residual and external load stresses may change in service, e.g. due to local plastic deformation, surface roughening or blunting of cutting edges. The current work quantitatively describes the buildup of tensile residual stresses in notched specimens made of WC-Co hard metal under uniaxial purely compressive fatigue loading conditions. The applied finite element simulation used a material model parameterised on the basis of experimentally attained stress-strain curves. The predicted shift in the residual stress state towards tension due to local plastification of the hard metal was compared for un-/coated specimens. The comparison showed good agreement for uncoated specimens but an overestimation in the case of coated specimens. This can be rationalised by the constraint to plastification by the high yield-strength coating and the emergence of open micro-cracks that were observed close and parallel to the hard metal-coating interface.