Contact damage induced by means of conical indentation in hardmetals: Microstructural effects on residual strength

Aiming to evaluate the influence of microstructure on residual strength of hardmetals, conical indentation is implemented and validated as a simple and practical methodology to introduce controlled damage. Work is carried out in three fine-grained WC-Co cemented carbides with different binder content. In doing so, a contact damage map is first constructed through combined use of a wide range of indentation loads, detailed inspection by means of optical/electron microscopy and sequential tomography. Threshold indentation load values, defining the transition from deformation into cracking phenomena, are then used as experimental variable for residual strength testing of indented specimens. Dummy indentations are included in single specimens for extracting additional post-mortem information. Microstructural effects on residual strength are determined by comparing the fracture resistance under four-point bending of indented specimens with respect to values determined for pristine samples. It is found that fracture toughness prevails over hardness as key parameter for inducing a gradual strength lessening, instead of abrupt resistance drops. Main reason behind it is the capability of tougher hardmetal grades to uniformly distribute and homogenize the damage introduced during conical indentation. This is supported by the differences observed in the interaction of imprint and cracking features along failure paths as well as in stable growth of fissures linked to dummy indentations under the application of an external load. This finally translates in enhanced resistance to crack extension and higher damage tolerance.