Damage tolerance and residual fatigue strength/life of WC-Co cemented carbides

Assessment of damage tolerance of WC-Co cemented carbides, also referred to as hardmetals, under cyclic loading requires not only the introduction of controlled damage, but also appropriated testing protocols on the damaged samples aiming to separate the influence of microstructure on both fatigue sensitivity and damage severity. Attempting to address such a challenge, conical indentation is here combined with flexural testing as well as detailed optical and electron microscopy inspection to evaluate the fatigue behavior of three fine-grained WC-Co cemented carbide grades with varying binder content. Experimental findings show opposite microstructural influence trends on the residual fatigue strength/life, depending on the absence or evidence of indentation-induced cracking features in the pre-existing damage scenario. Hence, as the binder content increases, susceptibility of hardmetals to strength lessening under cyclic loads rises when “just imprints without cracks” are induced, but diminishes when “imprints plus cracks” are introduced by means of conical indentation. Such differences are rationalized by considering that fatigue life of hardmetals is controlled by the subcritical propagation of flaws; and thus, depends upon the compromising effect of their crack growth law – same for both natural and artificial defects –, as well as the initial and final sizes – dependent indirectly and directly on fracture toughness, respectively. As a consequence, this intrinsic mechanical property emerges as the key parameter for tailoring effective damage tolerance in these materials because it defines both fatigue sensitivity as well as the initial size of the indentation-induced artificial flaws. Such statement is sustained by the observation for the toughest cemented carbide studied (and not for the other two more brittle grades) of fatigue strength/life data overlapping and a similar slope in the normalized applied stress – number of cycles to failure curves for all the specimens tested, independent of the damage scenario under consideration. This points out hardmetals with higher cobalt contents as preferable material choices for applications requiring mechanical reliability in terms of damage tolerance, particularly if the latter involves premature cracking when subjected to service-like conditions.