Contact fatigue behavior of PVD-coated hardmetals

There is an increasing usage of PVD-coated hardmetals in applications where contact loads of cyclic nature between curved surfaces are common, e.g. cold forming tools and machine components. Thus, knowledge of both the mechanical response and the damage mechanisms involved during contact fatigue of these systems is required if they want to be used effectively. In this investigation this is approached by implementing spherical indentation testing techniques. Experimental work includes the evaluation of monotonic indentation stress–strain curves, cyclic indentation stress–number of cycles diagrams and subsurface damage evolution. Emphasis is placed on determining the indentation stress affiliated to the emergence of circular cracks at the coating, event here proposed and proved to be the critical failure stage for the columnar TiN film and the low cobalt/fine grained WC–Co hardmetal substrate under study. It is found that the coated system investigated is susceptible to contact fatigue. However, the corresponding fatigue sensitivity, assessed on the basis of an indentation stress analysis of the relative contact response under monotonic and cyclic conditions, is rather low. Such a result is rationalized by considering the ceramic nature as well as the high residual compressive stress state of the hard coating, the system component where the damage defined as critical takes place.