Durability of hard protective coatings: Insights on fracture and fatigue mechanisms

Abstract

Tailoring the intrinsic fracture characteristics of hard protective coatings towards fatigue properties of state-of-the-art bulk materials is paramount for the application of innovative coating materials extending the fatigue-life of high-performance components. Thus, an in-depth knowledge on the failure pathways of ceramic-based thin films – typically lacking in intrinsic ductility – but also coated components under cyclic loading is imperative to extend their lifetime. Here, we present a novel approach using quasi-static and cyclic bending of pre-notched, unstrained micro-cantilever beams in conjunction with in-situ synchrotron X-ray diffraction to reveal the intrinsic fracture toughness (KIC) and critical failure aspects of thin films under various loading conditions. Up to the high-cycle fatigue regime (i.e., N = 107 cycles), the failure of monolithic coatings is shown to be dominated by the inherent fracture resistance, irrespective of the bonding character. The observed fracture behaviour is further correlated with large-scale dynamic-mechanical analysis of coated Ti6Al4V platelets to include residual stress states and the coating-substrate interface. The results are expected to provide key-insights into the underlying mechanisms promoting crack growth in PVD coated specimens.