Insights on fracture and fatigue mechanisms of hard protective coatings

Abstract

Tailoring the intrinsic fracture characteristics of hard protective coatings towards the 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 – generally associated with lack in intrinsic ductility – but also coated components under long-term mechanical and/or thermal loading is essential to extend their lifetime. In consequence, understanding and implementing coating concepts that allow for a controlled, and hence predictable crack propagation throughout their operating spectrum are of major interest for various industrial applications. Despite recent advances [1], literature reports on the fatigue resistance of especially hard ceramic coatings, but also coated components in general, are relatively rare with a strong approach via macroscopic test facilities. Within this study we present a methodical approach to understand the failure behaviour on different length scales utilizing model systems (i.e., Cr and Cr-based compounds) to consider the aspect of different bonding strengths and crystal structures, respectively. Using quasi-static and cyclic bending of pre-notched, unstrained micro-cantilever beams in conjunction with in-situ synchrotron X-ray diffraction the intrinsic fracture toughness (KIC) as well as the critical failure aspects of thin films under various loading conditions are presented. Up to the high-cycle fatigue regime (i.e., N = 107 cycles), the failure of monolithic sputter deposited PVD coatings is shown to be dominated by the inherent fracture resistance, irrespective of the bonding character. The recorded fatigue behaviour is further correlated with large-scale dynamic-mechanical analysis of coated Ti6Al4V platelets to step up in length scale and thus including residual stresses and changes in the elastic constants on the coating-substrate interface. The results are expected to provide key-insights into the underlying mechanisms promoting crack growth in PVD coated components.