Mechanical properties and interface toughness of FeCo thin films on Ti-6Al-4V

Magnetostrictive materials such as FeCo are attractive for applications as embedded thin-film sensors for detecting strains or cracks in structural components. For these applications, FeCo thin films must possess good adhesion and adequate mechanical strength such that the embedded sensors would have adequate durability, and not degrade the structural integrity of the underlying components. In this paper, we investigate the effects of thin-film thickness and the surface condition of the substrate on the mechanical properties and interface toughness of FeCo thin films deposited to a Ti-6Al-4V substrate. The Young's modulus, hardness, and yield strength of FeCo thin films were measured by nanoindentation. Furthermore, the interface toughness of FeCo/Ti-6Al-4V was determined by cross-section nanoindentation on the edge and by brale indentation on the face of the thin films. The results indicate that the interface toughness exhibit spatial variations as well as dependence on layer thickness and the surface condition of the substrate. The experimental results are combined with a theoretical analysis to predict the debonding strength of the thin films and to assess the propensity of thin-film cracking versus interface debonding. The theoretical predictions are evaluated against experimental data of the debonding strength of FeCo thin films and notch fatigue strength of Ti-6Al-4V.