EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

In order to comprehend the fracture resistance of nanocrystalline protective thin films, it is vital to elucidate the multiaxial stress fields throughout irreversible deformation. In this work, a notched double-clamped cantilever with dimensions of 200×23.7×40μm³ was cut by focused ion beam milling from a 21.7µm thick thin film composed of four alternating CrN and Cr layers on high-speed steel. The cantilever was loaded to 460mN in two steps and multiaxial strain distributions were retrieved by in situ cross-sectional X-ray nanodiffraction.
Characterization of the film in as-deposited state revealed the depth variation of fibre texture and residual stress across the layers. In detail, residual stress magnitudes up to -4 and -1GPa were determined for CrN and Cr sublayers, respectively. The stress fields evaluated from the double-clamped cantilever in as-fabricated state revealed preservation of the residual stress. Consequently, an effective negative stress intensity of −5.9±0.4MPa m^½ accompanied by a plastic zone around the notch tip arose in the notched Cr sublayer. The in situ experiment indicated a strong influence of the residual stresses on the cross-sectional stress fields evolution and crack arrest capability at the CrN-Cr interface. In detail, crack growth in the notched Cr layer to the adjacent CrN-Cr interface occurred at a critical stress intensity of 2.8±0.5MPa m^½. After crack growth, the cracks influence on the stress fields vanished, indicating crack tip blunting at the CrN-Cr interface.
The results were complemented by two-dimensional finite-element modelling to gain further insight into the elastic-plastic deformation processes. The quantitative experimental and modelling results illustrate the stepwise nature of fracture progress across the alternating brittle and ductile layers and their interfaces.