Cracking resistance of nanostructured freestanding tungsten films

Abstract

The fracture toughness K_c of freestanding tungsten films is explored using a MEMS-based crack-on-chip method and multiscale finite element modelling, in the context of miniaturised testing of structural materials for nuclear fusion applications. The primary ambition is to determine to what extent testing thin nanostructured tungsten films can provide relevant data with respect to bulk tungsten fracture behavior, particularly in view of irradiation testing. The second objective is to enhance fundamental knowledge on the cracking behavior of thin metallic films with a quasi-brittle response. Tungsten films with 370 nm thickness are deposited by magnetron sputtering under different pressures and characterized using grazing incidence X-ray diffraction, surface curvature measurements, scanning electron microscopy and nano-indentation. Microstructure evolution, residual stresses, and tensile properties are analyzed to confirm the BCC α-phase. The fracture toughness of the tungsten films is determined on-chip using a crack arrest approach and finite element modelling to extract K_c. The analysis conducted on 90 successful test structures provides an average fracture toughness value of 3.2 ± 0.36 MPa √m. This value is typically, 50 % lower than for bulk tungsten, despite the submicron thickness, while similar intergranular fracture mechanism is observed. The link with crack tip plasticity is further unravelled by XFEM-based simulations relying on a cohesive zone model. Care is taken to properly resolve the mechanical behavior of the nanometer scale fracture process zone. The calibrated peak strength is equal 7.8 GPa, which is less than two times the large yield stress of the nanocrystalline film. With such a ratio, the impact of plasticity outside the fracture process zone is limited, corresponding to negligible R curve effect and extra dissipation upon crack growth in contrast with bulk specimens for which a ratio above four is expected.