In-situ stress gradient evolution and texture-dependent fracture of brittle ceramic thin films under external load

Abstract

Brittle ceramic thin films failure problems induced by film stress during service have been recently getting more attention. Thus, it is crucial to know the stress to understand correlated film fracture. In this work, in-situ stress gradient evolution during cracking of TiN films under external load was investigated by grazing-incidence X-ray diffraction using the optimized cos2α sin2ψ method. Counterintuitively, cross-sectional scanning electron microscopy clearly demonstrated that cracks initiated in the middle of the film thickness and propagated to the surface and/or the interface with increased load. Eventually, film through-thickness crack and even delamination occurred. Preferred orientation evolution and texture transition along the film cross-section were characterized by selected area electron diffraction in transmission electron microscopy and the Kikuchi diffraction analysis. Thermodynamics modeling revealed that system energy was the driving force for dominant texture transition. The texture-dependent film fracture toughness accounts for the film cracking behavior. A model of fracture toughness is proposed to consider and evaluate the effect of the preferred orientation. The fracture toughness in the middle of the film, where the texture transition zone is located, is smaller than at the surface or the interface. Consequently, the texture transition zone of the film becomes the crack initiation point, which finally leads to the whole film-substrate system failure. Consequently, the texture transition zone of the film becomes the crack initiation point, which finally leads to the whole film-substrate system failure.