On crack evolution with texturization of bonding layer in thermal barrier coating

Abstract

The interface morphology of the bonding layer has a considerable effect on the damage and failure of sandwich-structured thermal barrier coatings. This work investigated the comprehensive effects of a grooved texture produced using laser ablation on the local surface strain, interfacial stress and strain, and crack behavior of the bonding layer in a thermal barrier coating system. The distribution and evolution of the local surface strain was obtained using the digital image correlation method. The interfacial stress, and the strain between the ceramic and bonding layers, were determined through a simulation of the plane-strain model, and the morphology and propagation of cracks were observed in thermal barrier coatings under an external tensile load. The results indicated that the local surface strain of the thermal barrier coating increased with the texturization of the bonding layer, whereas the fluctuation decreased. There were two inflection points in the local surface strain–time curves, corresponding to the initiation of surface cracks and that of interfacial transverse cracks. The surface cracks were initiated earlier than those without the texturization of the bonding layer. However, the behavior of the interfacial cracks was more complicated. If the roughness of the texture, defined as Rc, was small, the surface cracks propagated vertically to the interface between the ceramic and bonding layers, and turned into transverse cracks, leading to a separation of the ceramic layer. If Rc was greater than 22 μm, the surface cracks went further down to the interface between the bonding layer and substrate, and propagated horizontally, resulting in the separation of both the ceramic and bonding layers. Meanwhile, interfacial cracking and separation were deferred. A large roughness resulted in good cohesion between the ceramic and bonding layers, and a high stiffness for the coating, which improved the damage resistance and extended the life of the coating.