MULTIPLE CRACKS LOCALIZATION FOR THIN COATING FILM LAYERS IN PURE BENDING

Abstract

Recently, accurate small scale four-point bending flexural tests, for superalloy specimens with multiple ceramic type external coating film layers, have shown that under high loading, the failure mechanism is characterized by the development of complex multiple surface cracks, followed by interior interlayers debonding mechanisms. The final stage is then the complete debonding of the fragmented thermal coating, which leave the interior superalloy completely uncoated. The mechanical problem has an high technological relevance, since failure, or just severe damage, of the ceramic coating, can jeopardize the functionality of structural elements working at high temperature and under severe loadings, such as blades of gas turbines of aircraft engines or high performance electricity generators. This paper presents a nonlinear finite element analysis of the thermomechanical problem modelled as a 2D structure subjected to pure bending. The structural element is composed by an elastic substrate with several thin layers of quasibrittle coating materials, disposed as films in the tensile side of the element. The layers are modelled by an elastic damage constitutive relation and, in order to describe the interlayer decohesion effects, along the layer boundaries and between the first coating layer and the elastic substrate, zero-thickness cohesive-frictional interface elements are disposed. The presented nonlinear finite element analysis results show the evolution of the fracture patterns and the different possibilities of cracks distribution. It is also analyzed the effects of the thickness of the coating films and the qualitative and quantitative response as function of the fracture energy of the interface compared to the fracture energy in mode I of the coating film. Finally a critical review of the nonlinear response and a general comment on the most efficient strategy for improving the mechanical resistance to failure will be discussed.