Finite Element Analysis Of Failure Process Of Thermal Barrier Coatings By Using Cohesive Element

Thermal barrier coatings (TBCs) have been widely applied in nuclear technologyand aircraft engine due to their excellent wear resistance, corrosion resistance, andthermal insulation as engine components. However, most of TBC systems, frequentlyused in turbine blades and vans, have complex and irregular geometrical structures, itis inconvenient and difficult to use experimental methods and theoretical models toestimate their reliability and durability under severe environments. Thus, we have toresort to numerical analysis such as the finite element method (FEM). In this paper,finite element analysis and experimental verification were used to determinenumerical model parameter in predicting the damage, cracking and spallation of TBCssystem. The obtained material properties in this work would play a crucial role inpredicting the reliability and durability of TBCs with irregular geometry in future. Themain research contents are summarized as follows.Firstly, with the help of a digital image correlation technique, fracture strengthand fracture toughness of freestanding8wt%Y2O3-ZrO2(8YSZ) coatings weredirectly measured by single edge notched bending (SENB) tests. An extended finiteelement model (XFEM) was established to simulate the fracture process of notched8YSZ samples. Its energy release rate was estimated by the known fracture strengthand Young’s modulus. Within the linear elastic brittle fracture, the calculated energyrelease rate was transformed into the corresponding fracture toughness, which is ingood agreement with the experimental results by SENB. The obtained materialproperties of XFEM in this work would play a crucial role in predicting the reliabilityand durability of TBCs with irregular geometry in future.Secondly, a two-dimensional finite element model with cohesive zone elementwas developed to predict cracking in TBC system that is subjected to residual stressesand external tensile loads. In the first, the residual stress distributions of TBC systemwere analyzed by FEM during the deposition process. Effects of depositiontemperature and ceramic coating thickness on stress distribution were discussed,which can provide an important basis of stress analysis for evaluating the mechanicalproperties of TBC system during tensile tests in analytical solution. And then theevolution of normal stress in the coating and shear stress at the ceramic coating andbond coat interface were obtained and discussed with the increasing of external tensile loads in substrate. As tensile loads increase, the compressive stress in coatinggradually transformed into the tensile state and vertical cracks firstly appeared in thecoating. When strain in the coating approaches to about0.27%, the coating begins tofracture，which is consistent well with the experimental results.Thirdly, with the similar method, the parameters of cohesive element modelabout the interface fracture of TBC/bond coat were evaluated by simulating arepresentative segmented coating unit under tensions. The whole fracture processincluding vertical cracks, interface delamination and spallation of the coating werepredicted with these determined material properties. The simulation results show thatwhen the substrate strain is0.43%, a crack at the TBC/bond coat interface occurs andpropagates. As the external tensile loads increase more large, more and more cracksquickly nucleate and extend along the interface direction. Finally, different cracksconnect with each other at the interface region and a large delamination forms. Theresults are in agreement with with the corresponding experimental results by digitalimage correlation technique.