| Thermal barrier coatings(TBCs) have been widely used in aircraft and gas turbines due to their excellent thermal insulation, which can improve the thermal efficiency of engines. When TBCs are applied to a gas turbine, the turbine blade with TBCs will inevitably experience impurity and/or foreign particles during service, leading to the delamination and spallation of the coating. The typical features caused by FOD include a permanent impression, a zone of densification, shear bands penetrating from the impact site to the interface, and delamination cracks extending away from the impact zone in the TBC adjacent to the interface. But till now, most published works about particle erosion treat such problem based on single columnar grains. The role of complex morphologies of the TC coatings is seldom analyzed. However, the real geometry of the thermal barrier coatings exerts an influence on the formation of kink bands and the stress state. Thus, this work will try to address such shortcoming. The main consents are as follows,Firstly, a real morphology finite element model was developed with the help of image process technology and CAD software. Since there is isolated porosity in the columnar grain, the columns can be represented by a model for a minimally porous material, such as the GTN model. Because there exit a material between the columns, as well as a finite in-plane tensile modulus, this zone is modeled as low-density foam The simulations have been performed in conjunction with a non-dimensional analysis to facilitate the presentation of penetration depth and stress state.Secondly, the differences of particle penetration depth and stress state of the simplified model and real model have been analyzed. Results indicates that due to the fact that TBC columnar materials is treated as elastic-perfectly plastic and with the same material parameters, the time evolution of velocity and instantaneous penetration depth between the simple model and real model are almost the same. However, the residual tensile stress along the column direction in the real geometric model was found to be much larger than that in the simplified one. The critical dimensionless energy to produce a crack was about 0.168 and 0.209 for the real micro-structure and the simplified model, respectively.Lastly, as the kink band is one of the typical failure modes of EB-PVD erosion, thus a simplified model was developed to analyze the influence of erosion parameters on the formation of kink band. An empirical correlation between the depth of penetration and the erosion particle parameter was obtained by means of simulation. Simulation results indicate that kink bands are easily formed at high particle velocity, big particle radius and a large slenderness ratio, corresponding to a small threshold of particle energy. When the coatings are impacted at normal incidence, the bands extent symmetrically and the band width is the same as the contact width of particle. However, the bands extend downward and outside at the same time when the particle impact the coatings at oblique incidence. Moreover, the simulations have been carried out to analyze the differences of kink bands formation of the real morphology model and simplified model. Results indicate that owing to non-uniform size of real morphology models, kink bands are more irregular and unsymmetrical, which is consistent with the experiments. Moreover, the threshold of kinetic energy for the real morphology model is smaller than that of simplified model. These results indicate that the real morphology would be more susceptible to erosion failure compared to the results of previous theoretical or numerical estimations. |
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