Performance Prediction And Failure Mechanism Analysis Of Thermal Barrier Coatings Based On A 3D Micro-Structural Model

As the potential surface protection materials, thermal barrier coatings(TBCs) have a bright future in the application fields of aviation and spaceflight. However, due to the complicated structures and severe environment of TBCs, it is very difficult to accurately and reliably predict their performance based on an actual three-dimensional(3D) micro-structural model. In order to solve this technical problem and break the limitation of present 2D models or simplified models, in this thesis, a 3D finite element(FE) model reflecting the actual microstructures of plasma-sprayed TBCs is built for the first time using microcomputer tomography(micro-CT) and FE techniques. On this basis, the failure mechanism of TBCs during uniaxial tension and thermal cycling is analyzed deeply combining with a real-time 3D visualization of crack initiation, crack propagation and eventual failure.First, Micro-CT technique is applied to obtain the inner microstructures of TBCs. The microscopic slices are then processed through the software Simpleware so as to reconstruct a 3D image model reflecting the microstructures of actual coatings. On this basis, a 3D FE model is successfully established with the aid of automatic mesh generation techniques. The model can reflect the real interface roughness and pore distribution of TBCs and the porosity of the ceramic coating is calculated to be 9.48%, which is consistent with the value obtained by means of scanning electron microscope(SEM) image analysis and statistic method. The proposed methodology provides a key technical support for the performance calculations and predictions based on the actual 3D microstructures of TBCs.On the basis of this 3D FE model, considering sufficiently the real interface roughness and pore distribution inside the coatings, the tensile bond strength and fracture path of TBCs are simulated followed by a quantitative comparison with relevant experimental data. The simulation results show that the fracture path is near or at the top coat(TC)/bond coat(BC) interface and the distance between the lowest and the highest point of the fracture surface is 24.5μm, which is in good agreement with the results measured with surface profilometer, 20~35μm. According to the corresponding external load at the peak value of internal energy, the tensile bond strength of TBCs by FE method is 44.4MPa, which agrees well with the experimental data, 44.78 MPa. In addition, the crack initiation and propagation path in 3D space is clearly observed in real time, which cannot be recorded through the tensile experiment of TBCs. It is found that at time t=40s from initial application of the load, local stress concentration induces two types of crack sources located either near the TC/BC interface or along the pore boundaries. At t=44.4s, only the crack sources near the interface amalgamate and begin to form a primary crack. With the increase of load, the primary crack propagates rapidly along the interface direction due to the vertical tensile stress at the crack tip, thus eventually inducing an undulating fracture morphology near the TC/BC interface.A fitting kinetics curve of thermally grown oxide(TGO) growth is obtained according to the experimental data of thermal cycling. Subsequently, a 3D TGO growth model is generated combining with the real TC/BC interface roughness. On this basis, considering the TGO growth, phase transition, TC sintering, creep effects and so on, a multi-factor coupling method is proposed to achieve a 3D visualization of crack initiation, crack propagation and crack connection inside TBCs during thermal cycling. The results show that ‘stress inversion’ takes place at the TC/TGO interface as the TBCs crack during thermal cycling. Before cracking, tensile stress exists in the peaks and compressive stress exists in the valleys within the TC. When the crack sources initiate continuously near the peaks and relieve the high stress at the 51 th cycle, new stress concentration will develop at the crack tip. With the propagation of cracks, the tensile stress area transfers gradually from peaks to valleys and, eventually, compressive stress exists in the peaks while tensile stress exists in the valleys. In this case, a crack starts from the peak and propagates to the adjacent valley joining the corresponding crack, thus leading to the crack connection. Further research indicates that, in the earlier stage of thermal cycling, the peak stress of TC increases quickly, thus inducing numerous cracks. The volume growth rate of cracks is the highest, about 10~20μm3/min. In the medium stage of thermal cycling, the increase of the peak stress slows down, and accordingly the growth rate of cracks slows from 10.9μm3/min to 3.07μm3/min. At the later stage of thermal cycling, the peak stress of TC decreases slightly. The growth rate of cracks is below 2μm3/min and basically keeps constant.