| A set of basic rules that encapsulate the main physical features of the splat formation, coating build-up and pores formation were proposed based on the actual characteristics of plasma spraying process. A stochastic coating model was developed based on Monte Carlo method to predict the microstructure, porosity and surface roughness. The influence of particle impact velocity and diameter, spray distance and torch scanning speed on the porosity and surface roughness was investigated. The simulated microstructures were taken as the "object" of object-oriented finite element code (OOF) to estimate the effective thermal conductivity and Young’s modulus of zirconia coatings. The stress distribution in plasma sprayed coatings was analyzed by using ANSYS Workbench.The deposition of particles was simulated by applying different impact velocities from 120-200 m/s, while keeping the torch parameters constant. The simulation results show the porosity and surface roughness decreased with the increase of impact velocity and the influence of particle impact velocity weakened gradually when the velocity exceeds 160m/s. When increasing the particle diameter from 35μm to 40μm, the porosity and surface roughness were evidently reduced in low-speed range, while nearly the same with impact velocity over 160m/s. For a constant powder feed rate, increasing the spray distance would decrease the coating porosity and roughness. Increasing the torch scanning speed was expected to produce little effect on porosity and roughness.A transient heat transfer analysis was carried out within the ZrO2 coating using object-oriented finite element code. The results show heat flux above and below the pores was significantly lower than beside the pores. The narrow passages between neighboring pores exhibit concentrations of the heat flux. The predicted longitudinal thermal conductivity and Young’s modulus of coatings with porosity of 5.3%、7.5% and 12.41% were 2.08W·m-1·K-1,1.87W·m-1·K-1,1.54W·m-1·K-1 and 155.8GPa, 127.5GPa,100.7GPa, respectively. The anisotropic effects can be observed in the thermal conductivity and Young’s modulus, which means that the longitudinal thermal conductivity and Young’s modulus were lower than the transverse. The differences between the transverse and longitudinal thermal conductivity and Young’s modulus were 10.3%,11.8%,17.2% and 17.2%,24.0%,39.4%, respectively. The difference appears more obvious with the increase of porosity, and the difference in Young’s modulus was more significant than in thermal conductivity.The stress distribution within the ZrO2 coatings under thermal loading was investigated by using the thermoelasticity finite element method. The influence of interface roughness, thermally grown oxide (TGO) and its thickness on the stress distribution in TBCs was analyzed. The results show that tensile stresses are present at the peaks and compressive stresses are present at the valleys in the topcoat without TGO. When growing TGO layer, tensile stresses appear at the valleys and compressive stresses appear at the peaks. The stress inversion took place near the peak and valley in the topcoat after reaching a certain TGO thickness. And this stress inversion is delayed with the increase of interface roughness. For coatings with roughness of 10.5μm and 14.2μm, the stress inversion would occur only when TGO thickness reached 4μm and 5-6μm, respectively. |
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