| Summary: | The two-dimensional finite element microscopic model of NiCoCrAlY/YSZ gradient thermal barrier coating was established by using the representative volume element method to calculate the thermophysical properties of the gradient layer under different composition ratios. The parameter results were extended to the three-dimensional multi-layer solid model to study the thermodynamic properties of the double-layer coating and gradient structured coating under thermal cycling condition. The results show that the elastic modulus, Poisson's ratio, coefficient of thermal expansion and thermal conductivity of the gradient layer are approximately linear with the component proportion of each phase, and the thermal conductivity is also affected by the distribution pattern of each phase. The thermal conductivity is low and the highest value is 2.91 W·m−1·K−1 when the proportion of NiCoCrAlY phase in the gradient layer is below 0.7 at room temperature. Compared with the double-layer coating, the proportion of YSZ in gradient coatings is reduced by 20%, the insulation temperature is reduced by 14%, the radial tensile stress, axial tensile stress, and shear stress of the ceramic surface layer at high temperature are reduced respectively by 47%, 32% and 37%, and the residual stress after cooling is reduced by 50%. The results are attributed that the gradient of the coating structure can effectively reduce the thermal mismatch stress caused by the difference in the thermal expansion coefficient between coating and substrate. According to the results of coating stress distribution, the coating is inclined to form vertical cracks in the centre region and horizontal cracks near the outer edge of the TC/BC interface.
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