Application Principles of Thermal Barrier Coating Materials

       Thermal barrier coatings are generally used in high temperature environments above 1000°C to reduce the heat transferred to the base material and increase the service temperature of the base material. The most widely used thermal barrier coating material is yttria or magnesia-stabilized zirconia. The preparation method of thermal spraying generally adopts atmospheric pressure plasma spraying. There is a certain porosity in the coating to reduce the thermal conductivity of the coating and reduce the stress in the coating. Thermal barrier coatings are mainly used in engine combustion chambers, hot end components, exhaust passages, cylinder piston rod heads, nozzles, metallurgical furnaces, etc.



Since the phase transition of pure ZrO2 will bring about volume change, it is necessary to stabilize the high temperature phase of ZrO2.



The earliest ceramic material used to prepare TBCs was ZrO2 fully stabilized with 22% MgO. At 1400 °C, the equilibrium structure of the coating is t-phase or m-phase. During thermal cycling, MgO will precipitate out of the solid solution, which increases the thermal conductivity of the coating and reduces the performance of the coating. After further research, the improved Y2O3 stabilized ZrO2 coating material has become an important TBCs material that is widely studied and widely used today. It has comprehensive properties such as low thermal conductivity, high thermal expansion coefficient, phase stability at 1200°C, and corrosion resistance.



The content of Y2O3 has little effect on the thermal conductivity of ZrO2, but has a great effect on the thermal expansion coefficient of the ceramic layer. When Y2O3<6%, the t-m transition with volume change will occur during the thermal cycle, resulting in coating peeling off. When Y2O3 is 7%-8%, the coating structure has good stability.



In order to find a thermal barrier coating material suitable for higher temperature, scholars at home and abroad based on the partial stabilization of ZrO2 by Y2O3, made ZrO2(Y2O3) + HfO2, ZrO2(Y2O3) + CeO2, ZrO2(Y2O3) + Si and other materials The performance has been studied, and great progress has also been made. For example, the ZrO2-CeO2-Y2O3 structure with CeO2 addition has better thermal cycle performance than ZrO2-Y2O3 due to almost no tm transformation accompanied by volume change; the thermal stress generated at the alloy bonding layer is small; after adding CeO2 The thermal shock performance of the coating is significantly improved due to three reasons: the thermal expansion coefficient of ZrO2-Y2O3 is significantly higher than that of ZrO2-Y2O3.



Due to the high brittleness of the ceramic material and the mismatch with the thermal expansion coefficient of the matrix material, a bonding layer is usually added between the ceramic matrix to improve the physical compatibility between the ceramic and the alloy matrix, that is, a general double-layer structure system. The outer layer is a ceramic layer with a thickness of about 0.25mm, and the lower layer is a metal bonding layer with a thickness of about 0.1mm. The metal bonding layer plays the role of anti-oxidative corrosion and tightly bonding the ceramic layer to the substrate. At present, MCrAIY alloy is generally used as the bonding layer material, which can significantly improve the oxidation resistance of the coating; the ceramic layer is 7% Y203 partially stabilized ZrO2 (YPSZ), and this thermal barrier coating system has good oxidation resistance and heat insulation effect. , simple structure, good heat resistance.