Silicon oxide, aluminum oxide modified thermal barrier coating materials

       Mullite is composed of 3Al2O3 2SiO2 and is an important ceramic material. It has low density, high thermal stability, chemical corrosion resistance, low thermal conductivity, excellent mechanical properties and good creep properties. Compared with YSZ, mullite has higher thermal conductivity, lower thermal expansion coefficient and oxygen permeability, and its antioxidant capacity is much stronger than that of YSZ. In environments where the surface temperature is lower than that of the gas turbine and the temperature difference between the two sides of the coating is large, such as in diesel engines, mullite is superior to zirconia as a thermal barrier coating material. Engine testing of both materials showed that the engine life with the mullite coating was significantly longer than that with the zirconia coating. Above 1000°C, the thermal cycle life of mullite coating is much shorter than that of YSZ coating. Since mullite has close thermal expansion coefficients to SiC, it is the most promising coating material for SiC substrates.



Compared with YSZ, Al2O3 has higher thermal conductivity (5.3W·m-1·K-1, 1100℃) and lower thermal expansion coefficient (9.6x10-6K-1, 1000℃). a-Al2O3 is the only stable phase among all oxides of aluminum, which has high hardness and chemical inertness. The corrosion resistance of Al2O3 coatings by vacuum plasma spraying and atmospheric plasma spraying is comparable to that of Al2O3 bulk materials. Adding a certain amount of Al2O3 to YSZ can improve the hardness and bonding strength of the coating, while the elastic modulus and toughness are basically unchanged. In addition, the hardness of the coating can be improved by preparing another layer of Al2O3 on the YSZ coating. However, plasma sprayed Al2O3 coatings contain unstable phases such as γ and δ-Al2O3. During thermal cycling, these unstable phases transform into a-Al2O3 accompanied by a significant volume change (γ→a, about 15%), which leads to cracks in the coating. Doping transition metal oxides such as Cr2O3, Fe2O3 and TiO2 into Al2O3 can partially stabilize the a-phase. Toughening with SiC fibers can greatly improve the mechanical properties of Al2O3. Although Al2O3 is not a good thermal barrier coating material, adding it to YSZ can improve the life of the coating and the oxidation resistance of the substrate. The thermal cycle life of the 8YSZ-Al2O3 gradient coating is much longer than that of the 8YSZ coating.