Types and Functions of Stabilizers in Thermal Barrier Coating (TBC) Ceramic Surface Materials

       At present, the ceramic surface layers in TBC coatings are mainly fully stabilized or partially stabilized zirconia ceramics. Since pure zirconia crystals have different crystal types with temperature changes, when the temperature exceeds 1000 °C, the transformation from monoclinic crystals to tetragonal crystals occurs, accompanied by a 7% volume change, while in the subsequent cooling process, the monoclinic crystals are transformed into tetragonal crystals. The oblique crystal structure will be restored, but the volume cannot be restored to the original state, that is, the volume will be irreversibly transformed before and after heating and cooling. This crystal transformation and volume change, under the condition of thermal cycling, will generate a large thermal stress inside the coating, resulting in early cracking of the coating and even spalling failure. Therefore, it is necessary to add stabilizers to pure ZrO2 crystals.



After adding the stabilizer to the pure ZrO2 crystal, it is sintered or melted to form a solid solution to obtain a cubic stable ZrO2 that is stable in the entire temperature range below the melting point and has a low expansion coefficient. However, at high temperature, although the expansion and contraction of fully stabilized cubic crystal ZrO2 can be simulated, its linear expansion and contraction are large, which is not conducive to improving the thermal shock resistance life. Partially stable zirconium oxide composed of cubic crystal mixed structure. In this crystal structure, at high temperature, the monoclinic crystal part will undergo volume shrinkage phase transition, while the cubic crystal part will undergo volume expansion with the increase of temperature. Low average thermal expansion coefficient, with better thermal shock resistance.



Stabilizers added to zirconia include calcium oxide (CaO), magnesium oxide (MgO), yttrium oxide (Y2O3), and cerium oxide (CeO), among others. Among them, the addition amount of CaO stabilizer is 5%, 6%, 8%, 10%, 15% and 30%. With the increase of CaO content, the hardness of the coating increases. Coatings with a CaO content of up to 30% have relatively high hardness and good resistance to high temperature particle erosion. However, if the CaO-stabilized ZrO2 coating is exposed to a high temperature environment above 1093 °C for a long time or periodically, the CaO tends to diffuse out of the stabilized ZrO2 crystal, resulting in the limited use temperature of the coating, which can only be used at 845 °C. It can be used for a long time in a high temperature environment above 1093℃, and can only be used for a short time when it exceeds 1093℃. The amount of MgO stabilizer added is usually 20% to 30%. At this time, ZrO2 can keep the crystal form stable at different temperatures, especially during high temperature thermal cycles. MgO-stabilized ZrO, below 1400C, its equilibrium structure is tetragonal or monoclinic plus MgO. During thermal cycling, MgO will precipitate from solid solution, resulting in an increase in thermal conductivity of the coating and a decrease in thermal insulation capacity. Widely used. limit. And Y2O3 partially stabilized ZrO2, when used for a long time at up to 1650°C, Y2O3 will not diffuse out of the crystal like CaO, and its chemical stability and thermal stability are better than CaO partially stabilized ZrO2 and MgO partially stabilized ZrO2 is a thermal barrier coating material with excellent performance and the highest operating temperature. Its addition amount is 6% ~ 8%, 13% and 20%. The first two are partially stabilized ZrO2, and the latter is fully stabilized ZrO2. For thermal barrier coatings, partially stabilized zirconia has better resistance. Therefore, zirconia partially stabilized by 6% to 8% yttria has become the material of choice for the ceramic surface layer in thermal barrier coatings.



In recent years, studies on partial stabilizers (PSZ) such as Y2O3, Nd2O3, and Sc2O3 have found that under rapid cooling conditions, some or all of the "non-transformed" tetragonal phases exist in the ZrO2 ceramic layer, although they are still metastable phases. However, it does not decompose into equilibrium tetragonal and cubic phases under high temperature cycling conditions of 1100-1200 °C. The 6%~8% Y2O3-ZrO2 (YSZ) coating did not decompose at 1100~1200℃. In CeO-Y2O3-ZrO2, the stability of t' phase is better than 8% YSZ, but its resistance to gas containing V, S and other corrosive media is poor, while Sc2O3-Y2O3-ZrO2 (SYSZ) has a high temperature (1400℃) ) higher t' phase stability and resistance to hot salt corrosion.