In the periodical table of elements, the elements whose oxides find applications in TBCs are mainly dis-tributed in IIIB (rare earth elements), IVB (Ti, Zr and Hf), IIIA (Al) and IVA (Si). The IIA elements (Mg and Ca) can only be used as stabilizers of zirconia. Rare earth oxides are promising materials for TBCs because of their low thermal conductivity, high thermal expansion coefficients chemical inertness. By the doping of certain elements, the thermal expansion coefficient of La2Zr2O7 can be increased. When its thermal expansion coefficient is increased to a value similar to that of YSZ, the La2Zr2O7 coating can be hopefully used in high temperature applications.
On the other hand, as the dual layer MCrAlY/YSZ system seems to reach their optimum performance, the development of TBCs must shift towards different materials combinations and deposition processes. It seems that the concept of multilayer is effective for the improvement of the thermal shock life of TBCs because no single material satisfies all requirements for TBCs. The multilayer includes an erosion resistant layer as the outer layer, a thermal barrier layer, a corrosion-oxidation resistant layer, a thermal stress control layer and a diffusion resistant layer. Based on the multiplayer system, a double-ceramic- coating system is newly developed as shown in Fig. 1. On the top of 8YSZ coating, another coating whose thermal conductivity is lower than that of 8YSZ is formed. The top ceramic layer acts as thermal insulator to protect the 8YSZ layer. As reported by the authors, the thermal cycling performance of La2Zr2O7/ 8YSZ coating is excellent, and the CeO2/8YSZ coating showed a good thermal shock resistance. Taken in total, several decades of TBC development indicate that when all of the necessary properties for successful TBCs considered, YSZ remains difficult to be replaced.
Fig. 1. Double-ceramic-coating system.
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