Multilayer Coatings as a New Stage in the Development of Modern Highly Effective Thermal Barrier Coatings I. Two-Layer La2Zr2O7 (LZ2)/YSZ Thermal Barrier Coatings

Abstract

At the beginning of the 21st century, the classical yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) reached their maximum operating temperature (1200°C). A further increase in the operating temperature and associated gain in the efficiency of gas turbine engines became possible through the complex stabilization of ZrO₂; the use of other compounds (oxides with fluorite (La₂Ce₂O₇), pyrochlore (La₂Zr₂O₇, La₂Hf₂O₇), magnetoplumbite (LaMgAl₁₁O₁₉), perovskite (SrZO₃), and other structures); and the design of functionally graded and two-layer TBCs. In the latter case, the bottom layer is mainly YSZ, whose coefficient of thermal expansion (CTE) matches that of the bound coat. Various materials with low thermal conductivity are proposed as the topcoat (an overview of the efforts focusing on the development of La₂Zr₂O₇ (LZ₂)/YSZ TBCs is presented in the first paper of the series). Two-layer LZ₂/YSZ TBCs can be successfully produced by all available application methods: atmospheric plasma spraying (APS), electron-beam physical vapor deposition (EB-PVD), spark plasma sintering (SPS), etc. The LZ₂ and YSZ phases exhibit good chemical compatibility even after sintering at 1400°C for 24 h. The mismatch in the CTEs of the layers remains a serious problem. The LZ₂/YSZ coatings demonstrate better thermal shock resistance compared to nanostructured and conventional YSZ TBCs. To produce high-quality two-layer coatings with the LZ₂ topcoat, APS parameters should be lowered to avoid lanthanum losses. Reducing the elastic modulus of two-layer TBCs is useful for decreasing the crack propagation tendency. The LZ₂ phase reduces the oxygen permeability of the ceramic coat in two-layer TBCs, thus preventing the bond coat oxidation and the rapid growth of thermally grown oxides. The twolayer LZ₂/YSZ TBCs show higher resistance to hot corrosion caused by Ca–Mg–Al silicates (CMAS) and V₂O₅+Na₂SO₄ compared to their single-layer counterparts. The doping of LZ₂ with CeO₂ increases the CTE of two-layer APS coatings, improving their efficiency. The overview of two-layer La₂Zr₂O₇(LZ₂)/YSZ TBCs confirms their advantages over single-layer ones, because each layer contributes to improving the properties and mitigating the weaknesses of the coatings, ultimately enabling higher operating temperatures and longer service lives of power equipment.