IntroductionOne of the major reasons for the failure of thermal barrier coating materials is the corrosion caused by the reaction between aeroengine thermal barrier coating materials (TBC) and V₂O₅+Na₂SO₄ in fuel under high-temperature service conditions. Conventional TBCs are difficult to meet the increasingly harsh high-temperature service environment. It is thus necessary to develop new and more corrosion-resistant thermal barrier coating materials. Among the materials to replace conventional TBC, La₂(Zr_0.7Ce_0.3)₂O₇(LZC) ceramics are considered as potential TBC candidates due to their high sintering resistance, low thermal conductivity and high thermal expansion coefficient. Also, the thermal conductivity of LZC can be effectively reduced by using Re³⁺ with large mass and small radius doping at La³⁺. Therefore, the modified (La_0.5Gd_0.5)₂(Zr_0.7Ce_0.3)₂O₇ (LGZC) doped with Gd³⁺, which is a rare-earth element and has a smaller ionic radius, can further reduce the thermal conductivity of the material and improve the mechanical properties of the material through fine-grain strengthening. However, the existing reports on the hot corrosion resistance of V₂O₅+Na₂SO₄ molten salt of LZC and LGZC materials are more limited to the strong corrosive properties at < 1050 ℃, while the actual service temperature of the coating often exceeds 1200 ℃.MethodsIn this work, ZrO₂, La₂O₃, Gd₂O₃, CeO₂ and other oxide powders were used as raw materials, and LZC and LGZC ceramic samples were prepared by a high-temperature solid-state reaction method. A typical mixed salt of V₂O₅+Na₂SO₄ (in a molar ratio of 1:1) was used as a corrosive medium. V₂O₅ and Na₂SO₄ powders were mixed. The mixed powder was evenly spread on the surface of the ceramic sample at a concentration of 10 mg/cm², and then the coated ceramic sample was placed in a box-type resistance box and treated at 900, 1000, 1100 and 1250 ℃ for 5 h for thermal corrosion, respectively.The phase composition of LZC and LGZC samples before and after hot corrosion was determined by a model D8 X-ray diffractometer (XRD, Bruker Co., Germany). The microstructure and morphology were characterized by a model Sigma 500scanning electron microscope (SEM, ZEISS Co., Germany) equipped with energy dispersive spectrometer (EDS).Results and discussionLZC and LGZC ceramic specimens with a single pyrochlorite structure were synthesized. After corrosion, the diffraction peak of LGZC reduces, compared to that of LZC ceramic samples. There are mainly (La, Ce, Gd) VO₄, t-ZrO₂ and m-ZrO₂ on the surface according to the SEM images and EDS results. The content of (La,Ce,Gd) VO₄ increases with the increase of temperature in the range of 900-1100 ℃, and decreases after corrosion at 1250 ℃. From the micromorphology after corrosion, the ceramic surface of LZC specimens after corrosion at 900-1100 ℃ is mainly rod-like grains and granular clusters, while LGZC has more rod-like grains rather than granular grains. After corrosion at 1250 ℃, the surface of the LGZC specimen is granular crystals with intracrystalline pores. LZC and LGZC both forman obvious corrosion layer at 900-1100 ℃, and the thickness of the corrosion layer increases with the increase of temperature, and the corrosion layer becomes the thickest at 1100 ℃ (i.e., 38 μm and 35 μm), respectively. However, after corrosion at 1250 ℃, the corrosion layer does not form and the corrosion depth decreases, and the darker molten salt penetrates further downward, showing two completely different microscopic morphologies. Also, the corrosion depth of LGZC is smaller than that of LZC at different temperatures.ConclusionsLZC and LGZC ceramic materials reacted with V₂O₅+Na₂SO₄ at 900-1100 ℃ to form a relatively dense corrosion reaction layer, and the corrosion reaction degree increased with the increase of temperature, and the reaction was most intense at 1100 ℃. The hot corrosion mechanism of LZC and LGZC ceramic materials at 900-1250 ℃ followed Lewis’s acid-base law and Gibbs's free energy. The relative alkalinity of LGZC decreased, and the corrosion depth of LGZC was smaller than that of LZC at different temperatures due to the doping of Gd₂O₃. The corrosion resistance of ceramic materials was reflected in the corrosion depth, the smaller the corrosion depth, the stronger the corrosion resistance of the ceramic materials. Therefore, LGZC was more resistant to V₂O₅+Na₂SO₄ corrosion than LZC in this case. After the temperature increased to 1250 ℃, neither of the two samples formed a dense corrosion reaction layer, and the corrosion depth was smaller than that after corrosion at 1100 ℃ because the viscosity of NaVO₃ decreased with the increase of the corrosion temperature to 1250 ℃ and blocked the pores in a short time, thus preventing a further penetration of the molten salt.