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The aircraft engine is considered the "heart" of the aircraft. The development of advanced aircraft engines focuses on achieving a high thrust-to-weight ratio, high efficiency, low fuel consumption, and extended operational life. High-temperature functional coatings, such as thermal barrier coatings (TBCs), thermal/environmental barrier composite coatings (T/EBCs), and high-temperature stealth coatings, are applied to critical hot-end components of aircraft engines. These coatings play a significant role in enhancing the service performance, operational life, safety and reliability of the engine. Taking TBCs, T/EBCs, and high-temperature stealth coatings as examples, this paper provides a systematic overview of recent research progress in the design of high-temperature functional coating materials, coating preparation science and technology, and coating performance evaluation and characterization, both domestically and internationally, with a specific focus on the advancements made at Beihang University. Furthermore, challenges and development trends faced by new high-temperature functional coatings of advanced aircraft engines in the future are also discussed. In the future, the research focus of advanced high-temperature functional coatings will shift toward multifunctional composite coatings, enhanced adaptability to extreme environments, and improved process compatibility. [ABSTRACT FROM AUTHOR]

Abstract: Bulk material and coatings of Lanthanum–Cerium Oxide (La2Ce2O7) with a fluorite structure were studied as a candidate material for thermal barrier coating (TBC). It has been showed that such material has the properties of low thermal conductivity about four times lower than YSZ, the difference in the thermal expansion coefficient between La2Ce2O7 and bond coat is smaller than that of YSZ in TBC systems, high phase stability between room temperature and 1673 K, about 300 K higher than that of the YSZ. The coating prepared by electron beam physical vapor deposition (EB–PVD) showed that it has good thermal cycling behavior, implying that such material can be a promising thermal barrier coating material. The deviation of coating composition from ingot can be overcome by the addition of excess La2O3 during ingot preparation and/or by adjusting the process parameters. [Copyright &y& Elsevier]

Abstract: The phase stability of lanthanum cerium oxide (La2Ce2O7), which is stable up to 1400°C, and the thermal expansion coefficient of La2Ce2O7 doped with Ta2O5 or WO3 were studied. The thermal expansion coefficient of La2Ce2O7 below 400°C was increased by adding more CeO2 or doping with either Ta2O5 or WO3. [Copyright &y& Elsevier]

Ni3Al based alloy IC6, a high performance cast alloy with approximately 14 wt.% Mo addition, has been developed for turbine blades and vanes of advanced aero-engines and other high temperature structural components. However, because of the high Mo content, during high temperature use, Mo will diffuse into the protective coating layer, which will affect the thermal cycling performance directly. In order to block the diffusion and improve the thermal cycling behavior of thermal barrier coatings (TBCs), a Si chemical gradient MCrAlY&z.sbnd;Si bond coat was prepared by changing the Si content from 0.35 wt.% at the interface of substrate and bond coat to almost 0 near to the surface of the bond coat. For comparison, conventional two-layered structural TBCs with MCrAlY as bond coat were also prepared. The fabricated coatings were evaluated for the microstructures and the composition distribution properties, such as thermal stability and resistance to oxidation. For the bond coat without Si addition, Mo content near the surface was higher than 7 wt.% after isothermal oxidation at 1373 K for 60 h and 12.5 wt.% after thermal cycling tested at 1373 K for 300 h (600 cycles). However, in the Si chemical gradient bond coat, only 0.4 wt.% Mo was detectable after isothermal oxidation at 1373 K for 100 h and approximately 0.66 wt.% after thermal cycling tested at 1373 K for 340 h (680 cycles). [Copyright &y& Elsevier]

Abstract Optimization of the resistance of thermal barrier coatings (TBCs) to environmental particles is an area of considerable current research. Here, we present a novel methods, incorporating plasma spray physical vapor deposition (PS-PVD), in order to fabricate a novel yttria-stabilized zirconia (YSZ) TBC which is highly phobic (i.e. non-wettable) with respect to CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) particles. At 1250 °C, CMAS particles form liquid droplets which are readily repelled by the TBC surface, diminishing adherence to and penetration into the TBC. This effectiveness of the TBC in resisting wetting by molten CMAS deposits is interpreted to result from its lotus-leaf-like dual-scale microstructure. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Yttria-stabilized zirconia (YSZ) thermal barrier coatings with quasi-columnar structure were produced by plasma spray physical vapor deposition (PS-PVD). To form this typical structured coating, the process-structure relationships response for substrate temperature, carrier gas flow and spray distance were investigated. Furthermore, this study revealed its characterization in coating microstructures. It was found that the substrate temperature played a governor role in determining the formation of quasi-columnar structure partially because it affected the nucleation rate of vapors during deposition. Appropriate carrier gas flow and spray distance were helpful to remove the un-melted particles and droplets from the plasma plume and promote the formation of quasi-columnar microstructure with high-ratio vapors. The quasi-columnar coating is featured with feather-like polycrystal structure with average misorientation angle of ~ 35.4° and average diameter of ~ 1.17 μm. [ABSTRACT FROM AUTHOR]

Plasma spray physical vapor deposition (PS-PVD) was used to deposit yttria stabilized zirconia (YSZ) coatings with microstructures varying from dense lamellae to quasi-columns out of vapor, as well as hybrid microstructure resulting from liquid droplet and vapor co-deposition. Among the coatings investigated, the quasi-columnar coating exhibited the lowest thermal conductivity of around 1.1 W/mK at 1200 °C due to high porosity and defects. The quasi-columnar coating also had an average life of around 2000 cycles during flame shock testing with a coating surface temperature of ~ 1200 °C. The failure of the coating occurred through cracking at the interface between the thermally grown oxide (TGO) layer and YSZ top coat. [ABSTRACT FROM AUTHOR]

As one of promising thermal barrier coating (TBC) candidates, La2Ce2O7 (LC) has attracted increasing attention because of its low thermal conductivity and potential capability to be operated above 1250°C. In this paper, the microstructure evolution and mechanical properties of the plasma-sprayed LC TBC with calcium–magnesium–alumina–silicate (CMAS) glassy deposits at 1250°C were investigated. Due to chemical reaction between the CMAS deposits and LC coating, a dense sealing layer, mainly composed of Ca2(La x Ce1−x )8(SiO4)6O6−4x and CeO2, was formed on the coating after heat-treatment at 1250°C and effectively prevented CMAS from further penetration. The interaction layer had the micro-hardness of ∼10–12GPa, relatively harder than the LC coating. [ABSTRACT FROM AUTHOR]

Abstract: Rare earth oxides doping has been extensively investigated as one of the effective methods to lower thermal conductivity of 4.55mol% Y2O3 stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs). In the present work, 5–6mol% Yb2O3 and Y2O3 co-doped ZrO2 ceramics were synthesized by solid reaction sintering at 1600°C. The phase stability of the samples after heat treatment at 1500°C was investigated. Yb2O3 and Y2O3 co-doped zirconia, especially when Yb2O3/Y2O3≥1, contained less monoclinic phase than single Yb2O3 or Y2O3 phase doped zirconia, indicating that co-doped zirconia was more stable at high temperature than YSZ. The thermal conductivity of the 3mol% Yb2O3+3mol% Y2O3 co-doped ZrO2 was 1.8Wm−1 K−1 at 1000°C, which was more than 20% lower than that of YSZ. [Copyright &y& Elsevier]

Abstract: Multi-rare earth elements doping is one of effective methods to improve the phase stability and thermal barrier property of zirconia for thermal barrier coatings (TBCs) application. In this paper, the phase stability and thermo-physical properties of 7.5mol% Sc2O3 and Gd2O3 co-doped ZrO2 (ScGdSZ) were investigated. ScGdSZ after 150h heat-treatment at 1400°C exhibited no monoclinic phase, showing high phase stability. The substitution of Sc2O3 with Gd2O3 increased the fraction of c-phase, however, t′-phase was still the dominant phase when the substitution amount was less than 2mol%. The thermal conductivities of ScGdSZ gradually decreased with the increasing Gd2O3 proportion. 3.7 Sc2O3 and 3.7 Gd2O3 co-doped ZrO2 (in mol%) showed the lowest thermal conductivity, which was 20% lower than 7.5 Sc2O3-doped ZrO2 and 40% lower than 4.5 Y2O3-doped ZrO2. [Copyright &y& Elsevier]

Abstract: Considerable efforts are being invested to explore new thermal barrier coating (TBC) materials with higher temperature capability to meet the demand of advanced turbine engines. In this work, LaTi2Al9O19 (LTA) is proposed and investigated as a novel TBC material for application at 1300°C. LTA showed excellent phase stability up to 1600°C. The thermal conductivities for LTA coating are in a range of 1.0–1.3Wm−1 K−1 (300–1500°C) and the values of thermal expansion coefficients increase from 8.0 to 11.2×10−6 K−1 (200–1400°C), which are comparable to those of yttria stabilized zirconia (YSZ). The microhardness of LTA and YSZ coatings were in the similar level of ∼7GPa, however, the fracture toughness value was relatively lower than that of YSZ. The lower fracture toughness was compensated by the double-ceramic LTA/YSZ layer design, and the LTA/YSZ TBC exhibited desirable thermal cycling life of nearly 700h at 1300°C. [Copyright &y& Elsevier]

Abstract: As a potential thermal barrier coating (TBC) material, lanthanum titanium aluminum oxide (LaTi2Al9O19, LTA) possesses excellent phase stability between room temperature and 1600°C and desirable thermo-physical properties, but its thermal cycling performance is rather poor partially due to the low fracture toughness. In this work, LTA/yttria stabilized zirconia (YSZ) double ceramic layer TBC was prepared and its thermal cycling lifetime was assessed by a gas burner facility at a coating surface temperature of 1300±50°C. Chipping of LTA layer, which was different from the failure mechanism of the traditional plasma-sprayed TBCs, was visible in the center part of the TBC after about 3000cycles, equaling to a holding time of 500h at 1300±50°C. The failure of the LTA layer was mainly due to the decomposition and sintering of LTA exposed to extremely high-temperature gas flame. Followed by another 1000cycles, the YSZ layer was also cracked along its interface to the bond coat. [Copyright &y& Elsevier]

Abstract: Novel thermal barrier coatings based on La2Ce2O7/8YSZ double-ceramic-layer (DCL) systems, which were deposited by electron beam physical vapor deposition (EB-PVD), were found to have a longer lifetime compared to the single layer La2Ce2O7 (LC) system, and even much longer than that of the single layer 8YSZ system under burner rig test. The DCL coating structure design can effectively alleviate the thermal expansion mismatch between LC coating and bond coat, as well as avoid the chemical reaction between LC and Al2O3 in thermally grown oxide (TGO), which occurs above 1000 °C as determined by differential scanning calorimetry (DSC) analysis. The failure mechanism of LC/8YSZ DCL coating is mainly due to the sintering of LC coating surface after long-term thermal cycling. [Copyright &y& Elsevier]

Abstract: Thermal barrier coatings (TBCs) with nano-multilayer structure were investigated by thermal shock test. The change of insulation effect during thermal shock test was studied by in-situ temperature monitor with a thermal couple set into the substrate. Microstructure and electrical properties of TBCs were characterized by SEM and Impedance Spectroscopy, respectively. Initial increase in insulation effect was observed and related to the formation and growth of perpendicular microcracks in top coat and transversal microcracks in TGO. With thermal shock, the insulation effect decreased due to the further growth of microcracks in top coat and TGO which induced the failure of TBCs. [Copyright &y& Elsevier]

Abstract: In this work, by electron-beam physical vapour deposition (EB-PVD), grit blasting and vacuum heat treatment, a two-phase [NiAl+Ni3Al] coating was fabricated. Rather than the column grain boundaries, the small voids formed on the surface of the columnar grains serve as the fast diffusion route for chromium of the substrate. In order to evaluate the oxidation resistance of this coating, an isothermal oxidation test at 1100°C in air was conducted. The oxidation resistance of the as-coated specimen was improved approximately twice that for a substrate without coating. [Copyright &y& Elsevier]

Abstract: In this investigation, the oxidation of two-layer thermal barrier coatings (TBCs) prepared by EB-PVD (electron beam physical vapor deposition) in different oxygen concentration environments at 1323 K was evaluated by impedance spectroscopy and microstructural observation (SEM). The higher partial pressure of oxygen accelerated the oxidation rate of bond coating and faster oxidation caused cracking at interface between the TGO and the bond coat. Impedance spectroscopy was used to evaluate the microstructural changes which occurred in the TBC system non-destructively. The relationship between the electrical conductivity of the YSZ layer and its microstructure including the parameters of microcracks was established. [Copyright &y& Elsevier]