Collection of Thermal Barrier and Enviromental Barrier Coating(202412)
With the rising of the gas inlet temperature in front of the turbine of aero-engine, ceramic matrix composites (CMCs) have emerged as the preferred matrix material for the new generation of high-temperature components in aero-engine due to their light weight, high strength, oxidation resistance, insensitivity to crack, and excellent temperature durability. However, because of their limited resistance to high temperature water vapor and oxygen erosion, development of thermal spray coating technology for hot-end components of CMCs engines has become an urgent challenge to be overcome. In this paper, based upon changes of material selection strategies and application examples of foreign aero-engines, technical limitations of the employed superalloys + film cooling + thermal barrier coatings (TBCs) for hot-end components of aero-engines were analyzed, and technical advantages of the utilized CMCs + appropriate film cooling + environmental barrier coatings (EBCs) were consolidated. Thermal and environmental barrier coatings (TEBCs) and environmental barrier coatings-abradable sealing coatings (EBCs-ASCs) for CMCs were reviewed on the basis of recent research findings from domestic and oversea scholars. Finally, opportunities and challenges associated with thermal spraying EBCs for higher temperature gas flow were analyzed, and the direction of design and preparation on a certain composition and structure for TEBCs was clarified, among which the focal points of future research endeavors were prospected.
In order to improve the ablation resistance of carbon-based materials in elevated-temperature and oxygenated environments, Ti-doped HfB2-SiC and ZrB2-SiC composite coatings were prepared on the surface of graphite via a hybrid method involving slurry dipping and reactive infiltration. Phase compositions, microstructures, and element distributions of the composite coatings were studied, and anti-ablation ability of the coating was evaluated at 2300 ℃. Results show that structures of Ti-doped Hf(Zr)B2-SiC composite coatings are very dense after silicon infiltration. Both HfTiB2 and ZrTiB2 ceramic phases are embedded in the coatings, which exhibit no defects and establish robust bonds with the graphite substrates. Residual silicon continuously distributes around Hf(Zr)B2 and SiC particles. After undergoing ablation at 2300 ℃ for 480 s, the mass ablation rates of HfTiB2-SiC and ZrTiB2-SiC composite coating samples are -2.71×10-3 and -4.20×10-1 mg/s, respectively, indicating a slight weight gain. The corresponding line ablation rates are 1.88×10-4 and 3.70×10-4 μm/s, respectively. Following ablation, a Hf-Ti-Si-O multiphase oxide layer composed of HfTiO4-HfO2 as the skeleton and TiO2-SiO2 as the filling phase forms on the surface of HfTiB2-SiC coating. In contrast, a Zr-Ti-Si-O multiphase oxide layer with some micropores, comprising embedded ZrTiO4 and ZrO2 phases and a semi-continuous SiO2 glass phase, develops on the ablative surface of ZrTiB2-SiC coating. High-melting-point phases, such as HfTiO4, HfO2, ZrTiO4, and ZrO2, effectively counteract high-temperature flame erosion. Meanwhile, TiO2 and SiO2, possessing high-temperature fluidity, can seal the pore defects generated by erosion and thereby preventing oxygen from diffusing into the coatings and substrates. Therefore, the synergy between high-temperature skeletons and filling phases significantly enhances the anti-ablation protection of coatings.
The integration of ceramic matrix composites with environmental barrier coatings (CMC-EBC) represents the most promising thermal structural material system in the aerospace field. This paper provides an overview of the advancements in research on the failure mechanisms and numerical models of CMC-EBC. It commences with a concise review of the evolution and primary fabrication techniques of CMC-EBC material system. Subsequently, it summarizes the typical damage modes and failure mechanisms of CMC-EBC under operational conditions, identifying that the interplay between the CMC preform structure, porosity defects, and EBC inner cracks is a critical determinant of the material’s lifespan. However, current mechanistic studies are chiefly focused on the performance evaluation of the coating itself and its susceptibility to environmental factors, disregarding the synergistic effects of the coating and composite architecture during damage progression. This review proceeds with an examination of the history and current status of research on failure simulation and prediction models for CMC-EBC, highlighting issues related to modeling environmental factors and simulating coupled damage evolution. Though much effort has directly developed separate failure models for CMC and EBC, predicting the failure of CMC-EBC components should account for the coupling effects between damage evolution and microstructure. In conclusion, this review offers a perspective on development and service performance prediction methods for CMC-EBC system, which points out that considering the interdependent failure modes of the CMC substrate and EBC is pivotal. Integrated design and analysis of structural and functional aspects are emerging trends in CMC-EBC component research.
Environmental barrier coating (EBC) is a key material for high power-to-weight ratio aero engine, which can provide effective protection for the hot end components of ceramic matrix composites, and prevent the erosion of gas and environmental corrosive media. At present, high entropy rare earth disilicates ((xRE1/x)2Si2O7) are the most promising next-generation environmental barrier coatings. In order to enhance the CMAS corrosion resistance of high entropy rare earth disilicates, a novel high entropy (Y0.25Yb0.25Er0.25Tm0.25)2Si2O7/RE-Si-Al-O (RE=Yb, Y, and La) multiphase ceramic was designed and prepared. The results show that the RE-Si-Al-O glass phase can not only wrap the ceramic grains, but also exist at the grain boundaries. Moreover, this multiphase ceramics can promote the growth of rare earth disilicate grains, reduce the number of grain boundaries, and decrease the number of diffusion channel of CMAS melt. As the radius of rare earth ion in the RE-Si-Al-O glass phase increases, the glass phase is more prone to react with Ca2+ ion in the CMAS melt, generating apatite, reducing the activity of the CMAS melt, inhibiting the erosion of high entropy rare earth disilicate grains by the CMAS molten salt, and thus improving the CMAS corrosion resistance of high entropy rare earth disilicates. After corrosion at 1500 ℃ for 48 h, there is still a residual CMAS layer on the surface of (Y0.25Yb0.25Er0.25Tm0.25)2Si2O7/La-Si-Al-O multiphase ceramics, indicating that the multiphase ceramics have good resistance to CMAS corrosion. In conclusion, the microstructure design of this multiphase ceramic provides a new approach to improve the long-term application of EBC materials in high-temperature CMAS environments.
The investigation of novel materials exhibiting exceptional resistance to calcium-magnesium-aluminum- silicate (CMAS) corrosion at temperatures of 1300 ℃ and above has emerged as a pivotal objective in the advancement of environmental barrier coatings for aircraft engines in recent years. In this study, atmospheric plasma spraying (APS) technology was employed to fabricate YAG(Y3Al5O12)/Al2O3 coatings with eutectic composition, which was acknowledged as a promising material possessing outstanding CMAS corrosion resistance, thereby rendering it suitable for application in environmental barrier coatings. The as-deposited coatings were annealed at 1100, 1300, and 1500 ℃ to obtain different microstructures, and the corrosion resistance as well as mechanism of YAG/Al2O3 coatings against CMAS were investigated by comparing the corrosion results after exposure to CMAS at 1300 ℃. The reaction products between YAG/Al2O3 coatings and CMAS were found to be garnet-structure solid solution, CaAl2Si2O8, and Ca2MgSi2O7. The nearly continuous distribution of the garnet-structure solid solution layer at the reaction interface between YAG/Al2O3 coating annealed at 1100 ℃ and CMAS effectively impedes the diffusion of CMAS corrosion elements. For YAG/Al2O3 coating annealed at 1500 ℃, the increase in grain size and decrease in grain boundaries reduce the dissolution rate of the coating. Both of the above can affect the competitive precipitation of various products by influencing the ion transport rate in the corrosion process, and then improve the CMAS corrosion resistance of the coating. Moreover, heat-treatment temperature can tailor grain size, which influences both dissolution-precipitation rate and competitive precipitation of reaction products during CMAS corrosion. These findings provide guidance for selecting appropriate heat-treatment temperature and offer a novel approach to optimize CMAS corrosion resistance of YAG/Al2O3 coatings through microstructure optimization.
8% (molar fraction) Y2O3 stabilized ZrO2 (8YSZ) ceramics have important applications in fuel cells, thermal barrier coatings, as well as thermal insulation due to their excellent oxygen ionic conductivity and low thermal conductivity. However, their corrosion resistance to water and their behaviors as thermal insulation or structural material in pressurized water reactors during accidents are not fully understood. This study systematically examined the mass, crystal phase, microstructure, mechanical properties, and solution composition of 8YSZ ceramics over time in a dynamic water environment at 350 ℃/17.4 MPa with 0.3 μg/L dissolved oxygen, aiming to simulate a pressurized water reactor environment. It is found that the mass of 8YSZ ceramics increases firstly and then decreases with corrosion duration time. The mass change is influenced by the surface roughness. The weight gain is attributed to the formation of Zr-OH and Y-OH clusters by the entry of water molecules into the ceramics, whereas the weight loss is caused by the metal cations leaching and the dissolution of grains. Phase analysis demonstrates that the cubic 8YSZ after corrosion does not undergo any phase transformation towards tetragonal or monoclinic phases, which is different from the degradation mechanism of tetragonal or partially stabilized zirconia. Changes in surface and cross-section morphology indicate that water molecules enter the interior of the ceramics along defects or microcracks, producing grain boundary damage and changing the fracture mode in the corrosion-affected region from transgranular to intergranular fracture. Compressive and flexural strengths of this ceramics after corrosion do not change significantly, while the Vicker’s hardness decreases slightly, which are related to the formation of pits in the surface layer. As a consequence, depth of the corrosion pit after 1050 h is only 30.8 μm, and the mass change rate of per unit surface area is -0.108×10-3 mg∙cm-2∙h-1, consolidating excellent water corrosion resistance of 8YSZ ceramic. Therefore, 8YSZ ceramics are promising for thermal insulation or structural materials in pressurized water reactors.
With improvement in service temperature of thermal structural components for the new generation hypersonic aircraft, higher requirements are put forward for the phase stability and ablation resistance of the thermal protection coatings (TPCs). Carrying out high-entropy design for traditional transition metal oxide ZrO2 and HfO2 coatings, solid-phase reaction and supersonic atmosphere plasma spraying (SAPS) were applied to prepare (Hf0.125Zr0.125Sm0.25Er0.25Y0.25)O2-δ (M1R3O), (Hf0.2Zr0.2Sm0.2Er0.2Y0.2)O2-δ (M2R3O), (Hf0.25Zr0.25Sm0.167Er0.167Y0.167)O2-δ (M3R3O) high-entropy oxide (HEO) coatings. The effects of rare earth content on phase structure evolution, phase stability and ablative resistance of HEO coatings were investigated. M2R3O coating and M3R3O coating possessed excellent phase stability and ablation resistance, which maintained stable phase structure after ablation by oxygen-acetylene flame with heat flux density of 2.38-2.40 MW/m2, without decomposition of solid solution and precipitation of rare earth components. Mass ablation rate and linear ablation rate of M2R3O coating after cyclic ablation for 180 s are 0.01 mg/s and -1.16 μm/s, respectively. Compared with M1R3O coating (0.09 mg/s, -1.34 μm/s) and M3R3O coating (0.02 mg/s, -4.51 μm/s), the reductions of ablation rate are 88.9%, 13.4%, respectively, and 50.0%, 74.3% for M2R3O coatings, respectively, presenting the best ablation resistance. M2R3O coating exhibits excellent ablation resistance due to its high melting point (>2200 ℃) and low thermal conductivity ((1.07±0.09) W/(m·K)), which effectively protects the internal SiC transition layer and C/C composites from oxidation damage, avoiding interface cracking caused by the formation of SiO2 phase.
For the development of high-performance laser protective coatings, this work innovatively proposes the idea of recombining PCS ablative coatings on the surface of traditional yttrium stabilized zirconia (YSZ) thermal insulation coatings. Based on characteristics of the consumption of large amount laser energy through decomposition of polycarbosilane (PCS) and production of high-temperature ceramic protective phases, NiCrAlY/YSZ/PCS-TiO2 (YPT) and NiCrAlY/YSZ/PCS-Y2O3 (YPY) coatings were prepared on the surface of Ni-based alloy by slurry method combined with atmospheric plasma spraying (APS). Based on the effect of TiO2 and Y2O3 addition on the pyrolysis behavior of PCS, the laser ablation resistance of YPT and YPY composite coatings on 10.6 μm CO2 laser were systematically studied and compared with that of single layer YSZ coating. The results show that YPY and YPT composite coatings have better laser protection effect than traditional YSZ coating, because in the early stage of laser ablation, the PCS decomposition on the coating surface can consume laser energy, and the residual Y2SiO5, SiC and SiO2 phases after ablation can be deposited on the YSZ coating, forming a dense protective layer, which can continue to play a role in laser protection of the YSZ coating. YPY coating has better laser protection performance than YPT coating, because Y2O3 has higher thermal conductivity and lower coefficient of thermal expansion. Smaller temperature gradient can be produced in YPY coating, which can alleviate thermal stress. Moreover, Y2O3 participates in the pyrolysis of PCS to generate Y2SiO5 phase, which inhibits more volume expansion in PCS pyrolysis than that in TiO2. In addition, the core ablation temperature of YPY coating is higher, and the formation of PCS pyrolysis products SiC and SiO2 phase is faster, which can timely protect the lower coating, showing better laser ablation resistance. This study is expected to provide a new idea for the design of novel laser resistant composite coatings.
TiAl alloy has excellent properties of low density and high specific strength, which is a potential structural material for aero-engine. The service temperature of TiAl alloy is limited in the range of 700-900 ℃, which is further improved by preparation of high temperature thermal protection coating. In the present work, a new type of TiAlCrY/YSZ coating was prepared on the TiAl alloy by plasma spray technologies, of which long-term service performance at high temperature was compared with the traditional NiCrAlY/YSZ coating. It was found that the TiAlCrY/YSZ coating remained intact after served 300 h at 1100 ℃ in the air, showing excellent high temperature performance, while the service lifetime of the NiCrAlY/YSZ coating at 1100 ℃ was less than 100 h. The microstructure observation found that a continuous and dense TGO composed of Al2O3 was continuously formed on the TiAlCrY bonding coating, suggesting that TiAlCrY coating has good interfacial compatibility with the YSZ top coating. Moreover, the thickness of TGO was still less than 8 μm after served 300 h at 1100 ℃ in the air. As compared with traditional NiCrAlY/YSZ coating, the TiAlCrY/YSZ coating is a more suitable high temperature thermal coating ever for TiAl alloys.
Environmental barrier coatings (EBCs) are expected to be applied to the hot-section components of a new generation of high thrust-to-weight ratio aero-engines. Rare-earth silicates have been acknowledged as promising alternatives to EBC materials due to their superior high-temperature phase stability, suitable coefficient of thermal expansion, and long durability in water vapor. However, the calcium-magnesium-alumino-silicates (CMAS) molten salt corrosion under service conditions has become a bottleneck that limits the application of rare-earth silicates in EBCs. Factors such as the composition of CMAS and the crystal structures of rare-earth silicates have a significant impact on their corrosion behavior. In this paper, X1-Gd2SiO5 and X2-RE2SiO5 (RE=Y, Er) coatings with different crystal structures, were prepared by atmospheric plasma spraying (APS) technique. Their corrosion behaviors and mechanisms of the three kinds of coatings under CMAS melt environment at 1400 ℃ were explored. Results showed that the corrosion resistance of X2-RE2SiO5 coatings were better than that of X1-Gd2SiO5 coating due to their phase compositions and stability of crystal structure. After corrosion by CMAS, X1-Gd2SiO5 coating dissolved in CMAS melt and formed apatite phase, while the X2-RE2SiO5 coatings not only formed apatite phase, but also formed garnet phase from reaction of the RE2O3 in the coatings with Al2O3 in CMAS. Formation of generate garnet phase could increase relative content of CaO and SiO2 in CMAS, and promote formation of dense apatite layer, thereby improving corrosion resistance. This study provides a strategy for designing EBC systems with excellent CMAS corrosion resistance.