Water Vapor Corrosion Research Articles

Thermochemical Stability of Yb2Si2O7 Environmental Barrier Coatings During Thermal Cycling and Water Vapor Corrosion at 1350 °C

Thermochemical Stability of Yb2Si2O7 Environmental Barrier Coatings During Thermal Cycling and Water Vapor Corrosion at 1350 °C

Al-modified environmental barrier coatings for protection against water vapor corrosion

High-temperature water vapor corrosion is a great challenge for long lifetime of Si/mullite/Yb2SiO5 Environmental Barrier Coatings. In this work, the Yb3Al5O12 modified layer was in-situ synthesized on the surface of Yb2SiO5 layer to improve the steam resistance. The reaction mechanism of the Al-modified layer was explored. The steam corrosion experimental results indicated that the defects in Yb2SiO5 layer healed, and the oxidation rate of Si-bonded coating was slowed down under the protection of modified layer. Additionally, the calculation results of first-principle molecular dynamics confirmed that Yb3Al5O12 has excellent steam corrosion resistance.

Investigation on behavior and mechanism of enhanced water vapor corrosion resistance for (Lu0.25Yb0.25Er0.25Y0.25)2SiO5 environmental barrier coating

Rare-earth monosilicate (RE2SiO5) have been considered to be a promising material for the environmental barrier coating because of their superior thermal and mechanical properties. However, the water vapor corrosion resistance of single-component RE-silicate materials, such as Y2SiO5, should be further improved. The high-entropy design is one of the most suitable methods to enhance the corrosion resistance for single-component RE-silicate materials. In this work, the multicomponent RE-silicate ((Lu0.25Yb0.25Er0.25Y0.25)2SiO5, (4HES)) and single-component RE-silicate (Y2SiO5) coatings were investigated with regard to its water vapor corrosion behaviors at 1350 °C for 300 h. A thinner and denser corrosion layer was generated in the 4HES coating, indicating that the 4HES coating possessed better corrosion resistance than the Y2SiO5 coating. The improved corrosion resistance is attributed to the better hydrophobic property as well as the more stable crystal structure of the rare-earth oxide and 4HES phase which was resulted from the high-entropy design.

Feasibility research of Yb3Al5O12 garnet as environmental barrier coating materials

In this work, Yb3Al5O12 (YbAG) garnet, as a new material for environment barrier coating (EBC) application, was synthesized and prepared by atmospheric plasma spraying (APS). The phases and microstructures of the coatings were characterized by XRD, EDS and SEM, respectively. The thermal stability was measured by TG-DSC. The mechanical and thermal-physical properties, including Vickers hardness (Hv), fracture toughness (KIC), Young's modulus (E), thermal conductivity (κ) and coefficient of thermal expansion (CTE) were also measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb3Al5O12 and amorphous phase which crystallized at around 917 °C. Moreover, it has a hardness of 6.81 ± 0.23 GPa, fracture toughness of 1.61 ± 0.18 MPa m1/2, as well as low thermal conductivity (0.82–1.37 W/m·K from RT-1000 °C) and an average coefficient of thermal expansion (CTE) (∼6.3 × 10−6 K−1 from RT to 660 °C). In addition, the thermal shock and water-vapor corrosion behaviors of the Yb3Al5O12-EBC systems on the SiCf/SiC substrates were investigated and their failure mechanisms were analyzed in details. The Yb3Al5O12 coating has an average thermal shock lifetime of 72 ± 10 cycles as well as an excellent resistance to steam. These combined properties indicated that the Yb3Al5O12 coating might be a potential EBC material. Both the thermal shock failure and the steam recession of the Yb3Al5O12-EBC systems are primarily associated with the CTE mismatch stress.

Microstructure and water corrosion behavior of (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7 high-entropy rare-earth disilicate coating for SiC coated C/C composites

In order to make carbon/carbon composites suitable for application in gas turbine engine, it is necessary to develop environmental barrier coatings (EBCs) to protect them from reacting with water vapor. In our previous work, a novel high-entropy rare-earth disilicate (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7 ((5RE0.2)2Si2O7) has been developed and verified as a promising candidate for EBCs. In this work, the (5RE0.2)2Si2O7 coating was synthesized on the surface of SiC coated C/C composites by supersonic atmospheric plasma spraying method. The protective performance and mechanism of this coating under high temperature water vapor environment was explored in detail. Results showed that the weight change of the sample coated with (5RE0.2)2Si2O7 was only 0.2% after corrosion for 100 h at 1500 ºC, which proved that (5RE0.2)2Si2O7 coating could significantly improve the resistance of C/C composites against water vapor corrosion. This work may provide theoretical basis for the design and application of high-entropy rare-earth silicates as EBCs.

Water vapor corrosion behavior and failure mechanism of air sprayed bi-layer Yb2Si2O7/SiC and tri-layer Yb2Si2O7/ (SiCw-Mullite)/SiC environmental barrier coating

To improve the corrosion resistance of Cf/SiC composites in a water vapor oxygen coupled environment, the bi-layer Yb2Si2O7/SiC and tri-layer Yb2Si2O7/(SiCw-Mullite, SM)/SiC environment barrier coatings were designed and prepared on the surface of Cf/SiC composites by chemical vapor deposition and sol-gel method united with air spraying. Results show that the mass loss of bi-layer Yb2Si2O7/SiC coating coated samples was 17.24 ​× ​10−3 ​g ​cm−2 after 200 ​h oxidation at 1673 ​K, and the average compression strength retention rate was only 79.44% due to the formation of wide penetrating cracks. Comparatively, SiC whiskers in SM middle coating can not only increase the crack propagation resistance of mullite middle coating, but also alleviate the difference of coefficient of thermal expansion between Yb2Si2O7 outer coating and mullite middle coating. Therefore, the mass loss of tri-layer Yb2Si2O7/SM/SiC coating coated samples was only 2.93 ​× ​10−3 ​g ​cm−2 after 200 ​h oxidation at 1673 ​K, and the average compression strength retention rate was up to 98.79%.

Corrosion behavior of Yb4Hf3O12 ceramics exposed to Calcium-Ferrum-Alumina-Silicate (CFAS) coupled with water vapor at 1400 °C

In this work, the corrosion behavior of Yb4Hf3O12 ceramics under CFAS coupled with water vapor environment at 1400 °C for 2–50 h was investigated. Results show that the CFAS were rapidly dissolved into ceramics under test environment, and then the Yb-Ca-Si apatite and Hf-rich fluorite phases were re-precipitated. Simultaneously, the water vapor corrosion mainly induced grain surface corrosion, resulting in the formation of corrosion pits and ridge-like structures. Moreover, the pores and microcracks caused by water vapor corrosion offer additional penetration path for CFAS melt.

Preparation and water-vapour corrosion behaviour of BSAS environmental barrier coatings fabricated on ceramic matrix composites

Environmental barrier coatings (EBCs) are commonly utilized to protect ceramic matrix composite components from water vapour attacks in commercial gas turbine engines. In this work, Si/3Al2O3·2SiO2 (Mullite)/BaO-SrO-Al2O3-SiO2 (BSAS) tri-layered EBCs were fabricated on SiC-based ceramic matrix composites (CMC) substrate via atmospheric plasma spraying technique. The phase composition, microstructure, and bond strength of the environmental barrier coatings were studied by scanning electron microscope, energy dispersive spectrometer, X-ray diffraction analyzer, and mechanical test machine. The water vapour resistance of the BSAS coating and CMC was evaluated with a self-assembled water-vapour corrosion test device. The results indicate that the as-sprayed BSAS coating with a porosity of (5.9 ± 0.8) % mainly comprises the monoclinic celsian BSAS phase, hexacelsian BSAS phase, and SiO2 phase. The consumption velocity of the BSAS coating is about 12 % that of the naked CMC material in a 1300 °C water vapour environment. After corrosion for 300 h, the average flexural strength of naked samples was (481 ± 9) MPa, which was about 64 % of the coated ones. In addition, the bond strength of the as-sprayed coating is (18.7 ± 1.6) MPa. Even after 300 h of water vapour corrosion, the bond strength of the coating does not decay. Evidently, BSAS coating can protect CMC materials from water vapour corrosion at 1300 °C.

Preparation and water vapor corrosion behavior of AlN polytype bonded SiC bricks

SiC bricks were adopted as lining materials in waste incinerator because of their good slag resistance and mechanical performance. However, the process of waste treatment by an incinerator will produce H2O (g), which can severely corrode the SiC bricks and form Si(OH)4 (g) at high temperature. Due to extremely poor sinterability of SiC, the bonding phases have crucial influence on properties and must be generated in SiC based products during preparation. In this article, the Al-Si alloy was introduced into SiC bricks to form in-situ rod-shaped AlN polytype bonding phase under coke embedding atmosphere at 1700 ℃, which contributed to decrease apparent porosity, improve mechanical properties and water vapor corrosion resistance. The AlN polytype bonded SiC bricks performed good water vapor corrosion resistance because the mullite formed after vapor corrosion. The vapor corroded SiC bricks witnessed an increase of COMR after thermal shock test from 1.82 MPa to 28.46 MPa with the additive amounts of Al-Si alloy reaching to 12 wt %.

A SiC nanowires/Ba0.75Sr0.25Al2Si2O8 ceramic heterojunction for stable electromagnetic absorption under variable-temperature

The application of semiconductor materials was limited as electromagnetic absorption materials, due to the unstable absorption performance caused by the temperature sensitivity. In this work, a structurally controllable ceramic heterojunction was developed by assembling structural units of SiC nanowires (SiCnws) and Ba 0.75 Sr 0.25 Al 2 Si 2 O 8 (BSAS). Benefiting from the optimization of the energy gap and the construction of heterogeneous interfaces, the ceramic heterojunction can achieve stable electromagnetic absorption from room temperature to 600 °C, the effective absorption bandwidth is almost unchanged. And the minimum reflection loss value reached -63.6 dB at 600 °C. Meanwhile, the SiCnws/BSAS ceramic heterojunction shows stable electromagnetic absorption performance in various simulated outdoor environments, including acid rain, seawater, high temperature and water vapor corrosion. This result provides a fire-new idea to realize the environmental adaptability of electromagnetic absorption materials and largely broadens its application prospect.

Cyclic oxidation and water vapor corrosion behaviors of multi-layer environmental barrier coatings with HfO2-Si bond coat for SiCf/SiC

A novel multi-layer HfO2-Si/YbSi1.5/Yb2SiO5/LaMgAl11O19 environmental barrier coatings (EBCs) was prepared using atmospheric plasma spraying method to restrain spallation failure and improve lifetimes of coating. The thermal cycling behaviors of SiCf/SiC completely covered with multi-layer EBCs were comparatively investigated at 1300 °C and 1350 °C in air. Meanwhile the water vapor corrosion behavior of coated sample was also investigated at 1300 °C for 100 h in 90% H2O-10% O2 steam. The coating at 1300 °C showed better thermal shock resistance than the coating at 1350 °C, due to the increase of tensile stresses at 1350 °C. The coating showed different forms of crack penetration due to the influence of thermal cycling temperature and water vapor. The formation of HfSiO4 phase thanks to solid-state reaction between HfO2 and SiO2 alleviate the β→α-cristobalite phase change and avoid the crack initiation. However, the thermal mismatch stress was contributed to the spallation of coating along the surface of the SiCf/SiC substrate.

(Ho0.25Lu0.25Yb0.25Eu0.25)2SiO5 high-entropy ceramic with low thermal conductivity, tunable thermal expansion coefficient, and excellent resistance to CMAS corrosion

Low thermal conductivity, compatible thermal expansion coefficient, and good calcium—magnesium—aluminosilicate (CMAS) corrosion resistance are critical requirements of environmental barrier coatings for silicon-based ceramics. Rare earth silicates have been recognized as one of the most promising environmental barrier coating candidates for good water vapor corrosion resistance. However, the relatively high thermal conductivity and high thermal expansion coefficient limit the practical application. Inspired by the high entropy effect, a novel rare earth monosilicate solid solution (Ho0.25Lu0.25Yb0.25Eu0.25)2SiO5 was designed to improve the overall performance. The as-synthesized (Ho0.25Lu0.25Yb0.25Eu0.25)2SiO5 shows very low thermal conductivity (1.07 W·m−1·K−1 at 600 °C). Point defects including mass mismatch and oxygen vacancies mainly contribute to the good thermal insulation properties. The thermal expansion coefficient of (Ho0.25Lu0.25Yb0.25Eu0.25)2SiO5 can be decreased to (4.0–5.9)×10−6 K−1 due to severe lattice distortion and chemical bonding variation, which matches well with that of SiC ((4.5–5.5)×10−6 K−1). In addition, (Ho0.25Lu0.25Yb0.25Eu0.25)2SiO5 presents good resistance to CMAS corrosion. The improved performance of (Ho0.25Lu0.25Yb0.25Eu0.25)2SiO5 highlights it as a promising environmental barrier coating candidate.

Improved water vapor resistance of environmental barrier coatings densified by aluminum infiltration

A dense environmental barrier coating (EBC) was designed via aluminum infiltration into open pores or cracks to eliminate premature coating failure induced by the rapid permeation of oxidants through these pores. The water vapor corrosion performance of the EBC with and without aluminum infiltration was investigated at 1350 °C for 300 h under flowing 50%H2O–50%O2 gas. The infiltrated coating exhibited decrease silicon loss by more than 80%, almost no phase decomposition, and no continuous thermally grown oxide (TGO), furthermore, TGO thickness at the crack-root decreased by ∼90%. The excellent resistance is primarily attributed to the channel pores being filled with aluminum, which eliminates the rapid permeation of oxidants. In addition, a continuous refractory ytterbium aluminum garnet (YbAG) layer was formed in situ on the EBC surface, this layer exhibited sufficient stability in steam, rendering the EBC less permeable to oxidants.

Phase composition and property evolution of (Yb1−xHox)2Si2O7 solid solution as environmental/thermal barrier coating candidates

RE disilicates are good candidates as environmental/thermal barrier coating for SiCf/SiC composite in harsh gas turbine engines. We designed (Yb1−xHox)2Si2O7 solid solutions and studied mechanical properties, thermal properties, and water vapor resistance. Powders with different compositions were synthesized by pressureless sintering, and bulk samples were prepared by Spark Plasma Sintering (SPS). Polymorphic changes with temperature and composition of the solid solutions were examined. Through doping Ho into Yb2Si2O7, water vapor corrosion resistance is significantly promoted, and thermal expansion coefficient is maintained close to that of Si-based ceramics. Compared with host disilicates, thermal conductivity of solid solutions are decreased, and mechanical properties, including Vickers hardness and fracture toughness, are increased. A two-phase domain is found at (Yb1/2Ho1/2)2Si2O7, and the γ to δ phase transition of Ho2Si2O7 is observed during SPS. Among all samples, γ-(Yb1/3Ho2/3)2Si2O7 possesses superior high temperature stability, and excellent water vapor resistance, indicating its performance as environmental/thermal barrier coating.

A DFT-Based Descriptor to Predict the Water Vapor Corrosion Resistance of Rare-Earth Monosilicates.

Rare-earth monosilicates are used as environmental barrier coatings (EBCs) due to their excellent water vapor corrosion resistance. However, existing experimental studies on the water vapor corrosion behavior of rare-earth monosilicates are discrepant and even contradictory. Previous theoretical investigations on water vapor corrosion resistance mainly focus on a Mulliken analysis of Si-O bonds in the monosilicates. In this study, the structural and electronic properties of rare-earth monosilicates have been studied by density functional theory (DFT) calculations, and a descriptor correlated to the corrosion resistance has been developed. The maximum isosurface value of the valence band maximum (VBMFmax) can be used to predict the water vapor corrosion resistance of RE2SiO5. The results show that RE2SiO5 with a smaller VBMFmax may have better water vapor corrosion resistance.

Water vapor corrosion behaviors of high-entropy pyrosilicates

In this study, the water vapor corrosion resistance of two types of high-entropy pyrosilicates ((Yb0.2Y0.2Lu0.2Ho0.2Er0.2)2Si2O7 ((5RE1/5)2Si2O7) and (Yb0.25Lu0.25Ho0.25Er0.25)2Si2O7 ((4RE1/4)2Si2O7)) and two single-component pyrosilicates (Yb2Si2O7 and Lu2Si2O7) were evaluated at 1350 °C for 50–100 h, and the initial corrosion behaviors of these pyrosilicates were studied. The results showed that the final corrosion products of the four types of pyrosilicates were all X2-type monosilicates, exhibiting similar corrosion phenomena. However, (4RE1/4)2Si2O7 generated many nanoscale monosilicate grains during corrosion. The corrosion resistance of Lu2Si2O7 was clearly better than those of the others, and (4RE1/4)2Si2O7 exhibited the worst corrosion resistance. The corrosion mechanism of the pyrosilicate blocks was analyzed from the perspectives of grain size, bulk hydrophobicity, and binding energy. This study potentially provides a theoretical basis for the preparation of high-entropy pyrosilicates with different atomic ratios according to the different properties of the various rare earth elements.

Static water vapor corrosion behavior of MoSi2/Mullite composite coating on Nb based alloy at 1500 °C

Water vapor corrosion behavior of MoSi2/Mullite composite coating on Nb based alloy at 1500 °C were investigated compared with its static dry-oxygen oxidation behavior. The oxidation parabolic rate of samples in water vapor environment was − 0.39 mg/(cm2 h1/2), ~ 4 times higher than that in dry oxygen. Water vapor accelerated the penetration of O2 and resulted in second Mo (sp) and MoO2 (s), causing stronger volatilization of gaseous species out of the aluminosilicate scale than that in dry oxygen. Moreover, the crystallization of SiO2 in the scale was inhibited which accelerated the corrosion degradation of coating in water vapor environment at 1500 °C.

High-entropy rare-earth disilicate (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7: A potential environmental barrier coating material

A new high-entropy ceramic (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7 ((5RE0.2)2Si2O7) was proposed as a potential environmental barrier coating (EBC) material for ceramics matrix composites in this work. Experimental results showed that the (5RE0.2)2Si2O7 synthesized by solid-phase sintering was a monoclinic solid solution and had good phase stability proved by no obvious absorption/exothermic peak in the DSC curve from room temperature to 1400 °C. It performed a lower coefficient of thermal expansion (2.08 ×10−6-4.03 ×10−6 °C−1) and thermal conductivity (1.76–2.99 W•m−1•°C−1) compared with the five single principal RE2Si2O7. In water vapor corrosion tests, (5RE0.2)2Si2O7 also exhibited better water vapor corrosion resistance attributed to the multiple doping effects. The weight loss was only 3.1831 × 10−5 g•cm−2 after 200 h corrosion at 1500 °C, which was lower than that of each single principal RE2Si2O7. Therefore, (5RE0.2)2Si2O7 could be regarded as a remarkable candidate for EBCs.

Water vapor corrosion resistance and failure mechanism of SiCf/SiC composites completely coated with plasma sprayed tri-layer EBCs

In this work, the corrosion behaviors of as-plasma sprayed Yb2SiO5 and LaMgAl11O19 coatings under an environment with a volumetric ratio of 90% H2O-10% O2 were investigated at 1300 °C. The results showed that Yb2SiO5 and LaMgAl11O19 coatings had good water vapor corrosion resistance. Therefore, Si/Yb2SiO5/LaMgAl11O19 tri-layer EBCs were designed and prepared to completely cover the surface of SiCf/SiC by using atmospheric plasma spraying (APS). The water vapor corrosion behaviors of SiCf/SiC with and without EBCs at 1300 °C were investigated. The results showed that tri-layer coating can provide better corrosion resistance for SiCf/SiC at a high-temperature steam environment at 1300 °C. By comparing the water vapor corrosion behaviors of single-layer coatings and the tri-layer EBCs under same corrosion environment, it was found that the thermal mismatch stress and the formation of TGO is the main causes of tri-layer coating failure.

Mechanical-Chemical Coupling Effects on an Environmental Barrier Coating System under High-Temperature Water Vapour Conditions.

The degradation mechanisms for environmental barrier coatings (EBCs) under high-temperature water vapour conditions are vital for the service of aero-engine blades. This study proposes a theoretical model of high-temperature water vapour corrosion coupled with deformation, mass diffusion and chemical reaction based on the continuum thermodynamics and the actual water vapour corrosion mechanisms of an EBC system. The theoretical model is suitable for solving the stress and strain fields, water vapour concentration distribution and coating corrosion degree of an EBC system during the water vapour corrosion process. The results show that the thickness of the corrosion zone on the top of the EBC system depended on water vapour diffusion, which had the greatest influence on the corrosion process. The top corroded area of the rare-earth silicate EBC system was significantly evident, and there was a clear dividing line between the un-corroded and corroded regions.