SiC-based Ceramic Matrix Composites Research Articles

Enhanced steam thermal cycle resistance of Yb2Si2O7 environmental barrier coatings via Al modification

SiC-based ceramic matrix composites are susceptible to performance losses in steam environments, necessitating the use of environmental barrier coatings for their protection. The high Si activity of Yb2Si2O7 material may induce the emergence of Si(OH)4 in steam environments, resulting in the development of a pore structure within Yb2Si2O7. This pore structure weakens the service life of the coating, highlighting the importance of enhancing the steam corrosion resistance of environmental barrier coatings as a key factor in improving their overall longevity. In the present study, we employed Al modification technique to improve the steam corrosion resistance of Yb2Si2O7 coatings. The results indicated that Al-modified Yb2Si2O7 coatings exhibited superior steam thermal cycle corrosion resistance compared to the non-modified Yb2Si2O7 coatings after testing performed at 1523 K for 1000 h (10 h per cycle). This improvement was attributed to the presence of Yb3Al5O12 and Al2O3 composite phases within the Al-modified Yb2Si2O7 coatings, which exhibited low steam activity and effective steam corrosion resistance. Moreover, Al-modified Yb2Si2O7 coatings obtained the dual excellent performance of improving the steam corrosion resistance of Yb2Si2O7 coating and not weakening the thermal cycle ability, providing a guarantee for the longer life of Yb2Si2O7 coating.

Investigation on the Thermal–Mechanical Properties of YbRESiO5 (RE = Yb, Eu, Gd, Ho, Tm, Lu, Y, Sc): First-Principles Calculations and Thermal Performance Experiments

Environmental barrier coatings are critically needed in the future to safeguard SiC-based ceramic matrix composites (SiC CMCs) used in gas turbines. Element doping of rare earth monosilicates could further improve the properties of the coating. The crystal structure, elastic properties, and resistance to water vapor corrosion of Yb2SiO5 and YbRESiO5 (where RE = Sc, Y, Eu, Gd, Ho, Tm, Lu) were examined in this study using first-principles calculations. When RE is Yb, the material is Yb2SiO5. Based on the outcomes of the calculation, we prepared YbRESiO5 (RE = Sc, Yb, Eu) and studied the thermodynamic properties. The findings show that YbReSiO5’s resistance to water vapor corrosion is as follows: YbLuSiO5 < YbEuSiO5 < YbGdSiO5 < YbYSiO5 < Yb2SiO5 < YbTmSiO5 < YbHoSiO5 < YbScSiO5. YbScSiO5 has a lower unit cell volume, average Re-O bond length, and thermal expansion coefficient than Yb2SiO5, while YbEuSiO5 has the reverse pattern. Moreover, of the eight materials, YbScSiO5 has the greatest elastic modulus and lattice distortion. After doping with Eu, YbEuSiO5 exhibits a decrease in thermal conductivity by nearly thirty percent compared to Yb2SiO5, due to the formation of oxygen vacancies. The development of environmental barrier coating materials may benefit from these discoveries.

Anti-oxidation properties and phase composition evolution of the MoSi2-HfO2 duplex bond coat in air and steam/oxygen atmosphere at 1450 °C

Environmental Barrier Coatings (EBCs) can prevent SiC-based ceramic matrix composites from corrosion at high temperatures. However, the low melting point of conventional Si bond layers limits the operating temperature of EBCs to 1350 °C. Herein, a MoSi2-HfO2 duplex bond coat was designed and showed robust adhesion on SiC substrate after corrosion at 1450 °C for 100 h in steam/oxygen atmosphere. A mixed oxide layer was observed to in-situ form at the interface between the MoSi2 and HfO2 layers, containing SiO2, Hf-Si-O glass and HfSiO4. This oxide layer was proven to effectively inhibit further oxidation/corrosion of the coating and the substrate. The phase composition evolution and anti-oxidation mechanism were systematically analyzed.

Advanced Thermal/Environmental Barrier Coatings of High-Entropy Rare Earth Disilicates Tuned by Strong Anharmonicity of Eu2Si2O7

Advancing thermal/environmental barrier coating (TEBC) materials with integrated thermal-mechanical functions is paramount for safeguarding SiC-based ceramic matrix composites (CMCs) in high-efficiency gas turbines. Herein, we employ a synergistic approach, combining...

Design rules for the thermal and elastic properties of rare-earth disilicates

Rare-earth silicates are the current standard material for use as environmental barrier coatings for SiC-based ceramic matrix composites as hot-section components in gas-turbine engines. Expanding the design space to all available rare-earth elements to facilitate optimizing functionality requires an understanding of systematic trends in RE2Si2O7 properties. In this work, we combine first-principles calculations with experimental measurements of Young’s modulus, coefficient of thermal expansion, and thermal conductivity for a range of different RE2Si2O7 compositions and phases. Clear trends are observed in these properties as a function of the radius of the rare-earth cation. In the case of Young’s modulus and thermal expansion, these trends also hold for multi-component systems; while the thermal conductivity of multi-component systems is noticeably lower, indicating the potential of such materials to also act as thermal barriers. These results provide design rules for developing new thermal and environmental barrier coatings with stiffness and thermal expansion engineered to match that of the substrate, while simultaneously having reduced thermal conductivity.

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.

Near-seamless joining of Cf/SiC composites using Y3Si2C2 via electric field-assisted sintering technique

A novel Y3Si2C2 material was synthesized at a relatively low temperature (900 °C) using a molten salt method for the first time, and subsequently used as the joining material for carbon fiber reinforced SiC (Cf/SiC) composites. The sound near-seamless joints with no obvious remaining interlayer were obtained at 1600 °C using an electric field-assisted sintering technique (FAST). During joining, a liquid phase was formed by the eutectic reaction among Y3Si2C2, γ(Y—C) phase, and SiC, followed by the precipitation of SiC particles. The presence of the liquid promoted the sintering of newly formed SiC particles, leading to their complete consolidation with the Cf/SiC matrix. On the other hand, the excess of the liquid was pushed away from the joining area under the effect of a uniaxial pressure of 30 MPa, leading to the formation of the near-seamless joints. The highest shear strength (τ) of 17.2±2.9 MPa was obtained after being joined at 1600 °C for 10 min. The failure of the joints occurred in the Cf/SiC matrix, indicating that the interface was stronger than that of the Cf/SiC matrix. The formation of a near-seamless joint minimizes the mismatch of thermal expansion coefficients and also irradiation-induced swelling, suggesting that the proposed joining strategy can be potentially applied to SiC-based ceramic matrix composites (CMCs) for extreme environmental applications.

Process optimization for Hafnia‐doped silicon bond coats fabricated by plasma spraying for SiC‐CMC

HfO2-doped Si bond layer is expected to work at higher temperature with better oxidation resistance of environmental barrier coatings for SiC-based ceramic matrix composites. Establishing the relationship between process parameters and powder feeders with the property of coatings could help to further optimizing their performance. Three sets of plasma spray processes and two kinds of HfO2 powders were used to fabricate Si-HfO2 bond coats with different Si/HfO2 ratio. Particle size of HfO2 greatly affect the composition of coat, where smaller one induced uncontrollable Si/HfO2 ratio inside coatings. Larger HfO2 powder greatly improved the composition uniformity with a Si/HfO2 ratio similar to the starting feeder. However, phase segregation occurred for both cases, indicating the limit of mechanical mixing method to make composite feedstock. More strategies should be explored for precise control the phase and composition of composite coatings from spray technique. This article is protected by copyright. All rights reserved

Thermal shock behavior of LaMgAl11O19/Yb2Si2O7/Si thermal/environmental barrier coatings with LaMgAl11O19-LiAlSiO4 transition layer

Thermal/environmental barrier coatings (T/EBCs) could be used to protect SiC-based ceramic matrix composites (CMCs) from chemical attack and lower the substrate temperature for improving their lifetime. To mitigate the mismatch of thermal expansion coefficient (CTE) between LaMgAl11O19 (LMA) TBC and Yb2Si2O7 (YbDS) EBC, a transition layer formed by positive thermal-expansion LMA and negative thermal-expansion LiAlSiO4 (LAS) was introduced. Thermo-mechanical performance of LMA-LAS composite coating was studied. LMA-LAS composite coating exhibited an intermediate CTE and an excellent fracture toughness, revealing that LMA-LAS coating was an acceptable transition layer. Thermal shock behavior of LMA/YbDS/Si without and with LMA-LAS transition layer were compared. The thermal cycling lifetime of T/EBC was improved by about 20% with LMA-LAS intermediate layer. The final failure of T/EBC coating system was mainly associated with thermal mismatch and thermally grown oxide (TGO) formation.

Effect of the HfO2/SiO2 pre-mixing ratio and heating temperature on the formation of HfSiO4 as a bond coat of environmental barrier coatings

As the operating temperature of advanced gas turbines typically exceeds 1400 °C, it has been required to replace conventional Si bond coat in environmental barrier coatings (EBCs) with materials possessing higher thermal stability. Since HfSiO4 has excellent thermal properties such as a high melting point, phase stability over 1400 °C, and CTE matches with that of the SiC-based ceramic matrix composites, it has attracted much attention as a next-generation bond coat material. In this study, HfSiO4 bond coat was successfully formed by atmospheric plasma spray with pre-mixed HfO2-SiO2 powders (molar ratios: 7:3 and 5:5) followed by heat treatment. Effect of molar ratios of the HfO2-SiO2 and post-heat treatment temperature (1375 and 1475 °C) on the formation of HfSiO4 were studied. An oxidation test of the HfSiO4 coating was carried out at 1475 °C with the conventional Si bond coat to verify whether the new bond coat was suitable for use in a thermal environment of 1400 °C or higher. From the results, the HfO2/SiO2 ratio of 5:5 was suitable for the formation of HfSiO4 than that of 7:3. After heat treatment at 1475 °C, the ratio of HfSiO4 phase was 84.35%. The higher content of HfSiO4 formed under 1475 °C, meaning the higher heat treatment temperature accelerated the HfSiO4 formation. In the oxidation test at 1475 °C, the new HfSiO4 bond coat showed no cracks and maintained its integrity, but the Si bond coat was oxidized and cracked severely. Therefore, it can be concluded that the new HfSiO4 bond coat formed from 5HfO2–5SiO2 coating is a potential candidate as a next-generation bond coat material in EBCs.

Failure mechanisms of (Gd0.9Yb0.1)2Zr2O7/Yb2SiO5/Si thermal/environmental barrier coatings during thermal exposure at 1300 °C/1400 °C

A novel tri-layer (Gd0.9Yb0.1)2Zr2O7/Yb2SiO5/Si (GYbZ/YbMS/Si) thermal and environmental barrier coatings (TEBCs) was first proposed for protecting SiC-based ceramic matrix composites (CMCs). Wherein, the GYbZ layer by plasma spray physical vapor deposition (PS-PVD) was quasi-columnar structured while the YbMS and the Si layers by atmospheric plasma spray (APS) were lamellar structured. The oxidation behavior and the failure mechanisms of the GYbZ/YbMS/Si TEBCs at 1300 °C/1400 °C are revealed. At 1300 °C, the mud-cracks penetrated through the GYbZ/YbMS layer and transversely deflected in the Si layer are responsible for the oxidation at YbMS/Si interface. When the temperature increased to 1400 °C, the propagation of mud-cracks, cavities, and TGO channel cracks occurred due to the sintering of GYbZ and the fast growth of cristobalite. Eventually, these defects caused delaminating failure at interface. Moreover, another de-bonding failure of the coating was observed resulting from the significant thickening of oxide scale at the edge region.

Dry air cyclic oxidation of mixed Y/Yb disilicate environmental barrier coatings and bare silica formers

Mixed Y and Yb disilicate coatings (Y/Yb)DS have been proposed as dual function thermal and environmental barrier coatings (EBCs) for protecting SiC-based ceramic matrix composites in gas-turbine environments. As an initial step, the 1350 °C dry air cyclic oxidation of atmospheric plasma sprayed (Y1.2/Yb0.8)DS and ytterbium disilicate/ytterbium monosilicate (YbDS/YbMS) EBCs deposited onto Si bond coatings was compared. As a baseline for evaluating EBC oxidant permeability, the dry air cyclic oxidation scale growth rates for bare silica formers (SiC, Si) were also measured and were consistently higher than rates previously measured after isothermal oxidation. Regarding Si bond coat oxidation rates underlying (Y/Yb)DS and YbDS/YbMS EBCs, the thinner silica scale formed under the thinner and denser (Y/Yb)DS coatings suggested a lower oxidant permeability than YbDS/YbMS. After 500 1-h cycles, the (Y/Yb)DS coating was comprised of only the β-polymorph disilicate and minor amounts of the X-2 phase monosilicate phase. Negligible differences in oxidation kinetics for (Y/Yb)DS coatings over the 90 – 240 µm thickness range were observed.

Constituent and Ply-Level Understanding of Electrical Resistance in Si-Containing SiC/SiC Composites

Abstract Electrical resistance (ER), also known as direct current potential drop, has been demonstrated as an enabling means to monitor damage evolution in SiC-based ceramic matrix composites (CMCs). For laminate composites, it has become apparent that the location and orientation of SiC fibers, free Si, and in some cases insertion of C rods can greatly affect the measured resistance. In addition, the nature of crack growth through the different plies which consist of different constituents will have different effects on the change in resistance. Therefore, both experimental and modeling approaches as to the resistance and change in resistance for different laminate architectures based on the nature of constituent content and orientation are needed to utilize and optimize ER as a health-monitoring technique. In this work, unidirectional and cross-ply laminate composites have been analyzed using a ply-based electrical model. Based on a ply-level circuit model, the change in resistance was modeled for damage development. It is believed that this can serve as a basis for tailoring the architecture/constituent content to create a “smarter” composite.

The Versatility of HVOF Burner Rig Testing for Ceramic Matrix Composite Evaluation

Effective testing of ceramic matrix composites (CMCs) and CMC/coating systems for high temperature, high stress, high velocity and/or severe oxidation/corrosion environments is a critical need in materials/coatings evaluation for extreme environments of hot section parts in jet engine and hypersonic applications. Most current technology can evaluate two or three of the extreme conditions for a given application; however, incorporating as many of the extreme thermo-mechanical-environmental factors is highly advantageous to understand combinatorial effects. A high velocity oxygen fuel (HVOF) burner rig offers an excellent platform to evaluate many of these extreme conditions. In this work, the following three different thermo-mechanical-environmental test conditions using an HVOF rig on SiC-based CMCs are highlighted: (1) fatigue at temperature for >Mach 1 velocity and high temperature compared to typical stagnant air test environment, (2) high temperature hard particle erosion at temperature for ≤Mach 1 conditions and (3) ~Mach 5 near-hypersonic velocity conditions at very high temperature exposure.

Thermal durability of ytterbium silicate environmental barrier coating prepared by suspension plasma spray

Owing to their superior high-temperature stability, SiC-based ceramic matrix composites are widely used in turbine engine components for launch vehicles and other applications in the aerospace industry. However, because of the deterioration that occurs in these materials in high-temperature wet operating environments, researchers have developed environmental barrier coating technologies to protect the substrate. In this study, a top coat was prepared using Yb2SiO5 and Yb2Si2O7 as the third-generation coating of a rare-earth silicate with high thermal, chemical, and mechanical stability in high-temperature environments above 1400 °C. A new bond coat was prepared, incorporating Si, whose thermal expansion coefficient is similar to that of the SiC substrate, and Hf, which is expected to significantly contribute to high-temperature stability. The coating was prepared by suspension plasma spraying rather than atmospheric plasma spraying to realize a fine coating particle structure; to densify the coating layers, secondary densification coatings were created using ytterbium (III) nitrate pentahydrate (Yb(NO3)3∙5H2O) and tetraethyl orthosilicate (C8H20O4Si). The prepared specimens were measured in terms of weight change and weight change ratio with respect to temperature under high-temperature flames (1400 ℃ and 1700 ℃). After the secondary densification coating, the microstructure of the specimen was observed to be denser and the surface layer of the coating exhibited crystallinity. For the tested samples, a decrease in weight was observed. The largest weight loss was observed under the 1700 ℃ flame.

Low thermal conductivity in high-entropy rare-earth pyrosilicate solid-solutions for thermal environmental barrier coatings

Dense thermal environmental barrier coatings (TEBCs) with low thermal conductivities are needed for the protection and insulation of SiC-based ceramic matrix composites (CMCs) in future gas-turbine engines. Here we demonstrate experimentally that the high-temperature thermal conductivity of dense β-Yb2Si2O7 pyrosilicate ceramic can be reduced progressively in β-(Y0.1Yb0.9)2Si2O7 and β-(Y0.5Yb0.5)2Si2O7 solid-solutions for TEBC application. We also demonstrate that by going to a dense five-component equi-atomic single-phase β-(Y0.2Yb0.2Sc0.2Gd0.2Lu0.2)2Si2O7 pyrosilicate high-entropy ceramic (HEC), the thermal conductivity can be reduced further to 1.52 W.m.K−1 (at 1000 °C). This is attributed to enhanced phonon scattering due to the increased mass-disorder with the incorporation of the additional RE3+ cations in the HEC. Although the high-entropy aspect and bonding-disorder do not appear to contribute significantly to the phase stability or the low thermal conductivity in such HECs, they are strong candidates for future TEBCs for CMCs.

High-strength SiC joints with a novel in-situ formed SiC/Al4SiC4 joining filler

The in-situ formed SiC/Al4SiC4 joining layer was used to join monolithic SiC using an electric field-assisted sintering technique. A multiphase powder of Al4C3/SiC/Al4SiC4 was used as the initial joining material to obtain the in-situ reaction layer of SiC/Al4SiC4 via the appropriate interface reactions. The bending strength as high as 240.5 ± 6.6 MPa was obtained for the sample joined at 1800 °C, which was higher than the strength of the un-joined SiC matrix. Sound joints were obtained when Al4C3 was completely transformed to Al4SiC4, and a fully dense SiC/Al4SiC4 joining layer was consolidated. The integration of the joining layer with the SiC matrix was improved by a high amount of liquid phase formed at the interface. The proposed SiC/Al4SiC4 joining layer, with good thermal matching with SiC matrix, shows a great potential to be applied as a joining material for SiC-based ceramic matrix composites.

An investigation on burner rig testing of environmental barrier coatings for aerospace applications

In this study, burner rig testing of Si/Yb2Si2O7 environmental barrier coating protected SiC-based ceramic matrix composites was conducted. Tests were performed at standard conditions as well as with liquid water injection to the flame. Furthermore, the influence of the impingement angle of the flame (45° vs. 90°) on water vapor corrosion was explored. Gas flow rates were adapted in each test to adjust 1250 °C at the sample surface. The comparison of test results showed that water injection advances the corrosion of the Yb2Si2O7 topcoat and the impingement angle affects the size and shape of the corroded area on the sample surface.

Crystal growth behavior and sintering-resistance property of nano-sized β-Yb2Si2O7 for environmental barrier coatings

In this article, as an attractive candidate of EBCs for SiC-based ceramic matrix composites, nano-sized β-Yb2Si2O7 was synthesized by hydrothermal method. The sintering-resistance property and crystal growth behavior of β-Yb2Si2O7 powders and compacted bodies were evaluated via various techniques. Quantity analyses demonstrate that β-Yb2Si2O7 was formed at about 1100 °C, and possessed well thermal stability at even higher temperatures. The determined coefficient of thermal expansion and thermal conductivity are 4.63 × 10-6 K-1 and 1.72 W m-1 K-1, respectively. The microstructures of compacted bodies under different temperatures reveal the critical temperature of anti-sintering is about 1400 °C. Based on different growth mechanisms, the activation energy of crystal growth (Q) can be segmented into two sections with a turning point of 1076 °C. Furthermore, the relative relationship between activation energy and sintering was bridged.

Almost seamless joining of SiC using an in-situ reaction transition phase of Y3Si2C2

A new design of seamless joining was proposed to join SiC using electric field-assisted sintering technology. A 500 nm Y coating on SiC was used as the initial joining filler to obtain a desired transition phase of Y3Si2C2 layer via the appropriate interface reactions with the SiC matrix. The phase transformation and decomposition of the transition phase of Y3Si2C2 was designed to achieve almost seamless joining of SiC. The decomposition of the joining layer to SiC, followed up by the inter-diffusion and complete densification with the initial SiC matrix, resulted in the formation of an almost seamless joint at the temperature of 1900 °C. The bending strength of the seamless joint was 134.8 ± 2.1 MPa, which was comparable to the strength of the SiC matrix. The proposed design of seamless joining could potentially be applied for joining of SiC-based ceramic matrix composites with RE3Si2C2 as the joining layer.