Environmental Barrier Coatings Research Articles

Compositional design and phase formation capability of high-entropy rare-earth disilicates from machine learning and decision fusion

A key strategy for designing environmental barrier coatings is to incorporate multiple rare-earth (RE) components into β- and γ-RE2Si2O7 to achieve multifunctional performance optimization. However, the polymorphic phase presents significant challenges for the design of multicomponent RE disilicates. Here, employing decision fusion, a machine learning (ML) method is crafted to identify multicomponent RE disilicates, showcasing notable accuracy in prediction. The well-trained ML models evaluated the phase formation capability of 117 (RE10.25RE20.25Yb0.25Lu0.25)2Si2O7 and (RE11/6RE21/6RE31/6Gd1/6Yb1/6Lu1/6)2Si2O7, which are unreported in experiments and validated by first-principles calculations. Utilizing model visualization, essential factors governing the formation of (RE10.25RE20.25Yb0.25Lu0.25)2Si2O7 are pinpointed, including the average radius of RE3+ and variations in different RE3+ combinations. On the other hand, (RE11/6RE21/6RE31/6Gd1/6Yb1/6Lu1/6)2Si2O7 must take into account the average mass and the electronegativity deviation of RE3+. This work combines material-oriented ML methods with formation mechanisms of multicomponent RE disilicates, enabling the efficient design of superior materials with exceptional properties for the application of environmental barrier coatings.

Composition-driven superior CMAS corrosion resistance of high-entropy rare-earth disilicates

Achieving superior calcium-magnesium-aluminosilicate (CMAS) corrosion resistance in high-entropy rare-earth disilicates (HEREDs) is crucial to their applications as environmental barrier coatings in aerospace. Stimulated by the vast compositional field of HEREDs, we achieve exceptional CMAS corrosion resistance in HEREDs at elevated temperatures via a composition-driven strategy for the first time. Specifically, the equimolar 3- to 8-cation high-entropy rare-earth disilicate (3–8HERED) samples are first successfully designed and fabricated using a high-throughput pressure-less sintering technique and then their CMAS corrosion resistance up to 1773 K is evaluated. Among all the as-fabricated samples, the as-fabricated 7HERED samples are found to exhibit the best CMAS corrosion resistance. The outstanding CMAS corrosion resistance of the as-fabricated 7HERED samples is primarily attributed to pronounced sluggish diffusion induced by configurational entropy, with the synergistic effect of their chemical activity, which facilitates the formation of dense apatite protective layers. Our work provides valuable ways to design remarkable anti-CMAS corrosion high-entropy ceramics.

Nanostructured Yb2SiO5/mullite-SiC/Si environmental barrier coating with long-term stability at 1250 °C

In this work, nanostructured Yb2SiO5/mullite-SiC/Si environmental barrier coatings (EBCs) were prepared to investigate the effect of heat treatment on the microstructure and phase transition of nanostructured Yb2SiO5/mullite-SiC/Si coatings. The Yb2SiO5 content and microstructure evolution in the coatings were thoroughly analyzed before and after heat treatment. The results demonstrate a slight decrease in the porosities of nanostructured Yb2SiO5 and mullite-SiC coatings, accompanied by a gradual growth in grain sizes with increasing heat treatment time, indicating the nanostructured EBCs show good resistance to sintering. Moreover, the thickness of thermal growth oxidation layer is only 200 nm at 1250 °C for 500 h of heat treatment. In particular, the presence of the Yb2O3 phase has been observed to exert a favorable influence on the thermal stability of nanostructured EBCs. This work provides an innovative strategy for designing advanced EBCs with excellent high-temperature thermal stability.

Mechanism of stress evolution in environmental barrier coatings considering the presence of crack channels

The roles of crack channel transitions and thermally grown oxides (TGO) in the failure of environmental barrier coatings (EBCs) should be clarified. Hence, in this study, the laws of penetration and delamination crack evolution were investigated using a virtual crack extension method for multilayer materials to gain insights into the transformation conditions with a framework for a coupled thermal/mechanical/chemical numerical model of TGO containing channel cracks developed based on this law. The framework integrates diffusion, TGO growth, and phase transformation of the oxides. A theoretical model for high-temperature gas corrosion of EBCs coupled with deformation, mass diffusion, and chemical reactions was proposed, with the results indicating that the cracks transitioned from vertical to exfoliation cracks in the ytterbium dibasic silicate (YbDS) layer. The coupling effect of cracks and stresses led to non-uniform growth of the TGO by affecting the oxygen diffusion and inflow rates. The y-direction stresses at the YbDS/TGO interface near the crack channel changed from compressive to tensile stresses after cooling, making the first location to undergo delamination failure. The proposed model can provide better guidance for designing EBCs systems and supporting the life prediction of EBCs in service environments.

An investigation into the erosion and wear mechanisms observed in abradable ytterbium disilicate environmental barrier coatings

Abradable environmental barrier coatings (EBCs) are used to reduce clearances between blades and casings in the hottest parts of aero gas turbines, increasing the overall efficiency of the turbine, however, research into the mechanical performance of such coatings is limited. The aim of this work was to better understand the relationship between microstructure and erosion and wear performance in abradable EBCs, since publicly available literature regarding such coatings is scarce and the potential efficiency gains in gas turbines are significant. In this study, ytterbium disilicate abradable EBCs containing three different porosity levels (determined by fugitive polyester phase addition) were deposited using atmospheric plasma spraying with porosity levels of 8, 15 and 21.5% by area. These coatings were then characterised in numerous ways; the porosity was quantified, the thermal conductivity was measured, the superficial hardness and erosion resistance were measured, and finally, the coatings were subjected to a rig test designed to simulate in-service cutting mechanisms against a tipped turbine blade. The results show that increasing the level of porosity via increasing the amount of pore forming phase in the feedstock, led to reduced erosion resistance and improved cutting by a turbine blade.

Bimodal nanomechanical performance of nanostructured Lu2Si2O7 environmental barrier coating

Lutetium disilicate (Lu2Si2O7) is considered a promising material for environmental barrier coatings because of its remarkable high-temperature stability and low thermal conductivity. In this study, nanostructured Lu2Si2O7 coating was prepared successfully using as-prepared feedstocks via atmospheric plasma spraying. The nanomechanical performance of the coating was studied. The results showed that the coating exhibited a bimodal nanomechanical performance attributed to the bimodal distribution of its microstructure; i.e., in the coating, the melted lamella exhibited higher elastic modulus and nanohardness, whereas the unmelted and semi-melted parts processed low elastic modulus and nanohardness. The elastic modulus and nanohardness of the coating were 143.438 ± 25.449 GPa and 9.239 ± 3.514 GPa, respectively. Furthermore, the elastic and plastic works were 9.018–14.752 nJ and 12.792–25.793 nJ, respectively. The average value of the microhardness dissipation parameter of the coating was 0.615, demonstrated that the remarkable wear resistance of the coating.

Effect of calcium–magnesium–aluminum silicate composition on reaction behavior for X1‐Gd2SiO5 and X2‐Er2SiO5 environmental barrier coatings

AbstractRare‐earth (RE) silicates environmental barrier coatings (EBCs) are prone to accelerate failure resulted from molten calcium–magnesium–aluminum silicate (CMAS), which presents significant challenges to the design and development of durable EBCs. In this study, X1‐Gd2SiO5 and X2‐Er2SiO5 coatings were explored on their corrosion resistance to CMAS melts with different Ca/Si ratios (1.1, 0.75, 0.64, and 0.42) at 1350°C for 25 and 50 h, respectively. Results showed that with the Ca/Si ratio increasing, CMAS viscosity decreased and a more intense corrosion reaction with coatings occurred. Garnet phase was only observed in X2‐Er2SiO5 coating corroded by CMAS with high Ca/Si ratios, which could be beneficial for improving corrosion resistance. The X2‐Er2SiO5 phase, which is composed of [SO4] and [REO6]/[REO7] polyhedrons, exhibits better corrosion resistance due to its more stable crystal structure, lower reactivity with CMAS, and the radius difference between the RE ions and Ca2+.

Thermal shock resistance of a novel (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 environmental barrier coating

AbstractMulticomponent rare earth (RE) disilicates have been recognized as promising candidates for environmental barrier coatings (EBCs). In this work, a novel EBC system with (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 as top coat and Si as bond coat was deposited on SiC‐based substrates by atmospheric plasma spraying technology and subjected to 200 thermal shock cycles at 1350°C. The composition and microstructure evolution in the coating during thermal cycling were investigated using x‐ray diffraction analysis and scanning electron microscopy. The results show that the as‐prepared EBC remains dense and integrated, and no obvious spalling or penetrating cracks could be observed in the coating after 200 thermal shock cycles. Furthermore, the average thickness of thermal growth oxide layer between (6RE1/6)2Si2O7 top coat and Si bond coat is 0.71 ± 0.17 µm and 0.98 ± 0.23 µm before and after the thermal cycling, respectively, indicating the excellent thermal shock resistance of (6RE1/6)2Si2O7 EBC. The present study presents a novel EBC with excellent thermal shock ability and is expected to contribute to the development of EBC for advanced SiCf/SiC ceramic matrix composites.

Mechanical and Thermal Properties of the Hf–Si System: First-Principles Calculations

The relatively low melting point of a traditional Si bonding layer limits the upper servicing temperature of environmental barrier coatings (EBC). To explore suitable high temperature bonding layers and expedite the development of EBC, first-principles calculation was used to evaluate the mechanical properties and thermal conductivity of HfSi2, HfSi, Hf5Si4, Hf3Si2, and Hf2Si with much higher melting points than that of Si. Among them, HfSi2 has the lowest modulus capable of good modulus matching with SiC substrate. In addition, these Hf-Si compounds have much lower high temperature thermal conductivity with Hf2Si being the lowest of 0.63 W m−1 K−1, which is only half of Si, capable of improved heat insulation.

(Gd0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 and (Sc0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 high-entropy rare-earth disilicates as promising materials for environmental barrier coatings

The novel potential environmental barrier coating materials, two high-entropy rare earth disilicates (HEDs) (Gd0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 and (Sc0.2Ho0.2Er0.2Yb0.2Lu0.2)2Si2O7 were prepared by the solid-phase method. Both the HEDs showed remarkable high-temperature phase stability and an appropriate thermal expansion coefficient (CTE) that matched that of SiCf/SiC. In addition, both the HEDs possessed above 20% lower thermal conductivity at 1000 °C (1.37 and 1.42 W·m−1·K−1, respectively) compared to traditional single-component rare earth disilicate material YbDS, and they also exhibited higher mechanical properties and good resistance to water vapour corrosion at 1300 °C. Moreover, the corresponding free-standing coatings obtained after deposition by atmospheric plasma spraying (APS) showed excellent resistance to CMAS corrosion, primarily because of the formation of an apatite layer.

Isothermal Oxidation and Thermal Shock Resistance of Thick and Porous LaMgAl11O19 Abradable Topcoat

An exploration of the plasma-sprayed abradable sealing coatings (ASCs) of a thick and porous LaMgAl11O19 topcoat onto SiC/SiC ceramic matrix composites (CMCs) is detailed in this study. Interlayers comprising Si/Si + Yb2Si2O7/Yb2SiO5 environmental barrier coatings (EBCs) were strategically employed, considering their function in protecting the SiC/SiC CMCs from recession and mitigating thermal expansivity misfit. An isothermal oxidation test was conducted at 1300 °C and resulted in the formation of bubble and glassy melt on the side surface of the coated sample, while a significant reaction layer emerged at the Yb2SiO5/LaMgAl11O19 interface near the edge. The localized temperature rise caused by the exothermic oxidation of the SiC/SiC substrate was determined to be the underlying factor for bubble generation. The temperature-dependent viscosity of the melt contributed to various bubble characteristics, and due to the enrichment of Al ions, the glassy melt exacerbated the degradation of the Yb2SiO5 layer. After a thermal shock test at 1300 °C, the substrate on the uncoated backside of the sample experienced fracture, while the front coating remained intact. However, due to the presence of a through-coating crack, an internal crack network also developed within the substrate.

Thermal properties and calcium-magnesium-alumino-silicate (CMAS) interaction of novel γ-phase ytterbium-doped yttrium disilicate (γ-Y1.5Yb0.5Si2O7) environmental barrier coating material

Rare-earth disilicates are promising candidates for thermal and environmental barrier coatings (TEBC) in gas turbines that safeguard SiCf/SiC ceramic matrix composites (CMCs) from thermal degradation and environmental attacks. Here, we report a systematic investigation on novel TEBC material, γ-Y1.5Yb0.5Si2O7. The γ-phase quarter molar ytterbium–doped yttrium disilicate exhibited low thermal conductivity (1.72 W·m−1·K−1 at 1200 °C) and reduced intrinsic thermal expansion (3.17 ± 0.22 × 10−6 K−1 up to 1000 °C), ensuring promisingly effective thermal insulation and minimized thermal stress with CMC substrates. Using density functional theory (DFT), the heat capacity of γ-Y1.5Yb0.5Si2O7 was predicted higher than that of undoped γ-Y2Si2O7. Comparing these predictions to results calculated using the Neumann–Kopp (NK) rule revealed only minor variations. A metastable CMAS interaction byproduct, cyclosilicate phase Ca3RE2(Si3O9)2, was identified based on energy dispersive X-ray spectrometer (EDS) and electron backscatter diffraction (EBSD) techniques, appearing at 1300 °C but disappearing at 1400 °C. The γ-Y1.5Yb0.5Si2O7 exhibited good CMAS resistance on both dense pellets and sprayed coatings, forming a protective apatite (Ca2RE8(SiO4)6O2) interlayer that effectively hindered CMAS infiltration at evaluated temperatures. The relatively higher Y:Yb atomic ratio (> 3) in the apatite grains indicate differential reactivity with molten CMAS and provides crucial insights into the CMAS corrosion mechanism. These findings highlight the potential of γ-Y1.5Yb0.5Si2O7 as a CMC coating material, emphasizing the need for tailored microstructural optimization as a thermal sprayed coating to enhance long-term performance in extreme gas turbine environments.

Phase and properties evolution of plasma sprayed rare earth silicate coatings

Rare-earth silicates have been pursued for many years as candidates for Environmental Barrier Coatings for the protection of silicon carbide (SiC) based components in the new generation of aero and power production turbine engines. The properties and behavior of these materials are usually reported based on theoretical studies or measurements done on bulk specimens, but their prospective use is in the form of coatings. One of the most extended ways of depositing these coatings is plasma spray which produces partially or totally amorphous deposit and changes towards SiO2 lean chemical composition. In this work is comparatively discussed the effect of plasma spray deposition on Yb2Si2O7, Y2Si2O7 and the mixed cation silicate (Y0.5Yb0.5)2Si2O7 coatings. The phase evolution with temperature and its impact on the coefficient of thermal expansion, microstructure and thermal conductivity are described. The stability of crystalline phases, properties and absence of cracks in its microstructure makes (Y0.5Yb0.5)2Si2O7 a more optimum candidate among the studied rare-earth silicates for environmental barrier applications.

Holistic comparison of environmental barrier coating material candidates through design of a figure of merit

AbstractThe first figure of merit for environmental barrier coating (EBC) materials was designed through a ranking system for material properties pertaining to established EBC failure modes in service. Seven past, present, and novel EBC candidate materials were used in the figure of merit design: SiO2, Ba0.75Sr0.25Al2Si2O8 (BSAS), HfSiO4, Yb2Si2O7, Yb2SiO5, Yb2O3, and YbPO4. Utilizing compiled data from the literature, the presented figure of merit verified Yb2Si2O7 as the state‐of‐the‐art candidate with optimal EBC properties and inferred that YbPO4 should be considered as a potentially viable EBC material candidate. The figure of merit allows for a holistic comparison of EBC candidates and informs experimental and computational search efforts for next‐generation complex EBCs. Clear knowledge gaps found through this work include CaO–MgO–Al2O3–SiO2 (CMAS)‐resistant coatings, EBC lifetimes before delamination, and oxidant diffusion rates in relevant EBC microstructures. It was shown that while some materials show promise for solving a single key failure mode for EBCs (i.e., CMAS reactivity), a community‐wide goal should be placed on materials development to achieve acceptable resistance against all major failure modes, which are interconnected. Novel compositionally complex EBC materials, in addition to layered EBC architectures, show promise for the optimization of material properties for long‐lifetime EBCs in combustion environments.

Design, synthesis and characterization of multilayer environmental barrier coatings

The formation of cristobalite thermally grown oxides (TGO) is a major life limiting factor for environmental barrier coatings (EBCs) with Si bond coat exposed to high temperature steam environments due to its reversible phase transformation and the resultant cracking behaviour during thermal cycling. Therefore, controlling or avoiding cristobalite TGO formation by engineering coating chemistry and architecture will be desirable for a highly durable EBC system. In this work, a multilayer Si/Al-Si-N/Yb2Si2O7 EBC was produced using a hybrid vapor deposition process. The oxidation behavior of the EBC was evaluated in flowing 90% H2O (g) + 10% air at 1316 °C. After 100 h and 510 h steam test, crack-free mullite and Al-Si-O-N layers formed above Al-Si-N layer and no SiO2 scale formation of Si bond coat was observed. Detailed TEM analyses show that the Al-Si-N layer successfully prevented the oxidation of underlying Si bond coat and avoided the formation of detrimental cristobalite phase.

The Manufacturing of Environmental Barrier Coatings by HV-APS Plasma Spraying Using Er<sub>2</sub>O<sub>3</sub> and SiO<sub>2</sub> Powder Mixture

The Manufacturing of Environmental Barrier Coatings by HV-APS Plasma Spraying Using Er<sub>2</sub>O<sub>3</sub> and SiO<sub>2</sub> Powder Mixture

Investigation of Yb2SiO5/BSAS/Si coating on Cf/SiC composites: processing and steam corrosion behavior

Environmental Barrier Coatings (EBCs) are used to protect silicon carbide-based composites in the high-temperature steam combustion environment of the latest generation of high-efficiency gas turbine aero-engines. The Yb2SiO5/BSAS/Si coatings with gradient coefficient of thermal expansion were prepared using atmospheric plasma spraying (APS) method on the carbon fiber reinforced silicon carbide ceramic matrix composites (Cf/SiC) composites surface. The coating preparation process parameters were optimized by evaluating the influence of spraying power on the bonding strength, deposition efficiency, and porosity. The corrosion behaviors and mechanism of the optimized Yb2SiO5/BSAS/Si coating was investigated at 1200 ℃ by thermal cycle test in steam conditions. The results indicated that the decomposition of Yb2SiO5 and the conversion of SiO2 to Si(OH)4 were the main corrosion reactions of Yb2SiO5 top coating. Noticeably, the SiO2 component acted as an obvious sign of the reaction front diffusing, forming the clear SiO2-gradient layers. The gradient Yb2SiO5/BSAS/Si coating remained intact after thermal cycles for 500 h at 1200 ℃ in steam conditions which effectively enhanced the resistance of Cf/SiC composites to high temperature steam corrosion.

Reaction of Yb2SiO5 EBCs against CMAS melts with different composition

Ytterbium monosilicate (Yb2SiO5) has been recognized as environmental barrier coatings (EBCs) material with potential applications for SiCf/SiC. However, the corrosion caused by calcium-magnesium-alumino-silicate (CMAS) has posed significant challenges to the design and application of EBCs. In this study, reaction of Yb2SiO5 EBCs against CMAS melts with different Ca/Si ratios (1.1, 0.75, 0.64, and 0.42) at 1350 °C was investigated. Results showed that the components of CMAS have a significant impact on the corrosion behaviors of the Yb2SiO5 coating. The difference in Ca/Si ratio of CMAS changes the phase equilibrium during the reaction process. The CMAS melts with the highest Ca/Si ratios can form a combination of silico-carnotite (Ca3Yb2(SiO4)3), garnet and two kinds of apatite (Ca2Yb8(SiO4)6O2 and Ca4Yb6(SiO4)6O), whereas the CMAS melts with the lowest Ca/Si ratio is expected to form Yb2Si2O7 phase. With the Ca/Si ratio increasing, the viscosity of CMAS decreased and the aggressiveness of CMAS to Yb2SiO5 coating increased.

Influence of chemistry and surface roughness of various thermal barrier coatings on the wettability of molten sand

High temperature wettability experiments were carried out to investigate how chemistry and surface roughness affect wetting characteristics of molten sand several promising thermal/environmental barrier coatings. Contact angle experiments were performed on varying as-sprayed and polished yttria-stabilized zirconia (YSZ), rare-earth oxide-YSZ (REO-YSZ), and rare-earth silicate coatings subjected to molten synthetic sand at 1260 °C. The results showed that the spreading rate of molten sand was higher on silicate-based environmental barrier coatings (~60°/min) than on Zr-based thermal barrier coatings (~25°/min). Increased surface roughness had opposite effects on the wettability depending on the reactivity of the coatings. Wettability decreased on coatings such as YSZ (which did not react with sand) but increased for REO-YSZ coatings (which formed reaction products with sand). It was found that while the REO-YSZ coatings were able to promote formation of the apatite that performed as a sealing layer to reduce further infiltration, molten sand wetting increased. YSZ was found to have greater wetting resistance in the roughened state while the REO-YSZ coatings had greater wetting resistance in the polished state. Surface energy, microstructure, and molten sand penetration also play a key role in the wetting kinetics and are discussed. This work reveals the dependency of chemistry and surface roughness on influencing the “sandphobicity” (analogous to hydrophobicity) of a material and provides insights for optimizing these variables to realize more “sandphobic” coatings.

Design of an Environmental Barrier Coating Bond Strength Test Coupon

Abstract The bond strength of an environmental barrier coating (EBC) to a substrate such as a silicon carbide fiber/silicon carbide matrix (SiC/SiC) ceramic matrix composite (CMC) is a function of coating application parameters, matrix surface roughness, and environmental exposure, which results in the formation of a thermally grown oxide layer (TGO) layer. Current estimates of the EBC bond strength to a substrate are made assuming that the applied force divided by pull tab area is a representative metric. Although this is an expedient method for rapid strength estimates, the process of bonding the EBC to a pull-tab creates thermo-elastic residual stresses that are superimposed with the applied tensile load. Minimization of these stresses via test specimen design should give the most realistic estimate of EBC bond strength. We examine the residual stresses imposed on the EBC by cooling of an adhesive layer bonded to various metal and ceramic tabs. The results are extended to examine the stress distribution in a TGO layer formed on a Plasma Spray-Physical Vapor Deposited (PS-PVD) Yb2Si2O7 (ytterbium disilicate) EBC. An EBC/substrate coupon that overhangs a titanium or Kovar® pull tab is recommended in order to minimize thermal stresses, avoid edge effects, and provide an acreage strength measurement. The test method is sensitive to changes in substrate surface finish and oxidation time.