High temperature behavior of HfSiO4 subjected to calcium-magnesium-alumina-silicate (CMAS) and high-velocity water vapor

HfSiO₄ is considered as a candidate for environmental barrier coating (EBC), but there is a lack of comprehensive evaluation of its resistance against corrosive medium. We herein study the behavior of HfSiO₄ against CMAS melt and high-velocity water vapor. HfSiO₄ shows poor resistance to CMAS attack. Si diffusion occurs during CMAS attack, which leads to the formation of HfO₂ and CaSi₂O₅. HfSiO₄ decomposes to form SiO₂ and HfO₂ under the scouring of water vapor, in which SiO₂ forms volatile hydroxide and is taken away by high-velocity steam. HfSiO₄ is not the preferred system for surface layer of EBC system and is expected to be used as intermediate transition layer.     

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.     

Property evolutions of Si/mixed Yb2Si2O7 and Yb2SiO5 environmental barrier coatings completely wrapping up SiCf/SiC composites under 1300 °C water vapor corrosion

A bi-layer environmental barrier coating (EBC) consisting of silicon(Si) bond coat/mixed ytterbium disilicate (Yb₂Si₂O₇) and ytterbium monosilicate (Yb₂SiO₅) topcoats has been successfully prepared to completely wrap up the SiC_f/SiC composites and the protective effects of such EBC have been evaluated by soaking them in a mixed 50% O₂ and 50% H₂O corrosive gases at 1300 °C for various times. In topcoats, Yb₂Si₂O₇ is the major phase, providing good thermal expansion coefficient (CTE) matching with composite substrate and thus excellent thermal shock resistance, whereas Yb₂SiO₅ is the dispersing minor phase, providing improved water vapor corrosion resistance. The completely wrapping up of SiC_f/SiC composites by above EBC system is employed to avoid direct exposure to the corrosive conditions, making it possible to evaluate the genuine protection effects of current EBCs. Under 1300 °C water vapor corrosion, the mass change, the phase composition and the evolution of microstructure are investigated, which suggest that the bi-layer EBC has excellent performance on protecting SiC_f/SiC composites from water vapor corrosion at 1300 °C.     

Failure mechanisms in model thermal and environmental barrier coating systems

The durability of environmental barrier coating (EBC) systems in gas turbine engine environments depends upon their temperature dependent rates of degradation by processes such as steam volatilization and bond coat oxidation. While addition of a thermal barrier coating (TBC) reduces the temperature within the EBC system, it introduces new failure mechanisms. Deposition of a segmented HfO₂ TBC with a reduced in-plane Young’s modulus is essential to avoid bifurcated TBC channel cracking into a Yb₂Si₂O₇ EBC, and delamination, as a result of an approximately 50% difference in coefficients of thermal expansion (CTE) of the coating layers. During prolonged high temperature steam cycling, a thin fluorite phase reaction layer is observed to develop at the HfO₂-YbDS interface consistent with recent thermochemical assessments. The CTE of the fluorite phase is shown to be substantially higher than that of either of the layers to which it is bonded, resulting in tunnel cracking of the fluorite, and eventual coating delamination of the TBC at either the fluorite-HfO₂ or YbDS-fluorite interfaces upon cooling. The study highlights the importance of matching the CTEs of the TBC and EBC layers during coating system design, and those of the reaction products that may form between them.     

Interactions of hafnia/hafnon composites with molten silicate deposits

The article examines the chemical interactions between HfO₂/HfSiO₄ composites and melts that originate from siliceous debris ingested into gas turbine engines. Pellets with hafnon volume fractions of 50%, 70% and 100% were synthesized from powders of the pure components and exposed to two types of quinary siliceous deposits (one acidic and one basic) at 1400 °C for times ranging from 10 min to 4 h. Scanning and transmission electron microscopy examinations of reacted pellets show extensive melt penetration without evidence of an effective mitigating mechanism. Acidic melts preferentially react with hafnia to form hafnon while basic melts dissolve hafnon to form hafnia; in both cases, however, the melts penetrate extensively along grain and interphase boundaries. These processes are accompanied by swelling of the reaction layer followed by blistering and exfoliation of the affected coating material. The thermodynamics of the reactions, mechanisms of melt penetration, and implications for coating applications are discussed.     

Hafnium silicate formation during the reaction of β-cristobalite SiO2 and monoclinic HfO2 particles

Hafnium silicate (HfSiO₄; hafnon) is under consideration as an environmental barrier coating material for high-temperature applications. However, its rate of formation from mixtures of monoclinic HfO₂ and crystalline (β-cristobalite) SiO₂ powders is unknown. Here it has been synthesized and its formation rate measured during their solid-state reaction at temperatures from 1250°C to 1400°C. Rietveld refinement of X-ray diffraction patterns indicates that at 1250°C the hafnon phase fraction increases linearly with time, while at the highest reaction temperature, the hafnon phase fraction exhibited a parabolic dependence upon time. Between these two limiting temperatures, a region of linear behavior preceded a transition to parabolic kinetics, with the transition occurring at an earlier time as the reaction temperature increased. Arrhenius relations fitted the kinetics of hafnium silicate formation in both the linear and parabolic regimes. Scanning electron microscopy indicated that the reaction proceeded by diffusion of SiO₂ into HfO₂, similar to the mechanism by which zirconium silicate has been formed from vitreous SiO₂ and tetragonal ZrO₂. The initial linear rate of reaction is consistent with the growth of the contact area between the SiO₂ and HfO₂ particles combined with rapid permeation of the Si⁴⁺ and O²⁻ through the initial, incompletely formed hafnon. After a thin hafnon layer had formed between the reactants, the rate of hafnium silicate growth slowed and further growth was governed by the rate of diffusion of Si⁴⁺ and O²⁻ through the reaction product consistent with the observed parabolic dependence of the phase fraction upon time.     

High-temperature water-vapor reaction mechanism of barium strontium aluminosilicate (BSAS)

Environmental barrier coatings (EBCs) are used in commercial turbine engine applications as protection for ceramic matrix composites, yet the high-temperature water vapor reaction mechanism for EBC materials is not fully understood. Here, the water vapor reaction mechanism for barium strontium alumino-silicate (BSAS), an early generation EBC candidate, was determined from the time and temperature dependences of material loss. BSAS water vapor exposures were performed at 1200 °C, 1300 °C, and 1400 °C for 24, 48, and 72 h, at maximum gas velocities of ~ 240 m/s. FactSage thermodynamic calculations were shown to support the experimental findings, where the steam reaction mechanism consisted of volatilization of all BSAS oxide constituents as gaseous metal hydroxide species, i.e. Ba(OH)₂, Sr(OH)₂, Al(OH)₃, and Si(OH)₄ (g).     

Phase evolution of SiOC-based ceramic nanocomposites derived from a polymethylsiloxane modified by Hf- and Ti-alkoxides

SiOC/HfO₂-based ceramic nanocomposites with in situ formed HfO₂ nanoparticles were prepared via a single-source precursor (SSP) approach starting from a polymethylsilsesquioxane (PMS) modified by Hf- and Ti-alkoxides. By varying the alkyl-group of the employed Hf-alkoxides, SiOC/HfO₂-based ceramic nanocomposites with different HfO₂ polymorphs formed via thermal decomposition of the SSP under the same heat-treatment conditions. Using PMS chemically modified by Hf(OⁿBu)₄, tetragonal HfO₂ phase was formed after the synthesis at 1100°C in Ar, whereas both, tetragonal and monoclinic HfO₂ nanocrystals, were analyzed when replacing Hf(OⁿBu)₄ by Hf(OⁱPr)₄. After oxidation of the synthesized nanocomposites in air at 1500°C, a facile formation of oxidation-resistant HfSiO₄ (hafnon) phase occurred by the reaction of HfO₂ nanocrystals with silica present in the SiOC nanocomposite matrix derived from Hf(OⁱPr)₄-modified SSPs. Moreover the amount of hafnon is dramatically increased by the additional modification of the polysiloxane with Ti-alkoxides. In contrast, ceramic nanocomposites derived from Hf(OⁿBu)₄-modified SSPs, almost no HfSiO₄ is detected after oxidation at 1500°C even though in the case of Ti-alkoxide-modified single-source precursor.     

Thermochemical degradation of HfSiO4 by molten CMAS

The thermochemical degradation of hafnium silicate (HfSiO₄) was investigated with a molten calcium-magnesium-aluminosilicate (CMAS) glass relevant to gas turbine engine applications. Sintered HfSiO₄ coupons were fabricated, within which wells were drilled and filled with CMAS glass powder at a loading of ～35 mg/cm². Samples were heat treated at 1200°C, 1300°C, 1400°C, and 1500°C for 1 h, 10 h, and 50 h. At 1200°C and 1300°C, slow formation of a Ca₂HfSi₄O₁₂ cyclosilicate phase was observed at the HfSiO₄-CMAS interface. At 1300°C and higher, rapid infiltration of CMAS into the material along the grain boundaries was observed. Initial conjecture into CMAS degradation mechanisms of HfSiO₄ are presented herein.     

Exploration of YPO4 as a potential environmental barrier coating

A non-silicate material, yttrium phosphate (YPO₄), is developed for the application as environmental barrier coatings. The key issues of environmental durability, phase stability, chemical compatibility, and coefficient of thermal expansion (CTE) are considered for the selection of YPO₄. The water corrosion behaviors for the sol–gel prepared YPO₄ were investigated in an atmosphere of 50% H₂O–50% O₂ water vapor at 1350 °C. The hot corrosion study of YPO₄ was carried out at 900 °C with Na₂SO₄ melt. The results demonstrate that YPO₄ has excellent environmental durability. During these tests, YPO₄ had no phase change and decomposition. Moreover, the reactions between YPO₄ and silica at high temperatures were not detected, indicating the good chemical compatibility of YPO₄. The measured CTE of YPO₄ is close to that of SiC. The suitable CTE, good environmental durability and chemical compatibility, and excellent phase stability of YPO₄ indicate that it is a potential environmental barrier coating material.     

Resistance of pure and mixed rare earth silicates against calcium-magnesium-aluminosilicate (CMAS): A comparative study

Rare earth silicate environmental barrier coatings (EBCs) are state of the art for protecting SiC ceramic matrix composites (CMCs) against corrosive media. The interaction of four pure rare earth silicate EBC materials Yb₂SiO₅, Yb₂Si₂O₇, Y₂SiO₅, Y₂Si₂O₇ and three ytterbium silicate mixtures with molten calcium-magnesium-aluminosilicate (CMAS) were studied at high temperature (1400°C). The samples were characterized by SEM and XRD in order to evaluate the recession of the different materials after a reaction time of 8 hours. Additionally, the coefficient of thermal expansion (CTE) was determined to evaluate the suitability of Yb silicate mixtures as EBC materials for SiC CMCs. Results show that monosilicates exhibit a lower recession in contact with CMAS than their disilicate counterparts. The recession of the ytterbium silicates is far lower than the recession of the yttrium silicates under CMAS attack. Investigation of the ytterbium silicate mixtures exposes their superior resistance to CMAS, which is even higher than the resistance of the pure monosilicate. Also their decreased CTE suggests they will display better performance than the pure monosilicate.     

Thermal and Oxidation Response of UHTC Leading Edge Samples Exposed to Simulated Hypersonic Flight Conditions

Sharp leading edge (LE) samples of UHTC (20 vol%SiC–HfB₂) and SiC were exposed to simulated hypersonic flight conditions using a direct‐connect scramjet rig and their thermal and oxidation responses measured. The measured back‐wall temperatures and scale thicknesses were significantly smaller than might be expected from stagnation temperatures at the LE. Furthermore, the scale that formed around the LE was more uniform than expected from the steep drop in cold wall heat flux with distance from the tip. These results were interpreted and rationalized using physics‐based models. An aerothermal model in combination with an oxidation model accounted for the observed scale thicknesses at the tip and their slight variation with distance. The scale thicknesses were similar to values reported for exposures in furnaces at temperatures calculated for the tip, but less than those reported in arc jet tests. The formation of hafnon (HfSiO₄) and the absence of external glassy layer and of silica in the outer portions of the oxide region are unique to scramjet tested samples, presumably due to the high fluid flow (high shear and evaporation) rates.     

Environmental barrier coatings on enhanced roughness SiC: Effect of plasma spraying conditions on properties and performance

Environmental barrier coatings for SiC/SiC composites are limited by the melting temperature of the Si bond coating near 1414 °C. Systems without a bond coating may be required for future turbine applications where material temperatures go beyond 1350 °C. Enhanced roughness SiC substrates were developed to assess coating adhesion without the bond coating. Two EBCs with different YbMS/YbDS ratios were produced via modified plasma spraying parameters. Coating microstructure, thermal expansion, and modulus were measured for comparison of coating properties. Cyclic steam exposures at 1350 °C were performed to assess oxidation resistance. The EBC with increased concentration of Yb₂SiO₅ secondary phase displayed a higher CTE, which is typically expected to decrease adhesion lifetimes due to an increase in stress upon thermal cycling. Yet, the EBC chemistry with increased Yb₂SiO₅ concentration was able to experience longer cycling times prior to coating delamination, likely due to interface interactions with the substrate and the thermally grown oxide.     

Oxidation protection and mechanism of the HfB2-SiC-Si/SiC coatings modified by in-situ strengthening of SiC whiskers for C/C composites

To improve the oxidation resistance and alleviate the thermal stress of the HfB₂-SiC-Si/SiC coatings for C/C composites, in-situ formed SiC whiskers (SiC_w) were introduced into the HfB₂-SiC-Si/SiC coatings via chemical vapor deposition (CVD). Effects of SiC_w on isothermal oxidation and thermal shock resistance for the HfB₂-SiC-Si/SiC coatings were investigated. Results showed that the SiC_w-HfB₂-SiC-Si/SiC coatings exhibited excellent oxidation resistance for C/C composites with only 0.88% weight loss after oxidation for 468?h at 1500?°C, which was markedly superior to 4.86% weight loss for coatings without SiC_w. Meanwhile, after 50 times thermal cycling, the weight loss of the SiC_w-HfB₂-SiC-Si/SiC coated samples was 4.48%, which showed an obvious decrease compared with that of the HfB₂-SiC-Si/SiC coated samples. The SiC_w-HfB₂-SiC-Si/SiC coatings exhibited excellent adhesion to the C/C substrate and had no penetrating cracks after oxidation. The improved performance of the SiC_w-HfB₂-SiC-Si/SiC coatings could be ascribed to the SiC_w, which effectively relieved CTE mismatch and remarkably suppressed the cracks through toughening mechanisms including whiskers pull-out and bridging strengthening. The above results were confirmed by thermal analysis based on the finite element method, which demonstrated that SiC_w could effectively alleviate thermal stress generated by temperature variation. Furthermore, the SiC_w-HfB₂-SiC-Si/SiC coating can provide a promising fail-safe mechanism during the high temperature oxidation by the formation of HfSiO₄ and SiO₂, which can deflect cracks and heal imperfections.     

Ultra‐High‐Temperature Ceramic HfB2‐SiC Coating for Oxidation Protection of SiC‐Coated Carbon/Carbon Composites

To protect the carbon/carbon (C/C) composites from oxidation, an outer ultra‐high‐temperature ceramics (UHTCs) HfB₂‐SiC coating was prepared on SiC‐coated C/C composites by in situ reaction method. The outer HfB₂‐SiC coating consists of HfB₂ and SiC, which are synchronously obtained. During the heat treatment process, the formed fluid silicon melt is responsible for the preparation of the outer HfB₂‐SiC coating. The HfB₂‐SiC/SiC coating could protect the C/C from oxidation for 265 h with only 0.41 × 10^?2 g/cm² weight loss at 1773 K in air. During the oxidation process, SiO₂ glass and HfO₂ are generated. SiO₂ glass has a self‐sealing ability, which can cover the defects in the coating, thus blocking the penetration of oxygen and providing an effective protection for the C/C substrate. In addition, SiO₂ glass can react with the formed HfO₂, thus forming the HfSiO₄ phase. Owing to the “pinning effect” of HfSiO₄ phase, crack deflecting and crack termination are occurred, which will prevent the spread of cracks and effectively improve the oxidation resistance of the coating.     

Residual Stresses and Their Effects on the Durability of Environmental Barrier Coatings for SiC Ceramics

Qualitative residual stresses in current environmental barrier coatings (EBCs) were inferred from the curvature of EBC-coated SiC wafers, and the effects of EBC stresses on the durability of EBC-coated SiC were evaluated. The magnitude of substrate curvature correlated fairly well with the EBC–SiC coefficient of thermal expansion (CTE) mismatch, EBC modulus, and thermally induced physical changes in EBC. BSAS (1− x BaO· x SrO·Al₂O₃·2SiO₂, 0≤ x ≤1) components in the current EBCs, i.e., Si/mullite or mullite+BSAS/BSAS or yttria-stabilized zirconia (YSZ: ZrO₂–8 wt% Y₂O₃), were the most beneficial for reducing the EBC stress in as-sprayed as well as in post-exposure EBCs. The reduced stress was attributed to the low modulus of BSAS. The addition of a YSZ top coat significantly increased the substrate curvature because of its high CTE and sintering in thermal exposures. There were clear correlations between the wafer curvature and the EBC durability. The Si/mullite+20 wt% BSAS/BSAS EBC maintained excellent adherence, protecting the SiC substrate from oxidation, while the Si/mullite+20 wt% BSAS/YSZ EBC suffered delamination, leading to severe oxidation of the SiC substrate, after a 100 h −1300°C exposure in a high-pressure burner rig.     

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 (H_v), fracture toughness (K_IC), 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 m^1/2, 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 SiC_f/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.     

Effects of the chemistry of coating and substrate on the steam oxidation kinetics of environmental barrier coatings for ceramic matrix composites

Environmental barrier coatings (EBCs) are an enabler for SiC/SiC ceramic matrix composites (CMCs) in gas turbines by protecting CMCs from environmental degradation. A critical EBC failure mode is the EBC spallation due to a build-up of elastic strains caused by the formation of SiO₂ scale, known as TGO (thermally grown oxide). H₂O, a byproduct of combustion reactions, accelerates the TGO-induced EBC failure by increasing TGO growth rates by orders of magnitude. NASA’s approach to improve the EBC life, therefore, is to reduce TGO growth rates. NASA discovered that modifying the TGO chemistry by modifying the EBC chemistry of Gen 2 EBC (Si / Yb₂Si₂O₇) reduces the TGO thickness by up to ～80 %. A study was undertaken to understand the oxidation mechanism of modified Gen 2 EBCs as well as to investigate the effect of EBC and CMC chemistry on TGO growth rates. This study confirmed the previously proposed TGO-controlled oxidation mechanism of modified Gen 2 EBCs and determined the correlations between the EBC and CMC chemistry, TGO chemistry, and TGO growth rates.     

Steam oxidation of ytterbium disilicate environmental barrier coatings with and without a silicon bond coat

The current generation of multilayer Si/Yb₂Si₂O₇ environmental barrier coatings (EBCs) are temperature limited by the melting point of Si, 1414°C. To investigate higher temperature EBCs, the cyclic steam oxidation of EBCs comprised of a single layer of ytterbium disilicate (YbDS) was compared to multilayered Si/YbDS EBCs, both deposited on SiC substrates using atmospheric plasma spray. After 500 1-h cycles at 1300°C in 90 vol%H₂O-10 vol%air with a gas velocity of 1.5 cm/s, both multilayer Si/YbDS and single layer YbDS grew thinner silica scales than bare SiC, with the single layer YbDS forming the thinnest scale. Both coatings remained fully adherent and showed no signs of delamination. Silica scales formed on the single layer coating were significantly more homogeneous and possessed a markedly lower degree of cracking compared to the multilayered EBC. The single layer EBC also was exposed at 1425°C in steam with a gas velocity of 14 cm/s in an alumina reaction tube. The EBC reduced specimen mass loss compared to bare SiC but formed an extensive 2nd phase aluminosilicate reaction product. A similar reaction product was observed to form on some regions of the bare SiC specimen and appeared to partially inhibit silica volatilization. The 1425°C steam exposures were repeated with a SiC reaction tube and no 2nd phase reaction product was observed to form on the single layer EBC or bare SiC.     

Synthesis and characterization of ytterbium oxide: A novel CMAS-resistant environmental barrier coating material

Demand for more powerful aircraft promotes development of ceramic matrix composites and environmental barrier coating (EBC). A promising EBC material, ytterbium oxide (Yb₂O₃), was fabricated by hot pressing, and its properties were systemically investigated. The evaluation of thermal properties provides a baseline for the application of Yb₂O₃ on SiC_f/SiC or Al₂O_3f/Al₂O₃ composites. The performance in water vapor and molten calcium–magnesium–aluminosilicate (CMAS) environments indicates its excellent durability in harsh environment. Compared with rare-earth silicates, the thermochemical interactions between ytterbium oxide and CMAS changed greatly with the absence of silicon oxide. Reactions of ytterbium oxide with CMAS form several reaction products, including apatite, garnet, and silicocarnotite. The crystallization of garnet and silicocarnotite could effectively consume and solidify the CMAS melt, which prevents the melt infiltration and mitigates the further corrosion.