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.     

Reactivity between rare‐earth oxides based thermal barrier coatings and a silicate melt

The reactivity between rare‐earth (RE‐) oxide stabilized ZrO₂ or HfO₂ thermal barrier coatings (TBCs) and a calcium‐magnesium‐aluminum‐silicate (CMAS) melt was studied at 1310°C. These reactions are representative of the ingestion of siliceous materials by the intake air of gas turbines (e.g., in aircraft engines) at high temperatures (>1200°C). These materials can melt and react with coated components in the hot section, resulting in premature failure. The goal of this work was to probe the effect of various RE (RE = Y, Yb, Dy, Gd, Nd, and Sm) oxides in the melt phase equilibrium and stability of the top‐coating system. Thermodynamic calculations of the phase assemblage of the (1?x) ZrO₂‐xY₂O₃ coating materials and CMAS melt are compared with the experimental findings. CMAS was found to penetrate the samples at the grain boundaries and dissolve the coating materials to form silicate phases containing the RE elements. Furthermore, apatite and garnet crystalline phases formed in the samples with total RE‐oxide content higher than 16 mol% in the reaction zone for the ZrO₂ system. In general, samples with nominal compositions ZrO₂‐9Dy₂O₃, HfO₂‐7Dy₂O₃, ZrO₂‐8Y₂O₃, HfO₂‐6Er₂O₃, ZrO₂‐9.5Y₂O₃‐2.25Gd₂O₃‐2.25Yb₂O₃, and ZrO₂‐30Y₂O₃ exhibited lower reactivity, or more resistance, to CMAS than the other coating compositions.     

Phase equilibria in the ZrO2-YO1.5-TaO2.5 system at 1250 °C

The phase relationships in the ZrO₂-YO_1.5-TaO_2.5 (ZYTO) system at 1250?°C—a temperature of interest for thermal barrier coatings applications—were investigated using precursor-derived powders. The system is bisected by a quasi-binary joining the tetragonal (t) ZrO₂ and monoclinic (M’) YTaO₄ solid solutions. The mutual solubility limits for these phases differ significantly depending on whether the third phase in equilibrium with t and M’ is fluorite or the orthorhombic (O) Zr₆Ta₂O₁₇. More importantly, the tetragonal phase in equilibrium with fluorite is non-transformable to the monoclinic form upon cooling, whereas that in equilibrium with O, which has lower stabilizer content, is transformable. The behavior contrasts with that previously observed for the equilibrium at 1500?°C, wherein the t phase on both sides of the quasi-binary is non-transformable. The fluorite solid solution extends from the ZrO₂-YO_1.5 binary to the YO_1.5-TaO_2.5 binary at 1250?°C, as previously shown for 1500?°C.     

Thermodynamics of reaction between gas-turbine ceramic coatings and ingested CMAS corrodents

The thermodynamic stability of ceramic coatings with respect to their reaction products is crucial to develop more durable coating materials for gas-turbine engines. Here, we report direct measurements using high-temperature solution calorimetry of the enthalpies of reaction between some relevant ceramic coatings and a corrosive molten silicate. We also report the enthalpy of mixing between the coatings and molten silicate after combining the results measured by high-temperature solution calorimetry with enthalpies of fusion measured by drop-and-catch calorimetry and differential thermal analysis. The enthalpies of solution of selected silicate and zirconia-based coatings and apatite reaction products are moderately positive except for 7YSZ, yttria-stabilized zirconia. Apatite formation is only favorable over coating dissolution in terms of enthalpy for 7YSZ. The enthalpies of mixing between the coatings and the molten silicate are less exothermic for Yb₂Si₂O₇ and CaYb₄Si₃O₁₃ than for 7YSZ, indicating lower energetic stability of the latter against molten silicate corrosion. The thermochemical results explain and support the very corrosive nature of CMAS melts in contact with ceramic coatings.     

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.     

Corrosion of RE2Si2O7 (RE=Y,Yb, and Lu) environmental barrier coating materials by molten calcium-magnesium-alumino-silicate glass at high temperatures

With the increased demand for high operating temperature of gas turbine engines, corrosion by molten calcium-magnesium-alumino-silicate (CMAS) exhibits a significant challenge to the development of durable environmental barrier coatings (EBCs). EBC candidates, γ-Y₂Si₂O₇, β-Yb₂Si₂O₇, and β-Lu₂Si₂O₇ were explored on their corrosion resistance to CMAS melts at 1300 °C and 1500 °C for 50 h. Interaction and degradation mechanisms were investigated and the corrosion behaviors showed different trends at high temperatures. At 1300 °C, RE₂Si₂O₇ dissolves into CMAS melts and apatite phases reprecipitate forming a thick recession layer. However, when the temperature increases to 1500 °C, CMAS melts vigorously penetrate through the grain boundary of RE₂Si₂O₇ and ‘blister’ cracks form throughout the samples. The reduced grain boundary stability at 1500 °C promotes the penetration of CMAS melts in RE₂Si₂O₇. Grain boundary engineering is critically demanded to optimize CMAS corrosion at high temperatures.     

Influence of Yb:Hf Ratio on Ytterbium Hafnate/Molten Silicate (CMAS) Reactivity

This study investigated the influence of YbO_1.5 concentration on the reactivity between YbO_1.5–HfO₂ thermal barrier coating (TBC) materials and silicate melts [calcium–magnesium aluminosilicate (CMAS), specifically 33CaO–9MgO–13AlO_1.5–45SiO₂, all in mol%]. Three thermal barrier oxides (TBO) with YbO_1.5 concentration between 30 and 80 mol% were tested at 1300°C and 1500°C. Porous pellets were used to study reaction layer growth and the behavior within infiltrated porosity. Complementary experiments examined the phase equilibria in melts containing varying quantities of the candidate TBOs. The effectiveness of reactive crystallization to limit interaction depth increased with the YbO_1.5 concentration in the TBO. The results indicate that the TBO must contain sufficient YbO_1.5 to satisfy the equilibrium between the melt and HfO₂‐based fluorite phase before YbO_1.5‐bearing silicates can precipitate. Increasing the YbO_1.5 availability leads to the crystallization of apatite, garnet, silicocarnotite, and/or cuspidine (alumino)silicates. Apatite was observed at 1300°C and 1500°C, garnet and silicocarnotite only appeared at 1300°C, and cuspidine was only evident at 1500°C. The implications for the design of CMAS‐resistant coatings are discussed in the context of the experimental results.     

Effect of microstructure on the performance of Zr6Ta2O17 ceramics as thermal barrier coatings

In this study, Zr₆Ta₂O₁₇ ceramics with porous, fine-grained, and coarse-grained structures were obtained via in situ solid-state reactions, and their mechanical characteristics were examined. The significantly low thermal conductivity of dense Zr₆Ta₂O₁₇ ceramics (1.0 W m⁻¹ K⁻¹) was due to the grain boundary gap caused by superstructured grains. A calcium–magnesium–alumina–silicate (CMAS) corrosion experiment demonstrated that the formation of an interlocking structure composed of ZrO₂, CaTa₂O₆, and ZrSiO₄ prevented the penetration of CMAS impurities, thereby revealing the application potential of porous ceramics. In dense Zr₆Ta₂O₁₇ ceramics, the low-volume diffusion induced by an entropy-stable structure is conducive for corrosion resistance; however, the grain boundary is vulnerable to attacks by CMAS, which can be mitigated by the formation of a coarse crystal structure, thereby effectively improving the corrosion performance. This work provides a critical perspective on the thermal barrier coating design of A₆B₂O₁₇ (A = Zr, Hf; BNb, Ta) ceramics.     

Effect of HfO2 framework on steam oxidation behavior of HfO2 doped Si coating at high temperatures

HfO₂ doped Si is designed as bond coat material in thermal/environmental barrier coatings (TEBCs). In this work, the HfO₂-Si composite coatings with different HfO₂ contents were prepared by atmospheric plasma spraying (APS). The steam oxidation behavior of the coatings was comparatively studied at 1300 °C and 1400 °C. Volatilization of Si occurred during spraying, leading to the deviation of coating compositions. The sprayed coatings contained different HfO₂ structures. During steam oxidation, HfSiO₄ phase was formed at the SiO₂/HfO₂ interface by solid-state reaction between them. The HfSiO₄ or HfO₂/HfSiO₄ mixture particles worked to deflect or pin micro-cracks, thus improving the resistance of the coating to cracking. At 1300 °C, a protective oxide scale was formed on the traditional Si coating or the HfO₂-Si coating with isolated HfO₂ particles. However, the HfO₂-Si coating with inter-connected HfO₂ framework revealed poor oxidation-resistance. At 1400 °C, accelerated oxidation degradation, steam corrosion volatilization, interface reaction and sintering occurred. The HfO₂ framework structure played a dominating role in determining the steam oxidation resistance of the HfO₂-Si coating, and the connected HfO₂ framework and TGO network provided a rapid diffusion path for oxidants (H₂O, O^2? and OH^?) and deteriorated the oxidation resistance.     

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.     

Effect of Ta5+ doping on the thermal physical properties of defective fluorite Y3NbO7 ceramics

The effects of substituting the B cation in A₃BO₇ ceramics on their thermal physical properties were investigated by applying a large mass difference. Y₃(Nb_1-xTa_x)O₇ (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized, and their structural characteristics were determined. All the fabricated Y₃(Nb_1-xTa_x)O₇ ceramics showed defective fluorite structures and glass-like low thermal conductivity (1.18−2.04 W/m∙K at 25°C) because of the highly distorted crystal structure and significant mass difference. Substitution with Ta⁵⁺ enhanced the sintering resistance, leading to superior thermal-insulating performance via grain boundary scattering. Furthermore, the ceramics exhibited excellent coefficients of thermal expansion, implying the promising applicability of Y₃(Nb_1-xTa_x)O₇ as new thermal barrier materials. The effect of mass difference on the thermomechanical properties of the ceramics was examined, suggesting a simple strategy for engineering the chemical composition of new thermal barrier materials.     

High-temperature interface stability of tri-layer thermal environmental barrier coatings

Thermal environmental barrier coatings (TEBCs) are capable of protecting ceramic matrix composites (CMCs) from hot gas and steam. In this paper, a tri-layer TEBC consisting of 16 mol% YO_1.5 stabilized HfO₂ (YSH16) as thermal barrier coating, ytterbium monosilicate (YbMS) as environmental barrier coating, and silicon as the bond coating was designed. Microstructure evolution, interface stability, and oxidation behavior of the tri-layer TEBC at 1300 °C were studied. The as-sprayed YSH16 coating was mainly comprised of cubic phase and ～3.4 vol% of monoclinic (M) phase. After 100 h of heat exposure, the volume fraction of the M phase increased to ～27%. The YSH16/YbMS interface was proved to be very stable because only slight diffusion of Yb to YSH16 was observed even after thermal exposure at 1300 °C for 100 h. At the YbMS/Si interface, a reaction zone including a Yb₂Si₂O₇ layer and a SiO₂ layer was generated. The SiO₂ grew at a rate of ～0.039 μm²/h in the first 10 h and a reduced rate of 0.014 μm²/h in the subsequent exposure.     

High-temperature performances of Si-HfO2-based environmental barrier coatings via atmospheric plasma spraying

To improve high-temperature bearing capability of coatings, novel agglomerated Si-HfO₂ powders were prepared by adding HfO₂ powders into original Si powders by spray drying method. Three-layer environmental barrier coatings (EBCs) with Si-HfO₂ bond layer, Yb₂Si₂O₇ intermediate layer and Yb₂SiO₅ surface layer were prepared on SiC ceramic substrates by atmospheric plasma spraying (APS). The high temperature properties of coatings were systematically investigated. The results indicated that the coatings had good high temperature oxidation resistance, and remained intact after being oxidized or steam corrosion at 1400 °C for 500 h, so the addition of HfO₂ improved the thermal cycling performances of the coating. The HfO₂ in Si bond coating could effectively inhibit the growth of thermal grown oxide at high temperatures. This work indicates that the high temperature properties of the coatings are improved by this novel EBCs using the novel agglomerated Si-HfO₂ powders.     

Research status of bond coats in environmental barrier coatings

Bond coats in environmental barrier coatings (EBCs) prevent oxidants from penetrating the substrate, mediate the mismatch of the coefficient of thermal expansion (CTE), and improve the adhesion strength between adjacent layers. However, the development of bond coats is rarely studied systematically. In this paper, the research status of the bond coats in EBCs is introduced in detail, including the materials and deposition methods. Thus far, Si, modified-Si, mullite, etc., have been employed as bond coats. Nevertheless, visible drawbacks of each bond coat limit their application at high-temperatures in extreme environments. Si bond coat is easily oxidized and forms thermally grown oxides that form cracks, resulting in delamination, spallation, and failure of EBCs. In the Si–HfO₂ bond coat, the optimal ratios of Si/HfO₂, deposition methods, distribution of Si and HfO₂, and oxidation of Si remain completely unsolved. For mullite bond coat, SiO₂ suffers selective evaporation in the water vapor environment, and the ratios of the Al₂O₃ and SiO₂ in mullite coatings restrict its service lifetime. HfSiO₄ is a potential candidate acting as a next-generation bond coat in EBCs is proposed. Furthermore, choosing reasonable deposition methods is beneficial to improve the performances of the bond coats in EBCs.     

Y-doped HfO2 deposited by atomic layer deposition using a cocktail precursor for DRAM capacitor dielectric application

A Y-doped HfO₂ thin film deposited using a cocktail precursor for a DRAM capacitor dielectric application was investigated. It has been difficult to adapt HfO₂, a potential high-dielectric-constant material, deposited by a typical thin-film deposition technique to actual devices owing to its low dielectric constant of approximately 20, resulting from its monoclinic-phase crystal structure. Although several methods have been investigated to increase the dielectric constant by crystal structure transformation to the tetragonal phase, which has a dielectric constant as high as approximately 40, the formation of the monoclinic phase was not successfully suppressed. In this study, the tetragonal-phase formation of HfO₂ thin films was investigated using a cocktail precursor consisting of Y and Hf precursors. The monoclinic formation suppression mechanism in the Y-doped HfO₂ thin film was determined from the physical and chemical analyses results. Moreover, the leakage current change caused by the introduced oxygen vacancy with respect to the Y dopant concentration was investigated. Improved electrical properties of the dielectric constant and leakage current were achieved with Y-doped HfO₂.     

Phase equilibria in the ZrO2-YO1.5-TaO2.5 system at 1500 °C

The ZrO₂-YO_1.5-TaO_2.5 (ZYTO) system is of interest in phosphors, fuel cells, photocatalysis, and thermal barrier coatings. However, there is inadequate information on the ternary phase equilibria, especially in the ZrO₂-lean compositions. Precursor derived ZYTO compacts were heat treated at 1500 °C to elucidate the relevant phase relations. Chemical compositions measured using TEM/EDS and crystallographic information via x-ray diffraction, Raman spectroscopy and selected area electron diffraction were used to construct the isothermal section at 1500 °C, which was found to vary significantly from previous reports. It is shown for the first time that the fluorite field extends from the YO_1.5-ZrO₂ binary to the YO_1.5-TaO_2.5 binary, revealing a large YTaO₄ + fluorite phase field with potential engineering implications. The mutual solubility of ZrO₂ and YTaO₄ was quantified, and the presence of both YTa₃O₉ and YTa₇O₁₉ at 1500 °C was demonstrated. The emerging understanding helps identify compositions with potentially attractive properties for further investigation.     

Phase transformation and thermal conductivity of the APS Y2O3 doped HfO2 coating with hybrid structure

Hafnia-based materials are very promising to serve as thermal protecting coatings at temperature above 1200 °C. In this work, two kinds of 8 mol% Y₂O₃ stabilized HfO₂ ceramic coatings (YSH-SN and YSH-MX) with conventional and hybrid structures were prepared by air plasma spray (APS) method. The microstructure, thermal conductivity and the mechanical properties of the coatings before and after thermal exposure at 1300 °C were compared in detail. Results show that the as-sprayed YSH-MX has a hybrid laminated structure of monoclinic HfO₂ and cubicY₂O₃ splats, and transforms to monoclinic HfO₂ and cubic YSH after thermal exposure, while the YSH-SN is composed of major tetragonal YSH phase and transforms to monoclinic HfO₂ and cubic YSH afterward. Thermal conductivities at ultra-high temperature (1600 °C) before and after thermal exposure for those two coatings are close, and the fracture toughness in the direction parallel to the interface exceeds 2.1 MPa m^0.5. The YSH-MX coating with a hybrid structure provides insights to conveniently prepare gradient coating or other coatings with complex structures.     

Microscale fracture mechanisms of HfO2-Si environmental barrier coatings

Environmental barrier coatings (EBCs) play a vital role in protecting advanced turbine components subjected to extreme service environments, where mechanical, thermal and chemical behaviors typically dominate the design criteria. This study focuses on investigating the mechanical properties of individual phases within a HfO₂-Si ceramic matrix composite (CMC) structure through implementation of notched microcantilevers and micropillar splitting experiments. The microcantilever experiments resulted in fracture toughnesses ranging between 1.38–1.52 MPa-m^1/2 for the Si-rich phase and 2.26–2.38 MPa-m^1/2 for the HfO₂-rich phase depending on the method of analysis. From our micropillar splitting experiments, we found fracture toughnesses of 1.13 ± 0.39 MPa-m^1/2 for the Si-rich phase and 1.18 ± 0.26 MPa-m^1/2 for the HfO₂-rich phase. Comparisons with bulk single edge V-notched beams (SEVNBs) suggest the micropillar results are accurate whereas the microcantilever experiments may overestimate K_IC of the Si-rich and HfO₂-rich phases by ?0.2–1.2 MPa-m^1/2, most likely, due to dimensional errors that affect specimen and model compliances.     

Cyclic oxidation performances of new environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 coated SiC at 1375 °C and 1475 °C in the air environment

The objective of this work is to study the cyclic oxidation performances of the environmental barrier coatings (EBCs) containing the novel HfO₂-SiO₂ bond coats in the air environment. Bi-layer HfO₂-SiO₂/Yb₂Si₂O₇ (50HfO₂-50SiO₂, 70HfO₂-30SiO₂ bond coats) and conventional Si/Yb₂Si₂O₇ EBCs were deposited on SiC substrate using atmospheric plasma spray. The effect of the pre-mixing ratios of HfO₂/SiO₂ on the cyclic oxidation behavior of HfO₂-SiO₂/Yb₂Si₂O₇ EBCs was examined. The results showed that the higher content of the HfSiO₄ formed from the 50HfO₂-50SiO₂ bond coats, and it remained intact. A thermally grown oxide (TGO) SiO₂ layer was formed at the bond coat/SiC interface. The parabolic oxidation rate constant (k_p, μm²/h) of the TGO has been reduced 2 orders of magnitude in 50HfO₂-50SiO₂/Yb₂Si₂O₇ EBCs coated SiC compared to the bare SiC at 1475 °C, indicating that the 50HfO₂-50SiO₂/Yb₂Si₂O₇ EBCs effectively protected the SiC substrate at 1475 °C.     

Thermochemistry of calcium rare-earth silicate oxyapatites

The calcium rare-earth (RE) silicate oxyapatite, Ca₂RE₈(SiO₄)₆O₂ (RE = Yb, Er, Y, Dy, Nd, Gd, and Sm), powders were synthesized by the solid-state reaction method and characterized by X-ray diffraction (XRD), Raman spectroscopy, and elemental composition analysis. The thermodynamic properties of the oxyapatites have been investigated using high-temperature oxide melt calorimetry in molten 2PbO–B₂O₃ solvent at 805°C. The energetics of the oxyapatites related to ionic substitution on two crystallographic sites, M(1) and M(2), are discussed. The enthalpy of formation from the oxides becomes more exothermic as the ionic potential decreases or the ionic radius of the REs increases, which indicates increasing energetic stability in this order.