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.     

Research progress on hafnium-based thermal barrier coatings materials

Thermal barrier coatings (TBC) are important materials applied to hot part components of aero-engines in order to improve their service temperature. Increasing inlet temperature is an important factor to achieve elevated thrust-to-weight ratio and high heat engine efficiency. In recent years, traditional TBC materials have gradually reached their operating limits due to the increase in turbine operating temperature. Hafnium-based materials become promising new candidates for TBC because of the similar structure, higher temperature phase stability and lower thermal conductivity compared to traditional zirconium-based materials. In this review, recent progresses in the research and development for hafnium-based TBC materials are summarized. The phase stability, thermal and mechanical properties of rare-earth (RE)-doped HfO₂ and RE hafnate materials are introduced. RE-doped HfO₂ has good thermal properties and phase stability at high temperatures whereas relatively low fracture toughness. The RE hafnates possess the advantages of a higher phase transition temperature, lower thermal conductivity and superior fracture toughness than RE zirconates. However, the thermal expansion coefficients of most RE hafnates are quite different from the alloy matrix. Finally, further research directions for hafnium-based TBC materials are prospected in this study.     

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.     

Lanthanum cerate thermal barrier coatings generated from thermal plasma synthesized powders

Lanthanum cerate (La₂Ce₂O₇, LC) is one of the promising advanced thermal barrier coating (TBC) materials due to its high melting point, no phase transformation between room temperature and operating temperature, low thermal conductivity, comparable coefficient of thermal expansion (CTE) with metallic substrate. The present study investigates plasma transferred arc synthesis of LC powder, its subsequent spheroidization in a thermal plasma jet and plasma spray deposition. The PTA-synthesized LC powder, spheroidized as well as the plasma sprayed coatings was found to possess excellent phase stability; the single phase cubic fluorite structure of LC was found to be retained even after prolonged arc-melting, corroborating that the material was stable from room temperature up to its melting point. It was observed that PTA melting for longer duration resulted in small deviation from stoichiometry, although the phase structure of LC was retained. Spheroidization efficiency was found to increase with the input power of the torch. Very good adherent LC coatings could be deposited on nickel super alloy with reasonably good deposition efficiency.     

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.     

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.     

A computational modeling framework for reaction and failure of environmental barrier coatings under silicate deposits

Environmental barrier coatings (EBCs) for use with SiC-based composites in gas turbine engines may fail following reaction with molten silicate deposits. The processes involved may include dissolution of the EBC material into the deposit, reactions that produce new phases, and cracking or spallation upon cooling, the latter driven by thermal expansion mismatch between the reaction products and the underlying EBC and substrate. Here, we describe an integrated computational framework to simulate the processes and to predict the conditions leading to coating loss through reactive consumption and/or spallation. The framework integrates distinct models to determine: (a) the nature and quantity of phases resulting from dissolution and chemical reactions; (b) thermo-physical properties of those phases; and (c) energy release rates for penetration cracking and spallation upon cooling. We demonstrate the use of the framework by computing critical deposit thicknesses for one specific EBC material (Y₂Si₂O₇) as a function of deposit composition. In this system, the critical deposit thickness for typical coating thicknesses is dictated mainly by spallation, not by consumption, and may vary by orders of magnitude, depending on deposit composition and coating thickness and toughness. With respect to deposit composition, the key parameter governing coating failure is the Ca:Si ratio.     

Reactive crystallization in HfO2 exposed to molten silicates

Hafnia is of interest in thermal and environmental barrier coatings, but little is known about its response to molten silicate attack. This article investigates that response using two model silicate melts, compares it with pure ZrO₂ and examines the effect of YO_1.5 additions. HfO₂ was found to form HfSiO₄ with acidic melts but undergoes grain boundary penetration in basic melts, which do not exhibit reactive crystallization. The latter can be exacerbated by microcracking resulting from the thermal expansion anisotropy of monoclinic HfO₂. Y additions generally degrade the ability to form hafnon (and zircon), and exacerbate grain boundary penetration, especially in HfO₂ where Y is present as a fluorite second phase. The fluorite controls grain growth in monoclinic HfO₂ and suppresses microcracking, but dissolves faster, especially in basic melts. The results are presented in the context of the relevant thermodynamics and kinetics. The implications for coating applications are discussed.     

The toughening design of multi-layer antioxidation coating on C/C matrix via SiC-SiCw transition layer grown in-situ

Poor antioxidant and thermal-shock capacities of C/C composites thermal barrier coating (TBC) caused by cracking and shedding of coatings has been a major obstacle blocking the development of C/C composites. Herein, in-situ growth of whisker reinforced silicon carbide transition layer and inter-embedding mechanism of multi-gradient coatings were brought into the design of TBC to enhance the antioxidant and thermal-shock capacities. A three-layer gradient coating SiC-SiC_w/ZrB₂-SiC/ZrSiO₄-aluminosilicate glass (ZAG) from inside to outside, in which ZrB₂-SiC/ZAG serve as oxygen barrier layers with self-healing ability and SiC-SiC_w provides thermal stress buffering and bonding against cracking and shedding of coatings, is designed. The ZAG mainly forms a dense oxygen blocking frontier with self-healing ability through fluidized glass, while the ZrB₂-SiC can react actively with infiltrated oxygen in a way of self-sacrifice, preventing oxygen erosion to C/C matrix and SiC-SiC_w transition layer. As a result, the collaborative work among layers endows this coating with excellent high temperature service performance. This work provides a new insight for the design of excellent TBC.     

Thermomechanical and thermochemical stability of HfSiO4 for environmental barrier coating applications

Thermomechanical and thermochemical stability of hafnon (HfSiO₄) was evaluated for application as an environmental barrier coating (EBC) candidate for SiC/SiC CMCs. High-temperature X-ray diffraction (XRD) analysis showed that hafnon has an excellent coefficient of thermal expansion (CTE) match with SiC as well as minimal CTE anisotropy, which supports hafnon as an EBC candidate. Alternatively, high-velocity water vapor testing at 1,200-1,400°C showed large amounts of silica depletion from chemical reaction at all velocities and excessive material erosion at steam velocities greater than 190 m/s. HfSiO₄ is therefore not suitable as an EBC for turbine applications due to insufficient thermochemical stability in water vapor-containing combustion environments.     

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.     

High-entropy thermal barrier coating of rare-earth zirconate: A case study on (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 prepared by atmospheric plasma spraying

We report a double-ceramic-layer (DCL) thermal barrier coating (TBC) with high-entropy rare-earth zirconate (HE-REZ) as the top layer and yttria stabilized zirconia (YSZ) as the inner layer sprayed on Ni-based superalloy by atmospheric plasma spraying. La₂Zr₂O₇ (LZ) was selected as a reference for the HE-REZ. Thermal cycling test results demonstrate that the HE-REZ/YSZ DCL coating exhibited obviously improved thermal stability when compared to the LZ/YSZ DCL coating. The reasons for the improvement of the thermal shock resistance are considered to be the anti-sinterability of the HE-REZ ceramics during the thermal cycling test attributed to the sluggish diffusion effect and as well as the better match in the coefficient of thermal expansion of HE-REZ coating with the YSZ inner layer. In addition, the HE-REZ coating maintains fluorite structure after thermal cycling test. This study makes one step forward in the development and application of high-entropy rare-earth zirconate ceramic thermal barrier coatings.     

Thermal shock resistance and mechanical properties of La2Ce2O7 thermal barrier coatings with segmented structure

La₂Ce₂O₇ (LCO) is a promising candidate material for thermal barrier coatings (TBCs) application because of its higher temperature capability and better thermal insulation property relative to yttria stabilized zirconia (YSZ). In this work, La₂Ce₂O₇ TBC with segmentation crack structure was produced by atmospheric plasma spray (APS). The mechanical properties of the sprayed coatings at room temperature including microhardness, Young's modulus, fracture toughness and tensile strength were evaluated. The Young's modulus and microhardness of the segmented coating were measured to be about 25 and 5 GPa, relatively higher than those of the non-segmented coating, respectively. The fracture toughness of the LCO coating is in a range of 1.3–1.5 MPa m^1/2, about 40% lower than that of the YSZ coating. The segmented TBC had a lifetime of more than 700 cycles, improving the lifetime by nearly two times as compared to the non-segmented TBC. The failure of the segmented coating occurred by chipping spallation and delamination cracking within the coating.     

Numerical study of residual stresses in environmental barrier coatings with random rough geometry interfaces

To clarify the role of the coating interface geometry and thermally grown oxide (TGO) layer in the failure of environmental barrier coatings (EBCs) and to further understand the cracking and spalling mechanisms of coatings, in this study, the thermomechanical properties of the multilayer coating system (Yb₂SiO₅/Yb₂Si₂O₇/Si), the morphology of the coating interface and the influence of the oxide layer on the local stresses during cooling were considered based on a random rough interface geometry model. The results showed that the rough geometry increased the magnitude of residual stresses at the interface and that the stress distribution away from the interface was less affected than the coating without roughness. The cracks on the outer surface of the Yb₂SiO₅ layer initiate in the valley region and spread with a stress value independent of the TGO thickness, and this failure may occur by cracking under tensile stress. The overall stress intensity at the TOP/EBC interface was lower than that at the upper surface of the TOP layer. The presence of TGO increased the magnitude of residual stresses in the BC and EBC layers, which caused cracks at the TGO/BC and TGO/EBC interfaces to occur at opposite locations. The phase change of the TGO layer from β-cristobalite to α-cristobalite cause a rapid increase in the overall level of coating stress, which may be a direct factor in coating failure. The calculation results provide a theoretical basis for the coating design and manufacturing process.     

Stress analysis of the thermal barrier coating system near a cooling hole considering the free-edge effect

Due to the thermal mismatch between layers and the free-edge effect, interfacial peeling and shear stresses are generated locally around the edges of cooling holes in a thermal barrier coating (TBC)–film cooling system. These interfacial peeling and shear stresses may lead to modes I and II edge delamination, resulting in TBC spallation around the cooling hole. In this study, analytical and numerical models were built to study the stress and interfacial cracking behaviors of TBCs near the cooling hole. Analytical solutions for interfacial peeling moment and shear force at each layer were obtained to analyze the free-edge effect on the stress distributions in TBCs, and they were verified by the finite element calculations. The results showed that interfacial peeling moment and shear force were functions of the hole radius and thicknesses of top coat and oxide layer. The increase of interfacial peeling moment and shear force raised the likelihood of edge cracking around the hole. Derived by the local stresses, the interfacial cracks in TBCs initiated and propagated from the hole edge upon cooling.     

Cracking behavior and delamination mechanism of lamellar structured TBC with localized mixed oxides

Mixed oxide (MO) with localized growth feature and high growth rate remarkably affects the lifetime of thermal barrier coatings (TBCs), which indicates that clarifying the ceramic cracking mechanism induced by MO is critical for developing new coatings with high durability. Two kinds of TBC models involving spherical and layered mixed oxides are created to explore the influence of MO growth on the local stress state and crack evolution during thermal cycle. The growth of α-Al₂O₃ is also included in the model. The undulating interface between ceramic coat and bond coat is approximated using a cosine curve. Dynamic ceramic cracking is realized by a surface-based cohesive interaction. The ceramic delamination by simulation agrees with the experimental observation. The effects of MO coverage ratio and growth rate on the TBC failure are also discussed. The results show that the MO growth causes the local ceramic coat to bear the normal tensile stress. The failure mode of coating is turned from α-Al₂O₃ thickness control to MO growth control. Once the mixed oxide appears, local ceramic cracking is easy to occur. When multiple cracks connect, ceramic delamination happens. Suppressing MO formation or decreasing MO growth can evidently improve the coating durability. These results in this work can provide important theoretical guidance for the development of anti-cracking TBCs.     

Thermal cycling performance of La2Ce2O7/50 vol.% YSZ composite thermal barrier coating with CMAS corrosion

La₂Ce₂O₇ (LC) is receiving increasing attention due to its lower thermal conductivity, better phase stability and higher sintering resistance than yttria partially stabilized zirconia (YSZ). However, the low fracture toughness and the sudden drop of CTE at approximately 350?°C greatly limit its application. In this study, the LC/50?vol.% YSZ composite TBC was deposited by supersonic atmospheric plasma spraying (SAPS). Compared to YSZ or double layered LC/YSZ coating, the thermal cycling life of LC/50?vol.% YSZ coating with CMAS attack increased by 93% or 91%. The latter possessed higher fracture toughness (1.48?±?0.26?MPa?m^1/2) than LC (0.72?±?0.15?MPa?m^1/2) and better CMAS corrosion resistance than YSZ owing to the formation of Ca₂(La_xCe_1-x)₈(SiO₄)₆O_6–4x with <001> orientation perpendicular to the coating surface. The sudden CTE decrease of LC was fully suppressed in LC/50?vol.% YSZ coating due to the change of temperature dependent residual stresses induced by YSZ.     

A new approach to protect and extend longevity of the thermal barrier coating by an impermeable layer of silicon nitride

A sample representation of a gas turbine engine blade, consisting of a nickel superalloy substrate with a deposited thermal barrier coating (TBC), was covered with silicon nitride, Si₃N₄, as an impermeable layer using plasma enhanced chemical vapor deposition (PECVD). The silicon nitride layer was used to seal the topcoat of yttria-stabilized zirconia (YSZ) surface of the TBC to mitigate calcium–magnesium–aluminum–silicon oxide (CMAS) attack. CMAS testing was carried out on the covered and uncovered surfaces by melting a ratio of 25 mg/cm² of CMAS powder onto the surface of each sample in a furnace at 1100°C for 1 h. The conformal surface reaction of the sealed layer confirmed no cracking or delamination at high temperatures. Scanning electron microscopy (SEM) micrographs confirmed that the surface of YSZ was successfully sealed. The new coating of silicon nitride was shown to be a viable solution and technique to significantly block CMAS infiltration in porous thermal barrier coatings.     

Thermal shock resistance of air plasma sprayed thermal barrier coatings

The spallation resistance of an air plasma sprayed (APS) thermal barrier coating (TBC) to cool-down/reheat is evaluated for a pre-existing delamination crack. The delamination emanates from a vertical crack through the coating and resides at the interface between coating and underlying thermally grown oxide layer (TGO). The coating progressively sinters during engine operation, and this leads to a depth-dependent increase in modulus. Following high temperature exposure, the coating is subjected to a cooling/reheating cycle representative of engine shut-down and start-up. The interfacial stress intensity factors are calculated for the delamination crack over this thermal cycle and are compared with the mode-dependent fracture toughness of the interface between sintered APS and TGO. The study reveals the role played by microstructural evolution during sintering in dictating the spallation life of the thermal barrier coating, and also describes a test method for the measurement of delamination toughness of a thin coating.     

Thin single-phase yttrium-based environmental barrier coating systems for SiC/SiC CMCs

Two thin yttrium silicate-based EBC systems for SiC/SiC CMCs that comprised of three layers were produced via (reactive) magnetron sputtering with pure yttrium disilicate (YDS) and yttrium mono silicate (YMS) layers. The systems survived 1000 1-hour furnace cycle tests at 1250 °C in air interrupted by active cooling for 10 min. One system showed partial delamination of the YMS. The YDS intermediate layer stopped deleterious reactions among YMS and silicon and the silica formation did not cause spallation of the silicates. Some vertical cracks in YMS left possible attack paths for water vapor. Rapid water vapor testing was performed at 1250 °C in 100 vol% water vapor at 10 m/s for 2 h. This test proved that a thin but hermetic and phase pure YMS top layer can protect the system from volatilization: single phase X2-YMS was not attacked by steam whereas a mixed β-YDS/X2-YMS coating showed volatilization of SiO₂.