High temperature performances of tri-layer environmental barrier coating with Si bond layer modified by Yb2O3

Environmental barrier coatings (EBCs) have been expected to be applied on the surface of ceramic matrix composites (CMCs). However, the oxidation and propagation cracking of the silicon bond layer are the most direct causes to induce the failure of EBCs under high temperature service environment. The modification of silicon bond layer has become an important method to prolong the service life of EBCs. In this work, the Yb₂O₃ have been introduced to the silicon bond layer, and three kinds of tri-layer Yb₂SiO₅/Yb₂Si₂O₇/(Si-xYb₂O₃) EBCs with modified Si bond layer by different contents of Yb₂O₃ (x = 0, 10 vol%, 15 vol%) were prepared by vacuum plasma spray technique. The thermal shock performance and long-term oxidation resistance of the EBCs at 1350 °C were investigated. The results showed that the addition of appropriate amount of Yb₂O₃ (10 vol%) can improve the structural stability and reduce the cracks of the mixed thermal growth oxide (mTGO) layer by forming the oxidation product of Yb₂Si₂O₇ during long-term oxidation. The excessive addition of Yb₂O₃ increased the stress during thermal shock as well as accelerated the oxygen diffusion during long-term oxidation, leading to the failure of EBCs. Moreover, the distribution uniformity of Yb₂O₃ deserves further consideration and improvement.     

Oxidation behaviors and mechanisms of Yb2O3-doped silicon coatings fabricated by vacuum plasma spray

Environmental barrier coatings (EBCs) have been widely studied for the protection of ceramic matrix composites (CMCs). The phase transition of silica thermal growth oxide (TGO) has been proved to be an important factor for the durability of EBCs. Yb₂O₃ could react with SiO₂ TGO and form silicate which may improve the stability of TGO and prolong the service life of EBCs. In the present work, Si coatings doped with different contents of Yb₂O₃ were fabricated by vacuum plasma spray. The oxidation behaviors of the composite coatings were evaluated at 1350 °C and compared with the pure Si coating. The evolution of phase composition and microstructure of mixed thermal growth oxide (mTGO) was characterized in detail. The results showed that the newly formed oxidation product, namely Yb₂Si₂O₇, could reduce the vertical cracks in mTGO layer and the mTGO/coating interface cracks, leading to a better binding performance of the mTGO layer. The oxidation mechanisms of the Yb₂O₃-doped Si coatings were analyzed based on microstructure and phase composition observations.     

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.     

Environmental barrier coatings using low pressure plasma spray process

Yb₂Si₂O₇/Si bilayer environmental barrier coatings (EBCs) on SiC ceramic substrate were produced by low pressure plasma spray (LPPS) process. Phase composition, microstructure, and thermal durability of LPPS Yb₂Si₂O₇/Si coating were investigated. XRD analysis indicated that the coating is mainly composed of Yb₂Si₂O₇ with ~15.5v% Yb₂SiO₅ phases. The LPPS EBCs have a dense microstructure with porosity less than 4%. Adhesion strength measurement indicated the LPPS EBCs have an average adhesion strength of 29.1 ± 0.8 MPa. Furnace cycle test (FCT) on the coatings in air at 1316°C was performed and the test ran for 900 cycles and there was no coating spallation/failure for LPPS Yb₂Si₂O₇/Si EBCs. The FCT results demonstrated the excellent thermal cycle durability of LPPS EBCs. Oxidation kinetics investigation of LPPS EBCs in flowing 90% H₂O (g)+10% air at 1316°C showed that the thermally grown oxide (TGO) growth rate is close to the oxidation rate of pure Si in dry air and is significantly lower than that in water vapor environment. The LPPS process is promising in making highly durable Yb2Si2O7-based dense EBCs by impeding diffusion and ingression of water vapor/O₂.     

Exploration of the oxidation behavior and doping ratio of the Si–HfO2 bond layer used in environmental barrier coatings

Hafnia doping is expected to improve the performance of the silicon-bond layer of environmental barrier coatings (EBCs) for SiC-based ceramic matrix composites. The optimal doping ratio, distribution of HfO₂, and oxidation mechanism of the bond layer have not yet been fully addressed. A prototype Si–HfO₂ bond layer with a designed HfO₂-rich area was used to examine its oxidation behavior. A random dispersion model was developed to calculate the optimal HfO₂ doping ratio and its appropriate distribution state. The simulation results recommended that 20–30 vol% is the optimal doping ratio, where HfO₂ is well dispersed inside Si without forming networks. This enables HfO₂ to react with and consume SiO₂ without accelerating oxygen diffusion inside the bond layer. This was confirmed by oxidation experiments on Si–xHfO₂ tablets, in which the thinnest thermally grown oxide was achieved for the 20 vol% HfO₂-doped Si tablet. Both the microstructure design and material composition selection are highly important to further boost the performance of the EBCs.     

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.     

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.     

Interface evolution of Si/Mullite/Yb2SiO5 PS-PVD environmental barrier coatings under high temperature

Plasma spray-physical vapor deposition (PS-PVD) was used to prepare tri-layer environmental barrier coatings (EBCs) Si/mullite/Yb₂SiO₅ on SiC_f/SiC substrate. Isothermal oxidation tests of EBCs were performed at 1300 ℃ for 1000 h. The thermochemical and thermomechanical interface interaction among EBCs were investigated. The results show that more dense EBCs can be obtained through PS-PVD process, which is attributed to the mixed deposition of liquid/gas states. After isothermal oxidation, many pores were observed in the Yb₂SiO₅ coating near the interface of Yb₂SiO₅/mullite coating, which results from the diffusion of Yb₂O₃ phase dissociated from Yb₂SiO₅ into mullite coating at high temperature. In the mullite coating, the Yb₂O₃ reacted with Al₂O₃ generating rod-like Yb₃Al₅O₁₂ phase. Additionally, due to the thermal expansion mismatch and high temperature oxidation, cracks were formed at the interfaces of mullite/Si coating. Those interface cracks resulted in buckling in the mullite coating.     

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.     

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

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

Mechanical properties and microstructure of Yb2SiO5 environmental barrier coatings under isothermal heat treatment

Yb₂SiO₅ (ytterbium monosilicate) top coatings and Si bond coat layer were deposited by air plasma spray method as a protection layer on SiC substrates for environmental barrier coatings (EBCs) application. The Yb₂SiO₅-coated specimens were subjected to isothermal heat treatment at 1400 °C on air for 0, 1, 10, and 50 h. The Yb₂SiO₅ phase of the top coat layer reacted with Si from the bonding layer and O₂ from atmosphere formed to the Yb₂Si₂O₇ phase upon heat treatment at 1400 °C. The oxygen penetrated into the cracks to form SiO₂ phase of thermally grown oxide (TGO) in the bond coat and the interface of specimens during heat treatment. Horizontal cracks were also observed, due to a mismatch of the coefficient of thermal expansion (CTE) between the top coat and bond coat. The isothermal heat treatment improves the hardness and elastic modulus of Yb₂SiO₅ coatings; however, these properties in the Si bond coat were a little bit decreased.     

Thermal cycle test performances of environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 in the steam environment at 1475 °C

The novel bi-layer environmental barrier coatings (EBCs) with HfO₂-SiO₂/Yb₂Si₂O₇ structure (70HfO₂-30SiO₂/Yb₂Si₂O₇: 70HS/YbDS, 50HfO₂-50SiO₂/Yb₂Si₂O₇: 50HS/YbDS, molar ratios) was tested in 90%H₂O–10%O₂ conditions between room temperature and 1475 °C in an Al₂O₃ tube furnace, then its performance was evaluated. The YbDS layer was contaminated by alumina impurities under steam conditions. After 22 cycles, the 70HS/YbDS completely separated from the SiC substrate, while the 50HS/YbDS and SiC did not separate, even though cracks formed at the 50HS/SiC interface and the TGO layer. Furthermore, the thermally grown oxide (TGO) layer formed at the HfO₂-SiO₂/SiC interface. Formation and growth of the TGO led to the formation and propagation of cracks at the HfO₂-SiO₂/TGO interface and TGO interior, which was the culprit leading to the failure of EBCs. These results demonstrated that the 50HS/YbDS EBCs have the potential to protect SiC in steam conditions at 1475 °C.     

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.     

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

Mixed Y and Yb disilicate coatings (Y/Yb)DS have been proposed as dual function thermal and environmental barrier coatings (EBCs) for protecting SiC-based ceramic matrix composites in gas-turbine environments. As an initial step, the 1350 °C dry air cyclic oxidation of atmospheric plasma sprayed (Y_1.2/Yb_0.8)DS and ytterbium disilicate/ytterbium monosilicate (YbDS/YbMS) EBCs deposited onto Si bond coatings was compared. As a baseline for evaluating EBC oxidant permeability, the dry air cyclic oxidation scale growth rates for bare silica formers (SiC, Si) were also measured and were consistently higher than rates previously measured after isothermal oxidation. Regarding Si bond coat oxidation rates underlying (Y/Yb)DS and YbDS/YbMS EBCs, the thinner silica scale formed under the thinner and denser (Y/Yb)DS coatings suggested a lower oxidant permeability than YbDS/YbMS. After 500 1-h cycles, the (Y/Yb)DS coating was comprised of only the β-polymorph disilicate and minor amounts of the X-2 phase monosilicate phase. Negligible differences in oxidation kinetics for (Y/Yb)DS coatings over the 90 – 240 µm thickness range were observed.     

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.     

Characterization of Yb2SiO5-based environmental barrier coating prepared by plasma spray–physical vapor deposition

Due to the high-input power compared to atmospheric plasma spraying (APS), plasma spray-physical vapor deposition (PS-PVD) can primarily achieve a splat-like deposition, allowing for the preparation of high-density environmental barrier coatings (EBCs). In this paper, dense Yb₂SiO₅-based coatings are prepared by PS-PVD at different substrate temperatures. It was found that the coating deposited at the substrate temperature of 700 °C contained a large amount of silicon-rich amorphous phase. When the substrate temperature increased to 1100 °C and a slow cooling process after deposition was involved, a coating with high crystallinity of ～77% and low porosity of less than ～2% was achieved. Phase evolution of the coatings was studied by a semi-in-situ high-temperature X-ray diffractometer. During the heating process, metastable phases X1-Yb₂SiO₅ and α-Yb₂Si₂O₇ emerged and transformed into stable phases following high-temperature treatment. Furthermore, the effects of long-term thermal aging at 1300 °C on the microstructure, phase composition, thermal conductivity, and hardness of the coating prepared at the substrate temperature of 1100 °C were found to be limited.     

Enhancing CMAS corrosion resistance of PS-PVD Si/Yb2Si2O7 EBCs via Al-modification

Enhancing the resistance to molten silicate corrosion is crucial for the long service life of environmental barrier coatings (EBCs). In this study, we used the Al-modification technique to enhance the CMAS corrosion resistance of Si/Yb₂Si₂O₇ coatings prepared by plasma spray-physical vapor deposition. The results show that the Al-modified Yb₂Si₂O₇ coating had higher resistance to CMAS corrosion than the Yb₂Si₂O₇ coating annealed at 1300 ℃ for 100 h, which is related to the refractory mullite and Yb₂Mg(AlO₂)₂O₃ generated during the CMAS exposure of Al-modified Yb₂Si₂O₇ coating. The Al-modified Yb₂Si₂O₇ coating also exhibited excellent resistance to oxygen penetration. The Al-modification technology provides the direction for the corrosion resistance of Yb₂Si₂O₇ system to CMAS.     

Characterization of Yb2Si2O7–Yb2SiO5 composite environmental barrier coatings resultant from in situ plasma spray processing

Plasma spraying of multicomponent materials produces shifts in coating composition associated with differential vaporization of constituent elements within the strong thermal gradients of the process. This effect is quite noticeable in rare-earth silicates which are now widely being employed as Environmental Barrier Coatings (EBCs) for SiC based ceramic components of turbine engines. Of particular interest is the preferential volatilization of SiO₂ during thermal plasma spraying Yb₂Si₂O₇ (ytterbium disilicate) coatings which leads to the deviation from stoichiometry of the desired disilicate composition resulting in a mixed phase coating consisting of Yb₂Si₂O₇ plus Yb₂SiO₅ (ytterbium monosilicate). Recent work has shown that presence of monosilicate can be beneficial as its evolution from amorphous, metastable to stable crystalline phase can lead to crack healing during high temperature exposure, however, careful control of the chemistry and architecture may be needed. In this work a 50/50 mol% Yb₂Si₂O₇–Yb₂SiO₅ composite coating has been targeted through in situ decomposition during plasma spray from stoichiometric Yb₂Si₂O₇ powder. The as sprayed amorphous coating reverts to crystalline upon thermal treatment passing through a metastable state identified by XRD and Raman spectroscopy. The transition to the final stable phases results in a mixed phase coating comprising of 46/54 mol% Yb₂Si₂O₇–Yb₂SiO₅ composite that is thermo-mechanically stable with the underlying bond coated silicon coated SiC substrate.     

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.     

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.