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.     

Accelerated oxidation during 1350°C cycling of ytterbium silicate environmental barrier coatings

Environmental barrier coatings (EBCs) will be needed to protect SiC-based ceramic matrix composite components for the next generation of high-efficiency industrial gas turbines (IGTs). The IGT application will require ≥25 kh lifetimes, and little data are available on EBC failure mechanisms, particularly at ≥1300°C. Initial 1-h furnace cycle testing at 1350°C in 90 vol% H₂O/10 vol% air was conducted ≥1000 cycles on thermally sprayed ytterbium disilicate (YbDS) coatings with and without an Si bond coating. By ≥1000 h, both EBCs formed thick, highly cracked, and fully crystalline cristobalite scales. Comparison of thermally grown oxide (TGO) microstructure and kinetics to isothermal rates of Si and SiC steam oxidation indicated a departure from slow-growing parabolic growth to more rapid rates of silica formation. Possible mechanisms and implications for this acceleration are discussed.     

Steam oxidation and microstructural evolution of rare earth silicate environmental barrier coatings

A primary failure mode for environmental barrier coatings (EBCs) on SiC ceramic matrix composites (CMCs) is the oxidation of the intermediate Si-bond coating, where the formation of SiO₂ at the bond coating–EBC interface results in debonding and spallation. This work compares the microstructure evolution and steam oxidation kinetics of the Si-bond coating beneath yttrium/ytterbium disilicate ((Y/Yb)DS) and ytterbium disilicate/monosilicate (YbDS/YbMS) EBCs to better understand the impact of EBC composition on oxidation kinetics. After 500 1-h cycles at 1350°C, (Y/Yb)DS displayed a decreasing concentration of the monosilicate minor phase and increasing concentration of porosity as furnace cycling time increased, whereas the YbDS/YbMS EBC displayed negligible microstructural evolution. For both EBC systems, thermally grown oxide growth rates in steam were found to increase by approximately an order magnitude compared to dry air oxidation. The (Y/Yb)DS EBC displayed a reduced steam oxidation rate compared to YbDS/YbMS.     

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.     

Water vapor corrosion test using supersonic gas velocities

Testing of the corrosion resistance of environmental barrier coating (EBC) systems is necessary for developing reliable coatings. Unfortunately tests under realistic gas turbine conditions are difficult and expensive. The materials under investigation as well as parts of the test setup have to withstand high temperatures (≥1200°C), high pressure (up to 30 bar) as well as the corrosive atmosphere (H₂O, O₂, NO_x). Therefore most lab scale test-rigs focus on simplified test conditions. In this work water vapor corrosion testing of EBCs with a high velocity oxy fuel (HVOF) facility is introduced which combines high temperatures and high gas velocities. It leads to quite high recession rates in short periods of time, which are comparable to results from literature. It was found that high flow velocities can easily compensate low gas pressures. HVOF-testing is a simple and fast way to measure the recession rate of an EBC-system. As proof of concept the recession rates of an oxide/oxide CMC with and without EBC were measured.     

High-temperature mechanical properties and oxidation resistance of SiCf/SiC ceramic matrix composites with multi-layer environmental barrier coatings for turbine applications

Continuous silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) ceramic matrix composites are considered promising materials as high-temperature components of advanced aero-engines. However, due to their susceptibility to oxidation and corrosion at high temperature, environmental barrier coatings (EBCs) must be applied on the surface of SiC_f/SiC. In this study, Si/Y₂SiO₅/LaMgAl₁₁O₁₉ (LMA) multi-layer EBCs were fabricated to protect SiC_f/SiC by using atmospheric plasma spraying (APS). The high-temperature tensile fatigue performance of SiC_f/SiC with and without EBCs was evaluated. The results indicated that EBCs significantly improved the tensile fatigue properties of SiC_f/SiC at high temperature in air atmosphere. Meanwhile the bending strength of specimens after isothermal aging or not was also tested. The multi-layer EBCs in this study may be a promising EBCs system for SiC_f/SiC after some improvements.     

Effect of the thermally grown oxide and interfacial roughness on stress distribution in environmental barrier coatings

To clarify the role of interface morphology and thermally grown oxide (TGO) in the failure of environmental barrier coatings (EBCs). In this study, the effect of chemical expansion on free energy was considered based on the continuous thermodynamic framework. The effects of roughness and TGO growth on the stress distribution of EBCs were investigated. The results showed that the stress coupling effect led to the inhomogeneous growth of TGO by affecting the gas diffusion and gas inflow rate. The TGO thickness at the peak increased with increasing roughness, and the TGO thickness at the valley and the middle position decreased with increasing roughness. The y-direction at the TGO/EBC valley and the TGO/BC peak under tensile stress increased with the TGO thickness and roughness and may be the first to fail in delamination. The calculation results of the model can provide a theoretical basis for the coating design and manufacturing process.     

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.     

Rare earth silicate environmental barrier coatings: Present status and prospective

Due to drastic decreasing in mechanical properties at relative high temperature, traditional nickel based super alloys are replaced by Si-based non-oxide ceramics in the application of high temperature aero-engines. In order to reduce the spallation and deformation of aero-engine blades in the environment containing high temperature water vapor and oxygen, protection coatings on the surface of the ceramics are required. Owing to high temperature stability, superior oxidation resistance and corrosion resistance properties, rare earth (RE) silicates are promising as candidates and play an important role in improving the high-temperature mechanical/thermal properties of Si-based non-oxide ceramics. In this review, recent progress in the research and development of environmental barrier coatings (EBCs) are summarized. Development of EBCs is presented, and the multi-scale structures and properties of each part are introduced. In addition, the merits and demerits of each preparation technique are discussed. As a promising candidate for the application in high temperature aero-engines, Si/mullite/Lu₂Si₂O₇–Lu₂SiO₅ EBCs are highlighted.     

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.     

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.     

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.     

Polymer-derived yttrium silicate coatings on 2D C/SiC composites

Yttrium silicate environmental barrier coatings (EBCs) on C/SiC composites were fabricated by using polysiloxanes-derived ceramic process. In order to reduce the free silica in the resultant ceramic coatings, Y₂O₃ was added as an active filler. The materials with different weight ratios of polysiloxanes to Y₂O₃ were synthesized. Their coefficient of thermal expansion (CTE) and water-vapor resistance were tested. The results indicated that the composition of 50% Y₂O₃–50% polysiloxanes (Y50) was suitable to be the EBCs for C/SiC composites. The C/SiC composites coated with Y50 were tested in the water-vapor environments at 1400 °C for 200 h. The results revealed that such a coating could effectively protect the C/SiC composites.     

Oxidation resistance of SiCf/SiC composites with three-layer environmental barrier coatings up to 1360 °C in air atmosphere

Atmospheric plasma spraying (APS) was used to prepare three-layer environmental barrier coatings (EBCs) Si/Yb₂SiO₅/LaMgAl₁₁O₁₉ (LMA) on a SiC_f/SiC substrate. Isothermal aging test of the specimens were performed between 1000 and 1360 °C for 500 h. The flexural strength of the specimens after isothermal aging was investigated. Microcracks and holes were observed in the as-sprayed EBCs because of the shock cooling during the APS process, but reduced after isothermal aging, and the EBCs became denser. At least 80.07% of the flexural strength of the SiC_f/SiC substrate with EBCs was maintained after isothermal aging, but only 35.91% strength was maintained without EBCs. In particular, the retention ratio of flexural strength was 90.72% after isothermal aging at 1360 °C, despite a reaction between the layers of the EBCs. All the specimens with EBCs showed “pseudo-plastic” fracture, compared with the brittle fracture of specimens without EBCs.     

Water vapor volatilization and oxidation induced surface cracking of environmental barrier coating systems: A numerical approach

A numerical model is developed for surface crack propagation in brittle ceramic coatings, aiming at the intrinsic failure of rare-earth silicate environmental barrier coating systems (EBCs) under combustion conditions in advanced gas turbines. The main features of progressive degradation of EBCs in such conditions are captured, including selective silica vaporization in the top coat due to exposure to water vapor, diffusion path-dependent bond coat oxidation, as well as crack propagation during cyclic thermal loading. In light of these features, user-defined subroutines are implemented in finite element analysis, where surface crack growth is simulated by node separation. Numerical results are validated by existing experimental data, in terms of monosilicate layer thickening, thermal oxide growth, and fracture behaviors. The experimentally observed quasi-linear oxidation in the early stage is also elucidated. Furthermore, it is suggested that surface crack undergoes rapid propagation in the late stage of extended thermal cycling in water vapor and leads to catastrophic failure, driven by both thermal mismatch and oxide growth stresses. The latter is identified as the dominant mechanism of penetration. Based on detailed analyses of failure mechanisms, the optimization strategy of EBCs composition is proposed, balancing the trade-off between mechanical compliance and erosion resistance.     

Modeling microvoiding kinetics of rare-earth disilicates in flowing atmospheres containing water vapor

Rare-earth disilicate (REDS, RE₂Si₂O7) layers that may be used as environmental barrier coatings (EBCs) on ceramic matrix composite (CMC) components in high-temperature stages of turbine engines are subject to microvoiding to form porous rare-earth monosilicate (REMS, RE₂SiO₅) layers in flowing atmospheres containing water vapor. A simple microvoiding kinetic model that incorporates both mass transfer of the reaction product Si(OH)₄(g) through the external gas phase boundary layer and pore diffusion of Si(OH)₄(g) through the microvoided layer has been developed. Model predictions are in good agreement with measured growth rates of microvoided layers under low-flowrate steam furnace test and high-flowrate burner rig conditions. Since pore diffusion is generally the rate-limiting step for EBC microvoiding in turbine engines, furnace testing under conditions of kinetic control by gas phase mass transfer is not generally capable of predicting REDS microvoiding rates under engine conditions. The kinetic model can be extended to incorporate changes in pore size and distribution, cracking of the microvoided layer, and introduction and cracking of an additional REMS topcoat to the system. The model can be used to generate a reasonable prediction of the time required to fully microvoid a REDS EBC layer on a CMC component in the hot gas path of an aircraft engine or stationary gas turbine.     

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.     

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.     

Evaluation of environmental barrier coatings: A review

Environmental barrier coatings (EBCs) greatly improve the service performance of SiC-based ceramic matrix composites (CMCs) in high-temperature combustion chambers. Working environments with physical ablation, high temperature, and chemical corrosion require the performance of designed EBC materials and/or structures to be properly evaluated before their real applications. In this paper, EBCs’ lifetime-related phase stability, chemical compatibility, and microstructure retainability are discussed. And then, evaluation methods for basic and environmental properties of EBCs are thoroughly reviewed with newly proposed methods and improved techniques. Pros and cons of each method along with some potential strategies/techniques are also provided. We hope this article can give a timely and overall review for efficient and effective evaluation of EBCs and provide guidance not only for beginners but also for seasoned researchers when they design and develop high-performance EBC systems.     

Properties of gas detonation ceramic coatings and their effect on the osseointegration of titanium implants for bone defect replacement

An optimal performance of bone implants with bioceramic coatings is closely related to the surface modification technology. For the first time, we have evaluated a gas detonation deposition (GDD) approach to obtain biocompatible ceramic coatings based on bioglass (BG) and calcium phosphates on Ti-based alloys as prospective materials towards their application for the development of bone implants. For the production of the coatings, hydroxyapatite (HA), HA metal-substituted (containing Ag⁺, Cu²⁺, or Zn²⁺) and tricalcium phosphate (TCP) were synthesized and characterized. Pure powders and their combination with BG were used to obtain coatings on a Ti–6Al–4V alloy using the developed automatized GDD setup. The microstructure, phase and chemical composition of the produced coatings were studied using XRD, SEM-EDS and Raman spectroscopy. The produced coated materials were evaluated in vivo in Wistar rats to analyze a reparative osteogenesis over a period of 12 weeks. The results regarding the optimization of the GDD method indicate its high productivity, as confirmed by high deposition rates. The highest deposition rate was observed for the coatings obtained from the HA metal-substituted powders. The results revealed a partial transformation of a HA phase to an α-TCP phase during the deposition, with a prevalence of the HA-phase in the coatings. According to the histological evaluation, the reparative osteogenesis occurs through the perimeter of the titanium implants, whereas the regeneration level increases from the 4th to the 12th week. The highest osteointegration level was detected for the implants coated with a biocomposite consisting of BG, HA and TCP. The results of the current study demonstrate an effectiveness of the GDD method to produce biocompatible coatings on Ti-based alloys. This provides excellent prerequisites towards the application and standardization of the GDD technology to manufacture bone implants for bone fixation and defect replacement, as well as the development of dental implants.