Environmental Barrier Coating Systems Research Articles

Mechanism of stress evolution in environmental barrier coatings considering the presence of crack channels

The roles of crack channel transitions and thermally grown oxides (TGO) in the failure of environmental barrier coatings (EBCs) should be clarified. Hence, in this study, the laws of penetration and delamination crack evolution were investigated using a virtual crack extension method for multilayer materials to gain insights into the transformation conditions with a framework for a coupled thermal/mechanical/chemical numerical model of TGO containing channel cracks developed based on this law. The framework integrates diffusion, TGO growth, and phase transformation of the oxides. A theoretical model for high-temperature gas corrosion of EBCs coupled with deformation, mass diffusion, and chemical reactions was proposed, with the results indicating that the cracks transitioned from vertical to exfoliation cracks in the ytterbium dibasic silicate (YbDS) layer. The coupling effect of cracks and stresses led to non-uniform growth of the TGO by affecting the oxygen diffusion and inflow rates. The y-direction stresses at the YbDS/TGO interface near the crack channel changed from compressive to tensile stresses after cooling, making the first location to undergo delamination failure. The proposed model can provide better guidance for designing EBCs systems and supporting the life prediction of EBCs in service environments.

Thermal shock resistance of a novel (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 environmental barrier coating

AbstractMulticomponent rare earth (RE) disilicates have been recognized as promising candidates for environmental barrier coatings (EBCs). In this work, a novel EBC system with (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 as top coat and Si as bond coat was deposited on SiC‐based substrates by atmospheric plasma spraying technology and subjected to 200 thermal shock cycles at 1350°C. The composition and microstructure evolution in the coating during thermal cycling were investigated using x‐ray diffraction analysis and scanning electron microscopy. The results show that the as‐prepared EBC remains dense and integrated, and no obvious spalling or penetrating cracks could be observed in the coating after 200 thermal shock cycles. Furthermore, the average thickness of thermal growth oxide layer between (6RE1/6)2Si2O7 top coat and Si bond coat is 0.71 ± 0.17 µm and 0.98 ± 0.23 µm before and after the thermal cycling, respectively, indicating the excellent thermal shock resistance of (6RE1/6)2Si2O7 EBC. The present study presents a novel EBC with excellent thermal shock ability and is expected to contribute to the development of EBC for advanced SiCf/SiC ceramic matrix composites.

Design, synthesis and characterization of multilayer environmental barrier coatings

The formation of cristobalite thermally grown oxides (TGO) is a major life limiting factor for environmental barrier coatings (EBCs) with Si bond coat exposed to high temperature steam environments due to its reversible phase transformation and the resultant cracking behaviour during thermal cycling. Therefore, controlling or avoiding cristobalite TGO formation by engineering coating chemistry and architecture will be desirable for a highly durable EBC system. In this work, a multilayer Si/Al-Si-N/Yb2Si2O7 EBC was produced using a hybrid vapor deposition process. The oxidation behavior of the EBC was evaluated in flowing 90% H2O (g) + 10% air at 1316 °C. After 100 h and 510 h steam test, crack-free mullite and Al-Si-O-N layers formed above Al-Si-N layer and no SiO2 scale formation of Si bond coat was observed. Detailed TEM analyses show that the Al-Si-N layer successfully prevented the oxidation of underlying Si bond coat and avoided the formation of detrimental cristobalite phase.

Synthesis and characterization of single-phase X2-Lu2SiO5 nanostructured feedstocks for advanced plasma-sprayed environmental barrier coatings

The objective of this work is to synthesize the single-phase X2-Lu2SiO5 nanostructured feedstocks for plasma-sprayed environmental barrier coatings (EBCs). Generally speaking, high-performance feedstocks are a prerequisite for obtaining high-performance coatings. The single-phase X2-Lu2SiO5 nanostructured feedstocks were fabricated by a nanopowder reconstitution method for the first time. The morphology and phase structure of X2-Lu2SiO5 nanostructured feedstocks were examined. The physical properties of X2-Lu2SiO5 feedstocks were evaluated. In addition, the thermal conductivity of nanostructured X2-Lu2SiO5 coatings was measured. Results show that the single-phase X2-Lu2SiO5 nanostructured feedstocks are successfully synthesized when the mole ratio of Lu2O3 and SiO2 nanopowder is 1:1.1 and the agglomerated powders are sintered at 1300 °C for 2 h. The nanocrystalline size of the X2-Lu2SiO5 feedstocks ranges from 40 to 70 nm. Moreover, the d10, d50 and d90 of X2-Lu2SiO5 feedstocks are 24.69 μm, 37.78 μm and 54.75 μm, respectively. The apparent density and tap density of X2-Lu2SiO5 feedstocks are 1.76 g/cm3 and 2.09 g/cm3, respectively. The thermal conductivity of as-deposited Lu2SiO5 coating is 0.78–1.20 W m−1 K−1 (30–1200 °C). Overall, the single-phase X2-Lu2SiO5 nanostructured feedstocks are the most promising feedstocks for advanced EBCs system in the future.

Ytterbium Disilicate/Monosilicate Multilayer Environmental Barrier Coatings: Influence of Atmospheric Plasma Spray Parameters on Composition and Microstructure

SiC/SiC ceramic matrix composites (SiCf/SiC CMCs) are regarded as the new materials for the hot-section components of aircraft gas turbine engines, since they have one-third of the density of metallic superalloys, a higher temperature capability, good mechanical strength, and excellent thermal shock resistance. However, high-temperature water-vapor-rich combustion gases can induce severe surface recession phenomena in SiC/SiC leading to component failure. For this reason, it is necessary to design protective coatings, i.e., environmental barrier coatings (EBCs), able to protect the SiC/SiC surface in combustion environments. In the present work, ytterbium monosilicate (Yb2SiO5), stable when exposed to water vapor at high temperatures, and ytterbium disilicate (Yb2Si2O7), characterized by a thermal expansion coefficient closer to that of the substrate, were selected for a multilayer EBC system. EBCs were processed using the atmospheric plasma spray (APS) technique. A set of deposition parameters were tested, varying the power of the torch, and the composition and microstructure of the deposited coatings were studied in terms of porosity, crack density, and post-deposition phase retention by performing SEM, EDS, and XRD analysis. The results allow for the definition of the influence of deposition parameters on the final properties of multilayer EBC coatings.

Effect of heat treatment on hardness and fracture toughness of Si/BSAS/Yb2SiO5 EBCs system under 1200 °C

High-temperature oxidation environments have an important influence on phase transition and thermal/residual stresses of Si/BSAS/Yb2SiO5 environmental barrier coatings (EBCs). To further reveal the failure mechanisms of Yb2SiO5 EBCs, different kinds of Si, BSAS, Yb2SiO5 coatings deposited on Cf/SiC composites were prepared by air plasma spraying (APS) technique, respectively. The content and microstructure variations of all coatings before and after different heat treatments were analyzed by X-ray diffraction and scanning electron microscope methods, respectively. The corresponding Vickers hardness, fracture toughness and residual stresses were evaluated by indentation tests at room temperature. The hardness and fracture toughness of the coating after heat treatment at 1200 °C were systematically studied. It is found that with the increase of heat treatment, the porosity of the coating decreases, and the hardness and fracture toughness of the coating increases. Porosity of the coating is affected by phase transformation and grain growth. The residual compressive stress of the coating is positively correlated with the hardness and fracture toughness of the coating.

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

AbstractA 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.

Failure mechanism of ytterbium silicate/silicon bi-layer environmental barrier coatings on SiCf/SiC composites upon long-time water vapor and oxygen corrosion test

Environmental barrier coatings (EBCs) are crucial for SiCf/SiC composites to resist water vapor-oxygen corrosion and maintain the stability of their structure and properties in a complex combustion environment at high temperatures. In this work, an EBC system, composing Yb2Si2O7 topcoats and Si bond coats, has been prepared by an air plasma spraying (APS) route to completely wrap up SiCf/SiC composites. Then the corrosion and failure mechanisms of the EBC system in a water vapor-oxygen environment are systematically studied. The results indicate that the SiCf/SiC composites exhibit excellent stability under the full protection of the current EBCs. With the progression of corrosion, due to the difference of the thermal expansion coefficients (TECs) between the topcoat and bond coat materials, the vertical mud cracks generated by the thermal cycles become the intrusion passages for the corrosion species and then form transverse cracks and pores, resulting in the peeling off of the Yb2Si2O7 topcoat. Moreover, in the later stage of thermal cycles, the residue of the Si bond coat is shown to play an important protective role against the rapid corrosion of the substrates.

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.

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.

Microstructure and phase composition evolution of dual-phase ytterbium silicate coatings plasma sprayed from stoichiometric Yb2Si2O7 feedstock powder

The main challenges of atmospheric plasma spraying (APS) when dealing with rare earth silicate are the amorphous phase formation and silica volatilization in as-sprayed coatings. In this study, the microstructure and phase composition evolution of dual-phase ytterbium silicate environmental barrier coatings (EBCs) plasma sprayed from stoichiometric Yb2Si2O7 feedstock powder are investigated. The unavoidable silica volatilization leads to the coating composition deviate from stoichiometry of disilicate composition and result in the presence of Yb2SiO5 in coatings. By controlling the phase fraction of Yb2SiO5 through modulation of plasma character allows minimizing the effects of thermal residual stress on the coating durability in thermal cycling. The high crystallinity and crack-free coatings were achieved through optimized crystallization heat treatment. The presence of dislocation and deformation twins endow the dual-phase coating with intrinsic quasi plasticity and improvement in reliability. The present study provides useful inspirations on the optimization of spraying parameters and design of advanced EBC systems.

Micromechanical analysis and theoretical predictions towards thermal shock resistance of HfO2-Si environmental barrier coatings

As HfO2-doped Si bond coat acts a pivotal part in application of progressive environmental barrier coating (EBC) system owing to improving the high-temperature capability and creep properties of bond coat, it is essential to control the doping content and to balance the mechanical and thermal properties of doped coatings challenge in terms of upgrading service life and operating temperature of EBC system. In this study, the physical properties and thermal shock performance at 1723 K of HfO2-Si composite coatings with three different compositions were investigated via first-principle calculations and micromechanical model. Evidently, the composite coatings deposited by magnetron sputtering exhibit a better resistance towards crack initiation and propagation than pure Si bond coat up to 100 h thermal cycling time. Such improvement is attributed to the dispersion toughening effect caused by the characteristic structure of dispersed HfSiO4 particle within cristobalite “mortar”. With thermal cycling, the continuous Ostwald ripening of HfSiO4 particles decreased the elastic modulus and effective thermal expansion coefficient gap at room and high temperature of composite coatings, which was advantageous to thermal shock resistance. Unfortunately, as a consequence of excessive volume expansion during particle ripening, the composite coatings encountered varying degrees of interface separation after 100 cycles and hence not protective anymore. The results suggested that the further modification for low-frequency vibration patterns of HfSiO4 and realizing the interfacial metallurgical bonding can reduce the thermal expansion coefficient of HfO2-doped Si composite coatings and strengthen interface adhesion.

Mechanical-Chemical Coupling Effects on an Environmental Barrier Coating System under High-Temperature Water Vapour Conditions.

The degradation mechanisms for environmental barrier coatings (EBCs) under high-temperature water vapour conditions are vital for the service of aero-engine blades. This study proposes a theoretical model of high-temperature water vapour corrosion coupled with deformation, mass diffusion and chemical reaction based on the continuum thermodynamics and the actual water vapour corrosion mechanisms of an EBC system. The theoretical model is suitable for solving the stress and strain fields, water vapour concentration distribution and coating corrosion degree of an EBC system during the water vapour corrosion process. The results show that the thickness of the corrosion zone on the top of the EBC system depended on water vapour diffusion, which had the greatest influence on the corrosion process. The top corroded area of the rare-earth silicate EBC system was significantly evident, and there was a clear dividing line between the un-corroded and corroded regions.

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 (SiCf/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 SiCf/SiC. In this study, Si/Y2SiO5/LaMgAl11O19 (LMA) multi-layer EBCs were fabricated to protect SiCf/SiC by using atmospheric plasma spraying (APS). The high-temperature tensile fatigue performance of SiCf/SiC with and without EBCs was evaluated. The results indicated that EBCs significantly improved the tensile fatigue properties of SiCf/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 SiCf/SiC after some improvements.

Solid particle erosion of a plasma spray – physical vapor deposition environmental barrier coating in a combustion environment

Environmental barrier coatings (EBCs) were developed to reduce the susceptibility of SiC-based composites to the rapid volatilization and surface recession that occurs in the presence of water vapor. Extensive research and development has been performed on EBCs, and their corresponding failure mechanisms under different environmental conditions. However, one key failure mechanism, solid particle erosion (SPE), has not been thoroughly investigated. As a result, the present work investigates the SPE behavior of a ytterbium disilicate (Yb2Si2O7) environmental barrier coating (EBC) deposited via plasma spray-physical vapor deposition (PS-PVD). Erosion testing was performed at elevated temperature (1200 °C) in a simulated combustion environment at the NASA Glenn Research Center Erosion Burner Rig Facility. Alumina (Al2O3) particles were used as the eroding media. A range of particle kinetic energies and impingement angles were investigated along with the effect of coating surface roughness. The post-erosion damage morphology was characterized using scanning electron microscopy (SEM). The SPE behavior of the PS-PVD EBC was shown to be comparable to other EBC systems reported in the literature.

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

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

Anisotropic crack propagation behavior for the silicon‐bond coat layer in a multilayer coated system

AbstractThis study sought to examine the relationship between the degradation mechanism, thermal stress, and crack propagation behavior in environmental barrier coating (EBC) systems. An EBC system composed of a mullite topcoat (TC), Si‐bond coat (BC), and SiC substrate was prepared by atmospheric plasma spraying. Heat exposure tests were conducted to evaluate the microstructure of the EBC system at 1300°C for 1, 10, 50, and 100 h. The fracture resistance of the Si BC for the in‐plane (direction parallel to each layer, 0.4–0.6 MP) and through‐thickness directions (direction from the TC to substrate, 1.7–2.1 MP) differed because a thermal compressive stress was induced for the in‐plane direction owing to the mismatch of the thermal expansion coefficients for each layer, which acted as a barrier for crack propagation. However, cracks tended to propagate in the in‐plane direction because they were not affected by the in‐plane compressive stress. These results clearly showed that Si BC exhibited in‐plane anisotropy and crack propagation after heat exposure, which were the major sources of delamination of the EBC system.

Oxidation Performance of Ytterbium Disilicate/Silicon Environmental Barrier Coating via Optimized Air Plasma Spraying

Environmental barrier coatings (EBCs) play a critical role in mitigating the degradation of SiCf/SiC ceramic matrix composites (CMCs) in complex combustion environment, and improve the service life of thermal engine components. In this paper, by adjusting the parameters of atmospheric plasma spraying (APS), the spraying process of ytterbium disilicate (Yb2Si2O7) under a lower power has been optimized. A two-layer EBC system consisting of ytterbium disilicate and silicon is prepared on the SiCf/SiC composite substrate by using optimized technological parameters. The thermal resistance and water oxygen corrosion resistance of such two-layer EBC system are investigated. The results indicate that the current ytterbium disilicate/silicon EBC system exhibits good phase stability, excellent water vapor and oxygen corrosion resistance. However, the exposed silicon bonding layer tends to generate an excessive thermal growth oxide (TGO) layer known as SiO2, leading to an early spallation of the coating.

Diffusivity of Oxygen in Yttrium Silicates Estimated By Using High-Temperature Oxidation of Their Composites Dispersed with SiC Particles

Yttrium silicates, Y2SiO5 and Y2Si2O7, have been studied as candidate materials for the environmental barrier coating (EBC) system on SiC/SiC composites for advanced gas turbine blades because of low oxygen permeability, low volatility in steam conditions, and comparable thermal expansion with SiC/SiC composites. In order to design the EBC system, diffusion properties of yttrium silicates are important. In this work, diffusivity of oxygen in yttrium silicates is estimated by using kinetics of high-temperature oxidation of their composites dispersed with SiC particles. Assuming oxidation of SiC particles dispersed in yttrium silicates is promoted mainly with inward diffusion of oxygen, diffusion of oxygen in yttrium silicates can be estimated. Y2SiO5 powder was synthesized by using the solid-state reaction technique with Y2O3 and SiO2 powders at 1400ºC for 20 h in air. Y2Si2O7 powder was prepared with the solid-state reaction process from powder mixture of Y2O3 and SiO2 at at 1500ºC for 24 h in air. Densified bulks of yttrium silicates dispersed with 5 vol% SiC particles were obtained with the pulsed electric current sintering of powder mixture of yttrium silicate and SiC. The bulk samples were oxidized at 1300ºC in air. Depth of the zone in which SiC particles are oxidized was measured by observing the cross-section with scanning electron microstructure. Thickness of the oxidized zone obeys the parabolic law. In the both cases, the oxidation product layers were not observed significantly on the surface of the composites after high temperature oxidation. These results mean that outward diffusion of Y or Si did not occur significantly for oxidation of SiC dispersed in the yttrium silicates. That is, the oxidation of the SiC dispersoid was conducted with inward diffusion of oxygen. Diffusion coefficient of oxygen in yttrium silicates can be obtained as shown in Table 1. The value for Y2SiO5 is larger than that for Y2Si2O7. This fact means that the protection performance of oxygen diffusion on Y2Si2O7 is better than that on Y2SiO5 for the EBC applications.Table 1 Diffusion coefficients of oxygen in yttrium silicates at 1300°C. Y2SiO5 Y2Si2O7 D O/ m2s-1 2.8×10-16 1.3×10-18

Evolution of microstructure and thermal conductivity of multifunctional environmental barrier coating systems

Environmental barrier coating (EBC) systems are applied to the surface of silicon-based composites exposed to high temperature combustion gas flow paths in gas turbine engines. They reduce the rate of composite oxidation, its volatilization by reactions with water vapor, and the temperature of the composite as their thermal conductivity decreases. Current EBC systems consist of a silicon bond coat covered by an ytterbium disilicate (Yb2O3⋅2SiO2: YbDS) permeation resistant and low silica activity barrier. When applied by atmospheric-plasma spray deposition, this outer layer contains 10–15% of a secondary ytterbium monosilicate (Yb2O3⋅3SiO2: YbMS) phase. YbDS has a coefficient of thermal expansion (CTE) of 4–5×10−6 °C−1, similar to those of the silicon and the silicon-based composite, but a relatively high thermal conductivity of 5–7 W m−1 K−1. YbMS has a higher steam volatility resistance than YbDS, and it has a much lower thermal conductivity (∼2–2.5 W m−1 K−1) at ambient temperature compared to YbDS, but its higher, highly anisotropic CTE (3–11×10−6 °C−1) results in channel cracking which reduces environmental protection. Here, we use a combination of scanning electron beam and laser-based thermoreflectance methods to spatially map the distribution of the silicate phases and thermal conductivity at ambient temperature in a Si-ytterbium disilicate EBC system exposed to thermal cycling in water vapor. We show that during thermal cycling, diffusion of silica from the thermally grown oxide on the Si bond coat surface to nearby YbMS regions transforms this phase to YbDS, thereby reducing the risk of thermomechanical coating failure but decreasing its effective thermal resistance. We also show that as silica is volatilized at the water vapor–ytterbium silicate interface, YbDS is transformed to YbMS, restoring some of the thermal protection of the coating system lost by its reduction in thickness and the YbMS to YbDS transformation near the bond coat.