Environmental Barrier Coatings Research Articles

CMAS corrosion resistance of rare earth phosphates at high temperatures for environmental barrier coatings

CMAS corrosion resistance of rare earth phosphates at high temperatures for environmental barrier coatings

Tensile properties of R512E coated Nb[sbnd]C103 manufactured using laser powder bed fusion (L-PBF)

C103 is a niobium based refractory alloy commonly used in the aerospace industry. To protect the metal from high temperature, oxidizing environments, C103 is often coated with an environmental barrier coatings. A common coating for this material is known as R512E, a disilicide slurry applied and cured with heat. This paper aims to determine the effects on mechanical properties of common post processing techniques used on C103 produced via laser powder bed fusion. Tensile specimens were produced in both horizontal and vertical orientations, post processed via hot isostatic pressing, coated, and further heat treatment to simulate a typical operating environment for the coated material. Tensile tests were then conducted at room temperature and compared to ASTM B654 standards for wrought C103. Strength decreased after HIP and coating but remained unchanged after further heat treat. Grain growth was evident after HIP and coat applications which is in direct correlation with decreasing strength and increasing ductility.

Phase-Field Modeling of Thermally-Grown Oxide and Damage Evolution in Environmental Barrier Coatings

Phase-Field Modeling of Thermally-Grown Oxide and Damage Evolution in Environmental Barrier Coatings

Water vapor corrosion behaviours of nanostructured Yb2O3-Yb2SiO5/mullite/Si environmental barrier coatings

In this work, the nanostructured Yb2O3-Yb2SiO5/mullite/Si environmental barrier coatings (EBCs) were designed and prepared by atmospheric plasma spraying. The water vapor corrosion behavior of coatings was investigated at a temperature of 1350 °C for durations ranging from 100 to 500 hours, and the influence of phase compositions, microstructure on this behavior was systematically investigated. The results indicate that phase transformation occurs during the water vapor corrosion process, and severe corrosion takes place at the Yb2O3-Yb2SiO5 top coating and the mullite intermediate layer. Obvious diffusion reaction between Yb2O3-Yb2SiO5 and mullite layers occurred without water vapor, while the diffusion reaction cannot be observed under water vapor. The water vapor corrosion mechanism of EBCs is mainly attributed to residual thermal mismatch stresses, phase transformation resulting from sintering of Yb2O3-Yb2SiO5, as well as the formation of TGO by oxidation.

β‐Cristobalite thermal expansion and stability in environmental barrier coating systems

AbstractSiC‐based ceramic matrix composites (CMCs) for turbine engine applications require environmental barrier coatings (EBCs) to protect against water‐vapor induced corrosion. A significant limiting factor of EBC operating lifetime is the formation of a SiO2 thermally grown oxide (TGO) at the EBC–CMC interface at high temperature. Its rapid growth as well as displacive β→α cristobalite phase transformation upon cooling induces strains at this interface, which can result in coating spallation. Recent work has explored upon the use of modifier oxides in EBCs to reduce oxidizing specie transport and possibly stabilize the SiO2 TGO. This work utilizes EBC–glass mixtures to characterize the properties of modifier‐stabilized SiO2 and assess its impact in future EBC systems.

Synthesis of high-entropy Ti-Zr-Nb-Hf-Ta carbides and carbonitrides in high-speed arc discharge plasma jet

The interest in high-entropy materials has increased rapidly in recent decades due to their applications in various fields such as environmental barrier coatings, superhard and wear resistant coatings, nuclear energy, batteries, catalysts, thermoelectrics, supercapacitors, biocompatible structures, and microelectronics. In the present work, comprehensive theoretical and experimental studies are carried out to discover a new way to prepare high-entropy ceramic nanopowders of carbides and carbonitrides of IV-V transition metals. The possibility of (TiZrNbHfTa)CxNy formation is investigated using both ab initio and machine learning approaches. The chosen single-stage plasma dynamic technique allowed us to synthesize high-entropy carbide TiZrNbHfTaC5 and the corresponding carbonitrides (N up to 8 wt%) in the form of single-crystalline nanoparticles. By varying the experimental system conditions, we demonstrate not only the production of pure powders, but also the ability to apply different precursors, including pure metals and their oxides. The presented technique provides a simple and universal way to produce high-entropy nanomaterials and opens the door to the synthesis of many functional ceramic powders composed of other carbonitrides with selective nitrogen content.

Corrosion behaviors and failure mechanism of plasma sprayed Yb2Si2O7/Si environmental barrier coatings exposed to CMAS+ NaVO3

Aero-engine service environments are complex and variable, environmental barrier coatings are usually exposed to the coupled corrosion of CMAS and molten salt. Clarifying the relevant corrosion mechanisms is of guiding significance for the protection of environmental barrier coatings. In this study, the CMAS+NaVO3 (CN) corrosion behavior of a typical Yb2Si2O7/Si coating prepared by atmospheric plasma spray (APS) technique was explored. Results showed that CN was completely melted and uniformly spread at 1200℃, a temperature lower than the melting point of CMAS. Interaction between the Yb2Si2O7/Si coating and CN produced a discontinuous non-stoichiometric garnet phase at 1200℃. At 1300 °C, the reaction produced an apatite reaction layer, but when the duration was extended to 50 h, the apatite layer was gradually dissolved. Moreover, the CN melt infiltrated into the entire Yb2Si2O7 coating, causing buckling of the Yb2Si2O7 layer and debonding with the Si layer, ultimately leading to failure of the Yb2Si2O7/Si coating.

Understanding Environmental Barrier Coating Lifetimes and Performance for Industrial Gas Turbines

Abstract Hydrogen or hydrogen blend fuels are expected to replace natural gas in land-based industrial gas turbines (IGTs) to support a greener power economy. Silicon carbide (SiC) base ceramic matrix composites (CMCs) are considered for replacement of Ni-based superalloys to facilitate future efficiency improvements. SiC CMCs require environmental barrier coatings (EBCs) to mitigate volatilization from high-temperature steam, thus making the EBC lifetime critical information for identifying CMC component lifetimes. The goal of this project is to determine the maximum bond coating temperature underneath the EBC for achieving an IGT component lifetime goal of 25,000 h, which is far greater than current CMC component lifetime requirements for aeroturbine applications. To provide data for the lifetime model, laboratory testing used atmospheric plasma-sprayed rare-earth silicate EBCs on monolithic SiC substrates with an intermediate Si bond coating. Specimens exposed to 1-h thermal cycles in flowing air–steam environments and reaction kinetics were assessed from 700 °C to 1350 °C by measuring the thickness of the thermally grown silica scales. The silica growth and phase transformation appear critical in predicting EBC lifetime and several strategies have been explored to reduce the oxide growth rate and improve EBC durability at elevated temperatures. Advanced characterization using Raman spectroscopy has helped clarify this system.

Design and Manufacture of SiC/SiC Nozzle Guide Vanes With Environmental Barrier Coatings for High-Pressure Turbines

Abstract Increasing the efficiency of jet engines is essential to meet the demanded climate targets. Ceramic matrix composites (CMCs) are strong candidates for aircraft applications because they withstand high temperatures, while their density is two-thirds lower than that of conventional nickel-based alloys. This leads to cooling air savings and a lower overall engine weight, resulting in a potential reduction of emissions. To investigate the potential benefits and manufacturing techniques required for the introduction of CMC to the high-pressure turbine (HPT) of a modern jet engine, the geometry of a nozzle guide vane of an existing turbine was redesigned considering ceramic specific constraints. Then, the liquid silicon infiltration (LSI) process was used to manufacture silicon carbide fiber-reinforced silicon carbide (SiC/SiC) nozzle guide vanes (NGV). Hi-Nicalon S woven fabric was used together with a chemical vapor infiltration (CVI)-based fiber coating. The outer surface of the vane was ground to meet the requirements for surface roughness, and geometric and positional tolerances. Cylindrical, laser-drilled cooling holes were introduced for trailing edge cooling. In the final step, an environmental barrier coating (EBC) system consisting of yttrium disilicate (Y-DS) and yttrium monosilicate (Y-MS) layers was applied using physical vapor deposition (PVD) processing. Wind tunnel testing under technology readiness level (TRL) 4 will be performed and vane performance will be evaluated.

Expanding the Capability of a Legacy Combustion Flame Tube to Test High-Temperature Engine Materials in Relevant Environments

Abstract New materials and component designs are needed to advance gas turbine engine technology and provide the performance and efficiency needs for future applications. In order to advance these materials, testing in combustion environments is a critical step prior to engine testing. In this work, we detail the design and the fabrication of a materials test sector in a flame tube combustor facility. The facility simulates a combustion environment similar to that experienced by components in gas turbine engines. The flow regime is characterized by a combination of high-temperature, high-heat flux, and high-velocity that components experience in gas turbine engines. Exposure of components in this facility allows for the study of combined environmental effects and the impact on both coating and substrate durability. The test facility may operate across a wide range of pressures from 275 to 400 psig (1896–2758 kPa) and an air flowrate of 5 lb/s (2.27 kg/s). While combustion gas temperature is expected in excess of 3000 °F (1649 °C), 900 °F (482 °C) cooling air may be supplied to the backside of components or test articles. The flame tube combustor was previously used to evaluate fuel injectors and combustion products, and the new test configuration will also allow for materials exposure to complex, engine-like conditions. The interior of the test section was additively manufactured from GRCop-84 and cryogenically fit and brazed to a stainless-steel 304 housing. The use of a copper liner minimizes welds and, with active cooling, is expected to provide better durability over traditional hardware using stainless-steel or Inconel with a ceramic liner. The test section has two opposing 9.5 in × 3.125 in (241 mm × 80 mm) removable windows that can accommodate articles up to 3.5 in (89 mm) tall. This modular design allows for custom platforms to hold coupons, panels, or airfoil shapes to be tested with minimal re-engineering or fabrication. The bolted joint and sealing remains consistent, so any new testing only needs to work within the existing design footprint. This paper will provide an overview of the facility capabilities, design considerations, as well as thermal and structural analysis of the hardware. Future testing of ceramic matrix composite (CMC) airfoils and advanced environmental barrier coatings (EBCs) will also be discussed.

Thermal cycling and steam corrosion behavior of Yb3Al5O12‐based multilayered environmental barrier coatings for SiCf/SiC

AbstractTo alleviate thermal mismatch problem of environmental barrier coatings (EBCs) on SiC fiber‐reinforced SiC ceramic matrix composites (SiCf/SiC CMCs) surface and improve their high‐temperature durability for aircraft engines, by fully utilizing the appropriate thermal expansion coefficient, low oxygen transmittance, low thermal conductivity, and other advantages of ytterbium aluminum garnet (Yb3Al5O12), the double ceramic layered Yb3Al5O12/Yb2Si2O7 (DCL‐Yb3Al5O12) and the triple ceramic layered Yb3Al5O12/Yb2SiO5/Yb2Si2O7 (TCL‐Yb3Al5O12) EBC systems were prepared on SiCf/SiC CMCs by atmospheric plasma spraying (APS). Their phase composition and microstructures were investigated comparatively. The thermal cycling and water vapor/oxygen corrosion behavior of these coating systems were compared at 1300°C. The results showed that the thermal cycling life of the DCL‐Yb3Al5O12 and TCL‐Yb3Al5O12 EBC systems were 295 and 320 times, respectively. The failure of both the coating systems occurred between the Yb2Si2O7 layer and Si bond coat. The strength retention rate of DCL‐Yb3Al5O12 and TCL‐Yb3Al5O12 EBC systems after water vapor/oxygen corrosion for 70 h was 12.8% and 23.1%, respectively, and the fracture modes of both the systems exhibited “pseudoplastic” characteristic. TCL‐Yb3Al5O12 EBC systems with a gradient structure of more layers exhibit more excellent high‐temperature durability than DCL‐Yb3Al5O12 EBCs.

Computational design of high-entropy rare earth aluminum garnets for advanced thermal and environmental barrier coatings

Computational design of high-entropy rare earth aluminum garnets for advanced thermal and environmental barrier coatings

Environmental barrier coatings on alumina-reinforced CMCs: Experimental validation and numerical modeling of thermal stability

Recent advancements in ceramic matrix composites (CMCs), including SiC/SiC, C/SiC, C/C, Al2O3 reinforced Al2O3, face various challenges at high temperature applications, including grain growth, sintering, and creep deformation which are leading to strength loss. The new generation of Environmental Barrier Coatings (EBCs) must allow the protection of these CMCs when operating under harsh conditions, which can reach up to 1100 °C in some applications, such as gas turbines and certain reforming processes. In this work, 8 wt% yttria stabilized zirconia (8YSZ) applied by Atmospheric Plasma Spraying (APS) on top of a porous alumina oxide matrix with reinforced alumina fibers Al2O3/Al2O3-CMC (OCMC), considering different interfaces, has been studied. Surface morphology and pre-treatment of CMCs is the most challenging part for thermal spray applications. Hence, two different surface structure were used in this work. The results show that the roughness of OCMC can be reduced from Rz = 41.8±6.8 μm to 20.3±1.1 μm with the new porous bond coat and plasma sprayed coating, resulting in a homogeneous surface morphology.. In terms of thermal stability, a numerical model that combines Object Oriented FEM (OOF) and Cohesive Zone Model CZM tools, has been developed to evaluate the effect of the porosity of the real microstructure and the characteristics of the interfacial roughness profile, respectively, in CMC/EBC structures. In addition, other materials, such as La2Zr2O7 and Y2O3, have been numerically studied in order to evaluate their suitability as EBC.

Novel anti-oxidation coating prepared by polymer-derived ceramic for harsh environments up to 1200 °C

Environmental barrier coatings (EBCs) are protective barriers that could prevent internal materials from oxygen and harmful agents. However, the combination between EBCs and polymer-derived ceramics (PDCs) is relatively hard but essential. In this work, an anti-oxidation coating based on PDCs is fabricated on target SiBCN ceramics by spraying. The coating reveals a uniform and continuous structure with an approximate thickness of 11.7 μm. Experiments and characterization such as oxidation, thermal shock, scratch, and electrical resistance tests were carried out to investigate the performance of coating. The results show that during the high-temperature oxidation process, the coating surface undergoes a molten phase transformation that effectively seals pores and cracks, preventing oxygen diffusion into the ceramic interior. Moreover, the coating can withstand 20 thermal shocks without failure, and firmly adheres to the substrate even after prolonged oxidation and repeated thermal cycling. Comparative analysis of temperature-resistance results demonstrates the efficacy of the coatings in protecting against oxidation up to 1200 °C. The developed PDC-EBCs here exhibit remarkable resistance to oxidation and thermal shock, along with high insulation resistivity. This strategy offers a straightforward and efficient solution for protecting electronic materials in harsh environments.

Enhanced thermal shock resistance of SiCf/SiC composites with Y-Al-Si-O glass ceramic environmental barrier coatings in water-oxygen conditions at 1350°C

In this study, Y-Al-Si-O(YAS) glass-ceramic environmental barrier coatings were prepared after modifying CVI-SiCf/SiC composites with microcrystalline glass. The resistance of both SiCf/SiC and SiCf/SiC with YAS-EBC to thermal shock was systematically examined using a water quenching method. The findings demonstrate that the SiCf/SiC composites with YAS-EBC exhibits outstanding thermal shock resistance. The coefficient of thermal expansion (CTE) of the YAS coating exhibits minimal deviation from that of SiCf/SiC and possesses a low modulus of elasticity. Upon experiencing thermal shock, the YAS coating is subjected to tensile stress, with an average thermal stress of merely ∼4.4 MPa. After undergoing 100 cycles, there was no evidence of debonding between the coating and the substrate, nor was a thermally grown oxide (TGO) layer observed at the interface. This research provides a credible method and theoretical foundation for enhancing the thermal shock resistance behavior of CVI-SiCf/SiC composites.

Corrosion resistance of multicomponent disilicate (Ho0.2Er0.2Tm0.2Yb0.2Lu0.2)2Si2O7 against CMAS and volcanic ash

With the increasing operating temperature of gas turbine engines, calcium-magnesium-aluminosilicate (CMAS) poses a serious threat on environmental barrier coatings (EBCs) applied on hot-sections of aero-engines. Here, we have synthesized a novel multicomponent disilicate—(Ho0.2Er0.2Tm0.2Yb0.2Lu0.2)2Si2O7 (brief to (5RE0.2)2Si2O7), and comparatively studied its performance in the presence of synthesized CMAS and natural volcanic ash at 1400ºC. In comparison with Yb2Si2O7, (5RE0.2)2Si2O7 has a shorter Si-O bond length and a larger RE-O bond length because of the larger average RE3+ radius. After CMAS corrosion, some apatite grains precipitate at the CMAS/(5RE0.2)2Si2O7 interface to develop a loose reaction layer, exhibiting a higher corrosion resistance than Yb2Si2O7. Meanwhile, the consumption of CaO and release of SiO2 during the chemical reaction process increase the viscosity of CMAS to some extent and thus weaken its infiltration propensity. For the volcanic ash case, it directly infiltrates into the interior of (5RE0.2)2Si2O7 along grain boundaries without any reaction due to the relatively low CaO content, exhibiting a more serious attacking behavior. In addition, (5RE0.2)2Si2O7 effectively increases the contact angle of molten volcanic ash due to its lower surface energy. These finds here provide a better understanding for the design and application of next-generation EBC material.

Influence of annealing on the CMAS corrosion behaviour of ytterbium disilicate environmental barrier coatings: A Raman imaging study

The study examined the CMAS corrosion behaviour of ytterbium disilicate EBCs using Raman imaging before and after annealing. Annealing transformed the coatings to crystalline, healing pores and cracks, and reducing corrosion depth and rate. Raman spectroscopy showed increased residual stresses with corrosion time, while sprayed coatings had lower stresses and higher susceptibility to damage. This research enhances the use of Raman spectroscopy in non-destructively characterizing EBCs degradation under CMAS conditions, offering insights for aeroengine combustion chamber design improvements.

Enhanced steam thermal cycle resistance of Yb2Si2O7 environmental barrier coatings via Al modification

SiC-based ceramic matrix composites are susceptible to performance losses in steam environments, necessitating the use of environmental barrier coatings for their protection. The high Si activity of Yb2Si2O7 material may induce the emergence of Si(OH)4 in steam environments, resulting in the development of a pore structure within Yb2Si2O7. This pore structure weakens the service life of the coating, highlighting the importance of enhancing the steam corrosion resistance of environmental barrier coatings as a key factor in improving their overall longevity. In the present study, we employed Al modification technique to improve the steam corrosion resistance of Yb2Si2O7 coatings. The results indicated that Al-modified Yb2Si2O7 coatings exhibited superior steam thermal cycle corrosion resistance compared to the non-modified Yb2Si2O7 coatings after testing performed at 1523 K for 1000 h (10 h per cycle). This improvement was attributed to the presence of Yb3Al5O12 and Al2O3 composite phases within the Al-modified Yb2Si2O7 coatings, which exhibited low steam activity and effective steam corrosion resistance. Moreover, Al-modified Yb2Si2O7 coatings obtained the dual excellent performance of improving the steam corrosion resistance of Yb2Si2O7 coating and not weakening the thermal cycle ability, providing a guarantee for the longer life of Yb2Si2O7 coating.

High-temperature oxidation behavior and mechanical properties of PS-PVD sprayed Yb2Si2O7 environmental barrier coatings

The high-temperature oxidation behavior and mechanical properties of the Yb2Si2O7/Si environmental barrier coatings (EBCs) prepared by plasma spray-physical vapor deposition at 1250 °C and 1350 °C were investigated, and obvious delamination was observed after oxidation for 4000 h. The results showed that element diffusion occurred during oxidation, and the diffusion of oxygen led to the formation of thermal growth oxides (TGO), which induced pores and microcracks. The TGO played a role in hindering element diffusion, but the mismatching coefficients of thermal expansion and phase transition caused vertical and transverse cracks in the TGO layer. These cracks became channels for oxygen diffusion and ultimately resulted in delamination between TGO and the bonding coating (BC) layer. Furthermore, temperature significantly affects the oxidation kinetics of EBCs, and the oxidation within the BC layer alters its elastic modulus. The elastic modulus of the BC layer first increased and then decreased. The oxidation damage was more pronounced at 1350 °C.

Thermal properties and anti-corrosion behavior of multicomponent (Dy1/5Er1/5Tm1/5Yb1/5Y1/5)2SiO5 monosilicate against water vapor attack at high temperature

Materials with a thermal expansion coefficient compatible with silicon carbide based ceramic matrix composites and low thermal conductivity at higher temperatures are crucial for developing environmental barrier coatings (EBCs) for aero-engines. Lately, high entropy silicate engineering has been a promising approach to develop optimized EBC materials. The solid solution reaction method was adopted for the fabrication of high entropy multicomponent (Dy1/5Er1/5Tm1/5Yb1/5Y1/5)2SiO5 or (5RE1/5)2SiO5 monosilicate. Scanning electron microscopy and X-ray diffraction show the homogenous distribution and a single-phase solution of (5RE1/5)2SiO5 monosilicate. Differential scanning calorimetry testing was done at 1450 °C which shows no exo/endothermic reaction has occurred. The value of the coefficient of thermal expansion (6 ppm/°C) up to temperature 1500 °C and low thermal conductivity values for (5RE1/5)2SiO5 monosilicate were observed (1.1–1.4 W/m°C) in the temperature range between 25 and 1500 °C. The water vapor corrosion resistance of (5RE1/5)2SiO5 monosilicate further confirms that the material is suitable for EBC applications due to the synergistic effect of different rare earth radii and stable crystal structure with a negligible weight loss value of 0.0005 mg/cm2 after 150 h corrosion time under 90 % H2O and 10 % O2 at 1300 °C. The results of this work suggest that (5RE1/5)2SiO5 monosilicate can be used as a promising candidate for environmental barrier coatings.