Multilayer Environmental Barrier Coatings Research Articles

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.

Multi-layered environmental barrier coatings designed on basis of YbTaO4-Ta2O5 composites for protecting SiCf/SiC composites

YbTaO4-Ta2O5 composites were designed and different multi-layered YbTaO4-Ta2O5/Yb2Si2O7/Si (YbT-T/YbDS/Si) as well as YbTaO4-Ta2O5/Si (YbT-T/Si) environmental barrier coatings (EBCs) were put forward to protect SiC fiber reinforced silicon carbide (SiCf/SiC) ceramic matrix composites (CMCs). The thermal cycling and the water-vapor/oxygen corrosion behavior of different multi-layered EBCs were compared. Results indicated that the thermal cycling lifetime of the bi-layered YbT-T/Si EBCs was similar with the tri-layered YbT-T/YbDS/Si ones. The ultimate failure of these two EBCs was induced by the cracking of the SiCf/SiC substrates as well as the vertical cracks associated with the thermal mismatch stress among different layers. However, the tri-layered EBCs exhibited greater resistance to water-vapor/oxygen corrosion than the bi-layered ones, primarily due to that the tri-layered YbT-T/YbDS/Si EBCs contained significantly less cracks than the bi-layered ones, limiting the transport of reactant (oxygen and/or water vapor) to reach Si bond coat.

Cyclic oxidation and water vapor corrosion behaviors of multi-layer environmental barrier coatings with HfO2-Si bond coat for SiCf/SiC

A novel multi-layer HfO2-Si/YbSi1.5/Yb2SiO5/LaMgAl11O19 environmental barrier coatings (EBCs) was prepared using atmospheric plasma spraying method to restrain spallation failure and improve lifetimes of coating. The thermal cycling behaviors of SiCf/SiC completely covered with multi-layer EBCs were comparatively investigated at 1300 °C and 1350 °C in air. Meanwhile the water vapor corrosion behavior of coated sample was also investigated at 1300 °C for 100 h in 90% H2O-10% O2 steam. The coating at 1300 °C showed better thermal shock resistance than the coating at 1350 °C, due to the increase of tensile stresses at 1350 °C. The coating showed different forms of crack penetration due to the influence of thermal cycling temperature and water vapor. The formation of HfSiO4 phase thanks to solid-state reaction between HfO2 and SiO2 alleviate the β→α-cristobalite phase change and avoid the crack initiation. However, the thermal mismatch stress was contributed to the spallation of coating along the surface of the SiCf/SiC substrate.

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.

Determining Interface Fracture Toughness in Multi Layered Environmental Barrier Coatings with Laser Textured Silicon Bond Coat

In the high temperature combustion atmosphere inside of aircraft turbines, the currently used ceramic matrix composites require a protective environmental barrier coating (EBC) to mitigate corrosion of the turbine parts. Besides thermomechanical and thermochemical properties like matching thermal expansion coefficient (CTE) and a high resistance against corrosive media, mechanical properties like a high adhesion strength are also necessary for a long lifetime of the EBC. In the present work, the adhesion between an air plasma sprayed silicon bond coat and a vacuum plasma sprayed ytterbium disilicate topcoat was aimed to be enhanced by a laser surface structuring of the Si bond coat. An increase in interface toughness was assumed, since the introduction of structures would lead to an increased mechanical interlocking at the rougher bond coat interface. The interface toughness was measured by a new testing method, which allows the testing of specific interfaces. The results demonstrate a clear increase of the toughness from an original bond coat/topcoat interface (8.6 J/m2) compared to a laser structured interface (14.7 J/m2). Observations in the crack propagation indicates that the laser structuring may have led to a strengthening of the upper bond coat area by sintering. Furthermore, in addition to cohesive failure components, adhesive components can also be observed, which could have influenced the determined toughness.

Recycling of a Healing Agent by a Water‐Vapor Treatment to Enhance the Self‐Repair Ability of Ytterbium Silicate‐Based Nanocomposite in Multiple Crack‐Healing Test

Yb2Si2O7/Yb2SiO5 composites dispersed with silicon carbide (SiC) possess a self‐crack‐healing ability that makes them promising top‐coat materials for multilayered environmental barrier coatings (EBCs) of SiC/SiC gas turbine blades. Stress‐induced surface cracks can be fully healed at high temperatures by the volume expansion of SiO2 glass and the newly formed Yb2Si2O7 in the composite. The reaction between SiO2 and Yb2SiO5 to form Yb2Si2O7 is considered a critical step that determines the high healing efficiency of this composite, therefore, Yb2SiO5 is considered as the secondary healing agent apart from the primary one, SiC. However, once all the healing agents have reacted, the composite can no longer promote its crack‐healing ability. To retain this property, in this work, Yb2SiO5 is regenerated by a heat treatment in water‐vapor atmosphere at 1073 K. X‐ray diffraction (XRD) and energy‐dispersive X‐ray spectroscopy (EDS) analyses show that the healing agent can be partially recycled after the treatment. In addition, the composite treated in water vapor demonstrates a greater crack‐healing ability compared with the untreated composite. These results open a new path for the development of gas turbine blades and high‐temperature components possessing permanent crack‐healing ability.

Crack Initiation Criteria in EBC under Thermal Stress

For design of multi-layered environmental barrier coatings (EBCs), it is essential to assure mechanical reliability against interface crack initiation and propagation induced by thermal stress owing to a misfit of the coefficients of thermal expansion between the coating layers and SiC/SiC substrate. We conducted finite element method (FEM) analyses to evaluate energy release rate (ERR) for interface cracks and performed experiment to obtain interface fracture toughness to assess mechanical reliability of an EBC with a function of thermal barrier (T/EBC; SiC/SiAlON/mullite/Yb-silicate gradient composition layer/Yb2SiO5 with porous segment structure) on an SiC/SiC substrate under thermal stress due to cooling in fabrication process. Our FEM analysis revealed that a thinner SiAlON layer and a thicker mullite layer are most suitable to reduce ERRs for crack initiation at the SiC/SiAlON, SiAlON/mullite and mullite/Yb2Si2O7 interfaces. Interface fracture tests of the T/EBC with layer thicknesses within the proposed range exhibited fracture at the SiC/SiAlON and SiAlON/mullite interfaces. We also estimated the approximate fracture toughness for the SiC/SiAlON and SiAlON/mullite interfaces and lower limit of fracture toughness for the mullite/Yb2Si2O7 interface. Comparison between ERR and fracture toughness indicates that the fabricated T/EBC possesses sufficient mechanical reliability against interface crack initiation and propagation.

Structural stabilization of multilayered EBCs at high temperatures

Abstract The optimum structure for multi-layered environmental barrier coatings (EBC) with enhanced environmental shielding and structural stability was proposed based on the mass-transfer mechanisms of the EBC constituent oxides such as polycrystalline Yb2Si2O7 (Y2S2) and Al4+2xSi2-2xO10-x (mullite), as determined using the oxygen permeation technique under oxygen-chemical-potential gradient (dµO) at high temperatures. The oxygen permeation proceeded via grain boundary (GB) diffusion of oxygen ions in the mullite layer from the higher oxygen-partial-pressure PO2 surface to the lower PO2 surface, concurrently with GB diffusion of aluminum ions in the opposite direction, while oxygen ions and ytterbium ions were interdiffused along GBs for the Y2S2 layer. Thus, control of the outward diffusion of cations is essential to improve the oxygen shielding and structural stability of the layers under a dµO. For example, when the Y2S2 layer is set on the mullite layer and has limited oxygen shielding, the aluminum flux in the mullite layer will be significantly reduced. Another approach is to build a layer with an aluminum reservoir function under the mullite layer. Thus, even if an outward diffusion of aluminum ions occurs in the mullite layer, the underlying layer containing aluminum can supply aluminum to the aluminum-deficient zone in the mullite layer

Optimal rare-earth disilicates as top coat in multilayer environmental barrier coatings

In last decade, rare-earth disilicates have been considered as candidate materials for top coat in multilayer environmental barrier coatings (EBCs). However, optimal coating material has not yet been found. Previous studies revealed that parabolic rate constant of thermally grown silica scale (silica-TGO) beneath EBCs played a key role in the durability of SiC matrix ceramic composites coated with EBCs. To determine optimal coating material, in this study, Lu2Si2O7 and Sc2Si2O7 coatings were first corroded under quasi-static water-vapor corrosion conditions. The evolution of silica-TGO beneath coatings was then examined in detail. Accordingly, parabolic rate constants of silica-TGO growth were calculated. Compared with parabolic rate constants of silica-TGO growth beneath barium-strontium aluminosilicate (BSAS), Yb2Si2O7, and Y2Si2O7-BSAS coatings in previous studies, Y2Si2O7-BSAS coating shows the lowest parabolic rate constant and growth rate of silica-TGO. Crystal structure of rare-earth disilicates was also studied. Results showed that bond lengths of Y–O and Si–O in Y2Si2O7 were short, indicating that it possessed low ionic oxygen permeability. This resulted in its low parabolic rate constant of silica-TGO growth. Therefore, Y2Si2O7 is the optimal material as top coat in multilayer EBCs.

Preparation of a dense ytterbium disilicate layer via dual electron beam physical vapor deposition at high temperature

Ytterbium disilicate (Yb2Si2O7) is promising material for multilayered environmental barrier coatings (EBCs) on SiC fiber reinforced SiC matrix composites (SiC/SiC), because of its excellent volatilization resistance in a water vapor environment and the conformity of its coefficient of thermal expansion to SiC/SiC. A Yb2Si2O7 layer was formed on a mullite substrate, of which the combination was applied to the multilayered EBCs, via electron beam physical vapor deposition (EB-PVD) with dual electron guns using Yb2O3 and SiO2 ingots as vapor sources, simultaneously heating on the substrate at 1523K. The resulting layer consisted of a single-phase of Yb2Si2O7, with the (110) oriented structure. The layer had a highly dense columnar structure extending in the direction of thickness, but not the feather-like structure typical of layers synthesized via conventional EB-PVD techniques. It is expected that the application of dual EB-PVD during heating is useful for the construction of EBCs with excellent gas barrier performance.

The influence of calcium–magnesium–aluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer environmental barrier coatings

The interaction of calcium-magnesium-aluminosilicates (CMAS) with Yb2Si2O7-based multilayer environmental barrier coatings was studied using a combination of high-energy synchrotron X-ray techniques. Changes in the phase-specific stresses and lattice spacings as a function of temperature were investigated with wide-angle X-ray scattering. Small-angle X-ray scattering was used to study the interaction of CMAS with nanoporosity, while X-ray computed tomography was used to study the formation of cracks in the coating. It was found that CMAS interacts strongly with the Yb2Si2O7 topcoat, leading to changes in lattice spacing, alterations of the ambient temperature stresses, and large cracks in the CMAS that extend through the topcoat.

Mullite–YSZ multilayered environmental barrier coatings tested in cycling conditions under water vapor atmosphere

Plasma sprayed mullite/YSZ graded coatings on SiC substrates were examined as environmental barrier coatings (EBCs). Several coating systems with various layer arrangements were subjected to thermal aging at 1300°C in cycling (50 and 100cycles, i.e. 100 and 200h) conditions under rich water vapor atmosphere. The level of porosity and the crack population were quantified using cross section SEM images of these coatings. The thickness of the thermally grown silica (TGS) layer developed was estimated and the presence of cristobalite in its structure was evidenced by micro-Raman spectroscopy. Hardness and elastic modulus were also determined by the use of instrumented indentation for each layer in the different coatings. The evolution of all these parameters confirmed that EBC architectures formed by compliant porous layers resulted more effective in reducing cracks and the effects of aging than EBCs with dense graded compositions.

Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics — II. Oxidation resistance

In part I of this study, the dip-coat processing of mullite/gadolinium silicate (Gd 2SiO 5) environmental barrier coatings (EBCs) applied on α-SiC and SN282™ Si 3N 4 through alcohol based and sol based slurries was presented. Here, the performance of selected EBCs by evaluating their oxidation resistances during thermal cycling in simulated combustion (90% H 2O–balance O 2) environment between 1350 °C and RT for up to 400 cycles is being reported. Oxidation of un-coated α-SiC was severe, leading to aligned and layered porous silica scale formation (~ 17 μm thick) on its surface with frequent scale spallation when exposed to 100 cycles. Mullite/Gd 2SiO 5/B 2O 3 (83.5/11.5/5 wt.%) EBCs remained adherent to α-SiC substrate with an underlying porous silica layer formed at substrate/coating interface, which was ~ 12 μm after 100 cycles, ~ 16 μm after 200 cycles, and ~ 25 μm after 400 cycles. In contrast, α-SiC substrate coated with mullite/Gd 2SiO 5 (88/12 wt.%) EBC had only limited oxidation of ~ 10 μm even after 1350 °C/400 cycles. The sol based mullite/Gd 2SiO 5 (88/12 wt.%) EBC on α-SiC substrate after 400 cycles was adherent, but showed more interfacial damages (~ 20 μm after 400 cycles) though it had increased coating density. However, the mullite/Gd 2SiO 5 (88/12 wt.%) EBC (alcohol based) delaminated from the SN282™ Si 3N 4 substrate after 1350 °C/100 cycles, because of the formation of interconnected interfacial voids and hairline cracks. Parabolic growth kinetics for the underlying silica was observed for both the alcohol and sol based coated samples.

Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics — I. Processing

Multilayer mullite/gadolinium silicate (Gd2SiO5) environmental barrier coatings (EBCs) were deposited on α-SiC (Hexaloy) and Si3N4 (SN282) substrates through cost-effective slurry based dip-coat processing. Coatings applied by two approaches, alcohol and sol-based slurries, were examined and optimized in terms of their recipes and air sintering temperatures. A significant increase in densification rates was found for the sol-based EBCs applied on both SiC and SN282 substrates due to the fine mullite particles formed during reaction sintering of well-mixed silica and alumina sols. Mechanical alloying of the starting powder mixtures instead of their simple rotary-blending was found to be beneficial in terms of enhanced coat sintering kinetics. Dense thick coatings that were well-bonded to the substrate were obtained.

Residual Stress Analysis of Multilayer Environmental Barrier Coatings

Silicon‐based ceramics (SiC, Si3N4) are promising materials systems for high‐temperature structural applications in gas turbine engines. However, the silica layer that forms on these materials is susceptible to attack from water vapor present in combustion environments. To protect against this degradation, environmental barrier coatings (EBCs) have been developed to shield the underlying substrate and prevent degradation. Here we report on elastic and thermal properties, as well as internal stresses of candidate multilayer coatings, as measured in situ using microfocused high‐energy X‐rays in a transmission diffraction geometry. Doped aluminosilicate coatings were investigated for their stability on a SiC/SiC melt‐infiltrated substrate. The coatings consisted of a Ba1−xSrxAl2Si2O8 topcoat with a mullite or mullite+SrAl2Si2O8 interlayer, and a silicon bond coat. A numerical model was used to compare the stress results with an ideal coating system. Experiments were carried out on as‐sprayed and heat‐treated samples in order to analyze the strain and phase evolution as a function of multilayer depth and temperature. The phase transformation of the topcoat promoted healing of cracks in the EBC and reduced stresses in the underlying layers and the addition of SAS to the interlayer reduced stresses in thermally cycled coatings, but did not stop cracks from forming.

D-54 In Situ Heating Studies on the Internal Stresses of Multilayer Environmental Barrier Coatings

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