Failure Mechanism Associated with the Thermally Grown Silica Scale in Environmental Barrier Coated C/SiC Composites

The barium strontium aluminosilicate and Y₂Si₂O₇‐BSAS‐coated C/SiC composites were corroded in 50%H₂O–50%O₂ environments at 1250°C, respectively. It was found that the coated composites suddenly lost their strength as the corrosion time was up to 250 and 750 h, respectively. During the water vapor corrosion, a continuous silica scale was formed between the SiC bond coat and environmental barrier coatings, leading to the growth stress. The thickness of silica scale grew with the prolonged corrosion time, accompanied with the accumulation of growth stress in the silica scale. When the growth stress was greater than the bond strength between silica scale and SiC bond coat, the cracks would form and propagate along their interface, resulting in the spallation of EBCs, followed by the failure of the composites.     

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.     

High-temperature interface stability of tri-layer thermal environmental barrier coatings

Thermal environmental barrier coatings (TEBCs) are capable of protecting ceramic matrix composites (CMCs) from hot gas and steam. In this paper, a tri-layer TEBC consisting of 16 mol% YO_1.5 stabilized HfO₂ (YSH16) as thermal barrier coating, ytterbium monosilicate (YbMS) as environmental barrier coating, and silicon as the bond coating was designed. Microstructure evolution, interface stability, and oxidation behavior of the tri-layer TEBC at 1300 °C were studied. The as-sprayed YSH16 coating was mainly comprised of cubic phase and ～3.4 vol% of monoclinic (M) phase. After 100 h of heat exposure, the volume fraction of the M phase increased to ～27%. The YSH16/YbMS interface was proved to be very stable because only slight diffusion of Yb to YSH16 was observed even after thermal exposure at 1300 °C for 100 h. At the YbMS/Si interface, a reaction zone including a Yb₂Si₂O₇ layer and a SiO₂ layer was generated. The SiO₂ grew at a rate of ～0.039 μm²/h in the first 10 h and a reduced rate of 0.014 μm²/h in the subsequent exposure.     

Effects of crystal structure and cation size on molten silicate reactivity with environmental barrier coating materials

Rare earth (RE) disilicates are utilized in environmental barrier coatings to protect Si-based engine components from destructive reactions with water vapor and other combustion species. These coating materials, however, degrade when exposed to molten silicate deposits in the engine. Four RE-disilicates (RE₂Si₂O₇, RE = Er, Dy, Gd, Nd) are analyzed herein in thermochemical interactions with glassy calcium-magnesium-aluminosilicate (CMAS) compositions at 1400°C. Crystalline reaction products included RE₂Si₂O₇, SiO₂, and a Ca_2+yRE_8+x(SiO₄)₆O_2+3x/2+y apatite-type silicate. RE₂Si₂O₇ formation was favored in interactions with CMAS having low CaO:SiO₂ ratios. Increased reactivity was observed for higher CaO:SiO₂ ratios in CMAS combined with larger RE³⁺ cation size, resulting in apatite formation of varying stoichiometry and changes in lattice parameters. The crystallization of SiO₂ was dependent on both thermodynamic equilibrium at low CaO:SiO₂ ratios and sequestration of silicate modifiers at higher CaO:SiO₂ ratios, although residual amorphous content after CMAS exposure in both cases was still substantial.     

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.     

Tailorable thermal expansion in leucite-pollucite materials derived from geopolymers for environmental barrier coatings

The K[AlSi₂O₆]-Cs[AlSi₂O₆] pseudo-binary system was synthesized by geopolymer crystallization. The thermal expansion properties of these materials were studied by in situ high-temperature X-ray diffraction to characterize thermal expansion behavior for potential application as environmental barrier coatings. Tailorable thermal expansion through changing cation stoichiometry allowed reduced thermal expansion mismatch with SiC_f/SiC composites compared to rare-earth-based coatings.     

Non-destructive evaluation of thermally grown oxides in thermal barrier coatings using impedance spectroscopy

The growth of thermally grown oxide (TGO) layers in thermal barrier coatings (TBCs) due to the oxidation of the bondcoat alloy is a critical factor affecting the durability of TBCs. In the present study, diverse TBC specimens were subjected to long-term oxidation at various temperatures. The TGO growth mechanism was investigated according to cross-sectional images of the oxidized specimens. Impedance spectroscopy (IS) was performed to measure the electrical properties of the integrated TBCs non-destructively. Considering the influence of the TGO composition, the derived TGO electrical capacitance was found to have a good correspondence to the observed TGO thickness over a wide range 0–18.3 μm, regardless of the diverse specimens and oxidation conditions. The error was less than ±2.0 μm. With a certain design of the electrode size, IS is generalized and is recommended as an accurate and practical non-destructive evaluation method for the determination of TGO thickness within a very wide range in TBC systems under real operating conditions.     

Protocol for selecting exemplary silicate deposit compositions for evaluating thermal and environmental barrier coatings

A protocol for selecting representative silicate compositions for comparative testing of gas turbine coating materials is presented. It begins with a curated dataset of compositions of engine deposits and naturally occurring siliceous debris including volcanic ashes, sands, and dusts. The compositions are first reduced to the five major oxides—those of Ca, Mg, Fe, Al, and Si—and then distilled further using principal component analysis and k-means clustering. The process ultimately yields four classes of possible deposits with common chemical characteristics. Each class is represented by a composition centroid and a range in Ca:Si ratios. Key thermophysical properties of the possible deposits are calculated and related to the glass network connectivity, characterized by the Si:O ratio. Finally, deposits from each of these classes are compared in terms of their reactions with prototypical thermal and environmental barrier oxides, with due consideration of the effects of composition variations within each deposit class. The protocol is, in principle, adaptable to datasets compiled by OEMs and researchers in gas turbine coatings.     

Modeling oxygen permeability through topcoat and thermally grown oxide in dense Yb2Si2O7 environmental barrier coatings

The oxygen permeability of ytterbium disilicate (YbDS) topcoat (TC) and silicon dioxide (SiO₂) thermally grown oxide (TGO) is evaluated. The primary goal is to elucidate the oxidation mechanisms in environmental barrier coatings (EBCs). For this purpose, oxidant diffusion is investigated using physics-based and numerical modeling. The oxygen permeability constants are systematically evaluated and quantified in terms of thermodynamics using defect reactions and the parabolic rate constant (kp), respectively. Dry oxygen and wet oxygen conditions as well as different temperatures, partial pressures, and topcoat modifiers are investigated. The results offer evidence that the oxygen permeability constant for the YbDS topcoat is an order of magnitude higher than for the TGO. As such, the TGO hinders the oxidant diffusion stronger, proving to be the diffusion rate-controlling layer. Moreover, water vapor strongly increases the oxidant permeation with defect reactions playing a key role. It is suggested that the mass transfer through the topcoat is primarily by outward ytterbium ion diffusion and inward oxygen ion movement, with the latter being dominant, particularly in wet environments. The effect of topcoat modifiers on oxidant permeation is composition sensitive and seems to be related to their interaction with oxygen ions and their mobility.     

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.     

Thermochemistry of calcium rare-earth silicate oxyapatites

The calcium rare-earth (RE) silicate oxyapatite, Ca₂RE₈(SiO₄)₆O₂ (RE = Yb, Er, Y, Dy, Nd, Gd, and Sm), powders were synthesized by the solid-state reaction method and characterized by X-ray diffraction (XRD), Raman spectroscopy, and elemental composition analysis. The thermodynamic properties of the oxyapatites have been investigated using high-temperature oxide melt calorimetry in molten 2PbO–B₂O₃ solvent at 805°C. The energetics of the oxyapatites related to ionic substitution on two crystallographic sites, M(1) and M(2), are discussed. The enthalpy of formation from the oxides becomes more exothermic as the ionic potential decreases or the ionic radius of the REs increases, which indicates increasing energetic stability in this order.     

Plasma spray deposition of tri-layer environmental barrier coatings

An air plasma spray process has been used to deposit tri-layer environmental barrier coatings consisting of a silicon bond coat, a mullite inter-diffusion barrier, and a Yb₂SiO₅ top coat on SiC substrates. Solidified droplets in as-deposited Yb₂SiO₅ and mullite layers were discovered to be depleted in silicon. This led to the formation of an Yb₂SiO₅ + Yb₂O₃ two-phase top coat and 2:1 mullite (2Al₂O₃*SiO₂) coat deposited from 3:2 mullite powder. The compositions were consistent with preferential silicon evaporation during transient plasma heating; a consequence of the high vapor pressure of silicon species at plasma temperatures. Annealing at 1300 °C resulted in internal bond coat oxidation of pore and splat surfaces, precipitation of Yb₂O₃ in the top coat, and transformation of 2:1 mullite to 3:2 mullite + Al₂O₃. Mud-cracks were found in the Yb₂SiO₅ layer and in precipitated Al₂O₃ due to the thermal expansion mismatch between these coating phases and the substrate.     

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.     

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.     

An innovative model coupling TGO growth and crack propagation for the failure assessment of lamellar structured thermal barrier coatings

The failure of plasma-sprayed thermal barrier coating (TBC) is often caused by the coating spallation due to crack propagation. In this study, a new model with stacking lamellae is developed based on the cross-section micrograph to explore crack propagation behavior within the ceramic top coat (TC) during isothermal cycling. The dynamic growth process of thermally grown oxide (TGO) is simulated via material properties change step by step. The stress profiles in the lamellar model are first evaluated, and the pore and lamellar interface crack effects on the stress state are further explored. Then, the successive crack growth, linkage, and ultimate coating spallation process is simulated. The results show that the stress intensity in TC enhances with thermal cycling. Large stress concentration always occurs near the pore and lamellar interface crack, which can result in the incipient crack growth. Moreover, the lamellar interface crack also changes the stress distribution within the TC and at the TC/bond coat interface. The multiple crack propagation upon temperature cycling is explored, and the possible coalescence mechanism is proposed. The lamellar crack steadily propagates at the early stage. The crack length sharply increases before the occurrence of coating spallation. The simulated coat spalling path is in line with the experimental result. Therefore, the new lamellar model developed in this work is beneficial to further reveal coating failure mechanism and predict coating lifetime.     

Microstructure and nanomechanical properties of plasma-sprayed nanostructured Yb2SiO5 environmental barrier coatings

In this study, nanostructured and conventional Yb₂SiO₅ coatings were prepared by atmospheric plasma. The microstructure and nanomechanical properties of these coatings were compared before and after heat treatment. The results show that the nanostructured Yb₂SiO₅ coatings have a mono-modal distribution, and the conventional Yb₂SiO₅ coatings have a bimodal distribution. Both types of coatings had improved nanomechanical properties after heat treatment. However, the increased elastic modulus and nanohardness of the nanostructured Yb₂SiO₅ coating were more apparent than those of the conventional Yb₂SiO₅ coatings. The nanostructured Yb₂SiO₅ coating had a higher elastic modulus than the conventional Yb₂SiO₅ coating, reflecting its high density. Subsequently, the microscopic morphology and micromechanical properties of the coatings were analyzed after heat treatment. Defects in the coatings, including pores, and microcracks, were significantly reduced with grain growth after thermal treatment, and the nanostructured Yb₂SiO₅ coatings had improved healing ability and micro-mechanical properties.     

Failure mechanisms of calcium magnesium aluminum silicate affected thermal barrier coatings

Depth profiles of the phase composition of two examples of calcium magnesium aluminum silicate (CMAS) affected thermal barrier coatings (TBCs) from an aero gas turbine engine were obtained using a monochromatic and collimated beam of synchrotron radiation. One TBC was deposited by plasma spray and the other by electron beam physical vapor deposition. These examples were complemented with an X‐ray diffraction (XRD) study of mixtures of TBC zirconia powder and sand heated in a furnace. The XRD results were compared with electron backscatter images and energy dispersive spectroscopy studies of the cross sections and mixtures. It was found that when liquid, the CMAS enhances mass transport leading to the densification of the zirconia, which then leads to spalling because of the increased residual stresses generated on cooling. Even without spalling densification will reduce a TBC's ability to thermally insulate. The enhanced mass transport can also lead to destabilization of the zirconia if yttrium ions preferentially transfer to the liquid or greater stabilization if calcium or magnesium ions transfer from the liquid to the zirconia. Zircon also precipitates when the zirconium from the TBC reacts with the silicon in the liquid CMAS.     

Effect of oxide growth on the stress development in double-ceramic-layer thermal barrier coatings

A numerical study is conducted to investigate the effect of oxide growth on the stress development within the plasma sprayed double-ceramic-layer thermal barrier coatings. The roles of oxide morphology, growth rate, and oxidation duration are discussed. A two-dimensional periodical unit-cell model is developed, taking into account the different interfacial roughnesses among the coatings layers. Thermal gradient conditions are imposed during the high-temperature period to represent the non-uniform temperature distributions throughout the coatings thickness. It is found that stresses in the regions that close to the interface of the ceramic layers result from the thermal expansion mismatch and the non-uniform temperature field, in which the oxide growth reveals negligible influence on the development of the stresses. The gradually thickening thermally grown oxide (TGO) mainly contributes to the variations of stress and inelastic strain evolutions in its nearby regions. The residual stress fields in the coatings are almost unaffected by the oxide thickness after operating for a sufficiently long time. During long-term operation, the large inelastic deformation is found to be the intrinsic reason responsible for the cracking in the vicinity of TGO.     

Crystallization behavior of air-plasma-sprayed ytterbium-silicate-based environmental barrier coatings

A combination of characterization techniques has been used to provide new understanding of the complex crystallization behavior of as-sprayed amorphous Yb₂Si₂O₇-based air-plasma-sprayed environmental barrier coatings (EBCs). During crystallization heat-treatment, initially a mixture of metastable α-Yb₂Si₂O₇ and X1-Yb₂SiO₅ phases form, along with stable β-Yb₂Si₂O₇ and X2-Yb₂SiO₅ phases. Eventually the metastable phases transform to the stable β-Yb₂Si₂O₇ (major) and X2-Yb₂SiO₅ (minor) phases. The significant volume expansion associated with these transformations partially contributes towards the anomalous expansion measured in these EBCs after crystallization, but it does not account for all the measured expansion. In this context, in similar EBCs, it is also observed that the porosity increases upon crystallization heat-treatment, primarily in the form of thin, interconnected pores, which also contributes to the measured anomalous expansion. Based on this understanding, guidelines are provided for ‘near-net-shape’ crystallization of phase-pure, dense β-Yb₂Si₂O₇ EBCs that are free of vertical cracks.