Mechanisms of Ytterbium Monosilicate/Mullite/Silicon Coating Failure During Thermal Cycling in Water Vapor

An air plasma spray process has been used to apply a model tri‐layer Yb₂SiO₅/Al₆Si₂O₁₃/Si environmental barrier coating system on SiC test coupons. Significant differences in the thermal expansion of the component layers resulted in periodically spaced mud cracks in the Yb₂SiO₅ and Al₆Si₂O₁₃ layers. Upon thermal cycling between 1316°C and 110°C in a 90% H₂O/10% O₂ environment flowing at 4.4 cm/s, it was found that partial delamination occurred with the fracture plane located within a thermally grown oxide (TGO) at the Al₆Si₂O₁₃–Si interface. Delamination initiated at test coupon edges where the gaseous environment preferentially oxidized the exposed Si bond coat to form β‐cristobalite. Simultaneous ingress of the gaseous environment through mud cracks initiated local formation of β‐cristobalite (SiO₂), the thickness of which was greatest directly below mud cracks. Upon cooling, cristobalite transformed from the β to α phase with a large, constrained volume contraction that resulted in severe microfracture of the TGO. Continued thermal cycling eventually propagated delamination cracks and caused partial spallation of the coatings. Formation of the cristobalite TGO appears to be the delamination life‐determining factor in protective coating systems utilizing a Si bond coat.     

Effect of the HfO2/SiO2 pre-mixing ratio and heating temperature on the formation of HfSiO4 as a bond coat of environmental barrier coatings

As the operating temperature of advanced gas turbines typically exceeds 1400 °C, it has been required to replace conventional Si bond coat in environmental barrier coatings (EBCs) with materials possessing higher thermal stability. Since HfSiO₄ has excellent thermal properties such as a high melting point, phase stability over 1400 °C, and CTE matches with that of the SiC-based ceramic matrix composites, it has attracted much attention as a next-generation bond coat material. In this study, HfSiO₄ bond coat was successfully formed by atmospheric plasma spray with pre-mixed HfO₂-SiO₂ powders (molar ratios: 7:3 and 5:5) followed by heat treatment. Effect of molar ratios of the HfO₂-SiO₂ and post-heat treatment temperature (1375 and 1475 °C) on the formation of HfSiO₄ were studied. An oxidation test of the HfSiO₄ coating was carried out at 1475 °C with the conventional Si bond coat to verify whether the new bond coat was suitable for use in a thermal environment of 1400 °C or higher. From the results, the HfO₂/SiO₂ ratio of 5:5 was suitable for the formation of HfSiO₄ than that of 7:3. After heat treatment at 1475 °C, the ratio of HfSiO₄ phase was 84.35%. The higher content of HfSiO₄ formed under 1475 °C, meaning the higher heat treatment temperature accelerated the HfSiO₄ formation. In the oxidation test at 1475 °C, the new HfSiO₄ bond coat showed no cracks and maintained its integrity, but the Si bond coat was oxidized and cracked severely. Therefore, it can be concluded that the new HfSiO₄ bond coat formed from 5HfO₂–5SiO₂ coating is a potential candidate as a next-generation bond coat material in EBCs.     

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.     

Oxidation behaviors and mechanisms of Yb2O3-doped silicon coatings fabricated by vacuum plasma spray

Environmental barrier coatings (EBCs) have been widely studied for the protection of ceramic matrix composites (CMCs). The phase transition of silica thermal growth oxide (TGO) has been proved to be an important factor for the durability of EBCs. Yb₂O₃ could react with SiO₂ TGO and form silicate which may improve the stability of TGO and prolong the service life of EBCs. In the present work, Si coatings doped with different contents of Yb₂O₃ were fabricated by vacuum plasma spray. The oxidation behaviors of the composite coatings were evaluated at 1350 °C and compared with the pure Si coating. The evolution of phase composition and microstructure of mixed thermal growth oxide (mTGO) was characterized in detail. The results showed that the newly formed oxidation product, namely Yb₂Si₂O₇, could reduce the vertical cracks in mTGO layer and the mTGO/coating interface cracks, leading to a better binding performance of the mTGO layer. The oxidation mechanisms of the Yb₂O₃-doped Si coatings were analyzed based on microstructure and phase composition observations.     

Effect of deposition temperature on phase composition,morphology and mechanical properties of plasma-sprayed Yb2Si2O7 coating

Phase composition, microstructure and mechanical properties of ytterbium disilicate (Yb₂Si₂O₇, YbDS) coatings fabricated via atmospheric plasma spray (APS) at different deposition temperatures were studied. Two layers with distinct pore distributions were observed in the YbDS top coat, the formation of pores correlated fairly well with the thermal gradients of splats after the droplets impinged on the substrate or deposited splats. Microstructure evolution with the thicknesses of YbDS top coat resulted from splat stacking and temperature accumulation. In addition, the significant crystallinity improvement and cracks reduction of YbDS top coat arose with the increased deposition temperature. X-ray diffraction patterns indicated that the metastable ytterbium monosilicate (Yb₂SiO₅, YbMS) appeared in YbDS coatings and they disappeared after thermal-treatment due to the phase transformation. The mechanical properties of the YbDS coatings were affected notably by the phase composition and microstructure.     

High-temperature Na2SO4 interaction with air plasma sprayed Yb2Si2O7 + Si EBC system: Topcoat behavior

The high-temperature interaction between ~2.5 mg/cm² of Na₂SO₄ and an atmospheric plasma sprayed (APS) Yb₂Si₂O₇ topcoat–Si bond coat system on SiC CMC substrates was studied for times up to 240 h at 1000°C–1316°C in a 0.1% SO₂–O₂ gaseous environment. Yb₂Si₂O₇ reacted with Na₂SO₄ to form Yb₂SiO₅ and an intergranular amorphous Na-silicate phase. Below 1200°C, the reaction was sluggish, needing days to cause morphological changes to the “splat microstructure” associated with APS coatings. The reaction was rapid at 1200°C and above, needing only a few hours for the entire topcoat to transform into a granulated microstructure consisting of Yb₂SiO₅ and Yb₂Si₂O₇ phases. Na₂SO₄ deposits infiltrated the Yb₂Si₂O₇ topcoat and transformed into an amorphous Na-silicate in less than 1 h at all exposure temperatures. Quantitative assessment of the Yb₂SiO₅ area fraction in the topcoat showed a linear decrease over time at 1316°C, attributed to reaction with the SiO₂ thermally grown oxide (TGO) formed on the Si bond coat and rapid transport through the interpenetrating amorphous Na-silicate grain boundary phase. It was predicted that nearly 2 weeks is needed for complete removal of Yb₂SiO₅ from the topcoat at 1316°C for a single applied loading of Na₂SO₄.     

Interface evolution of Si/Mullite/Yb2SiO5 PS-PVD environmental barrier coatings under high temperature

Plasma spray-physical vapor deposition (PS-PVD) was used to prepare tri-layer environmental barrier coatings (EBCs) Si/mullite/Yb₂SiO₅ on SiC_f/SiC substrate. Isothermal oxidation tests of EBCs were performed at 1300 ℃ for 1000 h. The thermochemical and thermomechanical interface interaction among EBCs were investigated. The results show that more dense EBCs can be obtained through PS-PVD process, which is attributed to the mixed deposition of liquid/gas states. After isothermal oxidation, many pores were observed in the Yb₂SiO₅ coating near the interface of Yb₂SiO₅/mullite coating, which results from the diffusion of Yb₂O₃ phase dissociated from Yb₂SiO₅ into mullite coating at high temperature. In the mullite coating, the Yb₂O₃ reacted with Al₂O₃ generating rod-like Yb₃Al₅O₁₂ phase. Additionally, due to the thermal expansion mismatch and high temperature oxidation, cracks were formed at the interfaces of mullite/Si coating. Those interface cracks resulted in buckling in the mullite coating.     

High-temperature performances of Si-HfO2-based environmental barrier coatings via atmospheric plasma spraying

To improve high-temperature bearing capability of coatings, novel agglomerated Si-HfO₂ powders were prepared by adding HfO₂ powders into original Si powders by spray drying method. Three-layer environmental barrier coatings (EBCs) with Si-HfO₂ bond layer, Yb₂Si₂O₇ intermediate layer and Yb₂SiO₅ surface layer were prepared on SiC ceramic substrates by atmospheric plasma spraying (APS). The high temperature properties of coatings were systematically investigated. The results indicated that the coatings had good high temperature oxidation resistance, and remained intact after being oxidized or steam corrosion at 1400 °C for 500 h, so the addition of HfO₂ improved the thermal cycling performances of the coating. The HfO₂ in Si bond coating could effectively inhibit the growth of thermal grown oxide at high temperatures. This work indicates that the high temperature properties of the coatings are improved by this novel EBCs using the novel agglomerated Si-HfO₂ powders.     

Failure mechanisms of (Gd0.9Yb0.1)2Zr2O7/Yb2SiO5/Si thermal/environmental barrier coatings during thermal exposure at 1300 °C/1400 °C

A novel tri-layer (Gd_0.9Yb_0.1)₂Zr₂O₇/Yb₂SiO₅/Si (GYbZ/YbMS/Si) thermal and environmental barrier coatings (TEBCs) was first proposed for protecting SiC-based ceramic matrix composites (CMCs). Wherein, the GYbZ layer by plasma spray physical vapor deposition (PS-PVD) was quasi-columnar structured while the YbMS and the Si layers by atmospheric plasma spray (APS) were lamellar structured. The oxidation behavior and the failure mechanisms of the GYbZ/YbMS/Si TEBCs at 1300 °C/1400 °C are revealed. At 1300 °C, the mud-cracks penetrated through the GYbZ/YbMS layer and transversely deflected in the Si layer are responsible for the oxidation at YbMS/Si interface. When the temperature increased to 1400 °C, the propagation of mud-cracks, cavities, and TGO channel cracks occurred due to the sintering of GYbZ and the fast growth of cristobalite. Eventually, these defects caused delaminating failure at interface. Moreover, another de-bonding failure of the coating was observed resulting from the significant thickening of oxide scale at the edge region.     

Cyclic oxidation performances of new environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 coated SiC at 1375 °C and 1475 °C in the air environment

The objective of this work is to study the cyclic oxidation performances of the environmental barrier coatings (EBCs) containing the novel HfO₂-SiO₂ bond coats in the air environment. Bi-layer HfO₂-SiO₂/Yb₂Si₂O₇ (50HfO₂-50SiO₂, 70HfO₂-30SiO₂ bond coats) and conventional Si/Yb₂Si₂O₇ EBCs were deposited on SiC substrate using atmospheric plasma spray. The effect of the pre-mixing ratios of HfO₂/SiO₂ on the cyclic oxidation behavior of HfO₂-SiO₂/Yb₂Si₂O₇ EBCs was examined. The results showed that the higher content of the HfSiO₄ formed from the 50HfO₂-50SiO₂ bond coats, and it remained intact. A thermally grown oxide (TGO) SiO₂ layer was formed at the bond coat/SiC interface. The parabolic oxidation rate constant (k_p, μm²/h) of the TGO has been reduced 2 orders of magnitude in 50HfO₂-50SiO₂/Yb₂Si₂O₇ EBCs coated SiC compared to the bare SiC at 1475 °C, indicating that the 50HfO₂-50SiO₂/Yb₂Si₂O₇ EBCs effectively protected the SiC substrate at 1475 °C.     

Dry air cyclic oxidation of mixed Y/Yb disilicate environmental barrier coatings and bare silica formers

Mixed Y and Yb disilicate coatings (Y/Yb)DS have been proposed as dual function thermal and environmental barrier coatings (EBCs) for protecting SiC-based ceramic matrix composites in gas-turbine environments. As an initial step, the 1350 °C dry air cyclic oxidation of atmospheric plasma sprayed (Y_1.2/Yb_0.8)DS and ytterbium disilicate/ytterbium monosilicate (YbDS/YbMS) EBCs deposited onto Si bond coatings was compared. As a baseline for evaluating EBC oxidant permeability, the dry air cyclic oxidation scale growth rates for bare silica formers (SiC, Si) were also measured and were consistently higher than rates previously measured after isothermal oxidation. Regarding Si bond coat oxidation rates underlying (Y/Yb)DS and YbDS/YbMS EBCs, the thinner silica scale formed under the thinner and denser (Y/Yb)DS coatings suggested a lower oxidant permeability than YbDS/YbMS. After 500 1-h cycles, the (Y/Yb)DS coating was comprised of only the β-polymorph disilicate and minor amounts of the X-2 phase monosilicate phase. Negligible differences in oxidation kinetics for (Y/Yb)DS coatings over the 90 – 240 µm thickness range were observed.     

High temperature performances of tri-layer environmental barrier coating with Si bond layer modified by Yb2O3

Environmental barrier coatings (EBCs) have been expected to be applied on the surface of ceramic matrix composites (CMCs). However, the oxidation and propagation cracking of the silicon bond layer are the most direct causes to induce the failure of EBCs under high temperature service environment. The modification of silicon bond layer has become an important method to prolong the service life of EBCs. In this work, the Yb₂O₃ have been introduced to the silicon bond layer, and three kinds of tri-layer Yb₂SiO₅/Yb₂Si₂O₇/(Si-xYb₂O₃) EBCs with modified Si bond layer by different contents of Yb₂O₃ (x = 0, 10 vol%, 15 vol%) were prepared by vacuum plasma spray technique. The thermal shock performance and long-term oxidation resistance of the EBCs at 1350 °C were investigated. The results showed that the addition of appropriate amount of Yb₂O₃ (10 vol%) can improve the structural stability and reduce the cracks of the mixed thermal growth oxide (mTGO) layer by forming the oxidation product of Yb₂Si₂O₇ during long-term oxidation. The excessive addition of Yb₂O₃ increased the stress during thermal shock as well as accelerated the oxygen diffusion during long-term oxidation, leading to the failure of EBCs. Moreover, the distribution uniformity of Yb₂O₃ deserves further consideration and improvement.     

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 SiC_f/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 SiC_f/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 SiC_f/SiC composites from water vapor corrosion at 1300 °C.     

Research status of bond coats in environmental barrier coatings

Bond coats in environmental barrier coatings (EBCs) prevent oxidants from penetrating the substrate, mediate the mismatch of the coefficient of thermal expansion (CTE), and improve the adhesion strength between adjacent layers. However, the development of bond coats is rarely studied systematically. In this paper, the research status of the bond coats in EBCs is introduced in detail, including the materials and deposition methods. Thus far, Si, modified-Si, mullite, etc., have been employed as bond coats. Nevertheless, visible drawbacks of each bond coat limit their application at high-temperatures in extreme environments. Si bond coat is easily oxidized and forms thermally grown oxides that form cracks, resulting in delamination, spallation, and failure of EBCs. In the Si–HfO₂ bond coat, the optimal ratios of Si/HfO₂, deposition methods, distribution of Si and HfO₂, and oxidation of Si remain completely unsolved. For mullite bond coat, SiO₂ suffers selective evaporation in the water vapor environment, and the ratios of the Al₂O₃ and SiO₂ in mullite coatings restrict its service lifetime. HfSiO₄ is a potential candidate acting as a next-generation bond coat in EBCs is proposed. Furthermore, choosing reasonable deposition methods is beneficial to improve the performances of the bond coats in EBCs.     

Corrosion behavior of calcium–magnesium–aluminosilicate (CMAS) on sintered Gd2SiO5 for environmental barrier coatings

The Gd₂SiO₅ performed high-temperature corrosion behavior on calcium–magnesium– aluminosilicate (CMAS) for environmental barrier coatings (EBCs). The synthesized Gd₂O₃-SiO₂ powder was prepared to fabricate a sintered Gd₂SiO₅ by spark plasma sintering (SPS) at 1400°C for 20 min. CMAS was sprinkled on the sintered Gd₂SiO₅ surface and exposed for 2, 12, and 48 h at 1400°C by isothermal heat treatment. The main corrosion factor is Ca, and Ca₂Gd₈(SiO₄)₆O₂ phase is formed by reacting with Gd₂SiO₅. Extended morphology of Ca₂Gd₈(SiO₄)₆O₂ particles observed in the reaction area become thicker as the heat treatment time increases as the CMAS is dissolved. According to the results of high-temperature X-ray diffraction (HT-XRD) and differential scanning calorimetry (DSC), CMAS melted at 1243°C or a higher temperature formed the reaction area. The Ca₂Gd₈(SiO₄)₆O₂ phase was recrystallized and grown due to the reaction of Gd₂SiO₅ and Ca of the CMAS components.     

Thermal shock resistance of tri‐layer Yb2SiO5/Yb2Si2O7/Si coating for SiC and SiC‐matrix composites

A new tri‐layer Yb₂SiO₅/Yb₂Si₂O₇/Si coating was fabricated on SiC, C/SiC, and SiC/SiC substrates, respectively, using atmospheric plasma spray (APS) technique. All coated samples were subjected to thermal shock test at 1350°C. The evolution of phase composition and microstructure and thermo‐mechanical properties of those samples before and after thermal shock test were characterized. Results showed that adhesion between all the 3 layers and substrates appeared good. After thermal shock tests, through microcracks which penetrated the Yb₂SiO₅ top layer were mostly halted at the Yb₂SiO₅‐Yb₂Si₂O₇ interface and no thermal growth oxide (TGO) was formed after 40‐50 quenching cycles, implying the excellent crack propagation resistance of the environmental barrier coating (EBC) system. Transmission electron microscopy analysis confirmed that twinnings and dislocations were the main mechanisms of plastic deformation of the Yb₂Si₂O₇ coating, which might have positive effects on crack propagation resistance. The thermal shock behaviors were clarified based on thermal stresses combined with thermal expansion behaviors and elastic modulus analysis. This study provides a strategy for designing EBC systems with excellent crack propagation resistance.     

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.     

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

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

Numerical study of residual stresses in environmental barrier coatings with random rough geometry interfaces

To clarify the role of the coating interface geometry and thermally grown oxide (TGO) layer in the failure of environmental barrier coatings (EBCs) and to further understand the cracking and spalling mechanisms of coatings, in this study, the thermomechanical properties of the multilayer coating system (Yb₂SiO₅/Yb₂Si₂O₇/Si), the morphology of the coating interface and the influence of the oxide layer on the local stresses during cooling were considered based on a random rough interface geometry model. The results showed that the rough geometry increased the magnitude of residual stresses at the interface and that the stress distribution away from the interface was less affected than the coating without roughness. The cracks on the outer surface of the Yb₂SiO₅ layer initiate in the valley region and spread with a stress value independent of the TGO thickness, and this failure may occur by cracking under tensile stress. The overall stress intensity at the TOP/EBC interface was lower than that at the upper surface of the TOP layer. The presence of TGO increased the magnitude of residual stresses in the BC and EBC layers, which caused cracks at the TGO/BC and TGO/EBC interfaces to occur at opposite locations. The phase change of the TGO layer from β-cristobalite to α-cristobalite cause a rapid increase in the overall level of coating stress, which may be a direct factor in coating failure. The calculation results provide a theoretical basis for the coating design and manufacturing process.     

Degradation of ytterbium disilicate environmental barrier coatings in high temperature steam atmosphere

The use of Ceramic Matrix Composites (CMCs) in the hottest part of an aero engine promises great improvements in fuel efficiency by decreasing component weight and allowing higher gas inlet temperatures. However, an environmental barrier coating (EBC) is required to protect the CMC from the corrosive water vapour contained in the combustion environment.Here, CMC specimens were coated with a silicon bond coat and ytterbium disilicate (Yb₂Si₂O₇) layer using air plasma spraying. The specimens were subsequently exposed to a water steam environment at 1350 °C for hundreds of hours. Stress evolution and phase stability were measured throughout to observe possible degradation. Cross-sectioning of the samples revealed the occurrence of sintering, the formation of a thermally grown oxide along the silicon/EBC interface, and a reaction between the ytterbium disilicate and silica. However, no coating failure was observed, even after 750 h of isothermal exposure to the hot steam environment.