Effect of SiC whiskers on the anti-oxidation properties of Yb2Si2O7/mullite/SiC tri-layer environmental barrier coating for CMCs

The mullite and ytterbium disilicate (β-Yb₂Si₂O₇) powders as starting materials for the Yb₂Si₂O₇/mullite/SiC tri-layer coating are synthesized by a sol–gel method. The effect of SiC whiskers on the anti-oxidation properties of Yb₂Si₂O₇/mullite/SiC tri-layer coating for C/SiC composites in the air environment is deeply studied. Results show that the formation temperature and complete transition temperature of mullite were 800–1000 and 1300°C, respectively. Yb₂SiO₅, α-Yb₂Si₂O₇, and β-Yb₂Si₂O₇ were gradually formed between 800 and 1000°C, and Yb₂SiO₅ and α-Yb₂Si₂O₇ were completely transformed into β-Yb₂Si₂O₇ at a temperature above 1200°C. The weight loss of Yb₂Si₂O₇/(SiC_w–mullite)/SiC tri-layer coating coated specimens was 0.15 × 10⁻³ g cm⁻² after 200 h oxidation at 1400°C, which is lower than that of Yb₂Si₂O₇/mullite/SiC tri-layer coating (2.84 × 10⁻³ g cm⁻²). The SiC whiskers in mullite middle coating can not only alleviate the coefficient of thermal expansion difference between mullite middle coating and β-Yb₂Si₂O₇ outer coating, but also improve the self-healing performance of the mullite middle coating owing to the self-healing aluminosilicate glass phase formed by the reaction between SiO₂ (oxidation of SiC whiskers) and mullite particles.     

Strength improvement and purification of Yb2Si2O7‐SiC nanocomposites by surface oxidation treatment

A two‐step processing was developed to prepare Yb₂Si₂O₇‐SiC nanocomposites. Yb₂Si₂O₇‐Yb₂SiO₅‐SiC composites were first fabricated by a solid‐state reaction/hot‐pressing method. The composites were then annealed at 1250°C in air for 2 hours to activate the oxidation of SiC, which effectively transformed the Yb₂SiO₅ into Yb₂Si₂O₇. The surface cracks purposely induced can be fully healed during the oxidation treatment. The treated composites have improved flexural strength compared to their pristine composites. The mechanism for crack healing and silicate transformation have been proposed and discussed in detail.     

Crack-healing behaviour of MoSi2 dispersed Yb2Si2O7 environmental barrier coatings

MoSi₂ doped Yb₂Si₂O₇ composites were designed to extend the lifetime of Yb₂Si₂O₇ environmental barrier coatings (EBCs) via self-healing cracks during high-temperature applications. Yb₂Si₂O₇–Yb₂SiO₅–MoSi₂ composites with different mass fractions were prepared by applying spark plasma sintering. X-ray diffraction results confirmed that the composites consisted of Yb₂Si₂O₇, Yb₂SiO₅, and MoSi₂. The thermal expansion coefficients (CTEs) of the composites increased with an increase in the MoSi₂ content. The average CTE of the 15 wt% MoSi₂ doped Yb₂Si₂O₇ composite was 5.24 × 10^?6 K^?1, indicating that it still meets the CTE requirement of EBC materials. After being pre-cracked by using the Vickers indentation technique, the samples were annealed for 0.5 h at 1100 or 1300 °C to evaluate the crack-healing ability. Microstructural studies showed that cracks in 15 wt% MoSi₂ doped Yb₂Si₂O₇ composites were fully healed during annealing at 1300 °C. Two mechanisms may be responsible for crack healing. First, the cracks were filled with SiO₂ glass formed by MoSi₂ oxidation. Second, the formed SiO₂ continued to react with Yb₂SiO₅ to form Yb₂Si₂O₇, which can cause cracks to heal owing to volumetric expansion. The Yb₂Si₂O₇ formation with smaller volume expansion is more beneficial.     

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.     

SiC modified self-healing ytterbium disilicate materials for potential environmental barrier coating application

Environmental barrier coatings (EBCs) are crucial to the reliability and durability of SiC_f/SiC composite components seeking applications in hot sections of next-generation advanced aero-engines. The cracks initiated and developed in EBCs owing to various reasons during service greatly undermine their lifespans. To address this problem, in this work, silicon carbide (SiC) in the forms of particles and whiskers with various amounts have been introduced to ytterbium disilicate (Yb₂Si₂O₇), the mainstream EBC topcoat materials, so as to gain some self-healing potential. The results reveal that, the SiC inclusions in Yb₂Si₂O₇ in the presence of ytterbium monosilicate (Yb₂SiO₅) can trigger the following reactions. Specifically, SiC self-healing agents are oxidized to form viscous SiO₂, which actively reacts with Yb₂SiO₅ upon encountering it, forming Yb₂Si₂O₇. This has brought twofold beneficial effects including ① silicon supplementation of disilicate topcoat, whose silicon element tends to be “dragged out” by water vapor, leading to the deterioration of thermal mismatch; as well as ② crack self-healing resulting from the volume expansion induced by the above reactions. Then the two aspects of self-healing agents, namely the “promptness” and “sustainability,” have been discussed in detail. The former is unveiled to be more pertinent to the repairing of large cracks, whilst the latter is more relevant to the self-healing of tiny cracks at initiation or early stage of propagation. The current work sheds some lights on the design and development of more durable and robust EBCs with self-healing capability.     

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.     

Self-healing behavior and strength recovery of ytterbium disilicate ceramic reinforced with silicon carbide nanofillers

Environmental barrier coatings (EBCs) are necessary to protect SiC/SiC ceramic components against oxidation and hot corrosion in high-temperature applications. The volatilization of SiO₂ in SiC-reinforced materials is a major obstacle for the implementation of these self-crack-healing ceramics. The Yb₂Si₂O₇-Yb₂SiO₅-SiC composite is known as a self-healing material that can help to avoid this SiC recession. In this research, the crack-healing behavior of this composite is investigated by using pre-cracking followed by annealing in an oxidizing environment. The crack-healing mechanism is explored and elucidated as a function of the filler morphology, crack size, annealing time, and annealing temperature. The two main crack-healing mechanisms are the filling of cracks with SiO₂ glass and the volume expansion of Yb₂Si₂O₇ induced by the reaction between SiO₂ and Yb₂SiO₅. Full crack recovery is achieved with only 10 vol% SiC, with evidence from XRD and EDS analyses. SiC nanoparticulates are more efficient fillers than nanofibers and nanowhiskers.     

Environmental barrier coatings using low pressure plasma spray process

Yb₂Si₂O₇/Si bilayer environmental barrier coatings (EBCs) on SiC ceramic substrate were produced by low pressure plasma spray (LPPS) process. Phase composition, microstructure, and thermal durability of LPPS Yb₂Si₂O₇/Si coating were investigated. XRD analysis indicated that the coating is mainly composed of Yb₂Si₂O₇ with ~15.5v% Yb₂SiO₅ phases. The LPPS EBCs have a dense microstructure with porosity less than 4%. Adhesion strength measurement indicated the LPPS EBCs have an average adhesion strength of 29.1 ± 0.8 MPa. Furnace cycle test (FCT) on the coatings in air at 1316°C was performed and the test ran for 900 cycles and there was no coating spallation/failure for LPPS Yb₂Si₂O₇/Si EBCs. The FCT results demonstrated the excellent thermal cycle durability of LPPS EBCs. Oxidation kinetics investigation of LPPS EBCs in flowing 90% H₂O (g)+10% air at 1316°C showed that the thermally grown oxide (TGO) growth rate is close to the oxidation rate of pure Si in dry air and is significantly lower than that in water vapor environment. The LPPS process is promising in making highly durable Yb2Si2O7-based dense EBCs by impeding diffusion and ingression of water vapor/O₂.     

Characterization of Yb2Si2O7–Yb2SiO5 composite environmental barrier coatings resultant from in situ plasma spray processing

Plasma spraying of multicomponent materials produces shifts in coating composition associated with differential vaporization of constituent elements within the strong thermal gradients of the process. This effect is quite noticeable in rare-earth silicates which are now widely being employed as Environmental Barrier Coatings (EBCs) for SiC based ceramic components of turbine engines. Of particular interest is the preferential volatilization of SiO₂ during thermal plasma spraying Yb₂Si₂O₇ (ytterbium disilicate) coatings which leads to the deviation from stoichiometry of the desired disilicate composition resulting in a mixed phase coating consisting of Yb₂Si₂O₇ plus Yb₂SiO₅ (ytterbium monosilicate). Recent work has shown that presence of monosilicate can be beneficial as its evolution from amorphous, metastable to stable crystalline phase can lead to crack healing during high temperature exposure, however, careful control of the chemistry and architecture may be needed. In this work a 50/50 mol% Yb₂Si₂O₇–Yb₂SiO₅ composite coating has been targeted through in situ decomposition during plasma spray from stoichiometric Yb₂Si₂O₇ powder. The as sprayed amorphous coating reverts to crystalline upon thermal treatment passing through a metastable state identified by XRD and Raman spectroscopy. The transition to the final stable phases results in a mixed phase coating comprising of 46/54 mol% Yb₂Si₂O₇–Yb₂SiO₅ composite that is thermo-mechanically stable with the underlying bond coated silicon coated SiC substrate.     

Self-crack healing ability and strength recovery in ytterbium disilicate/silicon carbide nanocomposites

The SiO₂ volatility and SiC recession in hydrocarbon-rich combustion atmosphere is the main problem of SiC-reinforced ceramics when they are utilized as coating materials for gas turbine blades. The composite of Yb₂Si₂O₇-Yb₂SiO₅/SiC is expected as a self-crack healing material that can avoid this problem because the monosilicate can react with the SiO₂ glass to form disilicate and hence further reinforce the composite. In this study, the composites, fabricated with various morphology of SiC nanofillers, were precracked and then heat-treated in an oxidizing environment to activate their crack healing behavior. The healing effect as a function of filler morphology and annealing time was investigated. Healing mechanism and future applications of these composites were also discussed.     

Yttrium Bearing Silicon Carbide Matrices for Robust Ceramic Composites

Application of SiC‐based ceramic matrix composites (CMCs) in combustion environments demands the use of an environmental barrier coating (EBC) to prevent volatilization of the protective SiO₂ scale in flowing water vapor. The EBC only provides protection while present on the surface; cracking and spallation of the coating leaves the underlying SiC vulnerable to the oxidation–volatilization processes. A robust matrix material chemically tailored to regrow a yttrium silicate scale in the event of EBC loss has been developed by incorporating yttrium bearing species including YB₂, Y₂O₃, and Y₅Si₃ into the SiC. During oxidation a borosilicate glass helps seal cracks while Y₂O₃ and SiO₂ react to form Y₂Si₂O₇ for environmental protection. Candidate compositions were oxidized for 10 min to 100 h at 1400°C and for 24 h at 1500°C to understand the scale growth. The prospects for effectively applying this approach in CMCs are discussed.     

Microstructure and water corrosion behavior of (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7 high-entropy rare-earth disilicate coating for SiC coated C/C composites

In order to make carbon/carbon composites suitable for application in gas turbine engine, it is necessary to develop environmental barrier coatings (EBCs) to protect them from reacting with water vapor. In our previous work, a novel high-entropy rare-earth disilicate (Lu_0.2Yb_0.2Er_0.2Tm_0.2Sc_0.2)₂Si₂O₇ ((5RE_0.2)₂Si₂O₇) has been developed and verified as a promising candidate for EBCs. In this work, the (5RE_0.2)₂Si₂O₇ coating was synthesized on the surface of SiC coated C/C composites by supersonic atmospheric plasma spraying method. The protective performance and mechanism of this coating under high temperature water vapor environment was explored in detail. Results showed that the weight change of the sample coated with (5RE_0.2)₂Si₂O₇ was only 0.2% after corrosion for 100 h at 1500 ºC, which proved that (5RE_0.2)₂Si₂O₇ coating could significantly improve the resistance of C/C composites against water vapor corrosion. This work may provide theoretical basis for the design and application of high-entropy rare-earth silicates as EBCs.     

Enhanced wet-oxidation resistance of Y2O3 modified SiC ceramics by the formation of Y2Si2O7 protective layer

The poor wet-oxidation resistance limits the long-life service of SiC_f/SiC composites as the hot end components of aero-engines. The stability of SiC_f/SiC composites under high-temperature wet oxygen environment can be promoted by more robust SiC matrix. In this work, the effect of Y₂O₃ on the corrosion behaviors of SiC ceramics in flowing O₂/H₂O atmosphere at 1400 ℃ was studied. Duo to the continuous Y₂Si₂O₇ layer formed on the surface, SiC-Y₂O₃ ceramics exhibit much better wet-oxidation resistance than original SiC ceramics. During the oxidation process, Y₂O₃ dispersed in the ceramics migrates to the surface and reacts with SiO₂ to form β-Y₂Si₂O₇. Subsequently, the β-Y₂Si₂O₇ aggregates and grows to form a continuous Y₂Si₂O₇ layer, inhibiting the corrosion from oxidizing medium to the inner SiC matrix. This study is expected to provide important ideas for the design and structure regulation of wet-oxidation resistant SiC_f/SiC composites.     

Preparation of crystalline ytterbium disilicate environmental barrier coatings using suspension plasma spray

In high-speed modern industries, high-temperature stability of materials is essential. A promising high-temperature material currently attracting attention is silicon carbide (SiC)-based ceramic matrix composites (CMC). However, a disadvantage of these materials is their reduced lifetime in an oxidizing atmosphere. To overcome this, environmental barrier coating can be employed. In this study, we aimed to fabricate an environmental barrier coating using suspension plasma spray with Yb₂Si₂O₇, which exhibits excellent oxidation resistance and a similar thermal expansion coefficient to SiC. To prepare the crystalline Yb₂Si₂O₇ coating layer, the gas concentration of the plasma spray was adjusted, and then the suspension manufacturing solvent was adjusted and sprayed. The prepared coating samples were analyzed by X-ray diffraction, scanning electron microscope, transmission electron microscopes, and energy dispersive X-ray spectroscopy to determine phase and microstructure changes. Highly crystalline ytterbium disilicate was observed at low plasma enthalpy with no hydrogen and 20% addition of water.     

Enhancing CMAS corrosion resistance of PS-PVD Si/Yb2Si2O7 EBCs via Al-modification

Enhancing the resistance to molten silicate corrosion is crucial for the long service life of environmental barrier coatings (EBCs). In this study, we used the Al-modification technique to enhance the CMAS corrosion resistance of Si/Yb₂Si₂O₇ coatings prepared by plasma spray-physical vapor deposition. The results show that the Al-modified Yb₂Si₂O₇ coating had higher resistance to CMAS corrosion than the Yb₂Si₂O₇ coating annealed at 1300 ℃ for 100 h, which is related to the refractory mullite and Yb₂Mg(AlO₂)₂O₃ generated during the CMAS exposure of Al-modified Yb₂Si₂O₇ coating. The Al-modified Yb₂Si₂O₇ coating also exhibited excellent resistance to oxygen penetration. The Al-modification technology provides the direction for the corrosion resistance of Yb₂Si₂O₇ system to CMAS.     

Constrained sintering and thermal ageing behaviour of electrophoretically deposited Yb2Si2O7 environmental barrier coating

The oxidation of SiC and the formation of a thermally grown oxide layer (TGO) limit the lifetime of environmental barrier coatings. Thus, this paper focuses on the deposition of denser Yb₂Si₂O₇ coatings using electrophoretic deposition to reduce the TGO growth rate. The findings showed densification for Yb₂Si₂O₇ can be achieved with an optimized sintering profile (heating/cooling rate, temperature, and time). However, the addition of 1.5 wt% of Al₂O₃ to Yb₂Si₂O₇ promoted densification and lowered the required sintering temperature, 1380 °C using 2 °C/min heating/cooling rate for 10 h provided efficient coating density. Moreover, adding Al₂O₃ reduced the TGO growth rate by more than 70 % compared to the Al₂O₃-free coatings, without cracking in TGO after 150 h of thermal ageing at 1350 °C. Results within this study suggest electrophoretic deposition with Al₂O₃ addition produces promising Yb₂Si₂O₇ environmental barrier coatings on SiC substrate with low oxidation rates and increased lifetime.     

Phase composition and property evolution of (Yb1−xHox)2Si2O7 solid solution as environmental/thermal barrier coating candidates

RE disilicates are good candidates as environmental/thermal barrier coating for SiC_f/SiC composite in harsh gas turbine engines. We designed (Yb_1?xHo_x)₂Si₂O₇ solid solutions and studied mechanical properties, thermal properties, and water vapor resistance. Powders with different compositions were synthesized by pressureless sintering, and bulk samples were prepared by Spark Plasma Sintering (SPS). Polymorphic changes with temperature and composition of the solid solutions were examined. Through doping Ho into Yb₂Si₂O₇, water vapor corrosion resistance is significantly promoted, and thermal expansion coefficient is maintained close to that of Si-based ceramics. Compared with host disilicates, thermal conductivity of solid solutions are decreased, and mechanical properties, including Vickers hardness and fracture toughness, are increased. A two-phase domain is found at (Yb_1/2Ho_1/2)₂Si₂O₇, and the γ to δ phase transition of Ho₂Si₂O₇ is observed during SPS. Among all samples, γ-(Yb_1/3Ho_2/3)₂Si₂O₇ possesses superior high temperature stability, and excellent water vapor resistance, indicating its performance as environmental/thermal barrier coating.     

Oxidation resistance of hot-pressed SiC–BN composites

The oxidation resistance of SiC–BN composites with different BN content hot-pressed from Si₃N₄, B₄C and C was investigated. The oxidized products of SiC and BN were identified to be SiO₂, C and B₂O₃, N₂. SiO₂ and B₂O₃ could further form a borosilicate glass which covered the surfaces of the samples and withstood oxidation because of its flowability and self-healing. The oxidation resistance of the SiC–BN composites in static air atmosphere deteriorated with the increase of temperature as well as of the BN content.     

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.     

More effective crack self-healing capability of SiCf/SiC-B4C with Al2O3 modified under wet environment

Aluminum oxide has been introduced into SiC_f/(SiC + B₄C) composites to improve the crack self-healing property in O₂/H₂O atmosphere. The observation of the surface and interior morphologies of the oxidized composites reveal that Al₂O₃ can lead to rapid self-healing of cracks, which will effectively block the oxidation of interior interphase and fiber. Further investigation reveals that Al₂O₃ plays an important role in impeding the crystallization of SiO₂ and limiting the volatilization of B₂O₃. It is beneficial to form self-healing fluid glass phase and improve glass phase stability. Under these circumstances, the pre-crack can be healed more effectively, which is advantageous for composites to remain high strength retention rate.