Silicate Environmental Barrier Research Articles

Resistance of ytterbium silicate environmental barrier coatings against molten calcium-magnesium-aluminosilicate (CMAS): A comprehensive study

Molten calcium‑magnesium-aluminosilicate (CMAS) poses a considerable threat to the environmental barrier coatings (EBCs) service life. Here, we have prepared ytterbium monosilicate (YbMS) and ytterbium disilicate (YbDS) coatings using air plasma spraying (APS), and comprehensively investigated their high-temperature response to two CMAS relevant to real sediments within gas turbine and Beijing sand-dust, which are defined as CaO-rich CMAS and CaO-lean CMAS respectively. In the CaO-rich CMAS case, either YbMS or YbDS coating exhibits a vigorous reaction and rapid precipitation of apatite phase, constructing a dense reaction layer and inhibiting the molten CMAS infiltration. Note that the YbMS coating always possesses a smaller infiltration depth than YbDS coating because of the synergistic effect of its higher reactivity and more stable crystal structure. In comparison, the molten CaO-lean CMAS infiltrates along the grain boundaries of both coatings without any barrier, only some re-precipitated YbDS grains exist at the CMAS/coating interface. This phenomenon is closely related to the relatively high Yb3+ solubility due to its lower CaO content relative to the CaO-rich CMAS. These finds in this work are expected to provide a better understanding for the design of next-generation EBCs material and proposal of effective methods to mitigate the molten CMAS attack.

A review on multicomponent rare earth silicate environmental barrier coatings

Rare earth (RE) silicate environmental barrier coatings (EBCs), with superior physicochemical properties and resistance to hot water-oxygen and calcium-magnesium-aluminosilicate (CMAS) corrosion, are ideal candidates for safeguarding the next-generation aviation engines made of SiC ceramic matrix composites (CMCs). However, improving the corrosion resistance of RE silicates remains a critical challenge due to severe environmental degradation at 1500 °C. Herein, by doping multiple RE elements and designing high entropy formulations, researchers found that multicomponent RE silicates could establish stable lattice structures, phase compositions, and reaction products while impeding the infiltration of corrosive agents. This paper reviews the modulation of their phase stability, physicochemical properties, wettability, and corrosion behaviors. It looks at how state-of-the-art optimization solutions of multicomponent and high entropy doping strategies affected corrosion resistant behaviors. In conclusion, the authors suggest further studies to optimize the corrosion resistance of multicomponent RE silicates and highlight the importance of integrating multicomponent doping with high-throughput and other optimization methods, thereby unlocking an array of possibilities for advancing RE silicate EBCs.

Integrating State of the Art Zirconia Thermal Barriers with Ytterbium Silicate Environmental Barriers for Silicon-Based Ceramic Turbine Components

AbstractAs gas turbine firing temperatures continue to increase for the sake of improved operating efficiency, the material's transition from Ni-based superalloy components toward ceramic matrix composites (CMCs) is concurrently in progress. Due to the complex nature of the turbine operating environment (envisaged ultrahigh temperatures, presence of water vapor, etc.), coating solutions for these CMCs are still on the forefront of design optimizations. Typically, rare-earth (RE) silicate environmental barrier coatings (EBCs) have been utilized to protect the CMCs from impinging water vapor; however, they lack the thermal insulation properties to enable continued use of simple and/or easily accessible bond coat materials (i.e., silicon). Combined thermal-environmental barrier coatings (T-EBCs) are such a multifaceted surface solution. T-EBCs have been considered in the past, but to this point have not been demonstrated to be technologically robust either due to high implementation costs or complex processing. This study utilizes and combines straightforward and well-established processes—such as plasma-sprayed 7 wt.% yttria-stabilized zirconia—to demonstrate the feasibility of MultiLayered T-EBCs comprised of zirconia-based oxides and RE silicate EBCs in a single coating. The results show that despite high thermal mismatch strains, the structures cannot only be deposited, but also in certain circumstances sustain cyclic thermomechanical loading.

Estimates of permeability of water vapor in rare earth silicate environmental barrier coatings using the modified theory of deal and grove

The salient elements of the theory of oxide growth under an environmental barrier coating are reviewed. The results from steam oxidation tests recently conducted on environmental barrier coated specimens are reported. The oxide thickness between the ceramic topcoat and Si substrate are reported as a function of time. Also, the results from previously conducted steam oxidation tests are reviewed. An approach to estimate the permeability of water vapor through the various coating systems using the oxide thickness versus time measurements and the theory of oxide growth under an environmental barrier coating is demonstrated. The approach allows one to rank the protection efficiency of the various coating systems against steam oxidation.

Corrosion Behavior and Mechanism of Aluminum-rich CMAS on Rare-earth Silicate Environmental Barrier Coatings:

Corrosion Behavior and Mechanism of Aluminum-rich CMAS on Rare-earth Silicate Environmental Barrier Coatings:

Steam oxidation and microstructural evolution of rare earth silicate environmental barrier coatings

AbstractA 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.

Reactive Sputtered Ytterbium Silicate Environmental Barrier Coatings for Protection of Mo-Si-Based Alloys

The implementation of a structural material into the hot section of a turbine engine requires the protection against oxidation and water vapour. Since Mo-Si-based alloys do not provide this protection on their own over a long period of exposure time, the necessity for protective coatings becomes inevitable. Rare earth silicates, especially Yb-silicates, are known for their great potential to protect silica against water vapour recession and also for their oxidation protective behaviour due to their low silica reactivity. Reactive pulsed DC co-sputtering was investigated and used to produce a two-layered coating of Yb2Si2O7 and Yb2SiO5. The layers were implemented into a four-layered environmental barrier coating system combined with a bond coat consisting of a graded Mo-Si interlayer and a thin Si top layer intended to form slow-growing silica. The EBCs were tested up to 100 h at 1200 °C and 800 °C isothermally in air on top of three different Mo-Si-based alloys and show promise for oxidation protection. The pesting phenomenon was completely suppressed.

Microstructure and phase composition evolution of dual-phase ytterbium silicate coatings plasma sprayed from stoichiometric Yb2Si2O7 feedstock powder

The main challenges of atmospheric plasma spraying (APS) when dealing with rare earth silicate are the amorphous phase formation and silica volatilization in as-sprayed coatings. In this study, the microstructure and phase composition evolution of dual-phase ytterbium silicate environmental barrier coatings (EBCs) plasma sprayed from stoichiometric Yb2Si2O7 feedstock powder are investigated. The unavoidable silica volatilization leads to the coating composition deviate from stoichiometry of disilicate composition and result in the presence of Yb2SiO5 in coatings. By controlling the phase fraction of Yb2SiO5 through modulation of plasma character allows minimizing the effects of thermal residual stress on the coating durability in thermal cycling. The high crystallinity and crack-free coatings were achieved through optimized crystallization heat treatment. The presence of dislocation and deformation twins endow the dual-phase coating with intrinsic quasi plasticity and improvement in reliability. The present study provides useful inspirations on the optimization of spraying parameters and design of advanced EBC systems.

Molten Salt Corrosion Behaviors and Mechanisms of Ytterbium Silicate Environmental Barrier Coating

Molten Salt Corrosion Behaviors and Mechanisms of Ytterbium Silicate Environmental Barrier Coating

Accelerated oxidation during 1350°C cycling of ytterbium silicate environmental barrier coatings

AbstractEnvironmental barrier coatings (EBCs) will be needed to protect SiC‐based ceramic matrix composite components for the next generation of high‐efficiency industrial gas turbines (IGTs). The IGT application will require ≥25 kh lifetimes, and little data are available on EBC failure mechanisms, particularly at ≥1300°C. Initial 1‐h furnace cycle testing at 1350°C in 90 vol% H2O/10 vol% air was conducted ≥1000 cycles on thermally sprayed ytterbium disilicate (YbDS) coatings with and without an Si bond coating. By ≥1000 h, both EBCs formed thick, highly cracked, and fully crystalline cristobalite scales. Comparison of thermally grown oxide (TGO) microstructure and kinetics to isothermal rates of Si and SiC steam oxidation indicated a departure from slow‐growing parabolic growth to more rapid rates of silica formation. Possible mechanisms and implications for this acceleration are discussed.

Design of a new yttrium silicate Environmental Barrier Coating (EBC) based on the relationship between microstructure, transport properties and protection efficiency

Yttrium mono- and di-silicates are excellent candidate materials for Environmental Barrier Coatings (EBC), to protect Ceramic-Matrix Composites (CMC). A large distribution of microstructures can be generated by varying the duration of sintering, the particle size distribution of the precursor and/or the deposition method. In the present work, investigations of the corrosion resistance of yttrium silicate coatings with various microstructures in the high temperature range, under different oxidizing environments are reported. An original experimental methodology has allowed quantifying the influence of the microstructure (especially grain size and grain boundary surface area) on properties such as ionic conductivity, resistance to oxygen diffusion, volatilization rate and surface recession, and showing that they directly affect their protection efficiency against oxidation. Based on these results, guidelines for the design of an optimized yttrium silicate EBC featuring microstructural and compositional variations are proposed.

Corrosion behaviors and mechanisms of ytterbium silicate environmental barrier coatings by molten calcium-magnesium-alumino-silicate melts

The ytterbium silicate coatings with different contents of Yb2O3, Yb2SiO5 and Yb2Si2O7 phases were explored on their corrosion resistance to CMAS melts at 1400 ℃. Results showed that Yb2O3-rich coatings dissolved into CMAS melts and reprecipitated garnet and apatite phases, the reaction of producing garnet was beneficial to the formation of dense apatite layer, which can prevent the molten CMAS from penetrating into the coating. However, no dense apatite layer was observed in the corroded Yb2Si2O7-rich coatings. The accelerated recession in the Yb2Si2O7-rich coatings might be ascribed to the increased fraction of CMAS melts within the reaction layer.

Water vapor volatilization and oxidation induced surface cracking of environmental barrier coating systems: A numerical approach

A numerical model is developed for surface crack propagation in brittle ceramic coatings, aiming at the intrinsic failure of rare-earth silicate environmental barrier coating systems (EBCs) under combustion conditions in advanced gas turbines. The main features of progressive degradation of EBCs in such conditions are captured, including selective silica vaporization in the top coat due to exposure to water vapor, diffusion path-dependent bond coat oxidation, as well as crack propagation during cyclic thermal loading. In light of these features, user-defined subroutines are implemented in finite element analysis, where surface crack growth is simulated by node separation. Numerical results are validated by existing experimental data, in terms of monosilicate layer thickening, thermal oxide growth, and fracture behaviors. The experimentally observed quasi-linear oxidation in the early stage is also elucidated. Furthermore, it is suggested that surface crack undergoes rapid propagation in the late stage of extended thermal cycling in water vapor and leads to catastrophic failure, driven by both thermal mismatch and oxide growth stresses. The latter is identified as the dominant mechanism of penetration. Based on detailed analyses of failure mechanisms, the optimization strategy of EBCs composition is proposed, balancing the trade-off between mechanical compliance and erosion resistance.

Thermal durability of ytterbium silicate environmental barrier coating prepared by suspension plasma spray

Owing to their superior high-temperature stability, SiC-based ceramic matrix composites are widely used in turbine engine components for launch vehicles and other applications in the aerospace industry. However, because of the deterioration that occurs in these materials in high-temperature wet operating environments, researchers have developed environmental barrier coating technologies to protect the substrate. In this study, a top coat was prepared using Yb2SiO5 and Yb2Si2O7 as the third-generation coating of a rare-earth silicate with high thermal, chemical, and mechanical stability in high-temperature environments above 1400 °C. A new bond coat was prepared, incorporating Si, whose thermal expansion coefficient is similar to that of the SiC substrate, and Hf, which is expected to significantly contribute to high-temperature stability. The coating was prepared by suspension plasma spraying rather than atmospheric plasma spraying to realize a fine coating particle structure; to densify the coating layers, secondary densification coatings were created using ytterbium (III) nitrate pentahydrate (Yb(NO3)3∙5H2O) and tetraethyl orthosilicate (C8H20O4Si). The prepared specimens were measured in terms of weight change and weight change ratio with respect to temperature under high-temperature flames (1400 ℃ and 1700 ℃). After the secondary densification coating, the microstructure of the specimen was observed to be denser and the surface layer of the coating exhibited crystallinity. For the tested samples, a decrease in weight was observed. The largest weight loss was observed under the 1700 ℃ flame.

Water vapor corrosion behaviors of plasma sprayed ytterbium silicate coatings

In this work, the water vapor corrosion behaviors of plasma sprayed ytterbium silicate coatings with different contents of Yb2O3, Yb2SiO5 and Yb2Si2O7 phases were evaluated at 1400oC for 100–300 h and the influence of phase compositions on the corrosion behaviors was analyzed. The results showed that severe corrosion took place in the Yb2Si2O7-rich coating with a porous reacted layer with a thickness about 20 μm after 300 h corrosion. The Yb2O3 phase in the Yb2O3-rich coatings volatilized, lefting a relatively dense reacted layer with a thickness about 5–6 μm after the same corrosion time. A layer composed of Yb2SiO5 was formed on the surface of all the corroded ytterbium silicate coatings. This study may provide theoretical basis for selecting rare-earth silicate environmental barrier coatings (EBCs) with excellent performance.

Resistance of pure and mixed rare earth silicates against calcium‐magnesium‐aluminosilicate (CMAS): A comparative study

AbstractRare earth silicate environmental barrier coatings (EBCs) are state of the art for protecting SiC ceramic matrix composites (CMCs) against corrosive media. The interaction of four pure rare earth silicate EBC materials Yb2SiO5, Yb2Si2O7, Y2SiO5, Y2Si2O7 and three ytterbium silicate mixtures with molten calcium‐magnesium‐aluminosilicate (CMAS) were studied at high temperature (1400°C). The samples were characterized by SEM and XRD in order to evaluate the recession of the different materials after a reaction time of 8 hours. Additionally, the coefficient of thermal expansion (CTE) was determined to evaluate the suitability of Yb silicate mixtures as EBC materials for SiC CMCs. Results show that monosilicates exhibit a lower recession in contact with CMAS than their disilicate counterparts. The recession of the ytterbium silicates is far lower than the recession of the yttrium silicates under CMAS attack. Investigation of the ytterbium silicate mixtures exposes their superior resistance to CMAS, which is even higher than the resistance of the pure monosilicate. Also their decreased CTE suggests they will display better performance than the pure monosilicate.

Calcium–magnesium aluminosilicate (CMAS) interactions with ytterbium silicate environmental barrier coating material at elevated temperatures

Thermochemical interactions between a calcium-magnesium aluminosilicate (CMAS) glass and ytterbium disilicate (Yb2Si2O7), a candidate environmental barrier coating (EBC) material, have been investigated. Pellets of Yb2Si2O7 and CMAS glass powder were heat treated at 1200, 1300, 1400 and 1500 °C for 1, 10 and 50 h in air. Powder X-ray diffraction was employed to identify the resulting phases. In a second series of experiments, Yb2Si2O7 substrates were prepared by hot pressing, and cylindrical wells were drilled into the material surface. CMAS glass powder was added to the wells to achieve a loading of ~35 mg/cm2 and subsequently heat treated at 1200, 1300, 1400 and 1500 °C for durations of 1, 10 and 50 h in air. Sample cross-sections were characterized using scanning electron microscopy, X-ray energy-dispersive spectroscopy, X-ray diffraction, electron microprobe analysis and transmission electron microscopy to evaluate the resulting microstructure, phases and compositions at the CMAS/Yb2Si2O7 interface. Dissolution of ytterbium silicate into molten CMAS followed by precipitation of Yb2Si2O7 coupled with CMAS grain boundary penetration at elevated temperatures were the dominant mechanisms by which CMAS effectively infiltrated and altered the Yb2Si2O7 substrate microstructure.

Accurate residual stress measurement as a function of depth in environmental barrier coatings via a combination of X-ray diffraction and Raman spectroscopy

This study examined the properties of the latest generation of rare earth silicate environmental barrier coating system (EBCs). By combining X-ray diffraction and Raman spectroscopy, a novel and efficient method was developed to measure residual stress in EBCs produced by slurry coating. A 633-nm-wavelength laser beam from the confocal microlaser of a Raman spectrometer was focused on the cross-section of samples, and the spectra were recorded. We performed non-linear multi-peak spectrum fitting with the Lorenz function to obtain the peak shift of the EBC top coat at 5 μm intervals as a function of depth. We then obtained the stress distribution along the depth by calibrating the stress value on the surface via X-ray diffraction. The residual stress in the coating which endured high temperature, water, and oxygen corrosion without serious damage, was non-uniform and compressive. The stress at the bottom of the coating was approximately −275 MPa, decreasing towards the surface. A sudden shift in stress value was observed in the distribution of the residual stress, which was attributed to residual stress relaxation caused by microscopic pores contained within the coating.

Mass Transfer in Yb Silicate Environmental Barrier Coatings at High Temperatures

The oxygen permeabilities of polycrystalline Yb2Si2O7 wafers cut from sintered bodies to serve as models of environmental barrier coatings were determined at temperatures up to 1673 K under various oxygen potential gradients (dµO), produced by exposing the upper and lower surfaces of the wafer to atmospheres with different oxygen partial pressures (PO2). Oxygen permeation proceeded via grain boundary (GB) diffusion of oxygen from the higher PO2 surface to the lower PO2 surface, concurrently with GB diffusion of ytterbium in the opposite direction. Mass transfer analysis allowed for a design guideline for selectively enhancing the structural stability of the ytterbium silicate layer with applying a dµO.

Effect of Deposition Parameter and Mixing Process of Raw Materials on the Phase and Structure of Ytterbium Silicate Environmental Barrier Coatings by Suspension Plasma Spray Method

Effect of Deposition Parameter and Mixing Process of Raw Materials on the Phase and Structure of Ytterbium Silicate Environmental Barrier Coatings by Suspension Plasma Spray Method