Effects of the chemistry of coating and substrate on the steam oxidation kinetics of environmental barrier coatings for ceramic matrix composites

Environmental barrier coatings (EBCs) are an enabler for SiC/SiC ceramic matrix composites (CMCs) in gas turbines by protecting CMCs from environmental degradation. A critical EBC failure mode is the EBC spallation due to a build-up of elastic strains caused by the formation of SiO₂ scale, known as TGO (thermally grown oxide). H₂O, a byproduct of combustion reactions, accelerates the TGO-induced EBC failure by increasing TGO growth rates by orders of magnitude. NASA’s approach to improve the EBC life, therefore, is to reduce TGO growth rates. NASA discovered that modifying the TGO chemistry by modifying the EBC chemistry of Gen 2 EBC (Si / Yb₂Si₂O₇) reduces the TGO thickness by up to ～80 %. A study was undertaken to understand the oxidation mechanism of modified Gen 2 EBCs as well as to investigate the effect of EBC and CMC chemistry on TGO growth rates. This study confirmed the previously proposed TGO-controlled oxidation mechanism of modified Gen 2 EBCs and determined the correlations between the EBC and CMC chemistry, TGO chemistry, and TGO growth rates.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Thermal cycle test performances of environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 in the steam environment at 1475 °C

The novel bi-layer environmental barrier coatings (EBCs) with HfO₂-SiO₂/Yb₂Si₂O₇ structure (70HfO₂-30SiO₂/Yb₂Si₂O₇: 70HS/YbDS, 50HfO₂-50SiO₂/Yb₂Si₂O₇: 50HS/YbDS, molar ratios) was tested in 90%H₂O–10%O₂ conditions between room temperature and 1475 °C in an Al₂O₃ tube furnace, then its performance was evaluated. The YbDS layer was contaminated by alumina impurities under steam conditions. After 22 cycles, the 70HS/YbDS completely separated from the SiC substrate, while the 50HS/YbDS and SiC did not separate, even though cracks formed at the 50HS/SiC interface and the TGO layer. Furthermore, the thermally grown oxide (TGO) layer formed at the HfO₂-SiO₂/SiC interface. Formation and growth of the TGO led to the formation and propagation of cracks at the HfO₂-SiO₂/TGO interface and TGO interior, which was the culprit leading to the failure of EBCs. These results demonstrated that the 50HS/YbDS EBCs have the potential to protect SiC in steam conditions at 1475 °C.     

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.     

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.     

Effect of Yb2SiO5 addition on the physical and mechanical properties of sintered mullite ceramic as an environmental barrier coating material

In this study, ytterbium monosilicate (Yb₂SiO₅)-added sintered mullite ceramics are prepared as candidate materials for environmental barrier coatings (EBCs). The effect of adding Yb₂SiO₅ on the physical and mechanical properties of the sintered mullite ceramics is investigated. The Yb₂O₃–SiO₂–Al₂O₃ ternary phase diagram indicates that adding Yb₂SiO₅ to the mullite goes beyond simply mixing; instead, liquid sintering occurs. Therefore, when we add Yb₂SiO₅ to the mullite, the sintered body possesses a denser microstructure and faster densification rate than does pure mullite. The density rapidly increases with the addition of 6 wt% Yb₂SiO₅ in the mullite, and almost full densifications are achieved with the addition of 12 wt% and 18 wt% Yb₂SiO₅. In this study, mullite ceramic that contains 12 wt% Yb₂SiO₅ exhibits the smallest plastic deformation and the highest elastic modulus among ceramics containing 6, 12, and 18 wt% Yb₂SiO₅, according to Hertzian indentation results. The results suggest that 12 wt% Yb₂SiO₅-doped mullite may be expected to act as a potential EBC material based on its excellent elastic properties, dense microstructure, and appropriate coefficient of thermal expansion.     

Autonomous crack healing ability of SiC dispersed Yb2Si2O7 by oxidations in air and water vapor

Yb₂Si₂O₇ is a popular environmental barrier coating; however, it decomposes into Yb₂SiO₅ in high-temperature steam environments. The thermal mismatch between Yb₂Si₂O₇ and Yb₂SiO₅ leads to the cracking and failure of the disilicate coating via oxidation. Dispersing SiC nanofillers into the Yb₂Si₂O₇ matrix is suggested to maintain the Yb₂Si₂O₇ matrix and promote crack self-healing. This study is aimed at clarifying the effect of water vapor on the self-healing ability of such composites. X-ray diffraction analysis and scanning electron microscopy were used to monitor the surface composition and the crack formation, respectively, in 10 vol% SiC-dispersed Yb₂Si₂O₇ composites. Annealing at temperatures higher than 750 °C in air or in a water vapor rich atmosphere led to strength recovery and the self-healing of indentation-induced surface cracks owing to volume expansion during the oxidation of SiC. The self-healing effect was influenced by the oxidation time and temperature. Rapid diffusion of H₂O as an oxidizer into the SiO₂ layer promoted self-healing in a water vapor rich atmosphere. However, accelerated oxidation at temperatures higher than 1150 °C formed bubbles on the surface. Fabricating composites with a small amount of Yb₂SiO₅ will be a solution to these problems.     

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.     

Isothermal oxidation and TGO growth behaviors of YAG/YSZ double-ceramic-layer thermal barrier coatings

In this study, yttrium aluminum garnet/yttria-stabilized zirconia (YAG/YSZ) double-ceramic-layer thermal barrier coatings (DCL TBC) and yttria-stabilized zirconia (YSZ) single-ceramic-layer thermal barrier coatings (SCL TBC) were deposited by atmosphere plasma spray (APS) on the Inconel 738 alloy substrate, and isothermal oxidation tests were performed to investigate the formation and growth behavior of thermally grown oxide (TGO). Results showed that the Al₂O₃ TGO thickness of both TBC groups increased by increasing the isothermal oxidation time，and then slowly decreased with the appearance and growth of the adverse multilayer structure comprising CoCr₂O₄, (Ni,Co)Al₂O₄, NiCr₂O₄, and NiO mixed oxides. However, since the significant inhibition effect of the YAG coating to oxygen ionic diffusion, the mixed oxides appearance time and TGO growth behaviors were delayed in the DCL TBC. As a result, the TGO thickness of the DCL TBC was always smaller than that of the SCL TBC in the entire oxidation process. And the Al₂O₃ layer thickness proportion in the total TGO of the DCL TBC was greater than or equal to that of the SCL TBC after oxidation for the same period. The results of weight gain showed that compared with the SCL TBC, the parabolic oxidation rate of the DCL TBC was decreased approximately 35%. Consequently, the DCL TBC has better high-temperature oxidation resistance than the SCL TBC.     

Interaction of Yb2Si2O7 environmental barrier coating material with Calcium-Ferrum-Alumina-Silicate (CFAS) at high temperature

Experimental investigations of Yb₂Si₂O₇ pellet exposed to Calcium-Ferrum-Alumina-Silicate (CFAS) at 1400 °C in ambient air were carried out to reveal corrosion reaction between molten silicate deposit and Yb₂Si₂O₇. Phase transformation, microstructure evolution and reaction mechanism were evaluated. Results indicated that the corrosion process was accompanied by the infiltration of CFAS melt, the dissolution of Yb₂Si₂O₇ and the reprecipitation of Yb₂Si₂O₇ and Ca₂Yb₈(SiO₄)₆O₂ apatite as reaction product. The formation of apatite decreased the concentration of Ca²⁺ in the melt. After CFAS exposure at 1400 °C for 30 h, the thickness of the apatite layer stopped increasing due to insufficient Ca²⁺ content, and remained at about 115.4 μm. However, the infiltration depth of CFAS melt increased with the extending corrosion duration and increasing deposit content. And the infiltration rate was preliminarily found to first decrease and then increase with time. Most of the residual CFAS were crystallized into garnet (Ca₃Fe₂(SiO₄)₃ and Yb₃Fe₅O₁₂) and mayerite (Ca₁₂Al₁₄O₃₃), while a small volume of amorphous glass was dispersed among the garnet and mayerite grains.     

Thermochemical stability and microstructural evolution of Yb2Si2O7 in high-velocity high-temperature water vapor

Thermochemical stability and microstructural evolution of Yb₂Si₂O₇ was studied in high-temperature high-velocity water vapor at temperatures between 1200–1400 °C. Two reactions were shown to occur in the steam environment: Yb₂Si₂O₇ reaction to form Yb₂SiO₅, and further Yb₂SiO₅ reaction to form Yb₂O₃. Parabolic rates of both reactions were observed, and similar reaction enthalpies were determined for each reaction; 207 kJ/mol and 205 kJ/mol, respectively. Densification of the product phase Yb₂SiO₅ shut off pore connectivity for gas transport to the reaction interface at gas velocities exceeding 115?125 m/s and for temperatures of 1300 °C and 1400 °C, resulting in reduced reaction rates at higher velocities. Outward gas diffusion by a silicon hydroxide species is predicted to govern ytterbium silicate reactions with high temperature water vapor. Microstructure changes at high temperatures and velocities were shown to greatly impact the long-term stability of Yb₂Si₂O₇.     

High-Temperature thermochemical interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating materials

The thermochemical behavior of EBC candidate materials yttrium disilicate (Y₂Si₂O₇) and ytterbium disilicate (Yb₂Si₂O₇) was evaluated with three calcium-magnesium-aluminosilicate (CMAS) glasses possessing CaO:SiO₂ ratios relevant to gas turbine systems. Pellet mixtures of 50 mol% EBC powder to 50 mol% CMAS glass powder were heat treated at 1200°C, 1300°C, and 1400°C. The products of these interactions were evaluated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Above glass melting temperatures, exposure of the disilicates primarily resulted in dissolution into the molten glass followed by precipitation of a Ca₂RE₈(SiO₄)₆O₂ (RE = Yb³⁺, Y³⁺) apatite-type silicate and/or rare earth disilicate. In glasses with high CaO concentrations, apatite readily forms while the disilicate material is consumed by the reaction. As CaO content decreases, the disilicate phase becomes the main reaction product. Overall, reactions with yttrium disilicate favored more apatite crystallization than ytterbium disilicate. The viability of using these disilicates in various operating environments is discussed.     

Mechanical properties and microstructure of Yb2SiO5 environmental barrier coatings under isothermal heat treatment

Yb₂SiO₅ (ytterbium monosilicate) top coatings and Si bond coat layer were deposited by air plasma spray method as a protection layer on SiC substrates for environmental barrier coatings (EBCs) application. The Yb₂SiO₅-coated specimens were subjected to isothermal heat treatment at 1400 °C on air for 0, 1, 10, and 50 h. The Yb₂SiO₅ phase of the top coat layer reacted with Si from the bonding layer and O₂ from atmosphere formed to the Yb₂Si₂O₇ phase upon heat treatment at 1400 °C. The oxygen penetrated into the cracks to form SiO₂ phase of thermally grown oxide (TGO) in the bond coat and the interface of specimens during heat treatment. Horizontal cracks were also observed, due to a mismatch of the coefficient of thermal expansion (CTE) between the top coat and bond coat. The isothermal heat treatment improves the hardness and elastic modulus of Yb₂SiO₅ coatings; however, these properties in the Si bond coat were a little bit decreased.