Calcium-magnesium-alumino-silicate Research Articles

High temperature corrosion of (Er0.25Tm0.25Yb0.25Lu0.25)2Si2O7 environmental barrier coating material subjected to water vapor and molten calcium–magnesium–aluminosilicate (CMAS)

Environmental barrier coatings (EBCs) are being developed to improve the durability of SiCf/SiC CMC components against harsh combustion environment. Among the most promising EBC candidates, rare-earth (RE) disilicates attract attentions for the good thermal and chemical compatibility with SiC CMCs. We herein investigate the high temperature corrosion resistances of a new multicomponent β-(Er0.25Tm0.25Yb0.25Lu0.25)2Si2O7 disilicate subjected to hot water vapor and molten CMAS. Multicomponent (Er0.25Tm0.25Yb0.25Lu0.25)2Si2O7 shows superior water vapor corrosion resistance at 1400 °C and significantly enhanced CMAS resistance at 1500 °C compared with single principal RE disilicates. The present results highlight the merit of multiple RE engineering in advanced EBCs for the improvement of crucial high temperature corrosion resistances in engine applications.

Mixed phase ytterbium silicate environmental-barrier coating materials for improved calcium–magnesium–alumino-silicate resistance

Abstract

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.

Mechanical properties and calcium-magnesium-alumino-silicate (CMAS) corrosion behavior of a promising Hf6Ta2O17 ceramic for thermal barrier coatings

Hf6Ta2O17 ceramics are successfully prepared by solid state reaction and pressureless sintering. The mechanical properties and the calcium-magnesium-alumino-silicate (CMAS) corrosion behavior of Hf6Ta2O17 ceramics are studied. The hardness, elastic modulus and fracture toughness of Hf6Ta2O17 ceramics are 18.45 GPa, 273.42 GPa and 2.6–3.1 MPa m1/2, respectively. As the Hf6Ta2O17 ceramics are attacked by CMAS, the reaction layer and dense layer are formed on the ceramic surface, which prevents the further infiltration of molten CMAS. HfSiO4 and Ca2Hf7O16 are confirmed as the main reaction products of Hf6Ta2O17 and CMAS. Hf6Ta2O17 ceramics exhibit better CMAS corrosion resistance than 8 wt% yttria stabilized zirconia (8YSZ) ceramics, which is attributed to the dense structure formed by corrosion products (hafnium tantalum oxide and Ca2Hf7O16) and lower theoretical optical basicity (OB) value. Furthermore, Hf6Ta2O17/YSZ double ceramic top coat thermal barrier coatings (TBCs) are successfully prepared by plasma spraying, and the thermal cycling performance is investigated. Hf6Ta2O17/YSZ double-layers TBCs has good thermal cycling performance as 8YSZ single-layer TBCs.

Binary collision of CMAS droplets—Part I: Equal-sized droplets

Abstract

Rare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for resistance against attack by molten calcia–magnesia–aluminosilicate (CMAS) glass

Abstract

Interaction between Y-Al-Si-O glass-ceramics for environmental barrier coating materials and Ca-Mg-Al-Si-O melts

Y2O3-Al2O3-SiO2 (Y-Al-Si-O, YAS) glass is a highly promising material offering resistance against calcium magnesium aluminosilicate (Ca-Mg-Al-Si-O, CMAS) corrosion, and it is an excellent candidate for environmental barrier coating (EBC). In this study, influence of composition on YAS glass properties and CMAS corrosion resistance at 1300 °C was investigated, and the result indicated that most sets of YAS glass possessed the superior CMAS resistance than yttrium disilicate. The increase of Y2O3 content and decrease of Al/Si ratio both resulted in a higher nucleation rate during heating. Moreover, when YAS was subjected to the molten CMAS corrosion, two types of corrosion restraining behaviors were observed. One is the generation of anorthite layer on YAS with high-optical basicity, and the recession depth becomes less than that of yttrium disilicate. Stacking of anorthite grains obstructs the diffusion of Y to the corrosion front, which inhibits the corrosion. The other mechanism involves the generation of calcium yttrium cyclosilicate layer at ~1270 °C first on YAS with low-optical basicity. Next, with the increase in the temperature, the reaction changes to generate an amorphous phase. Moreover, the layer acts as a barrier to restrain the corrosion. Furthermore, the difference between the two corrosion behaviors was explained based on the optical basicity (OB) theory calculation, and a YAS composition region with excellent resistance toward CMAS corrosion was predicted. The region with an OB higher than that of CMAS shows superior CMAS resistance, in particular, for the portion with a lower difference in values of OB between the two substances.

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.

Phase field model for diffusion-reaction stress field in the thermal barrier coatings corroded by the molten CMAS

Thermal barrier coatings (TBCs) are prone to attacking by molten calcium-magnesium-alumino-silicates (CMAS) in service. Reactions of molten CMAS and the ceramic coating are solution-reprecipitation ones that generate a new phase, ZrSiO4. Swelling caused by these reactions result in accumulation of localized stresses in the ceramic coating, which subsequently contributes to the premature failure of TBCs. In this work, the CMAS diffusion-reaction with TBCs is investigated based on the phase field theory. A two-dimensional phase field model is developed, which fully couples the Cahn-Hilliard equation for the diffusion of CMAS and the constitutive equation for the prediction of the chemical reaction-induced stresses. The results indicate that very high compressive stresses emerged in the coating, which may lead to the spallation failure of the coating. Furthermore, it is found that the presence of mechanical action accelerates the corrosion kinetics in TBCs and the evolution of CMAS concentration in the coating is consistent with previous reports. Finally, approximately 3.4% volume strain caused by CMAS corrosion reaction is predicted.

Pore filling behavior of air plasma spray thermal barrier coatings under CMAS attack

The pore filling behavior in yttria-stabilized zirconia air plasma sprayed (APS) thermal barrier coatings (TBCs) during calcium-magnesium-alumino-silicate (CMAS) infiltration process was investigated. After fully infiltration, the total porosity of the coating decreased. However, the total porosity of the top region increased due to severe microstructural change. For the bottom region, whose microstructure did not change significantly, its total porosity dropped and almost all crack network disappeared, but 48 vol. % of globular pores still existed and larger globular pores were more likely to remain. This study indicates that introducing relatively large globular pores into APS TBCs may mitigate CMAS damage.

Hot corrosion behavior of calcium magnesium aluminosilicate (CMAS) on the Yb2SiO5-8YSZ composite as a candidate for environmental barrier coatings

Dense Yb2SiO5 and three types of Yb2SiO5-YSZ composite specimens, as an environmental barrier coating, were prepared using pressureless sintering process at 1600 °C for 10 h. Hot corrosion behaviors of the sintered specimens were investigated in the presence of calcium–magnesium-aluminosilicate (CMAS) at 1400 °C for 4, 8, 24 and 48 h. Phase change identification, cross-sectional microstructure, reaction layer thickness, and elemental distribution, of the different samples during hot corrosion was investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) equipped by EDS. Mechanical properties of synthesized samples include elastic modulus, hardness and microhardness were measured by the nanoindentation method. The results showed that the reaction layer of all composite samples was smaller than that of the Yb2SiO5 sample. Formation of dense Al5Yb3O12 and Al5Y3O12 phases in the reaction layer can reduce the infiltration of molten CMAS into the samples, which mean better hot corrosion resistance of composite samples. Yb2SiO5-30 wt%YSZ composite sample showed better performance in contrast to the composite samples because of the high concentration of dissolved Y3+ ion in the glassy CMAS which promotes the formation of the barrier oxide phases (Ca2Al2SiO7, Ca3ZrSi2O9) and protective garnet phases (Al5Yb3O12) in the reaction layer.

The corrosion behaviors of thermal barrier material of M-YTaO4 attacked by CMAS at 1250 °C

The corrosion of YSZ TBCs attacked by calcium–magnesium–aluminosilicate (CMAS) is a serious problem. Yttrium tantalite (YTaO4), a new kind of potential thermal barrier ceramic material, was expected to replace the YSZ to manufacture the TBCs because of its great thermophysical characteristics. In this study, porous YTaO4 ceramic pellets, instead of actual TBCs, were used to investigate the CMAS corrosion resistance at 1250 °C. Results indicated that CMAS couldn't cover the whole surface of YTaO4 pellets homogeneously because of low wettability between liquid CMAS and YTaO4, in addition, there was almost no reaction layer after 4 h reaction. The XRD results showed that M-YTaO4, M′-YTaO4, Ca2Ta2O7 and Y2Si2O7 were the main four phases after reaction and there was no phase containing the elements of Mg and Al. Compared with YSZ TBCs, this new kind of potential thermal barrier ceramic material showed well resistance to CMAS corrosion.

Thermal insulation of CMAS (Calcium-Magnesium-Alumino-Silicates)- attacked plasma-sprayed thermal barrier coatings

The impact of the penetration of small quantities of calcium-magnesium-alumino- silicates (CMAS) glassy melt in the porous plasma-sprayed (PS) thermal barrier coatings (TBCs) is often neglected even though it might play a non-negligible role on the sintering and hence on the thermal insulation potential of TBCs. In this study, the sintering potential of small CMAS deposits (from 0.25–3 mg.cm−2) on freestanding yttria-stabilized zirconia (YSZ) PS TBCs annealed at 1250 °C for 1 h was investigated. The results showed a gradual in-depth sintering with increasing CMAS deposits. This sintering was concomitant with local transformations of the tetragonal YSZ and resulted in an increase in the thermal diffusivity of the coatings that reached a maximum of ∼110 % for the fully penetrated coating.

Interactions between zirconia–yttria–tantala thermal barrier oxides and silicate melts

The effects of TaO2.5 additions to Y4Zr3O12 (YZO) on its interactions with calcium-magnesium alumino-silicate (CMAS) melts are investigated. YZO is a candidate material for the protection of novel ZrO2-YO1.5-TaO2.5 based thermal barrier coatings (TBCs) against CMAS attack, but thermochemical considerations dictate that YZO contain TaO2.5 for interfacial compatibility. Oxides based on Y4Zr3-xTaxO12+0.5x (x = =0, 0.7, 1.4) were mixed with various ternary and quinary silicate compositions ranging in Ca:Si ratio from 0.35 to 1.13 and equilibrated at 1300 °C. The results show that all combinations form the desired apatite phase except for the mixture of the higher Ta5+ content with the lowest Ca:Si melt. It is found that the addition of Ta5+ increases the capture of Y3+ by the reprecipitated fluorite, decreasing the amount available to form apatite and eventually suppressing it at low Ca:Si. The effect is ascribed to the attractive interactions between Y3+ and Ta5+ in solid solution, similar to that found in tetragonal zirconia, but with concurrent incorporation of anion vacancies associated with excess Y3+ that stabilize the cubic form. As the Ca2+ content of the melt increases, so does the amount of Ca2+ incorporated in fluorite. The latter also interacts attractively with Ta5+ and leads to a reduction of captured Y3+ in fluorite as the Ca content in the reacting deposit increases. This serves to increase the amount of Y3+available in the melt to form apatite in TBC-CMAS interactions.

Thermal cycling performances of multilayered yttria‐stabilized zirconia/gadolinium zirconate thermal barrier coatings

AbstractGadolinium zirconate (Gd2Zr2O7, GZO) as an advanced thermal barrier coating (TBC) material, has lower thermal conductivity, better phase stability, sintering resistance, and calcium‐magnesium‐alumino‐silicates (CMAS) attack resistance than yttria‐stabilized zirconia (YSZ, 6‐8 wt%) at temperatures above 1200°C. However, the drawbacks of GZO, such as the low fracture toughness and the formation of deleterious interphases with thermally grown alumina have to be considered for the application as TBC. Using atmospheric plasma spraying (APS) and suspension plasma spraying (SPS), double‐layered YSZ/GZO TBCs, and triple‐layered YSZ/GZO TBCs were manufactured. In thermal cycling tests, both multilayered TBCs showed a significant longer lifetime than conventional single‐layered APS YSZ TBCs. The failure mechanism of TBCs in thermal cycling test was investigated. In addition, the CMAS attack resistance of both TBCs was also investigated in a modified burner rig facility. The triple‐layered TBCs had an extremely long lifetime under CMAS attack. The failure mechanism of TBCs under CMAS attack and the CMAS infiltration mechanism were investigated and discussed.

Tracking the calcium-magnesium-alumino-silicate (CMAS) infiltration into an air-plasma spray thermal barrier coating using X-ray imaging

The progressive infiltration of a free-standing air plasma spray (APS) thermal barrier coating (TBC) by calcium-magnesium-alumino-silicate (CMAS) at 1250 °C is followed non-destructively by time-lapse X-ray micro-computed tomography (µ-CT). The top ∼50 µm of the TBC expands rapidly (within the first 20 min) by as much as 70% due to the physical ingress of the CMAS along the splat boundaries and becomes exfoliated. The ingress and resulting exfoliation are shown to be driven by capillary forces with the exfoliated depth dependent on the CMAS loading. The remainder of the TBC swells at a slower rate (ultimately by 15%) but remains intact.

A chemo-thermo-mechanically constitutive theory for thermal barrier coatings under CMAS infiltration and corrosion

High temperature ceramic corroded by calcium-magnesium-alumino-silicates (CMAS) deposits is an inevitable part of severe degradation of thermal barrier coatings (TBCs). Based on thermodynamic laws, a mechanism-based constitutive theory is proposed for describing CMAS corrosion process at high temperature in TBCs. Concentration of metal cations in CMAS and degree of corrosion dissolution of TBCs are defined respectively. The constitute of free energy which decides the driving force for governing equation of field variables includes energy contribution from the infiltration of CMAS, the subsequent corrosion dissolution of TBCs by CMAS and elastic strain energy. Also, some coupling terms are added to the expression of free energy to describe coupled effects between field variables. We further establish two 3D plate models for TBCs with and without constraint by substrate to predict the chemo-mechanical response of corroded TBCs. A corrosion experiment is conducted to confirm deformational behavior of corroded TBCs and transient distribution of CMAS mass density. In addition, coupled kinetics capture that out-plane tensile stress piles up near the bottom of corroded coating with constraint by substrate, and disastrous delamination can occur from the interface under the CMAS covered region. The constitutive theory presented here provides a great potential for modeling chemo-thermo-mechanical corrosion process in TBCs at high temperature.

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

AbstractRare earth (RE) disilicates are utilized in environmental barrier coatings to protect Si‐based engine components from destructive reactions with water vapor and other combustion species. These coating materials, however, degrade when exposed to molten silicate deposits in the engine. Four RE‐disilicates (RE2Si2O7, RE = Er, Dy, Gd, Nd) are analyzed herein in thermochemical interactions with glassy calcium‐magnesium‐aluminosilicate (CMAS) compositions at 1400°C. Crystalline reaction products included RE2Si2O7, SiO2, and a Ca2+yRE8+x(SiO4)6O2+3x/2+y apatite‐type silicate. RE2Si2O7 formation was favored in interactions with CMAS having low CaO:SiO2 ratios. Increased reactivity was observed for higher CaO:SiO2 ratios in CMAS combined with larger RE3+ cation size, resulting in apatite formation of varying stoichiometry and changes in lattice parameters. The crystallization of SiO2 was dependent on both thermodynamic equilibrium at low CaO:SiO2 ratios and sequestration of silicate modifiers at higher CaO:SiO2 ratios, although residual amorphous content after CMAS exposure in both cases was still substantial.

Cover Image, Volume 102, Issue 10

Cover Photograph: Button type specimen of a thermal barrier coating (TBC) for gas turbine application under test in a burner rig environment. Precursors of calcium‐magnesium‐aluminosilicates (CMAS) are injected to the flame to form corrosive deposits. The orange coloring indicates atomic emission spectra of dissolved CMAS constituents. Image Credit: Ms. Hiltrud Moitroux (Forschungszentrum Jülich GmbH). DOI: 10.1111/jace.16465 image

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.