Indirect Dissolution of Sapphire into Calcia-Magnesia-Alumina-Silica Melts: Electron Microprobe Analysis of the Dissolution Process

Spinel, MgAl₂O₄, has been observed to form on sapphire during sapphire dissolution into CaO-MgO-Al₂O₃-SiO₂ (CMAS) melts at 1450°. and 1550°C. Electron microprobe analysis was used to characterize the sapphire/melt interface for cases in which spinel did (indirect dissolution) or did not (direct dissolution) form on the sapphire during dissolution into CMAS melts. The concentrations of Al₂O₃, MgO, CaO, and SiO₂ were determined as a function of position within the spinel reaction product and in the adjacent melt. The rate-limiting steps for direct and indirect sapphire dissolution into CMAS melts are discussed.     

Direct and Indirect Dissolution of Sapphire in Calcia–Magnesia–Alumina–Silica Melts: Dissolution Kinetics

The dissolution rate of sapphire into CaO-MgO-Al₂O₃-SiO₂ (CMAS) melts was investigated at 1450° and 1550°C. The effects of immersion time, specimen rotation rate, and magnesia concentration in the bulk melt on the sapphire dissolution rate and on the growth rate of magnesium aluminate spinel at the sapphire/melt interface were determined. The processes of direct and indirect sapphire dissolution into CMAS melts are discussed, with the support of kinetic and microprobe data.¹ A model of indirect sapphire dissolution^1,2 is used along with the equations of Tedmon³ to describe the kinetics of indirect dissolution and spinel layer growth during forced-convective and static sapphire dissolution.     

Hot-corrosion behavior of calcium-magnesium aluminosilicate melts on Ba1/3Sr1/3Ca1/3Al2Si2O8

Hot-corrosion behavior of Ba_1/3Sr_1/3Ca_1/3Al₂Si₂O₈ (BSCAS) in the presence of molten calcium-magnesium-aluminum-silicate (CMAS) is investigated in the temperature range of 1250–1350 °C. In comparison, the hot corrosion behavior of Ba_0.5Sr_0.5Al₂Si₂O₈ (BSAS) is also studied under the same conditions. The results indicate that CMAS corrosion of both BSCAS and BSAS is caused by the interdiffusion of Ba/Sr and Ca between CMAS and corroded samples. The presence of Ca cations in BSCAS lowers the diffusion driving force of Ca cations between CMAS and BSCAS, resulting in a reduced diffusion rate of Ca cations from CMAS into BSCAS. Moreover, the sluggish diffusion effect of multi-component cations hinders the outward diffusion of Ba/Sr cations from BSCAS. Thus, the BSCAS shows a better CMAS corrosion resistance than BSAS.     

Indirect Dissolution of (Al, Cr)2O3 in CaO—MgO—Al2O3—SiO2 (CMAS) Melts

The dissolution of (Al, Cr)₂O₃ into CaO—MgO—Al₂O₃—SiO₂ melts, under static and forced-convective conditions was investigated at 1550°C in air. With sufficient MgO in the melt, or sufficient Cr₂O₃ in (Al, Cr)₂O₃, a layer consisting of a spinel solid solution, Mg(Al, Cr)₂O₄, formed at the (Al, Cr)₂O₃/melt interface. The dissolution kinetics of 1.5 and 10 wt% Cr₂O₃ specimens were determined as a function of immersion time, specimen rotation rate, and magnesia content of the melt. Electron microprobe analysis was used to characterize concentration gradients in the (Al, Cr)₂O₃ sample, the Mg(Al, Cr)₂O₄ spinel, or in the melt after immersion of specimens containing 1.5 to 78 mol% Cr₂O₃. The dissolution kinetics and microprobe analyses indicated that a steady-state condition was reached during forced-convective, indirect (Al, Cr)₂O₃ dissolution such that spinel layer formation was rate limited by solid-state diffusion through the spinel layer and/or through the specimen, and spinel layer dissolution was rate limited by liquid-phase diffusion through a boundary layer in the melt. This is consistent with a model previously developed for the indirect dissolution of sapphire in CMAS melts.     

The influence of thermal barrier coating dissolution on CMAS melt viscosities

Glassy deposits, largely consisting of CaO-MgO-Al₂O₃-SiO₂ (CMAS), are a common product on thermal barrier coatings (TBCs) within gas-turbines after an interaction with airborne particles. Here, in order to facilitate the quantification and modelling of the spreading and infiltration behavior of CMAS melts onto and into TBCs we have determined the high temperature viscosities of four widely used synthetic “CMAS” melts and the influence of TBC materials (yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO)) dissolution upon them. After a dissolution of 6.5 wt% YSZ or GZO one out of four CMAS melts shows no significant change in viscosity, while the other three melts exhibit a viscosity increase at lower temperatures that continuously changes to a decrease in viscosity towards higher temperatures. The influence of the doping amount on the viscosity was investigated in detail for one CMAS melt (C₃₅M₁₀A₇S₄₈) and parametrized.     

Dissolution and diffusion kinetics of yttria-stabilized zirconia into molten silicates

The degradation of thermal barrier coatings (TBCs) by molten silicates (CMAS) represents a fundamental barrier to progress in gas turbine technology, requiring a mechanistic understanding of the problem to guide the development of improved coatings. This article investigates the dissolution of yttria-stabilized zirconia (7YSZ and 20YSZ) into two model silicate melts at 1300–1400 °C. The approach involves the 1D dissolution of YSZ into a semi-infinite melt, characterizing the dissolution rates of YSZ and the diffusion rates of Zr⁴⁺ and Y³⁺ therein. The assessed kinetics of YSZ dissolution and diffusion were then applied to modeling the same phenomena on TBC-relevant length scales. These findings provide fundamental insight into (i) the dissolution mechanism of YSZ, (ii) the subsequent reprecipitation upon saturation, (iii) the quantitative effects of temperature and melt composition on the dissolution and diffusion kinetics, and (iv) how the measured kinetics manifests on the scale of flow channels present in TBCs.     

Synthesis and characterization of ytterbium oxide: A novel CMAS-resistant environmental barrier coating material

Demand for more powerful aircraft promotes development of ceramic matrix composites and environmental barrier coating (EBC). A promising EBC material, ytterbium oxide (Yb₂O₃), was fabricated by hot pressing, and its properties were systemically investigated. The evaluation of thermal properties provides a baseline for the application of Yb₂O₃ on SiC_f/SiC or Al₂O_3f/Al₂O₃ composites. The performance in water vapor and molten calcium–magnesium–aluminosilicate (CMAS) environments indicates its excellent durability in harsh environment. Compared with rare-earth silicates, the thermochemical interactions between ytterbium oxide and CMAS changed greatly with the absence of silicon oxide. Reactions of ytterbium oxide with CMAS form several reaction products, including apatite, garnet, and silicocarnotite. The crystallization of garnet and silicocarnotite could effectively consume and solidify the CMAS melt, which prevents the melt infiltration and mitigates the further corrosion.     

Wetting,infiltration and interaction behavior of CMAS towards columnar YSZ coatings deposited by plasma spray physical vapor

Thermal barrier coatings (TBCs) produced by electron beam physical vapor deposition (EB-PVD) or plasma spray (PS) usually suffer from molten calcium-magnesium-alumino-silicate (CMAS) attack. In this study, columnar structured YSZ coatings were fabricated by plasma spray physical vapor deposition (PS-PVD). The coatings were CMAS-infiltrated at 1250?°C for short terms (1, 5, 30?min). The wetting and spreading dynamics of CMAS melt on the coating surface was in-situ investigated using a heating microscope. The results indicate that the spreading evolution of CMAS melt can be described in terms of two stages with varied time intervals and spreading velocities. Besides, the PS-PVD columnar coating (～100?μm thick) was fully penetrated by CMAS melt within 1?min. After the CMAS attack for 30?min, the original feathered-YSZ grains (tetragonal phase) in both PS-PVD and EB-PVD coatings were replaced by globular shaped monoclinic ZrO₂ grains in the interaction regions.     

CMAS corrosion behavior of a LaPO4 ceramic prepared by spark plasma sintering

At high temperatures in gas turbines, traditional yttria stabilized zirconia materials fail prematurely owing to CMAS (calcium–magnesium–alumina–silicate) corrosion. Thus, new materials need to be developed urgently. In this study, LaPO₄ powder was synthesized by chemical coprecipitation and heat treatment using lanthanum nitrate (La(NO₃)₃∙6H₂O) and ammonium dihydrogen phosphate (NH₄H₂PO₄) as starting materials, and LaPO₄ bulk was prepared by spark plasma sintering. The surface of the LaPO₄ bulk was coated with CMAS (CaO–MgO–Al₂O₃–SiO₂) powder, and the CMAS interaction with the LaPO₄ bulk at different temperatures was investigated. The phase and microstructure of the LaPO₄ powder and bulk, as well as the CMAS corrosion products, were characterized using X-ray diffraction and scanning electron microscope. The superior CMAS resistance of the LaPO₄ bulk was attributed to the low wettability of LaPO₄ by the CMAS melt and the development of dense layers of new corrosion products, which effectively protected the LaPO₄ bulk from CMAS infiltration.     

Mechanical properties and calcium–magnesium–alumino–silicate corrosion behaviour of Ce/Gd co-doped SrZrO3 ceramics

Sr(Zr_1-2xCe_xGd_x)O_3-0.5x (x = 0, 0.05, 0.1 and 0.15) ceramics were prepared by pressureless sintering using powders that were synthesized by solid-state reaction. The mechanical properties and calcium–magnesium–alumino–silicate (CMAS) early corrosion behaviour of the prepared ceramics were reported. The mechanical properties of rare-earth-doped SrZrO₃ improved significantly. The reaction products of the Sr(Zr_1-2xCe_xGd_x)O_3-0.5x ceramics after CMAS corrosion were similar: zirconia, SrAl₂O₄, akermanite, and anorthite. The mechanism of CMAS corrosion resistance is summarized as follows: elemental Sr easily enters the CMAS melt, because of its high diffusivity, and promotes crystallization. Rare-earth elements can prevent melt infiltration because of their low diffusivity.     

Molten silicate interactions with plasma sprayed thermal barrier coatings: Role of materials and microstructure

Ingestion of siliceous particulate debris into both propulsion and energy turbines has introduced significant challenges in harnessing the benefits of enhanced operation efficiencies through the use of higher temperatures and thermal barrier coatings (TBCs). The so-called CMAS (for calcium-magnesium alumino-silicate) particles can melt in the gas path at temperatures greater than 1200C, where they will subsequently impact the coating surface and infiltrate through the carefully engineered porosity or cracks in a TBC. Ultimately, this CMAS attack causes premature spallation through its solidification and stiffening the ceramic during cooling. It has been noted in recent years, that TBCs based on yttria stabilized zirconia (YSZ) are completely non-resistant to CMAS attack due to their lack of reactivity with infiltrant liquid. New TBC ceramics such as Gadolinium Zirconate (GZO) show promise of CMAS resistance through rapid reaction-induced crystallization and solidification of the infiltrant, leading to its arrested infiltration. In both situations, the microstructure (porosity, micro and macro cracks) can be important differentiators in terms of the infiltration and subsequent failure mechanisms. This paper seeks to examine the interplay among microstructure, material, and CMAS attack in different scenarios. To do so, different types of YSZ & GZO single and multilayer coatings were fabricated using Air Plasma Spray (APS) and exposed to CMAS through isothermal and gradient mechanisms. In each of the cases, beyond their unique interactions with CMAS, it was observed the inherent microstructure and character of the porosity of the coating will have an additional role on the movement of the melt. For instance, vertical cracks can provide pathways for accelerated capillaric flow of the melt into both YSZ and GZO coatings. Based on these observations multilayer coatings have been proposed and realized toward potentially reducing complete coating failure and supporting multiple CMAS attack scenarios.     

Hot corrosion of RE2SiO5 with different cation substitution under calcium–magnesium–aluminosilicate attack

Rare earth (RE) silicates have been applied as advanced environmental barrier coatings (EBCs) to protect silicon carbide fibers reinforced silicon carbide ceramic matrix from water vapor and molten salt corrosion in engines. This process, however, is limited by volcanic ash corrosion as assessment of ash-induced corrosion is anecdotal and quantitative data are insufficient. In this account, the corrosion behavior of RE monosilicates (RE₂SiO₅, RE = Y, Lu, Yb, Eu, Gd, and La) by calcium–magnesium–aluminosilicate (CMAS), with similar composition as volcanic ash, was comprehensively investigated. Results indicated that RE₂SiO₅ could react with CMAS at 1200 °C at the interface, where the products crystallized in CMAS glass. RE₂Si₂O₇ was formed by the reaction between RE₂SiO₅ and silica (SiO₂) in CMAS, which was followed by corrosion of RE₂Si₂O₇ by CMAS. RE₂SiO₅ with Type B structure showed better resistance toward CMAS than RE₂SiO₅ with Type A structure. Moreover, RE₂SiO₅ with larger radii of RE³⁺ cations led to easy formation of oxyapatite phase; however, RE₂SiO₅ with smaller radii of RE³⁺ cations easily formed garnet phase. Besides, smaller radii RE³⁺ cations induced slower reactions. These findings can contribute to identifying, preventing, and minimizing the damage to matrix components with EBCs caused by volcanic ash.     

EB-PVD alumina (Al2O3) as a top coat on 7YSZ TBCs against CMAS/VA infiltration: Deposition and reaction mechanisms

Al₂O₃ was deposited as a top coat on a standard 7YSZ layer (or layers) by means of EB-PVD technique and the corresponding morphology of the Al₂O₃/7YSZ coatings was studied in detail. This multi-layer TBC system was tested against calcium-magnesium-aluminium-silicate (CMAS) recession by performing infiltration experiments for different time intervals from 5?min to 50?h at 1250?°C using two types of synthetic CMAS compositions and Eyjafjallajökull volcanic ash (VA) from Iceland. The results show that the studied EB-PVD Al₂O₃/7YSZ coatings react quickly with CMAS or VA melt and form crystalline spinel (MgAl_2-xFe_xO₄) and anorthite (CaAl₂Si₂O₄) phases. The presence of Fe-oxide in the CMAS has been found to be key element in influencing the spinel formation which was proved to be more efficient against CMAS sealing in comparison to the Fe-free CMAS compositions. Even though a rapid crystallization was assured, shrinkage cracks in the EB-PVD alumina layer produced during the crystallization heat treatment have proven to be detrimental for the CMAS/VA infiltration resistance. To overcome these microstructural drawbacks, an additional alumina deposition method, namely reaction-bonded alumina oxide (RBAO), was applied on top of EB-PVD Al₂O_3. RBAO acts as a sacrificial layer forming stable reaction products inhibiting further infiltration.     

2ZrO2·Y2O3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS: Part II,Interactions with Sand and Fly Ash

Based on the application of OB considerations (Part I) to various major thermal barrier coating (TBC) compositions and two types of important calcium–magnesium–alumino–silicates (CMAS)—desert sand and fly ash—the 2ZrO₂·Y₂O₃ solid solution (ss) TBC composition, with high CMAS‐resistance potential, is chosen for studying molten‐CMAS/TBC interactions. It is demonstrated that 2ZrO₂·Y₂O₃(ss) air plasma sprayed (APS) TBCs are highly resistant to high‐temperature attack by both sand‐CMAS and fly‐ash‐CMAS. Despite the differences in the compositions of the two CMASs, the overall CMAS‐attack mitigation mechanisms in both cases appear to be similar, viz reaction between 2ZrO₂·Y₂O₃(ss) APS TBC and the CMAS, and the formation of main reaction products of Y‐depleted c‐ZrO₂ and nonstoichiometric Ca–Y apatite. Large differences in the OBs (ΔΛ) between the 2ZrO₂·Y₂O₃(ss) and the CMASs are good predictors of ready reaction between this TBC and these CMASs. While the details of the CMAS‐mitigation mechanisms can depend critically on various other aspects, the OB difference (ΔΛ) calculations could be used for the initial screening of CMAS‐resistant TBC compositions.     

High-temperature interactions of desert sand CMAS glass with yttrium disilicate environmental barrier coating material

A calcium-magnesium aluminosilicate (CMAS) glass was prepared by melting a sample of desert sand to evaluate the high-temperature interactions between molten CMAS and yttrium disilicate (Y₂Si₂O₇), an environmental barrier coating (EBC) candidate material. Cold-pressed pellets of 80?wt% Y₂Si₂O₇ powder and 20?wt% CMAS glass powder were heat treated at 1200?°C, 1300?°C, 1400?°C and 1500?°C for 20?h in air. The resulting phases were evaluated using powder X-ray diffraction. In the second set of experiments, free standing hot-pressed Y₂Si₂O₇ substrates with cylindrical wells were filled with CMAS powder to a loading of ~35?mg/cm² and heat treated in air at 1200?°C, 1300?°C, 1400?°C and 1500?°C for 20?h. Scanning electron microscopy, energy-dispersive spectroscopy and electron microprobe analysis were used to evaluate the microstructure and phase compositions of specimens after heat treatment. An oxyapatite silicate (Ca₂Y₈(SiO₄)₆O₂) phase was identified in all specimens after CMAS exposure regardless of heat treatment temperature. Apatite appeared to form by dissolution of Y₂Si₂O₇ into molten CMAS, reacting with CaO in the melt according to the reaction 4Y₂Si₂O₇ +?2CaO → Ca₂Y₈(SiO₄)₆O₂ +?2SiO₂, and followed by precipitation of the apatite phase.     

Infrared luminescence in Er3+:Yb3Al5O12 bulk ceramics prepared by sol–gel method

Erbium doped and pure ytterbium aluminium garnet (YbAG, Yb₃Al₅O₁₂) bulk ceramics were successfully prepared by a chelating sol–gel route based on the polyesterification of ethylenediaminetetraacetic acid (EDTA) with triethanolamine (TEA). The gel decomposition and phase formation in precursor powders were studied using XRD and TG/DTA. Amorphous precursor was directly converted to YbAG phase after calcinations at 800 °C. The influence of intermediate grinding on microstructure and luminescent properties was investigated. The discrete luminescence bands of the ⁴I_13/2 → ⁴I_15/2 transition were observed in the infrared emission spectra of erbium doped samples. The lifetime of luminescence at 1530 nm was 2.82 ms and 1.82 ms for the doped samples. This may be attributed to the different efficiency of surface recombination channel, caused by different grain size distribution. Prepared samples are suitable as a standard for photoluminescence measuring of Er-doped YbAG thin films.     

Corrosion resistance and thermal-mechanical properties of ceramic pellets to molten calcium-magnesium-alumina-silicate (CMAS)

Because gas turbine engines must operate under increasingly harsh conditions, the degradation of thermal barrier coatings (TBCs) by calcium-magnesium-alumina-silicate (CMAS) is becoming an urgent issue. Mullite (3Al₂O₃·2SiO₂) is considered a potential material for CMAS resistance; however, the performance of mullite in the presence of CMAS is still unclear. In this study, mullite and Al₂O₃–SiO₂ were premixed with yttria stabilized zirconia (YSZ) in different proportions, respectively. Porous ceramic pellets were used to conduct CMAS hot corrosion tests, and the penetration of molten CMAS and its mechanism were investigated. The thermal and mechanical properties of the samples were also characterized. It was found that the introduction of mullite and Al₂O₃–SiO₂ mitigated the penetration of molten CMAS into the pellets owing to the formation of anorthite, especially at 45 wt% mullite/55 wt% YSZ. Compared with Al₂O₃–SiO₂, mullite possesses a higher chemical activity and undergoes a faster reaction with CMAS, thus forming a sealing layer in a short time. Additionally, the thermal expansion coefficient, thermal conductivity, and fracture toughness of different samples were considered to guide the architectural design. Considering the CMAS corrosion resistance, thermal and mechanical performance of TBCs systematically, a TBC system with a multilayer architecture is proposed to provide a theoretical and practical basis for the design and optimization of the TBC microstructure.     

A novel CMAS-resistant material based on thermodynamic equilibrium design: Apatite-type Gd10(SiO4)6O3

Calcium-magnesium-alumino-silicates (CMAS) melt attack has been a critical issue for the thermal barrier coatings (TBCs) with ever-increasing engine operating temperature. In this study, a novel CMAS-resistant material apatite-type Gd₁₀(SiO₄)₆O₃ is developed for TBCs application based on thermodynamic equilibrium design. The chemical reaction of Gd₁₀(SiO₄)₆O₃ bulk and CMAS melt is investigated at 1300°C. The CMAS corrosion resistance of Gd₁₀(SiO₄)₆O₃ bulk is evaluated and compared with the well-studied CMAS-resistant material Gd₂Zr₂O₇ (GZO). It is found that Gd₁₀(SiO₄)₆O₃ shows a significantly enhanced CMAS resistance, including lower intrinsic CMAS infiltration rate (~1.09 μm/h^1/2) and smaller infiltration upper limit (50-62 μm) for a 20 mg/cm² CMAS deposition. More importantly, for Gd₁₀(SiO₄)₆O₃, the CMAS infiltration only alters the composition but does not change the crystal structure or destroy microstructural integrity. The reaction mechanism is elucidated as following two stages: (a) surface Gd₁₀(SiO₄)₆O₃ quickly transforms into Ca₂Gd₈(SiO₄)₆O₂ in suit by interdiffusion with CMAS melt and then is thermodynamically stable with CMAS melt, thereby effectively inhibiting the further CMAS infiltration and (b) with the ongoing interdiffusion of Gd/Ca, the CMAS-infiltrated layer slowly thickens and follows a parabolic law. Meanwhile, the CMAS melt gradually precipitates Ca₂Gd₈(SiO₄)₆O₂ and CaAl₂Si₂O₈ (anorthite) until the melt is exhausted.     

Thermochemical Interaction of Thermal Barrier Coatings with Molten CaO–MgO–Al2O3–SiO2 (CMAS) Deposits

Thermal barrier coatings (TBCs) are increasingly susceptible to degradation by molten calcium–magnesium alumino silicate (CMAS) deposits in advanced engines that operate at higher temperatures and in environments laden with siliceous debris. This paper investigates the thermochemical aspects of the degradation phenomena using a model CMAS composition and ZrO₂–7.6%YO_1.5 (7YSZ) grown by vapor deposition on alumina substrates. The changes in microstructure and chemistry are characterized after isothermal treatments of 4 h at 1200°–1400°C. It is found that CMAS rapidly penetrates the open structure of the coating as soon as melting occurs, whereupon the original 7YSZ dissolves in the CMAS and reprecipitates with a different morphology and composition that depends on the local melt chemistry. The attack is minimal in the bulk of the coating but severe near the surface and the interface with the substrate, which is also partially dissolved by the melt. The phase evolution is discussed in terms of available thermodynamic information.     

Melting and Crystallization of Silicate Systems Relevant to Thermal Barrier Coating Damage

The melting and crystallization behaviors of model calcium–magnesium–alumino‐silicate (CMAS) compositions relevant to the degradation of thermal barrier coatings (TBCs) were investigated. A primary goal was to establish a baseline for studies on CMAS reactions with TBC materials, reported separately, and their potential to mitigate degradation. Ternary calcium alumino‐silicate (CAS) compositions investigated melt below their equilibrium solidus owing to their metastable phase constitution. Additions of MgO or FeO_x have significant effects on the melting behavior, depending on the C:A:S proportions. Amorphization on cooling is commonplace, with MgO, AlO_1.5, and especially FeO_x promoting crystallization. The behaviors of amorphous and crystalline versions of the same CMAS are different and depend on heating/cooling rates, with attendant implications for their interaction with TBCs.