Calcium-magnesium-alumino-silicate Research Articles

Calcium magnesium aluminosilicate (CMAS) corrosion behaviors of apatite Ca2La8(SiO4)6O2 thermal barrier coating material

The calcium magnesium aluminosilicate (CMAS) attack is becoming a critical obstacle to the application of thermal barrier coatings (TBCs). In this work, we prove that Ca2La8(SiO4)6O2 is an effective TBCs material for resisting CMAS attack, and investigates its CMAS corrosion behaviors. The results indicate that no evidence of chemical reactions was found between CMAS and Ca2La8(SiO4)6O2 heat-treated at 1300 °C and 1500 °C. The corrosion of apatite by CMAS occurs via dissolution and re-precipitation processes. At 1300 °C, the apatite Ca2La8(SiO4)6O2 substrates have little evidence of damage. Moreover, the apatite can also effectively block the infiltration of CMAS up to 1500 °C.

Corrosion behaviour of rare-earth monosilicates in CMAS exposure

The thermochemical reactions between calcium-magnesium-aluminosilicate (CMAS) and three different rare earth (RE)-based (Yb, Lu and Er) environmental barrier coatings (EBCs) were studied at 1300 °C for 1–48 h. The EBCs were applied on a reaction bonded SiC-B4C substrate using dip coating technique. The coating’s layout consists of a dense RE2SiO5 bottom coat and a protective RE2O3 topcoat. The microstructure and phase composition of the CMAS-EBC specimens were examined using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results showed that CMAS dissolved in RE2O3 to form protective Ca-RE garnet phase in the case of Yb and Lu-based coatings and unprotective apatite in the case of Er-based coating. CMAS melts penetrated the RE2O3 topcoat after 6 h. Yb and Lu-silicate EBCs offer higher resistance to CMAS attack compared to Er-silicate coat. The use of RE2O3 topcoat proves beneficial strategy to limit CMAS corrosion to the RE2SiO5 coating.

Investigation on improving corrosion resistance of rare earth pyrosilicates by high-entropy design with RE-doping

To combine the advantages of the thermal expansion coefficient and excellent corrosion resistance to calcium-magnesium-alumino-silicate (CMAS), two types of high-entropy pyrosilicates ((Yb0.2Y0.2Lu0.2Ho0.2Er0.2)2Si2O7 and (Yb0.25Lu0.25Ho0.25Er0.25)2Si2O7) and two single-component pyrosilicates (Yb2Si2O7 and Lu2Si2O7) were prepared by the solid-phase method. CMAS corrosion resistance test results showed that the high-entropy pyrosilicates that form a stable β phase could precipitate apatite (Ca2RE8(SiO4)6O2) more easily than Yb2Si2O7 and Lu2Si2O7 during the corrosion process to protect the block from CMAS. The results provide a theoretical basis for the selection of rare earth elements in high-entropy pyrosilicate according to the radius of different rare earth cation.

Enhanced calcium–magnesium–aluminosilicate (CMAS) resistance of GdAlO3 (GAP) for composite thermal barrier coatings

AbstractThe impact of calcium–magnesium–alumino‐silicate (CMAS) degradation is a critical factor for development of new thermal and environmental barrier coatings. Several methods of preventing damage have been explored in the literature, with formation of an infiltration inhibiting reaction layer generally given the most attention. Gd2Zr2O7 (GZO) exemplifies this reaction with the rapid precipitation of apatite when in contact with CMAS. The present study compares the CMAS behavior of GZO to an alternative thermal barrier coating (TBC) material, GdAlO3 (GAP), which possesses high temperature phase stability through its melting point as well as a significantly higher toughness compared with GZO. The UCSB laboratory CMAS (35CaO–10MgO–7Al2O3–48SiO2) was utilized to explore equilibrium behavior with 50:50 mol% TBC:CMAS ratios at 1200, 1300, and 1400°C for various times. In addition, 8 and 35 mg/cm2 CMAS surface exposures were performed at 1425°C on dense pellets of each material to evaluate the infiltration and reaction in a more dynamic test. In the equilibrium tests, it was found that GAP appears to dissolve slower than GZO while producing an equivalent or higher amount of pore blocking apatite. In addition, GAP induces the intrinsic crystallization of the CMAS into a gehlenite phase, due in part to the participation of the Al2O3 from GAP. In surface exposures, GAP experienced a substantially thinner reaction zone compared with GZO after 10 h (87 ± 10 vs. 138 ± 4 μm) and a lack of strong sensitivity to CMAS loading when tested at 35 mg/cm2 after 10 h (85 ± 13 versus 246 ± 10 μm). The smaller reaction zone, loading agnostic behavior, and intrinsic crystallization of the glass suggest this material warrants further evaluation as a potential CMAS barrier and inclusion into composite TBCs.

Thermochemical degradation of HfSiO4 by molten CMAS

The thermochemical degradation of hafnium silicate (HfSiO4) was investigated with a molten calcium-magnesium-aluminosilicate (CMAS) glass relevant to gas turbine engine applications. Sintered HfSiO4 coupons were fabricated, within which wells were drilled and filled with CMAS glass powder at a loading of ∼35 mg/cm2. Samples were heat treated at 1200°C, 1300°C, 1400°C, and 1500°C for 1 h, 10 h, and 50 h. At 1200°C and 1300°C, slow formation of a Ca2HfSi4O12 cyclosilicate phase was observed at the HfSiO4-CMAS interface. At 1300°C and higher, rapid infiltration of CMAS into the material along the grain boundaries was observed. Initial conjecture into CMAS degradation mechanisms of HfSiO4 are presented herein.

High-temperature corrosion of HfSiO4 environmental barrier coatings exposed to water vapor/oxygen atmosphere and molten calcium magnesium aluminosilicate

The hot corrosion behavior of plasma-sprayed HfSiO4 coatings exposed to molten calcium magnesium aluminosilicate (CMAS) and steam was investigated. No reactive crystallization was observed in the CMAS-attacked HfSiO4 coating which consist of tetragonal HfSiO4 and monoclinic HfO2. The penetration did not bring obvious volume expansion thanks to the buffering effect of the pores/microcracks. The bilayer environmental barrier coating (EBC) systems using HfSiO4 as top coat and Si as bond coat have the great resistance against steam corrosion. The coating degradation at high temperatures was associated with TGO volatilization and phase transformation as well as thermomechanical incompatibility.

Compatible performance of low thermal conductivity and high infrared radiation of Sm2O3-HfO2 series ceramics applied for thermal protection coatings

Sm2O3-HfO2 series ceramics were synthesized at high temperature using the solid-state reaction. The phase stability, thermo-physical and infrared emission properties of Sm2Hf2O7 (SHO) and Sm2Hf2O7-44.83 wt%HfO2 (25S/H) composite ceramics were comparatively investigated. Furthermore, their calcium magnesium aluminosilicate (CMAS) corrosion was conducted at 1250°C for different times. The results reveal that both SHO and 25S/H ceramics have excellent phase stability at 1600°C as well as excellent sintering resistance. SHO still exhibits slightly lower thermal conductivity and lower hardness and Young's modulus, higher thermal expansion coefficient (CTE) and fracture toughness as well as higher infrared emittance (0.899 at 800°C) than 25S/H composite with the excessive HfO2 inside. Both SHO and 25S/H ceramics react with CMAS to form a relatively compact reaction layer, which can effectively prevent the penetration of CMAS. These results preliminarily indicate that SHO ceramic can be proposed as an alternative material of the traditional YSZ for high-temperature thermal protective applications thanks to its compatible performance of low thermal conductivity and high infrared radiation, etc.

Interaction of multicomponent disilicate (Yb0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 with molten calcia-magnesia-aluminosilicate

Environmental barrier coating (EBC) materials that are resistant against molten calcia-magnesia-aluminosilicate (CMAS) corrosion are urgently required. Herein, multicomponent rare-earth (RE) disilicate ((Yb0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7, (5RE)2Si2O7) was investigated with regard to its CMAS interaction behavior at 1400 °C. Compared with the individual RE disilicates, the (5RE)2Si2O7 material exhibited improved resistance against CMAS attack. The dominant process involved in the interaction of (5RE)2Si2O7 with CMAS was reaction-recrystallization. A dense and continuous reaction layer protected the substrate from rapid corrosion at high temperatures. The results demonstrated that multicomponent strategy of RE species in disilicate can provide a new perspective in the development of promising EBC materials with improved corrosion resistance.

A case study on CMAS corrosion of an in-situ microstructure modification YSZ TBCs fabricated by EB-PVD

At elevated temperature, the inter-columnar pores of electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) may act as infiltration channels for the molten calcium–magnesium–alumino-silicate (CMAS) and, therefore, makes it susceptible to be attacked by CMAS. To enhance the CMAS corrosion resistance of EB-PVD TBCs, a microstructure modification yttria-stabilized zirconia (YSZ) top coat consisting of a low porosity fibrous structure layer on the conventional columnar structure layer was fabricated by in-situ parameter modification. The anti-CMAS corrosion performance of the in-situ modified YSZ coating was evaluated at 1250 °C and the evolution of the microstructure was investigated in detail. The results show that the inter-columnar pores were sealed by the low-porosity and smooth fibrous structure layer. Due to the porosity and the size of the pores of the fibrous structure layer are dramatically reduced, the penetration rate of CMAS is significantly lower in the in-situ microstructure modification coating than in the conventional columnar structure coating, and the interaction between CMAS and YSZ coating was also reduced.

Graceful behavior during CMAS corrosion of a high-entropy rare-earth zirconate for thermal barrier coating material

The corrosion resistance to calcium-magnesium-alumino-silicates (CMAS) is critically important for the thermal barrier coatings (TBCs). High-entropy zirconate (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 (HEZ) ceramics with low thermal conductivity, high coefficient of thermal expansion and good durability to thermal shock is expected to be a good candidate for the next-generation TBCs. In this work, the CMAS corrosion of HEZ at 1300°C was firstly investigated and compared with the well-studied La2Zr2O7 (LZ). It is found that the HEZ ceramics showed a graceful behavior to CMAS corrosion, obviously much better than the LZ ceramics. The HEZ suffered from CMAS corrosion only through dissolution and re-precipitation, while additional grain boundary corrosion existed in the LZ system. The precipitated high-entropy apatite showed fine-grained structure, resulting in a reaction layer without cracks. This study reveals that HEZ is a promising candidate for TBCs with extreme resistance to CMAS corrosion.

Effect of particle deposition on film cooling from fan-shaped holes

Film cooling from fan-shaped holes is an essential cooling technology in modern gas turbines. The cooling performance degradation and life reduction of high-temperature components caused by calcium-magnesium-alumino-silicates (CMAS) blockage in film cooling holes have received little attention in the public literature. The emphasis of this study lies in using the large eddy simulation (LES) method to reveal the effects of blockage on the flow, mixing, and heat transfer physics of fan-shaped hole film cooling. Blockages were predicted by the improved composite deposition model. The blowing ratio of 0≤M≤3 and the blockage level of 0≤H/D≤0.75 represent the typical practice in gas turbines. The conjugate heat transfer analysis shows that the particle deposition was closely related to the blowing ratio and the film cooling effectiveness, and the fan-shaped holes were blocked for more than 0.5D after 500 hours of service under the studied conditions. LES results indicate that blockages significantly change the organization of turbulent coherent structures and the mixing between coolant and hot gas. Spiral and lateral reorient structures and pressure gradients play important roles in coolant streamwise extension, lateral coverage, and vortex evolution. At the low blowing ratio, small blockages induced the differentiation of “hairpin vortex forest” and the formation of anti-counter-rotating vortex pairs (Anti-CRVP), and the downwash effect of coolant improved the film cooling effectiveness. At the high blowing ratio, the blockage promoted the fusion of hairpin vortices to the midspan, resulting in the dominance of large-scale CRVP and the attenuation or even complete dissipation of Anti-CRVP. The strong hot gas entrainment effect of CRVP leads to the intensification of vertical mixing and the weakening of lateral dispersion of coolant, and the dramatic decrease of film cooling effect. It was found that the maximum degradation rate of the area-averaged effectiveness caused by blockages can reach more than 90%. The discharge coefficient and aerodynamic loss increase significantly under large blockage and blowing ratios. Whether blocked or not, operating near the optimal effective momentum flux ratio (0.2 ∼ 0.32) is helpful to achieve efficient cooling.

Modeling molten droplet spreading and infiltration into non-isothermal thermal barrier coatings

Molten calcium-magnesium-alumino-silicate (CMAS) droplets impact and infiltrate porous thermal barrier coatings (TBCs) of gas turbines, thereby causing loss of strain tolerance and delamination of the ceramic topcoat. To develop efficient mitigation strategies, it is crucial to understand CMAS infiltration dynamics into the porous topcoat. An integrated model is introduced incorporating simultaneous droplet spreading, wetting interactions, heat transfer, and liquid infiltration with temperature-dependent viscosities in unsaturated porous media. The model is applied to CMAS droplet interactions with homogeneous/heterogeneous anisotropic TBC topcoats grown by the electron beam physical vapor deposition (EB-PVD) method. Simulation results show that the droplet height and contact diameter dynamics exhibit three stages - initial-stage fast decrease, slow quasi-linear intermediate decrease, and late-stage fast decrease. The first two stages are dominated by the dynamics of infiltration. The initial temperature of the droplet has insignificant effect on the infiltration dynamics. Rather, the temperature gradient in the topcoat is critical to the infiltration rate. The anisotropy determines the final infiltration diameter and depth. Further, bilayer and multilayer coating structures with alternating fine and coarse columns can delay the infiltration rate and promote lateral spreading during the early stage of the droplet infiltration compared to single-layer structures. The results demonstrate that heterogeneous structures provide a viable approach to mitigate fast infiltration and reduce damage to the TBCs during the early stage of droplet infiltration.

Experimental and Simulation Analysis of the Evolution of Residual Stress Due to Expansion via CMAS Infiltration in Thermal Barrier Coatings

The failure behavior of thermal barrier coatings (TBCs) involves multilayered systems infiltrated with calcium–magnesium–alumino-silicates (CMAS). The metastable tetragonal phase is mainly composed of 7YSZ (7 mol.% Y2O3-stabilized ZrO2), and it destabilizes into the Y-lean tetragonal phase, which may be induced by CMAS infiltration, and transforms into a monoclinic phase during cooling. The phase transformation leads to volume expansion around the CMAS-rich layer. Furthermore, it is shown that the spalling of the coating system emerges when the surface of the coating system is subjected to significant residual stress. In this study, a double-cantilever beam model is established to describe the macroscopic phenomenon of thermal buckling induced via CMAS. The result of the buckle height is used to demonstrate the consistency of the experiment and finite element simulation. The experimental parameters are imported into a multilayer cantilever beam model to analyze the interfacial stresses due to CMAS infiltration. The finite element results indicate that the phase transformation leads to damage in the coating system wherein the interfacial stresses due to phase transformation are 27% higher than those in the model without phase transformation.

Effect of Dolomite Filler on the Sulfuric Acid Resistance of Alkali-Activated Slag Binders

In this work, the influence of a dolomite filler, in combination with pulverized fly ash, on the sulfuric acid (H2SO4) resistance of alkali-activated slag (AAS) is studied. The evolution of compressive strength and mass loss, as well as mineralogical, compositional, and molecular structural alteration of hardened AAS binders containing 0%, 20%, and 40% dolomite filler, or a combination of 20% dolomite and 20% fly ash, exposed to 0% (deionized water), 1%, 2.5%, and 5% H2SO4 solutions, are investigated. The results show that the dolomite and fly ash incorporation in AAS pastes exacerbates the alkalis leaching and alkalinity loss and tends to enhance the strength loss in the H2SO4 solutions, likely due to a coarsened pore structure and, thus, enlarged permeability. Under the H2SO4 attack, AAS is decomposed into gypsum, glauberite, and amorphous calcium-magnesium aluminosilicate hydrates, with little thaumasite, ettringite, or magnesium-bearing mineral formations. While decalcification and the silica polymerization of gel products take place in AAS upon an acid attack, it has strong dealumination resistance. In low and intermediate acid concentrations, the dolomite incorporation reduces the amount of gypsum and glauberite formed in AAS due to the dilution effect, while in strong acid media, dolomite dissolution occurs and contributes to the deleterious sulfate minerals formation.

Preparation and corrosion resistance of high-entropy disilicate (Y0.25Yb0.25Er0.25Sc0.25)2Si2O7 ceramics

With the increasing demand for high thrust-weight ratio aero-engines, the development of environmental barrier coatings (EBCs) for SiC-based ceramics is an enormous challenge, which requires both high temperature stability and resistance to molten calcium-magnesium-aluminosilicate (CMAS) corrosion. In this work, a novel high-entropy disilicate (Y0.25Yb0.25Er0.25Sc0.25)2Si2O7 ((4RE0.25)2Si2O7) EBC materials was prepared by a two-step process. The high-entropy disilicate powder was synthesized by sol-gel method, and then Y-Si-Al-O silicate glass was used as sintering aids to facilitate densification of the (4RE0.25)2Si2O7 ceramic by liquid sintering. The results of XRD, SEM and EDS showed that as-synthesized powders were pure high-entropy disilicate (4RE0.25)2Si2O7, and bulk (4RE0.25)2Si2O7 ceramic exhibited a dense structure, in which (4RE0.25)2Si2O7 grains were uniformly distributed into Y-Si-Al-O glass. Compared with single disilicate RE2Si2O7 (RE = Y and Er) ceramics, the high entropy (4RE0.25)2Si2O7 ceramic showed a good phase stability and almost no change in grain size during sintering at 1600 °C for in the holding time range of 5–15 h, suggesting a good high temperature stability. During molten CMAS corrosion, Ca2+ ions can diffuse into the Y-Si-Al-O glass phase wrapped on the surface of (4RE0.25)2Si2O7 grains, and then corrode (4RE0.25)2Si2O7 grains, thereby forming the corrosion zone consisting of reaction layer and diffusion layer. The Y-Si-Al-O glass can play a blocking layer role to suppress Ca2+ diffusion and mitigate molten CMAS corrosion, so that as-prepared (4RE0.25)2Si2O7 ceramic showed a relatively good ability for resisting molten CMAS corrosion. After molten CMAS corrosion at 1500 °C for 48 h, thickness of the reaction layer was only 73 µm. The results will be a solid foundation for the application of environmental barrier coating materials in long-period high temperature molten silicate environment.

A coupled theory for deformation and phase transformation due to CMAS infiltration and corrosion of thermal barrier coatings

This work proposes a mechanism-based phase-field approach for tetragonal-to-monoclinic transformation in thermal barrier coatings (TBCs) corroded by calcium-magnesium-alumino-silicates (CMAS). A 2D plate model is established and verified by experiments results of the distribution of CMAS mass density and the extension of the monoclinic phase after different heating times. In addition, the simulation results show that the tensile stress accumulates in the interface between two phases, which can be seen as the motivation for horizontal cracks in TBCs. The theoretical model presented here shows excellent capacities to capture the evolution of the monoclinic phase in TBCs corroded by CMAS.

Comparative study of EB-PVD gadolinium-zirconate and yttria-rich zirconia coatings performance against Fe-containing calcium-magnesium-aluminosilicate (CMAS) infiltration

This detailed study compare and contrasts the calcium-magnesium-aluminosilicate (CMAS) infiltration resistance behavior of electron-beam physical vapor deposition (EB-PVD) produced gadolinium zirconate (GZO) and yttria rich zirconia (65YZ, 65 wt % Y2O3 rest zirconia) coatings. The infiltration kinetics, as well as the stability and protective nature of different reaction products, were studied by performing long term infiltration tests (up to 50 h) at 1250 °C. The results reveal that for the specific microstructures used in this study, 65YZ has a higher infiltration resistance and forms a thinner reaction layer compared to GZO. The analysis indicates that the better performance of 65YZ is associated with a synergetic reaction mechanism, which includes the formation of Ca-rich apatite and a uniform layer of a garnet phase. The formation of apatite requires more rare-earth (RE) in the case of GZO than its 65YZ counterpart, meaning that more Gd would be dissolved before forming apatite crystals, which leads to higher consumption of the GZO layer compared to that of 65YZ. The implications of these mechanisms are discussed in detail concerning the tendency of garnet formation, equilibration of the apatite phase with Ca and RE content, and the effects of the reduction in viscosity due to the RE dissolution into the glass. However, microstructural differences in the coatings used in this study might also affect the diverging infiltration resistance and reaction kinetics and need to be considered.

A Critical Review of Physical Models in High Temperature Multiphase Fluid Dynamics: Turbulent Transport and Particle-Wall Interactions

Abstract This review article examines the last decade of studies investigating solid, molten, and liquid particle interactions with one another and with walls in heterogeneous multiphase flows. Such flows are experienced in state-of-the-art and future-concept gas turbine engines, where particles from the environment, including volcanic ash, runway debris, dust clouds, and sand, are transported by a fluid carrier phase and undergo high-speed collisions with high-temperature engine components. Sand or volcanic ash ingestion in gas turbine engines is known to lead to power-loss and/or complete engine failure. The particle-wall interactions that occur in high-temperature sections of an engine involve physics and intrinsic conditions that are sufficiently complex that they result in highly disparate and transient outcomes. These particles, which often times are made up of glassy constituents called calcium–magnesium–alumino–silicate (CMAS), are susceptible to phase change at combustor temperatures (1650°), and can deposit on surfaces, undergo elastic and plastic deformation, rebound, and undergo breakup. Considerable research has been put into developing empirical and physics-based models and numerical strategies to address phase interactions. This article provides a detailed account of the conceptual foundation of physics-based models employed to understand the behavior of particle-wall interaction, the evolution of numerical methods utilized for modeling these interactions, and challenges associated with improving models of particle-particle and particle-wall interactions needed to better characterize multiphase flows. It also includes description of a testbed for acquiring canonical data for model validation studies.

Influence of pore characteristics of air plasma sprayed thermal barrier coatings on calcia-magnesia-alumino-silicate (CMAS) attack behavior

The pore characteristics of air plasma sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs) were tailored by Al2O3-sol impregnation, and then its influence on CMAS attack behavior was investigated. Cross-sectional inspection revealed that the pore-modifier Al2O3 can reduce the width of the infiltration path, increase tortuosity, or even block the path by forming connection arrays along with the path. CMAS attack test showed that the pore-tailored TBCs had a better CMAS infiltration resistance. This study implies that tailoring the pore characteristics of TBCs by impregnation may be a potential way to mitigate CMAS damage.

A potential ZrO2 doped scandium tantalite ceramics for thermal/environmental barrier coating material

Despite scandium tantalite (ScTaO4) has great potential application for thermal/environmental barrier coating system, it is necessary to reduce the thermal expansion coefficient difference between them to lower the thermal stress in practical applications. In this paper, (ZrO2)x-(ScTaO4)1-x (x = 0.1, 0.2) material was prepared by doping ZrO2 into scandium tantalite ceramics, and its thermal expansion coefficient and calcium–magnesium–alumino-silicate (CMAS) corrosion behavior were studied. After the CMAS corrosion experiments of (ZrO2)x-(ScTaO4)1-x material, the phase composition and microstructures of samples were detected by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis. The main results show that the alloying effect of ZrO2 doping can reduce the thermal expansion coefficient of ScTaO4 and then alleviate the thermal expansion coefficient difference in thermal/environmental barrier coating system. Besides, Zr4+ ions are separated from (ZrO2)x-(ScTaO4)1-x lattice by molten CMAS under high-temperature corrosion condition and (ZrO2)x-(ScTaO4)1-x material is decomposed into ScTaO4 and t-ZrO2 finally. In addition, only t-ZrO2 is defected in the corrosion products without other crystal type of ZrO2 in the XRD results of the corrosion products, which indicate that ScTaO4 can increase high-temperature phase stability of t-ZrO2 and prevent phase transformation from t-ZrO2 to m-ZrO2.