Calcium-magnesium-alumino-silicate Research Articles

Composition-driven superior CMAS corrosion resistance of high-entropy rare-earth disilicates

Achieving superior calcium-magnesium-aluminosilicate (CMAS) corrosion resistance in high-entropy rare-earth disilicates (HEREDs) is crucial to their applications as environmental barrier coatings in aerospace. Stimulated by the vast compositional field of HEREDs, we achieve exceptional CMAS corrosion resistance in HEREDs at elevated temperatures via a composition-driven strategy for the first time. Specifically, the equimolar 3- to 8-cation high-entropy rare-earth disilicate (3–8HERED) samples are first successfully designed and fabricated using a high-throughput pressure-less sintering technique and then their CMAS corrosion resistance up to 1773 K is evaluated. Among all the as-fabricated samples, the as-fabricated 7HERED samples are found to exhibit the best CMAS corrosion resistance. The outstanding CMAS corrosion resistance of the as-fabricated 7HERED samples is primarily attributed to pronounced sluggish diffusion induced by configurational entropy, with the synergistic effect of their chemical activity, which facilitates the formation of dense apatite protective layers. Our work provides valuable ways to design remarkable anti-CMAS corrosion high-entropy ceramics.

Corrosion resistance and microstructural evolution of Yb-Al-Si-O glass-ceramics under molten Ca-Mg-Al-Si environment at 1350 °C

The intrinsic characteristics of Yb2O3-Al2O3-SiO2 (YbAS) glass-ceramics and their resistance to corrosion by molten calcium magnesium aluminosilicate (CMAS) at 1350 °C were systematically investigated. YbAS glass-ceramics, characterized by diverse compositions, maintaining phase stability over extended durations at elevated temperatures. After CMAS corrosion for 50 h, the main reaction phases were CaAl2Si2O8 and Ca2Yb8(SiO4)6O2, accompanied by multiple ion solid solution garnet in certain components. The glass component positioned along the eutectic line of Yb2Si2O7 and mullite, with an OB value marginally exceeding that of CMAS, exhibits superior corrosion resistance to CMAS. This finding offers valuable insights for the subsequent design of environmental barrier coatings (EBCs).

CaO–MgO–Al2O3–SiO2 corrosion resistance of multi-rare-earth-oxide-doped zirconia thermal barrier coating

As a thermal barrier coating (TBC) material, 8 wt% yttria-stabilized zirconia (8YSZ) exhibits poor resistance against calcium–magnesium–aluminosilicate (CMAS) corrosion. This is caused by the penetration of molten CMAS glass and the depletion of Y3+, which limits the lifetime of 8YSZ. In this study, a multi-rare-earth-oxide-doped zirconia (15 wt% (Lu0.2Yb0.2Dy0.2Gd0.2Y0.2)–ZrO2, denoted as 15LYDGY–SZ) ceramic was prepared to examine its interaction with CMAS. The sluggish diffusion effect and the competition between various dopant elements toward reaction with CMAS effectively slow down the atomic diffusion and limit the transformation of tetragonal phase into monoclinic phase in 15LYDGY–SZ ceramic. Compared with the currently used 8YSZ ceramic, the corrosion resistance of 15LYDGY–SZ to CMAS and its retention rate of fracture toughness after CMAS corrosion are improved. The results show that the multi-rare-earth-oxide-doped zirconia ceramic prepared in this study is a promising candidate material for the development of future TBCs.

Medium-Entropy Monosilicates Deliver High Corrosion Resistance to Calcium-Magnesium Aluminosilicate Molten Salt.

For decreasing the global cost of corrosion, it is essential to understand the intricate mechanisms of corrosion and enhance the corrosion resistance of materials. However, the ambiguity surrounding the dominant mechanism of calcium-magnesium aluminosilicate (CMAS) molten salt corrosion in extreme environments hinders the mix-and-matching of the key rare earth elements for increasing the resistance of monosilicates against corrosion of CMAS. Herein, an approach based on correlated electron microscopy techniques combined with density functional theory calculations is presented to elucidate the complex interplay of competing mechanisms that control the corrosion of CMAS of monosilicates. These findings reveal a competition between thermodynamics and kinetics that relies on the temperatures and corrosion processes. Innovative medium-entropy monosilicates with exceptional corrosion resistance even at 1500°C are developed. This is achieved by precisely modulating the radii of rare earth ions in monosilicates to strike a delicate balance between the competition in thermodynamics and kinetics. After 50 and 100h of corrosion, the thinnest reactive layers are measured to be only 28.8 and 35.4µm, respectively.

Corrosion behavior of Al4SiC4/SiC ceramics exposed to molten calcium‐magnesium‐alumino‐silicate at 1350°C in air

AbstractAl4SiC4 shows excellent heat resistance, thermal shock resistance, machinability, and oxidation resistance. We focused on Al4SiC4‐based ceramics with SiC as a non‐oxide matrix for ceramic matrix composites for aircraft jet engines. In this study, monolithic Al4SiC4 and Al4SiC4/SiC ceramics were fabricated by hot‐pressing, and a corrosion test against molten calcium‐magnesium‐alumino‐silicate (CMAS) was conducted at 1350°C for 12–100 h in air, and their corrosion behavior was investigated. Scanning electron microscopy and energy‐dispersive X‐ray spectroscopy results revealed that severe damage was not observed at the interface between CMAS and the samples after the CMAS corrosion test. The recession of Al4SiC4‐100, ‐10, and SiC‐100 after corrosion for 100 h was 80–90 µm, and that of Al4SiC4‐50 was the highest of all samples and the value was 130 µm. The dissolution behavior of the oxidation layer into molten CMAS via a corrosion reaction was dependent on the composition of both the sample and the oxidation layer, the thickness, and the microstructure of the oxidation layers. The dominant mechanism of reaction between CMAS and Al4SiC4‐100, ‐90, and ‐50 samples was concluded to be the dissolution of the oxidation products, while in SiC‐100 and Al4SiC4‐10 samples, the dominant reaction was determined to be direct corrosion of the surface with CMAS.

Thermal properties and calcium-magnesium-alumino-silicate (CMAS) interaction of novel γ-phase ytterbium-doped yttrium disilicate (γ-Y1.5Yb0.5Si2O7) environmental barrier coating material

Rare-earth disilicates are promising candidates for thermal and environmental barrier coatings (TEBC) in gas turbines that safeguard SiCf/SiC ceramic matrix composites (CMCs) from thermal degradation and environmental attacks. Here, we report a systematic investigation on novel TEBC material, γ-Y1.5Yb0.5Si2O7. The γ-phase quarter molar ytterbium–doped yttrium disilicate exhibited low thermal conductivity (1.72 W·m−1·K−1 at 1200 °C) and reduced intrinsic thermal expansion (3.17 ± 0.22 × 10−6 K−1 up to 1000 °C), ensuring promisingly effective thermal insulation and minimized thermal stress with CMC substrates. Using density functional theory (DFT), the heat capacity of γ-Y1.5Yb0.5Si2O7 was predicted higher than that of undoped γ-Y2Si2O7. Comparing these predictions to results calculated using the Neumann–Kopp (NK) rule revealed only minor variations. A metastable CMAS interaction byproduct, cyclosilicate phase Ca3RE2(Si3O9)2, was identified based on energy dispersive X-ray spectrometer (EDS) and electron backscatter diffraction (EBSD) techniques, appearing at 1300 °C but disappearing at 1400 °C. The γ-Y1.5Yb0.5Si2O7 exhibited good CMAS resistance on both dense pellets and sprayed coatings, forming a protective apatite (Ca2RE8(SiO4)6O2) interlayer that effectively hindered CMAS infiltration at evaluated temperatures. The relatively higher Y:Yb atomic ratio (> 3) in the apatite grains indicate differential reactivity with molten CMAS and provides crucial insights into the CMAS corrosion mechanism. These findings highlight the potential of γ-Y1.5Yb0.5Si2O7 as a CMC coating material, emphasizing the need for tailored microstructural optimization as a thermal sprayed coating to enhance long-term performance in extreme gas turbine environments.

Reaction of Yb2SiO5 EBCs against CMAS melts with different composition

Ytterbium monosilicate (Yb2SiO5) has been recognized as environmental barrier coatings (EBCs) material with potential applications for SiCf/SiC. However, the corrosion caused by calcium-magnesium-alumino-silicate (CMAS) has posed significant challenges to the design and application of EBCs. In this study, reaction of Yb2SiO5 EBCs against CMAS melts with different Ca/Si ratios (1.1, 0.75, 0.64, and 0.42) at 1350 °C was investigated. Results showed that the components of CMAS have a significant impact on the corrosion behaviors of the Yb2SiO5 coating. The difference in Ca/Si ratio of CMAS changes the phase equilibrium during the reaction process. The CMAS melts with the highest Ca/Si ratios can form a combination of silico-carnotite (Ca3Yb2(SiO4)3), garnet and two kinds of apatite (Ca2Yb8(SiO4)6O2 and Ca4Yb6(SiO4)6O), whereas the CMAS melts with the lowest Ca/Si ratio is expected to form Yb2Si2O7 phase. With the Ca/Si ratio increasing, the viscosity of CMAS decreased and the aggressiveness of CMAS to Yb2SiO5 coating increased.

Thermochemical interactions between Ca2Sm8(SiO4)6O2 apatite and molten calcium–magnesium-aluminosilicate (CMAS)

Thermochemical interactions between Ca2Sm8(SiO4)6O2 (CSmS) apatite and molten calcium-magnesium-aluminosilicate (CMAS) glass having CaO/SiO2 mole ratio 0.37 have been evaluated. CSmS apatite-CMAS diffusion couples were annealed at 1200, 1300, and 1400 °C for 1–50 h. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high angle annular dark field imaging, selected area electron diffraction and energy dispersive x-ray spectroscopy were used to characterize the phases formed during the reaction. Formation of a distinct cyclosilicate [Ca3Sm2(Si3O9)2] layer was observed at apatite-CMAS interface in couples heat treated at 1200 °C but not at higher temperatures. The cyclosilicate layer nucleated at the apatite-CMAS reaction front, continued to grow into the residual melt and eventually detached from the apatite substrate. Residual CMAS melt became Ca-lean when cyclosilicate was present. Formation of MgCaSi2O6 diopside and dendrites of CaSiO3 wollastonite was also observed within the residual CMAS at 1200 °C. At 1300 and 1400 °C molten CMAS, due to its much lower viscosity, infiltrated the apatite substrate through open pores and along the grain boundaries. Ca2Sm8(SiO4)6O2 apatite thermal/environmental barrier coating has the potential to provide protection against CMAS attack up to about 1200 °C but not at higher temperatures.

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

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

A thermodynamic study of the influence of the Al2O3 content on the CaO-SiO2-YO1.5 system

In aircraft engines applications, the interactions between rare earth silicates (mainly yttrium silicates) used as Environmental Barrier Coatings (EBCs) on turbine blades and liquid calcium-magnesium-aluminosilicates (CMAS) due to the particles ingested by the engines may lead to severe damages at high temperature. Due to the large range of composition of CMAS, simplified model systems must be used to have a good comprehension of the thermochemical phenomena leading to degradation. The interaction between ceramics and CMAS can lead to the precipitation of phases as apatite Ca2Y8(SiO4)6O2 and cyclosilicate Ca3Y2(Si3O9)2 which might provide protection to the EBC. However, their thermodynamic stability depends on many parameters. In the present work, the influence of alumina Al2O3 addition to the ternary system CaO-SiO2-YO1.5 has been studied in order to verify the equilibrium with the liquid field at 1300 °C.

Corrosion behavior of YxYb(2−x)Si2O7 environmental barrier coating materials against molten calcium-magnesium-aluminosilicate (CMAS) at 1475 °C

The molten calcium-magnesium-aluminosilicate (CMAS) corrosion resistance of a series of YxYb(2−x)Si2O7 (x = 0.3, 0.5, 0.8, 1.0) environmental barrier coating (EBC) materials were systematically investigated at 1475 °C. The results have shown that the precipitate apatite Ca2(Y/Yb)8(SiO4)6O2 formed after CMAS corrosion of YxYb(2−x)Si2O7 pellets. The reactivity of YxYb(2−x)Si2O7 pellets and CMAS increased with the increase of Y3+/Yb3+ ratio, leading to the thickest reaction layer formed in Y1.0Yb1.0Si2O7. Furthermore, “blister” cracks were not found at the zone of uncorroded YxYb(2−x)Si2O7 pellets. It demonstrated that the YxYb(2−x)Si2O7 materials have excellent CMAS corrosion resistance, which can serve as promising candidates for EBC against CMAS corrosion.

Suspension High‐Velocity Oxy‐Fuel–Sprayed Dense Vertically Cracked and Suspension‐Plasma‐Sprayed Columnar Yttria‐Stabilized Zirconia Coatings: Calcia Magnesia Alumino Silicates Infiltration and Thermal Cycling Performance

The quest to increase the surface temperatures and resistance to the corrosive environment of thermal barrier coatings topcoats mean that newer coating design strategies are needed. In this study, a performance evaluation of suspension high‐velocity oxy‐fuel (SHVOF)‐sprayed dense vertically cracked (DVC) and suspension‐plasma‐sprayed (SPS) columnar structure (CS) topcoats is conducted. The calcia magnesia alumino silicate (CMAS) evaluation is conducted at 1300 °C for 30 min, whereas the furnace cycling tests (FCT) is conducted at 1135 °C for 45 min cycle dwell time. The CMAS infiltrates down to the bond coat layer, but does not induce partial or complete topcoat spallation on all studied topcoat layers. In terms of CMAS infiltration, the CMAS appears to be restricted along the vertical cracks. The FCT of the SPS CS structure indicates a failure largely at the thermally grown oxide (TGO)–topcoat interface, while the DVC topcoat layers indicate a mix‐mode failure, i.e., both material‐associated cracking and localized spallations at the TGO–topcoat interface. Overall, the SHVOF‐sprayed ethanol‐based DVC topcoat seems to offer a balanced trade‐off, i.e., a majority of the topcoat is still intact after 100 thermal cycles and exceeds the material durability and performance offered by the SPS CS structure.

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

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

Steam oxidation performance of Yb2Si2O7 environmental barrier coatings exposed to CMAS

Environmental barrier coatings (EBCs) are necessary for use of SiC/SiC ceramic matrix composites (CMCs) in gas turbine engines. Due to the complex operational environments, it is increasingly important to understand the interplay of damage modes to project the durability of the EBC/CMC system. Two life-limiting failure mechanisms of EBCs are steam oxidation and molten calcium-magnesium-aluminosilicate (CMAS) attack. Steam oxidation results in the formation of a thermally grown oxide (TGO) that acts as a weak interface and CMAS/EBC interactions cause thermomechanical and thermochemical changes that compromise coating integrity. EBCs consisting of a Yb2Si2O7 topcoat and a silicon bond coat were subjected to low (2 mg/cm2) loadings of two CMAS compositions prior to steam oxidation at 1316 °C for up to 300 h. Exposure to CMAS reduced the TGO thickness and the low CaO content glass resulted in the thinnest oxide scales. However, glass exposure had mechanical consequences that caused premature coating delamination.

Calcium-Magnesium-Aluminosilicate (CMAS) corrosion resistance of high entropy rare-earth phosphate (Lu0.2Yb0.2Er0.2Y0.2Gd0.2)PO4: A novel environmental barrier coating candidate

Single phase (Lu0.2Yb0.2Er0.2Y0.2Gd0.2)PO4 was synthesized, and its thermal properties and CMAS resistance were investigated to explore its potential as an environmental barrier coating (EBC) candidate. The high entropy phosphate (Lu0.2Yb0.2Er0.2Y0.2Gd0.2)PO4 displays a lower thermal conductivity (2.86 W m−1 K−1 at 1250 K) than all the single component xenotime phase rare-earth phosphates. Interaction of (Lu0.2Yb0.2Er0.2Y0.2Gd0.2)PO4 pellets with CMAS at 1300 °C led to the formation of a dense and uniformed Ca8MgRE(PO4)7 reaction layer, which halted the CMAS penetration into the bulk pellet. At 1400 and 1500 °C the (Lu0.2Yb0.2Er0.2Y0.2Gd0.2)PO4-CMAS corrosion showed CMAS penetrating beyond the reaction layer into the bulk pellet via the grain boundaries, and SiO2 precipitates remaining at the pellet surface. The effects of duration, temperature, and compositions on the resistance against CMAS corrosion are discussed within the context of optimizing materials design and performance of high entropy rare-earth phosphates as candidates for advanced EBC applications.

CMAS corrosion behavior of dual-phase composite Gd2Si2O7/Sc2Si2O7 as a promising EBC material

Environmental barrier coatings (EBCs) applied to gas-turbine components require excellent corrosion resistance to molten siliceous debris such as sand or volcanic ash in high-temperature environments while maintaining mechanical integrity. To date, most research has focused on single-phase rare-earth (RE) disilicates as candidate EBC materials, but here we report the superior corrosion resistance of a dual-phase disilicate composite, namely Gd2Si2O7/Sc2Si2O7 (70/30 vol%). EBSD measurements of cross-sections of the EBC after exposure to a calcium magnesium alumino-silicate (CMAS) for 0.5, 2, 12, and 48 h at 1400°C reveal that, unlike in single-phase systems, the CMAS reaction layer consists of two distinct sublayers. The inner sublayer consists of a mixture of Ca2Gd8(SiO4)6O2 and Sc2Si2O7 crystals in a Ca-depleted glassy matrix, whereas the thinner outer region contains larger, elongated Ca2Gd8(SiO4)6O2 crystals oriented perpendicular to the composite surface and devoid of any Sc2Si2O7 crystals. The total thickness of the reaction layer is found to be about 20% less compared to that of single-phase Gd2Si2O7 under the same conditions, indicating that dual-phase RE-disilicate composites are a promising materials system for increasing the lifetime performance of EBCs.

Spreading and reaction behavior of CMAS-type silicate melts with multiphase Y and Gd aluminate-zirconate T/EBC materials

Thermal and environmental barrier coatings must be robust against attack by calcium magnesium aluminosilicate (CMAS, or CMFAS with iron) deposits, and coatings that produce thin, uniform reaction layers are desirable. The reactions of four multiphase coating compositions in the AlO1.5–REO1.5–ZrO2 systems (RE = Gd or Y) with three model CMFAS deposits were studied. Following CMFAS exposure for 1 h at 1400 ºC, some samples exhibited thin, uniform CMFAS reaction product layers, while others were less uniform with pockets of deep reactions. Coating materials with higher AlO1.5 content produced uniform reaction layers, while the most SiO2-rich CMFAS was most likely to produce a non-uniform response. Apatite was formed in all cases, while the Y was more likely than Gd to form aluminosilicate garnet and cuspidine reaction products. The results are discussed in terms of implications for CMFAS-resistant T/EBC design.

Initial corrosion mechanism of Hf6Ta2O17 ceramic for TBCs against CMAS infiltration: Experiment and first-principle simulation

The initial calcium–magnesium–alumino–silicate (CMAS) corrosion mechanism of Hf6Ta2O17 ceramic at 1250 °C for TBC was investigated by experiment and first–principle simulation. The results showed that the Hf atom, originating from Hf6Ta2O17, substituted the Si atom of the CMAS structure, increasing the melting point of the CMAS by changing its elastic constant, which tended to prevent further penetrating of the CMAS. Meanwhile, the Ca atom, stemming from CMAS, diffused into the Hf6Ta2O17 structure and occupied the interstitial site, increasing the optical basicity value of the Hf6Ta2O17, which was advantageous to develop a dense corrosion product layer to block the CMAS corrosion attack.

High temperature corrosion behaviour of calcium magnesium aluminosilicate coated oxide-oxide ceramic matrix composites

Corrosion on turbine blades in calcium magnesium alumino-silicate (CMAS) environment is a crucial failure for turbine engines and its components. In this study, oxide-oxide (O-O) ceramic matrix composites (CMCs) (AS-N610), the potential materials for gas turbine components are examined for its corrosion behaviour at high temperatures at various intervals of time in presence of CMAS. The corrosion studies indicated that dip coated with CMAS revealed a weight gain of ∼3% owing to the formation of α-Al2O3 at 1400 °C. The SE images indicated cracks at the interface due to thermal mismatch between CMAS and O-O substrate. With increase in corrosion time, cracks at the interface propagated onto the matrix and fibres of O-O CMCs. This crack propagation is attributed to the diffusion of calcium aluminosilicate (CAS) with small traces of Mg which wicks the columns of O-O CMCs. Indentation fracture toughness of O-O CMCs degraded by ∼22% for 1400 °C in presence of CMAS compared to un-corroded sample.

Diffusion behaviour and corrosion rate of rare earth monosilicate-based EBCs under CMAS exposure

The thermochemical corrosion behaviour of different rare earth (RE)-based (Yb, Lu and Er) environmental barrier coatings (EBCs) between 1 and 48 h in low calcium–magnesium–aluminosilicate (CMAS) load environments at 1300 °C was studied. The EBCs were coated on reaction-bonded SiC–B4C substrates using the dip-coating method. Each coating is composed of a dense RE2SiO5-based bottom coat and a porous RE2O3 topcoat. The microstructures and compositions of the corroded samples were examined using scanning electron microscopy and energy dispersive spectroscopy. CMAS infiltrated the topcoat through pores and cracks, settled in these defects, and dissolved in the neighbouring coating areas to precipitate as a Ca–Mg–Si–RE garnet. The diffusion coefficients of Ca and Si in the CMAS–EBC corroded specimens were calculated. Yb and Lu silicates have the lowest diffusion coefficient values, whereas Er silicate has the highest values. The formed garnet filled the pores and inhibited further diffusion of corrosive species in the coat. The corrosion rates were calculated based on the physical properties of the coat and found to be lower than those of coatings produced using atmospheric plasma spraying. The diffusion coefficient and corrosion rate were successfully used to evaluate various corrosion-resistant coatings. The results show that single-layer dip-coated Yb silicate is a cost-effective CMAS-resistant RE-based EBC.