Calcium-magnesium-alumina-silicate Research Articles

High‐entropy engineering promotes the thermal properties and corrosion resistance of rare‐earth hafnates

AbstractRare‐earth hafnates are gaining attention due to their excellent high‐temperature phase stability and low thermal conductivity. However, they still have shortcomings of low thermal expansion and poor calcium‐magnesium‐aluminum‐silicate (CMAS) corrosion resistance. In this study, we employed high‐entropy engineering and component design to synthesize three high‐entropy hafnates (La0.2Ce0.2Nd0.2Gd0.2T0.2)2Hf2O7 (T = Dy, Ho, Tm) as well as a single‐component hafnate Nd2Hf2O7, with the aim of preparing thermal barrier coatings with an excellent comprehensive performance. Test results indicate that the high‐entropy compositions have excellent thermal properties. The focus is on elucidating the corrosion process and failure mechanism of CMAS at 1300°C. Moreover, the analysis of residual CMAS and corrosion products was conducted to evaluate the discrepancies in CMAS corrosion behavior among the various compositions.

Numerical modelling of CMAS infiltration and dimensionless numbers of anti-infiltration designs of thermal barrier coatings

Resistance to calcium–magnesium–alumina–silicate (CMAS) infiltration and corrosion is a crucial problem for safely applying thermal barrier coatings (TBCs) in advanced aero engines. This work proposes a CMAS infiltration model based on the phase-field method to simulate the CMAS infiltration process in TBCs and analyse the influence factor. Further, a key dimensionless number called relative driving force K=rσcos(θc)tsμϛ2hTC2 was found to determine the CMAS infiltration depth using dimensional analysis. A criterion of K<2 was derived and was considered to be the evaluation standard of TBCs with excellent CMAS infiltration resistance. Based on K minimization, novelty TBCs with microstructure of S-shaped intercolumnar gaps and spherical pores are expected to significantly improve the resistance to CMAS infiltration and corrosion.

CMAS resistance characteristics of Gd2(Hf0.7Ce0.3)2O7 thermal barrier material at 1300 °C and 1400 °C

The corrosion resistance to calcium‑magnesium‑aluminum-silicate (CMAS) is a key property affecting the long-term durability of thermal barrier coatings (TBCs). Meanwhile, rare-earth hafnates are considered promising TBC candidates due to their low thermal conductivity and suitable thermal expansion coefficients. Thus, this study explores the CMAS corrosion resistance of Gd2(Hf0.7Ce0.3)2O7 (GH7C3) ceramic material under different conditions and compares it with the binary system Gd2Hf2O7 (GH) ceramic material. The findings indicate that GH7C3 can react chemically with CMAS and rapidly form a dense apatite crystalline layer at the interface between GH7C3 ceramic and CMAS, providing excellent CMAS resistance. Furthermore, the composition of the remaining CMAS melt Furthermore, the composition of the remaining CMAS melt undergoes compositional changes upon interaction, ultimately forming spinel in Mg and Al-enriched regions. In the meantime, CeO2 can promote the rapid formation of apatite crystals, significantly enhancing the CMAS resistance of GH7C3 to CMAS at high temperatures compared to GH.

CMAS corrosion resistance of YSZ thermal barrier coatings with Al2O3 and YSZ-Al2O3 protective layers

Molten calcium-magnesium-alumina-silicate (CMAS) corrosion has become a major constraint for the application of YSZ thermal barrier coatings (TBCs). In this paper, Al2O3 and YSZ-Al2O3 protective layers with a thickness of 50 μm were prepared on the surface of 8YSZ TBCs using air plasma spraying technology, and the progressive damage modes of the different TBCs under CMAS exposure were investigated, and the effects of the composition and microstructure of the protective layers on the infiltration and wetting behaviour of CMAS were compared. When the exposure time was shorter, the dense YSZ-Al2O3 had a better impermeability barrier effect. As the interaction time between CMAS and the coating was prolonged, the structure and phase stability of YSZ-Al2O3 were damaged, the thickness was thinned, and Al2O3 had better stability and showed better protective effect. CMAS wettability was mainly related to the roughness of the coating surface, and the wettability was poorer on rough surfaces.

CMAS corrosion characteristics of YSZ and Al2O3-YSZ thermal barrier coatings at 1250 °C–1350 °C

The calcium-magnesium-aluminum-silicate (CMAS) corrosion is now a great threat to thermal barrier coatings (TBCs), inducing coating spallation readily. The CMAS corrosion characteristics of both 7YSZ and 20 wt.%Al2O3-YSZ TBCs at a broad temperature range of 1250 °C to 1350 °C were investigated. 7YSZ coatings were completely degraded by CMAS at 1250 °C while the increased corrosion temperatures induced more serious destruction. This is due to that the viscosity of CMAS melt significantly decreased with corrosion temperature increasing, promoting melt fluidity and infiltration. The Al2O3-YSZ composite TBCs demonstrated superior resistance to CMAS corrosion at 1250°C-1350 °C. The intensive chemical reactions between Al2O3 and CMAS melt promoted the formation of anorthite products with either short rod-shaped or blocky morphologies. These products closely interconnected and stacked up to generate a dense layer, physically blocking CMAS melt infiltration and effectively improving the anti-CMAS corrosion properties of TBCs at higher temperatures. Furthermore, the corrosion temperatures critically influenced anorthite formation and present morphologies.

Cracking mechanism of CMAS-corroded thermal barrier coatings based on a coupled thermo-chemo-mechanically phase-field fracture model

A thermo-chemo-mechanically coupled theoretical framework is proposed to describe the calcium-magnesium-alumina-silicate (CMAS) corrosion process during the cooling process. A phase-field fracture model is developed to investigate the effect of cooling temperature and CMAS concentration on the degree of corrosion reaction, the stress evolution and the crack initiation and propagation. σ11 concentrates in the region beneath the overlay of CMAS and σ22 appears at the interface between top ceramic coating (TC) and bond coating (BC). The higher stress concentration of σ11 and σ22 contribute to the formation of both vertical and transverse cracks. Transverse cracks first emerge at the interface between TC and BC in the edge region, followed by the formation of vertical cracks in the CMAS-coated region. Vertical cracks propagate to the interface and deflect into transverse cracks. The transverse cracks at the interface further propagate and merge, ultimately leading to the coating delamination. The higher initial cooling temperature and CMAS concentration contribute to the accelerated development of vertical cracking and the increase of the quantity and length of transverse and vertical cracks. The model provides a significant advantage in predicting the failure of TBCs during the cooling stage of CMAS corrosion.

Investigation of the CMAS corrosion resistance of high-entropy oxides through B-site cation doping

High-temperature erosion caused by calcium magnesium aluminum silicate (CMAS) poses a significant challenge to the advancement of thermal barrier coatings (TBCs). In this investigation, four multi-component A2B2O7 type oxide TBC materials, namely Eu2(Y0.2Ce0.2Zr0.2Hf0.2Ta0.2)2O7, Eu2(Y0.2Yb0.2Ce0.2Nb0.2Ta0.2)2O7, Eu2(Y0.2Yb0.2Zr0.2Nb0.2Ta0.2)2O7, and Eu2(Y0.2Yb0.2Hf0.2Nb0.2Ta0.2)2O7, were synthesized using the solid-state method. We investigated the corrosion behavior of molten CMAS on the samples at 1300 °C for durations of 1 h, 20 h, and 50 h. The resulting corrosion products consist of apatite and cubic fluorite structures. Our analysis reveals that as the ionic radius of RE3+ increases, the stability of the apatite structure improves. Furthermore, molten CMAS exhibits varied wetting behaviors on different substrates at the same temperature. A higher contact angle suggests that the CMAS melt will encounter difficulty in diffusing and penetrating the substrate surface. This research will contribute to the development of TBC materials with enhanced resistance to CMAS corrosion.

High-throughput composition screening of high-entropy rare-earth monosilicates for superior CMAS corrosion resistance up to 1873 K

Superior calcium-magnesium-aluminum-silicate (CMAS) corrosion resistance is the key to the applications of high-entropy rare-earth monosilicates (HEREMs) as environmental barrier coatings (EBCs) for next-generation gas turbine engines. Here, we report a high-throughput composition screening strategy to achieve superior CMAS corrosion resistance in HEREMs for the first time. Specifically, we first fabricate ten kinds of (Ho0.25Yb0.25Lu0.25X0.25)2SiO5 (X = Sc, Tm, Er, Y, Dy, Gd, Eu, Sm, Nd, and La) HEREM samples using a high-throughput pressure-less sintering technique, and subsequently screen their CMAS corrosion resistance at 1673 K via a high-throughput testing apparatus. Among all the as-fabricated samples, the HEREM-Er samples are found to have the best CMAS corrosion resistance, even up to 1873 K, which outperforms the EBC materials that have been reported. Such superior CMAS corrosion resistance of the HEREM-Er samples is primarily attributed to the formation of a dense apatite barrier layer, which results from its decreased melting point, as confirmed by first-principles calculations and molecular dynamic simulations. Our work provides a guide for designing exceptional anti-CMAS corrosion high-entropy rare-earth silicates.

Investigation of CMAS corrosion resistance for gradient Y2O3 stabilized ZrO2

The threat of calcium‑magnesium-alumina-silicate (CMAS) melt corrosion in thermal barrier coatings (TBCs) is significant at elevated working temperatures. In this study, a gradient Y2O3 stabilized ZrO2 coating (gradient YSZ coating) was designed and prepared by electron beam physical vapor deposition (EB-PVD) and its CMAS corrosion resistance was examined. The CMAS corrosion resistance test on the YSZ pellets highlighted the significance of dissolved Y2O3 and identified the critical Y2O3 content. Compared with the classical 8 wt% Y2O3 stabilized ZrO2 (8YSZ) coating, new crystal phases generated in the gradient YSZ coatings effectively blocked the CMAS melt infiltration for a short time; further, even after a long time, the CMAS melt was primarily distributed in the upper part of the gradient YSZ coating. In addition, the impact of coating microstructure on the CMAS resistance was investigated. The results showed the dense-gradient YSZ coating outperformed the 8YSZ coating on the resistance to CMAS infiltration. Finally, it is demonstrated that the gradient YSZ coating has potential applications against CMAS attacks, providing valuable design guidelines for TBCs with balanced corrosion resistance and promising mechanical properties.

CMAS corrosion resistance of scandia, ceria, yttria‐stabilized zirconia ceramic

AbstractSc2O3–CeO2–Y2O3– stabilized zirconia (ScCeYSZ) nanoparticles with different percentages of stabilizer agents [sample1: 1.8 wt.% (Sc2O3) 8.3 wt.% (CeO2) 1.9 wt.% (Y2O3), sample 2: 1.1 wt.% (Sc2O3) 9.0 wt.% (CeO2) 1.9 wt.% (Y2O3), sample 3: .5 wt.% (Sc2O3) 9.6 wt.% (CeO2) 1.9 wt.% (Y2O3) stabilized zirconia] were synthesized with Pechini method and consolidated by spark plasma sintered method. The results showed that despite the [(sample)1: 1.8 wt.% (Sc2O3) 8.3 wt.% (CeO2) 1.9 wt.% (Y2O3)] had lower density and higher porosity percentage compared to other samples, it had better calcium–magnesium–alumina–silicate (CMAS) corrosion resistance compared to other samples and the yttria‐stabilized zirconia nanopowders (nano‐YSZ) sample. It was due to the higher acidic nature and tetragonality of the (sample)1 sintered body compared to other samples and YSZ ceramic in the CMAS corrosive medium. Moreover, the results of phase and microstructural analysis following CMAS corrosion revealed the formation of the monoclinic phase and rod‐shaped CaAl2Si2O8 particles on the surface of the sampled sintered sample. However, the nano‐YSZ sample corroded homogenously and delamination occurred after the CMAS corrosion test.

Effect of calcium–magnesium–aluminum silicate composition on reaction behavior for X1‐Gd2SiO5 and X2‐Er2SiO5 environmental barrier coatings

AbstractRare‐earth (RE) silicates environmental barrier coatings (EBCs) are prone to accelerate failure resulted from molten calcium–magnesium–aluminum silicate (CMAS), which presents significant challenges to the design and development of durable EBCs. In this study, X1‐Gd2SiO5 and X2‐Er2SiO5 coatings were explored on their corrosion resistance to CMAS melts with different Ca/Si ratios (1.1, 0.75, 0.64, and 0.42) at 1350°C for 25 and 50 h, respectively. Results showed that with the Ca/Si ratio increasing, CMAS viscosity decreased and a more intense corrosion reaction with coatings occurred. Garnet phase was only observed in X2‐Er2SiO5 coating corroded by CMAS with high Ca/Si ratios, which could be beneficial for improving corrosion resistance. The X2‐Er2SiO5 phase, which is composed of [SO4] and [REO6]/[REO7] polyhedrons, exhibits better corrosion resistance due to its more stable crystal structure, lower reactivity with CMAS, and the radius difference between the RE ions and Ca2+.

Corrosion resistance characteristics of silicide coating on Nb[sbnd]Si based alloy exposed to CMAS at high temperature

Niobium disilicide shows good oxidation resistance in high-temperature oxidizing atmosphere and can be used as oxidation resistant coatings. However, these coatings are also required to have good environmental resistance, where corrosion by calcium‑magnesium-alumina-silicate (CMAS) represents a potential threat to the coating during the application. The aim of this work is to clarify the interaction between the AlY modified silicide coating on NbSi based alloy and the CMAS. The results indicate that the coating consists mainly of a disilicide layer and a transitional layer. The disilicide layer is mainly composed of (Nb,X)Si2 with (Zr,X′)Si2 particles dispersed in it. During the CMAS exposure, both the silicide coating and the as-formed SiO2-based composite oxide scale can be dissolved by the melted CMAS. The increase of temperature accelerates the CMAS corrosion process. The accumulation of dissolved Si at the interface between the CMAS and coating leads to the in-situ crystallization of Cristobalite-SiO2, which can arrest the melted CMAS, offering a good protection against CMAS attack. The (Nb,X)Si2 coating exhibits a two stage of CMAS corrosion process. The first one is the dissolution of the coating until SiO2 crystallization. The second one is the growth of the Cristobalite-SiO2 layer, during which the coating elements will be oxidized and can then diffuse into the melted CMAS through the Cristobalite-SiO2 layer.

Deposition characteristics, sintering and CMAS corrosion resistance, and mechanical properties of thermal barrier coatings by atmospheric plasma spraying

Atmospheric plasma spraying (APS) was used to fabricate 4.5 mol% Y2O3 stabilized ZrO2 (YSZ) and 4.0 mol% Yb2O3 and 0.5 mol% Y2O3 co-stabilized ZrO2 (YbYSZ) series coatings with different spraying parameters. This study presents the effects of particle size of feed powder and substrate temperature on the microstructure and performance of thermal barrier coatings. Single-pass and multi-pass experiments were conducted to explore the relationship between the splat and the coatings. The experimental results indicate that adjusting particle size of feed powders and deposition temperature can alter the splat behaviour and microstructure, improve anti-corrosion and anti-sintering abilities, and enhance the hardness and elastic modulus. Changes in parameters can cause changes in the microstructure of as-sprayed coatings. Lower substrate temperatures can lead to vertical cracks, finer powders can introduce more microdefects, and conventional sized powders can form larger pores. A coating deposited using a conventional feed powder at a substrate temperature of 800 °C shows excellent sintering resistance. A coating deposited using a fine feed powder on 800 °C substrate exhibits the best calcium‑magnesium-alumina-silicate (CMAS) corrosion resistance.

Processing-Microstructure-Properties of Columns in Thermal Barrier Coatings: A Study of Thermo-Chemico-Mechanical Durability.

Contemporary gas turbine engines rely on thermal barrier coatings (TBCs), which protect the structural components of the engine against degradation at extremely high operating temperatures (1300-1500 °C). The operational efficiencies of aircraft engines have seen significant improvement in recent years, primarily through the increase in operating temperatures; however, the longevity of TBCs can be potentially impacted by several types of degradation mechanisms. In this comprehensive study, a wide range of novel columnar suspension plasma sprayed (SPS) coatings were developed for their erosion, calcium-magnesium-aluminum-silicate (CMAS), and furnace cycling test (FCT) performance. Through a comprehensive investigation, the first of its kind, we achieved a range of SPS microstructures by modifying the spray parameters and measuring their microhardness, fracture toughness, column densities, and residual stresses using Raman spectroscopy. We were able to produce dendritic, lateral, branched, and columnar microstructures with a unique set of processing parameters. Coatings enhanced with a refined columnar microstructure, achieved by modulating the distance from the plasma torch, exhibited superior thermal cycling resilience. Conversely, the development of a columnar microstructure with dendritic branches, obtained by decreasing the robot's traversal speed during deposition, bolstered resistance to erosion and minimized damage from molten CMAS infiltration, thereby notably augmenting the coating's lifespan and robustness. The pursuit of the optimal columnar microstructure led to the conclusion that for each SPS coating, a general framework of optimization needs to be conducted to achieve their desired thermo-chemico-mechanical resistance as the properties required for TBCs are intertwined.

Reaction mechanisms of (RE0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 (RE = La or Yb) under CaO-MgO-Al2O3-SiO2 (CMAS) attack

In this study, two high-entropy ceramics, namely (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 (LaHZ) and (Nd0.2Sm0.2Eu0.2Gd0.2Yb0.2)2Zr2O7 (YbHZ), were prepared and investigated, in comparison to La2Zr2O7 (LZ). This investigation focused on their interactions with calcium-magnesium-alumina-silicate (CMAS), revealing noteworthy findings. High entropy samples particularly YbHZ exhibit significantly reduced infiltration depths compared to LZ. After CMAS corrosion, the reaction zones of LaHZ and LZ show a similar microstructure characterized by the presence of apatite and fluorite embedded in CMAS residue. In contrast, YbHZ forms a dense and nanoscale dual-phase zirconia (fluorite + pyrochlore) layer, with no visible CMAS residue. The different corrosion behavior is associated with the competition between fluorite and apatite phases, which is strongly related to the ionic radius of RE3+. Apart from the apatite, the formation of a dense and continuous fluorite Ca&RE-ZrO2 layer could also provide excellent CMAS resistance. These findings provide a viable strategy for designing CMAS-resistant materials.

Calcium–magnesium–aluminum‐silicate melt viscosities influenced by lanthanides, yttrium, and zirconium

AbstractLanthanides (Ln2O3) and elements like Zr and Y find application in the highest temperature‐resistant thermal and/or environmental barrier coatings. Such coatings are routinely exposed to silicate particles (e.g., sand, dust, volcanic ash), leading to chemical reactions that degrade the coating. The dissolution of 13.5 wt.% Ln2O3 into a calcium–magnesium–aluminum‐silicate (CMAS) melt leads to a viscosity reduction for the light lanthanides, while viscosity increases toward heavier lanthanides. For Gd, Y, and Zr, various amounts up to 13.5 wt.% (Gd2O3, Y2O3, ZrO2) were added to the CMAS melt, showing a tendency of increased viscosity for low concentrations (2.5‐3 wt.%) and a decreasing viscosity for higher values of the added component.

Corrosion of tri-layer LaMgAl11O19/Yb2SiO5/Si environmental barrier coatings with molten calcium-magnesium-alumina-silicate (CMAS)

Corrosion behavior of calcium-magnesium-alumina-silicate (CMAS) coated on LaMgAl11O19/Yb2SiO5/Si environmental barrier coatings are studied. The results show that a dense layer of anorthite is formed under short-term corrosion at 1300 °C (>5 h). After 25 h, CMAS melt penetrates and dissolves the LaMgAl11O19 layer along the pores, crystallizing with the Yb2SiO5 front. At 1400 °C for 5 h, the CMAS melt reacts vigorously with the coating and penetrates the LaMgAl11O19 layer, forming large pores and the production of garnet solid solution and Ca2(Yb, La)8(SiO4)6O2. After 50 h, the Yb2SiO5 layer completely dissolves, leaving residual CMAS on top of the thermally grown oxide layer.

Improvement on corrosion resistance of molten calcium-magnesium-alumina-silicate (CMAS) by Yb2O3-Y2O3 co-stabilized ZrO2 ceramic pellets

Five types of ceramic pellets with different doping contents of Yb2O3-Y2O3 co-stabilized ZrO2 (YbYSZ) (x mol.% Yb2O3 - (4.5-x) mol.% Y2O3-ZrO2; X = 0 (YSZ), 1, 2, 3, 4) were designed to investigate corrosion behavior under CMAS attack. The results show that the corrosion resistance of YbYSZ increased with increasing Yb2O3 content, which was attributed to the excellent phase stability and lower diffusion of Yb3+. More degradation with microstructural destruction was observed in the YSZ pellets, and less degradation was observed in YbYSZ with 4 mol.% Yb2O3 (YbYSZ-4). Further, YbYSZ-4 performs a larger contact angle than others, promoting CMAS resistance.

Impact of bond coat types on calcium-magnesium-alumina-silicate and hot corrosion behavior in thermal barrier coatings

This study examines the corrosion behavior of thermal barrier coatings (TBCs) with bond coats (BCs) fabricated using Air plasma spraying (APS) and High velocity oxygen fuel (HVOF) against molten salt and Calcium-magnesium-alumina-silicate (CMAS) corrosion. Results show that TBCs with the APS-BC have higher resistance and experience fewer failures than those with the HVOF-BC due to the further growth of the aluminum-rich oxide layer in APS-BC coatings, causing the salt to be neutralized during the reaction with BC. The products of the reaction between CMAS and TGO lead to changes in coating interface morphology and degradation in coatings with HVOF BC.

Evolution of microstructure and properties of Ba(Mg1/3Ta2/3)O3 thermal barrier material exposed to CMAS corrosion

Calcium-magnesium-alumina-silicate (CMAS) resistance has become a key parameter to evaluate the properties of thermal barrier coating (TBC) materials. The CMAS corrosion behavior of BMT ceramic as a novel candidate material for TBC is poorly understood. In this paper, the solid-state sintering process was used to prepare the BMT bulks, and the evolution of microstructure, phase composition, thermophysical properties, and mechanical properties of the BMT bulks exposed to CMAS corrosion were studied. The results indicate that obvious elemental interdiffusion between the CMAS melt and BMT occurred at 1250 °C, which changed the original phase composition and formed multiple substances including Ba(Al, Ca)Si2O8, Ba2MgSi2O7, BaTa2O6, Ba2Si4O10, etc. The microstructure of BMT also underwent changes due to differences in the diffusion coefficients of different elements. As corrosion time prolongs, an obvious porous columnar reaction layer formed on the sample surface, causing the CMAS melt to sustainably corrode the deep BMT, resulting in intragranular corrosion. In terms of properties, the thermophysical and mechanical properties of the BMT bulks corroded by CMAS deteriorated. The thermal conductivity at 1100 °C increased from 1.43 W m−1 · k−1 to 1.67 W m−1 · k−1. The average coefficient of thermal expansion at 200–1400 °C decreased from 11.60 × 10−6 K−1 to 10.95 × 10−6 K−1. The hardness dropped by 21 % compared to the original BMT bulk. Therefore, it is necessary to improve the CMAS resistance of BMT ceramic in future studies.