Calcium-magnesium-alumina-silicate Research Articles

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.

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.

Integrating APS TBCs with a built-in Al2O3 protection network for superior CMAS resistance

Calcium–magnesium–alumina–silicate (CMAS) attack poses a great threat to the durability of thermal barrier coatings (TBCs). We introduce a novel Al2O3-modified TBC architecture to counter this threat. The architecture integrates an Al2O3 protection network into the microstructure of an atmospheric plasma sprayed (APS) TBC by drip coating method, which modifies the vulnerable infiltration pathway of CMAS melt and promotes the self-crystallization of CMAS at the infiltration front. Moreover, during sintering, the built-in Al2O3 can inhibit inter-splat crack healing. This innovative TBC design achieves robust CMAS resistance and strain tolerance, while also offering advantages during sintering.

CMAS corrosion resistance of YSZ thermal barrier coatings enhanced by Pt–Al films

Corrosion of calcium magnesium aluminum silicate (CMAS) severely affects the durability of thermal barrier coatings (TBCs). In this study, 8 wt% Y2O3 partially stabilized ZrO2 (8YSZ) coatings were prepared using atmospheric plasma spraying (APS) technique, and a Pt–Al film was magnetron-sputtered onto the surface of the YSZ coating (Pt–Al-YSZ) to enhance its resistance against CMAS. The results indicated that the Pt–Al-YSZ coating displayed superior non-wetting performance against CMAS, as well as stronger resistance to CMAS corrosion. The results evidenced by Raman spectra and X-ray diffraction showed that phase transformation was smaller and corrosion depth was lower after CMAS corrosion at 1250 °C for 2 h. The thermal cycling test was also performed. The result demonstrated that the Pt–Al coating did not have a negative impact on the thermal cycling life of the YSZ TBCs. However, platinum tended to soften at high temperatures and became easily penetrated by molten CMAS. Therefore, it is necessary to further improve the strength of platinum layer to fully utilize its protective effect against CMAS attack.

Engineering stoichiometry of high entropy fluorite oxide to obtain optimized thermophysical property and improved corrosion resistance

Engineering stoichiometry of materials is an effective strategy to optimize the comprehensive properties. Here we prepare high-entropy Er2(Y0.2Yb0.2Nb0.2Ta0.2Cex)2Oδ with single-phase fluorite structure by a solid-state reaction. The effects of stoichiometry of cerium on thermophysical properties and corrosion resistance to calcium−magnesium−alumina−silicate (CMAS) of the high-entropy oxides have been investigated. By engineering the cerium stoichiometry, thermal conductivity increases slightly from 1.21 to 1.36 W·m−1·K−1 and thermal expansion coefficients also increases from 10.56 × 10−6 to 11.61 × 10−6 °C−1 at 1000 °C. Furthermore, corrosion resistance of the high-entropy Er2(Nb0.2Ta0.2Y0.2Yb0.2Cex)2Oδ improves with the increase of cerium content. The effects of cerium content on CMAS resistance have been discussed in detail. Our work reveals that engineering stoichiometry of high-entropy oxides is expected to be an effective strategy to broaden the compositional space and optimize the thermophysical properties and CMAS corrosion resistance.

Preparation and characterization of La-doped Y3Al5O12 as a potential protective coating material against CMAS corrosion

Mitigating calcium‑magnesium‑aluminum-silicate (CMAS) corrosion attack on thermal barrier coatings (TBCs) has attracted increasing interest. In this study, (Y1-xLax)3Al5O12 (x = 0, 0.1, 0.2, 0.3) were synthesized by the chemical co-precipitation method. The La-doped Y3Al5O12 was composed of Y3Al5O12 and LaAlO3 phases. As the La3+ doping concentration increased, the content of the LaAlO3 phase gradually increased. Compared to un-doped Y3Al5O12, the fracture toughness and Vickers hardness of Y3Al5O12 doped with 20 mol% La3+ increased by 54.9 % and 72.5 % respectively, reaching 2.88 MPa·m1/2 and 18.03 GPa. The thermal expansion coefficient of Y3Al5O12 doped with 30 mol% La3+ reached 9.79 × 10−6/K at 1000 °C, which was 7. 34 % higher than that of the un-doped Y3Al5O12. The CMAS corrosion resistance of (Y1-xLax)3Al5O12 gradually improved with the increase of LaAlO3 content. LaAlO3 promoted the formation of a dense apatite-spinel reaction layer and effectively inhibited the penetration of CMAS. (Y1-xLax)3Al5O12 (x = 0.1, 0.2, 0.3) can act as a potential protective coating material against CMAS corrosion.

The high-temperature CMAS corrosion behavior of high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Hf2O7 hafnate thermal barrier coating material with fluorite structure

High-temperature calcium-magnesium-alumina-silicate (CMAS) attack has become a fatal factor hindering the development of thermal barrier coatings (TBCs). In this study, a promising entropy-stabilized (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Hf2O7 (5LaH) hafnate was synthesized and prepared successfully, and then the CMAS corrosion behavior was investigated. During the corrosion process, the precipitation of the apatite phase causing the aggregation of Hf4+ promotes the formation of the (Hf, Si)O2 phase to generate a dense structure layer, which has a great barrier effect on the penetration of CMAS melt, and apatite phase with the entropy effect proves the stability of the reaction layer. Moreover, molten CMAS is less inclined to wetting and spreading on 5LaH surfaces compared to YSZ, which is also beneficial to the CMAS corrosion resistance. In general, the 5LaH exhibits great anti-CMAS corrosion performance.

Corrosion behavior of CMAS-infiltrated Ba(Mg1/3Ta2/3)O3 ceramic based on material characteristics

Calcium–magnesium–alumina–silicate (CMAS) corrosion is inevitable in thermal barrier coating applications. Herein, Ba(Mg1/3Ta2/3)O3 (BMT) ceramic, which is a promising thermal barrier coating material, was prepared using a solid-state reaction sintering method. The effects of the surface roughness and bulk density on the CMAS corrosion behavior of BMT ceramic were analyzed by investigating the surface morphology, cross-sectional microstructure, compositional evolution, and CMAS wetting behavior before and after CMAS corrosion. The results indicate that the wettability of CMAS on the BMT bulk material with a low surface roughness was lower than that of the material with a high surface roughness owing to the smaller driving capillary force, actual contact area between CMAS and the surface, and CMAS infiltration direction in BMT with a low surface roughness. This effectively inhibited the infiltration of CMAS and reduced its corrosion effect on BMT ceramic. The infiltration of CMAS into the BMT bulk materials with a high density was weaker because there were few channels, such as pores and microcracks, for infiltration. Therefore, reducing the surface roughness and increasing the density of BMT ceramic can be considered an effective method for improving its CMAS corrosion resistance.

Effects of GdTaO4 content on the thermal properties and CMAS corrosion of Gd2Zr2O7-GdTaO4 composite ceramics

Thermal barrier coating materials play a crucial role ensuring the efficient and stable operation of gas turbines. The thermophysical and mechanical properties of GZ1-x-GTx (x = 0, 0.1, 0.3, 0.5) composite ceramics, as well as the corrosion resistance properties against calcium magnesium aluminum silicate (CMAS) at 1250 °C, were investigated. The results indicated that the overall properties of Gd2Zr2O7-GdTaO4 composite ceramics significantly improved with an increase in the second phase GdTaO4 content. Among all the ceramics designed, GZ0.5-GT0.5 exhibited the lowest thermal conductivity (0.920 W m−1 K−1 at 800 °C) and the highest fracture toughness (2.02 MPa m1/2). Additionally, the molten CMAS interacted with the Gd2Zr2O7-GdTaO4 composite ceramics, forming a dense apatite layer, which positively contributed to inhibiting CMAS infiltration. Considering its excellent thermal, mechanical, and corrosion resistance properties, Gd2Zr2O7-GdTaO4 composite ceramics are considered a promising material for thermal barrier coatings.

CMAS resistance characteristics of multi-components rare earth phosphate materials at 1250 °C and 1350 °C

Calcium-magnesium-alumina-silicate (CMAS) attack is a major challenge for thermal barrier coatings (TBCs) during the application. In this work, four rare earth phosphates materials including (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)PO4, (La0.5Nd0.5)PO4, (La0.5Gd0.5)PO4 and (Nd0.5Gd0.5)PO4 were synthesized by chemical co-precipitation. The resistance behaviors to CMAS of the materials were investigated at 1250 °C and 1350 °C for 5 h, 10 h and 20 h, respectively. Results show that the main corrosion products are the apatite compounds (Ca2+xRe8-x(PO4)x(SiO4)6-xO2) and the Ca3(PO4)2. Increasing the corrosion temperature from 1250 °C to 1350 °C will promote the penetration of the melted CMAS. It would not lead to the more serious corrosion by prolonging the corrosion time at 1250 °C and 1350 °C. The P–Ca layer composed by the Ca3(PO4)2, which relates to the rare earth elements compositions, prevent the CMAS corrosion to the materials effectively. Preparing multi-components rare earth phosphates and optimizing their element compositions will be a new way to improve the resistance to the CMAS for these materials.

Doped effect of Yb on enhancing resistance to calcium-magnesium-alumina-silicate corrosion of Sm2(Zr0.7Ce0.3)2O7

Four kinds of (Sm1−xYbx)2(Zr0.7Ce0.3)2O7 (x = 0, 0.1, 0.2, 0.3) ceramics with different contents of Yb were prepared by the solid-phase reaction. Their thermochemical reaction behaviors with calcium-magnesium-alumina-silicate (CMAS) at 1300 °C were investigated. Among them, (Sm0.7Yb0.3)2(Zr0.7Ce0.3)2O7 has the best corrosion resistance to CMAS. The increase of Yb content can accelerate the formation rate of apatite, preventing the ceramic layer from erosion of CMAS and improving the strain tolerance of the ceramic. In addition, the CMAS wetting, corrosion process and failure mechanisms were discussed by high-temperature field wetting and transmission electron microscopy.