Calcium-magnesium-alumina-silicate Corrosion Research Articles

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 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.

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.

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.

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.

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.

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.

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.

CMAS infiltration behavior of atmospheric plasma-sprayed thermal barrier coating with tailored pore structures

The degradation of atmospheric plasma spraying (APS) thermal barrier coatings (TBCs) due to calcium-magnesium-alumina-silicate (CMAS) corrosion is considered as a critical issue. In present work, mitigation of CMAS infiltration of APS TBCs has been achieved by tailoring pore characteristics based on a novel APS structure regulation method, and its thermal conductivity, thermal shock lifetime and infiltration behavior were evaluated. The results revealed that the laminar pore distributed TBC formed by horizontally aligned porous embedded particle clusters (PEPC) showed longest thermal shock lifetime and lowest thermal conductivity. Characterization of CMAS corrosion suggested that whilst all the TBCs samples with different pore structures suffered corrosion by CMAS melt, the severity fluctuated greatly. Compared with randomly distributed PEPC areas, these horizontally aligned PEPC areas have a noticeable potential for restricting CMAS infiltration. This unique structure consumes a large amount of CMAS melt and creates anti-CMAS filling pores, thus improving the overall CMAS infiltration resistance of the coating. The alternating dense layers outside the porous layers also provide a layer-by-layer barrier to CMAS infiltration. This study emphasizes that the modulation of the pore structure of APS TBCs is a promising strategy to mitigate CMAS infiltration.

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

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

CMAS Corrosion Resistance Behavior and Mechanism of Hf6Ta2O17 Ceramic as Potential Material for Thermal Barrier Coatings

Thermal barrier coatings (TBCs) have been seriously threatened by calcium-magnesium-alumina-silicate (CMAS) corrosion. The search for novel ceramic coatings for TBCs with excellent resistance to CMAS corrosion is ongoing. Herein, CMAS corrosion resistance behavior and the mechanism of a promising Hf6Ta2O17 ceramic coating for TBCs are investigated. The results show that temperature is the most important factor affecting the CMAS behavior and mechanism. At 1250 °C, the corrosion products are composed of dense reaction products (HfSiO4, CaXHf6−xTa2O17−x) and CMAS self-crystallization products. At 1300 and 1400 °C, the corrosion products are mainly dense CaTa2O6 and HfO2, which prevent further CMAS infiltration.

The CMAS corrosion behavior of high-entropy (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 hafnate material prepared by ultrafast high-temperature sintering (UHS)

High-temperature molten calcium-magnesium-alumina-silicate (CMAS) corrosion has become a fatal factor for the failure of aero-engine thermal barrier coatings. In this study, a promising entropy-stabilized (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 (5YH) hafnate was prepared by the emerging ultrafast high-temperature sintering (UHS), and its corrosion and wetting behavior of molten CMAS were investigated. For the corrosion mechanism, the precipitation of the high-entropy apatite phase promotes the formation of the HfO2 phase, and it can improve the density and stability of the slow-growing reaction layer, hindering the further penetration of molten CMAS. At 1300 ℃, a reaction layer with a three-layered morphology is generated, resulting from the decreased viscosity of the molten CMAS. Moreover, computational analysis shows that molten CMAS on the 5YH surface has a larger contact angle (17°) than traditional YSZ (13°), and the spreading area is about 90 % of traditional YSZ, which benefits for its good CMAS corrosion resistance.

Ti4+-incorporated fluorite-structured high-entropy oxide (Ce,Hf,Y,Pr,Gd)O2−δ: Optimizing preparation and CMAS corrosion behavior

Environmental barrier coatings (EBCs) with excellent chemical resistance and good high-temperature stability are of great significance for their applications in next-generation turbine engines. In this work, a new type of high-entropy fluorite-structured oxide (Ce0.2Hf0.2Y0.2Pr0.2Gd0.2)O2−δ (HEFO-1) with different Ti4+ contents were successfully synthesized. Minor addition of Ti4+ could be dissolved into a high-entropy lattice to maintain the structure stable, effectively reducing the phase formation temperature and promoting the shrinkage of bulk samples. Heat treatment experiments showed that all the samples remained a single phase after annealing at 1200–1600 °C for 6 h. In addition, high-entropy (Ce0.2Hf0.2Y0.2Pr0.2Gd0.2Ti0.2x)O2−δ demonstrated great resistance to calcium—magnesium—alumina—silicate (CMAS) thermochemical corrosion. When the content of Ti was increased to x = 0.5, the average thickness of the reaction layer was about 10.5 after being corroded at 1300 °C for 10 h. This study reveals that high-entropy (Ce0.2Hf0.2Y0.2Pr0.2Gd0.2Ti0.2x)O2−δ is expected to be a candidate for the next-generation EBC materials with graceful resistance to CMAS corrosion.

Failure Mechanism of EB-PVD Thermal Barrier Coatings under the Synergistic Effect of Thermal Shock and CMAS Corrosion

Thermal barrier coatings (TBCs) suffer from the thermo-chemo-mechanical coupling action of thermal shock and calcium–magnesium–alumina–silicate (CMAS) corrosion. However, the failure mechanism of TBCs under the synergistic effect of thermal shock and CMAS corrosion is still unclear due to a lack of an environmental simulator. Herein, an 8YSZ ceramic coating is deposited on a PtAl bond coating/DD419 nickel-based single crystal superalloy substrate using the electron beam physical vapor deposition (EB-PVD) method. The thermo-chemo-mechanical coupling effect of TBCs is achieved in a self-developed environmental simulator. The interaction of volume expansion induced by the phase transition of ZrO2, structural degradation and thermal fatigue further increases the out-of-plane tensile stress and in-plane shear stress in the ceramic coating, which accelerates the initiation and propagation of surface vertical cracks and horizontal cracks. As multiple surface vertical cracks propagate to the interface and merge with interfacial cracks, the ceramic coating spalls from the substrate.

Finite element analysis on temperature field and stress distribution of thermal barrier coatings by laser modification and CMAS corrosion

Laser glazing to produce a modified layer on the surface of thermal barrier coatings (TBCs) is a promising method to alleviate calcium-magnesium-alumina-silicate (CMAS) attacks to coatings. In this study, finite element analysis is carried out to investigate the temperature field and stress distribution of TBCs after the laser glazing process and the modified TBCs after CMAS corrosion. Results revealed that along the direction of laser scanning, the principal stress was in a tensile state, which increased along the direction of the laser spot movement; while perpendicular to the laser scanning direction, the maximum principal stress appeared at the interface between the glazed layer and the unmodified coating, where could be a potential danger zone for crack initiation and coating spallation. For the modified TBCs with CMAS attack, in regions that are located in the range of 0–6 mm along the radial direction, the radial stress and maximum principal stress were high (∼1094 MPa), and the coating edge had complex shear stress state; as a result, these areas were easy to crack.

Calcium-magnesium-alumina-silicate (CMAS) corrosion behavior of A6B2O17 (A=Zr, Hf; B=Nb, Ta) as potential candidate for thermal barrier coating (TBC)

Developing new systems with excellent properties is of great significance to the progress of TBC. We herein study the CMAS corrosion behavior of four A6B2O17 ceramics to evaluate the feasibility for TBC application. Zr6Nb2O17 has the worst corrosion resistance and Hf6Ta2O17 shows the best. The effects of elements A and B are reflected in the differences of corrosion products and reverse dissolution process, respectively. The dense corrosion layer containing CaTa2O6 and HfSiO4 is an important factor for Hf6Ta2O17 to prevent further corrosion. The present results highlight the merit of Hf6Ta2O17 as a promising candidate for CMAS-resisted TBC.