Molten CMAS Research Articles

CMAS Corrosion Behavior of Mid-Entropy Rare-Earth Hafnate (Y0.3Gd0.3Yb0.4)4Hf3O12 as Thermal Barrier Coating Candidate.

High-temperature CMAS corrosion has become a crucial factor inhibiting the further development of thermal barrier coatings (TBCs) because of the increasing service temperature of aero-engines. Herein, a novel mid-entropy rare-earth hafnate (Y0.3Gd0.3Yb0.4)4Hf3O12 (YGYbH) was prepared by ultrafast high-temperature sintering (UHS) technology, and its CMAS corrosion behavior and mechanism were investigated. During corrosion, the Ca2RE8(SiO4)6O2 apatite phase with a lower formation enthalpy and entropy-stabilized effect had a more intense tendency to be generated, which improves the density and stability of the reaction layer, hindering the further penetration of molten CMAS. Moreover, the significant lattice distortion caused by the rare-earth ions with different radii impeded the ionic diffusion, which delayed the CMAS corrosion reaction. In general, YGYbH, with excellent CMAS corrosion resistance, has the potential to serve as a next-generation TBC material.

Chemical Degradation of the Ternary Al2O3–YAG–ZrO2 Eutectic Ceramic by Molten CMAS

Chemical Degradation of the Ternary Al2O3–YAG–ZrO2 Eutectic Ceramic by Molten CMAS

Influence of Yttria content in YSZ: An evaluation of water-based SPS coatings and spark plasma sintered pellets

The inherent phase instability of the state-of-the-art 7–8 mol% partially stabilised Yttria-Stabilised Zirconia (YO1.5, 8 YSZ, tetragonal) under a molten CMAS attack lacks the technological readiness needed to increase the gas inlet temperatures for more thermally efficient gas turbine engines. In this study, the concentration of Yttria in YSZ was systemically increased to impart CMAS resistance and their applicability via emerging Suspension Plasma Sprayed (SPS) coatings and Spark Plasma Sintered (SpPS) pellets under simulated conditions was evaluated. The fully stabilised higher Yttria YSZ compositions (21.4 mol% and 50.2 mol%, cubic) severely restricted the CMAS infiltration and interaction areas, with the later composition forming an apatite layer and the trans-granular cracking and chipping of YSZ into layers reduced with an increase in Yttria content. However, their application into water-based SPS coatings resulted in a complete coating failure following the high-temperature CMAS tests. During Furnace Cycling Test (FCT) tests, the 8 YSZ coating survived 34 thermal cycles compared to just one on the 50.2 mol% YSZ coatings. The premature delamination of the higher Yttria coating seems to have been caused by the spallation associated with horizontal cracks within the coating. The YSZ composition could be modified into CMAS-resistant chemistry; however, their applicability as a functioning TBC with inherent microstructural features, such as the Dense Vertically Cracked (DVC) coating strategy examined in this study, requires new coating design strategies that impart sustainable thermal cycling performance.

Microstructural, thermal characterization and CMAS corrosion resistance of novel quaternary (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 high entropy disilicate material

A novel (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 high entropy disilicate quaternary composition was synthesized from commercial oxide powders using ball milling and sintering processes as a candidate material for environmental barrier coatings (EBC). As-synthesized high entropy disilicate powders were sintered at different durations (12, 18, and 24 h) at 1600 °C in a muffle furnace before characterization. The XRD and SEM analyses revealed the single-phase monoclinic structure (β-type) with homogeneous elemental distribution for the sintered samples. The (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 samples exhibited low thermal diffusivity coefficient, low thermal conductivity, a close coefficient of thermal expansion (CTE) to SiC and a high temperature stability. The (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 samples were subjected to CMAS corrosion tests at 1300 °C with different durations (2, 12, and 24 h) to evaluate CMAS corrosion resistance. Additionally, Yb2Si2O7 samples were prepared and subjected to molten CMAS in the same way for comparison. Based on the results, the CMAS corrosion resistance was improved with (Y0,25Er0,25Tm0,25Yb0,25)2Si2O7 composition.

Characterization of partial wetting by CMAS droplets using multiphase many-body dissipative particle dynamics and data-driven discovery based on PINNs

The molten sand that is a mixture of calcia, magnesia, alumina and silicate, known as CMAS, is characterized by its high viscosity, density and surface tension. The unique properties of CMAS make it a challenging material to deal with in high-temperature applications, requiring innovative solutions and materials to prevent its buildup and damage to critical equipment. Here, we use multiphase many-body dissipative particle dynamics simulations to study the wetting dynamics of highly viscous molten CMAS droplets. The simulations are performed in three dimensions, with varying initial droplet sizes and equilibrium contact angles. We propose a parametric ordinary differential equation (ODE) that captures the spreading radius behaviour of the CMAS droplets. The ODE parameters are then identified based on the physics-informed neural network (PINN) framework. Subsequently, the closed-form dependency of parameter values found by the PINN on the initial radii and contact angles are given using symbolic regression. Finally, we employ Bayesian PINNs (B-PINNs) to assess and quantify the uncertainty associated with the discovered parameters. In brief, this study provides insight into spreading dynamics of CMAS droplets by fusing simple parametric ODE modelling and state-of-the-art machine-learning techniques.

Unveiling the protection mechanism of LaPO4 against CMAS attack

This work evaluated the resistance of LaPO4 ceramics to CMAS attack at 1250 °C and explored their interaction mechanisms. The chemical reaction between molten CMAS and LaPO4 involves two stages leading to formation of a continuous inner layer and outer polyhedron grains, which effectively prevents the infiltration of CMAS. The reaction layer in this study is formed by inter-diffusion between LaPO4 and CMAS melt in the interface region, which involves a series of events including solution, substitution and recrystallization. The mitigating mechanism differs significantly from the typical 'dissolution-precipitation' process observed in rare-earth zirconate or alumina-based materials during CMAS attack. Because of this novel mechanism, the reaction layer has a crack-free microstructure and seals the surface of the underlying LaPO4 ceramic, which consequently results in enhanced resistance to CMAS attack.

Phase and structural evolution of dysprosia stabilized zirconia ceramics under CMAS corrosion environment

In this study, 10 wt % dysprosia stabilized zirconia (10DySZ) ceramics were prepared by pressureless sintering method. The CMAS corrosion behaviors of DySZ ceramics were investigated and the corrosion mechanism was also discussed in detail. Results show that the CMAS corrosive agent can induce phase transformation of DySZ ceramic due to the stronger ability to capture Dy stabilizers than Zr ions. During the corrosion process, molten CMAS would preferentially corrode the grain boundaries and then infiltrated into the interior of DySZ ceramic, resulting in the formation of loose structure on the ceramic surface. Sandy-shaped DySZ particles were also formed in the residual CMAS layers based on dissolution and precipitation mechanism. The corrosion temperature plays a crucial role in accelerating the phase transformation and thermal-mechanical damage of DySZ ceramic. Mismatch of thermal expansion coefficient between CMAS and DySZ ceramics as well as composition and structural difference between dense layer and porous layer are all responsible to the formation of destructive cracks.

Comprehensive microstructural characterization and CMAS infiltration resistance of ytterbium disilicate coatings with lamellar and quasi-columnar structures

Degradation by CMAS deposits represents a considerable threat to the EBC service life. Current work contributes to understanding the response of Yb2Si2O7 coatings with lamellar and quasi-columnar structures to CMAS attack. It is revealed that the dual-phase structure in APS coating drastically improved the overall coating resistance to CMAS infiltration by enhancing the apatite-forming capability. The amorphous silica in PS-PVD coatings limited the formation of apatite “sealing” layer. With a continuous degradation of sand-phobic morphology, molten CMAS rapidly penetrated inside the entire coating. This study highlights that microstructure modulation of Yb2Si2O7 coating is a promising strategy for CMAS infiltration mitigation.

Mechanism of enhanced corrosion resistance against molten CMAS for pyrosilicates by high‐entropy design

AbstractMeeting service requirements at temperatures above 1400°C is challenging for the CMAS corrosion resistance of single‐component pyrosilicates. This research presents a high‐entropy design approach for pyrosilicates using ionic radius modulation. This method enhances pyrosilicates’ resistance to CMAS corrosion by regulating the apatite's quantity formed to obstruct CMAS melt infiltration while avoiding excessive reactions. We investigated the corrosion behavior of two types of single‐component pyrosilicates (Lu2Si2O7 and Yb2Si2O7) with a small ionic radius of rare‐earth elements (REEs), three types of β‐type pyrosilicates ((Ho1/4Er1/4Yb1/4Lu1/4)2Si2O7, (Y1/5Ho1/5Er1/5Yb1/5Lu1/5)2Si2O7 and (Y1/6Ho1/6Er1/6Tm1/6Yb1/6Lu1/6)2Si2O7), and one γ‐type pyrosilicate ((Gd1/4Dy1/4Yb1/4Lu1/4)2Si2O7) with a larger average ionic radius of REEs at 1450–1550°C. The analysis of the residual CMAS and apatite compositions showed the differences in the behavior of different REEs in the reaction with CMAS and the conditions required for the reaction to proceed.

Corrosion behavior and mechanism of ytterbium monosilicate by molten calcium-magnesium-alumino-silicate melts at 1400 °C and 1500 °C

Thermochemical interactions of Yb2SiO5 block, a potential environmental barrier coating (EBC) material and a synthetic CMAS with the composition of 33CaO–9MgO–13AlO1.5-45SiO2 at 1400 °C and 1500 °C have been investigated. Corrosion tests showed that CMAS dissolved Yb2SiO5 and caused precipitation of Ca2Yb8 (SiO4)6O2 crystalline phase. At 1400 °C, Ca2Yb8 (SiO4)6O2 tended to form a continuous reaction layer at the CMAS/Yb2SiO5 interface. This reaction layer became thicker and denser with increasing time, which effectively inhibited molten CMAS penetration into Yb2SiO5. However, at 1500 °C, a dense and continuous layer could hardly form. Due to the low viscosity of the CMAS, it quickly infiltrated Ca2Yb8 (SiO4)6O2 along grain boundaries, and Ca2Yb8 (SiO4)6O2 dissolved in molten CMAS followed by re-precipitation in the form of tiny Ca2Yb8 (SiO4)6O2 crystals during cooling. The results show that Yb2SiO5 has good CMAS corrosion resistance at 1400 °C, but its protective capabilities are limited at temperatures above this threshold.

Phase transformation failure in YSZ TBCs induced by component-dependent CMAS corrosion

Generally, CMAS corrosion is a key factor leading to the spalling failure in thermal barrier coatings (TBCs). Figuring out the component-dependent failure mechanism is the premise to improve the service life of TBCs. Here, CMAS corrosion behaviors with different Ca/Si components have been investigated through analyzing microstructure evolution in 8-YSZ coatings by EB-PVD. Capillary force from the gap between columnar crystals could accelerate the infiltration speed of the molten CMAS, leading to a fully populated coating with lower strain tolerance in 4 h. Subsequently, CMAS infiltration in the coatings led to preferential dissolution of yttrium ions and precipitation of yttria lean YSZ, and secondary CMAS phases. The corrosiveness of CMAS could be further tailored by optical basicity and viscosity. TBC failure could be enhanced in high-Ca case, resulting in the morphology evolution from columnar crystals to coarsening grain structures. Raman spectra indicates CMAS corrosion with high-Ca content will facilitate the phase transformation of t-YSZ to harmful monoclinic phase. In addition, DFT calculation demonstrates Ca doping into 8-YSZ will make monoclinic phase more stable and generate a 4.1 % volume expansion from the view point of energy. Consequently, the combination of the above low strain tolerance, new phases and volume expansion will accelerate the harmful morphology evolution. This work provides direct insight into the failure mechanism of CMAS corrosion and paves the way to design of new TBCs.

Al2O3-modified 7YSZ thermal barrier coatings for protection against volcanic ash corrosion

To prevent volcanic ash corrosion using thermal barrier coatings (TBCs), a novel method using Al2O3-modification was proposed to reduce molten CMAS wettability on TBCs surface through a lotus effect. Plasma spray-physical vapor deposition (PS-PVD), a third-generation method for TBCs fabrication, was adopted to deposit 7YSZ TBCs with a feather-like microstructure. Then, Al2O3 modification was introduced to fabricate a dense Al2O3 overlay with nano/micro-sized grains on the TBCs surface. The wetting ability of CMAS on APS/EB-PVD/PS-PVD 7YSZ TBCs was comparatively in situ observed at 1250 °C for 3600 s. The results indicated that the dense Al2O3 overlay inhibited penetration of molten CMAS. Additionally, the micro/nano dual-sized structure, which is similar to the papillary structure of a lotus leaf, reduced molten CMAS wettability on the TBCs surface. The results demonstrated that the Al2O3-modified TBCs had better CMAS corrosion resistance than the as-deposited APS/EB-PVD/PS-PVD TBCs.

Effect of HfO2 content on CMAS corrosion resistance of a promising Hf6Ta2O17 ceramic for thermal barrier coatings

The effect of HfO2 content on CMAS corrosion resistance of Hf6Ta2O17 ceramics at different temperatures is investigated. The results show that Hf6Ta2O17 ceramics with different HfO2 content exhibit no difference in CMAS corrosion resistance at 1250 °C and short-term corrosion (< 16 h). But Hf6Ta2O17 ceramic with a higher HfO2 content shows the best corrosion resistance at 1300 °C and 1400 °C under long-term corrosion (> 50 h). The increase in HfO2 content can reduce the reactivity of Hf6Ta2O17 ceramic with molten CMAS and increase the generation rate of dense product layers.

Preparation and CMAS Wettability Investigation of CMAS Corrosion Resistant Protective Layer with Micro-Nano Double Scale Structure

Solution precursor plasma spray (SPPS) can prepare thermal barrier coatings (TBCs) with nanostructures, which can modify the adhesion and wettability of molten silicate environmental deposits (CMAS) on the surface of TBCs, thereby improving the resistance of TBCs to CMAS corrosion. In this study, SPPS layers with micro-nano double scale structures were prepared on the surface of conventional atmospheric plasma spraying (APS) coatings. The effect of process parameters on the micro-nano double scale structures and the wetting and infiltration behavior of molten CMAS on the surface of coatings were investigated. The results show that micron structure is more sensitive to process parameters. Lower precursor viscosity, closer spraying distance, and smoother APS layer are favorable to form more typical and dense micron structures. After covering the SPPS layer, the CMAS wetting diameter is reduced by about 40% and the steady-state contact angle increased up to three times. The reason is that the micro-nano double scale structures can effectively trap air and form an air layer between the coating surface and the molten CMAS. In addition, nano-particles play a more important role in the formation of the air layer, which in turn determines the steady-state wettability properties. While micron structures can influence the time needed to reach the steady state. However, the SPPS layers composed of nano-particles have a very loose structure and weak cohesion, and they degrade and fail rapidly after the infiltration of molten CMAS. Therefore, maintaining the excellent CMAS wetting resistance of the SPPS layers while taking into account their lifetime and reliability has become the focus of further research.

CMAS corrosion and thermal cycling fatigue resistance of alternative thermal barrier coating materials and architectures: A comparative evaluation

The corrosion of ceramic thermal barrier coatings (TBCs) by molten silicate deposits, usually known as “CMAS” from their main constituents (CaO-MgO-Al2O3-SiO2), is an issue of increasing concern in modern gas turbines as the turbine inlet temperatures are increased to enhance thermodynamic efficiency. Because conventional ZrO2-7wt%Y2O3 (7YSZ) dissolves quite readily in a CMAS melt, many alternative materials have been proposed, but there are not many comparative studies among them. Multi-layer architectures featuring a tougher 7YSZ bottom layer and a more brittle, but more CMAS corrosion-resistant top layer have also been proposed; therefore, a comparison among these architectures is also in order. In this paper we studied comparatively the resistance to CMAS corrosion and to thermal cycling fatigue (an essential pre-requisite for any TBC system) of Gd2Zr2O7, ZrO2–55wt%Y2O3 and Gd/Yb/Y co-doped ZrO2, both in the form of single, dense-vertically cracked (DVC) layers deposited by plasma spraying onto an MCrAlY bond coat, and as top layers with a bottom layer of either porous or DVC 7YSZ. It was found that Gd2Zr2O7 resists CMAS corrosion, without any grain-boundary dissolution, slightly better than does ZrO2–55wt%Y2O3. They both develop a solid Gd- or Y-apatite layer (respectively) at the interface with the CMAS melt, driven by the rather large difference in optical basicity between these compounds and CMAS itself, but the Y-apatite layer is less continuous and, therefore, a bit less protective. Gd/Yb/Y co-doped ZrO2, instead, suffers as much grain-boundary dissolution in contact with molten CMAS as does 7YSZ. A Gd2Zr2O7/porous 7YSZ system would therefore exhibit simultaneously high resistance to CMAS dissolution and to thermal cycling fatigue, although there is a risk that the CMAS melt might infiltrate the segmentation macro-cracks and the microcracks of the Gd2Zr2O7 layer and undermine the porous 7YSZ bottom layer.

Improving molten CMAS resistance of thermal barrier coatings by modified laser remelting method

Improving molten CMAS resistance of thermal barrier coatings by modified laser remelting method

YbPO4: A novel environmental barrier coating candidate with superior thermochemical stability

YbPO4 was synthesized to study thermomechanical and thermochemical stability as a novel EBC material candidate. Thermal expansion coefficients were measured by high temperature XRD and determined to be a = 5.4 ± 0.4 × 10−6 / °C, c = 7.3 ± 0.1 × 10−6 / °C for the tetragonal structure with a linear CTE = 6.0 ± 0.3 × 10−6 / °C for the 100 °C – 1200 °C temperature range. High-velocity water vapor exposure at 1400 °C for 60, 125, and 250 h resulted in the formation of Yb2O3 product layer at a rate of 8.7 µm2/h for the 80–200 m/s steam velocity range, which was a slower reaction rate than state of the art EBC Yb2Si2O7 under similar conditions. YbPO4 – molten CMAS exposures at 1300 °C for 4, 24, and 96 h resulted in a compositionally variant reaction product composed primarily of a continuous Ca8MgYb(PO4)7 layer that inhibited CMAS penetration into the bulk of the samples. YbPO4 was additionally found to be chemically stable in the presence of SiO2 at 1400 °C, which is important for compatibility with SiC/Si substrates at elevated temperatures. Initial thermomechanical and thermochemical analysis indicate that YbPO4 should be considered a viable material candidate for next-generation EBC systems.

The thermophysical properties and the molten CMAS resistance performance of Ytterbium Tantalate

The development of YSZ TBCs has been limited by the CMAS corrosion and thermophysical characteristics. The Ytterbium Tantalate (YbTaO 4 ) ceramic was selected in this study to evaluate its potential to replace traditional YSZ. The thermal and mechanical properties of YbTaO 4 show great advantages over YSZ, especially the hardness (191.3 ± 7.6 Hv 0.3 ). The CMAS hot corrosion results indicate that the maximum thickness of the reaction layer at 1350 °C after 24 h is ~16.8 μm, which is much thinner than that of YSZ. Moreover, the porosity of the reaction layer does not increase significantly compared with the YSZ TBCs. The main product after the reaction is the Ca 2 Ta 2 O 7 phase, and there is no anorthite phase after the reaction, mainly because the formation of the Ca 2 Ta 2 O 7 phase consumes a lot of Ca atoms. In addition, the equilibrium contact angle of YbTaO 4 is higher than that of YSZ, which indicates that the YbTaO 4 possesses worse wettability . In actual conditions, low wettability is conducive to the YbTaO 4 TBCs get rid of the liquid CMAS, which can effectively reduce the corrosion effect of CMAS. Furthermore, the corrosion depth and area of YbTaO 4 causing by residual CMAS are also smaller due to the poor wettability. Based on the above results, YbTaO 4 exhibits excellent potential to replace YSZ in preparing TBCs for the next generation aero engines. • The on-line contact angle was measured by the high-temperature contact angle analyzer. • The CMAS glass on the YbTaO 4 samples has not disappeared after different time reaction. • The reaction layer thickness of YbTaO 4 after 24 h CMAS attacking is thinner than YSZ TBCs. • The YbTaO 4 ceramics possess worse CMAS wettability compared with the YSZ.

Corrosion behaviors and mechanisms of ytterbium silicate environmental barrier coatings by molten calcium-magnesium-alumino-silicate melts

The ytterbium silicate coatings with different contents of Yb2O3, Yb2SiO5 and Yb2Si2O7 phases were explored on their corrosion resistance to CMAS melts at 1400 ℃. Results showed that Yb2O3-rich coatings dissolved into CMAS melts and reprecipitated garnet and apatite phases, the reaction of producing garnet was beneficial to the formation of dense apatite layer, which can prevent the molten CMAS from penetrating into the coating. However, no dense apatite layer was observed in the corroded Yb2Si2O7-rich coatings. The accelerated recession in the Yb2Si2O7-rich coatings might be ascribed to the increased fraction of CMAS melts within the reaction layer.

The spreading behavior of CMAS melt on YSZ single crystal with low index orientation

The spreading behavior of CMAS melt on cubic crystalline YSZ with low index orientation is studied by observing the variation of contact angles of CMAS melt on YSZ surface and the microstructure change caused by thermal chemical corrosion. In this study, the empirical formula of CMAS spreading on YSZ is established based on the experimental results. Whether in the vacuum or in the air, the tendency of the equilibrium contact angle of molten CMAS on the surface of single crystalline cubic YSZ is 〈1 0 0〉 > 〈1 1 1〉 > 〈1 1 0〉, and the spreading speed increases as 〈1 0 0〉 < 〈1 1 0〉 < 〈1 1 1〉. However, the specific spreading behavior of sectional samples varies with the atmosphere. Compared to the air environment, YSZ 〈1 0 0〉 obviously shows faster spreading speed and smaller equilibrium contact angle than in the vacuum. In this spreading process, the microstructures of invert trapezoid formed at the interface between CMAS and YSZ 〈1 0 0〉 and the semicircle (or the rounded triangle) formed on YSZ 〈1 1 0〉 and 〈1 1 1〉 play more important role than the surface energy which is calculated by first-principles.