Thermochemical interactions between CMAS and Ca2Y8(SiO4)6O2 apatite environmental barrier coating material

Thermochemical interactions between Ca₂Y₈(SiO₄)₆O₂ apatite, a potential environmental barrier coating (EBC) material, and a synthetic CMAS having the composition 23.3 CaO - 6.4 MgO - 3.1 Al₂O₃ - 62.5 SiO₂ - 4.1 Na₂O - 0.5 K₂O - 0.04 Fe₂O₃ mole % were investigated. Pellets of apatite + CMAS powder and hot-pressed apatite disc-CMAS couples were annealed at 1200–1500 °C for 1–50 hours in air. Powder X-ray diffraction (XRD) was used to identify the phases present. Polished cross-sections of the heat treated pellets and diffusion couples were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark field (HAADF) imaging, selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). Ca₃Y₂(Si₃O₉)₂ cyclosilicate, apatite, and amorphous phases were present in the samples heat treated at 1200 and 1300 °C, whereas no cyclosilicate was detected in samples annealed at 1400 and 1500 °C. A distinct cyclosilicate layer was observed at the apatite-CMAS interface in the diffusion couples heat treated at 1200 and 1300 °C. However, at 1400 and 1500 °C, due to its much lower viscosity, CMAS quickly infiltrated the apatite substrate through pores and along the grain boundaries and no cyclosilicate was observed; the apatite grains dissolved in molten CMAS followed by re-precipitation of apatite needles within an amorphous phase on cooling.     

Significantly improved corrosion resistance of high-entropy rare-earth silicate multiphase ceramics against molten CMAS

To improve the ability of rare-earth (RE) silicates to resist molten calcium–magnesium–aluminosilicate (CMAS) at high temperature, a novel high-entropy (4RE_0.25)₂Si₂O₇/(4RE_0.25)₂SiO₅ (RE = Y, Yb, Er, and Sc) multiphase ceramic was prepared by a two-step process. During sintering, (4RE_0.25)₂SiO₅ can react with SiO₂ at the grain boundaries of (4RE_0.25)₂Si₂O₇, which can not only purify the grain boundary but also promote the growth of the original (4RE_0.25)₂Si₂O₇ grains, thereby significantly improving the ability to resist molten CMAS corrosion at high temperature. After corroding at 1500°C for 48 h, the reaction layer of the multiphase ceramic was only 55 μm thick. Our results confirm that the high-entropy RE silicate multiphase ceramics represent an effective way to improve the ability to resist molten CMAS corrosion at high temperature.     

Interactions between Ca2Gd8(SiO4)6O2 apatite and calcium-magnesium-aluminosilicate (CMAS) at elevated temperatures

High temperature corrosion behavior of Ca₂Gd₈(SiO₄)₆O₂ (CGdS) apatite has been investigated in the presence of molten calcium-magnesium-aluminosilicate (CMAS) glass having the composition 21.9 CaO - 4.3 MgO - 5.4 Al₂O₃ - 63.0 SiO₂ - 4.3 Na₂O - 0.8K₂O - 0.1 Fe₂O₃ (weight %). CGdS apatite powder was prepared by solid state synthesis from constituent oxides. Pellets of CGdS apatite + CMAS mixed powder and CGdS-CMAS diffusion couples were annealed at 1200, 1300, 1400, and 1500 °C for 1 and 20 h in ambient atmosphere. Development of phases in heat treated specimens was characterized using various analytical techniques as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high angle annular dark field imaging, selected area electron diffraction and energy dispersive X-ray spectroscopy. In both pellets and diffusion couples, monoclinic cyclosilicate Ca₃Gd₂(Si₃O₉)₂ formed from reaction of apatite with CaO in the CMAS melt only in samples heat treated at 1200 °C for 1 and 20 h or at 1300 °C for 1 h. Triclinic CaSiO₃ and monoclinic diopside MgCaSi₂O₆ were also observed in samples annealed at 1200 and 1300 °C. At 1400 and 1500 °C, because of its low viscosity, CMAS infiltrated along the pores and grain boundaries of the apatite substrates in diffusion couples. Phase compositions predicted from thermochemical computation were in good agreement with those observed experimentally. Ca₂Gd₈(SiO₄)₆O₂ apatite has the potential for being an effective T/EBC in circumventing the penetration of molten CMAS up to about 1300 °C but not at higher temperatures.     

High temperature behavior of HfSiO4 subjected to calcium-magnesium-alumina-silicate (CMAS) and high-velocity water vapor

HfSiO₄ is considered as a candidate for environmental barrier coating (EBC), but there is a lack of comprehensive evaluation of its resistance against corrosive medium. We herein study the behavior of HfSiO₄ against CMAS melt and high-velocity water vapor. HfSiO₄ shows poor resistance to CMAS attack. Si diffusion occurs during CMAS attack, which leads to the formation of HfO₂ and CaSi₂O₅. HfSiO₄ decomposes to form SiO₂ and HfO₂ under the scouring of water vapor, in which SiO₂ forms volatile hydroxide and is taken away by high-velocity steam. HfSiO₄ is not the preferred system for surface layer of EBC system and is expected to be used as intermediate transition layer.     

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

Meeting 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 ((Ho_1/4Er_1/4Yb_1/4Lu_1/4)₂Si₂O₇, (Y_1/5Ho_1/5Er_1/5Yb_1/5Lu_1/5)₂Si₂O₇ and (Y_1/6Ho_1/6Er_1/6Tm_1/6Yb_1/6Lu_1/6)₂Si₂O₇), and one γ-type pyrosilicate ((Gd_1/4Dy_1/4Yb_1/4Lu_1/4)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.     

CMAS degradation of ytterbium aluminum garnets

The degradation of ytterbium aluminum garnet (YbAG) exposed to molten Ca–Mg–Fe–Al–Si–O (CMAS) at 1673 K was investigated for two kinds of dense polycrystalline YbAG with compositions deviating slightly from stoichiometry, referred to as Al- and Yb-rich. The mitigation of the CMAS attack for Yb-rich YbAG was markedly superior to that for the Al-rich one. For both types of YbAG, corrosion progressed due to the preferential penetration of the CMAS melt along grain boundaries in the thickness direction and the simultaneous dissolution of crystal grains into the melt. The lower of the corroded region consisted of YbAG crystals with a core/shell-I/shell-II structure. Shell-I contained alkaline earth, silicon, and iron cations, whereas these cations were hardly detected in shell-II. Growth of the shell-I region was considered to progress by dissolution and reprecipitation through the melt existing around it, and finally, the melt disappeared, resulting in the formation of a thin shell-II region containing little of these ions. The formation and growth of the shell-I region were found to be promoted by making the YbAG Yb-rich, resulting in enhancement of the resistance to CMAS.     

Gadolinium oxide solubility in molten silicate: dissolution mechanism and stability of Ca2Gd8(SiO4)6O2 and Ca3Gd2(Si3O9)2 silicate phases

Improvement of the calcium-magnesium aluminosilicate (CMAS) infiltration mitigation concept in thermal barrier coatings (TBC) requires fundamental data on thermochemical reaction involving rare-earth oxide candidate as ZrO₂ alloying element. This study investigates, through a model approach, Gd₂O₃ dissolution at 1200 °C in a synthetic model CAS melt (64.4SiO₂–9.3Al₂O₃–26.4CaO mol. %) and the stability of the precipitated Gd-apatite Ca₂Gd₈(SiO₄)₆O₂ and Ca₃Gd₂(Si₃O₉)₂ cyclosilicate phases. The two Gd-rich silicates have been synthesized by solid-state reaction and then characterized by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The interactions of Gd₂O₃, Gd-apatite and Gd-cyclosilicate with CAS have been observed by scanning electron microscope (SEM) after various times of contact at 1200 ̊C, giving information about dissolution/precipitation processes. Dissolution kinetics has been evaluated by electron probe microanalysis (EPMA) measurements in the CAS melt. A discussion is finally provided concerning the thermodynamic stability of all phases of the system and confronted with kinetics considerations.     

High-temperature interactions of desert sand CMAS glass with yttrium disilicate environmental barrier coating material

A calcium-magnesium aluminosilicate (CMAS) glass was prepared by melting a sample of desert sand to evaluate the high-temperature interactions between molten CMAS and yttrium disilicate (Y₂Si₂O₇), an environmental barrier coating (EBC) candidate material. Cold-pressed pellets of 80?wt% Y₂Si₂O₇ powder and 20?wt% CMAS glass powder were heat treated at 1200?°C, 1300?°C, 1400?°C and 1500?°C for 20?h in air. The resulting phases were evaluated using powder X-ray diffraction. In the second set of experiments, free standing hot-pressed Y₂Si₂O₇ substrates with cylindrical wells were filled with CMAS powder to a loading of ~35?mg/cm² and heat treated in air at 1200?°C, 1300?°C, 1400?°C and 1500?°C for 20?h. Scanning electron microscopy, energy-dispersive spectroscopy and electron microprobe analysis were used to evaluate the microstructure and phase compositions of specimens after heat treatment. An oxyapatite silicate (Ca₂Y₈(SiO₄)₆O₂) phase was identified in all specimens after CMAS exposure regardless of heat treatment temperature. Apatite appeared to form by dissolution of Y₂Si₂O₇ into molten CMAS, reacting with CaO in the melt according to the reaction 4Y₂Si₂O₇ +?2CaO → Ca₂Y₈(SiO₄)₆O₂ +?2SiO₂, and followed by precipitation of the apatite phase.     

Corrosion resistance and thermal-mechanical properties of ceramic pellets to molten calcium-magnesium-alumina-silicate (CMAS)

Because gas turbine engines must operate under increasingly harsh conditions, the degradation of thermal barrier coatings (TBCs) by calcium-magnesium-alumina-silicate (CMAS) is becoming an urgent issue. Mullite (3Al₂O₃·2SiO₂) is considered a potential material for CMAS resistance; however, the performance of mullite in the presence of CMAS is still unclear. In this study, mullite and Al₂O₃–SiO₂ were premixed with yttria stabilized zirconia (YSZ) in different proportions, respectively. Porous ceramic pellets were used to conduct CMAS hot corrosion tests, and the penetration of molten CMAS and its mechanism were investigated. The thermal and mechanical properties of the samples were also characterized. It was found that the introduction of mullite and Al₂O₃–SiO₂ mitigated the penetration of molten CMAS into the pellets owing to the formation of anorthite, especially at 45 wt% mullite/55 wt% YSZ. Compared with Al₂O₃–SiO₂, mullite possesses a higher chemical activity and undergoes a faster reaction with CMAS, thus forming a sealing layer in a short time. Additionally, the thermal expansion coefficient, thermal conductivity, and fracture toughness of different samples were considered to guide the architectural design. Considering the CMAS corrosion resistance, thermal and mechanical performance of TBCs systematically, a TBC system with a multilayer architecture is proposed to provide a theoretical and practical basis for the design and optimization of the TBC microstructure.     

The mechanical and thermal properties,CMAS corrosion resistance,and the wettability of novel thermal barrier material GdTaO4

Because the CMAS corrosion and phase transformation at elevated temperatures above 1250 °C have limited the applications of traditional YSZ, the design of novel thermal barrier materials is a hotspot. GdTaO₄ is considered as a type of potential novel thermal barrier material owing to its low thermal conductivity. In this study, the mechanical and thermal properties, CMAS corrosion resistance, and the wettability of the GdTaO₄ were studied and compared with that of YSZ. The results show that the coefficient of thermal expansion and hardness of GdTaO₄ are 14.1 × 10⁻⁶ K⁻¹ (1350 °C) and 534.2 Hv_0.3 respectively. The thickness of CMAS reaction layer of GdTaO₄ is ~30.8 μm after 24 h reaction at 1350 °C, which is thinner than that of YSZ. After corrosion reaction, the CMAS glass aggregated instead of completely disappearing or continuously extending over the surface of GdTaO₄. The main reaction product is Ca₂Ta₂O₇, and the anorthite phase may not be detected, which is similar to YTaO₄. By comparison, the dense substrate of YSZ became porous and CMAS glass has disappeared after 10 h. CMAS corrosion at 1350 °C. The on-line contact angle results show that the wettability of CMAS on GdTaO₄ is worse than that on YSZ at 1350 °C, while the opposite of the work of adhesion, which indicates that GdTaO₄ can remove liquid CMAS more easily than YSZ TBCs during the service. Furthermore, the corrosion depth and areas of GdTaO₄ are smaller than those of YSZ in the same situation. These findings suggest that GdTaO₄ possesses better high-temperature properties and CMAS corrosion resistance than YSZ as a kind of potential of thermal barrier material.     

CMAS penetration-induced cracking behavior in the ceramic top coat of APS TBCs

The penetration of calcium-magnesium-alumino-silicate (CMAS) is one of the most vital factors inducing the failure of air plasma sprayed thermal barrier coatings (APS TBCs). In present study, a two-dimensional periodical model considering the microstructures in ceramic top coat (TC) is built to study the cracking behavior in the TC of APS TBCs penetrated by CMAS during the cooling process. The CMAS penetration process is considered by filling the microstructures with the same shape of CMAS. The results show that CMAS penetration into the microstructures of the TC changed the stress distribution around the microstructures and induced a mixed crack type here. A microstructure with a relatively sharper geometry will experience a more severe stress state when penetrated by CMAS. The material discontinuity due to CMAS penetration also causes a slightly higher stress level around the microstructure at the CMAS deposit/TC interface, the CMAS penetrated layer and TC/BC interface. Thus, the horizontal cracks are easier to initiate from the microstructures with sharper geometry in these three regions.     

Hot corrosion behaviour of barium-strontium aluminosilicates in a molten Na2SO4 environment

The hot corrosion behaviour of barium-strontium aluminosilicates (B_1−xS_xAS) attacked by Na₂SO₄ was investigated in the temperature range from 900 to 1100 °C and the weight change was measured as a function of the corrosion time. The surfaces and cross-sections of the corroded samples were observed by scanning electron microscopy in backscattered electron mode and energy-dispersive X-ray spectroscopy. The phase composition was characterized by X-ray diffraction. The results indicate that the hot corrosion of B_1−xS_xAS by molten Na₂SO₄ was controlled by a diffusion-reaction mechanism. The strontium and/or barium cations diffused out of their aluminosilicate network, and the vacant sites were filled by sodium cations diffusing into the structure to form a NaAlSiO₄ on the top. Due to their smaller radius, the strontium atoms showed a faster diffusion rate than the barium atoms. The corrosion depth significantly increased with the temperature and the strontium concentration in the B_1−xS_xAS.     

Crystallization behavior of CMAS and NaVO3+CMAS mixture and its potential effect to thermal barrier coatings corrosion

Calcium-magnesium-alumina-silicate (CMAS) and molten salt corrosion pose great threats to thermal barrier coatings (TBCs), and recently, a coupling effect of CMAS and molten salt has been found to cause even severer corrosion to TBCs. In this study, the crystallization behavior of CMAS and CMAS+NaVO₃ is investigated for potentially clarifying their corrosion mechanisms to TBCs. Results indicated that at 1000 °C and 1100 °C, CMAS was crystallized to form CaMgSi₂O₆, while at 1200 °C, the crystallization products were CaMgSi₂O₆, CaSiO₃ and CaAl₂Si₂O₈. The introduction of NaVO₃ in CMAS reduced the crystallization ability, and as the NaVO₃ content increased, glass crystallization occurred at a lower temperature, with crystallization products mainly consisting of CaAl₂Si₂O₈ and CaMgSi₂O₆. At 1200 °C, CMAS+10 wt% NaVO₃ was in a molten state without any crystallization, which suggested that NaVO₃ addition in CMAS could reduce its melting point, indicating enhanced penetration ability in TBCs and thus increased corrosiveness.     

Influence of the grain size on CMAS attack of Sm2Zr2O7 ceramic

The temperature resistance of thermal barrier coatings (TBCs) has increased with the continuous development of the aviation industry. This increase in temperature resistance has resulted in a new challenge for TBCs, namely, calcium-magnesium-aluminum-silicate (CMAS) attack. As a new generation of thermal barrier coating candidate materials, Sm₂Zr₂O₇ has good CMAS resistance properties. However, this material cannot meet the actual needs of aero-engines. Therefore, a change in the structure of Sm₂Zr₂O₇ was used to improve the CMAS resistance properties in this paper. The relationship between the grain size of the ceramic and its resistance to CMAS penetration in the microstructure was investigated in detail.Nonpressure and SPS sintering processes were used to prepare Sm₂Zr₂O₇ ceramics with different grain sizes that were then tested at high temperatures with CMAS. With the extension of penetration, the depth of CMAS penetration in microscale Sm₂Zr₂O₇ ceramics increased sharply with increasing reaction time, while the penetration depth of CMAS into nanoscale Sm₂Zr₂O₇ ceramics increased slowly. After 48 h of penetration, the penetration depth of the microscale Sm₂Zr₂O₇ ceramics was 86 μm, and the penetration depth of the nanoscale Sm₂Zr₂O₇ ceramics was only 47 μm. Compared with the microscale Sm₂Zr₂O₇ ceramics, the nanoscale Sm₂Zr₂O₇ ceramics had better CMAS resistance because the lower diffusion activation energy of the nanocrystalline grains accelerated the formation of a dense barrier layer.     

Phase equilibria in the CaO-TaO2.5-YO1.5 system and its implication for YTaO4 reactivity with CMAS

Phase equilibria in the CaO-TaO_2.5-YO_1.5 system were experimentally investigated and the isothermal section at 1400 °C was constructed. Ten three-phase equilibrium fields were determined, and the solid solution regions of the binary compounds were analyzed. The Ca₄Ta₂O₉ in the ternary system is marked as (YO_3/2)_x(Ca_2/3Ta_1/3O_3/2)_1?x. Its maximum solubility is the formula of Ca₂YTaO₆. The solubilities of CaO in the M′-YTaO₄ and fluorite phases reach up to about 2.0 mol% and 8.9 mol%, respectively. A ternary pyrochlore-type phase was found, which was expressed as the chemical formula of Ca_0.5–0.5xY_0.75xTa_0.5–0.25xO_1.75. The morphology of pyrochlore was polygonal, and it was the only reaction product when the YTaO₄ oxides were corroded by the molten silicate (CMAS). Since the CaO is the main reactant, the CaO-TaO_2.5-YO_1.5 phase diagram was used to successfully explain the corrosion behavior of YTaO₄. The current experimental phase diagram is important to understand the CMAS degradation of the thermal barrier coatings.     

航空发动机用热障涂层的CMAS侵蚀及防护

热障涂层作为航空发动机的关键技术,一旦在使用过程中失效将导致严重的后果。然而,热障涂层在使用过程中不可避免地会接触到钙镁铝硅酸盐(CMAS),引发涂层剥落,使高温合金直接暴露在高温燃气中,带来巨大的危险。因此,热障涂层的CMAS侵蚀及防护问题近年来得到了广泛关注。本文在介绍传统氧化钇稳定氧化锆(YSZ)涂层受CMAS侵蚀现状的基础上,明确了CMAS侵蚀YSZ的化学作用过程,阐明了YSZ涂层的失效机制,比较了不同种类CMAS的侵蚀效果,总结了目前热障涂层抵抗CMAS侵蚀的主要方法,并阐述了基于自损型防护原理开展的新型热障涂层材料的CMAS侵蚀行为研究进展,以期为未来航空发动机用热障涂层陶瓷材料的选择和CMAS防护提供有益参考。     

Wetting,infiltration and interaction behavior of CMAS towards columnar YSZ coatings deposited by plasma spray physical vapor

Thermal barrier coatings (TBCs) produced by electron beam physical vapor deposition (EB-PVD) or plasma spray (PS) usually suffer from molten calcium-magnesium-alumino-silicate (CMAS) attack. In this study, columnar structured YSZ coatings were fabricated by plasma spray physical vapor deposition (PS-PVD). The coatings were CMAS-infiltrated at 1250?°C for short terms (1, 5, 30?min). The wetting and spreading dynamics of CMAS melt on the coating surface was in-situ investigated using a heating microscope. The results indicate that the spreading evolution of CMAS melt can be described in terms of two stages with varied time intervals and spreading velocities. Besides, the PS-PVD columnar coating (～100?μm thick) was fully penetrated by CMAS melt within 1?min. After the CMAS attack for 30?min, the original feathered-YSZ grains (tetragonal phase) in both PS-PVD and EB-PVD coatings were replaced by globular shaped monoclinic ZrO₂ grains in the interaction regions.     

BAS, CMAS and CZAS glass coatings deposited by plasma spraying

In this study, three different industrial frits BaO–Al₂O₃–SiO₂ (BAS), CaO–MgO–Al₂O₃–SiO₂ (CMAS), CaO–ZrO₂–Al₂O₃–SiO₂ (CZAS) have been deposited on porcelainized stoneware tiles by plasma spraying. In the as-sprayed conditions, the microstructure of the coatings is defective because of pores, microcracks and low intersplat cohesion. Hot stage microscope and differential thermal analysis measurements made on the glass powders allowed to characterize the frits thermal behaviour. Post process thermal treatments have been arranged, following these indications as well as preliminary tests, in order to achieve the lowest porosity and the highest resistance to abrasion. At the chosen temperatures, a microstructural improvement has been induced, but in the BAS specimens, an optimal sintering has not been accomplished because of the unavoidable full overlapping of the sintering and crystallization processes.     

CMAS and ST3GAL4 Play an Important Role in the Adsorption of Influenza Virus by Affecting the Synthesis of Sialic Acid Receptors

Influenza A viruses (IAVs) initiate infection by attaching Hemagglutinin (HA) on the viral envelope to sialic acid (SA) receptors on the cell surface. Importantly, HA of human IAVs has a higher affinity for α-2,6-linked SA receptors, and avian strains prefer α-2,3-linked SA receptors, whereas swine strains have a strong affinity for both SA receptors. Host gene CMAS and ST3GAL4 were found to be essential for IAV attachment and entry. Loss of CMAS and ST3GAL4 hindered the synthesis of sialic acid receptors, which in turn prevented the adsorption of IAV. Further, the knockout of CMAS had an effect on the adsorption of swine, avian and human IAVs. However, ST3GAL4 knockout prevented the adsorption of swine and avian IAV and the impact on avian IAV was more distinct, whereas it had no effect on the adsorption of human IAV. Collectively, our findings demonstrate that knocking out CMAS and ST3GAL4 negatively regulated IAV replication by inhibiting the synthesis of SA receptors, which also provides new insights into the production of gene-edited animals in the future.