Microstructure evolution of CMAS glass below melting temperature and its potential influence on thermal barrier coatings

CaO–MgO–Al₂O₃–SiO₂ (CMAS) deposition significantly degrades the performance of thermal barrier coatings (TBCs). In this study, the microstructure evolution of CMAS glass at temperatures below its melting point was investigated in order to study the potential influence of temperature on the applicability of CMAS glass in TBCs. The CMAS glass fabricated in this study had a melting point of 1240 °C, became opaque, and underwent self-crystallization when the temperature reached 1000 °C. After heat treatment at 1050 °C, diopside and anorthite phases precipitated from the glass; at a higher temperature (1150 °C), diopside, anorthite, and wollastonite were formed as the self-crystallization products. An increase in the dwelling time resulted in the transformation of diopside to wollastonite and anorthite. At 1250 °C, all products formed a eutectic microstructure and melted. The results indicate that even at low temperatures, CMAS glass underwent microstructure evolution, which could influence the coating surface and stress distribution when deposited on TBCs.     

CMAS corrosion resistance in high temperature and rainwater environment of double-layer thermal barrier coatings odified by rare earth

In order to explore the difference of CMAS corrosion resistance in high temperature and rainwater environment of single-layer and double-layer thermal barrier coatings (TBCs), and further reveal the mechanism of CMAS corrosion resistance in above environment of double-layer TBCs modified by rare earth, two TBCs were prepared by air plasma spraying, whose ceramic coating were single-layer ZrO₂–Y₂O₃ (YSZ) and double-layer La₂Zr₂O₇(LZ)/YSZ, respectively. Subsequently, CMAS corrosion resistance tests at 1200 °C and rainwater environment of two TBCs were carried out. Results demonstrate that after high temperature CMAS corrosion for the same time, due to phase transformation, the volume of YSZ ceramic coating in single-layer TBCs shrank and surface cracks formed, which would lead to coating failure. When LZ ceramic coating of double-layer TBCs reacted with CMAS, compact apatite phases and fluorite phases formed, the penetration of CMAS into ceramic coating was inhibited effectively. Raman analysis and calculation results show that both of the surface residual stress of ceramic coating in two TBCs were compressive stress, and the residual stress of ceramic coating in double-layer TBCs were smaller than that of single-layer TBCs. Atomic force microscopy of TBCs after CMAS corrosion show that surface of double-layer TBCs was more uniform and compact than that of single-layer TBCs. The electrochemical properties in simulated rainwater of two TBCs after high temperature CMAS corrosion showed that double-layer TBCs possessed higher free corrosion potential, lower corrosion current and higher polarization resistance than those of single-layer TBCs. Consequently, the presence of LZ ceramic coating effectively improved CMAS corrosion resistance in high temperature and rainwater environment of double-layer TBCs.     

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。     

The effect of TiO2 additions on CaO–MgO–Al2O3–SiO2 (CMAS) crystallization behavior from the melt

Titania (TiO₂) was introduced into a model calcium-magnesium aluminosilicate (CMAS) glass in additions of 5-20 wt%. The crystallization behavior of the mixtures was characterized over a series of temperature profiles and compared to that of CMAS alone. X-ray diffraction, differential scanning calorimetry, light and scanning electron microscopy, and energy dispersive spectroscopy were used to characterize glass and crystalline products. Titania additions in the amount of approximately 12.5-20 wt% aided in the formation of CaTiO₃ from melts equilibrated at either 1300 or 1500°C and cooled at 10°C/min. Holding CMAS + TiO₂ (TiO₂ ≥ 10 wt%) at 900°C after cooling from 1300/1500°C resulted in the formation of additional crystalline phases including melilite, paqueite, and diopside. Implications for CMAS interactions with thermal and environmental barrier coatings are discussed.     

LaMeAl11O19(Me=Cu,Zn)陶瓷体材料的抗CMAS性能研究

随着燃气涡轮机的应用温度不断提升,陶瓷材料的抗CaO-MgO-Al₂O₃-SiO₂(CMAS)性能越来越重要。通过X射线衍射(XRD)、扫描电镜(SEM)等测试方法,研究了LaMeAl₁₁O₁₉(Me=Cu,Zn)陶瓷体材料在不同温度和时间条件下的抗CMAS腐蚀行为。结果表明,LaZnAl₁₁O₁₉(LZA)和LaCuAl₁₁O₁₉(LCA)体材料的腐蚀产物都包括透辉石(Ca(Mg,Al)(Si,Al)O₇)和钙长石(CaAl₂Si₂O₈)。随着腐蚀温度的提高和时间的延长,腐蚀深度增加,Ca(Mg,Al)(Si,Al)O₇逐渐转变为CaAl₂Si₂O₈。LZA和LCA体材料的CMAS腐蚀可以用“溶解-析出”机制解释。体材料逐渐溶解到CMAS中,形成Ca(Mg,Al)(Si,Al)O₇,进而逐渐转变为CaAl₂Si₂O₈,使难以结晶的透辉石相转变为易结晶的钙长石相。La原子为析晶的晶核,CMAS玻璃相与体材料之间存在界面能,这些因素共同促进了CaAl₂Si₂O₈在CMAS内部以及两者的界面处析出厚板状晶体。     

Characterization of stress in a thermal barrier coating during CMAS corrosion using Ce3+ photoluminescence spectroscopy

The stress caused by calcium–magnesium–alumino–silicate (CMAS) corrosion is a critical factor in thermal barrier failure of thermal barrier coatings (TBCs). For the service safety of TBCs, it is important to characterize the stress inside TBCs during CMAS corrosion using a nondestructive and accurate method. In this study, photoluminescence spectroscopy technology was applied to characterize the stress in TBCs during CMAS corrosion. First, TBC specimens containing yttrium–aluminum–garnet doped with trace Ce³⁺ ions (YAG:Ce³⁺)/yttrium oxide partially stabilized zirconia double-ceramic-layer were prepared by atmospheric plasma spraying. Then, CMAS corrosion experiments were performed using the TBC specimens, and a mechanical model was derived based on Ce³⁺ photoluminescence spectroscopy to investigate the stress in the TBCs. Finally, the microstructure, extent of CMAS corrosion and stress field in TBC specimens, was characterized. The results reveal that the penetration of CMAS leads to local stress concentration and a nonlinear stress distribution from the outside surface to the inside of the YAG:Ce³⁺ layer. In addition, an increase in corrosion time, temperature, and CMAS concentration can significantly influence the evolution of the stress field in TBCs.     

Preparation of nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings and their calcium-magnesium-alumina-silicate (CMAS) resistance

Nanostructured 30 mol% LaPO₄ doped Gd₂Zr₂O₇ (Gd₂Zr₂O₇-LaPO₄) thermal barrier coatings (TBCs) were produced by air plasma spraying (APS). The coatings consist of Gd₂Zr₂O₇ and LaPO₄ phases, with desirable chemical composition and obvious nanozones embedded in the coating microstructure. Calcium-magnesium-alumina- silicate (CMAS) corrosion tests were carried out at 1250 °C for 1–8 h to study the corrosion resistance of the coatings. Results indicated that the nanostructured Gd₂Zr₂O₇-LaPO₄ TBCs reveals high resistance to penetration by the CMAS melt. During corrosion tests, an impervious crystalline reaction layer consisting of Gd-La-P apatite, anorthite, spinel and tetragonal ZrO₂ phases forms on the coating surfaces. The layer is stable at high temperatures and has significant effect on preventing further infiltration of the molten CMAS into the coatings. Furthermore, the porous nanozones could gather the penetrated molten CMAS like as an absorbent, which benefits the CMAS resistance of the coatings.     

Melting and Crystallization of Silicate Systems Relevant to Thermal Barrier Coating Damage

The melting and crystallization behaviors of model calcium–magnesium–alumino‐silicate (CMAS) compositions relevant to the degradation of thermal barrier coatings (TBCs) were investigated. A primary goal was to establish a baseline for studies on CMAS reactions with TBC materials, reported separately, and their potential to mitigate degradation. Ternary calcium alumino‐silicate (CAS) compositions investigated melt below their equilibrium solidus owing to their metastable phase constitution. Additions of MgO or FeO_x have significant effects on the melting behavior, depending on the C:A:S proportions. Amorphization on cooling is commonplace, with MgO, AlO_1.5, and especially FeO_x promoting crystallization. The behaviors of amorphous and crystalline versions of the same CMAS are different and depend on heating/cooling rates, with attendant implications for their interaction with TBCs.     

Solid‐State CMAS Corrosion of an EB‐PVD YSZ Coated Turbine Blade: Zr4+ Partitioning and Phase Evolution

The corrosion of an ex‐service 7‐YSZ coated high‐pressure turbine blade by Si‐undersaturated, Fe‐,Ti‐rich CMAS (FT‐CMAS) was investigated. The constituents of the FT‐CMAS deposit and its synthetic laboratory counterpart are identified by means of analytical SEM and XRD. FT‐CMAS mainly consists of rhönite‐, melilite‐, and clinopyroxene‐type solid solutions. FT‐CMAS reacts with the 7‐YSZ thermal barrier coating to a continuous layer of a Ca,Zr‐rich garnet phase also known as kimzeyite. The microstructural analysis indicates that kimzeyite has formed via solid‐state reaction. The formation of kimzeyite was studied in laboratory using YSZ/FT‐CMAS diffusion couples and powder compacts. Experiments provide strong evidence that kimzeyite is forming predominantly from YSZ and melilite, whereas clinopyroxene and especially rhönite appear relative stable versus YSZ. The preferred kimzeyite formation from melilite and YSZ is explained by the availability of sixfold coordinated lattice sites for Zr⁴⁺ on melilite decomposition and subsequent garnet crystallization.     

Graceful behavior during CMAS corrosion of a high-entropy rare-earth zirconate for thermal barrier coating material

The corrosion resistance to calcium-magnesium-alumino-silicates (CMAS) is critically important for the thermal barrier coatings (TBCs). High-entropy zirconate (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ (HEZ) ceramics with low thermal conductivity, high coefficient of thermal expansion and good durability to thermal shock is expected to be a good candidate for the next-generation TBCs. In this work, the CMAS corrosion of HEZ at 1300°C was firstly investigated and compared with the well-studied La₂Zr₂O₇ (LZ). It is found that the HEZ ceramics showed a graceful behavior to CMAS corrosion, obviously much better than the LZ ceramics. The HEZ suffered from CMAS corrosion only through dissolution and re-precipitation, while additional grain boundary corrosion existed in the LZ system. The precipitated high-entropy apatite showed fine-grained structure, resulting in a reaction layer without cracks. This study reveals that HEZ is a promising candidate for TBCs with extreme resistance to CMAS corrosion.     

CaO-MgO-Al2O3-SiO2 corrosion behavior of air-plasma-sprayed (LaxYb1−x)2Zr2O7

The CaO-MgO-Al₂O₃-SiO₂ (CMAS) corrosion of thermal barrier coatings (TBCs) is a crucial problem for the lifetime of blades and vanes of jet engine and gas turbine at high operating temperature. Although many new alternative materials for TBCs have been developed in recent years, their application is limited by the CMAS corrosion. On the other hand, the composition difference of CMAS between regions makes solving this problem very difficult. Therefore, in this study, the yearly composition changes of sand-dust around Beijing area were investigated. The high-temperature corrosion behavior of air-plasma-sprayed 8YSZ and newly developed (La_xYb_1−x)₂Zr₂O₇ TBCs by the representative sand-dust of Beijing was investigated. In comparison, a universally used CaO-riched composition of simulated silicate deposit was also adopted for the TBCs corrosion test. It is found that the (La_xYb_1−x)₂Zr₂O₇ TBCs performs much better anti-corrosion behavior than that of 8YSZ, both by Beijing sand-dust and simulated one. Particularly, Yb₂Zr₂O₇ TBCs appear to be the optimum material against silicate deposits attack. The mechanism of silicate deposits corrosion has also been discussed.     

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.     

Erosion behavior of EB-PVD 7YSZ coatings under corrosion/erosion regime: Effect of TBC microstructure and the CMAS chemistry

Aero-engines operating in dust-laden environments often encounter a lot of dust/sand that causes a severe problem to the TBCs by means of erosion. As the turbine entry temperatures are rising, molten sand is also a big concern to the life-time of TBCs.This paper deals with the TBC behavior under the combined influence of erosion and corrosion attack. Variations in TBC morphology, CMAS infiltration time and CMAS composition and their influence on the erosion resistance at room temperature were investigated. Two different EB-PVD 7YSZ morphologies consisting of a different porosity arrangement were tested in the erosion/corrosion regime. The more ‘Feathery’ structure has a better resistance to erosion compared to a more columnar ‘Normal’ structure, which leads to less degradation of the TBC. However, under the influence of CMAS infiltration the effect was found to be reversed. In general, CMAS-infiltrated EB-PVD TBCs exhibit a higher erosion resistance than the non-infiltrated ones.     

High-temperature interactions between Yttria-stabilized zirconia thermal barrier coatings and Na-Rich calcia-magnesia-aluminosilicate deposits

We have evaluated the effectiveness of optical basicity, a chemical model, to predict and categorize the reaction behavior between calcia-magnesia-aluminosilicate (CMAS) deposits and ZrO₂-based thermal barrier coatings (TBCs), which are used to insulate and protect metallic components in gas-turbine engines. The attack behavior of two CMAS compositions (Na-lean and Na-rich) with 7 wt% Y₂O₃-partially-stabilized ZrO₂ (7YSZ) and 2ZrO₂·Y₂O₃(ss) free-standing TBCs at 1340 °C were evaluated and compared to previous studies. The behavior of Y³⁺ in the reaction is correlated with the optical basicity of the CMAS; more basic Na-rich CMAS melt resulted in lower Y-solubility and higher Y-content in the reprecipitated ZrO₂ grains than observed in the highly acidic CMAS attack for both tested TBCs. This behavior is consistent with previous studies of basic and acidic melts, and suggests that less acidic CMASs pose a unique danger to ZrO₂-based TBCs that rely on Y-rich secondary phases for CMAS mitigation.     

Corrosion of the crystalline phases of matte glazes in aqueous solutions

Chemical durability was determined for fast and conventionally fired matte raw glazes in acidic to alkaline aqueous solutions. According to XRD, the main crystalline phases in the glazes were diopside, plagioclase, anorthite, wollastonite, and pseudowollastonite. SEM-EDXA and whitelight confocal microscopy were used to analyze the surfaces after immersion in the aqueous solutions. The ionic concentrations in selected solutions after immersion were determined by ICP-AES. Diopside was not attacked by the test solutions. Plagioclase started to corrode along the crystal interfaces in the most acidic environments. Wollastonite crystals with different crystal habitus and slightly different chemical composition formed depending on firing cycle and composition of the glaze. In fast firing, tiny columnar wollastonite crystals were formed. These crystals were attacked in acidic to slightly alkaline environments. In conventionally fired glazes the wollastonite crystals were dendritic. These crystals were attacked only by acidic solutions. Pseudowollastonite with poor chemical resistance formed only in magnesia-free glazes.     

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.     

Plasma sprayed nanostructured GdPO4 thermal barrier coatings: Preparation microstructure and CMAS corrosion resistance

Nanostructured GdPO₄ thermal barrier coatings (TBCs) were prepared by air plasma spraying, and their phase structure evolution and microstructure variation due to calcium–magnesium–alumina–silicate (CMAS) attack have been investigated. The chemical composition of the coating is close to that of the agglomerated particles used for thermal spraying. Nanozones with porous structure are embedded in the coating microstructure, with a percentage of ~30%. CMAS corrosion tests indicated that nanostructured GdPO₄ coating is highly resistant to penetration by molten CMAS at 1250°C. Within 1 hour heat treatment duration, a continuous dense reaction layer forms on the coating surface, which are composed of P–Si apatite based on Ca_2+xGd_8?x(PO₄)_x(SiO₄)_6?xO₂, anorthite and spinel phases. This layer provides effective prevention against CMAS further infiltration into the coating. Prolonged heat treatment densifies the reaction layer but does not change its phase composition.     

The corrosion behaviors of thermal barrier material of M-YTaO4 attacked by CMAS at 1250 °C

The corrosion of YSZ TBCs attacked by calcium–magnesium–aluminosilicate (CMAS) is a serious problem. Yttrium tantalite (YTaO₄), a new kind of potential thermal barrier ceramic material, was expected to replace the YSZ to manufacture the TBCs because of its great thermophysical characteristics. In this study, porous YTaO₄ ceramic pellets, instead of actual TBCs, were used to investigate the CMAS corrosion resistance at 1250 °C. Results indicated that CMAS couldn't cover the whole surface of YTaO₄ pellets homogeneously because of low wettability between liquid CMAS and YTaO₄, in addition, there was almost no reaction layer after 4 h reaction. The XRD results showed that M-YTaO₄, M′-YTaO₄, Ca₂Ta₂O₇ and Y₂Si₂O₇ were the main four phases after reaction and there was no phase containing the elements of Mg and Al. Compared with YSZ TBCs, this new kind of potential thermal barrier ceramic material showed well resistance to CMAS corrosion.     

Firing transformations of Chilean clays for the manufacture of ceramic tile bodies

This contribution is focused on the study of the mineralogical changes occurring in the ceramic body after heating ceramic clays. Chile has an important local ceramic industry. Five deposits of clays with industrial applications were studied. The clays came from San Vicente de Tagua-Tagua (SVTT), Litueche (L), Las Compañías-Río Elqui (LC), La Herradura-Coquimbo (LH) and Monte Patria-Coquimbo (MP). The samples were heated to 830, 975, 1080 and 1160 °C keeping at the maximum temperature for 35 min. The bending strength of each ceramic body was determined at 1100 °C. Mineralogical analysis of the fired samples was carried out by X-ray diffraction. The SVTT contained quartz, spinel, cristobalite, microcline, albite, anorthite, hematite and enstatite; the LC clays quartz, mullite, spinel, microcline, albite, anorthite, hematite, diopside, enstatite, illite/muscovite and talc; the LH clays quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, illite and augite; the MP clays quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, gehlenite, enstatite and wollastonite and the L clays quartz, microcline and mullite. The persistence of illite at at least 900 °C was observed for LC and LH. SVTT and LH showed the required specifications for earthenware. The L clays were refractory clays with very low bending strength.     

Production of ceramic pigments with diopside and anorthite structure using the gel method

The possibility of producing ceramic pigments with the diopside and anorthite structures by the sol-gel method using wollastonite is studied. It is established that the use of the gel method intensifies the synthesis of anorthite and diopside crystal structures by means of better homogenization of the batch components at the mixing stage. A positive factor is the effective formation of anorthite and diopside structures at relatively low temperatures (about 1100°C), as well as the mineralizing effect of the chromophores on the crystallization of these minerals. __________ Translated from Steklo i Keramika, No. 8, pp. 26–28, August, 2006.