Transient thermal stress due to the penetration of calcium-magnesium-alumino-silicate in EB-PVD thermal barrier coating system

Electron beam-physical vapor deposited (EB-PVD) thermal barrier coating system (TBCs) are vulnerable to the degradations induced by the penetration of calcium- magnesium-alumino-silicate (CMAS). In this work, we conduct a numerical study to investigate the effect of CMAS penetration on the development of transient thermal stress in EB-PVD TBCs with the columnar microstructure. A two-dimensional periodical model is developed, taking into account the columnar microstructure of EB-PVD TBCs and the CMAS penetration. We found that the CMAS penetration would induce a field of high in-plane tensile stress in TC upon the rapid cooling, promoting the initiation of the vertical cracks from top surface toward to the bottom of TC. Meanwhile, the accumulation of out-of-plane tensile stress tends to occur at the side edges of EB-PVD columns near three main regions: closely beneath the top surface of TC, at the interface between CMAS penetrated and non-penetrated zone, and close to the TC/BC interface. Therefore, the horizontal cracks are likely to initiate from the side edges of EB-PVD columns at these three regions, which agrees well with experiments.     

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.     

Thermal insulation of CMAS (Calcium-Magnesium-Alumino-Silicates)- attacked plasma-sprayed thermal barrier coatings

The impact of the penetration of small quantities of calcium-magnesium-alumino- silicates (CMAS) glassy melt in the porous plasma-sprayed (PS) thermal barrier coatings (TBCs) is often neglected even though it might play a non-negligible role on the sintering and hence on the thermal insulation potential of TBCs. In this study, the sintering potential of small CMAS deposits (from 0.25–3 mg.cm⁻²) on freestanding yttria-stabilized zirconia (YSZ) PS TBCs annealed at 1250 °C for 1 h was investigated. The results showed a gradual in-depth sintering with increasing CMAS deposits. This sintering was concomitant with local transformations of the tetragonal YSZ and resulted in an increase in the thermal diffusivity of the coatings that reached a maximum of ∼110 % for the fully penetrated coating.     

Lanthanum magnesium hexaluminate thermal barrier coatings with pre-implanted vertical microcracks: Thermal cycling lifetime and CMAS corrosion behaviour

Atmospheric plasma-sprayed (APS) coatings have a layered structure as well as lower strain tolerance and a shorter lifetime than EB-PVD coatings. In this study, TBCs composed of a LaMgAl₁₁O₁₉ (LMA) top coat and a NiCrAlY bond coat were prepared by APS coupled with dry-ice blasting to implant vertical microcracks in the top coat. The thermal cycling lifetime and CMAS corrosion behaviour of LMA-TBCs with pre-implanted vertical microcracks were investigated in detail. The results show that the LMA top coat possesses an improved proportion of vertical microcracks and that the corresponding TBC has an improved thermal cycling lifetime. The vertical microcracks in the top coats, which not only reduce the thermal stress but also improve the strain tolerance of TBCs, dramatically contribute to the improvement in the thermal cycling lifetime. Surprisingly, the CMAS corrosion resistance of LMA-type TBCs with implanted vertical microcracks is better than that of conventional TBCs with a typical layered structure.     

Effects of CMAS penetration on the delamination cracks in EB-PVD thermal barrier coatings with curved interface

During the high-temperature operation of electron beam physical vapor deposited (EB-PVD) thermal barrier coating (TBC), the penetration of environmental calcium-magnesium-alumina-silicate (CMAS) compositions into the ceramic top-coat would affect the growth of delamination cracks. In this work, the effects of CMAS penetration on the delamination cracks in EB-PVD TBC with curved interface are investigated by finite element analysis. In the numerical model, the curved interface evolves as the cyclic displacement instability of the thermally grown oxide (TGO) layer. The penetration of CMAS into the columnar gaps of EB-PVD TBC mainly increases the in-plane modulus of TC layer. It is demonstrated that, with the increase of in-plane modulus in an intact TC, the level of tensile stress, which mainly occurs in the region above the curved interface and responsible for initiating the delamination cracks, presents a decrease; meanwhile, the level of shear stress, which mainly occurs in the region at the periphery of the curved zone to drive the delamination crack when it propagates into this region, presents a increase. Furthermore, the calculation of the strain energy release rate shows that, for the crack located above the curved interface, the increase of in-plane modulus in TC layer can prevent the accumulation of strain energy release rate, and therefore make it more difficult for delamination initiation. However, once the crack propagates into the flat periphery, CMAS penetration would begin to enhance its growth.     

Influence of interface morphology on erosion failure of thermal barrier coatings

The thermal barrier coating system (TBCs) has complex structure and works in severe service environment. Erosion is one of the main factors causing the failure of TBCs. In the present study, the particle erosion process of atmospheric plasma sprayed (APS) thermal barrier coatings at elevated temperature was simulated by the finite element method. The effects of interface morphology on the penetration depth, particle ricochet velocity and interface stress state were studied, and the key parameters such as particle size, initial velocity and erosion position were also considered. The cosine curve with constant wavelength and varying amplitude was used to represent different interface roughness of TBCs. The results show that the interface morphology has little effect on the penetration depth of top coat (TC) and the particle ricochet velocity. The influence of particle erosion position related to the interface morphology is obvious. Basically, the greater the interface roughness is, the more violent the interfacial stress fluctuation is. During the erosion process, the stress in the middle of the interface is significantly higher than that at other positions. These results facilitate understanding of the particle erosion failure mechanism of APS TBCs. The influence of interface morphology should be considered in erosion research.     

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.     

Corrosion behavior of air plasma spraying zirconia-based thermal barrier coatings subject to Calcium–Magnesium–Aluminum-Silicate (CMAS) via burner rig test

Three different kinds of thermal barrier coatings (TBCs) — 8YSZ, 38YSZ and a dual-layered (DL) TBCs with pure Y₂O₃ on the top of 8YSZ were produced on nickel-based superalloy substrate by air plasma spraying (APS). The Calcium–Magnesium–Aluminum-Silicate (CMAS) corrosion resistance of these three kinds of coatings were researched via burner rig test at 1350 °C for different durations. The microstructures and phase compositions of the coatings were characterized by SEM, EDS and XRD. With the increase of Y content, TBCs exhibit better performance against CMAS corrosion. The corrosion resistance against CMAS of different TBCs in descending was 8YSZ + Y₂O₃, 38YSZ and 8YSZ, respectively. YSZ diffused from TBCs into the CMAS, and formed Y-lean ZrO₂ in TBCs because of the higher diffusion rate and solubility of Y³⁺ in CMAS than Zr⁴⁺. At the same time, 38YSZ/8YSZ + Y₂O₃ reacts with CAMS to form Ca₄Y₆(SiO₄)₆O/Y_4·67(SiO₄)₃O with dense structure, which can prevent further infiltration of CMAS. The failure of 8YSZ coatings occurred at the interface between the ceramic coating and the thermally grown oxide scale (TGO)/bond coating. During the burner rig test, the Y₂O₃ layer of the DL TBCs peeled off progressively and the 8YSZ layer exposed gradually. DL coatings keep roughly intact and did not meet the failure criteria after 3 h test. 38YSZ coating was partially ablated, the overall thickness of the coating is thinned simultaneously after 2 h. Therefore, 8YSZ + Y₂O₃ dual-layered coating is expected to be a CMAS corrosion-resistant TBC with practical properties.     

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.     

CMAS corrosion resistance,thermal shock resistance and numerical simulation of novel surface micromesh thermal barrier coatings

Thermal barrier coatings (TBCs) are widely used as insulating layers to protect the underlying metallic structure of gas turbine blades. However, the thermal cycling performance of TBCs is affected by their complex working environments, which may shorten their service life. Previous studies have shown that preparing a mesh structure in the bonding layer can relieve thermal stress and improve the bonding strength, thereby prolonging the service life of TBCs. In this paper, a micromesh structure was prepared on the surface of the bonding layer via wet etching. The microstructure and failure mechanism of the micromesh TBCs after CMAS (CaO-MgO-Al₂O₃-SiO₂) thermal erosion were investigated. Numerical simulation was combined with thermal shock experiments to study the stress distribution of the micromesh-structured TBCs. The results showed that the circular convex structure can effectively improve the CMAS corrosion resistance and thermal shock resistance of TBCs.     

Evolution of porosity,crack density,and CMAS penetration in thermal barrier coatings subjected to burner rig testing

Degradation of thermal barrier coatings (TBCs) in gas-turbine engines due to calcium–magnesium–aluminosilicate (CMAS) glassy deposits from various sources has been a persistent issue since many years. In this study, state of the art electron microscopy was correlated with X-ray refraction techniques to elucidate the intrusion of CMAS into the porous structure of atmospheric plasma sprayed (APS) TBCs and the formation and growth of cracks under thermal cycling in a burner rig. Results indicate that the sparse nature of the infiltration as well as kinetics in the burner rig are majorly influenced by the wetting behavior of the CMAS. Despite the obvious attack of CMAS on grain boundaries, the interaction of yttria-stabilized zirconia (YSZ) with intruded CMAS has no immediate impact on structure and density of internal surfaces. At a later stage the formation of horizontal cracks is observed in a wider zone of the TBC layer.     

Molten silicate interactions with plasma sprayed thermal barrier coatings: Role of materials and microstructure

Ingestion of siliceous particulate debris into both propulsion and energy turbines has introduced significant challenges in harnessing the benefits of enhanced operation efficiencies through the use of higher temperatures and thermal barrier coatings (TBCs). The so-called CMAS (for calcium-magnesium alumino-silicate) particles can melt in the gas path at temperatures greater than 1200C, where they will subsequently impact the coating surface and infiltrate through the carefully engineered porosity or cracks in a TBC. Ultimately, this CMAS attack causes premature spallation through its solidification and stiffening the ceramic during cooling. It has been noted in recent years, that TBCs based on yttria stabilized zirconia (YSZ) are completely non-resistant to CMAS attack due to their lack of reactivity with infiltrant liquid. New TBC ceramics such as Gadolinium Zirconate (GZO) show promise of CMAS resistance through rapid reaction-induced crystallization and solidification of the infiltrant, leading to its arrested infiltration. In both situations, the microstructure (porosity, micro and macro cracks) can be important differentiators in terms of the infiltration and subsequent failure mechanisms. This paper seeks to examine the interplay among microstructure, material, and CMAS attack in different scenarios. To do so, different types of YSZ & GZO single and multilayer coatings were fabricated using Air Plasma Spray (APS) and exposed to CMAS through isothermal and gradient mechanisms. In each of the cases, beyond their unique interactions with CMAS, it was observed the inherent microstructure and character of the porosity of the coating will have an additional role on the movement of the melt. For instance, vertical cracks can provide pathways for accelerated capillaric flow of the melt into both YSZ and GZO coatings. Based on these observations multilayer coatings have been proposed and realized toward potentially reducing complete coating failure and supporting multiple CMAS attack scenarios.     

Fast stress evaluation of the top coat of thermal barrier coatings under CaO–MgO–Al2O3–SiO2 penetration based on image recognition and an artificial neural network

An image recognition and artificial neural network (ANN)-based method for fast stress evaluation of thermal barrier coatings (TBCs) is proposed. Random two-phase images of ceramic top coats (TCs) with penetrated CaO–MgO–Al₂O₃–SiO₂ (CMAS) are established, and the TC–CMAS image boundaries are recognized. Image-restoration finite element models are constructed based on boundaries optimized using the average coordinate method. The isothermal thermal stress in the restoration models is calculated. The structural parameters and local maximum stress are extracted to establish sample databases on which the ANN is trained. The thermal stress states in a new group of samples are predicted by the trained ANN. The root-mean-square error between the stress calculated by the new stress evaluation method and those of results calculated using the finite element method is approximately 6%. The new method also has a higher calculation efficiency and its stress evaluation speed is accelerated by a factor of approximately 77 compared with that of the finite element method.     

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.     

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.     

Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are used to protect hot-components in aero-engines from hot gases. In this paper, the microstructure and thermo-physical and mechanical properties of plasma sprayed YSZ coatings under the condition of calcium-magnesium-alumina-silicate (CMAS) deposits were investigated. Si and Ca in the CMAS rapidly penetrated the coating at 1250 °C and accelerated sintering of the coating. At the interface between the CMAS and YSZ coating, the YSZ coating was partially dissolved in the CMAS, inducing the phase transformation from tetragonal phase to monoclinic phase. Also, the porosity of the coating was reduced from ∼25% to 5%. As a result, the thermal diffusivity at 1200 °C increased from 0.3 mm²/s to 0.7 mm²/s, suggesting a significant degradation in the thermal barrier effect. Also, the coating showed a ∼40% increase in the microhardness. The degradation mechanism of TBC induced by CMAS was discussed.     

Self-crystallization characteristics of calcium-magnesium-alumina- silicate (CMAS) glass under simulated conditions for thermal barrier coating applications

Understanding self-crystallization characteristics of calcium-magnesium-alumina- silicate (CMAS) glass is of great significance for seeking for solution to its corrosion to thermal barrier coatings (TBCs). Here, we design a series of experiments to investigate the relationship between CMAS self-crystallization behavior and cooling/heating rates and dwell temperature, and emphasize the potential influence of self-crystallization on CMAS corrosion behavior to TBCs. With the cooling rate decreasing, crystalline phases formed in a sequence of diopside, wollastonite and anorthite, and the thickness of the crystalline layer increased. During the heating process, diopside and melilite phases formed when the temperature was lower than 1050 °C; while at higher temperatures, melilite transformed to anorthite and wollastonite, independent on the heating rate. Although self-crystallization can slow molten CMAS penetration, the function on protecting TBCs from damage is limited, and other strategies alleviating CMAS corrosion are necessitated to be developed.     

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.     

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。     

Heat-treated lanthanum magnesium hexaaluminate coatings exposed to molten calcium-magnesium-alumino-silicate

LaMgAl₁₁O₁₉ (LMA) coatings usually have a large amount of amorphous phase due to the rapid cooling during spraying, which will not benefit the service lifetime in aeroengine. Fortunately, the thermal cycling lifetime of LMA TBCs can be improved by heat treatment. Moreover, the hot corrosion degradation with calcium-magnesium-alumino-silicate (CMAS) attracts much attention recently. Therefore, to clarify the influence of heat treatment on CMAS corrosion behaviour, plasma-sprayed LMA coatings were isothermally heat-treated and then exposed to CMAS. Results indicate that heat treatment promoted a crystallization of LMA coatings, but the molten CMAS had completely penetrated into LMA coatings along the increased and widened vertical microcracks. It is the isotropic pores that determine the penetration kinetics of CMAS to LMA. CaAl₂Si₂O₈ and MgAl₂O₄ were the predominant reaction products in the superficial layer, while Ca₃La₆(SiO₄)₆ interpenetrated with the residual LMA in the inner layer, forming a corroded bilayer for LMA coatings.