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.     

Tailoring the EB-PVD columnar microstructure to mitigate the infiltration of CMAS in 7YSZ thermal barrier coatings

The CMAS associated degradation of 7YSZ TBC layers is one of the serious problems in the aero engines that operate in dusty environments. CMAS infiltrates into TBC at high temperatures and stiffens the TBC which ultimately loses its strain tolerance and gets delaminated. The EB-PVD technique is used to coat TBCs exhibiting a columnar microstructure on parts such as blades and on vanes. By varying the EB-PVD process parameters, columnar morphology and porosity of the 7YSZ coating is changed and its effect on the CMAS infiltration behaviour is studied in detail. Two different TBC pore geometries were created and infiltration experiments were carried out at 1250 °C and 1225 °C for different time intervals. The 7YSZ coating with more ‘feathery’ features has resulted in higher CMAS resistance by at least by a factor of 2 than its less ‘feathery’ counterpart. These results are explained on the basis of a proposed physical model.     

Microstructural characterization of GZ/CYSZ thermal barrier coatings after thermal shock and CMAS+hot corrosion test

In this study, first, Gd₂Zr₂O₇/ceria–yttria stabilized zirconia (GZ/CYSZ) TBCs having multilayered and functionally graded designs were subjected to thermal shock (TS) test. The GZ/CYSZ functionally graded coatings displayed better thermal shock resistance than multilayered and single layered Gd₂Zr₂O₇ coatings. Second, single layered YSZ and functionally graded eight layered GZ/CYSZ coating (FG8) having superior TS life time were selected for CMAS + hot corrosion test. CMAS + hot corrosion tests were carried out in the same experiment at once. Furthermore, to generate a thermal gradient, specimens were cooled from the back surface of the substrate while heating from the top surface of the TBC by a CO₂ laser beam. Microstructural characterizations showed that the reaction products were penetrated locally inside of the YSZ. On the other hand, a reaction layer having ∼6 μm thickness between CMAS and Gd₂Zr₂O₇ was seen. This reaction layer inhibited to further penetration of the reaction products inside of the FG8.     

Interaction of CMAS on thermal sprayed ytterbium disilicate environmental barrier coatings: A story of porosity

Molten calcium magnesium alumina-silicates (CMAS) represent a challenge for the current generation of rare earth silicates environmental barrier coatings (EBCs). Their interaction with ytterbium disilicate (Yb₂Si₂O₇) free-standing coatings deposited using thermal spraying technique has been studied to further understand the reaction mechanisms. Three coatings, deposited with different porosity levels and thickness, representing traditional EBCs (<3% porosity and ～350 μm thickness) and abradable coatings (～20% porosity and 500–1000 μm thickness) were exposed to CMAS at 1350 °C. The results show that higher porosity levels facilitates CMAS infiltration in the first hour of exposure, in combination with infiltration through the inter-splat boundaries. Preferential dissolution of ytterbium monosilicate (Yb₂SiO₅) takes place, forming a 10–15 μm Ca₂Yb₈(SiO₄)₆O₂ apatite layer as the reaction product, producing a network of fine porosity (<10 μm) as the inter-splat boundary material is consumed. After exposure for 48 h, CMAS has completely infiltrated all three coatings, with apatite crystals present across the coatings, up to a depth of ～550 μm. Despite the extensive CMAS infiltration and apatite formation, no damage could be observed in any of the coatings, providing a promising first step for environmental barrier abradable coatings.     

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.     

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.     

CMAS corrosion of YSZ thermal barrier coatings obtained by different thermal spray processes

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al₂O₃-SiO₂ (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.     

Enhanced properties of Al-modified EB-PVD 7YSZ thermal barrier coatings

7 wt% yttria-stabilized zirconia (7YSZ) thermal barrier coating (TBC) prepared by electron beam-physical vapor deposition (EB-PVD) has been used in gas turbine engines for many years, where the TBC must successfully withstands the damage caused by a variety of environmental and mechanical aspects. The primary failure modes for TBC are oxidation of bond coating, particle erosion and CMAS (calcium-magnesium-alumina-silicates) corrosion. The lifetime of TBC associated with above three failure factors will be reduced significantly. In order to prolong the operation time, an alternative approach depositing Al film on 7YSZ TBC surface by magnetron sputtering is proposed. An α-Al₂O₃ overlay was in-situ synthesized on each 7YSZ column through reaction of Al and ZrO₂ during vacuum heat treatment. And the results indicate that the Al-modified EB-PVD 7YSZ TBC shows better oxidation resistance, as well as lower particulate erosion and CMAS corrosion.     

Comparative study on failure behavior of promising CMAS-resistant plasma-sprayed thermal barrier coatings in burner rig test with/without CMAS deposition

Along with continuous progress in inlet temperature of turbine engine, calcium-magnesium-aluminum-silicate (CMAS) deposition has become one of serious challenges for traditional yttria partially stabilized zirconia thermal barrier coatings at elevated temperature. Although lots of materials with superior CMAS resistance have been proposed, there is few comparative research on performance of corresponding coatings reported especially when subjected to thermal cycling and CMAS simultaneously. To this end, some coatings were prepared in present study, and thereafter failure behavior in condition of thermal cycling and thermal cycling-CMAS was systemically investigated and compared. Experimental results showed a varied lifetime and cracking behavior in thermal cycling test and thermal cycling-CMAS test, indicating that CMAS infiltration affected failure behavior of coatings. Besides, it was numerically found that CMAS penetration would lead to a promotion of thermal stress, which increased the tendency for cracking during thermal cycling. And the phenomenon that channel crack was the precondition of delamination crack was revealed.     

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.     

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.     

Friction delamination mechanism of EB-PVD thermal barrier coatings in high-temperature and high-speed rotating service environment

High-speed rotation is an indispensable working state in the service process of aero-engines, therefore, the centrifugal load cannot be ignored in the failure analysis of thermal barrier coatings. However, due to the lack of service environment simulators that can realize high-temperature as well as high-speed rotation, the failure mechanism of high-speed rotation thermal barrier coatings is still unclear. Here, the effects of rotational speed variation on the service life and failure mode of thermal barrier coatings at high temperatures are studied by experiments and finite element method (FEM). The results show that the service life of high-speed rotating thermal barrier coatings decreases with the increase of rotational speed. The failure is mainly governed by the thinning and spalling of the columnar crystal region of the ceramic layer and the delamination and exfoliation of the equiaxed crystal region, rather than the abnormal growth of TGO. Further in-depth analysis shows that the failure of high-speed rotating thermal barrier coatings is mainly due to the joint driving of centrifugal force and wall shear stress, as well as the contribution of thermal fatigue at high temperatures. This work adds to the understanding of the failure mechanism of thermal barrier coatings under extreme working conditions, and also provides guidance for the safe and reliable service of thermal barrier coatings on working blades.     

Thermal shock resistance of air plasma sprayed thermal barrier coatings

The spallation resistance of an air plasma sprayed (APS) thermal barrier coating (TBC) to cool-down/reheat is evaluated for a pre-existing delamination crack. The delamination emanates from a vertical crack through the coating and resides at the interface between coating and underlying thermally grown oxide layer (TGO). The coating progressively sinters during engine operation, and this leads to a depth-dependent increase in modulus. Following high temperature exposure, the coating is subjected to a cooling/reheating cycle representative of engine shut-down and start-up. The interfacial stress intensity factors are calculated for the delamination crack over this thermal cycle and are compared with the mode-dependent fracture toughness of the interface between sintered APS and TGO. The study reveals the role played by microstructural evolution during sintering in dictating the spallation life of the thermal barrier coating, and also describes a test method for the measurement of delamination toughness of a thin coating.     

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.     

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.     

Research progress on hafnium-based thermal barrier coatings materials

Thermal barrier coatings (TBC) are important materials applied to hot part components of aero-engines in order to improve their service temperature. Increasing inlet temperature is an important factor to achieve elevated thrust-to-weight ratio and high heat engine efficiency. In recent years, traditional TBC materials have gradually reached their operating limits due to the increase in turbine operating temperature. Hafnium-based materials become promising new candidates for TBC because of the similar structure, higher temperature phase stability and lower thermal conductivity compared to traditional zirconium-based materials. In this review, recent progresses in the research and development for hafnium-based TBC materials are summarized. The phase stability, thermal and mechanical properties of rare-earth (RE)-doped HfO₂ and RE hafnate materials are introduced. RE-doped HfO₂ has good thermal properties and phase stability at high temperatures whereas relatively low fracture toughness. The RE hafnates possess the advantages of a higher phase transition temperature, lower thermal conductivity and superior fracture toughness than RE zirconates. However, the thermal expansion coefficients of most RE hafnates are quite different from the alloy matrix. Finally, further research directions for hafnium-based TBC materials are prospected in this study.     

Calcium-magnesium-alumina-silicate (CMAS) resistant Ba2REAlO5 (RE = Yb,Er, Dy) ceramics for thermal barrier coatings

Calcium-magnesium-alumina-silicate (CMAS) attack has been a great challenge for the application of thermal barrier coatings (TBCs) in modern turbine engines. In this study, a series of prospective TBC candidate materials, Ba₂REAlO₅ (RE = Yb, Er, Dy), are found to have high resistance to CMAS attack. The rapid formation of a continuous crystalline layer on sample surface contributes to this desirable attribute. At 1250 °C, Ba₂REAlO₅ dissolve in the molten CMAS, accumulating Ba, RE and Al in the melt, which could trigger the crystallization of celsian, apatite and wollastonite crystals. Especially, the formation of the crystalline layer in the Ba₂DyAlO₅ sample is the fastest. This study also reveals that Ba is a useful element for altering CMAS composition to precipitate celsian. Thus, doping Ba²⁺ in yttria partially stabilized zirconia or other novel TBCs might be an attractive way of mitigating CMAS attack.     

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.     

Effect of TGO thickness on the thermal barrier coatings life under thermal shock and thermal cycle loading

Effect of thermally grown oxide (TGO) thickness on thermal shock resistance of thermal barrier coatings (TBCs) and also their behavior under a cyclic loading (including aging at maximum temperature) was evaluated experimentally. In order to form different thicknesses of TGO, coated samples experience isothermal loading at 1070?°C for various periods of times. Heat-treated samples were heated to 1000?°C and cooled down rapidly in water from the substrate side using a mechanical fixture. The life of samples was investigated as a function of TGO thickness. Furthermore, by performing an experiment the simultaneous effect of the TGO growth and thermal expansion mismatch– on the failure of thermal barrier coatings was evaluated. The results demonstrated that the presence of TGO with a thickness of 2–3?µm has a positive effect on the resistance against thermal shock.