Degradation Of Thermal Barrier Coatings Research Articles

Thermochemical interaction of (Yb0.7Gd0.3)4Hf3O12 ceramic with CMAS melts

The molten calcium-magnesium alumino-silicate (CMAS) deposits degradation of thermal barrier coatings (TBCs) is increasingly severe in advanced turbine engines. In this study, the thermochemical interaction of a promising TBC candidate material, (Yb0.7Gd0.3)4Hf3O12, with CMAS melts at 1300 ℃ and 1500 ℃ was systematically investigated. The apatite and reprecipitated fluorite with high melting points were crystallized rapidly when the ceramic reacted with CMAS melts, resulting in the formation of a dense crystalline layer to suppress further infiltration of CMAS melts. Moreover, as the reaction proceeded, the formation of garnet and cuspidine occurred at 1300 ℃ and 1500 ℃, respectively. The CMAS resistance of (Yb0.7Gd0.3)4Hf3O12 was significantly higher compared to that of Yb4Hf3O12, which could be attributed to the greater volume fraction of apatite and reprecipitated fluorite in the reaction products. The (Yb0.7Gd0.3)4Hf3O12 exhibited high effectiveness in mitigating CMAS infiltration, making it a potential candidate material for thermal barrier coatings.

Highly-durable plasma-sprayed Al2O3-YSZ/YSZ double ceramic layer TBCs against CMAS corrosion

The addition of Al2O3 inside thermal barrier coatings (TBCs) is highlighted effective on mitigating TBCs degradation induced by CMAS corrosion. In this study, the cost-effective 20 wt.%Al2O3-7YSZ/7YSZ double ceramic layer (DCL) TBCs were fabricated and their long-term performance resisting CMAS corrosion was investigated. Significant bending and CMAS infiltration occurred readily for the single layer 7YSZ coatings just after 5 h duration. The top sacrificial Al2O3-7YSZ layer of DCL coatings greatly constrained coating deflection and CMAS infiltration. This is due to the chemical reaction between alumina splats and CMAS, generating interconnected anorthites for effectively blocking CMAS infiltration and improving resistance to CMAS attack. However, it is highlighted that the role of alumina splats on mitigating CMAS degradation was found different between the short-term corrosion and long-term corrosion. In addition, interfacial delamination/cracking was not observed between the top Al2O3-7YSZ layer and bottom 7YSZ layer after 100 h CMAS corrosion, demonstrating superior interface integrity and durability.

Hot corrosion behaviour of mullite thermal barrier coatings for marine diesel engines

Mullite's inherent qualities have made it a potential material for the application of thermal barrier coatings (TBCs) for diesel engine components. Hot corrosion at 600–800 °C can cause TBC degradation, thus significantly affecting the performance of engine components and reducing their service life. This work examines the hot corrosion behaviour of atmospheric plasma-sprayed (APS) mullite coating over NiCrAlY bond coat on mild steel substrates. The coated specimen surface was covered with a mixture of Na2SO4 (sodium sulphate) and V2O5 (vanadium pentoxide) in the form of paste and heated in a muffle furnace at 700 °C for up to 300 h. SEM, EDS, and XRD characterisations were used to investigate the mechanism of hot corrosion. Coatings remained intact after corrosion tests; however, it had reacted with corrosive salts, particularly sodium sulphate. This was evidenced by the removal of amorphous silica, followed by the formation of nosean as a major phase. During the reaction between sodium sulphate and mullite coating, vanadium pentoxide was found to be acting as a flux and mineraliser.

Laser surface modification to improve the resistance of CMAS + molten salt coupling corrosion to thermal barrier coatings

In marine environment, thermal barrier coatings (TBCs) face coupling corrosion of calcium magnesium aluminum silicate (CMAS) and molten salt leading to premature failure. In this study, microstructure evolution of TBCs resulting from CMAS + NaVO3 (CN) coupling corrosion is investigated, and a corrosion protection method based on laser glazing is proposed. TBC degradation by the CN coupling corrosion is mainly due to the high penetration ability of the melt. Laser glazed TBCs have improved resistance to the coupling corrosion, however, the vertical cracks in the glazed layer provide paths for melt penetration. TBCs with double laser-glazed layer are designed, with vertical cracks in the glazed layer being bifurcated and staggered, which could largely suppress CN melt infiltration, and also exhibits excellent phase and microstructure stability in the coupling corrosion condition. This type of laser modified TBC is confirmed to be attractive against the coupling corrosion.

The Effect of Bond Coat Roughness on the CMAS Hot Corrosion Resistance of EB-PVD Thermal Barrier Coatings

In a high-temperature, high-flame-velocity, and high-pressure gas corrosion environment, the intercolumnar pores and gaps of electron beam–physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) may serve as infiltration channels for molten calcium–magnesium–alumino–silicate (CMAS), leading to the severe degradation of TBCs. In order to clarify the relationship between the roughness of the bond coat and the CMAS corrosion resistance of the EB-PVD TBCs, 7 wt.% yttria-stabilized zirconia (7YSZ) TBCs were prepared on the surfaces of four different roughness-treated bond coats. The effect of the bond coat roughness on the columnar microstructure of the EB-PVD YSZ was investigated. The effect of the change of the bond coat’s microstructure on the CMAS corrosion resistance of the EB-PVD YSZ was studied in detail. The results showed that the reduction in the roughness of the bond coat contributes to the improved formation of the EB-PVD YSZ columns. The small and dense columns are similar to a lotus leaf-like structure, which could reduce the wettability of CMAS and minimize the spread area between the coating and the CMAS melt. Thus, the CMAS corrosion resistance of the coating can be greatly improved. This preparation process also provides a reference for the preparation of other TBC materials, improving the resistance to CMAS hot corrosion.

Impedance spectroscopy of thermal barrier coatings as non-destructive evaluation tool for failure detection

Abstract Since thermal barrier coatings (TBCs) for turbine blades suffer from bond coat oxidation and sintering of the ceramic top coat during service, quantification of TBC degradation by non destructive evaluation methods (NDE) is essential. IN617 substrates with standard NiCoCrAlY/PYSZ EBPVD TBCs were annealed at 1100 °C and measured by impedance spectroscopy. Parameters such as phase angle, total impedance, real and imaginary part were analyzed. To clearly separate bond coat oxidation from aging of the zirconia, freestanding ceramic top coats and ceramic free samples were analyzed as well. A straight correlation between the changes in the impedance spectra and the measured thickness of the thermally grown oxide was found.

Characterization of isothermal and cyclic thermal damage of EB‐PVD TBCs with the help of the 3D‐DIC technique

AbstractThe thermal barrier coatings (TBCs) are working under complex elevated temperature loading conditions. In the present work, a damage model for the isothermal and cyclic thermal loads was developed to quantify the failure process of the coatings subjected to isothermal and cyclic thermal exposures. Effects of different damage mechanisms, such as thermal exposure, thermal cycling, aluminum migration, and thermal dwell times, were experimentally and computationally studied. The digital image correlation (DIC) technique was introduced to evaluate degradation of TBCs. The complex damage can be quantified with the help of the DIC strain variations. The introduced model provides a general method to estimate the remaining life.

Deposition of Fuel Impurities Within Thermal Barrier Coatings in Gas Turbine Hot Gas Paths

Abstract For the reliable operation of modern gas turbines, thermal barrier coatings (TBCs) need to withstand a wide range of ambient conditions resulting from impurities in inlet air or fuels. When analyzing the deposition of detrimental hot gas constituents, previous efforts largely focus on the investigation of solid and molten deposit interaction with TBCs. Recent literature and observations in gas turbines indicate that not only liquids can penetrate porous TBCs, but the deposition from gas phase inside of pores and cracks is also an aspect of TBC degradation. To investigate this vapor deposition process, a diffusion model has been coupled with a thermodynamic equilibrium solver. The diffusion model calculates vapor transport of trace elements through pores and gaps in the TBC, where the thermodynamic equilibrium solver calculates local thermodynamic equilibria to predict whether deposition takes place. In this work, the model is applied to discuss the deposition properties of calcium. In recent literature, calcium has—in some cases—been reported to deposit inside of TBCs as pure anhydrite (CaSO4). An actual anhydrite finding in the TBC of a stationary gas turbine blade was reproduced applying the introduced model. The vapor deposition is shown to occur within and on top of the TBC, depending on several factors, such as pressure, temperatures, calcium-to-silicon ratio, and calcium-to-sulfur ratio. The successful alignment of conditions in real engines with model results will allow addressing the increasing demand for more fuel- and operational flexibility of current and future gas turbines.

The relation between the interface undulation and oxide film visco-elasticity in thermal barrier coatings

Interface undulation is one of the important mechanisms that are responsible for the damage and degradation of thermal barrier coatings. This work is aimed at analyzing the relation between the visco-elasticity of thermally grown oxide (TGO) and the interface undulation in thermal barrier coating system. A two-dimensional thermal barrier coating model was built for this reason, with the Fortran subroutines for the nonlinearity and anisotropy of TGO film integrated with the numerical model. Additionally, TGO growth stress is calculated from a three-dimensional theoretical model in order to show the further understanding of the interface undulation. It's revealed that the viscosity of TGO film is one decisive factor for the interface undulation. Like the decreasing TGO viscosity, the increasing TGO elasticity is able to promote the growth of the interface amplitude. Nonetheless, the elasticity of TGO film is not a source of the interface undulation.

Degradation characteristics of thermal barrier coatings for hot corrosion and CaO–MgO–Al2O3–SiO2

Hot corrosion and CaO–MgO–Al 2 O 3 –SiO 2 (CMAS) resistance of thermal barrier coatings (TBCs) are required in an industrial gas turbine employed with impure fuels and high turbine inlet temperatures . Therefore, various TBC systems with different structural combinations were investigated in thermochemical aspects of hot corrosion and CMAS causing the degradation of TBCs. Four types of 8 wt% yttria-stabilized zirconia (8YSZ), namely dense microstructure (8YSZ), porous microstructure (C–8YSZ), no monoclinic phase (NoM–8YSZ), and low thermal conductivity (low-k), were employed to prepare topcoats using an atmospheric plasma spray on the two bondcoats coated by a vacuum plasma spray with feedstocks of NiCoCrAlY and NiCoCrAlY+Hf + Si. Hot corrosion components penetrated through the whole topcoat layer at 1000 °C however did not infiltrate to the bondcoat layers because of the thermally grown oxide layer formed between the topcoat and the bondcoat. In addition, the elements Hf and Si contained in the bondcoat composition suppressed diffusion of β phase aluminum. In the visual inspection after the hot corrosion at 1000 °C and CMAS test at 1250 °C, the TBC system with the NoM–YSZ topcoat layer showed a sound condition without failure unlike other TBCs, and no monoclinic phase after hot corrosion and the CMAS test, indicating that the phase stability plays a critical role in TBCs' lifetime in the hot corrosion and CMAS environments. • Superiority of Hf and Si elements in MCrAlY bondcoat was observed through hot corrosion test. • Conventional and advanced TBCs failed with a similar trend independent of chemicals and porosity. • low-k TBCs showed a negative dependency on both hot corrosion and CMAS • Phase stability of topcoat layer can also be proposed to have more life time under the environment of hot corrosion and CMAS.

The morphology, thermal property, and failure mechanism of GdNdZrO thermal barrier coatings by EB‐PVD

AbstractNowadays, the Gd2Zr2O7 thermal barrier coatings (TBCs) have been evaluated as a promising alternative to yttria‐stabilized zirconia (YSZ). Thus, this investigation focuses on the thermal property, morphology, and failure mechanism of double ceramic layers (DCLs) GdNdZrO/YSZ advanced TBCs. The GdNdZrO coatings with columnar morphology have been deposited on NiCoCrAlYHf bond coating using an electron beam physical vapor deposition method. Material characterizations mainly include X‐ray diffraction, scanning electron microscope, and transmission electron microscopy. The thermal conductivity of GdNdZrO ceramic material is 0.494 W/mK at 1200°C. The thermal shock life of GdNdZrO/YSZ TBCs shows an average shock life of 5235 cycles. The TBC degradation occurs on the crack area within thermally grown oxide layer leading to the interface instability. The interface broken might play an important role in the failure mechanism of TBCs.

Dissolution and diffusion kinetics of yttria-stabilized zirconia into molten silicates

The degradation of thermal barrier coatings (TBCs) by molten silicates (CMAS) represents a fundamental barrier to progress in gas turbine technology, requiring a mechanistic understanding of the problem to guide the development of improved coatings. This article investigates the dissolution of yttria-stabilized zirconia (7YSZ and 20YSZ) into two model silicate melts at 1300–1400 °C. The approach involves the 1D dissolution of YSZ into a semi-infinite melt, characterizing the dissolution rates of YSZ and the diffusion rates of Zr4+ and Y3+ therein. The assessed kinetics of YSZ dissolution and diffusion were then applied to modeling the same phenomena on TBC-relevant length scales. These findings provide fundamental insight into (i) the dissolution mechanism of YSZ, (ii) the subsequent reprecipitation upon saturation, (iii) the quantitative effects of temperature and melt composition on the dissolution and diffusion kinetics, and (iv) how the measured kinetics manifests on the scale of flow channels present in TBCs.

Pulsed thermal imaging for non-destructive evaluation of hot gas path coatings in gas turbines

ABSTRACT To address global research in energy technologies, a UK–US collaboration was setup to look at implications in the use of alternative gas turbine liquid/gaseous fuels and first-generation IGCC-derived syngas. The development of predictive models for deposition, corrosion and component living in gas turbines operating on such novel fuel gases is now being developed. Linked to this, the non-destructive inspection of hot gas path components needs to be refined and the life prediction models utilising thermal barrier coatings need to be validated to enable the on-going assessment of the remnant lives of components during their use. Thermal barrier coatings (TBCs) have been critical to the operation of gas turbines in aircraft and land-based applications. As the engine demands increase with respect to operational temperature and efficiency, so has the requirement for TBCs, both from the perspective of their performance (thermal conductivity, durability) as well as their reliability. Because TBCs play critical role in protecting the underlying super alloy, their failure (spallation) cause accelerated component damage leading to unplanned outage. Therefore, it is important to inspect and monitor the TBC condition to assure its quality and reliability. Non-destructive evaluation (NDE) methods would allow for direct determination of TBC thermal conductivity on coated components and, therefore, they can be used to inspect the quality of as processed components and monitor TBC degradation during service. This paper presents a new method, the pulsed thermal imaging–multilayer analysis (PTI–MLA) method, which can measure the coating thermal conductivity and heat capacity distributions for coatings on different substrates and thickness. NDE methods were also evaluated based on preliminary experimental results for TBC samples that were thermal cycled to various lifetimes to monitor TBC degradation and predict TBC life. Initial insight into the TBC response with painted and unpainted surfaces is also discussed to demonstrate next steps.

Infiltration Behavior of CMAS in LZ-YSZ Composite Thermal Barrier Coatings

Calcium magnesium alumina-silicate (CMAS) corrosion on thermal barrier coatings (TBCs) is one of the reasons for the early delamination of TBCs. In this study, the effects of CMAS on the microstructure evolution of TBCs were investigated for yttria-stabilized zirconia (YSZ), lanthanum zirconate (LZ), and composite coating with a 50:50 volume ratio of LZ and YSZ (LZ-YSZ). The LZ showed the mitigated penetration of CMAS by the formation of the reaction products produced by the high reactivity between the LZ and the molten CMAS. The LZ-YSZ showed a denser microstructure on the surface, resulting in retarding the penetration of CMAS. Therefore, the LZ-YSZ composite TBC is expected to prolong the lifetime performance and protect against further corrosive degradation of TBCs in high-temperature environments. The infiltration behavior and reaction mechanism of molten CMAS were discussed based on the microstructures after the corrosion test.

Real-time Detection of CMAS Corrosion Failure in APS Thermal Barrier Coatings Under Thermal Shock

Calcium-magnesium-alumina-silicate (CMAS) corrosion has been regarded as the most important factor that leads to the degradation of thermal barrier coatings (TBCs). The failure mechanism of TBCs attacked by CMAS corrosion in the actual service conditions is still not clear due to the lack of an environmental simulator and nondestructive testing techniques. To solve the above problems, a real-time acoustic emission method combined with infrared thermography are developed to investigate the failure mechanism of TBCs attacked by CMAS corrosion. The results show that the acoustic emission signal spectrum only depends on the failure mode of the TBCs, and five failure modes are identified: surface vertical cracks, sliding interfacial cracks, opening interfacial cracks, substrate deformation and noise. The lifetime of TBCs attacked by CMAS corrosion is 40 thermal shock cycles, which is nearly six times lower than that of TBCs without CMAS corrosion (350 cycles). Conclusions: The failure mechanism of the former is interlaminar cracking and delamination in the ceramic coating; while that for the latter is interfacial delamination in the vicinity of thermal growth oxide.

Investigation into the evolution of interface fracture toughness of thermal barrier coatings with thermal exposure treatment by wedge indentation

Abstract

Investigation of CMAS Resistance of Sacrificial Suspension Sprayed Alumina Topcoats on EB-PVD 7YSZ Layers

Molten calcium–magnesium–aluminum–silicate (CMAS) mineral particles cause significant degradation of thermal barrier coatings (TBCs) in aero-engines. One approach to protect the TBC coating against the CMAS attack is the application of a sacrificial coating on top of the TBC coating. In this work, Al2O3 coatings were deposited on EB-PVD 7YSZ layers using suspension plasma spraying (SPS) and suspension high velocity oxy-fuel spraying (SHVOF), in order to produce sacrificial topcoats with two different microstructures and porosity levels. The coating systems were tested under CMAS attack with one natural volcanic ash and two artificial CMAS powders by conducting infiltration tests at 1250 °C in the time intervals between 5 min and 10 h. It was found that the porosity and morphology of suspension sprayed alumina topcoats, the chemical composition of the deposits and the infiltration conditions strongly influence the CMAS infiltration, reaction kinetics and formation of the reaction products. While the porous SPS coatings offer limited resistance against CMAS infiltration, the dense SHVOF coatings show promising CMAS sealing behavior. Among the formed reaction products, only (Fe, Mg) Al spinel acted as an efficient barrier against CMAS infiltration. However, the formation of uniform spinel layers strongly depends on the pore morphology of the sacrificial coating and the CMAS chemistry.

A chemo-thermo-mechanically constitutive theory for thermal barrier coatings under CMAS infiltration and corrosion

High temperature ceramic corroded by calcium-magnesium-alumino-silicates (CMAS) deposits is an inevitable part of severe degradation of thermal barrier coatings (TBCs). Based on thermodynamic laws, a mechanism-based constitutive theory is proposed for describing CMAS corrosion process at high temperature in TBCs. Concentration of metal cations in CMAS and degree of corrosion dissolution of TBCs are defined respectively. The constitute of free energy which decides the driving force for governing equation of field variables includes energy contribution from the infiltration of CMAS, the subsequent corrosion dissolution of TBCs by CMAS and elastic strain energy. Also, some coupling terms are added to the expression of free energy to describe coupled effects between field variables. We further establish two 3D plate models for TBCs with and without constraint by substrate to predict the chemo-mechanical response of corroded TBCs. A corrosion experiment is conducted to confirm deformational behavior of corroded TBCs and transient distribution of CMAS mass density. In addition, coupled kinetics capture that out-plane tensile stress piles up near the bottom of corroded coating with constraint by substrate, and disastrous delamination can occur from the interface under the CMAS covered region. The constitutive theory presented here provides a great potential for modeling chemo-thermo-mechanical corrosion process in TBCs at high temperature.

Influence of microstructure on the erosion behaviour of suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are applied on the surface of hot parts of gas turbine engines to increase the turbine efficiency by providing thermal insulation and to protect the engine parts from the harsh environment. Typical degradation of TBCs can be attributed to bond coat oxidation, thermal stress etc. In addition to this, erosion can also lead to partial or complete removal of the TBCs especially when the engine operates under erosive environment such as flying over desert area, near active volcanic or offshore ocean environment. Suspension Plasma Spraying (SPS) is a promising technique for TBC applications by virtue of its ability to produce a strain-tolerant porous-columnar microstructure that combines the benefits of both electron beam physical vapor deposited (EB-PVD) as well as atmospheric plasma sprayed (APS) coatings. This work investigates the influence of various coating microstructures produced by SPS on their erosion behavior. Six different coatings with varied microstructures produced using different suspensions with distinct characteristics were studied and their erosion resistance was compared. Results showed significant influence of SPS TBCs microstructures on the erosion resistance. Furthermore, the erosion resistance of SPS TBCs showed a close correlation between fracture toughness and the erosion rate, higher fracture toughness favours superior erosion resistance.

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 (3Al 2 O 3 ·2SiO 2 ) 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 Al 2 O 3 –SiO 2 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 Al 2 O 3 –SiO 2 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 Al 2 O 3 –SiO 2 , 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.