Thermal Barrier Coating Damage Research Articles

Detection mechanism of delamination in thermal barrier coatings of turbine blade using a Rayleigh wave EMAT

Nondestructive testing (NDT) for damage in thermal barrier coatings (TBCs) is significant for the safety guarantee of gas turbine blades. As a new NDT technology, electromagnetic acoustic transducer (EMAT) is widely applied for NDT of conductive structural components due to its advantages of coupling-free and high adaptability. In this paper, numerical simulations are conducted to study the wave propagation and interaction with delamination defects in TBCs inspected with a Rayleigh wave EMAT of the Lorentz force mechanism. Based on the numerical results, the wave structure in TBC, wave conversion at delamination defect, time domain EMAT signals, and its B-scan images are evaluated and the feasibility of Rayleigh wave EMAT to inspect delamination in TBCs was theoretically clarified.

Methodology for predicting the life of plasma-sprayed thermal barrier coating system considering oxidation-induced damage

The lifespan of plasma-sprayed thermal barrier coating (TBC) systems is difficult to predict owing to the variety of microstructures and deterioration histories. In this study, we developed a novel TBC damage model to reflect deterioration histories; thus, it can be applied to various TBCs. Damage to TBCs is classified into oxidation and mechanical damage; therefore, a detailed deterioration history can be reflected. In addition, by introducing a virtual S–N diagram, a life prediction model that can be applied to TBCs with various microstructures was established. We used the proposed damage and life prediction models in isothermal aging and thermal cycle tests with different aging cycles. The predicted lifespan of TBCs by using the proposed models was within 95% of the results obtained by performing actual tests in the temperature range of 1150–1350 °C.

Multiscale modeling of chemo-thermo-mechanical damage of EB-PVD thermal barrier coatings

The thermal barrier coatings (TBCs) are serviced in elevated thermal conditions characterized by chemo-thermo-mechanical (C-T-M) failure mechanism and multiscale failure modes. In the present work, isothermal, cyclic thermal and mechanical induced spalling tests were carried out. Since the contribution of aluminum migration in TBCs damage evolution and failure was confirmed, the C-T-M life prediction at the micro level, TGO respiratory growth process at the meso level and the C-T-M damage at the macro level have been developed and modeled. Multiscale modeling of C-T-M damage and clarification of EB-PVD TBCs (electron beam physical vapor deposition thermal barrier coatings) failure criterion provides an effective and feasible way to systematically analyze the essential mechanism of TBCs failure and quantitatively evaluate its service performance.

Oxidation and hot corrosion resistance of HVOF/EB-PVD thermal barrier coating system

Hot corrosion and oxidation cause very destructive damage in thermal barrier coatings (TBCs) during service conditions. In hot corrosion, TBCs exposed to molten salts lose their integrity easily due to the phase transformations while oxygen easily penetrates from the TBCs to bond coats and forms thermally grown oxide (TGO) layer which causes higher stresses at the interface of bond and top coating. In the current study, CoNiCrAlY powders were sprayed by high-velocity oxygen fuel (HVOF) technique on Inconel 718, and then yttria-stabilized zirconia (YSZ) and YSZ/Gd2Zr2O7 ingots were deposited by electron beam physical vapor deposition (EB-PVD) technique on the bond coated substrates. Isothermal oxidation tests were carried out at 1000 °C for 8, 24, 50, and 100 h, while hot corrosion tests were carried out at 1000 °C in the presence of NaCl, Na2SO4, and V2O5 molten salts with 5, 10, 15, and 20 h cycles. The produced coatings, as well as the oxidation and hot corrosion test results, were examined using SEM, EDS, XRD, and image analysis techniques. After the tests, the Gd2Zr2O7 layer was found to exhibit superior oxidation and hot corrosion performance as compared to the conventional YSZ TBC system.

Computational method for the analysis of erosion-induced stress and damage in thermal barrier coatings

In this study, we study the effects of erosion on the distribution of stress and cracking behaviors in thermal barrier coatings (TBCs) using the finite element method. The distribution of stress and failure processes of models that include effects of erosion were different from those of non-erosion models. Erosion increased the average temperature in thermal growth oxidation layers and caused interfacial cracks to appear earlier. Moreover, a high erosion rate can accelerate stress growth in TBCs and induce greater interfacial damage during the first few cycles. For TBCs subjected to various thermal cycle fatigue loading waveforms, the difference between the maximum stresses for erosion-included and non-erosion models increases with dwelling time at elevated temperature. Therefore, the effect of erosion was deemed worth consideration in future lifetime prediction studies on TBCs.

Nondestructive inspection of thermal barrier coating of gas turbine high temperature components

A thermal barrier coating (TBC) is applied to high‐temperature components in gas turbines, and consists of a ceramic topcoat and a metallic bondcoat. Various kinds of TBC degradation and damage occur in high‐temperature components during service, such as topcoat thinning, topcoat delamination, and formation of a thermally grown oxide (TGO) layer below the topcoat, each of which can be examined using a suitable nondestructive inspection technique. Topcoat thinning can be detected by topcoat thickness measurement using terahertz waves, which are electromagnetic waves in the frequency region between optical and radio waves. The measurement resolution is about 10 μm, which is comparable to microscopic observation of the cross section in destructive inspection. Topcoat delamination can be detected by active thermography, in which the topcoat surface is scanned by a heating laser and the surface temperature distribution is measured by a thermal infrared camera. The combination of temperature peak and residual thermal image detection is effective in eliminating false detection. The TGO layer can be detected using photoluminescence, in which the Cr3+ ions included as an impurity in Al2O3 are detected. Since delamination tends to occur at locations at which the TGO layer has grown, TGO layer detection provides an effective method to select regions where delamination has occurred or is likely to occur. An inspection flow based on these techniques is proposed, which is expected to aid the establishment of condition‐based maintenance strategies of high‐temperature components. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

The use of experimental bending tests to more accurate numerical description of TBC damage process

Thermal barrier coatings (TBCs) have been extensively used in aircraft engines to protect critical engine parts such as blades and combustion chambers, which are exposed to high temperatures and corrosive environment. The blades of turbine engines are additionally exposed to high mechanical loads. These loads are created by the high rotational speed of the rotor (30 000 rot/min), causing the tensile and bending stresses. Therefore, experimental testing of coated samples is necessary in order to determine strength properties of TBCs. Beam samples with dimensions 50×10×2 mm were used in those studies. The TBC system consisted of 150 μm thick bond coat (NiCoCrAlY) and 300 μm thick top coat (YSZ) made by APS (air plasma spray) process. Samples were tested by three-point bending test with various loads. After bending tests, the samples were subjected to microscopic observation to determine the quantity of cracks and their depth. The above mentioned results were used to build numerical model and calibrate material data in Abaqus program. Brittle cracking damage model was applied for the TBC layer, which allows to remove elements after reaching criterion. Surface based cohesive behavior was used to model the delamination which may occur at the boundary between bond coat and top coat.

CMAS damage in thermal barrier coatings: an exploration via single crystal bulk YSZ specimen

CMAS damage in thermal barrier coatings: an exploration via single crystal bulk YSZ specimen

調和振動加熱を受ける多孔質セラミックコーティングの熱伝導特性に関する研究

It is necessary for a damage evaluation of Thermal Barrier Coatings (TBCs) to understand temperature and thermal stress fields in turbine blade. In the gas turbine, turbine blade surface temperature vary randomly due to fluctuation in combustion temperature. For the purpose of improving reliability of TBC and Porous-TBC (P-TBC) damage evaluation, the effect of harmonic oscillation on an unsteady state temperature field of TBC and P-TBC was clarified. As a result, it was found that the heating wave was reflected from pores, and surface temperature amplitude was attenuated. Furthermore, it is confirmed that the temperature amplitude decreased toward the interface.

調和振動加熱を受ける多孔質コーティングの熱応力解析

It is necessary for a damage evaluation of Thermal Barrier Coatings (TBCs) to understand temperature and thermal stress in turbine blade. In the gas turbine, turbine blade surface temperature vary randomly due to fluctuation in combustion temperature. For the purpose of improving reliability of TBC and Porous-TBC (P-TBC) damage evaluation, the effect of harmonic oscillation on an unsteady state thermal stress of TBC and P-TBC was clarified. As a result, it was found that the position and geometry of pore is related to thermal stress.

Degradation study of 7 wt.% yttria stabilised zirconia (7YSZ) thermal barrier coatings on aero-engine combustion chamber parts due to infiltration by different CaO–MgO–Al2O3–SiO2 variants

A damage assessment of TBC (thermal barrier coatings) coated combustion chamber parts of aero-engines after service in CaO–MgO–Al2O3–SiO2 (CMAS) loaded areas was performed in this study. Parts from different aero-engines were investigated by scanning electron microscopy (SEM) and the extent of damage of the 7wt.% yttria stabilised zirconia (7YSZ) plasma-sprayed TBC due to CMAS deposition and infiltration was estimated. With the help of SEM and energy-dispersive spectroscopy (EDS), CMAS chemical composition was analysed and two different typical chemical compositions of CMAS were derived. These model CMAS compositions were synthesised in laboratory and their melting behaviour was determined by means of differential scanning calorimetry (DSC). Both CMAS variants were subsequently deposited on air plasma sprayed yttria stabilised zirconia (APS-7YSZ) TBC specimens. Samples were subjected to isothermal heat treatments in air at temperatures ranging from 1200°C to 1250°C for times between 10h and 100h. The phase formations and microstructural changes were examined by SEM and X-ray diffractometry (XRD). Results from the CMAS/TBC interaction experiments could duplicate with the damage patterns on the real combustion chamber parts.Furnace cycle tests (FCT) were conducted at 1135°C on TBC coated buttons with and without CMAS. The extent of TBC damage strongly correlates to the CMAS composition. The presence of Ca-sulphate (CaSO4, anhydrite) in the CMAS plays a large role in damaging the 7YSZ and infiltration depth of CMAS. With raising temperature, the depth of infiltration increases rapidly and the CMAS has penetrated completely throughout the TBC thickness at 1250°C. In addition, the time to failure of the TBCs strongly depended on the type of CMAS deposited. The life time of the samples with CaSO4 containing CMAS was found to be the lowest compared to the samples without CMAS and with the CaSO4 free CMAS variant.

Stress measurements via photoluminescence piezospectroscopy on engine run thermal barrier coatings

Optical measurements of stress in a thermally grown oxide (TGO) layer that has formed between a metallic bond coat and thermal barrier coating (TBC) have been previously demonstrated and shown useful in understanding aspects of TBC failure. These measurements have promise for nondestructive evaluation of coated turbine components. However, engine-run turbine parts collect significant surface contamination consisting of calcium-magnesium-alumina-silicate (CMAS). The deposited CMAS both blocks optical measurement of stress in the TGO and produces false stress signals. A recently developed laser ablation technique has enabled contaminant removal with minimal TBC damage. The locally cleaned engine run parts can then have their TGO stresses optically determined. In the present paper, it is shown that sporadic signals from contamination are still present due to CMAS infiltration into the TBC and due to local regions that cannot be cleaned without damaging the TBC. Methods for separating the contamination signal from the TBC signals are presented as needed for successful use of optical stress measurement on engine run turbine components. The stress measurements censored for false signals are compared to other destructive approaches and the methods are shown to provide robust nondestructive evaluation of the coating stress.

Compressive Creep Testing of Thermal Barrier Coated Nickel-Based Superalloys

Turbine airfoils have complex geometries and, during service operation, are subjected to complex loadings. In most publications, results are typically reported for either uniaxial, isothermal tensile creep or for thermal cyclic tests. The former generally provides data for creep of the superalloy and the overall performance, and the later provide data for thermal barrier coating (TBC) spallation as a function of thermally grown oxide thickness, surface roughness, temperature, and thermal mismatch between the layers. Both tests provide valuable data but little is known about the effect of compressive creep strain on the performance of the superalloy/protective system at elevated temperatures. In conjunction with computational model development, laboratory-scale experimental validation was undertaken to verify the viability of the underlying damage mechanics concepts for the evolution of TBC damage. Nickel-based single crystal René N5 coupons that were coated with a ∼150–200 μm MCrAlY bond coat and a ∼200–240 μm 7-YSZ APS top coat were used in this effort. The coupons were exposed to 900°C, 1000°C, and 1100°C, for periods of 100 h, 300 h, 1000 h, and 3000 h in slotted silicon carbide fixtures. The difference in the coefficients of thermal expansion of the René N5 substrate and the test fixture introduces thermally induced compressive stress in the coupon samples. Exposed samples were cross sectioned and evaluated using scanning electron microscopy. Performance data were collected based on image analysis. Energy-dispersive X-ray was employed to study the elemental distribution in the TBC system after exposure. To better understand the loading and failure mechanisms of the coating system under loading conditions, nanoindentation was used to study the mechanical properties (Young’s modulus and hardness) of the components in the TBC system and their evolution with temperature and time. The effect of uniaxial in-plane compressive creep strain on the rate of growth of the thermally grown oxide layer, the time to coating failure in TBC systems, and the evolution in the mechanical properties are presented.

Residual stress trend in thermal barrier coatings in through thickness direction measured by photoluminescence piezospectroscopy

Photoluminescence piezospectroscopy (PLPS) has been used to determine residual stresses in sapphire, alumina in the yttria stablised zirconia (YSZ)/Al2O3 composite and alumina in thermal barrier coatings (TBCs). The TBC of YSZ containing 0·5 wt-% alumina has been produced using electron beam physical vapour deposition. The stress profile through the TBC thickness was measured using a depth sensing method. Reasonable residual stress profiles have been obtained using PLPS with the confocal system for all three material systems. Measurements of TBCs suggest that stress distribution in a TBC system is not uniform in general. However, uniform stress distribution has been found in some positions where damage in TBCs might occur.

Damage growth triggered by interface irregularities in thermal barrier coatings

The efficiency and reliability of modern jet engines strongly depend on the performance of thermal barrier coatings (TBCs), which prevent melting and oxidation of the turbine blades’ structural core. The system’s lifetime is limited by cracks appearing in and in the vicinity of an oxide layer that grows in the TBC under thermal cycling. High replacement costs have led to an increased demand to identify, quantify and remedy damage in TBCs. An integrated experimental–numerical approach is presented for studying the main factors that contribute to damage, particularly interfacial irregularities. Damage at several stages of oxidation in TBCs is analyzed in samples with predefined interfacial irregularities. The model predicts the experimentally observed crack patterns, clearly quantifying the influence of imperfections and indicating that damage can be delayed by surface treatment.

STUDY FOR BLADE CERAMIC COATING DELAMINATION DETECTION FOR GAS TURBINE

The component of the hot gas path in gas turbines can survive to very high temperatures because they are protected by ceramic Thermal Barrier Coating (TBC); the failure of such coating can dramatically reduce the component life. A reliable assessment of the Coating integrity and/or an Incipient TBC Damage Detection can help both in optimizing the inspection intervals and in finding the appropriate remedial actions. This study gives the TBC integrity; so other methods are required, like thermography to obtain indications of TBC delamination. Pulsed Thermography detects coating detachments and interface defects, with a large area of view but a spatial resolution of few mm. The mentioned techniques as a whole constitute a powerful tool for the life assessment of thermal barrier coating.

Effects of Temperature and Particle Size on Deposition in Land Based Turbines

Four series of tests were performed in an accelerated deposition test facility to study the independent effects of particle size, gas temperature, and metal temperature on ash deposits from two candidate power turbine synfuels (coal and petcoke). The facility matches the gas temperature and velocity of modern first stage high pressure turbine vanes while accelerating the deposition process. Particle size was found to have a significant effect on capture efficiency with larger particles causing significant thermal barrier coating (TBC) spallation during a 4 h accelerated test. In the second series of tests, particle deposition rate was found to decrease with decreasing gas temperature. The threshold gas temperature for deposition was approximately 960°C. In the third and fourth test series, impingement cooling was applied to the back side of the target coupon to simulate internal vane cooling. Capture efficiency was reduced with increasing mass flow of coolant air; however, at low levels of cooling, the deposits attached more tenaciously to the TBC layer. Postexposure analyses of the third test series (scanning electron microscopy and X-ray spectroscopy) show decreasing TBC damage with increased cooling levels.

Damage mechanisms, life prediction, and development of EB-PVD thermal barrier coatings for turbine airfoils

Thermal barrier coatings (TBCs) must successfully resist damage from a variety of environmental and mechanical mechanisms to be viable on turbine airfoils. This paper reviews engine-operative TBC damage mechanisms and the requirements for life methods that enable a designer and engine operator to achieve acceptable TBCed component lives. Understanding TBC damage mechanisms facilitates development of advanced TBCs. The following damage mechanisms are discussed: • growth and cracking of the bond coating's thermally grown oxide and its interfaces • molten deposit (sulfate salt and calcia–magnesia–alumina–silicate) wicking into TBC • sintering shrinkage, particle impact, bond coating creep

A physics-based life prediction methodology for thermal barrier coating systems

A novel mechanistic approach is proposed for the prediction of the life of thermal barrier coating (TBC) systems. The life prediction methodology is based on a criterion linked directly to the dominant failure mechanism. It relies on a statistical treatment of the TBC’s morphological characteristics, non-destructive stress measurements and on a continuum mechanics framework to quantify the stresses that promote the nucleation and growth of microcracks within the TBC. The last of these accounts for the effects of TBC constituents’ elasto-visco-plastic properties, the stiffening of the ceramic due to sintering and the oxidation at the interface between the thermally insulating yttria stabilized zirconia (YSZ) layer and the metallic bond coat. The mechanistic approach is used to investigate the effects on TBC life of the properties and morphology of the top YSZ coating, metallic low-pressure plasma sprayed bond coat and the thermally grown oxide. Its calibration is based on TBC damage inferred from non-destructive fluorescence measurements using piezo-spectroscopy and on the numerically predicted local TBC stresses responsible for the initiation of such damage. The potential applicability of the methodology to other types of TBC coatings and thermal loading conditions is also discussed.

Development of residual stress and damage in thermal barrier coatings

Photo-luminescence piezo-spectroscopy through the electron beam deposited (EBPVD) yttria-stabilised zirconia (YSZ) thermal barrier coating has been used to study the evolution of residual stress due to the alumina thermally grown oxide (TGO) formed on Pt–Al bond coats on CMSX-4 substrates. The coated specimens were thermally cycled between room temperature and 1150 °C, holding for 1 h at the top temperature. The experiments explored the effects of two different bond coat surface finishes (normal and polished) and thickness of YSZ from 70 to 270 μm. In most cases the mean TGO stress was found to gradually decrease with thermal cycling in agreement with previously published results. This is mainly due to rumpling increasing the waviness of the TGO and hence the ability to relieve stress by bending. For the same reason, the TGO residual stress in specimens with polished bond coats was higher initially than in those with a normal surface finish. Stress mapping showed that there are regions, approximately 100 μm in size, where the stress is particularly low (< 1 GPa compressive) and these were attributed to local TGO damage or delamination. The incidence of these low stress “events” increased markedly just before coating spallation. The failure was in the TGO, or in the YSZ close to the TGO, probably as a result of weakening in this region by the damage induced by rumpling and driven by the stored energy in the YSZ.