Thermal Barrier Coatings Interface Research Articles

Research Progress in the Failure Behavior on the Interface of Thermal Barrier Coatings

This paper mainly introduces the research progress on interface failure behavior in hightemperature alloy surface thermal barrier coating systems. The degradation failure and structural evolution behavior during high-temperature service were analyzed for the matrix/bonding layer interface, bonding layer/TGO interface, and TGO/ceramic layer interface in thermal barrier coatings. The research focus and direction that affect the interface performance of thermal barrier coatings were proposed.

Numerical and experimental study on double coaxial gas assisted water jet guided laser machining of superalloy

To achieve more uniform gas film and higher working temperature, the design of film holes on combustion section parts of aero engine is developing towards three trends: smaller diameter, higher aspect ratio and more quantity. To realize the high quality manufacture of small film holes with high aspect ratio, a new water jet guided laser (WJGL) machining method based on double coaxial assistant gas was proposed in this article. An outer coaxial gas jet with high density was added to the existing coaxial jet of water and helium. That outer coaxial gas jet can weaken the interference effect of reverse water and sweep out the low speed water in deep blind hole. The sweep effect of outer coaxial assistant gas was confirmed by simulation and experimental results. The penetration time of Ф0.7 mm holes with depth of 4 mm decreased from 410 s to 173 s with the help of outer coaxial assistant gas. A series of Ф0.7 mm holes with the aspect ratio of 14.6:1 were machined using double coaxial assistant gas WJGL method on superalloy specimens with thermal barrier coatings (TBCs). The roughness of side wall was better than 0.6 μm, and no breakage or stratification was found at the interface of TBCs and matrix. This method can also be used in the manufacture of SiC/SiC ceramic matrix composites (CMCs) parts with deep small holes.

Infrared thermal wave detection of interfacial debonding defects of thermal barrier coatings based on non-linear frequency modulation

In order to improve the reliability of detecting the debonding defects at the interface of thermal barrier coatings (TBC), a non-linear frequency modulated (NLFM) infrared thermal wave detection method is proposed. A NLFM infrared thermal wave detection system is built, and zirconia TBC specimens are prepared and tested. The effects of defect diameter, excitation power, initial frequency and termination frequency on the defect detection effect are analyzed. Three algorithms such as principal component analysis (PCA) are used to process the image sequence, and the signal-to-noise ratio (SNR) of each sequence processing algorithm is calculated and compared. The results show that the larger the diameter of the defect, the easier it is to be detected, and the appropriate adjustment of the excitation power or the reduction of the initial and termination frequencies is beneficial to the detection of defects. Compared with the other two algorithms, the PCA method is more effective for image sequence processing. It offers a reference for detecting debonding flaws at the TBC interface.

Experimental Study of the Thermally Grown Oxide and Interface of Thermal Barrier Coatings Using TEM In-Situ Heating.

Thermal barrier coating (TBC) materials play important roles in gas turbine engines to protect the Ni-based superalloys from high-temperature airflow damage. In this work, the nano-mechanism of TBC failure is analyzed. A scanning transmission electron microscopy-energy dispersive spectrometer (STEM-EDS)-based analysis method was used to study the influence of element migration on the deformation behavior of the bond-coat (BC) layer during heating. The content of elements in the same region varied greatly at different temperatures, which could prove the contribution of element migration to the deformation of the BC layer. TEM in-situ heating experiments were designed and carried out to study the deformation behavior near the ceramic topcoat (TC)/thermally grown oxide (TGO) and the TGO/BC interface. The TC/TGO interface was deformed violently during heating, and obvious deformation occurred at 100 °C, while the TGO/BC interface was relatively stable. A subset geometric phase analysis method was used for full field-strain measurement. The strain value near the TGO/BC interface was relatively small and did not change significantly at lower temperatures. The TC/TGO interface is more unstable and easier to deform than the TGO/BC interface. The stress and strain evolution in the internal region of TGO at high temperatures was quantitatively analyzed. The TGO layer has a tensile stress of GPa magnitude along the interface direction at the peak position, and the shear stress is small.

Microstructure evolution and thermal cyclic failure behavior of thermal barrier coatings with interface modification

Thermal barrier coatings (TBCs) interface between bond coat and ceramic top coat is critical since most of degradation presented in this region as TBCs failure analysis. Shot peening, as a valuable surface engineering, has been applied for TBCs interface modification. The interface modification by shot peening process to the morphology evolution and degradation optimization still need to be studied. In this work, three different shot peening parameters have been selected for interface modification of as-prepared TBCs. The divergent column microstructure has been reduced and surface roughness improved with the shot peening parameters changing. Thermal cyclic failure behavior has been summarized as two mode, thermally grown oxide (TGO) growth induced spalling and interface premature delamination. TGO growth induced spalling dominated failure at roughly interface condition, while interface delamination dominated failure at smooth interface condition. This failure mode has explained the thermal cyclic life tendency, climbing up and then declined, of TBCs.

Experimental and Computational Investigation of Integrated Internal and Film Cooling Designs Incorporating a Thermal Barrier Coating

Abstract Few studies in the open literature have studied the effect of thermal barrier coatings (TBC) when used in combination with shaped hole film cooling and enhanced internal cooling techniques. The current study presents Reynolds-averaged Navier–Stokes (RANS) conjugate heat transfer simulations that identify trends in cooling design performance as well as experimental measurements of overall effectiveness using a flat-plate matched-Biot number model with a simulated TBC layer of 0.42D thickness, where D is the film cooling hole diameter. Coolant is fed to the film cooling holes in a co-flow configuration, and the results of both the smooth and rib-turbulated channels are compared. At a constant coolant flow rate, enhanced internal cooling was found to provide a 44% increase in spatially-averaged overall effectiveness, ϕ¯¯, without a TBC. The results show that the addition of a TBC can raise ϕ¯¯ on a film-cooled component surface by 47%. The optimum velocity ratio was found to decrease with the addition of enhanced cooling techniques and a TBC as the film provided minimal benefit at the expense of reduced internal cooling. While the computational results closely identified trends in overall system performance without a TBC, the model over-predicted effectiveness on the metal–TBC interface. The results of this study will inform turbine component design as material science advances increase the reliability of the TBC.

Thermal property and failure behaviors of Gd doped LaZrCeO coatings with feathery microstructure

LaZrCeO coatings are promising candidates to substitute Y2O3-stabilized ZrO2 in advanced gas turbine engines. In this study, Gd doped LaZrCeO coatings were deposited by electron beam physical vapor deposition. This study focuses on the phase, microstructure, thermal property, and thermal durability of (La1-xGdx)2(Zr0.7Ce0.3)2O7 coatings. The as-deposited coatings show relatively good thermal shock life and thermal cycling life. The broken regions are observed on the interface of thermal barrier coatings. The failure behaviors are relevant with crack evolution and thermally grown oxide growth. This study might guide the investigation of advanced coatings under high temperature.

Thermo-structural behavior of thermal barrier coatings for thrust chamber applications

The investigation of the failure mechanism is essential for the application of thermal barrier coating in the thrust chamber of liquid rocket engines. In this work, a four-layer thermal barrier coating system in a typical regeneratively-cooled thrust chamber was simulated by finite element method, and the interface morphology between layers was represented by an ideal cosinusoidal curve. A thermo-structural analysis, the boundary conditions of which were calculated by a fluid-thermal coupling method, was carried out to study the influence of the interface roughness and thermally grown oxide on the temperature and stress distributions under thermal cycles. Elasto-plastic deformation and creep behavior were also taken into consideration. The computed results reveal that the roughness parameters including interface amplitude and wavelength as well as the thermally grown oxide thickness have limited effects on the temperature distribution in the thrust chamber wall, but a significant impact on the residual stress distribution in thermal barrier coatings. As these parameters grow, the magnitude of residual stress generally increases, meanwhile, the dangerous regions transform in a complicated way. Much attention should be paid to the interface of thermal barrier coatings in thrust chambers.

Intelligent Life Prediction of Thermal Barrier Coating for Aero Engine Blades

The existing methods for thermal barrier coating (TBC) life prediction rely mainly on experience and formula derivation and are inefficient and inaccurate. By introducing deep learning into TBC life analyses, a convolutional neural network (CNN) is used to extract the TBC interface morphology and analyze its life information, which can achieve a high-efficiency accurate judgment of the TBC life. In this thesis, an Adap–Alex algorithm is proposed to overcome the problems related to the large training time, over-fitting, and low accuracy in the existing CNN training of TBC images with complex tissue morphologies. The method adjusts the receptive field size, stride length, and other parameter settings and combines training epochs with a sigmoid function to realize adaptive pooling. TBC data are obtained by thermal vibration experiments, a TBC dataset is constructed, and then the Adap–Alex algorithm is used to analyze the generated TBC dataset. The average training time of the Adap–Alex method is significantly smaller than those of VGG-Net and Alex-Net by 125 and 685 s, respectively. For a fixed number of thermal vibrations, the test accuracy of the Adap–Alex algorithm is higher than those of Alex-Net and VGG-Net, which facilitates the TBC identification. When the number of thermal vibrations is 300, the accuracy reaches 93%, and the performance is highest.

Thermal shock life and failure behaviors of La2Zr2O7/YSZ, La2Ce2O7/YSZ and Gd2Zr2O7/YSZ DCL TBCs by EB-PVD

Double layer ceramic (DCL) thermal barrier coatings (TBCs) have attracted extensive attention due to their high thermal stability under high temperature. The thermal shock life and failure behaviors of DCL TBCs still remain a challenge. This work focuses on crystal structure, thermal expansion coefficients, thermal conductivity, microstructure and failure behaviors of the La2Zr2O7/YSZ (LZ/YSZ), La2Ce2O7/YSZ (LC/YSZ) and Gd2Zr2O7/YSZ (GZ/YSZ) DCL TBCs. The advanced ceramic material exhibits low thermal diffusivity and thermal conductivity. Furthermore, the advanced DCL TBCs exhibit good thermal shock life at 1100 °C due to the feathery microstructure with inter-columnar gaps and inter-columnar pores. After thermal shock test, the broken regions occur on the cracks area within ceramic coatings, TGO and interface leading to the TBCs instabilities. The TBCs interface instabilities play a key role on the failure behaviors of DCL TBCs.

Y–Er–ZrO2 thermal barrier coatings by EB-PVD: Thermal conductivity, thermal shock life and failure mechanism

Rare earth stabilized ZrO2 thermal barrier coatings (TBCs) have attracted much attention due to their low thermal conductivity and high thermal stability under high temperature. The investigation of thermal shock life and failure mechanism of rare earth stabilized ZrO2 TBCs still remain a challenge. This work focuses on structure, thermal conductivity, thermal shock life and failure mechanism of the rare earth stabilized ZrO2 advanced TBCs. The advanced ceramic material exhibits low thermal diffusivity and thermal conductivity. Furthermore, the advanced TBCs also exhibit good thermal shock life at 1100 °C. After thermal shock test, the broken regions occur on the crack area within ceramic coating, TGO and interface. The TBCs interface instabilities play a key role on the failure mechanism of rare earth stabilized ZrO2 TBCs.

Phosphor-Doped Thermal Barrier Coatings Deposited by Air Plasma Spray for In-Depth Temperature Sensing.

Yttria-stabilized zirconia (YSZ)-based thermal barrier coating (TBC) has been integrated with thermographic phosphors through air plasma spray (APS) for in-depth; non-contact temperature sensing. This coating consisted of a thin layer of Dy-doped YSZ (about 40 µm) on the bottom and a regular YSZ layer with a thickness up to 300 µm on top. A measurement system has been established; which included a portable; low-cost diode laser (405 nm); a photo-multiplier tube (PMT) and the related optics. Coating samples with different topcoat thickness were calibrated in a high-temperature furnace from room temperature to around 900 °C. The results convincingly showed that the current sensor and the measurement system was capable of in-depth temperature sensing over 800 °C with a YSZ top layer up to 300 µm. The topcoat thickness was found to have a strong effect on the luminescent signal level. Therefore; the measurement accuracy at high temperatures was reduced for samples with thick topcoats due to strong light attenuation. However; it seemed that the light transmissivity of YSZ topcoat increased with temperature; which would improve the sensor’s performance at high temperatures. The current sensor and the measurement technology have shown great potential in on-line monitoring of TBC interface temperature.

Effect of Pt on adherence of γ′-Ni3Al/Al2O3 interface of thermal barrier coatings investigated by first-principle molecular dynamics

While the effect of Pt on the adhesive strength of the γ′-Ni3Al/Al2O3 (coating/oxide) interface of thermal barrier coatings (TBCs) has gradually been verified, the exact underlying mechanism is not yet fully understood. The present study investigates the chemical bonding mechanism of the interface by means of first-principle molecular dynamics, modelling the room temperature interfacial microstructure after a temperature induced phase transition and utilising the deformation energy to evaluate the adhesive strength of the coating/oxide interface. A theoretical benchmark is provided, which is that the adhesive strength of 12 wt-% Pt addition triples that of Pt-free. According to the analysis of chemical bonds, the Pt induced reinforcement of the Al oxidation plays an important role in enhancing the interface’s adhesive strength. A theoretical interpretation of the unit cell phase transition mechanism on the TBCs’ failure is also illustrated.

Explanation of the effect of rapid cycling on oxidation, rumpling, microcracking and lifetime of air plasma sprayed thermal barrier coatings

The effect of cycle duration on thermally grown oxide (TGO) growth kinetics, bond coat rumpling kinetics, microcracking and failure life was examined by cyclic furnace tests at three dwell times and at three temperatures for an air plasma sprayed (APS) thermal barrier coating (TBC) samples with 7wt.% yttria stabilized zirconia (YSZ) top coat and an APS NiCoCrAlY bond coat. Total hot time to failure decreased with increasing cyclic rate, resulting in lower TGO thickness at failure for higher cycling rates. The TBC interface experienced progressive shape changes for the NiCoCrAlY bond coat. The activation energy for TGO growth and the rumpling rate were determined and neither was affected by cyclic rates. Hence the influence of cycling on rumpling and TGO growth does not provide an explanation for the observed effect of cycling on hot time to failure. Data collected from scanning electron microscopy (SEM) images showed that the failure induced in cyclic furnace test was caused by progressive cracking in the top coat. Using over 1000 micrographs, crack length, crack density and crack opening statistics as a function of dwell time and hot time were collected. Average crack length and average crack density were approximately a single valued function of life fraction despite the 33X variation of failure lives in these tests. Crack density decreased and crack length increased after approximately half life, indicating a transition to a crack linking dominated phase. Linear elastic fracture mechanics (LEFM) calculation based on the measured crack opening displacement versus crack length relationship predicted stresses that are impossibly large, indicating that there must be very significant inelasticity present in crack formation and crack growth in TBC.

A real time deformation evaluation method for surface and interface of thermal barrier coatings during 1100 °C thermal shock

A simple high temperature resistant speckle manufacturing technology and a high temperature micro digital image correlation method were developed and applied to study the real time deformation of thermal barrier coatings’ (TBCs) near-interface regions and surfaces during a thermal shock of 1100 °C. This method was used to measure the displacement and strain fields of TBCs and to analyze the strain evolution of TBCs at high temperature. During the thermal shock heating stage, tensile and shear strain concentrations were found in the ceramic coat near the ceramic coat/bond-coat interface, indicating that the inner regions of the ceramic coat near the interface are a weak link because of the tensile and shear stress concentrations here during the process of thermal shock cycles. Experimental results show that this method can be applied to strain distribution regularity investigation of TBCs’ near-interface regions and surfaces in an 1100 °C thermal shock environment.

Determining a critical strain for APS thermal barrier coatings under service relevant loading conditions

Despite the huge progress made in recent years in analysing the degradation behavior and the reliability of thermal barrier coating systems, there is still some deficit in the capability to predict damage evolution in terms of crack initiation and crack growth, which ultimately leads to macroscopic delamination and spallation of the coating system. In order to obtain this prediction capability, a fundamental understanding of the damage evolution processes under isothermal, thermo-cyclic and under thermo-mechanical loading conditions has to be developed.The aim of the presented work is to determine the critical strain, i.e. the strain at which cracking initiates, and to analyse the evolution of a network of cracks for widely used atmospheric plasma sprayed (APS) thermal barrier coating (TBC) systems. The TBC system has been exposed in our study to service relevant loading conditions, namely to thermal gradient mechanical fatigue (TGMF). TGMF tests for in-phase as well as out-of-phase loading cycles were performed on hollow cylindrical specimens made of the single crystal super alloy CMSX-4, loaded mechanically in 〈001〉 orientation, and being coated with a duplex system comprised of a CoNiCrAlY bond coat and a 8wt.% Yttria partially stabilized Zirconia (YSZ) TBC. The CoNiCrAlY bond coat was deposited by Low Pressure Plasma Spraying (LPPS), while the ceramic top coat was deposited using the APS process. The loading cycles were chosen to represent an industrial gas turbine engine. Critical strains measured for delamination (within the ceramic coating or at the CoNiCrAlY – TBC interface) and through cracking, i.e. segmentation of the ceramic top coat was determined using a special compression test equipped with in situ acoustic emission technique. The mechanical testing was performed at room temperature after TGMF exposure. In order to study the impact of thermally grown oxide (TGO), specimens have been TGMF tested in the “as received” conditions as well as after isothermal aging (up to 3000h at 1000°C). To correlate the signal obtained by acoustic emission (AE) with the evolution of (micro-) cracks, the specimens have been carefully sectioned and investigated by standard metallographic means.The measured critical strains are used as a data basis for a strain-based lifetime model developed for isothermal and cyclic oxidation as well as thermo-mechanical loading. The lifetime model considers two failure modes, namely delamination and (vertical) through cracking.Metallographically obtained crack patterns within the TBC system have been incorporated into finite element models to quantify stress–relaxation as a consequence of damage evolution in the TBC system.The observations show that thermal gradient fatigue loading under in-phase loading leads to a shorter lifetime compared to out-of-phase loading.For the delamination mode, the critical strain values of the model are in good agreement with the experimental data of the TGMF experiments. The modeled critical strain for through cracking, on the other hand, is consistently lower than the experimentally determined failure strains, implying that the model describes the failure situation in a conservative manner.

Effect of Vanadium Contaminated Fuel on Bond Coat and Bond Coat/Top Coat Interface of Thermal Barrier Coatings

In the present paper, an attempt has been made to discuss the degradation mechanism of bond coat and bond coat/top coat interface in thermal barrier coating system in the presence of corrosive salts such Na2SO4 and V2O5.These salts come from impurities in low grade fuel used in gas turbine industry. Salt mixture of Na2SO4 + V2O5 was prepared and applied on surface of thermal barrier coating (TBC) specimens. The specimens were exposed isothermally to 900oC for 200, 400 and 700 hours. SEM analysis revealed the formation of thermally grown oxide (TGO) in specimens sprayed with corrosive salts. Results revealed that there was no degradation of either bond coat or bond coat/top coat interface up to 200 hours of isothermal exposure .Interface cracking and spallation was observed after 400 hours of isothermal exposure owing to depletion of zirconia stabilizer i.e yttria and phase transformation of tetragonal zirconia to monoclinic zirconia.

Non-destructive thermographic monitoring of crack evolution of thermal barrier coating coupons during cyclic oxidation aging

The diffused microscopic cracking of the interface between a thick (>300μm) air plasma spray (APS) thermal barrier coating (TBC) and a bond coat cannot be clearly identified by non-destructive evaluation and testing techniques. In this work, a semi-quantitative estimation of cracks at the TBC interface is obtained from thermal diffusivity values measured on coupons subjected to thermal cycling using a single-side thermographic technique. In fact, during thermal cycling, two phenomena occur: sintering, which promotes a significant increase in thermal diffusivity, and cracking, representing an additional thermal resistance which causes an apparent decrease in thermal diffusivity. The results refer to an experimental activity carried out on 28 APS TBC samples cyclic aged at six different fractions of their lifetime. For each sample, the thermal diffusivity was measured at fixed lifetimes, and the evolution of the cracked fraction of the interface was estimated by adopting a two-dimensional inversion model. Furthermore, at each of the six lifetime fractions, some samples were destructively characterized by image analysis, and the results were compared with the estimations given by the inversion model. Good agreement between the non-destructive estimations and image analysis results was obtained. Moreover, a figure of merit incorporating both the cracked fraction and the crack thickness is also proposed for ranking the damage.

Mitigation of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings

Abstract Ceramic thermal barrier coatings (TBCs) used in gas-turbine engines afford higher operating temperatures, resulting in enhanced efficiencies and performance. However, in the case of syngas-fired engines, fly ash particulate impurities that may be present in syngas can melt on the hotter TBC surfaces and form glassy deposits. These deposits can penetrate the TBCs leading to their failure. In experiments using lignite fly ash to simulate these conditions we show that conventional TBCs of composition 93 wt% ZrO 2 + 7 wt% Y 2 O 3 (7YSZ) fabricated using the air plasma spray (APS) process are completely destroyed by the molten fly ash. The molten fly ash is found to penetrate the full thickness of the TBC. The mechanisms by which this occurs appear to be similar to those observed in degradation of 7YSZ TBCs by molten calcium–magnesium–alumino–silicate (CMAS) sand and by molten volcanic ash in aircraft engines. In contrast, APS TBCs of Gd 2 Zr 2 O 7 composition are highly resistant to attack by molten lignite fly ash under identical conditions, where the molten ash penetrates ∼25% of TBC thickness. This damage mitigation appears to be due to the formation of an impervious, stable crystalline layer at the fly ash/Gd 2 Zr 2 O 7 TBC interface arresting the penetrating molten-fly-ash front.

Effect of Time and Temperature on Thermal Barrier Coating Failure Mode Under Oxidizing Environment

Thermal barrier coatings (TBCs) have been recently introduced to hot section components, such as transition pieces and the first two stages of turbine blades and vanes of advanced F, G, and H class land-based turbine engines. The TBC is typically applied on metallic-coated components. The metallic bond coat provides oxidation and/or corrosion protection. It is generally believed that the primary failure mode of TBCs is delamination and fracture of the top ceramic coating parallel to the bond coat in the proximity of the thermally grown oxide (TGO) between coatings. One of the concerns associated with the use of a TBC as a prime reliant coating is its long-term stability. The effect of long-term operation at typical land based turbine operating temperatures of below 1010°C (1850°F) of the failure mode of TBCs is unknown. Long-term isothermal tests were conducted on the thermal barrier-coated specimens at three temperatures, 1010°C (1850°F), 1038°C (1900°F), and 1066°C (1950°F), to determine the effects of long term exposure on the TBC failure location (mode). Following the isothermal testing, the samples were destructively examined to characterize the degradation of the TBC and determine the extent of TGO cracking, TGO growth, bond coat oxidation, and TBC failure location after long term exposure for up to 18,000h. Optical microscopy and a scanning electron microscope (SEM) attached with an energy dispersive spectroscopy (EDS) system were used to study the degradation of the TBC and bond coatings. The results showed that long term isothermal exposure leads to a change in the TBC failure mode from the delamination of the TBC at the TGO∕TBC interface to the internal oxidation of the bond coat and bond coat delamination. In this paper, the effect of long-term exposure on the delamination of TBC and the bond coat failure mode is discussed.