Thermal Barrier Coatings Surface Research Articles

Structure and Performance of YSZ Thermal Barrier Coatings Irradiated by High Intensity Pulsed Ion Beam

The thermal barrier coatings (TBC) of the yttria-stabilized zirconia (YSZ) has been deposited by the atmospheric plasma spraying (APS),followed by the irradiation of high intensity pulsed ion beam (HIPIB) with the voltage of 250 KV and the ion current density of 300 A/cm2 and pulsed times of 2, 5, 10 and 20, respectively. The X-ray diffraction (XRD) analysis reveals that the coating is characterized by the tetragonal ZrO2 phase instead of the cubic phase and the original monoclinic phase after the irradiation. The scanning electron micros cope analysis demonstrates that the HIPIB treatment leads to a smooth TBC surface, but produces micro-cracks and round grain at the surface. This implies that the plasma erupts during the ion beam interaction with the coatings with poor thermal conductivity, and the micro-cracks were produced in the cooling process. The isothermal oxidation experiment performed at 1050°C in air and suggests that the oxidation resistance of the coating can be largely enhanced after HIPIB treatment.

Effect of Hole Spacing on Deposition of Fine Coal Flyash Near Film Cooling Holes

Particulate deposition experiments were performed in a turbine accelerated deposition facility to examine the nature of flyash deposits near film cooling holes. Deposition on both bare metal and thermal barrier coating (TBC) coupons was studied, with hole spacing (s/d) of 2.25, 3.375, and 4.5. Sub-bituminous coal ash particles (mass mean diameter of 13 μm) were accelerated to a combustor exit flow Mach number of 0.25 and heated to 1183°C before impinging on a target coupon. The particle loading in the 1 h tests was 310 ppmw. Blowing ratios were varied in these experiments from 0 to 4.0 with the density ratio varied approximately from 1.5 to 2.1. Particle surface temperature maps were measured using two-color pyrometry based on the red/gree/blue (RGB) signals from a camera. For similar hole spacing and blowing ratio, the capture efficiency measured for the TBC surface was much higher than for the bare metal coupon due to the increase of surface temperature. Deposits on the TBC coupon were observed to be more tenacious (i.e., hard to remove) than deposits on bare metal coupons. The capture efficiency was shown to be a function of both the hole spacing and the blowing ratio (and hence surface temperature). Temperature seemed to be the dominant factor affecting deposition propensity. The average spanwise temperature downstream of the holes for close hole spacing was only slightly lower than for the large hole spacing. Roughness parameters Ra and Rt decreased monotonically with increased blowing ratio for both hole spacing analyzed. The roughness for s/d=3.375 was lower than that for s/d=4.5, especially at high blowing ratio. It is thought that these data will prove useful for designers of turbines using synfuels.

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.

Jet Engine Coatings for Resisting Volcanic Ash Damage

The severe disruption in air travel and the economic loss caused by the explosive eruption of the Eyjafjallajokull volcano in Iceland reminds us of the enormous threat posed to aircraft jet engines by volcanic ash clouds over vast airspace. The nature of damage to gas-turbine jet engines flying through ash clouds depends on several factors, one of which is the temperature in the hottest parts of the engine. The maximum achievable operating temperature in modern jet engines has been increasing steadily in response to a strong demand for higher power output which scales with that temperature This is enabled by: (1) improvements in metal-alloys used to make hot-section engine components; (2) advanced air-cooling technologies that cool them; and (3) the use of ceramic thermal barrier coatings (TBCs) to insulate them. Higher operating temperatures also make modern jet engines more vulnerable to ash damage because ingested ash melts and adheres to TBC surfaces that can be as hot as 1200°C. This can result in buildup of a molten-glass deposit that penetrates into the TBCs causing them to spall-off, exposing the bare metal to dangerously hot gases. In extreme cases where the ash concentration is very high, catastrophic engine failure can occur due to heavy ash blockage of engine components. It is recognized that ingestion of some amount of ash by engines may be inevitable. A similar threat to engines is posed by airborne silicate sand particles which, although lower in concentration at flight altitudes, are more pervasive. There is a growing need to build protective measures within modern jet engines against damage from a broad range of undesirable silicate deposits. Jet engine coatings are discussed in this paper.

Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature attack by glassy deposits

Thermal barrier coatings (TBCs) used in gas-turbine engines afford higher operating temperatures, resulting in enhanced efficiencies and performance. However, at these high operating temperatures, environmentally ingested airborne sand/ash particles melt on the hot TBC surfaces and form calcium–magnesium–aluminosilicate (CMAS) glass deposits. The molten CMAS glass penetrates the TBCs, leading to loss of strain tolerance and TBC failure. Here we demonstrate the use of the commercial manufacturing method of air-plasma-spray (APS) to fabricate CMAS-resistant yttria-stabilized zirconia (YSZ)-based TBCs containing Al and Ti in solid solution. Results from thermal stability studies of these new TBCs and CMAS/TBC interaction experiments are presented, together with a discussion of the CMAS mitigation mechanisms. The ubiquity of airborne sand/ash particles and the ever-increasing demand for higher operating temperatures in future high efficiency/performance gas-turbine engines will necessitate CMAS resistance in all hot-section components of those engines. In this context the versatility, ease of processing, and low cost offered by the APS method has broad implications for the design and fabrication of next-generation CMAS-resistant TBCs for future engines.

Microscopic observation of laser glazed yttria-stabilized zirconia coatings

Thermal barrier coatings (TBCs) are frequently used as insulation system for hot components in gas-turbine, combustors and power plant industries. The corrosive gases which come from combustion of low grade fuels can penetrate into the TBCs and reach the metallic components and bond coat and cause hot corrosion and erosion damage. Glazing the top coat by laser beam is advanced approach to seal TBCs surface. The laser beam has the advantage of forming a dense thin layer composed of micrograins. Plasma-sprayed yttria-stabilized zirconia (YSZ) coating was glazed with Nd-YAG laser at different operating conditions. The surface morphologies, before and after laser treatment, were investigated by scanning electron microscopy. Laser beam assisted the densification of the surface by remelting a thin layer of the exposed surface. The laser glazing converted the rough surface of TBCs into smooth micron-size grains with size of 2–9 μm and narrow grain boundaries. The glazed surfaces showed higher Vickers hardness compared to as-sprayed coatings. The results revealed that the hardness increases as the grain size decreases.

Damage evolution of TBC system under in-phase thermo-mechanical tests

In-phase thermo-mechanical tests (TMF) of EB-PVD Y 2 O 3 –ZrO 2 thermal barrier coating (TBC) system (8 wt% Y 2 O 3 –ZrO 2 /CoNiCrAlY/IN-738 substrate) were done under a through-the-thick-direction thermal gradient from TBC surface temperature at 1150 °C to substrate temperature at 1000 °C. Deformation and failure behaviors of the TBC system were observed at the macroscopic and microscopic scales and damage evolution of the system under in-phase thermo-mechanical test was discussed. Special attention was paid to TBC layer cracking, thermally grown oxide (TGO) layer formation and void formation in bond coat and substrate. Effect of TMF conditions on the damage evolution behaviors was also discussed.

Numerical study on the thermo-stress of ZrO2 thermal barrier coatings by high-intensity pulsed ion beam irradiation

This paper studies numerically the thermo-mechanical effects of ZrO2 thermal barrier coatings (TBCs) irradiated by a high-intensity pulsed ion beam in consideration of the surface structure. Taking the deposited energy of ion beams in TBCs as the source term in the thermal conduction equation, the distribution of temperature in TBCs was simulated. Then, based on the distribution, the evolution of thermal stress was calculated by the finite element method. The results show that tensile radial stress formed at the valley of TBC surfaces after irradiation by HIPIB. Therefore, if cracks happen, they must be at valleys instead of peaks. As for the stress waves, no matter whether through peak or valley position, tensile and compressive stresses are present alternately inside TBCs along the depth direction, and the strength of stress decreases with time.

Microstructure and mechanical properties of zirconia-based thermal barrier coatings with starting powder morphology

The effects of starting powder morphology on the microstructure and mechanical properties of thermal barrier coatings (TBCs) prepared by an air plasma spray (APS) were investigated after thermal exposure of the TBCs. Three kinds of powder were prepared by a spray drying process. They showed mixed morphologies and microstructures, consisting of the following ratios of deformed hollow type to filled spherical type: 30:70 vol.% in water solvent (WS-1), 70:30 vol.% in methanol solvent (MS), and fully filled spherical type in water solvent (WS-2). Partial cracking was observed at the interface in the cases of the MS and WS-2 powders after 400 h. After 800 h, the defects such as interlamellar and vertical cracks were newly reproduced with the partial cracking at the interface in all samples tested. In the case of WS-1, there were relatively thin and uniform “splat” boundaries/interlamellar cracks and vertical cracks. The adhesive strength of TBCs with MS was higher than with the WS powders, because of the relatively dense microstructure. The hardness values depended on the plane of the TBC section and surface. Higher values were recorded on both planes of MS. The toughness values on the surface were higher than on the section. The highest toughness values on the section were achieved with MS. The feedstock powder morphology and microstructure affected the microstructure evolution and mechanical properties of the TBCs.

Thermal Analysis in a Film Cooling Hole with Thermal Barrier Coating

F ILMcooling and thermal barrier coatings (TBC) arewidely used cooling techniques, especially in a gas turbine, which are used to protect a material surface exposed to a high-temperature environment. Increasing the turbine inlet temperature causes the thermal expansion mismatch between the TBC and the substrate, which generates high thermal stresses and thereby TBC delamination as noted in previous reports [1,2]. In addition, TBC delamination has caused thermal cracks on the sides of film cooling holes because temperature difference between regions with and without TBC increased, as reported by the Electric Power Research Institute [3]. Furthermore, the flow temperature on the TBC surface is varied by introducing secondary cooling flow from film cooling holes. Thus, the local thermal loads on the TBC surfacewithfilm cooling holes are different from convectional cooling system. However, most previous studies [4–11] of TBC delamination have mainly focused on a convectional cooling system without a film cooling hole, and have also investigated the effects of thermal cycles and thermally grown oxide (TGO) on TBC delamination. To design hot components such as combustors, vanes, and blades andtopredict their lifetimeandsafety, it isnecessarytoestimateproper TBC thickness as well as to study the thermal cycles in a film cooling systemunderanappropriatethermalenvironment.ThinTBCresults in higher metal temperature, whereas thick TBC results in higher TBC temperature,whichcanbethesourceoftheTBCfailure.Therefore, the objective of the present paper is to determine the tolerable TBC thicknessateachmainhotflowtemperaturewithafilmcoolingsystem of normal injection. For the calculation, we conduct thermal analysis using numericalmethods [12] and experimental heat transfer data in a nondimensional form (Nusselt numberNu) from the previous studies [13,14]. The test materials of TBC and substrate are yttria-stabilized zirconia (YSZ) coating and IN738-superalloy, respectively. As results, we present maximum temperature and maximum debonding stressvaluesat theedgeof coolingholes asa functionof twovariables, that is, thickness of TBC and temperature of main flow. This work helps to design actual TBC systems and to understand mechanics of interface segregation between TBC and substrate.

The effect of start-up cycle in ceramic coating used as thermal barrier for a gas turbine bucket

The unsteady aerodynamic and aero-thermal performance of a first stage gas turbine bucket with thermal barrier coating (TBC) and internal cooling configuration were investigated by application of a three dimensional Navier–Stokes commercial turbomachinery oriented CFD-code. Convection and conduction were modeled for a super alloy blade with TBC. The CFD simulations were configured with a mesh domain including the nozzle and bucket interstage in order to accurately predict the fluid parameters at inlet and outlet of bucket. Comparisons to the gas turbine manufacturer data have permitted validation of the flow conditions at the inlet of the rotor. The effects of blade TBC surface temperature changes during a start-up cycle were simulated by means of an unsteady simulation, with unsteady inlet/outlet boundary conditions specified according to test data. The calculations include not only the fluid but also the solving of conduction within the blade, allowing for a correct modeling of the large difference of thermal inertia between the fluid and solid. The role of thermal barrier coatings (TBC) is, as their name suggests, to provide thermal insulation of the blade. A coating of about 100–400 μm can reduce the temperature by up to 200 °C. A TBC can be used either to reduce the need for blade cooling (by about 36%) increasing the turbine efficiency, while maintaining identical creep life of the substrate; or to increase considerably the creep life of the blade while maintaining level of blade cooling (and therefore allowing the blade to operate at a lower temperature for an identical turbine inlet temperature).

Microstructural Features of EB-PVD Thermal Barrier Coatings Irradiated by High-Intensity Pulsed Ion Beam

Thermal barrier coatings (TBCs) fabricated by electron-beam physical-vapor deposition (EB-PVD) were irradiated by high-intensity pulsed ion beam (HIPIB) at an ion current density of 100 A/cm2 with a shot number of 1-10. Microstructural features of the irradiated EB-PVD TBCs were characterized by using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. All the HIPIB-irradiated EB-PVD TBC surfaces present smooth and densified features. The originated intercolumnar channels growing out to the top-coat surface and nanometer-scale gaps inside each single column were sealed after the remelting of TBC surface induced by HIPIB, resulting in formation of a continuous remelted layer about 1-2 μm in thickness. The dense remelted layer can work as a barrier against the heat-flow and corrosive gases, and gives the possibility of improving thermal conductivity and oxidation resistance of the HIPIB irradiated EB-PVD TBC.

Failure characteristics during cyclic oxidation of yttria stabilized zirconia thermal barrier coatings deposited via electron beam physical vapor deposition on platinum aluminide and on NiCoCrAlY bond coats with processing modifications for improved performances

In this study, the cyclic oxidation lives of the current state-of-the-art thermal barrier coating (TBC) systems (heavy grit-blasted Pt aluminide and NiCoCrAlY bond coats with EBPVD TBCs) were investigated first, followed by TBC systems that were modified based on the results obtained on the failure of the state-of-the-art TBC systems. The specimens were subjected to cyclic oxidation testing, mostly at 1100 °C, in a bottom-loading furnace in laboratory air. Optical and scanning electron microscopy techniques were used to characterize the as-processed and failed specimens. The state-of-the-art TBC systems with NiCoCrAlY bond coats failed as the result of defects that were identified as TBC defects, transient oxides, surface defects, and reactive element-rich oxide protrusions. On the other hand, the failures of the state-of-the-art TBC systems with Pt aluminide bond coats were due to deformation of the bond coat by a mechanism known as ratcheting. The stored strain energy in the thermally grown oxide (TGO) was also a factor that contributed to the failure of both systems. Most of the modifications performed on the state-of-the-art TBC systems improved their lives to some extent. In the case of NiCoCrAlY systems, elimination or at least minimization of the identified defects was responsible for the improvements, whereas the prevention of the ratcheting type of failure was the main reason for the improvement in lives in the case of Pt aluminide systems. On the other hand, other issues, such as slower growth of the TGO as well as improved TGO/bond coat interfacial toughnesses with some of the modifications, were observed to be contributing factors in the improved lives. Based on the observations on the failure of both the state-of-the-art as well as the modified TBC systems, the surface condition of the bond coats and the morphology of the TBCs close to the TGO were found to have a first-order effect on the failure of TBC systems. The characteristics of the TGO, such as composition, growth rate, and adherence both to the bond coat and the TBC, as well as the characteristics of the bond coats were also observed to have an effect on the failures. Recommendations for future work that should be pursued to better define the conditions necessary for optimized TBC performances are given.

High-Pressure Turbine Deposition in Land-Based Gas Turbines From Various Synfuels

Ash deposits from four candidate power turbine synfuels were studied in an accelerated deposition test facility. The facility matches the gas temperature and velocity of modern first-stage high-pressure turbine vanes. A natural gas combustor was seeded with finely ground fuel ash particulate from four different fuels: straw, sawdust, coal, and petroleum coke. The entrained ash particles were accelerated to a combustor exit flow Mach number of 0.31 before impinging on a thermal barrier coating (TBC) target coupon at 1150°C. Postexposure analyses included surface topography, scanning electron microscopy, and x-ray spectroscopy. Due to significant differences in the chemical composition of the various fuel ash samples, deposit thickness and structure vary considerably for each fuel. Biomass products (e.g., sawdust and straw) are significantly less prone to deposition than coal and petcoke for the same particle loading conditions. In a test simulating one turbine operating year at a moderate particulate loading of 0.02 parts per million by weight, deposit thickness from coal and petcoke ash exceeded 1 and 2mm, respectively. These large deposits from coal and petcoke were found to detach readily from the turbine material with thermal cycling and handling. The smaller biomass deposit samples showed greater tenacity in adhering to the TBC surface. In all cases, corrosive elements (e.g., Na, K, V, Cl, S) were found to penetrate the TBC layer during the accelerated deposition test. Implications for the power generation goal of fuel flexibility are discussed.

Improved oxidation life of segmented plasma sprayed 8YSZ thermal barrier coatings

Unconventional plasma sprayed thermal barrier coating (TBC) systems were produced and evaluated by interrupted or cyclic furnace oxidation life testing. First, approximately 250 µm thick 8YSZ coatings were directly sprayed onto grit blasted surfaces of PWA 1484, without a bond coat, to take advantage of the excellent oxidation resistance of this superalloy. For nominal sulfur (S) contents of 1 ppmw, total coating separation took place at relatively short times (200 h at 1100°C). Reductions in the S content, by melt desulfurization commercially (0.3 ppmw) or by hydrogen (H2) annealing in the laboratory (0.01 ppmw), improved scale adhesion and extended life appreciably, by factors of 5–10. However, edge-initiated failure persisted, producing massive delamination as one sheet of coating. Secondly, surfaces of melt desulfurized PWA 1484 were machined with a grid of grooves or ribs (∼250 µm wide and high), resulting in a segmented TBC surface macrostructure, for the purpose of subverting this failure mechanism. In this case, failure occurred only as independent, single-segment events. For grooved samples, 1100 °C segment life was extended to ∼1000h for 5 mm wide segments, with no failure observed out to 2000 h for segments ≤2.5 mm wide. Ribbed samples were even more durable, and segments ≤6 mm remained intact for 2000 h. Larger segments failed by buckling at times inversely related to the segment width and decreased by oxidation effects at higher temperatures. This critical buckling size was consistent with that predicted for elastic buckling of a TBC plate subject to thermal expansion mismatch stresses. Thus, low S substrates demonstrate appreciable coating lives without a bond coat, while rib segmenting extends life considerably.

Estimating the fracture resistance of functionally graded thermal barrier coatings from thermal shock tests

Characterizing the fracture resistance of thermal barrier coatings (TBC) is of technological importance in TBC design. It can enable coating designers predict the expected thermal fracture response under various thermo-mechanical loading conditions. A methodology is presented wherein results from previously reported laser thermal shock experiments are utilized in conjunction with principles of crack arrest to estimate the resistance to crack growth in a functionally graded yttria stabilized zirconia (YSZ) -bond coat (BC) alloy (NiCoCrAlY) TBC. Fracture resistance curves for cracks initiating at the TBC surface and propagating through its thickness are presented. The results show that fracture resistance increases with crack extension for the single-layer YSZ TBC as well as for the YSZ–BC alloy graded TBCs. Furthermore, the addition of BC alloy to the TBC layers (in the graded coatings) results in a substantial increase to their fracture resistance. The approach used provides a means of estimating the fracture resistance of such coatings at elevated temperatures.

Morphological evolution of thermal barrier coatings induced by cyclic oxidation

High temperature oxidation behavior of EB-PVD thermal barrier coatings (TBC) deposited on a single-crystal nickel-base superalloy with platinum aluminide bond coat has been studied using optical microscopy and luminescence spectroscopy. Examination of the TBC morphology in the course of cyclic oxidation at 1150 °C reveals that the TBC surface may develop undulations, which grow with oxidation time and eventually link together leading to TBC failure. The undulations are shown to be associated with local separations near the interface between the TBC and the thermally grown alumina scale. The observations of the pre-failure TBC evolution made by a low-magnification optical microscopy can serve as a reliable method of non-destructive evaluation of the coating. The relationship between residual stress in the thermally grown oxide and morphological evolution occurring at the TBC-bond coat interface is discussed.

723 軸荷重負荷バーナー加熱熱サイクル試験装置による TBC 評価試験

Burner heat type thermal cycle testing machine under axial loading was newly developed in order to evaluate the life of thermal barrier coating (TBC). This test machine is able to load both mechanical stress and thermal stress independently. The outer surface of hollow shape specimen is heated by ring shape gas burner, while inner surface is cooled by the air. When the surface temperature was elevated at 1100℃, the temperature differnce between TBC surface and inner surface was about 200℃. Thermal stress could be loaded by this temperature gradient, which is equivalent to the actual blade with TBC and cooling hole. This testing machine is also able to load axial tensile and axial compression stress that simulates mechanical stress of TBC components. It is concluded that this testing machine is able to simulate the operation conditions, which is high temperature, thermal stress by temperature gradient, and mechanical stress like centrifugal force.

Anti-coking air injection apparatus for an internal combustion engine

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% ZrO2 + 7 wt% Y2O3 (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 Gd2Zr2O7 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/Gd2Zr2O7 TBC interface arresting the penetrating molten-fly-ash front.

Applications on High‐Energy X‐Rays to Stress Measurements of Thermal Barrier Coatings

High‐energy X‐rays from a synchrotron radiation source, SPring‐8, were applied to the stress measurements of thermal barrier coating (TBC). The specimen had a zirconia top coat on a bond coat of NiCoCrAlY sprayed on the substrate of Ni‐base super alloys. The stress in the bond coat was measured through the top coat using the diffraction of Ni3Al 311 by high‐energy X‐rays with an energy of 72 keV. The sin2ψ method was used to determine the stress value. A specially designed furnace with a wide beryllium window was developed to conduct in-situ measurements of the internal stress in the bond coating at the room temperature, 773, 1073, and 1373 K. The internal stress was tensile at the room temperature, and decreased with increasing temperature. At 1073K or higher, the internal stress in the bond coat was released due to softening of the bond coat. The normal stress perpendicular to the coating surface of TBC was evaluated by a new hybrid method. Since the penetration depth of low‐energy X‐ray is very small around a few micrometers for zirconia, the stress value measured by the sin2ψ method is the in‐plane stress, σ1, and the stress perpendicular to the surface was zero. On the other hand, the penetration depth of high‐energy X‐rays is very deep, so the measured stress value will be the in‐plane stress minus the out‐of‐plane stress, i.e. σ1 − σ3. The normal stress perpendicular to the surface, σ3, i.e. the spalling stress, was estimated from these two measurements. The specimens were exposed in air atmosphere at 1373 K for 500, 1000, and 2000 h. The distribution of the spalling stress in the top coat was estimated by the hybrid method. The spalling stress near the interface to the bond coat changed to a large tension after long‐time exposure.