热喷涂复合结构MCrAlY/8YSZ热障涂层的抗剥落能力

在陶瓷涂层与金属粘接层之间制备一层NiCoCrAlTaY/YSZ复合过渡层和通过半熔化团聚YSZ粉末制备层状/多孔团状复合结构YSZ隔热层,用SEM表征了涂层的显微组织;依照ASTM C633标准测试了涂层的结合强度;用压痕法测试了陶瓷层的弹性模量和断裂韧性。用激光脉冲法测试了陶瓷层的热导率。用高温水淬快速冷却实验验证涂层的抗剥落性能。结果表明,在不降低涂层隔热效果的前提下复合过渡层和和隔热层显著提高了涂层的抗剥落能力。HVOF制备的NiCoCrAlTaY粘接层组织致密,没有明显的氧化物;APS制备的NiCoCrAlTaY/YSZ复合过渡层内层间的结合良好,组织致密,金属与陶瓷粒子呈现出典型的层状交替分布特征;陶瓷层由典型层状结构内包含约11%未完全熔化团聚粉末形成的弥散分布多孔团状组织构成。复合结构使等离子喷涂TBC的结合强度由25.8 MPa提高到38.6 MPa,陶瓷层的弹性模量和热导率没有明显的变化,但是断裂韧性提高了1倍以上,涂层出现30%剥落的平均水淬周次由19.7次提高到72.1次,表明抗剥落能力显著提高。     

Lifetime‐determining spalling mechanisms of NiCoCrAlRE / EB‐PVD zirconia TBC systems

The mechanisms that control the lifetime of thermal barrier coating (TBC) systems have been traced by two particular overlay bondcoats serving as model systems: superalloy pins (IN100, CMSX‐4) with two alternative NiCoCrAlRE (RE: Hf, Y) bond coat compositions (i) NiCoCrAlY without and (ii) with co‐dopants of silicon and hafnium. On top an electron‐beam physical‐vapor deposited (EB‐PVD) yttria partially stabilized zirconia (YPSZ) TBC commonly mixed with 2 wt.% hafnia, or, rarely with 10 wt.%, was applied. The test pins were thermo‐cycled at 1100 and 1150 °C until failure. Identical lifetimes in cyclic tests on YPSZ TBCs with 2 (relatively high sintering rate) and 10 wt.% hafnia (relatively low sintering rate) preclude an effect of diffusion mechanisms of the YPSZ TBC on lifetime. The fit of lifetimes and test temperatures to Arrhenius‐type relationships gives activation energies for failure. These energies agree with the activation energies for anion and cation diffusion in alumina for the respective bondcoat variant: (i) for the NiCoCrAlY/TBC system for O^2‐ diffusion in alumina, (ii) for the NiCoCrAlYSiHf/TBC system for Al³⁺ diffusion in alumina. SEM and EDS investigations of the thermally grown oxides (TGOs) confirm the mechanisms responsible for TBC failure as indicated by activation energies. Two categories of failure can be distinguished: (i) NiCoCrAlY coatings fail by an “adhesive mode of failure” along smooth bond coat/TGO interfaces driven by a critical TGO thickness. (ii) NiCoCrAlYSiHf coatings fail later and more reluctantly by a “cohesive” crack mode via de‐cohesion at the TGO/TBC interface. In the latter case a quasi‐integrity of the crack‐affected TGO is lengthily maintained up to failure by a crack‐pinning mechanism which runs via Al³⁺ supply from the bondcoat.     

Oxide layer development under thermal cycling and its role on damage evolution and spallation in TBC system

The nature and cause of failure of thermal barrier coatings (TBCs) consisting of physical vapor deposited (PVD) yttria stabilized zirconia (YSZ, 8 wt.% Y₂O₃) and a diffusion aluminide bond coat (Pt-Al) were investigated after oxidative thermal cycling and isothermal heat treatment at 1177 °C in air. Experiments were conducted for 45 and 10-minute hold times and for isothermal condition for disk specimens with and without TBC. It is found that microcracks starts in the oxide scales at the bond coat grain boundary protrusions. Total number of thermal cycles affect the density of microcracks within the TGO layer. Evidence is presented that higher density of microcracks in the 10-min hold-time experiments tend to separate the TBC from the TGO layer via extensive coating micro-decohesion and promotes 'complete' TBC separation as opposed to traditional 'partial' spallation of TBC from the substrate as in the 45-min hold-time and isothermal experiments.     

The effect of bond coat oxidation on the microstructure and endurance of a thermal barrier coating system

Abstract

Isothermal oxidation tests have been carried out on a thermal barrier coating (TBC) system consisting of a nickel-based superalloy, CoNiCrAlY bond coat applied by HVOF and yttria-stabilised zirconia (YSZ) top coat applied by EB-PVD. Bond coat microstructure, coating cracking and failure were characterised using high resolution scanning electron microscopy complemented with compositional analyses using energy dispersive X-ray spectrometry. A protective alumina layer formed during the deposition of the YSZ top coat and this grew with sub-parabolic kinetics during subsequent isothermal oxidation at temperatures in the range 950 to 1150°C. After short exposures at 1050°C and final cooling, small sub-critical cracks were found to exist within the YSZ but adjacent to bond coat protuberances. Their formation is related to the development of local tensile strains associated with the growth of an alumina layer (TGO) on the non-planar bond coat surface. However, for the specimens examined, these cracks did not propagate, in contrast to other TBC systems, and final spallation was always found to have occurred at the bond coat/TGO interface. This shows that the strain energy within the TGO layer made a significant contribution to the delamination process.     

Effects of heat treatment on adhesion strength of thermal barrier coating systems

Thermal barrier coatings (TBCs) are widely used as protective and insulative coatings on hot section components of gas turbines and their applications, like blades and combustion chambers. The quality and performance properties of TBCs are of great importance in terms of their resistance to service conditions. In a TBC system, there is a close relationship between the adhesion properties of coating layers. The adhesion strength of TBCs varies depending on the coating technique used and the surface treatments. In this study, CoNiCrAlY and YSZ (ZrO₂ + Y₂O₃) powders were deposited on stainless steel substrate. High Velocity Oxy-Fuel (HVOF) and Atmospheric Plasma Spraying (APS) techniques were used to produce the bond coats. The ceramic top layers on CoNiCrAlY bond coats were produced by the APS technique. The TBC specimens were subjected to heat-treatment tests. Adhesion strength for top coat/bond coat interface of as-sprayed and heat-treated samples was investigated. The results showed that the heat treatment of the coatings in different temperatures led to an increase in the adhesion strength of TBCs.     

Evaluation of cyclic oxidation of thermal barrier coatings exposed to NaCl vapor by finite element method

The cyclic oxidation of NiCrAlY + YSZ coating exposed to NaCl vapor has been investigated under atmospheric pressure at 1050 °C, 1100 °C and 1150 °C. The result showed that the cyclic oxidation life of NiCrAlY + YSZ coating in the presence of NaCl vapor was shortened compared with that in air. The failure of the TBC exposed to NaCl vapor occurred within the top coat and close to the YSZ/thermal growth oxide (TGO) interface. A finite element analysis was employed to analyze the stress distribution in the coatings. The computed result showed that maximum stresses occurred at the interface between the bond coat and TGO near the edge of the sample and the increased thickness of TGO caused the value of stress in TGO/YSZ interface to increase. The comparison of the maximum stresses indicated that the spinel TGO resulted in significantly higher stresses than Al₂O₃ TGO. This implies that the formation of spinel plays a dominant role in shortening the coating cycling lifetime.     

Study of failure of EB-PVD thermal barrier coating upon near-α titanium alloy

The cracking failure of a conventional thermal barrier coating (TBC), consisting of a near-α titanium substrate, a NiCoCrAlY bond coat (BC), and a 8 wt.% yttria-stabilized zirconia ceramic layer deposited by electron beam-physical vapor deposition (EB-PVD) method, was studied by cyclic furnace testing and isothermal exposure. The scanning electron microscope, electron probe microanalysis, and microhardness indentation were used to probe the failure mechanism. It is found that due to the mismatch of the coefficient of thermal expansion, the as-deposited BC is suffered the long-term tensile creeping at room temperature. During the high-temperature exposure, the TBC locally rumples, bringing in-plane tensile stress at the shoulders, and out-of-plane tensile stress at the peak of the rumpled BC, where primal cracks are originated. During the cooling period, the ridges of substrate pulled by the local rumpling of the BC blocks the contracting of the BC, originating new cracks in planar BC, and aggravating the original cracks. Furthermore, the oxidation products pushed into the BC and the 8YSZ enlarges the TBC and cracks the substrate along the weakest diffused grain boundaries. The cracking failure related to the diffusion of the BC to the substrate is also discussed.     

The effect of superalloy substrate on the behaviour of high-purity low-density air plasma sprayed thermal barrier coatings

Abstract

Several superalloy-bond coat couples were prepared without ceramic topcoat layers to better understand the effects of superalloy substrate on the oxidation behaviour of NiCoCrAlY bond coats. The same composition NiCoCrAlY bond coats were deposited on three superalloy substrates (Inconel 718, Haynes 188 and Rene’ N5) via argon-shrouded plasma spraying. The specimens were exposed to cyclic oxidation in laboratory air at 1100°C in a bottom loading furnace. Scaling behaviour and rate of aluminum depletion were compared between the various specimens. The bond coats on all three superalloys experienced some form of chemical failure after an extended number of cycles. The number of cycles until chemical failure was shortest for the IN718 specimen followed by the HA188 specimen, both of which experienced complete bond coat chemical failure, and then the Rene’ N5 specimen, which experienced localized chemical failure. The cycles to chemical failure coincide with the cycles to thermal barrier coating (TBC) spallation from previous work, indicating chemical failure of the bond coat is a critical event in the lifetime of TBCs. The effect of bond coat surface finish and porosity on the scaling behaviour has been investigated using specimens with the same superalloy substrate but with different bond coat surface finishes and porosity levels which were produced by utilizing two separate sized starting bond coat metallic powders. Bond coats with minimal porosity and smooth surface finishes did not experience chemical failure, at least in the time frame they were tested; however, oxide scale spallation was more apparent in the smooth bond coats as compared to the specimens with the rough surface finishes and high levels of porosity.     

Modification of NiCoCrAlY with Pt: Part Ⅱ. Application in TBC with pure metastable tetragonal(t’) phase YSZ and thermal cycling behavior

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4mol.% yttria stabilized zirconia(4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4 YSZ thermal barrier coatings(TBCs) with and without Pt modification was evaluated in ambient air at 1100?C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide(TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top(Ni,Co)(Cr,Al)_2O_4 spinel and underlying α-Al_2O_3, was formed at the NiCoCrAlY/4 YSZ interface with thickness of 8.4μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al_2O_3 scale with thickness of 5.6μm was formed at the interface of Pt-modified NiCoCrAlY and 4 YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al_2O_3 and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.     

On the performance and failure mechanism of thermal barrier coating systems used in gas turbine blade applications: Influence of bond coat/superalloy combination

Surface engineering plays a major role in achieving the performance and design lives of gas turbine components such as the high pressure turbine aerofoils which operate under the most arduous conditions of temperature and stress leading to a wide range of thermal and mechanical loading during service. In this study, emphasis is placed upon the role of composite systems consisting of bond coat and superalloy substrate in determining the performance and useful life of thermal barrier coatings using yttria-stabilized zirconia as top coat processed by electron-beam physical vapor deposition. Three platinum-modified bond coats of the diffusion type and three nickel-based superalloys are included in the study. Thermal exposure tests at 1150 °C in air with a 24-hour cycling period to room temperature have been used to rank the performance of the coating systems. Various electron-optical techniques have been used to characterize the sequence of events leading to coating failure as marked by spallation of the top ceramic coat. It is shown that for a given superalloy substrate, the coating performance is dependent upon the type of bond coat. Conversely, for a given bond coat, the performance becomes a function of the superalloy composition used in the application. However, in both cases, coating failure is found to be predominated by loss of adhesion between the thermally grown oxide and bond coat indicating that the respective interface is the weakest link in the system. The results are interpreted in terms of the phase transformations which occur in the bond coats during exposure at elevated temperatures and the corresponding effects on their oxidation behavior.     

On the opening of a class of fatigue cracks due to thermo-mechanical fatigue testing of thermal barrier coatings

The evolution of fatigue cracks observed in thermal barrier coatings (TBCs) subjected to an accelerated test scheme is investigated via numerical simulations. The TBC system consists of a NiCoCrAlY bond coat and partially yttria stabilized zirconia top coat with a thermally grown oxide (TGO) between these two coatings. The cracks of interest evolve in the bond coat parallel and near the interface with the TGO during thermo-mechanical fatigue testing. In their final stage, the cracks lead to partial spallation of the TBC. This study focuses on why the cracks open to their characteristic shape. To this end, finite element simulations are utilized. The crack surface separation is monitored for a range of material properties and oxidation rates. The simulations show that the inelastic response of the bond coat and the oxidation rate of the TGO govern the crack surface separation. Most interestingly, permanent separation of the crack surfaces is caused by a structural ratcheting in the vicinity of the crack.     

Interfacial fracture toughness of APS bond coat/substrate under high temperature

Thermal barrier coating (TBC) is an essential requirement of a modern gas turbine engine. The TBC failure is the delamination and spallation. The failure mechanism is interfacial expansion mismatch and oxidation of bond coat (BC). The oxidation damage under high temperature results in the reduction of interfacial adhesion. The interfacial fracture toughness is an important property to analyze the TBC failure. Using the simple tensile test, pushout test method and three-point or four-point-bending test and so on, the interfacial fracture toughness of ceramic top coat/BC has been researched in the past. However, the fracture toughness of the BC/substrate due to the Al depletion was very few studied. In this study, a NiCrAlY bond coat by air plasma spray (APS) was deposited. The substrate is directionally solidified superalloy (DZ40M). The Young’s modulus of bond coat was obtained by the nanoindentation and average Young’s modulus of bond coat is 66.9 GPa. Isothermal oxidation was performed at 1,050^?C for 100 h. Using the HXZ-1000 micro-hardness equipment and fracture mechanics approach, the five different times was chosen to test the hardness and the crack length, and then the fracture toughness was obtained. While the oxidation exposure time increased at 1,050^?C, the hardness of the substrate close to the bond coat decreased with the increase of the bond coat in hardness. Meanwhile, the interfacial fracture toughness of the bond coat–substrate decreased because of the Al depletion.     

Failure morphologies of cyclically oxidized ZrO2-based thermal barrier coatings

Abstract

Advanced and baseline thermal barrier coatings (TBCs) were thermal cycle tested in air at 1163°C until delamination or spallation of the ceramic top coat. The top coat of the advanced TBC’s consisted of ZrO₂ with various amounts of Y₂O₃, Yb₂O₃, Gd₂O₃, or Nd₂O₃ dopants. The composition of the top coat of the baseline TBC was ZrO₂-8wt.%Y₂O₃. All top coats were deposited by air plasma spraying. A NiCrAlY or NiCoCrAlY bond coat was deposited by low pressure plasma spraying onto a single-crystal, Ni-base superalloy. The TBC lifetime for the baseline coatings was approximately 190 cycles (45 minutes at 1163°C per cycle) while the lifetime for the advanced coatings was as high as 425 cycles. The fracture surfaces and sample cross sections were examined after TBC failure by SEM and optical microscopy, and the top coats were further examined by X-ray diffraction. These post-test studies revealed that the fracture path largely followed splat boundaries with some trans-splat fracture. However, there were no obvious distinguishing features which explained the difference in TBC lifetimes between some of the advanced and baseline coatings.     

Influence of the Thermal Barrier Coatings Design on the Oxidation Behavior

The properties of two different types of thermal barrier coatings(TBCs) were compared to improve the surface characteristics on high temperature components.These TBCs consisted of a duplex TBC and a five-layered functionally graded TBC.NiCrAlY bond coats were deposited on a number of Inconel-738LC specimens using high velocity oxy-fuel spraying(HVOF) technique.For duplex coating,a group of these specimens were coated with yttria stabilized zirconia(YSZ) using plasma spray technique.Functionally graded NiCrA...     

Local stress around cap-like portions of anisotropically and nonuniformly grown oxide layer in thermal barrier coating system

The intrinsic deformation accompanying the growth of thermally grown oxide (TGO) can induce significant local stress potentially causing interfacial delamination and coatings fracture in a thermal barrier coating system (TBCs). Multiple mechanisms can be involved in a TGO growth process which is sensitive to the reactive elements contained in the coatings, and as a result anisotropic and nonuniform growth deformation can be produced in the TGO layer. The objective of this study is to analytically and numerically investigate the oxide-growth-induced local stress around the cap-like portions of a TGO layer having grown to a certain thickness and furthermore demonstrate the associated micro-crack patterns. A sphere model is proposed to analytically derive the elastic and elastic–plastic solutions of the stress field, which takes into account the anisotropy and nonuniformity of growth strain as well as the yielding of coating materials. On the other hand, finite element analysis is carried out to consider more realistic undulation morphology of the TGO layer and to verify the analytical prediction. It is seen that the through-thickness and lateral components of the anisotropic growth strain compete in the stress generation and there exist critical conditions for the dominance of different growth strains. The effect of growth strain gradient is examined to disclose the consequence of TGO dominant growth at the TGO/BC (bond coat) interface. The effects of the roughness and thickness of TGO and the plastic behaviour of different coating layers are also analysed. Finally, the possible micro-crack patterns due to TGO growth in typical TBC systems are illustrated with suggestions about how to reduce the driving force for the related structural failure.     

PVD thermal barrier coating systems for Mo–Si–B alloys

Abstract

Oxidation protective layers with chemical compositions of Mo–70Al, Mo–46Si–24B, Mo–37Si–15B and Mo–47Si–24Al (at.-%) were deposited on Mo–9Si–8B specimens by magnetron sputtering. After pre-oxidation of the coated samples, ceramic topcoats of yttria partially stabilized zirconia (YSZ) and gadolinium zirconate (GZO) were applied using electron-beam physical vapour deposition. Both as-deposited YSZ and GZO topcoats exhibited good adhesion to the pre-oxidised bond coats. The different thermal barrier coating (TBC) systems were exposed to air at 1000 °C for periods between 20 and 100 h. The YSZ topcoat was tightly-adherent to the borosilicate scale grown on the Mo–46Si–24B bond coat after 20 h of exposure. Similar results were obtained for GZO topcoats deposited on Mo–46Si–24B and Mo–37Si–15B bond coats. The TBC system consisting of GZO topcoat and Mo–47Si–24Al bond coat, which formed a mixed scale of silica and mullite-like oxides, survived 100 h at 1000 °C. However, after this exposure time, the bond coats were approaching their lifetime due to the low layer thickness (5–10 μm). Oxidation of the Mo–Si–B substrate at unprotected areas around the suspension hole of the samples caused severe deterioration of the Mo–70Al bond coat and substantial degradation of the outer region of the GZO topcoat due to chemical reactions with MoO₃.     

Analytical electron microscopy of the mixed zone in NiCoCrAlY-based EB-PVD thermal barrier coatings: as-coated condition versus late stages of TBC lifetime

Abstract

The microstructural evolution of the alumina-zirconia mixed zone in a NiCoCrAlY-based electron beam physical vapor deposited (EB-PVD) yttria partially stabilized zirconia (Y-PSZ) thermal barrier coating (TBC) system from the as-coated condition into the advanced stages of TBC lifetime is monitored by analytical transmission electron microscopy (TEM). In the as-coated condition yttria-rich islands at the thermally-grown oxide (TGO)/TBC interface locally impede zirconia uptake of the scale during TBC deposition and give rise to the formation of an “off-plane” alumina-zirconia mixed zone textured perpendicular to the TGO/TBC interface. During prolonged isothermal/cyclic oxidation an increased chromium diffusion through the TGO scale turns the mixed zone into a reaction zone introducing a morphological instability of the mixed zone/TBC interface due to solutioning of the bottom TBC layer.

This microstructural pattern is corroborated by a triple-stage growth model for the mixed zone during three successive stages in TBC lifetime: (i) during TBC deposition, the thickness of the mixed zone increases due to predominant outward aluminum diffusion and uptake of zirconia. No columnar alumina zone (CAZ) has formed at this stage, (ii) upon completion of the transition alumina-to-corundum phase transformation the thickness of the mixed zone remains constant while the change in diffusion mechanism for an inward oxygen diffusion process now initiates parabolic growth of the columnar alumina sublayer of the TGO scale, (iii) in the late stage of TBC lifetime an marked outward chromium diffusion from the bond coat causes the mixed zone to resume growth due to TBC destabilization and the formation of a (Al, Cr)₂O₃ mixed oxide matrix phase.

A transient YCrO₃ phase is proposed for driving the destabilization of yttria-rich sections of the bottom TBC layer.     

Einfluss einer PVD‐Al‐Zwischenschicht auf die Eigenschaften eines thermisch gespritzten Wärmedämmschichtsystems nach Temperaturwechselbelastung

Thermal barrier coatings (TBC) generally consist of a metallic bond coat (BC) and a ceramic top coat (TC). Co–Ni–Cr–Al–Y metallic super alloys and Yttria stabilised zirconia (YSZ) have been widely used as bond coat and top coat for thermal barrier coatings systems, respectively. As a result of long‐term exposure of thermal barrier coatings systems to oxygen‐containing atmospheres at high temperatures, a diffusion of oxygen through the porous ceramic layer occurs and consequently an oxidation zone is formed in the interface between ceramic top coat and metallic bond coat. Alloying components of the BC layer create a so‐called thermally grown oxides layer (TGO). One included oxide type is α‐Al₂O₃. α‐Al₂O₃ lowers oxygen diffusion and thus slows down the oxidation process of the bond coat and consequently affects the service life of the coating system positively. The distribution of the alloying elements in the bond coat layer, however, generally causes the formation of mixed oxide phases. The different oxide phases have different growth rates, which cause local stresses, micro‐cracking and, finally, delamination and failure of the ceramic top coat layer. In the present study, a thin Al inter‐layer was deposited by DC‐Magnetron Sputtering on top of the Co–Ni–Cr–Al–Y metallic bond coat, followed by thermal spraying of yttria‐stabilised zirconia (YSZ) as a top coat layer. The deposited Al inter‐layer is meant to transform under operating conditions into a closed layer with high share of α‐Al₂O₃ that slows down the growth rate of the resulting thermally grown oxides layer. Surface morphology and microstructure characteristics as well as thermal cycling behaviour were investigated to study the effect of the intermediate Al layer on the oxidation of the bond coat compared to standard system. The system with Al inter‐layer shows a smaller thermally grown oxides layer thickness compared to standard system after thermal cycling under same conditions.     

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 〈0 0 1〉 orientation, and being coated with a duplex system comprised of a CoNiCrAlY bond coat and a 8 wt.% 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 3000 h 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.     

Investigation on Failure in Thermal Barrier Coatings on Gas Turbine First-Stage Rotor Blade

Failure in turbine blades can affect the safety and performance of the gas turbine engine. Results of coating decohesion, erosion and cracking at the first-stage high-pressure (HPT) blade working in gas turbine engine are being reported in this paper. This investigation was carried out for the possibility of various failure mechanisms in the thermal barrier coating exposed to high operating temperature. The blade was made of nickel-based superalloy, having directionally solidified grain structure coated with thermal barrier coatings of yttria-stabilized zirconia with EB-PVD process and platinum-modified aluminum (Pt–Al) bond coat with electro-deposition. The starting point of analysis was apparent coating decohesion close to the leading edge on the suction side of blade. The coating decohesion was found to be widening of interdiffusion zone toward the bond coat at higher operating temperature which could change the composition and induce thermal stresses in the bond coat. The erosion, cracking and decohesion of the coating on the pressure side was also observed during failure investigation. The erosion of the coating was coupled by two factors: one by increase in temperature as demonstrated by change in microstructure of the substrate and second by increase in coating inclination toward the trailing side. As a result of high operating temperature, swelling and thickening of TGO was observed due to outward diffusion of aluminum from the bond coat to form alumina (non-protective oxide) which causes internal stresses that leads to top coat decohesion and cracking. The possibility of hot corrosion was also investigated, and it was found that top coat decohesion did not involve this failure mechanism. Visual inspection, optical microscopy, scanning electron microscopy and energy-dispersive spectroscopy have been used as characterization tools.