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Failure of Thermal barrier coatings (TBCs) caused by interface oxidation directly determines the ultimate durability of TBCs. The interfacial delamination and brittle fracture will co-occur during the interface oxidation of the TBCs. To study the interaction between the two failure modes, we develop a thermodynamically-consistent coupling framework combining the phase-field model of the brittle fracture in the bulk material and the cohesive zone model of the pre-existing adhesive interface for TBCs. We divide the critical energy release into two parts to distinguish interface and bulk material failure. According to the variational principle, the phase-field crack evolution equation is obtained. Moreover, the other variables' coupled constitutive and evolving equations are obtained through the dissipation inequality and balance equations. Models and experiments show interface oxidation of APS (air plasma spraying)-TBCs is tending to cracks in the valley of the TC (top coating)/TGO (thermally grown oxide) interface and the peak of the TGO/BC (bond coating) interface. The newly generated TGO will turn the crack at the TGO/BC interface peak into TGO inner crack and eventually lead to equidistant cracks in the TGO peak. Besides, by studying the interaction between cracks at two locations, we find that the TGO/BC interface cracks promote the TC/TGO interface crack initiation and inhibit its propagation. The TC/TGO interface crack very weakly promoted the TGO/BC interface cracks initiation and propagation. The framework present here provides excellent potential for modeling the oxidation failure process in TBCs. [ABSTRACT FROM AUTHOR]

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

7% yttria stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) prepared by air plasma spray were laser-remelted, and subsequently pre-heated and Al2O3 sol-gel repaired for restraining the crack growth in the remelted coatings. The as-prepared coatings were exposed to high temperature molten CaO-MgO-Al2O3-SiO2 (CMAS) to explore their corrosion resistance. The results show that both the laser remelted and the pre-heated, the laser-remelted coatings are densified by CMAS attack. In addition, the thickness of the densified layers is on the same order of that of the non-remelted coatings. Despite of this, the densified layer in the laser remelted coating repaired by Al2O3 sol-gel is much thinner than the other coatings. This indicates that Al2O3 sol-gel repairing coupling with laser-remelted method can effectively improve the CMAS resistance of 7YSZ TBCs due to the refractory anorthite generated during corrosion process. This refractory compound produced between CMAS and Al2O3 sol-gel is capable to decrease the mobility and corrosivity of the CMAS.

Due to the serious consequences that calcium‑magnesium-alumino-silicate (CMAS) attack has on reducing the service life of thermal barrier coatings (TBCs), it is important to understand the degradation mechanisms of CMAS attack on TBCs. Air plasma sprayed yttria-stabilized zirconia (YSZ) coatings were tested with 20 mg/cm2 real volcanic ash (VA) deposits at 1150 °C and 1250 °C for 8 h. The thermo-physical and mechanical properties of the coatings subjected to VA attack were investigated. It was found that VA reacted top coat exhibited apparently higher coefficient of thermal expansion, thermal conductivity, hardness and Young's modulus values than the as-sprayed condition. The microstructure and phase evolution of the coatings were characterized by X-ray diffraction, scanning electron microscope, Raman and transmission electron microscope. The results indicated that Y was leached from YSZ and into the VA melt, which interacted with VA to form yttrium iron garnet, resulting in the detrimental phase transformation from tetragonal to monoclinic. Unlabelled Image • The molten VA penetrated the top coat and filled the pores and interlamellar cracks. • Thermal conductivity, hardness and Young's modulus increased after VA-corroded. • Yttria is leached from YSZ and reacted with VA to form YIG. • YIG accelerated the phase transformation from t-ZrO 2 to m-ZrO 2. [ABSTRACT FROM AUTHOR]

The erosion behavior of EB-PVD thermal barrier coatings (TBCs) with real morphology was investigated based on finite element (FE) simulation. Firstly, an FE modeling method was developed to establish a geometric numerical model using a scanning electron microscope. Secondly, an empirical correlation between the depth of penetration and some characteristic parameters of the erodent particle (i.e. diameter, impact velocity and angle) was obtained by means of simulation. There is almost no difference in the depth of non-dimensional penetration between the real morphology and the simplified model; however, there are obvious differences in the dynamic kink band process in these two models. Due to the non-uniform size of the columnar structure of the EB-PVD TBCs, kink bands are irregular in the real morphology model, which redisplays this experimental phenomenon perfectly. Based on the plastic instability model, the relationship between particle impacting energy and kink band structure evolution in TBCs was set up. Lastly, the crack driving force to produce a surface crack was analyzed. The tensile stress along the column direction is small for TBCs under erosion conditions, with the critical dimensionless impact energy to start a crack being below 0.16 and about 0.195 for the real micro-structure and the simplified model, respectively. These results indicate that the real morphology would be more susceptible to erosion failure compared to the results of previous theoretical or numerical estimations. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBCs) with high brittleness usually exhibit uncertain lifetime due to their scattered mechanical properties and severe service conditions. Here we establish a method of assessing the reliability and the dominant parameters affecting the reliability of TBCs deposited onto a turbine blade substrate during erosion by a hot gas stream. An erosion rate model is established to analyse the failure of TBCs induced by a single solid particle. The velocity, angle, and erosion location of foreign particles attacking the TBCs in a hot gas stream are calculated via numerical simulation, and the distribution of erosion rate is obtained according to the erosion rate model of a single particle. Additionally, reliability methods along with sensitivity analysis are used to quantitatively assess the reliability of TBCs. The results indicate that the most significant parameter affecting the reliability is the pressure at turbine outlet, followed by the coating thickness and the rotational speed of rotor, while both exhibit more obvious effect than the lamella thickness. Moreover, possible approaches to improve the reliability of TBCs are also proposed based on the modeling study. [ABSTRACT FROM AUTHOR]

Thermally grown oxide (TGO), besides bringing extra mismatch between layers in thermal barrier coatings (TBCs), induces volumetric expansion and local large deformation in constrained interface, which results in huge growth stress and then causes the spallation of coatings. Therefore, characterization on the dynamic growth of oxide and stress evolution during oxidation is desirable, but still intractable, especially considering local finite deformation. In the present research, the effects of growth stress evolution in the thermally grown oxide(TGO) on failure mechanism of TBCs are investigated through developing both finite deformation mechanics-oxidation growth model and corresponding finite element method (FEM). Results demonstrate that the TGO growth rate decreases as oxidation time increases and is uneven in different positions, which is consistent with the experimental observations. The maximum tensile stress and compressive stress are situated at the peak and valley regions of bond coating layer, respectively. However, the stress distribution in top coating is opposite to that of bond coating. It implies that the thermal stress in the TGO layer dominates the failure of the TBCs. Compared with small deformation analysis, the maximum stress value in finite deformation description is closer to the experimental result, and it is more suitable to evaluate the growth stress induced by the growth of TGO layer. [ABSTRACT FROM AUTHOR]

Oxidation is often regarded as a key factor to cause the failure of thermal barrier coating system (TBCs), which arouses uneven growth of thermal grown oxide (TGO) layer. During the process of oxidation, the rough interface between TGO and bond coat (BC) influences the large growth of TGO and the stress concentration in TBCs. However, theoretical models to describe the effect are not many previously. In the present research, firstly, based on the finite deformation analysis, a three-concentric-circle model considering growth effect of TGO layer is presented to characterize stress concentration around the TGO induced by the oxidation of TBCs. Secondly, another growth model for TGO layer growth is presented based on the creep constitutive theory. From two models, the results indicate that the maximum tensile stress in the valley region locates at the interface of top coating (TC)/TGO, while the maximum normal stress locates at the BC/TGO interface in the peak region. Additionally, the thickness of TGO layer in the valley region decreases as roughness increases, while it increases as the roughness increases in the peak region. The theoretical models can be used to explain the uneven growth of TGO. [ABSTRACT FROM AUTHOR]

The strain field of the thermal barrier coatings (TBCs) corroded by molten CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) is investigated using the digital image correlation (DIC) techniques. The results indicate that the strain in TBCs is compressive and it increases during cooling from 1250 to 25 °C. The cracks induced by CMAS form at the YSZ/BC interface at 300–400 °C. These cracks can be observed even without CMAS as temperature decreases to 100 °C. The DIC measurements show that the critical strains for the failure of TBCs with and without CMAS are − 0.7% and − 1.6%, respectively, and these results can be predicted by using a theoretical model. [ABSTRACT FROM AUTHOR]

In impedance spectroscopy testing of metal/ceramic multi-layers, such as thermal barrier coatings (TBCs), in order to avoid electric leakage, asymmetric electrode is widely adopted. However, how is electric field distribution in asymmetric electrode system, what error does it bring to measurement results, and how to modify these errors etc., are still not understood. In this study, electric field divergence in TBCs measurement induced by asymmetric electrode is investigated through solving a three-dimensional electric field equations numerically. The results show that the asymmetric electrode inevitably triggers divergence of electric field line, and as frequency decreases the divergence increases considerably. When Pt electrode diameter is chosen as 1 mm in ceramic coating, the errors in thickness measurement for both YSZ and TGO are 26.20% and 89.3%, respectively. The errors rise as both YSZ and TGO thicknesses increase, while the errors decrease as Pt electrode size increases. The error in thickness measurement for TGO layer is much larger than that for YSZ. Through present research, a scheme to eliminate the effect of electric field divergence on measurement error is proposed. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBCs), such as 7–8 wt.% yttria stabilized zirconia (YSZ), are widely used to reduce the temperature of coated materials. However, when a turbine operates in a harsh environment, for example a volcanic ash attack, sand and ash particles ingested by the engine could be deposited on the TBC surfaces as molten calcium-magnesium-alumino-silicate (CMAS). CMAS melts and penetrates into TBCs at high temperatures, which causes a loss of strain tolerance and results in the premature failure of the top coat. It is recognized that the formation of CMAS is inevitable due to exposing the turbine to sand and ash; therefore, CMAS mitigation solutions are among the top challenges for materials scientists. To some extent, CMAS is the biggest weakness of the traditional 7-8YSZ TBC material. The thermochemical interactions between YSZ and real volcanic ash are investigated in this paper as a means to alleviate the volcanic ash attack by understanding the underlying mechanisms. 7YSZ, 0.5Gd-8YSZ and 3Er-7YSZ powders and free-standing plates were prepared, respectively. The effect of the volcanic ash on the three different samples was investigated using an X-ray diffraction, scanning electron microscopy, a scanning transmission electron microscopy, and a differential scanning calorimetry. The phase transformations and reaction of TBCs materials subjected to a volcanic ash attack were examined and the volcanic ash degradation and mitigation mechanisms were discussed. It was found that rare earth-doped YSZ samples suffered less damage from the volcanic ash attack than the standard 7YSZ. The results demonstrate that rare earth-doped YSZ may be effectively utilized to mitigate a volcanic ash attack. [ABSTRACT FROM AUTHOR]

Molten calcium-magnesium-alumino-silicate (CMAS) corrosion is the most dangerous failure mode for thermal barrier coatings (TBCs). The real-time monitoring and mode discrimination of the corrosion failure process are desirable to understand the corrosion failure mechanism of TBCs. In this paper, the failure mechanism of the TBCs subjected to molten CMAS attack is investigated with acoustic emission (AE). Based on the number and amplitude, and compared to the substrate and the TBCs without CMAS, more serious damage is found to occur in the TBCs attacked by the molten CMAS. Possible failure modes are discriminated by wavelet analysis, and the results indicate that there are four distinct frequency bands corresponding to surface vertical cracks, sliding interface cracks, opening interface cracks, and substrate deformation in the TBCs suffered high temperature molten CMAS corrosion. In fact, the frequency band of sliding interface crack is associated with two crack types, including the crack at the YSZ/BC interface and the parallel crack in the ceramic coating induced by the compressive stress. These two cracks dominate the whole failure process, and ultimately result in the spallation of coatings. [ABSTRACT FROM AUTHOR]

This study enhances the cohesive zone model to describe the propagation of interfacial cracks in thermal barrier coatings (TBCs) corroded by calcium-magnesium-alumino-silicates (CMAS) at high temperatures. The finite element method is used to build a multi-layer 2D plate model that simultaneously simulates stress distributions, infiltration of CMAS, corrosion of ceramic coat and interface crack growth in TBCs. Furthermore, this study provides a detailed elaboration of the growth mechanism of interface cracks and the performed corrosion experiment. The theoretical model presented here validates the spalling phenomena of TBCs caused by CMAS corrosion at high temperatures. • The cohesive zone model is modified to describe the crack growth in corroded interface of TBCs. • The evolution of interfacial crack is captured by both model and experiment. • The development of stress with the interfacial crack and corrosion strain are predicted. [ABSTRACT FROM AUTHOR]

A key parameter in discriminating the failure types of thermal barrier coatings (TBCs) was found out by using the k -means cluster analysis of acoustic emission (AE) signals. It is shown that there are five classes of mechanisms, including surface vertical cracks, opening interface cracks, sliding interface cracks, substrate deformation and macroscopic cleavage or spallation. Except for the last one, the other four classes can be clearly distinguished from their peak frequency distributions in the ranges of 170–250, 400–500, 260–350 and 40–150 kHz, respectively. However, AE signals overlap with each other in other parameter spaces, e.g., amplitude, energy, rise time, and duration time. The results indicate that the frequency can be applied to identify the AE source mechanisms in TBCs. [ABSTRACT FROM AUTHOR]

Determination of interfacial adhesion energies of thermal barrier coatings is important for understanding failure mechanisms and predicting their lifetime. Combined compression test with a cohesive zone finite element model, it is shown that the interfacial adhesion energy is in the range of 100–130 J/m 2 . Based on the nonlinear delamination theory, the critical interfacial adhesion energy of delamination is 120 J/m 2 and the corresponding loading phase angle is −56°. With the increase of the half-length of the crack, the crack propagation tends to be steady with a steady-state interface energy release rate of 150 J/m 2 , and delamination experiences almost pure mode II. These results obtained from finite element simulations and theoretical analyses are in good agreement with the available values determined by other testing methods reported in the literatures, which confirms the validity of this method. [ABSTRACT FROM AUTHOR]

The interaction of surface cracking and interfacial delamination in thermal barrier coatings under tension is investigated by using a cohesive zone finite element model. It is found that the surface crack density has a significant effect on the initiation and propagation of interfacial delamination. The interfacial delamination length decreases with increase of the surface crack density. The influence of ceramic coating thickness and interfacial adhesion parameters on surface cracking and interfacial delamination is discussed. It is shown that the saturated crack densities decrease with increase of the ceramic coating thickness and interfacial delamination length, and the critical surface crack density without interfacial delamination decreases as the interfacial adhesion energy increases. The results imply that the larger the surface crack density and interfacial adhesion energy are, the less the probability of interfacial delamination. [ABSTRACT FROM AUTHOR]

Abstract: The real-time assessment on the details in damage evolution of thermal barrier coatings (TBCs) is desirable, especially if the key coating performance parameters, such as, the surface and interface fracture toughness, could be accurately characterized. In this paper, the fracture details of as-sprayed and pre-oxidized TBCs under three-point bending are monitored by an acoustic emission (AE) combined with digital image correlation (DIC) methods. The surface and interface toughness of TBCs can be accurately determined on the basis of AE signals and strain images. A linear relationship is found between the energy released from coating failure and that of AE signals, whose slope depends on the fracture modes and properties of TBCs. [Copyright &y& Elsevier]

Abstract: The real-time testing and quantitative assessment of damage evolution in thermal barrier coatings (TBCs) is desirable, but still intractable, especially at elevated temperature. In this paper, the fracture process of TBCs subjected to cyclic heating and cooling is monitored using an acoustic emission method. Based on the wavelet analysis of acoustic emission signals, damage modes in TBCs are discriminated. The results show that, due to thermal stress, vertical cracks tend to occur in the heating stage and interface cracks in the cooling stage. The surface crack density and interface crack length are calculated to obtain the quantitative correlation of damage evolution in TBCs and acoustic emission parameters. The rupture time of TBCs can be predicted from the statistical analysis of acoustic emission signals. [Copyright &y& Elsevier]

• A model of corrosion failure in porous ceramics is proposed. • The coupled effects of infiltration, corrosion, and deformation are considered. • The corrosion process and delamination mechanism of TBCs are successfully predicted. • Cracks tend to initiate from the side edges of TBCs columns at the infiltration interface. Corrosion failure leads to huge economic losses and potential safety issues in many fields. With a focus on corrosion failure in porous ceramics infiltrated by molten silicates, we developed a mechanical model based on thermodynamic laws that considers infiltration, diffusion, corrosion, and deformation. The model was used to analyze the failure mechanism of thermal barrier coatings (TBCs) corroded by molten silicates. The modeled corrosion process was consistent with experimental results. The delamination mechanism of TBCs due to corrosion was successfully revealed. The model has great potential in analyzing corrosion failure in porous ceramics. [ABSTRACT FROM AUTHOR]

• A damage phase field based FEM model considering thermal cycling and growth stresses is developed. • The interaction between the thermal cycling and growth stresses accelerates TBCs damage. • The delamination mechanisms of TBCs under different interface roughness are obtained. The growth of thermally grown oxide (TGO) is often regarded as a key factor causing failures of thermal barrier coatings (TBCs) because it leads to thermal cycling and growth stresses. However, the delamination mechanism of TBCs induced by the growth process of TGO is still unclear. Here, a finite element method (FEM) considering both thermal cycling and growth stresses is developed. The accumulation of damage and crack nucleation, propagation, and coalescence are obtained using a phase field damage model. The results indicate that there is a critical TGO thickness of 8–10 μm, which may result in delamination of the TBCs. Both thermal cycling and growth stresses induce the premature failure of TBCs. The delamination mechanism induced by thermal cycling and growth stresses is obtained. When the interface roughness is A / L ≥ 1/4, the cracks pass through the TGO/TC interface and propagate in the TC layer. When the interface roughness is A / L ≤ 1/8, the cracks propagate along the vicinity of the TGO and will not pass through the TC/TGO interface. When the interface roughness is 1/8 < A / L less than 1/4, the cracks bifurcate at the vicinity of TGO interface, and one branch propagates in the TC and the other propagates along the vicinity of the TGO. [ABSTRACT FROM AUTHOR]

The evolutions of elastic modulus and fracture toughness are the key factors affecting the failure mechanism and durability of thermal barrier coatings (TBCs). Combined the high temperature three-point bending with the digital image correlation (DIC) method, the variations of high temperature elastic modulus and fracture toughness of air plasma sprayed TBCs with temperature are determined. The surface and interfacial cracking information can be monitored real-time by DIC system. The results show that when the temperature rises from 30 °C to 800 °C, the elastic modulus and fracture toughness of TC decrease from 20.3 GPa to 13.1 GPa and from 1.31 MPa m1/2 to 1.16 MPa m1/2, respectively. And the interfacial fracture toughness increases from 83.7 J/m2 to 156.3 J/m2. These results are consistent with the available values determined in literatures, which ensures the validity of this method. [ABSTRACT FROM AUTHOR]

In this study, Zr 6 Ta 2 O 17 ceramics with porous, fine-grained, and coarse-grained structures were obtained via in situ solid-state reactions, and their mechanical characteristics were examined. The significantly low thermal conductivity of dense Zr 6 Ta 2 O 17 ceramics (1.0 W m−1 K−1) was due to the grain boundary gap caused by superstructured grains. A calcium–magnesium–alumina–silicate (CMAS) corrosion experiment demonstrated that the formation of an interlocking structure composed of ZrO 2 , CaTa 2 O 6 , and ZrSiO 4 prevented the penetration of CMAS impurities, thereby revealing the application potential of porous ceramics. In dense Zr 6 Ta 2 O 17 ceramics, the low-volume diffusion induced by an entropy-stable structure is conducive for corrosion resistance; however, the grain boundary is vulnerable to attacks by CMAS, which can be mitigated by the formation of a coarse crystal structure, thereby effectively improving the corrosion performance. This work provides a critical perspective on the thermal barrier coating design of A 6 B 2 O 17 (A = Zr, Hf; B Nb, Ta) ceramics. [ABSTRACT FROM AUTHOR]

Abstract Characterization of the evolution of pores in thermal barrier coatings (TBCs) under volcanic ash (VA) corrosion is essential for assessing coating premature failure. As a nondestructive three-dimensional visualization method, X-ray computed tomography (CT) is used to quantitative investigate the pore structure in ceramic layer. An image morphological method combining Top-Hat and region growing method is applied to separate the pores and background. The change of porosity, pore size and shape resulting from VA corrosion were obtained, which is consistent with the results of mercury infiltration porosimetry (MIP) and scanning electron microscope (SEM). Graphical abstract Unlabelled Image Highlights • The image morphological method has a greater advantage in extracting pore features. • The reconstructed images show a predominantly globular porosity. • The porosity decreases with the increase of corrosion temperature and VA coated amount. • The VA coated amount has a more obvious effect on porosity. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract The durability of thermal barrier coatings (TBCs) is controlled by the thermally grown oxide (TGO) growth, which is sustained by continuous diffusion of oxygen in TBCs. At the same time, stresses are induced due to volumetric change when TBCs are oxidized. Such stress may in return affect the diffusion of oxygen in the TGO layer, thus changing the TGO growth kinetics. In the present research, a continuum thermodynamic model is developed to account for such stress–diffusion interaction in the oxidation of TBCs. Then we numerically implement our chemo-mechanically-coupled constitutive theory into the widely used finite element software to simulate the oxidation behavior of TBCs. The results demonstrate that the maximum tensile stresses locate at the peak regions of bond coating (BC) layer and the valley region of top coating, respectively. It implies that the failure of the TBCs may occur at the peak of BC/TGO and the valley of TC/TGO interface, which is consistent with the experimental observations. It is also found the stress significantly slows down the rate of oxidation. Consequently, the TGO growth kinetic is not strictly parabolic. [ABSTRACT FROM AUTHOR]

High-speed rotation is an indispensable working state in the service process of aero-engines, therefore, the centrifugal load cannot be ignored in the failure analysis of thermal barrier coatings. However, due to the lack of service environment simulators that can realize high-temperature as well as high-speed rotation, the failure mechanism of high-speed rotation thermal barrier coatings is still unclear. Here, the effects of rotational speed variation on the service life and failure mode of thermal barrier coatings at high temperatures are studied by experiments and finite element method (FEM). The results show that the service life of high-speed rotating thermal barrier coatings decreases with the increase of rotational speed. The failure is mainly governed by the thinning and spalling of the columnar crystal region of the ceramic layer and the delamination and exfoliation of the equiaxed crystal region, rather than the abnormal growth of TGO. Further in-depth analysis shows that the failure of high-speed rotating thermal barrier coatings is mainly due to the joint driving of centrifugal force and wall shear stress, as well as the contribution of thermal fatigue at high temperatures. This work adds to the understanding of the failure mechanism of thermal barrier coatings under extreme working conditions, and also provides guidance for the safe and reliable service of thermal barrier coatings on working blades. [ABSTRACT FROM AUTHOR]

Experimental observations display that there exist two types of cracks induced by thermally grown oxide (TGO) in electron beam physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs), which will lead to the coating spallation. Up to now, the failure mechanism induced by TGO is not clear. In this work, a finite element model with real thermally grown oxide morphology for crack propagation in TBCs is established by taking the advantage of commercial finite element package (ABAQUS) and image processing technique (Coreldraw). The coating failure mechanism under thermal mismatch stress is investigated. Present analysis and discussions show that the horizontal crack tends to propagate through the top coat layer toward the interface during the cooling stage, while the interface crack initiates at the points located in the middle region and then propagates in two side directions simultaneously, and mode II fracture plays a dominant role in the whole crack propagation process. Finally, two failure mechanisms of the TC/TGO interface are proposed in this paper. [ABSTRACT FROM AUTHOR]

Prediction of the stress distribution on a 3D turbine blade coated with thermal barrier coatings (TBCs) plays a key role in analyzing the failure of TBCs. In this work, a 3D finite element model of turbine blade coated with multilayer-structure TBCs is developed, in which conjugate heat transfer analysis and the decoupled thermal-stress calculation method are adopted. To obtain a closer to the actual temperature field, the external flow field performed by three turbulence model (RNG k - ε , realizable k - ε and SST k - ω turbulence model) are analyzed. It is found that the temperature and pressure distribution of the flow field performed by realizable k - ε turbulence model are much closer to the experiment data. TBCs present an excellent insulating effect. The heat insulation performance at the leading and trailing edges is relatively better than the suction and pressure sides. Based on the thermal-stress simulation results, the dangerous regions of TBCs are predicted. It is shown that the maximum principal stress of ceramic layer locate at both suction and pressure surfaces near leading edge and trailing edge. Furthermore, the stress level of ceramic layer is higher than that of bond coating, TBCs may spall off at these regions, especially the ceramic layer. [ABSTRACT FROM AUTHOR]

H f 6 T a 2 O 17 has shown great promise as a high-performance thermal barrier coating (TBC) for aeroengines because of its low thermal conductivity and strong resistance against calcium-magnesium-alumino-silicate (CMAS). Investigating the mechanical characteristics and failure mechanisms of H f 6 T a 2 O 17 ceramic materials at high temperatures is crucial for developing novel TBCs. In this study, H f 6 T a 2 O 17 was prepared by hot pressing 16 mol% Ta-doped H f O 2 powders in a vacuum, followed by annealing and pressureless sintering in air. The fracture toughness of the synthesised H f 6 T a 2 O 17 gradually decreased from 3.8136 M P a m 1 / 2 to 0.5806 M P a m 1 / 2 with the increase in the temperature from 25°C to 1600°C. Furthermore, the superstructure toughening mechanism and brittle-ductile transition mechanism were discussed in this paper. The results are beneficial in evaluating the effects of high temperature on the mechanical properties of H f 6 T a 2 O 17. [ABSTRACT FROM AUTHOR]

To study the effect of morphology of thermally grown oxide on the stress distribution and evolution under cyclic thermal loading, a three-dimensional finite element model of a turbine blade with thermal barrier coatings is developed, in which the coating deposition process, high temperature creep and elastic–plastic behavior are taken into account. Based on the simulation results, dangerous regions in thermal barrier coatings can be predicted. It is shown that, during the cooling stage, tensile stress occurs at the peak of thermally grown oxide/bond coating interface, and compressive stress lies in the valley. With the increase of thickness and amplitude of thermally grown oxide, both the maximum tensile and compressive stresses increase, and the stress distribution is more sensitive to its amplitude than thickness. [ABSTRACT FROM AUTHOR]

Abstract: Thermal barrier coatings (TBCs) are commonly used as protective coatings for engine metal components to improve performance. Many investigations have shown that residual stresses in TBCs applications play an important role, but the residual stresses are mainly obtained by simulation method. As we know, there are a few analytical solutions of residual stress in TBCs system. In this paper, a new two-dimensional analytical solution has been obtained under the condition of non-linear coupled effects of temperature gradient, thermal fatigue, deposited residual stress, thermally grown oxide (TGO) thickening, elasto-plasticity deformation and creep deformation of TBC. Moreover, the influences of bending moment and curvature on stress variation in TBCs are considered during thermal cycling. The calculated results are in agreement with the prior experimental results. [Copyright &y& Elsevier]

Understanding the thermal insulation performance and thermal stress of thermal barrier coating (TBC) coupled with internal cooling and film cooling plays a key role in designing and predicting the failure of TBC. In this work, A one-dimensional theoretical model is presented to analyze the trends of performance of TBC with respect to various parameters. Based on the fluid-solid coupling method, a three-dimensional (3D) finite element model of a guide vane coated with TBC is performed to evaluate the thermal insulation effectiveness and thermal stress of TBC. Furthermore, the effect of film hole geometries on the thermal insulation performance and thermal stress distribution of TBC are investigated. It is found the thermal insulation effectiveness decreases with the increase of film cooling effectiveness but increases with external/internal convective heat transfer coefficients. According to the thermal stress results, the dangerous regions of TBCs are predicted. Stress concentration occurs at the upstream position of film hole. Compared with the round and fan holes, the thermal stress of TBC with trench hole is lowest. Comprehensive consideration the thermal insulation performance and stress distribution of TBC with different film hole geometries, the trench hole is a better design for a longtime service of TBC. • Increased external/internal convective heat transfer coefficients increases the thermal insulation performance of TBC. • Design of film hole should take both the performance and life of TBC into account. • Adding a trench to rows of film cooling holes reduces mechanical stresses for a longer service life of TBC. [ABSTRACT FROM AUTHOR]

Hf 6 Ta 2 O 17 ceramics are successfully prepared by solid state reaction and pressureless sintering. The mechanical properties and the calcium-magnesium-alumino-silicate (CMAS) corrosion behavior of Hf 6 Ta 2 O 17 ceramics are studied. The hardness, elastic modulus and fracture toughness of Hf 6 Ta 2 O 17 ceramics are 18.45 GPa, 273.42 GPa and 2.6–3.1 MPa m1/2, respectively. As the Hf 6 Ta 2 O 17 ceramics are attacked by CMAS, the reaction layer and dense layer are formed on the ceramic surface, which prevents the further infiltration of molten CMAS. HfSiO 4 and Ca 2 Hf 7 O 16 are confirmed as the main reaction products of Hf 6 Ta 2 O 17 and CMAS. Hf 6 Ta 2 O 17 ceramics exhibit better CMAS corrosion resistance than 8 wt% yttria stabilized zirconia (8YSZ) ceramics, which is attributed to the dense structure formed by corrosion products (hafnium tantalum oxide and Ca 2 Hf 7 O 16) and lower theoretical optical basicity (OB) value. Furthermore, Hf 6 Ta 2 O 17 /YSZ double ceramic top coat thermal barrier coatings (TBCs) are successfully prepared by plasma spraying, and the thermal cycling performance is investigated. Hf 6 Ta 2 O 17 /YSZ double-layers TBCs has good thermal cycling performance as 8YSZ single-layer TBCs. [ABSTRACT FROM AUTHOR]

Thermal shock failure is the most common type occurring in service. The thermal shock life is overestimated and the failure process of thermal barrier coatings (TBCs) is still not clear due to lack of environmental simulator and the real-time detection methods. In this work, the damage evolution and failure mechanism of EB-PVD TBCs coated on a turbine vane under thermal shock is studied by acoustic emission combined with digital image correlation (DIC) method. Four damage modes are discriminated by spectrum of the acoustic emission signals, which are surface vertical cracks (200–220 kHz), sliding interfacial cracks (300–325 kHz), opening interfacial cracks (400–450 kHz) and substrate deformation (90–110 kHz). The principal strain of TBCs surface varies from compressive strain to tensile strain with increasing the thermal shock cycles. Furthermore, the thermal shock life is 710 cycles, and the exfoliation area is located in the middle of the leading edge. The failure mechanism is the propagation and coalescence of surface and interfacial cracks during the cooling stage due to the huge thermal gradient and stress concentration around the film holes. • The heating rate at the leading edge of a turbine vane is faster than that of other locations. • Four damage modes are discriminated by spectrum of the acoustic emission signals. • The principal strain of ceramic coating changes from compressive to tensile strain. [ABSTRACT FROM AUTHOR]

• A phase field model fully couples the CMAS diffusion and the reaction-induced stresses is developed. • The presence of mechanical action accelerates the corrosion kinetics in TBCs. • Approximately 3.4% volume strain caused by CMAS corrosion reaction is predicted. Thermal barrier coatings (TBCs) are prone to attacking by molten calcium-magnesium-alumino-silicates (CMAS) in service. Reactions of molten CMAS and the ceramic coating are solution-reprecipitation ones that generate a new phase, ZrSiO 4. Swelling caused by these reactions result in accumulation of localized stresses in the ceramic coating, which subsequently contributes to the premature failure of TBCs. In this work, the CMAS diffusion-reaction with TBCs is investigated based on the phase field theory. A two-dimensional phase field model is developed, which fully couples the Cahn-Hilliard equation for the diffusion of CMAS and the constitutive equation for the prediction of the chemical reaction-induced stresses. The results indicate that very high compressive stresses emerged in the coating, which may lead to the spallation failure of the coating. Furthermore, it is found that the presence of mechanical action accelerates the corrosion kinetics in TBCs and the evolution of CMAS concentration in the coating is consistent with previous reports. Finally, approximately 3.4% volume strain caused by CMAS corrosion reaction is predicted. [ABSTRACT FROM AUTHOR]

The interface of atmospheric plasma spray (APS) thermal barrier coatings (TBCs) is extremely rough and uneven, causing extremely complicated laws in thermally grown oxide (TGO) growth, stress distribution and evolution, as well as crack initiation and expansion. In this paper, a fracture phase field theory of TBCs interfaces with considerations of growth and thermal stresses is established. A two-dimensional geometric model of APS TBCs was constructed based on the real interface morphology. Next, the TGO growth laws and interface spallation mechanism during the service of TBCs were studied through numerical simulation and experiment. Results demonstrated that the TGO growth rate at the convex position on the interface is higher than those at other positions, especially at the local convex positions. TGO at the left and right sides of concave positions approach continuously with oxidisation of the interface, causing the TGO to wrinkle and the interface to become rougher. With an increase in TGO thickness, stresses in the TGO at peak areas perpendicular to the interface increase significantly and their maximum reaches to 1.58 GPa. Interface cracks initiated in peak areas around the TGO/BC interface at 877 h, and then formed in flat positions in the TGO at 912 h as well as valley areas around the TGO/TC interface at 928 h. Final, TBCs failed at 959 h due to the coalescence of these cracks. All four types of interface cracks could be found in the simulation results. • A TGO growth simulation was developed based on a real interface morphology of TBCs. • The TGO growth rate at the convex of interface is higher than those at other positions. • Stresses in TGO at peak areas perpendicular to the interface increase significantly. • The initiation, propagation and coalescence laws of interface cracks was obtained. • All four types of interface cracks could be found in the simulation results. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBCs) are key to improving aero-engine performance. The lifetime assessment of TBCs in a simulation environment is an essential consideration for successful application. Environmental simulators for rotor blade TBCs are still scarce, largely owing to the difficulty in obtaining real-time TBC failure data in extreme environments. In this work, an environmental simulator with high-speed rotation and gas thermal shock characteristics was developed and tested. High-temperature resistant strain gauges and thermocouples were fixed onto the surface of turbine blades by a flame-sprayed Al 2 O 3 layer. The damage evolution and failure mechanism of TBCs were analysed by the real-time detection of strain damage and temperature field. The results reveal only a little fluctuation in the temperature distribution of the TBC surface (122 °C) under the synergistic effect of high-speed rotation and thermal shock, and the strain at each measuring point is tensile. During thermal cycling tests of the TBCs, the strain at a specific point varied from 0.21% to 0.79%. The final thermal shock life was determined to be 156 cycles, thus proving that the coating enables good thermal shock resistance. The spalled area was mainly distributed from the suction surface to the leading edge, and TBC failure could be attributed to crack initiation and propagation caused by the impact of the high-temperature flame and accumulation of fatigue load. The present work paves the way for coating preparation process optimisation and real service performance evaluation. • A turbine rotor blade environment simulator with real-time detection function was developed. • The temperature of the suction surface is greater than that of the pressure surface. • The strain gradient is the largest in the 0° direction. • When the speed is increased, the surface temperature decreases but the principal strain increases. • The principal strain of the thermal barrier coatings during the thermal shock cycle varied from 0.21% to 0.79%. [ABSTRACT FROM AUTHOR]

Abstract: An acoustic emission technique was used to monitor the cracking behavior and fracture process of thermal barrier coatings subjected to tensile loading. Acoustic emission signals were extracted and preformed by fast Fourier transform, and their characteristic frequency spectrums and dominant bands were obtained to reveal fracture modes. Three different characteristic frequency bands were confirmed, corresponding to substrate deformation, surface vertical cracking and interface delamination, with the aid of scanning electronic microscopy observations. A map of the tensile failure mechanism of air plasma-sprayed thermal barrier coatings was established. The fracture strength and interfacial shear strength were estimated as 45.73±3.92MPa and 20.51±1.74MPa, respectively, which are well in agreement with available results. [Copyright &y& Elsevier]

Abstract: By using specially designed freestanding 8wt.% Y2O3 stabilized ZrO2 (8YSZ) specimens, a linear relationship is found between the Raman peak shift, Δω, and applied uniaxial compressive stress, σ̅ u , i.e., Δω =Π u σ̅ u with Π u being the piezo-spectroscopic coefficient. The linear relationship is used to determine the in-plane residual stress in air plasma-sprayed 8YSZ thermal barrier coatings (TBCs). It is shown that the Π u of the 8YSZ is an exponential function of the thermal cycle N for a given thermal cycling process: Π u =37.6exp(− N/25.9)+13. Based on these two relationships, the actual residual stress of 8YSZ TBCs determined by the Raman spectroscopy method is quantitatively consistent with that obtained by the X-ray diffraction. [Copyright &y& Elsevier]