EB-PVD Thermal Barrier Coatings Research Articles

Reliability analysis of thermal barrier coatings under CMAS deposition and penetration

To address the issue of thermal barrier coating (TBC) failure and spalling caused by the deposition and infiltration of environmental particulate matter, a thermal-mechanical coupling model for EB-PVD TBC considering CMAS deposition failure was developed. Based on this model, the effects of CMAS deposition thickness and infiltration depth on TBC spalling failure were numerically studied. First, the deposition characteristics of CMAS were obtained using a critical velocity model. Second, the corresponding TBC failure criteria were established. Then, the effects of CMAS deposition thickness and infiltration depth on TBC spalling failure were specifically studied under different CMAS particle sizes and service times. The results show that the CMAS deposition thickness is the greatest at the leading edge and the pressure surface near the trailing edge of the blade. The CMAS infiltration depth is influenced by the temperature field, with higher temperatures leading to greater infiltration depth. With the simultaneous increase of service time and particle size, the average damage value of the coating surface (D¯) initially increases slightly to 0.2, then rapidly to 0.9, and finally slowly to 0.97, resulting in almost complete failure and spalling of the coating.

Assessment of the microstructural features and electrochemical corrosion behaviour of nanostructured Gd2Zr2O7 EB-PVD TBCs under three salt conditions

Improving corrosion resistance is a challenging task, as it determines the life time of thermal barrier coatings (TBCs). In this regard, this study was aimed to characterize the electrochemical corrosion behaviour of nanostructured Gadolinium Zirconate in three different corrosive salt conditions such as Na2SO4 + 55 wt% V2O5 (SM1), Na2SO4 + 10 wt% NaCl (SM2), and Na2SO4 + 7.5 wt% NaCl +7.5 wt% V2O5 (SM3). The uniformity of the coated surface was verified by mapping the vibrational bands associated with Gd and Zr using FT-IR and Raman spectroscopy. The coated surface possessed a uniform columnar structure with leafy appearance. Potentiodynamic polarisation studies and electrochemical impedance spectroscopic data demonstrated that the nanostructured GZ exhibited superior corrosion resistance properties under SM1 salt condition as compared to SM2 and SM3 conditions. SEM and EDS analysis were used to evaluate the nanostructured GZ coatings that were subjected to electrochemical corrosion. In the case of SM2 and SM3 conditions, the surfaces were degraded with microcracks. The formation of corrosion products and the exposure of succeeding layers as a result of corrosion were also characterized using FT-IR, Raman spectroscopy, XRD technique and 3D optical microscope.

Analysis of infrared emissivity of EB-PVD thermal barrier coatings at the microscopic level

Thermal barrier coatings (TBCs) applications of yttria-stabilized (YSZ) are widely utilized on turbine blades of aviation engines due to its low thermal conductivity and high electrical conductivity. Predicting the infrared emissivity aids in monitoring blade operating temperatures, ensuring safe engine operation. This paper innovatively introduces a novel three-dimensional columnar EB-PVD coating model and utilizes the FDTD method to predict emissivity. The model exhibits strong consistency with experimental results for predicted emissivity in the short-wave infrared band (1.4–3 μm). The unique microstructure of the EB-PVD thermal barrier coatings significantly influence the infrared emissivity. The results indicate that vertically uniform arranged feathers exhibit higher emissivity levels compared to feathers with varying densities. Additionally, increasing the thickness of the ceramic top coating and reducing spherical closed porosity also enhance the emissivity levels. This research provides guidance for controlling emissivity at the microscale in the preparation of thermal barrier coatings.

Microstructures evolution, corrosion and oxidation mechanisms of EB-PVD thermal barrier coatings exposed to molten salt corrosion

Hot corrosion caused by molten salt attack has been one of the main factors leading to the thermal barrier coating (TBC) degradation and premature failure. In this work, the molten salt corrosion behaviours of EB-PVD TBCs exposed to V2O5 + Na2SO4 at 1050 ℃ were and characterized. Hot corrosion mechanisms, microstructures evolution, and failure behaviours of TBCs under hot corrosion were investigated. The molten salt would react with the stabilizer Y2O3, which caused the depletion of Y2O3 phase in the ceramic coating, promoting the destabilization of metastable zirconia (t′-ZrO2) into monoclinic zirconia (m-ZrO2), resulting in obvious volume expansion of ceramic coating. The corrosion and formation of new products would densify the ceramic coating and generate high level stress in the TBCs, leading to the formation of micro cracks and pores in the ceramic layer. In addition, the growth behaviour and elemental composition of thermally grown oxide (TGO) under molten salt corrosion were determined. Hot corrosion significantly accelerated the growth of TGO, as the thickness of TGO layer under corrosion was about five times that after isothermal oxidation. Hot corrosion also destroyed the integrity of TGO layer, and the loose and porous TGO layer formed during corrosion would lead to cracking at the interface between the ceramic layer and bonding layer. The formation of cracks and pores under corrosion finally resulted in the premature failure of TBCs.

Reliability Evaluation of EB-PVD Thermal Barrier Coatings in High-Speed Rotation and Gas Thermal Shock

The uncertain service life of thermal barrier coatings (TBCs) imposes constraints on their secure application. In addressing this uncertainty, this study employs the Monte Carlo simulation method for reliability evaluation, quantifying the risk of TBC peeling. For reliability evaluation, the failure mode needs to be studied to determine failure criteria. The failure mode of high-speed rotating TBCs under gas thermal shock was studied by combining fluid dynamics simulations and experiments. Based on the main failure mode, the corresponding failure criterion was established using the energy release rate, and its limit state equation was derived. After considering the dispersion of parameters, the reliability of TBCs was quantitatively evaluated using failure probability and sensitivity analysis methods. The results show that the main mode is the fracture of the ceramic layer itself, exhibiting a distinctive top-down “step-like” thinning and peeling morphology. The centrifugal force emerges as the main driving force for this failure mode. The failure probability value on the top side of the blade is higher, signifying that coating failure is more likely at this location, aligning with the experimental findings. The key parameters influencing the reliability of TBCs are rotation speed, temperature, and the thermal expansion coefficient. This study offers a valuable strategy for the secure and reliable application of TBCs on aeroengine turbine blades.

Research on crack propagation behaviour of EB-PVD TBCs based on TGO evolution

Thermal Barrier Coatings (TBCs) are functional coatings used to protect high-temperature components that are prone to early damage and premature failure under the influence of complex working conditions. This paper examines the crack propagation behaviour of 8% yttria-stabilized zirconia (8YSZ) EB-PVD TBCs under different oxidation conditions at 1100 °C. The morphology of interfacial cracks after oxidation was summarized and the evolution of thermally grown oxide (TGO) was quantified. Based on the evolution of TGO, the causes of crack propagation were analyzed. For the specimens after oxidation experiment, the interfacial crack propagation behaviour was observed and analyzed by SEM, and the reason of lateral crack propagation was explained from the perspective of interfacial fracture toughness. The reason for crack deflection is analyzed from the perspective of energy release rate. The equivalent thickness, normalized rumpling index and two-dimensional roughness index were calculated, then the TGO growth behaviour was comprehensively analyzed and related to the crack propagation.

Enhancing the performances of EB-PVD TBCs via overlayer Al-modification

Thermal barrier coatings (TBCs) guarantee the service life of high-temperature alloy substrates under harsh service environments. Premature failure of TBCs due to coating fracture, high-temperature oxidation, and calcium oxide–magnesium oxide–aluminum oxide–silicon oxide (CMAS) corrosion, which significantly reduce their service lifespan. In this study, Al film was applied on the surface of EB-PVD TBCs using arc ion plating, and then converted it to α-Al2O3 by an in situ reaction between Al and ZrO2. The α-Al2O3 have toughening effect resulting in a higher bending resistance of EB-PVD TBCs. Because α-Al2O3 could seal the columnar inter-crystal gaps on the surface of EB-PVD TBCs, it reduced oxygen penetration, which enhanced the high-temperature oxidation resistance. CMAS reacted with α-Al2O3 to generate spinel and calcium‑aluminum yellow feldspar compounds at 1350 °C, which prevented following CMAS penetration and corrosion. Furthermore, Al-modification reduced the surface roughness, which made Al-modified TBCs have better wetting resistance.

Thermal shock behaviors and failure mechanisms of EB-PVD thermal barrier coatings using radiant heating

In this work, the thermal shock behaviors and failure mechanisms of the thermal barrier coatings (TBCs) were investigated with the self-built thermal shock platform. The results showed that the thermal shock cycles significantly could affect the microstructure and element distribution of TBCs. Firstly, the size of columnar gaps in the top coating (TC) layer increased with the increase of the thermal shock cycles. And, the transverse micro-cracks generated near the tip of the columnar gaps under the action of gradient stress. Moreover, Al elements in the bond coating (BC) significantly affected the growth mechanism of thermally grown oxide (TGO). The growth rate of the mixed oxide layer would be accelerated when the concentration of Al element in the BC layer dropped to a certain value. Finally, the accumulation of the thermal mismatch stress induced transverse cracks initiation and propagation in the porous mixed oxides layer, and the adhesion strength of TBCs decrease. When the transverse crack was connected with the columnar gaps, the TBCs would peeling off and failure at the interface of TC and TGO.

Friction delamination mechanism of EB-PVD thermal barrier coatings in high-temperature and high-speed rotating service environment

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.

Quantitative Characterization of the Interfacial Damage in EB-PVD Thermal Barrier Coating

Considering the influence of non-equibiaxial stress state and initial residual strain on the compressive buckling of the ceramic layer, a quantitative characterization method of the damage generated at the interface between the top coat and bond coat in thermal barrier coating based on uniaxial compression was developed. It was verified by the axial compression tests of the single crystal specimens with EB-PVD thermal barrier coating after undergoing various isothermal oxidation times and thermal cycles. On this basis, the correlations between the measured interfacial damage and the thermal loads experienced as well as the thickness of thermally grown oxide (TGO) were analyzed. The results show that the critical compressive strain inducing the spallation of thermal barrier coating at room temperature can effectively characterize the accumulation of interfacial damage caused by isothermal oxidation and thermal fatigue. Under the same TGO thickness, the damage caused by thermal fatigue is greater than that caused by isothermal oxidation. The total damage generated in thermal barrier coating can be divided into three parts: oxidatively driven damage related to TGO thickness, mechanically driven damage related to stress–strain cycles in the coating, and their interaction, where the interaction term is negative.

Real-time detection of damage evolution and failure of EB-PVD thermal barrier coatings using an environmental simulator with high-temperature and high-speed rotation

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 Al2O3 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.

Impedance spectroscopy of thermal barrier coatings as non-destructive evaluation tool for failure detection

Abstract Since thermal barrier coatings (TBCs) for turbine blades suffer from bond coat oxidation and sintering of the ceramic top coat during service, quantification of TBC degradation by non destructive evaluation methods (NDE) is essential. IN617 substrates with standard NiCoCrAlY/PYSZ EBPVD TBCs were annealed at 1100 °C and measured by impedance spectroscopy. Parameters such as phase angle, total impedance, real and imaginary part were analyzed. To clearly separate bond coat oxidation from aging of the zirconia, freestanding ceramic top coats and ceramic free samples were analyzed as well. A straight correlation between the changes in the impedance spectra and the measured thickness of the thermally grown oxide was found.

Contemporary Materials Issues for Advanced EB-PVD Thermal Barrier Coating Systems

AbstractThe key issues for thermal barrier coating (TBC) development are high temperature capability and durability under thermal cyclic conditions as experienced in the hot section of gas turbines. Due to the complexity of the system and the interaction of the constituents, performance improvements require a systems approach. However, there are issues closely related to the ceramic top coating and the bond coat, respectively. Reduced thermal conductivity, sintering, and stresses within the ceramic coating are addressed in the paper as well as factors affecting failure of the TBC by spallation. The latter is primarily governed by the formation and growth of the thermally grown oxide scale and therefore related to the bond coat. A strategy for lifetime assessment of TBCs is discussed.

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

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

Effect of pre-oxidation on the failure mechanisms of EB-PVD thermal barrier coatings with (Ni,Pt)Al bond coats

In order to clarify the effect of pre-oxidation for (Ni,Pt)Al bond coat on failure modes of TBCs, cyclic oxidation behavior of thermal barrier coatings system with both pre-oxidized and normal (Ni,Pt)Al bond coats was investigated at 1100 °C. The results indicated that thermal cycling resistance of TBCs was dramatically enhanced by pre-oxidation, where thinner and lower residual stress oxide scale, lighter rumpling of the BC/TGO interface were achieved pre-oxidized. The failure modes of these two TBCs were completely diverse. Besides, the lower oxygen partial pressure effectively enhanced transformation from θ to α-Al2O3 during pre-oxidation.

Microstructure Dependence of Effective Thermal Conductivity of EB-PVD TBCs.

Microstructure dependence of effective thermal conductivity of the coating was investigated to optimize the thermal insulation of columnar structure electron beam physical vapor deposition (EB-PVD coating), considering constraints by mechanical stress. First, a three-dimensional finite element model of multiple columnar structure was established to involve thermal contact resistance across the interfaces between the adjacent columnar structures. Then, the mathematical formula of each structural parameter was derived to demonstrate the numerical outcome and predict the effective thermal conductivity. After that, the heat conduction characteristics of the columnar structured coating was analyzed to reveal the dependence of the effective thermal conductivity of the thermal barrier coatings (TBCs) on its microstructure characteristics, including the column diameter, the thickness of coating, the ratio of the height of fine column to coarse column and the inclination angle of columns. Finally, the influence of each microstructural parameter on the mechanical stress of the TBCs was studied by a mathematic model, and the optimization of the inclination angle was proposed, considering the thermal insulation and mechanical stress of the coating.

Thermal cycling of EB-PVD TBCs based on YSZ ceramic coat and diffusion aluminide bond coat

The objective of this research was to optimize the surface morphology and element compositional distribution of the bond coat by adjusting the hydrogen flow, and to fabricate the 6–8 wt% yttria partially stabilized zirconia (YSZ) ceramic topcoat via EB-PVD. The interfacial stability and spalling failure behavior of thermal barrier coatings (TBCs) were evaluated by cyclic oxidation test at 1373 K. The results show that with the increment of hydrogen flow, the X-ray diffraction peak belonging to the β phase tends to be obvious and the high temperature oxidation resistance becomes correspondingly excellent. There are no microcracks, spalling, peeling, edge warping and cloud spatter on the ceramic coating surface. The cross section of the ceramic layer exhibits a typical columnar grain microstructure. The columnar crystal grows evenly perpendicular to the interface between the ceramic and bonding layers. Although it has been exposed to long-term thermal exposure, the ceramic coat has densified and sintered locally, and the grain boundary between the columnar grains is basically disappeared, the ceramic coat surface still has the typical columnar crystal “cauliflower” shape. Lots of transverse cracks and voids have been found at the interface of the bond coat and TGO layer, and within a dozen micrometers approach the bond coat surface. Internal fracture has occurred in the bonding layer and bridging structures have been developed due to internal oxidation in the bond coat.

Chemo-thermo-mechanical modeling of EB-PVD TBC failure subjected to isothermal and cyclic thermal exposures

The thermal barrier coatings (TBC) are working under chemo-thermo-mechanical loading conditions and the failure process is characterized by complex aluminum migration, oxidation reactions and cyclic thermo-mechanical degradation. In the present work, the aluminum migration in EB-PVD TBCs subjected to isothermal and cyclic thermal exposure was studied and integrated into fracture mechanics description of the TBC system failure. Experimental and analytical investigations revealed that the damage of the TBC cannot be uniquely characterized by the thickness of the thermal growth oxide layer (TGO) and is furthermore related to the chemical reactions around TGO as well as evolutions of residual stresses. The expansion of the TGO played a key role in the mechanical performance of TBC. It was confirmed that the residual stress evolution is linearly correlated with the rumpling surface length of the TGO layer. Failure of the TBC system was characterized by degradation of the TGO interface and increment of residual stresses. A life model was developed based on the concept of the fracture toughness depending on interface degradation and residual stress evolution.

Real-time detection of damage evolution and fracture of EB-PVD thermal barrier coatings under thermal shock: An acoustic emission combined with digital image correlation method

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.

Experimental and Modeling Studies of Bond Coat Species Effect on Microstructure Evolution in EB-PVD Thermal Barrier Coatings in Cyclic Thermal Environments

In this work, the effects of bond coat species on the thermal barrier coating (TBC) microstructure are investigated under thermal cyclic conditions. The TBC samples are prepared by electron beam-physical vapor deposition with two species of bond coats prepared by either air-plasma spray (APS) or high-velocity oxygen fuel (HVOF) methods. The TBC samples are evaluated in a variety of thermal cyclic conditions, including flame thermal fatigue (FTF), cyclic furnace thermal fatigue (CFTF), and thermal shock (TS) tests. In FTF test, the interface microstructures of TBC samples show a sound condition without any delamination or cracking. In CFTF and TS tests, the TBCs with the HVOF bond coat demonstrate better thermal durability than that by APS. In parallel with the experiments, a finite element (FE) model is developed. Using a transient thermal analysis, the high-temperature creep-fatigue behavior of the TBC samples is simulated similar to the conditions used in CFTF test. The FE simulation predicts a lower equivalent stress at the interface between the top coat and bond coat in bond coat prepared using HVOF compared with APS, suggesting a longer cyclic life of the coating with the HVOF bond coat, which is consistent with the experimental observation.