Local residual stress evolution of highly irregular thermally grown oxide layer in thermal barrier coatings

Local residual stress in thermally grown oxide (TGO) layers is the primary cause of failure of thermal barrier coating (TBC) systems, especially TBCs prepared by air plasma spray (APS) with a highly irregular TGO. Herein, the distribution of residual stress and the evolution of the irregular TGO layer in APS TBCs were investigated as a function of oxidation time. The stress was measured from cross-sectional micrographs and converted to the actual stress inside the coatings before sectioning. The TGO exhibited significant inhomogeneity at different locations. Stress conversion occurred across the TGO thickness; the layer near the yttria-stabilised zirconia (YSZ) component exhibited compressive stress, whereas that along the bond coat was under tensile stress. The evolution of the compressive stress is also discussed. These analyses may provide a better understanding of the mechanism of APS TBCs.     

Effect of TGO Thickness on Thermal Cyclic Lifetime and Failure Mode of Plasma‐Sprayed TBCs

Gradient thermal cycling test was performed on atmospheric plasma‐sprayed (APS) thermal barrier coatings (TBCs) with different thermally grown oxide (TGO) thicknesses. The TBCs with a thickness of TGO from 1.3 μm to 7.7 μm were prepared by controlling isothermal oxidation time of cold‐sprayed MCrAlY bond coat. The gradient thermal cycling test was performed at a peak surface temperature of 1150°C with 150°C difference across 250 μm thick YSZ with a duration of 240 s for each cycle. Results indicate that the thermal cyclic lifetime of APS TBCs is significantly influenced by TGO thickness. When initial TGO thickness increases from 1.3 μm to 7.7 μm, the thermal cyclic lifetime decreases following a power functions by a factor of about 20. It was revealed that there exists a critical TGO thickness over which the thermal cyclic lifetime is reduced more significantly with the increase in TGO thickness. Moreover, two typical failure modes were observed. The failure mode changes from the cracking within APS YSZ at a TGO thickness less than the critical value to through YSZ/TGO interface at TGO thickness range higher than the critical value.     

Photoluminescence Piezospectroscopy: A Multi-Purpose Quality Control and NDI Technique for Thermal Barrier Coatings

Many thermal barrier coating (TBC) applications have been hampered by the lack of accurate quality control, nondestructive inspection, remaining life assessment, and lifetime prediction tools for TBCs. Photoluminescence piezospectroscopy (PLPS) has the potential to provide these capabilities. The PLPS technique provides for the measurement of stress in the thermally grown oxide (TGO) and for the determination of TGO type (α, θ, γ alumina) and distribution. The capability of PLPS for these various applications will be described based on specimen and engine component measurements.     

Residual stress distribution along interfaces in thermal barrier coating system under thermal cycles

The residual interfacial stress plays an important role in crack initiating and propagating along the interface, which could result in delamination failure of the thermal barrier coatings (TBCs). In this study, the finite element model of air plasma spraying(APS) TBCs was established to assess the level and distribution of residual stress along top coat(TC)/thermally grown oxide (TGO) and bond coat (BC)/TGO interfaces under thermal cycles. Instead of using vertical stress S₂₂ in global coordinate system, the normal and tangential components in the local system along the interfaces, transformed from stress components S₁₁, S₂₂, and S₁₂ in the global one, were used to evaluate the way the cracks initiate and propagate along the interfaces. Firstly, the effect of the number of thermal cycles on residual stress was investigated. It was found that, for the TBCs model without TGO growth and crack, the impact of the number of thermal cycles on the stress is very insignificant and could be ignored. So the present study only chose to focus on the first thermal cycle. Then the influence of the TGO thickness and the interface amplitude on the normal and tangential residual stresses for both homogeneous and inhomogeneous temperature fields was explored. The results show that the TGO thickness, interface amplitude and temperature field affect the residual stress level and distribution, leading to different fracture mechanisms along TC/TGO and TGO/BC interfaces. Finally, the difference between the vertical stress in the global coordinate system and the normal stress in the local coordinate system was studied. Compared with vertical stress S₂₂, the stress components normal and tangential to the TC/TGO and TGO/BC interfaces are more appropriate to describing the stress distribution along the interfaces and predicting the propensity of crack initiating and propagating along the interfaces.     

Stress states in plasma-sprayed thermal barrier coatings upon temperature cycling: Combined effects of creep,plastic deformation,and TGO growth

To ascertain material parameter effects on the stress states is beneficial to comprehend the crack growth behavior and delamination mechanism in thermal barrier coatings (TBCs). In this work, numerical models are established to explore the combined effects of material parameters including creep, plastic deformation, and thermally grown oxide (TGO) growth on the stress states upon temperature cycling. For all layers, thermal-physical properties reliant on temperature are incorporated into the model. The process of bond coat (BC) oxidation, namely TGO growth, is materialized by changing material properties with cycles. Based on the principle of a single variable, the residual stress states are explored using many different material combinations. The results indicate that the tensile stress in the ceramic top coat (TC) decreases with the increase in the TGO lateral strain distribution gradient. Increasing the BC yield strength or decreasing the TGO growth stress can reduce the tensile stress in TC if there is no creep in the model. When BC yield strength is relatively high (≥150 MPa), BC creep will strengthen the TC tensile stress. TGO creep can decrease the tensile stress in TC irrespective of TGO growth stress and BC creep. When TGO creep rate is higher than 10B^tgo, an exceedingly small tensile stress can always be achieved. This work could provide significant theory direction for material selection and composition control towards advanced TBCs with prolonged lifetime.     

Stress states and crack behavior in plasma sprayed TBCs based on a novel lamellar structure model with real interface morphology

To ascertain the crack growth behavior and coalescence mechanism in thermal barrier coatings (TBCs) is beneficial for understanding the failure of TBCs and proposing the probable optimization methods. In this work, a novel lamellar structure model with real interface morphology is developed to explore the crack growth behavior and the failure mechanism of TBCs during thermal cycling. Three typical defects which include pore, inter-splat crack, and intra-splat are incorporated in the model. To simulate the oxidation process of the bond coat (BC) realistically, The oxidation growth process is simulated via changing the BC properties to thermally grown oxide (TGO) properties layer by layer. The effects of the lateral growth strain distribution through TGO thickness on the stress states are executed. Moreover, the influences of BC creep on the crack growth and coating lifetime are further elaborated. The results show that the larger the lateral growth strain gradient, the smaller the residual tensile stress. The irregular interface morphology results in the redistribution of residual stresses. Although the pores and cracks can alleviate the tensile stress near the valley, large stress concentration will occur near them. At the early phase of thermal cycling, the cracks grow steadily. After more cycles, the cracks propagate rapidly and merge with others. The simulated delamination path is in agreement with the experiment results. Not only does BC creep change the crack coalescence mechanism, it also decreases the thermal cyclic lifetime of TBCs. The coating optimization method proposed in this study provides another option for developing advanced TBCs with longer lifetime.     

Thermal shock resistance of air plasma sprayed thermal barrier coatings

The spallation resistance of an air plasma sprayed (APS) thermal barrier coating (TBC) to cool-down/reheat is evaluated for a pre-existing delamination crack. The delamination emanates from a vertical crack through the coating and resides at the interface between coating and underlying thermally grown oxide layer (TGO). The coating progressively sinters during engine operation, and this leads to a depth-dependent increase in modulus. Following high temperature exposure, the coating is subjected to a cooling/reheating cycle representative of engine shut-down and start-up. The interfacial stress intensity factors are calculated for the delamination crack over this thermal cycle and are compared with the mode-dependent fracture toughness of the interface between sintered APS and TGO. The study reveals the role played by microstructural evolution during sintering in dictating the spallation life of the thermal barrier coating, and also describes a test method for the measurement of delamination toughness of a thin coating.     

Effect of CeO2 doping on high temperature oxidation resistance of YSZ TBCs

In the present work, YSZ TBCs and 10 wt% CeO₂-doped YSZ thermal barrier coatings (CeYSZ TBCs) were prepared via atmospheric plasma spraying(APS) respectively, whereupon high temperature oxidation experiment was carried out at 1100 °C to compare the high temperature oxidation behavior and mechanism of the two TBCs. The results showed that the doping of CeO₂ reduced the porosity of YSZ TBCs by 23%, resulting in smaller oxidation weight gain and lower TGO growth rates for CeYSZ TBCs. Besides, the TGO generated in CeYSZ TBCs was obviously thinner and there were fewer defects inside it. For YSZ TBCs, as the oxidation process proceeded, Al, Cr, Co and Ni elements in the bonding coating were oxidized successively to form loose and porous spinel type oxides (CS), which was apt to cause the spalling failure of TBCs. While, the Al₂O₃ layer of the TGO generated in CeYSZ TBCs ruptured later than that in YSZ TBCs, which delayed the oxidation of Cr, Co, and Ni elements and the formation of CS accordingly. Therefore, CeO₂ doping can effectively improve the high temperature oxidation resistance of YSZ TBCs.     

Thermal cycling behavior of EB-PVD rare earth oxides co-doping ZrO2-based thermal barrier coatings

Novel ceramic topcoat of Gd₂O₃–Yb₂O₃–Y₂O₃ co-stabilized ZrO₂ (GYbYSZ) thermal barrier coatings were fabricated via EB-PVD technique. The phase structural stability, phase constituent, chemical composition, morphology and cyclic oxidation of the thermal barrier coatings (TBCs) were systematically studied. Based on the XRD results, the GYbYSZ ceramics has not undergone phase transformation upon long-term annealing at 1373 K and 1523 K. Although the chemical content of the GYbYSZ ceramic coat deviates from the stoichiometric value, the coating is mostly composed of cubic phase, which is accord with the XRD pattern of the original ingot. A pyramidal-like morphology appears in the microtexture of the column tips and the measured diameters of the pyramids are about 2.5~4 μm. After thermal cycling, the surface of the coating presents a multi-layer structure, which is followed by layer-by-layer spallation. The failure zone of the ceramic coats is possible to occur the interior of the thermally grown oxide (TGO) layer, or within the top ceramic coat at the interface of bond coat/TGO layers. The degradation of GYbYSZ TBCs is primarily attributed to the accumulation and relaxation of residual stress, propagation of vertical through microcracks, the growth rumpling of TGO layer, the ridges of grain boundary and the abnormal oxidation of bond coat.     

Comparison of stress evolution under TGO growth simulated by two different methods in thermal barrier coatings

The growth of thermally grown oxide (TGO) is a significant factor affecting the failure mechanism of thermal barrier coatings (TBCs) during cyclic high temperature service. In this work, a complicated finite element model with two semicircles reflecting the undulation of TGO interfaces was proposed, and four representative shapes of TGO interfaces were selected. There are mainly two methods to simulate TGO growth under high temperature, and each method was achieved by implementation of user subroutines in finite element method. A total of 100 thermal cycle loads were applied to the TBCs continuously. The stress evolution in the layers of Top Ceramic Coating (TC) and Bond Coating (BC) at the end of each thermal cycle load was obtained, the influence of TGO growth on stress evolution was analyzed, the differences between two methods of TGO growth were discussed. The results show that under TGO growth simulated by the first method, the stress distribution in the y direction does not change in both TC and BC layer, and the maximum stress decreases a lot in TC layer but nearly remains the same in BC. When the growth of TGO was simulated by the second method, stress evolution is complex and undergoes up to five stages with a small undulation or convex of TGO interfaces. Stress evolution in BC layer remains as the same as in the first method. Moreover, the maximum stress increases continually in BC layer. The comparison of these two simulation method would help to study the failure of TBCs caused by TGO growth.     

Residual stress trend in thermal barrier coatings in through thickness direction measured by photoluminescence piezospectroscopy

Abstract

Photoluminescence piezospectroscopy (PLPS) has been used to determine residual stresses in sapphire, alumina in the yttria stablised zirconia (YSZ)/Al₂O₃ composite and alumina in thermal barrier coatings (TBCs). The TBC of YSZ containing 0·5?wt-% alumina has been produced using electron beam physical vapour deposition. The stress profile through the TBC thickness was measured using a depth sensing method. Reasonable residual stress profiles have been obtained using PLPS with the confocal system for all three material systems. Measurements of TBCs suggest that stress distribution in a TBC system is not uniform in general. However, uniform stress distribution has been found in some positions where damage in TBCs might occur.     

Sintering‐induced delamination of thermal barrier coatings by gradient thermal cyclic test

Lifetime is crucial to the application of advanced thermal barrier coatings (TBCs), and proper lifetime evaluation methods should be developed to predict the service lifetime of TBCs precisely and efficiently. In this study, plasma‐sprayed YSZ TBCs were subjected to gradient thermal cyclic tests under different surface temperatures, with the aim of elucidating the correlation between the coating surface temperature and the thermal cyclic lifetime. Results showed that the thermal cyclic lifetime of TBCs decreased with the increasing of surface temperatures. However, the failure modes of these TBCs subjected to thermal cyclic tests were irrespective of different surface/BC temperatures, that is, sintering‐induced delamination of the top coat. The thickness of thermally grown oxide (TGO) was significantly less than the critical TGO thickness to result in the failure of TBCs through the delamination of top coat. There was no phase transformation of the top coat after failure. In contrast, in the case concerning the top coat surface of the failure specimens, the elastic modulus and microhardness increased to a comparable level due to sintering despite of the various thermal cyclic conditions. Consequently, it is conclusive that the failure of TBCs subjected to gradient thermal cyclic test was primarily induced by sintering during high‐temperature exposure. A delamination model with multilayer splats was developed to assist in understanding the failure mechanism of TBCs through sintering‐induced delamination of the top coat. Based on the above‐described results, this study should aid in facilitating the lifetime evaluation of the TBCs, which are on active service at relatively lower temperatures, by an accelerated thermal cyclic test at higher temperatures in laboratory conditions.     

Lanthanum magnesium hexaluminate thermal barrier coatings with pre-implanted vertical microcracks: Thermal cycling lifetime and CMAS corrosion behaviour

Atmospheric plasma-sprayed (APS) coatings have a layered structure as well as lower strain tolerance and a shorter lifetime than EB-PVD coatings. In this study, TBCs composed of a LaMgAl₁₁O₁₉ (LMA) top coat and a NiCrAlY bond coat were prepared by APS coupled with dry-ice blasting to implant vertical microcracks in the top coat. The thermal cycling lifetime and CMAS corrosion behaviour of LMA-TBCs with pre-implanted vertical microcracks were investigated in detail. The results show that the LMA top coat possesses an improved proportion of vertical microcracks and that the corresponding TBC has an improved thermal cycling lifetime. The vertical microcracks in the top coats, which not only reduce the thermal stress but also improve the strain tolerance of TBCs, dramatically contribute to the improvement in the thermal cycling lifetime. Surprisingly, the CMAS corrosion resistance of LMA-type TBCs with implanted vertical microcracks is better than that of conventional TBCs with a typical layered structure.     

Modelling of catastrophic stress development due to mixed oxide growth in thermal barrier coatings

In thermal barrier coatings (TBCs) of heavy-duty gas turbines, thermally grown oxide (TGO) develops in two stages, i.e. firstly, a thin layer of dense protective α-Al₂O₃ forms slowly, and then, a layer of porous detrimental mixed oxide (MO) between top coat (TC) and α-Al₂O₃ appears. During long-term isothermal oxidation at high temperature, the failure of TBCs usually occurs when a critical thickness of MO is reached, but the exact failure mechanism is still largely unclear, let alone the related stress development. In this paper, we analyze the stress evolution and the resultant failure modes due to the whole-layer growth of uniform MO. The results show that it is MO, rather than α-Al₂O₃, that is mainly responsible for the micro-cracking and/or delamination in TBCs. The fast growth of expansive MO induces catastrophic stresses, which leads to micro-cracking in the α-Al₂O₃ layer. The cracking of α-Al₂O₃ layer reduces the oxidation resistance and further accelerates the MO growth. Our theoretical analysis provides a reasonable explanation of the experimental results.     

Evolution of internal cracks and residual stress during deposition of TBC

Thermal barrier coating (TBCs) are ceramic coatings that are deposited on metallic substrates to provide high thermal resistance. Residual stress is among the critical factors that affect the performance of TBCs. It evolves during the process of coating deposition and in-service loading. High residual stresses result in significant cracking and premature delamination of the TBC layer. In the present study, a hybrid computational approach is used to predict the evolution of internal cracks and residual stress in TBC. Smooth particle hydrodynamics (SPH) is first used to model the deposition of yttria-stabilized zirconia (YSZ) layer that contains various interfaces and micropores on a steel substrate. Then, three-dimensional (3D) finite element analysis is utilized to predict the evolution of internal cracks and residual stress in the ceramic coating layer. It is found that multiple cracks emerge during the solidification of the coating layer due to the development of high tensile (quenching) stresses. The cracking density is higher at regions near the coating interface. It is also found that compressive (residual) stresses are developed when the deposited coating is cooled to room temperature. The residual stress state is equibiaxial and nonlinear across the thickness/width of the TBC layer. The residual stress profile predicted compares well with that of hole drilling experiments.     

Non-destructive evaluation of thermally grown oxides in thermal barrier coatings using impedance spectroscopy

The growth of thermally grown oxide (TGO) layers in thermal barrier coatings (TBCs) due to the oxidation of the bondcoat alloy is a critical factor affecting the durability of TBCs. In the present study, diverse TBC specimens were subjected to long-term oxidation at various temperatures. The TGO growth mechanism was investigated according to cross-sectional images of the oxidized specimens. Impedance spectroscopy (IS) was performed to measure the electrical properties of the integrated TBCs non-destructively. Considering the influence of the TGO composition, the derived TGO electrical capacitance was found to have a good correspondence to the observed TGO thickness over a wide range 0–18.3 μm, regardless of the diverse specimens and oxidation conditions. The error was less than ±2.0 μm. With a certain design of the electrode size, IS is generalized and is recommended as an accurate and practical non-destructive evaluation method for the determination of TGO thickness within a very wide range in TBC systems under real operating conditions.     

Testing method to assess lifetime of EB-PVD thermal barrier coatings on tubular specimens in static and cyclic oxidation tests

A testing methodology is presented to assess the delamination failure of an yttria stabilized zirconia electron beam physical vapor deposited (EB-PVD) top-coat as studied for tubular specimens applying short thermal cycles with high heating and cooling rates as well as for oxidation tests at constant temperature. The influence of maximum temperature and specimen diameter were investigated. Higher maximum temperatures as well as a smaller specimen diameter resulted in shorter lifetimes. No significant difference was observed between the static and cyclic oxidation tests. The failure relevant time at elevated temperatures is correlated with the thickness of the thermally growing oxide (TGO) scale developing on the bond coat. The experimental results can be described using a residual stress based analytical approach. In the model the lifetime is assumed to be terminated when a critical stress is reached.     

Thermomechanical simulations of the transient coupled effect of thermal cycling and oxidation on residual stresses in thermal barrier coatings

Failures in thermal barrier coatings (TBCs) are associated with the build-up of residual stresses that result from thermal cycling, growth strain, and stress relaxation associated with high temperatures. To address these highly coupled processes, three aspects were examined. The first was concerned with the effect of thermal cycling and thermal gradients on the resulting residual stress fields. The second with the dynamic growth of thermally grown oxide (TGO) layer using novel finite volume-finite element algorithms. In the third, we examined the effect of stress relaxation on the (TC/TGO) interface. We modelled these highly coupled processes using transient thermomechanical finite element simulations. The temperature profile and state of oxidation variation with time were imported as a predefined field and solved in ANSYS nonlinear platform. Our results revealed that stress relaxation of the TGO stresses at high temperatures leads to a reduction in the TC/TGO interfacial stresses. They also revealed that the use of the isotropic hardening rule limits the increase in plastic deformation of the bond coat (BC), while the use of kinematic hardening rule leads to ratcheting. Furthermore, we highlighted the importance of considering uneven growth of TGO on the resulting stress field.     

Simulation of thermal barrier coating spallation induced by the initiation/growth/coalescence of internal crack and interfacial crack based on a real image model

In the furnace cycle test, the growth of oxide film leads to the propagation and coalescence of multiple cracks near the interface, which should be responsible for the spallation of thermal barrier coatings (TBCs). A TBC model with real interface morphology is created, and the near-interface large pore is retained. The purpose of this work is to clarify the mechanism of TBC spallation caused by successive initiation, propagation, and linkage of cracks near the interface during thermal cycle. The dynamic growth of thermally grown oxide (TGO) is carried out by applying a stress-free strain. The crack nucleation and arbitrary path propagation in YSZ and TGO are simulated by the extended finite element method (XFEM). The debonding along the YSZ/TGO/BC interface is evaluated using a surface-based cohesive behavior. The large-scale pore in YSZ near the interface can initiate a new crack. The ceramic crack can propagate to the YSZ/TGO interface, which will accelerate the interfacial damage and debonding. For the TGO/BC interface, the normal compressive stress and small shear stress at the valley hinder the further crack propagation. The growth of YSZ crack and the formation of through-TGO crack are the main causes of TBC delamination. The accelerated BC oxidation increases the lateral growth strain of TGO, which will promote crack propagation and coalescence. The optimization design proposed in this work can provide another option for developing TBC with high durability.     

Introducing segmentation cracks in air plasma-sprayed thermal barrier coatings by controlling residual stress

Segmentation cracks are crucial for enhancing the strain tolerance and decreasing the propensity of delamination for thermal barrier coatings (TBCs). In this study, segmentation cracks were prepared in air plasma-sprayed TBCs by controlling the residual stress. The evolution of the stress in the coating was characterized via photoluminescence piezospectroscopy using trace α-Al₂O₃ impurities as stress sensor. Tensile stress (~170 MPa) formed in the as-deposited coating was converted into compressive stress through further thermal exposure. The relationship between the formation of the segmentation cracks and stress in the coating was investigated. It was demonstrated that the segmentation cracks could be formed when a critical coating thickness is achieved. The critical coating thickness and spacing of the segmentation cracks dependent on the tensile stress in the as-deposited coating, and they could be manipulated by controlling the deposition and substrate temperatures. In addition, the evolution of the microstructure and phase composition of the yttria-stabilized zirconia coating was examined.