Numerical study of residual stresses in environmental barrier coatings with random rough geometry interfaces

To clarify the role of the coating interface geometry and thermally grown oxide (TGO) layer in the failure of environmental barrier coatings (EBCs) and to further understand the cracking and spalling mechanisms of coatings, in this study, the thermomechanical properties of the multilayer coating system (Yb₂SiO₅/Yb₂Si₂O₇/Si), the morphology of the coating interface and the influence of the oxide layer on the local stresses during cooling were considered based on a random rough interface geometry model. The results showed that the rough geometry increased the magnitude of residual stresses at the interface and that the stress distribution away from the interface was less affected than the coating without roughness. The cracks on the outer surface of the Yb₂SiO₅ layer initiate in the valley region and spread with a stress value independent of the TGO thickness, and this failure may occur by cracking under tensile stress. The overall stress intensity at the TOP/EBC interface was lower than that at the upper surface of the TOP layer. The presence of TGO increased the magnitude of residual stresses in the BC and EBC layers, which caused cracks at the TGO/BC and TGO/EBC interfaces to occur at opposite locations. The phase change of the TGO layer from β-cristobalite to α-cristobalite cause a rapid increase in the overall level of coating stress, which may be a direct factor in coating failure. The calculation results provide a theoretical basis for the coating design and manufacturing process.     

Effects of the chemistry of coating and substrate on the steam oxidation kinetics of environmental barrier coatings for ceramic matrix composites

Environmental barrier coatings (EBCs) are an enabler for SiC/SiC ceramic matrix composites (CMCs) in gas turbines by protecting CMCs from environmental degradation. A critical EBC failure mode is the EBC spallation due to a build-up of elastic strains caused by the formation of SiO₂ scale, known as TGO (thermally grown oxide). H₂O, a byproduct of combustion reactions, accelerates the TGO-induced EBC failure by increasing TGO growth rates by orders of magnitude. NASA’s approach to improve the EBC life, therefore, is to reduce TGO growth rates. NASA discovered that modifying the TGO chemistry by modifying the EBC chemistry of Gen 2 EBC (Si / Yb₂Si₂O₇) reduces the TGO thickness by up to ～80 %. A study was undertaken to understand the oxidation mechanism of modified Gen 2 EBCs as well as to investigate the effect of EBC and CMC chemistry on TGO growth rates. This study confirmed the previously proposed TGO-controlled oxidation mechanism of modified Gen 2 EBCs and determined the correlations between the EBC and CMC chemistry, TGO chemistry, and TGO growth rates.     

Analysis of interfacial crack initiation mechanism of thermal barrier coatings in isothermal oxidation process based on interfacial stress state

In this paper, the interfacial stress state is used to analyze the interfacial crack initiation mechanism of the thermal barrier coatings (TBCs) during isothermal oxidation. The influence of thermal growth stress, initial residual stress, and creep behavior on the stress distribution is considered to have an accurate simulation result. A parameter that integrates the effects of interfacial normal and tangential stress is modified for evaluating interfacial crack initiation. It is found that, in the cooling stage, the interfacial cracks sprout at the top coat (TC)/thermally grown oxide (TGO) interface valley region and the TGO/bond coat (BC) interface peak region, which agrees with the experimental results. Furthermore, the influence of interfacial roughness on crack initiation is investigated. The result shows that different interfacial roughness affects the sprouting region of interfacial cracks and cracks within the TC layer.     

Finite element analysis of TGO thickness on stress distribution and evolution of 8YSZ thermal barrier coatings

According to the experimental research results of the thermally grown oxide (TGO) layered growth during the pre-oxidation process of 8 wt.% yttria-stabilized zirconia thermal barrier coating (TBC), a two-dimensional sinusoidal TC/bonding coat (BC) curve interface model of the longitudinal section of TBCs based on finite element simulation was constructed; the thickness and composition of the TGO layer relative to the TC/BC curve interfacial stress distribution and its evolution during the thermal cycling process were studied. The results show that when the TGO layer uses α-Al₂O₃ as the main oxide (black TGO), the thicker the black TGO layer, the more uniform the stress distribution of the TC/BC interface. When the TGO layer is dominated by spinel-structured Co and Cr oxides (gray TGO), the stress “band” of the TC/BC interface is destroyed; it shows the alternating phenomenon of tensile stress zone and compressive stress zone, and after the rapid random growth of TGO, the concentrated tensile stress increased by a large jump. Affected by the thickness of the prefabricated black TGO layer, there is a limit peak in the thickness of the black TGO layer, the normal stress at the TC/BC boundary is minimized, and the magnitude of the stress change is also minimized.     

Effect of TGO thickness,pores, and creep on the developed residual stresses in thermal barrier coatings under cyclic loading using SEM image-based finite element model

Thermal barrier coatings (TBC) allow the metallic internal components of gas turbine engines to operate at elevated temperatures near its melting points. Formation of thermally grown oxide (TGO) layers at the top coat (TC) and bond coat (BC) interface induces cracks in the TC that may lead to complete TBC failure due to spallation. An SEM image-based finite element (FE) model is developed using commercial finite element package ABAQUS to investigate the development of residual stresses resulting from cyclic loading of TBCs. The model includes thermo-mechanical material properties and considers the real interface between the coating layers. The model includes real pores based on an SEM image, taking advantage of image processing techniques. Effect of TC surface roughness and pores on the developed residual stresses during thermal cycling is investigated with respect to different TGO thicknesses. The analysis shows that presence of TC roughness causes stress concentration sites during heating that may force horizontal cracks to initiate and propagate with stress values that are indifferent to the TGO thickness. The pores are found to shift stress concentration regions from the TC/TGO interface to the vicinity of the pores during cooling, and that may cause horizontal cracks to start from within the TC with stresses that increase with TGO thickness. Moreover, the effect of creep for all layers on the generated residual stresses is studied. Considering creep gives lower stresses at the end of cooling, however, stress distribution remains the same with and without creep.     

Raman spectroscopic characterization of SiO2 phase transformation and Si substrate stress relevant to EBC performance

To accurately model the long-term durability of environmental barrier coatings (EBCs), a more complete understanding of the phase composition and transformations of the thermally grown oxide SiO₂ (TGO) is desired. For the TGO formed during thermal cycling in steam, cristobalite formation and the subsequent β- to α-cristobalite transformation has been identified as a potentially life-limiting mechanism. In this study, Raman micro-spectroscopy was used to quantify the cristobalite transformation on a polycrystalline Si coupon that was exposed to steam at 1350°C for 100 h. The phase transformation was mapped at 200–260°C on the TGO surface at different ramp rates using a heating stage and a micro-positioning stage. The stress in the Si substrate was also determined using Raman spectroscopy by measuring the stress induced peak shift. The α→β phase transformation produced a 300–500 MPa tensile stress in the Si substrate, which compared well to the stress predicted from the volumetric expansion of the cristobalite. Quantifying the phase transformation and residual stress are critical tools in developing the next generation of high performance EBCs.     

Influence of surface roughness on the electrochemical behavior of carbon steel

The effect of the surface roughness of carbon steel on corrosion properties was investigated using electrochemical tests, and surface and Kelvin probe force microscopy (KPFM) analyses. The results of electrochemical tests show that the corrosion rate of carbon steel is increased as the surface roughness increases. It was estimated using KPFM measurement that the difference in the Volta potential between the peak and the valley increased with increasing surface roughness. As the peak has a lower potential than that of the valley, the peak acts as an anode. The surface roughness affects the Volta potential, and the Volta potential difference is inversely proportional to electron work function (EWF). The larger difference in Volta potential between the peak and valley on the rougher surface and the smaller EWF accelerated the micro-galvanic corrosion between them. The surface analyses reveal that corrosion initiated along the peak lines. The results from this study suggest that an increase of surface roughness leads to a decrease of the corrosion resistance.     

Comprehensive dynamic failure mechanism of thermal barrier coatings based on a novel crack propagation and TGO growth coupling model

Comprehensive understanding of failure mechanism of thermal barrier coatings (TBCs) is essential to develop the next generation advanced TBCs with longer lifetime. In this study, a novel numerical model coupling crack propagation and thermally grown oxide (TGO) growth is developed. The residual stresses induced in the top coat (TC) and in the TGO are calculated during thermal cycling. The stresses in the TC are used to calculate strain energy release rates (SERRs) for in-plane cracking above the valley of undulation. The overall dynamic failure process, including successive crack propagation, coalescence and spalling, is examined using extended finite element method (XFEM). The results show that the tensile stress in the TC increases continuously with an increase in an undulation amplitude. The SERRs for TC cracks accumulate with cycling, resulting in the propagation of crack toward the TC/TGO interface. The TGO cracks nucleate at the peak of the TGO/bond coat (BC) interface and propagate toward the flank region of the TC/TGO interface. Both TC cracks and TGO cracks successively propagate and finally linkup leading to coating spallation. The propagation and coalescence behavior of cracks predicted by this model are in accordance with the experiment observations. Therefore, this study proposed coating optimization methods towards advanced TBCs with prolonged thermal cyclic lifetime.     

Oxidation behaviors and mechanisms of Yb2O3-doped silicon coatings fabricated by vacuum plasma spray

Environmental barrier coatings (EBCs) have been widely studied for the protection of ceramic matrix composites (CMCs). The phase transition of silica thermal growth oxide (TGO) has been proved to be an important factor for the durability of EBCs. Yb₂O₃ could react with SiO₂ TGO and form silicate which may improve the stability of TGO and prolong the service life of EBCs. In the present work, Si coatings doped with different contents of Yb₂O₃ were fabricated by vacuum plasma spray. The oxidation behaviors of the composite coatings were evaluated at 1350 °C and compared with the pure Si coating. The evolution of phase composition and microstructure of mixed thermal growth oxide (mTGO) was characterized in detail. The results showed that the newly formed oxidation product, namely Yb₂Si₂O₇, could reduce the vertical cracks in mTGO layer and the mTGO/coating interface cracks, leading to a better binding performance of the mTGO layer. The oxidation mechanisms of the Yb₂O₃-doped Si coatings were analyzed based on microstructure and phase composition observations.     

Experimental and numerical life evaluation of TBCs with different BC and diffusion coating under cyclic thermal loading

This paper experimentally and numerically investigates the thermally grown oxide (TGO), lifetime, and stress values in thermal barrier coatings with different bond coat (BC), without top coat (TC), and diffusion coating under cyclic thermal loading. Scanning electron microscope (SEM) analysis shows that the atmospheric plasma spraying (APS) and high-velocity oxygen fuel (HVOF) fabricated sample has the highest and lowest TGO thickness and growth rate, respectively. The new coating with two BCs has a maximum lifetime of 102 cycles. After that, the lifetime of the coatings with HVOF-BC, diffusion coating, and APS-BC reach 84, 56, and 44 cycles, respectively. The diffusion coating does not have much effect on the TGO thickness; however, it delays the Al interdiffusion to the substrate. In the sample without the TC layer, oxygen contact with the BC layer has increased, leading to a rise in the BC oxidation rate. The numerical analysis of the stress values based on SEM images shows that the more intense TGO layer growth in APS coating caused an increase in TC layer stress values. Furthermore, the results show that the new coating with two-layer BC has the lowest stress values. The TC absence causes the loss of compressive stresses caused by TC on TGO and increases the tensile stress values in this layer.     

Effects of TGO growth on the stress distribution and evolution of three-dimensional cylindrical thermal barrier coatings based on finite element simulations

Based on the ultrasonic C-scan results of 8YSZ coatings after thermal cycles, three-dimensional cylindrical numerical simulations of the physical geometry model of the thermal barrier coating (TBC) sinusoidal surfaces were conducted with finite elements to estimate the stress distribution and evolution law of the top coat (TC)/bond coat (BC) interface, including the centre and edge of the specimen affected by the dynamic growth of the thermally grown oxide (TGO). The results show that when a layer of TGO is grown on the TC/BC interface, compressive stress is uniformly distributed on the TGO interface, and the stress value decreases as a function of the TGO layer thickness. When the thickness of the TGO exceeds a certain value, the compressive stress of all parts of the interface gradually changes to tensile stress; meanwhile, the edges of the model affected by the crest and trough effects of the wave are reflected in the radial and circumferential directions, especially along the axial direction, with alternating concentrated tensile and compressive stresses. TGO growth imposes a minor influence on the magnitude and distributions of the radial and circumferential stresses at the BC interface. The linear elasticity, creep, fatigue, and stress accumulation effects of each layer of TBCs in each thermal cycle were fully considered in this model. The model not only interprets the crest and trough effects of the TC/BC surface interface during the growth of TGO, but also interprets the effects of the core and edge of the cylindrical model, further revealing the reason for which the core and edge of the TBC will most likely form cracks.     

Influences of interface morphology and thermally grown oxide thickness on residual stress distribution in thermal barrier coating system

Calculation of residual stress with finite element method is a basic work in failure mechanism investigation in thermal barrier coating (TBC) system because the residual stress is main driving force for crack nucleation and propagation. In this work, a complicated cosine curve with gradually increasing amplitude was used to simulate interface morphologies between layers so as to study the residual stress behavior during the cooling process in air plasma spraying TBC system by finite element method. The substrate, thermally grown oxide (TGO) and top coat (TC) are considered to be elastic and bond coat (BC) elastic-perfectly plastic. The material properties are all temperature dependent. The stress result comparison between models with and without substrate shows the effect of substrate on the residual stress distribution around layers interfaces should not be ignored as the substrate influences the value of normal residual stress as well as the stress distribution along undulating interfaces. Then the model with substrate was used to study the residual stress evolution along interfaces during cooling down from the temperature of 1000 °C to room temperature. The influences of the thickness of TGO and the amplitude and wavelength of interface on the residual stress distributions near interfaces were considered. The results show that these influences are very complicated. Meanwhile, it's found that the hybrid roughness parameter containing information for height and spacing is more suitable to describe the interface complicacy. The results facilitate understanding the failure mechanism relevant to interface morphology and TGO thickness.     

High temperature creep and tensile properties of alumina formed on Fecralloy foils doped with yttrium

The three batches of Fecralloy foils, which differ from each other in contents of yttrium, that is, 10, 280 and 560 ppm, respectively, were chosen as the thermally grown oxide (TGO), alumina (α-Al₂O₃) forming substrate. The creep tests were performed with the Fecralloy foils, which have the α-Al₂O₃ TGO of 0–4 μm thickness, on the surfaces. The creep rates decreased as the TGO thickness increased. The yttrium content above 280 ppm delayed the creep rate of the Fecralloy substrate. The higher creep rate than that of the stand-alone polycrystalline alumina and the dependency of the creep rate on TGO thickness agrees with the hypothesis that the high temperature creep of the TGO is a consequence of the inter-grain growth of the TGO. The yttrium content of lower than 560 ppm did not affect on the creep rate of TGO. Tensile tests were performed with the same alloys, which have the α-Al₂O₃ TGO of 0–3 μm thickness. The tensile strength of the substrate itself increased with Y content by ～20%. The tensile stress of the α-Al₂O₃ TGO decreased with Y-content but it is almost constant, regardless of the TGO thickness. The peak stresses were found at the strain range of ? = 0.7–1.5%, regardless of the TGO thickness, and the batches, thereafter, the parallel cracks perpendicular to the loading direction, formed on the surface. The obtained stress–strain curves of TGO fluctuated. It showed a common feature, that is, a sharp stress drop after the initial yield point at σ_Y = 40–85 MPa, but the stress increased again until the peak points, σ_UTS = 50–110 MPa.     

Accelerated oxidation during 1350°C cycling of ytterbium silicate environmental barrier coatings

Environmental barrier coatings (EBCs) will be needed to protect SiC-based ceramic matrix composite components for the next generation of high-efficiency industrial gas turbines (IGTs). The IGT application will require ≥25 kh lifetimes, and little data are available on EBC failure mechanisms, particularly at ≥1300°C. Initial 1-h furnace cycle testing at 1350°C in 90 vol% H₂O/10 vol% air was conducted ≥1000 cycles on thermally sprayed ytterbium disilicate (YbDS) coatings with and without an Si bond coating. By ≥1000 h, both EBCs formed thick, highly cracked, and fully crystalline cristobalite scales. Comparison of thermally grown oxide (TGO) microstructure and kinetics to isothermal rates of Si and SiC steam oxidation indicated a departure from slow-growing parabolic growth to more rapid rates of silica formation. Possible mechanisms and implications for this acceleration are discussed.     

Model construction and effect of thermally grown oxide dynamic growth on distribution of thermal barrier coatings

A physical geometric model of the dynamic growth of thermally grown oxide (TGO) was established based on an analysis of the TGO growth of 8YSZ thermal barrier coatings during thermal cycling. Finite-element simulation was used to simulate the evolution law between the coating residual stress and thermal cycling, and the linear elasticity, creep effect, and stress accumulation in each thermal cycle were studied. The interface between the top coat (TC) and the bond coat (BC) was covered with a TGO layer that grew vertically and slowly in a layer-like manner. The stress in the TGO was distributed with a “layer” zonal gradient, and the TGO/BC boundaries were distributed uniformly with a large compressive stress, which decreased the TGO layer thickening. With the longitudinal rapid random TGO growth, the boundaries were subjected to a tensile stress, and a high tensile stress concentration area developed at the boundaries. The internal stress consisted of an alternating and mixed distribution of concentrated compressive and tensile stresses. The concentration area of the maximum equivalent stress was distributed in the one-layer TGO near the TC/TGO interface. When a microcrack formed at the TGO/BC boundaries, the crack was subjected to a tensile stress of different size, with a higher tensile stress at both ends, which facilitated crack expansion. Thus, the 8YSZ thermal barrier coating was prone to crack formation and expansion at the TGO/BC boundaries and in the TGO layer near the TC/TGO boundaries.     

Comprehensive understanding of horizontal and vertical crack effects on failure mechanism of lamellar structured thermal barrier coatings

The local spalling induced by the propagation and coalescence of cracks in the ceramic layer is the fundamental reason for the thermal barrier coatings (TBCs) failure. To clarify the effects of horizontal and vertical cracks on the coating failure, an integrated model combining dynamic TGO growth and ceramic sintering is developed. The effects of cracks on the normal and shear stress characteristics are analyzed. The driving force and propagation ability of cracks under different configurations are evaluated. The interaction between horizontal and vertical cracks is explored by analyzing the variation of the crack driving force. The results show that TGO growth causes the ratcheting increase of σ₂₂ tensile stress above the valley, and the σ₁₂ shear stress is on both sides of the peak. Ceramic sintering mainly contributes to the ratcheting increase of σ₁₁ tensile stress. There is minimum strain energy when the horizontal crack extends to the peak. The vertical cracks on the surface of the ceramic layer are easier to propagate through the coating than that of other locations. When the horizontal and vertical cracks simultaneously appear near the valley, they can promote the propagation of each other. The present results can offer theoretical support for the design of an advanced TBC system in the future.     

Effect of segmented cracks on TGO growth and life of thick thermal barrier coating under isothermal oxidation conditions

This paper is devoted to a comparative study on the isothermal oxidation of thick thermal barrier coating (TTBC) with and without segmented cracks produced by atmospheric plasma spray (APS) process. Accordingly, the growth of thermally grown oxide (TGO) and its effect on the degradation of the coating were investigated. Thick top coat in both segmented crack and conventional thick TBC reduced the double layered TGO growth rate slightly. The segmented crack thick TBC demonstrated longer isothermal oxidation life in comparison with that of the conventional thick TBC at 1100 °C. The dominant failure mechanism was spallation due to lateral cracking within the TGO and/or within TBC near the TGO layer, called mixed failure. Stress, and consequently strain, induced on the TTBC due to progressive TGO growth, seems to be primarily responsible for the crack initiation and propagation leading to the coating failure. Increment of elastic energy stored within the top coat due to the increasing of TGO thickness, finally causes thick thermal barrier coating failure in high temperature isothermal oxidation.     

Yb2Si2O7 Environmental barrier coatings with reduced bond coat oxidation rates via chemical modifications for long life

Spallation of environmental barrier coating (EBC) induced by thermally grown oxide (TGO) resulting from steam oxidation is a key EBC failure mode. A logical approach to improve EBC life, therefore, is to reduce TGO growth rates. A study was undertaken to investigate whether TGO growth rates can be reduced by adding modifier oxides. It was based on a hypothesis that modifier oxides dissolve in SiO₂ TGO and modify the SiO₂ structure, making the TGO less permeable to oxidants. Using a current state-of-the-art EBC (Si/Yb₂Si₂O₇) as the baseline, the Yb₂Si₂O₇ layer was modified by adding Al₂O₃ or Al₂O₃-containing oxide compounds, such as mullite and YAG (Y₃Al₅O₁₂), and TiO₂. EBCs were processed using air plasma spraying. Steam oxidation tests and post-oxidation test oxidation kinetics, chemistry, microstructure, and phase analysis were used to test the hypothesis. The best modified EBC reduced the TGO thickness by ~87% compared with that of the baseline EBC in 90% H₂O + 10% O₂ at 1316°C under thermal cycling. Correlations between oxidation kinetics, chemistry, and microstructure of EBC and TGO were used to explain the effect of modifier oxides on reducing TGO growth rates.     

Thermal cycle test performances of environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 in the steam environment at 1475 °C

The novel bi-layer environmental barrier coatings (EBCs) with HfO₂-SiO₂/Yb₂Si₂O₇ structure (70HfO₂-30SiO₂/Yb₂Si₂O₇: 70HS/YbDS, 50HfO₂-50SiO₂/Yb₂Si₂O₇: 50HS/YbDS, molar ratios) was tested in 90%H₂O–10%O₂ conditions between room temperature and 1475 °C in an Al₂O₃ tube furnace, then its performance was evaluated. The YbDS layer was contaminated by alumina impurities under steam conditions. After 22 cycles, the 70HS/YbDS completely separated from the SiC substrate, while the 50HS/YbDS and SiC did not separate, even though cracks formed at the 50HS/SiC interface and the TGO layer. Furthermore, the thermally grown oxide (TGO) layer formed at the HfO₂-SiO₂/SiC interface. Formation and growth of the TGO led to the formation and propagation of cracks at the HfO₂-SiO₂/TGO interface and TGO interior, which was the culprit leading to the failure of EBCs. These results demonstrated that the 50HS/YbDS EBCs have the potential to protect SiC in steam conditions at 1475 °C.     

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.