Thermally Grown Oxide Thickness Research Articles

Residual stress in air-plasma-sprayed thermal barrier coatings under long-term high-temperature oxidation

The accumulated residual stress in yttria-stabilized zirconia top coat (YSZ TC) and thermally grown oxide (TGO) is regarded as the key driving force for the delamination of air-plasma-sprayed thermal barrier coating system (APS-TBCs). In this study, the synchronous evolution of residual stress of both YSZ TC and TGO, as well as the TGO thickness, are characterized via the Raman spectroscopy (RS) and photoluminescence piezo-spectroscopy (PLPS) methods. For the APS-TBCs where one surface is coated with YSZ TC, while the other surface remains directly exposed to high temperature, the service life can be categorized into four distinct stages: the early stage with tensile stress state (0–40 h), the medium stage with reversal compressive stress state (40–100 h), the late stage with re-inverted tensile stress (100–140 h), and the final stage with compressive stress state (140–300 h). Such stress reversal phenomenon may be attributed to the competition between the growth of TGO and substrate oxide (SO). It is advisable to apply an oxidation-resistant coating on the inner wall of high-temperature blade cooling passages, to prevent the formation of SO and consequently avoid complex stress transitions during high-temperature service of the APS-TBC, ultimately preventing local stress concentration-induced cracking and delamination.

Steam oxidation performance of Yb2Si2O7 environmental barrier coatings exposed to CMAS

Environmental barrier coatings (EBCs) are necessary for use of SiC/SiC ceramic matrix composites (CMCs) in gas turbine engines. Due to the complex operational environments, it is increasingly important to understand the interplay of damage modes to project the durability of the EBC/CMC system. Two life-limiting failure mechanisms of EBCs are steam oxidation and molten calcium-magnesium-aluminosilicate (CMAS) attack. Steam oxidation results in the formation of a thermally grown oxide (TGO) that acts as a weak interface and CMAS/EBC interactions cause thermomechanical and thermochemical changes that compromise coating integrity. EBCs consisting of a Yb2Si2O7 topcoat and a silicon bond coat were subjected to low (2 mg/cm2) loadings of two CMAS compositions prior to steam oxidation at 1316 °C for up to 300 h. Exposure to CMAS reduced the TGO thickness and the low CaO content glass resulted in the thinnest oxide scales. However, glass exposure had mechanical consequences that caused premature coating delamination.

Evaluation of high temperature oxidation behavior of LZ, CSZ, and LZ-CSZ composite thermal barrier coatings

La2Zr2O7 (LZ) compound is a promising candidate for the new generation of thermal barrier coatings (TBCs). Despite its unique properties, this material suffers from a quite low coefficient of thermal expansion (CTE) and also brittleness which restricts its potential application. In this study, LZ was modified with ceria stabilized zirconia (CSZ), and LZ-CSZ composite powders with different compositions were prepared and applied with the air plasma spraying (APS) method. The oxidation behavior of samples was studied at 1100 °C, and the tests were continued until coatings spallation. Microstructural studies showed that the thermally grown oxide (TGO) layer in all samples consists of alumina and mixed spinel oxide layers. Due to lower oxygen diffusivity, the samples with a higher fraction of LZ contain a narrower TGO layer with a small spinel phase. The inward oxygen penetration in coatings with higher CSZ percentages was considerable, that led to the evolution of Al-depletion zone in the bond coat area, and increased the portion of spinel oxides in the TGO. The 50CSZ-50LZ coating, due to its better combination of CTE compatibility, thermal stability, and fracture toughness, showed better oxidation resistance and collapsed after 264 h with a TGO thickness of 5.4 μm.

Failure behaviour of EB-PVD YSZ thermal barrier coatings under simulated aero-engine operating conditions

Thermal shock test simulating aero-engine operating conditions was carried out on electron beam physical vapor deposition (EB-PVD) yttria partially stabilized zirconia (YSZ) thermal barrier coatings (TBCs) using advanced multi factor coupling test equipment. Phase composition, thermally grown oxide (TGO) residual stress, surface morphology and cross-sectional microstructure evolution in coatings were systematically studied. Based on XRD results, coatings possessed tetragonal phase structure without any further transformation during testing. After 5000 cycles, the thickness of TGO gradually increased from 0.3 to 2 μm. TGO residual compressive stress also gradually increased and then drastically dropped when the coating spalled. Horizontal cracks first initiated from TGO/bond coat (BC) interface, then arose from top coat (TC)/TGO interface and finally coalesced through TGO to cause delamination. Repeated thermal shock and gas impact conditions promoted the emergence of vertical cracks in ceramic top coat. Once communicated with transverse delaminations, cracks eventually led to spalling failure of the coating system. The contribution to spallation failure was mainly from thermal stress as a result of thermal expansion mismatch between the coating and the substrate. In turn, elastic stress induced by high speed impact of gas and TGO growth stress promoted initiation and propagation of coating cracks.

Oxidation behaviors and mechanisms of Yb2SiO5-Yb2O3-Si-SiC ceramic fabricated by tape casting and reactive melt infiltration

The phase transition of thermally grown oxide (TGO) has been proven to be an important factor affecting the durability of EBCs. Yb2O3 and Yb2SiO5 can react with silica to form silicate, which can reduce the TGO thickness and extend the lifetime of EBCs. In this paper, the Yb2SiO5-Yb2O3-Si-SiC ceramic was prepared by tape casting combined with silicon reactive melt infiltration. The oxidation properties of the ceramic were evaluated at 1200–1400 °C. With the increase in temperature, the content of SiO2 in the oxide layer decreased and the content of Yb2Si2O7 increased. A dense and continuous Yb2Si2O7 layer was formed at 1400 °C. Yb2Si2O7 is formed at the edge of Yb2SiO5, then Yb3+ diffuses outwards, Si4+ diffuses inwards and gradually forms Yb2Si2O7 from the outside to the inside.

Effect of the thermally grown oxide and interfacial roughness on stress distribution in environmental barrier coatings

To clarify the role of interface morphology and thermally grown oxide (TGO) in the failure of environmental barrier coatings (EBCs). In this study, the effect of chemical expansion on free energy was considered based on the continuous thermodynamic framework. The effects of roughness and TGO growth on the stress distribution of EBCs were investigated. The results showed that the stress coupling effect led to the inhomogeneous growth of TGO by affecting the gas diffusion and gas inflow rate. The TGO thickness at the peak increased with increasing roughness, and the TGO thickness at the valley and the middle position decreased with increasing roughness. The y-direction at the TGO/EBC valley and the TGO/BC peak under tensile stress increased with the TGO thickness and roughness and may be the first to fail in delamination. The calculation results of the model can provide a theoretical basis for the coating design and manufacturing process.

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

AbstractThis 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.

High-temperature oxidation and TGO growth behaviors of laser-modified YAG/YSZ double-ceramic-layer TBC

Yttrium—aluminum garnet/yttria-stabilized zirconia (YAG/YSZ) double-ceramic-layer thermal barrier coating (TBC) with NiCoCrAlY bond coating was deposited by atmospheric plasma spraying (APS) on Inconel 738 alloy substrate, and laser modification was carried out on the surface of YAG ceramic coating to improve the high- temperature oxidation resistance of TBC. Then, isothermal oxidation tests were designed to study the high-temperature oxidation and thermally grown oxide (TGO) growth behaviors of the as-sprayed (AS) and the laser modified (LM) YAG/YSZ TBC. Results show that TGO thickness and structure of both AS-TBC and LM-TBC exhibit the similar evolution trend, TGO thickness increases with the increasing of isothermal oxidation time, and TGO structure develops from single Al2O3 layer to the double-layer structure of upper mixed oxides and lower Al2O3. Because of the crucial inhibition effect of the laser modification on oxygen permeation, the appearance time of mixed oxides in LM-TBC is postponed, and the total TGO thickness of LM-TBC is reduced at the middle and final oxidation stages. The Al2O3 layer thickness proportion of total TGO in LM-TBC is always greater than or equal to that in AS-TBC at the same isothermal oxidation time. Compared with the AS-TBC, the parabolic oxidation rate of LM-TBC is decreased by 18.42%. Therefore, YAG/YSZ LM-TBC presents better oxidation resistance and lower TGO growth rate than AS-TBC.

Deformation Prediction Theory of Thermal Barrier Coatings near Cooling Holes under Thermal Cycling.

Thermal barrier coating (TBC) systems are widely adopted in gas turbine blades to improve the thermal efficiency of gas turbine engines. However, TBC failure will happen due to the thermal stress between the different layers of the TBC systems. The traditional two-layer theoretical model only considers TGO (thermally grown oxide) and a substrate in the inner cooling hole with the surface uncoated, which results in poor prediction of the deformations of the TBC systems. It should be mentioned that the effect of TBC is very important because the thickness of TBC is much larger than the TGO thickness. In this study, a new three-layer theoretical model was derived, which is composed of the cylindrical TGO and TBC mounted in the substrate with a circular hole, and the stress and strain of TGO near the cooling hole under the condition of the thermal cycles were calculated. The high temperature characteristics of TGO and the substrate including the high temperature strength and growth ratio were from the experiments. The results show that the strain of the developed three-layer model is irrelevant with increasing number of cycles, which indicates that TBC in the cooling hole significantly inhibits the deformation of TGO near the cooling hole. Therefore, aimed at confirming the feasibility of the three-layer theoretical model, the finite element analysis with coating in the cooling hole and on the surface was carried out with a three-layer axisymmetric model, which proves that the 3-layer theoretical model can predict the deformation trend near the cooling hole.

Spallation mechanism of thermal barrier coatings with real interface morphology considering growth and thermal stresses based on fracture phase field

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.

Cracking Behavior of Gd2Zr2O7/YSZ Multi-Layered Thermal Barrier Coatings Deposited by Suspension Plasma Spray

A new multi-layered thermal barrier coating system (TBCs) containing gadolinium zirconate (GZ, Gd2Zr2O7) and yttria-stabilized zirconia (YSZ) was developed using suspension plasma spray (SPS) to improve the overall thermal cycling performance. This study focuses on the cracking behavior of the GZ/YSZ TBC after thermal exposure to find out the key factors that limit its lifetime. Different cracking behaviors were detected depending on the thermal treatment condition (i.e., horizontal cracks within the ceramic layer and at the thermally grown oxide (TGO)/YSZ interface) which can be related to stresses developed through thermal expansion mismatch and increased TGO thickness beyond a critical value, respectively. A reduction in hardness of bond coat (BC) was measured by nanoindentation and linked with the thermally activated grain growth mechanism. The hardness and elastic modulus of ceramic layers (GZ and YSZ) showed an increased trend after treatment that contributed to the interfacial cracks.

Creep Effect on Chemo-Mechanical Coupling Oxidation of Thermal Barrier Coatings

In this paper, a chemo-mechanical coupling model considering the creep effect during high-temperature oxidation is developed based on the diffusion-reaction equation. A sinusoidal curve model is developed by the finite element method to simulate the thickness and stress distribution of thermally grown oxide (TGO). The thickness and stress distribution of the TGO in the thermal barrier coating was investigated by changing the coupling effect and creep effect conditions of the model. The results show that with the increase of the coupling coefficient, the thickness of TGO gradually becomes smaller. In the late stage of oxidation, this effect diminishes, and the compressive stress located in the BC/TGO layer groove keeps decreasing, while the tensile stress located in the TC layer groove is the opposite. As the creep rate increases, the maximum tensile/compressive stress in the Y-direction within the thermal insulation coating slowly decreases, and the oxidation rate of the oxide layer moderately decreases. This study is important to find the corresponding solutions and preventive measures for the failure of the thermal barrier coating.

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.

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

AbstractAccording 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.

Numerical Simulation of Initial Residual Stress in Thermal Barrier Coatings: Planar geometry model

Thermal barrier coatings (TBCs) are one of the key materials of turbines for propulsion and power generation. A typical TBCs system generally contains four layers, i.e. top ceramic coat (TC), thermally grown oxide (TGO), bond coat (BC) and substrate. The material parameters （such as Young’s modulus, Poisson’s ratio and thermal expansion coefficient） of each layer will result in the different cooling velocity of thermal barrier coatings (TBCs) system during the deposition process. Initial residual stresses in TBCs occur after cooling to the ambient, which play an important role for evaluating the failure and reliability of TBCs. In this paper, the initial residual stresses of a planar TBCs model were obtained by finite element method, the effects of cooling kinds and the thickness of TC and TGO on the fields of initial residual stresses were also considered. The results may be useful for the optimization of TBCs system design and preparation.

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.

Improved water vapor resistance of environmental barrier coatings densified by aluminum infiltration

A dense environmental barrier coating (EBC) was designed via aluminum infiltration into open pores or cracks to eliminate premature coating failure induced by the rapid permeation of oxidants through these pores. The water vapor corrosion performance of the EBC with and without aluminum infiltration was investigated at 1350 °C for 300 h under flowing 50%H2O–50%O2 gas. The infiltrated coating exhibited decrease silicon loss by more than 80%, almost no phase decomposition, and no continuous thermally grown oxide (TGO), furthermore, TGO thickness at the crack-root decreased by ∼90%. The excellent resistance is primarily attributed to the channel pores being filled with aluminum, which eliminates the rapid permeation of oxidants. In addition, a continuous refractory ytterbium aluminum garnet (YbAG) layer was formed in situ on the EBC surface, this layer exhibited sufficient stability in steam, rendering the EBC less permeable to oxidants.

The Thermally Grown Oxide Thickness and Substrate Film Coefficient Effect on Residual Stresses within Thermal Barrier System

Aims: This work is to give a further insight into the residual stress distribution within APS TBC system within TBC system by coupled thermo-mechanic finite element analysis. Background: Thermal barrier coatings are a typical example of ceramic-metal system. Thermal stress could be produced in the thermal barrier coatings on account of the difference in coefficient of thermal expansion between the ceramic and metal and poses a serious threat to the performance and lifetime of the thermal barrier coatings. Objective: Many previous studies used out-of-plane stresses to estimate the delamination of the bond-coat/TGO and TGO/top-coat interfaces. However, in the present work, normal and tangential stresses under thermal-mechanic coupling analysis are adopted to investigate the interface delamination Methods: A thermo-mechanic coupled model comprised of a top-coat, thermally grown oxide, bond-coat, and substrate, is built for the stress evolution within the thermal barrier coatings using finite element method. The thermal conduction is carried out inside the thermal barrier coating model according to Fourier's law, with the temperature gradient forming in the model. Results: The top-coat is subjected to tensile stresses around the peak, with the stress magnitudes decreasing with the oxide thickness. Normal tension acts across the oxide/bond-coat interface around the peak while normal compression acts around the valley. Their magnitudes are dependent on the oxide thickness but are not so sensitive to the substrate surface heat transfer coefficient Conclusion: The TC/TGO interface might be subjected to tensile or compressive stresses around the peak, which depends on the TGO thickness. The TGO/BC interface is subjected to tensile stresses around the peak and compressive stresses around the valley, the magnitudes of which increase with the TGO thickness. The influence of substrate film coefficient on the stress across the TGO/BC interface is not so significant. Discussion: The thermo-mechanic coupled model is employed to analyze the top-coat stress, bond-coat plastic strain and interface stress. Simultaneously, a parametric study is performed on the effect of thermally grown oxide thickness and substrate surface heat transfer coefficient on the above mechanical responses.

Factors affecting cyclic lifetime of EB-PVD thermal barrier coatings with various bond coats

Abstract Standard EB-PVD (electron beam physical vapor deposition) thermal barrier coatings (TBCs) of partially yttria-stabilized zirconia were applied on Ni-base substrate alloys and thermally cycled with T max = 1100 °C. Two different bond coats were investigated: NiCoCrAlY and NiPtAl. The longest lifetimes have been achieved on the Hf-containing Rene142 substrate. A rough interface between TGO (thermally grown oxide) and bond coat with hafnia pegs is characteristic for these alloys. A bond coat heat treatment in Ar–H significantly improved the lifetime of the TBC system with IN100 + PtAl bond coats, whereas reduced lifetimes were found on IN100 + NiCoCrAlY. Spallation of the TBCs was correlated to TGO thickness and microstructure. The TGO growth is faster for NiCoCrAlY than for NiPtAl bond coats, while TBC lifetime is longer for the NiCoCrAlY systems. Tests with reduced cycle length yielded an increased number of cycles to failure for IN100+NiCoCrAlY, but left the time to failure at temperature unchanged. TBC thickness had only a limited influence on cyclic lifetime.

Review of Numerical Simulation of TGO Growth in Thermal Barrier Coatings

Thermally grown oxide (TGO) is a critical factor for the service life of thermal barrier coatings (TBC). Numerical simulations of the growth process of TGO have become an effective means of comprehensively understanding the progressive damage of the TBC system. At present, technologies of numerical simulation to TGO growth include two categories: coupled chemical-mechanical methods and mechanical equivalent methods. The former is based on the diffusion analysis of oxidizing elements, which can describe the influence of bond coat (BC) consumption and phase transformation in the growth process of TGO on the mechanical behavior of each layer of TBC, and has high accuracy for the thickness evolution of TGO, but they cannot describe the lateral growth of TGO and the rumpling phenomenon induced. The latter focuses on describing the final stress and strain state after the growth of a specific TGO rather than the complete growth processes of TGO. Based on the measured TGO thickness growth curve, simulations of thickening and lateral growth can be achieved by directly applying anisotropic volumetric strain to oxidized elements and switching elements properties from the BC to the TGO.