Metallic Bond Coat Research Articles

The Effect of Shot Blasting Abrasive Particles on the Microstructure of Thermal Barrier Coatings Containing Ni-Based Superalloy

Grit particles remaining on the substrate surface after grit blasting are generally considered to impair the thermal performance of thermal barrier coatings (TBCs). However, the specific mechanisms by which these particles degrade the multilayer structure of TBCs during thermal cycling have not yet been fully elucidated. In this study, the superalloy substrate was grit-blasted using various processing parameters, followed by the deposition of thermal barrier coatings (TBCs) consisting of a metallic bond coat (BC) and a ceramic top coat (TC). After thermal shock tests, local thinning or discontinuities in the thermally grown oxide (TGO) layer were observed in TBCs where large grit particles were embedded at the BC/substrate interface. Moreover, cracks originated at the concave positions of the TGO layer and propagated vertically towards BC; these cracks may be associated with additional stress imposed by the foreign grit particles during thermal cycling. At the BC/substrate interface, crack origins were observed in the vicinity of large grit particles (~50 μm).

Ab initio calculations of Nb-based MAX phases as bond coats for thermal barrier coatings

Thermal barrier coatings (TBCs) are essential to the reliable high-temperature operation of gas turbines and engines. They comprise a ceramic top coat (TC), a metallic bond coat (BC), and a superalloy substrate. Metallic BCs are common, but they require a minimal difference in the coefficient of thermal expansion (CTE) between the TC and substrate. Among ceramic materials, MAX phases have high CTE. This study reports our use of ab initio calculations to assess the suitability of MAX phases as TBCs. We model Nb-based MAX phases with 211 and 312 structures that have Al or Si at the A sites and C or N at the X sites. We use the quasi-harmonic approximation to calculate the Young's moduli and CTEs of the materials with respect to temperature. The Nb2SiN MAX phase appears as the most suitable for use as a BC between various ceramic TCs and an Inconel-718 substrate. It is predicted to be effective in relieving thermal stresses due to its high CTE of 10.882 × 10−⁶ K−1 at 1273 K. The results indicate that carbide MAX phases with high Young's modulus and low CTE should be used with caution. They may not accommodate thermal stresses as effectively, potentially leading to material failure or reduced performance. Overall, our study indicates the potential of Nb-based MAX structures for use as BCs.

Ti3AlC2-modified NiCrAlY as a metallic bond coat for thermal barrier coatings: a study on high-temperature oxidation resistance

Ti3AlC2-modified NiCrAlY as a metallic bond coat for thermal barrier coatings: a study on high-temperature oxidation resistance

Mullite/YSZ hybrid thermal barrier coatings plasma sprayed on IN-738LC: Microstructure, phase analysis and oxidation resistance

In the current study, the low carbon Ni-based superalloy Inconel-738 (IN-738LC) was used as the substrate and a metallic bond coat (NiCrAlY) was deposited on the substrate by air plasma spray (APS) method. Subsequently, the layer was coated by a ceramic interlayer Yttria-stabilized zirconia (YSZ) by APS. Finally, as the last ceramic layer, a hybrid ceramic top layer containing mullite and YSZ was plasma deposited on the YSZ interlayer in various compositions such as: 25 wt% mullite/75 wt% YSZ, 50 wt% mullite/50 wt% YSZ, 75 wt% mullite/25 wt% YSZ, and 100 wt% mullite. In order to investigate the oxidation resistance of the produced TBCs, they were heated at 1000 °C for 50 and 100 h. The results showed that the presence of mullite phase is advantageous to hybrid coatings, so that in the 50 wt% mullite/50 wt% YSZ hybrid coating, the percentage of destructive spinel oxides forming the thermally grown oxide (TGO) layer reduced compared to the 25 wt% mullite/75 wt% YSZ coating. Moreover, aluminum oxide is the dominant oxide in the 50 wt% mullite/50 wt% YSZ hybrid coating. However, the further increase in the mullite percentage had no noticeable effect on the reduction of spinel oxides. In addition, the presence of mullite alongside YSZ led to an increase in the hardness of the 25 wt% mullite/75 wt% YSZ hybrid coating (845 HV) compared to the conventional coating including a YSZ layer (680 HV) or a mullite layer (655 HV). The X-ray diffraction (XRD) analysis revealed that the presence of mullite beside YSZ in the coating increases the stability of the coating after oxidation test so that, in the XRD pattern, monoclinic phase was not detected in the hybrid coatings. In general, 50 wt% mullite/50 wt% YSZ hybrid coating provided the best results compared to other coatings. Moreover, with increasing thickness of the hybrid ceramic layer containing 50 wt% mullite to 350 μm, the thickness of the TGO layer decreased to approximately 7 μm. Additionally, it was shown that due to the decrease of chromium and nickel elements in the TGO layer, the level of the spinel oxides in the thick coatings decreases. This led to an increase in the stability of the TGO layer and an extension of the thermal system's lifespan.

Theoretical Evaluation of YSZ and Alumina-YSZ Thermal Barrier Coatings in a Hydrogen Enriched Combustion Environment

Abstract Yttria-stabilized zirconia (YSZ) ((ZrO2)0.93(Y2O3)0.07) and alumina-yttria-stabilized zirconia ((Al2O3)0.853 + (ZrO2)0.93(Y2O3)0.07) thermal barrier coatings (TBCs) were modeled in the presence of hydrogen-enriched combustion product gases to evaluate phase composition and thermal expansivity (coefficient of thermal expansion). Thermal equilibrium simulations for various equivalence ratios (0.5–0.75) and hydrogen enrichment percentages (0−50%) were conducted to determine the product gas composition for various combustor operating conditions. The obtained product gases were then used in a second thermal equilibrium simulation to demonstrate their effect on the defined thermal barrier coatings. The modeling predictions showed that hydrogen enrichment percentage and equivalence ratio were positively correlated to thermal expansivity for both the thermal barrier coatings examined. The alumina-YSZ composite coating exhibited a higher CTE, more closely matching the CTE of a metallic bond coat, for the studied conditions. This closer match of thermal expansivity results in less significant thermal stresses than the YSZ thermal barrier coating. An increase in hydrogen enrichment percentage and equivalence ratio yielded increased percentages of phase transitions from tetragonal zirconia (t-ZrO2) to cubic zirconia (c-ZrO2). The YSZ thermal barrier coating had a larger percentage of phase transitions throughout the operating range examined, which renders concerns for potential failure from thermal cycling and creep. Theoretical examination of the phase composition and thermal expansivity provided further insights into the fate and behavior of the thermal barrier coatings.

Thermal shock behavior of EB-PVD thermal barrier coatings based on YSZ ceramic coat and (Ni, Pt)Al metallic bond coat

Three different chemical vapor deposition (CVD) aluminizing processes were adopted for preparing of (Ni, Pt)Al metallic coatings, whose microstructure, element content, phase constituent and gas hot corrosion performance were evaluated and comparatively analyzed. YSZ ceramic coatings were subsequently fabricated via electron beam physical vapor deposition (EB-PVD) technique on top of the optimized (Ni, Pt)Al bond coat surface, and thermal barrier coating (TBC) specimens were subjected to 1373 K long-term thermal shock. The surface of aluminized sample with a single external reaction generator is relatively compact, and no defects such as micropores and microcracks are observed. Only a large number of protruding sediments are detected at grain boundary as well as the degree of rumpling and cross-links of these sediments is more obvious. A large amount of Pt is enriched at that grain boundary, while elemental content of Al is lower at the same location. Meanwhile, such sample basically displays β-(Ni, Pt)Al phase, and only three very weak diffraction peaks belonging to ζ-PtAl2 coexist. After long-term thermal shock, the exposed region of bond coat shows two kinds of micro-morphology. The biggest distinction between them is that the composition of Pt and Al elements is evidently different. The interfacial separation of TBCs is mainly concentrated at interface between thermally grown oxide (TGO) and bonding layers, and a very small amount of TGO layer is merely adhered to bond coat surface. It demonstrates that TGO layer is densified and thickened with the extension of thermal exposure period, and then the mismatching of thermal expansion between Al2O3 and (Ni, Pt)Al bond coat occupies the dominant factor. In addition, the grain boundary ridges and undulations on bonding layer surface can also induce accumulation of the tensile stress and further accelerate degradation of the critical interlayer interface during cooling stage of thermal shock.

Interfacial delamination of porous thermal barrier coatings based on SEM image processing in finite element model

Numerical analysis of interfacial delamination and the effect of microstructural pores and rough interfaces on the field variables in thermal barrier coatings (TBCs) based on a real scanning electron microscope (SEM) image is performed. The interfacial delamination is modelled by the utilization of cohesive zone method (CZM) in finite element (FE) analysis. The empirical TBC sample considered in this study is constructed via air plasma spray (APS) method and comprises ZrO27-8wt.%Y2O3 ceramic top coat (TC), thermally grown oxide (TGO) made of Al2O3 and NiCrAlY metallic bond coat (BC). A compatible geometry is provided by applying image processing technique to SEM image of the sample's cross-section. SEM image-based geometry imported into FE software to scrutinize the interplay effects of porosity and rough interfaces on temperature fields, thermal stresses and interfacial debonding of TC/TGO and TGO/BC interfaces of the TBC structure. FE analysis reveal that porosity distribution has a prominent role on the stress concentration zones at the sharp corners of pores especially adjacent to undulations of the TC/TGO interface, which is responsible for possible internal cracks and interfacial fracture, respectively. The main findings demonstrate that the porosity near the TC/TGO interface has detrimental effect as leads to aggravation in the TC/TGO interface damage, which causes an increment in TC/TGO interfacial crack length. While, the porosity within the BC has the ability to reduce the stress and damage level along the TGO/BC interface. In addition, considering creep for all layers leads to lower stress levels up to 24.82 % and consequently alleviates the TC/TGO interface damage.

Driving forces in thermal barrier coatings blistering

Thermal barrier coating (TBC) systems are known to be extremely sensitive to the applied thermo-mechanical loading in terms of both microstructure evolution and of subsequent damage, leading to final failure by ceramic top-coat layer spallation. The analysis of the relationship between mechanical behavior and final failure is limited by the scatter in the experimental results. Based on a typical TBC system for columnar top-coat, this study focuses on blister evolution for pure thermal cycling. The blister is processed by LAser Shock Adhesion Test (LASAT) method. It is observed experimentally that prior to significant further interfacial debonding, the height of the blister increases with the number of applied cycles. To clarify this, a finite element analysis was developed to account for multilayer system behavior and growth strain associated to a thermal-grown oxide layer evolution. Finally, this study demonstrates that rough interfaces, elasto-viscoplastic behavior of the metallic bond-coat, and oxide growth strain are needed to model the experimentally observed evolution of the blister. Thus, the driving forces acting on bond-coat rumpling, are similar to driving forces for blister height increase and final failure of the TBC.

The behavior of plasma sprayed thermal barrier coating with laser microtextured bond coat under high temperature testing

In this work, the laser microtexturing is exploratory studied for controlling the interface between the oxidation resistant metallic bond coat and thermally insulating ceramic top coat in Thermal Barrier Coatings.The 80 to 100 μm thick NiCrAlY layer was deposited by Atmospheric Plasma Spraying over the nickel-based super alloy substrate. Then, the infrared fiber nanosecond laser was used for shaping the topography of bond coat prior to the top coat deposition. The surface geometry was adapted by selecting the laser set-up operational conditions, mainly: (i) laser power, (ii) frequency, (iii) scanning velocity and (iv) inter-pass spacing. The two groove-based patterns were finally selected for further work together with one as-sprayed bond coat, as a reference. The 8 wt% yttria stabilized zirconia (8YSZ) was deposited by means of Suspension Plasma Spraying over all three types of bond coats. The samples were then subjected to isothermal oxidation and thermal cyclic fatigue testing in order to study the influence of bond coat topography on the overall behavior of coatings under high temperature conditions. The microstructural studies revealed that the as-sprayed bond coat topography may be progressively modified by multistep laser microtexturing. The uniform bond coat topography is found to be beneficial in promoting formation of homogeneous, columnar-like top coat. However, the modification of bond coat topography affects the behavior of the bond coat/top coat interface during high temperature testing. The study shows that the microtexture depth should be precisely controlled, otherwise the through bond coat thickness oxidation is observed, which has a detrimental effect on the overall durability of the TBC system. The bond coat asperity width plays also an important role. If it is too narrow, then the failure may propagate locally through the bond coat or the thermally grown oxide layer.

Effect of Thermal Growth Oxide Composition and Morphology on Local Stresses in Thermal Barrier Coatings

The failure of thermal barrier coatings (TBCs) during operation depends mainly on the thermal mismatch between the ceramic top coat (TC) and the metal bond coat (BC). The thermal mismatch at the interface is influenced by the dynamic changes in the composition and morphology of the thermally grown oxide (TGO) between TC and BC during thermal cycling. This work focuses on the establishment of a TGO dynamic growth model, which considers the changes in TGO composition and morphology for investigating the effect of dynamic growth of TGO on local mismatch stresses during thermal cycling. The results show that the sharp locations at the TGO/BC interface are more prone to high tensile stresses during thermal cycling due to the uneven growth behavior of TGO, leading to crack initiation. The valley region of the interface is in a state of compressive stress σxx during the early stages of thermal exposure. The peak region preferentially forms a concentration of tensile stress σyy. Once large-scale "layer" (Ni, Co)Al2O4-based spinel-like mixed oxides(MO) growth occurs in TGO, the stress σxx changes from compressive stress to tensile stress in the valley region, eventually forming high tensile stress (Max: +158 MPa). The maximum tensile stress σyy in the peak region is increased to 256 MPa, which is more than two times larger than the early period of thermal exposure. As a result, the dramatic changes in local stresses seriously affect the time and location of microcracks.

The Model for Estimating the Failure Mechanism of Tensioned Plasma-Sprayed Zirconia Ceramic Hard Coating

Loading flat coated specimens with tensile forces is a universal method to determine the nature of fracture of hard coatings in maritime applications. Such an approach allows the determination of the shear strength at the interfaces between layers and the cohesive properties. Thus, the authors studied the fracturing processes of plasma-sprayed yttria-stabilized zirconia two-layer coatings under the action of tensile forces. The results revealed cracking characteristics in the outer ceramic layer and delamination of flat metal specimens. An analytical method led to normal stress determination in the coating. It also exhibited shear stresses at the bond coat-substrate interface and the top coat-bond coat interfaces. The results include normal and shear stress distributions before and after fragmentation of the ceramic top layer. Tested substrate, the metal bond coat, and the ceramic top coat exhibit a modulus of elasticity of 196, 142, and 47 GPa, respectively. Engineers could apply the proposed methodology and the mathematical analysis to study the causes of cracks and delamination between layers.

Critical scale grain size for optimal lifetime of TBCs

• The critical scale grain size d c of ∼2.5 μm is identified for optimal TBC lifetime. • When d < d c , the rapid growth of the scale causes delamination failure of TBCs. • When d > d c , the growth stress causes vertical cracking and early failure of TBCs. Thermal barrier coatings (TBCs) suffer from spalling failure resulting from the rapid growth of thermally grown oxides (TGO) on the metallic bond coat. In this study, to achieve the optimal lifetime of TBCs, a lifetime phase diagram was constructed for the entire service of TBCs. Herein, both the thermal mismatch stress during thermal cycling and the TGO growth stress during thermal exposure were considered. First, thickening behavior of the Al 2 O 3 scale related to grain size was investigated. Second, transverse delamination failure due to the thermal mismatch stress was analyzed. Third, vertical cracking failure related to early scale growth was explored. Finally, a critical scale grain size of ∼2.5 μm was identified for achieving the optimal lifetime of TBCs. It was experimentally proved that vertical cracking of the Al 2 O 3 scale occurred during early oxidation as the scale grain size was larger than ∼4 μm after pre-oxidation. The proposed lifetime phase diagram and critical scale grain size effect serve as novel criteria for the design of next-generation durable TBCs.

Design and Optimization of Interface Morphology of Thermal Barrier Coatings Based on Regulation of Residual Stress: A Finite Element Simulation Study

Laser surface texture (LST) technology can be used to increase the adherence of thermal barrier coating (TBC). The primary research method is to conduct a large number of laser experiments to determine the optimal texture parameters. To minimize costs and enhance efficiency, in the current work, five types of circular pit textures were summarized; the plane strain model was established using the transient thermomechanical coupling finite element method; the residual stress field after spraying was used as the prestress field; the influence of different textures on the distribution of the residual stress field after a thermal cycling was analyzed; and the propagation law of cracks in the coating was predicted. The current work focuses on: (1) The two-dimensional cross-sectional morphology of texture; (2) the principal stress s22 perpendicular to the interface (resulting in mode I interface crack) and the shear stress s12 parallel to the interface (resulting in mode II interface crack); (3) texture variables—diameter, depth, and spacing. The results revealed that after thermal cycling, the texture in the ceramic top coat (TC) bore tensile stress of around 350 MPa. Both sides of the pit in the metallic bond coat (BC) bore tensile stress, while the bottom bore compressive stress. Among them, the positive tensile stress of the texture with a sinusoidal section was the greatest, whereas the shear stress was the least. The maximum stress in texture increased as the diameter and depth increased, while the minimum principal stress was obtained by adjusting the spacing among the adjacent textures. The stress level in the coating was reduced by selecting the appropriate texture morphology, and the crack propagation was more complex, that is, it took a longer time before reaching failure, which is expected to improve the life.

Thermal conductivity study of ZrO2-YO1.5-NbO2.5 TBC

The reduction of thermal conductivity provided by the zirconia ceramic for TBC application improves protection of turbine blades during the thermal cycles and the high temperature operations. The TBC is a complex system composed of different layers: a metal substrate, a metallic bond coat, thermally grown oxide (TGO), and ceramic top coat. The present study determined thermal conductivity properties by theoretical calculation of the thermal diffusivity, measured by laser flash technique, at 800 °C and 1250 °C for ZrO2-YO1.5-NbO2.5 (TC) with 14.5, 16.0, and 17.5 mol% equimolar yttria and niobia in zirconia, by CO2 laser on the substrate of titanium. Although microscopy and EDS showed that specific parameters of the laser parameters must be defined to lager thickness of the TBC layers, zirconia co-doped with yttria and niobia promoted a significant reduction of the thermal conductivity by approximately 52 % when compared to zirconia with 7.6 % molar with yttria, currently used in gas turbine engine industry. That TC composition with laser processing could be explored as a candidate thermal insulation material for TBC application.

Effect of Low-CTE Oxide-Dispersion-Strengthened Bond Coats on Columnar-Structured YSZ Coatings

Thermal barrier coatings (TBCs) are commonly used to protect gas turbine components from high temperatures and oxidation. Such coatings consist of ceramic top coats and metallic bond coats. The mismatch in thermal expansion of the top coat, the bond coat and the component material is one main factor leading to the failure of the coating system. Columnar-structured top coats offer an enhanced tolerance to the strain during thermal cycling. On a flat bond coated surface, these TBCs reach higher thermal cycling performance. However, on rough surfaces, as used for thermal spray coatings, the performance of these thermal barrier coatings seems to be restricted or even stays below the performance of atmospheric-plasma-sprayed (APS) thermal barrier coatings. This low performance is linked to out-of-plane stresses at the interface between the top coat and the bond coat. In this study, a thin additional oxide-dispersion-strengthened (ODS) bond coat with high alumina content provides a reduced mismatch of the coefficient of thermal expansion (CTE) between the top coat and the bond coat. Columnar suspension plasma sprayed (SPS), yttria-stabilized zirconia (YSZ) TBCs were combined with low-CTE ODS bond coats. The behavior of these TBCs was characterized with respect to thermal cycling performance and degradation in a burner-rig facility. The comparison showed an up-to-four-fold increase in the performance of the new system.

Nano-agglomerated powder and thermal shock cycling property of 8YSZ nano-structured thermal barrier coating

Based on the excellent thermodynamic and service performances of nano-structured coatings, 8YSZ nano-agglomerated spraying powder and its nano-structured thermal barrier coating (TBC) were successfully prepared by the integrated application of nanotechnology and atmospheric plasma spraying. The influences of spraying granulation process (solid content of slurry and inlet air temperature) on the spraying powder characteristics and the water quenching-thermal shock cycling behavior at 1050 °C of 8YSZ nano-structured TBC were investigated, respectively. The results showed that the nano-agglomerated powders exhibit apple-like, hollow spheroids with relatively uniform particle size when the spraying granulation process involved a 40% solid content of slurry, a 240 °C inlet air temperature, and a 35 Hz atomizer rotation frequency. The fluidity and apparent density were 57.53 s/50 g and 1.39 g/cm3, respectively. The nano-agglomerated 8YSZ spraying powders met the requirements of plasma-spraying technology. The plasma-sprayed 8YSZ nano-structured coating was composed of a single t-ZrO2 phase, had a layered structure and a certain amount of nanostructures, and presented a special “bimodal microstructure”. The average porosity of the coating was 7.49%. 8YSZ nano-structured TBC failed layer-by-layer after water quenching-thermal shock 264 cycles at 1050 °C. The main reasons for the failure of 8YSZ nano-structured TBC were the large thermal stress and cracks in the substrate caused by the water quench-thermal shock cycle, and the mismatch of thermal expansion between the metal bond coat and the ceramic coat.

Durability of nanolayer Ti–Al–O–N hard coatings under simulated polycarbonate melt processing conditions

To protect forming tools and components from abrasion, adhesion and corrosion, the application of thin hard coatings by physical vapor deposition is state-of-the-art. The use of a hybrid coating technology, consisting of high power pulse magnetron sputtering (HPPMS) and direct current Magnetron Sputtering (dcMS), allows a combination of the advantageous properties of both. HPPMS coatings for examples show a dense morphology, high hardness and smooth surface. Using dcMS, higher deposition rates can be achieved, resulting in a higher economic efficiently. Within the scope of this work, a novel multilayer coating concept of the material system Ti–Al–O–N was developed. The coatings were deposited using an industrial coating unit. Cold work steel X42Cr13 was used as substrate material. The coatings consist of a metallic titanium bond coat and a nitride TiN/AlN nanolayer as an interlayer. The nanolayer coating architecture leads to a dense, compact and fine crystalline morphology with smooth surfaces. Finally, an oxynitride toplayer with two different oxygen contents was applied. In order to investigate the coating durability relevant for polycarbonate melt processing, electrochemical impedance spectroscopy and linear sweep voltammetry were carried out in borate buffer, NaClO4 and benzoic acid electrolyte. An increased oxygen content in the oxynitride toplayer led to a decreased interfacial current density.

Understanding the influence of microstructure on hot corrosion and erosion behavior of suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are bilayer systems comprising a 7–8 wt% yttria partially stabilized zirconia (YSZ) top coat deposited over a metallic bond coat. Suspension plasma spraying (SPS) is an advanced and attractive top coat processing technique due to its capability to yield a variety of microstructures, including the desired columnar microstructure for enhanced strain tolerance and durability. This work attempts to investigate the desirable microstructural features in an SPS processed TBCs to mitigate hot corrosion and minimize erosion related losses that are often responsible for coating degradation. SPS processed TBCs were deposited utilizing three different spray conditions to obtain distinct microstructural features (column density, interpass [IP] porosity bands, column width), porosity content, and mechanical properties. Apart from comprehensive characterization utilizing SEM, XRD and micro-indentation tests, the as-deposited TBCs were subjected to hot-corrosion tests in the presence of vanadium pentoxide and sodium sulfate as corrosive salts. Post-corrosion analysis revealed complete infiltration of the molten salts in all the investigated TBCs. However, the delamination cracks generated due to the infiltrated corrosive species were minimal in case of TBCs with higher fracture toughness. The differences in microstructure and mechanical properties also led to differences in erosion performance, with TBCs possessing minimal total porosity content and high fracture toughness best resisting erosion related damage. Post-erosion analysis revealed that the TBCs with higher fracture toughness and micro-hardness showed superior erosion resistance. Based on the erosion and corrosion results and subsequent post-mortem of failed specimens, plausible damage mechanisms are proposed. Findings from this work provide new insights on developing damage tolerant TBCs microstructures with enhanced durability when exposed to erosion and hot corrosion environments. • SPS TBCs with varying columnar microstructural features and mechanical properties were deposited. • Influence of parameters (suspension feed rate, spray distance, etc.) on microstructural features in SPS TBCs examined. • Desirable microstructural features were identified to minimize erosion and hot corrosion.

Stress models for electron beam-physical vapor deposition thermal barrier coatings using temperature-process-dependent model parameters

This article presents a new stress model for EB-PVD TBC to provide insight into TBC failure mechanisms. Cycling-induced and temperature-process dependent model parameters are incorporated into stress analysis of EB-PVD TBC and then used to simulate the variation of mechanical, thermal and inelastic behaviour from metallic bond coat to thermally grown oxide (TGO). This gives a smooth evolution of residual stresses and is more realistic than prior finite element (FE) work. Two types of interfacial roughness approximate profiles are presented and implemented in the FE model. Geometrical parameters are used to model the interface roughness, and residual stresses are then evaluated at specific positions within TBCs. This article's stress analysis establishes a link between stress distribution and the evolution of interfacial roughness during thermal cycles. Consequently, the results are expected to provide insight into the failure modes related to localized interfacial roughness evolution.

An analytical model for predicting residual stress in TBC-film cooling system considering non-uniform temperature field

Residual stress is an important parameter to evaluate and predict the interfacial peeling and failure of a thermal barrier coating (TBC) system in the thermal cycle. Considering a non-uniform temperature field, an analytical model for predicting residual stress of the TBC-film cooling system was established. Based on the double-layer TBC-film cooling system, the analytical solutions were validated by comparing with the previous results. Then, the four-layer system, including the superalloy substrate (SUB), the metallic bond coat (BC), the thermally grown oxide (TGO), and the ceramic top coat (TC), was analyzed. The distribution of residual stress was discontinuous at the interface among the layers. The maximum peeling moment occurred at the BC/SUB interface, and the maximum shear stress occurred at the TGO/BC interface. Therefore, the probabilities, of which opening edge cracks (mode I) appeared at the BC/SUB interface and shearing edge cracks (mode II) appeared at the TGO/BC interface, were higher than that at other interfaces. The deflection and bending direction of the system were affected by the temperature difference and coefficient of thermal expansions. The large thickness of the coating could effectively improve the thermal insulation capacity of the system, but the peeling moment and shear stress also increased. Therefore, preventing the thermal growth of the TGO layer and reducing the thickness of the TC layer can improve the stability of the system and extend service lifetime of the system while ensuring that the cooling requirements for hot section components are met.