This work develops an integrated theoretical-experimental approach to investigate the effect of TC porosity and TC/TGO interface roughness on oxidation-induced failure in APS thermal barrier coating system (TBCs) under long-term isothermal exposure. A chemo-mechanical-damage coupled framework is proposed, integrating entire analyses of oxygen transport in porous TC, TGO formation, stress evolution, and crack initiation/propagation. Experimental and numerical results show that TC porosity and interface roughness jointly regulate interfacial oxidation behavior. TC porosity below 4 % significantly reduces interfacial oxidation rate by impeding oxygen diffusion, thereby delaying crack initiation and propagation at the TC/TGO interface. Interface with amplitude exceeding 15 mu m introduces complex stress distributions, increasing TGO thickness non-uniformity and driving crack paths to deviate from the TC/TGO interface, which effectively prolongs TC spallation time. This study establishes microstructure-property relationships for TBCs, providing a direct guide for evaluating TBCs reliability in high-temperature environments.

Thermal barrier coating system (TBCs) at elevated temperature are susceptible to the formation of thermally growth oxide (TGO) at the TC/BC interface, which limits the lifespan of TBCs. In TBCs, the porosity of the top coat (TC) can significantly alter the diffusion pathway and diffusion coefficient of oxygen, thereby leading to the variations in the TGO growth rate and the resulting growth stress. In this work, we focus on the oxidation behavior of atmospheric plasma-sprayed (APS) 8YSZ/NiCoCrAlY TBCs under isothermal oxidation at 1373 K in static air. The influence of porous TC on the oxygen diffusion and oxidation reaction is investigated. The variations of the TGO thickness in the TBCs with different TC porosities are identified via isothermal oxidation experiments. We also develop a theoretical model to investigate the interplays between the oxygen transport, oxidation reaction, and stress evolution in TBCs during their service. We reveal that the TC porosity can regulate the oxidant supply to the TC/BC interface, which further induces the variation of the TGO growth rate. Based on the trends of TGO thickness variation with TC porosity, we identify a critical TC porosity (15 %) modulating the transition of TGO growth mechanism, where the TGO growth is predominantly controlled by the oxidation reaction for the TC porosity higher than 15 % while the TGO growth is dominated by oxygen diffusion for the TC porosity lower than 15 %.

Strain-tolerant thermal barrier coatings achieve superior thermal shock resistance by incorporating ceramic top coats with vertically cracked, segmented or columnar structures. In hydrogen-fueled gas turbines, the elevated operating temperatures intensify the sintering process, rendering the interactions among multiple cracks in these complex architectures particularly pronounced but still insufficiently understood. In this work, mechanisms underlying multi-crack competition driven by differential sintering were investigated through a combined experimental-numerical approach. Experimental characterization under both uniform/nonuniform temperature fields was conducted to capture sintering-induced structural and mechanical evolution. Based on these findings, a temperature-dependent constitutive model was developed within a variational principle framework and implemented in finite element simulations for fracture analyses. The model predictions were validated by thermal shock and sintering experiments under various thermomechanical boundary conditions. The results show that enhanced interfacial strength and differential sintering promote branching crack propagation in different regions of the ceramic top coat. Although interfacial delamination remains the dominant fracture mode, this failure mechanism can be mitigated through controlled interfacial strengthening and sintering gradients. A three-dimensional fracture mechanism map is further proposed to elucidate the relationships among sintering behavior, crack competition, and fracture modes in strain-tolerant ceramic coatings.

The presence of cracks in brittle film plays a crucial role in determining the functionality and reliability of film-substrate system. It is challenging to predict and control the cracking in brittle film during layer-by-layer deposition process. Here, we develop a thermo-mechanical discrete element method that incorporates a thermal-conductive beam model to capture the cracking in brittle film during deposition process. Taking thermal barrier coating system as an example, we reveal that the temperature difference between the film and the substrate during deposition process is a key factor regulating the cracking in the film. As the temperature difference increases, there is a transition from no cracking to channel surface cracking, eventually leading to mixed cracking with coexisting channel surface cracks, interfacial cracks, and intralayer horizontal cracks, which are experimentally validated. Additionally, we elucidate how the in-plane and out-of-plane strengths of the film affect the competition of various cracks during deposition process. Then, we can create a selection map for crack regulation.

The cracking behavior of the top coat (TC) during fabrication is closely associated with its microstructures and characteristics in air plasma sprayed thermal barrier coating systems (APS-TBCs). In this study, the evolution and competition between two major cracking behaviors, namely segmental cracking and interfacial cracking, are investigated during the continuous deposition process of TC. By experimentally measuring the peak quenching stress of TC and theoretically calculating the propensity for both cracking behaviors, the competition between both behaviors is revealed. The experimental and theoretical results demonstrate that the propensity for both cracking behaviors increases with increasing peak quenching stress, however, their competition leads to a decreased propensity for each other. A reduction in surface crack density caused by interfacial cracking occurs once the threshold value for interfacial cracking is satisfied. Furthermore, regulation of interfacial cracking behavior is achieved by adjusting the interfacial bonding strength at the TC-BC interface, enabling the fabrication of segmented APS-TBCs with a controllable surface crack density over a wider range of peak quenching stress. The conclusions drawn in this article provide a better understanding of the competition between segmental cracking and interfacial cracking during plasma spraying process and offer guidance for fabricating APS-TBCs with desired crack patterns. (c) 2025 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

As a key component of heavy-duty gas turbines, the compressor blades typically have complex free-form surfaces. These blades are widely made of martensitic stainless steel, which has excellent high strength, high corrosion resistance, and wear resistance, making it a typical difficult-to-machine material. To address these challenges, a novel green chemical mechanical polishing (CMP) slurry has been developed. Its components include silica, zirconia, malic acid, hydrogen peroxide, and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na). By applying this environmentally friendly CMP process, the surface roughness (Sa) of the stainless steel is reduced to 0.271 ± 0.04 nm, and the material removal rate reaches 127.11 nm/min. To verify its practical application effect, this formulation was applied to the polishing of compressor blades using an immersion CMP equipment. Finally, a surface roughness of 103.0 ± 0.5 nm was achieved, and the material removal rate reached 650.67 nm/min. The thickness of the damaged layer after polishing decreased to 28.78 nm. Results from both scanning electron microscopy and transmission electron microscopy indicated no chemical residues and no abnormal elemental enrichment after polishing.

Residual stresses in top coat (TC) and thermal grown oxide (TGO) of segmented air plasma sprayed thermal barrier coating system (APS-TBCs) are crucial for its durability. However, the equivalent residual stress in this TC-TGO sub-system of one-surface-coated samples is complex and abnormal during isothermal cyclic service. This article presents a theoretical investigation on the underlying mechanisms governing the stress reversal phenomenon by exploring the influence of each driving force during service and the impact of service cycle lifetime ratio on the evolutions of equivalent residual stress in TC-TGO film. By combining a shear-lag model and an equivalent equilibrium model, the equivalent residual stress in TC-TGO film is calculated. The analysis confirms that this phenomenon results from competition between growth of TGO and substrate oxide (SO) as well as interlayer mismatch, which can be avoided through quick isothermal cyclic service with one cycle lifetime no more than 0.25 h.

The capability to maintain operational temperature in safe limit is pivotal for the durability of thermal barrier coating system (TBCs). However, the spallation of TBCs during service may cause local over temperature in TBCs, thereby leading to the catastrophic failure. In this work, we investigate the over temperature in locally spalled TBCs through a combination of numerical calculation and experiment. We numerically calculate the temperature fields in locally spalled TBCs of different spallation depths and spallation diameters, and study the relationship between the maximum temperature in the spalled TBCs and the spallation size. Subsequently, we experimentally study the local spallation induced over temperature in TBCs, and validate by the relationship between the maximum temperature and the spallation size. We assess the effects of gas temperature (1700 K similar to 1800 K), spallation depth (0.2 mm similar to 0.6 mm), and spallation diameter (0 similar to 20 mm) on the maximum temperature in locally spalled TBCs. Furthermore, we identify a critical spallation size of TBCs, beyond which over temperature occurs. The results show that the spallation size of TBCs is vitally important to the thermal management of gas turbine.

Thermally grown oxide (TGO) is important for the failure of thermal barrier coating system (TBCs). It is significant to nondestructively measure the thickness of TGO and residual stress in TGO. However, it is challenging to simultaneously measure both of them. Here, we extend the photo-luminescence piezo-spectroscopy (PLPS) method to simultaneously measure both the thickness of TGO and the residual stress in TGO. We develop an experimental setup for nondestructive detection of Cr3 + luminescence spectrum within TGO in air-plasmasprayed (APS) TBCs. By extracting the intensity of the R2 peak in Cr3+ luminescence spectrum and TGO thickness through SEM observation, we reveal a linear relationship between them. Using the relation, we calculate the TGO thickness of 3600 points with their peak intensities within an experimental zone, and obtained the spatial distribution of TGO thickness. Furthermore, we calculate the residual stress in TGO based on the peak shift data of the Cr3+ luminescence spectrum, and obtain the spatial distribution of residual stress in TGO. Subsequently, we obtain the simultaneous variations of residual stress and thickness of TGO. This work enables the simultaneous measurement of the thickness of TGO and residual stress in TGO, providing a potential approach for evaluating failure process of APS-TBCs.

Air plasma sprayed thermal barrier coating systems (APS-TBCs) are widely employed in both land-based and aerospace turbine engines, featuring a heterogeneous multilayer structure. The failure of APS-TBCs has been confirmed to involve multiple contributing factors. Due to its porous lamellar structure, APS-TBCs exhibit limitations such as inferior strain tolerance and short service lifetime under extreme temperature. This study systematically investigates the service performance degradation of APS-TBCs to establish correlations between the failure process and relevant mechanical properties. Through non-destructive residual stress measurements and cross-sectional morphological observations using disk-shaped samples, combined with evaluations of TC cracking strength and interfacial fracture toughness using dog-bone-shaped samples, the degradation mechanisms of conventional porous APS-TBCs are investigated. Experimental results demonstrate that APS-TBC degradation involves progressive accumulation of residual stress in YSZ TC and TGO layers, as well as temperature-dependent and time-dependent reduction in TC cracking strength. Notably, interfacial fracture toughness shows an increasing trend with service duration, which induces a transition in TC cracking mode from interfacial delamination to vertical segmentation during tensile testing. Simple prediction models for residual-life and residual-strength have been established based on the experimental results. These findings provide fundamental understanding of APS-TBC degradation mechanisms and support the development of life-prediction models.

Residual particles embedded at the bond coat/substrate (BC/SUB) interface after grit blasting can affect the failure behavior of thermal barrier coatings (TBCs) under thermal cycling. This study employed a 2D finite element model combining the cohesive zone method (CZM) and extended finite element method (XFEM) to analyze the effect of interfacial grit particles. Specifically, the CZM was used to simulate crack propagation at the BC/thermally grown oxide (TGO) interface, while XFEM was applied to model the arbitrary crack propagation within the BC layer. Three models were analyzed: no grit inclusion, 20 μm grit particles, and 50 μm grit particles at the BC/SUB interface. This systematic variation allowed isolating the influence of particle size on the location of crack propagation onset, stress distribution, and crack growth behavior. The results showed that grit particles at the SUB/BC interface had negligible influence on the crack propagation location and rate at the BC/TGO interface, due to their spatial separation. However, their presence significantly altered the radial tensile stress distribution within the BC layer. Larger grit particles induced more intense stress concentrations and promoted earlier and more extensive vertical crack propagation within the BC. However, due to plastic deformation and stress redistribution in the BC, the crack propagation was progressively suppressed in the later stages of thermal cycling. Overall, grit particles primarily promoted vertical crack propagation within the BC layer. Optimizing grit blasting to control grit particle size is crucial for improving the durability of TBCs.

This article investigates the equivalent residual stress of the top coat (TC) and thermal grown oxide (TGO) during isothermal cyclic service and experimentally confirms the existence of stress reversal phenomenon in onesurface-coated segmented APS-TBCs for the first time. An in-situ beam curvature measurement equipment is employed to experimentally measure equivalent residual stress, while global curvature is estimated using photographic images. According to the experimental observations, this complex phenomenon is governed by competition between TGO-induced and interlayer-mismatch-induced compressive stresses and substrate oxide (SO)-induced tensile stresses. The discovery of this phenomenon in one-surface-coated APS-TBC samples suggests that traditional laws governing residual stress accumulation are not suitable for describing its evolution during isothermal cyclic service. Therefore, we recommend either using fully-coated samples for long-term isothermal cyclic services or employing one-surface-coated samples for gradient cyclic services (keeping the uncoated substrate surface below 700 degrees C to prevent SO growth).

Segmented structure featuring channeled surface cracks has been identified as a highly exceptional type of air plasma spray (APS) thermal barrier coating system (TBCs) with remarkable strain tolerance and long-term serviceability. However, the underlying mechanisms governing the relationship between deposition parameters and segmental cracking responsible for surface crack formation in APS-TBCs remain incompletely elucidated. In this article, we conducted experimental analysis to investigate the sensitivity of five chosen deposition parameters on the induced peak quenching stress in top coat (TC) during fabrication. Subsequently, we established a quantitative relationship between the five deposition parameters and the peak quenching stress, enhancing our understanding of how these parameters influence segmental cracking. During our investigation, we monitored inflight particle parameters using the DVP-2000 equipment and coating mechanical parameters using a self- developed real-time curvature measurement equipment under different deposition conditions. This allowed us to further analyze the correlations between particle and coating parameters. Based on their relative impact on peak quenching stress, we determined that the sensitivity of these five deposition parameters on segmental cracking is as follows: deposition power > deposition speed > preheating cycles > deposition distance > feed rate. Furthermore, we recommend setting optimum deposition parameters as follows: highest power, lowest speed, highest preheating cycles, lowest distance, and lowest feed rate to improve the possibility of fabricating segmented APS-TBCs with higher density of surface cracks.

Surface cracks are recognized as an effective approach for enhancing the strain tolerance of air plasma sprayed thermal barrier coating system (APS-TBCs). Therefore, segmented APS-TBCs with channeled surface cracks is considered a superior coating system for improving the combustion efficiency of advanced engines. However, the underlying mechanism responsible for its enhanced strain tolerance and service durability compared to conventional ones have not yet been fully elucidated. This article reveals this mechanism by investigating the residual stress evolutions in segmented and conventional APS-TBCs. We fabricate both types of APS-TBC samples and conduct isothermal cyclic oxidation service on them. Then, we measure the residual stresses of YSZ TC and TGO using Raman spectroscopy (RS) method and Photoluminescence piezo-spectroscopy (PLPS) method, respectively. Experimental results indicate that the segmented sample exhibited 2.5 times of service durability compared to the conventional sample. Through detailed analysis, we find that the macro-compliance mechanisms played a crucial role in governing the enhanced service performance of segmented samples. This macro-compliance effect is achieved by lowering the residual stress at given stain level through surface cracks, and thus results in improved interfacial integrity compared to conventional ones. The discoveries in this study contribute towards comprehending microstructure design and selection of APS-TBCs.

Thermal barrier coatings (TBCs) applied in hydrogen gas turbines encounter a critical challenge of water vapor corrosion at higher operating temperatures. However, the degradation mechanism of ceramic top coats exposed directly to high-temperature water vapor environments is still unclear. In this study, the degradation of YSZ coatings with feather-like structures was found to have depth and temperature dependence and could be accelerated by water vapor. The non-uniform phase transformation in the through-thickness direction driven by oxygen vacancy gradient was characterized utilizing the difference in detection depths between neutron and X-ray diffraction. Meanwhile, accelerated sintering was observed throughout the whole depth due to the diffusion and reaction of hydroxyl groups dissociated from water vapor, which was elucidated by computational analysis. Furthermore, direct proofs of transition in diffusion mechanisms were obtained, which were responsible for the severe degradation of YSZ coatings at elevated temperatures. In comparison, the excellent phase/chemical stability and enhanced sintering resistance of the (Gd0.9Yb0.1)2Zr2O7 (GYbZ) and the novel high-entropy (Gd0.2Dy0.2Ho0.2Tm0.2Yb0.2)2Zr2O7 (GDHTY) coatings were evaluated, which demonstrated the potential as alternatives to YSZ applied in hydrogen gas turbines.

Yttria-stabilized zirconia hollow spherical powder (YSZ-HOSP) is a widely utilized ceramic material in thermal barrier coatings (TBCs). Within the category of YSZ-HOSP, yttria-stabilized zirconia spherical thin -walled hollow -shell powder (YSZ-STHS) displays immense potential for producing TBCs with minimal inter -lamellar porosity and cracks. Arc plasma torch equipped with flow -shaping nozzle shows promise for preparing YSZSTHS. However, there is limited research on the impact of the nozzle geometry on plasma flow field characteristics and the resulting effect on the quality of YSZ-HOSP, particularly in terms of spheroidization ratio, hollow -shell powder ratio, and shell thickness. To achieve controllable preparation of YSZ-STHS in arc plasma spheroidization, this paper proposed a novel model for the formation of YSZ-HOSP under different plasma flow characteristics, including high-speed compressing flow field, intermediate -speed critical flow field, and lowspeed expanding flow field. These different plasma flow characteristics were achieved by changing the nozzle diameter and investigated by both numerical simulation and experiments in terms of plasma flow characteristics, electro-thermal characteristics, and the quality of the YSZ-HOSP. Results reveal that YSZ-HOSP prepared with high-speed compressing flow field resulted in a low hollow -shell powder ratio, while low -speed expanding flow field led to an over -thick shell. Conversely, the intermediate -speed critical flow field demonstrates relatively high thermal efficiency and enthalpy, along with moderate jet temperature and velocity, resulting in YSZ-STHS with a spheroidization ratio of 96.6 %, a hollow -shell powder ratio of 92.5 %, and a mean shell thickness of 3.49 mu m. Therefore, it is evident that manipulating plasma flow characteristics allows for the controlled preparation of YSZ-STHS.

In this study, turbine dovetail tenon specimens made of iron-based superalloy were composite treated by shot peening and CuNiIn coating, and the fretting fatigue performance at room temperature and 500 degrees C high temperature was investigated. The surface integrity of the composite-treated dovetail specimens and the wear, fracture morphology, and microstructure after the fretting fatigue tests were characterized. The results showed that the composite treatment of shot peening and CuNiIn coating made the surface roughness of iron-based superalloy from 0.405 mu m to 11.279 mu m, 46 % reduction in surface hardness and the residual compressive stress layer of about 100 mu m was introduced. Compared with the as-received (AS) specimens, the fretting fatigue lifetime of shot peening and CuNiIn coating composite treatment (SC) specimens was increased by 437 % at room temperature, and the fretting fatigue lifetime of SC specimens at high temperature was reduced by 54 % compared with that at room temperature. The cracks in SC specimens were still initiated by multiple fatigue sources, but the number of crack sources decreased and the position of crack sources moved down. At room temperature, CuNiIn coating first underwent shear grinding and then entered delamination wear, while at high temperature, the presence of a large number of coating oxides would lead to serious abrasive wear of CuNiIn coating. Fretting fatigue resulted in obvious orientation differences in the contact region, and the formation and propagation of cracks were related to the plastic deformation and dislocation accumulation of the contact region. The good plasticity of CuNiIn coating is an important reason why it can improve the fretting fatigue performance. The surface hardening caused by shot peening and the introduction of residual compressive stress layer can effectively inhibit crack initiation and propagation. The composite treatment of shot peening and CuNiIn coating can effectively improve the fretting fatigue performance of the dovetail structure of 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).

Segmented vertical cracks in high-strain-tolerance thermal barrier coating (TBC) systems provide fast diffusion pathways for oxygen, leading to the accelerated growth of thermally grown oxide (TGO) interfaces. This results in a competition between enhanced strain tolerance and accelerated oxidation. Here, the effects of crack width on oxygen diffusion and TGO growth in TBCs with segmented vertical cracks and underlying mechanism were investigated experimentally and numerically. The findings reveal a critical crack width of approximately 4 μm, underscoring the importance of selecting an appropriate segmented vertical crack width to design TBCs with superior strain tolerance and sufficient oxidation resistance.