Plasma-sprayed Thermal Barrier Coatings Research Articles

Interfacial delamination mechanism of thermal barrier coatings with real microstructure considering gradient thermal and growth strains based on the fracture phase field

The interface of atmospheric plasma spray (APS) thermal barrier coatings (TBCs) is highly irregular and rough, while their microstructure is porous and extremely uneven. This complexity leads to intricate patterns during the growth of thermally grown oxide (TGO), the distribution and evolution of the generated stress, as well as the initiation and expansion of cracks. Along these lines, in this work, a phase-field theory considering both the thermal gradient strain and TGO growth was established to thoroughly investigate the interface oxidation cracking in TBCs. More specifically, a two-dimensional geometric model of APS TBCs was established, incorporating real interface morphology and microstructure. Numerical simulations and experiments were also conducted to study the temperature and stress distribution, as well as the underlying mechanism of interface cracking during the service of TBCs. The results indicated that at high temperatures, the bottom temperature of the top coating (TC) layer with a porosity of 3.1% and a thickness of 200 μm was approximately 5 °C lower than that of the TC layer without porosity. Compared to the ideal model without pores, the model considering the real microstructure, with the presence of microcracks in the TC, makes it easier for the normal stress in the TGO at the peak region to be released. The inter-lamella cracks (Type A) in the TC near the peaks initiated earliest, followed by the creation of inter-lamella cracks at other locations, accompanied by the appearance of lamella-fracture cracks (Type B), inter-lamella to interfacial cracks (Type C), inter-lamella to TGO cracks (Type D), and internal TGO cracks (Type E). The coalescence of all these types of cracks will ultimately lead to coating failure.

Enhanced CMAS and hot corrosion degradation of YSZ thermal barrier coating with nano powders

Thermal barrier coatings are used to protect hot section parts of gas turbine engines. These coatings are subject to corrosion due to impurity elements in the fuel and dust absorbed from the atmosphere into the engine. The rough and porous surface nature of plasma-sprayed coatings facilitates the penetration of these factors into the coating. Coatings are damaged prematurely as a result of corrosion. In this study, nano-structure YSZ, YSZ + Al2O3, YSZ + TiO2, and YSZ + Al2O3 + TiO2 coatings were deposited on plasma sprayed YSZ coating by electrophoretic deposition method. Hot corrosion and CMAS resistance were investigated. The coatings were characterized by SEM, XRD, AFM, and microhardness before corrosion tests. The changes in the coatings after corrosion tests were characterized by SEM and XRD. The penetration behavior of corrosive factors was examined by elemental mapping with EDS. Nanostructure coatings deposited on APS YSZ made the surfaces of as-sprayed coatings more stable. The ability of these nanolayers to prevent pure penetration into coatings during hot corrosion and CMAS is limited. However, the Al2O3 + TiO2-doped nanostructure YSZ reduced the bond-layer oxidation. In addition, the addition of Al2O3 + TiO2 to the EPD deposited layer also reduced the tetragonal-monoclinic phase transformation that occurred in YSZ as a result of corrosion. Al2O3 and TiO2 additives alone are not effective in improving the corrosion behavior of coatings. Nanostructured coatings deposited with the EPD technique can help protect plasma-sprayed thermal barrier coatings against corrosion.

SEM-Guided Finite Element Simulation of Thermal Stresses in Multilayered Suspension Plasma-Sprayed TBCs

This study presents novel insights into thermal stress development and crack propagation mechanisms in single- and multilayered suspension plasma-sprayed (SPS) coatings of gadolinium zirconate (GZ) and yttria-stabilized zirconia (YSZ), thermally treated at 1150 °C. By combining image processing with finite element simulation, we pinpointed sites of high-stress concentration in the coatings, leading to specific cracking patterns. Our findings reveal a dynamic shift in the location of stress concentration from intercolumnar gaps to pores near the top coat/thermally grown oxide (TGO) interface with TGO thickening at elevated temperatures, promoting horizontal crack development across the ceramic layers. Significantly, the interface between the ceramic layer and TGO was found to be a critical area, experiencing the highest levels of both normal and shear stresses. These stresses influence failure modes: in double-layer SPS structures, relatively higher shear stresses can result in mode II failure, while in single-layer systems, the predominant normal stresses tend to cause mode I failure. Understanding stress behavior and failure mechanisms is essential for enhancing the durability of thermal barrier coatings (TBCs) in high-temperature applications. Therefore, by controlling the interfaces’ roughness along with improving interfacial toughness, the initiation and propagation of cracks can be delayed along these interfaces. Moreover, efforts to optimize the level of microstructural discontinuities, such as intercolumnar gaps and pores, within the creaming layer and close to the TGO interface should be undertaken to reduce crack formation in the TBC system.

Effects of thermal barrier coating porosity on combustion and heat losses in a light duty diesel engine

Plasma sprayed thermal barrier coatings (TBCs), applied in internal combustion engines, can have a profound impact on the apparent rate of heat release. Upon fuel jet contact with the piston wall, the heat loss seems to increase compared to an uncoated piston and combustion appears delayed. Permeable porosity of plasma sprayed TBCs is identified as one of the possible causes for this unwanted effect. In this article the impact of open porosity and sealing of the TBC surface are investigated with single cylinder engine experiments and with simulations. Based on the results, a mechanism is presented to explain the observations.

Prediction of Porosity in Atmospheric Plasma Sprayed Thermal Barrier Coatings Using Terahertz Pulse and Hybrid Machine Learning Algorithms

Prediction of Porosity in Atmospheric Plasma Sprayed Thermal Barrier Coatings Using Terahertz Pulse and Hybrid Machine Learning Algorithms

Plasma sprayed TBCs with controlled intra-splat spherical pores and enhanced blocking of radiative heat transfer

Thermal barrier coatings (TBCs) utilized for thermal insulation in gas turbines are typically semitransparent to infrared thermal radiations, resulting in a limited thermal insulation performance. In this study, plasma spraying TBCs with intra-splat spherical pores (SPs) are designed and prepared by the oxidation of micro carbon spheres particles. SPs with diameter of 0.5–1.5 µm remarkably improve the scattering coefficients (1.9 times vs conventional coating) and reduce the thermal conductivity of TBCs, leading to a temperature drop of 79.8 K at coating/substrate interface. The coatings with SPs in micrometers also exhibit excellent anti-sintering behavior. The average reflectance in 1.0–1.8 µm of coatings with SPs keep almost unchanged after heated at 1300 °C for 80 h. As SPs are wrapped by YSZ laminar splats, the embedded SPs shows neglectable decrease (<10%) in Young’s modulus and hardness to the coating.

Integrating APS TBCs with a built-in Al2O3 protection network for superior CMAS resistance

Calcium–magnesium–alumina–silicate (CMAS) attack poses a great threat to the durability of thermal barrier coatings (TBCs). We introduce a novel Al2O3-modified TBC architecture to counter this threat. The architecture integrates an Al2O3 protection network into the microstructure of an atmospheric plasma sprayed (APS) TBC by drip coating method, which modifies the vulnerable infiltration pathway of CMAS melt and promotes the self-crystallization of CMAS at the infiltration front. Moreover, during sintering, the built-in Al2O3 can inhibit inter-splat crack healing. This innovative TBC design achieves robust CMAS resistance and strain tolerance, while also offering advantages during sintering.

Stiffening-resistant composite TBCs counteracting the substrate constraint effect during thermal exposure

Thermal barrier coatings (TBCs) endow metal components with an increased capacity to endure exposure to high temperatures. Plasma-sprayed TBCs exhibit excellent thermal insulation performance and are widely used in land-based thermal protection systems. However, plasma-sprayed TBCs are more prone to cracking during usage, significantly limiting their applications. The main issue is the sintering-induced stiffening that enhances the cracking driving force. In this work, stiffening-resistant TBCs were fabricated using a composite structure that counteracts the substrate constraint effect. First, the substrate constraint effect on the TBCs was studied based on the surface roughness. The results revealed that stiffening was the primary cause of substrate constraint-related failure. Two-dimensional (2D) interface gaps were spontaneously formed during thermal exposure, which could reduce stiffening by 20%. Moreover, the healing behavior of the 2D gaps was analyzed and used to explain the stiffening-resistant behavior. Finally, the effect of the newly formed 2D gaps on the lifespan of the TBCs was analyzed. Detailed simulations suggested that the newly formed 2D gaps were capable of reducing the strain energy release rate and led to the growth of large in-plane cracks. For future applications, it is necessary to fully take advantage of the newly-formed 2D gaps by avoiding their negative effects on TBC life span.

A matching design co-enhancing thermal barrier and thermal cyclic performances of sintering-resistant coatings

Sintering-induced healing of intrinsic micropores is the main cause of thermal insulation degradation in plasma-sprayed thermal barrier coatings (TBCs). Although large pores can effectively retain the thermal-insulation performance, they can lower the mechanical performance during thermal cyclic tests. Therefore, balancing the tradeoff between the thermal and mechanical performances is a challenging task. Herein, sintering-resistant coatings were achieved through a matching design that co-enhances the thermal insulation and thermal cycling span. TBCs with different contents of sintering-resistant pores were prepared. The thermal-insulation performance was found to be positively correlated with the contents of sintering-resistant pores. However, in thermal cyclic tests, the lifespan increased as the pore content increased from 0 vol% to 20 vol% but drastically decreased as the pore content increased further above 20 vol%. Next, the competing effects of the newly formed pores on the crack driving force and fracture toughness, which might hasten TBC failure, are discussed. Based on the critical pore content, the matching design enhances the thermal insulation and lifespan of the resultant TBCs by 50% and 40%, respectively, from those of conventional TBCs. Therefore, the sintering-resistant structure is expected to considerably contribute to next-generation applications of TBCs.

CMAS infiltration behavior of atmospheric plasma-sprayed thermal barrier coating with tailored pore structures

The degradation of atmospheric plasma spraying (APS) thermal barrier coatings (TBCs) due to calcium-magnesium-alumina-silicate (CMAS) corrosion is considered as a critical issue. In present work, mitigation of CMAS infiltration of APS TBCs has been achieved by tailoring pore characteristics based on a novel APS structure regulation method, and its thermal conductivity, thermal shock lifetime and infiltration behavior were evaluated. The results revealed that the laminar pore distributed TBC formed by horizontally aligned porous embedded particle clusters (PEPC) showed longest thermal shock lifetime and lowest thermal conductivity. Characterization of CMAS corrosion suggested that whilst all the TBCs samples with different pore structures suffered corrosion by CMAS melt, the severity fluctuated greatly. Compared with randomly distributed PEPC areas, these horizontally aligned PEPC areas have a noticeable potential for restricting CMAS infiltration. This unique structure consumes a large amount of CMAS melt and creates anti-CMAS filling pores, thus improving the overall CMAS infiltration resistance of the coating. The alternating dense layers outside the porous layers also provide a layer-by-layer barrier to CMAS infiltration. This study emphasizes that the modulation of the pore structure of APS TBCs is a promising strategy to mitigate CMAS infiltration.

Unraveling the Cracking Mechanisms of Air Plasma-Sprayed Thermal Barrier Coatings: An In-Situ SEM Investigation

In this research work, real-time three-point bending coupled with the scanning electron microscopy (SEM) technique were used to study the crack formation and growth of air plasma spraying (APS) thermal barrier coatings (TBCs). The acquired micrographs were then used to study the strain fields in the vicinity of the cracking region using digital image correlation (DIC) analysis. Fractography analysis for the fractured regions of the APS coatings was also discussed. Based on real-time observation, it was found that roughness of the coatings’ free surface (e.g., valleys) can promote the initiation of cracks since it acts as stress concentration points. Pores and splats features of the coating microstructure contribute to crack branching and crack path deflection, respectively. The former phenomenon (i.e., crack branching) negatively affects the lifetime of the TBC system as it results in an increased fracture area, while the latter can improve the fracture toughness of the coatings and its durability through improving the coating’s ability to dissipate the energy required for crack propagation.

Microstructure and Deposition Mechanism of Suspension Plasma Spraying Thermal Barrier Coatings

Microstructure and Deposition Mechanism of Suspension Plasma Spraying Thermal Barrier Coatings

Systematic Approach to Optimize Technological and Economical Aspects of Atmospheric Plasma Sprayed Thermal Barrier Coatings

Plasma‐sprayed yttria‐stabilized zirconia coatings have been used in gas turbines for decades. They are applied for thermal insulation to increase operating temperature and hence efficiency and component's lifetime. To keep manufacturing costs low, especially deposition efficiency is important. However, increasing it is also related to a reduction in porosity, affecting the insulating properties of the layer. To find an optimal combination of efficiency and technological performance, a systematic study of the most affecting parameters of the atmospheric plasma spraying process is conducted, using response surface methodology. In detail, the influence of current, spraying distance, and hydrogen gas flow is investigated with respect to the deposition efficiency, porosity, microstructure, and mechanical properties of the coatings. Characterization is carried out by scanning electron microscopy, microindentation tests, and three‐point bending tests. The models generated based on these measured properties allow predictions of the system responses for any parameter variation in the investigated design space. In addition, a numerical model is developed for targeted optimization of the coating properties. This can be used to produce optimized coatings for load‐flexible gas turbines with high deposition efficiency, high porosity, and at the same time advantageous mechanical properties.

A modified single edge V-notched beam method for evaluating surface fracture toughness of thermal barrier coatings

The surface fracture toughness is an important mechanical parameter for studying the failure behavior of air plasma sprayed (APS) thermal barrier coatings (TBCs). As APS TBCs are typical multilayer porous ceramic materials, the direct applications of the traditional single edge notched beam (SENB) method that ignores those typical structural characters may cause errors. To measure the surface fracture toughness more accurately, the effects of multilayer and porous characters on the fracture toughness of APS TBCs should be considered. In this paper, a modified single edge V-notched beam (MSEVNB) method with typical structural characters is developed. According to the finite element analysis (FEA), the geometry factor of the multilayer structure is recalculated. Owing to the narrower V-notches, a more accurate critical fracture stress is obtained. Based on the Griffith energy balance, the reduction of the crack surface caused by micro-defects is corrected. The MSEVNB method can measure the surface fracture toughness more accurately than the SENB method.

Numerical investigation of thermal stresses induced interface delamination in plasma-sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are designed to be able to withstand a severe environment with coupled conditions including high temperature and thermal stresses. However, the complexity of their microstructure and the severity of these conditions give rise to premature TBCs failure and consequently reduce their service lifetime. This work aims to investigate the mechanisms of failure in plasma-sprayed TBCs and one of the most important failure mechanisms is related to thermal residual stresses developed in TBCs which leads to coating delamination at the interface contact between layers. A discrete element-based model has been developed to investigate thermal stresses generated at the interface on either side of the thermally grown oxide (TGO) layer due to the mismatch of coefficients of thermal expansion (CTE) between layers, and their material properties are assumed to vary with temperature. Thermal stresses induced in local areas due to the TGO interface irregularities are considered as well in the model by using the image analysis, which consists of a segmentation processing on actual microstructure images of TBCs, thereof allow to account for the undulation of the TGO interface which has been found to be associated with the failure of TBCs.

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.

Thermal Aging Effect on In-Plane and Cross-Plane Thermal Diffusivities of Plasma-Sprayed Thermal Barrier Coating

Thermal barrier coatings (TBCs) are used to shield from heat and wear of components which are used at high temperature situation, for example, the blades of gas turbines. There is a pressing need to evaluate the thermal conductivity of TBCs in the thermal design of advanced gas turbines with high energy efficiency. In the case of gas turbines, these TBCs consist of a ceramic-based top-coat (TC) and a bond-coat (BC) on a superalloy substrate. TBCs are commonly prepared by plasma spraying process. The TC is expected to play the role of a thermal shield. Thus, thermal conductivity of thickness direction of TBC has been investigated. However, plasma-sprayed TCs were known that they have anisotropic microstructure. We have been investigating in-plane and cross-plane thermal diffusivities of plasma-sprayed Yttria-stabilized zirconia (YSZ) TCs at 300 K in detail in a previous study. From the results of thermal diffusivity measurements for four kinds of TCs, it was found that plasma-sprayed TCs had significantly anisotropic thermal diffusivity depending on their microstructures. In this study, we measured in-plane and cross-plane thermal diffusivities of typical two plasma-sprayed TCs from 300 K to 1200 K in vacuum. And we also measured in-plane and cross-plane thermal diffusivities of annealed TC with different aging time. It was found that anisotropy estimated from in-plane and cross-plane thermal diffusivities did not change from 300 K to 1200 K. There was a tendency that thermal diffusivity value and anisotropy slightly increased by thermal aging effects.

Cracking Behavior of Atmospheric Plasma-Sprayed 8YSZ Thermal Barrier Coatings during Thermal Shock Test

The failure of plasma-sprayed thermal barrier coatings (TBCs) during service is usually related to the cracking behavior. In this study, plasma-sprayed TBCs were prepared with two kinds of agglomerated sintered yttria-stabilized zirconia (YSZ) powders with different particle sizes. The evolution of mechanical properties and crack propagation behavior of the coatings during the whole life stage were studied by a thermal shock test. The effect of powder particle size on the cracking behavior of the TBCs during thermal shock was analyzed from the aspect of pore structure, mechanical properties, and stress state of the coatings. The crack propagation and coalescence in the direction parallel to the substrate in the coating is the main factor leading to the spalling failure of the coating during thermal shock. Although the coating prepared by fine YSZ has higher fracture toughness, the lower strain tolerance will increase the cracking driving force on the crack tip of the coating during thermal shock, and the cracks in the coating propagate merge at a faster rate during thermal shock. The larger porosity and pore size of the coating prepared by coarse YSZ help the coating suffer less thermal stress during thermal shock. Although the existence of pores reduces the fracture toughness of the coating to a certain extent, the increase of strain tolerance reduces the crack growth rate in the coating, so the coating has a longer life.

Comparison of Hot Corrosion Behavior of YSZ, YSZ/Mullite and YSZ/Mullite Gradient Coatings on Nickel Base Superalloy Prepared by Plasma Spray Method

Three types of thermal barrier coatings, namely NiCrAlY/YSZ double-layer, NiCrAlY/YSZ/mullite three-layer and NiCrAlY/YSZ/gradient of YSZ and mullite coatings, were deposited on a nickel-base superalloy using a plasma spray method. Hot corrosion studies of the plasma-sprayed thermal barrier coatings were conducted in 45wt% Na2SO4 + 55 wt% V2O5 molten salt at 1050 °C. Microstructure and morphology of the coatings were examined and compared by scanning electron microscopy and X-ray diffraction. The addition of mullite in thermal barrier coating results in the mullite phase reacting with corrosive salt and oxygen to form NaAlSiO4 phase. In addition to this phase, alumina (Al2O3) and Na2O phases are also formed. The presence of alumina phase (Al2O3) along with Na2O leads to formation of NaAlO2. A dense layer of NaAlO2 can increase the resistance to hot corrosion of the coating compared to YSZ coating.

Long lifespan thermal barrier coatings overview: Materials, manufacturing, failure mechanisms, and multiscale structural design

A long lifespan is a basic support for the thermal protection of thermal barrier coatings (TBCs). Plasma-sprayed TBCs provide high thermal insulation and cost-effective advantages; thus, they are used in cutting-edge areas, such as land-based gas turbines. However, following thermal exposure, sintering causes structural changes and significant degradation in thermal insulation and strain tolerance. Thus, plasma-sprayed TBCs often show poor performance in thermal cycling tests. Therefore, extending the lifespan of plasma-sprayed TBCs is crucial. This study reviews the structural design of TBCs for a long lifespan. The main contents reviewed include the following aspects: (1) the basic structural features of plasma-sprayed TBCs with high thermal insulation and strain tolerance in an as-deposited state; (2) the dynamic structural evolution during thermal exposure to reveal the degradation mechanism in thermal and mechanical properties; and (3) multiscale structural design of TBCs to achieve a long lifespan. This review aims to contribute to the development of advanced gas turbines.