Ceramic Thermal Barrier Coatings Research Articles

Impact of interfacial texture on failure behaviour of 8YSZ thermal barrier coatings under thermal cyclic loading

Thermal barrier coatings (TBCs) provide thermal protection for high-temperature components of aero-engines and steam turbines, but are susceptible to failure under thermal cyclic loads. The effective design of interfacial texture is one of the approaches to enhance the lifetime of ceramic TBCs. In this study, four types of interfacial textures were designed on the surface of substrates, and 8YSZ coatings of equal thickness were prepared using atmospheric plasma spraying, and thermal cycling tests were carried out. Meanwhile, the thermal cycle lifetime, microstructure evolution, residual stress, and energy release rate criterion were also studied. The optimal morphology and parameters of the texture are determined. It is found that the lifetime of the concave trapezoidal texture is the longest, with a deflection angle of 67°, diameter of 700 μm, depth of 500 μm, and spacing of 5 mm. This study can provide theoretical guidance for the interfacial texture design of ceramic TBCs.

The effect of ceramic YSZ powder morphology on coating performance for industrial TBCs

The increasing turbine inlet temperatures in modern gas turbines have raised concerns about the corrosion of ceramic thermal barrier coatings (TBCs) caused by molten silicate deposits, commonly referred to as “CMAS” due to their main constituents (CaO-MgO-Al2O3-SiO2). The objective of this study was to investigate the combined influence of powder morphology and chemical composition on the CMAS resistance and thermal cycling resistance of ceramic monolayer and bi-layer coatings created through Atmospheric Plasma Spraying (APS). Three powder morphologies were examined: porous Agglomerated and Sintered (A&S) granules, Hollow Spherical (HOSP) powders, and dense, irregular Fused and Crushed (F&C) particles. Monolayer 7-8YSZ coatings with both porous and dense vertically cracked (DVC) microstructures, and bi-layer coatings consisting of a bottom layer of porous standard 7-8YSZ and a top layer composed of a porous high‑yttrium ZrO2–55 wt% Y2O3 were obtained using all three powder types (A&S, HOSP, or F&C). Furthermore, the bi-layer systems were deposited with different ratios between the individual layer thicknesses and/or different total thickness. FEG-SEM, EDX, and micro-Raman analyses, were conducted to assess the coatings' performance. Nanoindentation high-speed mapping and pillar splitting test were performed to evaluate the mechanical behaviour. The study on 8YSZ monolayers shows that coatings from a F&C feedstock exhibit higher density, reducing the CMAS penetration. However, these coatings demonstrate poorer thermal cycling performance due to increased stiffness and thermal stresses. Coatings from HOSP and A&S powders allow CMAS penetration but offer stress relief pathways, enhancing the coating's ability to withstand thermal stresses. Bi-layer coatings with a 55YSZ top coat show superior CMAS resistance compared to 7-8YSZ monolayer coatings, with limited penetration causing top coat peeling. The thickness ratio between the layers also affects thermal cycling resistance, where a thinner 55YSZ layer extends the TBC lifetime.

Investigations on the water-jet guided laser scribing of thermal barrier coated IC21 nickel-based superalloy

Ceramic thermal barrier coatings (TBCs) are widely recognized as state-of-the-art for ensuring the stability and reliability of turbine blades operating in extreme conditions. However, the significant disparities between the thermal and mechanical properties of TBCs, bonding layers (BCs), and nickel-based superalloy substrates pose considerable challenges for machining film cooling structures on turbine blades while minimizing coating delamination, heat-affected zones (HAZs), and other potential defects. The utilization of Water-Jet Guided Laser (WJGL) has gained significant attention as a promising approach for the machining of advanced materials, including ceramics and multi-layer composites. In this study, a theoretical model is proposed to describe the transient interaction between WJGL and TBC IC21 nickel-based superalloy. A numerical simulation using the deformation geometry method is employed to investigate the evolution of ablation morphology, which is further validated by experimental data. Specifically, the formation mechanism of the shoulder structure at the material interface is explained. Furthermore, orthogonal experiments are conducted to establish the fundamental influence rule and significance level of machining parameters on the microscopic groove morphology. Based on these findings, multi-row cutting experiments are performed to identify the optimized scanning trajectory for achieving the high-quality thorough cutting of the substrate with a thickness of 2.7 mm, while minimizing oxidation, residue deposition, and delamination. The outcomes of this study contribute to the knowledge of WJGL machining, and could potentially improve the efficiency and accuracy of advanced material processing techniques.

Infrared irradiation to drive phosphate condensation as a route to direct additive manufacturing of oxide ceramics

AbstractThis paper introduces a fast, low‐temperature, pressureless process to chemically bind ceramic parts with the help of infrared (IR) irradiation and phosphate binder condensation. Ceramic components are synthesized from slurries of ceramic powders and Al(H2PO4)3 binder that are irradiated with short‐waved IR light capable of heating the system to 350°C. This irradiation is found to be sufficient to drive phosphate condensation, binding the ceramic powders together within a matter of seconds. The IR‐irradiated components show an increase in density and Vickers hardness. Layer‐by‐layer spraying and irradiation is demonstrated as a route to additive manufacturing using various ceramic chemistries. While further optimization is needed to control desired microstructure, this process of using chemically bonded ceramic binders with IR heating for additive manufacturing shows the potential to find applications in various ceramic systems, including refractories, bone implants, electronics, and thermal barrier coatings.

Effect of microstructure on CMAS corrosion behavior of (Gd0.8Yb0.2)2Zr2O7/YSZ thermal barrier coatings prepared by EB-PVD

Three types of (Gd0.8Yb0.2)2Zr2O7 (GYbZ)/YSZ double ceramic layer thermal barrier coatings (TBCs) were prepared with different microstructures by electron beam physical vapor deposition. The CMAS corrosion performance of the three TBCs was studied at 1250 ℃ compared with YSZ-TBC. The results showed that the CMAS corrosion resistance strongly depends on their microstructures. TBCs consisting of smaller inter-column gaps and larger specific surface area exhibited superior CMAS corrosion performance. The reaction products of GYbZ-TBCs are mainly composed of Gd-apatite and fluorite c-ZrO2 based fluorite. The effect of microstructure on CMAS infiltration depth was discussed.

Evaluation of functionally graded YSZ - IN625 clad without bond coat using laser directed energy deposition

M-CrAlY or M-Cr-based bond coats are used as a buffer layer to apply ceramic thermal barrier coatings (TBCs) to metal turbine blades. However, due to oxygen diffusion, thermally grown oxides grow over the bond coat material, leading to coating failure in the components. Therefore, this study attempts to fabricate a novel TBC-coated IN625 without bond coat material using a Functional grading approach. The findings from this study may pave the way for processing functionally graded ceramic materials using Laser Additive manufacturing techniques. This study also evaluates the performance of functional grading in joining dissimilar materials using small spot-size laser sources. In the present study, samples were fabricated for three sample conditions: S1: direct clad, S2: 25% linear grading, and S3: 50% linear grading. The interface between Yttria Stabilized Zirconia (YSZ) and IN625 for the S1 sample showed discontinuous and horizontal cracks along the interface due to steep variations in thermal properties. However, the interface of S2 and S3 samples showed good adhesion and a smooth transition in microstructure between IN625 and YSZ as a result of functional grading. SEM micrographs showed homogeneous YSZ distributions without segregation within the IN625 matrix. This was attributed to strong Marangoni flow as a result of the small spot-size laser beam used in this study.

Unveiling the underlying mechanism of unusual thermal conductivity behavior in multicomponent high-entropy (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 ceramics

Previously, the research on thermal conductivity of ceramic thermal barrier coatings mainly focused on phonon and photon thermal conductivity (thermal radiation effect). However, electrical conductivity is remarkable in some systems. Hence, the contribution of phonon, photon and electronic heat conduction to thermal conductivity of high-entropy systems was evaluated in this study. The (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 (x = 0–0.5) high-entropy ceramics with single defective fluorite structure were successfully prepared via a solid reaction method. Below 600 °C, the thermal conductivities decrease with increasing temperature for x = 0.1–0.5 components, then reveal a drastic temperature dependent increase. Moreover, the composition dependent thermal conductivities are also unusual based on the conventional phonon thermal conduction mechanism. The increased electronic thermal conductivity, improved photon thermal conductivity (at high temperatures) and reduced phonon-grain boundary scattering should be responsible for the unusual thermal conductivity behavior. This can be verified by the significantly increased electrical conductivity, optical transmittance and grain size, as well as reduced emissivity for (La0.2Gd0.2Y0.2Yb0.2Er0.2)2(Zr1-xCex)2O7 high-entropy ceramics. The present study also broadens the way to investigate the thermal conductivity of ceramic thermal barrier coatings, and is helpful to design thermal barrier coatings with low thermal conductivity.

Novel Thermal Barrier Coatings with Phase Composite Structures for Extreme Environment Applications: Concept, Process, Evaluation and Performance

In this paper, a novel concept in the field of phase composite ceramics has been proposed and applied for creating the topcoats of durable thermal barrier coatings (TBCs), which is one of the most critical technologies for advanced high-efficiency gas turbine engines in extreme environments. The phase composite ceramic TBCs were designed to demonstrate superior and comprehensive performance-related merits, benefits, and advantages over conventional single-phase TBCs with a topcoat of 8YSZ or Gd2Zr2O7, including thermal phase stability, thermal shock durability, low thermal conductivity, and solid particle erosion resistance. In this paper, we review and summarize the development work conducted so far related to the phase composite ceramic concept, coatings processing, and experimental investigation into TBC behaviors at elevated temperatures (typically, ≥1250 °C) using different evaluation and characterization methods, including isothermal sintering, a burner rig test, a solid particle-impinging erosion test, and a CMAS corrosion test. Two-phase (t’+c) zirconia-based TBCs demonstrated improved thermal shock and erosion resistance in comparison to conventional single-phase (t’), 8YSZ TBC, and Gd2Zr2O7 TBC, when used separately. Additionally, a triple-phase (t’+c+YAG) TBC sample demonstrated superior CMAS resistance. The TBC’s damage modes and failure mechanisms for thermal phase stability, thermal cycling resistance, solid particle erosion behavior, and CMAS infiltration are also characterized and discussed in detail, in terms of microstructural characterization and performance evaluation.

LaYZrO/YSZ double ceramic layer thermal barrier coatings by EB-PVD: Thermal performance, morphology and failure behavior

Thermal barrier coatings have been widely investigated over the last decade. Lanthanum zirconate is known for its high melting point and good thermal stability in aircraft engine environment. Owing to the distinctive thermophysical properties, Y doping in pyrochlore La2Zr2O7 might be a promising TBCs material as the top ceramic layer. It remains a challenge to completely understand the LaYZrO/YSZ double ceramic layer TBCs properties and microstructure evolution during thermal cyclic. In this paper, LaYZrO/YSZ double ceramic layer TBCs have been prepared by EB-PVD. The phase structure, thermal performance, microstructure and thermal cyclic lifetime have been researched. Thermal conductivity has been decreased compared with YSZ and La2Zr2O7, simultaneously, coefficient of thermal expansion has been increased compared with La2Zr2O7. The failure behavior of LaYZrO/YSZ double ceramic layer TBCs have been elucidated as typical TGO growth mode and specific sintering crack evolution mode. This study of properties and failure behaviors might provide the valuable investigation and thought for other advanced TBCs.

Plasma spraying porous thermal barrier coatings with high deposition efficiency: A solvable dilemma?

In addition to the selection of suitable materials, another way to lower the thermal conductivity and improve thermal cyclic performance of ceramic thermal barrier coatings is to produce microstructures with high porosity reaching up to 20 vol%, sometimes even more. The way to achieve this is by selecting suitable process parameters for plasma spraying, e.g. a low plasma enthalpy or a large spraying distance. The choice of the powder feedstock also plays a role. This attempts to melt the powder particles only partially, so that voids are not completely filled between the deposited splats. Hereby, the desired porosity is formed in the layer. Naturally, however, this is typically associated with losses in deposition efficiency. This dilemma is difficult to solve.In this work, spraying experiments were performed in which several process parameters were varied. An argon‑hydrogen mixture was used as plasma gas for the cascaded three-cathode torch. Porosities were obtained by image analysis and, for some samples, also by mercury intrusion; deposition efficiencies were determined gravimetrically. The resulting porosities and deposition efficiencies were combined in an evaluation function to compare different process parameters with respect to their effects on these two target parameters. The results show that the dilemma of optimizing porosity and deposition efficiencies simultaneously could only be solved well to a limited extent. This makes a systematic approach all the more important in order to identify optimum parameter sets within the bounds of what is possible.

Deposition of Next-Generation Ceramic Thermal Barrier Coatings Based on the Sm2O3–ZrO2–HfO2 System

Deposition of Next-Generation Ceramic Thermal Barrier Coatings Based on the Sm2O3–ZrO2–HfO2 System

CMAS corrosion and thermal cycling fatigue resistance of alternative thermal barrier coating materials and architectures: A comparative evaluation

The corrosion of ceramic thermal barrier coatings (TBCs) by molten silicate deposits, usually known as “CMAS” from their main constituents (CaO-MgO-Al2O3-SiO2), is an issue of increasing concern in modern gas turbines as the turbine inlet temperatures are increased to enhance thermodynamic efficiency. Because conventional ZrO2-7wt%Y2O3 (7YSZ) dissolves quite readily in a CMAS melt, many alternative materials have been proposed, but there are not many comparative studies among them. Multi-layer architectures featuring a tougher 7YSZ bottom layer and a more brittle, but more CMAS corrosion-resistant top layer have also been proposed; therefore, a comparison among these architectures is also in order. In this paper we studied comparatively the resistance to CMAS corrosion and to thermal cycling fatigue (an essential pre-requisite for any TBC system) of Gd2Zr2O7, ZrO2–55wt%Y2O3 and Gd/Yb/Y co-doped ZrO2, both in the form of single, dense-vertically cracked (DVC) layers deposited by plasma spraying onto an MCrAlY bond coat, and as top layers with a bottom layer of either porous or DVC 7YSZ. It was found that Gd2Zr2O7 resists CMAS corrosion, without any grain-boundary dissolution, slightly better than does ZrO2–55wt%Y2O3. They both develop a solid Gd- or Y-apatite layer (respectively) at the interface with the CMAS melt, driven by the rather large difference in optical basicity between these compounds and CMAS itself, but the Y-apatite layer is less continuous and, therefore, a bit less protective. Gd/Yb/Y co-doped ZrO2, instead, suffers as much grain-boundary dissolution in contact with molten CMAS as does 7YSZ. A Gd2Zr2O7/porous 7YSZ system would therefore exhibit simultaneously high resistance to CMAS dissolution and to thermal cycling fatigue, although there is a risk that the CMAS melt might infiltrate the segmentation macro-cracks and the microcracks of the Gd2Zr2O7 layer and undermine the porous 7YSZ bottom layer.

Degradation and lifetime of self-healing thermal barrier coatings containing MoSi2 as self-healing particles in thermo-cycling testing

Yttria-stabilized zirconia (YSZ) is the state-of-the-art top coat material for thermal barrier coatings (TBCs) applied on highly loaded gas turbine parts. During operation at high temperatures, stresses are induced by the thermal expansion coefficient mismatch between the ceramic TBC and the metallic substrate. As a consequence cracks can grow, propagate and finally lead to a spallation of the top coat. Using atmospheric plasma spraying (APS), so-called self-healing MoSi2 particles can be incorporated into the YSZ matrix to mitigate the propagation of cracks leading to a lifetime gain and possibly higher temperature capability of the TBC. In the present work, the healing process is realized by the oxidation of the self-healing particles, which introduces a volume expansion by a formation of reaction products, which can seal the cracks. The self-healing particles were introduced within the first 150 μm of the YSZ coating matrix immediately on top of the bond coat. The degradation and lifetime of such systems were studied in furnace cycling and in burner rig tests, in which a temperature gradient through the sample was applied. The lifetime of the self-healing coatings was then compared to the lifetime of an YSZ coating without self-healing particles. In burner rig tests a clear lifetime extension of the self-healing TBCs was observed. The origin of this different behavior was investigated by microstructural analysis in scanning electron microscopy. A further insight into the failure mechanisms was gained by the analysis of a self-healing TBC cycled in a furnace cycling test only for about 55% of its expected lifetime.

Degradation of thermally sprayed Al2O3 coatings in reactor-grade liquid-sodium and its mitigation by laser treatment

Laser treatment (LT) to enhance the performance of atmospheric plasma spray (APS) deposited Al2O3 thermal barrier coatings (TBCs) was attempted for applications in extreme liquid-sodium environments of a sodium-cooled fast reactor (SFR). Thermally sprayed sacrificial ceramic TBCs are being proposed and investigated for installation on the stainless steel (SS) 316LN core catcher assembly of future SFRs to combat the core disruptive accidents (CDAs), wherein the primary criterion to be satisfied by the TBC is its long-term compatibility with the liquid-sodium coolant over a whole reactor life. Sacrificial ceramic coatings are susceptible to liquid-metal induced degradation by reactor-grade sodium. This study provides new insights into the failure mechanisms of APS Al2O3 coatings in liquid-sodium environments. Subsequently, an attempt is made to improve the performance of coatings by a surface modification approach using a laser treatment of the TBC topcoat. APS coatings exposed to liquid-sodium failed by spallation and delamination of the incrementally deposited topcoat, which is attributed to the formation and growth stress of ternary Na–Al oxides between the lamellar layers of the APS coatings. Compared to the APS coatings, LT coatings successfully retarded the liquid-sodium ingression and thereby exhibited improved degradation resistance and structural stability in reactor simulated sodium environments. The enhanced performance of LT coatings is attributed to its carefully re-engineered architecture of the APS TBC that could beneficially control the kinetics of interaction with liquid-sodium.

Isothermal Oxidation TGO Growth Behaviors of Laser-Remolten LZO/YSZ Thermal Barrier Coatings

Laser scanning modification was applied to secondarily melt the top ceramic coating surface of lanthanum zirconate/yttria-stabilized zirconia double ceramic thermal barrier coatings (LZO/YSZ TBCs) to reduce the gas oxygen diffusion and improve the TBCs service life. Isothermal oxidations with different times were carried out on the as-sprayed (AS) TBCs and laser-remolten (LR) TBCs at 1100 °C to investigate thermally growth oxide (TGO)growth mechanisms and isothermal oxidation behaviors. The results showed that the laser-remolten top-ceramic-coating dense layer with a columnar crystal structure of LR TBCs presented a 96.3% and 59.1% lower surface roughness and porosity, respectively, than those of the top ceramic coating of AS TBCs, and the TGO growth rate of LR TBCs decreased by 46.2% compared to that of AS TBCs. The mixed-oxides appearance time of LR TBCs (50 h) was later than that of AS TBCs (25 h). After 100 h of isothermal oxidation, the total TGO thickness of LR TBCs was only 77.2% of that of AS TBCs, and the effects of the laser-remolten TBCs on gas oxygen diffusion inhibition and high-temperature oxidation resistance were promising in LZO/YSZ TBCs.

LaYZrO/YSZ Double Ceramic Layer Thermal Barrier Coatings by EB-PVD: Thermal Performance, Morphology and Failure Behavior

LaYZrO/YSZ Double Ceramic Layer Thermal Barrier Coatings by EB-PVD: Thermal Performance, Morphology and Failure Behavior

Proposal of a Thermal Fatigue Damage Evaluation Method for Ceramic Thermal Barrier Coating Systems.

In recent years, the application of thermal barrier coatings to the components used in gas turbines for thermal power generation has become indispensable as higher inlet temperatures for requirement to further improve the efficiency of thermal power generation. Thermal barrier coatings, generally referred to as TBCs, consist of a top coat, which is a thermal barrier layer, and a bond coat, which improves the adhesion strength between the TC and the base material. As the development of new thermal barrier materials is progressed, it is also demanded to clarify the stress and damage behavior of TBCs under the complex thermal fatigue loading conditions in actual gas turbine operation. In this study, comprehensive finite element analysis is performed on a TBC system under cyclic thermal fatigue loading conditions with temperature and stress variations to clarify the deformation behavior and crack initiation life of the TBC, and to develop a fatigue life evaluation method for TBCs.

CMAS Issues in Jet Engines: The Less, the Better

Jet engines are susceptible to contamination by environmental debris (volcanic ash, sand, and dust, called CMAS). When CMAS ash is ingested into gas turbines, it melts and can attach to hot components of the jet engines that may clog engine parts and damage protective ceramic thermal barrier coatings (TBCs). The engine’s performance may be impeded and can even cause catastrophic failure when this contamination is in excess. This research presents the current understanding of these CMAS challenges in nature and deposit-induced failure mechanisms of TBCs. The strategy mitigation of new functional TBCs to stop the wetting of CMAS explained the details of experimental and theoretical analysis of the melting and impacting processes of CMAS ash in jet engines.

Review of ceramic materials that used as a thermal barrier in diesel engine pistons

The diesel engines is used in many industrial applications in addition to that in cars. In this type of engine the fuel-air mixture will burn behind the piston inside the cylinder and will produce high temperature. Developing diesel engines to be more efficient, less fuel consumption, low gas emissions, and high performance that has been pursued for more than two decades. Increased combustion temperature can enhance engine power and efficiency while lowering specific fuel consumption and CO emission rates. Ceramic thermal barrier coatings have been identified as the most promising method for achieving these goals. Due to the abundance of materials and technologies, it is necessary to understand the best material suitable for depositing and the techniques required to complete the process. Many researchers have studied a wide range of materials like ZrO2, Al2O3, mullite, YSZ, and forsterite to develop suitable systems that use thermal barrier coatings. Additionally, many techniques such as EB-PVD and ASP have been developed to meet the requirements of these systems. This paper will examine the materials and technologies that can be used to improve internal diesel engine performance.

Effects of Ta2O5 content on mechanical properties and high‐temperature performance of Zr6Ta2O17 thermal barrier coatings

AbstractZr6Ta2O17/YSZ double ceramic top coat thermal barrier coatings (TBCs) with different Ta2O5 content are prepared by atmospheric plasma spraying (APS). The effects of Ta2O5 content on mechanical properties and high‐temperature performance of Zr6Ta2O17 TBCs are investigated. With the decrease in Ta2O5 content, the hardness and fracture toughness of the Zr6Ta2O17 coating increases from 12.833 to 15.117 GPa and from 3.3592 to 3.7753 MPa m1/2, respectively. The Zr6Ta2O17 coating specimens with different Ta2O5 content do not delaminate from the substrate except the edge peeling after 180 h of oxidation or 2000 thermal cycles at 1150°C. The Zr‐12Ta specimen (the molar ratio of ZrO2/Ta2O5 of 0.88:0.12) presents the more excellent mechanical properties and high‐temperature performance. The reduction in Ta2O5 content will improve the mechanical properties, high‐temperature oxidation, and thermal cycle performance of Zr6Ta2O17/YSZ double ceramic top coat TBCs.