Thermal Spray Research Articles

Inside Ceramics and Between MgO Grains: Solid-State Synthesis of Intergranular Semiconducting or Magnetic Spinels.

Configurations of composite metal oxide nanoparticles are typically far off their thermodynamic equilibrium state. As such they represent a versatile but so far overlooked source material for the intergranular solid-state chemistry inside ceramics. Here, it is demonstrated how the admixture of Fe3+ and In3+ ions to MgO nanoparticles, as achieved by flame spray pyrolysis, can be used to engage ion exsolution, phase separation, and subsequent spinel formation inside the network of diamagnetic and insulating MgO grains. Extremely high uniformity in the distribution of intergranular ferrimagnetic MgFe2O4 films and grains with resulting magnetic coercivity values that depend on the nanoparticles' initial Fe3+ concentration is achieved. Moreover, percolating networks of semiconducting MgIn2O4 are derived from MgO nanoparticles with admixtures of 20 at% In3+ that gives rise to an enhancement of dc conductivity values by more than five orders of magnitude in comparison to the insulating MgO host. The here presented approach is general and applicable to the synthesis of a variety of functional spinel nanostructures embedded inside ceramic matrices. Nanoparticle loading with aliovalent impurity ions, the level of nanoparticle powder density after compaction, and sintering temperature are key parameters for this novel type of solid-state chemistry in between the host grains.

Tribomechanical characterization of ceramic reinforced composite coatings for modification of cylinder liner of heavy-duty diesel engines

Heavy-duty diesel engines are essential mechanic power generation tools for industry and transporting freight and passengers. Therefore, increasing engine efficiency and minimizing maintenance and repair costs is crucial. This study examines and characterizes three thermal spray coatings (Cr3C2/NiCr, NiCr, and Al2O3/TiO2) utilized on cylindrical gray cast iron substrate specimens. The coatings were tested for their microstructural (optical microstructure, SEM, EDS, XRD, and AFM topography), mechanical (nanoindentation and nano scratch), and tribological (short and long-distance wear test) properties. Microstructural tests showed that an average coating thickness of 120 to 170 µm was successfully achieved on the cast iron base substrate using different thermal coating methods. The Cr3C2/NiCr coating, observed through the AFM mapping method, achieved the most uniform roughness distribution on the surface. The elasticity of the coatings was measured with a nanoindentation test, and Al2O3/TiO2 coating exhibited high rigidness (177 GPa). The nano scratch test showed that the highest load-carrying capacity was achieved with the Cr3C2/NiCr coated sample. NiCr-coated sample exhibited approximately similar rigidness (61 GPa) to cast iron (67 GPa). Short- and long-distance wear tests showed that the coatings provided significantly better wear resistance than the cast iron base. The coatings’ efficacy was evident. At the long-distance tests, while the cast iron base sample had a wear rate of 19.874 (10−7mm3/Nm), the wear rates of Al2O3/TiO2, Cr3C2/NiCr, and NiCr coatings were 3.835, 4.020, and 6.466 (10−7mm3/Nm), respectively. The Al2O3/TiO2 and Cr3C2/NiCr coatings had superior tribological performance than the NiCr coating, yet the NiCr coating showed much higher wear performance than the uncoated base. The Cr3C2/NiCr coating exhibited superior tribomechanical performance in all the coatings.

Terahertz Non-Destructive Testing of Porosity in Multi-Layer Thermal Barrier Coatings Based on Small-Sample Data

Accurately characterizing the internal porosity rate of thermal barrier coatings (TBCs) was essential for prolonging their service life. This work concentrated on atmospheric plasma spray (APS)-prepared TBCs and proposed the utilization of terahertz non-destructive detection technology to evaluate their internal porosity rate. The internal porosity rates were ascertained through a metallographic analysis and scanning electron microscopy (SEM), followed by the reconstruction of the TBC model using a four-parameter method. Terahertz time-domain simulation data corresponding to various porosity rates were generated employing the time-domain finite difference method. In simulating actual test signals, white noise with a signal-to-noise ratio of 10 dB was introduced, and various wavelet transforms were utilized for denoising purposes. The effectiveness of different signal processing techniques in mitigating noise was compared to extract key features associated with porosity. To address dimensionality challenges and further enhance model performance, kernel principal component analysis (kPCA) was employed for data processing. To tackle issues related to limited sample sizes, this work proposed to use the Siamese neural network (SNN) and generative adversarial network (GAN) algorithms to solve this challenge in order to improve the generalization ability and detection accuracy of the model. The efficacy of the constructed model was assessed using multiple evaluation metrics; the results indicate that the novel hybrid WT-kPCA-GAN model achieves a prediction accuracy exceeding 0.9 while demonstrating lower error rates and superior predictive performance overall. Ultimately, this work presented an innovative, convenient, non-destructive online approach that was safe and highly precise for measuring the porosity rate of TBCs, particularly in scenarios involving small sample sizes facilitating assessments regarding their service life.

Development of laser felling modes of gas-thermal coating

The use of copper and its alloys to create parts for metallurgical equipment is associated with an increase in abrasive wear and high-temperature corrosion. In this regard, there is a need to apply a protective coating. In particular, to prevent wear and premature chipping of the metal of copper tuyeres, the surface is hardened with a coating of zirconium dioxide stabilized with yttria oxide by thermal spraying in an air atmosphere. Due to the difference in the coefficient of thermal expansion of copper (at T = 300 K: 16.7 µm/m оС and at T = 750 K: 19.7 µm/m оС) and its low resistance to gas corrosion, the application of zirconium oxide (produced by a preapplied intermediate layer that plays a role in matching the coefficient of thermal expansion (CTE) between the copper base and the ceramic coating. In addition, the intermediate layer protects copper from gas corrosion. In this case, The use of copper and its alloys to create parts for metallurgical equipment is associated with an increase in abrasive wear and high-temperature corrosion. In this regard, there is a need to apply a protective coating. In particular, to prevent wear and premature chipping of the metal of copper tuyeres, the surface is hardened with a coating of zirconium dioxide stabilized with yttria oxide by thermal spraying in an air atmosphere. Due to the difference in the coefficient of thermal expansion of copper (at T = 300 K: 16.7 pm/m °G and at T = 750 K: 19.7 pm/m оG) and its low resistance to gas corrosion, the application of zirconium oxide (produced by a pre-applied intermediate layer that plays a role in matching the coefficient of thermal expansion (CTE) between the copper base and the ceramic coating. In addition, the intermediate layer protects copper from gas corrosion. In this case, nickel-based alloys were used as intermediate layers. The use of nickel as the basis of intermediate layers is due to the fact that copper and nickel form a continuous series of solid solutions, such as cupronickel or monel metal-like structures. This, in turn, assumes a smooth transition of thermophysical properties from copper to nickel alloy. To ensure increased adhesion of the transition layer to copper by increasing the area of mutual contact between copper and the sublayer (dagger penetration) and significantly increasing the homogeneity of the material of the intermediate layer made of a nickel alloy, laser melting of the intermediate sublayer (Ni–B–Si system) was used on a laser complex based on laser LS-5 with a power of 5 kW with a KUKA KR-60HA robot in an argon atmosphere. To test the modes, experiments were carried out on copper samples of a flat shape and a body of rotation. The optimal parameters for the process of melting flat samples were: processing speed 33 mm/s, power from 400 to 3900 W, focal length from 200 to 230 mm, pitch between tracks: 0.25, 0.5 and 1 mm. The optimal parameters for the process of melting rotating samples were: laser radiation power 400–450 W, processing step 0.125; 0.5, focal length from 200 to 210 mm.

Solidification process of hollow metal droplets impacting a substrate

Recently, there has been a renewed interest in hollow droplet impact, as the internal bubble makes a difference in the spreading morphology during thermal spraying and three-dimensional printing. This paper is dedicated to the solidification process of a hollow metal droplet impacting a cold substrate. The influence of initial impact velocity and substrate temperature on the spreading process is numerically investigated by employing a phase-field method coupled with a solidification model. Solidification was shown to strongly alter droplet spreading and the flow state of the center liquid column. To describe the flow state of the center liquid column, three patterns, namely ‘no-jet’, ‘transition region’, and ‘counter-jet’ are defined. The simulation results show that the formation of the center counter-jet is inhibited by the solidification layer when Péclet number Pe<357, where Pe is the ratio of convection rate to diffusion rate. In addition, the critical Pe at which the phenomenon of counter-jet occurs gradually rises with the substrate temperature falling. Finally, a quantitative analysis of the maximum spreading diameter is proposed by establishing an energy conservation equation. The predicted diameters are in good agreement with the simulated results. It allows us to provide a general expression for the hollow droplets' maximum spreading diameter, using the impact and temperature parameters.

Impact of Flame Conditions on the Pd‐O Structure and Methane Oxidation Activity over Ceria Support

AbstractIn this work, we employed flame spray pyrolysis (FSP), a high‐temperature synthesis method, to control the formation of Pd structures on the CeO2 support. Multiple types of Pd structures deposited on CeO2 are observed on FSP‐made samples. Our results show that the oxidizing environment during FSP synthesis facilitates the formation of incorporated Pd2+ structures, along with highly dispersed Pd2+, Pd0 nanoparticles, and Pd° clusters formed under the reducing synthesis condition. Notably, these Pd2+ species remained stable at temperatures up to 400 °C. The catalysts containing both highly dispersed Pd2+ nanoparticles and incorporated Pd2+ species demonstrated superior methane oxidation activity, with higher turnover frequencies than those containing only one type of Pd2+ structure. However, hydrothermal pretreatment in the presence of water vapor led to partial deactivation, likely due to structural alterations in the Pd species or the interaction with the CeO2 support, which reduced the stability and effectiveness of the active sites. This study underscores the importance of both highly dispersed and incorporated Pd2+ species in enhancing catalytic performance and highlights the challenges posed by water‐induced deactivation in practical applications.

Corrosion behaviors and failure mechanism of plasma sprayed Yb2Si2O7/Si environmental barrier coatings exposed to CMAS+ NaVO3

Aero-engine service environments are complex and variable, environmental barrier coatings are usually exposed to the coupled corrosion of CMAS and molten salt. Clarifying the relevant corrosion mechanisms is of guiding significance for the protection of environmental barrier coatings. In this study, the CMAS+NaVO3 (CN) corrosion behavior of a typical Yb2Si2O7/Si coating prepared by atmospheric plasma spray (APS) technique was explored. Results showed that CN was completely melted and uniformly spread at 1200℃, a temperature lower than the melting point of CMAS. Interaction between the Yb2Si2O7/Si coating and CN produced a discontinuous non-stoichiometric garnet phase at 1200℃. At 1300 °C, the reaction produced an apatite reaction layer, but when the duration was extended to 50 h, the apatite layer was gradually dissolved. Moreover, the CN melt infiltrated into the entire Yb2Si2O7 coating, causing buckling of the Yb2Si2O7 layer and debonding with the Si layer, ultimately leading to failure of the Yb2Si2O7/Si coating.

Adhesion-Related Phenomena of Stellite 6 HVOF Sprayed Coating Deposited on Laser-Textured Substrates.

The focus of this research is to examine the feasibility of using laser texturing as a method for surface preparation prior to thermal spraying. The experimental part includes the thermal spraying of a Stellite 6 coating by High Velocity Oxygen Fuel (HVOF) technology on laser-textured substrates. The thermal spraying of this coating was deposited both on conventional substrate material (low carbon steel) and on substrates that had been previously heat treated (nitrided steel). The properties of the coatings were analysed using scanning electron microscopy (SEM), optical microscopy (OM) and Raman spectroscopy. Adhesion was assessed through a tensile adhesion test. The results showed the usability of laser texturing in the case of carbon steel, which was comparable or even better than traditional grit blasting. For nitrided steel, the problem remains with the hardness and brittleness of the nitrided layer, which allows for the propagation of brittle cracks near the interface and thus reduces the adhesion strength.

Treatment of turbine blades in electric power stations by adding nano oxides to the matrix material (Al80-Ni20)

Abstract A base material of Al80-Ni20 was used and mixed with variable proportions of Nano chromium oxide (Cr2O3) at percentages of 2, 4, 6, 8 and 10%, as well as with Nano magnesium oxide (MgO) at percentages of 2, 4, 6, 8 and 10%. These mixtures were then applied using the thermal spraying method with a flame. This particular method is commonly used for repairing cracks and protecting turbine blades in electric power stations from external corrosion. However, it is important to note that this method may face challenges when exposed to high-temperature water vapour, salts and other working conditions experienced by turbine blades. Samples were prepared by thermally sintering the coating at 1000 °C for two hours. Various measurements were performed to assess the structural properties using a scanning electron microscope (SEM), as well as other physical tests such as porosity, hardness, adhesion strength and frictional wear. The SEM analysis revealed that the presence of 10% Cr2O3 resulted in a surface that was uniformly free from external defects, whereas the addition of MgO led to a less homogeneous surface. The physical data obtained indicated a preference for Cr2O3, as evidenced by the porosity results (4.5%) observed after thermal sintering at 10%, as well as the hardness (193 HV), adhesion strength (40 MPa) and wear (2.90 × 10−5 g cm−1) measurements. Moreover, the analyses of the properties of MgO under the same conditions included porosity (10%), hardness (155 HV), adhesion strength (35 MPa) and wear (5.50 × 10−5 g cm−1).

Thermal cycling and steam corrosion behavior of Yb3Al5O12‐based multilayered environmental barrier coatings for SiCf/SiC

AbstractTo alleviate thermal mismatch problem of environmental barrier coatings (EBCs) on SiC fiber‐reinforced SiC ceramic matrix composites (SiCf/SiC CMCs) surface and improve their high‐temperature durability for aircraft engines, by fully utilizing the appropriate thermal expansion coefficient, low oxygen transmittance, low thermal conductivity, and other advantages of ytterbium aluminum garnet (Yb3Al5O12), the double ceramic layered Yb3Al5O12/Yb2Si2O7 (DCL‐Yb3Al5O12) and the triple ceramic layered Yb3Al5O12/Yb2SiO5/Yb2Si2O7 (TCL‐Yb3Al5O12) EBC systems were prepared on SiCf/SiC CMCs by atmospheric plasma spraying (APS). Their phase composition and microstructures were investigated comparatively. The thermal cycling and water vapor/oxygen corrosion behavior of these coating systems were compared at 1300°C. The results showed that the thermal cycling life of the DCL‐Yb3Al5O12 and TCL‐Yb3Al5O12 EBC systems were 295 and 320 times, respectively. The failure of both the coating systems occurred between the Yb2Si2O7 layer and Si bond coat. The strength retention rate of DCL‐Yb3Al5O12 and TCL‐Yb3Al5O12 EBC systems after water vapor/oxygen corrosion for 70 h was 12.8% and 23.1%, respectively, and the fracture modes of both the systems exhibited “pseudoplastic” characteristic. TCL‐Yb3Al5O12 EBC systems with a gradient structure of more layers exhibit more excellent high‐temperature durability than DCL‐Yb3Al5O12 EBCs.

Corrosion resistance analysis of Al2O3-TiO2 composite ceramic coatings on carbon steel pipe surfaces

This study investigates the corrosion resistance of Al2O3-TiO2 composite coatings reinforced with multi-walled carbon nanotubes (MWCNTs) on carbon steel pipe surfaces. Coatings were deposited using atmospheric plasma spraying (APS) and high velocity oxy-fuel (HVOF) techniques. Microstructural analysis revealed that HVOF-sprayed coatings exhibited denser and more homogeneous structures compared to APS-sprayed coatings. Electrochemical tests in 3.5 wt% NaCl solution showed that the HVOF-sprayed Al2O3-13 wt% TiO2-1.5 wt% MWCNT coating demonstrated the lowest corrosion current density of 7.6 × 10−9 A/cm2 and the most positive corrosion potential. Hot corrosion tests in a simulated boiler environment (500°C, 1000 h) revealed that this coating also exhibited minimal weight gain (0.9 mg/cm2) and thickness loss (2 μm). The corrosion rate of the optimal coating was 0.05 mm/year, significantly lower than the bare carbon steel (2.45 mm/year). XRD analysis of corroded samples showed that HVOF-sprayed coatings maintained their original phase composition without detectable substrate corrosion products. The superior corrosion resistance of the HVOF-sprayed Al2O3-13 wt% TiO2-1.5 wt% MWCNT coating is attributed to its optimized composition, dense microstructure, and the reinforcing effect of MWCNTs. These findings provide valuable insights for developing advanced coating solutions to mitigate corrosion-related challenges in the oil and gas industry and other harsh industrial environments.

Effect of Process Parameters on Yttria‐Stabilized Zirconia Particles In‐Flight Behavior and Melting State in Atmospheric Plasma Spraying

Effect of Process Parameters on Yttria‐Stabilized Zirconia Particles In‐Flight Behavior and Melting State in Atmospheric Plasma Spraying

Methods of active friction control in the presence of lubricant compositions with mesogenic additives

Purpose of research. The aim of the study is to systematize the latest literature data related to the modulation of the friction coefficient by external fields when using lubricant compositions with liquid crystal mesogens and polymer composites.Methods. The article considers the methods of tribological tests using various schemes of friction pairs (cylinder-disk, pin-on-disk). Some commonly used methods of applying coatings to the elements of friction pairs are considered: ion beam-assisted deposition, chemical and physical thermal spraying, molecular layer deposition, photopolymerization, etc. Of the discussed methods of characterizing tribosystems, the following are indicated: dielectric spectroscopy, Raman scattering, polarization optical microscopy, nuclear-physical methods (positron annihilation spectroscopy and Xray diffraction).Results. The article systematizes modern trends in the development of modulated modes of operation of tribological circuits with lubricant compositions containing liquid crystals and polymer composites. The following approaches are used to implement active control of the friction coefficient: 1) modulation of friction by an electric field, 2) modulation of friction by a temperature field, and 3) modulation of friction on optical gratings under light irradiation. These approaches take into account the changed characteristics of the lubricant associated with the use of mesogenic additives, modes of exposure to electromagnetic and thermal fields, electrical characteristics, geometry of the surface of friction pairs, and provide the values of tribological characteristics (friction coefficient and wear) that are achieved as a result of friction modulation.Conclusion. A positive effect of mesogenic additives of liquid crystals and polymers on tribological characteristics with active control of the friction coefficient has been established: a decrease in the friction coefficient is observed, which helps to reduce material wear.

Environmental barrier coatings on alumina-reinforced CMCs: Experimental validation and numerical modeling of thermal stability

Recent advancements in ceramic matrix composites (CMCs), including SiC/SiC, C/SiC, C/C, Al2O3 reinforced Al2O3, face various challenges at high temperature applications, including grain growth, sintering, and creep deformation which are leading to strength loss. The new generation of Environmental Barrier Coatings (EBCs) must allow the protection of these CMCs when operating under harsh conditions, which can reach up to 1100 °C in some applications, such as gas turbines and certain reforming processes. In this work, 8 wt% yttria stabilized zirconia (8YSZ) applied by Atmospheric Plasma Spraying (APS) on top of a porous alumina oxide matrix with reinforced alumina fibers Al2O3/Al2O3-CMC (OCMC), considering different interfaces, has been studied. Surface morphology and pre-treatment of CMCs is the most challenging part for thermal spray applications. Hence, two different surface structure were used in this work. The results show that the roughness of OCMC can be reduced from Rz = 41.8±6.8 μm to 20.3±1.1 μm with the new porous bond coat and plasma sprayed coating, resulting in a homogeneous surface morphology.. In terms of thermal stability, a numerical model that combines Object Oriented FEM (OOF) and Cohesive Zone Model CZM tools, has been developed to evaluate the effect of the porosity of the real microstructure and the characteristics of the interfacial roughness profile, respectively, in CMC/EBC structures. In addition, other materials, such as La2Zr2O7 and Y2O3, have been numerically studied in order to evaluate their suitability as EBC.

Microstructures, phase and mechanical characterisation of Al2O3-ZrO2-TiO2 coating produced by atmospheric plasma spraying

The microstructure, crystallographic phases, and mechanical properties of a newly developed Al2O3 – TiO2 – ZrO2 ternary ceramic coating were characterized. The coatings were produced by atmospheric plasma spraying as a preblended powder on Ti-6Al-4 V substrates using the new generation of the Debye-Larmor cascaded plasma torch. The 400 μm thick as-sprayed ternary ceramic coating is compact and neither delamination nor inter-/trans-granular cracks were found. The coating consists of single phase α-Al2O3, monoclinic m-ZrO2, and a nanocrystalline dual phase structure of α-Al2O3 and m-ZrO2. Ti is either present as ZrTiO4 or as solute in the dual phase. Cracking from the tip of the indent is rare and delamination was not observed after the progressive scratch test. The coating has potential in high wear applications for example in medical devices.

A comparison of cold spray, atmospheric plasma spray and high velocity oxy fuel processes for WC-Co coatings deposition through LCA and LCCA

In this study, an environmental and economic assessment of WC-Co coatings deposited by Cold Gas Spray (CGS), Atmospheric Plasma Spray (APS) and High Velocity Oxy Fuel (HVOF) spray technologies is carried out. Using SimaPro LCA software, several environmental impact categories are analyzed to compare their environmental performance. The economic analysis includes capital and operating expenditures. The results have highlighted that all three processes exhibit low environmental impact in terms of CO2 emissions but the performance of the CGS process is heavily influenced by the low deformability of WC-Co, while the APS process is affected by high electricity consumption. In terms of economic analysis, the HVOF process exhibits the best performance, while the CGS process requires most time to deposit the coating, and consequently, it is the process where the workforce component is most significant. These results depend on the fact that CGS might not be the most suitable deposition technique for fabricating WC-Co coatings.

Comprehensive study of the effect of Hf and Ta co-doped MCrAlY bond coat on the high-temperature properties of thermal barrier coating

The MCrAlY bond coat is modified by reactive element (RE) Hf and refractory element Ta, and the high-temperature oxidation resistance of the thermal barrier coating (TBC) before and after the modification are investigated by comparative experiments. The experiments use high-velocity oxygen-fuel (HVOF) to prepare NiCoCrAlY and NiCoCrAlYHfTa bond coats on the surface of GH4099 high-temperature alloy, and then yttrium stabilized zirconia (YSZ) ceramic layers are prepared on the surface of the bond coat by atmospheric plasma spraying (APS), and cyclic oxidation is carried out in air at 1050 °C until failure. It is shown that the oxidation lifetime of the modified TBC reached 1992 h, which is one time higher than that of the unmodified TBC. The doping of Hf and Ta elements reduces the growth rate of the oxide layer, and these diffuse externally and combine with the internally diffused O to form HfO2 and Ta2O5 in the thermally grown oxide (TGO) layer, thereby improving the adhesion and mechanical properties of the oxide layer. In addition, the bond strength and thermal shock resistance of the TBCs are significantly improved after doping with Hf and Ta elements, which prolongs the lifetimes of the TBCs. In this paper, the morphology, structure, and properties of the modified TBCs are characterized to investigate the mechanism of the oxidation lifetime extension.

Impact of atmospheric plasma spraying parameters on microstructure, mechanical properties and thermal cycling performance of YSZ coatings

Yttria-stabilized zirconia (YSZ) coatings are widely utilized thermal protective layers for metal and superalloy surfaces. These coatings are employed as thermal barrier coating (TBCs) to insulate metallic parts from higher thermal energy, thereby minimizing the cooling need for the topcoat ceramic layer. The choice of material is 7 wt% YSZ due to its exceptional capability to withstand challenging conditions characterized by extremely high temperatures and pressures, typically employed by the atmospheric plasma spraying (APS) in gas turbines, effectively extending their lifespan and improving performance. The deposition process of TBCs is significantly influenced by various spraying parameters, including gas flow rate and current (amp). These key parameters mutually play a significant role in controlling thermal stability, phase composition, and improved mechanical and microstructure properties. The aim of this research is to evaluate and contrast the impact of various spraying parameters on YSZ TBC performance. Accordingly, the influence of Hydrogen (H2), Argon (Ar) flow rate, and Current (amp) was investigated. Microstructure and elemental mapping studying was conducted using Field Emission Scanning Electron Microscopy FESEM/EDS and phase studying was examined with X-ray diffraction (XRD). Porosity was measured by IMAGE J software while hardness measurement was calculated by Vickers hardness tester. Additionally, thermal cycling tests were conducted between 700 °C and 1200 °C. The porous coatings exhibited poor performance, delaminating early during the tests. In contrast, dense coatings performed significantly better, enduring up to 140 cycles before failure.

Enhancing anti corrosion properties of plasma-sprayed ZrO2-25 wt%CeO2-2.5 wt%Y2O3 thermal barrier coatings via nanostructure engineering

The current research aimed to enhance the corrosion resistance of (i) the CaO-MgO-Al2O3-SiO2 (CMAS) and (ii) the hot corrosion of the Na2SO4-55 wt%V2O5 environments. Conventional thermal barrier coatings (TBCs) with a composition of ZrO2-25 wt%CeO2-2.5 wt%Y2O3 (micro-CYSZ) were replaced by nanostructured coatings (nano-CYSZ) with a similar composition. This replacement was achieved by applying the desired coatings using the atmospheric plasma spraying (APS) process on IN738LC/CoNiCrAlY. Corrosion and hot corrosion tests were conducted in the CMAS and Na2SO4-55 wt%V2O5 environments at 1100 °C and 950 °C with 4-h cycles. In terms of resistance to CMAS corrosion and hot corrosion in the Na2SO4-55 wt%V2O5 environment, the nano-CYSZ coating exhibited better performance compared to the micro-CYSZ with a similar composition. The use of nanostructuring, including the formation of dual structures consisting of unmelted nano-sized particles and melted particles in the top layer of the TBC, resulted in delaying the penetration of molten salts and consequently improving the resistance to the CMAS and Na2SO4-55 wt%V2O5 environments.

Microstructure evolution and oxidation resistance of plasma sprayed AlSi-doped AlCoCrFeNi coatings with post-annealing

High entropy alloy (HEA) coatings of equimolar AlCoCrFeNi typically exhibit a lower oxidation rate at high temperatures by forming a protective passivation film. However, the metal elements consumption during long-term oxidation limitted the application. In this work, AlCoCrFeNi HEA coatings doped by AlSi as a supplement to passivation elements were prepared by atmospheric plasma spraying (APS), and AlSi capsules were diffused uniformly into the coating through annealing treatment to offset the element consumption during high-temperature oxidation. Results showed that annealing promoted Al and Si atoms diffusing into the solid solution, which stabilized BCC and inhibited FCC formation. During the oxidation at 900 °C, a protective Al2O3 film was formed on the coating surface, and AlSi capsules continuously transported Al ions to the consumption zone and reduced oxidation rate to 0.0015 g/cm2. The HEA coating doped by passivation element capsules provided a new approach for the design of novel antioxidant coatings.