Graceful behavior during CMAS corrosion of a high-entropy rare-earth zirconate for thermal barrier coating material

The corrosion resistance to calcium-magnesium-alumino-silicates (CMAS) is critically important for the thermal barrier coatings (TBCs). High-entropy zirconate (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ (HEZ) ceramics with low thermal conductivity, high coefficient of thermal expansion and good durability to thermal shock is expected to be a good candidate for the next-generation TBCs. In this work, the CMAS corrosion of HEZ at 1300°C was firstly investigated and compared with the well-studied La₂Zr₂O₇ (LZ). It is found that the HEZ ceramics showed a graceful behavior to CMAS corrosion, obviously much better than the LZ ceramics. The HEZ suffered from CMAS corrosion only through dissolution and re-precipitation, while additional grain boundary corrosion existed in the LZ system. The precipitated high-entropy apatite showed fine-grained structure, resulting in a reaction layer without cracks. This study reveals that HEZ is a promising candidate for TBCs with extreme resistance to CMAS corrosion.     

Corrosion resistant polymer derived ceramic composite environmental barrier coatings

Corrosion resistant coatings are a promising solution to protect structural metals in harsh environments. Ceramic composite coatings made from polymer-derived ceramics are highly attractive due to the ease of their processing and the ability to work in various environments. This paper is focused on the performance of a TiSi₂-filled SiOC ceramic composite coating system on 316 stainless steel (SS) substrates as a corrosion resistant coating. The best-performing quadruple-dip coatings were shown to be able to reduce the weight loss due to hot sulfuric acid (95+%, 104–107 °C) corrosion by 85% over a 30-day period. Coatings from the same system were also examined under 800 °C static (100 h) and cyclic (10 cycles) oxidation. Our results indicate that the coatings perform well under both conditions of prolonged high temperature oxidation and thermal cycling, suggesting the strong potential of this system as an environmental barrier coating (EBC).     

Investigation on behavior and mechanism of enhanced water vapor corrosion resistance for (Lu0.25Yb0.25Er0.25Y0.25)2SiO5 environmental barrier coating

Rare-earth monosilicate (RE₂SiO₅) have been considered to be a promising material for the environmental barrier coating because of their superior thermal and mechanical properties. However, the water vapor corrosion resistance of single-component RE-silicate materials, such as Y₂SiO₅, should be further improved. The high-entropy design is one of the most suitable methods to enhance the corrosion resistance for single-component RE-silicate materials. In this work, the multicomponent RE-silicate ((Lu_0.25Yb_0.25Er_0.25Y_0.25)₂SiO₅, (4HES)) and single-component RE-silicate (Y₂SiO₅) coatings were investigated with regard to its water vapor corrosion behaviors at 1350 °C for 300 h. A thinner and denser corrosion layer was generated in the 4HES coating, indicating that the 4HES coating possessed better corrosion resistance than the Y₂SiO₅ coating. The improved corrosion resistance is attributed to the better hydrophobic property as well as the more stable crystal structure of the rare-earth oxide and 4HES phase which was resulted from the high-entropy design.     

Novel polymer-derived ceramic environmental barrier coating system for carbon steel in oxidizing environments

Polymer-derived ceramic environmental barrier coatings (EBCs) in combination with active fillers are highly attractive due to their facile processing and applicability at elevated temperatures. In this study, several kinds of active and passive fillers were added to polymer-derived ceramics and then coated onto carbon steel, using cheap and simple lacquer methods (such as dip or spray coating). The resultant coating, investigated by Thermal Gravimetric Analysis (TGA), Scanning Electron Microscopy with X-ray microanalysis (SEM/EDS), X-ray Diffraction (XRD), adhesion tests and oxidation tests, showed that it acted as an excellent film to withstand thermal cycling, and prevented carbon steel from being oxidized at elevated temperatures. The low-cost and effective coating method described in this paper can be used widely to protect carbon steel used at high temperatures (e.g. steel boiler tubes in waste-to-energy plants).     

Feasibility research of Yb3Al5O12 garnet as environmental barrier coating materials

In this work, Yb₃Al₅O₁₂ (YbAG) garnet, as a new material for environment barrier coating (EBC) application, was synthesized and prepared by atmospheric plasma spraying (APS). The phases and microstructures of the coatings were characterized by XRD, EDS and SEM, respectively. The thermal stability was measured by TG-DSC. The mechanical and thermal-physical properties, including Vickers hardness (H_v), fracture toughness (K_IC), Young's modulus (E), thermal conductivity (κ) and coefficient of thermal expansion (CTE) were also measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb₃Al₅O₁₂ and amorphous phase which crystallized at around 917 °C. Moreover, it has a hardness of 6.81 ± 0.23 GPa, fracture toughness of 1.61 ± 0.18 MPa m^1/2, as well as low thermal conductivity (0.82–1.37 W/m·K from RT-1000 °C) and an average coefficient of thermal expansion (CTE) (∼6.3 × 10⁻⁶ K⁻¹ from RT to 660 °C). In addition, the thermal shock and water-vapor corrosion behaviors of the Yb₃Al₅O₁₂-EBC systems on the SiC_f/SiC substrates were investigated and their failure mechanisms were analyzed in details. The Yb₃Al₅O₁₂ coating has an average thermal shock lifetime of 72 ± 10 cycles as well as an excellent resistance to steam. These combined properties indicated that the Yb₃Al₅O₁₂ coating might be a potential EBC material. Both the thermal shock failure and the steam recession of the Yb₃Al₅O₁₂-EBC systems are primarily associated with the CTE mismatch stress.     

High-entropy rare-earth disilicate (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7: A potential environmental barrier coating material

A new high-entropy ceramic (Lu_0.2Yb_0.2Er_0.2Tm_0.2Sc_0.2)₂Si₂O₇ ((5RE_0.2)₂Si₂O₇) was proposed as a potential environmental barrier coating (EBC) material for ceramics matrix composites in this work. Experimental results showed that the (5RE_0.2)₂Si₂O₇ synthesized by solid-phase sintering was a monoclinic solid solution and had good phase stability proved by no obvious absorption/exothermic peak in the DSC curve from room temperature to 1400 °C. It performed a lower coefficient of thermal expansion (2.08 ×10^?6-4.03 ×10^?6 °C^?1) and thermal conductivity (1.76–2.99 W?m^?1?°C^?1) compared with the five single principal RE₂Si₂O₇. In water vapor corrosion tests, (5RE_0.2)₂Si₂O₇ also exhibited better water vapor corrosion resistance attributed to the multiple doping effects. The weight loss was only 3.1831 × 10^?5 g?cm^?2 after 200 h corrosion at 1500 °C, which was lower than that of each single principal RE₂Si₂O₇. Therefore, (5RE_0.2)₂Si₂O₇ could be regarded as a remarkable candidate for EBCs.     

High-temperature mechanical properties and oxidation resistance of SiCf/SiC ceramic matrix composites with multi-layer environmental barrier coatings for turbine applications

Continuous silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) ceramic matrix composites are considered promising materials as high-temperature components of advanced aero-engines. However, due to their susceptibility to oxidation and corrosion at high temperature, environmental barrier coatings (EBCs) must be applied on the surface of SiC_f/SiC. In this study, Si/Y₂SiO₅/LaMgAl₁₁O₁₉ (LMA) multi-layer EBCs were fabricated to protect SiC_f/SiC by using atmospheric plasma spraying (APS). The high-temperature tensile fatigue performance of SiC_f/SiC with and without EBCs was evaluated. The results indicated that EBCs significantly improved the tensile fatigue properties of SiC_f/SiC at high temperature in air atmosphere. Meanwhile the bending strength of specimens after isothermal aging or not was also tested. The multi-layer EBCs in this study may be a promising EBCs system for SiC_f/SiC after some improvements.     

Calcium-Magnesium-Aluminosilicate (CMAS) corrosion resistance of high entropy rare-earth phosphate (Lu0.2Yb0.2Er0.2Y0.2Gd0.2)PO4: A novel environmental barrier coating candidate

Single phase (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ was synthesized, and its thermal properties and CMAS resistance were investigated to explore its potential as an environmental barrier coating (EBC) candidate. The high entropy phosphate (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ displays a lower thermal conductivity (2.86 W m⁻¹ K⁻¹ at 1250 K) than all the single component xenotime phase rare-earth phosphates. Interaction of (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄ pellets with CMAS at 1300 °C led to the formation of a dense and uniformed Ca₈MgRE(PO₄)₇ reaction layer, which halted the CMAS penetration into the bulk pellet. At 1400 and 1500 °C the (Lu_0.2Yb_0.2Er_0.2Y_0.2Gd_0.2)PO₄-CMAS corrosion showed CMAS penetrating beyond the reaction layer into the bulk pellet via the grain boundaries, and SiO₂ precipitates remaining at the pellet surface. The effects of duration, temperature, and compositions on the resistance against CMAS corrosion are discussed within the context of optimizing materials design and performance of high entropy rare-earth phosphates as candidates for advanced EBC applications.     

Hot-corrosion of refractory high-entropy ceramic matrix composites synthesized by alloy melt-infiltration

Carbon fiber-containing refractory high-entropy ceramic matrix composites (C/RHECs) were fabricated through a reaction with carbon powders, transition metal carbides, and Zr–Ti alloys as a novel heat resistant material used for components of hypersonic vehicles cruising at Mach 7–10. With the infiltration of alloys at 1750 °C into a composite preform containing carbon and carbide powders for 15 min, a high-entropy matrix was successfully formed in situ. Arc-jet tests were conducted in the temperature range of 1800–1900 °C. Results showed the formation of an oxidized region composed of complex oxides, such as (Zr, Hf)O₂, (Nb, Ta)₂(Zr, Hf)₆O₁₇, (Zr, Hf)TiO₄, and Ti(Nb, Ta)₂O₇, with an average thickness of ~600 μm, under which an unoxidized region remained. The porous oxidized region resulted from the evolution of CO_(g) during oxidation, while a dense oxide region formed as the outermost region. This indicates that the dense oxide region acted as a barrier to oxygen diffusion for the unoxidized region during oxidation.     

Effect of Yb2SiO5 addition on the physical and mechanical properties of sintered mullite ceramic as an environmental barrier coating material

In this study, ytterbium monosilicate (Yb₂SiO₅)-added sintered mullite ceramics are prepared as candidate materials for environmental barrier coatings (EBCs). The effect of adding Yb₂SiO₅ on the physical and mechanical properties of the sintered mullite ceramics is investigated. The Yb₂O₃–SiO₂–Al₂O₃ ternary phase diagram indicates that adding Yb₂SiO₅ to the mullite goes beyond simply mixing; instead, liquid sintering occurs. Therefore, when we add Yb₂SiO₅ to the mullite, the sintered body possesses a denser microstructure and faster densification rate than does pure mullite. The density rapidly increases with the addition of 6 wt% Yb₂SiO₅ in the mullite, and almost full densifications are achieved with the addition of 12 wt% and 18 wt% Yb₂SiO₅. In this study, mullite ceramic that contains 12 wt% Yb₂SiO₅ exhibits the smallest plastic deformation and the highest elastic modulus among ceramics containing 6, 12, and 18 wt% Yb₂SiO₅, according to Hertzian indentation results. The results suggest that 12 wt% Yb₂SiO₅-doped mullite may be expected to act as a potential EBC material based on its excellent elastic properties, dense microstructure, and appropriate coefficient of thermal expansion.     

A method for testing the interface toughness of ceramic environmental barrier coatings (EBCs) on ceramic matrix composites (CMCs)

A simple interface fracture test for ceramic environmental barrier coatings (EBCs) on ceramic matrix composites (CMCs) was developed. A variation on the asymmetric double cantilever beam (ADCB) test was proposed so that the interface toughness could be measured in a small specimen of simple shape without applying interlaminar loading to the CMC substrate. The proposed test was applied to an EBC consisting of a mullite layer and Si bond coat on a monolithic SiC substrate. A pre-crack was introduced by pop-in cracking, and then a notch overlapping the pre-crack was machined. The pre-crack was opened by inserting a wedge into the notch. From the critical notch opening displacement the crack starts to propagate, interface toughness is calculated. The measured interface toughness was 4.1?J/m2. Finally, the application range of the test was discussed and suggestions were made for introduction of the notch and pre-crack.     

Hot corrosion behavior of multialkaline-earth aluminosilicate for environmental barrier coatings

Multialkaline-earth aluminosilicate Ba_1/3Sr_1/3Ca_1/3Al₂Si₂O₈ (BSCAS) were synthesized to serve as new environment barrier coatings. Their hot corrosion behavior in an Na₂SO₄ environment was studied in the temperature range of 900–1100 °C over a period of 100 h. The phase and cross-sectional morphology evolutions of the corroded samples were characterized via X-ray diffraction and scanning electron microscopy. Combined with the thermodynamic analysis of the possible reactions occurring during hot corrosion, the competitive out-diffusion of the alkaline-earth elements to react with Na₂SO₄ is believed to have a considerable influence on the hot corrosion behavior of BSCAS. The sluggish diffusion and the dense Ca₂Al₂SiO₇ layer, which originate from the competitive reactions of the multialkaline earth elements, lead to an improvement in the hot corrosion resistance of BSCAS. A model is proposed to describe the hot corrosion process.     

Rare-earth high-entropy aluminate-toughened-zirconate dual-phase composite ceramics for advanced thermal barrier coatings

Superb toughening is achieved by incorporating a secondary ferroelastic phase in high-entropy rare-earth zirconate 5RE₂Zr₂O₇ (HZ). Here, we report an enhancement of 64% in fracture toughness through the addition of 30mol% high-entropy rare-earth aluminate 5REAlO₃ (HA) to the HZ matrix (30HA). The aforementioned rare-earth elements RE are La, Sm, Eu, Gd, and Yb. The present dual-phase composite ceramic 30HA has a large fracture toughness of 2.77 ± 0.14 MPa m^1/2, along with excellent high-temperature phase stability, resulting in good usage for potential thermal barrier coating applications. Particularly, the fracture toughness of the dual-phase composite ceramics at first increases to a maximum and then drops suddenly, as the mole fraction of HA increases from 0 to 50%. A clear definition of fitting parameters and their physical significance is provided for a better interpretation of the experimental data. The present toughening mechanism sheds light on microstructure engineering in high-entropy ceramics for excellent mechanical properties.     

Thin single-phase yttrium-based environmental barrier coating systems for SiC/SiC CMCs

Two thin yttrium silicate-based EBC systems for SiC/SiC CMCs that comprised of three layers were produced via (reactive) magnetron sputtering with pure yttrium disilicate (YDS) and yttrium mono silicate (YMS) layers. The systems survived 1000 1-hour furnace cycle tests at 1250 °C in air interrupted by active cooling for 10 min. One system showed partial delamination of the YMS. The YDS intermediate layer stopped deleterious reactions among YMS and silicon and the silica formation did not cause spallation of the silicates. Some vertical cracks in YMS left possible attack paths for water vapor. Rapid water vapor testing was performed at 1250 °C in 100 vol% water vapor at 10 m/s for 2 h. This test proved that a thin but hermetic and phase pure YMS top layer can protect the system from volatilization: single phase X2-YMS was not attacked by steam whereas a mixed β-YDS/X2-YMS coating showed volatilization of SiO₂.     

Plasma spray deposition of tri-layer environmental barrier coatings

An air plasma spray process has been used to deposit tri-layer environmental barrier coatings consisting of a silicon bond coat, a mullite inter-diffusion barrier, and a Yb₂SiO₅ top coat on SiC substrates. Solidified droplets in as-deposited Yb₂SiO₅ and mullite layers were discovered to be depleted in silicon. This led to the formation of an Yb₂SiO₅ + Yb₂O₃ two-phase top coat and 2:1 mullite (2Al₂O₃*SiO₂) coat deposited from 3:2 mullite powder. The compositions were consistent with preferential silicon evaporation during transient plasma heating; a consequence of the high vapor pressure of silicon species at plasma temperatures. Annealing at 1300 °C resulted in internal bond coat oxidation of pore and splat surfaces, precipitation of Yb₂O₃ in the top coat, and transformation of 2:1 mullite to 3:2 mullite + Al₂O₃. Mud-cracks were found in the Yb₂SiO₅ layer and in precipitated Al₂O₃ due to the thermal expansion mismatch between these coating phases and the substrate.     

Study on sintering behaviour of ytterbium disilicate and ytterbium monosilicate for environmental barrier coating applications

Yb₂Si₂O₇ (YbDS) and Yb₂SiO₅ (YbMS) are two promising materials being developed as environmental barrier coatings (EBC) for the protection of Ceramic Matrix Composites (CMC) applied to gas-turbine engines operating in high-temperature corrosive environments. In this work, sintering behaviours of YbDS, YbMS, and YbDS/YbMS composites compacts were investigated. The effect of the thermal ageing at 1350 °C on microstructural characteristics as well as crack healing and elastic properties were examined. It was found that YbDS had a lower critical sintering temperature and higher grain growth rate than that of YbMS due to lower activation energy. The sintering behaviour of the YbDS/YbMS composites showed that the addition of YbMS retarded the grain growth rates and contributed to the stabilisation of the elastic properties. In addition to this, composite containing 22 wt %YbMS displayed a crack healing behaviour during high-temperature exposure, which was attributed to the generation of compressive stress, consequently accelerated diffusion in the YbDS matrix.     

Multi-component strategy for remarkable suppression of thermal conductivity in strontium diyttrium oxide: The case of high-entropy Sr(Y0.2Sm0.2Gd0.2Dy0.2Yb0.2)2O4 ceramic

Anti-spinel oxide SrY₂O₄ has attracted extensive attention as a promising host lattice due to its outstanding high-temperature structural stability and large thermal expansion coefficient (TEC). However, the overhigh thermal conductivity limits its application in the field of thermal barrier coatings. To address this issue, a novel high-entropy Sr(Y_0.2Sm_0.2Gd_0.2Dy_0.2Yb_0.2)₂O₄ ceramic was designed and synthesized for the first time via the solid-state method. It is found that the thermal conductivity of Sr(Y_0.2Sm_0.2Gd_0.2Dy_0.2Yb_0.2)₂O₄ is reduced to 1.61 W·m⁻¹·K⁻¹, 53 % lower than that of SrY₂O₄ (3.44 W·m⁻¹·K⁻¹) at 1500 °C. Furthermore, reasonable TEC (11.53 ×10⁻⁶ K⁻¹, 25 °C ∼ 1500 °C), excellent phase stability, and improved fracture toughness (1.92 ± 0.04 MPa·m^1/2) remained for the high-entropy Sr(Y_0.2Sm_0.2Gd_0.2Dy_0.2Yb_0.2)₂O₄ ceramic, making it a promising material for next-generation thermal barrier coatings.     

Protecting the MoSi2 healing particles for thermal barrier coatings using a sol-gel produced Al2O3 coating

A successful sol-gel process to encapsulate molybdenum di-silicide MoSi₂ particles with a closed and thermally stable Al₂O₃ layer using aluminium tri-sec-butoxide as a precursor is presented. The processing conditions such as precursor selection and temperature were optimized through analysing the interaction of the MoSi₂ particles with the sol. The application of the sol-gel based coating was followed by calcining the coated particles at temperatures between 900 and 1200?°C in Ar. The shell composition and the mechanical stability of the microcapsule were analysed by means of X-ray diffraction, scanning electron microscopy and thermogravimetric analysis. Upon calcining at 1200?°C in Ar, the MoSi₂ core remains intact as it is, covered by an α - alumina shell with a thickness of about 0.6?μm. The stability tests proved that the encapsulate particles are about five times more oxidation resistant than the uncoated MoSi₂ particles.     

Effect of environmental pressure on the microstructure of YSZ thermal barrier coating via suspension plasma spraying

Suspension plasma spraying (SPS) as a potential technique to prepare thermal barrier coatings (TBCs) has been attracting more and more attention. However, most reports on SPS were carried out in the atmosphere. Given the unique features of in-flight particles and plasma jets under low pressure, the resulting coatings are expected to be different from those under atmospheric pressure. In this article, yttria-stabilized zirconia (YSZ) thermal barrier coatings were prepared using suspension plasma spraying under different environmental pressures. The results show that as the environmental pressure decreased, the column-like structural coating turned into a vertical crack segmented structure, as well as a dramatic decrease in surface roughness. More nanoparticle agglomerates were formed in the coating under lower environmental pressures. The real porosity of the coating increased with a decrease in environmental pressure.