Superior Performance of Ablative Glass Coatings Containing Graphene Nanosheets

Glass compositions in the Y₂O₃–Al₂O₃–SiO₂ (YAS) system are envisaged as promising coatings for high‐temperature protection, in particular for the thermal protection systems (TPS) looked for aerospace applications. Recently, thermally sprayed YAS hybrid coatings containing a small amount of graphene nanoplateletes (GNPs) showed enhanced performance as compared to the blank YAS coating, demonstrated by the occurrence of unusual electrical conductivity for these glasses and the development of better mechanical compliance, both phenomena associated with the presence of GNPs. Nevertheless, a crucial issue is to demonstrate if these kinds of coatings would also have superior behavior under ablation conditions, particularly regarding the mentioned TPS applications. This work goes far beyond, exploring the ablative behavior of new YAS/GNPs coatings flame sprayed over SiC substrates. These essential tests were carried out under laboratory conditions, reaching limit temperatures of 1350°C while blowing gas. Results evidence that hybrid coatings having just 1.05 vol% GNPs show enhanced ablation resistance, actually withstanding up to 30 thermal cycles (between 200°C and 1350°C) without apparent damage. This satisfactory performance is linked to the benefits of the GNP additions, and fundamentally to the higher emissivity and the directional thermal conduction characteristics of the hybrid coatings—produced by the formation of a GNP network with a preferential surface parallel arrangement—that preclude the creation of hot spots and also hinder heat propagation toward the substrate; accordingly, coating degradation is constrained to the uppermost layer of these coatings.     

Passive filler loaded polysilazane-derived glass/ceramic coating system applied to AISI 441 stainless steel,part 1: Processing and characterization

This study describes an oxidation and corrosion resistant environmental barrier coating (EBC) applied to an AISI 441 stainless steel substrate. For this purpose, four polymer-derived ceramic (PDC) coating systems were developed. These coating systems consisted of a bond coat applied by dip coating, and a top-coat that was loaded with passive fillers and deposited by spray coating. The microstructures of the coatings were investigated using optical microscopy and scanning electron microscopy, including energy dispersive spectroscopy (EDS). X-ray powder diffraction (XRD) was used to investigate the phase composition of the coatings. The optimized composite top coatings were prepared from the preceramic polymer HTT1800, filled with yttria-stabilized zirconia and a specially tailored Al₂O₃–Y₂O₃–ZrO₂ (AYZ) passive filler, and commercial barium silicate glasses were used as sealing agents. After thermal treatment in air at 750°C, uniform and crack-free composite coatings on stainless steel substrates were developed, with thicknesses of up to 93 μm. Oxidation tests, which were performed at 850°C in synthetic air, showed that every tested coating system remained undamaged by oxidation and showed good bonding to the metal substrate.     

Infrared radiative performance and anti-ablation behaviour of Sm2O3 modified ZrB2/SiC coatings

To improve the emissivity of ZrB₂/SiC coatings for serving in more serious environment, ZrB₂/SiC coatings with varying contents of high emissivity Sm₂O₃ were fabricated using atmospheric plasma spraying. The microstructure, infrared radiative performance and anti-ablation behaviour of the modified coatings were investigated. The results showed that as the content of Sm₂O₃ increased, the density of the coatings increased because of the low melting point of Sm₂O₃. When the content of Sm₂O₃ was 10 vol%, the coating had the highest emissivity in the 2.5–5 μm band at 1000 °C, up to 0.85, because of the oxygen vacancies promoting additional electronic transitions. Due to the high emissivity, the surface temperature of the coating modified with 10 vol% Sm₂O₃ decreased by 300 °C, which led to little volatilisation of the sealing phase. Further, the mass ablation ratio of the above coating was 3.19 × 10^?4 g/s, decreasing 31% compared to that of a ZrB₂/SiC coating. The formed dense surface structure of the coatings showed considerable oxygen obstructive effects. These findings indicate that the modified coatings show considerable anti-ablation performance, which provides effective anti-ablation protection for the C/C composite substrate.     

Evaluation of Yttria Coated High Density Graphite with Silicon Carbide Interlayer for Uranium Melting Applications

Yttrium Oxide (Y₂O₃) deposited over High Density Graphite (HDG) by Atmospheric Plasma Spray (APS) process is highly desirable as a chemical barrier coating for reusable Uranium (U) melting crucibles in the pyrochemical reprocessing of spent metallic fuels. In the present study, an oxidation protective Silicon Carbide (SiC) interlayer coating over HDG has been achieved by pack cementation process. The high-temperature oxidation resistance and resistance to thermal fatigue failure of Y₂O₃ coating with and without SiC interlayer were evaluated by performing repeated thermal cycling studies at 1450, 1500 and 1550?°C. The durability performance of Y₂O₃ coating with SiC interlayer in the actual working environment was simulated by performing U melting studies using miniature size HDG coated crucibles. The microstructural, chemical and phase characterization of coatings prior and post thermal cycle failure were carried out by SEM/EDS and XRD techniques. It is observed that the SiC interlayer developed by novel pack cementation technique for the Y₂O₃ top coat extended the thermal cycling and life of the coating with U melting in inert argon gas environment significantly. The occurrence of micro-cracking over Y₂O₃ top coat with SiC interlayer perceived after 33, 30 and 25 thermal cycles at 1450, 1500 and 1550?°C, respectively.     

Effects of Y2O3–RE2O3 (RE = Sm,Gd, Lu) Additives on Electrical and Thermal Properties of Silicon Carbide Ceramics

In this study, we investigated the electrical and thermal properties of SiC ceramics with 2 vol% equimolar Y₂O₃–RE₂O₃ (RE = Sm, Gd, Lu) additives. The three SiC ceramics with 2 vol% equimolar Y₂O₃–RE₂O₃ additives showed electrical conductivities on the order of ~10³ (Ω·m)^?1, which is one order of magnitude higher than that of the SiC ceramics sintered with 2 vol% Y₂O₃ only. The increase in electrical conductivity is attributed to the growth of heavily nitrogen‐doped SiC grains during sintering and the confinement of oxide additives in the junction area. The thermal conductivities of the SiC ceramics were in the 176–198 W·(m·K)^?1 range at room temperature. The new additive systems, equimolar Y₂O₃–RE₂O₃, are beneficial for achieving both high electrical conductivity and high thermal conductivity in SiC ceramics.     

Preparation of transparent Y2O3 ceramic via gel casting: Realization of high solid volume via surface modification

In this article, isocyanate was adopted to modify Y₂O₃ powder for the purpose of preparing transparent Y₂O₃ ceramics via gel casting. The modification could enhance the hydration resistance of Y₂O₃ powder through the steric hindrance effect. The coating mechanism can be proved by the infrared spectrum of the surface-modified Y₂O₃ powder. Modification could not only prevent Y₂O₃ particles from reacting with water, but also prevents agglomeration between particles. The viscosity of the slurry with a solid content of 52.7 vol% is only 0.48 Pa·s at the shear rate of 100 s⁻¹, which is suitable for preparing high-density compacts by gel casting. The transmittance of the sample (1840°C × 8 h, 1 mm thickness) at 1100 nm reaches 75%. The microstructure of the sintered body is dense with the average grain size of 6.5 μm without obvious impurities nor pores. Five mol% ZrO₂-doped Y₂O₃ transparent ceramic fairing with the diameter of 5 cm without defects was successfully fabricated by gel casting (52.7 vol% solid volume) and vacuum sintering (1840°C × 8 h).     

Evolution behavior of rare-earth yttria modified silicide oxidation-resistant coating at 1700 oC

Yttria-modified silicide (MoSi₂-Y₂O₃) oxidation-resistant coatings with multiple Y₂O₃ contents were prepared using supersonic atmospheric plasma spraying, and the oxidation resistance was investigated at 1700 °C with static atmosphere. Experimental results indicated that the sprayed MoSi₂-Y₂O₃ coating possessed good compactness, which adhered well to the SiC transition layer. Meanwhile, the Y₂O₃ addition greatly enhanced the bonding strength of the coating, and extended its service life at 1700 °C. Wherein the MoSi₂-20 wt.%Y₂O₃ coating exhibited the highest adhesive strength and best oxidation resistance with the lowest mass loss among all the coatings. In practice, the Y₂O₃ changed the microstructures of formed oxide glass scale during oxidation, and then modified the oxidation resistance of the coating. The action mechanism of Y₂O₃ on the oxidation behavior of MoSi₂-Y₂O₃ coating was analyzed.     

Phase and microstructure evolution in plasma sprayed Yb2Si2O7 coatings

Yb₂Si₂O₇ coatings were deposited on Si/SiC substrates by atmospheric plasma spray (APS). The different power and plasma chemistries used in this work produced mainly amorphous crack-free coatings with compositions shifted to lower SiO₂ content with higher power and H₂ flow. Differences in microstructure and thermomechanical properties (crystallization behavior, thermal expansion coefficient and thermal conductivity) of as-deposited and thermally treated coatings were directly related to the evolution from amorphous to crystalline phases. A Yb₂SiO₅ metastable phase was identified after thermal treatments at temperatures ～ 1000 °C that transformed to its stable isomorph at 1220 °C. This transformation, followed by the growth of the crystal cell volume, promoted the coating expansion and the “healing” of microcracks present in the amorphous as-sprayed coating.     

Polymer-derived yttrium silicate coatings on 2D C/SiC composites

Yttrium silicate environmental barrier coatings (EBCs) on C/SiC composites were fabricated by using polysiloxanes-derived ceramic process. In order to reduce the free silica in the resultant ceramic coatings, Y₂O₃ was added as an active filler. The materials with different weight ratios of polysiloxanes to Y₂O₃ were synthesized. Their coefficient of thermal expansion (CTE) and water-vapor resistance were tested. The results indicated that the composition of 50% Y₂O₃–50% polysiloxanes (Y50) was suitable to be the EBCs for C/SiC composites. The C/SiC composites coated with Y50 were tested in the water-vapor environments at 1400 °C for 200 h. The results revealed that such a coating could effectively protect the C/SiC composites.     

Yttrium Bearing Silicon Carbide Matrices for Robust Ceramic Composites

Application of SiC‐based ceramic matrix composites (CMCs) in combustion environments demands the use of an environmental barrier coating (EBC) to prevent volatilization of the protective SiO₂ scale in flowing water vapor. The EBC only provides protection while present on the surface; cracking and spallation of the coating leaves the underlying SiC vulnerable to the oxidation–volatilization processes. A robust matrix material chemically tailored to regrow a yttrium silicate scale in the event of EBC loss has been developed by incorporating yttrium bearing species including YB₂, Y₂O₃, and Y₅Si₃ into the SiC. During oxidation a borosilicate glass helps seal cracks while Y₂O₃ and SiO₂ react to form Y₂Si₂O₇ for environmental protection. Candidate compositions were oxidized for 10 min to 100 h at 1400°C and for 24 h at 1500°C to understand the scale growth. The prospects for effectively applying this approach in CMCs are discussed.     

Assessment of the performance of Y2SiO5-YSZ/YSZ double-layered thermal barrier coatings

Y₂SiO₅ is a promising material for the thermal barrier coatings due to its low thermal conductivity, high temperature stability and exceptional resistance for molten silicate attack. However, it suffers low fracture toughness and low coefficient of thermal expansion compared with yttria-stabilized zirconia (YSZ). In this study, a composite coating approach, i.e., incorporating YSZ into Y₂SiO₅ coating, was employed to overcome those limitations. The double-layered Y₂SiO₅-YSZ/YSZ coatings were fabricated using atomospheric plasma spraying and tested under thermal cycling at 1150 °C. The phase compositions, microstructure, mechanical properties and the failure behavior were evaluated. It was found that the amorphous phase during spraying would crystallize at high temperature accompanied by volume shrinkage, leading to cracks and spallation in the coating. With YSZ addition, the composite coatings exhibited a much longer lifetime than the single phase Y₂SiO₅ coating due to a lower volume shrinkage and enhanced toughness.     

Corrosion behavior of air plasma spraying zirconia-based thermal barrier coatings subject to Calcium–Magnesium–Aluminum-Silicate (CMAS) via burner rig test

Three different kinds of thermal barrier coatings (TBCs) — 8YSZ, 38YSZ and a dual-layered (DL) TBCs with pure Y₂O₃ on the top of 8YSZ were produced on nickel-based superalloy substrate by air plasma spraying (APS). The Calcium–Magnesium–Aluminum-Silicate (CMAS) corrosion resistance of these three kinds of coatings were researched via burner rig test at 1350 °C for different durations. The microstructures and phase compositions of the coatings were characterized by SEM, EDS and XRD. With the increase of Y content, TBCs exhibit better performance against CMAS corrosion. The corrosion resistance against CMAS of different TBCs in descending was 8YSZ + Y₂O₃, 38YSZ and 8YSZ, respectively. YSZ diffused from TBCs into the CMAS, and formed Y-lean ZrO₂ in TBCs because of the higher diffusion rate and solubility of Y³⁺ in CMAS than Zr⁴⁺. At the same time, 38YSZ/8YSZ + Y₂O₃ reacts with CAMS to form Ca₄Y₆(SiO₄)₆O/Y_4·67(SiO₄)₃O with dense structure, which can prevent further infiltration of CMAS. The failure of 8YSZ coatings occurred at the interface between the ceramic coating and the thermally grown oxide scale (TGO)/bond coating. During the burner rig test, the Y₂O₃ layer of the DL TBCs peeled off progressively and the 8YSZ layer exposed gradually. DL coatings keep roughly intact and did not meet the failure criteria after 3 h test. 38YSZ coating was partially ablated, the overall thickness of the coating is thinned simultaneously after 2 h. Therefore, 8YSZ + Y₂O₃ dual-layered coating is expected to be a CMAS corrosion-resistant TBC with practical properties.     

Microstructure and properties of porous SiC ceramics with AlN–RE2O3 (RE = Sc,Y, Lu) additives

The intrinsic microstructure and crystalline phases of porous SiC ceramics with 5 vol% AlN–RE₂O₃ (RE = Sc, Y, Lu) additives were characterized by high-resolution transmission microscopy with energy-dispersive spectroscopy and X-ray diffraction. The homophase (SiC/SiC) and heterophase (SiC/junction) boundaries were found to be clean; that is, amorphous films were not observed in the specimens. In addition, ScN, YN, and LuN were formed as secondary phases. The flexural strength and thermal conductivity of the ceramics were successfully tuned using different additive compositions. The flexural strength of the ceramics improved by a factor of ~3, from 11.7 MPa for the specimen containing Y₂O₃ to 34.2 MPa for that containing Sc₂O₃, owing to the formation of a wide necking area between SiC grains. For the same reason, the thermal conductivity improved by ~56%, from 9.2 W·m^?1·K^?1 for the specimen containing Lu₂O₃ to 14.4 W·m^?1·K^?1 for that containing Sc₂O₃.     

Preparation of Pd-doped Y3Al5O12 thermal barrier coatings using cathode plasma electrolytic deposition

Here, noble metal Pd-doped Y₃Al₅O₁₂ thermal barrier coatings (TBCs) were efficaciously prepared by means of cathode plasma electrolytic deposition (CPED). The formation mechanism of the Y₃Al₅O₁₂ coatings and the difference in coating performance before and after doping with Pd were analyzed. The results indicated that the preparation of the Y₃Al₅O₁₂ TBCs by using the CPED method could be divided into three stages, and the phase compositions of the coatings obtained with different deposition times were different. A single-phase Y₃Al₅O₁₂ TBCs with a 115-μm thickness was obtained after a deposition time of 20 min. After Pd doping, the average surface roughness of the TBCs decreased from 27.72 to 13.84 μm, and the high-temperature oxidation resistance and thermal shock resistance at 1050 °C improved significantly.     

Phase, microstructure, and oxidation resistance of yttrium silicates coatings prepared by a hydrothermal electrophoretic deposition process for C/C composites

Oxidation-resistant yttrium silicates coatings for SiC precoated carbon/carbon composites were prepared by a novel hydrothermal electrophoretic deposition process. Sonochemical-synthesized yttrium silicates nanocrystallites, isopropanol, and iodine were respectively used as source materials, solvent, and charging agent during the deposition. Phase compositions, surface and cross-section microstructures of the as-prepared multilayer coatings were characterized by an X-ray diffractometer (XRD) and a scanning electron microscopy (SEM). The influence of deposition temperatures on the phase, microstructure, and oxidation resistance of the multilayer coated C/C composites was particularly investigated. Results show that the as-prepared outer coatings are composed of yttrium silicates crystallites with a main phase of Y₂Si₂O₇ and Y₂SiO₅. The thickness and density of the yttrium silicates coatings are improved with the increase of deposition temperature. Compared with SiC coating prepared by pack cementation, the multilayer coatings prepared by pack cementation with a later hydrothermal electrophoretic deposition process exhibit better antioxidation properties. The as-prepared multilayer coatings can effectively protect C/C composites from oxidation at 1773 K in air for 35 h with a weight loss of 0.32 × 10⁻³ g/cm².     

In situ Y2Si2O7 coatings on SiC fibers: Thermodynamic analysis and processing

It is shown using thermodynamic analysis and kinetic modeling that a processing window exists for the formation of Y₂Si₂O₇ coatings on SiC. The proposed method is validated using an experimental procedure in which the in situ formation of Y₂Si₂O₇ on a commercial SiC-based fiber is demonstrated. The method involves the deposition of YPO₄ on preoxidized fine diameter SiC-based fibers, and heat treating the coated fibers within a calculated processing window of oxygen partial pressure, temperature, degree of preoxidation, and coating thickness. The results are promising for the development of environmentally resistant interfacial coatings for SiC-fiber reinforced SiC-based matrix composites. The proposed and validated approach allows a low-cost method to obtain continuous hermetic coatings on SiC fibers with interfacial properties adequate for tough composite behavior that resists degradation under turbine engine conditions.     

MgO-Y2O3:Eu composite ceramics with high quantum yield and excellent thermal performance

MgO-Y₂O₃:Eu composite ceramics with high quantum yield and excellent thermal performance were successfully synthesized by vacuum sintering. All samples exhibited uniform composite microstructures and pure binary phase. Eu³⁺ ions were completely incorporated into Y₂O₃ phase, and the optimal Eu concentration is 15 at%. Sintered at 1800 °C, the fluorescent properties of MgO- z vol% Y₂O₃: Eu (z = 30, 40, 50, 60, 70, 100) composites proved to be independent on component proportion, including the similar fluorescence lifetimes (953–983 μs), quantum yield (70%−80%), and activation energy (ΔE) of thermal quenching (0.163 eV). Significantly, thermal conductivity of composites with 30 vol%, 50 vol% and 70 vol% MgO attained 11.58, 17.45, and 29.65 W/(m∙K) at room temperature, which are nearly 2, 3, and 5 times as high as that of 15 at% Eu:Y₂O₃ ceramic (5.90 W/(m∙K)), respectively, demonstrating their potential for application in high-power-density display and lighting technology.     

Effect of Y2O3 on the oxidation properties of ZrSi2/SiC coating prepared by SAPS on the carbon-carbon composites

In order to improve the oxidation resistance of carbon-carbon (C/C) composites at high temperature, different content of Y₂O₃ modified ZrSi₂/SiC coating for C/C composites were prepared by pack cementation and supersonic atmosphere plasma spraying (SAPS). Microstructure observation and phase identification of the coatings were analyzed by SEM, XRD, DSC/TG and EDS. Experimental results shown that the coating with 10?wt% Y₂O₃ effectively protected C/C composites from oxidation at 1500?°C in air for 301?h with a mass loss of 0.13% and experienced 18 thermal shock times from room temperature (RT) to 1500?°C. First, Y₂O₃ could restrain the phase transition of ZrO₂ to reduce the formation of thermal stresses of the coating; second, the random distribution of ZrO₂ ceramic particles and the formation of ZrSiO₄ enhanced the stability of the SiO₂; third, the formation of Y₂Si₂O₇ and Y₂SiO₅ could relieve the thermal mismatch between ZrSi₂-Y₂O₃ outer layer and the inner layer.     

Inhibition of grain growth in liquid-phase sintered SiC ceramics by AlN additive and spark plasma sintering

SiC ceramics were prepared from nanosized β-SiC powder with different compositions of AlN and Y₂O₃ sintering additives by spark plasma sintering (SPS) at 1900 °C for 600 s in N₂. The relative density of the sintered SiC specimens increased with increasing amount of AlN, reaching a relative density higher than 99%, while at the same time grain size decreased significantly. The smallest average grain size of 150 nm was observed for SiC sample sintered with 10 vol% of additives consisting of 90 mol% AlN and 10 mol% Y₂O₃. Fully dense nanostructured SiC ceramics with inhibited grain growth were obtained by the AlN additive and SPS technique. The flexural strength of the SiC body containing 70 mol% AlN and 30 mol% Y₂O₃ additives reached the maximum value of 1000 MPa. The SiC bodies prepared with AlN and Y₂O₃ additives had the fracture toughness of around 2.5 MPam^1/2.     

Synthesis and characterization of yttrium and ytterbium silicates from their oxides and an oligosilazane by the PDC route for coating applications to protect Si3N4 in hot gas environments

Environmental barrier coatings are required to protect Si₃N₄ against hot gas corrosion and enable its application in gas turbines, among which yttrium and ytterbium silicate-coatings stand out. Thus, the polymer-derived ceramic route was used to synthesize these silicates for basic investigations regarding their intrinsic properties from a mixture of Y₂O₃ or Yb₂O₃ powders and the oligosilazane Durazane 1800. After pyrolysis above 1200 °C in air, the silicates are predominant phases. The corrosion behaviour of the resulting composites was tested at 1400 °C for 80 h in moist environments. The material containing x₂-Yb₂SiO₅ and Yb₂Si₂O₇ undergoes the lowest corrosion rate (−1.8 μg cm⁻² h⁻¹). Finally, the processing of Y₂O₃/Durazane 1800 as well-adherent, crack-free and thick (40 μm) coatings for Si₃N₄ was achieved after pyrolysis at 1400 °C in air. The coating consisted of an Y₂O₃/Y₂SiO₅ top-layer and an Y₂O₃/Y₂Si₂O₇ interlayer due to the interaction of the coating system with the substrate.