Microstructure and water corrosion behavior of (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7 high-entropy rare-earth disilicate coating for SiC coated C/C composites

In order to make carbon/carbon composites suitable for application in gas turbine engine, it is necessary to develop environmental barrier coatings (EBCs) to protect them from reacting with water vapor. In our previous work, a novel high-entropy rare-earth disilicate (Lu_0.2Yb_0.2Er_0.2Tm_0.2Sc_0.2)₂Si₂O₇ ((5RE_0.2)₂Si₂O₇) has been developed and verified as a promising candidate for EBCs. In this work, the (5RE_0.2)₂Si₂O₇ coating was synthesized on the surface of SiC coated C/C composites by supersonic atmospheric plasma spraying method. The protective performance and mechanism of this coating under high temperature water vapor environment was explored in detail. Results showed that the weight change of the sample coated with (5RE_0.2)₂Si₂O₇ was only 0.2% after corrosion for 100 h at 1500 ºC, which proved that (5RE_0.2)₂Si₂O₇ coating could significantly improve the resistance of C/C composites against water vapor corrosion. This work may provide theoretical basis for the design and application of high-entropy rare-earth silicates as EBCs.     

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.     

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.     

Interaction of Yb2Si2O7 environmental barrier coating material with Calcium-Ferrum-Alumina-Silicate (CFAS) at high temperature

Experimental investigations of Yb₂Si₂O₇ pellet exposed to Calcium-Ferrum-Alumina-Silicate (CFAS) at 1400 °C in ambient air were carried out to reveal corrosion reaction between molten silicate deposit and Yb₂Si₂O₇. Phase transformation, microstructure evolution and reaction mechanism were evaluated. Results indicated that the corrosion process was accompanied by the infiltration of CFAS melt, the dissolution of Yb₂Si₂O₇ and the reprecipitation of Yb₂Si₂O₇ and Ca₂Yb₈(SiO₄)₆O₂ apatite as reaction product. The formation of apatite decreased the concentration of Ca²⁺ in the melt. After CFAS exposure at 1400 °C for 30 h, the thickness of the apatite layer stopped increasing due to insufficient Ca²⁺ content, and remained at about 115.4 μm. However, the infiltration depth of CFAS melt increased with the extending corrosion duration and increasing deposit content. And the infiltration rate was preliminarily found to first decrease and then increase with time. Most of the residual CFAS were crystallized into garnet (Ca₃Fe₂(SiO₄)₃ and Yb₃Fe₅O₁₂) and mayerite (Ca₁₂Al₁₄O₃₃), while a small volume of amorphous glass was dispersed among the garnet and mayerite grains.     

High-entropy thermal barrier coating of rare-earth zirconate: A case study on (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 prepared by atmospheric plasma spraying

We report a double-ceramic-layer (DCL) thermal barrier coating (TBC) with high-entropy rare-earth zirconate (HE-REZ) as the top layer and yttria stabilized zirconia (YSZ) as the inner layer sprayed on Ni-based superalloy by atmospheric plasma spraying. La₂Zr₂O₇ (LZ) was selected as a reference for the HE-REZ. Thermal cycling test results demonstrate that the HE-REZ/YSZ DCL coating exhibited obviously improved thermal stability when compared to the LZ/YSZ DCL coating. The reasons for the improvement of the thermal shock resistance are considered to be the anti-sinterability of the HE-REZ ceramics during the thermal cycling test attributed to the sluggish diffusion effect and as well as the better match in the coefficient of thermal expansion of HE-REZ coating with the YSZ inner layer. In addition, the HE-REZ coating maintains fluorite structure after thermal cycling test. This study makes one step forward in the development and application of high-entropy rare-earth zirconate ceramic thermal barrier coatings.     

New class of high-entropy defect fluorite oxides RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Y,Ho, Er,or Yb) as promising thermal barrier coatings

High-entropy ceramics exhibit great application potential as thermal barrier coating (TBC) materials. Herein, a series of novel high-entropy ceramics with RE₂(Ce_0.2Zr_0.2Hf_0.2Sn_0.2Ti_0.2)₂O₇ (RE₂HE₂O₇, RE = Y, Ho, Er, or Yb) compositions were fabricated via a solid-state reaction. X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) mapping analyses confirmed that RE₂HE₂O₇ formed a single defect fluorite structure with uniform elemental distribution. The thermophysical properties of the RE₂HE₂O₇ ceramics were investigated systematically. The results show that RE₂HE₂O₇ ceramics have excellent high-temperature phase stability, high thermal expansion coefficients (10.3–11.7 × 10^?6 K^-1, 1200 ℃), and low thermal conductivities (1.10-1.37 W m^-1 K^-1, 25 ℃). In addition, RE₂HE₂O₇ ceramics have a high Vickers hardness (13.7–15.0 GPa) and relatively low fracture toughness (1.14-1.27 MPa m^0.5). The outstanding properties of the RE₂HE₂O₇ ceramics indicate that they could be candidates for the next generation of TBC materials.     

Ultrafast high-temperature sintering of (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 high-entropy ceramics with defective fluorite structure

An entropy-stabilized rare earth hafnate (Y_0.2Dy_0.2Er_0.2Tm_0.2Yb_0.2)₄Hf₃O₁₂ (5RH) with defective fluorite structure was successfully prepared by the emerging ultrafast high-temperature sintering (UHS) in less than six minutes. The 5RH ceramic possessed a higher thermal expansion coefficient (11.23 ×10^?6/K, 1500 °C) and extremely low thermal conductivity (0.94 W/(m·k), 1300 ℃) owing to the larger lattice distortion of high-entropy materials. After high-temperature annealing at 1500 ℃, the 5RH showed extremely sluggish grain growth characteristics and excellent high-temperature phase stability, mainly attributed to the non-equilibrium sintering characteristic of the UHS and the sluggish diffusion effect of high-entropy materials. Therefore, (Y_0.2Dy_0.2Er_0.2Tm_0.2Yb_0.2)₄Hf₃O₁₂ has excellent potential as a next-generation thermal barrier coating material to replace traditional Y₂O₃ stabilized ZrO₂. Finally, using the UHS to prepare high-entropy ceramics provides a new technique for fast-sintering and developing next-generation thermal barrier coating materials.     

Novel (Sm0.2Lu0.2Yb0.2Y0.2Dy0.2)3TaO7 high-entropy ceramic for thermal barrier coatings

A novel (Sm_0.2Lu_0.2Dy_0.2Yb_0.2Y_0.2)₃TaO₇ (SLT-5RE_0.2) oxide with a single-fluorite structure was synthesized via an optimized sol-gel and sintering method, and its crystal structure, mechanical and thermophysical properties were investigated. The results indicate that the calcined nanoscale powder is of high crystallinity, and bulk sample is of a uniform elemental distribution. Compared to YSZ (6–8 wt.% Y₂O₃ partially stabilized by ZrO₂), SLT-5RE_0.2 exhibits lower Young's modulus, less mean acoustic velocity, and higher Vickers microhardness. Owing to the strengthened anharmonic vibration and phonon scattering, SLT-5RE_0.2 exhibits low thermal conductivity (1.107 W K^?1·m^?1, 900 °C). The high thermal expansion coefficient (11.3 × 10^?6 K^?1, 1200 °C) of SLT-5RE_0.2 ceramic can be ascribed to the reduced lattice energy and ionic spacing as well as the cocktail effect of high-entropy ceramics. The excellent mechanical and thermophysical properties, and excellent phase steadiness during the whole testing temperature cope, indicate that SLT-5RE_0.2 high-entropy ceramic can be a candidate material for thermal barrier coatings.     

High-Temperature thermochemical interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating materials

The thermochemical behavior of EBC candidate materials yttrium disilicate (Y₂Si₂O₇) and ytterbium disilicate (Yb₂Si₂O₇) was evaluated with three calcium-magnesium-aluminosilicate (CMAS) glasses possessing CaO:SiO₂ ratios relevant to gas turbine systems. Pellet mixtures of 50 mol% EBC powder to 50 mol% CMAS glass powder were heat treated at 1200°C, 1300°C, and 1400°C. The products of these interactions were evaluated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Above glass melting temperatures, exposure of the disilicates primarily resulted in dissolution into the molten glass followed by precipitation of a Ca₂RE₈(SiO₄)₆O₂ (RE = Yb³⁺, Y³⁺) apatite-type silicate and/or rare earth disilicate. In glasses with high CaO concentrations, apatite readily forms while the disilicate material is consumed by the reaction. As CaO content decreases, the disilicate phase becomes the main reaction product. Overall, reactions with yttrium disilicate favored more apatite crystallization than ytterbium disilicate. The viability of using these disilicates in various operating environments is discussed.     

Constrained sintering and thermal ageing behaviour of electrophoretically deposited Yb2Si2O7 environmental barrier coating

The oxidation of SiC and the formation of a thermally grown oxide layer (TGO) limit the lifetime of environmental barrier coatings. Thus, this paper focuses on the deposition of denser Yb₂Si₂O₇ coatings using electrophoretic deposition to reduce the TGO growth rate. The findings showed densification for Yb₂Si₂O₇ can be achieved with an optimized sintering profile (heating/cooling rate, temperature, and time). However, the addition of 1.5 wt% of Al₂O₃ to Yb₂Si₂O₇ promoted densification and lowered the required sintering temperature, 1380 °C using 2 °C/min heating/cooling rate for 10 h provided efficient coating density. Moreover, adding Al₂O₃ reduced the TGO growth rate by more than 70 % compared to the Al₂O₃-free coatings, without cracking in TGO after 150 h of thermal ageing at 1350 °C. Results within this study suggest electrophoretic deposition with Al₂O₃ addition produces promising Yb₂Si₂O₇ environmental barrier coatings on SiC substrate with low oxidation rates and increased lifetime.     

High temperature performances of tri-layer environmental barrier coating with Si bond layer modified by Yb2O3

Environmental barrier coatings (EBCs) have been expected to be applied on the surface of ceramic matrix composites (CMCs). However, the oxidation and propagation cracking of the silicon bond layer are the most direct causes to induce the failure of EBCs under high temperature service environment. The modification of silicon bond layer has become an important method to prolong the service life of EBCs. In this work, the Yb₂O₃ have been introduced to the silicon bond layer, and three kinds of tri-layer Yb₂SiO₅/Yb₂Si₂O₇/(Si-xYb₂O₃) EBCs with modified Si bond layer by different contents of Yb₂O₃ (x = 0, 10 vol%, 15 vol%) were prepared by vacuum plasma spray technique. The thermal shock performance and long-term oxidation resistance of the EBCs at 1350 °C were investigated. The results showed that the addition of appropriate amount of Yb₂O₃ (10 vol%) can improve the structural stability and reduce the cracks of the mixed thermal growth oxide (mTGO) layer by forming the oxidation product of Yb₂Si₂O₇ during long-term oxidation. The excessive addition of Yb₂O₃ increased the stress during thermal shock as well as accelerated the oxygen diffusion during long-term oxidation, leading to the failure of EBCs. Moreover, the distribution uniformity of Yb₂O₃ deserves further consideration and improvement.     

Corrosion of RE2Si2O7 (RE=Y,Yb, and Lu) environmental barrier coating materials by molten calcium-magnesium-alumino-silicate glass at high temperatures

With the increased demand for high operating temperature of gas turbine engines, corrosion by molten calcium-magnesium-alumino-silicate (CMAS) exhibits a significant challenge to the development of durable environmental barrier coatings (EBCs). EBC candidates, γ-Y₂Si₂O₇, β-Yb₂Si₂O₇, and β-Lu₂Si₂O₇ were explored on their corrosion resistance to CMAS melts at 1300 °C and 1500 °C for 50 h. Interaction and degradation mechanisms were investigated and the corrosion behaviors showed different trends at high temperatures. At 1300 °C, RE₂Si₂O₇ dissolves into CMAS melts and apatite phases reprecipitate forming a thick recession layer. However, when the temperature increases to 1500 °C, CMAS melts vigorously penetrate through the grain boundary of RE₂Si₂O₇ and ‘blister’ cracks form throughout the samples. The reduced grain boundary stability at 1500 °C promotes the penetration of CMAS melts in RE₂Si₂O₇. Grain boundary engineering is critically demanded to optimize CMAS corrosion at high temperatures.     

Study on properties of Hf6Ta2O17/Ta2O5 system: A potential candidate for environmental barrier coating (EBC)

Ta₂O₅ doped Hf₆Ta₂O₁₇ system (Hf₆Ta₂O₁₇/Ta₂O₅) is considered to have potential application prospect in the field of aero-engine. We herein focus on the thermo-physical, mechanical properties and CMAS corrosion resistance of Hf₆Ta₂O₁₇/Ta₂O₅ to systematically evaluate the possibility for the application of environmental barrier coating (EBC). By changing the content of Ta₂O₅, the gradient adjustment of thermal expansion coefficient can be realized while maintaining low thermal conductivity (1.5–2.2 W/(m·K)). The introduction of Ta₂O₅ significantly reduces the modulus and improves the fracture toughness. Single-phase Hf₆Ta₂O₁₇ shows excellent corrosion resistance against molten calcium-magnesium-alumina-silicate (CMAS). The crystallization of CaTa₂O₆ and HfSiO₄ is the important factor to prevent further corrosion. The introduction of Ta₂O₅ weakens the ability to prevent Si penetration and greatly increases the thickness of the corrosion layer. The results highlight the merit of Hf₆Ta₂O₁₇/Ta₂O₅ system as potential candidate for multi-layer gradient coating on the surface of ceramic matrix composites.     

Reaction between environmental barrier coatings material Er2Si2O7 and a calcia-magnesia-alumina-silica melt

The suitability of sintered erbium disilicate (Er₂Si₂O₇) as an environmental barrier coatings (EBCs) for gas turbine applications was assessed by characterizing its high-temperature corrosion behavior in contact with a synthetic calcia-magnesia-alumina-silica (CMAS) melt. Er₂Si₂O₇ was fabricated using spark plasma sintering at 1400 °C for 20 min. Corrosion tests were performed by coating sintered Er₂Si₂O₇ pellets with CMAS and heating them to 1400 °C for 2, 12, and 48 h. High-temperature X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis were used to identify and analyze the reaction products. The two materials were found to react chemically to form an apatite phase, Ca₂Er₈(SiO₄)₆O₂, at their interface. The Ca₂Er₈(SiO₄)₆O₂ grains were observed to have shard-like morphologies oriented perpendicular to the Er₂Si₂O₇ surface; the reaction layer thickened with increasing heat-treatment time, with the thickness after exposure for 48 h approximately three times the thickness after 2 h.     

The CMAS corrosion behavior of high-entropy (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 hafnate material prepared by ultrafast high-temperature sintering (UHS)

High-temperature molten calcium-magnesium-alumina-silicate (CMAS) corrosion has become a fatal factor for the failure of aero-engine thermal barrier coatings. In this study, a promising entropy-stabilized (Y_0.2Dy_0.2Er_0.2Tm_0.2Yb_0.2)₄Hf₃O₁₂ (5YH) hafnate was prepared by the emerging ultrafast high-temperature sintering (UHS), and its corrosion and wetting behavior of molten CMAS were investigated. For the corrosion mechanism, the precipitation of the high-entropy apatite phase promotes the formation of the HfO₂ phase, and it can improve the density and stability of the slow-growing reaction layer, hindering the further penetration of molten CMAS. At 1300 ℃, a reaction layer with a three-layered morphology is generated, resulting from the decreased viscosity of the molten CMAS. Moreover, computational analysis shows that molten CMAS on the 5YH surface has a larger contact angle (17°) than traditional YSZ (13°), and the spreading area is about 90 % of traditional YSZ, which benefits for its good CMAS corrosion resistance.     

Thermophysical properties of Yb2O3 doped Gd2Zr2O7 and thermal cycling durability of (Gd0.9Yb0.1)2Zr2O7/YSZ thermal barrier coatings

(Gd_1−xYb_x)₂Zr₂O₇ compounds were synthesized by solid reaction. Yb₂O₃ doped Gd₂Zr₂O₇ exhibited lower thermal conductivities and higher thermal expansion coefficients (TECs) than Gd₂Zr₂O₇. The TECs of (Gd_1−xYb_x)₂Zr₂O₇ ceramics increased with increasing Yb₂O₃ contents. (Gd_0.9Yb_0.1)₂Zr₂O₇ (GYbZ) ceramic exhibited the lowest thermal conductivity among all the ceramics studied, within the range of 0.8–1.1 W/mK (20–1600 °C). The Young's modulus of GYbZ bulk is 265.6 ± 11 GPa. GYbZ/YSZ double-ceramic-layer thermal barrier coatings (TBCs) were prepared by electron beam physical vapor deposition (EB-PVD). The coatings had an average life of more than 3700 cycles during flame shock test with a coating surface temperature of ∼1350 °C. Spallation failure of the TBC occurred by delamination cracking within GYbZ layer, which was a result of high temperature gradient in the GYbZ layer and low fracture toughness of GYbZ material.     

Thermo-mechanical properties and CMAS resistance of (Ho0.4Yb0.3Lu0.3)2SiO5 solid solution for environmental barrier coating applications

Rare earth monosilicate (RE₂SiO₅) is one of the most promising candidates as an environmental barrier coating (EBC) for SiC_f/SiC ceramic matrix composites. But single-component RE₂SiO₅ is hard to meet the multiple and harsh performance requirements of EBC which brings a significant challenge to their applications. Based on our previous research on single-component RE₂SiO₅ ceramics, (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ solid solution was designed and successfully fabricated in this work. Doping of multiple RE elements endows (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ with excellent thermal insulation properties and matched thermal expansion coefficient with SiC_f/SiC substrates. In addition, it exhibits lower elastic modulus and comparable hardness than that of single-component RE₂SiO₅. (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ also presents good resistance to calcium-magnesium alumino-silicates (CMAS) corrosion. Rational composition design allows (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ to retain the merits of single-component RE₂SiO₅ while taking advantage of the solid solution effect. The results of this work suggest (Ho_0.4Yb_0.3Lu_0.3)₂SiO₅ as a promising EBC candidate.     

Structure,thermal properties and hot corrosion behaviors of Gd2Hf2O7 as a potential thermal barrier coating material

In this work, Gd₂Hf₂O₇ ceramics were synthesized and investigated as a potential thermal barrier coating (TBC) material. The phase composition, microstructure and associated thermal properties of Gd₂Hf₂O₇ ceramics were characterized systematically. Results show that the thermal conductivity of Gd₂Hf₂O₇ ceramics is 1.40 Wm⁻¹K⁻¹ at 1200 °C, ~25% lower than that of 8 wt% yttria partially stabilized zirconia (8YSZ). Gd₂Hf₂O₇ ceramics also present large thermal expansion coefficients, which decrease from 12.0 × 10⁻⁶ K⁻¹ to 11.3 × 10⁻⁶ K⁻¹ (300–1200 °C). Besides, the hot corrosion behaviors of Gd₂Hf₂O₇ ceramics exposed to V₂O₅ and Na₂SO₄ + V₂O₅ salts at temperatures of 900–1200 °C were discussed in great detail. We pay much attention on the corrosion process, corrosion mechanism and corrosion damage of Gd₂Hf₂O₇ ceramics subjected to molten V₂O₅ and Na₂SO₄ + V₂O₅ salts at different temperatures.     

Investigation on bi-layer coating with La2(Ti0.2Zr0.2Sn0.2Ce0.2Hf0.2)2O7/YSZ prepared by laser cladding

Thermal barrier coatings are an effective technology for improving the high-temperature performance of hot section components in gas turbine engine. Due to their excellent properties, high-entropy oxides are considered to be promising materials for thermal barrier coatings. Laser cladding is a coating preparation technology and the top coat prepared by laser cladding technology has an important application value for thermal barrier coatings. In this work, to improve the thermal cycling behavior of the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide coating, a bi-layer coating with the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide layer and the YSZ layer was designed and fabricated by laser cladding on the NiCoCrAlY alloy surface. The microstructure, phase and mechanical properties of the coating were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and micro-hardness and nanoindentation tests, respectively. The results show that a bi-layer La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇/YSZ coating was successfully prepared by the laser cladding method, and shows good bonding at the interface between the layers. The high-entropy oxide layer maintains a relatively stable defective fluorite structure and its microstructure exists in the stable cellular and dendrite crystalline state after laser cladding. The high-entropy oxide layer prepared by laser cladding showed an average elastic modulus of 167 GPa and an average hardness of 1022.8HV in nanoindentation tests. Thermal cycling of the coating was carried out at 1050 °C. Failure of the bi-layer coating occurred after 60 thermal cycles at 1050 °C. Thermal stresses between different layers are calculated during thermal cycling. Due to its excellent mechanical properties, the bi-layer coating with the La₂(Ti_0.2Zr_0.2Sn_0.2Ce_0.2Hf_0.2)₂O₇ high-entropy oxide and YSZ layers is expected to become an effective high-entropy oxide thermal barrier coating.     

High-temperature Na2SO4 interaction with air plasma sprayed Yb2Si2O7 + Si EBC system: Topcoat behavior

The high-temperature interaction between ~2.5 mg/cm² of Na₂SO₄ and an atmospheric plasma sprayed (APS) Yb₂Si₂O₇ topcoat–Si bond coat system on SiC CMC substrates was studied for times up to 240 h at 1000°C–1316°C in a 0.1% SO₂–O₂ gaseous environment. Yb₂Si₂O₇ reacted with Na₂SO₄ to form Yb₂SiO₅ and an intergranular amorphous Na-silicate phase. Below 1200°C, the reaction was sluggish, needing days to cause morphological changes to the “splat microstructure” associated with APS coatings. The reaction was rapid at 1200°C and above, needing only a few hours for the entire topcoat to transform into a granulated microstructure consisting of Yb₂SiO₅ and Yb₂Si₂O₇ phases. Na₂SO₄ deposits infiltrated the Yb₂Si₂O₇ topcoat and transformed into an amorphous Na-silicate in less than 1 h at all exposure temperatures. Quantitative assessment of the Yb₂SiO₅ area fraction in the topcoat showed a linear decrease over time at 1316°C, attributed to reaction with the SiO₂ thermally grown oxide (TGO) formed on the Si bond coat and rapid transport through the interpenetrating amorphous Na-silicate grain boundary phase. It was predicted that nearly 2 weeks is needed for complete removal of Yb₂SiO₅ from the topcoat at 1316°C for a single applied loading of Na₂SO₄.