Cyclic oxidation performances of new environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 coated SiC at 1375 °C and 1475 °C in the air environment

The objective of this work is to study the cyclic oxidation performances of the environmental barrier coatings (EBCs) containing the novel HfO₂-SiO₂ bond coats in the air environment. Bi-layer HfO₂-SiO₂/Yb₂Si₂O₇ (50HfO₂-50SiO₂, 70HfO₂-30SiO₂ bond coats) and conventional Si/Yb₂Si₂O₇ EBCs were deposited on SiC substrate using atmospheric plasma spray. The effect of the pre-mixing ratios of HfO₂/SiO₂ on the cyclic oxidation behavior of HfO₂-SiO₂/Yb₂Si₂O₇ EBCs was examined. The results showed that the higher content of the HfSiO₄ formed from the 50HfO₂-50SiO₂ bond coats, and it remained intact. A thermally grown oxide (TGO) SiO₂ layer was formed at the bond coat/SiC interface. The parabolic oxidation rate constant (k_p, μm²/h) of the TGO has been reduced 2 orders of magnitude in 50HfO₂-50SiO₂/Yb₂Si₂O₇ EBCs coated SiC compared to the bare SiC at 1475 °C, indicating that the 50HfO₂-50SiO₂/Yb₂Si₂O₇ EBCs effectively protected the SiC substrate at 1475 °C.     

Effect of HfO2 framework on steam oxidation behavior of HfO2 doped Si coating at high temperatures

HfO₂ doped Si is designed as bond coat material in thermal/environmental barrier coatings (TEBCs). In this work, the HfO₂-Si composite coatings with different HfO₂ contents were prepared by atmospheric plasma spraying (APS). The steam oxidation behavior of the coatings was comparatively studied at 1300 °C and 1400 °C. Volatilization of Si occurred during spraying, leading to the deviation of coating compositions. The sprayed coatings contained different HfO₂ structures. During steam oxidation, HfSiO₄ phase was formed at the SiO₂/HfO₂ interface by solid-state reaction between them. The HfSiO₄ or HfO₂/HfSiO₄ mixture particles worked to deflect or pin micro-cracks, thus improving the resistance of the coating to cracking. At 1300 °C, a protective oxide scale was formed on the traditional Si coating or the HfO₂-Si coating with isolated HfO₂ particles. However, the HfO₂-Si coating with inter-connected HfO₂ framework revealed poor oxidation-resistance. At 1400 °C, accelerated oxidation degradation, steam corrosion volatilization, interface reaction and sintering occurred. The HfO₂ framework structure played a dominating role in determining the steam oxidation resistance of the HfO₂-Si coating, and the connected HfO₂ framework and TGO network provided a rapid diffusion path for oxidants (H₂O, O^2? and OH^?) and deteriorated the oxidation resistance.     

Research status of bond coats in environmental barrier coatings

Bond coats in environmental barrier coatings (EBCs) prevent oxidants from penetrating the substrate, mediate the mismatch of the coefficient of thermal expansion (CTE), and improve the adhesion strength between adjacent layers. However, the development of bond coats is rarely studied systematically. In this paper, the research status of the bond coats in EBCs is introduced in detail, including the materials and deposition methods. Thus far, Si, modified-Si, mullite, etc., have been employed as bond coats. Nevertheless, visible drawbacks of each bond coat limit their application at high-temperatures in extreme environments. Si bond coat is easily oxidized and forms thermally grown oxides that form cracks, resulting in delamination, spallation, and failure of EBCs. In the Si–HfO₂ bond coat, the optimal ratios of Si/HfO₂, deposition methods, distribution of Si and HfO₂, and oxidation of Si remain completely unsolved. For mullite bond coat, SiO₂ suffers selective evaporation in the water vapor environment, and the ratios of the Al₂O₃ and SiO₂ in mullite coatings restrict its service lifetime. HfSiO₄ is a potential candidate acting as a next-generation bond coat in EBCs is proposed. Furthermore, choosing reasonable deposition methods is beneficial to improve the performances of the bond coats in EBCs.     

High-temperature performances of Si-HfO2-based environmental barrier coatings via atmospheric plasma spraying

To improve high-temperature bearing capability of coatings, novel agglomerated Si-HfO₂ powders were prepared by adding HfO₂ powders into original Si powders by spray drying method. Three-layer environmental barrier coatings (EBCs) with Si-HfO₂ bond layer, Yb₂Si₂O₇ intermediate layer and Yb₂SiO₅ surface layer were prepared on SiC ceramic substrates by atmospheric plasma spraying (APS). The high temperature properties of coatings were systematically investigated. The results indicated that the coatings had good high temperature oxidation resistance, and remained intact after being oxidized or steam corrosion at 1400 °C for 500 h, so the addition of HfO₂ improved the thermal cycling performances of the coating. The HfO₂ in Si bond coating could effectively inhibit the growth of thermal grown oxide at high temperatures. This work indicates that the high temperature properties of the coatings are improved by this novel EBCs using the novel agglomerated Si-HfO₂ powders.     

Thermal cycle test performances of environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 in the steam environment at 1475 °C

The novel bi-layer environmental barrier coatings (EBCs) with HfO₂-SiO₂/Yb₂Si₂O₇ structure (70HfO₂-30SiO₂/Yb₂Si₂O₇: 70HS/YbDS, 50HfO₂-50SiO₂/Yb₂Si₂O₇: 50HS/YbDS, molar ratios) was tested in 90%H₂O–10%O₂ conditions between room temperature and 1475 °C in an Al₂O₃ tube furnace, then its performance was evaluated. The YbDS layer was contaminated by alumina impurities under steam conditions. After 22 cycles, the 70HS/YbDS completely separated from the SiC substrate, while the 50HS/YbDS and SiC did not separate, even though cracks formed at the 50HS/SiC interface and the TGO layer. Furthermore, the thermally grown oxide (TGO) layer formed at the HfO₂-SiO₂/SiC interface. Formation and growth of the TGO led to the formation and propagation of cracks at the HfO₂-SiO₂/TGO interface and TGO interior, which was the culprit leading to the failure of EBCs. These results demonstrated that the 50HS/YbDS EBCs have the potential to protect SiC in steam conditions at 1475 °C.     

Mechanical properties and microstructure of Yb2SiO5 environmental barrier coatings under isothermal heat treatment

Yb₂SiO₅ (ytterbium monosilicate) top coatings and Si bond coat layer were deposited by air plasma spray method as a protection layer on SiC substrates for environmental barrier coatings (EBCs) application. The Yb₂SiO₅-coated specimens were subjected to isothermal heat treatment at 1400 °C on air for 0, 1, 10, and 50 h. The Yb₂SiO₅ phase of the top coat layer reacted with Si from the bonding layer and O₂ from atmosphere formed to the Yb₂Si₂O₇ phase upon heat treatment at 1400 °C. The oxygen penetrated into the cracks to form SiO₂ phase of thermally grown oxide (TGO) in the bond coat and the interface of specimens during heat treatment. Horizontal cracks were also observed, due to a mismatch of the coefficient of thermal expansion (CTE) between the top coat and bond coat. The isothermal heat treatment improves the hardness and elastic modulus of Yb₂SiO₅ coatings; however, these properties in the Si bond coat were a little bit decreased.     

Cyclic ablation behavior of mullite-modified C/C-HfC-SiC composites under an oxyacetylene flame at about 2400 °C

Mullite-modified C/C-HfC-SiC composites were prepared via precursor infiltration and pyrolysis (PIP). The phase composition, microstructure, and cyclic ablation behavior under oxyacetylene flame with a heat flux of 4.18 MW/m² were investigated, and a comparison with the C/C-HfC-SiC composites showed that the mullite-modified composites have better ablation resistance. Results displayed a HfO₂ skeleton structure wrapped by SiO₂ and a dense layer of HfO₂-SiO₂ in the center and transition regions after the first ablation for 30 s, respectively. The structures transformed into HfSiO₄-wrapped carbon fibers and "island" shape HfO₂-HfSiO₄-SiO₂ layer after the second ablation for 30 s. Then both structures underwent severer peeling of HfSiO₄ and consumption of SiO₂ after the third ablation for 30 s. The modified composites exhibited better mass ablation rates after forming HfSiO₄, which were 0.36 mg/s cm² and 0.38 mg/s cm², respectively.     

Exploration of the oxidation behavior and doping ratio of the Si–HfO2 bond layer used in environmental barrier coatings

Hafnia doping is expected to improve the performance of the silicon-bond layer of environmental barrier coatings (EBCs) for SiC-based ceramic matrix composites. The optimal doping ratio, distribution of HfO₂, and oxidation mechanism of the bond layer have not yet been fully addressed. A prototype Si–HfO₂ bond layer with a designed HfO₂-rich area was used to examine its oxidation behavior. A random dispersion model was developed to calculate the optimal HfO₂ doping ratio and its appropriate distribution state. The simulation results recommended that 20–30 vol% is the optimal doping ratio, where HfO₂ is well dispersed inside Si without forming networks. This enables HfO₂ to react with and consume SiO₂ without accelerating oxygen diffusion inside the bond layer. This was confirmed by oxidation experiments on Si–xHfO₂ tablets, in which the thinnest thermally grown oxide was achieved for the 20 vol% HfO₂-doped Si tablet. Both the microstructure design and material composition selection are highly important to further boost the performance of the EBCs.     

Preparation,properties and high-temperature oxidation resistance of MoSi2-HfO2 composite coating to protect niobium using spent MoSi2-based materials

Industrial spent MoSi₂-based materials and HfO₂ were recycled as raw materials to fabricate MoSi₂-HfO₂ composite coating by spark plasma sintering (SPS). The microstructural evolution of the coatings was characterized and the 1500 °C oxidation behavior was explored. Cracks penetrated through the MoSi₂ coating while no cracks can be found in the HfO₂-containing composite coating owing to the reduction of the mismatch of thermal expansion coefficient (CTE). Good metallurgical bonding was exhibited since (Mo,Nb)₅Si₃ diffusion layer was found in the HfO₂-containing coating by the diffusion of Nb and Si across the interface without gaps. After 1500 °C oxidation of 20 h, cracks appeared in the surface of SiO₂ layer on MoSi₂ coating while the HfO₂-containing composite coating possessed crack-free oxide scale. HfSiO₄ with high temperature (>2900 °C) is formed during oxidation and it inlays in the silica oxide scale to improve the stability. Compared to MoSi₂ coating, Nb coated MoSi₂-HfO₂ has thinner oxide scale with lower mass gain during oxidation, thus presenting better high-temperature anti-oxidation properties.     

Thermochemical degradation of HfSiO4 by molten CMAS

The thermochemical degradation of hafnium silicate (HfSiO₄) was investigated with a molten calcium-magnesium-aluminosilicate (CMAS) glass relevant to gas turbine engine applications. Sintered HfSiO₄ coupons were fabricated, within which wells were drilled and filled with CMAS glass powder at a loading of ～35 mg/cm². Samples were heat treated at 1200°C, 1300°C, 1400°C, and 1500°C for 1 h, 10 h, and 50 h. At 1200°C and 1300°C, slow formation of a Ca₂HfSi₄O₁₂ cyclosilicate phase was observed at the HfSiO₄-CMAS interface. At 1300°C and higher, rapid infiltration of CMAS into the material along the grain boundaries was observed. Initial conjecture into CMAS degradation mechanisms of HfSiO₄ are presented herein.     

Nitridation of silicon powder studied by XRD, 29Si MAS NMR and surface analysis techniques

The nitridation of elemental silicon powder at 900–1475 °C was studied by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), XRD, thermal analysis and ²⁹Si MAS NMR. An initial mass gain of about 12% at 1250–1300 °C corresponds to the formation of a product layer about 0·2 μm thick (assuming spherical particles). XPS and XAES show that in this temperature range, the surface atomic ratio of N/Si increases and the ratio O/Si decreases as the surface layer is converted to Si₂N₂O. XRD shows that above 1300 °C the Si is rapidly converted to a mixture of α- and β-Si₃N₄, the latter predominating >1400 °C. In this temperature range there are only slight changes in the composition of the surface material, which at the higher temperatures regains a small amount of an oxidised surface layer. By contrast, in the interval 1400–1475 °C, the ²⁹Si MAS NMR chemical shift of the elemental Si changes progressively from about −80 ppm to −70 ppm, in tandem with the growth of the Si₃N₄ resonance at about −48 ppm. Possible reasons for this previously unreported change in the Si chemical shift are discussed. ©     

Dry air cyclic oxidation of mixed Y/Yb disilicate environmental barrier coatings and bare silica formers

Mixed Y and Yb disilicate coatings (Y/Yb)DS have been proposed as dual function thermal and environmental barrier coatings (EBCs) for protecting SiC-based ceramic matrix composites in gas-turbine environments. As an initial step, the 1350 °C dry air cyclic oxidation of atmospheric plasma sprayed (Y_1.2/Yb_0.8)DS and ytterbium disilicate/ytterbium monosilicate (YbDS/YbMS) EBCs deposited onto Si bond coatings was compared. As a baseline for evaluating EBC oxidant permeability, the dry air cyclic oxidation scale growth rates for bare silica formers (SiC, Si) were also measured and were consistently higher than rates previously measured after isothermal oxidation. Regarding Si bond coat oxidation rates underlying (Y/Yb)DS and YbDS/YbMS EBCs, the thinner silica scale formed under the thinner and denser (Y/Yb)DS coatings suggested a lower oxidant permeability than YbDS/YbMS. After 500 1-h cycles, the (Y/Yb)DS coating was comprised of only the β-polymorph disilicate and minor amounts of the X-2 phase monosilicate phase. Negligible differences in oxidation kinetics for (Y/Yb)DS coatings over the 90 – 240 µm thickness range were observed.     

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.     

Environmental barrier coatings using low pressure plasma spray process

Yb₂Si₂O₇/Si bilayer environmental barrier coatings (EBCs) on SiC ceramic substrate were produced by low pressure plasma spray (LPPS) process. Phase composition, microstructure, and thermal durability of LPPS Yb₂Si₂O₇/Si coating were investigated. XRD analysis indicated that the coating is mainly composed of Yb₂Si₂O₇ with ~15.5v% Yb₂SiO₅ phases. The LPPS EBCs have a dense microstructure with porosity less than 4%. Adhesion strength measurement indicated the LPPS EBCs have an average adhesion strength of 29.1 ± 0.8 MPa. Furnace cycle test (FCT) on the coatings in air at 1316°C was performed and the test ran for 900 cycles and there was no coating spallation/failure for LPPS Yb₂Si₂O₇/Si EBCs. The FCT results demonstrated the excellent thermal cycle durability of LPPS EBCs. Oxidation kinetics investigation of LPPS EBCs in flowing 90% H₂O (g)+10% air at 1316°C showed that the thermally grown oxide (TGO) growth rate is close to the oxidation rate of pure Si in dry air and is significantly lower than that in water vapor environment. The LPPS process is promising in making highly durable Yb2Si2O7-based dense EBCs by impeding diffusion and ingression of water vapor/O₂.     

Interaction of Gd2Si2O7 with CMAS melts for environmental barrier coatings

Environmental barrier coatings (EBCs) prevent the oxidation of ceramic matrix composites (CMC), which are used as components in gas turbines. However, EBCs deteriorate more rapidly in real environments, molten silicate deposits accelerate the deterioration of EBCs. In this study, high-temperature behavior sintered Gd₂Si₂O₇ with calcia-magnesia-alumina-silica (CMAS) melt at 1400 °C for 0.5, 2, 12, 48, and 100 h was investigated. HT-XRD results showed that at 1300 °C, CMAS and Gd₂Si₂O₇ chemically reacted to form Ca₂Gd₈(SiO₄)₆O₂ (apatite). The reaction layer became thicker as the heat-treatment time increased, and the thickness of the reaction layer has increased following a parabolic curve. With the extension of the reaction time from 0.5 to 100 h, the thickness of the reaction layer increased from approximately 98 to 315 µm. It was confirmed that Ca₂Gd₈(SiO₄)₆O₂ grew vertically on the Gd₂Si₂O₇ surface. Vertical and horizontal cracks were found after reacting at 1400 °C for 100 h, but no interfacial delamination occurred in this study. In addition, the effects of CaO:SiO₂ molar ratios, monosilicates (RE₂SiO₅) and disilicates (RE₂Si₂O₇), heat-treatment time, and cation size were determined and compared with the results of previous studies (Gd₂SiO_5, Yb₂SiO_5, and Er₂Si₂O₇).     

Low temperature pressureless sintering of dense silicon nitride using BaO-Al2O3-SiO2 glass as sintering aid

Dense Si₃N₄ ceramic with BaO-Al₂O₃-SiO₂ low temperature glass powders as sintering aids were prepared by pressureless sintering techniques at a relatively low temperature (1550 °C). Four kinds of glass powders of compositions melting at 1120 °C, 1300 °C, 1400 °C and 1500 °C, respectively, have been introduced as sintering aids. XRD results demonstrate that the BaO-Al₂O₃-SiO₂ glass powders reacted with BaAl₂O₄ and converted into hexagonal celsian, which is a high-temperature phase with melting point of 1760 °C, so being beneficial to the high temperature properties of the materials. In addition, a portion of α-Si₃N₄ transformed to rod like β-Si₃N₄ with high aspect ratio as shown by XRD and SEM analysis. The bulk density increased with the rise of the melting temperature of the BaO-Al₂O₃-SiO₂ glass powders, the sample obtained with the BaO-Al₂O₃-SiO₂ glass powder melting at 1500 °C reaching a maximum density of 98.8%, an high flexural strength (373 MPa) and a fracture toughness (4.8 MPa m^1/2).     

High-temperature water-vapor reaction mechanism of barium strontium aluminosilicate (BSAS)

Environmental barrier coatings (EBCs) are used in commercial turbine engine applications as protection for ceramic matrix composites, yet the high-temperature water vapor reaction mechanism for EBC materials is not fully understood. Here, the water vapor reaction mechanism for barium strontium alumino-silicate (BSAS), an early generation EBC candidate, was determined from the time and temperature dependences of material loss. BSAS water vapor exposures were performed at 1200 °C, 1300 °C, and 1400 °C for 24, 48, and 72 h, at maximum gas velocities of ~ 240 m/s. FactSage thermodynamic calculations were shown to support the experimental findings, where the steam reaction mechanism consisted of volatilization of all BSAS oxide constituents as gaseous metal hydroxide species, i.e. Ba(OH)₂, Sr(OH)₂, Al(OH)₃, and Si(OH)₄ (g).     

Corrosion behaviour of chromium-free ceramics for liquid slag removal in Pressurized Pulverized Coal Combustion

The corrosion behaviour of HfO₂, HfSiO₄, ZrSiO₄, NiAl₂O₄ and, for comparison, a commercial Cr₂O₃-containing ceramic was investigated under Pressurized Pulverized Coal Combustion (PPCC) conditions at 1450 °C in the presence of liquid slag. Although Cr₂O₃-containing ceramics show sufficient corrosion resistance, they have the main disadvantage of vaporisation of toxic Cr(VI)-species. Among the investigated chromium-free ceramics, HfO₂ and HfSiO₄ showed relatively good corrosion resistance in laboratory investigations. The stability of the latter one depends not only on the microstructure of the ceramic but also on the slag composition. Technically produced ceramic balls of HfO₂ and ZrSiO₄ performed very well in the liquid slag separator of a PPCC test facility.     

Environmental barrier coatings on enhanced roughness SiC: Effect of plasma spraying conditions on properties and performance

Environmental barrier coatings for SiC/SiC composites are limited by the melting temperature of the Si bond coating near 1414 °C. Systems without a bond coating may be required for future turbine applications where material temperatures go beyond 1350 °C. Enhanced roughness SiC substrates were developed to assess coating adhesion without the bond coating. Two EBCs with different YbMS/YbDS ratios were produced via modified plasma spraying parameters. Coating microstructure, thermal expansion, and modulus were measured for comparison of coating properties. Cyclic steam exposures at 1350 °C were performed to assess oxidation resistance. The EBC with increased concentration of Yb₂SiO₅ secondary phase displayed a higher CTE, which is typically expected to decrease adhesion lifetimes due to an increase in stress upon thermal cycling. Yet, the EBC chemistry with increased Yb₂SiO₅ concentration was able to experience longer cycling times prior to coating delamination, likely due to interface interactions with the substrate and the thermally grown oxide.     

Microstructure,mechanical and optical properties of MgAl2O4/MgAl2O4 joints bonded using CaO-Al2O3-SiO2 glass filler

Transparent MgAl₂O₄ ceramics were bonded by using CaO-Al₂O₃-SiO₂ (CAS) glass filler. The CAS glass filler exhibited the same thermal expansion behavior as MgAl₂O₄ ceramic and excellent wetting ability on the surface of MgAl₂O₄ ceramic. When the cooling rate of 15 °C/min was used, no interfacial reaction was observed and the amorphous brazing seam could be obtained. However, low joining temperature (1250 °C) led to the formation of pores and high joining temperature (1400 °C) resulted in the formation of cracks. Furthermore, the slow cooling rate of 5–10 °C/min induced the crystallization of CaAl₂Si₂O₈ and Mg₂Al₄Si₅O₁₈ due to the dissolution of MgAl₂O₄ substrate. The optimal flexural strength of 181–189 MPa was obtained when the joining temperature and cooling rate were 1300–1350 °C and 15 °C/min respectively. Moreover, the in-line transmittance of the joint at 1000 nm was 82.1%, which was slightly lower than that of MgAl₂O₄ ceramic (85.6%).