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Ytterbium disilicate (Yb2Si2O7) is a potential environmental barrier coating (EBC) material. In this paper, the microstructure and phase composition of plasma‐sprayed Yb2Si2O7 coatings with different spraying process were systematically investigated to optimize the spraying power and spraying distance. Furthermore, in order to obtain Yb2Si2O7 coating with single phase, a certain proportion of SiO2 was added to the Yb2Si2O7 powder to compensate for the volatilization of SiO2 during plasma‐spraying process, and the influence of spraying powder composition on the microstructure and phase structure (composition and content) of Yb2Si2O7 coatings were also characterized in detail. The results showed that the Yb2Si2O7 coating was dense with fewer pores and cracks at the spraying power of 40 kW and the spraying distance of 100 mm. Yb2Si2O7 coating with the "sandwich" structure was composed of Yb2Si2O7, Yb2Si2O7 + Yb2SiO5, and Yb2SiO5 three microstructures, which was mainly caused by the volatilization of SiO2 in Yb2Si2O7 spraying powder. The Yb2Si2O7 coating contained 90.68 mol.% Yb2Si2O7 phase and 9.32 mol.% Yb2SiO5 phase after crystallization at 1300°C for 20 h. The coating was composed of Yb2Si2O7 phase and amorphous phase after SiO2 addition. Yb2SiO5 phase precipitated from the coating after heat treatment at 1300°C, and its content decreased with the increase of SiO2 content. The content of the Yb2Si2O7 phase was as high as 99.41 mol.% when SiO2 content was 10 mol.%, which would provide a scientific basis for further research on stoichiometric Yb2Si2O7 EBCs with excellent properties. [ABSTRACT FROM AUTHOR]

In this study, Al2O3-based coatings with varying TiO2 contents (0, 3, 13, and 40%) were fabricated using atmospheric plasma spraying technique. To compare the superiority of the samples, their mechanical (Vickers hardness and adhesion strength) and thermal properties (thermal conductivity and thermal shock resistance) were characterized. As observed, Al2O3-40%TiO2 (A-40T) coating exhibited relatively inferior mechanical properties but superior thermal insulation and thermal shock resistance at 600°C. According to the microstructure and phase analysis, this finding is attributed to the special phase, Al2TiO5, and preexisting microcracks inside the coating. Thus, A-40T manifested excellent characteristics for thermal insulation application compared with pure Al2O3 and low-TiO2 content coatings. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The novel bi-layer environmental barrier coatings (EBCs) with HfO 2 -SiO 2 /Yb 2 Si 2 O 7 structure (70HfO 2 -30SiO 2 /Yb 2 Si 2 O 7 : 70HS/YbDS, 50HfO 2 -50SiO 2 /Yb 2 Si 2 O 7 : 50HS/YbDS, molar ratios) was tested in 90%H 2 O–10%O 2 conditions between room temperature and 1475 °C in an Al 2 O 3 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 HfO 2 -SiO 2 /SiC interface. Formation and growth of the TGO led to the formation and propagation of cracks at the HfO 2 -SiO 2 /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. [ABSTRACT FROM AUTHOR]

Environmental barrier coating systems (EBCs) have been developed to protect SiC-based ceramics exposed to harsh environments. A series of novel EBCs containing 50HfO 2 -50SiO 2 (molar ratio, 50HS) bond coat and Y x Yb (2−x) Si 2 O 7 (x = 0.3, 0.5, 0.8, and 1.0) topcoat layers were deposited on SiC substrate using atmospheric plasma spraying in this work. Cyclic oxidation of EBCs was performed between room temperature and 1475 °C under dry air and 90%H 2 O-10%O 2 conditions, respectively. Monosilicate (Y/Yb) 2 SiO 5 formed during the Y x Yb (2−x) Si 2 O 7 deposition. The Re 2 O 3 -stabilized HfO 2 solid solution with a high coefficient of thermal expansion formed at the Y x Yb (2−x) Si 2 O 7 /50HS interface due to the reaction between monosilicate (Y/Yb) 2 SiO 5 and HfO 2 originating from the 50HS bond coat, which was detrimental to EBCs. Furthermore, the thermally grown oxide layer was found at the bond coat/SiC interface, and its formation mechanism has been studied. The results showed that the novel 50HS/Y x Yb (2−x) Si 2 O 7 EBCs could effectively protect the SiC substrate at 1475 °C. [ABSTRACT FROM AUTHOR]

The objective of this work is to study the cyclic oxidation performances of the environmental barrier coatings (EBCs) containing the novel HfO 2 -SiO 2 bond coats in the air environment. Bi-layer HfO 2 -SiO 2 /Yb 2 Si 2 O 7 (50HfO 2 -50SiO 2 , 70HfO 2 -30SiO 2 bond coats) and conventional Si/Yb 2 Si 2 O 7 EBCs were deposited on SiC substrate using atmospheric plasma spray. The effect of the pre-mixing ratios of HfO 2 /SiO 2 on the cyclic oxidation behavior of HfO 2 -SiO 2 /Yb 2 Si 2 O 7 EBCs was examined. The results showed that the higher content of the HfSiO 4 formed from the 50HfO 2 -50SiO 2 bond coats, and it remained intact. A thermally grown oxide (TGO) SiO 2 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 50HfO 2 -50SiO 2 /Yb 2 Si 2 O 7 EBCs coated SiC compared to the bare SiC at 1475 °C, indicating that the 50HfO 2 -50SiO 2 /Yb 2 Si 2 O 7 EBCs effectively protected the SiC substrate at 1475 °C. [ABSTRACT FROM AUTHOR]

A key strategy to design novel bond coating of advanced environmental barrier coatings (EBC) focuses on enhancing its service temperature. The SiO 2 -HfO 2 /Y 0.5 Yb 1.5 Si 2 O 7 EBC system consists of the novel SiO 2 -HfO 2 composite bond coating using electron beam-physical vapor deposition and Y 0.5 Yb 1.5 Si 2 O 7 top coating with atmospheric plasma spraying. EBC was exposed to steam environments at ultra-high temperature of 1475 °C. Microstructure and service performance of EBC after exposure to steam environment were systematically analyzed. The results show that the novel EBC improves the service temperature of EBC and effectively protects SiC substrate, which is a promising candidate for EBC enhancing the service temperature. • The novel advanced EBC composites of Electron beam-physical vapor deposition (EB-PVD) SiO 2 -HfO 2 bond coating and atmospheric plasma spraying (APS) top coating of Y 0.5 Yb 1.5 Si 2 O 7 is designed and prepared, the performance of EBC at the ultra-high temperature of 1475 °C in steam conditions is characterized. • HfSiO 4 successfully formed in SiO 2 -HfO 2 bond coating after heat-treatment at 1475 °C, which improves the operation temperature of current EBC from 1300 °C to 1475 °C. • Novel advanced EBC could provide effective protection for SiC substrate for 450 h (10 h = 1 cycle) at 1475 °C in steam conditions. The thermally grown oxide formation and failure mechanism of EBC are systemically studied. [ABSTRACT FROM AUTHOR]

As the operating temperature of advanced gas turbines typically exceeds 1400 °C, it has been required to replace conventional Si bond coat in environmental barrier coatings (EBCs) with materials possessing higher thermal stability. Since HfSiO 4 has excellent thermal properties such as a high melting point, phase stability over 1400 °C, and CTE matches with that of the SiC-based ceramic matrix composites, it has attracted much attention as a next-generation bond coat material. In this study, HfSiO 4 bond coat was successfully formed by atmospheric plasma spray with pre-mixed HfO 2 -SiO 2 powders (molar ratios: 7:3 and 5:5) followed by heat treatment. Effect of molar ratios of the HfO 2 -SiO 2 and post-heat treatment temperature (1375 and 1475 °C) on the formation of HfSiO 4 were studied. An oxidation test of the HfSiO 4 coating was carried out at 1475 °C with the conventional Si bond coat to verify whether the new bond coat was suitable for use in a thermal environment of 1400 °C or higher. From the results, the HfO 2 /SiO 2 ratio of 5:5 was suitable for the formation of HfSiO 4 than that of 7:3. After heat treatment at 1475 °C, the ratio of HfSiO 4 phase was 84.35%. The higher content of HfSiO 4 formed under 1475 °C, meaning the higher heat treatment temperature accelerated the HfSiO 4 formation. In the oxidation test at 1475 °C, the new HfSiO 4 bond coat showed no cracks and maintained its integrity, but the Si bond coat was oxidized and cracked severely. Therefore, it can be concluded that the new HfSiO 4 bond coat formed from 5HfO 2 –5SiO 2 coating is a potential candidate as a next-generation bond coat material in EBCs. [ABSTRACT FROM AUTHOR]

The molten calcium-magnesium-aluminosilicate (CMAS) corrosion resistance of a series of Y x Yb (2−x) Si 2 O 7 (x = 0.3, 0.5, 0.8, 1.0) environmental barrier coating (EBC) materials were systematically investigated at 1475 °C. The results have shown that the precipitate apatite Ca 2 (Y/Yb) 8 (SiO 4) 6 O 2 formed after CMAS corrosion of Y x Yb (2−x) Si 2 O 7 pellets. The reactivity of Y x Yb (2−x) Si 2 O 7 pellets and CMAS increased with the increase of Y3+/Yb3+ ratio, leading to the thickest reaction layer formed in Y 1.0 Yb 1.0 Si 2 O 7. Furthermore, "blister" cracks were not found at the zone of uncorroded Y x Yb (2−x) Si 2 O 7 pellets. It demonstrated that the Y x Yb (2−x) Si 2 O 7 materials have excellent CMAS corrosion resistance, which can serve as promising candidates for EBC against CMAS corrosion. • The prepared Y x Yb (2−x) Si 2 O 7 (x = 0.3, 0.5, 0.8, 1.0) disilicate pellets have a stable phase at high temperatures. • The Y3+/Yb3+ ratios in the Y x Yb (2−x) Si 2 O 7 pellets determined the reaction layer thickness at 1475 °C. • The reactivity of Y x Yb (2−x) Si 2 O 7 pellets with the molten CMAS was studied based on the optical basicity (OB) theory. • The Y x Yb (2−x) Si 2 O 7 pellets show excellent corrosion resistance with the molten CMAS at 1475 °C, which can serve as promising candidates for the next generation of EBCs. [ABSTRACT FROM AUTHOR]

The performance of 50HfO 2 -50SiO 2 /Y x Yb (2−x) Si 2 O 7 thermal environmental barrier coating prepared using atmospheric plasma spraying was investigated in the Al 2 O 3 tube furnace in 90%H 2 O-10%O 2 conditions at 1475 °C. The results showed that Y x Yb (2−x) Si 2 O 7 layer was contaminated by alumina impurities during the testing process. The thermally grown oxide layer was formed at the bond coat/SiC interface, it grew as the cycles increased. Furthermore, the (Y/Yb) 2 O 3 -HfO 2 solid solution with a higher coefficient of thermal expansion was formed at Y x Yb (2−x) Si 2 O 7 /50HfO 2 -50SiO 2 interface. The horizontal cracks were the principal factor that caused the failure of 50HfO 2 -50SiO 2 /Y x Yb (2−x) Si 2 O 7. 50HfO 2 -50SiO 2 /Y x Yb (2−x) Si 2 O 7 protected SiC for 25 cycles in steam conditions. • The APS prepared Y x Yb (2−x) Si 2 O 7 topcoat layer showed a lower thermal conductivity than the conventional YbDS. • The thermally grown oxide (TGO) layer was formed at the bond coat/SiC interface due to the oxidation of SiC substrate, it grew as the cycles increased. • 50HfO 2 -50SiO 2 /Y x Yb (2−x) Si 2 O 7 protected SiC for 25 cycles (250 h) in steam conditions at 1475 °C. [ABSTRACT FROM AUTHOR]

Oxidation resistance of TiC–SKD11 composite has been improved through SiC/TiB 2 based hybrid coating by a simple air spray process and subsequent transition to dense and stable phases in oxidation environment. As a result of the oxidation test at 700 °C for up to 50 h, the coated TiC–SKD11 composite exhibited excellent oxidation resistance characteristics compared with the uncoated TiC–SKD11 composite, and also compared to SUS431 and 17-4PH. The formation of a thermally stable, volume-expanded SiO 2 and TiO 2 coating layer with the aid of B 2 O 3 by phase transition of TiB 2 /SiC based hybrid coating under oxidation atmosphere is the origin of the improved anti-oxidation ability of the TiC–SKD11 composite. [ABSTRACT FROM AUTHOR]

The hot corrosion behavior of two kind of HVOF-sprayed NiCr-based coatings with different oxide-forming elements (such as Al and Si) was investigated and compared in the presence of KCl salt at 700 ℃. The results showed that the hot corrosion resistance of NiCrBSi coating was superior to that of NiCrAlY coating. NiCrAlY coating was seriously corroded and severe internal oxidation was produced along the splat boundaries, while the corrosion of NiCrBSi coating was relatively mild and the inner coating remained intact, which was ascribed to the denser structure and the generation of a SiO 2 protective layer. [Display omitted] • NiCrBSi coating offered better corrosion resistance to KCl than NiCrAlY coating. • Chromia and alumina failed to resist the hot corrosion ingress in coating. • Alumina is difficult to form a protective oxide film at lower temperatures in KCl salt. • The continuous SiO 2 layer effectively blocks the progress of activation oxidation. • SiO 2 -forming materials could be a promising material in alkali chloride salt. [ABSTRACT FROM AUTHOR]