Integrated Gasification Combined Cycle (IGCC) technology offers efficient and cleaner coal-based power generation but introduces corrosion challenges associated with high temperatures and molten slag. This study examines the corrosion behavior of ytterbium monosilicate (Yb2SiO5) and its composites containing 5 and 10 wt% Al2O3under IGCC slag conditions. The addition of Al2O3promotes the formation of garnet phases with higher thermal expansion, improving compatibility with the substrate. Samples exposed to molten slag were evaluated for microstructural evolution, thermal expansion, conductivity, and slag resistance. Yb2SiO5 exhibited high thermal stability and formed dense protective layers that limited slag infiltration. The composites enhanced thermal compatibility with carbon steel, increasing the coefficient of thermal expansion (CTE) to 9.8 x 10-6 and 10.9 x 10-6 K-1 , respectively. The formation of a protective Yb2Si2O7 layer underscores the potential of ytterbium silicates as effective high-temperature coatings for IGCC applications.

ZrO2-WC composite ceramics are promising structural materials that have excellent flexural strength and also complement fracture toughness. However, the sintering behavior that simultaneously controls grain size and density has not yet been fully elucidated. This leads to significant difficulties in increasing flexural strength. In this study, the densification and grain growth kinetics of a ZrO2-WC composite were systematically analyzed to establish a kinetics-guided design strategy for pressure-assisted two-step sintering. Log-log analysis of grain size and densification rate showed that bulk densification was more strongly correlated with the grain size evolution of ZrO2 than with that of WC, indicating that ZrO2 plays the dominant role in densification while WC mainly contributes through pinning and interfacial constraints. Arrhenius analysis of ZrO2 grain growth revealed two distinct temperature regimes separated near 1400 degrees C. In the range of 1200-1400 degrees C, the activation energy was 207 kJ/mol for grain growth of ZrO2, which suggests that the grain growth is strongly inhibited by Zener-pinning effect. Above 1400 degrees C, the lower apparent activation energy suggested reduced pinning effectiveness associated with porosity elimination and increased grain-boundary mobility. Based on this kinetic transition, two-step sintering performed within this range produced a more favorable density-grain size trajectory than normal sintering, resulting in a dense fine-grained microstructure and a maximum flexural strength of 1650.3 MPa. From this, we hope to contribute to the optimal process design for improving the flexural strength of ZrO2-WC composites.

Environmental barrier coatings (EBCs) are essential for protecting SiCf/SiC ceramic matrix composites from water vapor recession and calcia-magnesia-aluminosilicate (CMAS) corrosion in gas turbines. In this study, (YbxSc1_x)2Si2O7 solid solutions with varying Yb/Sc ratios are evaluated as CMAS-resistant EBC topcoat candidates. Five compositions are synthesized and tested at 1500 degrees C. Corrosion resistance improves as the optical basicity of the disilicate matches that of CMAS, minimizing chemical reactions and apatite formation; Sccontaining compositions exhibit the best performance. Increasing Sc content decreases the ionic radius and lattice parameters, further inhibiting Ca2+-to-RE3+ substitution. Microstructural analysis shows Yb-rich samples retain surface CMAS, whereas Sc-rich samples experience rapid grain-boundary infiltration with less reaction. Thermophysical measurements confirm low, stable thermal conductivity and coefficient of thermal expansion compatibility with SiCf/SiC substrates. These results indicate that (YbxSc1_x)2Si2O7 solid solutions offer a balanced combination of CMAS corrosion resistance, thermal compatibility, and low thermal conductivity for robust EBCs.

Thermal barrier coatings (TBCs) for hydrogen-fueled gas turbines withstand higher combustion temperatures and increased steam concentrations compared to conventional natural-gas systems. These harsh operating conditions significantly accelerate the thermal degradation of widely used YSZ coatings, emphasizing the need for alternative top-coat materials with improved phase stability and reduced thermal conductivity. In this study, rare-earth zirconate ceramics, Gd2Zr2O7 (GdZO), Tm2Zr2O7 (TmZO), and a mixed composition (Gd0.5Tm0.5)2Zr2O7 (Gd/TmZO), are synthesized and investigated as potential next-generation TBC candidates. Each material was comparatively examined with a focus on crystal structure, thermophysical properties, and thermal conductivity. Furthermore, high-temperature steam exposure experiments were performed to simulate hydrogen combustion environments. Microstructural analyses, high-temperature degradation behavior, and phase stability evaluations were carried out to obtain fundamental experimental data. This study provides essential baseline information for the design and development of high-performance TBC materials suitable for the hydrogen-fueled gas turbine systems.

Light-weight ceramic insulation materials and high-emissivity coatings were fabricated for reusable thermal protection systems (TPS). Alumina-silica fibers and boric acid were used to fabricate the insulation, which was heat treated at 1250 °C. High-emissivity coating of borosilicate glass modified with TaSi2, MoSi2, and SiB6 was applied via dip-and-spray coating methods and heat-treated at 1100°C. Testing in a high-velocity oxygen fuel environment at temperatures over 1100 °C for 120 seconds showed that the rigid structures withstood the flame robustly. The coating effectively infiltrated into the fibers, confirmed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction analyses. Although some oxidation of TaSi2 occurred, thereby increasing the Ta2O5 and SiO2 phases, no significant phase changes or performance degradation were observed. These results demonstrate the potential of these materials for reusable TPS applications in extreme thermal environments.

RBSC with a low residual Si less than 5 vol% was fabricated using SiC-C preform with diamond as a main carbon source by a conventional reaction sintering process at the temperature range between 1500 degrees C and 1800 degrees C. The diamond content in the SiC-C preform varied from 10 wt% to 17 wt%. The diamond as the main carbon source used in SiC-C preform effectively minimized the residual Si in RBSC by increasing the reaction of diamond due to the enhanced Si melt infiltration even in SiC-C preform with high carbon content. The Si melt infiltration should be performed above 1600 degrees C for the complete reaction of diamond particles with size of 0.5 mu m because of the relatively low dissolution rate of diamond in Si melt. The flexural strength of RBSC with residual Si content less than 7 vol% exceeded 400 MPa. The flexural strength of RBSC was increased with decreasing residual Si content, but sharply decreased by the residual carbon accompanying pores in RBSC. RBSC with low residual Si content displayed moderate flexural strengths around 200 MPa even above Si melt temperature because of the formation of continuous SiC structure in RBSC by the enhanced grain growth of SiC. Vickers hardness of RBSC was steadily increased from 17.7 GPa to 24.6 GPa with decreasing residual Si.

We studied a candidate TPS (thermal protection system) material for reusable re-entry space vehicle applications. The material was based on a high-temperature-resistant material called Cerakwool. A total of six specimens were fabricated with substrate densities of 0.45 g/cm3, 0.40 g/cm3, and 0.35 g/cm3, with two specimens for each density. All specimens were coated with high-emissivity TUFI (toughened unpiece fibrous insulation), with coating thicknesses ranging from 445 to 1606 µm. The specimens were tested using an HVOF (high-velocity oxygen fuel) material ablation test facility. For each density specimen pair, one specimen was tested at 1 MW/m2 and the remaining one was tested at 0.65 MW/m2. The average stagnation point temperature for specimens tested at 1 MW/m2 was ~893 °C, approximately 200 °C higher than those tested at 0.65 MW/m2. This suggests a ~200 °C increase in stagnation point temperature for a 0.35 MW/m2 rise in incident heat flux. During the tests, internal temperatures were measured at three locations. For all tested specimens, regardless of heat flux test conditions and density, the temperature at ~40 mm from each specimen’s stagnation point remained around or below 50 °C, well within the 180 °C design limit set for the TPS back face temperature. Post-test visual inspections revealed no signs of ablation or internal damage, confirming the material’s reusability.

During the machining of Inconel 718 using SiAlON ceramics, the tool life can be significantly influenced by diffusion wear. To investigate this crucial wear behavior, this study focuses on the thermochemical interactions in the diffusion couple between SiAlON ceramics and Inconel 718 superalloy. So, we examine the microstructural characteristics and phase formation behavior in the reacted layer between these two materials. The interfaces between these two materials exhibit distinguishable elemental contrasts and phase formations. Particularly, the phases indicate vitrification or devitrification in the diffusion layer, influenced by the chemical composition. This issue in the diffusion couple was analyzed with thermophysical properties using light flash analysis (LFA). After, it was confirmed that the vitrification of the diffusion layer can be induced by the conduction behavior of heat trapped between SiAlON and Inconel 718 during supercooling. These characterizations verify that the devitrification of the welding layer (diffusion layer), resulting from the reaction between SiAlON ceramics and Inconel 718 superalloy, can depend on the thermal conductivity of the prepared ceramics. Furthermore, we discovered a possibility that this could contribute to improving tool performance by suppressing diffusion wear.

This research presents a study on the fabrication and characterization of tungsten-zirconium carbide (W-ZrC), an ultra -high temperature ceramic, using an in -situ reaction sintering method. Tungsten carbide (WC) and zirconia (ZrO2) powders were used as precursors to synthesize composites with three different ratios. The thermodynamic phase stabilities of the precursor materials were analyzed, and the reaction products were determined based on the total pressure within the system. The fabricated composites were characterized by their microstructures, porosities, hardnesses, and thermal conductivities. The results show that the composites exhibited distinct crystal phases, including those of tungsten, tungsten carbide (W2C), zirconium carbide (ZrC), and tetragonal zirconia (tZrO2). The reaction pathways were determined based on the composition ratios and thermodynamic stability. The composites demonstrate good densification and interparticle bonding owing to the in -situ reaction sintering process, which simplifies the fabrication and lowers processing costs. The composites also exhibited desirable properties, such as high hardness and thermal conductivity (maximum 36.4 W & sdot;m-1 & sdot;K-1), making them suitable for high -temperature applications. The findings of this study contribute to the understanding of the synthesis process and properties of W-ZrC composites and provide insights for their potential applications to industries such as aerospace, defense, and nuclear energy.

Yttria-stabilized zirconia (YSZ)-based engineering ceramics are manufactured by adding non-oxide additives as the typical toughening mechanism of zirconia (ZrO2). In this study, YSZ-10/20 vol% WC composite ceramics were prepared by pressureless sintering and hot pressing to view reaction pathways based on different driving forces. During the reactions, the decarburization of WC and the formation of ZrC were investigated at temper-atures of >= 1500 degrees C. W was detected at 1700 degrees C, which is related to the thermodynamic stability of various chemical reactions in the ZrO2-WC binary system. The hardness of the composite ceramics decreased, whereas the fracture toughness and wear resistance increased. The phase evaluation and reaction pathways from the thermodynamic perspective derived from the YSZ-WC binary system suggest considerable potential for the future composition designs of ZrO2-based non-oxide composite ceramics.

This study investigated the relationship between the green/brown colors and Yb/Y-co-doped ternary eutectic reactions. A color difference was initially observed at 1600 degrees C. The green color indicates a relatively highly porous area compared to the brown color, and it was confirmed that the green color was spherical in all samples. This difference in color was analyzed through photoluminescence (PL), and it was confirmed that the reduction of Yb2+ increased at temperatures higher than 1550 degrees C owing to the reducing atmosphere generated by the carbon ambient. The reaction sequence of the phase formation of SiAlON exhibited a difference resulting from the reduction in Yb ions, and the sequence of phase formation improved with an increase in the content of Y substituted for Yb.

X-37b is a fairly new but very successful uncrewed technology demonstrator that can reenter Earth's atmosphere, partially replacing the reentry mission of the Space Shuttle program that retired after serving decades in the United States of America. For this, an improved, thermal protection system(TPS) was built with the heritage of previous reentry spacecraft, in particular based on silica ceramic tiles. In view of the TPS design, it is noteworthy that the leading edge of X-37b wings is made in new material called TUFROC instead of the flight-proven RCC(Reinforced Carbon-Carbon) of the Space Shuttle era. In this study, the technologies of Space Shuttle TPS and TUFROC are analyzed.

SiAlON ceramics are primarily employed in ceramic cutting tools, which exploit their stable physical properties in high-temperature cutting environments due to their excellent mechanical properties. Here, Yb/Y-co-doped SiAlON ceramics are prepared by adding Yb and Y rare-earth (RE) ions in the RExSi12-(m+n)Alm+nOnN16-n (m = 0.4, n = 1.0) composition. Yb2O3, the RE oxide, is the main sintering additive. For REx composition design with (Yb1-y + Y-y)(x), Yb2O3 is replaced by Y2O3 (y = 0.00, 0.25, 0.50, 0.75, 1.00). While Yb2O3 has excellent high temperature stability, it is limited by its microstructural characteristics, that is, the beta-SiAlON morphology and limited fracture toughness due to the small cation sizes. Thus, the changes in the above properties are investigated for various Y2O3 additive contents substituting for Yb2O3. The average grain width decreases, and the equiaxed beta-SiAlON grains are elongated with increasing Y2O3 content. Regarding the mechanical properties, the hardness and fracture toughness are evaluated using the indentation fracture method. The hardness decreases with increasing Y2O3 content; however, the fracture toughness exhibits a significant increase (~53.6%) from 4.6 to 7.0 MPa.m(1/2). Regarding crack propagation, intergranular fracture is mainly observed in the Yb/Y-co-doped SiAlON ceramics, whereas transgranular fracture is primarily observed in the Yb-single-doped SiAlON ceramic. Y(2)O(3 )substitution increases the alpha/beta-SiAlON phase ratio, and the grain boundary phase exhibits increasing vitrification with increasing Y2O3 content. Moreover, the thermal properties of the Yb/Y-co-doped compositions are analyzed and discussed regarding intrinsic properties such as phonon scattering. The microstructural characteristics and improved fracture toughness derived from the Yb/Y-co-doped system designed in this study suggest considerable potential for the future composition design of ceramic cutting tools.

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 degrees 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 O-2 from atmosphere formed to the Yb2Si2O7 phase upon heat treatment at 1400 degrees 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.

Environmental barrier coatings (EBCs) are essential to protect ceramic matrix composites against water vapor recession in typical gas turbine environments. Both oxide and non-oxide-based ceramic matrix composites (CMCs) need such coatings as they show only a limited stability. As the thermal expansion coefficients are quite different between the two CMCs, the suitable EBC materials for both applications are different. In the paper examples of EBCs for both types of CMCs are presented. In case of EBCs for oxide-based CMCs, the limited strength of the CMC leads to damage of the surface if standard grit-blasting techniques are used. Only in the case of oxide-based CMCs different processes as laser ablation have been used to optimize the surface topography. Another result for many EBCs for oxide-based CMC is the possibility to deposit them by standard atmospheric plasma spraying (APS) as crystalline coatings. Hence, in case of these coatings only the APS process will be described. For the EBCs for non-oxide CMCs the state-of-the-art materials are rare earth or yttrium silicates. Here the major challenge is to obtain dense and crystalline coatings. While for the Y2SiO5 a promising microstructure could be obtained by a heat-treatment of an APS coating, this was not the case for Yb2Si2O7. Here also other thermal spray processes as high velocity oxygen fuel (HVOF), suspension plasma spraying (SPS), and very low-pressure plasma spraying (VLPPS) are used and the results described mainly with respect to crystallinity and porosity.