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Carbon fiber-containing refractory high-entropy ceramic matrix composites (C/RHECs) were fabricated through a reaction with carbon powders, transition metal carbides, and Zr–Ti alloys as a novel heat resistant material used for components of hypersonic vehicles cruising at Mach 7–10. With the infiltration of alloys at 1750 °C into a composite preform containing carbon and carbide powders for 15 min, a high-entropy matrix was successfully formed in situ. Arc-jet tests were conducted in the temperature range of 1800–1900 °C. Results showed the formation of an oxidized region composed of complex oxides, such as (Zr, Hf)O2, (Nb, Ta)2(Zr, Hf)6O17, (Zr, Hf)TiO4, and Ti(Nb, Ta)2O7, with an average thickness of ~600 μm, under which an unoxidized region remained. The porous oxidized region resulted from the evolution of CO(g) during oxidation, while a dense oxide region formed as the outermost region. This indicates that the dense oxide region acted as a barrier to oxygen diffusion for the unoxidized region during oxidation.

The degradation behavior of Yb- and Gd-based ternary silicides (YbGdSi) was evaluated in Ar, 50 vol%air-H2O (air-H2O), and 100 vol%H2O (steam) environment and compared with the results obtained in a previous study conducted in air environment. Heat exposure at 1200 °C for 1, 2, 4, and 8 h was carried out in addition to thermogravimetric analysis up to 1200 °C. The results of these studies demonstrated that YbGdSi formed complex oxides in the (Yb- and/or Gd)-Si-O system and Yb2O3 was commonly formed for each environment. In addition, the resultant weight gain for YbGdSi in air, air-H2O and steam decreases by 2–5% in comparison with it for Yb3Si5. These results clearly indicate that the addition of Gd was effective in preventing the degradation of Yb-silicides.

Recent studies on carbon fiber-reinforced ultra-high temperature ceramic matrix (C/UHTC) composites fabricated by hot-pressing, chemical vapor infiltration, polymer impregnation and pyrolysis, and melt infiltration (MI) are reviewed. Various efforts have been made to improve these preparation processes and to combine two or more of these because they have both the advantages and disadvantages in terms of the processing time, operating temperature, and the porosity of the resulting C/UHTC composites. In addition, the parameters governing the fracture toughness, thermal conductivity, and recession behavior (in oxidizing environments) of these composites have been discussed. This review demonstrates that C/UHTC composites with Zr- or Hf-based UHTC matrices fabricated via MI are potential candidates for advanced heat-resistant structural materials.

Si-rich ytterbium silicide was fabricated by through an arc-melting technique for applying use as a bond coat material in an environmental barrier coating system. Evaluation of its potential was accomplished through oxidation tests in dry air and an inert atmosphere. The experimental results showed that the changes in weight and morphologies of ytterbium silicide observed after the tests depended on the oxygen partial pressure. Extensive oxidation and weight gain occurred after oxidation in air. In order to apply this material for hot structures, improvement of the oxidation resistance is needed.

Accepted: 2018-02-03, 資料番号: SA1170294000

Rapid oxidation testing for monolithic ZrB 2 and ZrB 2 –SiC binary composites with different SiC contents (0–30 vol%) was performed using an electric heating system above 2000 °C. The system used in this study achieved the high heating rate of 250 °C/s. The experimental results showed that the morphologies of the oxidized specimens depended strongly on the SiC content. The formation mechanism of SiC-depleted layers beneath the surface scale above 2000 °C differed completely from that below 2000 °C. Although the holding time was below 10 s, SiC-depleted layers were formed because the oxygen partial pressure of the air atmosphere was not enough to form SiO 2 by the oxidation of SiC. It was determined that ZrB 2 –20 vol% SiC showed the best oxidation resistance above 2000 °C at high heating rates.