Abstract

Dense Si₂N₂O was successfully synthesized using 2 mol% Li₂O as an additive by a hot-pressing method at 1500 ^oC. Compared to other metal oxide additives, Li₂O can significantly decrease the sintering temperature of Si₂N₂O, which is ascribed to the lower melting point of Li₂O–SiO₂ and the formation of less viscous liquid phase. Increasing Li₂O content has no apparent influence on the mechanical and dielectric properties of dense Si₂N₂O, which is due to the easy evaporation of Li₂O at sintering temperature. The mechanical properties of Si₂N₂O with Li₂O additive are comparable to those of Si₂N₂O synthesized with other additives. The as-prepared bulk Si₂N₂O with 2 mol% Li₂O additive exhibits both low dielectric constant (6.17 at 1 MHz) and loss tangent (0.0008 at 1 MHz) and combines good mechanical performance, indicating it is a potential high-temperature structural/functional material.

Abstract

Layered ternary T-Al-C ceramics containing early transition metal Sc, Zr, and Hf, crystallize with the T_nAl3C_n+2 formula, while others containing neighbor elements Ti, V, Cr, Nb, Mo, W, and Ta yield the T_n+1AlC_n formula. Ternary T_nAl₃C_n+2 ceramics are structurally characterized by NaCl-type TC slabs being separated by Al₄C₃-type AlC layers. In the present study, we suggest that the ability of forming the T_nAl₃C_n+2
carbide could be traced back to the structure mismatches between the TC, Al₄C₃ and T_nAl₃C_n+2 compounds. Ternary carbides following the T_nAl₃C_n+2 formula experience small lattice mismatches and strain energies. Moreover, the discrepancy between crystal structures of T_nAl₃C_n+2 and T_n+1AlC_n is interpreted by lattice mismatch and the produced strain energy for the ternary T-Al-C ceramics. We also present close relationships between the atomic radii of transition metal and lattice mismatch, as well as the strain energy. The proposed method is not only helpful to explain the trend in crystal structure of T-Al-C based ceramics, but may be also general to predict the crystal structure of layered compounds constructed by alternatively stacked structural units.

Abstract

Cr₂AlC is an unusual layered ternary ceramic that combines the merits of both metals and ceramics. The
salient properties of Cr₂AlC are strongly related to its bonding characteristics and microstructures. Synthesis,
microstructure, and property of Cr₂AlC are reviewed in this paper. First, theoretical calculations and physical
properties are introduced. Then, the processing of Cr₂AlC ceramic in both bulk form and thin films and their basic mechanical properties are summarized. Atomic-scale characterizations of Cr₂AlC, as well as the microstructural relationships among Cr₂AlC, Al₈Cr₅, and AlCr₂ were achieved using a series of transmission
electron microscopy (TEM) techniques. Moreover, high-temperature oxidation and hot corrosion behaviors
of Cr₂AlC were investigated by means of thermogravimetric analysis, X-ray diffraction, Raman spectroscopy,
scanning electron microscopy, and TEM. Mechanism of the excellent high-temperature corrosion resistance of Cr₂AlC is discussed based on systematic microstructural analyses. Finally, concise conclusions are drawn.

Abstract

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 ^oC in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr2AlC is about 800 C, which is 400 ^oC higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr2AlC displayed excellent hightemperature oxidation resistance with parabolic rate constants of 1.08×10 ^-12 and 2.96×10 ^-9 kg² m ^-4 s ^-1 at 800 and 1300 ^oC, respectively.  Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO4 salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.

Abstract

As-polished and preoxidized Ni–20Cr alloys were Ce-implanted with a dosage of 1×10 ¹⁷  ions/cm², then subsequently oxidized at 1050 ^oC in air. The oxide adhesion and the extent of sulfur segregation at the oxide–alloy interface were determined, respectively, using tensile pull testing and scanning Auger microscopy with an in situ scratch device. The critical load for oxide failure was the lowest on the unimplanted Ni–20Cr, and was slightly higher on those with implantation made into a preformed oxide. Oxides that formed directly on Ce-implanted Ni–20Cr never failed under the pull test, which showed the strongest scale adhesion; however, similar amounts of interfacial sulfur, which segregated from the alloy during oxidation, were found at all interfaces. Ce additions were also found to reduce the oxidation rate and affect the extent of voids at the scale–alloy interface. It is suggested that the change in the oxide growth mechanism reduces the number of interfacial voids and, unlike Al₂O₃, these factors are more important for Cr₂O₃ scale adhesion than sulfur segregation to the scale–alloy interface.