In the present paper, MoSi₂(Cr₅Si₃)-RSiC composites were prepared via a combination of precursor impregnation pyrolysis (PIP) and MoSi₂-Si-Cr alloy active melt infiltration (AAMI) process. Composition, microstructure, mechanical retention characteristics, and oxidation behaviors of the composites at elevated temperature were studied. X-ray diffraction (XRD) pattern confirms that the composites mainly compose of 6H-SiC, hexagonal MoSi₂, and tetragonal Cr₅Si₃. Scanning electron microscopy (SEM) image reveals that nearly dense MoSi₂(Cr₅Si₃)-RSiC composites exhibiting three-dimensionally (3D) interpenetrated network structure are obtained when infiltrated at 2173 K, and the interface combination of the composites mainly depends on the composition ratio of infiltrated phases. Oxidation weight gain rate of the composites is much lower than that of RSiC matrix, where MoSiCr2 possesses the lowest value of 0.1630 mg•cm^-2, about 78% lower than that of RSiC after oxidation at 1773 K for 100 h. Also, it possesses the highest mechanical values of 139.54 MPa (flexural strength σ_f and RT) and 276.77 GPa (elastic modulus E_f and RT), improvement of 73.73% and 29.77% as compared with that of RSiC, respectively. Mechanical properties of the composites increase first and then decrease with the extension of oxidation time at 1773 K, due to the cooperation effect of surface defect reduction via oxidation reaction and thermal stress relaxation in the composites, crystal growth, and thickness increase of the oxide film. Fracture toughness of MoSiCr2 reaches 2.24 MPa·m^1/2 (1673 K), showing the highest improvement of 31.70% as compared to the RT value.

In this paper, temperature dependence of nanoporous framework evolution process and variety of pore properties (pore volume, specific surface area (BET), and pore size) of SiO₂ aerogels were characterized by FTIR, XPS, XRD, SEM, TEM, BET, BJH, etc. Results show that SiO₂ aerogels treated at different temperatures all possess amorphous structure. With the increase of treated temperatures, BET values of SiO₂ aerogels increase initially and then decrease, and it reaches the maximum value of 882.81 m²/g when treated at 600 ℃ for 2 h due to the addition of the nanopores and shrinkage skeleton of SiO₂ aerogels. Higher temperatures may result in a framework transformation and particle growth; both factors could reduce the BET values of the aerogels. Nanoporous skeleton of SiO₂ aerogels at room temperatures is composed of tetrahedron with a pore size of about 22.28 nm. Higher treated temperatures result in an increase of octahedron amount in nanoporous framework and a decrease of pore size. When treated at 1000 ℃, an approximate dense SiO₂ bulk via the framework collapse and particle growth is obtained. These varieties are derived from the formed extra bonds of Si–O–Si, higher local stress, and liquid phase between particles during heat treatment process.