Abstract
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 σf and RT) and 276.77 GPa (elastic modulus Ef 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·m1/2 (1673 K), showing the highest improvement of 31.70% as compared to the RT value.
Highlights
Re-crystallized silicon carbide (RSiC) is well-known for its excellent creep resistance, high strength, high hardness, oxidation resistance, chemical stability at elevated temperature, high thermal conductivity, relatively low coefficient of thermal expansion, and low density [1,2]
The MoSi2(Cr5Si3)–RSiC composites mainly compose of 6H–SiC, hexagonal MoSi2, and tetragonal Cr5Si3, a small amount of 6H Mo4.8Si3C0.6 are found, newly formed Cr5Si3 and Mo4.8Si3C0.6 are derived from the reaction of MoSi2, pyrolytic C (PyC), Cr, and Si
It can be seen clearly that weight ratios between MoSi2, Cr5Si3, and Mo4.8Si3C0.6 possess different values in different composites as the intensity ratios of the corresponding peaks in X-ray diffraction (XRD) patterns exhibit different values for different composites [41], which may affect the microstructure of the composites as the infiltrated phase MoSi2, Cr5Si3, and Mo4.8Si3C0.6 possess different physical properties, such as coefficient of thermal expansion (CTE), melting point (m.p.) [14,42]
Summary
Re-crystallized silicon carbide (RSiC) is well-known for its excellent creep resistance, high strength, high hardness, oxidation resistance, chemical stability at elevated temperature, high thermal conductivity, relatively low coefficient of thermal expansion, and low density [1,2]. A new type of ceramic-metal composite, in which both phases are continuous and 3D interpenetrating in the whole microstructure, has drawn considerable attention [8,9] This kind of composites exhibits multi-functions for their special structure and remaining performance of each phase; the most general method for fabricating these composites is to impregnate the desired phases into the pre-existing open-cell porous preform, thereby producing the requisite connectivity and spatial distribution of two or more phases [1,10]. The obtained composites exhibit excellent mechanical properties at room and elevated temperatures In this infiltrated system, Si (low melting point, 1683 K) helps to reduce the infiltrated temperature via the formation of uniform pre-melted alloy, and the excessive Si can react with X or existed C to form the metal silicide and SiC to avoid the decline of high-temperature mechanical properties of the composites under oxidation surroundings [27,28].
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