High-temperature structural stability of MoSi2-based nanolayer composites
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A systematic study of the hightemperature structural stability is reported for the following three nanolayered composites: (1) MoSi2MoSi2Nx, (2) MoMoSi2Nx, and (3) MoMoSi2NxMoSi2, where x34. The alternating layers with layer thickness varying from 1 to 50 nm were synthesized by sputtering techniques. The structural evolution in these composites has been studied by crosssectional transmission electron microscopy as a function of annealing temperature. Asdeposited Mo layers exhibit nanocrystalline structure, while the other types of layers are amorphous in structure. With increasing annealing temperature, MoSi2 crystallizes to form a metastable C40 phase at 500C and then transforms to the stable C11b phase at 900C, while MoSi2Nx remains amorphous up to temperature as high as 1000C. No difference in crystallization behavior was observed for the constituents in either single phase or multilayered form. One way to improve the toughness of MoSi2 is through the addition of a ductile phase, e.g., Mo. However, the Mo and MoSi2 layers react and the layer structure deteriorates. Previous study has shown that the compound, MoSi2Nx, stays amorphous at 1000C. This suggests that it could function either as a stable secondphase reinforcement or as a diffusion barrier between Mo and MoSi2. The MoSi2MoSi2Nx, MoMoSi2Nx, and MoMoSi2MoSi2Nx nanolayers are found to remain stable up to 900C (highest temperature tested).
