Structure and mechanical properties of MoSi2-SiC nanolayer composites

A systematic study of the structure-mechanical properties relationship is reported for both single phase and nanolayer composites of MoSi2 and SiC. Single phase or alternating layers of MoSi2 and SiC were synthesized by d.c.-magnetron and r.f.-diode sputtering, respectively. Cross-sectional transmission electron microscopy was used to examine several distinct reactions in the specimens when annealed at progressively higher temperatures: crystallization and phase transformation of MoSi2, crystallization of SiC, layer spheroidization, and grain growth. Nanoindentation was employed to characterize the mechanical response of the materials as a function of the structural changes. As-sputtered material exhibits an amorphous structure in both single phase and multilayer forms. Heat treatment induces similar recrystallization behaviour in MoSi2 in both single phase and multilayers. SiC, in single phase form, remains amorphous up to 900 degrees-1 h anneal, while in multilayers it starts to crystallize at 700 degrees C. Annealing at 900 degrees C for 2 h causes the spheroidization of the layering which results in the formation of a nanocrystalline equiaxed microstructure. Abnormal grain growth is observed after the spheroidization. The crystallization process is directly responsible for the hardness and modulus increase in both single phase and multilayered films. A maximum hardness of 25.5 GPa and a modulus of 382 GPa can be achieved through crystallizing both MoSi2 and SiC layers. Prolonged high temperature exposure causes hardness degradation due to grain growth but the modulus remains almost constant. The layered geometry offers better elastic properties (higher modulus) than the single phase films