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 a nd SiC. Single phase or alternating layers of MoSi2 and SiC were synth esized by d.c.-magnetron and r.f.-diode sputtering, respectively. Cros s-sectional transmission electron microscopy was used to examine sever al 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. Nan oindentation was employed to characterize the mechanical response of t he materials as a function of the structural changes. As-sputtered mat erial exhibits an amorphous structure in both single phase and multila yer forms. Heat treatment induces similar recrystallization behaviour in MoSi2 in both single phase and multilayers. SiC, in single phase fo rm, remains amorphous up to 900 degrees-1 h anneal, while in multilaye rs 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 crystallizatio n process is directly responsible for the hardness and modulus increas e in both single phase and multilayered films. A maximum hardness of 2 5.5 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 almo st constant. The layered geometry offers better elastic properties (hi gher modulus) than the single phase films.