INTERFACES IN MOSI2-SIC IN-SITU COMPOSITES SYNTHESIZED BY MELT PROCESSING

Interfaces between the primary beta-SiC and the surrounding MoSi2 matr ix in melt-synthesized in situ composites have been investigated, with emphasis on the chemistry and crystallographic relationships develope d during solidification. Primary SiC growth occurs with {002} and {111 } facets, both of which are found to template the subsequent nucleatio n and epitaxial growth of the MoSi2 matrix. Eight independent orientat ion relationships (ORs) were identified, involving the following combi nations of planes: {002}(SiC) parallel to (001)(MoSi2) (3 rotational v ariants), or {101}(MoSi2) {111}Sic parallel to (001)(MoSi2), or {100)( MoSi2) (2 rotational variants), or {110)(MoSi2) The interfacial relati onships were rationalized using coincident site lattice arguments as w ell as energetic simulations based on the Grey-Bohr algorithm. The lat ter analysis suggests that the:multiplicity of relationships arises fr om local effects associated with the size and shape of the adsorbate l ayers preceding the formation of the MoSi2 nuclei. An amorphous carbon layer, 2- to 5-nm thick, was detected at all interfaces and some of t he matrix grain boundaries. This interphase is believed to evolve by s olid-state precipitation of C during postsolidification cooling and is , in principle, metastable. The C interphase enables easy debonding an d thus may have important implications for the mechanical performance of materials involving SiC/MoSi2 constituents.