Study Of MoSi₂-SiC Composites Synthesized In Situ Using Solid State Displacement Reactions

In this dissertation, the solid-state displacement in situ synthesizing reaction path between Mo2C and Si, the vacuum hot pressing of Mo2C, Si and graphite powder have been researched in preparing MoSi2-SiC composites. The mechanism of solid-state displacement reaction has been studied, the solid-state displacement reaction path has been analyzed, the mechanical properties of composites has been tested. The main research results are as follows:1.The Mo-Si-C ternary equilibrium phase diagrams are important tools that we analyze the solid-state displacement reaction path of Mo2C-Si quantificationally. A criterion on the evaluation of thermodynamic data of intennetallic compounds was derived on the basis of the minimum energy principle and the nature of convex-downward surface. As an example, we use the Mo-Si-C ternary equilibrium phase diagrams and thermodynamic data of some known materials at 1200C and 1600C respectively, estimate the unknown Gibbs free energy of formation of ternary-phase T(Mo5Si3C) in the Mo-Si-C system. The Gibbs free energy of formation of ternary-phase T(MosSi3C) is-883.1 +8.2KJ/mol at 1200C and -1112.7+ 10.8KJ/mol at 1600C. 2.The collected and the estimated unknown thermodynamic data of intemietalHc compounds and the equilibrium phase diagrams of Mo-Si-C ternary system at 1200C and 1600C were utilized to calculated and draw the stabilized chemical potential diagrams of Mo-Si-C ternary system. The stabilized chemical potential diagrams combined with the equilibrium phase diagrams of Mo-Si-C ternary system arid the principles of thermodynamic, kinetic, mass-balance could be applied to analyzing and judging reaction paths of in situ synthesis of MoSi2-SiC composites through solid-state displacement reactions theoretically.3. A new interface-perturbation model of solid-state displacement reactions ternary system is suggested and the interface-stability criterion is derived in the form of chemical potential If the chemical potential of rate-control-element at frontier of tiny perturbative zone goes up less than 20.7%, linear interface will grow up stablly and form layered structure; if it goes up more than 20.7%, linear interface is not stable and will form aggregate structure. The aggregate density of rod-aggregate structure is also calculated on the basis of a common presumption.4. Experimental data analysis and theoretical calculation indicated that the interface micro-structure of the solid-state displacement reaction between Mo2C and Si can be explained properly by the new theoretical model: rate-control-elements convergent diffusing model. Moreover, reaction path analysis was accomplished on the fundamental base of the theoretical calculation for MojSi/T phase-interface in the diffusion couple Mo2C-Si at 1200C.5. Interface stability analysis of solid-state displacement reaction shows that the layered structure of solid-state displacement reaction between Mo2C and Si (Fig.6.8) is not stable. Therefore, it is impossible that the microstructure of MoSi2-SiC composites prepared by solid-state displacementreaction are interpenetrating structure.6. A new type of structure of the solid-stale displacement reactions which named spiral-mtergrowth-aggregate structure was discovered in the Mo2C-Si diffusion couple and was considered as the result of spinodal-decomposition continuous phase-transformation. However, the spiral-intergrowth mechanism of SiC+Mo5Si3 two phase structure requires further study.7. The precise interface analysis shows that the growth of the parallel rod-aggregate SiC is self-organiae appearance of non- equilibrium system. The precise analysis of interlace model of solid-state displacement reaction needs method of non-equilibrium thermodynamic. The complicated non-linear mechanism of solid-state displacement reaction requires further study.8. The thermodynamic analyses in preparing MoSi2-SiC composites have been suggested: if MojC and Si as starting reaction materials, the four solid-state displacement reactions we designed can be proceed, and we can de...