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Polymer derived ceramics: Processing-structure-property relationships

Zunjarrao, Suraj C.
Silicon carbide (SiC) is an important ceramic, most commonly used because of its excellent properties such as high strength, high modulus, excellent creep resistance and its high temperature stability. Moreover, it is attracting wide attention as a promising candidate for several applications in nuclear reactors owing to its high thermal conductivity, high melting temperature, good chemical stability, and resistance to swelling under heavy ion bombardment. However, fabricating SiC by traditional powder processing route generally requires very high temperatures for pressureless sintering. Alternatively, polymer derived ceramic materials offer unique advantages such as ability to fabricate net shaped components, incorporate reinforcements, along with lower processing temperatures and relatively easy control over the microstructure. In this study, fabrication of polymer derived amorphous and nanograined SiC, by controlled pyrolysis of allylhydridopolycarbosilane (AHPCS) under inert atmosphere, is presented. Final processing temperatures and hold times were varied to study the influence of processing parameters on the structure and resulting properties. Chemical changes, phase transformations, and microstructural changes occurring during the pyrolysis process are studied as a function of the processing temperatures. Polymer crosslinking and polymer to ceramic conversion is studied using infrared spectroscopy (FTIR). Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are performed to monitor the mass loss and phase change as a function of temperature. X-ray diffraction studies are done to study the intermediate phases and microstructural changes. Variation in density is carefully monitored as a function of processing temperature. Hardness and modulus measurements are carried out using instrumented nanoindentation to establish processing-property-structure relationship for SiC derived from the polymer precursor. It is seen that the presence of nanocrystalline domains in amorphous SiC significantly influences the modulus and hardness. A non-linear relationship is observed in these properties with optimal mechanical properties for SiC processed to 1150°C for 1 hour hold duration, having average grain size of ~3 nm. In addition, bulk mechanical characterization, in terms of biaxial flexure strength, was done for SiC-SiC particulate composites purely derived from the polymer precursor. Finally, a simple finite element model is developed for nanocrystalline SiC.