Effective Activation Energy for the Solid-State Sintering of Silicon Carbide Ceramics
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SILICON carbide (SiC) is a material of immense technological importance as it exhibits outstanding properties including light-weight, high strength, moderate toughness, high wear, and oxidation resistance and strength retention at elevated temperature.[1,2] SiC is suitable for various applications including the main structural material in telescopes for space exploration, as abrasives for cutting and grinding, in wear parts such as bearings and atomization nozzles, for the production of seals and valves for demanding chemical environments in the process industry, high-temperature heat exchangers and in ceramic armor plates for ballistic protection.[1–3] Because of the high melting point and brittle behavior of SiC, normal melting and deformation processes are not suitable for the shaping of SiC components. Therefore, SiC components for various applications are produced by powder consolidation techniques involving shape formation followed by temperature-assisted densification.
DULAL CHANDRA JANA is with the Centre for Non-Oxide Ceramics, International Advanced Research Centre for Powder Metallurgy & New Materials (ARCI), Balapur PO, RCI Road, Hyderabad 500 005, India. Contact e-mail: [email protected] G. SUNDARARAJAN is with the Centre for Non-Oxide Ceramics, International Advanced Research Centre for Powder Metallurgy & New Materials (ARCI) and also with the Metallurgical and Materials Engineering, Indian Institute of Technology, Madras, Chennai, India. K. CHATTOPADHYAY is with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted March 27, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
SiC is difficult to sinter in the absence of sintering additives and/or external pressure.[4] The lack of inherent sinterability of SiC is due to the presence of covalent chemical bonds in it. The kinetic and thermodynamic phenomena associated with the non-sinterability can also be explained based on its strong and directional covalent bonding. The activation energy (AE) for Si and C diffusion in pure SiC is very high due to the high energy involved in the formation and movement of structural defects.[5] A powder compact undergoes densification and appreciable shrinkage is noticed only when the mass transfer occurs at the pore surfaces through volume or grain boundary diffusion during heat treatment at elevated temperatures. In contrary, the presence of other mass transfer mechanisms of low AE (e.g., surface diffusion and evaporation/condensation) by which material transport can occur from pore surfaces to the neck can lead to a reduction of the specific surface area through particle coarsening. Under these conditions, very little shrinkage or densification are observed.[4] Conventionally, SiC can be densified in the absence of sintering aids by pressure-assisted densification processes like hot pressing (HP) and hot isostatic pressing (HIP). However, pressure-assisted densification processes are limited to the fabrication of simple parts and not economical for mass production
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