Amorphous and Nanocrystalline Ultra-High-Temperature Ceramics; Materials Design and Solid State Synthesis

This paper reports the concepts and methodology for material design and solid state synthesis of amorphous and nanocrystalline ultra-high temperature ceramics (UHTCs) in widespread applications. Until now, the author has achieved the amorphous powder synthesis of SiC, ZrB₂ and B₄C with extremely high crystallization temperature, and a binary system of B₄C-SiC by planetary ball milling^[1]; is in attempt to provide a mass production of the amorphous powder in term of attrition milling on the basis of the proposed thermodynamics and kinetics of mechanically driven amorphization. Then, pulse electric current consolidation and millimeter wave pressure sintering can be powerful tools by which to provide a route to the bulk nanocrystalline synthesis of covalent-bonding type ceramics such as SiC^[1]. Especially, field assisted pressure sintering makes it possible to obtain full-density amorphous B₄C and B₄C-SiC products prior to major crystallization, following a rapid densification in heating according to an Arrhenius type equation of Newtonian viscous flow and an extensive structural stabilization as characterized by high temperature X-ray diffraction, during glass transition. For thus-obtained amorphous and nanocrystalline UHTCs with a nearly threshold crystallite size, strength, toughness and superplastic forming will be reviewed.

(1) H. Kimura: Ceramic Transactions, 203, 119-130 (2009).