Eutectic Ceramics by Melt-Solidification

Takashi Goto
New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8579, Japan
Nagaoka University of Technology, Nagaoka 940-2188, Japan
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 40070, China

Since many carbides, nitride and borides such as SiC, TiN and ZrB2 have high melting temperature, high mechanical strength and stability at high temperature, they are promising structural materials and often called as ultra-high-temperature ceramics (UHTCs). In order to improve the property and/or to add more functionality, multiphase materials, composites, have been fabricated by solid-state sintering such as hot pressing and spark plasma sintering. However, fully dense composites are often difficult to fabricate due to low diffusion coefficient and covalent nature. Although melt-solidification has been commonly used in metals and alloys, ceramics has hardly been manufactured by melting due to high-melting point. On the other hand, high-temperature technology is rapidly advancing, and high-temperature more than 3000 K can be easily produced. We can use arc-melting, floating-zone melting and laser irradiation to melt UHTCs even at more than 3000 K. The performance of UHTCs strongly depends on microstructure. Directionally solidification using eutectic reaction is particularly useful to control microstructure of ceramic composites improving mechanical properties. Since ceramics have conventionally been studied by solid-state sintering, phase relation of UHTCs above melting points has not well being investigated. We have prepared UHTCs composites by melt-solidification. SiC has high mechanical strength and oxidation resistance, and thus SiC is a promising UHTCs. Although SiC would sublimate (not melt) at high temperature, SiC-based composite can be melted by eutectic reaction. SiC-TiC, SiC-B4C, SiC-TiB2 were binary eutectic systems, and SiC-TiC-TiB2, SiC-B4C-TiB2 were ternary eutectic systems. The mechanical properties depend on the size of second phase, the small inter-lamellar spacing of the eutectic composites would cause high strength and hardness of UHTCs basically obeying Hall-Petch relation. We can easily explore high-performance UHTCs simply by arc-melting. Laser melting is also advantageous to melt and solidify UHTCs because of high cooling rate, and significantly fine microstructure of eutectic composites can be synthesized. CrB2-SiC was binary eutectic system, whose eutectic composition was 80 mass% CrB2. By scanning laser, CrB2-SiC eutectic composite was easily melted and solidified one-directionally along the scanning direction. At the eutectic composition, thin lamellar SiC phase dispersed in a CB2 matrix. The eutectic temperature of the CrB2-SiC system was relatively lower, and thus CrB2-SiC eutectic composite was melted by laser (Nd:YAG laser, 180 W). CrB2(0001) was perpendicular to the scanning direction. Laser was scanned at 1.8 mm/s, and laminar CrB2-SiC about 1 m in width formed 100 m in depth.