Chapter 2 - Structure and Properties of SiC Ceramics

Abstract

This chapter outlines the structure and properties of SiC and the connections between structure and properties. It also explores in detail the complex sintering mechanisms and the role of sintering aids for SiC. SiC is crystallographically similar to carbon. Silicon and carbon are both tetravalent, and both reside in Group 14 of the Periodic Table. Carbon is in Period 2 (S and P orbitals only), and silicon is in Period 3 (S and P orbitals only). Their electron structures are therefore analogous. Crystallographically, SiC is a diamond analogue with >150 known polytypes, although only five polytypes are commonly found in SiC ceramics. Diamond also has many polytypes. Like diamond, SiC comprises low atomic weight elements of small atomic radius, with high electropositivity. Moreover, SiC has a high degree of covalency in its bonding (88%). As a result, SiC is the fourth hardest known material, with a very high compressive strength, high flexural strength for ceramic, high elastic modulus, and a much lower density (3.21g⸱cm−3) than its primary competitor Al2O3 (4g⸱cm−3). These properties lead to SiC being the leading body armour ceramic in the world and a top-end wear-resistant material. SiC also has high hot strength, excellent oxidation resistance, high thermal shock resistance, excellent creep resistance, and outstanding corrosion resistance. These thermomechanical and chemical properties have led to SiC being a very widely used refractory. SiC is also significantly superior to silicon as a semiconductor by all important criteria, leading to its rapid growth in semiconductor market share since it was launched in 2001. However, pure SiC is very difficult to sinter. Many years of research and development in SiC sintering aids since the 1950s have now made it possible to pressureless sinter SiC to full density, although grain growth remains a perennial problem. The mechanisms of these sintering aids are explored in detail in this chapter.