Optimization of Mechanical Properties in SiC Control of the Microstructure

Abstract

The optimization of mechanical properties through microstructural design in metallurgical industries has been used successfully for many years. In comparison, the microstructural design in ceramics has been less successful, mainly due to the complexity and difficulties associated with low diffusivity (in their monolithic form) and to the ability to be sintered at elevated temperatures. For years, many structural ceramics could not be sintered to high densities (>95% of their theoretical density) without large quantities of sintering aids. A typical example is silicon carbide (SiC), which has a unique combination of properties, such as: high hardness and wear resistance, considerable strength at high temperatures, and excellent thermal conductivity, but, due to a high level of covalent bonding and consequently low diffusion rates, it cannot be sintered to high density (without the addition of sintering aids) even at temperatures approaching 3000°C.Since the first successful sintering of SiC with small additions of boron and carbon in the early 1970s, remarkable progress has been made in developing a wide range of properties for the microstructure, including fracture toughness from 3–4 MPa m½ to 12–14 MPa m½ four-point-bending strength from 300 MPa to over 900 MPa, and thermal conductivity from 60 to 260 W/m K.4This article presents the most recent developments in SiC ceramics, emphasizing optimization of mechanical properties through microstructure control. The discussion is limited to β-phase SiC pressureless sintering, using oxide additives.