Unique Microstructural Development in SiC Materials with High Fracture Toughness

Extract

Application of silicon carbide (SiC) as a structural material has been limited thus far by its low fracture toughness, even though, in comparison to other ceramic materials, SiC has superior high-temperature strength and creep, wear, corrosion, and oxidation resistance. For automotive applications, a higher fracture toughness is required. For example, the brittleness and catastrophic fracture behavior of SiC materials have resulted in limited use in automobile exhaust-valve systems and turbocharger rotors. High-density SiC bodies can be produced by a pressureless sintering process. However, the sintered bodies often include flaws which are related to processing, primarily, the presence of agglomerates and crystallographic defects in the starting powders. The importance of grain size and shape refinement in the improvement of mechanical properties has been recognized, and thus, processing procedures and sintering aid compositions have been examined extensively. However, one of the key factors is the “as-received” powder characterization (distribution of grain sizes, polytypes, and impurities) for producing sintered bodies of SiC with consistent physical properties.

A complexity in SiC materials is that SiC can form various crystal structures having essentially the same chemical composition but a differing number of stacking layers in the unit cell. This is commonly called a polytype. There is only one crystal structure with cubic symmetry, which is identified as 3C or the β-phase. At high temperature, the β-phase transforms to α-phases with hexagonal or rhombohedral symmetry, with 4H, 15R, and 6H (Ramsdell notation) being the major polytypes observed in SiC materials. Preference of the polytype selection during the β- to α-phase transformation is dependent on the chemistry of the sintering aids and metallic impurities in the grain boundaries.