Solid State Sintering of SiC-Ceramics

Abstract

The most interesting feature in silicon carbide is the structure-property relation where the formation of different types of microstructure due to different structural modifications (polytypism) and grain-boundary/interfacial phase chemistry dictate the final properties of the monoliths. Since synthesis of SiC in last century, several methods such as hot pressing with a sintering aid (B, C), pressureless sintering with a sintering aid (B, C, Al) and reaction bonded (Si-SiC) were used to fabricate dense SiC. A newer method of fast sintering (spark plasma sintering) using pulsed current is also employed to consolidate nano/submicron size SiC with or without additives. The solid state sintered SiC materials have fine-grained equiaxed microstructure (grain size 1 to 4 µm) with thin layer of intergranular phases (amorphous film), exhibit moderate high-temperature creep and oxidation resistance, fracture toughness (3 to 4 MPam1/2) and have highly flaw-sensitive strength at room temperature. The high temperature mechanical properties are highly influenced by the presence of free C, Al and B + C containing grain-boundary phases. Moreover, during prolong processing, abnormal grain growth occurs resulting in anisotropic -SiC phase formation. The Si-SiC materials are poor candidates for high-temperature applications due to the limit set by the melting point of silicon, and the limitations of hot pressing (HPSiC) as a densification technique are well known. SPSed SiC without sintering additive revealed inferior mechanical properties attributed to poor bonding between adjacent grains. In the present survey, an overview of the new developments in silicon carbide processing and properties will be presented together with the information on structure-properties correlationship. Information on the structure of the grain-boundary/secondary phases and interfaces until now was not comprehensively analyzed.

Effects of graphene on the thermal conductivity of pressureless-sintered SiC ceramics

“…For solid-state pressureless-sintered SiC ceramics using B 4 C and C as sintering additives, the densification process always accompanied with SiC grain coarsening. During the sintering process, smaller SiC grains, with small radii of curvature, tended to evaporate and condense on the larger particles, resulting in coarsening of the microstructure without the elimination of pores [20]. In addition, too much carbon (here means graphene) blocked the mass transport processes, and this was ineffective for densification.…”

Tyranno SA3 fiber–ZrB2 composites. Part I: Microstructure and densification

“…confirmed that the outer rim area, about 200 nm thick, had the tendency to coalesce and this region was characterized indeed by tight SiC grains, coarser than in the core of the fiber and with lower content of dark phase. This behavior is quite reasonable since C removal can induce coarsening and coalescence of the SiC crystallites fiber [11,17]. The fiber morphology is displayed in TEM images in Fig.…”

“…The fracture is mainly intergranular, owing to the presence of carbon itself acting as weak interphase. Excess of carbon is actually reported to inhibit grain growth and to maintain equiaxed microstructure [17].…”

Pressureless sintering of beta silicon carbide nanoparticles

“…23 Although many theories have been proposed to explain the means by which sintering aids such as boron or B 4 C activate sintering, no explanation has yet been clearly established. 12,13 According to Prochazka and Scanlan, 2 the high grain boundary to surface energy ratio (γ GB /γ SV ) makes it difficult to sinter pure silicon carbide. Boron was then thought to segregate to the grain boundary reducing their energy and thus decreasing γ GB /γ SV .…”

“…12,13 This study intends to report the pressureless sintering of nanocrystalline ␤ phase silicon carbide with addition of boron carbide and carbon. The activation energy of sintering was determined.…”