Silicon carbide (SiC) possesses excellent mechanical, electrical and thermal properties that have since made it the epicenter of much research. Material scientists have persistently been investigating the high hardness it possesses despite having low density. Lots of publications report on very hard SiC ceramics, which can be effectively applied in environments requiring high endurance, such as car brakes, car clutches and ceramic plates in bulletproof vests.
SiC can present many polytypic varieties ranging from cubic (β-SiC or 3C-SiC) to hexagonal (α-SiC) type, differing from each other by the stacking sequence of the bilayers constitutive of their structure. Many researchers have investigated the 3C-SiC polytype, which possesses superior electronic and mechanical properties, and generally concluded that it always destabilized under high temperature conditions to form one of the hexagonal polytypes denoted 6H-SiC. The isotropic properties of β-SiC have been shown to be of significance in very demanding applications, such as high power electronics and extreme specific stiffness materials. Nonetheless, in the literature, SiC developed for ballistic applications is never made of pure β-SiC since high temperature densification always induce the transition from the β-phase to the α-phase.
Recently, scientists at the French-German research Institute of Saint-Louis, Dr. Florimond Delobel, Dr. Sébastien Lemonnier, Dr. Élodie Barraud, and Dr. Julien Cambedouzou (ENSCM), investigated in detail the relationship between the hexagonal and cubic phase of SiC during sintering using X-ray diffraction and Raman scattering analyses. They focused on demonstrating that the pressure applied during spark plasma sintering had a strong influence on the polytypic transformation in SiC. Their work is currently published in the Journal of the European Ceramic Society.
For this purpose, two sources of β-SiC – i.e. a commercial powder and a powder derived from a preceramic polymer precursor – were chosen, and the investigation was undertaken at specific temperature and pressure ranges.
The analyses undertaken revealed that the 3C-6H phase transition depended on the sintering pressure, which was shown to stabilize the cubic phase. In addition, the stability domains of the cubic phase as a function of sintering pressure and temperature were determined for both powdery precursors and revealed that the cubic phase stability was also linked to the nature of powdery precursor and in particular to its crystallinity state.
In summary, the described approach enabled observation of the 3C-6H transition of SiC according to sintering parameters. Generally, application of high-pressure during sintering was confirmed to stabilize the 3C form of SiC and to increase the temperature of the 3C-6H transition. The team further showed that it was possible to obtain pure cubic SiC reaching a density of 95%. Altogether, these findings open interesting possibilities that could enable better control of SiC sintering, improve the densification of 3C-SiC pellets, and consequently increase their performance for highly demanding mechanical applications.
Florimond Delobel, Sébastien Lemonnier, Élodie Barraud, Julien Cambedouzou. Influence of sintering temperature and pressure on the 3C-6H transition of silicon carbide. Journal of the European Ceramic Society, volume 39 (2019) page 150–156.Go To Journal of the European Ceramic Society