Silicon Carbide (SiC) is a high temperature structural material with excellent chemical, mechanical and thermal properties. However, SiC has a high melting point and highly covalent bonding which make its sintering extremely difficult. Therefore, there are many studies on different techniques for the production of highly densified SiC.

This work focuses on the use of high pressure and high temperature (HPHT) to obtain densified SiC composites. The samples were made by mixing, in a high energy mill, microsized β-SiC powder (H.C. Starck, grade BF 12) with two different metal powders as additives: Aluminium (10 wt.%) or Titanium (12 wt.%).  The compacts were produced at high pressures (7.7 GPa) and high temperatures (up to ~ 2000°C) using a toroidal-type high-pressure chamber. The heating and cooling rates were about 300°C/min and the samples were kept at the highest temperature during 5 min. After surface polishing with diamond pastes down to 0.25 mm, the Vickers microhardness of the sintered bodies was measured. Their phase composition and microstructure were investigated by X-ray diffraction (XRD), and scanning electron microscopy (SEM) with X-ray energy dispersive spectrometry (EDS).

For samples produced using only the SiC powder as also for those produced with the addition of Ti, the XRD showed a very slight β-SiC to α-SiC phase transformation  for processing temperatures above 1900°C. The samples produced with the addition of Al showed a significant β-SiC to 4H-SiC phase transformation for processing temperatures above 1900°C.

The microstructure observed by SEM showed an important grain growth for the samples processed with Ti as additive. All the Ti reacted to form large agglomerates of TiC in the sample processed at 2000°C, which showed a hardness  of ~19 GPa.

For the samples produced with addition of Al, a residual quantity of the metallic additive could be observed for processing temperatures lower than 1500°C. The sample processed at 7.7 GPa/1500°C showed the highest hardness ( ~22 GPa) and only β-SiC was identified by XRD. In the samples sintered at higher temperatures the β-SiC to 4H-SiC transformation could be observed.

All the studied systems reached advanced sintering stages. The use of high pressure allowed the production of high hardness compacts without requiring very high temperatures and/or long sintering times.