Spark Plasma Sintering of Variable SiC alpha/beta Ratio with Boron and Carbon Additions-Microstructure Transformation

This study investigated the possibility of obtaining dense silicon carbide sinters with the use of a spark plasma sintering (SPS) process by changing the composition of SiC alpha/beta polymorphs in a starting powder mixture. Amorphous boron was used as the basic additive to activate the sintering processes. Some of the compositions were prepared with additional carbon in two different forms: multilayer graphene flakes and carbon black. The well-described effect of the beta-alpha transition in the form of elongated lamellar grains in the sintered structure was confirmed. The obtained sinters were analyzed qualitatively and quantitatively in terms of the microstructure and density. The hardness and the participation of the polytypes in the sinter structures were examined. During the study, SPS sintering allowed us to obtain a material with a density close to the theoretical (relative density of 99.5% and hardness of 27 MPa) without the addition of carbon. It was found that the role of carbon was not limited to the activation of the sintering process. Additional effects accompanying its presence, depending on the initial alpha/beta composition, included grain size reduction and an influence on the transformation kinetics.