Flash sintering of silicon carbide with boron and carbon sintering aids

Thesis

The aims of this doctoral thesis were to assess the feasibility of using flash sintering to manufacture silicon carbide and to understand how the technique affects its density, microstructure and mechanical properties.

An experimental apparatus was developed that allowed silicon carbide bar shaped specimens with boron and carbon sintering aids to be flash sintered to 96% density (with an average grain size of 4 µm) in 16 minutes in an alumina tube furnace at 1500 °C. Compared with samples of similar density that were conventionally sintered using the powder’s recommended sintering schedule, the production time of the flash sintered samples was reduced by more than 6 hours and with a furnace temperature lower by 700 °C. Both methods sintered silicon carbide it in the liquid phase. Of the investigated flash sintering parameters, higher power limits, higher furnace temperatures, slower power ramp rates, and longer hold times were all found to increase the bulk density and the average grain size of specimens. The addition of carbon sources in the furnace tube during sintering was beneficial for sample densification, but could be detrimental if in close proximity to the sample. The additions also retarded grain growth in specimens and increased the time that samples could be flash sintered before fracturing from 16 minutes to 1.5 hours.

The Vickers hardness of most of the silicon carbide samples in this work was ∼24 GPa. The inclusion of carbon additions in the furnace tube during sintering significantly increased the Vickers hardness of silicon carbide: 27.8 GPa ± 0.4 GPa. This increase in hardness was independent of density, grain size and polytype concentration but is suggested to be related to a carbon concentration threshold in the sintering atmosphere. Atom probe tomography analysis indicated that the super-hard silicon carbide samples had a greater concentration of boron rich clusters than silicon carbide of regular hardness. These clusters are suggested to cause dispersion hardening in these samples. Flash sintering did not degrade the strength or the fracture toughness of silicon carbide. If flash sintered silicon carbide was successfully scaled up, the material may outperform traditional silicon carbide armour plates in ballistic trials.

The flash event in a non-oxide ceramic was shown to be phenomenologically similar to that found in oxide ceramics: characterised by a rapid rise in sample temperature followed by densification and a decrease in sample resistivity. This behaviour was shown to be consistent with thermal runaway; the origin of which was found to be the early stages of sintering.

Authors: Gibson, AR

• DOI: 10.5287/ora-qqnom2rey
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: mechanical characterisation; silicon carbide; boron and carbon sintering additives; microstructure; flash sintering
• Subjects: Materials science