Reactive spark plasma sintering of SiC-TiC-diamond composites

Abstract

SiC-TiC-diamond composites were fabricated via spark plasma sintering at hold temperatures of 1600 °C, 1625 °C, and 1650 °C from a powder blend containing by weight 30 % Si, 30 % Ti, and either 40 % uncoated or TiC-coated diamond. Due to the sintering conditions, sp3-diamond decomposed into thermodynamically favorable sp2‑carbon, reacting with Si and Ti to form SiC and TiC at T ≥ 1600 °C. As the hold temperature increased, the composite density reduced due to excess graphite formation for the uncoated diamond composites. However, at too low of a temperature the porosity fraction increased in the SiC-TiC matrix for the TiC-coated diamond composites. Overall, the results revealed that composites with ~97 % theoretical density and minimal (up to 5 vol%) graphite can be achieved by sintering Si-Ti-uncoated diamond at 1600 °C and Si-Ti-TiC-coated diamond at 1625 °C. X-ray diffraction, electron and x-ray microscopies with imaging analyses were carried out to quantify the volume fractions of each composite phase and used to explain the composite densification behavior. Conventional and energy filtered transmission electron microscopy, in addition to electron energy loss spectroscopy, were conducted to understand the nature of diamond graphitization with respect to the nucleation and growth of SiC, TiC, and nanoscale graphite.