Solid solution formation during spark plasma sintering of ZrB₂–TiC–graphite composites

Densification and mechanical behavior of graphite-free and graphite-doped ZrB₂–TiC composites were investigated. Spark plasma sintering was used to achieve near fully-dense composites. Microstructural and phase analysis were carried out via scanning electron microscopy and X-ray diffraction spectroscopy, to illustrate the sintering and toughening mechanisms in the fabricated samples. Results indicated that 1 wt% graphite nano-flakes can improve the hardness of the composite. However, 3% drop in relative density and ~6% decrease in indentation fracture toughness were observed. The formation of TiB₂ and ZrC was verified in both TiC-contained composites, although B₄C was recognized as the byproduct of reactive sintering in graphite-doped composite. Moreover, the microstructural analysis and the peak shifts in XRD pattern indicated the formation of a solid solution between the ZrB₂ and TiB₂ phases. Higher hardness of the graphite-doped sample was also attributed to the formation of B₄C as a superhard interfacial phase. Toughening mechanisms as well as possible chemical reactions which result in the in-situ formed reinforcement phases were also discussed.