Influence of vanadium content on the characteristics of spark plasma sintered ZrB₂–SiC–V composites

Spark plasma sintering was applied on ZrB₂–25 vol% SiC composites doped with 2, 4 and 6 wt% vanadium at 1900 °C for 7 min under 40 MPa pressure. A comprehensive microstructure-mechanical properties correlation study was carried out on the obtained near-fully dense composites. The composite samples were microstructurally studied via scanning electron microscopy and energy dispersive spectroscopy. Phase analysis and mechanical investigations were carried out by X-ray diffraction spectrometry and Vickers indentation method, respectively. The results indicated that vanadium not only promotes reactive sintering mechanism, but also increases the fracture toughness via transformation toughening mechanism, majorly due to the in-situ synthesized reinforcement phases such as VB₂. It was also found that besides the reinforcing effect of VB₂, the obtained indentation fracture toughness may not follow a distinct trend, particularly due to internal microcracking of VB₂, derived by its volume changes. Although increasing the vanadium content from 2 to 4 wt% resulted in enhanced hardness (from 22.1 to 22.9 GPa) and fracture toughness (from 4.0 to 4.5 MPa m^1/2), further vanadium addition up to 6 wt% worsened the hardness. Based on microstructural and phase analyses, such a decreased hardness was attributed to the increased volume fraction of low-melting point reactive sintering byproducts, particularly SiO₂ and VSi₂.