Critical role of ZrO2 on densification and microstructure development in spark plasma sintered NbB2

The present work reports the densification and microstructure development in niobium diboride (NbB₂) containing ZrO₂, consolidated using four stage spark plasma sintering with final holding at 1750°C for 2 min. The adopted processing approach enables to obtain fully dense (99.6% ρ_th) NbB₂-ZrO₂ ceramic with good mechanical properties. The sintered microstructure is characterized by the uniform dispersion of intragranular ZrO₂ particles within the NbB₂ grains. Fine scale microstructure analysis using high resolution transmission electron microscopy (HRTEM) together with HAADF (high angle annular dark field) technique reveals the dispersion of t-ZrO₂ or o-ZrO₂ and the defect structure including edge dislocations and asymmetric twins in o-ZrO₂. An attempt has been made to rationalize the enhanced densification in terms of quantitative analysis of 'constriction resistance' induced localized heating and defect structure. The evaluation of basic mechanical properties revealed that a combination of high hardness (∼22 GPa) together with moderate indentation toughness (∼5.2 MPa m^1/2) can be achieved with solid state sintered NbB₂ containing ZrO₂. The present study, for the first time, conclusively establishes the role of zirconia in obtaining dense NbB₂ with good mechanical property combination at relatively lower sintering temperature of 1750 °C.