Sintering behavior and mechanical properties of spark plasma-sintered ZrB2 with the addition of boron nitride nanotubes

Zirconium diboride (ZrB₂) is a promising ultrahigh temperature ceramic (UHTC) for aerospace applications due to its high thermal conductivity and oxidation resistance, but its low fracture toughness restricts broader use. In this study, boron nitride nanotubes (BNNTs) were incorporated into ZrB₂ matrices using spark plasma sintering (SPS) across a wide range of temperatures (1300–2000 °C) and BNNT contents (0–5 wt%), extending beyond the narrow conditions explored in prior reports. X-ray diffraction and scanning electron microscopy revealed that BNNTs partially transformed into h-BN above 1300 °C, coexisting with residual nanotubes at intermediate temperatures, and were uniformly distributed along ZrB₂ grain boundaries at higher temperatures. The BN phases located at grain boundaries effectively suppressed ZrB₂ grain growth, leading to a refined microstructure. High-resolution TEM further identified a thin amorphous interfacial layer at the ZrB₂/BN boundary, which, together with h-BN platelets, promoted crack deflection, delamination, and interfacial debonding. Quantitative crack path analysis confirmed these mechanisms collectively enhanced toughness, raising the fracture toughness of the 5 wt% BNNT composite to 6.2 MPa·m^0.5—more than double that of monolithic ZrB₂. Despite reduced hardness and thermal conductivity at higher BNNT contents, the uniformly distributed BN phases provided effective toughening. This study systematically establishes the microstructural evolution and toughening mechanisms of ZrB₂–BNNT composites, offering practical guidelines for designing tougher UHTCs.