Achieving superior strength-toughness synergy in ZrC-based ceramics: An <i>in situ</i> multiscale construction strategy via a two-step reactive SPS process

Abstract To overcome the poor sinterability and low fracture toughness (KIC) of zirconium carbide (ZrC) ceramics, a novel multiscale microstructure design was proposed via a two-step in situ reactive spark plasma sintering (SPS) process using ZrC, TiSi2, and B4C powders. During sintering, TiSi2 preferentially reacted with B4C to form TiB2 and primary silicon carbide (SiC), while the released Si further reacted with ZrC to yield ZrSi2 and secondary SiC. The two-step SPS (1600 °C/3 min + 1800 °C/10 min) promoted complete in situ reactions, liquid-phase sintering, and interdiffusion of Zr/Ti, leading to the formation of (Zr,Ti)C and (Ti,Zr)B2 solid solutions. With the addition of 30 mol% TiSi2 and 15 mol% B4C, the multiphase ceramics exhibited a refined submicrostructure (grain size < 500 nm), achieving a high flexural strength of 824±46 MPa and KIC of 7.5±0.5 MPa·m1/2. The synergistic enhancement in strength and toughness is attributed to a multiscale strengthening/toughening mechanism: solid-solution strengthening at the atomic scale, effective grain boundary pinning by nanosized primary and secondary SiC particles at the nanoscale, and toughening through crack deflection and bridging by TiB2–SiC agglomerates and the higher-toughness ZrSi2 phase at the microscale. This work provides a viable and innovative approach for designing high-performance ultrahigh-temperature ceramics through tailored in situ reactions and microstructural control.