ZrB₂–SiC spiral fibers prepared by combining liquid rope effect with non-solvent-induced phase separation method: A promising toughening material for ultra-high temperature ceramics

Spiral fibers were considered to be an ideal toughening phase of ultra-high torsional release effect. In this work, ZrB₂ (Z)–20 vol% SiC (S) spiral fiber (ZS_sf) with controllable structure was prepared by a combination approach of liquid rope effect and non-solvent-induced phase separation. Dominantly depended on the kinematic viscosity (η), dropping height (H), and flow rate (Q), the geometric parameters of ZS_sf involving filament diameter (d) and coil diameter (D) were followed the relationship of d ≈ 0.516×10⁻³Q^1/2H^−1/4 and D ≈ 0.25×10^–3(Q/H)^1/3, respectively, within the optimized η of 10–15 Pa·s. Three different microstructures of ZS_sf were achieved by adjusting the polymer/solvent/non-solvent system assisted with phase diagram calculation, including dense, hollow, and hierarchical pore structures. The ZrB₂–SiC with 1 wt% ZS_sf composites prepared by hot isostatic pressing (HIP) exhibited a ~30% increase in fracture toughness (K_IC, 4.41 MPa·m^1/2) compared with the ZrB₂–SiC composite, where the microscopic fracture toughness of the ZS_sf was ~80% higher than that of the matrix. The fibers with a ~10 nm in-situ-synthesized graphite phase amongst grain boundaries of ZrB₂ and SiC changed the fracture mode, and promoted the crack deflection and pull-out adjacent the interface of matrix and the fiber.