Direct ink writing and pressureless spark plasma sintering of ultra-high temperature ceramics for the production of complex parts with internal microchannels

A novel manufacturing route combining direct ink writing (DIW) with pressureless spark plasma sintering (SPS) has been developed to produce dense ultrahigh temperature ceramics (UHTC) components of TiB 2 , ZrB 2 , and ZrC incorporating internal microchannels, using 20 vol% MoSi 2 as additive. It is shown that the production of microchanneled parts requires careful control of the printing and drying conditions to prevent cracking. Inks formulated with carboxymethyl-cellulose as binder proved advantageous as they enabled deposition under humid air conditions followed by freeze‑drying. Results show that, among the three UHTC compositions investigated, TiB 2 exhibited optimal sinterability, achieving full densification (>97%) at 1650 °C, whereas ZrB₂ required slightly higher temperatures (1750 °C) and ZrC reached comparable densification only after significantly increasing both the temperature (up to 1950 °C) and the MoSi₂ content (to 25 vol%). Vickers indentation tests yielded hardness and toughness values consistent with literature reports, with TiB₂–MoSi₂ showing the best overall performance (hardness 26 ± 1 GPa; indentation toughness 7 ± 1 MPa·m 1/2 ). This, together with its superior sinterability, lightweightness and higher thermal conductivity, make this UHTC a highly promising candidate for aerospace applications in which the material requires active cooling to survive extreme temperature and environmental conditions. Nevertheless, ZrB₂ and ZrC may offer advantages in terms of oxidation resistance, elastic modulus and/or strength. The successful fabrication of dense parts of arbitrary shape based on TiB 2 , ZrB 2 , and ZrC demonstrates the robustness and versatility of the proposed manufacturing route for producing UHTC-based complex structural elements or devices.