  Ultra-high-temperature ceramics (UHTCs) are candidate structural materials
for applications that require resiliency to extreme temperature (>2000{\deg}C),
high mechanical loads, or aggressive oxidizing environments. Processing UHTC
transition metal carbides as standalone materials using additive manufacturing
(AM) methods has not been fully realized due to their extremely slow atomic
diffusivities that impede sintering and large volume changes during indirect AM
that can induce defect structures. In this work, a two-step, reactive AM
approach was studied for the formation of the ultra-high temperature ceramic
TiCx. Readily available equipment including a polymer powder bed fusion AM
machine and a traditional tube furnace were used to produce UHTC cubes and
lattice structures with sub-millimeter resolution. This processing scheme
incorporated, (1) selective laser sintering of a Ti precursor mixed with a
phenolic binder for green body shaping, and (2) ex-situ, isothermal gas-solid
conversion of the green body in CH4 to form TiCx structures. Reactive
post-processing in CH4 resulted in up to 98.2 wt% TiC0.90 product yield and a
reduction in net-shrinkage during consolidation due to the volume expansion
associated with the conversion of Ti to TiC. Results indicated that reaction
bonding associated with the Gibbs free energy release associated with TiC
formation produced interparticle adhesion at low furnace processing
temperatures. The ability to bond highly refractory materials through this type
of process resulted in structures that were crack-free and resisted fracture
during thermal shock testing. Broadly, the additive manufacturing approach
presented could be useful for the production of many UHTC carbides that might
otherwise be incompatible with prevailing AM techniques that do not include
reaction synthesis.
