Compositionally complex (Hf,Ti,Zr,W)B2/(Hf,Ti,Zr,W)C-based ceramics were produced with different boride and carbide phase contents by varying the C/B4C molar ratio used in boro/carbothermal reduction. A monoboride phase based on WB was detected in all compositions. As the C/B4C ratio was varied, the WB content decreased from 23.7 vol% for a C/B4C ratio of 6.0-15.3 vol% for a C/B4C ratio of 10.3. Further increases in the C/B4C ratio increased the WB content. The monoboride phase contained mainly W and Ti in a solid solution with the nominal composition of (W0.86Ti0.14)B that change with variation in processing conditions. The highest hardness was 27.8 GPa for a ceramic containing 15.3 vol% WB. The Vickers hardness decreased as the content of the softer WB phase increased, indicating the hardness was governed by its content.

Dual-phase carbide-boride ceramics in different vanadium-Me (Me = Cr, Hf, Ti, and Zr) binary systems were synthesized by boro/carbothermal reduction under stoichiometric and carbon-deficient conditions and densified by spark plasma sintering. Thermodynamic analysis was used to evaluate the influence of composition on the phase stability and metal segregation between phases, along with the resulting effects on microstructure and hardness. Pairing vanadium with Group IV elements (Hf, Ti, and Zr) consistently formed one boride and one carbide phase, while the Cr-V system formed a monoboride phase and a carbon-deficient carbide. Metal segregation trends depended on composition. Vanadium preferentially segregated to the carbide phase in the Ti-V and Cr-V systems, while it was enriched in the boride phase in Hf-V and Zr-V systems. Hardness measurements showed a higher hardness for the Ti-V sub-stoichiometric system, reaching 27.7 +/- 0.9 GPa at 9.8 N, while the Cr-V sub-stoichiometric system presented the lowest hardness at 18.8 +/- 0.3 GPa at the same load. Notably, in the Zr-V system, thermodynamic predictions based solely on standard Gibbs energy deviated from experimental observations. However, when combined with first-principles calculations that accounted for non-stoichiometry in vanadium carbide, the predictions aligned more closely with experimental observations, qualitatively indicating a preference for vanadium segregation to the boride phase and zirconium to the carbide phase. These results suggest that elemental distribution between phases results from complex interactions beyond simple Gibbs free energy minimization, especially in systems like Zr-V with strong carbide-to-boride ratio dependence. The optimized systems were characterized with nominal compositions of (Ti0.61,V0.39)B2-(Ti0.39,V0.61)C0.9, (Hf0.12,V0.88)B2-(Hf0.88,V0.12)C0.9, (Zr0.29,V0.71)B2-(Ti0.71,V0.29)C0.9 and (Cr0.60,V0.40)B-(Cr0.40,V0.60)C0.8. Their thermodynamic interactions provided insights into metal segregation in dual-phase ceramics, demonstrating its strong composition dependence.

Interdiffusion of Zr and Hf in the diboride system was investigated. High‐purity zirconium diboride (ZrB 2 ) and hafnium diboride (HfB 2 ) were prepared by a combination of boro‐/carbothermal reduction synthesis and densification by spark plasma sintering. The bulk ceramics had relative densities of 99.3% for ZrB 2 and 99.7% for HfB 2 , and both ceramics were phase‐pure by x‐ray diffraction analysis. Diffusion couples of ZrB 2 and HfB 2 were annealed at temperatures ranging from 2000°C to 2200°C for times from 4 to 12 h. Zr and Hf elemental concentration profiles were measured using energy dispersive spectroscopy and analyzed using an algorithm based on a Boltzmann transformation of Fick's second law. The interdiffusion coefficients increased as the annealing temperature elevated, which indicates that the interdiffusion was a thermally activated process. The diffusion rate of Hf in ZrB 2 is smaller than that of Zr in HfB 2 due to the larger atomic mass and stronger bonding strength.

Interdiffusion of Zr and Hf in the diboride system was investigated. High-purity zirconium diboride (ZrB2) and hafnium diboride (HfB2) were prepared by a combination of boro-/carbothermal reduction synthesis and densification by spark plasma sintering. The bulk ceramics had relative densities of 99.3% for ZrB2 and 99.7% for HfB2, and both ceramics were phase-pure by x-ray diffraction analysis. Diffusion couples of ZrB2 and HfB2 were annealed at temperatures ranging from 2000 degrees C to 2200 degrees C for times from 4 to 12 h. Zr and Hf elemental concentration profiles were measured using energy dispersive spectroscopy and analyzed using an algorithm based on a Boltzmann transformation of Fick's second law. The interdiffusion coefficients increased as the annealing temperature elevated, which indicates that the interdiffusion was a thermally activated process. The diffusion rate of Hf in ZrB2 is smaller than that of Zr in HfB2 due to the larger atomic mass and stronger bonding strength.

The synthesis and characterization along with the resulting properties of fully dense (Cr,Mo,Ta,V,W)C high entropy carbide ceramics were studied. The ceramics were synthesized from metal oxide and carbon powders by carbothermal reduction, followed by spark plasma sintering at various temperatures for densification. Increasing the densification temperature resulted in grain growth and an increase in the lattice parameter. Thermal diffusivity increased linearly with testing temperature, resulting in thermal conductivity values ranging from -7 W/m center dot K at room temperature to -12 W/m center dot K at 200 degrees C. Measured heat capacity values matched theoretical estimates calculated using the Neumann-Kopp rule. Room temperature electrical resistivity decreased from -137 to -120 mu Omega center dot cm as the nominal excess carbon decreased from 5.4 to 0.1 vol%, suggesting an enhanced electronic contribution to thermal conductivity as excess carbon decreased. All specimens exhibited a Vickers hardness of -29 GPa under a 0.49 N load. These results underscore the tunability of this high entropy carbide system.

Understanding grain-boundary mobility during spark plasma sintering can enable microstructure control in high-entropy carbides, yet quantitative grain-growth kinetics remain scarce. In this work, grain growth kinetics and densification behavior were investigated for single-phase fully dense (Cr,Mo,Ta,V,W)C1-δ high-entropy carbide ceramics. Specimens were densified by spark plasma sintering for a constant dwell time of 10 min at temperatures between 1750 °C and 1950 °C to isolate the role of temperature on microstructural evolution. Increasing sintering temperature produced grain growth and increased lattice parameter, while maintaining a single-phase rock salt structure. Elemental mapping showed a progressive reduction of Ta segregation with increasing sintering temperature, suggesting enhanced chemical homogenization at elevated temperatures. Grain growth kinetics were analyzed using a normal grain growth model with an assumed growth exponent of n=3, physically reasonable for grain-boundary-controlled growth influenced by solute and vacancy pinning. Arrhenius analysis of the growth factor yielded an apparent activation energy of approximately 620 kJ mol-1, comparable to diffusion-controlled processes in refractory transition-metal carbides. Densification curves revealed rapid consolidation prior to reaching the peak temperature followed by temperature-dominated grain coarsening. These results establish quantitative relationships between densification temperature, grain growth, and diffusion kinetics in a carbide system, providing insight into the microstructural stability of high-entropy, ultra-high-temperature carbide ceramics.

High-entropy carbides (HECs) are often treated as a single material class defined by configurational entropy; however, their properties are governed by composition-specific defects and grain-boundary chemistry. This study demonstrates that substituting a single element can qualitatively change grain-boundary structure and fracture behavior in HEC ceramics. Two HECs, (Cr,Hf,Ta,Ti,Zr)C (HEC-Cr) and (Hf,Ta,Ti,W,Zr)C (HEC-W), were examined via electron microscopy, atom probe tomography, and molecular dynamics simulations. Simulations predict preferential segregation of Cr, W, and Zr to grain boundaries. Experiments confirmed Cr enrichment at grain boundaries and the formation of W-rich nanograins along boundary networks. Mechanical testing revealed that these compositional differences translated into grain-boundary cohesion differences: HEC-Cr exhibited intergranular fracture and lower compressive strength (2.66 GPa), whereas HEC-W exhibited transgranular fracture and higher strength (5.95 GPa). These findings establish grain-boundary segregation as a dominant mechanism linking composition to mechanical performance in HECs and underscore the advantages of integrated chemistry-microstructure design strategies rather than thermodynamics alone.

Fabricating large, monolithic ceramic parts using material-extrusion additive manufacturing remains challenging due to difficulty maintaining uniform moisture content during printing, which can lead to drying-induced defects such as warping and cracking, especially as part size and print time increase. Fabricated parts have trade-offs among print resolution, high throughput, and structural fidelity. Our study has shown that increasing the ratio of nozzle traverse speed vs. material extrusion speed increases filament stretching in viscoelastic ceramic paste, helping to overcome the trade-offs between resolution and throughput. Using aqueous ZrB2-SiC (70/30 vol.%) as a representative ultra-high temperature ceramic paste, rheological characterisation revealed viscoelastic yield-stress behaviour with strong shear-thinning properties. Filament behaviour was examined for different nozzle sizes, printing speeds, and layer heights. A critical balance was identified between the speed ratio and layer height to avoid filament instability such as necking and Rayleigh-Plateau instability while maintaining deposition continuity. Three different print approaches were evaluated to produce a representative compact heat exchanger as a large ceramic part with fine features, while addressing the part drying issue during the fabrication.

Dual-phase compositionally complex ultra-high temperature ceramics were formulated by incorporating different Groups V and VI metals such as V, Nb, Ta, Cr, Mo, or W into a base composition containing the Group IV elements, Hf, Ti, and Zr. Metal distribution was predicted using first-principles-based thermodynamics simulations and compared with experimental results. Moreover, phase stability, microstructure, and mechanical properties were evaluated for all of the ceramics. Compositions containing Cr, V, Nb, or Ta formed dual-phase ceramics containing only one boride and one carbide phase, while compositions containing Mo or W developed an additional third phase. The experimental metal distribution trends generally aligned with thermodynamic predictions, except for compositions containing V, which showed unexpected segregation behavior that was influenced by complex interactions of the coexistence of boride and carbide structures. From the dual-phase ceramics, the composition containing V exhibited the highest hardness (HV1 = 25.5 +/- 0.6 GPa) combined with smaller grain sizes (0.99 +/- 0.33 mu m for the boride and 1.15 +/- 0.31 mu m for the carbide phases). Our findings provide insights into phase formation and elemental segregation and help the design of next-generation dual-phase UHTCs with tailored properties.

Niobium (Nb) presents a challenge for achieving homogeneous solid solutions in high-entropy boride and carbide ceramics. In this work, we synthesized (Hf,Ta,Ti,Zr)B2 (Zr-HEB) and (Hf,Ta,Ti,Nb)B2 (Nb-HEB) ceramics by boro/carbothermal reduction, followed by densification via spark plasma sintering (SPS). Diffusion couples were prepared to investigate the interdiffusion behavior of Nb and zirconium (Zr) in these complex systems at temperatures between 2000 degrees C and 2200 degrees C. Aside from Nb and Zr, the other elements exhibited no significant concentration gradient across the diffusion interface. Nb exhibited lower diffusion coefficients ( Nb = 1.7 & times; 10-17-1.2 & times; 10-16 m2/s) compared to Zr ( Zr = 1.9 & times; 10-17-1.7 & times; 10-16 m2/s) over the temperature range of 2000 degrees C-2200 degrees C. However, at 2200 degrees C, their interdiffusion coefficients were comparable. The calculated activation energy for diffusion of Nb into Zr-HEB (Q Nb = 1008 +/- 76 kJ/mol) was nearly twice that of Zr into Nb-HEB (Q Zr = 565 +/- 54 kJ/mol). The slower diffusion of Nb is derived from the electronegativity difference between Nb and boron (B) and the bonding characteristics of NbB2 compared to the other diborides.

Zirconium carbide (ZrC) was fabricated by material extrusion additive manufacturing (AM) followed by pressureless sintering at 2000 degrees C for two hours, achieving a relative density of 90.3 %. SEM analysis revealed grains of 4.6 f 1.8 mu m; XRD confirmed single phase ZrC. A stoichiometry of ZrC0.92 was determined by XPS. Four-point flexure testing exhibited a strength of 331.3 f 57.1 MPa and Young's modulus of 232.8 f 13.1 GPa. Vickers hardness was 13.6 f 1.0 GPa and 11.6 f 0.5 GPa at 4.91 and 9.81 N, respectively. Indentation fracture resistance was 2.9 f 0.2 MPa center dot m1/2; Griffith analysis confirmed the largest grains as the critical flaw. Thermal conductivity increased with temperature and was 13.4 W/m center dot K at room temperature. Electrical resistivity was 122.4 f 0.5 mu Omega center dot cm. This work adds to the viability of additively manufactured ZrC and is the first to report detailed thermal and mechanical properties for ZrC produced by material extrusion AM.

Multi-metal compositionally complex (CC) AlB2-type diboride solid solutions (SS) containing IV-V-VI group elements are garnering a steadily growing research interest. The present work explores the composition dependence of oxidation resistance on nominally equiatomic (Hf,Me,Ta,Ti,Zr)B2 SS, Me = Nb or Cr, exposed to a dissociated air plasma (total pressure of about 1 kPa): specimens were exposed for 5 min at some selected set points between 1800 K and 2500 K using a solar furnace. The CC diboride with Nb showed superior resistance to oxidation compared to that with Cr. The experimental results showed good agreement with predictions based on a thermodynamic assessment.

A dual phase boride and carbide ceramic with the nominal composition (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 and (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C was prepared by reactive synthesis and consolidated by spark plasma sintering. The resulting microstructure contained about 30% (in volume) boride and 70% carbide. Compositional inhomogeneities were observed within single grains that had core-shell structures and preferential accumulation of specific metals in the boride or carbide phases. Specifically, Ti and Nb had higher concentrations in the boride, whereas Hf and Ta in the carbide. The Zr concentration was relatively equally distributed in the two phases. The dual phase ceramic had additional, distinctive features including nanosized inclusions, possibly related to local miscibility gaps and supersaturation, linear defects, and strain due to adjustment of the crystal structure. As a consequence, the fracture mode was transgranular with the crack path deviated by these nanometric microstructure alterations. Nanoindentation under 5 mN measured higher hardness and modulus for the boride, 30 GPa and 525 GPa, as compared to the carbide phase, 22 GPa and 425 GPa, due to a higher concentration of dislocation tangles and strains deriving from the introduction of metals with different sizes (and properties) in a less compliant hexagonal lattice.

A highly loaded, aqueous-based zirconium carbide (ZrC) paste was developed for ceramic on-demand extrusion (CODE). Commercial ZrC powder was ball milled to reduce the particle size to -1 mu m. Paste composition was determined by rheological characterization and observation of printing behavior; the final paste consisted of 46-47 vol% ZrC, 43.5 vol% distilled water, 8.1 vol% dispersant, and 1.6 vol% binder. Rheological characterization of the paste showed a yield stress of -8 Pa and shear thinning behavior (Delta G '/Delta sigma* = -55). The paste was printed using a 610 mu m nozzle; a test print showed shape fidelity using the developed paste. A pressureless sintering study was performed; a sintering temperature of 2000 degrees C with a 2-hour hold produced samples of 90 % relative density with 3.8 mu m grains, which was consistent with literature. The developed paste, in combination with the pressureless densification results, adds a new, ultra-high temperature material to ceramic extrusion additive manufacturing.

Ultrahigh temperature ceramic matrix composites (UHTCMCs) were fabricated from unidirectional prepreg tapes consisting of a matrix of ZrB 2 with 5, 10, and 15 vol.% ZrSi 2 additions and continuous polyacrylonitrile carbon fibers and were densified at 1600°C in a hot press. The relative matrix densities ranged from 88% to 93% with interlayer spacings of ∼72 µm and fiber volume fractions between 30% and 36%. Phenolic resin additions were utilized to react with ZrSi 2 acting as a transient sintering aid to form ZrC and SiC phases. Elastic moduli of the UHTCMCs decreased with increasing temperature during 4‐pt flexure testing. The elastic moduli values were 95–122 GPa at room temperature and ∼26 GPa for all three compositions at 1800°C. Strength increased as test temperature increased for each UHTCMC with varying rollover temperatures for each composition. Strength started decreasing above 1400°C for the 5 vol.% ZrSi 2 composition, above 1600°C for the 10 vol.% ZrSi 2 composition and did not decrease for the 15 vol.% ZrSi 2 composition over the range of temperatures tested. The work of fracture increased from 0.3 to 1.9 kJ/m 2 at room temperature to 18 to 20 kJ/m 2 for each UHTCMC at 1800°C. All UHTCMCs displayed graceful failure beyond the initial fracture event.

Dual phase mid entropy carbide/ boride ceramics were produced by boro/carbothermal reduction followed by spark plasma sintering. Based on the transition metal distribution in the dual phase ceramics, individual mid entropy boride and carbide ceramics were produced. The hardness value for the dual phase ceramic was 43.5 +/- 5.1 GPa at a load of 0.49 N while value calculated using a volumetric rule of mixtures was 36.2 GPa based on measured hardness values of the constituents. Thermal conductivity of the optimized dual phase ceramic was similar to 15 % higher than the constituent single phases. The dual phase ceramic also had an electrical resistivity that was similar to 35 % lower than the value calculated from the individual constituents. For the dual phase ceramic, the phonon contribution was approximately 44 % of the total conductivity, indicating that the mid entropy boride phase had a dominant effect on the electrical resistivity of the dual phase ceramic.

A range of (Zr,Nb)C ceramics containing 20-50 vol% NbC were spark plasma sintered at 2000 degrees C. The ceramics had relative densities greater than 97 %, and all compositions formed a complete solid solution. The average grain size was similar to 7.1 mu m over all compositions. Elastic moduli values ranged from 436 to 445 GPa, and the fracture toughness ranged from 0.8 to 2.8 MPam(1/2) over the compositional range, both increasing with increasing NbC content. The hardnesses of the (Zr,Nb)C ceramics ranged from 19.1 to 20.2 GPa at a load of 9.81 N. The flexural strength increased from 352 to 395 MPa as the NbC content decreased from 50 to 20 vol%. The successful solid solution formation and densification of the (Zr,Nb)C ceramics provide a surrogate framework for substituting UC for the NbC to produce (Zr,U)C ceramics for nuclear applications.

An ultra-high temperature ceramic based on ZrB2, TiB2, and SiC was hot pressed to full density at 1850 degrees C. Addition of 5 vol% of different metal-compounds, in the form of HfC, VC, NbC or CrB2, increased the densification temperature to 1910 degrees C. The resulting compositionally complex ceramics had homogeneous microstructures with boride grains exhibiting core-shell features. The shell was a solid solution containing variable amounts of the three metals. Notably, TiB2 remained as a discrete phase in the reference material and in the presence of the V- and Cr- based additions, whilst it dissolved into the main ZrB2-based grains for other additives. Thermodynamic simulations and atomic size factors were exploited to explain the different solubility in the various systems. The compositionally complex diborides exhibited excellent properties at room temperature, with hardness up to 25 GPa and strength up to 800 MPa, which was preserved up to 1500 degrees C. However, increasing the testing temperature to 1800 degrees C resulted in plastic deformation owing to residual carbide phases. Electrical resistivity ranged between similar to 13 and 70 mu Omegacm, with higher values in those ceramics where TiB2 remained as a discrete phase. The observed overall properties improvements in compositionally complex borides pave the way for tailored design of UHTC materials with multication non-equiatomic composition for applications in extreme environments.

The room and elevated temperature flexural strengths were measured for (Hf,Nb,Ta,Ti,Zr)B2-(Hf,Nb,Ta,Ti,Zr)C high-entropy dual-phase ceramics. The fracture of ceramics originated from two distinct flaw populations that were either surface defects introduced during specimen preparation or volume defects introduced during processing. The average strength at room temperature was 638 +/- 94 MPa with individual bars strengths as high as similar to 800 MPa. At 1800 degrees C, average strength increased to 786 +/- 265 MPa with individual bar strengths as high as similar to 1050 MPa. Above 1800 degrees C, creep dominated resulting in deformation of the bars without fracture. The fracture mode changed from completely transgranular at room temperature to mixed mode with increasing temperature and then to completely intergranular at 1800 degrees C. The room temperature strength was higher than the reported strengths of the individual constituent phases, which was attributed to fine grain sizes. At elevated temperatures, the increase in strength could be due to the relaxation of stresses in addition to the grain size. To our knowledge, this is the first study reporting the flexural strength of (Hf,Nb,Ta,Ti,Zr)B2-(Hf,Nb,Ta,Ti,Zr)C dual-phase ceramics at elevated temperatures.