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A nominally pure and dense (TiCr)B ceramic was produced by spark plasma sintering of powders synthesized by boro/carbothermal reduction of oxides. The synthesized powders were a single phase and had an average particle of 0.4 ± 0.1 ?m and an oxygen content of 1.2 wt%. Average Vickers hardness values of the resulting ceramics increased from 25.9 ± 0.8 GPa at a load of 9.81 N, to 46.3 ± 0.8 GPa at a load of 0.49 N. Compared to the nominally pure TiB ceramic obtained under the same processing conditions, the (TiCr)B ceramic had higher values under the same load due to the finer average grain size (2.4 ± 1.0 ?m), higher relative density, and solid solution hardening. The results indicated that the Cr addition promoted densification, suppressed grain growth, and improved the hardness of TiB ceramics. This is the first report for dense and single-phase (Ti,Cr)B ceramics as superhard materials.

Zirconium diboride ceramics were prepared with additions of up to 50 vol.% TiB2. The resulting (Zr,Ti)B2 ceramics formed complete solid solutions based on x-ray diffraction. The addition of TiB2 resulted in grain size decreasing from 22 μm for nominally pure ZrB2 to 7 μm for ZrB2–50 vol.% TiB2. The thermal conductivity at 25°C ranged from 93 W/m⋅K for nominally pure ZrB2 to 58 W/m⋅K for ZrB2–50 vol.% TiB2. Thermal conductivity was as high as 67 W/m⋅K for nominally pure ZrB2 at 2000°C, but dropped to 59 W/m K with the addition of 50 vol.% TiB2. Electrical resistivity measurements were used to calculate the electron contribution to thermal conductivity, which was 76 W/m⋅K for nominally pure ZrB2 decreasing to 57 W/m⋅K when 50 vol.% TiB2 was added. The phonon contribution to thermal conductivity did not change significantly for ≤10 vol.% TiB2. Additions of ≥25 vol.% TiB2 reduced the phonon contribution to nearly zero for all temperatures.

Zirconium Carbide (ZrCx) was irradiated with 10 MeV Au3+ ions to a dose of 10 displacements per atoms (dpa) and subsequently with 100 and 300 keV electrons in a transmission electron microscope (TEM). After ion irradiation, dislocation loops were observed in the microstructure and an increase in the number of carbon vacancies was revealed by Raman spectroscopy. Grazing incidence X-ray diffraction (GIXRD) analysis showed that neither amorphization nor oxidation occurred during ion irradiation of the specimen. Subsequent electron irradiation of the pre-implanted ZrCx foil led to formation of nanosized tetragonal ZrO2 precipitates (5−10 nm diameter) on the surface of the TEM lamella. The formation of the new oxide phase was not related to the electron beam-induced heating of the specimen, but to electron stimulated oxidation caused by the residual oxygen inside the transmission electron microscope. Changes in size and density of ZrO2 crystallites were observed between the pristine and ion irradiated ZrCx regions following electron irradiation, suggesting that the initial microstructure of the ZrCx substrate played a key role in the nucleation and growth of the oxide islands. The obtained results provide insights into the microstructural response of ZrCx to different types of radiation and the inadvertent effects of the electron beam during TEM analysis of in-situ and ex-situ ion irradiated ZrCx. Additionally, the findings of this work suggest a method to prepare local ZrO2 nanoprecipitates within ZrCx grains by selective electron beam irradiation.

Densification behavior, microstructure, and mechanical properties of zirconium diboride (ZrB2) ceramics modified with a complex Zr/Si/O-based additive were studied. ZrB2 ceramics with 5-20 vol.% additions of Zr/Si/O-based additive were densified to >95% relative density at temperatures as low as 1400 degrees C by hot-pressing. Improved densification behavior of ZrB2 was observed with increasing additive content. The most effective additive amount for densification was 20 vol.%, hot-pressed at 1400 degrees C (similar to 98% relative density). Microstructural analysis revealed up to 7 vol.% of residual second phases in the final ceramics. Improved densification behavior was attributed to ductility of the silicide phase, liquid phase formation at the hot-pressing temperatures, silicon wetting of ZrB2 particles, and reactions of surface oxides. Room temperature strength ranged from 390 to 750 MPa and elastic modulus ranged from 440 to 490 GPa. Vickers hardness ranged from 15 to 16 GPa, and indentation fracture toughness was between 4.0 and 4.3 MPa center dot m(1/2). The most effective additive amount was 7.5 vol.%, which resulted in high relative density after hot-pressing at 1600 degrees C and the best combination of mechanical properties.

Lattice thermal conductivities of zirconium carbide (ZrCx, x = 1, 0.75 and 0.5) ceramics with different carbon vacancy concentrations were calculated using a combination of first-principles calculations and the Debye-Callaway model. The Gruneisen parameters, Debye temperatures, and phonon group velocities were deduced from phonon dispersions of ZrCx determined using first-principles calculations. In addition, the effects of average atomic mass, grain size, average atomic volume and Zr isotopes on the lattice thermal conductivities of ZrCx were analyzed using phonon scattering models. The lattice thermal conductivity decreased as temperature increased for ZrC, ZrC0.75 and ZrC0.5 (Zr2C), and decreased as carbon vacancy concentration increased. Intriguingly, ZrCx can be tailored from a thermal conducting material for ZrC with high lattice thermal conductivity to a thermal insulating material for ZrC0.5 with low lattice thermal conductivity. Thus, it opens a window to tune the thermal properties of ZrCx by controlling the carbon vacancy content.

Mechanical properties and microstructure were compared for zirconium diboride and two zirconium diboride solid solutions containing 3 and 6 at% tantalum diboride. X-ray diffraction indicated that the ceramics were nearly phase-pure and that tantalum dissolved into the ZrB2 lattice to form (Zr,Ta)B2 solid solutions. Microstructural analysis indicated that samples achieved nearly full relative density with average grain sizes that ranged from 3−5 μm. The three compositions had similar values of Young’s modulus (510−531 GPa), shear modulus (225−228 GPa), Vickers hardness (15.2–16.4 GPa), and flexural strength (391−452 MPa). Fracture toughness ranged from 2.6 to 3.7 MPa m1/2 and with increasing tantalum content, the fracture mode changed from predominantly intergranular to predominantly transgranular. Diboride solid solution materials had comparable properties to the single metal diboride, but differences in microstructure, secondary phases, and strain state among the three ceramics partially obscured the actual effects of the solid solution on fracture behavior.

Various SiC-ZrB2-ZrC ceramics were joined by fusion welding to determine the maximum silicon carbide content that could be joined. Commercial powders were hot pressed, machined, and preheated to 1450 °C before joining with a tungsten inert gas welding torch at 160–200 A. Resulting welds were cross-sectioned and analyzed to determine which compositions were weldable and to characterize microstructural evolution in welded samples. As compositions approached the ternary eutectic, the welds had smaller SiC grains and exhibited better weldability. Penetration depth of welds was controlled by a combination of current input and welding speed. The ternary eutectic in the system was found at 36.9 ± 1.3 vol% SiC, 42.7 ± 1.5 vol% ZrB2, and 20.4 ± 1.9 vol% ZrC and its melting temperature was 2330 ± 23 °C. A ternary phase diagram for the SiC-ZrB2-ZrC was constructed and proposed via microstructural analysis of arc melted pellets on binary joins between each binary eutectic and the ternary eutectic in the system.

The Ceramic On-Demand Extrusion (CODE) process has been recently proposed for additive manufacturing of dense, strong ceramic components via extrusion with uniform layered drying. This study focuses on enabling CODE to fabricate functionally graded ceramics. A controlled volumetric flowrate for each ceramic paste was used to achieve a gradient between alumina and zirconia. A dynamic mixer was built to mix constituent ceramic pastes homogeneously. Functionally graded alumina/zirconia samples were printed, sintered, and tested to examine the capability of CODE in fabricating functionally graded components. The desired and actual material compositions were compared using energy dispersive spectroscopy. Dimensions of sintered samples were evaluated to study the deformation of functionally graded components during drying and sintering. Vickers hardness was also measured at different locations, corresponding to different material compositions. Finally, a case study was conducted to demonstrate the capability of the proposed method to build functionally graded ceramics with complex geometries.

Oxidation studies of ZrB2 were performed under wet air and dry air conditions at 1200, 1400 and 1500°C for 1, 4 and 10h. Compared to dry air, the presence of water vapor was found to enhance the oxidation kinetics by a factor of 7 to 30, depending on the temperature. Thermodynamic calculations suggested that water vapor promoted the formation of additional volatile species such as boric acid (HBO2), in addition to boria (B2O3) produced in dry air, which increased the evaporation rate of B2O3. Compared to dry air, the presence of water vapor leads to more rapid evaporation of boria and the transition from parabolic oxidation kinetic behavior (i.e., rate controlled by diffusion through boria) to linear (i.e., underlying ZrB2 is directly exposed to the oxidizing environment) at shorter times and lower temperatures.

Dense (Hf0.2,Zr0.2,Ti0.2,Ta0.2,Nb0.2)B2 high-entropy ceramics with high phase purity were produced by two-step spark plasma sintering of precursor powders synthesized by boro/carbothermal reduction of oxides. The reacted powders had low oxygen (0.404 wt%) and carbon (0.034 wt%) contents and a sub-micron average particle size (∼0.3 μm). Powders were synthesized by optimizing the excess B4C content of the reaction mixture and densified by a two-step spark plasma sintering process. The relative density increased from 98.9% to 99.9% as the final sintering temperature increased from 2000 °C to 2200 °C. The resulting ceramics were nominally single-phase (Hf,Zr,Ti,Ta,Nb)B2 with oxygen contents as low as 0.004 wt% and carbon as low as 0.018 wt%. The average grain size increased from 2.3 ± 1.2 μm after densification at 2000 °C to 4.7 ± 1.8 μm after densification at 2100 °C, while significant grain growth occurred during sintering at 2200 °C. The high relative densities, low oxygen and carbon contents, and fine grain sizes achieved in the present study were attributed to the use of synthesized precursor powders with high purity and fine particle size, and the two-step synthesis-densification process. These are the first reported results for dense high-entropy boride ceramics with high purity and fine grain size.

A tungsten carbide ceramic containing 5 vol% silicon carbide was hot pressed to full density at 1820 °C. A small amount of transient liquid phase based on W-C-Si-O facilitated oxide removal in the reducing environment and favoured the development of a bimodal microstructure containing sub-micrometric grains with square or rod-like morphologies. These microstructural features led to outstanding mechanical properties from room to elevated temperatures. For the first time, WC-materials were characterized up to 1500 °C exhibiting flexural strength over 1 GPa in the whole temperature range and fracture toughness from 7 to 15 MPa⋅√m.

The microstructural evolution was characterized for ZrC ceramics irradiated with 10 MeV Au3+ ions at 800 °C. Post-irradiation examination showed that ZrC did not amorphize at doses up to 30 displacement per atoms (dpa). Concurrent oxidation of ZrC was found to occur during ion irradiation. Coarsening of the defective microstructure, as a function of dose, was revealed by transmission electron microscopy analysis. Black dot defects were observed at low doses (0.5 dpa), and tangled dislocation networks were formed at 5 dpa and above. Diffraction analysis showed a change in the defect structure occurred at doses close to ∼2.5 dpa. The evolution of lattice parameter with dose indicated that uptake of adventitious oxygen could occur in specimens irradiated at high doses. Raman spectroscopy analysis indicated an increase in non-stochiometry after irradiaton. This work identified specific relationships between dose and microstructure after irradiation, revealing the mechanisms of damage production in ZrCx ceramics.

ZrC ceramics containing 30 vol% SiC-ZrB2 were produced by high-energy ball milling and reactive hot pressing. The effects of ZrB2 content on the densification, microstructure, and mechanical properties of ceramics were investigated. Fully dense ceramics were achieved as ZrB2 content increased to 10 and 15 vol%. The addition of ZrB2 suppressed grain growth and promoted dispersion of the SiC particles, resulting in fine and homogeneous microstructures. Vickers hardness increased from 23.0 ± 0.5 GPa to 23.9 ± 0.5 GPa and Young’s modulus increased from 430 ± 3 GPa to 455 ± 3 GPa as ZrB2 content increased from 0 to 15 vol%. The increases were attributed to a combination of the higher relative density of ceramics with higher ZrB2 content and the higher Young’s modulus and hardness of ZrB2 compared to ZrC. Indentation fracture toughness increased from 2.6 ± 0.2 MPa⋅m1/2 to 3.3 ± 0.1 MPa⋅m1/2 as ZrB2 content increased from 0 to 15 vol% due to the increase in crack deflection by the uniformly dispersed SiC particles. Compared to binary ZrC-SiC ceramics, ternary ZrC-SiC-ZrB2 ceramics with finer microstructure and higher relative densities were achieved by the addition of ZrB2 particles.

The densification behavior of transition metal diboride compounds was reviewed with emphasis on ZrB2 and HfB2. These compounds are considered ultra-high temperature ceramics because they have melting temperatures above 3000?C. Densification of transition metal diborides is difficult due to their strong covalent bonding, which results in extremely high melting temperatures and low self-diffusion coefficients. In addition, oxide impurities present on the surface of powder particles promotes coarsening, which further inhibits densification. Studies prior to the 1990s predominantly used hot pressing for densification. Those reports revealed densification mechanisms and identified that oxygen impurity contents below about 0.5 wt% were required for effective densification. Subsequent studies have employed advanced sintering methods such as spark plasma sintering and reactive hot pressing to produce materials with nearly full density and higher metallic purity. Further studies are needed to identify fundamental densification mechanisms and further improve the elevated temperature properties of transition metal diborides.

Transition metal diborides with core-shell microstructures have demonstrated excellent mechanical properties at elevated temperatures. Previous studies concluded that core-shell microstructures were formed by liquid-assisted mass transport mechanisms, but in this study, we propose a solid-state formation mechanism for core-shell microstructures in (Zr,Ta)B ceramics produced by reaction hot pressing and in ZrB-TaB diffusion couples. Diffusion couple experiments demonstrated that core-shell microstructures developed as a result of Ta diffusion along ZrB grain boundaries, which occurred concurrently with lattice diffusion of Ta into ZrB. These findings suggest that with optimization of batching and processing parameters, core-shell diboride materials may be formed through solid-state processes rather than liquid-assisted processes, which could assist in raising the upper temperature limits of use for these materials.

The influence of carbon fibre content on the mechanical behaviour of HfC/SiC composites was investigated up to 2100 degrees C for specimens containing 40 or 55 vol% fibres. Silicon carbide was added as a sintering aid during hot pressing. Increasing the fibre content made infiltration more difficult, which resulted in higher porosity in the specimen with 55 vol% fibres. The room temperature flexural strength ranged from 340 to 380 MPa, and it increased to more than 400 MPa at 1800 degrees C due to stress relaxation. Increasing temperature was accompanied by a decrease in the slope of the load-displacement curve, indicating a decrease in elastic modulus, but plastic deformation was not observed below 2100 degrees C. At 2100 degrees C, the specimen containing a higher fibre content underwent significant deformation due to low interfacial strength between the fibre plies, retaining a strength at the proportional limit of 290 MPa and an ultimate strength of 520 MPa.

Zirconium carbide (ZrCx) powders were synthesized at temperatures between 1300℃ and 2000℃ by solid state reaction of zirconium hydride (ZrH2) and carbon black. Crystal structure, lattice parameters, and grain sizes of the as-synthesized ZrCx powders were characterized for two different starting ZrH2:C ratios of 1:0.60 and 1:0.98. Powders with stoichiometry approaching ZrC0.98 were synthesized at temperatures as low as 1600℃ whereas ZrCx powders synthesized at lower temperatures had lower carbon contents regardless of the starting ZrH2:C ratio. Crystallite sizes as small as about 50 nm were obtained due to the low synthesis temperature. Oxygen dissolved into the ZrCx lattice when carbon vacancies were present. Neutron diffraction analysis was used to determine that carbon stoichiometry increased and dissolved oxygen content decreased as synthesis temperature increased.

The mechanical behavior of ZrB2-MoSi2 ceramics made of ZrB2 powder with three different particle sizes and MoSi2 additions from 5 to 70 vol% was characterized up to 1500 °C. Microhardness (12–17 GPa), Young’s modulus (450–540 GPa) and shear modulus (190–240 GPa) decreased with both increasing MoSi2 content and with decreasing ZrB2 grain size. Room temperature fracture toughness was unaffected by grain size or silicide content, whilst at 1500 °C in air it increased with MoSi2 and ZrB2 grain size, from 4.1 to 8.7 MPa m½. Room temperature strength did not trend with MoSi2 content, but increased with decreasing ZrB2 grain size from 440 to 590 MPa for the largest starting particle size to 700–800 MPa for the finest due to the decreasing size of surface grain pullout. At 1500 °C, flexure strength for ZrB2 with MoSi2 contents above 25 vol% were roughly constant, 400–450 MPa, whilst for lower content strength was controlled by oxidation damages. Strength for compositions made using fine and medium ZrB2 powders increased with increasing MoSi2 content, 250–450 MPa. Ceramics made with coarse ZrB2 displayed the highest strengths, which decreased with increasing MoSi2 content from 600 to 450 MPa.

This study systematically correlates processing with quantitative microstructural information over an extended compositional range for ZrB2-MoSi2 ceramics, with MoSi2 contents ranging from 5 to 70 vol% and diboride starting particle sizes ranging from 3 to 12 μm. Fifteen different ceramics were hot pressed between 1750 and 1925 °C. Plastic deformation of MoSi2 contributed to initial densification, but some of the MoSi2 decomposed during the later stages of hot pressing. Finer diboride particles required lower temperatures to densify (1750 to 1850 °C) compared to coarser diboride particles (1900 °C). Increasing MoSi2 content led to a decrease in sintering temperature. As MoSi2 content increased, ZrB2 grain size decreased and MoSi2 cluster size increased. Starting powders with lower impurity contents and isothermal vacuum holds contributed to lower oxide impurity contents in the final ceramics. A diboride core-shell microstructure involving (Zr1-x,Mox)B2 solid solutions formed in all compositions with Mo contents in the solid solution shells ranging from 3 to 6 at%. This work identified specific relationships between starting composition, processing conditions and final microstructure, showing how microstructure and properties could be tailored by processing. The outcomes of this extensive study will serve as guidelines for the design of other structural ceramics that have to attain determinate thermo-mechanical properties for targeted applications.