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A dual phase boride and carbide ceramic with the nominal composition (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2and (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.

[Display omitted] • In the present study, Debye-Callaway model was first employed to calculate the lattice thermal conductivities of zirconium carbide with different carbon vacancy contents. • Key parameters for calculating the lattice thermal conductivity which includes Grüneisen parameters, Debye temperatures, and phonon group velocities were calculated based on the phonon dispersion. • The effects of average atomic mass, grain size, average atomic volume and Zr isotopes on the lattice thermal conductivities of zirconium carbide were also calculated using phonon scattering models. • Debye-Callaway model correctly predicted the trends of ZrCx lattice thermal conductivities with increasing carbon vacancy content. Lattice thermal conductivities of zirconium carbide (ZrC x , 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 Grüneisen parameters, Debye temperatures, and phonon group velocities were deduced from phonon dispersions of ZrC x 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 ZrC x were analyzed using phonon scattering models. The lattice thermal conductivity decreased as temperature increased for ZrC, ZrC 0.75 and ZrC 0.5 (Zr 2 C), and decreased as carbon vacancy concentration increased. Intriguingly, ZrC x can be tailored from a thermal conducting material for ZrC with high lattice thermal conductivity to a thermal insulating material for ZrC 0.5 with low lattice thermal conductivity. Thus, it opens a window to tune the thermal properties of ZrC x by controlling the carbon vacancy content. [ABSTRACT FROM AUTHOR]

Equilibrium Gibbs' free energy calculations were used to determine metal segregation trends between boride and carbide solid solutions containing two metals that are relevant to dual phase high entropy ceramics. The model predicted that Ti had the strongest tendency to segregate to the boride phase followed by Zr, Nb, Mo, V, Hf, and Ta, which matches experimental results of measured compositions. The ratio of a metal in the carbide phase to the content of the same metal in the corresponding metal boride had a linear trend with the change in standard Gibbs' free energy of reaction for a metal carbide reacting with B4C to produce its corresponding metal boride and carbon. The proposed model was used to predict the changes in standard Gibbs' free energy for CrC→CrB2to be −260 kJ and WC→WB2to be 148 kJ, which indicates that Cr has the strongest segregation to the boride and W has the strongest segregation to the carbide. The proposed model can be used to estimate the segregation of metals in dual phase high entropy boride-carbide ceramics of any boride/carbide ratio or metal content.

An extrusion-based additive manufacturing process, called the Ceramic On-Demand Extrusion (CODE) process, for producing three-dimensional ceramic components with near theoretical density is introduced in this paper. In this process, an aqueous paste of ceramic particles with a very low binder content (<1vol%) is extruded through a moving nozzle at room temperature. After a layer is deposited, it is surrounded by oil (to a level just below the top surface of most recent layer) to preclude non-uniform evaporation from the sides. Infrared radiation is then used to partially, and uniformly, dry the just-deposited layer so that the yield stress of the paste increases and the part maintains its shape. The same procedure is repeated for every layer until part fabrication is completed. Several sample parts for various applications were produced using this process and their properties were obtained. The results indicate that the proposed method enables fabrication of large, dense ceramic parts with complex geometries.

Ceramic matrix composites (CMCs) based on zirconium diboride (ZrB2) reinforced by vapor grown carbon nanofibers are a potential constituent of reusable thermal protection systems. A manufacturing procedure was devised that involved the fabrication of thin films by tape casting to obtain a layer that could be integrated into a more complex system. Higher thermal conductivities and improved toughness can be expected for nanofiber additions, as compared to the matrix alone. Consolidation by hot‐pressing was more effective than pressureless sintering, in terms of the final relative density and flatness of specimens. Examination of microstructures showed that few carbon nanofibers were present in the matrix after consolidation by sintering, which was attributed to a reaction between the nanofibers and zirconium oxide present on the surface of the ZrB2powder. As a solution, oxygen impurities from the boride powders were removed by reduction with carbon coatings derived from phenolic resin. The deleterious reaction was avoided, but residual carbon remained at the grain boundaries, likely from decomposition of the binder. The use of an alternative binder (PMMA vs. PVB) will be used in future studies to reduce the residual carbon content. Further, consolidation by Spark Plasma Sintering (SPS) will be explored to further reduce the reaction of surface oxides with the nanofibers. Finally, characterization of the microstructure at the nanometric level and further determination of the mechanical and thermal properties will be conducted as part of future studies.

The effect of tungsten additions on the oxidation behavior of zirconium diboride-based ceramics was studied. Four mole percent tungsten carbide was added to ZrB2. The oxidation behavior was studied using thermal gravimetric analysis and isothermal testing in flowing air. Upon heating to 1500{degree sign}C, the mass gain decreased from ~14 mg/cm2 for nominally pure ZrB2 to ~4.5 mg/cm2 for tungsten containing ZrB2. After heating to 1500{degree sign}C for three hours, the scale thickness on nominally pure ZrB2 was ~500 um compared to ~100 um for tungsten containing ZrB2. Tungsten additions improved the oxidation resistance of ZrB2 by modifying the morphology of the ZrO2 scale by liquid phase sintering, providing an improved barrier to oxygen diffusion.

The effect of tungsten additions on the oxidation behavior of zirconium diboride-based ceramics was studied. Four mole percent tungsten carbide was added to ZrB2. The oxidation behavior was studied using thermal gravimetric analysis and isothermal testing in flowing air. Upon heating to 1500{degree sign}C, the mass gain decreased from ~14 mg/cm2 for nominally pure ZrB2 to ~4.5 mg/cm2 for tungsten containing ZrB2. After heating to 1500{degree sign}C for three hours, the scale thickness on nominally pure ZrB2 was ~500 µm compared to ~100 µm for tungsten containing ZrB2. Tungsten additions improved the oxidation resistance of ZrB2 by modifying the morphology of the ZrO2 scale by liquid phase sintering, providing an improved barrier to oxygen diffusion.