Optimal preparation of high-entropy boride-silicon carbide ceramics

High-entropy boride-silicon carbide (HEB-SiC) ceramics were fabricated using boride-based powders prepared from borothermal and boro/carbothermal reduction methods. The effects of processing routes (borothermal reduction and boro/carbothermal reduction) on the HEB powders were examined. HEB-SiC ceramics with > 98% theoretical density were prepared by spark plasma sintering at 2000 °C. It was demonstrated that the addition of SiC led to slight coarsening of the microstructure. The HEB-SiC ceramics prepared from boro/carbothermal reduction powders showed a fine-grained microstructure and higher Vickers’ hardness but lower fracture toughness value as compared with the same composition prepared from borothermal reduction powders. These results indicated that the selection of the powder processing method and the addition of SiC phase could contribute to the optimal preparation of high-entropy boride-based ceramics

Reaction Mechanisms and Microstructures of Ceramic-metal Composites Made by Reactive Metal Penetration

Ceramic-metal composites can be made by reactive penetration of molten metals into dense ceramic preforms. The metal penetration is driven by a large negative Gibbs energy for reaction, which is different from the more common physical infiltration of porous media. Reactions involving Al can be written generally as (x+2)Al + (3/y)MOy → Al2O3 + M3/yAlx, where MOy is an oxide, such as mullite, that is wet by molten Al. In low Po2 atmospheres and at temperatures above about 900°C, molten Al reduces mullite to produce Al2O3 and Si. The Al/mullite reaction has a AGr°(1200K) of-1014 kJ/mol and, if the mullite is fully dense, the theoretical volume change on reaction is less than 1%. A microstructure of mutually-interpenetrating metal and ceramic phases generally is obtained. Penetration rate increases with increasing reaction temperature from 900 to 1150°C, and the reaction layer thickness increases linearly with time. Reaction rate is a maximum at 1150°C; above that temperature the reaction slows and stops after a relatively short period of linear growth. At 1300°C and above, no reaction layer is detected by optical microscopy. Observations of the reaction front by analytical transmission electron microscopy show only Al and Al2O3 after reaction at 900°C, but Si is present in increasing amounts as the reaction temperature increases to 1100°C and above. The kinetic and microstructural data suggest that the deviation from linear growth kinetics at higher reaction temperatures and longer times is due to Si build-up and saturation at the reaction front. The activation energy for short reaction times at 900 to 1150°C varies from ∼90 to ∼200 kJ/mole, depending on the type of mullite precursor

High-Entropy Metal Diborides: A New Class of High-Entropy Materials and a New Type of Ultrahigh Temperature Ceramics

Seven equimolar, five-component, metal diborides were fabricated via high-energy ball milling and spark plasma sintering. Six of them, including (Hf(0.2)Zr(0.2)Ta(0.2)Nb(0.2)Ti(0.2))B(2), (Hf(0.2)Zr(0.2)Ta(0.2)Mo(0.2)Ti(0.2))B(2), (Hf(0.2)Zr(0.2)Mo(0.2)Nb(0.2)Ti(0.2))B(2), (Hf(0.2)Mo(0.2)Ta(0.2)Nb(0.2)Ti(0.2))B(2), (Mo(0.2)Zr(0.2)Ta(0.2)Nb(0.2)Ti(0.2))B(2), and (Hf(0.2)Zr(0.2)Ta(0.2)Cr(0.2)Ti(0.2))B(2), possess virtually one solid-solution boride phase of the hexagonal AlB(2) structure. Revised Hume-Rothery size-difference factors are used to rationalize the formation of high-entropy solid solutions in these metal diborides. Greater than 92% of the theoretical densities have been generally achieved with largely uniform compositions from nanoscale to microscale. Aberration-corrected scanning transmission electron microscopy (AC STEM), with high-angle annular dark-field and annular bright-field (HAADF and ABF) imaging and nanoscale compositional mapping, has been conducted to confirm the formation of 2-D high-entropy metal layers, separated by rigid 2-D boron nets, without any detectable layered segregation along the c-axis. These materials represent a new type of ultra-high temperature ceramics (UHTCs) as well as a new class of high-entropy materials, which not only exemplify the first high-entropy non-oxide ceramics (borides) fabricated but also possess a unique non-cubic (hexagonal) and layered (quasi-2D) high-entropy crystal structure that markedly differs from all those reported in prior studies. Initial property assessments show that both the hardness and the oxidation resistance of these high-entropy metal diborides are generally higher/better than the average performances of five individual metal diborides made by identical fabrication processing