Diborides of transition metals: Properties, application and production. review. Part 2. Chromium and zirconium diborides

The second part of the review considers properties, application and methods for producing chromium and zirconium diborides. These diborides are oxygen-free refractory metal-like compounds. As a result, they are characterized by high values of thermal and electrical conductivity. Their hardness is relatively high. Chromium and zirconium diborides exhibit significant chemical resistance in aggressive environments. They have found application in modern technology because of these reasons. Chromium diboride is used as a sintering additive to improve the properties of ceramics based on boron carbide and titanium diboride. Zirconium diboride is a component of advanced ultra-high temperature ceramics (UHTC) ZrB2 –SiC used in supersonic aircrafts and in gas turbine assemblies. Ceramics B4C–CrB2 and B4C–ZrB2 have high-quality performance characteristics, in particular, increased crack resistance. The properties of refractory compounds depend on the content of impurities and dispersion. Therefore, to solve a specific problem associated with the use of refractory compounds, it is important to choose the method of their preparation correctly, to determine the admissible content of impurities in the starting components. This leads to the presence of different methods for the borides synthesis. The main methods for their preparation are: a) synthesis from elements; b) borothermal reduction of oxides; c) carbothermal reduction (reduction of mixtures of metal oxides and boron with carbon; d) metallothermal reduction of metal oxides and boron mixtures; e) boron-carbide reduction. Plasma-chemical synthesis (deposition from the vapor-gas phase) is also used to obtain diboride nanopowders. Each of these methods is described.

Diborides of transition metals: Properties, application and production. Review. Part 1. Titanium and vanadium diborides

The properties, applications and methods for producing titanium and vanadium diborides are considered. These diborides are oxygen-free refractory metal-like compounds. As a result, they are characterized by high values of thermal and electrical conductivity. Their hardness is relatively high. Titanium and vanadium diborides exhibit significant chemical resistance in aggressive environments. For these reasons, they have found application in modern technics. So, they are used as surfacing materials when applying wear-resistant coatings on steel products. It is also possible to use vanadium diboride as a catalyst in organic synthesis and the anode in renewable electrochemical current sources. Perspective are ceramics B4C – TiB2 and B4C – VB2 , which make it possible to obtain products based on boron carbide with high-quality performance characteristics, in particular, with increased crack resistance. Such composite ceramics are obtained by means of hot pressing, spark plasma sintering and pressureless sintering. The properties of refractory compounds depend on the content of impurities and dispersion. Therefore, to solve a specific problem associated with the use of refractory compounds, it is important to choose the method of their preparation correctly, to determine the admissible content of impurities in the starting components. This leads to the presence of different methods for the synthesis of borides. The main methods for their preparation are: synthesis from simple substances (metals and boron); borothermal reduction of oxides; carbothermal reduction (reduction of mixtures of metal oxides and boron with carbon; metallothermal reduction of mixtures of metal oxides and boron; carbide-boron reduction. Plasma-chemical synthesis (deposition from the vapor-gas phase) is also used to obtain diboride nanopowders. Each of these methods is characterized in the article.

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.

Optimal preparation of high-entropy boride-silicon carbide ceramics

High-entropy boride-silicon carbide (HEB-SiC) ceramics were fabricated by using boride-based powders prepared from borothermal and boro/carbothermal reduction methods. The effects of processing routes (borothermal reduction and boro/carbothermal reduction) of HEB powders were examined. HEB-SiC ceramics with nearly relatively full density (>98%) were prepared by spark plasma sintering at 2000 °C. It was demonstrated that the addition of SiC led to slightly coarsening of the microstructure. The HEB-SiC ceramics prepared from boro/carbothermal reduction powders showed the fine-grained microstructure and higher Vickers’ hardness but lower fracture toughness values 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.

Optimal preparation of high-entropy boride-silicon carbide ceramics

High-entropy boride-silicon carbide (HEB-SiC) ceramics were fabricated by using boride-based powders prepared from borothermal and boro/carbothermal reduction methods. The effects of processing routes (borothermal reduction and boro/carbothermal reduction) of HEB powders were examined. HEB-SiC ceramics with nearly relatively full density (>98%) were prepared by spark plasma sintering at 2000oC. It was demonstrated that the addition of SiC led to slightly coarsening of the microstructure. The HEB-SiC ceramics prepared from boro/carbothermal reduction powders showed the fine-grained microstructure and higher Vickers’ hardness but lower fracture toughness values 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.

Optimal Preparation of High-entropy Boride-silicon Carbide Ceramics

High-entropy boride-silicon carbide (HEB-SiC) ceramics were fabricated by using boride-based powders prepared from borothermal and boro/carbothermal reduction methods. The effects of processing routes of HEB powders were examined between borothermal reduction and boro/carbothermal reduction. HEB-SiC ceramics with nearly relatively full density (>98%) were prepared after spark plasma sintering at 2000oC. It was demonstrated that the addition of SiC led to the slightly microstructure coarsening. The HEB-SiC ceramics prepared from boro/carbothermal reduction powders showed the fine-grained microstructure and higher Vickers’ hardness but lower fracture toughness values 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.