Properties, Applications, and Production of Diborides of Some Transition Metals: Review. Part 2. Chromium and Zirconium Diborides

Abstract

Abstract: Part two of the review considers the properties, applications, and methods of producing chromium and zirconium diborides. These diborides are oxygen-free, refractory metal-like compounds characterized by high values of thermal and electrical conductance and relatively high hardness. Chromium and zirconium diborides exhibit significant chemical resistance in aggressive environments. Thus, they have found application in modern engineering. Chromium diboride is used as a sintering additive to improve the properties of boron carbide and titanium diboride ceramics. Zirconium diboride is a component of advanced ultrahigh temperature ZrB2–SiC ceramics (UHTC) used in supersonic aircrafts and gas turbine assemblies. B4C–CrB2, and B4C–ZrB2 ceramics 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 and determine the admissible content of impurities in the primary components. This leads to the diversity of existing methods of synthesizing borides. The main methods of their preparation are synthesis from elements, boron thermal reduction of oxides, carbothermal reduction (carbon reduction of mixtures of metal oxides and boron), metallothermic reduction of metal oxides and boron mixtures, and boron-carbide reduction. There is also plasma-chemical synthesis (deposition from the vapor-gas phase) used to obtain diboride nanopowders. Each of these methods is described.