Synthesis features, structure and properties of promising high-temperature ceramics in the Hf-Ta-B-Ti-Si system

The study covers the elemental synthesis features of Hf-Ta-B-Ti-Si ceramic materials used to obtain promising high-temperature ceramics and analyze its structure and properties. The macrokinetics of self-propagating high-temperature synthesis (SHS) were studied. Combustion temperature and velocity as a function of initial temperature were plotted. It was established that chemical interactions occurring in the liquid phase play a pivotal role in the combustion process. Structure and phase formation processes were studied using the stopped combustion front technique. The mechanism of phase formation in the combustion wave was determined. The primary crystals of hafnium, titanium and tantalum diborides are precipitated from the super-saturated melt after the Si and Ti contact melting and B, Hf and Ta dissolution in the melt through the reactive diffusion process. A two-phase structure consisting of complex solid solutions based on diboride and borosilicide is formed due to the similarity of the crystal lattices. Porous synthesis products of the specified composition were milled into powders with the required particle size distribution for subsequent hot pressing (HP) or spark plasma sintering (SPS). It was found that specimens produced by HP, SPS, and SHS pressing feature a similar phase composition containing solid solutions based on diboride (Hf,Ti,Ta)B₂ and borosilicide (Hf,Ti,Ta)₅Si₃B. Specimens were made of ceramics produced using the above technologies for physical-mechanical testing. It was found that the hardness and elastic modulus of (Hf,Ti,Ta)B₂ solid solution are 2-3 times higher than that of (Hf,Ti,Ta)₅Si₃B borosilicide. Depending on composition, the density of ceramics produced varied from 8 to 6.5 g/cm³, which corresponds to a porosity of less than 5 %. Temperature dependences of heat capacity and diffusivity were determined. The heat conductivity of ceramics produced by HP and SPS was 24.05 and 23.1 W/(m•K), respectively.

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