National University of Science and Technology “MISiS”, Moscow, Russia

S. Vorotilo, Post-Graduate Student at the Department of Powder Metallurgy and Functional Coatings, e-mail: s.vorotilo@misis.ru
Yu. S. Pogozhev, Senior Researcher of the Scientific and Education Center SHS, e-mail: yspogozhev@mail.ru
A. Yu. Potanin, Researcher of the Scientific and Education Center SHS, e-mail: a.potanin@inbox.ru

In this work, macrokinetic features of the combustion of mixture Mo – Hf – Si – B, mechanisms of phase and structure formation in the combustion front as well as structure and properties of combustion products were investigated. It was demonstrated that self-heating is initiated when the reaction mixtures are pre-heated above 150 ^oС. At Т₀ = 25 ^oС the combustion temperature Т_c = 2050 ^oС, while the combustion velocity U_c = 22 mm/s. However, when Т₀ is increased to 230 ^oС, partial oxidation of the components (molybdenum and boron) occurs, leading to the self-heating of the reaction mixture and subsequent decrease of combustion temperature to 1770 ^oС and combustion velocity to 14.5 mm/s. Further increase of Т₀ leads to the linear increase of Т_c(Т₀) and U_c(Т₀). Using the time-resolved XRD method, mechanisms of the structural and phase transformations in the combustion front of Mo – Hf – Si – B mixture were studied. Primarily, in the pre-heating zone, the formation of HfB₂ takes place via gas-transport reaction. Subsequently, in the combustion zone, the silicon melts and spreads over the surface of particles of Mo, Hf, and B, with the dissolution of B in the Si melt. MoSi₂ grains are formed on the boundary between the melt and Mo via the reaction diffusion of Si into Mo, while the borides of Mo and Hf are formed as a result of diffusion of Mo and Hf into the melt. In the aftercombustion zone, the equilibrium structure is formed, comprised of MoSi₂ grains with the size up to 15 μm, surrounded by the interlayers of MoB with the size of 2–4 μm and needle-like grains of HfB₂ with the size of 0,5–1 μm. Hot pressing of the SHS powder produced the hierarchically organized ceramic composite MoSi₂ – MoB – HfB₂ with a high hardness HV₁₀ of 17.6 GPa and porosity less than 1%.

This research was funded under the Federal Target Programme “Research and development initiatives in the priority areas of Russian science and technology for 2014–2020”, Grant Agreement No. 14.578.21.0227 (Project ID: RFMEFI57817X0227).

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