Abstract
<p>This study is dedicated to investigation of the combustion mechanisms during the SHS of ceramic materials in multicomponent Mo–Si–B and Cr–Al–Si–B systems. It is concluded that the following processes are defined the SHS for Si-rich Mo–Si–B compositions: Si melting, its spreading over the surfaces of the solid Mo and B particles, followed by B dissolution in the melt, and formation of intermediate Mo<sub>3</sub>Si-phase film. The subsequent diffusion of Si into Mo results in the formation of MoSi<sub>2</sub> grains and MoB phase forms due to the diffusion of Mo into B-rich melt. The formation of MoB phase for B-rich compositions may occur via gas-phase mass transfer of MoO<sub>3</sub> gaseous species to B particles and B<sub>2</sub>O<sub>2</sub> to Mo particles. In Cr–Al–Si–B system firstly, the Al–Si eutectic mixture undergoes contact melting followed by formation of the reactionary surface as the eutectic melt spreads over the Cr and B particles surface. An increase in Al content increases the proportion of the Al–Si eutectic melt. The dissolution of Cr particles in this melt becomes the rate-limiting stage of the combustion process. The melt is saturated with these elements followed by crystallization of CrB and Cr(Si,Al)<sub>2</sub> grains. In the Cr- and B-rich areas and low melt concentration, the formation of CrB may occur via gas-phase mass transfer of B<sub>2</sub>O<sub>2</sub> gaseous species to Cr particles. Consecutive formation of chromium and molybdenum borides and silicides is established by means of dynamic X-ray diffraction analysis. Compact ceramic samples were produced using forced SHS pressing technique. Their structural investigations were conducted by XRD and SEM.</p>
Highlights
Borides and silicides of molybdenum and chromium are promising compounds because of their unique combination of mechanical, thermal and physical properties
Mo2) a detailed analysis of the composition of the different phases formed in the combustion zone, after-combustion zone and in the final products area, it was found that in the combustion front (Fig. 1a) the silicon particles are melting and this melt spreading over the surface of molybdenum particles with simultaneous chemical interaction, yielding the intermediate product in the form of an underlayer close to composition of the silicide Mo3Si
Grains of disilicide MoSi2 formed in the outer layer due to mutual reaction diffusion of molybdenum and silicon
Summary
Borides and silicides of molybdenum and chromium are promising compounds because of their unique combination of mechanical, thermal and physical properties. The high melting point, heat resistance, low oxidation resistance at temperatures above 1200 °C, relatively high thermal and electrical conductivity, make these compounds attractive for the development of advanced materials. These materials can be used as targets (cathodes) in the ion-plasma technologies for deposition of functional (heat-resistant and wear-resistant) nanostructured coatings, for example by magnetron sputtering (PVD). During combustion of two-component Mo–B and Cr–B mixtures in [1, 2] was noted the leading role of the gas-transport reactions which determine the supply of reagent to the reaction surface.
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