The features of combustion and structure formation of ceramic materials in the Cr–Al–Si–B system

Abstract

Kinetics of the SHS process, stages of chemical transformations and structure formation of ceramic materials in the Cr–Al–Si–B multicomponent system were investigated. The effect of reaction mixtures composition and initial temperature on the combustion rate Uc and combustion temperature Tc, which reduce with increasing Al content, was studied. An increase in the initial temperature of the SHS process causes a linear increase of Uc and Tc in the range of T0=290–750K. This is evidence to the fact that each composition is characterized by the similar combustion mechanism, when the stages of chemical reactions of product formation remain unchanged. However, an increase in T0 above 750K, probably, may lead to exponential character of Uc growth. Furthermore, 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, thus reducing the effective activation energy approximately from ~290 to ~110kJ/mol. The stages of chemical transformations in the combustion wave were studied; the mechanism of structure formation was proposed. Firstly, the Al–Si eutectic mixture undergoes contact melting followed by formation of the reactionary surface as the melt spreads over the Cr and B particles surface. The melt is saturated with these elements followed by crystallization of CrB and Cr(Si,Al)2 grains. In the Cr- and B-rich areas and low melt concentration, the formation of CrB may occur by solid-phase interaction supported via gas-transport reaction. Dynamic X-ray diffraction confirmed the sequential formation of chromium monoboride CrB and after chromium silicide Cr5Si3 or chromium alumosilicide Cr(Si,Al)2. Force SHS–pressing was used to fabricate ceramic targets for magnetron sputtering of multicomponent coatings.