Self-Propagating High-Temperature Synthesis of the Heterophase Materials in the Zr–Mo–Si–B System: Kinetics and Mechanisms of Combustion and Structure Formation

Abstract

The paper is devoted to the study of the combustion kinetics and mechanisms of elemental mixtures in the Zr–Mo–Si–B system, as well as the analysis of phase and structural transformations in the combustion wave. A thermodynamic analysis of possible chemical reactions occurring in combustion wave is carried out. In the range of 298–2500 K, the reaction of ZrB₂ formation is preferred. Above 2200 K, the formation of MoB becomes more thermodynamically advantageous compared to MoSi₂. Estimates of the phase stability of synthesis products have shown that the phases ZrB₂, MoSi₂, and MoB are in equilibrium. The experimental dependences T_c(T₀) and U_c(T₀) are linear, which implies an unchanged combustion mechanism at T₀ = 298–800 K. Preheating leads to an increase in U_c. Similarly, the increase in the proportion of Zr and B in the mixture has a similar effect, which increases heat emission and T_c. With a minimum content of Zr and B, the interaction of Mo with Si with the formation of MoSi₂ by the mechanism of reaction diffusion is decisive. With an increase in the proportion of Zr and B, the rise of T₀ to 750 K does not affect the T_c. The E_eff values (50–196 kJ/mol) confirm the significant influence of liquid-phase processes on the combustion kinetics. The mechanism of structure formation has been studied. In the combustion front, a Si–Zr–Mo melt is formed. The primary grains of ZrB₂ and MoB crystallize from this melt as it is saturated with boron. At the same time, the melt spreads over the surface of Zr and Mo particles. This leads to formation of ZrSi_x and MoSi_x films. Core-shell structures are formed behind the combustion front, which disappear as they move towards the post-combustion zone. The phase composition of the products is formed in the combustion front in less than 0.25 s.