Self-propagating high-temperature synthesis of heterophase materials in the Zr–Mo–Si–B system. Kinetics and mechanisms of combustion and structure formation

Abstract

The paper focuses on 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 transformation stages in the combustion wave. A thermodynamic analysis of potential chemical reactions occurring in the combustion wave was carried out. The reaction of ZrB2 formation is preferred in the range of 298–2500 K. Above 2200 K, the formation of MoB becomes more thermodynamically advantageous as compared to MoSi2. Phase stability estimates of combustion products showed that ZrB2, MoSi2 and MoB phases are in equilibrium. Experimental dependences Тc(Т0) and Uc(Т0) are linear, which implies an unchanged combustion mechanism at T0 = 298÷800 K. Preheating leads to an increase in Uc. Similarly, an increase in the proportion of Zr and B in the mixture has a similar effect, i.e. an increase in heat emission and Tc. With a minimum content of Zr and B, the interaction between Mo and Si with the formation of MoSi2 by the reaction diffusion mechanism is decisive. As the proportion of Zr and B increases, the rise of T0 to 750 K does not affect the Tc. Eeff values (50–196 kJ/mol) confirm the significant influence of liquid-phase processes on the combustion kinetics. The mechanism of structure formation was studied. A Si–Zr–Mo melt is formed in the combustion front. 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 the formation of ZrSix, MoSix films. Core-shell structures are formed behind the combustion front, which disappear as they move towards the post-combustion zone. The phase composition of products is formed in the combustion front in less than 0.25 s.