Study On The Mechanism Of Combustion Synthesis Of ZrB₂-Al₂O₃ Composite Powders

In this paper, mechanism of combustion synthesis(CS) of Zr B2-Al2O3 composite powders was systematically analyzed by a combustion front quenching method(CFQM). In the specimens, the combusting waves self-propagating were quenched, thus the initial, intermediate and end reaction products were frozen in the quenched sample. Thermodynamically, the feasibility of the reaction in the Zr O2-B2O3-Al system was analyzed by Fact-Sage software. The phase constituents of the CS product were examined by X-ray diffraction(XRD). The microstructural evolution during the CS process was investigated by field-emission scanning electron microscopy(FESEM), and the changes of constituents of different regions were analyzed by energy dispersive X-ray spectrometer(EDS). The combustion temperature and wave velocity were measured by the data acquisition system. The thermal behaviors of the stoichiometic powders in the B2O3-Al, Zr O2-Al and Zr O2-B2O3-Al system under the thermal exposure were respectively characterized using differential thermal analysis(DTA) and thermo-gravimetric(TG). Based on this, mechanism of CS of Zr B2-Al2O3 composite powders was proposed.The results showed that, in terms of thermodynamics, the adiabatic temperature of the Zr O2-B2O3-Al system was 2327 K. That is, a combustion wave can theoretically self-propagate in the compact proposed by Merzhanov. And the reactions in the B2O3-Al, Zr O2-Al and Zr O2-B2O3-Al system were feasible(ΔGo<0) in a certain temperature range(300~1800K). The analysis showed that reaction process of CS of Zr B2-Al2O3 composite powders could be described with a reaction-dissolution-precipitation mechanism. It could be obtained that the combustion reaction started from the melting of the B2O3 and Al particles, which was followed by the formation of Zr O2-B2O3-Al solution. The ignition temperature of this system was determined to be around 1073 K. B and Al2O3 were then precipitated from the solution. As the CS reaction proceeded, Zr and Al2O3 were produced by the thermit reaction between Zr O2 particles and Al and precipitated from the solution. Zr B2 could then be formed by the direct reaction between Zr and B. Finally, the Zr B2-Al2O3 composite powders were obtained. And the formation of Al2O3 has precedence over that of Zr B2. Furthermore, a model corresponding to the dissolution-precipitation mechanism was drawn.Moreover, the effect of the volatilization of starting materials on the phase constituents of final product of CS was very important. Results showed that, on the basis of stoichiometric ratio, the combustion-synthesized product consists of Zr B2, Al2O3 and small amounts of Zr O2 phases. In the case of the presence of extra B2O3 in raw materials, Al4B2O9 phases were also observed. With the increase of content of Al in the initial materials, the content of Al in the product increased. In addition, the formation of only Zr B2 and Al2O3 phases without any by-product in final product was feasible when the starting mixture of Zr O2, B2O3, and Al was in a molar ratio of 3:3:11.5.