Optimization Of Processing Parameters For Template Assisted Combustion Synthesis Of ZrB₂-Al₂O₃ Composite Powders Containing ZrB₂ Fibers

In this paper,ZrB₂-Al₂O₃ composite powders containing ZrB₂ fibers were prepared by combustion synthesis using ZrO₂ fibers as templates.The adiabatic temperature and Gibbs free energy of ZrO₂-B₂O₃-Al system were calculated theoretically,and the physical and chemical changes in the reaction process were analyzed by differential thermal-thermo gravimetric method,then the influence of argon pressure,ratio of raw materials and the particle size of ZrO₂ fibers on the combustion parameters,the phase composition and the microstructure of the combustion product were investigated.Finally,the optimum process conditions were obtained.According to thermodynamics,the adiabatic temperature of ZrO₂-B₂O₃-Al system is 2327 K,which is much higher than 1800 K,thus the self-propagating reaction could occur.The Gibbs free energies of the B₂O₃-Al,ZrO₂-Al,Zr-B and ZrO₂-B₂O₃-Al systems are less than 0 in the temperature range of T<2000K showing that all above reactions are possible.The physical and chemical changes in the reaction process were analyzed by DSC-TG method.It was found that B₂O₃ lose moisture at 105?C and melt at 450?C,then Al melt at 660?C.Firstly,the redox reaction between ZrO₂ and Al occurred at about 852?C,then ZrO₂ reacted with Al,finally the synthesis reaction between B and Zr occurred and formed ZrB₂.The vaporing of B₂O₃ was suppressed by increasing of Ar pressure.The reaction conduct completely,so the product has minimum weight loss and comprises of ZrB₂ and Al₂O₃ when the Ar pressure is 1.5 MPa.ZrB₂ and Al₂O₃ were formed under the argon gas pressure of 1.5MPa.A part of ZrB₂ with a rough surface and a relatively completestructure were formed as ZrB₂ fibers with ZrO₂ fibers as the template,another part of ZrB₂ were dispersed in ZrB₂/Al₂O₃ composite powder in granular form.The diffraction peak intensity of ZrO₂ in the product gradually weakened with an excessive of B₂O₃,and phase composition of products were ZrB₂ and Al₂O₃,the content of produced fiber were higher and the structure was relatively integrity when the molar ratio of ZrO₂,B₂O₃ and Al is 3:4.2:10.There were unreacted ZrO₂ with an excessive of Al,when the molar ratio of ZrO₂,B₂O₃ and Al is 3:3:11.5,the phase composition of products were ZrB₂ and Al₂O₃,and the phase composition of products and the quality of fibers were not changed by the increase of Al,thus the optimal molar ratio of ZrO₂,B₂O₃ and Al is 3:3:11.5.ZrO₂ fibers with different length were obtained by grinding raw materials at different time.The effects of the length of ZrO₂ fiber on the phase composition of the combustion products and the fiber structure were studied.The results showed that the length of ZrB₂ fibers became shorter with increasing of grinding time and was the shortest?<50?m?after grinding for 90 min.Although there were almost no significant effects of the length of ZrO₂ fibers on the phase composition of combustion products,shorter ZrO₂ fibers were more beneficial to the production of ZrB₂ fibers with complete fibrous structure.