Controlled combustion synthesis in the Ti–B system with ZrO2 addition

The effects of an addition of monoclinic ZrO2 into the Ti/2B system were studied in order to control the exothermic reaction. Thermal histories of the phase formation in both Ti/2B and Ti/2B/20 vol.% ZrO2 system were investigated by High Temperature X-ray Diffraction (HTXRD) accompanied with differential thermal analysis (DTA) under a flowing argon atmosphere. The results indicated that ZrB2 formation was initiated at about 900°C prior to TiB2 formation in the ZrO2 added compositions. A two-step borothermic reduction of ZrO2 is believed to yield liquid boron oxide as an intermediate phase. Actual combustion synthesis experiments were carried out for the same compositions under a vacuum atmosphere. Ti/2B samples ignited easily and the reaction front propagated quickly while the ZrO2 added samples were hard to ignite and the propagation was slow. Controlled reactions were achieved due to the formation of glassy B2O3 during the borothermic reduction of ZrO2. SEM microstructures showed that B2O3 flowed into the pores thus dissipating the heat away. X-Ray diffraction patterns taken at room temperature after the ignition, showed TiB2 as the major phase for the Ti/2B composition and ZrB2 and TiB2 as the major phases for the ZrO2 added compositions.