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 st udied 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 investig ated by High Temperature X-ray Diffraction (HTXRD) accompanied with differe ntial thermal analysis (DTA) under a flowing argon atmosphere. The results indicated that ZrB2 formation was initiated at about 900 degrees C prior to TiB2 formation in the ZrO2 added compositions. A two-step borothermic redu ction of ZrO2 is believed to yield liquid boron oxide as an intermediate ph ase. Actual combustion synthesis experiments were carried out for the same compositions under a vacuum atmosphere. Ti/2B samples ignited easily and th e 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 ZrO 2. SEM microstructures showed that B2O3 flowed into the pores thus dissipat ing the heat away. X-Ray diffraction patterns taken at room temperature aft er the ignition, showed TiB2 as the major phase for the Ti/2B composition a nd ZrB2 and TiB2 as the major phases for the ZrO2 added compositions. (C) 2 000 Elsevier Science S.A. All rights reserved.