EFFECTS OF TIO2 ADDITION ON THE SINTERING OF ZRO2.TIO2 COMPOSITIONS AND ON THE RETENTION OF THE TETRAGONAL PHASE OF ZIRCONIA AT ROOM-TEMPERATURE

The production of tetragonal zirconia polycrystalline (TZP) ceramics and the identification of factors controlling retention of the tetragonal phase in the ZrO2.TiO2 system have been investigated. In this binary system, it was not possible to retain tetragonal zirconia polycrystals at room temperature for a range of compositions sintered above 1200-degrees-C. A decrease in the martensitic transformation temperature of zirconia with titania addition was observed, but the effect was insufficient to retain the tetragonal phase at room temperature. In solid solution, the TiO2 additions act to suppress ZrO2 densification, this leading to grain growth when attempts are made to attain higher densities. The use of fine powders, fast firing or sintering in reducing conditions altered densification but was not able to generate a final grain size sufficiently small to avoid spontaneous tetragonal-->monoclinic transformation on cooling. Based on the results obtained for ZrO2.MO(x) systems, the main factors involved in the retention of tetragonal zirconia at room temperature are discussed in an attempt to incorporate thermodynamical and the stress field effects.