Tetragonal-to-rhombohedra1 stress-induced phase transformation was studied by X-ray diffraction on the ground sui$aces (of tetragonal zirconia polycrystals and partially stabilized zirconia containing 2.0 to 5.0 mol% Y203 prepared by hot isostatic pressing. The rhombohedral phase increased with Y203 content and also with hot isostatic pressing temperature. The stability of the rhombohedra1 phase was studied with regard to surface finish und thermal annealing. The subsequent heat treatment of the specimens was found to cause the reverse rhombohedral-totetragonal transformation.T IS well-known that zirconia has three Ipolymorphs: monoclinic, tetragonal, and cubic phases .' Transformation from tetragonal ( t ) to monoclinic (m) phase, which is believed to be martensitic, has been extensively studied in conjunction with the enhancement of toughness and strength in tetragonal zirconia polycrystals (TZP). Recently, the existence of an additional new phase, rhombohedral (r), was The r phase was formed from the cubic (c) phase through transformation under stress, not only on the abraded surface but also on
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