Tetragonal zirconia: Wet chemical preparation, mechanical and electrical properties

Yttria-stabilized zirconia powders were prepared in the composition range of 3 to 13 at% yttria. The hydrolysis-gel precipitation technique was used, starting from metal alkoxides or chlorides. In the composition range between 5 and 10 at% yttria, the materials sintered at 1250°C have a fully tetragonal structure. The density was higher than 95% and the grain sizes can be varied between 0.1 and 0.5μm depending on the preparation conditions. The fracture toughness KIC amounts 6 to 11 MPam but is not dependent on the composition. The transformation toughening mechanism and the properties of the tetragonal phase itself play an important role in the increase of KIC compared with cubic materials. The magnitude of the oxygen-ion conductivity value is comparable with that for cubic materials

Microstructure and sintering kinetics of highly reactive ZrO2-Y2O3 ceramics

Ultra-fine stabilized zirconia powders, which only contain extremely small aggregates were prepared. The control of agglomerates and aggregates in these powders is of utmost importance in order to obtain highly sinter-reactive ceramics. The very small aggregates appear to be the smallest microstructural units which determine the ultimate packing situation after compaction. Resulting green microstructures and sintering behaviour were studied extensively. The sintering process is seen to proceed via several stages of micro-structural development. During the most important stage, where the ceramic material approaches full density, the observed occurrence of abnormal grain growth strongly influences the ultimate grain size. The extent of abnormal growth is highly dependent on aggregate sizes present in the starting powder

TETRAGONAL ZrO2 - Y2O3 PART I : PREPARATION METHODS AND CHARACTERIZATION

Des échantillons polycristallins de zircone tétragonale (Y-TZP) sont préparés par des méthodes chimiques en voie humide. On obtient ainsi par frittage réactif, des céramiques avec des tailles de grains aussi faibles que 0,1 µm. Le vieillissement dans l'air et dans l'eau à haute température a été étudié par diffraction X. Les principaux facteurs du vieillissement sont la teneur en yttrium, le taux d'impureté et la taille des grains. Les Y-TZP avec 6,1 moles % Y01,5 et une taille de grains de 0,1 µm ne présentent pas de dégradation en phase monoclinique même après 5 heures à 450 K dans l'eau.Y2O3 - containing tetragonal ZrO2 polycrystals (Y-TZP) are prepared by wet chemical methods. This results in sinterreactive ceramics with grain sizes down to 0.1µm. The aging behaviour in air and water at high temperatures has been studied by X-ray diffraction. Important factors on aging are Y-concentration, impurity level and grain size. Y-TZP with 6.1 mol% YO1.5 and a grain size of 0.1µm do not show degradation to monoclinic phase even after 5 hours at 450 K in water

Tetragonal ZrO₂-Y₂O₃:Part II : The mechanical and electrical properties

Dense tetragonal zirconia-yttria ceramics has been prepared with grain sizes between 0.1 and 0.5 µm. The fracture toughness and flexure strength at room temperature are independent on grain size and composition in the tetragonal phase region with YO1.5 larger than 5 at.%. The ageing rate of the mechanical properties increases with increasing grain size, but for samples with grains of 0.1 um no ageing is observed. The bulk and grain boundary resistivities are 1.5-2 times higher than in cubic materials. A low impurity level decreases the grain boundary resistivity while the bulk resistivity is hardly affected

Wet-chemical preparation of zirconia powders: Their microstructure and behavior

Ultrafine homogeneous substituted zirconia powders are of importance for the preparation of ceramics for electronic devices. Such powders not only enable the production of dense ceramics of moderate temperatures but also improve thermo-mechanical behavior because of the resulting small grain sizes. Many methods are available for the chemical preparation of powders. However, the application of these methods to highly refractory compounds, such as zirconia, results in powder morphologies that frequently give poor sintering behavior. This is commonly caused by the inadequate control of agglomerate structures. This paper reviews the results of a number of modern synthesis methods applied to the preparation of zirconia, with emphasis on agglomeration control and microstructural development