Acid washing of milled stabilized zirconia and its influence on the properties of ceramics

Abstract

In the last decade ceramics made from tetragonal zirconia have found ever-increasing uses; these possess a high toughness and impact strength [1][2][3]. The most effective technology for their production is slip casting, which facilitates the shaping of dense pore-free blanks that contain no microheterogeneities [4, 5]. Traditionally the slip is obtained by the wet milling of powdered solid solutions based on zirconia in a water medium using steel ballmills followed by solution of the tramp iron in HCI, and separating the solution and the deposit [4][5][6].In the present paper we studied the influence of the form and amount of acid on the completeness of removing the tramp iron and the properties of the ceramic.The experiments involved the use of powders of tetragonal solid solution of 6.47%* yttrium oxide in the zirconia produced by the Verkhnedneprovsk Mining Metallurgical Combine (TU-48-0502-63/0-86) and also hydrochloric acid, nitric acid, perchloric acid, acetic acid, sulfuric acid, and oxalic acid of chemical-purity grades.The material was ground in a steel drum with balls made of ShKh-15 steel. The content of tramp iron was 3.05% (calculated as Fe203).The original reactive acids were added to the milled suspension in amounts of i mole per i0 moles of ZrO 2 (experiment I) and 3 moles per i0 moles ZrO 2 (experiment II), and then mixed and stored for 2 days. After settling the liquid phase was poured off, the residue was washed with distilled water and mixed and stored for another 2-3 days, after which the liquid was drained. Decantation washing was done until the pH of the suspension equaled 2-3 (preliminary experiments established that in this pH range the suspension of the zirconia powder has the least viscosity). The residue, solution, and wash waters were analyzed for the concentrations of ZrO2, Y202, and Fe203. The results are given in Table I. These experiments revealed the selective solubility of oxides from the solid solution powder. Although the total amount of Y203 in the solid phase is approximately 15 times lower than ZrO 2 its washing from the solid solution occurs much more rapidly.With an increase in the particle size of the powdered solid solution from 0.i to 0.5 #m (in the case of additional calcination of the original raw material) the solubility of ZrO 2 remains practically unaltered, while the solubility of Y203 is reduced proportionally to the reduction inthe specific surface. Calculation shows that during acid treatment Y203 is removed from the surface layer of the particles of solid solution powder to a depth of 0.3-0.7 nm, which is close to the lattice parameter of the solid solution (-0.5 nm).From the point of view of the completeness of the removal of the tramp iron all the acids studied can be recommended (with the exception of the oxalic) providing their concentrations are varied.It should be noted that dense residues of milled and washed powder were formed only when treated with hydrochloric perchloric, and acetic acids. In the remaining cases the residue was brittle and easily e...