Aspects of the chemical technology of ZrO2-Y2O3-Al2O3 powders and testing them in ceramics technology

Abstract

In recent years in the most technically advanced countries, the production and use of ceramics from partially stabilized ZrO 2 have seen rapid developments compared with other products made from highly refractory oxides [i].It is known [2] that a serious drawback of this ceramic with the use of yttria as stabilizer is the low stability of the phase composition of the surface layers in the 150-400~ range, especially under the action of strong shear stresses and moist atmospheres, which leads to a sharp reduction in the ceramic's strength. The most effective method of increasing the thermal-shock resistance of partially stabilized solid solution in the system ZrO 2-Y203, apparently, is the incorporation of additives (5-40%) of alumina [3].In making design ceramics from ZrO 2 it is advantageous to obtain the starting powders by chemical coprecipitation from solutions of salts [4]; this method has not been thoroughly investigated for the ternary system ZrO2-Y203-A1203.We made a study in order to develop an experimental technology for powders in this system with a controlled degree of crystallization and a stated phase chemical composition by chemical coprecipitation from aqueous solutions of salts. The powders were tested for obtaining design ceramics.Preliminary investigations established that after calcining the hydroxides obtained from salts of zirconium, yttrium, and aluminum, in addition to AI20 ~ and a solid solution of yttria in ZrO~, the material also contained alumino-yttrium garnet, which prevents sintering of the ceramic [5].The method of so-called potentiometric titration was used to solve the above problem; this enables us to determine certain important factors: pH at the point of equivalence, and range of pH suitable for precipitation. We studied the integral and differential curves of forward (introduction into the salts of ammonia) and reverse precipitation [6], first of the chlorides individually, and then mixtures of different composition and concentration with a solution of ammonia at different temperatures (20, 40, 60, 80~ In developing the experimental procedure for obtaining ZrO2-Y203-AI203 powder compositions we prepared laboratory equipment which yielded hydroxide compositions in conditions of direct and reverse precipitation.The apparatus shown in Fig. 1 consists of a reactor controlled at 80-85~ with a stirrer in which, using pumps and a capillary, solutions of salts and ammonia are fed at a controlled rate, continuously, and also providing continuous removal of the reaction products. This provides an adequately constant pH in the reaction mixture, control of which is effected with the ionometer, using the glass and silver-chloride electrodes. The temperature is regulated with a control thermometer and thermostat.*Here and subsequently mass parts are stated."Tekhnologiya" ONPO. Urals Polytechnical Institute.