Effect of the method of producing zirconium dioxide based ceramics on their mechanical strength

Abstract

The continuously decreasing reserves of the high-quality raw materials that are used in the field of automobile and machine building and the increasing stringency of the specifications concerning the high temperature strength characteristics of the components because of the increasing severity of their service conditions make it necessary to carry out extensive investigations for replacing metals with structural ceramics based on partially stabilized zirconium dioxide that exhibit improved strength characteristics owing to transformationinduced strengthening [i, 2]. This paper deals with a study of the dependence of the ultimate bend strength of the zirconium dioxide ceramics on the type of the additives used for stabilizing ZrO 2 and on the methods of stabilization and specimen shaping and firing.The powders prepared at the Verkhne-Dneprovsk Mining and Metallurgical Complex were used as the raw materials for obtaining the ceramics: the TsRO-2 grade powder (GOST 21907-76) containing more than 99.0 wt. % ZrO 2 and the TsI-6 grade powder (TU 48-0502-63/0-86) containing more than 92% ZrO 2 and a stabilizing Y=O 3 additive (approximately 6 wt. %) that is introduced according to the method of coprecipitation from aqueous solutions of the salts of zirconium hydroxy chloride and yttrium chloride.When using ZrO 2 of the TsRO-2 grade, stabilization was carried out in solid state by introducing 6% Y203 (the grade of the material: 7U) or 2% MgO (grade MgChSTs). For this purpose, the powders of TsRO-2 and the ITO-2 grade yttrium oxide (or magnesium chloride in an aqueous solution) were mixed in the required proportions. The mixture was compacted for obtaining briquettes that were subsequently fired at 1600~ maintaining a 2-h dwell. Thereafter, they were crushed and were subjected to vibrational milling until the particle size decreased up to less than 3 Dm. Subsequently, the particles were washed with hydrochloric acid in order to remove the iron contaminant and were used in the form of a slip for casting specimens or in the form of a dried powder in order to prepare the charges required for different methods of shaping.The TsI-6 grade powder was subjected to vibrational milling until a particle size finer than 2 ~m was obtained and was washed with hydrochloric acid for removing the iron contaminant. Subsequently, it was used in the same way as the powders obtained by stabilizing in the presence of solid phases.The following shaping methods were used for specimen preparation: semidry compaction, hydrostatic pressing, hot pressing*, hot isostatic pressing#, and slip casting.The ultimate bend strength was determined using the three-point bending scheme according to the procedure at the Ukrainian Scientific-Research Institute of Refractories [3]. Figure 1 shows the schematic of the apparatus. The test specimens were heated up to 1500~ in an electric furnace having a movable (roller) hearth and the K~N A 14/400/350 grade silicon heating elements. We placed i0 specimens on corundum rolls measuring 5 mm in diameter and I0 mm in l...