High-strength partially stabilized zirconia compact obtained by centrifugal consolidation

Abstract

It has been reported that agglomerates present in ceramics powder limit the final density of the sintered compact [1, 2]. This is shown [3] to be due to different sintering kinetics between the inside and outside of an agglomerate and because the interagglomerate pores do not easily disappear even at the final stage of sintering. A technique of powder dispersion in liquid has been investigated [4] as a method for obtaining dense packing of powder particles. Rhodes [2] reported that a highly dense compact is obtained when agglomerates are eliminated by means of ultrasonic dispersion of powder followed by centrifuge cast. In the present study, the starting powder containing agglomerates is dispersed in aqueous solution by means of a highpower ultrasonic vibrator; then a centrifugal force is applied to the slurry, not only for the forced sedimentation, but for the effective compaction of powder. This is aimed at obtaining highly dense bulkcompacts to carry out the measurement of their flexural strength. PSZ powder used (Toyo Soda Co.) is co-precipitated ZRO:2(2.5) mol % Y203 (called 2Y and 2.5Y) having particle size of about 30 to 50 nm. The powder contains agglomerates of several to several tens of micrometres diameter. Firstly, the as-received powder was gently ground with an agate mortar for 15min. Then the powder dispersion was carried out in powder-to-solution ratio of 15g/15ml. For promoting dispersion, ultrasonic vibration (at 1 kW output) was applied to the slurry for 1⁄2h. Besides methanol as an effective organic dispersing solvent [5], Pierre [6] rep~)rted that the lowest viscosity is attained at pH = 1.2. In the present study, a sedimentation experiment was carried out for aqueous-HNO3 solution with different pH; the result suggests that solution at pH = 3 suspends the powder well with the least amount of sediments, as shown in Fig. 1. (Dispersion at pH = 11, 7, 3, 1.5 was tried and the porosities of the sintered compacts examined. The lowest porosity was obtained in the case of pH = 3.) Because undispersed agglomerates still remain after dispersion treatment, a sedimentation-classifying procedure was performed for reasons given below. The slurry was diluted 10 times and, after stirring, it was sedimented and the suspension was decanted. The lower centrifugal force (< 10000G) was applied for 10min for collection of the powder and the higher (20 000 G) force was applied for 30 min to the condensed slurry for final compaction. Much higher centrifugal force (200 000G) has been shown to result in