JOURNAL OF MATERIALS SCIENCE 18 (1983) 109-1 13 Transfomation toughening of I "-alumina by incorporation of zirconia L. VISWANATHAN, Y. IKUMA, A. V. VIRKAR Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84106, USA It is shown that zirconia is chemically compatible with lithia-stabilized/3"-alumina and that incorporation of tetragonal zirconia in the matrix of/3"-alumina leads to an improve- ment in fracture toughness and four-point bend strength without significantly affecting ionic conductivity. Chemical compatibility is demonstrated by reacting c~-alumina with sodium zirconate at 1250 ~ C which leads to the formation of ~"-alumina and free zirconia. The fracture toughness as measured using the indentation technique is found to increase with increasing crack length. 1. Introduction It was first reported by Garvie [1] that partially stabilized zirconia containing finely dispersed metastable tetragonal zirconia in a matrix of cubic zirconia has significantly superior mechanical properties compared to fully stabilized cubic zirconia. Subsequently, several researchers [2-8] have shown that the strength and fracture tough- ness of ceramic bodies containing finely dispersed tetragonal zirconia are considerably higher when compared to the same ceramics without tetragonal zirconia. The origin of the toughening effect lies in the fact that the tetragonal zirconia particles convert to the monoclinic polymorph in the near stress field of a propagating crack. The strength of ceramic bodies containing tetragonal zirconia can also be improved substantially by surface grinding. The process of grinding converts the tetragonal zirconia particles in the near surface region to the monoclinic polymorph. This leads to the develop- ment of compressive stress on the surface due to the lower density of the monoclinic zirconia, which in effect increases fracture strength. While most of the work has been on partially stabilized zirconia, Claussen [9] has shown that the mech- anical properties of alumina can also be improved by incorporating fine particles of zirconia, which are retained metastably in the tetragonal form at room temperature. The magnitude of this toughen- ing effect, which depends upon: 1. the volume fraction of tetragonal zirconia, 2. the size of the zirconia particles and 3. the elastic properties of the matrix, also depends upon temperature. Below 500 ~ C, the improvement in toughness is signifi- cant but at elevated temperatures it is rather marginal. Thus, for high-temperature structural applications it is doubtful that transformation toughening would be important. However, there are likely to be several low temperature appli- cations for ceramics of technological importance whose performance could be significantly im- proved by transformation toughening. It is the object of the present paper to report such an application. Sodium /3"-alumina is currently being used as a solid electrolyte for application in the sodium- sulphur battery and the sodium heat engine [10, 11]. The sodium-sulphur battery operates at a temperature of about 350 ~ C. It has long been known that under certain conditions, the /3"- alumina solid electrolyte undergoes strength degradation. This degradation manifests itself in the form of cracks filled with liquid sodium. It has also been suggested that electronic conductivity may play a significant role in the onset of propa- gation of degradation. The sodium-filled cracks propagate through the thickness of the solid elec- trolyte leading to the failure of the battery. The degradation of the solid electrolyte has been modelled by several researchers [12-17]; common 0022-2461/83/010109-05503.10/0 9 Chapman and Hall Ltd. 109