Fracture properties of layered mullite/zirconia-toughened alumina composites

Abstract

Austral~n NuclearScience& ~chnologyO~an~adon, Priva~ MaflBagl, Mena~ N.S.~ ~34, Austral~ In spite of their good mechanical strength, hardness, chemical stability, and oxidation resistance, alumina ceramics are tyrannized by low intrinsic fracture toughness (about 3 MPam 1/2) which restricts their use in structural applications. Two methods have been successfully employed to improve the tough- ness. The first method involves the well-known process of phase-transformation toughening in zirco- nia. By dispersing metastable zirconia particles in an alumina matrix, one can obtain a considerable increase in the fracture toughness. Fracture tough- nesses up to 18MPam 1/2 and strengths up to 1.2 GPa have been reported in such zirconia-tough- ened alumina (ZTA) ceramics [1]. The second method of toughening is more radical, and involves encasing an alumina host body with a mullite (3A1203.2SIO2) protective layer [2-6]. This is done by infiltrating the alumina body with a silica-rich solution (e.g. ethyl-silicate), and then firing the composite at an appropriate temperature. Due to thermal expansion mismatch between mullite and alumina, desirable macroscopic compressive stresses can be induced in the casing. The protective layer inhibits crack growth: it thereby improves the effective fracture toughness and strength by a factor of about two [5]. Recently we successfully synthesized mullite-ZTA composites [7] with much improved fracture tough- ness by use of the two methods outlined above, i.e. encasing a ZTA body with a mullite layer using an infiltration technique. The source of toughening was attributed to the presence of desirable residual strains. In this letter we report the synthesis and properties of a suite of mullite-ZTA composites containing a substantial range of ZrO2 concentra- tion, and in which both tetragonal and monoclinic ZrO2 are present. The influence of mullite forma- tion on the fracture properties of the composites is discussed. The procedure for sample preparation was based on an infiltration method adopted by Marple and Green [2] in their processing of mullite-alumina composites. Instead of using alumina, ZTA pre- forms with 7.5, 15, and 20 wt % ZrO2 were used. The ZTA powder was obtained by wet ball milling the appropriate amount of zirconia (SF-Ultra 0.5, Z-Tech Australia) and alumina (A1000SG, Alcoa, USA) and spray drying it until free-flowing. The