Transformation plasticity and fracture behavior of ceria-TZP/alumina composites.

Abstract

A systematic study of transformation plasticity, R-curve behavior, and the strength characteristics in Ce-TZP/Al$\sb2$O$\sb3$ composites was conducted to explore the microstructural effects on the mechanical properties of transformation-toughened ceramics. Microstructures with different degrees of tetragonal phase connectivity and grain sizes were obtained by various amounts of alumina addition and different sintering temperatures. The test range, in terms of stress space for transformation plasticity and strength characteristics, was extended beyond those found in the literature. Four point bending and compression tests with a superimposed hydrostatic pressure constituted the experimental methods for that purpose. R-curves were measured by cycled loading-unloading in four point bending of fatigue-precracked specimens. Results from such a study indicate that a Mohr-Coulomb type of yield criterion can be used to describe the deformation behavior of the Ce-TZP/Al$\sb2$O$\sb3$ composites. Both alumina addition and grain refinement expand the yield surface outward. The evolution of the transformation curve from one with distinct two stages to one with continuously gradual transformation is facilitated by alumina addition or applied pressure. On the other hand, grain refinement favors perfectly plastic deformation but suppresses the burst-like transformation during cooling. Both the height and shape of the R-curve depend on the microstructure. The addition of alumina decreases K$\sb0$ (slightly), K$\sb{\rm m}$, and $\Delta$K, while grain refinement increases K$\sb0$ (slightly), but decreases K$\sb{\rm m}$, and $\Delta$K. Above a certain alumina content, the R-curve exhibits a peak followed by extensive crack growth due to subcritical crack growth. The increase of crack resistance, $\Delta$K, can be attributed to transformation toughening as the data fit well with a model proposed by Chen. An analysis of the bend strengths of the Ce-TZP/Al$\sb2$O$\sb3$ composites reveals a ductile-brittle-ductile-brittle transition map which serves as a framework to guide future materials development of transformation-toughened zirconia. A simple model was also developed to correlate strength with the R-curves in these materials.