SMALL-SCALE MECHANICAL BEHAVIORS OF ZIRCONIA PROCESSED BY DIFFERENT TECHNIQUES

Abstract

Zirconium oxide (zirconia, ZrO2) is one of the essential structural ceramics for industrial applications due to its superb strength and fracture toughness. ZrO2 has three main polymorphs: cubic, tetragonal, and monoclinic phase, depending on temperature, type, and concentration of dopants. Stabilized zirconia with metastable tetragonal phase can transform into monoclinic phase with ~ 4% volume expansion under an applied external stress. The tetragonal-to-monoclinic transformation can hinder crack propagations by generating a compressive stress field near crack field, thereby enhancing fracture toughness. In addition, other deformation mechanisms such as dislocation activities, crack deflection, and ferroelastic domain switching can further enhance its deformability. Bulk ZrO2 is typically prepared by sintering at high temperatures over a long period of time. Recently, field-assisted sintering techniques such as flash sintering and spark plasma sintering have been applied to effectively sinter ZrO2. These techniques can significantly decrease sintering temperature and time, and more importantly introduce a large number of defects in the sintered fine grains. The miniaturization of sample dimension can alter the mechanical properties of materials by increasing the surface-to-volume ratio and decreasing the likelihood of retaining process-induced flaws. The knowledge of mechanical properties of ZrO2 at micro and nanoscale is critical in that superelasticity and shape memory effect of ZrO2 can be utilized for applications of actuation, energy-damping, and energy-harvesting at small scale. Here, we performed in-situ microcompression tests at various temperatures inside a scanning electron microscope to examine and compare the mechanical properties of ZrO2 prepared by flash sintering, spark plasma sintering, plasma spray, and thermal spray. Detailed microstructural analyses were conducted by transmission electron microscopy. The unique microstructures in ZrO2 prepared by field-assisted sintering largely improved their plasticity. Temperature and processing technique-dependent underlying deformation mechanisms and fracture behavior of ZrO2 are discussed.