Ferroelastic and plastic behaviors in pseudo-single crystal micropillars of nontransformable tetragonal zirconia

The orientation-dependent micromechanical properties of nontransformable tetragonal (t’) zirconia, which underwent a diffusionless transformation from the fluorite cubic phase and does not exhibit a stress-induced phase transformation, were characterized via pseudo-single crystal micropillar compression and electron microscopy. The t’ zirconia sample was obtained via atmospheric plasma spraying of 4.5 mol% yttria-stabilized zirconia (YSZ) powders into liquid nitrogen and consolidated into a bulk state via hot pressing at 1100°C. Dense and cylindrical micropillars were fabricated using a focused ion beam from pseudo-single crystalline regions, which exhibited a nanodomain microstructure of three t’ variants partitioned by {1 0 1}c twin boundaries with 90° symmetry. These micropillars were compressed using a flat-end diamond indenter. Near-<0 0 1>c compressions were attributed to ferroelastic domain switching and subsequent {1 0 1}c and/or {1 1 1}c hard slips. In ferroelastic deformation, a certain t’ variant diminished, and a binary domain microstructure developed with c axes perpendicular to the compressive direction. Near-<1 1 1>c compressions were governed by {0 0 1}c soft slips accompanied by strain hardening with negligible ferroelasticity, which resulted in buckling deformation with rotational kinking. In both the hard- and soft-slip orientations, ferroelastic toughening was observed with certain t’ variants awaken around the crack tips. Contrarily, cleavage fractures subsequent to yielding were observed in near-<1 0 1>c compressions. In the cubic counterpart with a domain-free microstructure (8.0 mol% YSZ), ferroelastic toughening was not observed. Hence, it is viewed as the origin of enhanced toughness in t’ zirconia.