The use of ceramics is generally restricted due to their limited ductility and toughness, compromising their behavior as structural components. However, zirconia-based ceramics may be able to overcome this limitation by a martensitic phase transformation from tetragonal (t) to monoclinic (m). Ceramic materials similar to shape memory alloys can even be obtained, with a stress-induced phase transformation as a source of toughening that can be reversed by temperature increase. However, the most commonly used yttria-stabilized zirconia ceramic (3Y-TZP) shows transformation-toughening at the crack tip, increasing its crack resistance, but no transformation-induced plasticity. In other words, it still fails before any macroscopic sign of plasticity [1]. To overcome these limitations, ceria-stabilized tetragonal zirconia (Ce-TZP) polycrystal ceramics can be used. The possibility of transforming t into m at lower stresses and before any crack initiation in addition to a very high aging resistance make these materials suitable for achieving larger ductility [2]. Moreover, the optimization of the microstructure and composition of pure and Ce-TZP based-ceramics results in interesting combinations of strength and toughness values for a wide range of applications. For example, ceria-based zirconia composites have been developed in the framework of the European project Longlife [3] with a strength of 600 MPa, toughness (>10 MPa√m) and impressive Weibull modulus (≈ 30). In this formulation (called ZA8Sr8Ce11), the presence of Al2O3 fine grains hindered Ce-TZP grain growth during sintering to increase strength. On another hand, elongated secondary phase grains containing Sr and Al are formed during sintering to increase toughness by bridging/crack-deflection mechanisms. Still, the mechanism of the t-m phase transformation is not fully described at micro and nanoscopic scales. Hence, we propose to describe the potential of Ce-TZP ceramics through macro and micro mechanical tests coupled with structural analyses.
At the macroscale, four-point bending tests have been conducted on dense ceramic composites of the same ZA8Sr8Ce11 powder for different shaping process (CIP [4] and Stereolithography [5]). Samples exhibited high reliability (Weibull moduli>15). However, SLA printed samples were more transformable and displayed thinner but more numerous transformation bands on their tensile surface. This behaviour has been recognized coming from inhomogeneously distributed and larger SrxAlyOz grains. The formation of such agglomerated SrxAlyOz clusters has been recognized coming from the sintering cycle that needed more energy for SLA samples because of the lack of compression compared to CIP. Nevertheless, this difference is not redhibitory but opens prospects about tuning transformability in another way than by adjusting doping content.
At the microscale, in situ compression experiments were performed on 12Ce-TZP (12 mol.% CeO2) single-crystalline micropillars, milled by focused ion beam (FIB). The pillars were compressed using a FT-NMT04 nano-indenter (FemtoTools), and the deformation was followed by scanning electron microscopy (SEM), and Laue microdiffraction. The latter was performed in the BM32 beamline, at the European Synchrotron Radiation Facility (ESRF). These analyses enable to characterize the stress conditions for transformation onset, along with the anisotropic properties and plastic activity according to the different crystal orientations.
