Ana Morales Rodríguez

Uniaxial compression tests were conducted on pure 8 mol% yttria-stabilized zirconia (8YSZ) polycrystalline ceramics with micrometric grain size 4 mm), at temperatures from 1100 °C to 1300 °C and initial strain rates ranging from 5·10-6 to 5·10-4 s-1.True stress-strain curves characterized by initial work hardening typically exhibited a sharp yield point, followed by softening flow and a sudden drop in stress at 0.3-0.4 true strain, a plastic flow instability often correlated with shear band failure. Microstructural analysis revealed cavitation damage, formation of submicron pores, microstructure rearrangement involving the creation of new small grains at grain boundaries and triple junctions, and plastic deformation driven by dislocation activity. This deformation behavior with concurrent microstructural refinement indicated the occurrence of dynamic recrystallization (DRX) in this ceramic.A safe superplastic deformation window of 5·10-5-10-4 s-1 strain rate at 1200 °C was identified. Under optimal conditions, the DRX rate was high enough to induce damage self-healing rather than the fast interlinkage of voids along shear bands. DRX involved the widespread replacement of severely strained grains with new, dislocation-free small grains. This process substantially refined the microstructure by up to ten times and facilitated the achievement of elevated ductility up to 0.7-0.8 true strain without failure. Therefore, the DRX process enabled superplastic flow after a transient softening period, similar to the hot working of metallic alloys. These findings suggest that DRX can trigger significant plasticity, even in inherently brittle materials such as pure 8YSZ.

In this paper, 8 mol% yttria cubic stabilized zirconia (8YCSZ) composites with reduced graphene oxide (rGO) contents up to 10 vol% were consolidated by spark plasma sintering (SPS) at two different temperatures with the aim of evaluating the relationship of their electrical properties with the graphene content, the rGO crystallinity, and the microstructural features. Successful in situ reduction of GO was accomplished during SPS, and highly densified composites with homogeneous rGO distribution, even at the highest contents, were obtained. The electrical properties were analyzed using impedance spectroscopy. Measurements were taken up to 700 °C, revealing an inductive response for the composites with 5 and 10 vol% rGO and a capacitive response for the composites with 1 and 2.5 vol% rGO. The results indicate that, along with the ionic conduction typical of zirconia, there are additional polarization mechanisms associated with the presence of graphene at ceramic grain boundaries that substantially modify the impedance response. A minor electronic conductivity contribution was identified in the composites below the percolation threshold. These characteristics make the 8YCSZ composites promising candidates for application as SOFC components, as ceramic interconnects when the graphene content is above the percolation threshold, or as electrolytes when the graphene content is below this limit.

Graphene-based nanostructures, used as potential reinforcement in ceramic composites, have a great tendency to agglomerate. This requires the use of homogenization techniques during the powder processing, posing the need to evaluate how these techniques affect the microstructure and the mechanical properties of the resulting composites. The influence of the processing route on the properties of 3YTZP (3 mol % yttria tetragonal zirconia polycrystals) ceramic composites with 10 vol % cost-effective GNP (graphene nanoplatelets) has been addressed. Four different powder processing routines combining ultrasonic powder agitation (UA) and planetary ball milling (PBM) in wet and dry media have been used and all the composites were densified by spark plasma sintering (SPS). The mechanical properties at room temperature in the macroscale have been assessed by Vickers indentations, four-point bending tests and the impulse-echo technique, while instrumented indentation was used to measure the hardness and Young’s modulus at the nanoscale. The application of dry-PBM enhances greatly the mechanical and electrical isotropy of the composites, slightly increases the hardness and lowers the elastic modulus, independently of the application of UA. The combination of UA and dry-PBM enhances the flexure strength by 50%, which is desirable for structural applications.

In this work, the flexure strength and fracture propagation mechanisms in yttria tetragonal zirconia (3YTZP) dense composites with 1 and 5 vol.% exfoliated graphene nanoplatelets (e-GNP) were assessed. The composite powders were processed by dry planetary ball milling to exfoliate the as-received GNP, and then densified by spark plasma sintering (SPS). The hardness and Young’s modulus were measured by Vickers indentation and the impulse-echo technique, respectively. Flexural strength and modulus were estimated by four-point bending tests. Finally, cracks originated by Vickers indentations were analyzed by scanning electron microscopy (SEM). The Raman spectra and SEM observations showed a reduction in the number of graphene layers and most remarkably in the lateral size of the e-GNP, achieving a very homogeneous distribution in the ceramic matrix. The hardness, elastic modulus, and flexural strength of the 3YTZP matrix did not vary significantly with the addition of 1 vol.% e-GNP, but they decreased when the content increased to 5 vol.%. The addition of e-GNP to 3YTZP increased its reliability under bending, and the small lateral size of the e-GNP produced isotropic fracture propagation. However, the energy dissipation mechanisms conventionally attributed to the larger GNP such as fracture deflection or blocking were limited.