Dense and porous zirconia prepared by gelatine and agar gel-casting: Microstructural and mechanical characterization

Dense and cellular yttria-tetragonal zirconia polycrystal (Y-TZP) bodies were produced by using a natural gelatin and two different agars (Fluka and Sigma-Aldrich) as gelling agents, while commercial polyethylene (PE) spheres were added as pore formers. Casting of the suspensions was either carried out in air or under vacuum, depending on their viscosity, with and without anti-foaming agents, in the attempt of producing defect-free materials. The microstructure of all dense and cellular (40 vol%) ceramics was characterized by FEG SEM and Focused Ion Beam (FIB) techniques, and a common microstructure, consisting of fine hexagonal grains (200-500 nm) was identified. Dense samples prepared with the gelatin reached, after sintering, 97 % of theoretical density. Elastic modulus and fracture toughness measurements closely matched results found in the literature for similar materials. Macroporosities were evidenced, which may have been formed because of PE irregular structures and of beads incomplete dispersion into the suspension. Compressive testing evidenced a high scattering of results. In dense agar samples, the presence of a diffuse residual microporosity was observed by FIB-SEM section analyses, confirming an incomplete densification. On the contrary, porous samples exhibited a very homogeneous dispersion of the macropores. An extensive micro/macro mechanical characterization was carried out by means of nanoindentation and room temperature compressive/tensile testing, in the effort of correlating elastic modulus at different scales with the amount of residual microporosity present in both dense and cellular ceramics. Results indicated a certain decay of elastic modulus and bending strength, compared with values found in the literature

Dense and cellular yttria-tetragonal zirconia polycrystal (Y-TZP) bodies were produced by using a natural gelatine and two different agars as gelling agents, while commercial polyethylene (PE) spheres were added (125 to 300 μm diameter) as a volatile pore forming agent to create 50-65 vol% spherical macro-pores, uniformly distributed in a micro-porous matrix. The microstructure of all dense and cellular ceramics was characterized by FEG-SEM and Focused Ion Beam (FIB) techniques. The mechanical properties of both dense and porous samples were investigated at the microscale by nanoindentation testing, while the influence of microporosity was obtained by the analysis of hardness and modulus depth profiles, coupled with FIB-SEM section observations of selected indentation marks. Mechanical characterization at the macroscale consisted of uniaxial compression tests and four point bending tests.