Development of new cellular materials by gel-casting technique

In this PRISMA project new cellular materials will be developed by a gel casting technique. The process was initially set up to produce dense ceramics, andwill be adapted in a second stage to the formation of cellular solids with the addition of a pore forming agent (poly(ethylene)), which decomposes during thermal treatment at high temperature, leaving calibrated porosities. Commercial ZrO2 powders (Tosoh TZ-3Y and TZ-3YS, 3 mol % Y2O3) as a ceramic material and gelatine as a gelling agent were employed. Selected powders were preliminary characterised by Transmission Electron Microscopy (BF and SADP), in order to acquire information about size distribution and microstructure. The first step of the work was the optimisation of the zirconia suspension with respect to its stability, because while the gelling process is occurring there should be no powder sedimentation. The best results were obtained bywith dispersing the powders in distilled water under natural pH (ca. 4.7) and after 10 minutes of sonication by means of an u.s. probe, as evidenced by laser granulometry. Then, slurries with varying solid/liquid ratios and various amounts of gelatine were prepared. The influence of the dispersion method, of the temperature of gelatine dissolution and of casting, gelation and drying conditions were studied. In particular, an increase of the viscosity of the suspension was observed with the TZ- 3Y powder, which made the casting very difficult, then this powder was given up. The best results were reached with a solid content of 50 wt% and a gelatine content of 3 wt.% respect to the water amount. Knoop micro-hardness technique, performed on a section carefully cut and polished, was used to investigate the effectiveness of the drying phase: differences in hardness between surface and core were found, possibly as a consequence of differential drying rate which involves an internal gradient of density, useful for an effective comparison between different drying procedures. Density measurements by Archimede’s principle on fired samples at 1400°C for 1 hour indicated that it was possible to reach at least 95% t.d.. Then, poly(ethylene) granules (125-300 microns) were added to create porosities inside the gel cast pieces (50 vol.%). The thermal cycle was set up to allow the polymer to decompose, without foam collapsing prior to high temperature sintering. Dense products were machined to the final standard size of 10x30 mm for cylinders and 4x3x50 mm for prismatic bars, obtaining parallel planar surfaces and removing surface defects such as voids and irregular profiles. Samples were tested for mechanical properties by means of Instron 8033 (200 kN) and Zwick Roell Z010 (10kN) electromechanical testing machines. Uniaxial compressive strength was measured on cylinders, tensile strength and elastic modulus were measured by four points bending test on prismatic samples equipped with strain gauges. Hardness and fracture toughness of bulk samples were finally evaluated by Vickers micro-hardness testing adopting models available in literature.