MECHANICAL PROPERTIES OF STARCH-ZIRCONIA COMPOSITES - THEIR RELATION TO COMPOSITION AND MICROSTRUCTURE

The mechanical properties of starch-zirconia composites, prepared by starch consolidation casting, and their as-fired counterparts (porous zirconia ceramics) are investigated. It is shown that the rigidity (elastic modulus) and strength of starch-zirconia composites can be controlled by selecting the starch type and even fine-tuned by applying starch mixtures. In particular, it is shown that rice starch enhances both the Young's modulus and the flexural strength of such bodies. For composite bodies with 30 vol.% of starch (nominal content of dry starch related to zirconia), the bulk densities achieved for starch-zirconia composites are 2.17-2.47 g/cm(3) (lowest when using potato starch, highest when using rice starch). Young's moduli can attain values of up to approx. 7 GPa (range 2-7 GPa), while flexural strengths attain values up to approx. 12 MPa (range 3-12 MPa), with a close correlation to bulk density. Drying shrinkage is quite similar in all cases (range 11-14 %) and largely independent of the starch type used. After firing (and burnout of the gelatinized starch) porous zirconia ceramics are obtained (with essentially open porosity), in which the bulk density differences are retained, leading to total porosities of 34, 26 and 21 % when using 30 vol.% (nominally) of potato, corn and rice starch, respectively. Young's moduli are of order 60 120 GPa. Irrespective of the starch type used, they exhibit a strong correlation to porosity, which is best described by an exponential relation.