Abstract

The influence of the grade of agglomeration due to different interparticle electrostatic forces in the aqueous suspension as well as the liquid saturation on bending strength and fracture toughness of slip cast green bodies was examined, Different grades of agglomeration were achieved by varying the suspension's pH. The states chosen were well dispersed (pH 3.5), partly agglomerated (pH 4.75) and strongly agglomerated (pH 6.0). Solids loading was 35% volume alumina. Liquid saturations S examined were dry (ca. 0-1% volume), pendular state (ca. 20% volume) and capillary state (ca. 90% volume). Samples were slip cast on plaster-plates in dismantable plastic molds. A statistical Weibull analysis of the measured 4-point bending strengths was performed. Mode I Fracture toughness was determined using SENB (Single Edge Notched Beam) samples with pre-crack lengths between 0.5 and 4.5 mm. To account for the not ideally brittle fracture a plastic zone correction resulting in an effective crack length was used, based on a method by Adams. It was shown that the presence of water and the resulting long range capillary forces dominate the strength and fracture toughness. This leads to similar mechanical properties at corresponding liquid saturations: bending strength ca. 0.7 MPa, capillary state; 0.4 MPa, pendular state, fracture toughness ca. 0.021 MPam(0.5), capillary state, 0.018 MPam(0.5), pendular state. This is almost independent of the suspension's pH. For dry green bodies the properties were strongly influenced by the grade of agglomeration and the resulting green microstructure, with bending strength ranging from 0.05 MPa to 1.3 MPa. Fracture toughness was between 0.006 MPam(0.5) and 0.049 MPam(0.5).