Improved green properties of gelcast alumina through multiple synergistic interaction of polysaccharides

Carrageenans have been proposed for the aqueous gelcasting of ceramic and metallic powders because they form strong gels on cooling that allow fast consolidation and enhanced green strength. However, for additive contents above a threshold value, extensive plastic deformation occurs during fracture. Different gums can be combined with carrageenan to obtain the desired gel properties through synergistic interaction. In this work the combination of carrageenan with locust bean gum and with mixtures of locust bean gum and xanthan gum is investigated. The rheological behaviour of solutions (2 wt.%) of these three polysaccharides and their mixtures is studied at 60 °C, as well as the evolution of viscosity on cooling. The effect of mixtures of these gums on the rheological behaviour of deflocculated alumina slurries (50 vol.%, 80 wt.%) is also studied. The additions increase the viscosity of the suspensions and the deformability of the wet bodies. However, after drying, the green strength of the alumina pieces is enhanced and the plastic deformation during fracture is significantly reduced, thus improving machineability. The fracture strengths and Youngʹs modulus values of alumina bars prepared with the three polysaccharides are around 4 MPa and 2 GPa, respectively. Relative densities are >55% of the theoretical value in the green state and ∼98% after sintering at 1500 °C/2 h.