Improvement of the mechanical strength of alumina preforms by coating with montmorillonite/LDH gels

A composite material was prepared from montmorillonite and Mg–Al LDH. These oppositely charged particles form thixotropic gels with distinct flow and micro-structural properties. The stability of the gel was dependent on the montmorillonite/LDH ratio. Novel advanced ceramic materials were designed by coating ceramic preforms with these gels. During calcination these gels decompose to metal oxides, aluminosilicates and spinel. On calcination at 1400 °C, MgO formed from the LDH of the gel reacted with the aluminosilicate formed by the decomposition of the montmorillonite. A non-porous dense matrix was formed with well grown crystals of MgAl2O4 spinel embedded in the aluminosilicate matrix. When the ceramic preforms like porous alumina tube were dip coated with the gel and fired at 1400 °C, the molten aluminosilicate infiltrated into the pores of the aluminum oxide carrying spinel crystals. These spinel crystals settled in the pores and increased the mechanical strength of the alumina tubes. The compressive strength was increased by around 94% and the mechanical extension in the z-direction in the coated body by 4% compared to the uncoated counterpart.

Research highlights
► The thixotropic gel of montmorillonite and Mg-Al LDH has coat forming properties.
► At 1400 °C, the MgO from LDH reacted with the alumino-silicate of montmorrillonite.
► The alumino-silicate and spinel at 1400 °C infiltrates to pores of support.
► These spinels increase the mechanical strength of the supports by 94%.
► There is also a 4.0% increase of mechanical extension in the z-direction.