Influence of processing route on microstructure and mechanical properties of MgAl₂O₄ spinel

Abstract

This paper reports on process dependant microstructural and mechanical properties of MgAl₂O₄ spinel (MAS) ceramics. Two MAS powders with different chemical compositions were synthesized by solid-state reaction of alumina and calcined caustic magnesia at 1400 °C for 1 h. The surface of the as obtained MAS powders was passivated against hydrolysis by coating it with H₃PO₄ and Al(H₂PO₄)₃ species dissolved in ethanol at 80 °C for 24 h. The as protected powders could then be dispersed in aqueous solutions of tetramethylammonium hydroxide (TMAH) and Duramax D-3005 as dispersing agents to obtain stable slurries with 45 vol.% solids loading. The stable aqueous MAS slurries were consolidated by slip casting (SC), gelcasting (GC), hydrolysis assisted solidification (HAS) and hydrolysis induced aqueous gelcasting (GCHAS) routes, fully dried and then sintered for 1 h at 1650 °C. For comparison purposes, dense MAS ceramics were also prepared following a conventional dry-powder pressing (DP) and temperature induced gelation (TIG) routes. All the sintered MAS ceramics were thoroughly characterized for bulk density, apparent porosity, water absorption capacity, SEM microstructure, XRD phase, hardness, 3-point bend strength, and percentage of shrinkage to evaluate the suitability of the processing routes for fabricating defect free components with near-net shape. Among the various techniques employed, the GCHAS was found to be best for fabricating near-net shape MAS ceramics.