Phase evolution and properties of a porcelain body fabricated using different rapid firing techniques

Abstract

A porcelain powder was sintered using different rapid sintering techniques including direct sintering (DS), spark plasma sintering (SPS), microwave sintering (MWS), and flash sintering (FS). Densification, phase evolution, and physicomechanical properties of the fully sintered porcelain samples were investigated to improve understanding of the role of particular process parameters. Densification of the porcelains was controlled by liquid phase formation. Study of the DS revealed that formation of liquid glass is rapid even at 780 C/min heating rate, and the porcelain was fully densified within 15 min dwelling at 1175 °C. SPS enhanced densification rate about 10 times greater over that of conventional sintering, starting at 920 C. The dwelling step was negligible due to the rapid flow of the liquid glass filling the pores, assisted by the applied pressure. On using microwave energy, the sintering temperature of the porcelain was reduced by ~75 °C and dwell time from 15 min to 5 min compared to conventional sintering. Formation of the liquid phase was observed at 900-1000 °C and it was the key for the rapid densification because it promoted microwave absorbability. Densification in FS was difficult to resolve, but it could be deduced that by applying an electric potential of 1.5 kV/cm for 30 seconds, large amounts of glassy phase were produced. Various microstructures were observed as a result of the different processing routes. DS and MWS produced a typical porcelain microstructure which is dense but has numbers of residual pores embedded in the glassy phase. SPS produced a highly dense microstructure with a few residual pores assisted by applied pressure. FS, in contrast, produced a nonuniform microstructure containing under-sintered, well sintered, and over-sintered regions due to localised melting. Etched microstructures revealed special mullite morphologies. The applied pressure in SPS caused mullite orients perpendicular to the direction of the applied load. SPS also induced mullite dissolution at 1200 °C. Microwave produced fibre-like mullite via a vapour-liquid-solid mechanism having Fe(l) as a catalyst. Flash sintered samples contained dendritic mullite and some mullite needles were formed via a screw dislocation mechanism. However, mullite crystallite sizes calculated by the Scherrer equation revealed that in fully dense samples from each processing route, mullite crystallite sizes were in a narrow range of 25-40 nm. Physicomechanical of the porcelains produced using the different processing routes were similar; for example, apparent bulk density ranged 2.35-2.46 g/cm3 (MWS<DS<SPS), water absorption <0-0.5 wt.% (MWS>DS>SPS), hardness varied 5-7 GPa (MWS<SPS<DS) while fracture toughness varied from 3-6 MPa· m1/2 (MWS<DS<SPS). A comparison of energy consumption during the various sintering routes for this porcelain composition showed that DS consumed ~11 MJ/kg, MWS ~18 MJ/kg, FS ~3060 MJ/kg and SPS ~1612 MJ/Kg. While, the energy consumption listed here must be viewed with caution, the values are useful for comparison of the amounts of energy required for the different rapid sintering processes.