Pressureless Densification of Ultra-high Temperature Ceramics and Microstructure Tailoring

Pressureless sintering is the commonest and most economic method for sintering. The composition and microstructure of ceramics could be tailored by changing the additives, sintering technology and the history of heat treatment; these are the keys for improving the density and performance of the ceramic product during the pressureless sintering. The current review started from the research background, trend and significance of ultra-high temperature ceramics (UHTCs), most recent domestic and overseas progresses on the pressureless densification and microstructure tailoring of UHTCs were comprehensively reviewed. The surface energy of a powder compact was higher than the grain boundary energy in a polycrystalline sintered body, which was the driving force for sintering. Therefore, two approaches for accelerating the pressureless densification were apparent: increasing the surface energy for the powders and reducing the grain boundary energy in the sintered body. According to this principle, three approaches were adopted for enhancing the densification of ZrB2 based ceramics during pressureless sintering: (1) increasing the surface energy of the powders by reducing their particle size; (2) increasing the sinterability of the powders by removing the oxygen contamination from their surfaces; (3) introducing additives which could change into liquid phase at the sintering temperature, meanwhile the liquid could properly wet the grain boundary of the targeted ceramics. Numerous examples were used to demonstrate these principles in practices, the morphology and size of the ZrB2 grains could also be tailored during this process. Finally, an outlook of the future directions for the pressureless densification of UHTCs was summaried. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.