超高温陶瓷的无压烧结致密化与微结构调控

Pressureless Densification of Ultra-high Temperature Ceramics and Microstructure Tailoring

无压烧结,也称常压烧结,是获得致密且复杂形状的陶瓷材料最常用且经济的烧结方式。在无压烧结过程中,通过烧结助剂、工艺或制度对所获得陶瓷的微组成和结构进行调控,是提升陶瓷制品致密度和性能的关键。从超高温陶瓷材料的研究背景和意义出发,总结了国内外近年来超高温陶瓷的发展趋势,以及在超高温陶瓷无压烧结致密化和微结构调控方面的研究进展。陶瓷粉体的表面能大于多晶烧结体的晶界能,二者的差异是陶瓷烧结的驱动力。因此,加快陶瓷无压烧结致密化有两种途径,一种是增大原料粉体的表面能;另一种是降低多晶陶瓷的晶界能。以最重要的超高温陶瓷的体系二硼化锆(ZrB2)为例,展开论述。实现超高温陶瓷无压烧结致密化途径的主要方法有3种:(1)降低原料粒径,提高粉体表面能;(2)去除粉体表面氧污染,增大粉体的烧结活性;(3)引入能与陶瓷基体晶界润湿的液相,降低多晶陶瓷晶界能。文章围绕这些措施在实践中的具体应用而展开, ZrB2陶瓷的晶粒形态和尺寸也可以在这些过程中得以调控。最后,文章还对超高温陶瓷无压烧结的发展前景进行了展望。

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.