Mechanisms of pore formation in high-temperature carbides: Case study of TaC prepared by spark plasma sintering

A detailed analysis of the microstructure and pore evolution of TaC is presented and three pore-forming mechanisms have been discovered: (i) evolution of oxygen impurity; (ii) evolution of excess carbon, and (iii) incongruent sublimation of TaC. The evolution of oxygen impurities is the predominant mechanism and is thought to be the consequence of the high level of oxygen impurity typical of nanopowders; however, all mechanisms must be considered in order to completely eliminate porosity. We propose that the latter two mechanisms can be facilitated by local hot spots and discuss a possible source. The sphere-like morphology and distribution of isolated pores at grain interfaces, edges, and corners indicate a high surface energy of the pores with respect to the grain boundary energy so that the pores become trapped in the microstructure. The root cause of the three mechanisms indicates that a limiting sintering temperature (2173K) and carefully controlled chemistry are essential for producing TaC without trapped porosity. We propose that these three mechanisms define the sintering behavior of bulk TaC and that similar mechanisms need to be considered for the sintering of other carbide ceramics.