Interplay between decarburization, oxide segregation, and densification during sintering of nanocrystalline TaC and NbC

Abstract

The present study shows that ball-milled nanosized powders of TaC and NbC can be successfully sintered to high densities at 1300−1400 °C for 30 min under vacuum. Fabricated ceramics demonstrate hardness of 20−25 GPa due to decrease in carbon stoichiometry and submicron grain size. The main techniques to investigate the underlying phenomena during processing of initial powders were XRD, SEM and TGA-DSC. After milling, carbide particles demonstrate a significant amount of oxygen impurities but no signs of oxidation. During sintering, these impurities react with structural carbon and metal ions, which results in decarburization and segregation of oxides. Above 1000 °C, the oxide phases undergo partial reduction by structural carbon, promoting decarburization even further. Densification starts shortly after the reduction of oxides and provides dense microstructures. The effects of decarburization and oxide segregation can be compensated by carbon excess, however it can be difficult to control densification curve in such case.