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Abstract
The current research focuses on high-temperature nitrides, from binary and ternary to high entropy. The goals are to study the synthesis and processing of representative transition metal nitrides and understand the relationships between composition, processing, microstructure, and their properties.
The first part of the study is focusing on the binary nitride of ZrN. It aims to demonstrate the synthesis of ZrN powders by a facile and low-cost urea route without using any solvent and understand the formation mechanism including the effects of different processing parameters such as metal-to-urea molar ratio, heat treatment temperature, and dwelling time on the product phase and stoichiometry. In addition, the synthesized ZrN powder was consolidated into fully dense single-phase bulk ceramic in a few minutes with a new flash sintering technique and the microstructure as well as properties such as hardness and oxidation resistance of the sintered ZrN were characterized.
Next, reactive flash sintering (RFS) of ternary metal nitrides TiZrN and TiAlN from TiN-ZrN and TiN-AlN mixtures was demonstrated. Phase transformation was tracked using both conventional XRD and in situ synchrotron study. Uniform Ti0.57Zr0.43N solution formed in RFS and persisted upon cooling, while (Ti, Al) N solid solution formed at high temperature was unstable and went through a very quick phase separation in the cooling process.
Finally, novel bulk high entropy nitrides (HEN) such as, (Al0.17Nb0.17Ta0.17Ti0.32Zr0.17)N have been successfully synthesized in just a few minutes using the RFS technique. The formation of a single-phase nitride solid solution was confirmed using both ex situ XRD and in situ synchrotron. Microscopy revealed a dense microstructure without noticeable segregation of elements. The bulk HEN displays attractive mechanical property that combines high hardness with exceptional fracture toughness, a very attractive property in ceramics, which is attributed to layered microstructure despite cubic crystal lattice. Some other attractive properties include decent oxidation resistance and superconductivity.
This study provides a better understanding of these important nitride materials and the related new technologies, which might help future industrial applications in many fields.