Exploration of Hexagonal, Layered Carbides and Nitrides as Ultra-High Temperature Ceramics

TL;DR

This study uses high-throughput DFT calculations and machine learning to identify new thermally stable hexagonal layered carbides and nitrides with high melting points, expanding options for ultra-high temperature ceramics.

Contribution

It discovers 67 new stable layered materials and predicts several with melting points above 2500 K, broadening the material landscape for high-temperature applications.

Findings

Identified 67 new thermally stable layered carbides and nitrides.

Predicted several materials with melting points exceeding 2500 K.

Expanded the chemical and structural diversity of high-temperature ceramics.

Abstract

Layered, hexagonal crystal structures, like zeta and eta phases, play an important role in ultra-high temperature ceramics, often significantly increasing toughness of carbide composites. Despite their importance open questions remain about their structure, stability, and compositional pervasiveness. We use high-throughput density functional theory to characterize the thermodynamic stability and elastic constants of layered carbides and nitrides M$_{n+1}$X$_{n}$ with $n$ = 1, 2, and 3, $M$ = Ta, Ti, Hf, Zr, Nb, Mo, V, W, Sc, Cr, Mn and $X$ = C, N. The stacking sequences explored are inspired by the possible use of MXenes as precursors to enable relatively low temperature processing of high-temperature ceramics. We identified 67 new hexagonal, layered materials with thermal stability comparable or better than previously observed zeta phases. To assess their potential for high temperature applications, we used machine learning and physics-based models with DFT inputs to predict their melting temperatures and discovered several candidates on par with the current state of the art zeta-like phases and five with predicted melting temperatures above 2500 K. The findings expand the range of chemistries and structures for high-temperature applications.