Density-functional-theory predictions of mechanical behaviour and thermal properties as well as experimental hardness of the Ga-bilayer Mo2Ga2C

Xinxin Qi,Sen Jin,Weilong Yin,Xiaodong He,Yuelei Bai,Aiguo Zhou,Guangping Song,Yongting Zheng

doi:10.1007/s40145-021-0531-9

Abstract

Mo2Ga2C is a new MAX phase with a stacking Ga-bilayer as well as possible unusual properties. To understand this unique MAX phase structure and promote possible future applications, the structure, chemical bonding, and mechanical and thermodynamic properties of Mo2Ga2C were investigated by first-principles. Using the “bond stiffness” model, the strongest covalent bonding (1162 GPa) was formed between Mo and C atoms in Mo2Ga2C, while the weakest Ga-Ga (389 GPa) bonding was formed between two Ga-atomic layers, different from other typical MAX phases. The ratio of the bond stiffness of the weakest bond to the strongest bond (0.33) was lower than 1/2, indicating the high damage tolerance and fracture toughness of Mo2Ga2C, which was confirmed by indentation without any cracks. The high-temperature heat capacity and thermal expansion of Mo2Ga2C were calculated in the framework of quasi-harmonic approximation from 0 to 1300 K. Because of the metal-like electronic structure, the electronic excitation contribution became more significant with increasing temperature above 300 K.

Highlights

Over the past two decades, a class of ternary transition metal carbides or nitrides known as MnAXn+1 phases, are formed by inserting A-group atoms into the corresponding binary carbides or nitrides, and have attracted growing
All first-principles calculations were performed within the framework of density-functional-theory (DFT) as implemented within the Vienna Ab initio Simulation Package (VASP) [43]
The bulk Mo2Ga2C samples were prepared by hot pressing at 750 °C for 8 h, and X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images were shown in Ref. [50]

Summary

Introduction

For most of the MAX phases except Ti2SC, the ratio is between 1/3 and 1/2 [39], where weak bonding among the sublayers of grains can cause typical delamination and crack deflection to avoid catastrophic crack propagating when a crack encounters this weak bonding, with consuming a lot of external energy. The ratio can be used as a theoretical judgment for the macromechanical behaviour of ternary layered ceramics This finding shows that the MAX phases have extraordinary mechanical properties, which require that the M–A bond is weaker than the M–C bond, but is “weak enough”. The present work is to investigate the mechanical and thermal properties as well as phase stability of Mo2Ga2C with the Ga-bilayer by first-principles, which would provide a theoretical guidance for further understanding the influence of crystal structure on macro behaviour of MAX phases, and inspire future experimental research

Theoretical

Experimental

Results and discussion

Conclusions