Abstract: Metal dopant, such as aluminum, has a significant influence on the performance of amorphous Si₂BC₃N ceramics and is of particular interest. In this work, the structural responses of amorphous Si₂BC₃N to incorporating aluminum and related mechanical property modification are investigated employing ab initio molecular dynamics calculations. Considering different Al sources of Al and AlN, two models, i.e. Si₂BC₃NAl_0.6 and Si₂BC₃N_1.6Al_0.6 are constructed, respectively. It is found that the integration of Al engenders the Al-Si, Al-C, and Al-N chemical bonds within the amorphous framework, while the proportions of C-C and Si-Si bonds decrease, indicating that Al promotes a transition from nested polyhedra to independent polyhedral structures. The incorporation of Al induces an increase in tetrahedral arrangements and a decrease in sp²-like trigonal configurations compared to amorphous Si₂BC₃N. This structural transformation contributes to the enhancement of mechanical characteristics of Si₂BC₃NAl_0.6 ceramics. Conversely, Si₂BC₃N_1.6Al_0.6 shows a marginal increase in tetrahedral configurations, resulting in similar mechanical performance to Si₂BC₃N. This work elucidates a novel mechanism of local structure transformation in amorphous SiBCN ceramics with incorporated metal elements.

Key words: Ultra-high temperature ceramics, First-principles calculations, Elastic constants, Mechanical properties