Key Takeaways & Executive Findings
- •• • NNAP-guided simulations lower the energy barrier for β→α transformation by nearly 60% compared to DFT predictions, enabling large-scale atomistic simulations of phase transformations in Si3N4. • • The bilayer cooperative transformation pathway exhibits an energy barrier of 0.018 eV/Ų, which is 10% lower than the independent layer-by-layer pathway (0.020 eV/Ų), as quantified by CI-NEB calculations. • • The energy barrier for the bilayer cooperative pathway shows robust convergence with respect to system size, as shown in Fig. S11, ensuring reliability of the mechanism across scales. • • The bilayer cooperative mechanism involves concerted breaking and reformation of Si–N bonds with coordinated rotations of Si atoms, leading to vertically aligned nucleation domains that delocalize transformation strain and reduce kinetic barriers, aligning with experimentally observed abnormal plasticity.
Abstract
Silicon nitride (Si3N4) is a strong, thermally stable covalent ceramic typically regarded as brittle with limited deformability. Recent experimental and density functional theory (DFT) studies indicate that the α/β interface undergoes a β→α transformation via sliding followed by bond-switching, suggesting a pathway to achieve plasticity, but DFT's spatiotemporal reach prevents a full mechanistic picture. Here, we develop a physics-informed high-accuracy neural network interatomic potential (NNAP) model with DFT-level accuracy for phase transformations and use it to perform large-scale atomistic simulations. NNAP-guided simulations show that structural relaxation during relative sliding between α- and β-phases at the interface triggers pronounced atomic-layer rearrangements and lowers the energy barrier by nearly 60%. We further find that the ensuing phase transformation does not proceed by isolated layer-by-layer switching but instead follows in-plane nucleation and growth mediated by a bilayer cooperative mechanism, which further reduces kinetic barriers and facilitates the transformation. CI-NEB calculations reveal that the bilayer cooperative pathway has an energy barrier of 0.018 eV/Ų, lower than the independent layer-by-layer manner (0.020 eV/Ų), indicating enhanced kinetic accessibility. These results provide new atomistic insights into interface-driven phase transformations in dual-phase Si3N4 and offer guidance for designing more deformable covalent ceramics.
1. Introduction
Silicon nitride (Si3N4) is a covalent ceramic prized for its high strength, hardness, thermal-shock resistance, and chemical stability, enabling use in high-temperature structural components, cutting tools, rolling bearings, and electronic packaging. However, its strongly directional covalent bonding and dense lattice structure severely limit plastic deformation at ambient conditions. Unlike metals, where plasticity is mediated by dislocation glide, Si3N4 suppresses slip and twinning, leading to bond rupture and catastrophic failure. Achieving a balance between high strength and controllable plastic deformability remains a central challenge for covalent ceramics.
During conventional synthesis, α-Si3N4 transforms into the thermodynamically stable β-Si3N4 via a dissolution-reprecipitation mechanism, creating ubiquitous dual-phase α/β interfaces. Recent experimental and DFT studies indicate that the α/β interface undergoes a β→α transformation via sliding and bond-switching, offering a pathway to plasticity. However, DFT's computational cost restricts simulations to picosecond timescales and small system sizes, preventing a full mechanistic understanding. This work develops a physics-informed neural network interatomic potential (NNAP) with DFT-level accuracy, enabling large-scale atomistic simulations that reveal a bilayer cooperative transformation mechanism, which lowers energy barriers and facilitates the transformation, providing guidance for designing more deformable covalent ceramics.
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Yuxuan Chen, Guanchen Dong, Qing-an Li, Huanrong Liu, Rui Su, Pengfei Guan (2026). Unraveling the bilayer-cooperative transformation mechanism at the α/β-Si3N4 interface via machine-learning simulations. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4088-y
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Frequently Asked Questions
What is the quantitative reduction in energy barrier achieved by the bilayer cooperative mechanism compared to the independent layer-by-layer pathway, and how does this affect the feasibility of plastic deformation in Si3N4?
The bilayer cooperative pathway exhibits an energy barrier of 0.018 eV/Ų, which is 10% lower than the independent layer-by-layer pathway (0.020 eV/Ų), as determined by CI-NEB calculations. This reduction enhances kinetic accessibility, making the phase transformation more feasible under stress, which aligns with the pronounced abnormal plasticity observed experimentally.
How does the NNAP model achieve DFT-level accuracy, and what is the computational speedup enabling large-scale simulations?
The NNAP model is trained on DFT data to reproduce potential energy surfaces and forces with high accuracy. While the exact speedup is not specified in the text, the model enables simulations at length and time scales inaccessible to DFT, allowing the observation of collective atomic rearrangements and the bilayer cooperative mechanism.
What are the structural signatures of the bilayer cooperative mechanism, and how do they contribute to lowering the transformation barrier?
The bilayer cooperative mechanism is characterized by synchronized Si–N bond breaking and reformation across adjacent layers, producing temporally overlapped and spatially correlated atomic rearrangements. Nucleation clusters in the two layers are vertically aligned, delocalizing transformation strain and reducing the overall activation barrier, as evidenced by the lower energy barrier compared to independent layer-by-layer switching.
How does the energy barrier for the bilayer cooperative pathway converge with system size, and what implications does this have for extrapolating to macroscopic scales?
The energy barrier exhibits robust convergence with respect to system size, as shown in Fig. S11. This convergence indicates that the mechanism is intrinsic and not an artifact of finite-size effects, supporting its relevance to macroscopic polycrystalline Si3N4.
What experimental evidence supports the bilayer cooperative mechanism, and how does it correlate with the observed plasticity in Si3N4?
The bilayer cooperative mechanism aligns with the pronounced abnormal plasticity observed experimentally, as the low energy barrier (0.018 eV/Ų) facilitates the β→α transformation under stress. The atomistic simulations reveal that the transformation occurs via concerted bond breaking and reformation, which is consistent with the reconstructive-type phase transformation pathway suggested by prior experimental and DFT studies.
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