Comparative Molecular Dynamics Study of Graphitization Mechanisms in Nanosecond Laser Irradiation of Single-Crystal and Nanocrystalline Diamond

At a Glance

Metadata	Details
Publication Date	2025-04-25
Journal	Optics
Authors	Huixin Yuan, Liang Zhao, Weimian Guan, Yuqi Yang, Junjie Zhang
Institutions	Harbin Institute of Technology
Citations	1
Analysis	Full AI Review Included

Executive Summary

This Molecular Dynamics (MD) study provides a comparative analysis of graphitization mechanisms in Single-Crystal Diamond (SCD) and Nanocrystalline Diamond (NCD) under nanosecond pulsed laser irradiation, offering critical insights for precision laser processing.

Microstructural Influence: The material microstructure dictates the phase transition pathway. Graphitization in SCD is localized and thermally driven, while in NCD, it preferentially initiates and expands along Grain Boundaries (GBs).
Driving Mechanism: The primary mechanism for graphitization in NCD is the concentration of thermal stress and lower bonding energy at GBs, making these regions highly susceptible to the sp³-to-sp² transformation.
Stress Distribution: SCD exhibits high stress concentrated near the ablation pit, leading to localized graphitization. NCD shows a broader, higher overall stress distribution, particularly around GBs, promoting wider phase transformation.
Phase Transition Type: The simulation confirms a direct solid-state phase transition from diamond (sp³) to highly oriented graphite (sp²), without an intermediate molten carbon phase, driven by rapid thermal expansion and cooling.
Energy Density Effect: Increasing laser energy density (10 J/cm² to 30 J/cm²) significantly expands the ablation pit size, exacerbates volumetric expansion, and increases the quantity of layered graphite formed in both materials.
Theoretical Foundation: This research provides a theoretical basis for controlling surface modification processes in diamond materials by highlighting the crucial role of microstructure and thermal stress distribution.

Technical Specifications

Parameter	Value	Unit	Context
Simulated Laser Fluence Range	10 to 30	J/cm²	Energy density applied during irradiation stage.
MD Simulation Time Step	0.5	fs	Time step used in LAMMPS simulations.
Initial Equilibration Temperature	300	K	Temperature maintained during the relaxation stage.
C-C Bond Cutoff Distance	1.6	A	Used to define coordination number (slightly > first nearest neighbor distance).
High-Intensity Irradiation Phase	20	ps	Simulated duration approximating the peak intensity of the nanosecond pulse.
Ablation Pit Temperature (Peak)	> 2000	K	Temperature reached in the localized ablation zone.
Graphitization Initiation Temperature (Cited)	1475	K	Temperature threshold for graphite formation (sp² hybridization).
Maximum Von Mises Stress	250	GPa	Peak stress observed during laser irradiation.
Sample Dimensions (X x Y x Z)	10 x 5 x 10	nm	Dimensions of the SCD and NCD models.

Key Methodologies

MD Model Construction: Single-Crystal Diamond (SCD) was constructed along the (010) plane. Nanocrystalline Diamond (NCD) was modeled using Atomsk Beta 0.13.1 and the Voronoi algorithm to generate randomly oriented grains.
Boundary Conditions: Periodic boundary conditions were applied in the X and Z directions, with a free surface in the Y direction.
Interatomic Potential: The Second-Generation Reactive Empirical Bond Order (REBO) potential was utilized to accurately simulate atomic-scale non-equilibrium phase transitions and carbon-carbon bond hybridization (sp³ and sp²).
Laser Energy Deposition: Nanosecond pulsed laser energy was approximated by localized kinetic energy loading (velocity-rescaling algorithm) applied to surface atoms within a 1 nm diameter irradiated area.
Simulation Stages: The process was divided into three sequential stages:
- Relaxation (20 ps): Equilibration under NPT ensemble (300 K, 0 bar).
- Irradiation (20 ps): Energy deposition phase (no thermostat).
- Energy Decay (20 ps): Thermal relaxation under NPT ensemble.
Structural Analysis: Post-processing using Ovito software included analysis of:
- Coordination Number (CN) mapping to track sp³-to-sp² conversion.
- Radial Distribution Function (RDF) and Bond Angle Distribution (BAD) to confirm the solid-state transition and the coexistence of diamond and graphite phases.
- Von Mises stress distribution to track thermal stress evolution.

Commercial Applications

The findings regarding controlled graphitization and material response under laser irradiation are crucial for advanced diamond manufacturing and engineering applications.

Advanced Tooling and Wear Parts:
- Optimizing laser texturing and surface finishing of NCD cutting tools and tribological coatings. The high susceptibility of NCD GBs to graphitization requires precise fluence control to maintain structural integrity and hardness.
Diamond Semiconductor Fabrication:
- Creating highly conductive graphitic electrodes or interconnects on insulating SCD substrates via localized laser patterning. The localized stress in SCD allows for high-resolution feature creation.
Micro/Nano-Electromechanical Systems (MEMS/NEMS):
- Enabling precise internal scribing and material removal for fabricating diamond-based sensors and actuators, where control over subsurface phase transitions is paramount for device reliability.
High-Power Optical Components:
- Laser processing of diamond optical windows and heat spreaders. Understanding the thermal stress distribution prevents catastrophic failure (cracking, fragmentation) caused by uncontrolled graphitization and volumetric expansion.
Surface Functionalization:
- Developing methods for controlled surface graphitization to tailor diamond’s electronic or chemical properties for applications like field emission devices or electrochemical sensors.

View Original Abstract

The mechanisms of material removal and structural transformation under laser radiation differ significantly between single-crystal diamond (SCD) and nanocrystalline diamond (NCD). This study employs atomic simulations to investigate the material removal mechanisms and structural transformation behaviors of SCD and NCD when subjected to laser irradiation. We analyze the effects of temperature and stress changes induced by laser radiation on structural transformations, revealing the driving mechanism behind graphitization transitions. Specifically, the thermal-mechanical coupling effect induced by lasers leads to graphitization in SCD, while in NCD, due to the stress concentration effects at the grain boundaries, graphitization preferentially occurs at these boundaries. The material removal processes for both SCD and NCD are attributed to thermal stress concentrations in the regions where the laser interacts with the diamond surface. This investigation provides a theoretical foundation for a more profound understanding of the behavior of diamond materials during laser irradiation.

Tech Support

Original Source

DOI: https://doi.org/10.3390/opt6020016

References

2024 - Microstructure Control and Mechanical Properties of Ultra-Nanocrystalline Diamond Films [Crossref]
2024 - Carbon-Based Films: A Review on Mechanical and Tribology Properties [Crossref]
2025 - Microscopic Characterization and Tribological Properties of Boron-Doped Microtextured Spherical Diamond Composite Coatings [Crossref]
2024 - Nanodiamond Coating in Energy and Engineering Fields: Synthesis Methods, Characteristics, and Applications [Crossref]
2024 - Periodic Renucleation as an Approach to Improving the Tribological Properties of CVD Diamond Films [Crossref]
2024 - Solid Particle Erosion of CVD Diamond Coatings: A Review [Crossref]
2024 - The Effect of Microstructure and Film Composition on the Mechanical Properties of Linear Antenna CVD Diamond Thin Films [Crossref]
2024 - Investigation on Nanocrystalline Diamond Film with High Hardness [Crossref]
2021 - Plastic Deformation and Strengthening Mechanisms of Nanopolycrystalline Diamond [Crossref]
2012 - Laser in Micro and Nanoprocessing of Diamond Materials [Crossref]