Experimental Parametric Investigation of Nanosecond Laser-Induced Surface Graphitization of Nano-Crystalline Diamond

At a Glance

Metadata	Details
Publication Date	2024-06-03
Journal	Materials
Authors	Huixin Yuan, Liang Zhao, Junjie Zhang
Institutions	Harbin Institute of Technology
Citations	4
Analysis	Full AI Review Included

Executive Summary

This investigation details the optimization of nanosecond laser ablation parameters for inducing surface graphitization in Nano-Crystalline Diamond (NCD), a critical pre-treatment step to enhance its machinability.

Core Value Proposition: Laser-induced graphitization converts ultra-hard diamond (sp³) into softer graphite (sp²), significantly reducing the material’s hardness and brittleness prior to conventional grinding and polishing.
Material and Method: Bulk CVD NCD (99 GPa hardness) was processed using a 532 nm nanosecond pulsed YAG laser in both point and linear scanning modes.
Optimal Parameters Identified: Maximum graphitization with minimal surface damage (brittle cracks, spalling) was achieved at a Laser Power of 25 mW and a Pulse Repetition Rate of 1000 Hz.
Graphitization Quantification: Raman spectroscopy confirmed the phase transition, with the optimal microgroove exhibiting an I_G/I_D ratio of 1.342, indicating a high degree of graphitized carbon formation.
Mechanism Insight: The nanosecond laser’s dominant thermal effect creates a high temperature gradient, driving the sp³-to-sp² phase change and forming a deposited metamorphic layer and Laser-Induced Periodic Surface Structures (LIPSSs).
Ablation Threshold: The ablation threshold for NCD under these conditions was experimentally determined to be approximately 3.3 J/cm².
Outcome: The optimized process yields a machined surface with enhanced graphitization and high surface integrity, promoting efficient subsequent finishing operations.

Technical Specifications

Parameter	Value	Unit	Context
NCD Hardness	99	GPa	Bulk CVD NCD sample property
NCD Density	3.52	g/cm³	Bulk CVD NCD sample property
NCD Grain Size Range	50-100	nm	Bulk CVD NCD sample property
Laser Wavelength	532	nm	Nanosecond pulsed YAG laser source
Laser Pulse Width	5	ns	Fixed laser parameter
Laser Spot Diameter	10	µm	Nominal spot size
Pulse Repetition Rate (Optimal)	1000	Hz	Maximizes graphitization, minimizes cracking
Laser Power (Optimal)	25	mW	Used for optimal microgroove fabrication
Scanning Speed (Optimal)	0.01	mm/s	Used for linear microgroove fabrication
Ablation Threshold (NCD)	3.3	J/cm²	Determined using a 5.6 µm laser spot radius
Optimal Microgroove Depth	~5	µm	Achieved with optimized parameters
Diamond Raman Peak (sp³)	1332	cm^-1	Characteristic diamond vibration mode
Graphite G Peak (sp²)	1580	cm^-1	Characteristic graphite vibration mode
Optimal Microgroove I_G/I_D Ratio	1.342	N/A	Degree of graphitization in the groove region
Diamond Thermal Conductivity	Up to 2200	W/(m·K)	High conductivity reduces the heat-affected zone (HAZ)

Key Methodologies

The experimental investigation utilized systematic single-variable controlled experiments focusing on laser power and pulse repetition rate.

Experimental Setup:
- A nanosecond pulsed YAG laser (532 nm, 5 ns pulse width) was used, integrated with a Glan prism for precise power control (0.1 mW accuracy) and a CCD camera for in situ focal point calibration.
- Experiments were conducted in a constant temperature (20 °C) and dust-free environment.
Material Preparation:
- Bulk NCD samples (2 mm x 2 mm x 1 mm) prepared via Chemical Vapor Deposition (CVD) were used as the substrate.
Parameter Optimization (Point Scanning):
- Initial ablation tests were performed in point scanning mode (100 ms duration) to determine the influence of parameters on graphitization and morphology.
- Repetition Rate Variation: Tested rates included 500, 1000, 1500, 2000, 5000, and 10,000 Hz (fixed power 25 mW). Optimal rate identified was 1000 Hz (best surface integrity and graphitization).
- Laser Power Variation: Tested powers ranged from 5 mW to 40 mW (fixed repetition rate 1000 Hz). Optimal power identified was 25 mW (maximum I_G/I_D ratio).
Microgroove Fabrication (Linear Scanning):
- Microgrooves were fabricated using the optimized parameters: 25 mW power, 1000 Hz repetition rate, and 0.01 mm/s scanning speed.
Characterization Techniques:
- Morphology (SEM): Scanning Electron Microscopy was performed after depositing a thin (5 nm) Au film to enhance conductivity without altering microstructure.
- 3D Profile: A three-dimensional optical profilometer measured microhole and microgroove depth and cross-sectional profiles (~5 µm depth achieved).
- Graphitization Analysis (Raman Spectroscopy): A 532 nm Raman spectrometer was used to detect the sp³ (diamond) peak at 1332 cm^-1 and the sp² (graphite G and D) peaks. The degree of graphitization was quantified by the I_G/I_D ratio (Graphite intensity / Amorphous Carbon intensity).

Commercial Applications

This technology is directly applicable to industries requiring the precision machining and finishing of ultra-hard materials, particularly those utilizing NCD and polycrystalline diamond (PCD) composites.

Precision Tooling and Machining:
- Pre-treatment of high-precision NCD cutting tools, inserts, and dies to facilitate subsequent grinding and polishing, significantly reducing processing time and wear on conventional abrasives.
- Fabrication of micro-structures (microholes, microgrooves) in diamond components for fluidics or micro-electromechanical systems (MEMS).
Aerospace and Defense:
- Finishing of high-wear-resistance components used in extreme environments (e.g., components manufactured by Shenyang Aircraft Industry, mentioned in the paper).
Advanced Optics:
- Manufacturing and finishing of diamond optical components that require exceptional surface integrity and ultra-low roughness (less than 100 nm).
Laser-Assisted Manufacturing (LAM):
- Integration into Laser-Assisted Grinding (LAG) systems, where the laser acts as a localized softening mechanism immediately preceding mechanical removal.
High-Performance Electronics:
- Processing of diamond substrates used in high-power semiconductor devices, where precise surface modification is necessary for thermal management and device integration.

View Original Abstract

While nano-crystalline diamond (NCD) is a promising engineering composite material for its unique mechanical properties, achieving the ultrahigh surface quality of NCD-based components through conventional grinding and polishing is challenging due to its exceptional hardness and brittleness. In the present work, we experimentally investigate the nanosecond laser ablation-induced graphitization characteristics of NCD, which provides a critical pretreatment method of NCD for realizing its superlative surface finish. Specifically, systematic experimental investigations of the nanosecond pulsed laser ablation of NCD are carried out, in which the characteristics of graphitization are qualitatively characterized by the Raman spectroscopy detection of the ablated area of the microhole and microgroove. Subsequently, the influence of laser processing parameters on the degree and morphological characteristics of graphitization is evaluated based on experimental data and related interpretation, from which optimized parameters for maximizing the graphitization of NCD are then identified. The findings reported in the current work provide guidance for promoting the machinability of NCD via laser irradiation-induced surface modification.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma17112704

References

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