Effect of Concentration Modulation on Mechanical Properties of Diamond Films Synthesized via Microwave Plasma CVD

At a Glance

Metadata	Details
Publication Date	2025-05-12
Journal	Nanomanufacturing and Metrology
Authors	Ryota Ohnishi, Ippei Tanaka, Natsuki Kawaguchi, Yasunori Harada
Analysis	Full AI Review Included

Technical Documentation & Analysis: Concentration Modulation for High-Performance Diamond Films

Executive Summary

This research demonstrates a highly effective Microwave Plasma Chemical Vapor Deposition (MPCVD) technique—Concentration Modulation (CMD)—to synthesize Nanocrystalline Diamond (NCD) films with superior mechanical and tribological properties, overcoming the traditional trade-off between hardness and surface roughness.

Core Achievement: CMD films achieved a smooth surface (Sz as low as 0.15 µm) combined with high hardness (up to 61.1 GPa), exceeding the performance of standard NCD films (38.2 GPa).
Methodology: The process involved modulating the CH₄ concentration between 1% (etching/growth promotion) and 10% (nucleation) at high frequency (2-5 minute steps).
Tribological Performance: CMD films exhibited extremely low and stable friction coefficients (as low as 0.03) against Al₂O₃ counterparts under dry sliding conditions.
Wear Resistance: The specific wear rate of the Al₂O₃ counterpart was reduced by over 90% (down to 0.09 x 10^-6 mm³/Nm) compared to conventional Microcrystalline Diamond (MCD) and NCD films.
Material Classification: Raman spectroscopy confirmed the CMD films possess a fine, nanoscale diamond structure, classifying them as high-quality NCD.
Modulation Control: Hardness and graphite content (It-PA/I_G ratio) were shown to be controllable by adjusting the modulation frequency (Step 1 and Step 2 durations).

Technical Specifications

Parameter	Value	Unit	Context
Maximum Hardness (CMD3)	61.1	GPa	Measured via ultra-micro indentation
Standard NCD Hardness	38.2	GPa	For comparison
Minimum Surface Roughness (Sz)	0.15	µm	Achieved by CMD3 film
Standard MCD Roughness (Sz)	1.02	µm	Rough surface comparison
Minimum Friction Coefficient (Average)	0.03	N/A	Achieved by CMD3 film against Al₂O₃
Specific Wear Rate (Al₂O₃ Counterpart)	0.09 x 10^-6	mm³/Nm	Achieved by CMD4 film (90%+ reduction)
CVD Pressure	6	kPa	Constant operating pressure
Microwave Power	400	W	Constant operating power
H₂ Flow Rate	200	SCCM	Constant carrier gas flow
CH₄ Concentration Range	1% to 10%	N/A	Modulated concentration range
Deposition Rate (CMD)	0.56 to 0.63	µm/h	Minimal variation across modulation frequencies
Crystallite Size (CMD)	5.7 to 6.2	nm	Calculated using Scherrer’s equation

Key Methodologies

The high-performance diamond films were synthesized using a Microwave Plasma CVD (MPCVD) system with precise gas flow control via MATLAB and an interface board.

Substrate Preparation: Si (100) wafers (0.7 mm thick, 1x1 cm²) were pre-treated by scratching with 80-100 µm diamond powder (0.1 g/mL in ethanol) for 15 min, followed by ultrasonic cleaning in acetone.
Source Gas Supply: H₂ flow was maintained at 200 SCCM. CH₄ concentration was dynamically modulated between two steps:
- Step 1 (High CH₄): 10% CH₄ concentration (20 SCCM CH₄). Promotes nucleation.
- Step 2 (Low CH₄): 1% CH₄ concentration (2 SCCM CH₄). Promotes diamond growth and etching of non-diamond phases.
Modulation Cycle: The CMD films (CMD2-CMD5) were deposited by alternating Step 1 and Step 2 for equal durations (2, 3, 4, or 5 minutes per step) for a total deposition time of 60-64 minutes.
CVD Parameters: Pressure was maintained at 6 kPa, and microwave power was 400 W.
Characterization: Films were analyzed using SEM (surface/cross-section), Laser Scanning Microscopy (Sz roughness), XRD (crystallite size), and Raman Spectroscopy (structure/phase purity).
Tribology Testing: Friction tests were performed using a ball-on-disk tester against an 8 mm Al₂O₃ ball (1 N load, 500 rpm, 0.16 m/s sliding speed) for 60,000 cycles under 50% ± 3% relative humidity.

6CCVD Solutions & Capabilities

The research successfully demonstrates that precise control over the CVD environment, specifically through concentration modulation, is critical for engineering high-performance diamond films suitable for advanced tribological applications. 6CCVD is uniquely positioned to supply the necessary materials and customization required to replicate and advance this research into industrial applications.

Applicable Materials

The CMD films synthesized are high-quality Nanocrystalline Diamond (NCD) structures optimized for low friction and high wear resistance.

Tribological Grade PCD (NCD Structure): 6CCVD specializes in Polycrystalline Diamond (PCD) films, which includes the NCD morphology required for this application. We offer PCD optimized for mechanical and tribological uses, ensuring high density and structural uniformity necessary for stable, low-friction performance.
Custom Thickness: The paper utilized films around 0.5-0.7 µm thick. 6CCVD routinely supplies PCD films ranging from 0.1 µm up to 500 µm, allowing researchers to optimize film thickness for specific load-bearing requirements.

Customization Potential

6CCVD’s advanced manufacturing capabilities directly address the needs of researchers working on complex CVD recipes and specialized applications like the CMD technique.

Research Requirement	6CCVD Capability	Value Proposition
Substrate Size (1x1 cm²)	Plates/wafers up to 125 mm diameter	Scale-up capability for industrial prototyping and large-area deposition.
Surface Smoothness (Sz ~ 0.15 µm)	Guaranteed Polishing: Ra < 5 nm (for inch-size PCD)	6CCVD provides films with surface roughness significantly lower than the best results achieved in this paper, reducing or eliminating the need for costly post-processing.
Custom CVD Recipes (CMD)	Engineering Support & Custom Growth	Our in-house PhD team can assist in developing and optimizing complex MPCVD recipes, including concentration modulation or laminated structures, to achieve specific NCD/MCD layer properties.
Integration with Counterparts (Al₂O₃)	Custom Metalization Services	If the application requires integration or bonding, 6CCVD offers internal metalization services (Au, Pt, Pd, Ti, W, Cu) for enhanced adhesion or electrical contact layers.

Engineering Support

The success of Concentration Modulation relies on precise control of gas flow, pressure, and power to manage the transition between nucleation (high CH₄) and etching/growth (low CH₄). 6CCVD’s expertise ensures material quality is maintained even under dynamic synthesis conditions.

Material Selection for Tribology: 6CCVD’s in-house PhD team can assist with material selection and structural optimization for similar high-performance sliding parts projects, ensuring the optimal balance of hardness, smoothness, and phase purity (low sp² content).
Global Supply Chain: We offer reliable global shipping (DDU default, DDP available) to ensure rapid delivery of custom diamond materials for time-sensitive research and development projects worldwide.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Abstract Diamond films have excellent mechanical characteristics, such as high hardness and low friction, but have rough surfaces. Therefore, we developed a method for synthesizing diamond films while modulating carbon gas concentration using microwave plasma chemical vapor deposition. Then, the effects of concentration modulation on the mechanical properties of smooth diamond films were examined. A concentration-modulated diamond (CMD) film with a surface roughness (Sz) of 0.2 µm was synthesized by modulating the methane concentration from 1% to 10%. The diamond films synthesized by modulating CH 4 concentrations were classified as nanocrystalline diamond (NCD) films by Raman spectroscopy. The hardness of the CMD film ranged from 43.8 to 61.1 GPa, exhibiting higher values than those of NCD synthesized at 10%. In friction testing with Al 2 O 3 , the friction coefficient of the CMD film was below 0.1. The specific wear rate of the counterpart material subjected to dry conditions was 0.09 × 10 −6 mm 3 /Nm, demonstrating a reduction of over 90% compared with conventional diamond films.

Tech Support

Original Source

DOI: https://doi.org/10.1007/s41871-025-00255-y