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

Executive Summary

This research investigates the synthesis of Concentration-Modulated Diamond (CMD) films via Microwave Plasma Chemical Vapor Deposition (MPCVD) to optimize mechanical and tribological performance for engineering applications.

Core Achievement: CMD films successfully combine the high smoothness of Nanocrystalline Diamond (NCD) with significantly improved hardness and drastically reduced wear rates for counterpart materials.
Surface Quality: CMD films achieved a smooth surface roughness (Sz) ranging from 0.15 to 0.21 µm, comparable to NCD (0.19 µm) and substantially smoother than Microcrystalline Diamond (MCD) (1.02 µm).
Hardness Enhancement: CMD films exhibited hardness values up to 61.1 GPa, representing a substantial improvement over conventional NCD films (38.2 GPa).
Tribological Performance: Friction testing against an Al₂O₃ ball resulted in ultra-low friction coefficients (0.03-0.05) for CMD films.
Wear Reduction: The specific wear rate of the Al₂O₃ counterpart material was reduced by over 90% (down to 0.09 x 10^-6 mm³/Nm) compared to both MCD and NCD films.
Process Control: The structure, hardness, and graphite content (I_t-PA/I_G ratio) of the films were effectively modulated by adjusting the frequency and duration of the CH₄ concentration steps.
Structural Classification: Raman spectroscopy classified the CMD films as NCD, characterized by abundant nanoscale diamond particles and reduced non-diamond (graphitic) components compared to standard NCD.

Technical Specifications

Parameter	Value	Unit	Context
Synthesis Method	MPCVD	N/A	Microwave Plasma Chemical Vapor Deposition
Source Gas Modulation	1% to 10%	CH₄ concentration	Modulated between low (1%) and high (10%) steps.
H₂ Flow Rate	200	SCCM	Constant for all depositions.
Microwave Power	400	W	Constant during deposition.
Pressure	6	kPa	Constant during deposition.
CMD Film Hardness (Max)	61.1	GPa	Achieved by CMD3 film.
NCD Film Hardness (Avg)	38.2	GPa	Conventional NCD comparison.
CMD Surface Roughness (Sz)	0.15 to 0.21	µm	CMD films (CMD3 achieved 0.15 µm).
MCD Surface Roughness (Sz)	1.02	µm	Conventional MCD comparison.
CMD Friction Coefficient (Min)	0.03	N/A	Average value for CMD3 film against Al₂O₃.
CMD Specific Wear Rate (Min)	0.09 x 10^-6	mm³/Nm	Al₂O₃ counterpart wear rate (CMD3 film).
NCD Specific Wear Rate	17.9 x 10^-6	mm³/Nm	Al₂O₃ counterpart wear rate.
Diamond Raman Peak	1333	cm^-1	Characteristic sp³ diamond bond.
CMD Crystallite Size	5.7 to 6.2	nm	Calculated from XRD (Scherrer’s equation).
Deposition Rate (CMD)	0.56 to 0.63	µm/h	Growth rate showed minimal variation with modulation.

Key Methodologies

The diamond films were synthesized and characterized using the following procedures:

Substrate Preparation:
- Si (100) wafers (1x1 cm²) were used as substrates for fundamental investigation.
- Pre-treatment involved scratching in ethanol containing 80-100 µm diamond powder for 15 min, followed by ultrasonic cleaning in acetone for 10 min.
Film Deposition (MPCVD):
- A constant H₂ flow rate of 200 SCCM was maintained.
- Pressure was manually adjusted to 6 kPa, and microwave power was set to 400 W.
- Concentration Modulation: CMD films were deposited by alternating between Step 1 (10% CH₄ concentration) and Step 2 (1% CH₄ concentration).
- Modulation Frequency Variation: The duration of Steps 1 and 2 was varied (2 min, 3 min, 4 min, or 5 min per step) to produce CMD2, CMD3, CMD4, and CMD5 films, totaling 60-64 min deposition time.
Structural and Morphological Characterization:
- Morphology: Surface and cross-sectional images were obtained using Scanning Electron Microscopy (SEM).
- Roughness: Surface roughness (Sz) was calculated using a Laser Scanning Microscope.
- Crystallinity: Raman spectra (532 nm wavelength) were deconvoluted using a Gaussian function to analyze sp³ (diamond) and sp² (graphite/amorphous carbon) components.
- Crystallite Size: X-ray Diffraction (XRD) patterns were used, and crystallite size was calculated from the (111) peak using Scherrer’s equation.
Mechanical and Tribological Testing:
- Hardness: Measured at nine points using an ultra-micro indentation method with a Berkovich diamond pyramid (maximum load ≤ 5 mN).
- Friction Test: Conducted using a ball-on-disk tester (60,000 cycles).
- Counterpart: 8 mm diameter Al₂O₃ ball, 1 N normal load, 500 rpm rotation speed.
- Wear Analysis: Wear scars on the Al₂O₃ ball and diamond films were observed via optical microscopy to determine the specific wear rate of the counterpart material.

Commercial Applications

The synthesis of Concentration-Modulated Diamond (CMD) films, which offer superior smoothness, hardness, and drastically reduced counterpart wear, is highly relevant for high-performance tribological systems.

High-Performance Sliding Components: Ideal for mechanical seals, bearings, and gears where both low friction and high wear resistance are critical, particularly in dry or minimally lubricated environments.
Precision Manufacturing Tools: Used for coating cutting tools, molds, and dies, where the smooth surface finish reduces friction and heat generation, while high hardness extends tool life.
Aerospace and Automotive Components: Applicable to engine parts and high-stress sliding interfaces requiring coatings that minimize abrasive wear on mating ceramic or metal components.
Post-Processing Reduction: Since CMD films achieve high smoothness (Sz < 0.2 µm) during deposition, they significantly reduce or eliminate the costly and time-consuming post-polishing steps typically required for rougher MCD films.
Micro- and Nano-Scale Devices: Suitable for coating components in micro-electromechanical systems (MEMS) where surface smoothness and durability at the nanoscale are paramount.

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