Antibacterial and Film Characteristics of Copper-Doped Diamond-like Carbon Films via Sputtering Using a Mixed Target of Copper and Graphite

At a Glance

Metadata	Details
Publication Date	2025-05-07
Journal	Coatings
Authors	Kazuya Kanasugi, Takayoshi NAKAJIMA, Kenji Hirakuri
Institutions	Tokyo Denki University
Analysis	Full AI Review Included

Executive Summary

This research successfully developed a simplified method for depositing highly effective Copper-Doped Diamond-Like Carbon (Cu-DLC) films using a single, sintered C/Cu mixed target via Ar-sputtering.

Process Simplification: Cu-DLC films were fabricated using a single mixed C/Cu target, eliminating the complexity of dual magnetron or reactive sputtering methods, thus facilitating commercialization.
Controlled Doping: The Cu concentration in the resulting films (ranging from 40.0 to 54.1 at% via EPMA) was successfully controlled by adjusting the C/Cu ratio in the mixed target material.
Maintained DLC Quality: Despite high Cu incorporation, the films maintained excellent mechanical properties, exhibiting low dynamic friction coefficients (0.16-0.17) and smooth surfaces (Rq 4-6 nm), comparable to standard DLC films.
High Antibacterial Efficacy: All prepared Cu-DLC films demonstrated strong antibacterial activity against Staphylococcus aureus, achieving R values between 3.9 and 4.2 (R > 2.0 is considered effective).
Structural Integrity: Raman spectroscopy indicated that Cu incorporation increased the sp² content (ID/IG ratio higher than DLC), but the amorphous carbon structure remained largely unaffected across the tested Cu concentration range.
Mechanism Confirmation: ICP-OES confirmed that Cu ions (up to 0.56 ppm) are released from the film surface in wet environments, validating the mechanism responsible for the observed antibacterial effect.

Technical Specifications

Parameter	Value	Unit	Context
Deposition Method	Magnetron Sputtering	N/A	Using a single, sintered C/Cu mixed target.
Substrate Material	SUS304	N/A	Standard test piece material.
Target C:Cu Ratios	70:30, 60:40, 50:50	Ratio	Used for Cu-DLC (A), (B), and (C), respectively.
Ar Gas Flow Rate	20	sccm	Constant flow during sputtering.
Gas Pressure	3.5 - 3.7 x 10^-1	Pa	Operating pressure range.
Bias Voltage (Cu-DLC)	~530	V	Controlled discharge voltage.
Target Film Thickness	~1000	nm	Intended thickness for all samples.
Achieved Thickness	944 - 1287	nm	Range across Cu-DLC samples.
Deposition Rate (Max)	26.5	nm/min	Cu-DLC (C) (50:50 target), highest rate due to lower electrical resistance.
Film Cu Content (Max)	54.1	at%	Cu-DLC (C) measured via EPMA.
ID/IG Ratio (Max)	2.04	N/A	Cu-DLC (B), indicating increased sp² content compared to DLC (1.44).
Surface Roughness (Rq)	4.02 - 5.97	nm	Average RMS roughness for Cu-DLC films.
Dynamic Friction Coeff.	0.16 ± 0.02	N/A	Cu-DLC (A) and (C) (low friction, comparable to DLC).
Antibacterial Activity (R)	4.2 ± 0.00	N/A	Cu-DLC (B) and (C) (R > 2.0 is effective).
Cu Release Rate (Max)	0.56	ppm	Cu-DLC (C) after 24 h immersion in pure water (ICP-OES).

Key Methodologies

The Cu-DLC films were prepared using a unique Ar-sputtering process with custom-developed mixed targets, followed by comprehensive structural and functional characterization.

Target Preparation:
- C and Cu powders were mixed at specific ratios (70:30, 60:40, 50:50) to form three distinct mixed targets.
- The powders were then sintered to create solid, single-source targets.
Substrate Preparation:
- SUS304 substrates were ultrasonically cleaned in acetone and ethanol for 10 minutes each.
Sputtering Deposition (Magnetron Sputtering):
- The chamber was evacuated to 3.0 x 10^-3 Pa.
- Ar gas (99.9995% purity) was introduced at 20 sccm, stabilizing the pressure at approximately 3.5 x 10^-1 Pa.
- A bias voltage of ~530 V was applied to the substrate side (ground potential used to inhibit arcing).
- Deposition time was adjusted (40-80 min) to achieve a target film thickness of 1000 nm.
Structural and Compositional Analysis:
- EPMA (Electron Probe Microanalysis): Used to confirm bulk C, Cu, and O content in the films.
- XPS (X-ray Photoelectron Spectroscopy): Used for surface composition analysis (C 1s, Cu 2p, O 1s narrow spectra) and depth profiling (etching for 40 cycles) to confirm Cu continuity and surface oxidation.
- Raman Spectroscopy: Used to evaluate the amorphous carbon structure, calculating the I_D/I_G ratio and G peak position to assess sp² content.
Surface and Mechanical Characterization:
- AFM (Atomic Force Microscopy): Used in contact mode (10 µm x 10 µm scan size) to measure average root mean square roughness (Rq).
- Ball-on-Disk Test: Used to determine dynamic friction coefficient (load = 3 N, linear velocity = 5 cm/s, sliding distance = 200 m) using a SUS304 ball.
Functional Testing (Antibacterial and Release):
- Antibacterial Activity: Tested against Staphylococcus aureus (NBRC12732) according to ISO 22196 standards. Activity (R value) was calculated, with R > 2.0 indicating antibacterial properties.
- Cu Release: High-frequency inductively coupled plasma optical emission spectrometry (ICP-OES) was performed after immersing samples in pure water for 24 hours to quantify released Cu ions (ppm).

Commercial Applications

The successful development of highly antibacterial Cu-DLC films using a simplified, single-target sputtering process makes this technology highly relevant for hygiene-critical environments and surface modification of consumer goods.

Healthcare and Medical Fields:
- Surface coating for medical equipment, instruments, and implants requiring long-term hygiene resilience.
- Modification of hospital surfaces, such as bed rails, trays, and door handles, to reduce microbial transmission.
Residential and Public Environments:
- Coating for high-touch surfaces in public spaces (e.g., handrails, elevator buttons, doorknobs) to mitigate microbial threats.
- Application in food processing and preparation equipment where hygiene is paramount.
Industrial Surface Modification:
- Integration into industrial products, such as cutting tools and hard disks, where the Cu-DLC maintains the high hardness and wear resistance of standard DLC while adding antimicrobial function.
Consumer Products:
- Surface treatment for consumer electronics and everyday items that benefit from both durability and hygienic properties.

View Original Abstract

Copper-doped diamond-like carbon films (Cu-DLC) are effective antibacterial materials and are fabricated using different techniques. By controlling the ratio of the graphite and diamond structures as well as the hydrogen bonds, the biocompatibility, chemical stability, wear resistance, and high hardness of Cu-DLC can be regulated. In this study, three types of Cu-DLC films were deposited on SUS304 substrates using Ar-sputtering with mixed targets comprising different C/Cu ratios. The films’ structures, surface, and antibacterial properties were investigated using electron probe microanalysis, Raman and X-ray photoelectron spectroscopy, atomic force microscopy, and ball-on-disk tests. The Cu concentration in the Cu-DLC films increased with an increase in its content in the target; however, no significant differences were observed in the Raman spectra. The surface composition, roughness, and dynamic friction coefficients were similar across all Cu-DLC films, which displayed smoothness and friction properties similar to those of standard DLC films without Cu. The antibacterial activity (R value) was evaluated as per ISO 22196. Although DLC films exhibited no antibacterial activity (R < 2), all the prepared Cu-DLC films displayed good antibacterial activity (R ≥ 2). The proposed deposition process facilitated Cu-DLC coating, thus promoting its use in the healthcare fields.

Tech Support

Original Source

DOI: https://doi.org/10.3390/coatings15050559

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