Effects of Environmental Gas and Trace Water on the Friction of DLC Sliding with Metals

At a Glance

Metadata	Details
Publication Date	2017-07-11
Journal	Micromachines
Authors	Yoshihiro KURAHASHI, Hiroyoshi Tanaka, Masaya Terayama, Joichi Sugimura
Institutions	Kyushu University
Citations	19
Analysis	Full AI Review Included

6CCVD Technical Documentation: Tribology of MPCVD Diamond

Executive Summary

This study meticulously analyzes the critical role of trace water (in the parts-per-million range) on the tribological performance of Diamond-Like Carbon (DLC) sliding against aluminum in pure gas environments (Hydrogen, Nitrogen, Argon). These findings are highly relevant for advanced engineering applications utilizing diamond in high-ppurity hydrogen fuel systems, vacuum processing, and inert gas reactors.

Critical Impurity Control: The research confirms that extremely low concentrations of water vapor (0-160 ppm) drastically alter the friction dynamics of carbon coatings, necessitating ultra-high purity material solutions for reliable system operation.
Hydrogen Environment Superiority: Both hydrogenated (a-C:H) and hydrogen-free (ta-C) DLC demonstrated stable, super-low friction coefficients (as low as 0.05) in pure hydrogen gas, regardless of transfer film oxygen content.
Water-Assisted Lubricity in Inert Gas: In inert gases (Nitrogen, Argon), friction was exceptionally high (> 0.8) without trace water. However, the presence of 60-120 ppm water vapor facilitated the formation of a graphitic carbon transfer film on the aluminum counterface, dramatically reducing the friction coefficient for a-C:H DLC (down to ~0.05).
Mechanism Confirmation: Fourier Transform Infrared Spectroscopy (FT-IR) confirmed that water participation introduces H and OH species into the transfer film, forming C-O-C and C-OH bonds which are crucial for achieving super-low friction in non-hydrogen environments.
Material Differentiation: Hydrogenated DLC (a-C:H) proved significantly superior in utilizing trace water to achieve low friction in inert environments compared to hydrogen-free ta-C DLC.
Application Relevance: These results underscore the complexity of tribochemistry in ultra-clean environments, particularly for components in hydrogen storage, fuel cells, and vacuum systems where trace contamination levels must be engineered into the component design specifications.

Technical Specifications

Hard data extracted from the study detailing test conditions and material performance metrics.

Parameter	Value	Unit	Context
Max Trace Water (H₂O)	160	ppm	Range of controlled H₂O contamination
Max Trace Oxygen (O₂)	< 0.2	ppm	Controlled background O₂ concentration
Lowest Friction (a-C:H, H₂)	< 0.05	-	Best steady-state performance
Lowest Friction (a-C:H, Inert Gas)	~0.05	-	Achieved in Ar at 120 ppm H₂O
Highest Friction (ta-C, Inert Gas)	> 0.8	-	High friction observed without H₂O
Sliding Speed	0.0628	m/s	Pin-on-disk linear velocity
Normal Load	10	N	Applied load on the pin
Test Temperature	298	K	Standard room temperature (25 °C)
Test Pressure	0.12	MPa	Absolute gas pressure
a-C:H Hydrogen Content	30-40	atm%	Hydrogenated coating composition
ta-C Hydrogen Content	0-5	atm%	Hydrogen-free coating composition
a-C:H Surface Roughness (Ra)	0.003	µm	Disk surface finish (pre-test)
ta-C Vickers Hardness	2000-2300	HV	Measured hardness (10 mN load)

Key Methodologies

The experiment utilized highly specialized equipment for ultra-purity environmental control and surface analysis.

Preparation and Evacuation:
- Specimens (DLC disks on AISI 52100 steel, pure aluminum pins) were cleaned ultrasonically in acetone and hexane.
- The ultra-high vacuum chamber was evacuated (reaching 1 x 10^-5 Pa).
- Chamber and specimens were baked in two stages (353 K for 50 min, then 403 K for 50 min) to achieve 1-3 x 10^-6 Pa vacuum, minimizing initial contamination.
Environmental Gas Control:
- High-purity gases (H₂, N₂, Ar: 99.999% purity) were filtered, achieving initial O₂ and H₂O concentrations below 1 ppb.
- Trace water was introduced using a permeation method moisturizer.
- Concentrations were precisely monitored: Water (0-160 ppm) via Panametrics and HALO moisture analyzers; Oxygen (< 0.2 ppm guaranteed) via coulometric meter.
Tribological Testing:
- Pin-on-disk configuration conducted under dry contact conditions.
- Test duration: Sliding distance of 126 m, with continuous measurement of friction force.
- Temperature maintained at 298 K (ambient).
Post-Test Surface Analysis:
- FT-IR (Fourier Transform Infrared Spectroscopy): Used to identify chemical bond structures (C-H, O-H, C-O-C, C-OH) within the carbon transfer films formed on the aluminum pins.
- Laser Raman Spectroscopy: Used to analyze the structure of the amorphous carbon transfer film (G-band and D-band peaks) to determine graphitic content.

6CCVD Solutions & Capabilities

This research demonstrates the necessity of high-integrity, purity-controlled carbon materials for extreme tribological environments. 6CCVD’s specialized MPCVD diamond capabilities provide the optimal platform for replicating, extending, and industrializing this critical research.

Applicable Materials for High-Purity Tribology

The high purity and controlled structure of MPCVD diamond surpass the stability and chemical uniformity of conventional PECVD or Arc Ion Plating DLC coatings used in this study, making them ideal for rigorous, repeatable tribological studies in controlled atmospheres.

6CCVD Material	Relevance to Research Requirements	Advantage over Standard DLC
Optical Grade Single Crystal Diamond (SCD)	Ultra-high purity, extremely low defect density, ideal for fundamental friction mechanism studies requiring perfect stoichiometry.	Provides unparalleled thermal conductivity (up to 2000 W/mK) for managing localized sliding heat generation.
Tribological Grade Polycrystalline Diamond (PCD)	Excellent mechanical strength and hardness (up to 10,000 HV), suitable for high-wear components in hydrogen systems (e.g., valves/seals).	Available in large area wafers (up to 125 mm diameter) necessary for industrial scale testing environments.
Custom Thick Diamond Substrates	SCD/PCD can be grown up to 500 µm thick, providing a robust, bulk counterface that eliminates variables associated with DLC film-to-substrate adherence and residual stress.	Eliminates the substrate (e.g., AISI 52100 steel) as a contamination or diffusion source in ultra-high purity gas environments.

Customization Potential for Tribological Research

6CCVD offers the specific dimensional, material, and surface preparation controls required to replicate or advance complex tribology experiments like those presented.

Custom Dimensions: The study utilized 25 mm disks and 4 mm pins. 6CCVD provides custom laser cutting services to shape MPCVD diamond plates and wafers into precise dimensions required for pin-on-disk geometries, including:
- Wafers/Disks up to 125 mm (PCD).
- Custom Pins/Rods of specified diameter and curvature.
Surface Finish Control: The a-C:H DLC used had an extremely low roughness (Ra 0.003 µm). 6CCVD guarantees superior polishing:
- SCD Polishing: Ra < 1 nm (0.001 µm)
- PCD Polishing: Ra < 5 nm (0.005 µm) for inch-size wafers, ensuring repeatable contact mechanics.
Integrated Metalization Layers: While this study focused on carbon transfer films, future integration of diamond components into sensors or heated tribo-systems may require precise contacts. 6CCVD offers in-house deposition of standard metal stacks (Au, Pt, Pd, Ti, W, Cu) for bonding or electrical contact layers.

Engineering Support

Understanding the complex tribochemistry involving carbon termination (H, OH, C-O) in specific gas environments is critical for optimizing components in Hydrogen Energy Systems and Inert Gas Processing. 6CCVD’s in-house PhD engineering team specializes in diamond material science and can assist researchers and technical buyers with:

Selecting the optimal SCD or PCD grade based on required purity, wear resistance, and thermal management specifications for low-friction hydrogen applications.
Designing customized diamond substrates to reduce surface variables in transfer film formation studies.
Consulting on material compatibility in high-pressure or high-temperature systems where the effects of trace water may be accelerated.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We offer global shipping (DDU default, DDP available) to ensure timely delivery of high-purity diamond components for your next research phase.

View Original Abstract

This paper describes an experimental study on the friction of a-C:H diamond-like carbon (DLC) and ta-C DLC coatings in gas with different concentration of trace water. Pin-on-disk sliding experiments were conducted with DLC coated disks and aluminum pins in hydrogen, nitrogen, and argon. Trace oxygen was eliminated to less than 0.1 ppm, while water in the gas was controlled between 0 and 160 ppm. Fourier transform infrared spectroscopy (FT-IR) and laser Raman spectroscopy were used to analyze the transfer films on the metal surfaces. It was found that trace water slightly increased friction in hydrogen gas, whereas trace water caused a significant decrease in the friction coefficient in nitrogen and argon, particularly with a-C:H DLC. The low friction in hydrogen was brought about by the formation of transfer films with structured amorphous carbon, but no differences in the structure and contents of the films were found in the tests with and without trace water. In nitrogen and argon, the low friction with a-C:H DLC was achieved by the gradual formation of transfer films containing structured amorphous carbon, and FT-IR spectra showed that the films contained CH, OH, C-O-C, and C-OH bonds.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi8070217

References

2006 - Tribology of diamond-like carbon films: Recent progress and future prospects [Crossref]
2014 - History of diamond-like carbon films—From first experiments to worldwide applications [Crossref]
2014 - Achieving superlubricity in DLC films by controlling bulk, surface, and tribochemistry [Crossref]
1994 - An overview on the tribological behavior of diamond-like carbon in technical and medical applications [Crossref]
2014 - Diamond-Like Carbon Coating Applied to Automotive Engine Components [Crossref]
1991 - The effects of oxygen and humidity on friction and wear of diamond-like carbon films [Crossref]
1993 - Friction and wear performance of ion-beam-deposited diamond-like carbon films on steel substrates [Crossref]
1994 - Tribochemistry of diamond-like carbon coatings in various environment [Crossref]
1995 - Characterization of transfer layers on steel surfaces sliding against diamond-like hydrocarbon films in dry nitrogen [Crossref]
1996 - Characterization of transfer layers forming on surfaces sliding against diamond-like carbon [Crossref]