Humidity‐Resistant Ultralow Friction in Diamond‐Like Carbon Coatings Enabled by Graphitic Nanodiamonds
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-06-23 |
| Journal | Small Structures |
| Authors | Andrea Mescola, Giovanni Bertoni, Gian Carlo Gazzadi, Michał Bartkowski, Adalberto Camisasca |
| Institutions | Istituto Nanoscienze, Superconducting and other Innovative Materials and Devices Institute |
| Citations | 2 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research presents a disruptive, scalable strategy for achieving humidity-resistant ultralow friction in industrial hydrogenated diamond-like carbon (H-DLC) coatings using Graphitic Nanodiamonds (Gr-NDs).
- Core Achievement: Stable ultralow friction (Coefficient of Friction, CoF, less than 0.1) is achieved on micro-rough industrial DLC coatings (Sq ≈ 0.5 µm) under standard humid air conditions (Relative Humidity, RH 50%-60%).
- Material Innovation: Gr-NDs, pre-synthesized core-shell nanostructures (diamond core wrapped in few-layer graphene), are used as functional additives, enabling a robust single-step deposition process.
- Performance Enhancement: In humid air, Gr-ND functionalization stabilizes the CoF at ≈0.09, drastically outperforming pristine DLC and conventional Graphene Flakes + Nanodiamonds (GFs + NDs) mixtures (CoF ≈ 0.20).
- Wear Resistance: Gr-NDs reduce wear volume on the steel counterface by a factor of 4.5 compared to GFs + NDs functionalization (0.57 x 10-4 mm3 vs. 2.6 x 10-4 mm3).
- Mechanism Validation: HRTEM and EELS confirm that the Gr-ND core-shell structure is retained within the Transfer Layer (TL-I). Moderate tribochemical oxidation (formation of C-O-C and C-OH groups) stabilizes the graphitic shells, promoting water-mediated lubrication and durability.
- Scalability: The method uses industrially produced DLC and a simple drop-casting functionalization, offering a robust solution for demanding mechanical applications without requiring complex vacuum treatments or controlled atmospheres.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| CoF (Humid Air) | 0.09 | - | Gr-NDs functionalized DLC (RH 50%-60%) |
| CoF (Dry N2) | 0.07 | - | Gr-NDs functionalized DLC (RH 15%) |
| Wear Volume (Gr-NDs) | 0.57 x 10-4 | mm3 | Removed volume on steel counterface |
| Wear Volume (GFs + NDs) | 2.6 x 10-4 | mm3 | Removed volume on steel counterface |
| DLC Coating Type | a-C:H (≈23% H) | - | Hydrogenated amorphous carbon |
| DLC Thickness | 2.5 | µm | Deposited via PA-CVD |
| DLC Hardness | 2600 | Hv | Manufacturer specification |
| DLC Surface Roughness (Sq) | 0.50 ± 0.05 | µm | Measured by AFM |
| Normal Load (Tribology) | 1 | N | Ball-on-disc test |
| Max Hertzian Contact Pressure | ≈0.8 | GPa | Theoretical calculation |
| Gr-ND Synthesis Temperature | 1300 | °C | Thermal annealing under He atmosphere |
| Gr-ND sp2 Content (Pristine) | 46 | % | EELS analysis |
| TL-I Thickness (Gr-ND TL) | ≈85 | nm | Measured via HRTEM cross-section |
| O/C Fraction (TL-I) | 0.22 ± 0.04 | - | EELS analysis (indicates tribochemical oxidation) |
| Counterpart Material | 100Cr6 | - | Steel spherical ball (4 mm diameter) |
Key Methodologies
Section titled “Key Methodologies”The study employed a combination of material synthesis, industrial coating deposition, tribological testing, and advanced electron microscopy for structural and chemical analysis.
I. Material Synthesis and Deposition
Section titled “I. Material Synthesis and Deposition”- DLC Substrate: Industrial H-DLC coatings (a-C:H) were deposited on AlSi10Mg disks using Plasma-Assisted Chemical Vapor Deposition (PA-CVD).
- Gr-ND Synthesis: Pristine nanodiamonds (4-6 nm) were thermally annealed at 1300 °C for 60 minutes under a Helium (He) atmosphere to induce partial graphitization, forming the core-shell Gr-ND structure.
- Functionalization: Gr-NDs were deposited onto the DLC substrates via a simple drop-casting technique using an isopropanol suspension (1 mg mL-1 concentration), followed by drying under nitrogen flow.
II. Tribological Testing
Section titled “II. Tribological Testing”- Equipment: CSM ball-on-disc tribometer.
- Test Parameters: 1 N normal load, 10 cm s-1 linear velocity, 1800 s duration (180 m sliding distance).
- Environmental Control: Tests were conducted under two regimes:
- Humid Air: Relative Humidity (RH) 50%-60%.
- Dry Nitrogen (N2): RH 15%.
- Wear Analysis: Wear volume on the 100Cr6 steel counterface was calculated using top-view optical microscopy, modeling the wear scar as a spherical cap.
III. Structural and Chemical Characterization
Section titled “III. Structural and Chemical Characterization”- Nanomaterial Structure: Thermogravimetric analysis (TGA), X-ray diffraction (XRD), and High-Resolution Transmission Electron Microscopy (HRTEM) confirmed the core-shell structure of Gr-NDs (diamond core, graphitic shell).
- Transfer Layer (TL) Preparation: Cross-sectional lamellae of the TL formed on the steel counterface were prepared using Focused Ion Beam (FIB) milling via the lift-out technique.
- TL Analysis (HRTEM/EELS):
- HRTEM provided direct visualization of diamond nuclei and graphitic planes within the TL.
- Electron Energy-Loss Spectroscopy (EELS) was performed in STEM mode to analyze the C-K and O-K edges.
- EELS quantified the sp2 fraction (lubricating component) and identified specific oxygen-containing functional groups (C-O-C epoxy/ether and C-OH hydroxyl groups), confirming tribochemical oxidation.
Commercial Applications
Section titled “Commercial Applications”The development of humidity-resistant ultralow friction coatings using Gr-NDs is highly relevant for engineering sectors where mechanical reliability and efficiency are paramount under non-ideal atmospheric conditions.
- Automotive and Transportation:
- Engine components (e.g., piston rings, valve trains) and transmission systems requiring stable, low-friction operation despite exposure to moisture or varying humidity levels.
- Reducing energy consumption and extending the lifespan of critical moving parts.
- Aerospace and Defense:
- Mechanical actuators and sliding contacts in aircraft and satellite systems where components must maintain performance across a wide range of environmental conditions, including high humidity or condensation phases.
- Industrial Machinery and Manufacturing:
- High-precision bearings, gears, and tooling used in manufacturing plants where ambient humidity is not strictly controlled.
- Providing robust solid lubrication solutions that surpass the operational limits of conventional H-DLC films.
- Next-Generation Mechanical Devices:
- Enabling the design of highly durable and efficient micro-electromechanical systems (MEMS) and other demanding mechanical devices that rely on stable interfacial lubrication.
View Original Abstract
Hydrogenated diamond‐like carbon (H‐DLC) coatings are extensively employed in high‐performance tribological applications, yet their frictional behavior in humid environments remains a critical limitation. A scalable and industrially viable strategy based on graphitic nanodiamonds (Gr‐NDs) is here proposed, enabling humidity‐resistant ultralow friction (coefficient of friction <0.1) without requiring controlled atmospheres or complex surface treatments. This approach is distinct from many recent methods involving the deposition of nanoparticles and 2D materials, which often fail under humid conditions. Gr‐NDs leverage their intrinsic core-shell nanostructure to promote the in operando formation of a peculiar graphitic transfer layer (TL), directly enhancing interfacial lubrication. High‐resolution transmission electron microscopy and Raman spectroscopy confirm that these nanostructures are effectively retained within the TL, ensuring superior wear resistance and friction reduction. Furthermore, spectroscopic analysis reveals that moderate tribochemical oxidation stabilizes the TL, extending its durability under realistic operating conditions. This work establishes Gr‐NDs as a disruptive functional additive for H‐DLC coatings, offering a robust, scalable, and environmentally friendly solution for next‐generation tribological systems in demanding mechanical applications.