Tribological Performance of Silver- and Oxygen-Doped Diamond-Like Carbon Coatings Under Nitrogen-Based Copolymer Additives
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-07-05 |
| Journal | Tribology Letters |
| Authors | Ălvaro Diego Bedoya-Zapata, Takeru Omiya, R. Serra, N.M. Figueiredo, A. Cavaleiro |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research investigates hybrid tribological systems combining Silver (Ag) and Oxygen (O) doped Diamond-Like Carbon (DLC) coatings with nitrogen-based copolymer additives to enhance performance under boundary lubrication conditions.
- Core Value Proposition: The use of functionalized copolymer additives (PLMA95-b-PDMAEMA5) paired with doped DLC coatings significantly improves wear resistance and friction reduction compared to undoped DLC systems.
- Optimal Wear Resistance: The Ag7-DLC coating, when paired with the functionalized additive, achieved a 55% reduction in wear rate, attributed to the formation of a robust, protective transfer film anchored by silver sites.
- Optimal Friction Reduction: The O10-DLC/PDMAEMA system demonstrated the best overall friction performance, reducing the coefficient of friction by 6-26% across the boundary lubrication regime.
- Adhesion and Wettability: Increased doping levels (Ag7 and O10) improved the coatingâs adhesion (higher critical loads Lc1 and Lc2) and surface wettability, which is crucial for effective adsorption of the polar additive segments.
- Mechanism of Action: The PDMAEMA functional group interacts strongly with the homogeneously distributed oxygen sites (O-DLC) or the silver nanocrystallites (Ag-DLC), forming a dense anchoring layer that stabilizes the protective tribofilm.
- Wear Mechanism: Micrographic analysis confirmed that the dominant wear mechanism in all tested systems was pure polishing, which was significantly mitigated by the protective tribofilms formed by the doped-DLC/PDMAEMA systems.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Ag3-DLC Hardness (H) | 18.1 ± 1.2 | GPa | Nanoindentation |
| O10-DLC Hardness (H) | 13.5 ± 0.6 | GPa | Nanoindentation |
| Ag7-DLC Thickness | 1585 | nm | DLC Layer |
| O10-DLC Thickness | 870 | nm | DLC Layer |
| Ag7-DLC Silver Content | 7.6 | at.% Ag | Chemical Composition (EDS) |
| O10-DLC Oxygen Content | 10.3 | at.% O | Chemical Composition (EDS) |
| Copolymer Molecular Weight (Mw) | 38 | kDa | PLMA95-b-PDMAEMA5 Additive |
| Functional Group Fraction (vA) | 5 | mol% | PDMAEMA in Copolymer |
| Lubricant Kinematic Viscosity | 5.7 | mm2/s | KV100 (at 100 °C) |
| Wear Test Load | 100 | N | Ball-on-Disk Tribometer |
| Wear Test Contact Pressure | ~1.53 | GPa | Estimated Contact Pressure |
| Test Temperature | 80 | °C | Oil Temperature |
| Estimated Grafted Layer Thickness | 3.5 | nm | Calculated Boundary Film (Friction Test) |
| Undoped DLC Contact Angle (Water) | 79 | ° | Surface Wettability |
| O10-DLC Contact Angle (Water) | 62 | ° | Surface Wettability (Improved Wettability) |
Key Methodologies
Section titled âKey MethodologiesâThe study utilized a hybrid approach combining advanced coating deposition techniques, specialized lubricant formulation, and rigorous tribological testing under boundary conditions.
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Coating Deposition (DOMS/HiPIMS):
- Substrates: Silicon [100] wafers (for characterization) and M2 steel disks (for tribology).
- Deposition Method: Deep Oscillation Magnetron Sputtering (DOMS) for DLC layers, preceded by High Power Impulse Magnetron Sputtering (HiPIMS) for the interlayer.
- Interlayer: A Titanium (Ti) layer followed by a Nitrogen (N2) gradient layer and a Titanium Nitride (TiN) interlayer were deposited to optimize adhesion and stress distribution.
- Silver Doping (Ag-DLC): Pure silver pellets were placed in circular grooves machined into the graphite target. Two concentrations were tested (Ag3 and Ag7).
- Oxygen Doping (O-DLC): Oxygen gas (O2) was introduced into the chamber at partial pressures (7% and 40%) prior to DLC deposition.
-
Lubricant Formulation:
- Base Oil: Polyalphaolefin (PAO 4).
- Additives: 8% copolymer additives, synthesized via SARA ATRP.
- PLMA (Non-functionalized benchmark).
- PLMA95-b-PDMAEMA5 (Functionalized, 5 mol% PDMAEMA anchoring segment).
-
Tribological Testing:
- Apparatus: Ball-on-disk tribometer (Rtec MFT-5000) using a SiC ball counter body.
- Regime: Boundary lubrication conditions (Tallian parameter lambda less than 1).
- Conditions: Tests performed at 80 °C oil temperature. Friction tests used variable speed (0.006-0.15 m/s) and low load (3 N). Wear tests used constant speed (0.01 m/s) and high load (100 N).
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Characterization:
- Surface/Morphology: SEM, AFM (for roughness parameters Sa, Sq, Rpk, Rk, Rvk), and particle size distribution analysis (LND fitting).
- Mechanical/Chemical: Nanoindentation (H, E), Scratch tests (Adhesion Lc values), EDS (Chemical composition), and Goniometer (Surface Free Energy, Wettability).
Commercial Applications
Section titled âCommercial ApplicationsâThe development of hybrid tribological systems featuring doped DLC coatings and functionalized polymeric additives is highly relevant for industries requiring enhanced durability and energy efficiency in high-stress, boundary-lubricated contacts.
- Internal Combustion Engines (ICE):
- Piston Rings and Liners: The primary application focus, where components operate under mixed-to-boundary lubrication regimes right after top dead centers, requiring high wear resistance and low friction.
- Automotive and Transportation:
- Drivetrain Components: Gears, cams, and followers operating under high contact pressure where traditional AW/EP additives (like ZDDP) are being phased out due to environmental concerns.
- Electric Vehicles (EVs): While EVs have different lubrication needs, the high wear resistance and friction reduction capabilities are valuable for reducing energy losses in gearboxes and bearings.
- Industrial Machinery:
- High-Load Bearings and Sliding Contacts: Any machinery requiring robust boundary lubrication, such as heavy-duty pumps, compressors, and manufacturing equipment, where component life extension is critical.
- Sustainable Lubrication:
- The technology offers a pathway toward greener lubricant solutions by utilizing non-toxic, sulphur- and phosphorus-free polymeric additives, aligning with modern environmental regulations.
View Original Abstract
Abstract Additives made of nitrogen-functionalized copolymers were paired with diamond-like carbon (DLC) coatings doped with different amounts of oxygen and silver to form systems capable of improving tribological performance against undoped-DLC/additive systems. Initial characterisation indicated that silver doping reduced hardness and wettability in the surface, contrary to oxygen doping. Adhesion improved with higher doping levels. Tribological testing was done in boundary conditions, with silver-doped DLC coatings achieving a reduction in wear, but not friction. Oxygen-doped DLC coatings showed similar behaviour. Micrographs identified the wear mechanism as pure polishing and proved the protective effect of doped-DLC/additive systems. The findings suggest an across-scales effect of properties in the performance of the system and promising use in applications requiring wear resistance and friction reduction. Graphical abstract