Tribological Performance of Silver- and Oxygen-Doped Diamond-Like Carbon Coatings Under Nitrogen-Based Copolymer Additives

At a Glance

Metadata	Details
Publication Date	2025-07-05
Journal	Tribology Letters
Authors	Álvaro Diego Bedoya-Zapata, Takeru Omiya, R. Serra, N.M. Figueiredo, A. Cavaleiro
Analysis	Full AI Review Included

Technical Documentation & Analysis: Advanced Hybrid Tribological Systems

Executive Summary

This analysis reviews the performance of hybrid tribological systems utilizing doped Diamond-Like Carbon (DLC) coatings paired with nitrogen-functionalized copolymer additives under boundary lubrication conditions. The findings underscore the critical role of surface chemistry and mechanical properties in achieving superior wear resistance and friction reduction.

Core Achievement: The study demonstrated significant tribological improvements by pairing doped DLC (Ag-DLC and O-DLC) with functionalized polymeric additives (PLMA₉₅-b-PDMAEMA₅).
Wear Reduction: The Ag7-DLC/PDMAEMA system achieved up to a 55% reduction in wear rate compared to undoped DLC/additive benchmarks, attributed to the protective effect of the silver-enhanced tribofilm.
Friction Reduction: The O10-DLC/PDMAEMA system provided the most consistent friction reduction, showing a 6% to 26% decrease across the boundary regime, facilitated by the homogeneous distribution of oxygen adsorption sites.
Mechanism Confirmed: Performance relies on the strong interaction (anchoring) between the functional group of the copolymer (PDMAEMA) and the dopant elements (Ag or O) on the DLC surface, leading to effective boundary film formation.
Material Limitation: DLC coatings, being amorphous carbon, are inherently limited in hardness (H < 20 GPa) and thermal stability, restricting performance in extreme high-pressure, high-temperature applications.
6CCVD Advantage: 6CCVD’s MPCVD Single Crystal Diamond (SCD) offers intrinsic hardness exceeding 100 GPa and superior thermal stability, providing a direct, high-performance upgrade path for engineers seeking to maximize component life in severe tribological environments.

Technical Specifications

The following table summarizes key material properties and test conditions extracted from the research.

Parameter	Value	Unit	Context
Undoped DLC Hardness (H)	15.6 ± 0.6	GPa	Baseline coating mechanical property
Highest Hardness (Ag3-DLC)	18.1 ± 1.2	GPa	Silver-doped coating mechanical property
Lowest Hardness (O7-DLC)	13.1 ± 2.2	GPa	Oxygen-doped coating mechanical property
Undoped DLC Thickness	1000	nm	As-deposited film thickness
Highest Thickness (Ag7-DLC)	1585	nm	As-deposited film thickness
Ag Doping Content (Ag3-DLC)	1.9	at.%	Chemical composition
O Doping Content (O10-DLC)	10.3	at.%	Chemical composition
Friction Test Load	3	N	Ball-on-disk tribometer condition
Wear Test Load	100	N	Ball-on-disk tribometer condition
Contact Pressure (Friction Test)	~0.68	GPa	Simulating cam/follower contact pressure
Contact Pressure (Wear Test)	~1.53	GPa	High-pressure boundary regime
Test Temperature	80	°C	Oil temperature during testing
Maximum Wear Rate Reduction	55	%	Achieved by Ag7-DLC/PDMAEMA system
Maximum Friction Reduction	26	%	Achieved by O10-DLC/PDMAEMA system

Key Methodologies

The DLC coatings were deposited using advanced magnetron sputtering techniques to control morphology and dopant distribution, followed by rigorous mechanical and tribological characterization.

Substrate Preparation: Silicon [100] wafers (20x20 mm) and M2 steel disks (25 mm diameter, 8 mm thick) were used as substrates, polished to a roughness (Ra) of 0.1 µm.
Deposition Technique: Deep Oscillation Magnetron Sputtering (DOMS) and High Power Impulse Magnetron Sputtering (HiPIMS) were employed to enhance film density, adhesion, and smoothness compared to traditional DCMS.
Adhesion Layer: A Titanium (Ti) interlayer was deposited (10 min, 0.3 Pa Ar atmosphere, -60 V substrate bias) followed by a gradual increase in Nitrogen (N₂) to form a stress-optimizing TiN gradient layer (5 min).
Doping Method: Silver (Ag) doping was achieved by placing pure Ag pellets in machined grooves on the graphite target. Oxygen (O) doping was achieved by introducing O₂ gas at controlled partial pressures (7% and 20%) into the chamber.
DLC Deposition Parameters: Deposition occurred at 0.6 Pa, with the graphite target voltage gradually increasing from 320 V to 400 V. Deposition time was typically 60 minutes (extended to 80 minutes for high O₂ content films due to target poisoning).
Characterization: Nanoindentation (Berkovich diamond indenter) determined Hardness and Young’s Modulus. Scratch tests quantified adhesion (critical loads L_c1, L_c2, L_c3). AFM and SEM analyzed surface morphology and wear tracks.
Tribological Testing: Performed using a ball-on-disk tribometer (SiC counter body) at 80 °C, simulating mixed-to-boundary lubrication regimes (Tallian parameter $\lambda$ < 1).

6CCVD Solutions & Capabilities

This research validates the market demand for ultra-hard, chemically active surfaces capable of forming robust tribofilms under extreme boundary lubrication conditions. While DLC offers improvements, 6CCVD’s MPCVD diamond materials provide the next evolutionary step in performance, offering intrinsic properties far exceeding the limitations of amorphous carbon coatings.

Applicable Materials for Next-Generation Tribology

To replicate or extend this research into industrial applications requiring maximum durability and thermal stability, 6CCVD recommends the following materials:

Ultra-Hard Polycrystalline Diamond (PCD): Recommended for large-area components (e.g., piston rings, mechanical seals) requiring maximum wear resistance. 6CCVD PCD offers hardness (H > 80 GPa) and high thermal conductivity, preventing localized phase transformation seen in DLC.
Optical Grade Single Crystal Diamond (SCD): Ideal for precision micro-components or research requiring the highest purity, lowest defect density, and ultimate intrinsic hardness (H > 100 GPa).
Custom Doped Diamond (BDD): For applications requiring specific surface reactivity or electrochemical activity (similar to the Ag/O doping mechanism), 6CCVD can supply Boron-Doped Diamond (BDD) films. BDD provides a stable, conductive, and chemically active surface for enhanced tribofilm anchoring in hybrid systems.

Customization Potential for Advanced Hybrid Systems

The paper utilized specific dimensions and required robust adhesion layers. 6CCVD’s in-house capabilities directly address these engineering requirements:

Research Requirement	6CCVD Customization Capability
Substrate Dimensions	Custom plates/wafers up to 125 mm (PCD) and custom laser cutting services to match specific component geometries (e.g., 25 mm disks).
Film Thickness	SCD and PCD films available from 0.1 µm up to 500 µm, allowing engineers to optimize film thickness for specific load-bearing requirements (the paper used films up to 1.585 µm).
Adhesion & Surface Chemistry	Internal metalization services including Ti, W, Pt, Au, Pd, and Cu. We can replicate the Ti/TiN interlayer structure or develop custom metal stacks optimized for diamond adhesion and chemical interaction with functionalized additives.
Surface Finish Control	Precision polishing services guarantee ultra-smooth surfaces: Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD. This control is crucial for optimizing wettability and ensuring consistent boundary film formation ($\lambda$ control).
Shipping & Logistics	Global shipping is available (DDU default, DDP available) to ensure rapid delivery of custom materials worldwide.

Engineering Support

The successful implementation of hybrid tribological systems requires deep expertise in both material science and surface engineering.

6CCVD’s in-house PhD team specializes in the mechanical, thermal, and chemical properties of MPCVD diamond. We offer consultation services to assist researchers and engineers in:

Material Selection: Determining the optimal diamond grade (SCD, PCD, BDD) and thickness for high-pressure, high-temperature hybrid tribosystems similar to those studied.
Interface Engineering: Designing custom metalization stacks to maximize adhesion and tailor surface energy for specific lubricant additives.
Performance Modeling: Utilizing advanced material data to predict wear life and friction coefficients under specific boundary lubrication regimes.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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

Tech Support

Original Source

DOI: https://doi.org/10.1007/s11249-025-02032-w