Effect of sp3 Content on Adhesion and Tribological Properties of Non-Hydrogenated DLC Films

At a Glance

Metadata	Details
Publication Date	2020-04-18
Journal	Materials
Authors	Chao Li, Lei Huang, Juntang Yuan
Institutions	Nanjing University of Science and Technology
Citations	56
Analysis	Full AI Review Included

Technical Documentation & Analysis: Controlling sp³ Content in Diamond-Like Carbon (DLC) Films for Optimized Tribology

Analysis of MDPI Materials Article: “Effect of sp³ Content on Adhesion and Tribological Properties of Non-Hydrogenated DLC Films”

This technical document analyzes the mechanisms used to control carbon bonding ratios and the resulting performance metrics (adhesion and wear) in amorphous carbon films, translating these findings into actionable material solutions and customized product offerings from 6ccvd.com.

Executive Summary

The research provides a quantitative analysis of how manufacturing parameters, specifically substrate bias voltage, control the sp³/sp² bonding ratio in non-hydrogenated Diamond-Like Carbon (DLC) films and the subsequent impact on mechanical performance.

Controlled Synthesis: Non-hydrogenated DLC films were prepared via DC magnetron sputtering on cemented carbide (YG8), with the crucial sp³/sp² ratio deliberately tuned across a range of 0.74 to 0.98 by adjusting the negative substrate bias voltage.
Adhesion Mechanism: A Ti atom etching pretreatment was successfully implemented to enhance adhesion, achieving a maximum critical load (L_C1) of 31.5 N.
Adhesion Trade-off: Increasing the sp³ content beyond optimal levels negatively impacted film toughness, resulting in a reduction of adhesion strength.
Wear Mechanism Confirmation: Wear performance showed a superior resistance relationship with the applied bias voltage (linear decrease), indicating that enhanced film compactness was a more significant factor in wear resistance than the simple sp³/sp² ratio.
Counter-Material Dependency: Tribological results highlight that friction performance is highly dependent on the counter material: low friction against Si₃N₄ (promotes graphitic transfer layer) versus high friction against Ti6Al4V (oxidation of Ti6Al4V creates hard Al₂O₃ debris, breaking the lubrication layer).

Technical Specifications

Hard data extracted from the study detailing deposition conditions, material characteristics, and performance metrics.

Parameter	Value	Unit	Context
Substrate Bias Voltage Range	-175 to -300	V	Adjusted to control sp³/sp² ratio
DLC Film Thickness Range	337 to 645	nm	Total thickness across 10 layers
Sp³/Sp² Ratio Measured Range	0.74 to 0.98	N/A	Measured via XPS
Maximum Adhesion Strength (L_C1)	31.5	N	Achieved at -200 V bias
Minimum Adhesion Strength (L_C1)	18.4	N	Measured at -300 V bias
Deposition DC Power	600	W	Fixed constant power
Argon Gas Pressure	1.4	Pa	Standard deposition environment
Lowest Friction Coefficient (Si₃N₄)	0.116	N/A	Against Si₃N₄ ball, Sample 3 (-225 V)
Lowest Friction Coefficient (Ti6Al4V)	0.126	N/A	Against Ti6Al4V ball, Sample 6 (-300 V)
Friction Test Load	3	N	Reciprocating ball-on-disk testing
Surface Roughness (R_a) Range	14.82 to 20.70	nm	Post-deposition roughness
Ti Pre-etch Etching Depth	400	nm	Used to improve adhesion on YG8 substrate

Key Methodologies

A concise, ordered list of the primary manufacturing and testing processes applied in the study.

Substrate Preparation: Cemented carbide YG8 (10 mm x 10 mm) samples were ultrasonically cleaned and heated to a constant 80 °C.
Adhesion Pretreatment: A 30-minute etching process utilized mid-frequency magnetron sputtering with Ti targets, applying a high bias voltage of -1000 V to etch the substrate surface to a depth of 400 nm.
DLC Deposition Conditions: Non-hydrogenated DLC was prepared using DC magnetron sputtering with a 99.99% pure graphite target, maintaining a DC voltage of 520 V (approx. 600 W), Ar flow of 120 sccm, and chamber pressure of 1.4 Pa.
Multi-layer Structure: All DLC films consisted of 10 sequential layers, deposited for 15 minutes each, designed to enhance film integrity and manage internal stresses.
sp³ Control: The sp³/sp² ratio was varied by adjusting the substrate negative bias voltage from -175 V to -300 V.
Tribological Testing: Reciprocating ball-on-disk tests were performed under dry friction conditions using Si₃N₄ and Ti6Al4V counter balls (3 N load, 8 mm/s velocity).
Characterization: Adhesion strength (L_C1) was determined via scratch testing, composition was analyzed using Raman spectroscopy and XPS, and wear morphology/depth was analyzed using SEM and stylus profilers.

6CCVD Solutions & Capabilities

6CCVD specializes in high-purity, high-stability CVD Diamond, offering material platforms that build upon and fundamentally exceed the performance characteristics studied in metastable DLC films. We provide the robust, crystalline material solutions necessary for engineering applications that demand uncompromising hardness, thermal stability, and low friction.

Applicable Materials

To replicate the ultra-hard, wear-resistant characteristics achieved in this DLC study, or to extend the research into environments requiring extreme thermal stability, 6CCVD recommends the following core materials:

Optical/Electronic Grade Single Crystal Diamond (SCD): Offers the highest intrinsic sp³ purity and hardness available. Ideal for applications requiring ultimate wear resistance and thermal management far surpassing that of amorphous DLC, especially in high-speed or high-temperature tribological systems.
Mechanical Grade Polycrystalline Diamond (PCD): Suitable for large-area coating or substrate requirements (up to 125 mm wafers). PCD’s fine-grain structure allows researchers to study grain boundary effects on transfer layer formation, offering a stable and scalable platform for advanced tribological component development.
Boron-Doped Diamond (BDD): For applications where charge dissipation or electrochemical inertness is required alongside superior hardness. BDD provides metallic conductivity while maintaining diamond’s supreme mechanical properties, offering a solution for tribological tests involving electrical contacts or sensitive environments.

Customization Potential

The research highlights the critical role of surface pretreatment (Ti etch) and precise layer geometry. 6CCVD’s engineering capability allows direct integration of similar, or superior, features into our diamond products:

Custom Dimensions and Geometry: While the study used small (10 mm x 10 mm) samples, 6CCVD provides custom-sized diamond plates and wafers, including large-area PCD up to 125 mm.
Precision Thickness Control: We offer precise control over film thickness, providing SCD and PCD layers from 0.1 µm up to 500 µm, allowing for optimization of structural support and internal stress management, similar to the multilayer structure utilized in the paper.
Advanced Metalization: 6CCVD’s internal cleanroom facility can apply custom refractory and noble metal stacks (including Ti, Pt, Au, Pd, W, Cu) to diamond substrates, guaranteeing chemically inert and robust adhesion layers for integration into complex devices or for use as custom contact pads.
Ultra-Polishing Services: To minimize intrinsic friction and ensure a pristine interface for tribological studies, 6CCVD provides industry-leading polishing, achieving Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD wafers, significantly exceeding the roughness levels reported in the analyzed DLC films (R_a up to 20.7 nm).

Engineering Support & Global Logistics

In-House PhD Engineering Support: 6CCVD’s team of PhD material scientists can assist researchers and technical engineers in transitioning from theoretical DLC models to high-performance SCD/PCD specifications. We specifically offer expertise in selecting materials optimized for interaction with common counter materials like Si₃N₄ and Ti6Al4V to manage oxidation effects and promote desired transfer layer characteristics.
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View Original Abstract

Non-hydrogenated diamond-like carbon (DLC) films with various ratios of sp3/sp2 were prepared on cemented carbide YG8 with DC magnetron sputtering technology. A pure graphite target was selected as the carbon source. Before DLC deposition, a surface etching pretreatment was carried out by mid-frequency magnetron sputtering method, using Ti atoms to improve adhesion strength. The ratios of sp3/sp2 were adjusted by bias voltages. In order to investigate the effect of the ratio of sp3/sp2 on adhesion and tribological properties, Raman spectra, XPS spectra, adhesion scratch test and ball-on-disk dry friction tests were applied. The results indicated that the ratio of sp3/sp2 fluctuated with bias voltage, increasing in the range of 0.74 to 0.98. The adhesion strength decreased from 31.5 to 18.4 N with the increasing ratio of sp3/sp2, while the friction coefficient rose in DLC-Si3N4 and dropped in DLC-Ti6Al4V. For DLC-Ti6Al4V, the oxidation of Ti6Al4V had a greater influence than graphitization of DLC. The hard oxides of Ti6Al4V broke the graphite transfer layer leading to a high friction coefficient. The wear rate was approximately linearly related to bias voltage. The coefficients of the linear regression equation were influenced by different friction materials. The adhesion strength and the friction coefficient were fitted as a function of the ratio of sp3/sp2.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma13081911

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