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6CCVD Research

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

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2020-04-18
JournalMaterials
AuthorsChao Li, Lei Huang, Juntang Yuan
InstitutionsNanjing University of Science and Technology
Citations56
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This research investigates the quantitative relationship between the sp3/sp2 ratio and the mechanical/tribological performance of non-hydrogenated Diamond-Like Carbon (DLC) films prepared via DC magnetron sputtering.

  • sp3 Control: The ratio of sp3/sp2 content (ranging from 0.74 to 0.98) was precisely controlled by adjusting the substrate bias voltage (-175 V to -300 V).
  • Adhesion vs. Hardness Trade-off: Adhesion strength (LC1) decreased significantly (from 31.5 N to 18.4 N) as the sp3 content increased, confirming that higher diamond-phase content reduces the film’s overall toughness.
  • Wear Resistance Mechanism: Wear rate showed a strong linear decrease with increasing bias voltage, independent of the sp3/sp2 ratio. This is primarily attributed to the increased compactness and density of the film achieved at higher ion bombardment energies.
  • Counter-Material Dependence: Friction behavior was highly sensitive to the counter-material’s chemical stability:
    • Against Si3N4 (stable ceramic): Friction coefficient increased with higher sp3 content, as lower sp2 content inhibited the formation of a beneficial graphitic transfer layer.
    • Against Ti6Al4V (oxidizable alloy): Friction was dominated by the oxidation of the alloy, which produced hard oxide debris (e.g., Al2O3) that broke the protective graphitic transfer layer, leading to high abrasive wear.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Substrate MaterialYG8Cemented Carbide10 mm x 10 mm samples
Deposition MethodDC Magnetron SputteringN/ANon-hydrogenated DLC
Carbon SourcePure GraphiteN/A99.99% purity
Bias Voltage Range-175 to -300VUsed to control sp3/sp2 ratio
sp3/sp2 Ratio Range0.74 to 0.98N/AMeasured via XPS
Adhesion Strength (Max)31.5NAchieved at -200 V bias (sp3/sp2 = 0.75)
Adhesion Strength (Min)18.4NAchieved at -300 V bias (sp3/sp2 = 0.98)
Friction Coefficient (Min, Si3N4)0.116N/AAchieved at -225 V bias
Friction Coefficient (Min, Ti6Al4V)0.126N/AAchieved at -300 V bias
Deposition Pressure1.4PaArgon atmosphere (120 sccm)
Deposition Temperature80°CMaintained throughout deposition
Wear Rate Correlation (Si3N4)Linear Decreasemm3/N¡mWSi3N4 is proportional to -1.014 x 10-14v (bias voltage)
Wear Rate Correlation (Ti6Al4V)Linear Decreasemm3/N¡mWTi6Al4V is proportional to -4.8457 x 10-14v (bias voltage)
Ti Pre-treatment Etch Depth400nmUsed Ti atoms at -1000 V bias

Key Methodologies

Section titled “Key Methodologies”

The non-hydrogenated DLC films were prepared using a multi-chamber hybrid coating system, focusing on precise control of the sp3 content via bias voltage adjustment.

  1. Substrate Pre-treatment: Cemented carbide YG8 substrates were heated to 80 °C. A surface etching pretreatment was performed using mid-frequency magnetron sputtering (Ar flow, -1200 V bias).
  2. Adhesion Layer Deposition: A high-energy Ti atom etching step (-1000 V bias, 4.0 A current, 30 min) was performed to improve adhesion by exposing surface WC crystals and creating a chemically active interface.
  3. DLC Deposition: DC magnetron sputtering was used with a pure graphite target (520 V DC voltage, approx. 600 W power) in an Argon atmosphere (1.4 Pa pressure).
  4. sp3 Content Adjustment: The sp3/sp2 ratio was varied by adjusting the negative substrate bias voltage from -175 V to -300 V. Higher bias voltage increased ion energy, enhancing etching and compaction, which ultimately increased the sp3 content (up to 0.98).
  5. Layered Structure: To further improve adhesion strength, each DLC film was composed of 10 distinct DLC layers.
  6. Structural Analysis: The sp3/sp2 ratio was quantified using X-ray Photoelectron Spectroscopy (XPS). Raman spectroscopy was used to analyze the ID/IG ratio and G peak shift, confirming structural changes.
  7. Mechanical Testing: Adhesion strength (LC1) was determined using a scratch tester with a linear load (0 to 40 N).
  8. Tribological Testing: Dry friction tests were conducted using a reciprocating ball-on-disk tester (3 N load, 8 mm/s velocity) against two counter-materials: Si3N4 (stable ceramic) and Ti6Al4V (oxidizable alloy).

Commercial Applications

Section titled “Commercial Applications”

This research provides critical data for optimizing DLC coatings in applications where the balance between hardness, toughness, and specific tribological performance against different counter-surfaces is paramount.

  • Precision Cutting Tools: DLC coatings on cemented carbide (YG8) are essential for high-performance machining. The Ti pre-treatment strategy ensures robust adhesion, while the ability to tailor sp3 content allows optimization for specific material removal processes (e.g., higher sp3 for maximum hardness).
  • Aerospace and Medical Implants: Components involving titanium alloys (like Ti6Al4V) operating under dry friction conditions. The findings highlight the necessity of mitigating counter-surface oxidation, which is the dominant factor in wear and friction performance in these systems.
  • Dry Sliding Bearings and Seals: Applications requiring low friction against chemically stable ceramics (Si3N4). Coatings with lower sp3 content (higher sp2) are preferred here, as they readily form the self-lubricating graphitic transfer layer necessary for ultra-low friction coefficients (down to 0.116).
  • High-Stress Friction Components: General use in friction components where high normal and shear stresses are present. The linear correlation between high bias voltage and superior wear resistance confirms a robust method for producing highly compact, durable films for demanding environments.
  • High-End Equipment Manufacturing: Used in friction pairs where predictable wear rates and controlled adhesion strength are required, leveraging bias voltage control to fine-tune film properties.
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

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2004 - Historical developments and new trends in tribological and solid lubricant coatings [Crossref]
  2. 2002 - Diamond-like amorphous carbon [Crossref]
  3. 2010 - Micro-Raman Studies on DLC coatings [Crossref]
  4. 2010 - Quantitative measurements of sp 3 content in DLC films with Raman spectroscopy [Crossref]
  5. 1999 - Determination of the sp3/sp2 ratio of a-C:H by XPS and XAES [Crossref]
  6. 2000 - The cutting performance of diamond and DLC-coated cutting tools [Crossref]
  7. 2009 - Development of DLC Coated Tool For Cutting Of Aluminum Alloy: Influence Of Deposition Condition On Cutting Characteristic
  8. 2010 - Session 01: DLC coatings OA 01 P2009-245 evaluation of adhesion strength and wear resistance of DLC films [Crossref]
  9. 2017 - Nano-scale multi-layered coatings for improved efficiency of ceramic cutting tools [Crossref]
  10. 2017 - Effect of load on the friction-wear behavior of magnetron sputtered DLC film at high temperature [Crossref]

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