Properties of Diamond-like Coatings in Tribological Systems Lubricated with Ionic Liquid

At a Glance

Metadata	Details
Publication Date	2025-07-08
Journal	Coatings
Authors	Krystyna Radoń-Kobus, Monika Madej
Institutions	Kielce University of Technology
Citations	1
Analysis	Full AI Review Included

Executive Summary

This study investigates the synergistic tribological performance of tungsten-doped diamond-like carbon (a-C:H:W DLC) coatings when lubricated with 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF₆) ionic liquid (IL).

Synergistic Performance: The combination of the DLC coating and the BMIM-PF₆ ionic liquid demonstrated a strong synergistic anti-wear and anti-friction effect, achieving the lowest coefficients of friction (CoF).
Friction Reduction (Dry): The DLC coating alone significantly reduced the CoF by approximately 57% (at 10 N load) compared to the uncoated 100Cr6 steel substrate under technically dry friction.
Friction Reduction (Lubricated): When lubricated with BMIM-PF₆, the DLC coating further reduced the average CoF by 18-20% compared to the lubricated bare steel, reaching minimum values of 0.08 (10 N) and 0.09 (15 N).
Wear Mitigation: Under dry friction (10 N), the DLC coating reduced the maximum abrasion depth by over four times and the wear area by three times compared to the uncoated steel.
Wettability: The DLC coating exhibited better wettability (more hydrophilic) with distilled water than the bare steel (24% smaller contact angle), suggesting favorable interaction with polar lubricants like BMIM-PF₆.
Surface Integrity: Post-test analysis showed that samples lubricated with ionic liquid exhibited virtually no visible wear traces, confirming the exceptional anti-wear properties of the system.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	100Cr6 Steel	-	High wear resistance bearing steel
Coating Type	a-C:H:W DLC	-	Tungsten-doped hydrogenated amorphous carbon
Deposition Method	PECVD	-	Plasma-Enhanced Chemical Vapor Deposition
Interlayer Material	Chromium (Cr)	-	Applied for adhesion improvement
Coating Thickness	1.8 ± 0.2	µm	Measured via SEM cross-section
Substrate Roughness (S_a)	0.33	µm	100Cr6 steel, pre-coating
Coating Roughness (S_a)	0.44	µm	DLC coating, pre-test
Ionic Liquid (IL)	BMIM-PF₆	-	1-butyl-3-methylimidazolium hexafluorophosphate
IL Melting Point	-8	°C	-
IL Viscosity (20 °C)	300-400	cP	-
Tribometer Type	TRB³	-	Ball-on-Disc, Rotational Motion
Counter-Sample	100Cr6 Steel	-	6 mm diameter ball
Applied Load (L)	10, 15	N	Test condition
Sliding Speed	0.1	m/s	Test condition
Sliding Distance	1000	m	Test condition
Lowest CoF (DLC + IL)	0.08	-	Achieved at 10 N load
Max Wear Depth (Dry, 10 N, Steel)	3.39	µm	Uncoated 100Cr6 steel
Max Wear Depth (Dry, 10 N, DLC)	0.73	µm	DLC coated steel
Wear Area (Dry, 10 N, Steel)	494.5	µm²	Uncoated 100Cr6 steel
Wear Area (Dry, 10 N, DLC)	145.3	µm²	DLC coated steel
Water Contact Angle (DLC)	64	°	Indicates good wettability

Key Methodologies

Substrate Preparation: 100Cr6 steel discs (42 mm diameter) were ground and polished sequentially up to 1200 µm gradation, achieving an initial surface roughness (S_a) of 0.33 µm.
Cleaning: Samples were cleaned using an ultrasonic cleaner in ethanol before coating deposition.
Coating Deposition (PECVD): The DLC film was applied using Plasma-Enhanced Chemical Vapor Deposition (PECVD) at a temperature less than 250 °C, divided into two stages:
- Interlayer: Application of a Chromium (Cr) interlayer to enhance adhesion.
- DLC Layer: Application of the actual a-C:H:W coating, achieved by sputtering tungsten metal in a gas phase containing the non-metallic carbon component.
Pre-Test Characterization:
- Thickness and Composition: SEM (15 kV, 10,000x magnification) and EDS linear analysis confirmed the 1.8 µm thickness and the presence of Cr, C, and W.
- Surface Geometry: Confocal microscopy (DCM 8) analyzed amplitude parameters (S_a, S_sk, S_ku) to confirm similar geometric structures between the bare steel and the DLC coating.
- Wettability: An optical tensiometer measured the contact angle of 4 µL drops of distilled water, diiodomethane, and BMIM-PF₆ IL on both surfaces.
Tribological Testing: Performed on a TRB³ tribometer in ball-on-disc rotational motion. Tests were conducted under technically dry conditions and with BMIM-PF₆ lubrication, using 10 N and 15 N loads at 0.1 m/s sliding speed for 10,000 seconds.
Post-Test Wear Analysis: Wear traces were analyzed using a confocal microscope (DCM 8). Wear depth and wear area were quantified by averaging measurements from ten randomly selected 2D profiles.

Commercial Applications

The demonstrated performance characteristics—ultra-low friction, high wear resistance, and stability under load—make this DLC/Ionic Liquid system suitable for demanding engineering applications:

High-Speed and Precision Bearings: Use in systems requiring stable, ultra-low friction operation, such as high-speed spindles or precision robotics, where the IL provides superior boundary lubrication.
Aerospace and Aviation: Application in components subjected to extreme temperature variations and low vapor pressure environments, leveraging the inherent thermal and electrochemical stability of ionic liquids.
Automotive Powertrain: Coating engine components (e.g., piston rings, valve trains) to reduce friction losses and improve fuel efficiency, especially when paired with IL-based additives or pure IL lubricants.
Compressors and Pumps: Use in machinery handling sensitive or corrosive gases/fluids, where the chemical inertness of the DLC coating and the non-flammability of the IL are advantageous.
Medical Devices and Implants: Potential application in bio-tribological systems, utilizing the biotolerance and high wear resistance of DLC coatings.

View Original Abstract

The paper shows the effect of using a lubricant in the form of an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6), on the tribological properties of a hydrogenated diamond-like coating (DLC) doped with tungsten a-C:H:W. The coatings were deposited on 100Cr6 steel by plasma-enhanced chemical vapor deposition PECVD. Tribological tests were carried out on a TRB3 tribometer in a rotary motion in a ball-disc combination. 100Cr6 steel balls were used as a counter-sample. Friction and wear tests were carried out for discs made of 100Cr6 steel and 100Cr6 steel discs with a DLC coating. They were performed under friction conditions with and without lubrication under 10 N and 15 N loads. The ionic liquid BMIM-PF6 was used as a lubricant. Coating thickness was observed on a scanning microscope, and the linear analysis of chemical composition on the cross-section was analyzed using the EDS analyzer. The confocal microscope with an interferometric mode was used for analysis of the geometric structure of the surface before and after the tribological tests. The contact angle of the samples for distilled water, diiodomethane and ionic liquid was tested on an optical tensiometer. The test results showed good cooperation of the DLC coating with the lubricant. It lowered the coefficient of friction in comparison to steel about 20%. This indicates the synergistic nature of the interaction: DLC coating-BMIM-PF6 lubricant-100Cr6 steel.

Tech Support

Original Source

DOI: https://doi.org/10.3390/coatings15070799

References

1971 - Ion Beam Deposition of Thin Films of Diamond-like Carbon [Crossref]
2021 - Tribological Behaviour of the Sucaslide Diamond-like Carbon Coating [Crossref]
2014 - Effect of Tungsten on the Durability of Diamond-like Carbon Coatings in the Chemical Industry
1993 - Hardness and Elastic Modulus of Diamond and Diamond-like Carbon Films [Crossref]
2016 - Residual Stress and Its Effect on Failure in a DLC Coating on a Steel Substrate with Rough Surfaces [Crossref]
2001 - Differentiating the Tribological Performance of Hydrogenated and Hydrogen-Free DLC Coatings [Crossref]