Lubricating Properties of Cyano-Based Ionic Liquids against Tetrahedral Amorphous Carbon Film

At a Glance

Metadata	Details
Publication Date	2020-02-08
Journal	Coatings
Authors	Shouhei Kawada, H. Ōkubo, Seiya Watanabe, Chiharu Tadokoro, Ryo Tsuboi
Institutions	Daido University, Tokyo University of Science
Citations	6
Analysis	Full AI Review Included

Executive Summary

This research investigates the tribological performance and lubrication mechanism of cyano-based ionic liquids (ILs) against tetrahedral amorphous carbon (ta-C) films, targeting ultra-low friction regimes for advanced engineering applications.

Ultra-Low Friction Achieved: The ionic liquid 1-butyl-3-methylimidazolium tricyanomethane ((BMIM)(TCC)) demonstrated an exceptionally low friction coefficient of 0.03 when sliding against ta-C film at 90 °C.
Temperature Dependence: Friction reduction was highly dependent on ambient temperature. (BMIM)(DCN) achieved its lowest friction (0.07) at 170 °C, confirming that the anion structure and thermal environment critically influence reactivity.
Dual Lubrication Mechanism: The low friction state is attributed to two synergistic effects occurring at elevated temperatures:
1. Graphitization: The ta-C film surface graphitized (increased I_D/I_G ratio) within the wear track, forming a low-shear transfer layer.
2. Anion Adsorption: MALDI-TOF/MS confirmed that IL anions (DCN or TCC) strongly adsorbed onto the worn ta-C surface, while cation adsorption was minimal.
Tribo-Decomposition Indicated: TGA results showed that (BMIM)(TCC) was thermally more stable (344 °C) than (BMIM)(DCN) (300 °C), contradicting the sliding test where (BMIM)(TCC) achieved low friction at a much lower temperature (90 °C). This suggests that tribo-decomposition occurs even at 30 °C, but effective anion adsorption requires a minimum high ambient temperature.
Material Reactivity: The (TCC) anion reacted more easily with the ta-C film than the (DCN) anion, allowing low friction to be achieved at a lower temperature.

Technical Specifications

Parameter	Value	Unit	Context
Lowest Friction Coefficient	0.03	None	Achieved by (BMIM)(TCC) at 90 °C
Low Friction Coefficient	0.07	None	Achieved by (BMIM)(DCN) at 170 °C
ta-C Film Hardness	73	GPa	Measured by tribo-indenter
ta-C Film Roughness (R_a)	0.01	µm	As-deposited surface
ta-C Film Thickness	1.0	µm	Arc ion plated coating
ta-C Hydrogen Content	<1.0	at%	Tetrahedral amorphous carbon (ta-C)
(BMIM)(DCN) Viscosity	11.6	mPas	At 50 °C
(BMIM)(TCC) Viscosity	11.1	mPas	At 50 °C
TGA Decomposition T (DCN)	300	°C	Temperature for 5% weight loss in N₂
TGA Decomposition T (TCC)	344	°C	Temperature for 5% weight loss in N₂
Sliding Load	50	N	Reciprocating sliding test (SRV4)
Reciprocating Frequency	50	Hz	Reciprocating sliding test
Temperature Ramp Rate	10 K per 5 min	K/min	Elevated temperature friction test

Key Methodologies

The lubricating properties and mechanisms were evaluated using a reciprocating sliding tester (SRV4) combined with advanced surface analysis techniques.

Specimen Preparation: AISI 52100 steel disks and cylinders, coated with ta-C film (Cr interlayer), were ultrasonically cleaned for 10 min using a 1:1 mixed solution of petroleum benzine and acetone.
Lubricant Application: 90 µL of the neat ionic liquid ((BMIM)(DCN) or (BMIM)(TCC)) was applied to the disk surface prior to testing.
Friction Testing (Elevated Temperature): Tests were conducted under a 50 N load, 50 Hz frequency, and 1 mm amplitude. The temperature was programmed to heat from 30 °C up to 290 °C at a rate of 10 K per 5 min until a sharp friction increase (film failure) occurred.
Friction Testing (Constant Temperature): Sliding tests were repeated at specific temperatures (e.g., 90 °C for TCC, 170 °C for DCN) for 30 min to confirm the stability and reproducibility of the low friction state.
Raman Spectroscopy: Used a 532 nm YAG laser to analyze the worn surface. The degree of graphitization, which influences friction behavior, was quantified by calculating the ratio of the D-peak to G-peak intensities (I_D/I_G ratio).
MALDI-TOF/MS Analysis: Used to map and identify the chemical species (cations and anions) adsorbed on the worn ta-C surface. This confirmed the formation of an adsorption film derived from the ionic liquid decomposition.
Thermogravimetric Analysis (TGA): Performed under a nitrogen atmosphere (30-500 °C) to assess the intrinsic thermal stability of the neat ionic liquids, providing a baseline for comparison against tribo-chemical decomposition temperatures.

Commercial Applications

The demonstrated ability of cyano-based ionic liquids to achieve ultra-low friction against ta-C films at moderate temperatures opens avenues for high-performance lubrication systems in demanding environments.

High-Density Data Storage (HDD): ta-C films are standard protective coatings on hard disk drive platters. ILs offer non-volatile, ultra-low friction boundary lubrication, crucial for minimizing wear and improving the reliability of the head-disk interface.
Precision Manufacturing and Robotics: Components requiring extremely low friction coefficients (0.03) and high wear resistance, such as high-precision bearings, gears, and micro-actuators, can benefit from IL/ta-C pairings.
Vacuum and Aerospace Systems: The non-volatility and low vapor pressure of ILs make them superior lubricants for mechanical systems operating in high vacuum (e.g., space mechanisms or semiconductor processing equipment) where conventional oils would evaporate.
High-Performance Coatings: The findings support the development of tailored tribological systems where ta-C coatings are specifically designed to react with cyano-based ILs, maximizing the formation of protective graphitic and anion adsorption layers.
Automotive and Industrial Machinery: Use as high-performance additives or base oils in systems utilizing DLC/ta-C coatings to reduce friction losses, improving energy efficiency and component lifespan.

View Original Abstract

Ionic liquids have unique characteristics, which render them ideal candidates as new base oils or additives. In particular, there are great expectations from the combination of diamond-like carbon and cyano-based ionic liquids. Lubricating properties of cyano-based ionic liquids have been studied on specific tetrahedral amorphous carbon (ta-C) films. After lubrication, ta-C film/ta-C film contact interface exhibits exceedingly low friction. Therefore, it is necessary to understand this low friction phenomenon. The current study evaluated the lubricating mechanism of cyano-based ionic liquids against ta-C films. 1-Butyl-3-methylimidazolium dicyanamide ((BMIM)(DCN)) and 1-butyl-3-methylimidazolium tricyanomethane ((BMIM)(TCC)) were used as lubricants, with the latter exhibiting low friction coefficient of 0.03. Steel cylinders and disks with ta-C films were used as test specimens. Raman spectroscopy, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and thermogravimetric analysis (TGA) helped us understand the mechanism of low friction induced by (BMIM)(TCC). Graphitization of the ta-C film at high temperatures might have caused the reduction in friction between the films. Similarly, anion adsorption on the worn surface at high temperatures also led to reduced friction. However, the TGA result showed a different trend than that of the sliding test. Our results indicate that the cyano-based ionic liquids underwent tribo-decomposition at low temperatures. Further, a minimum temperature was required for the adsorption of anions onto the sliding surface.

Tech Support

Original Source

DOI: https://doi.org/10.3390/coatings10020153

References

2002 - A short history of ionic liquids—From molten salts to neoteric solvents [Crossref]
2003 - Ionic liquids—Solvents of the future? [Crossref]
2003 - A taste of the future [Crossref]
2013 - Effect of alkyl chain length and anion species on the interfacial nanostructure of ionic liquids at the Au (111)-ionic liquid interface as a function of potential [Crossref]
2007 - Effect and mechanism of additives for ionic liquids as new lubricants [Crossref]
2017 - Long range electrostatic forces in ionic liquids [Crossref]
2017 - Potential-induced restructuring dynamics of ionic liquids on a gold electrode: Steric effect of constituent ions studied by surface-enhanced infrared absorption spectroscopy [Crossref]
2007 - Tribological characteristics of imidazolium-based room temperature ionic liquids under high vacuum [Crossref]
1989 - Novel lubricants for magnetic thin film media
1990 - Frictional properties of novel lubricants for magnetic thin film media [Crossref]