In-situ TEM Studies of Tribo-Induced Bonding Modification in NFC Films

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Insitu transmission electron microscopy, Diamond like carbon, Near Frictionless Carbon, Wear, Friction, Tribology, Thin films

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In-situ TEM studies of tribo-induced bonding modification in near-frictionless carbon films

A.P. Merkle, A. Erdemir, O.I. Eryilmaz, J.A. Johnson, L.D. Marks

Deepak Rajput
Center for Laser Applications
University of Tennessee Space Institute
Tullahoma, Tennessee 37388-9700

* DLC: metastable disordered carbon with significant sp3 hybridization.
* Properties: high hardness, strength, chemically inert, electrically insulating, optically transparent, low static and kinetic friction.
* Applications: as protective coatings in automotive gears, magnetic storage disks, biological implants, MEMS devices.
* Common Methods: PVD and CVD !
* Plasma-enhanced chemical vapor deposition (PECVD) developed by Erdemir et al.
* Ultra-low friction hydrogenated films in a plasma of a 3:1 - H2:CH4 mixture.
* Friction coefficients as low as 0.001 (dry conditions).
* Named as ?near-frictionless carbon? (abbreviated NFC).
* DLCs have the largest range of wear and friction among solid lubricants ( ?= 0.001 to >0.5).
* Humidity, hydrogen and oxygen partial pressure affect the friction and wear rates.
* Hydrogen-free DLC: low friction in humid conditions.
* Hydrogenated DLC: low friction in dry or inert conditions.
* Hydrogenated DLC: hydrogen termination layer, which has resistance to tribochemical reactions on the surface of NFC films.
* Friction behavior: stabilized by doping with S, Ti, Fe, or Si.
* The presence of third-bodies or transfer layers affect the friction properties of DLC.
* Graphitized transfer layers maintain low and stable friction in humid conditions (just like graphite).
* TEM and Raman analysis verified the presence of graphitized debris particles on the surfaces of worn DLC.
* The formation and wear of transfer layers central to the study and understanding of self-lubricating carbon surfaces.

Objective: to reproduce DLC sliding conditions within the TEM and look for direct evidence of mechanically induced formation of a carbon rich transfer layer.
* Method: PECVD
* A capacitively coupled r.f. discharge plasma used to deposit films on a substrate.
* 30 mTorr, bias of -500 volts.
* A 30 nm bond layer of Si deposited to improve adhesion to the Cu-grid.
* Carbon film deposited at room temperature: 100 nm.
* NFC6 (1:3::CH4:H2) and NFC7 (Pure CH4).
* NFC7 less favorable tribological performance (high friction and wear).
* Sliding element: standard electropolishing techniques (0-5 V AC) from 0.25 mm polycrystalline tungsten (99.995%) wire in a 2 N NaOH solution to a radius of curvature on the order of 10 nm.
* An HS100 STM-HolderTM was used to carry out in-situ sliding experiments (designed for a 200KV SFE-TEM).
* Mid-10-7 Torr range vacuum / liquid N2 cooled anti-contamination finger used to check the contamination.
* The nanomanipulation holder configured to accept 3 mm TEM grids at a 30-degree inclination to the horizontal.

* A piezo elements based spatially controlled probe, capable of STM measurements was inserted.
* Resolution: 0.2? in XY and 0.025? in Z
* Coarse motion: ?1-2 mm in XY and ? 1 mm in Z.
* Sliding was performed after establishing a gentle contact with the sample.
* A track length of a few hundred nanometers at a sliding speed of 1 ?m/sec.
* EELS spectra collected every 50-100 passes.
* Direct measurement of the magnitude of the normal force applied to the samples could not be done.
* EELS: a post-column Gatan image filter was used to perform EELS measurements. Each spectrum was acquired from a region of approximately 100 nm length.
* TEM: bright field images were taken before and after sliding to record the microstructure of the sample.
Results and Discussion
* Early work:
* the steady-state coefficient of friction was due to wear-induced graphitization.
* Sliding velocity and loading level influence the graphitization process.
* This work was done ex-situ, which may have been affected by humidity, capillary forces, etc.
* Present study: In-situ studies done in high vacuum.
Results and Discussion
Results and Discussion
Results and Discussion
* Reason:
* The film debris attached to the tip appears both significantly brighter than the tungsten tip, and
* Nearly identical in structure to the standalone NFC film.

* The source of carbon: A combination of material worn from contamination layers built up due to the electron beam as well as worn material from the carbon sample.

Results and Discussion: NFC6

Results & Discussion: NFC7
* EELS showed successive increases in the 1s-?carbon peak ratio for NFC6, whereas no variations were observed for NFC7.
* Local mechanical excitation result in an increase in local relative sp2 bonding and graphitization effects.
* Superior lubricity properties of NFC6 as compared to NFC7 are a function of the higher hydrogen content of NFC6 films.

* NSF, Air Force Office of Scientific Research, Office of Energy Efficiency and Renewable Energy, Freedom Car and Vehicle Technologies Program.
* Electron Microscopy Center at Argonne National Laboratory.

Questions ??
Thanks !!