Synthesis of Diamond Like Carbon Films with Super-Low Friction and Wear Properties

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Synthesis, DLC, Diamond like carbon, Wear, Friction, Tribology, PECVD

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Synthesis of diamond-like carbon films with super-low friction and wear properties
A. Erdemir, O.L. Eryilmaz, and G. Fenske
J. Vac. Sci. Technol. A 18(4), Jul/Aug 2000 1987-1992

* Unique mechanical, chemical, optical, and electrical properties.
* Quite hard, strong, and stiff.
* Most DLC films are electronically insulating and can be made optically transparent to visible and ultraviolet light.
* DLC films are chemically inert and impervious to acidic and saline media.
* They are amorphous and made of sp2- and sp3- bonded carbon atoms.
* DLC films may also have large amounts of hydrogen in their amorphous structures.
* Hydrogen-free DLC films can also be deposited.
* Doping DLC films to achieve better electrical and mechanical properties is also possible.
* DLC films deposition range: subzero to 400oC.
* Processes: plasma or ion beam- PVD and CVD.
* Carbon source: hydrocarbon gas like CH4, C2H2.

* The mechanical and tribological properties depend on microstructures, chemistry, hydrogen content, sp2/sp3 bonded carbon.
* Test conditions strongly influence the friction and wear performance.
* Friction coefficients of the DLC films: 0.01 to >0.5
* Relative humidity has the greatest effect on the friction of DLC films.
* Low humidity: 0.01; high humidity: 0.1 ? 0.3
* Hydrogen-free DLC films: best in humid air
* Hydrogenated DLC films: best in dry or inert conditions.
* At high temperatures, most undoped DLC films undergo permanent chemical and microstructural changes that degrade their friction and wear behavior (e.g., graphitization).

A new DLC film with coefficient of friction 0.001 ? 0.003 in inert-gas environments.
* Process: Plasma Enhanced Chemical Vapor Deposition (PE-CVD) at room temperature.
* Coated with 50-70 nm silicon bond layer prior to deposition on AISI M50 balls, H13 steel disks, and sapphire balls and disks.
* Source gas:
? Pure methane
? Mixture of methane and increasing hydrogen
* Film thickness: 1 ?m
* Friction and wear test: Ball-on-disk tribometer
* Conditions: Dry nitrogen under a load of 10 N.
* Hardness of steel balls and substrates: 8 GPa.
* Hardness of sapphire: 35 GPa
* Surface roughness better than 0.05 ?m (steel).
* Wear volume determined:

Proposed Mechanism
* Hydrogen chemically bonds and effectively passivate the free ? bonds of carbon atoms in the DLC films and make them chemically very inert.
* C-H bond is covalent and stronger than single C-C, C-O, or C-N bonds.
* Increased hydrogen etches out or remove the sp2-bonded or graphitic carbon precursor from the film surface and thus prevent the formation of planar graphitic phases and/or cross-linking that can give rise to ? bonding (C=C double bonds gives rise to high friction).
* DLC films grown with pure CH4 exhibit relatively poor friction and wear performance.
* DLC films grown with CH4 + increasing H2 exhibit increasingly better friction and wear performance.
* DLC films grown on hard and highly rigid sapphire substrate have friction coefficient of ~ 0.001 for 25% CH4 + 75% H2.
* The main reason is the difference in hydrogen concentration on the sliding surfaces as well as within the bulk DLC structures.
* Higher hydrogen concentration on sliding surface is analogous to better shielding or passivation of carbon bonds and hence lower friction.