Synthesis of diamond-like carbon films with super-low friction and wear properties

A. Erdemir, O.L. Eryilmaz, and G. FenskeJ. Vac. Sci. Technol. A 18(4), Jul/Aug 2000 1987-1992

MSE 676 All Things Carbon / 09-29-2009

Deepak RajputCenter for Laser ApplicationsUT Space Institute, Tullahoma

Tennessee 37388, USAEmail: drajput@utsi.eduWeb: http://drajput.com

Introduction

Unique mechanical, chemical, optical, andelectrical properties.Quite hard, strong, and stiff.Most DLC films are electronically insulating andcan be made optically transparent to visible andultraviolet light.DLC films are chemically inert and impervious toacidic and saline media.They are amorphous and made of sp2- and sp3-bonded carbon atoms.

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Introduction

DLC films may also have large amounts ofhydrogen in their amorphous structures.Hydrogen-free DLC films can also be deposited.Doping DLC films to achieve better electrical andmechanical 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.

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Tribology

The mechanical and tribological propertiesdepend on microstructures, chemistry, hydrogencontent, sp2/sp3 bonded carbon.Test conditions strongly influence the frictionand wear performance.Friction coefficients of the DLC films: 0.01 to>0.5Relative humidity has the greatest effect on thefriction of DLC films.Low humidity: 0.01; high humidity: 0.1 – 0.3

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Tribology

Hydrogen-free DLC films: best in humid airHydrogenated DLC films: best in dry or inertconditions.At high temperatures, most undoped DLC filmsundergo permanent chemical andmicrostructural changes that degrade theirfriction and wear behavior (e.g., graphitization).

A new DLC film with coefficient of friction 0.001– 0.003 in inert-gas environments.

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Experimental

Process: Plasma Enhanced Chemical VaporDeposition (PE-CVD) at room temperature.Coated with 50-70 nm silicon bond layer prior todeposition 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 μm

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Experimental

Friction and wear test: Ball-on-disk tribometerConditions: Dry nitrogen under a load of 10 N.Hardness of steel balls and substrates: 8 GPa.Hardness of sapphire: 35 GPaSurface roughness better than 0.05 μm(steel).Wear volume determined:

rdWb 64

14.3 4

=d is the diameter of the wear scarr is the radius of the ball

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Results

Source gas: 25% CH4 + 75% H2SEM micrograph

TEM micrograph

Structurally amorphous, free of volume defects,and well bonded to the substrate 8

Results

Variation of coefficients of friction for different source gas compositions

0.015

0.003

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Results

Wear rate comparison of various DLC-coated M50 balls sliding against DLC-coatedH13 disks in dry nitrogen.

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Results

Friction coefficient of DLC film produced on sapphire substrates in a 25% CH4 +75% H2 plasma.

0.001

Substrate material influences frictional performance

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Proposed MechanismHydrogen chemically bonds and effectivelypassivate the free σ bonds of carbon atoms inthe DLC films and make them chemically veryinert.C-H bond is covalent and stronger than single C-C, C-O, or C-N bonds.Increased hydrogen etches out or remove thesp2-bonded or graphitic carbon precursor fromthe film surface and thus prevent the formationof planar graphitic phases and/or cross-linkingthat can give rise to π bonding (C=C doublebonds gives rise to high friction).

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SummaryDLC films grown with pure CH4 exhibit relativelypoor friction and wear performance.DLC films grown with CH4 + increasing H2exhibit increasingly better friction and wearperformance.DLC films grown on hard and highly rigidsapphire substrate have friction coefficient of ~0.001 for 25% CH4 + 75% H2.The main reason is the difference in hydrogenconcentration on the sliding surfaces as well aswithin the bulk DLC structures.Higher hydrogen concentration on slidingsurface is analogous to better shielding orpassivation of carbon bonds and hence lowerfriction. 13

Picture courtesy: http://thefutureofthings.comImage courtesy: www.diameterltd.co.uk/DLC.htm

Dr. Ali ErdemirArgonne National Laboratory, IL