Transcript Synthesis of diamond-like carbon films with superlow

Synthesis of diamond-like
carbon films with super-low
friction and wear properties
MSE 676 All Things Carbon / 09-29-2009
A. Erdemir, O.L. Eryilmaz, and G. Fenske
J. Vac. Sci. Technol. A 18(4), Jul/Aug 2000 1987-1992
Deepak Rajput
Center for Laser Applications
UT Space Institute, Tullahoma
Tennessee 37388, USA
Email: [email protected]
Web: http://drajput.com
Introduction
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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 sp3bonded carbon atoms.
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Introduction
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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.
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Tribology
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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
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Tribology
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).
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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 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 μm
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Experimental
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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 μm
(steel).
Wear volume determined:
3.14d
Wb 
64r
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d is the diameter of the wear scar
r is the radius of the ball
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Results
TEM micrograph
SEM micrograph
Source gas: 25% CH4 + 75% H2
Structurally amorphous, free of volume defects,
and well bonded to the substrate
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Results
0.003
0.015
Variation of coefficients of friction for different source gas compositions
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Results
Wear rate comparison of various DLC-coated M50 balls sliding against DLC-coated
H13 disks in dry nitrogen.
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Results
Substrate material influences frictional
performance
0.001
Friction coefficient of DLC film produced on sapphire substrates in a 25% CH4 +
75% H2 plasma.
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Proposed Mechanism
Hydrogen chemically bonds and effectively
passivate the free σ bonds of carbon atoms in
the DLC films and make them chemically very
inert.
 C-H bond is covalent and stronger than single CC, 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 π bonding (C=C double
bonds gives rise to high friction).
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Summary
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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.
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Dr. Ali Erdemir
Argonne National Laboratory, IL
Image courtesy: www.diameterltd.co.uk/DLC.htm
Picture courtesy: http://thefutureofthings.com