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

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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 drajput_at_utsi.edu Web
http//drajput.com
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Introduction

• 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.

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Introduction

• 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

• 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
  gt0.5
• 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).
• 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

• 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

d is the diameter of the wear scar r is the
radius of the ball
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Results
TEM micrograph
Source gas 25 CH4 75 H2
SEM micrograph
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 s 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 p bonding (CC double bonds
  gives rise to high friction).

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Summary

• 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

Picture courtesy http//thefutureofthings.com
Image courtesy www.diameterltd.co.uk/DLC.htm