Supporting Material (063732APL):
Effects of Energetic Ion Particles on Friction of Diamond-Like Carbon
K. Wang
Department of Physics Texas A&M University, College Station, TX 77843-4242
H. Lianga
Department of Mechanical Engineering, Texas A&M University, College Station, TX 778433123
J.M. Martin and T. Le Mogne
Ecole Central de Lyon, LTDS, 69134 Ecully Cedex, Lyon, France
Methods and Materials
In the present research, we use an in situ approach to change the morphology and to analyze the
surface properties of a DLC film. Specifically, we introduce energetic ion particles to the DLC
surface. The effects of energetic particles on friction of the DLC film are fundamentally
important for applications of MEMS devices at demanding conditions. The interaction of
energetic particles with a solid surface, such as DLC, has been studied in the process of film
deposition. 1 The durability of DLC has not been reported in terms of its resistance against
impact of energetic particles. We study the damage induced by Ar ions and the surface property
after ion etching.
A tetrahedral amorphous (ta-C) 2 DLC film was deposited on 52 100 bearing steel disks using a
physical vapor deposition process. The coating had a surface roughness of about 30 nm in root
mean square and contained less than 0.5 at. % of hydrogen. Ar ions were generated by x-ray
photon spectroscopy (XPS) integrated in the ultrahigh vacuum tribometer, in which a pressure as
low as 10-7 mPa could be achieved. The accelerating voltage was 5 keV. Etching times were 1
and 10 min, respectively.
a
Corresponding author can be reached at: hliang@tamu.edu.
The XPS analysis was carried out in situ with a VG 220i spectrometer using monochromatic MgKα line for DLC films with different etching times. The emitted photoelectrons were detected
using a hemispherical analyzer at a passing energy of 20 eV for the C 1s high resolution XPS
peaks. The energy resolution of the instrument at 20 eV was 0.1 eV as estimated from the full
width at half maximum of the XPS Ag 3d5/2 peak of a pure silver target.
The in situ friction tests were conducted by using the reciprocating pin-on-disk tribometer inside
the same vacuum chamber. The sliding pin made of a DLC-coated AISI 52100 steel has a
hemispherical head with a 5 mm diameter. The disk is DLC-coated stainless steel and has a
dimension of 10 x 10 x 5 mm3. The pin moves in a linear reciprocating motion at the speed of 0.1
mm/s. The friction data were collected during sliding except at low speed toward the ends of the
friction track. The applied normal force was 1 N. Topographic images and surface roughness
were obtained using an atomic force microscope (Nano-R, Pacific Nanotechnology, Inc.) in a
close contact mode.
Other Possible Aspects
Could friction and its heating induce graphitization? According to a previous study by Liu et al.,
3,4
there was no friction-induced graphitization when the sliding speed was lower than 60 mm/s
at a load of 1 N. In the present work, our sliding speed was 0.1mm/s and the load was 1 N. The
graphitization was not expected. In addition, thermally induced sp3 to sp2 phase transition was
reported to occur at ~500 K for DLC films during deposition.5,6,7 Recent theoretical study shows
that the transformation from ta-C to highly ordered sp2 material takes place at Tact=1,750 K.8 In
our work, this temperature is not likely to be reached. The only variable here is the etching time.
The stable friction observed on the reference sample indicates that there is no phase
transformation observed through either friction or frictional heating.
Overall, at 5 keV ion impact energies, the C network is sufficiently damaged throughout the bulk
that subsequent long-range reconstruction results in predominantly large graphitic sp2 structures.
This leads to the formation of an amorphous sp2 rich layer on the surface of the film.
Graphitelike sp2 structures have low shear strengths promoting low friction.
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