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Over 100 commercial
sources produce various types of DLC
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This
is an
analysis of a sample from one of them deposited upon 304 stainless
steel.
SampleB
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Notable is that the
debris from deposition of the coating not yet impacted into DLC could
be burnished away with light pressure. A play of interference colors
appeared on the cleaned surface. A section was cut from the sample for
analysis.
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first essential to determine is the thickness of the DLC
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A bore
hole can be "dug" with
ions and the elements streaming back out of the hole can be identified
and recorded for each depth into the coating as the hole is dug. The
concentration of one element, hydrogen in this case, can be
calibrated in terms of atoms/ cm3 at the
different depths into the coating of DLC.
The DLC coating is seen to be 1.0 mm thick.
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Secondary Ion Mass
Spectroscopy
(SIMS) Analysis
If
the sample coating were pure diamond with density of 3.5 g/ cm3
there would be 17.6 (x1022 ) carbon atoms/ cm3
. The amount of hydrogen shown by the calibrated part of the SIMS
measurement would be a fraction of 2.2/17.6 or 12.5%. However, the
sample is not pure diamond with a density of 3.5 g/ cm3
. While the amount of hydrogen is known absolutely the fraction that
represents depends upon the actual density of the DLC.
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next concern is for the fractional abundance of hydrogen in the DLC.
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From the
measured value of H
atom density the H atom fraction can be determined from formulas if the
mass density of the coating is separately measured.
The issue is that the mass density tells how much carbon is in the
coating with the measured amount of H. Carbon is heavier; the more the
carbon, the greater
the mass density. |
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Possible
fractions of hydrogen for different possible densities
For different possible values of coating (mass) density, the colored
zones show the ranges for possible fractions of hydrogen where the
zones cross the vertical line marking the measured value of H atom
density. |
| An independent
measurement of mass density can be made with Laser-excited Surface
Acoustic Waves (SAW) |
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Accurate
measurment of the mass density of the film "fixes" the fractional
content of hydrogen diluent in the DLC. |
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Density of
the coating on the same sample was measured by technical experts at the
Fraunhofer-IWS Institute in Dresden
by laser-acoustics excitation of surface acoustic waves (SAW). It is
the
technique best suited for thicker coatings where there might be surface
layers
of unrepresentative residue from the deposition process. Using the
value determined
from the SIMS measurement of coating thickness, a density of 2.25 g cm-3
was reported to us. The corresponding value of hydrogen content is:
Fractional
hydrogen content = 16.6 %
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| To completely locate
the type of DLC the fraction of sp3
must be discovered. Direct measurement is difficult. |
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The same
SAW measurements give the Young's Modulus of Elasticity (YME) of the
film.
Standards relate the value of YME to the sp3 %.
For SampleB YME was found to be 210 in metric units
of GPa. |
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Figure
reproduced from Fig. 5b in Shi Xu, et
al., Phil. Mag. 76, 351
(1997). Results for Sample B are shown. |
| Having the fractional contents of hydrogen
diluent and fraction of the conversion of graphite to diamond give the
precise type of DLC |
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The type
of DLC is definitively located. |
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This
type of DLC is good and has useful functionality, but it
could be made much stronger if the amounts of diluents of hydrogen and
of unconverted carbon were reduced.
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