22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Influence of the precursor gas on the process and layer properties of DLC films
J. Wöckel1, B. Dzur2, J. Emmerlich1, M. Müller1, S. Rath1 and D. Spreemann2
1
Robert Bosch GmbH, Stuttgart, Germany
2
TU Ilmenau, Ilmenau, Germany
Abstract: This study investigates the influence of the precursor gas on the layer properties
of diamond like carbon films. The examined gases are acetylene and butane. Butane as
precursor results in higher micro hardness and lower friction coefficient of the films
compared to acetylene. This behaviour indicates a different hydrogen content and
sp2/sp3-ratio.
Keywords: DLC, PACVD, acetylene, butane
1. Introduction
Diamond like carbon films (DLC) offer a variety of
applications because of their unique properties such as
low friction coefficient, high wear resistance and high
hardness. However, the technical requirements increase
with the engineering progress.
The variation of the precursor gas mixtures has been
subject to several investigations. It was lately discussed
by Vetter et al. [1]. Butane and acetylene differs in terms
of the hydrogen carbon ratio and the bond type.
Acetylene exhibits a low H/C-ratio. Butane chains are
linked with single bonds, however, acetylene possesses
two single bonds and one triple bond.
The different binding energies in Table 1 influence the
dissociation of both gases. Lackner reported, acetylene as
precursor forms preferentially double bonds and single
bonds in the layer. Double bonds support a sp2-structure
[2]. A C-C-bond is formed involving two hybrid orbitals.
For the second C-C-bond the sp3-hybrid orbitals are too
distant to form a bond by overlapping. One p-orbital is
uninvolved, which is perpendicular to the sp2-system. It
forms a π-bond [3].
Table 1. Binding energy [2].
Bond type
C–C
C–H
C≡C
Binding energy [kJ ⋅ mol-1]
345
416
811
Butane offers a higher H/C-ratio, which should yield a
higher hydrogen content in the layer. Besides, increasing
hydrogen content promotes a higher sp3-content.
Terminal bonds are formed. Consequently, the network is
weaker and gets a polymeric character. Accordingly,
acetylene derived coatings should possess higher hardness
and butane derived coatings should have a lower friction
coefficient [2, 4-7]. Otherwise hydrogen is involved in
different processes, for example Michaelson et al.
discussed the influence of hydrogen on the growth
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mechanism. It was analyzed that the hydrogen etched
non-diamond constituents and extracted hydrogen. More
C-species can be incorporated [8]. Accordingly, the
butane derived coatings should be harder compared to the
acetylene derived films, because of the higher H/C-ratio.
2. Experiment
Both layer types were deposited onto stainless steel by
PACVD. The gases used are acetylene and butane. For
both layer systems the process parameters (i) bias current,
(ii) bias voltage and (iii) gas flow were varied.
The layer thickness was measured by a reflectometer.
The adhesion of the film was analyzed with a Rockwelldiamond. The diamond was indented with a force of
1471 N. The resulting bulging indicates residual stress.
The adhesion was categorized in school grades according
to VDI 3198. With increasing delamination the adhesion
class deteriorates. The micro-hardness was measured
with the Fischerscope HCU with a test load of 50 mN.
The friction coefficient was measured with the model
SRV4 Optimol Instruments. The measurement was
performed in ambient air. The contact body is a steel ball
with a diameter of 4 mm. The ball was pressed against
the sample with a test load of 20 N and a frequency of
40 Hz after a running-in process. The amplitude was
0.2 mm.
3. Results and discussion
Acetylene and butane generate different process
windows. Because of the low ionization potential of
butane a dense plasma is formed with constant excitation
intensity compared to acetylene.
The resulting current is determined by the number of
charged species. With increasing excitation intensity
more charged species are generated. Hence, the current
increases. Additionally, the bias voltage influences the
current of the ions within the plasma sheath. With
increasing current more species impact on the sample
surface in the same time.
Because of the low ionization potential of butane the
gas generates a higher ionization density compared to
acetylene. Fig. 1 shows the correlation between the gas
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flow of both gases and the pressure. The pressure
increases significantly at low gas flows. The same
pressure under plasma conditions is established at a
higher gas flow for acetylene compared to butane.
Fig. 1. Correlation between the gas flow of butane and
acetylene and the plasma process pressure.
layer thickness. There are some possibilities to explain
this effect. On the one hand the growth rate depends on
different relationships: (i) ion/radical-ratio, (ii)
ion/electron-ratio at constant current, influenced by e.g.,
secondary electron emission, and (iii) degree of ionization
of the species. Due to the lower dissociation energy of
butane more radicals might be created. The radicals
participate at the deposition process. They are not
affected by the electric current. Consequently the
increase of the layer growth rate of butane is higher
compared to acetylene. On the other hand butane has the
higher molar mass compared to acetylene. Larger
fragments of butane molecules may impinge at the
surface. Accordingly the butane should result in coatings
of lower density and hardness compared to acetylene.
Otherwise the molecule bonds break at impact because of
the high energy. In this case the size of the molecule
should be irrelevant. These both theories should be
subject to further discussions.
Due to the different plasma properties the butane and
acetylene derived films were deposited in different
process parameter that were particularly adapted to each
precursor gas.
The variation of the bias current does not affect the
micro hardness, as shown in Fig. 2. The variations of the
measurement are within the standard deviation. The
butane derived layers exhibit 10 GPa harder films. This
indicates a lower hydrogen content and/or a higher C-C
sp3-content of the coating. This correlates with the
growth mechanism described by Michealson et al. [8].
Fig. 3. Comparison of layer thickness and adhesion of
butane derived coatings and acetylene derived films as a
function of bias current.
Fig. 2. Comparison of micro hardness of butane derived
films and acetylene derived films coatings as a function of
bias current.
In Fig. 3 the layer thickness and adhesion as a function
of the electric substrate current are shown. The layer
thickness increases with increasing current. Butane
derived films exhibits a significantly higher increase in
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The adhesion decreases with increasing layer thickness.
With increasing layer thickness the residual stress
increases. As a result, the layer delaminates more easily.
The bias current does not significantly influence the
friction coefficient (Fig. 4). The variations of the
measurement are also within the standard deviation. The
layer I2 shows a short lifetime. With decreasing layer
thickness the lifetime of the film decreases. The fretting
wear test procedure ran after 180 minutes.
The variation of the bias voltage leads to an increase in
hardness of the acetylene derived film. The hardness of
the butane layer seems to be unaffected. The voltage
determines the velocity of the species in the plasma
sheath. With increasing voltage the acceleration of the
species increases. The species impinge with higher
energy and densify the thin film. Consequently the
hardness increases within the analyzed limits. This effect
does not occur for butane.
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Fig. 4. Variation of friction coefficient and lifetime by
changing the bias current for butane derived DLC films
compared to acetylene derived film.
The hardness remains the same for butane with
increasing bias voltage, shown in Fig. 5. However, the
layer thickness for butane decreases with increasing
voltage. With decreasing layer thickness, the measured
hardness is increasingly influenced by the substrate.
Because of the decreasing layer thickness an increase in
hardness is not visible. The coating hardness seems to be
constant, probably due to the increasing influence of the
lower substrate hardness.
Fig. 6. Comparison of layer thickness and adhesion of
butane derived films and acetylene derived films as a
function of bias voltage.
With increasing bias voltage the friction coefficient
decreases (Fig. 7).
The running-in phase is very
important for friction wear resistant coatings. The load
and shear rates influence the friction coefficient.
Furthermore, the final friction behavior is affected by the
changes in surface roughness and microstructure during
the running in phase. Pastewka et al. detected the change
from sp3 to sp2 in the wear track. Hence, the decreasing
friction coefficient during the running-in phase results in
an increase in sp2 hybridization [9]. Adapted to our
investigations it can be assumed that the increasing bias
voltage gives rise to a modified surface roughness and
structure.
Further micro-structural properties (i.e.,
sp3/sp2-ratio, surface roughness and binding energy) must
be analyzed to explain this phenomenon.
Fig. 5. Comparison of micro hardness of butane derived
films and acetylene derived films as a function of bias
voltage.
Furthermore Fig. 6 shows that the layer thickness of
butane derived DLC films decreases with increasing
voltage and the thickness of acetylene derived coatings is
constant. Maybe the voltage also influences the size of
the impinging molecules due to the impact. With
increasing voltage the energy of the molecules increases.
Consequently more bonds crack leaving the species more
mobile and hence resulting in fewer voids and densify the
layer.
Another well known mechanism shows an
increasing impact speed of the species with increasing
voltage. There increasingly occur sputtering effects.
Species with high energy may sputter previously
deposited species. This mechanism is called resputtering.
Hence, the layer thickness also decreases.
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Fig. 7. Variation of friction coefficient and lifetime by
changing the bias voltage for butane derived DLC films
compared to acetylene derived film.
The lifetime for butane derived films decrease with
increasing bias voltage because of the decreasing layer
thickness.
Finally, the gas flow was varied. Fig. 8 shows the
comparison of hardness between acetylene derived films
and butane derived films. The hardness of butane derived
coatings is about constant with increasing gas flow. The
variations of the measurement are within the standard
deviation. The hardness of the acetylene film decreases.
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The hardness is mainly affected by the hydrogen and sp3content. These both properties are influenced by the
degree of ionization and species energy. With higher gas
flow rate in the vacuum chamber and constant pumping
rate the pressure increases. Consequently the mean free
path length decreases. As the bias current is constant the
ion/radical-ratio might decrease. Thus, the energy per
arriving species decreases with increasing gas flow
resulting in decreasing hardness.
Fig. 9. Comparison of layer thickness and adhesion of
butane derived DLC films and acetylene derived DLC
films as a function of gas flow.
Fig. 8. Comparison of layer thickness and adhesion of
butane derived films and acetylene derived films as a
function of gas flow.
The layer thickness of the butane derived films is
constant with increasing gas flow, for acetylene the
thickness increases (Fig. 9).
Because of the low
ionization potential of butane the ion/radical-ratio shifts
insignificantly.
To achieve the same current with
acetylene the excitation intensity must be reduced. This
reduces the ion/radical-ratio and hence more radicals
participate at the deposition process. Consequently, the
layer thickness increases with increasing gas flow for
acetylene derived coatings. This effect correlates with the
results of the hardness measurements.
With increasing gas flow the friction coefficient
increases, see Fig. 10. This indicates a decreasing
graphitization. However, the surface and layer properties
must be analyzed in detail.
4. Conclusions
The butane derived DLC films exhibit 10 GPa higher
hardness compared to the acetylene layer. A lower
friction coefficient can be achieved by parameter
variation.
The layer properties depend on the particular parameter
variation. In example with increasing bias voltage the
hardness increases. This could be explained by a higher
sp3-content or lower hydrogen content.
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Fig. 10. Variation of friction coefficient and lifetime by
changing the gas flow for butane derived DLC films
compared to acetylene derived film.
Furthermore both gases offer a different behavior by
variation of the process parameters. Now the generated
theories have to be confirmed by appropriate
measurements.
5. References
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(2012)
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