Note: Descriptions are shown in the official language in which they were submitted.
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METAL-CONTAINING DIAMOND-LIKE-CARBON COATING COMPOSITIONS
BACKGROUND OF THE INVENTION
[0003] Field of Invention. The present invention relates to
metal diamond-like carbon coating compositions, and more
particularly, to metal diamond-like-carbon coating
compositions containing an intermediate layer of a transition
metal, such as chromium, or transition-metal carbide, and an
exterior layer of carbon and a transition metal such as
titanium, tungsten or niobium and method of making the same.
[0004] For many years, it has been known that diamond-like
carbon (DLC) films are hard and have low friction coefficients
( ), especially against steel. These low g values (-0.2) were
revealed for amorphous hydrogenated (abbreviated as a-C:H or
DLC) as well as for metal containing hydrocarbon (Me-C:H or
Me-DLC) films. Today, for both types of coatings, several
applications, above all in the field of machine elements and
of tools, are known. Besides the coating properties, it is an
important aspect that DLC as well as Me-DLC coatings can be
deposited at low substrate temperatures (<200 C).
[0005] A comparison of DLC and Me-DLC shows that there are
advantages and disadvantages to both coating materials as well
as to the corresponding deposition techniques. Hard DLC
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coatings, consisting of a highly cross-linked network of
carbon atoms, have high compressive stress (a few GPa). The
mentioned high stress values often lead to poor adhesion with
the substrate, especially on steel and, therefore, limit its
use in practical applications.
[0006] Many methods for the preparation of DLC films have been
developed. The most commonly applied method is the radio
frequency (r.f.) glow discharge of hydrocarbon gases with
negatively self-biased substrates. However, there are several
problems in the scaling up of this r.f. technique to
industrially relevant dimensions and geometries.
[0007] Commonly Me-DLC films having low metal content (atomic
ratios of Me/C up to approximately 0.3) have markedly lower
compressive stress than a-C:H (<1 GPa). Such films consist of
a network of amorphous carbon (DLC) with incorporated metal
carbides. The friction coefficients of such coatings are
rather similar to those of DLC coatings. However, the wear
resistance of Me-DLC coatings generally is lower than that of
DLC. At one time, the lowest abrasive wear rates reported for
Me-DLC coatings were at least a factor of 2 higher than those
reported for metal-free DLC coatings.
[0008] Generally, Me-C:H (Me-DLC) coatings are prepared in
industrial batch coaters by reactive magnetron sputtering in
argon-hydrocarbon gas mixtures using metal or metal carbide
targets. When comparing the two types of coatings, it should
be noted that the electrical resistivity of DLC coatings (>106
n cm) is much higher than that of Me-DLC (10-3 - 1 Q cm).
[0009] For example, a magnetron-sputtering assisted pulsed
laser deposition technique was used to prepare titanium
carbide and tungsten carbide containing amorphous diamond-like
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carbon films in an article by Voevodin et al. In another
article, Wei et al. used a pulsed laser technique with a
special target configuration, which allowed to ablate graphite
and a dopant (Cu, Ti, Si). A few percent of these elements
incorporated into the carbon matrix caused markedly improved
adhesion. However, it seems to be difficult to compare the
properties of the mentioned composite films to Me-DLC films
containing hydrogen and prepared using different preparation
techniques as discussed in an article entitled "Effect of
target material on deposition and properties of metal-
containing DLC (Me-DLC) coatings", by K. Bewilogua, C. V.
Cooper, C. Specht, J. Schroder, R. Wittorf and M. Grischke,
Surface & Coatings Technology 127, 224-232, Elsevier (2000),
which is incorporated by reference herein in its entirety.
[0010] Consequently, there exists a need for a metal
diamond-like-carbon coating composition having a superior
hardness and resistance to failure in pure rolling compared to
commercially available metal diamond-like-carbon coating
compositions.
SUMMARY OF THE INVENTION
[0011] In accordance with the present invention, a
metal-containing diamond-like-carbon coating composition
broadly including a first layer having a first transition
metal composition, and having a first surface and a second
surface in contact with a substrate is provided. The coating
also includes a second layer having carbon and a second
transition metal selected from the group consisting of
tungsten, niobium, titanium and combinations thereof, and
having a first surface and a second surface in contact with
the first surface of the first layer. Preferably, the
thickness of coating is about 0.5 micrometer to 10
micrometers. In addition, the composition can optionally
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possess a low abrasive wear rate. Such low abrasive wear rate
should not exceed more than about 1 x 10-10 m3m-'N-1. Such a
coating may be deposited using substrate bias potential having
a bias potential range of about -50 to -750 volts DC.
[0012] It may therefore be seen that the present invention
teaches a metal-containing coating having an intermediate
layer comprising a transition metal and an exterior,
functional layer comprising a transition metal and carbon.
[0013] The details of one or more embodiments of the invention
are set forth in the accompanying drawings and the description
below. Other features, objects, and advantages of the
invention will be apparent from the description and drawings,
and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a representation of a metal
diamond-like-carbon coating having an intermediate layer and
an exterior layer deposited upon a substrate comprising a
rolling-contact-fatigue rod;
[0015] FIG. 2 is a graph showing the relationship between
metal concentration and abrasive wear rate for metal diamond-
like-coating compositions containing intermediate layers of
chromium and exterior layers of carbon and metals such as
tungsten, titanium and niobium;
[0016] FIG. 3 is a bar graph representing the adherence
performance of metal diamond-like-coatings as a function of
substrate bias potential;
[0017] FIG. 4A is a representation of a
rolling-contact-fatigue (RCF) experiment depicting a top view
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of the arrangement of three roughened balls positioned in
contact with a rotating rod coated with the compositions of
Samples 1-7 as described in the Examples; and
[0018] FIG. 4B is a representation of the
rolling-contact-fatigue (RCF) experiment of FIG. 4A depicting
a side view of the roughened balls positioned in contact with
the rotating rod.
[0019] Like reference numbers and designations in the various
drawings indicate like elements.
DETAILED DESCRIPTION
[0020] Me-DLC coatings are deposited via magnetron sputtering,
in which four targets composed of the metal of interest are
installed into the interior of the deposition chamber,
activated, and sputtered with Argon (Ar) and acetylene (C2H2)
gases. Deposition substrates, comprising
rolling-contact-fatigue rods, or substrates 10, composed of a
through-hardened ferrous alloy, e.g., AISI M50, are placed
into the deposition chamber and negatively biased using a
direct current (DC) potential ranging from about -50 to -750
volts DC. Sputtering target compositions include elemental
titanium (Ti), niobium (Nb), and tungsten (w).
[0021] Metal hydrocarbon (Me-DLC) coatings are prepared by
reactive dc magnetron sputtering in unbalanced mode housing
HTC 1000/4 (ABS) coater commercially available from Hauzer
Techno Coating, Venlo, The Netherlands. Before initializing
the deposition runs, the residual pressure in the vacuum
chamber is set at less than about 10-3 Pascal (Pa). During the
deposition runs, the total gas pressure in the vacuum chamber
is between about 0.3 Pa to 0.6 Pa. The substrates mentioned
herein are supported within the vacuum chamber using substrate
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holders maintained at temperatures up to about 200 C. These
substrate holders permitted the substrate to be rotated at a
rate of about twelve revolutions per minute of planetary,
two-fold rotation. Throughout the entire process, the
deposition rates are typically about 2 to 3 micrometers per
hour.
[0022] The Me-DLC deposition process generally comprises four
steps. First, the substrates are cleaned using an Argon
etching process at a pressure of about 0.3 Pa as known to one
of ordinary skill in the art. For this step, argon gas is
admitted to the chamber until the achievement of the desired
gas flow rate and/or chamber pressure, generally about 300
standard cubic centimeters per minute (sccm) and 0.3 Pa,
respectively, and ionized to produce Ar+ ions for sputter
cleaning the substrate surface.
[0023] Second, an intermediate layer 12 composed of a
transition metal, in this case chromium (Cr), is then sputter
deposited upon the Ar-ion-etched substrates. During this
second stage of the deposition process, the DC current applied
to one or more targets composed of chromium metal is held
constant at the target value. For this process step, argon gas
is introduced into the chamber and held constant until the
intermediate layer had grown to achieve its target thickness.
As for the Ar-ion-etching stage, the flow rate and partial
pressure of argon for this second process step are,
respectively, approximately 300 sccm and 0.3-0.6 Pa,
respectively. As these deposition conditions are maintained,
chromium metal is sputter deposited onto the substrate surface
until the achievement of a desired thickness. Typically, the
deposited transition-metal intermediate layer possesses a
thickness of at least about 10 nanometers (0.01 micrometers)
and no more than about 2000 nanometers (2.0 micrometers) and
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preferably about 100 nanometers (0.1 micrometers) to 300
nanometers (0.3 micrometers).
[0024] Third, the dc current applied to the chromium metal
targets is decreased until reaching zero. The mixture of
reactive gases is flushed from the vacuum chamber in
preparation for the sputter deposition of the exterior layer.
A quantity of argon gas is again introduced into the chamber,
and a quantity of acetylene gas is then introduced until the
ratio of argon gas to acetylene gas is approximately 1:1 or a
50/50 mixture. Generally, the amount of acetylene gas present
in the reactive gas mixture is about fifteen percent to
forty-five percent by volume of the chamber.
[0025] Fourth, an exterior layer 14 of the metal
diamond-like-carbon coating composition is then sputter
deposited upon the intermediate layer of the substrates. The
DC current is applied gradually, that is, starting from zero
and gradually increasing in intensity, to one or more targets
composed of a metal such as tungsten, niobium, titanium and
combinations thereof. Again, a quantity of argon gas is
initially introduced into the chamber and then a quantity of
acetylene gas is introduced until the ratio of argon gas to
acetylene gas is approximately 1:1. Again, the amount of
acetylene gas generally present in the mixture is about
fifteen percent to forty-five percent by volume of the
chamber. During the transition from pure inert gas to a
combination of inert gas and acetylene or following the
achievement of final acetylene-inert gas mixture, target metal
begins being sputter deposited upon a first surface, that is,
the exterior surface, of the intermediate layer of transition
metal on each substrate until a desired thickness is achieved.
These processing conditions result in the deposition of a
Me-DLC coating containing an atomic metal-to-carbon (Me/C)
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ratio in the range of 0.1 to O.B. For purposes of illustration
and not to be taken in a limiting sense, the deposited
exterior layer may possess a thickness of about 0.1 micrometer
to 10 micrometers. The resulting metal-containing diamond-
like-carbon coating composition deposited upon the substrate
material is illustrated in FIG. 1. As depicted, a second
surface, that is, the interior surface, of the exterior layer
is in contact with the first surface or exterior surface of
the intermediate layer whose second surface, that is, the
interior surface is in contact with the exterior surface of
the substrate.
[0026] EXPERIMENTAL RESULTS
[0027] Abrasive Wear Rate
[0028] Referring now to FIG. 2, an abrasive wear rate was
determined for several planar samples of substrates having
various metal diamond-like-carbon coating compositions
deposited thereupon in accordance with the method described
herein and compared to a substrate coated with a
vendor-supplied metal diamond-like-carbon coating (Balzerso
Balinit Cm) .
[0029] A Calo tester operating with an aluminum oxide (A1203)
and water suspension was utilized to calculate the abrasive
wear rate for Samples 1-7. To quantify the results, the volume
of coating removed by the Calo device, generated by a rotating
ball, was divided by the normal force and the track length of
the rotating ball.
[0030] Sample 1, representative of substrate 10, is a
rolling-contact-fatigue rod made of AISI MS0 having deposited
thereupon an intermediate layer of chromium metal (Cr) having
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a thickness of 0.1 micrometers and an exterior layer of
titanium (Ti) diamond-like-carbon composition having a
thickness of 1 micrometer.
[0031] Sample 2, representative of substrate 10, is a
rolling-contact-fatigue rod made of AISI M50 having deposited
thereupon an intermediate layer of chromium metal (Cr) having
a thickness of 0.3 micrometers and an exterior layer of
titanium (Ti) diamond-like-carbon composition having a
thickness of 3 micrometers.
[0032] Sample 3, representative of substrate 10, is a
rolling-contact-fatigue rod made of AISI M50 having deposited
thereupon an intermediate layer of chromium metal (Cr) having
a thickness of 0.1 micrometers and an exterior layer of
tungsten (W) diamond-like-carbon composition having a
thickness of 1 micrometer.
[0033] Sample 4, representative of substrate 10, is a
rolling-contact-fatigue rod made of AISI M50 having deposited
thereupon an intermediate layer of chromium metal (Cr) having
a thickness of 0.3micrometers and an exterior layer of
tungsten (W) diamond-like-carbon composition having a
thickness of 3 micrometers.
[0034] Sample 5, representative of substrate 10, is a
rolling-contact-fatigue rod made of AISI M50 having deposited
thereupon an intermediate layer of chromium metal (Cr) having
a thickness of 0.1 micrometers and an exterior layer of
niobium (Nb) diamond-like-carbon composition having a
thickness of 1 micrometer.
[0035] Sample 6, representative of substrate 10, is a
rolling-contact-fatigue rod made of AISI M50 having deposited
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thereupon an intermediate layer of chromium metal (Cr) having
a thickness of 0.3 micrometers and an exterior layer of
niobium (Nb) diamond-like-carbon composition having a
thickness of 3 micrometers.
[0036] Sample 7, representative of substrate 10, is a
rolling-contact-fatigue rod made of AISI M50 coated with an
intermediate layer of chromium metal (Cr) having a thickness
of 0.3 micrometers and an exterior layer of Balinit C~, from
Balzerso, Inc. of Elgin, Illinois, whose international
headquarters is located in the Principality of Liechtenstein,
a tungsten (W) diamond-like-carbon coating having a thickness
of 3 micrometers, deposited using a floating potential.
[0037] The abrasive wear rate of samples 3 and 4 (W-DLC) is
significantly lower than the abrasive wear rate for other
compositions. The metal-to-carbon ratio of both samples 3 and
4 is much lower than for the metal-to-carbon ratios of the
other samples. For the deposition of samples 3 and 4, the
resulting abrasive wear rate was strongly independent of the
bias potential over a range of -100 to -120 V DC, showing an
abrasive wear rate of less than or equal to 1 x 10 ls m3m-1N-1
Given the metal-to-carbon ratios of about 0.1 to 0.3 and
resulting high metal content, in the same range as the metal
content of the Balzers'~' Balinit C coating, the resulting W-DLC
coatings of samples 3 and 4 demonstrate unexpected results in
light of prior documented research by the inventors of record.
Moreover, these abrasive wear rate results constitute a
distinct advantage over the other compositions investigated.
When compared to the standard, vendor-supplied W-DLC coating
(Balzers Balinit Cm), samples 3 and 4 are also superior to
sample 7. The abrasive wear rates of samples 3 and 4 are
greater than one order of magnitude to the abrasive wear rate
range of 15-20 x 10-15 m3m-1N-1 measured for sample 7. As
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demonstrated, a Me-DLC coating, and more specifically a W-DLC
coating, applied using a negative bias potential as opposed to
a floating bias potential, for example, sample 7, Balinit C-T ,
from Balzers , Inc., exhibits far superior abrasive wear rate
results.
[0038] Adherence Performance
[0039] Referring now to FIG. 3, an adherence performance value
was determined for Samples 1-6.
[0040] The adherence performance of the coatings was carried
out by means of static Rockwell indentation at a 150 kilogram
load. According to the well known VDI classification, a value
of HF-1 indicates excellent adherence, and a value of HF-6
indicates complete delamination around the indentation.
[0041] Samples 3 and 4 demonstrate superior adherence over a
substrate bias potential range of -100 volts to -500 volts
when compared to both the. Ti-containing DLC and Nb-containing
DLC coatings. Moreover, the results indicate that the
adherence is essentially invariant with the substrate bias
potential, which is an advantage in the processing of larger
parts.
[0042] Generally, as the distance from target-to-substrate
increases, as may be the case for the processing of large
parts, for example, the atoms sputtered from the metal targets
lose kinetic energy as they traverse the distance from target
to substrate. This argues in favor of applying a higher
substrate bias potential when coating large parts. However,
with other compositions and deposition processes parameters,
the use of higher substrate bias potentials leads to coating
degradation; for example, the coating adherence becomes
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inferior. As shown in FIG. 3, the W-DLC coatings of samples 3
and 4 do not exhibit coating degradation.
[0043] Rolling-Contact-Fatigue
[00447 Finally, this family of W-DLC coatings exhibits a
dramatic improvement in rolling-contact-fatigue (RCF)
performance compared to the standard, vendor-supplied W-DLC
coating (Balzers'~ Balinit C~) .
[0045] A representation of an RCF experiment performed in
accordance with the present invention is illustrated in FIGS.
4A and 4B. Three roughened balls (B1, B2, B3), 0.5 inches
(12.7 mm) in diameter, are radially loaded within a pair of
bearing cups (Bc1and Bcz) against the rolling-contact-fatigue
rod (R1), 0.375 inches (9.525 mm) in diameter, of sample 1 (as
described above). A clamping force (F) is applied to the
bearing cups (Bc1, Bc2) causing a radial load to be applied
between the roughened balls and the rod. The rod is rotated at
three thousand six hundred revolutions per minute (3,600 rpm)
in a direction (Dl) on its longitudinal axis (L1) while being
drip lubricated with oil conforming to MIL-L-23699, a
synthetic turbine engine lubricant employed by the United
States military.
[0046] Each rolling-contact-fatigue rod of samples 1-7 was
subjected to 5 x 106 cycles to determine the highest amount of
applied contact stress, or the rolling-contact stress limit
(RCSL), required to induce failure. The RCSL for the standard,
vendor-supplied W-DLC coating (Balzers Balinit CO) was
determined to be 400 Ksi (2.76 GPa). The RCSL for the W-DLC
coating was 700 Ksi (4.83 GPa). The RCSL for the Ti-DLC
coating was 800 Ksi (5.52 GPa). The RCSL for the Nb-DLC
coating was 400 Ksi (2.76 GPa).
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[0047] When translated into a constant contact stress level of
approximately 400 Ksi (2.76 GPa), it is estimated that the
increase in RCSL for a W-DLC or Ti-DLC coated part would
result in an increase in the rolling-contact-fatigue life of
the part of between about 10 and 15 times greater than the
rolling-contact-fatigue life delivered by the standard,
vendor-supplied W-DLC coating (Balzers9 Balinit C ).
[0048] Although an exemplary embodiment of the present
invention has been shown and described with reference to
particular embodiments and applications thereof, it will be
apparent to those having ordinary skill in the art that a
number of changes, modifications, or alterations to the
invention as described herein may be made, none of which
depart from the spirit or scope of the present invention. All
such changes, modifications, and alterations should therefore
be seen as being within the scope of the present invention.
[0049] Although the foregoing description of the present
invention has been shown and described with reference to
particular embodiments and applications thereof, it has been
presented for purposes of illustration and description and is
not intended to be exhaustive or to limit the invention to the
particular embodiments and applications disclosed. It will be
apparent to those having ordinary skill in the art that a
number of changes, modifications, variations, or alterations
to the invention as described herein may be made, none of
which depart from the spirit or scope of the present
invention_ The particular embodiments and applications were
chosen and described to provide the best illustration of the
principles of the invention and its practical application to
thereby enable one of ordinary skill in the art to utilize the
invention in various embodiments and with various
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modifications as are suited to the particular use
contemplated. All such changes, modifications, variations,
and alterations should therefore be seen as being within the
scope of the present invention as determined by the appended
claims when interpreted in accordance with the breadth to
which they are fairly, legally, and equitably entitled.
14
