Diamond-like carbon (DLC)
exists in seven different forms[1] of amorphous carbon materials that
display some of the unique properties of diamond. They are usually
applied as coatings to other materials that could benefit from some of
those properties. All seven contain significant amounts of sp3
hybridized carbon atoms. The reason that there are different types is
that even diamond can be found in two crystalline polytypes. The usual
one has its carbon atoms arranged in a cubic lattice, while the very
rare one (lonsdaleite) has a hexagonal lattice. By mixing these
polytypes in various ways at the nanoscale level of structure, DLC
coatings can be made that at the same time are amorphous, flexible, and
yet purely sp3 bonded "diamond". The hardest, strongest, and slickest
is such a mixture, known as tetrahedral amorphous carbon, or ta-C. For example, a coating of only 2 μm thickness of ta-C increases the resistance of common (i.e. type 304) stainless steel against abrasive wear; changing its lifetime in such service from one week to 85 years. Such ta-C
can be considered to be the "pure" form of DLC, since it consists only
of sp3 bonded carbon atoms. Fillers such as hydrogen, graphitic sp2
carbon, and metals are used in the other 6 forms to reduce production
expenses or to impart other desirable properties.[2][3] The various
forms of DLC can be applied to almost any material that is compatible
with a vacuum environment. In 2006, the market for outsourced DLC
coatings was estimated as about 30,000,000 € in the European Union.
Naturally
occurring diamond is almost always found in the crystalline form with a
purely cubic orientation of sp3 bonded carbon atoms. Sometimes there
are lattice defects or inclusions of atoms of other elements that give
color to the stone, but the lattice arrangement of the carbons remains
cubic and bonding is purely sp3. The internal energy of the cubic
polytype is slightly lower than that of the hexagonal form and growth
rates from molten material in both natural and bulk synthetic diamond
production methods are slow enough that the lattice structure has time
to grow in the lowest energy (cubic) form that is possible for sp3
bonding of carbon atoms. In contrast, DLC is typically produced by
processes in which high energy precursive carbons (e.g. in
plasmas, in filtered cathodic arc deposition, in sputter deposition and
in ion beam deposition) are rapidly cooled or quenched on relatively
cold surfaces. In those cases cubic and hexagonal lattices can be
randomly intermixed, layer by atomic layer, because there is no time
available for one of the crystalline geometries to grow at the expense
of the other before the atoms are "frozen" in place in the material.
Amorphous DLC coatings can result that have no long-range crystalline
order. Without long range order there are no brittle fracture planes,
so such coatings are flexible and conformal to the underlying shape
being coated, while still being as hard as diamond. In fact this
property has been exploited to study atom-by-atom wear at the nanoscale
in DLC.[4]
[edit] Production
SEM
image of a gold-coated replica of a ta-C "diamond-like" coating.
Structural elements are not crystallites but are nodules of sp3-bonded
carbon atoms where lattice geometries randomly alternate between the
cubic and hexagonal polytypes of diamond. The grains are so small that
the surface appears mirror smooth to the eye.
There are several
methods for producing DLC, but all depend upon the fact that in carbon
the sp3 bond length is significantly less than the length of the sp2
bond. So the application of pressure, impact, catalysis, or some
combination of these at the atomic scale can force sp2 bonded carbon
atoms closer together into sp3 bonds. This must be done vigorously
enough that the atoms cannot simply spring back apart into separations
characteristic of sp2 bonds. Usually techniques either combine such a
compression with a push of the new cluster of sp3 bonded carbon deeper
into the coating so that there is no room for expansion back to
separations needed for sp2 bonding; or the new cluster is buried by the
arrival of new carbon destined for the next cycle of impacts. It is
reasonable to envision the process as a "hail" of projectiles that
produce localized, faster, nanoscale versions of the classic
combinations of heat and pressure that produce natural and synthetic
diamond. Because they occur independently at many places across the
surface of a growing film or coating, they tend to produce an analog of
a cobblestone street with the cobbles being nodules or clusters of sp3
bonded carbon. Depending upon the particular "recipe" being used, there
are cycles of deposition of carbon and impact or continuous
proportions of new carbon arriving and projectiles conveying the
impacts needed to force the formation of the sp3 bonds. As a result, ta-C
may have the structure of a cobblestone street, or the nodules may
"melt together" to make something more like a sponge or the cobbles may
be so small as to be nearly invisible to imaging. A classic "medium"
morphology for a ta-C film is shown in the figure.
[edit] Properties
As
implied by the name, diamond-like carbon (DLC), the value of such
coatings accrues from their abilities to provide some of the properties
of diamond to surfaces of almost any material. The primary desirable
qualities are hardness, wear resistance, and slickness (DLC film
friction coefficient against polished steel ranges from 0.05-0.20[5]).
However,
which properties are added to a surface and to what degree depends
upon which of the 7 forms are applied, and further upon the amounts and
types of diluents added to reduce the cost of production. In 2006 the
Association of German Engineers, VDI, the largest engineering
association in Western Europe issued an authoritative report VDI2840[6]
in order to clarify the existing multiplicity of confusing terms and
trade names. It provides a unique classification and nomenclature for
diamond-like-carbon (DLC) and diamond films. It succeeded in reporting
all information necessary to identify and to compare different DLC
carbon films which are offered on the market. Quoting from that
document:
These [sp3] bonds can occur not only with
crystals - in other words, in solids with long-range order - but also
in amorphous solids where the atoms are in a random arrangement. In
this case there will be bonding only between a few individual atoms and
not in a long-range order extending over a large number of atoms. The
bond types have a considerable influence on the material properties of
amorphous carbon films. If the sp2 type is predominant the film will be
softer, if the sp3 type is predominant the film will be harder.
A
secondary determinant of quality was found to be the fractional
content of hydrogen. Some of the production methods involve hydrogen or
methane as a catalyst and a considerable percentage of hydrogen can
remain in the finished DLC material. When it is recalled that the soft
plastic, polyethylene is made from carbon that is bonded purely by the
diamond-like sp3 bonds, but also includes chemically bonded hydrogen,
it is not surprising to learn that fractions of hydrogen remaining in
DLC films degrade them almost as much as do residues of sp2 bonded
carbon. The VDI2840 report confirmed the utility of locating a
particular DLC material onto a 2-dimensional map on which the X-axis
described the fraction of hydrogen in the material and the Y-axis
described the fraction of sp3 bonded carbon atoms. The highest quality
of diamond-like properties was affirmed to be correlated with the
proximity of the map point plotting the (X,Y) coordinates of a
particular material to the upper left corner at (0,1), namely 0%
hydrogen and 100% sp3 bonding. That "pure" DLC material is ta-C
and others are approximations that are degraded by diluents such as
hydrogen, sp2 bonded carbon, and metals. Valuable properties of
materials that are ta-C, or nearly ta-C follow.
[edit] Hardness
STM image of surfaces at the edge of a 1 μm thick layer of ta-C
"diamond-like" coating on 304 stainless steel after various durations
of tumbling in a slurry of 240 mesh SiC abrasive. The first 100 min
shows a burnishing away from the coating of an overburden of soft
carbons than had been deposited after the last cycle of impacts
converted bonds to sp3. On the uncoated part of the sample, about 5 μm
of steel were removed during subsequent tumbling while the coating
completely protected the part of the sample it covered.
Within the
"cobblestones", nodules, clusters, or "sponges" (the volumes in which
local bonding is sp3) bond angles may be distorted from those found in
either pure cubic or hexagonal lattices because of intermixing of the
two. The result is internal (compressive) stress that can appear to add
to the hardness measured for a sample of DLC. Hardness is often
measured by nanoindentation methods in which a finely pointed stylus of
natural diamond is forced into the surface of a specimen. If the
sample is so thin that there is only a single layer of nodules, then
the stylus may enter the DLC layer between the hard cobblestones and
push them apart without sensing the hardness of the sp3 bonded volumes.
Measurements would be low. Conversely, if the probing stylus enters a
film thick enough to have several layers of nodules so it cannot be
spread laterally, or if it enters on top of a cobblestone in a single
layer, then it will measure not only the real hardness of the diamond
bonding, but an apparent hardness even greater because the internal
compressive stress in those nodules would provide further resistance to
penetration of the material by the stylus. Nanoindentation
measurements have reported hardness as great as 50% more than values
for natural crystalline diamond. Since the stylus is blunted in such
cases or even broken, actual numbers for hardness that exceed that of
natural diamond are meaningless. They only show that the hard parts of
an optimal ta-C material will break natural diamond rather than
the inverse. Nevertheless, from a practical viewpoint it does not
matter how the resistance of a DLC material is developed, it can be
harder than natural diamond in usage. One method of testing the coating
hardness is by means of the Persoz pendulum.
[edit] Bonding of DLC coatings
The
same internal stress that benefits the hardness of DLC materials makes
it difficult to bond such coatings to the substrates to be protected.
The internal stresses try to "pop" the DLC coatings off of the
underlying samples. This challenging downside of extreme hardness is
answered in several ways, depending upon the particular "art" of the
production process. The most simple is to exploit the natural chemical
bonding that happens in cases in which incident carbon ions supply the
material to be impacted into sp3 bonded carbon atoms and the impacting
energies that are compressing carbon volumes condensed earlier. In this
case the first carbon ions will impact the surface of the item to be
coated. If that item is made of a carbide-forming substance such as Ti
or Fe in steel a layer of carbide will be formed that is later bonded
to the DLC grown on top of it. Other methods of bonding include such
strategies as depositing intermediate layers that have atomic spacings
that grade from those of the substrate to those characteristic of sp3
bonded carbon. In 2006 there were as many successful recipes for bonding
DLC coatings as there were sources of DLC.
[edit] Tribology
DLC
coatings are often used to prevent wear due to its excellent
tribological properties. DLC is very resistant to abrasive and adhesive
wear making it suitable for use in applications that experience
extreme contact pressure, both in rolling and sliding contact. DLC is
often used to prevent wear on razor blades and metal cutting tools,
including lathe inserts and milling cutters. DLC is used in bearings,
cams, cam followers, and shafts in the automobile industry. The
coatings reduce wear during the 'break-in' period, where drive train
components may be starved for lubrication.
DLCs may also be used
in chameleon coatings that are designed to prevent wear during launch,
orbit, and re-entry of land launched space vehicles. DLC provides
lubricity at ambient atmosphere and at vacuum, unlike graphite which
requires moisture to be lubricious.
Despite the favorable
tribological properties of DLC it must be used with caution on ferrous
metals. If it is used at higher temperatures, the substrate or counter
face may carburize, which could lead to loss of function due to a
change in hardness. This phenomenon prevents the use of DLC coated
machine tool on steel.
[edit] Electrical
If a DLC material is close enough to ta-C
on plots of bonding ratios and hydrogen content it can be an insulator
with a high value of resistivity. Perhaps more interesting is that if
prepared in the "medium" cobblestone version such as shown in the above
figure, electricity is passed through it by a mechanism of hopping
conductivity. In this type of conduction of electricity the electrons
move by quantum mechanical tunneling between pockets of conductive
material isolated in an insulator. The result is that such a process
makes the material something like a semiconductor. Further research on
electrical properties is needed to explicate such conductivity in ta-C
in order to determine its practical value. However, a different
electrical property of emissivity has been shown to occur at unique
levels for ta-C. Such high values allow for electrons to be emitted from ta-C
coated electrodes into vacuum or into other solids with application of
modest levels of applied voltage. This has supported important advances
in medical technology.
[edit] Applications
Applications
of DLC typically utilize the ability of the material to reduce abrasive
wear. Tooling components, such as endmills, drill bits, dies and molds
often use DLC in this manner. DLC is also used in the engines of modern
supersport motorcycles, Formula 1 racecars, NASCAR vehicles, and as a
coating on hard-disk platters and hard-disk read heads to protect
against head crashes. Virtually all of the multi-bladed razors used for
wet shaving have the edges coated with hydrogen-free DLC to reduce
friction, preventing abrasion of sensitive skin. Some forms have been
certified in the EU for food service and find extensive uses in the
high-speed actions involved in processing novelty foods such as "chips"
and in guiding material flows in packaging foodstuffs with plastic
wraps. DLC coats the cutting edges of tools for the high-speed, dry
shaping of difficult exposed surfaces of wood and aluminum, for example
on automobile dashboards.
The implantable human heart pump[7] can
be considered the ultimate biomedical application where DLC coating is
used on blood contacting surfaces of the key components of the device.
Other
medical applications such as Percutaneous coronary intervention
employing brachytherapy find additional benefits from the unique
electrical properties of DLC. At low voltages and low temperatures
electrodes coated with DLC can emit enough electrons to be arranged
into disposable, micro-X-ray tubes as small as the radioactive seeds
that are introduced into arteries or tumors in conventional
brachytherapy. The same dose of prescribed radiation can be applied from the inside, out with the additional possibility to switch on and off the radiation in the prescribed pattern for the X-rays being used.
[edit] Environmental effects of durable products
Peer-reviewed
research published in scholarly journals has established that the
increases in lifetimes of articles coated with DLC that wear out
because of abrasion can be described by the formula f = (g)µ, where g
is a number that characterizes the type of DLC, the type of abrasion,
the substrate material and μ is the thickness of the DLC coating in
μm.[8] For "low-impact" abrasion (pistons in cylinders, impellers in
pumps for sandy liquids, etc.), g for pure ta-C on 304
stainless steel is 66. This means that one-μm thickness (that is ~5%
of the thickness of a human hair-end) would increase service lifetime
for the article it coated from a week to over a year and two-μm
thickness would increase it from a week to 85 years. These are measured
values; though in the case of the 2 μm coating the lifetime was
extrapolated from the last time the sample was evaluated until the
testing apparatus itself wore out.
There are environmental
arguments that a sustainable economy ought to encourage articles not
engineered to lower performance or to fail prematurely. This in turn
will reduce the need to support greater production of units and their
frequent replacement, which might provide an economic disincentive to
manufacturers of such devices.
Currently there are about 100
outsource vendors of DLC coatings that are loaded with amounts of
graphite and hydrogen and so give much lower g-numbers than 66 on the
same substrates.
引用出處：
http://en.wikipedia.org/wiki/Diamond-like_carbon
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