 |
Description  |
|
|
The present invention relates to articles which are coated with pyrolytic
carbon, and more particularly, it relates to methods of coating articles
with pyrolytic carbon and to the resultant products of such methods.
It is already known to coat articles with pyrolytic carbon for a variety of
purposes. Generally, the coating process is carried out by the deposition
onto the surfaces of the articles of carbon that is formed by the high
temperature decomposition (pyrolysis) of combination substances, such as
volatile gaseous hydrocarbons. For example, U.S. Pat. No. 3,677,795, which
issued on July 18, 1972 to Jack C. Bokros and Willard H. Ellis, teaches
the making of prosthetic devices having coatings of pyrolytic carbon. U.S.
Pat. No. 3,676,179, issued on July 11, 1972 to Jack C. Bokros, teaches the
making of pyrolytic-coated articles having high strength characteristics
by methods using the high temperature decomposition of a hydrocarbon or
the like. A preferred method of coating disclosed in the foregoing patents
involves the employment of a fluidized bed in which the hydrocarbon, or a
mixture of the hydrocarbon with a carrier gas, is utilized to levitate the
particle bed and the articles being coated.
It is an object of the present invention to provide pyrolytic carbon-coated
articles having improved wear resistance. A further object is to provide
improved pyrolytic carbon-coated articles that are designed to withstand
concentrated wear. Another object is to provide an efficient and effective
method for producing pyrolytic carbon-coated articles having improved wear
resistance.
Generally, in accordance with the present invention, it is found that the
wear resistance of a pyrolytic carbon-coated article is markedly improved
if a thin layer or band is created near the outer surface which is made up
of an alloy of pyrolytic carbon plus a substantial amount of a carbide
additive which has good frictional wear characteristics. For example, the
provision of a layer made up of an alloy containing a substantial amount
of silicon carbide in a thickness as small as about 0.001 inch has been
found to markedly increase the resistance to frictional wear. It is also
important that there be some transition between the main pyrolytic carbon
portion of the coating and the wear resistant layer so there is no
interface appearing between the wear-resistant layer and the remainder of
the coating that could have a detrimental effect on the strength of the
composite article, as by causing the relatively thin layer to crack and/or
become unbonded. Because pyrolytic carbon is disposed exterior of the
wear-resistant layer, it reinforces the thin layer and presents an
exterior surface that is primarily pyrolytic carbon and is thus more
biocompatible.
It has been found that improved wear resistance is provided in a pyrolytic
carbon-coated article by creating, in a region adjacent the surface
thereof, a layer of an alloy having a high percentage of an additive
carbide that exhibits such wear-resistant characteristics. To achieve this
purpose, a metal or metalloid additive element is chosen which will form a
carbide having such characteristics and which will alloy with the
pyrolytic carbon. As pointed out in the aforementioned patents, it is
known to alloy pyrolytic carbon coatings with such carbide-forming and
alloying elements in relatively minor amounts in order to increase the
overall structural strength of the resultant pyrolytic carbon coating.
These additives may be similarly employed in such minor amounts throughout
the main portion of the pyrolytic carbon coating in respect of the present
invention. If it is desired to so alloy the main portion of the pyrolytic
carbon coating, an additive will usually be chosen that is suitable for
employment in a wear-resistant layer as well as for increasing structural
strength generally. However, it is conceivable that a change in additive
element could be effected when the wear-resistant layer is deposited.
Silicon is one example of an additive which may be so employed as it
provides increased general structural strength when used in minor amounts
and excellent wear resistance when provided in substantial proportions,
and silicon is the preferred additive element. Examples of other
carbide-forming elements which might be used as additives for increasing
the structural strength of the main portion of the pyrolytic carbon
coating include boron, hafnium, molybdenum, niobium, tantalum, titanium,
tungsten, and zirconium. Such an additive element is generally used in an
amount of about 10 to 15 weight percent to provide the desired increase in
structural strength.
For the wear-resistant layer alone, silicon is also the preferred element;
however, other carbide-forming metals and metalloids which produce a
carbide exhibiting good resistance to frictional wear and which will alloy
with carbon may also be used. Generally, elements from the foregoing group
which provide carbides having good hardness may be used. To obtain the
desired wear resistance, the additive is employed in an amount sufficient
to provide an alloy which contains between 25 and about 80 volume percent
of carbide, and preferably not more than about 50 volume percent. Optimum
amounts may vary slightly depending upon the element chosen and even with
regard to the ultimate use of the coated article; however, such amounts
can readily be determined by empirical methods. For example, a
wear-resistant layer may preferably contain silicon carbide in an amount
between about 30 and about 40 volume percent.
It is considered that the thickness of such a wear-resistant layer should
be at least about 0.001 inch. Although thicker layers can be employed, the
use of a layer thicker than about 0.003 inch is not considered to be
justified from the standpoint of improved wear-resistance. Usually a
thickness greater than 0.01 inch would not be employed for this purpose;
however, in some instances it may be advantageous to use the high carbide
content wear-resistant material in depositing the entire portion of the
coating interior of the exterior pyrolytic carbon layer. Redundancy in
this coating may also be of some value, as it is in other structural
applications. Accordingly, it may be desirable to provide a pair of spaced
wear-resistant layers, each about 0.001 inch in thickness, separated by a
layer of pyrolytic carbon containing a lesser amount of the alloying
carbide, which separating layer might be between about 0.001 and about
0.01 inch in thickness and should be at least 0.0005 inch thick.
As previously indicated, it is desirable that an interface adjacent the
wear-resistant layer be avoided inasmuch as the inclusion of such an
interface might have an adverse effect on the overall strength and
integrity of the coating. The amount of the additive element that is
supplied to the depositon zone is preferably gradually increased, either
from zero or from the minor amounts being used throughout the pyrolytic
carbon coating, until the amount desired for the wear-resistant layer is
reached. Operation in this manner assures that the desirable transition to
the wear-resistant layer is provided. A similar gradual decrease
preferably is employed upon the conclusion of deposition of the layer. It
is believed that the regions adjacent the wear-resistant layer, for a
distance of about 20 to 90 percent of the thickness of the layer, should
contain amounts of the additive carbide to such an extent that the
additive carbide is present in an average amount at least about 25 percent
of the amount in which it is employed in the wear-resistant layer and
preferably about 50 percent. These border regions should preferably be at
least about 0.0005 inch thick. It is also possible to create the layer by
increasing the amount of additive element to a peak and then similarly
decreasing the composition so long as the result provides a central band
at least about 0.001 inch thick having at least about 30 volume percent of
carbide.
The articles which are provided with these improved pyrolytic carbon
wear-resistant coatings may have any reasonable shape. However, the
invention is of particular advantage with respect to articles which will
be subjected to concentrated wear in particular locations. For example,
they may be rods or cylinders that rotate or that are subject to relative
motion axially thereof. Likewise, the invention is very effective in
protecting complex shapes that will be subject to relative motion where
the primary amount of wear will occur only in a few selected locations, as
for example, in the case of a disc-type occluder for an artificial heart
valve.
By the provision of a zone of pyrolytic carbon exterior of the
wear-resistant layer, the article presents the outward appearance of a
pyrolytic carbon-coated article which is important for biocompatibility.
As eariler indicated, any distinct interface is avoided by gradually
decreasing the amount of the additive element provided to the reaction
zone while the region just exterior of the wear-resistant layer is being
deposited. The overall article has the surface characteristics of a
pyrolytic carbon-coated article whereas, in the region very close to the
surface, preferably within about 0.003 inch thereof, although in some
instances the distance may be 5 mils or more, there is provided this
improved wear-resistant layer. Accordingly, although pyrolytic carbon
itself exhibits good resistance to frictional wear, extended operation
under concentrated wear conditions may cause the surface zone to be worn
away in certain locations, in which instance, the wear-resistant layer
will effectively halt further erosion in these regions of greatest
frictional wear.
Accordingly, the invention provides a coating for an article to be
protected which exhibits the desirable properties of pyrolytic carbon,
either unalloyed or alloyed with slight amounts of a strength-improving
carbide. However, disposed slightly below the exterior surface, there is
provided a thin wear-resistant layer which substantially improves the
performance of such an overall coated article in applications where it
will be subject to repeated relative motion and therefore frictional wear.
The effective life of the coated article in such applications is markedly
extended.
Because it is the objective that the coated article exhibit good structural
strength and wear-resistance, the pyrolytic carbon employed should
complement this objective. The density of the pyrolytic carbon deposited
should be at at least about 1.7 g/cm.sup.3, and usually the density will
be 1.8 g/cm.sup.3 or greater. The overall thickness of the pyrolytic
carbon coating should be thick enough to provide overall strength in a
generally monolithic structure. The overall thickness of the coating will
be at least about 0.006 inch, with the interior pyrolytic carbon layer
being at least about 0.003 inch and for many applications, the thickness
will be at least about 10 mils, with 15 mils often being an average
thickness. Either laminar or isotropic carbon may be employed to produce
coatings having good structural strength; however, isotropic carbon is
preferred because it exhibits no tendency to delaminate and because
stresses due to anisotropic expansion do not arise in isotropic coatings
on irregular shapes at locations where small radii of curvature are
encountered. Isotropic pyrolytic carbon having an apparent crystallite
size of about 50A or less and which is deposited at temperatures of about
1500.degree.C. and below is generally preferred.
The following Examples illustrate preferred processes for making
wear-resistant articles coated with pyrolytic carbon and the preferred
method of operation in a fluidized bed at a temperature at about
1500.degree.C. or below. The pyrolytic carbon is deposited from a mixture
of hydrocarbon and inert gas, and the additive element is conveniently
provided in the deposition zone by bubbling the inert gas portion of the
supply stream through a suitable reservoir containing the element as a
part of a volatile compound. Operation in this manner provides a
convenient way of changing the amount of the additive element being
supplied by simply altering the amount of inert gas flow through the
reservoir. The volatile compound can be injected directly into the mixed
gas stream for it will be vaporized quickly at the temperatures in the
deposition zone. A temperature at least above 1000.degree.C. is
contemplated. Although the illustrated processes employ propane as the
hydrocarbon in the temperature range of about 1350.degree. -
1400.degree.C., it should be understood that other hydrocarbons can be
employed and that higher temperatures may also be used. For example,
methane may be employed at a temperature of 1800.degree. to 2000.degree.C.
to deposit dense isotropic pyrolytic carbon.
EXAMPLE I
A wire strut approximately 0.03 inch in diameter is levitated in a vertical
graphite reaction tube together with 100 grams of zirconium dioxide
particles, having an average particle size of about 400 microns, which
provide additional depositon surface area. The strut is made of an alloy
of molybdenum and rhenium and is designed for use as a part of a heart
valve which employs a disc occluder. The strut and particles are heated to
a temperature of about 1350.degree.C. while maintaining a flow of helium
gas upward through the 3.5 inch diameter tube.
Propane gas is admixed with the helium to provide a total gas flow of about
18 liters per minute, having a partial pressure of propane of about 0.4
atmosphere (total pressure of 1 atmosphere). All of the helium is bubbled
through a reservoir containing methyltrichlorosilane. The propane and the
methyltrichlorosilane pyrolyze in the reaction zone and deposit onto the
strut as isotropic carbon containing a minor amount of silicon carbide
dispersed therein as an alloy. Deposition is continued until a coating
about 7 mils (0.007 inch) thick is obtained, a time of about 30 minutes.
At this time, the deposition of the wear-resistant layer is ready to begin,
and the propane flow is reduced to 3600 cc. per minute while the flow of
helium is increased by 4000 cc. per minute, so the total flow is
approximately 18.4 liters per minute, with all of the helium being bubbled
through the methyltrichlorosilane. The change takes place over about 10
seconds. Because of mixing of the gas throughout the system and the
deposition zone, the silicon carbide content being codeposited
continuously increases to the maximum. Once this condition is reached,
coating is continued for about 5 minutes, after which time a return to the
original deposition conditions is effected by reversing the
above-indicated procedure, and the coating is then continued for about 15
minutes. The coated strut is allowed to cool to about ambient temperature
in a levitating flow of helium alone before being removed from the
reaction tube.
Examination of the coated strut shows that the major portion of the coating
is isotropic pyrolytic carbon having about 10 weight percent of silicon
dispersed therein in the form of silicon carbide, as an alloy, and that
the coating has a density of about 2 grams/cm.sup.3. The pyrolytic carbon
is the continuous phase of this alloy with the silicon carbide being
present as the dispersed phase. The wear-resistant layer is about 0.002
inch in thickness with the zone bordering this layer where the transition
in the composition of the deposition atmosphere took place, being about
0.0005 inch in thickness. The exterior pyrocarbon coating is about 3 to 4
mils thick.
Testing of a strut made by the process described above shows that it
exhibits excellent resistance to frictional wear and that, in all other
respects, the coated strut performs in essentially the same manner as a
strut coated entirely with pyrolytic carbon of the type initially
deposited thereon. Because of its shape, the wear on such an article is
concentrated along a line near its base where it comes in contact with the
disc occluder.
EXAMPLE II
A generally disc-shaped item made of graphite and designed to serve as an
occluder in a heart valve, which has a major dimension of about 1 inch and
a maximum thickness of about 0.15 inch, is introduced into the same
reaction tube employed in Example I, together with a similar charge of 100
grams of zirconium oxide particles. The disc, particles and the reaction
zone of the tube are heated to a temperature of about 1400.degree.C. while
a flow of helium gas is maintained therethrough to levitate the disc and
particles.
Thereafter, propane is admixed with the helium to provide an atmosphere in
the reaction zone having a partial pressure of propane of about 0.4
atmosphere (total pressure of one atmosphere). The total gas flow is
maintained at about 20 liters per minute, and the propane undergoes
pyrolysis and deposits isotropic pyrolytic carbon. Deposition is continued
until an isotropic pyrolytic carbon coating about 10 mils thick is
obtained, a time of about 40 minutes.
At this time, a gradually increasing portion of the helium flow is bubbled
through a reservoir of methyltrichlorosilane, as in respect of Example I.
After about a time period of 5 minutes, all of the 12 liters per minute
flow of helium is passing through the methyltrichlorosilane. Thereafter,
while maintaining the total flow at about 20 liters per minute, the flow
of propane is decreased by 1000 cc. per minute every three minutes while
the flow of helium is simultaneously proportionally increased until the
flow constitutes 4000 cc. of propane and 16,000 cc. of helium per minute.
This condition is maintained for about three minutes while a
wear-resistant layer is deposited.
At the end of this time period, the foregoing steps are reversed until
original conditions of 8 l./min. of propane and 12 l./min. of helium
(without any bubbling through the methyltrichlorosilane) are achieved. The
total time during which methyltrichlorosilane is introduced is about 31
minutes. Thereafter, coating is continued until an outer substantially
pure pyrolytic carbon layer about 0.008 inch thick is deposited and
deposition is halted. The article is cooled, removed and tested as in
respect of Example I.
Examination shows that the pyrolytic carbon has a density of about 1.9
grams/cm.sup.3, a BAF of about 1.1 and an apparent crystallite size of
about 40A. The BAF is an accepted measure of preferred orientation of the
layer planes in the carbon crystalline structure. The technique of
measurement and a complete explanation of the scale of measurement is set
forth in an article by G. E. Bacon entitled "A Method for Determining the
Degree of Orientation of Graphite" which appeared in the Journal of
Applied Chemistry, Volume 6, page 477 (1956). For purposes of explanation,
it is noted that 1.0 (the lowest point on the Bacon scale) signifies
perfectly isotropic carbon. The overall wear-resistant layer includes a
band about 0.007 inch in thickness, wherein the centermost region contains
about 30 volume percent silicon carbide and 70 volume percent carbon. The
transition zones between the wear-resistant layer and the pure pyrolytic
carbon coating regions each measure about 0.003 inch in thickness and show
a gradual continuous increase (or decrease) in silicon carbide volume
percentage.
Actual testing of the article under simulated use conditions shows that it
exhibits excellent resistance to wear. In the regions where the maximum
concentrated wear occurs, although the exterior pyrolytic carbon layer is
very gradually worn away, the wear-resistant rich silicon carbide layer
proves excellently wear-resistant and maintains its integrity.
Although the invention has been described particularly with regard to
certain preferred embodiments, it should be understood that modifications
and changes as would be obvious to one having the ordinary skill in the
art may be made without departing from the scope of the invention which is
defined in the appended claims.
Various of the features of the invention are defined in the claims which
follow. What is claimed is:
* * * * *
|
|
 |
|
 |
Description |
|
