BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention pertains to precision coating techniques, or more particularly to chemical vapor deposition of materials uniformly on spherical objects. Still more particularly, the invention provides a method of uniformly depositing rhenium metal onto a light weight carbon ball. Such find use in valves for high temperature, corrosive environments such as are used in rocket exhaust gas steering valves.

DESCRIPTION OF THE PRIOR ART It has long been a problem in the art to apply a precise coating of a material onto a spherical object.

In the prior art, when a sphere is to be coated, it must either rest on a platform or be supported in a mounting fixture during the coating operation. It is inevitable that some small portion of the sphere either remains uncoated or is irregularly coated. In many industrial applications, such as in high temperature valves, these coating imperfections cannot be tolerated. One solution has been to apply an excess of the coating material and then remove the surplus in a

finishing operation. However, this method still results in a non-uniform coating. Currently, coating thickness variations must be machined to set the sphere diameter into desired specifications. This additional operation is costly and time consuming. One solution has been proposed by U. S. patent 5,098,483 wherein a sphere is randomly rotated within a concave fixture while a chemical vapor deposition or other coating treatment is conducted. A disadvantage of this approach is that at any given instant, the sphere is always in contact with the fixture, thus risking localized imperfections at the contact points. The present invention provides an improved method of coating spherical objects by levitating the sphere over an orifice by pressure from a stream of a gas which includes the material to be deposited on the sphere. The invention finds particular use for coating a carbon ball with rhenium metal in a chemical vapor deposition process.

Chemical vapor deposition is a process that uses the reaction of gaseous precursors on a surface to produce a solid coating. Generally, the method is capable of producing films of uniform thickness on surfaces of complex shapes. However, as previously mentioned, uniform coatings on spheres provide a unique problem. If the spherical substrate is held stationary in a chemical vapor deposition gas steam, the coating will be thin or nonexistent at the points of contact between the sphere and the fixture. In addition, nonuniformities in the gas flow pattern, such as eddy currents on the downstream side of the sphere, will result in the deposition of nonuniform thickness around

the circumference of the ball. For many applications, such as with ball bearings, these nonuniformities can degrade the performance of the coated component.

Mechanical methods have been used to rotate spherical components in a chemical vapor deposition gas stream, but these methods can be difficult to implement. For chemical vapor deposition conditions in high temperature and corrosive atmospheres, mechanical agitation of the sphere can damage the surface of the growing film. It has now been found that by vertically levitating and randomly rotating a carbon ball during chemical vapor deposition one can produce highly uniform films with controlled thickness. Thickness control and surface finish uniformity are such that no further machining of the coating is required. By placing the sphere to be coated into a tapered fixture with gas supply attached, it can be lifted from contact with the taper by flowing a gas up through the bottom of the taper. The gas flow volume can be adjusted to give a random motion to the ball, or flutes can be machined into the tapered surface in order to impart a predetermined rotation to the sphere. The method combines chemical vapor deposition with levitation by a gas stream to deposit uniform coatings on spherically shaped objects such as ball bearings and the like. A tapered fixture containing the ball to be coated is mounted in a vertical chemical vapor deposition reactor, such as a quartz tube, which is surrounded by a furnace to maintain the ball at the required temperature for the chemical vapor deposition process.

The chemical vapor deposition reactant gases are added to the gas stream used to levitate that ball, and form

a uniform film deposit on the ball as they flow past it through the tapered holder. Generally an inert gas, such as argon, is used to levitate the ball. However, the chemical vapor deposition gases themselves may be sufficient for levitation, and the addition of an inert gas would not be required. The fixture mounted over a CVD reactor gas nozzle and a gas flow is provided sufficient to raise the sphere. The gas flows are sufficient to raise the ball off of the fixture and induce rapid rotation of the ball. The ball rotation ensures a uniform deposit on the sphere circumference.

SUMMARY OF THE INVENTION The invention provides an apparatus and method for uniformly coating a sphere. The apparatus includes a levitation fixture disposed in a vacuum chamber with the sphere mounted on top of the fixture. The fixture receives a flow of two gasses. One gas is inert and lifts the sphere away from the fixture and the other gas contains the coating. A reactant gas is introduced into the chamber so that in the presence of heat, the reactant gas reacts with the gas containing the coating so that the coating is deposited on the sphere.

BRIEF DESCRIPTION OF THE DRAWINGS The sole figure is a schematic representation of a chemical vapor deposition levitation reactor according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred embodiment, the apparatus and method of present invention concerns chemical vapor deposition of fine grained rhenium on carbon spheres.

The coated sphere is light weight, and resistant to high temperatures, corrosive gases, erosion and oxidation. Uniform, fine grained, ductile rhenium having a small grain size is deposited which resists fracture and crack formation produced by differences in rhenium and carbon substrate thermal expansions. In the preferred embodiment, rhenium metal is coated onto a carbon sphere by a chemical vapor deposition from rhenium hexafluoride. Chemical vapor deposition reactors are well known in the art. A suitable reactor is a hot wall CVD reactor which comprises a 4 inch diameter quartz tube surrounded by an external furnace.

One suitable reactor is commercially available from Vactronic Equipment Labs of Bohemia, New York. The sphere is a substantially pure carbon such as AXF-5Q or ZXF-5Q commercially available as from Poco Graphite, Inc. of Decatur, Texas. Such a commercially available unit may be modified by providing a sphere retaining fixture on a gas inlet, and regulating a gas supply to provide sufficient continuous pressure to raise and rotate the sphere above the fixture while injecting rhenium hexafluoride and reducing hydrogen to cause a uniform coating of the sphere with rhenium metal under appropriate vacuum and temperature conditions.

The sole figure shows an apparatus according to the present invention. The apparatus includes a gas

supply means such as tubes 2 and 14. Tube 2 introduces gasses through through the vertically oriented levitation fixture 4 while tube 14 introduces to the annular region in a quartz vacuum chamber 8 surrounding the levitation fixture 14. Means are provided for flowing a stream of gas, preferably argon, upwardly through the levitation fixture 4 sufficient to levitate and rotate a sphere 6 above the fixture when a stream of gas is propelled through the fixture. Such means may be a simple gas supply and regulatory means such as gas flow controllers 16,18, and 20 that are well known in the art. The gas flow volume can be adjusted to give a random motion to the ball, or flutes can be machined into the tapered surface of the levitation fixture in order to impart a predetermined rotation to the sphere. The sphere is repositioned on the orifice by gravity when the stream of gas is disengaged. Means are provided for uniformly applying a coating composition to the sphere while the sphere is being levitated and rotated by the propelled stream of the gas. In the preferred embodiment, the coating composition is supplied as rhenium hexafluoride which is chemically reduced by a supply of hydrogen such that metallic rhenium is coated onto the sphere. In the preferred embodiment, both the argon gas used to levitate the sphere and the rhenium hexafluoride reactant gas are supplied through tube 2 and the levitation fixture 4. The levitation fixture 4, and sphere 6 are disposed in a chemical vapor deposition device which includes the quartz vacuum chamber 8. The separate gas supply tube 14 is connected to a hydrogen gas supply though the gas flow controller 20. The

vacuum chamber is provided with suitable pressure control valves 10 and vacuum pumps, not shown. The vacuum chamber 8 is disposed in furnace 12.

In the process, a carbon sphere 6 is placed in the vacuum chamber on levitation fixture 4. The chamber is sealed and evacuated to less than one millitorr of ambient background gas pressure. In the preferred embodiment, a flow of an inert gas, such as argon is established through tube 2 and the chamber is heated until it is stabilized to the desired reaction temperature and gas flow rate. The use of argon is preferred since it has been found to increase the ductility of the rhenium film by reduction of fluorine containing impurities in the grain boundary regions.

The argon gas flow ranges from 0 to about 5,000 sccm (standard cubic centimeters per minute measured at 0°C and atmospheric pressure) or preferably about 1,000 sccm. The furnace establishes a sphere temperature ranging from about 200 °C to about 1,150 °C and more preferably from about 450 °C to about 1050 °C and most preferably about 950 °C. The vacuum chamber is capable of establishing pressures ranging from about 0.1 torr to about 760 torr, more preferably from about 0.2 torr to about 400 torr and most preferably from about 0.25 to about 2.0 torr. After the temperature has stabilized, a hydrogen flow is introduced through tube 14 while maintaining a constant pressure. Sufficient hydrogen gas must be delivered to chemically reduce rhenium hexafluoride to rhenium metal. This requires at least 3 moles of hydrogen gas per mole of rhenium hexafluoride. Preferably hydrogen is introduce in a

large excess such as 10 or 20 moles of hydrogen per mole of rhenium hexafluoride. The hydrogen gas flow ranges from about 10 to about 1000 sccm or preferably from about 50 to about 500 sccm and most preferably about 500 sccm. After the hydrogen flow has stabilized, a rhenium hexafluoride flow is introduced through tube 2 combining with the argon stream while still maintaining a constant pressure to initiate the rhenium deposition. The rhenium hexafluoride gas flow ranges from about 1 to about 100 sccm or preferably from about 5 to about 20 sccm and most preferably about 10 sccm. The rhenium hexafluoride gas flow is a small fraction of the argon gas flow and does not perturb the levitation process that was established by the argon flow. The gas flow controllers 16,18 and 20 allow the individual gas flow to be controlled independently to regulate both the levitation and chemical vapor deposition processes. Physical separation of the rhenium hexafluoride and hydrogen gas flows prevents deposition of the rhenium metal except in the mixing zone around the levitated sphere. As such, the deposition is localized to the desirable area of the sphere and reduces loss of the rhenium metal to extraneous portions of the reactor. This configuration of gas streams also prevents the deposition of rhenium metal inside of levitation fixture 4 that can alter or block the gas flow into the fixture after sufficient rhenium metal build up thereby halting the deposition process. After the deposition process has run for the desired time, the rhenium hexafluoride and hydrogen gas flows are stopped and the sample is returned to room temperature while maintaining the argon flow to

maintain a constant gas pressure. After the sample has reached room temperature, the argon flow is stopped, sphere 6 is returned to levitation fixture 4 by gravity and the rhenium coated carbon sphere is removed from the vacuum chamber. Preferably the deposited rhenium has a thickness of from about 0.1 micrometer to about 5 mm on the substrate, more preferably from about 0.5 to about 5000 micrometers and most preferably about 500 micrometers. The foregoing flow conditions are suitable for a 4 inch diameter reactor. Flow conditions depend on the reactor diameter and a larger reactor will require greater flows. Average grain diameters of from about 0.1 to about 25 micrometers are obtained by the present invention. More preferably, the grain diameters range from about 1 to about 10 micrometers. It is within the contemplation of the invention that nonspherical objects can also be levitated for coating. Such can be accomplished by adding a bump rail or a cage structure around the fixture. The following non-limiting examples serve to illustrate the invention.

EXAMPLE 1 A spherical carbon ball that has been machined from Poco Graphite AXF-5Q or ZXF-5Q with a density of 1.8 grams/cc and total weight of 2.2 grams, is placed in a tapered fixture inside a quartz reaction chamber.

The reactor is sealed and evacuated to <0. 001 torr residual gas pressure. An argon flow of 500 sccm is initiated through the tapered fixture which is sufficient to raise the carbon ball out of physical

contact with the fixture. The pressure inside the reactor is set to 2.0 torr by means of an automatic control valve located between the reactor exit and a vacuum pump. The temperature of the carbon ball is raised to approximately 900°C by means of an external furnace that surrounds the quartz chamber. After the temperature has stabilized at approximately 900°C, a hydrogen flow of 500 sccm is added to the argon flow while maintaining the reactor pressure at 2.0 torr.

After stabilization of the hydrogen flow, a flow 10 sccm of rhenium hexafluoride is added to the argon and hydrogen flows while maintaining the 2.0 torr pressure.

These conditions are maintained for 2.5 hours, after which the rhenium hexafluoride and hydrogen flows are stopped and the furnace is turned off. After the sample has cooled to room temperature, the reactor is opened and the coated ball removed. Measurement of the coated ball diameter at random points on the ball, and the measured weight increase indicate a uniform coating of rhenium has been applied to a thickness of 75 micrometers.

EXAMPLE 2 A spherical carbon ball that has been machined from Poco Graphite AXF-5Q or ZXF-5Q with a density of 1.8 grams/cc and total weight of 2.2 grams, is placed in a tapered fixture inside a quartz reaction chamber.

The reactor is sealed and evacuated to < 0.001 torr residual gas pressure. An argon flow of 500 sccm is initiated through the tapered fixture which is sufficient to raise the carbon ball out of physical

contact with the fixture. The pressure inside the reactor is set to 2. 0 torr by means of an automatic control valve located between the reactor exit and a vacuum pump. The temperature of the carbon ball is raised to approximately 950°C by means of an external furnace that surrounds the quartz chamber. After the temperature has stabilized at approximately 950°C, a hydrogen flow of 500 sccm is added to the argon flow while maintaining the reactor pressure at 2.0 torr.

After stabilization of the hydrogen flow, a flow 10 sccm of rhenium hexafluoride is added to the argon and hydrogen flows while maintaining the 2.0 torr pressure.

These conditions are maintained for 1 hour, after which the rhenium hexafluoride and hydrogen flows are stopped and the furnace is turned off. After the sample has cooled to room temperature, the reactor is opened and the coated ball removed. Measurement of the coated ball diameter at random points on the ball, and the measured weight increase indicate a uniform coating of rhenium has been applied to a thickness of 32 micrometers.

EXAMPLE 3 A spherical carbon ball that has been machined from 5 Poco Graphite AXF-5Q or ZXF-5Q with a density of 1.8 grams/cc and total weight of 2.2 grams, is placed in a tapered fixture inside a quartz reaction chamber.

The reactor is sealed and evacuated to < 0.001 torr residual gas pressure. An argon flow of 500 sccm is initiated through the tapered fixture which is sufficient to raise the carbon ball out of physical

contact with the fixture. The pressure inside the reactor is set to 2.0 torr by means of an automatic control valve located between the reactor exit and a vacuum pump. The temperature of the carbon ball is raised to approximately 950°C by means of an external furnace that surrounds the quartz chamber. After the temperature has stabilized at approximately 950°C, a hydrogen flow of 500 sccm is added to the argon flow while maintaining the reactor pressure at 2.0 torr.

After stabilization of the hydrogen flow, a flow 10 sccm of rhenium hexafluoride is added to the argon and hydrogen flows while maintaining the 2.0 torr pressure.

These conditions are maintained for 20 minutes, after which the rhenium hexafluoride and hydrogen flows are stopped and the furnace is turned off. After the sample has cooled to room temperature, the reactor is opened and the coated ball removed. Measurement of the coated ball diameter at random points on the ball, and the measured weight increase indicate a uniform coating of rhenium has been applied to a thickness of 11 micrometers.