Coating processes – Coating by vapor – gas – or smoke
Reexamination Certificate
1999-08-17
2001-05-15
Meeks, Timothy (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
C427S251000, C427S255500, C118S730000
Reexamination Certificate
active
06231923
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention relates to means for producing monolithic ceramic parts during the chemical vapor deposition (CVD) process in a shape more closing approximating the desired final shape of a discreet part, and more particularly to the use of multiple mandrel substrates mounted on rotating shafts in a CVD furnace.
2. Background
The current technology of producing monolithic ceramic parts via the chemical vapor deposition (CVD) process consists of producing large sheets of the ceramic material in the CVD furnace, from which the final ceramic part or parts are then machined. The machining process includes cutting the rough shape out of a large sheet, grinding the piece to near thickness to produce a “blank” approximating the final form but with surplus material thickness on each face, and then machining the blank to the dimensions of the final form.
When CVD materials are deposited on large flat substrates, the material thickness profile is typically non-uniform. The CVD process time must be increased to bring the low deposition rate areas up to the minimum thickness requirements for the desired parts. The higher deposition rate areas then cause the pieces cut from the sheet to require more machining time in order to be ground to the required thickness.
There is limited flexibility in the geometry of a sheet and the pieces to be cut, so there is material waste due to the layout pattern of parts. There is also surplus material in the piece pattern that is cut or ground from between and within the pieces to make the blanks, such as edges and centers of rings, which is scrapped. Multiple depositions are required for some forms and shapes.
There are also occasional problems with cracking of a large, CVD-produced plate or sheet of ceramic material during the CVD process, which can reduce the yield significantly. The combination of cracks, the scrap material from the sheet and the material that is ground off the blanks lowers the average raw material-to-product conversion ratio significantly.
SUMMARY OF THE INVENTION
The invention, stated in the simplest terms, is a method and apparatus for producing near net shaped monolithic ceramic parts by chemical vapor deposition, where the resultant parts consume less deposit material and require less machining than previous methods and set ups. Double sided mandrel substrates are closely designed for producing a specific, near net shaped final part, ring parts being especially suitable for production by this methodology. Disks, rings, spheres, and cylindrical parts are produced using a rolling point of contact during the deposition that enables the entire surface area of the substrate to be coated in one cycle.
Several of the ring or disk shaped substrates are loosely suspended through their centers in individual, spaced grooves on one or more rotable shafts. The shafts are incorporated into the setup tooling for installation in the chemical vapor deposition furnace. The tooling is installed and configured so that the majority surface areas of the substrates are oriented parallel to the flow pattern of reactant gases. The shafts are rotated so that the mandrel substrates roll freely over the rolling shafts within the vapor flow so as to receive a uniformly distributed ceramic deposit in the near net shape of the final part.
The oversize center of the substrate creates the ability to have only a line or two point contact between the substrates and the support shaft. this line is moving along the substrates and therefore gives the ability to deposit on the full surface area, including the inside diameter, of the substrates without leaving “support marks” or uncovered areas. This allows deposition of parts ready for final machining, in one deposition cycle.
The apparatus used in the process of the invention is a furnace system, configured to produce CVD ceramic materials in the manner of the prior art, but with modified internal deposition tooling and with modified process conditions.
The CVD furnace system consists of a water cooled furnace chamber and covers, a vacuum pumping system in the cases where the process is conducted at lower than ambient pressure, a process gas preparation and distribution system, a heating element, power supply, thermal insulation, process controller, and an effluent scrubbing system.
The internal tooling consists of a square or rectangular enclosure, a rotation shaft or shafts that are supported by and passing through two walls of the enclosure, multiple mandrel substrates supported along the shaft, gear system, a rotary seal feed through port, and a mechanical rotation system for driving the shaft and gear system.
The shaft or shafts are configured with spaced apart grooves that may be square, radius, or v-groove in cross section, depending on the geometry of the edge or inner diameter of the mandrel substrate. The groove keeps the substrate in position on the shaft and adequately spaced from adjacent substrates. The shape and depth of the groove is sufficient to accommodate a fully deposited substrate.
The substrates are machined to size and include the negative shape of the features of the final part. The substrates rotate with the shaft.
The enclosure, shafts, substrates, and gear system are commonly fabricated or incorporate graphite, although other materials can be used. Substrates of substantially different sizes, in respect to outside diameter and inside diameter and detail, can be combined in the same batch, increasing the flexibility of the batch.
The function of the apparatus used to practice the invention is as follows. The design of the substrate tooling provides a total deposition surface area for the batch being processed, that is considerably greater than the deposition area of the current technology. By using multiple shafts loaded with substrates, more product sites can be utilized per deposition cycle. A similar amount of reactant materials will produce more usable solid material in the near net shape form.
Also, the rotating shafts and orientation of the substrates parallel to the reactant gas flow, promotes a more uniform rate of deposit over all surface areas, and therefore decreases the CVD process cycle time. The combination of a near net shape part and the uniform deposit thickness also decreases the machining operations needed to achieve a final part. By providing near net shape parts and eliminating some machining operations, the product yield is increased, and the total time and cost is decreased.
The method of the invention is as follows. The furnace is loaded with the internal tooling set up, sealed, evacuated, inerted, and is heated up to the deposition temperature. The reactant gases are injected into the heated deposition isolation box. The process controller controls the temperature, pressure, and reactant gas flows inside the furnace. As the mandrel substrates are rotating on the shaft, the reactant gases flow in parallel to the mandrel substrates, and deposit solid ceramic material on the entire exposed surfaces.
When the deposition is complete, the system is flushed, cooled, and unloaded. The mandrel substrates are removed from the shaft. The solid ceramic material is then separated from the substrate, depending on the application. The near net shape part is then machined to produce the final part.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein we have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by us in carrying out our invention.
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Burns Lee Erich
MacDonald James C.
Teverovsky Alexander
Asmus Scott J.
Maine Vernon C.
Meeks Timothy
Tevtech LLC
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