Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making composite or hollow article
Reexamination Certificate
2000-05-16
2001-03-20
Jones, Deborah (Department: 1775)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making composite or hollow article
C428S665000, C428S655000
Reexamination Certificate
active
06203752
ABSTRACT:
The present invention relates to coatings for tungsten-based articles. More particularly, this invention relates to rhenium coatings for tungsten-based alloy and composite articles such as tungsten alloy nozzles for drawing and extruding quartz, glass and other silica-based materials, and processes for forming rhenium-containing coatings on such articles.
BACKGROUND OF THE INVENTION
Quartz, glass and other silica-based materials are often processed in the form of rods and tubes by heating the material in a furnace and then drawing or extruding the material through a die or orifice. Because of its high temperature strength and creep resistance, tungsten has been used to form the furnace structure and components that contact silica-based materials during processing, including drawing dies and extrusion nozzles. However, tungsten tends to react with silica, generating undesirable tungsten-based inclusions and defects in the processed material, as well as leading to degradation of the tungsten-based component. Defects in materials generated by reactions with tungsten are highly undesirable for many quartz applications, particularly in the semiconductor and lamp industries. The reactivity of tungsten with silica also reduces the service life of the tungsten components.
In response, the prior art has used rhenium to protect tungsten nozzles on the basis that rhenium is less reactive with silica than tungsten. An existing method has been to form a rhenium tube by producing a blank of 90% theoretical density using a powder metallurgy (PM) process. The blank is then rolled to form a tube that can be inserted into an appropriately sized bore formed in a PM tungsten nozzle that was pre-sintered to near 100% density. After installation, the rhenium tube is diffusion bonded to the tungsten nozzle.
While being less reactive to silica-containing materials, rhenium tubes produced and installed in the manner described above are prone to cracking during use of the tungsten nozzles at high service temperatures. Accordingly, it would be desirable if improved components were available for use in the processing of silica-based products.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, a component having a tungsten-based substrate is provided whose surface is protected by a rhenium-based layer in order to render the component less reactive to quartz, glass and other forms of silica. The layer preferably consists essentially of rhenium or rhenium with alloying additions of tungsten. The substrate may be formed of concentric layers of different tungsten-based alloys or composites in order to tailor the physical and mechanical properties of the component. The rhenium layer and tungsten substrate of this invention have densities of at least 96% of theoretical, and preferably very near 100% of theoretical. Rhenium layer densities of at least 96% are preferred for this invention based on the determination that rhenium tubes of the prior art (with densities of less than 96%) were cracking as a result of the tubes continuing to sinter at high temperatures during use of the component in which they were installed. It was learned that sintering of a rhenium tube within a fully dense tungsten body caused the tube to further densify, which generated tensile stresses within the tube that led to cracking. Therefore, this invention produces a fully-sintered and dense rhenium layer on a fully-sintered and dense tungsten substrate.
In one embodiment, the rhenium layer and tungsten substrate are formed by simultaneously isostatically pressing and sintering rhenium and tungsten powders. In another embodiment the rhenium layer and tungsten substrate are formd by isostatically pressing a rhenium powder to form a rhenium preform and then isostatically pressing and sintering a tungsten powder around the rhenium preform so that the preform is a protective PM layer on a PM tungsten article. In this manner, the rhenium layer can be fully sintered to at least 96% theoretical density, and therefor much less prone to cracking when the component is exposed to high temperatures.
According to this invention, a typical bond between the rhenium layer and tungsten substrate is characterized by a reduced contact area—that is, the contact area between the layer and substrate is significantly less than the surface area of the substrate covered by the layer. A notable advantage of this bond interface is that interdiffusion between the rhenium layer and tungsten substrate is greatly reduced, so that the formation of brittle tungsten-rhenium intermetallics is inhibited. A typical method for producing a component of this type is to contact a tungsten-based substrate with a rhenium-based wire, and then heat the wire and substrate to sinter and bond the wire to the substrate. By this process, the resulting rhenium layer generally retains the macrostructure of the wire, such that the layer is segmented with distinct cross-sections corresponding in shape to the rhenium-based wire, with each cross-section being individually bonded to the substrate. A general advantage of this method of forming the rhenium layer is substantially lower processing costs as compared to more conventional methods, such as plasma spraying, physical vapor deposition (PVD), chemical vapor deposition (CVD) and electroplating. Other advantages of this invention include the ability to precisely control the thickness of the rhenium layer by appropriately sizing the wire, and the ability to selectively apply the wire to form the rhenium layer at only those locations requiring a protective coating. The resulting rhenium layer is also sufficiently machinable to enable all or part of it to be removed during subsequent fabrication or repair of the component.
Other advantages of this invention will be better appreciated from the following detailed description.
REFERENCES:
patent: 3780418 (1973-12-01), Hurst
patent: 4612162 (1986-09-01), Morgan et al.
patent: 4657822 (1987-04-01), Goldstein
patent: 4726927 (1988-02-01), Morgan et al.
patent: 5041041 (1991-08-01), Passmore et al.
patent: 5056209 (1991-10-01), Ohashi et al.
patent: 5376329 (1994-12-01), Morgan et al.
“Diffusion Layer at the Interface of W/Re-Composite Nozzles”, by Yin Weihong et al., High Temperatures-High Pressures, 1994, vol. 26, pp. 115-121 (No Month).
Bewlay Bernard Patrick
Dalpe Dennis Joseph
General Electric Company
Johnson Noreen C.
Jones Deborah
Santandrea Robert P.
Savage Jason
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