Abstract:
A furnace component having a tungsten-based substrate 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 in order to tailor the physical and mechanical properties of the component. A preferred method of forming the rhenium layer is to wrap the substrate with a rhenium-based wire, and then heat the wire and substrate to sinter and bond the wire to the substrate. Alternatively, the substrate and rhenium layer can be formed by isostatic pressing. Both methods are performed so that the substrate and layer have densities of at least about 96% of their respective theoretical densities.

Description:
This application is a division of application Ser. No. 09/204,230, filed Dec. 3, 1998, which is hereby incorporated by reference in its entirety. 
    
    
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a nozzle for extruding quartz and having a tungsten-based substrate with a rhenium-based coating in accordance with this invention; 
     FIG. 2 represents a magnified cross-sectional view of the nozzle of FIG.  1  and showing the rhenium-based coating as being formed by a rhenium-based wire bonded to the tungsten-based substrate; 
     FIGS. 3 through 5 and  6  through  7  are cross-sectional views depicting depict alternative processes for forming the tungsten-based substrate and the rhenium-based coating; and 
     FIG. 8 is a perspective view of an alternative configuration for a nozzle with axial layers of tungsten and rhenium. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a component of the type used with high temperature furnaces for processing quartz, glass and other forms of fused silica. An example is an axisymmetric extrusion nozzle  10  represented in FIG. 1 as having a generally tubular-shaped core  12 , a cylindrical external surface  14 , and an orifice  16  defining a cylindrical internal surface  18 . A quartz rod product can be drawn through the orifice  16 . Alternatively a quartz tube product can be extruded through the orifice  16  and around a mandrel (not shown) positioned within the orifice  16 . 
     The core  12  of the nozzle  10  is formed of tungsten or a tungsten-based alloy or composite, while the cylindrical interior and exterior surfaces of the core  12  are protected by inner and outer layers  20  and  22 , respectively, of rhenium or a rhenium alloy or composite. The rhenium-contianing layers  20  and  22  serve as protective coatings for the nozzle  10  to prevent reaction between the tungsten-based core  12  and quartz being processed through the nozzle  10 . While commercially pure rhenium can be used for the layers  20  and  22 , rhenium containing up to about 11 weight percent tungsten (near the solubility limit of tungsten in rhenium) may be used to reduce costs, though a disadvantage is a higher reactivity with silica. The core  12  is preferably formed by powder metallurgy, by which tungsten or a tungsten alloy powder is consolidated and sintered to 100% of theoretical density. An example of a suitable tungsten alloy contains up to 27 weight percent rhenium (near the solubility limit of rhenium in tungsten), the balance essentially tungsten and incidental impurities. Though the coefficient of thermal expansion of rhenium is lower than tungsten, rhenium is sufficiently more ductile than tungsten to accommodate tensile stresses that may be generated in the layers  20  and  22  during processing and service, so that the layers  20  and  22  can be resistant to cracking. 
     FIG. 2 represents a typical structure for the rhenium layers  20  and  22  achieved by the bonding of circular cross-section rhenium-based wire to the core  12 . As shown, the resulting rhenium layers  20  and  22  are characterized by circular segments  24  that have generally retained the cross-sectional shape of the wire. Each segment  24  is individually bonded to the core  12  by a neck formation  26 , such that the contact area between the core  12  and the layers  20  and  22  is much less (e.g., less than half as shown) of the entire surface of the core  12  covered by the layers  20  and  22 . A significant advantage of this aspect of the invention is that less interdiffusion occurs between the tungsten-based core  12  and the rhenium-based layers  20  and  22  during coating and subsequent high-temperature exposures. This less interdiffusion minimizes the formation of undesirable brittle W-Re intermetallics, such as, but not limited to, sigma (σ) and chi (χ) intermetallics, that could lead to spallation of the layers  20  and  22 . The amount of interdiffusion that will occur between the core  12  and the layers  20  and  22  is determined by the cross-sectional area of the neck formation  26 , which can be controlled by the diameter of the wire, the cross-sectional shape of the wire (e.g., circular, triangular or rectangular), and the temperature and duration of the heat treatment used to bond the wire to the core  12 . 
     In accordance with the embodiment of FIG. 2, rhenium-based wire (rhenium or a rhenium composite or rhenium alloy containing up to 11 weight percent tungsten) is tightly and closely wound onto the exterior or interior surfaces or both the exteriorand interior surfaces of the core  12  to achieve a mechanical bond with the core  12 . Typically, each wire turn intimately contacts the adjacent wire turn and the core  12 . One or more layers of wire can be simultaneously built up on the interior and exterior surfaces of the core  12  by winding the wire from the inside to the outside of the core  12  in longitudinal loops parallel to the symmetry axis of the core  12 . Alternatively, wire can be wound onto the core exterior with circumferential loops approximately perpendicular to the symmetry axis of the core  12 , and wire can be prewound and inserted into the interior of the core  12 . For the core interior, wire is typically wound on a mandrel (not shown) and then installed in the core  12  as a coil in much the same manner as a helical wire insert of the type used with threaded fasteners. The mandrel typically has a diameter slightly larger than the core interior so that, following insertion by winding the wire coil up slightly, the wire is secured by a compressive load that promotes contact with the core  12 . The bore of the core  12  can be threaded to match the pitch of the wire coil. The wire can be applied to uniformly and completely cover the interior and exterior surfaces of the core  12 . Alternatively, the wire can be applied at different axial locations along the length of the core  12  to produce a rhenium layer only at selected locations. For example, wire could be applied to only one axial end of the core  12  that in service will be subjected to higher temperatures due to a temperature profile within the component  12 . Selective placement of rhenium layers on a component also reduces material costs in view of the expense of rhenium. 
     Following application of the wire to the core  12 , the core and wire assembly undergoes a heat treatment to sinter the rhenium wire turns together to generate an integral coating and atomically bond the wire to the tungsten core  12 . A minimum heat treatment temperature is about 1800° C., with a more typical range being about 2300° C. to about 2800°C., the upper limit of which is below the eutectic temperature of rhenium and tungsten and therefore prevents melting. The rhenium layer  20 / 22  represented in FIG. 2 was formed by about 0.030 inch (about 0.75 mm) diameter rhenium wire heat treated at a temperature of about 2700° C. for about four hours, which produced the desirable bond structure between the wire and core  12  depicted in FIG.  2 . 
     From FIG. 2, it can be appreciated that the thickness of the layers  20  and  22  can be tailored by the diameter of the wire used, as well as by applying multiple layers of wire. Generally, layer thicknesses of as thin as 50 Fm and as thick as several millimeters can be produced with the technique described above. Following heat treatment, the rhenium layers  20  and  22  may be further consolidated to full density (at least about 96% of theoretical density) by hot isostatic pressing. The rhenium layers  20  and  22  may also be machined, ground or polished to improve their surface finish after heat treatment. 
     Alternative embodiments for producing the rhenium layers  20  and  22  on the tungsten core  12  are described below in reference to FIGS. 3 through 8. In FIGS. 3 through 5, a process is shown that simultaneously forms the core  12  and the rhenium layer  20  by powder metallurgy using cold isostatic pressing techniques. In FIG. 3, a flexible mold  28  is formed of urethane or another suitable material to define a cavity  30  in which the desired powders for the core  12  and layer  20  will be deposited. Centrally positioned within the cavity  30  is a mandrel  32 , around which two annular partitions  34  and  36  are positioned to define three concentric annular-shaped cavities. In one embodiment, the partitions  34  and  36  are solid wall sections that are removed prior to consolidation of the powders. To minimize intermixing of the powders, the partitions  34  and  36  are typically thin-walled to minimize the volume change that occurs when the partitions  34  and  36  are subsequently removed. Also to minimize intermixing, the partitions  34  and  36  may comprise polished surfaces to reduce friction with the powders as the partitions  34  and  36  are removed. Alternatively, the partitions  34  and  36  can be formed of a tungsten or rhenium mesh, enabling the partitions  34  and  36  to be left in place to become part of the core  12  upon consolidation and sintering of the powders. 
     In FIG. 4, the outermost annular cavity is filled with a tungsten powder  38 , the middle cavity is filled with a tungsten-rhenium alloy powder  40 , and the innermost cavity is filled with a rhenium powder  42 . A suitable mean particle size for the powders  38 ,  40  and  42  is less than about 4.5 Fm. The tungsten-rhenium alloy powder  40  provides a transition between the tungsten core  12  and the rhenium layer  20 , and reduces differences in thermal expansion coefficients and provides a reduction in thermal stresses that can be generated in thermal cycling. In FIG. 5, a mold cap  44  has been placed on the mold  28  to seal the mold  28  and sufficient pressure has been applied to compact the powders  38 ,  40  and  42 . As noted above, the partitions  34  and  36  are removed before sealing and compaction. Alternatively, the partitions  34  and  36  can be formed of tungsten or rhenium mesh and compacted with the powders  38 ,  40  and  42 . A typical consolidation process is wet-bag cold isostatic pressing at a pressure of up to about 280 MPa (about 40 ksi), in which the entire mold  28  is placed in a hydrostatic vessel filled with a hydraulic fluid, for example water, that is pressurized to compact the mold  28  and powders  38 ,  40  and  42  at room temperature, as known in the art. An alternative method is dry-bag isostatic pressing, which is also known to those skilled in the art. After consolidation, the resulting article is removed from the mold  28  and furnace presintered at a temperature of about 1000° C. to about 1400° C., machined to acquire the desired shape and dimensions for the article, and then sintered at a temperature of up to about 2450° C., preferably about 1800° C. to about 2200° C., to attain a density of at least about 96%. 
     In FIGS. 6 and 7, rhenium powder  50  is first consolidated in a flexible mold  46  with a central mandrel  48  at pressures up to about 210 MPa (about 30 ksi). The pressed rhenium article  52  and mandrel  48  are then placed in a larger diameter mold  54 . The gap between the pressed rhenium article  52  and the outer wall of the mold  54  is filled with a tungsten or tungsten alloy powder  58 . The diameters of the molds  46  and  54  are sized to accommodate the powder compaction ratios for the individual powders  50  and  58 . The second pressing operation (performed in the mold  54 ) must be performed at an equal or higher pressing pressure than that performed in the mold  46  to effect mechanical bonding of the tungsten powder  58  to the rhenium article  52 . For this purpose, dry-bag cold isostatic pressing is believed to be preferred for both pressing operations to preserve the length of the rhenium article  52  during consolidation of the tungsten powder  58 . The length of the rhenium article  52  would be difficult to maintain using wet-bag pressing due to the axial compaction that would occur, with the result that area fraction and spatial distribution of the powders  50  and  58  would be difficult to control, and the article  52  would be susceptible to being broken during the final pressing in the mold  54 . 
     Following consolidation, the resulting rhenium-coated tungsten article is presintered, machined, and then sintered in a high temperature furnace at a temperature of up to about 2450° C. The article achieves the desired final dimensions and a density of at least about 96% of theoretical. 
     FIG. 8 illustrats an alternative embodiment for a nozzle  60  for drawing or extruding quartz, in which tungsten and rhenium layers  62  and  64  are located axially instead of radially to each other. The nozzle  60  can be formed in a mold similar to that in FIG.  6 . The mold cavity is partially filled with a tungsten-based alloy powder, then filled with a rhenium-based alloy powder, compacted and sintered. This configuration is advantageous if the temperature profile within the nozzle  60  requires the presence of rhenium only within a limited region of the nozzle  60 . The axial thickness of the rhenium layer  64  may be as little as about 30 Fm or as thick as several centimeters using the processing methods of this invention. A nozzle  60  with tungsten and rhenium layers  62  and  64  as shown in FIG. 8, or with additional layers of tungsten or rhenium or thungsten and rhenium, can have aspect ratios (length:diameter) of up to twenty or more. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.