Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Reexamination Certificate
2001-11-01
2004-12-07
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C148S516000, C427S250000, C427S255700, C427S249150, C427S249180, C427S253000, C427S255150, C427S255180, C427S255380, C427S255390
Reexamination Certificate
active
06827796
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to heat-resistant, and wear-resistant alloys useful for coatings or free-standing bodies having favorable combinations of strength, hardness and/or toughness. It also relates to methods for making the alloys. More specifically, the invention relates to chemical vapor deposition processes and products therefrom, which have unique, substantially improved physical and mechanical characteristics.
BACKGROUND OF THE INVENTION
Group VIB transition metals include, for purposes of this application, tungsten, molybdenum and chromium. The Group VIB, transition metal elements, such as tungsten, molybdenum, and chromium, have characteristics that allow their incorporation into some new, high-performance alloys. Their high stiffness suggests that they have intrinsic high strength. This indicates that they should have high fracture energy and high specific resilience. It also suggests that they are capable of being made into hard and wear resistant alloys. They have high melting temperatures, as well. Unfortunately, the potentially superior mechanical properties of these materials are seldom realized because of their lack of toughness.
They are all used as pure metals and as important alloying species with base metals. As pure metals, or as the major species in alloys, tungsten and molybdenum are more important than chromium as structural materials. Chromium is used more frequently as a coating.
These Group VIB transition metals, such as tungsten, are used industrially as pure metals, sometimes containing small quantities of a finely divided dispersant; as an alloy with other high melting metals; or as a pure metal cemented into a body with small quantities of a lower melting metal matrix; or as a carbide, either pure or alloyed, cemented with a similar lower melting metal matrix. They are also used as a dilute alloying species in high strength and high hardness base metal alloys.
Among the most important applications of tungsten, for example, are for resistance wire as in lamp bulbs and vacuum tubes, extremely small conductors in microprocessors, x-ray targets, so-called heavy metal alloys, and cemented carbide tool and wear parts. The wire and x-ray target uses take advantage of tungsten's high melting temperature; the microprocessor use of its electrical conductivity and thermal expansion coefficient; the heavy metal alloys of its high specific gravity; and the cemented carbides of the hardness and wear resistance of its monocarbide.
In most instances, it is important for these Group VIB transition metals to have the highest strength and toughness, consistent with the maintenance of its other important properties.
Fine tungsten wire, for example, after the large amount of mechanical work which goes into its manufacture, exhibits high strength. Bulk metal parts of tungsten are usually much weaker, however. In all but a few instances, e.g., the fine wire, tungsten parts suffer from lack of toughness. Even the wire soon loses both strength and ductility on heating due to the work being a high driving force for re-crystallization and grain growth. The brittleness of x-ray targets and other larger bodies has been avoided, at considerable increase in cost, by the addition of the rare metal, rhenium, as an alloying species in quantities as high as twenty-five percent.
The heavy metal and cemented carbide parts rely on another approach to achieve acceptable toughness. They are made by pressing and sintering a mixture of pure metal powder, or of carbide powder, with a lower-melting, more ductile, base metal. The tungsten or tungsten carbide is thereby cemented by the small quantities of the ductile base metal.
Properties of the final product are achieved by the judicious selection of the matrix metal composition, the size of the metal powders, or the size and composition of the carbide powders. For many applications of tungsten and for most applications of tungsten carbide the base-metal-cemented, these pseudo-alloys are the only practical solutions. There are many instances, however, where the incorporation of the softer, lower-melting, less-stiff, and less corrosion-resistant cement substantially degrades the usefulness of the bodies. Pure tungsten, or alloys of tungsten with strengthening or hardening species which would not use such cement would be much more useful.
With regard to metals and other materials in general, it has been well known to materials engineers and scientists that refinement of the crystal habit of bodies increases yield strength, and hardness. Since ancient days mechanical working to reduce their grain size has strengthened metal parts. With more sophisticated understanding, the so-called Hall-Petch relationship has become generally accepted. This relationship teaches that the yield strength of materials varies inversely with the reciprocal of the square root of the grain size. In a more recent publication, Jundal and Armstrong (see Trans. AIME 1969 vol. 245, pg. 625) reported that the Hall-Petch relationship could be extended to treat the increase in material hardness with grain size reduction as well as yield strength. Additional verification, for the case of the hardness of tungsten, comes from Vashi, et al. (see Metallurgical Trans., Vol. 1, June 1970, pg. 1769-1771). (The entire contents of all publications and patents mentioned anywhere in this disclosure are hereby incorporated by reference.)
Within the last decade, research has demonstrated that the dramatic effects on properties can be extended in materials of much finer grain refinement than had been earlier possible. Progress in the manufacture of cemented tungsten carbide cutting tool materials discussed above is a particularly good example of such improvement. Two decades ago the most modern of these cemented carbides had WC crystallite sizes no smaller than about two microns. Today, they are made quite regularly, commercially, with 0.4 micron (400 nm) crystals; and even smaller, on an experimental basis. This has resulted in superior products from the point of view of strength and wear resistance.
This reduction in grain size is not accomplished without difficulty. There are practical limits to the fineness of powders which may be used in the pressing and sintering process. Very small powders have long been considered explosion and worker-ingestion hazards. Even more importantly, these powders tend to agglomerate in handling, thereby preventing the formation of a final product with a crystal refinement as small as might be desired.
Advances to reduce the agglomeration problems have been claimed to be effected by the use of a spray-reaction process from salts of tungsten and the matrix metal with subsequent gas-phase carburization. This process is described in U.S. Pat. Nos. 5,230,729 and 5,352,269. Further, however, even after these very fine powders have been pressed successfully to a so-called green body, there is a tendency toward grain growth upon sintering, although efforts have been made to alloy the cementing metals to allow lower temperature processing and to minimize this grain growth. This approach is described in U.S. Pat. No. 5,841,044.
For reasons which have not been totally explained, none of sub-micron-size or nanostructure cemented carbides, except those with grain sizes above about 0.4 &mgr;m, or even above 0.8 &mgr;m, has shown sufficiently good toughness to be generally accepted commercially.
In the materials science arena, however, investigators have become increasingly anxious to investigate the effects of nano-technology. Nano-technology is usually defined as dealing in microcrystalline sizes below 0.1 &mgr;m (100 nm). Because of the aforementioned limitations, and because they need only small samples, they have chosen to use deposition techniques to make their research samples. Deposition is an attractive way to make extremely fine-grain materials since the crystallites of the materials of interest may be grown and consolidated, simultaneously, at temperatures which are low relative to their fusion temperatures, or even to their sinter
Holzl Robert A.
Shinavski Robert J.
Canter, Esq. Bruce M.
Composite Tool Company, Inc.
King Roy
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