Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2000-02-29
2002-04-16
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S325000, C428S328000, C428S329000, C428S404000, C428S698000, C428S699000, C428S702000
Reexamination Certificate
active
06372346
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ceramic powders and sintered materials made from such powders. Such materials find particular utility as metal forming members such as metal cutting and forming tools.
2. Background of the Invention
During the mid-1930's, tool steel alloys began to be replaced by sintered tungsten carbide powder tools, which quickly became the standard because of their excellent hardness and inherent high toughness and transverse mechanical strength. The hardness of such materials improved tool life, and the toughness and strength helped increase productivity by allowing higher feeds, speeds, and more aggressive forging parameters. Carbide tool development and commercial availability increased significantly after World War II.
Even these materials eventually wear, and the mechanisms of such wear are not as yet fully understood. Progressive wear causes variation in the materials being shaped, and as a result of the need to hold part dimensional tolerances, the tool must be replaced when it is no longer able to form the part to the correct dimension. The time or number of parts formed before such an occurrence ultimately determines the limit of the tool's life. The resulting productivity loss during tool change out and process readjustment, nonconforming production, rework, and missed schedules have been a driving force for obtaining materials that provide longer tool life.
Tool life is determined by its resistance to several types of wear, its response to heavy loads, and to shock. In general, the higher the chip removal rate (high feeds and speeds), drawing and forming pressures, and the longer tool geometry is retained, the better the tool. Superior cutting and forming tools must be simultaneously hard, strong, stiff, and resistant to chipping, fracture, heat breakdown, fatigue, chemical reaction with the work piece, and attrition wear. Accordingly, the dominant desirable mechanical properties sought in a sintered tool are strength, hardness, high elastic modulus, fracture toughness, low chemical interaction with the work piece, and low coefficient of friction to aid work piece forming while reducing heat buildup.
In recent years, the powder metallurgy (PM) industry has increased significantly because of the ability of powders to flow cold into a precision mold. This allows the mold to be reused, often at high volume, while dramatically reducing machining, forming, and other process steps because the sintered part is already very close to its intended configuration, or “near net shape.” Increasingly these parts, now produced principally of aluminum, ferrous, and copper powders, require some of the same desirable attributes as tools. For this reason, many PM articles undergo additional forging, plating, or heat treatment operations to develop localized hardness, toughness, and strength. Many of these parts require resistance to shock and abrasion that call upon the same mechanical properties as are required for tools.
In tools and hard articles, wear resistance is increased at the expense of strength; today, the best tools exhibit the best compromises, and therefore are limited for use in special applications.
Beyond tungsten carbide, various alloys, coating techniques, and combinations of both have been found to permit not only longer tool life but also increased cutting speeds and feeds. Powder metallurgy and sintering have lead to the development of new materials with enhanced hardness and toughness, and adding a hard coating to the sintered alloy such as by chemical vapor deposition (CVD), physical vapor deposition (PVD), or plasma-assisted chemical vapor deposition (PACVD) has increased wear resistance.
Much is taught in the prior art about preparation of coatings on powders, coating substrates, and other hard material enhancements. The prior art of tool materials teaches six approaches that are currently known and in general use for the achievement of such enhanced wear resistance and toughness; each having significant benefits and significant drawbacks: (1) mixing hard and tough phase particles, (2) chemical vapor deposition (or other) coating of sintered substrates with hard phase layers, (3) combining approaches one and two, (4) cermetallic (cermet) compacts, (5) for a special type of tool (grinding and sanding media), chemically bonding low concentrations of large diamond or cBN particles into a hard but relatively weak abrasive substrate, and (6) Functionally Gradient Materials (FGM).
None of these solutions has brought about the essential combination of desired tool properties, and only the chemical vapor deposition (CVD or PVD) approach is today applicable for some mechanical parts requiring increased abrasion resistance.
Mixing Hard and Tough Ternary Systems.
In spite of the many ancillary treatments and variations that exist and that are taught in the art, mixing hard WC-TiN-Co alloy particles with the carbide powder before sintering has several disadvantages. Because these harder particles have low mutual solubility with the binder, substrate transverse strength drops quickly above 6-10 wt. percent hard particles. Surface hardness and wear resistance are accordingly reduced also, compared with a surface coating. The wear mechanism is also not greatly enhanced because the few hard particles (less than one in ten at the surface where needed), weakly bound to the binder, break away whole.
Chemical Vapor Deposited (CVD) Coatings.
These hard external coatings of hard intermetallic and cermet layers on tool steels or sintered article substrates (after sintering) are valued for the high surface hardness they impart, typically exhibiting values of 2400 Vickers (TiN) to 5000 Vickers (cubic boron nitride) to 9000 (diamond). Yet, for all the ancillary treatments, variations, and sintering aids that exist and are disclosed in the art, including additional coating layers, locally altered substrate structures, and grain-size reducing dopants or coatings, the external coating solution has several major disadvantages, including coating delamination and cracking in use (from different coating and substrate thermal expansion rates and from bending and surface loads) and the high CVD process temperatures required (900° C.-1200° C.) may not be consistent with the heat-treatment needed for the strength or the geometry of the sintered part.
Conventional CVD coating of already-sintered articles with several different coatings or layers allows them to resist two or three unique work piece challenges. But since each layer must be deposited sequentially the remaining one or two special coatings must remain covered until the outer layers have worn away. Therefore, only one of the concurrent substrate coating design challenges can be met at the same time.
Some categories of tools, such as drawing dies and nozzles, are even more prohibitively expensive because there is an additional cost to assure the CVD vapor is adequately circulated through the die orifice for the deposition of a coating, where it is most needed. Diffusion of the CVD gas is slow and penetration is typically 0.5 to 10 micrometers or less. First, at these thicknesses, the coating is worn through to the underlying carbide before most of the wire or tube diameter tolerance is used up. Second, the normal reutilization of the dies at larger diameters must be done without the hard coating. In many cases, tool total life prolongation may not be proportional to the added CVD cost.
Today, external coatings are the most common commercial solution to enhancing the performance of plain sintered tungsten carbide products. Increasing the deposition thickness of outer layers to gain greater life has diminishing returns; it tends to increase the propensity for cracking and to round off sharp tool edges, adversely affecting optimal cutting or die geometry.
Combined Mixes and Coatings.
CVD coating and mixing hard alloy particles, a combination of (1) and (2) above gives very limited added benefit while having the same drawbacks.
Cermets.
Cermets are ceramic p
EnDurAloy Corporation
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Jones Deborah
McNeil Jennifer
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