Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1998-07-07
2001-02-20
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S698000, C428S699000, C501S087000, C501S096100, C264S432000
Reexamination Certificate
active
06190762
ABSTRACT:
The present invention relates to an oxide-free composite body with a binder metal phase and at least one hard phase substantially consisting of:
a cermet composition with a binder metal phase of 2 to 30 mass % (weight %), balance at least one carbonitride phase or
a hard metal with at least one hard material phase of 65 to 99 mass %, balance binder metal phase.
The invention also relates to a process for producing the composite body.
INVENTION
Composite materials of the aforedescribed type are described in DE 43 40 652 A1. Such composite bodies are especially used as cutting plates for chip removal machining, but also as high temperature materials. According to the conventional technology, these bodies are prepressed from previously prepared hard material powders and metals, with addition of a plastifying medium, after mixing, to shaped bodies. The shaped bodies are then sintered in an electrically heated furnace which, for example, is equipped with graphite heating elements, whereby the heating of the samples is effected indirectly by means of radiation emitted from the heating elements or by convection or thermal conductivity. Usually a pretreatment involves intensive milling of the hard material powder, the mixing and milling of numerous additives and the binder metal for optimum shaping in combination with a pressure sintering, or a sintering hot isostatic pressing or hot isostatic pressing. The fine grained character of the starting powders used according to the state of the art are desirable also for additives which serve as grain-growth blockers. These additional substances are present, after the sintering, as brittle phases and reduce, correspondingly, the ductility and the corrosion resistance of the composite body.
To improve the bend-to-break strength and the hardness of such composite bodies, it has been proposed in DE 43 40 652 to carry out a sintering of the prepressed shaped body in a microwave field. In this case, with higher contents of the binder metal the effectiveness of the heating by microwave of the preformed pressed body is increased. The shaped body is directly heated by the microwave sintering.
OBJECT OF THE INVENTION
Based upon this state of the art, it is an object of the present invention to obtain hard metals and cermets with fine grain structures, whose ductility is improved with simultaneously greater hardness and strength.
SUMMARY OF THE INVENTION
This object is attained, in an oxide-free composite body of the type mentioned at the outset, in which the metals, the carbon as well as optionally other metals and metal carbides and other nitrides required for the formation of the hard phase, namely, the carbonitride phase or phases (with cermets) or the hard material phase or hard phase (in the case of hard metal) are exclusively each in a powder form, i.e. as a solid, compacted to a green body and then subjected to a microwave field in a pressure-free reaction sintering, whereby the hard phases which are formed with the remaining further materials which do not participate in the hard phase reaction, for a liquid phase. The thus obtained shaped bodies have in relation to the particle size of the educt powder used, a substantially reduced grain size in the final product. The lattice structure is fine grained and uniform; the composite body has an unusually high ductility while simultaneously maintaining its high hardness and strength. As a result of the formation of the liquid phase, a complete compaction of the composite body is achieved on sintering. Simultaneously in the production of the composite body according to the invention, hitherto required processing steps are eliminated, like the carburization of the starting materials and additional milling and processing steps. The hard phase is obtained in situ from the starting materials required for the carburization reaction during the same heat treatment which also gives rise to sintering of the shaped body. The liberated reaction heat from the reaction forming the hard phase can be used to apply the necessary activation energy for the sintering. The complete compaction of the shaped body is obtained without additional operations like the application of pressure, or encapsulation which hitherto have been necessary according to the state of the art.
Earlier attempts to synthesize materials like TiC by mixing the starting materials, namely, titanium and carbon utilizing the effect of heat in a furnace, fail because of the development of gaseous byproducts from carbothermic reductions, whereby the resulting gas byproducts, like CO or CO
2
can give rise to undesired inclusions and thus porous shaped bodies. Limited help can be obtained only by the application of a high external pressure which significantly increases the manufacturing costs. It has also been proposed previously to introduce metal powder like titanium and chromium or silicon, with inert auxiliary materials or organic compounds and to treat the resulting gas pyrolytically. In all of these cases the presence of a gas phase impedes or prohibits a sufficiently good compaction upon sintering.
Surprisingly the directed use of material specific characteristics by means of a microwave heating, can give rise to control of the reaction in a solid pressed body comprised of various metal powder type and of nonmetallic powders whereby apart from one or more hard phases, a liquid phase also arises which accelerates the sintering. The use of microwave radiation allows, by contrast with the conventional heating or ignition processes of the state of the art, the simultaneous carburization or carbonitriding and the sintering. The sinter charge can, by the microwave heating, be brought to reaction even independently of heat transfer within the charge by microwave dissipation in volumes of the sinter charge.
Furthermore, by means of a microwave heating a rapid after-heating is possible without loss of the reaction heat to the surroundings. Rather, the reaction heat is used in the binding of the starting materials so that the formation reaction of the hard phase takes place in the solid as well as in the liquid state together with the dissolving and reprecipitation of the hard phase from the binder metal in the presence of the microwave field, substantially more rapidly than without the electromagnetic alternating field. The extremely fine grained structure is possible with a rapid compaction in this manner. Thus, especially in the production of a composite body starting from tungsten powder with an average mean particle size of
1
Am, Co powder and carbon black, a WC-Co hard metal can be achieved with grain size of 0.4 to 0.8 &mgr;m, i.e., grain sizes which lie below the starting grain size of the tungsten powder. Special characteristics like, for example, the hardness, tendency to corrosion, magnetic, electronic and thermomechanical properties can be combined also by correspondingly selected starting mixtures.
Because of the effect of the microwave field, there is a direct reaction of metals with the carbon for hard phase formation as, for example, the reaction of tungsten with carbon to WC or a corresponding hard phase formation to TiC, ZrC, HfC, VC, NbC, TaC, Cr
2
C
3
or MO
2
C. This mentioned reaction is carried out substantially more rapidly and also at lower temperatures than is the case with conventional heating. In the microwave reaction sintering, initially the hard phase is formed which is partly dissolved in the binder metal. For example, the solubility of the hard phase in cobalt (as binder) at the eutectic temperature in mol % is: TaC, HfC:3; TiC:5; ZrC, NbC:6; VC:10; Cr
2
C
3
:12; WC:20; MC
2
C:30. Thus even with relatively small amounts of solubilized carbide, a crystallization of the hard phase can be expected. The eutectic melt which is formed is rapidly drawn into all capillaries of the dielectrically inhomogeneous body under the effect of the electromagnetic alternating field. As a result, the porous body compacts uniformly in volume as hitherto has only been the case when a high external pressure was used. The hard phase dissolved
Dreyer Klaus
Gerdes Thorsten
Rodiger Klaus
Willert-Porada Monika
Dubno Herbert
Jones Deborah
Savage Jason
Widia GmbH
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