Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2001-07-25
2003-02-18
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C428S336000, C428S472000, C428S216000, C428S208000, C427S248100, C407S119000
Reexamination Certificate
active
06521349
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a tool, especially a cutting insert, for the machining of metallic workpieces, composed of a hard metal base body, a cermet base body, a ceramic base body or a steel base body and at least one layer deposited thereon, in which the sole layer or in the case of a plurality of layers, the outermost layer or the layer lying beneath the outermost layer is constituted predominantly of molybdenum sulfide.
The invention also relates to a method of producing the molybdenum sulfide layer by means of CVD (chemical vapor deposition).
BACKGROUND OF THE INVENTION
The coating of substrate bodies of hard metal, a cermet, a ceramic, or steel with hard materials or hard material mixtures or ceramic materials is known from the state of the art. Especially, carbides, nitrides, carbonitrides, oxycarbonitrides and/or borides of at least one of the elements of groups IVa to VIa of the Periodic Table are used, such as, for example, TiC, TiN, Ti(C,N) or also ceramic materials like Al
2
O
3
or ZrO
2
. With large cutting speeds and/or large chip cross sections, high temperatures arise at the cutting edge of the machining tool especially in the case of materials which are difficult to machine, thereby giving rise to increased wear which can lead in part also to edge breakage. To minimize the temperatures which arise at the cutting edge of the tool during machining, usually so-called cooling lubricants have been used which contain either toxic and environmentally polluting substances and which must be disposed of at relatively high cost.
A further possibility, which can reduce the use of coolant lubricants drastically, is the principle of so-called minimum quantity lubrication, whereby very small amounts of coolant lubricants are supplied with point precision through fine passages to the location between the tool cutter and the workpiece. Apart from a high cost for carrying out the method and for the apparatus involved, this approach does not eliminate the waste treatment problem but only minimizes it.
On the other hand, there are so-called solid dry lubricant films, for example, molybdenum disulfide, known in the art. For example DE-A- 24 15 255 proposes to improve the adhesion of a layer which is preferably based upon MoS
2
or WS
2
and which is applied by so-called sputtering with a thickness of 1 to 2 &mgr;m and in which the application of a dry lubricant layer based also upon MoS
2
or WS
2
can be effected in a conventional manner by a polishing application. In the East German patent document (DD) 202 898 it has been proposed to apply layers or the like of molybdenum disulfide by sputtering to cutting tools, stamping tools, drawing tools and machining tools, whereby the layer thickness lies in the nm range. The layer which is applied by sputtering to a substrate body provided with a hard material layer should have a hexagonal lattice structure. EP 0 534 905 A2 also proposes the PVD (plasma vapor deposition) coating of machining tools, inter alia with molybdenum disulfide.
It has, however, been found that MoS
2
layers produced by cathodic atomization (sputtering) adhere poorly to the substrate body or to the outer hard material layer applied to the substrate body or have a predominantly unsatisfactory orientation of the MoS
2
crystallites. To obtain the lowest friction coefficients, the hexagonal components of the MoS
2
layer should be so oriented that the hexagonal base planes lie parallel to the surface. Thereby the oxidation resistance of the layer is also improved.
It is thus proposed in DE 35 16 933 A1 that the ratio of the water vapor partial pressure to the coating rate be adjusted to be smaller than 10
−7
mbar/1.6 mgm
−2
s
−1
which, in practice, can only be achieved with considerable technological cost.
Apart from the requirement which is customary for the PVD process for purity, i.e. that the gas atmosphere not contain foreign substances, there is a further drawback with the PVD process in that a directed particle stream is provided from the target source to the substrate whereby the substrate must be rotated about three axes to produce a uniform coating.
Furthermore WO 97/04142 describes a process for uniform coating of MoS
2
and a metal or a metal compound like titanium or titanium nitride through the use of two targets by means of a sputtering process.
While MoS2 is relatively soft and is not especially wear-resistant, the formation of an MoS
2
coating has a significant effect since by the use of a single MoS
2
layer or an outermost MoS
2
layer at the contact location of the tool with the workpiece, the MoS
2
may be quickly abraded but the friction force because of the penetration of the MoS
2
particles from the edge inwardly (entrainment) is still noticeably reduced for a period of time.
OBJECTS OF THE INVENTION
It is the object of the present invention to provide a single layer or an outermost layer or layer combination which has a dense lattice structure and a higher hardness of greater mechanical stability against wear. It is also an object to provide a method of producing such a layer.
DESCRIPTION OF THE INVENTION
These objects are achieved by a tool in which the molybdenum sulfide layer contains a mixture of the sulfide phases MoS
2
and MoS
2
S
3
with a substantially random orientation of the phase crystals. The layered lattice structure is comprised of randomly oriented MoS
2
grains which partly have a platelet-shaped configuration. The intervening spaces are filled with the Mo
2
S
3
phase. With these features a surprisingly improved wear resistance of the layer is achieved on machining.
Thus still further phases of molybdenum sulfur compounds and/or chlorine can be contained in the molybdenum sulfide layer but should not exceed 6 atomic %.
A single molybdenum sulfide layer or the outermost molybdenum sulfide layer can also be fabricated so as to be of a multilayer type, whereby the respective undermost layer or an underlying layer with a thickness respectively not exceeding 200 nm has a hexagonal lattice structure with a preferred orientation of the hexagonal planes parallel to the tool surface. The remaining layers however are of isotropic orientation. This applies especially for a substrate body which is coated with only a single molybdenum sulfide layer (and no other layer). The layer with the hexagonal MoS
2
crystals is thereby bounded directly by the substrate and has a thickness of 100 nm to 200 nm.
According to a further feature of the invention, the Mo
2
S
3
component in the molybdenum layer does not remain continuously homogeneous but with increasing distance from the substrate body surface is present in different proportions, preferably varying periodically from 5 to 30 volume %.
According to a further feature of the invention, the molybdenum sulfide layer contains successive individual layers of different compositions of which one type comprises the mixture of MoS
2
and Mo
2
S
3
sulfide phases and the other type consists substantially of pure MoS
2
. Each of the MoS
2
layers has a maximum thickness of 200 nm. The thicknesses of layers of the mentioned phase mixture (MoS
2
/Mo
2
S
3
) respectively lie between 200 nm and 400 nm.
In a further embodiment of the tool of the invention there are still further metallic phases and/or hard material phases in the molybdenum sulfide layer under consideration and whose volume proportion however is preferably not in excess of 10 volume %. The metals in question can be especially molybdenum and the materials especially molybdenum carbide, titanium carbonitride or chromium carbide. Such metal phases or hard material phases serve to increase the stability of this layer which however is in substance the molybdenum sulfide of the mixture described at the outset. The volume proportion of MoS
2
in this layer lies between 30 and 98 volume %, preferably between 40 to 95 volume %. The layer thickness lies between 0.5 to 8 &mgr;m, preferably between 2 and 4 &mgr;m.
Optionally the single or outermost layer of molybdenum sulfide can have still a further meta
Endler Ingolf
König Udo
Leonhardt Albrecht
Sottke Volkmar
Blackwell-Rudasill G. A.
Dubno Herbert
Jones Deborah
Widia GmbH
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