Cutting tool and method of making same

Details Cutting tool and method of making same Cutting tool and method of making same

1999-06-01

2001-09-04

Turner, Archene (Department: 1775)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C051S307000, C051S309000, C428S472000, C428S698000, C428S701000, C428S702000, C428S704000

Reexamination Certificate

active

06284366

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a cutting tool of the type in which a substrate, usually of a hard metal, cermet, ceramic or steel, is coated with at least one layer and in which the outermost layer of the coating is a molybdenum sulfide layer. The invention also relates to a method of making such a tool with an outermost molybdenum sulfide layer.
BACKGROUND OF THE INVENTION
The coating of substrate bodies of hard metal, cermets, ceramics or steel with hard-facing and wear-reducing layers of hard materials, mixtures of hard materials or ceramics, is known in the production of cutting tools. The coatings can be carbides, nitrides, carbonitrides, oxycarbonitrides, oxides and/or borides of an element from Groups IVa to Via of the Periodic Table. Examples of such wear-reducing materials include TiC, TiN, Ti(C,N) and ceramics like Al
2
O
3
and ZrO
2
. At high cutting speeds and/or large cutting cross sections, the high temperatures at the tool/workpiece interface give rise to enhanced wear, especially with workpieces which are difficult to machine, and ultimately to breakage of the tool. To minimize the temperatures which develop in machining of such workpieces, i.e. at the chip-forming region, coolants and lubricants can be used which are environmentally unsound and can produce noxious and toxic waste and which are costly because of the cost of the coolant and/or lubricant.
It is known to provide solid dry lubricant films, for example of molybdenum disulfide (DE-A 24 15 25)and to improve the adhesion of such layers to the substrate by first sputtering the base layer of MoS
2
or WS
2
onto the substrate with a thickness of 1 to 2 &mgr;m a and then applying the dry lubricant film of MOS
2
or WS2.
In DD 202 898 it has been proposed to sputter layers of molybdenum disulfide or the like on tools used for chip removal machining, punching, drawing or like shaping, the layer thickness being in the nm range. In these systems, a substrate body is provided with a hard layer by sputtering of a coating which has a hexagonal lattice structure. EP 0 534 905 A2 proposes a plasma vapor deposition (PVD) coating of chip removing tools with, for example molybdenum disulfide.
The cathodic sputtering of molybdenum disulfide layers on a substrate body or on hard material coating previously applied to the substrate body can provide MOS
2
crystallites with an unsatisfactory orientation. To keep the friction coefficient of such a layer as small as possible, the hexagonal lattice component of the MOS
2
layer should be so oriented that the hexagonal planes run parallel to the surface and the axes of the hexagonal lattice structures run perpendicular or normal to the surface. This has been found to improve the oxygen resistance of the layer.
DE 35 16 933 A1 proposes that the ratio of the water vapor partial pressure for such coating be less than 10
−7
millibar/1.6 mgm
−2
s
−1
which can be attained in high cost.
Apart from the fact that the PVD process requires pure gas atmospheres, i.e. gas atmospheres free from foreign substances, a drawback of the PVD process is that a directed particle stream must pass from the target source to the substrate, thereby requiring that the substrate be rotated about three axes for uniform coating.
OBJECTS OF THE INVENTION
It is the principal object of the present invention to provide a tool of the aforedescribed type which has an improved wear resistance, a wear layer which has better adhesion to the sputter or layers of hard materials on the substrate and improved oxidation resistance.
Another object of the invention is to provide a method of forming such a workpiece and, in particular, a method of coating a molybdenum disulfide layer onto a substrate or a substrate body coated with other layers such that complex apparatus and mechanisms can be avoided and especially well adhering oxidation resistant molybdenum disulfide layers will be deposited.
SUMMARY OF THE INVENTION
These objects and others which will become apparent hereinafter are attained, in accordance with the invention in a tool which comprises:
a substrate forming a tool body and composed of at least one material selected from the group which consists of hard metals, cermets, ceramics and steel; and
a coating comprised of at least one layer on the substrate and including an outermost molybdenum sulfide layer, the outermost molybdenum sulfide layer being composed predominantly of molybdenum sulfide and containing chlorine in an amount of 0.5 to 6 atomic percent.
The method of applying a molybdenum sulfide layer to a substrate can comprise the steps of:
(a) depositing molybdenum sulfide on a substrate by chemical vapor deposition from a gas mixture containing molybdenum chloride, hydrogen sulfide, an inert gas and hydrogen; and
(b) controlling the hydrogen content of the gas mixture so that it lies within the range of 1 volume percent to 90 volume percent.
The single layer on the substrate body or the outermost layer when a plurality of layers are provided should thus have according to the invention a chlorine content of 0.5 to 6 atomic %, preferably 0.5 to 3 atomic %. The molybdenum sulfide in the single layer or the outermost layer can have a hexagonal lattice structure and preferably has its hexagonal lattice cells with axes that are normal (perpendicular) to the plane of the hexagonal cell and thus to the tool surface. According to a further feature of the invention the volumetric proportion of the molybdenum sulfide with the hexagonal lattice structure should be greater than 70%. The balance of this layer can consist of rhombohedral molybdenum disulfide and/or MO
2
S
3
.
It is surprising that a chlorine content in the aforedescribed range can significantly improve the wear resistance of the tool.
The hexagonal cells of the molybdenum sulfide layer should lie in planes parallel to the substrate surface and the axes of these hexagonal cells should be perpendicular to the surface, to minimize the friction coefficient of the single molybdenum sulfide layer or the outermost molybdenum sulfide layer and simultaneously enhance the oxidation resistance.
The applied molybdenum sulfide layer can either be completely in the form of the hexagonal phase of MoS
2
or partially composed of rhombohedral molybdenum disulfide and/or MO
2
S
3
, with the preponderance bring the hexagonal molybdenum disulfide. Apart from the chlorine inclusions in the single layer or the outermost layer, inert gas, for example, argon can be included in small amounts up to two atomic percent.
Preferably the thickness of the exclusive outer layer or the outermost layer of a plurality of such layers is selected to be between 0.2 &mgr;m to 3 &mgr;m and especially about 2 &mgr;m.
In order to improve upon the resistance of the surface to frictional abrasion of the relatively soft MoS
2
layer a thin metallic protective film can be formed on the single or outermost MoS
2
layer in a thickness of at most 0.3 &mgr;m and preferably composed of molybdenum carbide, titanium carbide, titanium carbonitride or zirconium carbonitride.
It is within the scope of the present invention to make the single or outermost molybdenum disulfide layer itself a multilayer stack, in which case between each pair of molybdenum sulfide sublayers, there is a thin metal intermediate film of a thickness of at most 0.3 &mgr;m or a film of a hard material, preferably one of those last mentioned.
The outer layer according to the invention can either be applied to a substrate body of a hard metal, cermet, ceramic or steel directly or to a substrate body which itself has been coated with a hard material. Especially it has been found to be advantageous to provide between the substrate body and the outermost layer in the latter case, at least three layers which can be formed substantially from carbides, nitrides, carbonitrides, oxycarbonitrides, oxides and/or borides of at least one of the elements from Group IVa to VIa of the Periodic Table, or of a ceramic, preferably A1
2
O
3
. The hard material coating of the substrate is itself known in

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