Hard metal wearing part with mixed oxide coating

Details Hard metal wearing part with mixed oxide coating Hard metal wearing part with mixed oxide coating

2002-02-05

2004-04-27

Turner, Archene (Department: 1775)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C428S472000, C428S216000, C428S697000, C428S698000, C428S701000, C428S702000

Reexamination Certificate

active

06726987

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the field of materials, and relates, more specifically, to a wearing part, in particular a cutting insert, made from hard metal or cermet as the base material. The part has a hard-material coating which comprises one or more coats, a single-layer or lamellar multilayer mixed-oxide coat. The latter substantially determines the wear and comprises predominantly aluminum oxide.
The effect of hard-material coats on hard metal and on cermets of increasing the resistance to wear has been utilized at a commercial level for many years. Among the many hard-material phases which have by now been used to protect against wear, hard materials selected from the group consisting of the carbides and carbonitrides and nitrides, and also hard materials selected from the group consisting of the oxides, have proven particularly successful and are nowadays often used together, as protective layers which complement one another, in a layered coat sequence.
It is thereby customary for coats belonging to one hard-material group to be designed in lamellar form, as a plurality of individual layers alternately comprising different hard materials, in order to satisfy the various demands imposed on a wearing part, in particular also on a cutting insert for the machining of metallic workpieces, with regard to adhesion, toughness, and low wear.
There are a number of known difficulties which arise when producing usable aluminum oxide coats on hard metal cutting inserts. The coat becomes considerably more brittle as its thickness increases and as the coat deposition rate increases, on account of coat grain coarsening. The high coating temperatures which are required in order to achieve the desired aluminum oxide phase (proportion of &agr; and/or &kgr; Al
2
O
3
) and coat structure conceal the risk of permanent quality losses for the base material; moreover, they lead to increased thermal stresses in the coat.
The permanent demand for oxide coats with a wear resistance that is improved compared to that of pure Al
2
O
3
, in particular for cutting inserts, has in the past been satisfied in various ways.
For example, U.S. Pat. No. 4,052,530 (see German application DE A 27 18 647) describes the application of a mixed coating comprising Al
2
O
3
and TiO
2
phases on tools for material-removing machining. In accordance with the embodiment described in more detail in that document, the titanium oxide concentration in the coating is 2-10%, and this component is in the form of its own second phase in the aluminum oxide, as dispersed, hexagonal &agr;-titanium oxide. The increases in service life which can be achieved for cutting tools coated in this manner compared to pure Al
2
O
3
no longer match the standard wear resistance for cutting inserts which can nowadays be achieved in other ways.
U.S. Pat. No. 4,746,563 (see European patent EP 0 162 656) describes hard metals that are provided with a multilayer coating, divided into undercoat and top coat. It is possible for each of the two coats in turn to be divided into a plurality of layers of different hard-material composition. The multilayer outer coat, with an overall layer thickness of 3 to 20 &mgr;m, comprises a plurality of Al
2
O
3
layers each with a thickness of from 0.01 to 2 &mgr;m, it being possible for the Al
2
O
3
phase to contain up to 30% by volume of titanium dioxide, the individual Al
2
O
3
layers of the outer coat being interrupted or detached by interlayers of Ti(C, N, O, B) and SiC, AlN and AlON which are from 0.1 to 2 &mgr;m thick. According to the exemplary embodiment, the Al
2
O
3
CVD coating takes place at a furnace temperature of 1000° C.
In view of the coat thicknesses, which are given as 3 to 20 &mgr;m, these coat sequences and deposition conditions indicate long coating times and therefore a time-consuming, expensive coating process. Moreover, the long coating times lead to losses of quality in the sintered carbide base material.
U.S. Pat. No. 4,599,281 (see European patent EP 0 083 043), as a further example of the development of Al
2
O
3
coats, relates to a wearing part, in particular a hard metal cutting insert for material-removing machining, having a multicoat hard-material coating, at least one coat comprising oxides, nitrides, carbides and/or borides using individual metals such as titanium, zirconium and hafnium which are applied alternately with at least one aluminum/boron mixed oxide coat with boron contents of from 0.01 to 1% by weight.
Wearing parts which have been coated in this manner are now in commercial use in practice. Nevertheless, there is a need for a further increase in wear resistance or service life and for production costs for coated cutting inserts to be reduced. The cutting edge stability or resistance to flaking of the cutting edges is very important for reliable, nondestructive machining of workpieces and, compared to this prior art, is considered to be urgently in need of improvement.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a wearing part of hard metal with a mixed oxide coating, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which satisfies the ongoing demand for improved wear resistance and cutting edge stability for wearing parts, in particular for cutting inserts for material-removing machining, while further reducing production costs.
With the foregoing and other objects in view there is provided, in accordance with the invention, a wearing part, comprising:
a base material selected from the group consisting of hard metal and cermet;
a hard-material coating on said base material, said coating having at least one mixed-oxide layer predominantly formed of Al
2
O
3
and substantially determining a wear of said hard-material coating, said mixed-oxide layer having a thickness of 0.5 to 10 &mgr;m containing an addition of from 0.1 to <3% by weight of TiO
2
and 0.01 to 0.5% by weight of B
2
O
3
, and said addition is substantially dissolved and homogeneously distributed in the Al
2
O
3
phase.
In other words, a wearing part of the generic type specified in the introduction is further improved with the single-layer or multilayer Al
2
O
3
mixed oxide coat that is 0.5 to 10 &mgr;m thick and contains, substantially dissolved and homogeneously distributed in the Al
2
O
3
phase, an addition of 0.1 to ≦3% by weight of TiO
2
and 0.01 to 0.5% by weight of B
2
O
3
.
The proportions by weight of titanium oxide and boron oxide in the Al
2
O
3
mixed-oxide coat are based on the compounds TiO
2
and B
2
O
3
. Therefore, there is no specific statement made as to the compound which is actually present.
The Al
2
O
3
in the mixed-oxide coat may comprise one or more Al
2
O
3
modifications, for example &agr; Al
2
O
3
, &kgr; Al
2
O
3
or others.
The definition of the maximum additions of titanium oxide and boron oxide which can be introduced into the Al
2
O
3
phase and the production parameters to be applied are to be adapted to one another, using rules which are known to the person skilled in the art, in such a way that the additions are actually completely dissolved in the Al
2
O
3
or are to some extent homogeneously distributed in the Al
2
O
3
so finely that they cannot be detected under light microscopy as an independent titanium oxide or boron oxide phase in addition to the Al
2
O
3
phase.
The term “substantially” which is used for the additions within the feature “substantially dissolved and homogeneously distributed” therefore defines an Al
2
O
3
mixed oxide phase within which a Ti oxide or B oxide phase cannot be detected under light microscopy, irrespective of what proportion of the addition actually satisfies the conditions of a solid solution.
A wearing part with an Al
2
O
3
top coat formed in this manner is distinguished by relatively short production times and therefore lower production costs.
At the same time, the Al
2
O
3
mixed oxide coat which has been modified in accordance with the invention leads to an unexpe

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