Oxide coated cutting tool

Details Oxide coated cutting tool Oxide coated cutting tool

2002-05-28

2004-01-06

Pianalto, Bernard (Department: 1762)

Coating processes

Coating by vapor, gas, or smoke

Mixture of vapors or gases utilized

C427S255393, C427S255700, C427S419300

Reexamination Certificate

active

06673393

ABSTRACT:

This application claims priority under 35 U.S.C. §§119 and/or 365 to 0101902-5 filed in Sweden on May 30, 2001; the entire content of which is hereby incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to a coated cutting tool for chipforming machining. The coating includes at least one alumina (Al
2
O
3
) layer characterized by fine, equiaxed grains.
BACKGROUND OF THE INVENTION
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Cemented carbide cutting tools coated with various types of Al
2
O
3
layers using Chemical Vapour Deposition (CVD), e.g., pure &kgr;-Al
2
O
3
, mixtures of &kgr;- and &agr;-Al
2
O
3
coarse grained &agr;-Al
2
O
3
and finegrained textured &agr;-Al
2
O
3
have been commercially available for years generally in multilayer combinations with other metal carbide and/or nitride layers, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table.
Al
2
O
3
crystallises in several different phases: &agr;, &kgr;, &ggr;, &dgr;, &thgr; etc. The two most frequently occurring phases of CVD-produced wear resistant layers are the thermodynamically stable, &agr;-phase and the metastable &kgr;-phase or a mixture thereof. Generally, the &kgr;-phase exhibits a grainsize in the range 0.5-3.0 &mgr;m (dependent on the layer thickness) and the grains predominately grow through the whole coating forming a columnar type coating morphology.
Furthermore, the &kgr;-Al
2
O
3
layers are free from crystallographic defects and also free from micropores and voids. Coarse grained (3-6 &mgr;m) &agr;-Al
2
O
3
often possesses porosity and crystallographic defects, while finegrained textured &agr;-Al
2
O
3
are free of defects with very pronounced columnar-shaped grains.
In U.S. Pat. No. 5,674,564 a method is disclosed of growing a fine-grained &kgr;-Al
2
O
3
layer by employing a low deposition temperature and a high concentration of a sulphur compound.
In U.S. Pat. No. 5,487,625 a method is disclosed for obtaining a finegrained, (012)-textured &agr;-Al
2
O
3
layer consisting of columnar grains with a small cross section (about 1 &mgr;m).
In U.S. Pat. No. 5,766,782 a method is disclosed for obtaining a columnar fine-grained (104)-textured &agr;-Al
2
O
3
layer.
The lifetime and the performance of a coated cutting tool are closely related to the microstructure of the coating materials used. Although, coatings produced according to above-mentioned prior art patents show good cutting properties there is still a strong desire to further improve coating microstructures to suit specific cutting conditions and workpiece materials.
As has been mentioned above, all Al
2
O
3
layers produced by the CVD technique possess a more or less columnarlike grainstructure. An Al
2
O
3
layer with an equiaxed grainstructure is, however, expected to show some favourable mechanical properties, e.g., resistance to crack propagation and higher cutting edge toughness, as compared to a layer with a columnar grainstructure. In addition, finegrained layers generally have smoother surfaces than coarse grained layers. During cutting less workpiece materials will adhere onto a smooth coating surface which in turn will imply lower cutting forces and less tendency for the coating to flake off. Nowadays coated cutting inserts are often brushed with SiC based brushes or blasted with finegrained Al
2
O
3
powder in order to obtain a smooth coating surfaces, a rather costly production step.
One well-known and possible technique to produce a finegrained structure and to restrain a columnar grain growth is to deposit a so-called multilayer structure in which the columnar growth of, e.g., Al
2
O
3
, is periodically interrupted by the growth of a 0.05-1 &mgr;m thick layer of a different material such as disclosed in U.S. Pat. No. 4,984,940 and U.S. Pat. No. 5,700,569. The latter layer should preferably have a different crystal structure or at least different lattice spacings in order to be able to initiate renucleation of the original layer. One example of such a technique is when the Al
2
O
3
growth periodically is interrupted by a short TiN deposition process resulting in a (Al
2
O
3
+TiN)xn multilayer structure with a thickness of each individual TiN layer in the range 0.1-1 &mgr;m, e.g., see Proceedings of the 12th European CVD Conference page pr. 8-349. However such multilayer structures very often suffer from a low adherence between the two different types of layers.
In Swedish patent application SE 0004272-1 a method is disclosed for obtaining a finegrained &agr;-Al
2
O
3
layer consisting of essentially equiaxed grains with a grain size <1 &mgr;m. The grain refinement is accomplished by periodically interrupting the Al
2
O
3
process and treating the Al
2
O
3
surface with a mixture of TiCl
4
/H
2
. When the Al
2
O
3
process is restarted renucleation of the &agr;-Al
2
O
3
will take place.
It is not possible to use this method to produce finegrained &kgr;-Al
2
O
3
since only the &agr;-Al
2
O
3
phase will nucleate on the TiCl
4
/H
2
treated Al
2
O
3
surface.
&kgr;-Al
2
O
3
and &agr;-Al
2
O
3
coatings used as tool materials have slightly different wear properties when cutting different materials. It is therefore also desirable to have means to produce finegrained &kgr;-Al
2
O
3
with a controllable grain structure.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide onto a hard substrate, or preferably onto a hard substrate coated with a TiC
x
N
y
O
z
and/or ZrC
x
N
y
O
z
-layer (x+y+z=1 and x,y,z≧0) at least one finegrained single phase &kgr;-Al
2
O
3
layer with a microstructure which is different from the prior art columnar &kgr;-Al
2
O
3
CVD layers mentioned above.
It is also the object of the present invention to provide a high performance tool coating comprising the invented Al
2
O
3
layer.
It is a further object of the invention to provide an alumina coated cutting tool insert with improved cutting performance in steel and nodular cast iron.
According to one aspect of the present invention there is provided a cutting tool comprising a body of sintered cemented carbide, cermet or ceramic, and a hard and wear resistant coating applied on at least a functioning portion of a surface of the body. The coating comprises a structure of one or more refractory layers of which at least one layer consists essentially of an equiaxed finegrained &kgr;-Al
2
O
3
with a thickness of 0.5-25 &mgr;m and with a grain size of less than 0.5 &mgr;m, and said finegrained &kgr;-Al
2
O
3
layer comprises at least one sub-layer with a thickness between 0.02 and 3 &mgr;m containing Al, Si and O with a Si concentration between 4-34 at %, Al concentration of 0-37 at % and O concentration of 60-67 at %.
According to another aspect, the present invention provides a method of coating a body with at least one finegrained &kgr;-Al
2
O
3
layer comprising contacting the body with a reaction mixture comprising a hydrogen carrier gas, one or more halides of aluminium and a hydrolysing and/or oxidising agent at 800-1050° C., adding a sulphur agent to the reaction mixture to enhance the growth rate, and depositing at least one silicon enriched sublayer in the &kgr;-Al
2
O
3
layer by periodically introducing a silicon halide.


REFERENCES:
patent: 4984940 (1991-01-01), Bryant et al.
patent: 5487625 (1996-01-01), Ljungberg et al.
patent: 5674564 (1997-10-01), Ljungberg et al.
patent: 5700569 (1997-12-01), Ruppi
patent: 5763008 (1998-06-01), Sarin et al.
patent: 5766782 (1998-06-01), Ljungberg
patent: 5786069 (1998-07-01), Ljungberg et al.
patent: 6090476 (2000-07-01), Thysell et al.
patent: 2002/0122701 (2002-09-01), Ljungb

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