Coated cutting insert

There is provided a coated cutting insert particularly useful for finishing operations in milling of grey cast iron. The insert has a fine-grained WC-Co cemented carbide body and a coating of multiple layers of TiC.sub.x N.sub.y deposited at conditions aiming to minimize the decarburization of the cutting edge without any negative effects on the substrate. There is also disclosed a method of applying a TiC.sub.x N.sub.y layer by CVD technique on a cemented carbide cutting insert without eta phase formation and without etching of the binder phase.

BACKGROUND OF THE INVENTION 
The present invention relates to a coated cutting tool (cemented carbide 
insert) particularly useful for finishing operations in milling of grey 
cast iron. 
Grey cast iron is a material which, in general, is reasonably easy to 
machine with cemented carbide tools and long tool life can often be 
obtained. However, the machinability of cast iron can vary considerably. 
The tool life may be influenced significantly by small variations in the 
chemical composition within the material. These variations may be related 
to the casting technique used such as the cooling conditions. In the 
finishing operation, the surface finish of the machined work piece is 
extremely important. It is also as important to avoid frittering of the 
work piece on the exit side. This puts very high demands on the wear 
behavior of the cutting edge of the cutting tool, especially close to the 
corner between the main cutting edge and the parallel land which creates 
the surface of the work piece. 
Measures can be taken to improve the cutting performance with respect to a 
specific property, but often such action may have a negative influence on 
other cutting properties as indicated below: 
a very sharp cutting edge, i.e., with an edge radius (ER) of about 15 .mu.m 
or less, minimizes the risk of frittering of the work piece and normally 
gives a very good surface finish on the component. However, this makes the 
cutting edge very sensitive to microchipping and for coated cutting 
inserts also sensitive to flaking; 
uncoated cutting inserts with high toughness (e.g., a cemented carbide with 
a high binder phase content) can be used with a very sharp cutting edge 
but they have a poor wear resistance and a short tool life; 
coating of the inserts in order to improve the wear resistance properties 
are commonly used. But inserts with very sharp cutting edges are 
particularly susceptible to decarburization and eta phase formation when 
subjected to a conventional CVD coating process. The eta phase embrittles 
the edge line and decreases the flaking resistance and thus also the tool 
life due to bad surface finish of the work piece. Therefore, 
conventionally CVD coated inserts generally have an edge radius of about 
25 .mu.m as a minimum; and 
PVD coating, using known technique, is recognized to make possible the 
coating of very sharp edges without eta phase formation. However, in the 
milling of grey cast iron, severe flaking of this type of coating often 
occurs which deteriorates the wear resistance and tool life of the insert. 
The normally used process parameters in conventional CVD technique, e.g., 
temperature, pressure and gas composition are a compromise in order to 
obtain the best combination of growth rate, uniformity and cutting 
properties avoiding, e.g., undesirable reactions between gas phase and the 
cemented carbide body, such as etching of the binder phase or 
decarburization of the insert surface especially the edge line. 
A method to avoid decarburization using intermediate oxygen-containing 
layers is described in certain prior art instances, e.g., U.S. Pat. No. 
5,135,801. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of this invention to avoid or alleviate the problems of the 
prior art. 
It is further an object of this invention to provide a coated cutting tool 
particularly useful for finishing operations in milling of grey cast iron. 
In one aspect of the invention there is provided a cutting tool insert for 
finishing milling of grey cast iron comprising a cemented carbide body and 
a coating applied by CVD technique, said cemented carbide body being free 
of eta phase, particularly in the edge line and consisting essentially of 
WC with a mean grain size of &lt;2 .mu.m, 5-15 weight % Co and &lt;0.5 weight % 
cubic carbides of metals from groups IVb, Vb or VIb of the periodic table 
of the elements and said coating comprising a 0.5-3 .mu.m layer of 
TiC.sub.x N.sub.y with x+y=1 and x&gt;0.3 and y&gt;0.3. 
In another aspect of the invention there is provided a cutting tool insert 
for finishing milling of grey cast iron comprising a cemented carbide body 
and a coating applied by CVD technique, said cemented carbide body being 
free of eta phase, particularly in the edge line and consisting 
essentially of WC with a mean grain size of &lt;2 .mu.m, 5-15 weight % Co and 
&lt;0.5 weight % cubic carbides of metals from groups IVb, Vb or VIb of the 
periodic table of the elements and said coating comprising 
an inner layer of TiC.sub.x N.sub.y with x+y=1 with a thickness of 0.1-2 
.mu.m; 
a layer of TiC.sub.x N.sub.y where x+y=1 having a thickness of 0.5-3 .mu.m; 
and 
an outer layer of TiC.sub.x N.sub.y with x+y=1 with a thickness of 0.5-2.5 
.mu.m. 
In yet another aspect of the invention there is provided a method of 
depositing a TiC.sub.x N.sub.y layer on a cemented carbide insert by CVD 
technique without eta phase formation and without etching of the binder 
phase comprising operating a CVD coating process with a pressure during 
the whole process between 750 mbar and atmospheric and a temperature 
between 900 and 970.degree. C., introducing gases in the order H.sub.2, 
N.sub.2, optionally CH.sub.4 and TiCl.sub.4, followed by HCl, keeping the 
amount of TiCl.sub.4 between 1.5% and 5% and the amount of HCl between 
1.0% and 6%.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
It has surprisingly been found that by combining a straight WC-Co cemented 
carbide body having a very fine-grained WC structure with a coating 
consisting essentially of one or multiple TiC.sub.x N.sub.y layers, an 
excellent cutting tool for finishing operations in milling of grey cast 
iron can be obtained. Furthermore, it has been found that by using a 
carefully controlled CVD process, the inserts can be coated without 
decarburization and eta phase formation and without etching of the binder 
phase. The edge radius can be down to 20 .mu.m. 
According to the present invention, a cutting tool insert is provided 
consisting essentially of a cemented carbide body and a CVD coating 
whereby the cemented carbide has the composition 5-15 weight % Co, 
preferably 6-11 weight % Co, &lt;0.5 weight %, preferably &lt;0.2 weight %, 
cubic carbides of the metals Ti, Ta and/or Nb and balance WC. The mean 
grain size of the WC is &lt;2 .mu.m, preferably 0.5-1.5 .mu.m, most 
preferably 0.7-1.3 .mu.m. The cobalt binder phase may be alloyed with 
small amounts of, e.g., Cr&lt;0.5 weight % as a grain growth inhibitor. 
The coating comprises: 
a first (innermost) CVD layer of TiC.sub.x N.sub.y with x+y=1, y&gt;0.7, 
preferably y&gt;0.9, with a thickness of 0.1-2 .mu.m, preferably 0.5-1.5 
.mu.m; 
a second CVD layer of TiC.sub.x N.sub.y with x+y=1, with x&gt;0.3 and y&gt;0.3, 
with a thickness of 0.5-3.0 .mu.m, preferably 1-2.5 .mu.m; and 
a third, yellow CVD layer of TiC.sub.x N.sub.y with x+y=1 and x&lt;0.05, with 
a thickness of 0.5-2.5 .mu.m, preferably 0.5-1.5 .mu.m. 
The coating may contain oxygen as an impurity only. The total thickness of 
the coating should be between 2 and 6 .mu.m, preferably 3 and 5 .mu.m. The 
second layer is preferably thicker than each of the other two. Preferably, 
the coating contains only carbides, nitrides and/or carbonitrides of Ti. 
The coating may comprise only one or two of the above-mentioned layers. It 
is also within the scope of the present invention to make the coating a 
multiple of the above-mentioned layers in any combination. The total 
coating thickness should be between 2 and 6 .mu.m, preferably 3 and 5 
.mu.m. 
According to the method of the invention, a WC-Co based cemented carbide 
body having a very fine-grained WC structure is coated, using CVD 
technique, with: 
a first (innermost) layer of TiC.sub.x N.sub.y with x+y=1, y&gt;0.7, 
preferably y&gt;0.9, with a thickness of 0.1-2 .mu.m, preferably 0.5-1.5 
.mu.m, using a carefully controlled CVD process which enables the inserts 
to be coated without decarburization and eta phase formation except in 
occasional small spots and without etching of the Co binder phase. The 
pressure should during the whole process, be controlled to be between 750 
mbar up to atmospheric, preferably above 900 mbar and the temperature 
should be kept between 900.degree. C. and 970.degree. C. The gases are 
introduced into the reactor vessel in the order H.sub.2, N.sub.2 and 
TiCl.sub.4 followed by HCl. The amount of TiCl.sub.4 should be kept 
between 1.5% and 5%, preferably between 2.0% and 3.5% and the amount of 
HCl should be controlled to be between 1% and 6%, preferably 1.5%-4%. The 
exact conditions, however, depend to a certain extent on the design of the 
equipment used; 
a second layer of TiC.sub.x N.sub.y with x+y=1, with x&gt;0.3 and y&gt;0.3, with 
a thickness of 0.5-3 .mu.m, preferably 1-2.5 .mu.m. The pressure and 
temperature are kept the same as for the innermost coating layer, but 
CH.sub.4 is added to the gas mixture in an mount of 8-20%, preferably 
10-18%. The process may be performed with HCl concentrations between 0% 
and 3% and the amount of N.sub.2 shall be kept at 1%-40%, preferably 
5%-20%; and 
a third, yellow layer of TiC.sub.x N.sub.y with x+y=1 and x&lt;0.05, with a 
thickness of 0.5-2.5 .mu.m, preferably 0.5-1.5 .mu.m, deposited at process 
conditions mainly as for the first, innermost layer. 
A smooth coating surface layer with a surface roughness R.sub.max 
.ltoreq.0.4 .mu.m over a length of 10 .mu.m is obtained using a wet or dry 
blasting treatment of the coated cutting insert surface with fine-grained 
(400-150 mesh) alumina powder as has been disclosed in U.S. Ser. No. 
08/497,934 (our reference: 024444-144). 
It has been found that this blasting treatment considerably changes the 
X-ray spectrum of the coating, e.g., when using Cu K-.alpha..sub.1,2 as 
X-ray radiation source. This can be measured as a difference in peak 
height of the (422)-reflection of the TiN and/or TiCN of a blasted and a 
blasted and heat treated coated insert. The heat treatment may be 
performed at about 800.degree. C. in hydrogen atmosphere for 1 hour. 
Dividing the peak height values before and after the heat treatment, a 
ratio (from here on referred to as the "(422)-ratio") is obtained, see 
FIGS. 2 and 3. 
##EQU1## 
The (422)-ratio of the TiC.sub.x N.sub.y layer with x&gt;0.3 and y&gt;0.3, has 
been found to be &gt;1.5, preferably &gt;1.75. 
Furthermore, it has been found that the blasting treatment affects the 
presence of the so-called K.alpha..sub.2 reflection of the (422)-peak, 
especially for the TiC.sub.x N.sub.y layer with x&lt;0.05. Before the 
blasting treatment and also after the heat treatment of the blasted 
insert, this reflection appears as a shoulder or a small peak on the right 
side of the K.alpha..sub.1 reflection, whereas in the blasted condition, 
this K.alpha..sub.2 reflection cannot be detected due to the broadening of 
the main K.alpha..sub.1 peak, see FIGS. 2 and 3. Instead of blasting, 
other mechanical treatment giving the same result may be used. 
The invention is additionally illustrated in connection with the following 
Examples which are to be considered as illustrative of the present 
invention. It should be understood, however, that the invention is not 
limited to the specific details of the Examples. 
Example 1 
A. Cemented carbide milling inserts of a special finishing type 
N260.8-1204-F with the composition 6.0 weight % Co and balance WC with a 
mean WC grain size of about 1.3 .mu.m, were edge rounding treated to an ER 
size of about 20 .mu.m and coated with a TiN-TiC.sub.x N.sub.y -TiN-layer 
(about x=0.45 and y=0.55) with the individual coating thicknesses 1.4 
.mu.m, 2.0 .mu.m and 1.2 .mu.m, respectively. 
The CVD process conditions were as follows: 
______________________________________ 
Gas Mixture 
Step 1 Step 2 Step 3 
______________________________________ 
TiCl.sub.4 
2.5% 3.3% 2.5% 
N.sub.2 45% 8% 45% 
HCl 3.3% -- 3.3% 
CH.sub.4 -- 14% -- 
H.sub.2 balance balance balance 
Pressure atmospheric in all steps 
Temperature 
930.degree. C. 
930.degree. C. 
930.degree. C. 
Duration 1.5 hours 2 hours 1.5 hours 
______________________________________ 
The inserts were smoothed by wet basting after the coating procedure. No 
eta phase was found in the edge line. One insert was investigated by X-ray 
diffraction and then heat treated in hydrogen atmosphere at 800.degree. C. 
for 1 hour and the X-ray investigated again. The (422)-ratio of the TiCN 
layer was measured to be 2.0 and no K.alpha..sub.2 reflection could be 
detected for the TiN peak in the blasted condition. 
B. Cemented carbide milling inserts of the same type as in A, with the 
composition 10.0 weight % Co, 0.4 weight % Cr and balance WC with a mean 
WC grain size of about 0.9 .mu.m and with an ER size of about 20 .mu.m, 
were coated under the procedure given in A. The thicknesses of the 
individual coating layers TiN-TiC.sub.x N.sub.y -TiN were 1.4 .mu.m, 2.0 
.mu.m and 1.2 .mu.m, respectively. After coating, the inserts were 
smoothed by wet blasting. No eta phase was found in the edge line. One 
insert was investigated by X-ray diffraction and then heat treated in 
hydrogen atmosphere at 800.degree. C. for 1 hour and then X-ray 
investigated again. The (422)-ratio of the TiCN layer was measured to be 
1.8 and no K.alpha..sub.2 reflection could be detected for the 
TiN-(422)-peak in the blasted condition. 
C. Cemented carbide milling inserts with the composition and coating as in 
A, but without the blasting treatment of the coating. In the X-ray 
spectrum of these inserts, the K.alpha..sub.2 reflection for the 
TiN-(422)-peak could clearly be observed. 
D. Milling inserts from the same batch as in A were coated with an equiaxed 
2.0 .mu.m TiC coating and a 1.0 .mu.m Al.sub.2 O.sub.3 layer according to 
prior art CVD technique. The eta phase zone in the edge line was about 5 
.mu.m thick. 
E. Milling inserts from the same batch as in A were coated with a 3.5 .mu.m 
PVD-TiCN coating according to prior art technique. No eta phase was found 
in the edge line. 
F. Milling inserts from the same batch as in A were coated with a 4 .mu.m 
PVD-TiN coating according to prior art technique. No eta phase was found 
in the edge line. 
G. Cemented carbide milling inserts of the same type as in A with the 
composition 7.3 weight % Co, 7.7 weight % TiC and balance WC with a mean 
grain size for WC of about 1.5 .mu.m, were ER treated to an ER size of 
about 15 .mu.m and used uncoated. 
Example 2 
Inserts A-G from Example 1 were tested as below: 
Operation: face milling with SANDVIK NF (N260.8) 
Cutter Diameter: 200 mm 
Workpiece: specially designed component with pre-machined surface 
Material: SS0125 (grey cast iron HB=205) 
Cutting speed: 140 m/min and 290 m/min, respectively 
Feed rate: 0.18 mm/tooth 
Depth of cut: 0.5 mm 
Insert type: N260.8-1204-F 
The operation was run both with and without coolant (wet and dry). Tool 
life criterion was surface finish of the work piece and the tool life 
given below is a mean value of two or three tested edges/variant. 
______________________________________ 
Cutting Speed: 140 m/min 
RESULTS: Tool life, number of components 
______________________________________ 
Grade Variant Wet Dry 
A. (invention) 38 35 
B. (invention) 35 30 
C. (invention) 29 23 
D. (prior art, CVD) 
15 14 
E. (prior art, PVD-TiCN) 
8 30 
F. (prior art, PVD-TiN) 
5 27 
G. (prior art, uncoated) 
29 23 
______________________________________ 
Inserts D, E, and F were susceptible to flaking close to the edge line, 
especially on the parallel land. This behavior was most pronounced for 
inserts E and F in wet milling. Insert G got a more rapid flank wear than 
inserts A and B. 
______________________________________ 
Cutting Speed: 290 m/min 
RESULTS: Tool life, number of components 
______________________________________ 
Grade Variant Wet Dry 
A. (invention) 23 15 
B. (invention) 20 10 
C. (invention) 15 14 
D. (prior art, CVD) 
15 8 
E. (prior art, PVD-TiCN) 
13 13 
F. (prior art, PVD-TiN) 
10 7 
G. (prior art, uncoated) 
13 7 
______________________________________ 
At this higher cutting speed, inserts D, E, and F got flaking on the edge 
line, but they also got a more rapid normal flank wear than insert A. 
Insert G was also less wear resistant than insert A, especially in dry 
milling. At this high cutting speed, insert B was susceptible to plastic 
deformation during the dry milling operation. It is evident that inserts 
according to the invention perform well in both wet and dry machining in a 
wide cutting speed range. 
Example 3 
Inserts A-G from Example 1 were tested as below: 
Operation: face milling with SANDVIK NF (N260.8) 
Cutter Diameter: 200 mm 
Workpiece: specially designed component with several holes with cast skin 
at the surface inside the holes but pre-machined upper surface 
Material: SS0125 (grey cast iron HB=205) 
Cutting speed: 130 m/min 
Feed rate: 0.20 mm/tooth 
Depth of cut: 0.5 mm 
Insert type: N260.8-1204-F 
The operation was run with coolant. Tool life criterion was chipping of the 
edge line due to inclusions in the cast skin. The tool life given below is 
a mean value of the three tested edges/variant. 
______________________________________ 
RESULTS: Cutting Speed: 130 m/min 
Grade Variant Tool life, number of components 
______________________________________ 
A. (invention) 29 
B. (invention) 38 
C. (invention) 22 
D. (prior art, CVD) 
11 
E. (prior art, PVD-TiCN) 
19 
F. (prior art, PVD-TiN) 
21 
G. (prior art, uncoated) 
20 
______________________________________ 
Insert B was superior due to its outstanding toughness behavior. 
From these milling tests, it is evident that inserts according to the 
invention show a broader and higher performance level compared to inserts 
according to prior art. 
The principles, preferred embodiments and modes of operation of the present 
invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as limited to the particular forms disclosed, since these are to 
be regarded as illustrative rather than restrictive. Variations and 
changes may be made by those skilled in the art without departing from the 
spirit of the invention.