Ceramic cutting tool with improved toughness behavior

This invention relates to a cutting tool material with improved toughness behavior. The improved toughness behavior is obtained by using carefully selected reinforcing materials. The objectives of this invention are realized by simultaneously utilizing additives resulting in different toughening mechanisms. By the simultaneous utilization of different toughening additives, synergistic effects occur giving the tool excellent material properties. The tool material of the present invention comprises reinforcing whisker additives of one dimensional single crystals with preferably different diameter and aspect ratios, two dimensional single crystals with different thickness and aspect ratios, and, in combination therewith, phase transformation additives such as ZrO.sub.2.

The present invention relates to a cutting tool material. More 
specifically, the present invention relates to a cutting tool material 
exhibiting excellent toughness through the utilisation of different 
toughening additives. 
A metal cutting tool is subjected to fluctuating stresses and temperatures 
of dynamic nature. The conditions to which a cutting edge is subjected in 
operation are severe, the temperature of the tool surface often exceeding 
1000.degree. C. as the chip contacts the rake face of the tool at a 
pressure close to 150 MPa and at a relative speed in the range 0-1000 
mmin.sup.-1. Steep temperature and stress gradients also exist in the 
cutting tool inserts. A consequence of the dynamic conditions described 
above is that generally several different failure mechanisms operate 
simultaneously for given cutting conditions. However, for a specific set 
of cutting data, one mechanism is dominating and limiting of the tool 
life. 
One objective of the present invention is to obtain a cutting tool material 
having a unique combination of good resistance against several failure 
mechanisms. The result is a tool material showing improved tool life and 
wider application area. 
Known toughening mechanisms in cutting tool materials include i) 
transformation toughening, which utilises a phase transformation of 
ZrO.sub.2 -particles dispersed in a ceramic matrix, this type of material 
is described in U.S. Pat. No. 4,218,253; ii) whisker pull out which 
utilises the bridging effect of small diameter (typically 0.6 .mu.m), high 
aspect ratio and high strength single crystal whiskers in a ceramic matrix 
as described in U.S. Pat. No. 4,543,345, and the combined effect of 
ZrO.sub.2 and small diameter SiC-whisker as described in U.S. Pat. No. 
4,657,877. 
Both of these effects have led to substantial improvements of the toughness 
behaviour in certain metal cutting operations. However, the search for 
greater improvements in the properties of cutting tools is continuously 
ongoing. 
The primary objective of the present invention is to provide a material 
with an improved toughness behaviour and a wider application area than has 
heretofore been known. This objective is achieved through the simultaneous 
action of several toughening mechanisms. In different practical machining 
tests, it has been found that the maximum tool life is dependent on single 
crystal size and geometry of the reinforcing additives for a specified 
operation. Therefore, a particular prior art material has a relative 
narrow optimum application area. It has now surprisingly been found that 
by adding several different classes of reinforcing single crystals, with 
respect to geometry and size, not only a widening of the application area 
is obtained but also an improvement in tool life is realised. 
Generally, the present invention is directed to the fabrication of 
whisker-reinforced ceramic cutting tool materials characterised by 
increased tool life and a wider application area than prior art materials. 
The composites comprises a matrix based on A1.sub.2 O.sub.3 or Si.sub.3 
N.sub.4 and up to 35 weight-% single crystals or whiskers with different 
geometries and sizes. Preferably an adequate concentration (in the alumina 
based matrix preferably 3-20, most preferably 5-15 weight% and in the 
silicon nitride based matrix up to 10 weight-%) of unstabilized and/or 
partially stabilized zirconia increases the toughness behaviour of the 
composite in a wider range of metal cutting applications than obtainable 
with only one of the additives mentioned above. The alumina based matrix 
may further comprise chromium in amounts corresponding to a total of 1-20 
weight-% as Cr.sub.2 O.sub.3. The whiskers, preferably of SiC, used in the 
present invention are of a monocrystalline structure and are divided into 
three types characterized by different diameters and aspect 
(length/diameter or e.g., diameter/thickness) ratios at least 5 weight-% 
of each of at least two of the three types being present in the matrix: 
(i) Single crystal fibers with a diameter less than 1 .mu.m, typically 0.6 
.mu.m, and an aspect ratio of 15-150; 
(ii) Single crystal fibers with a diameter of 1-6 .mu.m, typically 3-4 
.mu.m, and an aspect ratio of 5-100; and 
(iii) Single crystal discs with an equivalent diameter (hypothetical 
diameter of a circle with the same area as the disc) of 5-50 .mu.m, 
typically 20 .mu.m and an aspect ratio (aspect ratio being defined as 
ratio of equivalent diameter through thickness of the disc) of 5-50, 
typically 10-20. 
The composite may further comprise refractory nitrides or carbides to 
increase hot hardness and thermal conductivity which is advantageous in 
certain metal cutting applications.

EXAMPLE 1 
Cutting tool materials are prepared from the following starting materials: 
A. Alumina with a grain size.ltoreq.1 .mu.m; 
B. SiC-single crystal with average diameter of 0.6 .mu.m and average aspect 
ratio 60; 
C. SiC-single crystal with average diameter of 4 .mu.m and average aspect 
ratio 10; 
D. SiC-single crystal with average equivalent diameter of 20 .mu.m and 
aspect ratio 10; 
E. ZrO.sub.2 with a grain size&lt;2 .mu.m. The SiC-single crystals are 
dispersed and wet milled with alumina powder. After drying, the mixture is 
hot pressed at 1725.degree. C. for 60 min., and then with ZrO.sub.2 at 
1650.degree. C. for 60 min. to 99.6 % of theoretical density. The 
composition variants are shown in Table 1. 
TABLE 1 
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Composition weight percent 
Var. A B C D E 
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1 bal 24 0 0 0 prior art 
2 bal 24 0 0 10 prior art 
3 bal 0 24 0 10 own application 
4 bal 8 8 8 10 invention 
5 bal 8 8 8 0 " 
6 bal 12 12 0 10 " 
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EXAMPLE 2 
The materials from Example 1 are tested as inserts SNGN 120412 in cast iron 
SS 0125 in an interrupted facing operation with high toughness demands. 
The following tool life ranking is obtained for different cutting 
conditions (Table 2). 
TABLE 2 
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Tool Life Ranking for Various Cutting Conditions 
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Cutting speed, 
400 400 700 700 Total 
mmin.sup.-1 
Feed rate, 0.3 0.5 0.3 0.5 Ranking 
mmrev.sup.-1 
Variant 
1 6 6 6 6 24 
2 5 5 4 5 19 
3 4 4 5 4 17 
4 l 2 2 1 6 
5 3 3 1 3 10 
6 2 1 2 2 7 
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The result shows that the variants 4, 5 and 6 according to the present 
invention utilise multiple toughening mechanisms and are superior to the 
prior art variants. 
EXAMPLE 3 
Inserts SNGN 120412 made according to Example 2 are tested in a continuous 
turning operation of cast iron SS 0125 with high demands of wear 
resistance (at a cutting speed of 700 mmin.sup.-1 and a feed rate of 0.3 
mmrev.sup.-1). The relative tool life ranking is shown in Table 3. 
TABLE 3 
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Variant 1 2 3 4 5 6 
______________________________________ 
Relative Ranking 
2 not tested 4 1 3 
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The result shows that improved toughening with ZrO.sub.2 additives results 
in a reduction of wear resistance.