Wear performance of ceramic cutting tool materials when cutting steel

1. JournalofMaterialsProcessingTechnology,28 (1991) 25-36
Elsevier
Wear Performance of Ceramic Cutting Tool Materials when
Cutting Steel
S. Lo Casto~, E. Lo Valvoa, E. Lucchinib, S. Maschiob, F. Micari~ and V.F. Ruisi~
~Dipartimento di Tecnologia e Produzione Meccanica, Universitd di Palermo,
Viale delIe Scienze, 90128 Palermo, Italy
bIstituto di Chirnica Applieata, Universit?t di Trieste,
Via A. Valerio, 34127 Trieste, Italy
Abstract
Some test cycles have been carried out in continuous cutting conditions, em-
ploying cutting parameters (feed, depth of cut and cutting speed) chosen follow-
ing experimental planes and suitabe test fields. The wear tests have been
carried out on AISI 1040 steel with cutting speeds from 5ndsec to llm/sec.
The silicon nitride, sintered carbide, cubic boron nitride and alumina rein-
forced with SiC whiskers inserts, have shown, at each assigned cutting
parameter, poor wear resistance when cutting steel.
Alumina and alumina in submicron grain, which has been toughened by
ZrO2 phase transformation, and the oxide-based alumina, have been the better
wear resistance. The mixed based alumina has shown a wear resistance, a little
lower than the previous materials. The low chemical stability seems to be the
reason for the poor performances of the silicon nitride and the silicon carbide
whiskers reinforced alumina inserts.
The high wear rate of the sintered carbide and the cubic boron nitride is
essentially due to the pull out of the ceramic particles. From the other hand, the
chemical stability, together with their good mechanical properties can explain
the appreciable results obtained with the other alumina based materials.
1. INTRODUCTION
Tool failure is also usually attributed to excessive wear on tool flank and
rake face, where the tool is in intimate contact with the workpiece and the chip,
respectively. The principal properties required of modern cutting tool material
for a high production rate and high precision machining include good wear
resistance, toughness and chemical stability under high temperatures. It is not
0924-0136/91/$03.50 © 1991ElsevierSciencePublishersB.V. Allrightsreserved.

2. 26
possible to achieve all these properties, some of which are mutually exclusive, in
a single material.
For this reason many high temperature resistant tool materials have been
developed in the last ten years, such as various coatings and ceramics.
Several refractory coatings have been developed. These include single coat-
ings of TiC, TiN, A1203,HfN or HfC, and multiple coatings of A12O3 or TiN on
top of TiC. Chemical vapour deposition is the technique commonly used in the
coating process. [1,2].
More recently, ceramics have been introduced as tool materials for a wide
range of high-speed finishing and high-removal rate machining operations.
Ceramics are predominantly alumina-based materials, although silicon-
based materials are just beginning to appear with attractive features for certain
applications. These alumina-based materials mainly include oxide-based mate-
rials, such as A1203-ZrO2 and mixed-based materials, such as A1203-TiN-TiC-
ZrO2 [3,4,5,6]. But relatively recently this group has been supplemented by
nonoxide ceramic cutting tool materials, those composed of silicon nitride
[4,7,8].
Moreover in the last three years, 'whiskers' reinforced ceramic material, has
been commercially available. This is a ceramic matrix reinforced with extremely
strong, maximum tensile strength 7GPa, stiff single silicon carbide crystals
(0.3pro diameter, 20-50~m long) commonly called 'whiskers' [9,10,11].
Another ceramic material, now available, is alumina in submicron grain
which is toughened by ZrO~. phase transformation. The effect of phase
transformation toughening is that cracks which starts to propagate in the
material is arrested before the crack grows large enough to allow insert
breakage [12]. Finally the cubic boron nitride can be used to efficently and
economically machine some materials which are difficult to machine at high
speeds and with a high removal rate [13,14].
A lot of the ceramic materials mentioned earlier have been tested for wear
with cast iron and some difficult to machine materials with different cutting pa-
rameter. Therefore it has been very difficult to make a wear performance com-
parison [3,4,5,6,7,15].
The quantity of the steel worked by machine-tools is comparable with the
quantity of cast iron also worked by machine-tools [16]. It is therefore
significant to understand the performance of ceramic material when cutting
steel.
Consequently the purpose of this paper is to refer to the performance of ce-
ramic materials when cutting steel in continuous turning at high speed.
2. I~PERIMENTAL SECTION
A set of tool life tests in continuous dry turning conditions was performed on
AISI 1040 steel whose characteristics are reported in Tab.1. The material

3. 27
worked was supplied as a commercial tube with an outer diameter of 244mm
and an inner one of 144ram. A piece of this tube, approximately 600ram long,
was fixed between chuck and tailstock.
Table 1
Characteristics of AISI 1040 steel
Chemical composition:
Tensile strength:
Hardness:
C=0.43%, Mn=0.76%, Si=0.28%, S=0.027%, P=0.016%
R=620 N/mm2
HBN(2.xls~.5) = 182
All tests were carried out with a Boehringer DM 550/1000 lathe.
The cutting tool materials, selected for tests, for inserts SNGN 120408 were
the following:
sintered carbide grade P10 (WC-TiC-Co), in the following called "C";
silicon nitride (SisN4), in the following "S";
alumina based (A12Oa-ZrO2),in the following "F";
- mixed-based alumina (A12Oa-TiN-TiC-ZrO2), in the following "Z";
alumina reinforced with SiC whiskers (A1203-SiCw), in the following "W";
alumina based material (A1203-ZrO0, in submicron grain in the following "G";
cubic boron nitride (BN), in the following "B".
The room temperature characteristics of such tool materials are reported
in Tab. 2
These inserts were mounted on a commercial tool holder having the
following geometry:
- rake angle 7=-6°
- clearance angle a= 6°
- side cutting edge angle ~V=75°
- inclination angle ~.=-6°
Each cutting tool material was tested three times, with each of the following
cutting conditions:
- depth of cut d=2.00mm;
- feeds f equal: 0.224mm/rev, 0.28mm/rev;
- speeds V equal: 5.5m/sec, 7.8m/sec, 11m/sec.
In each test the cutting tool wear level was periodically submitted first to a
classical control by profilometer and after to observation of rake face and flank
by a TV camera. Each image of the cutting tool observed in this way was
digitized by a real time video digitizer board. Finally the image so obtained was
stored in an optical W.O.R.M. disk. With this technique it is always possible, to
measure the flank wear and to observe and control the crater dimensions.
Moreover the cristallografic and microstructural features were determined
by the X-ray diffractometry and SEM microscopy.

4. 28
Table 2
Room temperature characteristics of tool materials employed
Characteristcs Units C S Z F W G B
Density Kg/m3 12000 3200 4200 3800 3700 4300 3500
15000 3400 4300 4500 3800 4200
Vickers GPa 17 15 20 17 19 40
Hardness 19 21 24 21 21 60
VHmeasured GPa 15.2 15.5 18 17 19.5 17 25
Young's GPa 550 300 360 340 390 366 680
Modulus 600 380 420 400 400
Poisson's 0.22 0.25 0.21 0.24 0.23 0.23 0.22
Ratio 0.27 030
UltimateTensile GPa 1.0 0.4 0.35 0.24 0.45
Strength 2.0 0.35
Ultimate Comp. GPa 4.5 2.5 4.4 2.8
Strength 5.0 4.0
Trasverse Rupt. GPa 1.7 0.60 0.6 0.6 0.55
Strength 2.8 0.95 0.9 0.9 0.72
Fracture MN/m~5 10 5 4.0 4 8.5 5.25
Toughness 17 7 5.5 6 9.0
Thermal 10~/oC 5 3.0 7 7.8 6 5.5 3.6
7 4.8 9 8.6 7 5.6
Expansion
Thermal Wm/m~°C 50 30 20 20 16.5 38
Conductivity 50 30 30
An EDAX equipment, coupled with a scanning electron microscope, was
used to perform qualitative chemical analysis of some microstructural details.
The mentioned techniquies were also used to study the aspect of the cutting
zones and the fracture surfaces.
In Tab. 3a and 3b tool life values and Taylor's constants for ceramic materi-
als "G"; "F" and "Z" which showed the best performances were reported.
In Tab. 4 the average tool lives for ceramic materials "S", "C", "B" and "W"
were reported. These latter ceramic materials have shown poor wear resistance.
Fig. 1 and 2 report the tool life - cutting speed relationship for both feeds,
for ceramic materials "G","F" and "Z" for both wear criteria.

5. 29
3. ANALYSIS OF THE RESULTS
From Tab. 3a, 3b and 4 we can infer that the ceramic materials, "F", "G" and
"Z" have shown the best performances. Moreover these ceramic materials at the
highest speed, llm/sec, have shown a tool life between 180 and 300 seconds.
Table 3a
Tool life, in seconds, and Taylor's constants for some ceramic materials, for vari-
ous cutting speeds and feeds, with crater depth criterion Kt/Km=0.1
Mat. Feeds CUTTING SPEEDS [m/sec] Taylor's Statistics
[ram/my] constants
5.5 7.8 11 n C R2 F P
G 0.280
0.224
F 0.280
0.224
Z 0.280
0.224
1552 1620 1710
1700 1800 1895
1423 1430 1598
2146 2150 2346
1128 1140 1146
1561 1690 1753 !
451 470 485
573 650 694
400 466 585
340 364 510
360 395 438
420 431 504
180 200 240
289 300 325
301 305 335
180 230 270
163 170 248
181 194 155
0.33 63 0.98 9.76o.o2
0.39 100 0.99 7.53o.o4
0.45 136 0.92 16.5o.oI
0.30 54 0.91 20.6o.oI
0.39 82 0.97 2.22o.19
0.31 56 0.99 6.970.04
Tab. 3b
Tool life, in seconds, and Taylor's constants for some ceramic materials, for vari-
ous cutting speeds and feeds, with flank wear criterion Vb=0.3
Mat. Feeds CUTTING SPEEDS [m/sec] Waylor's Statistics
[mm/rev] constants
5.5 7.8 11 n c R2 F P
G 0.280
0.224
F
Z
0.280
0.224
0.280
0.224
1108 1340 1537
1144 1236 1398
919 1047 1097
974 1017 1088
836 931 953
964 1024 1063
501 585 671
533 584 675
538 576 593
445 482 534
470 484 488
454 465 472
240 277 331
330 344 383
382 393 420
315 346 356
220 237 255
214 244 268
0.45 139 0.96 0.11o.75
0.54 256 0.97 2.12o.19
0.76 1064 0.97 5.44o.o6
0.63 4.03 0.95 13.7o.ol
0.52 187 0.99 1.01o.35
0.48 147 0.99 1.36o2.s
These perfomances are the best among all the ceramic materials tested but they
are not sufficient.The same ceramic materials, "F", "G" and "Z",have shown a

6. 30
good wear resistance at 7.8m/sec and 5.5m/sec, with both feeds. From Tab. 3a
and 3b we can also observe that the tool life of "F", "G" and "Z", with flank wear
criterion Vb=0.3 is longer, more or less equal or shorter than tool life with crater
depth criterion Kt/Km=0.1, for the highest medium and lowest speed
respectively. This is caused, as is known, by the more rapid growth of crater
depth than that of wear land.
Table 4
Average tool life, in seconds, for some ceramics materials, for various cutting
speeds and feeds with crater wear (Kt/Km=0.1) and flank wear (VB=0.3) criteria
Tool life criterion Kt/Km=0.1 Tool life criterion Vb=0.3
Cutting Feeds Cutting Speeds Feeds Cutting Speeds
Materials [mngrev] [m/sec] [mndrev] [ndsec]
5.5 7.8 11 5.5 7.8 11
S 0.280 10 3 1 0.280 13 4 2
0.224 15 4 1 0.224 15 4 2
C 0.280 150 10 1 0.280 500 30 4
0.224 200 27 3 0.224 800 90 12
B 0.280 180 100 20 0.280 240 150 30
0.224 400 110 21 0.224 350 120 28
W 0.280 400 300 150 0.280 400 300 180
0.224 500 350 200 0.224 480 300 140
In cutting tool "F" the X-ray diffractometer and the SEM examination
revealed the presence of approximately 7-10 vol.% of tetragonal zirconia; the
inclusions size ranged from 0.4 to 1 microns. The SEM analysis showed also the
presence of smooth fracture surfaces in the tool nose. The aspect of these
surfaces is similar to the one of the metallic particles pasted into the cutting
zone; moreover the EDAX analysis revealed, in these zones, a high iron
concentration. We suppose that these fractures can be imputed to the oxidation
of the steel penetrated into the cracks during the turning tests. This hypothesis
is corroborated by the examination of a macrocrack propagating from the
chamfer into the ceramic insert ( see Fig.3 and Fig.4 with distribution map of
the iron).
The microstructure analysis of ceramic material "G" showed the presence of
20-25 vol.% of tetragonal zirconia. The SEM examination of the cutting zone
revealed also in this case the presence of the chemical wear and fracture
surfaces covered by a steel layer. Large cracks in the crater bottom were also
observed.
The SEM examination of the microstructure of ceramic material "Z" revealed
the presence of approximatively 30-35 vol.% of inclusions in the alumina matrix.

7. 31
The X-ray diffractometry confirmed the presence of TiC and TiN, but it was
not possible to ascertain the presence of zirconia with this analytical method,
because probably its quantity is lower than 5 vol.%. After the turning tests the
surface fractures covered by a metallic layer are evident, but it was not possible
find evidence of cracks in the crater bottoms.
F
= ~x~
LJ
4
10 =
k
Kt/Km=0.1
10
Cutttng opood Em/ooo ]
010
m
6J
o
,.s
o
o
t--
10
|illl
~ 1 1 1
I ~ L I I I I
I I k 1 ~ l l
IlIl~l
I I 1 ~
IIIW
Vb=O. 3
10
Cut, ttng opood Em/ooo]
Figure 1. Tool life - cutting speed relationship for feed=0.28 mm/rev.
r~
°10
m
t_l
I
o
o
I--
10
3
k',
~ o
z
K t/Km=O. 1
10
CuLttng opood rm/ooo]
r-~
=°10=
m
o
o
o
b-
10 =
G
~'%
z
Vb=O. 3
10
Cutt, tng epeod Em/oeo ]
Figure 2. Tool life - cutting speed relationship for feed=0.224 mm/rev.
In Tab. 3a and 3b some relevant statistics related to the fitted curves are also
reported in the last two columns. The first one represents the squared
correlation coefficient; as it can be noted these values are highest in both
criteria, assuring that the two variables (cutting speed, life) are linked very

8. 32
Figure 3. Macrocracks in the insert
"F" filled by steel, after cutting at 7.8
m/sec for 540 sec using a feed rate of
0.28 mm/rev
Figure 4. Macrocracks in the insert
"F" of Fig. 3 with iron distribution.
Figure 5. Flank wear in insert "F",
after cutting at 11 m/sec for 330 sec
using a feed rate of 0.224 mm/rev
Figure 6. Pull out of the WC
particles in the insert "C", after
cutting at 7.8 m/sec for 30 sec using
a feed rate of 0.28 mm/rev

9. 33
strictly. The second one is a F-test which computes the divergency from linearity
in respect of the internal variability measured on the replications obtained with
same cutting conditions. Such F-test value is followed by the associated
significance level. As it can be seen, for the crater depth criterion the linearity of
the relationship must be questionable, in fact the reached significance levels are
below 5% for five curves on six. On the other hand, for the flank wear criterion,
only one fitted relation show a significative divergency from linearity (F=13.73,
P=0.01). In base of this, the measures of flank wear criterion give a best fitness
to the Taylor's curves for these cersmlc materials (G, F, Z).
On the other hand the measurement of flank wear evaluation, Fig. 5, on the
"F", "G" and "Z" ceramic materials is easy, exact quick and repeatable. For these
reasons for this ceramic materials, the flank wear must be measured to evaluate
wear.
As seen from Tab. 3a and 3b the tool life and Taylor's constants of ceramic
materials "C", "S", "B" and "W" are not reported. This is due to their poor wear
resistance. The average tool life of these ceramic materials are reported in
Tab.4.
The sintered carbide "C", WC - Co, exhibits a relatively high wear rate,
Tab.4. This behavior can be attributed to the high temperature attained during
the tests at high speeds. Under these conditions the binder phase (a Co based
alloy) is softened and the WC particles can be easily pulled out by the
mechanical action of the tuning metallic piece. In Fig. 6 is presented the view of
the inner edge of the crater: the pull out of the tungsten carbide particles and
the deformation of the metallic phase are evident. Also we can observe, from
Tab. 4, that only for sintered carbide "C" the tool life for each speed and feed,
with flank wear land criterion Vb=0.3, is three or four times that of analogous
tool life with crater depth criterion KtlKm = 0.1. This means that this material
has poor flank wear for the speeds, feeds, depth of cut and tool geometry
employed.
In the ceramic material "S" the wear rate is very high and the craters are
large and deep, Table 4. This behavior is due to the chemical reaction between
the steel and the binder phase which is present in the silicon nitride materials.
In Fig. 7 is shown the crater bottom after a chemical etching which has removed
the surface layer. It is evident that the metal (the grey and smooth phase) has
reacted with the ceramic material and has permeated in the SisN4. It was also
observed that, under the same test conditions, the width of the crater in the "C"
and in the "S" cutting tools is approximately the same, whereas the depth in the
SisN4 insert is higher. This behavior can be explained considering that the
chemical wear in the silicon nitride materials is preeminent.
The cutting tool "B" consist of a cutting edge, joined by brazing to a sintered
carbide holder. The cutting edge is based on cubic boron nitride (CBN) particles
and exhibits a poor wear resistance, Table 4 [17]. As in the case of the "C"
material, the performances of this tool are related to the high temperature
behavior of the binder phases.

10. 34
Figure 7. Crater bottom of the insert
"S", after cutting at 7.8 m/sec for 10
sec using a feed rate of 0.28 mm/rev.
Deep penetration of the steel in
Si3N4 are evident.
Figure 8. Crater wear of the insert
"W", after cutting at 7.8 m/sec for
300 sec using a feed rate of 0.28
mm/rev.
Figure 9. Insert "W" of Fig. 8. The
holes previously occupied by the
whiskers are evident.
Figure 10. Spherical particles
deposited on the surface of the insert
"W" of Fig. 8

11. 35
Despite of the room temperature high toughness of the silicon carbide
whiskers the insert "W" exhibits a relatively high wear rate, Table 4. This
behavior can be explained examining the altered microstructure of the crater
borders. The ceramic surface under the deposited metallic layer is completely
free from the SiC whiskers and holes are also evident where the whiskers were
previously set, Fig. 9. The pull out of the reinforcements is essentially due to a
chemical reaction between the steel and SiC with formation of silicides and
eutectics [18]. The presence of many particles with a spherical shape is the
indication of the liquid formation during the cutting tests, Fig. 10. No cracks
were found in this insert and its behavior can be explained considering its high
toughness and its high thermal conductivity, Table 2. On the basis of the above
mentioned observations it is possible infer that in this cutting tool the chemical
wear is predominant on the mechanical.
4. CONCLUSION
On cutting steel, with speeds from 5ndsec to 11ndsec and feeds from
0.2mm/rev to 0.3mm/rev with ceramic materials, previous mentioned, we can
conclude that:
-no ceramic materials tested have sufficient performance at llrrgsec;
-oxide based alumina, alumina in submicron grain and the mixed based
alumina, have shown the better wear resistance: the latter has shown a wear
resistance that is a little lower than the previous materials;
-the highest wear rate was exhibited by the silicon nitride cutting tools "S",
this behavior is due essentially to the chemical reactions between the steel and
the binder phase;
-the WC-TiC-Co cutting tools "C", showed poor performances because of the
heavy pull out of the tungsten carbide particles. In this case the wear can be
essentially imputed to the plastic behavior of the metallic matrix at high
temperatures;
-better performances exhibited the ceramic materials "F" and "G", despite
the chemical stability of alumina, evidence of reactions between the steel chips
and the ceramic material were observed; the presence of cracks filled of metal
and fracture surfaces with a high iron concentration suggest that the wear
mechanism is of mixed type: mechanical and chemical;
-the wear resistance of the insert "Z" is comparable with that of the two
above mentioned alumina based tools;
-SEM examination of the cutting tool "W" showed the presence of an
important chemical reaction between the steel and silicon carbide whiskers;
-the presence of a binder, also influences the performances of the "B" cutting
tool, which exhibits a poor wear resistance, despite the hardness of the CBN
particles.

12. 36
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