Note: Descriptions are shown in the official language in which they were submitted.
~332~06
-1- K-0933Y
`:
; ALUMINA-ZIRCONIA-SILICON CARBIDE-MAGNESIA CERAMIC
; ~ ! CUTTING TOOLS '
;`[~ BACKGROUND OF THE INVENTION
'~ 5 The present invention relates to alumina
based ceramic cutting tools containing zirconia and
silicon carbide. It especially relates to ceramic ~`'
cutting tools useful in the high speed, rough machining
of ferrous and nonferrous metals and alloys.
10 ~ In the past, zirconia has been added in ~'
specified amounts to selected alumina-silicon carbide
whisker reinfqrced compositions described in United
States Patent Nos. 4,534,345 and 4,507,224 to provide
enha~nced fracture toughness andjor flexural strength `"
15~ sée: United~States Patent Nos. 4,657,877 and ''
4,749,667;~Japane~se Patent Publication No.
Sho~62-265182;~Clausen et al, "Whisker-Reinforced Oxide
Ceramic;e," Journal de Physique Colloque Cl, Supplement
au~No. 2, Tome~47, February 1986, Pages Cl-693 to ~;
20~ Cl-702; Becher et;al, "Toughening of Ceramics by
Whisker Reinforcement," Fracture Mechanics of -~
Ceramics~7,` ed. by Bradt e~al, Plenum P~ess, New York ' '~
(1986)~, Pages'61-73)~
It has been indicated that the æirconia
25~ should be in~the monoclinic and/or ~metastable) ~'~
;;tetragcnal phase to obtain improved fracture toughness ~`
and/or flexural strength. It has been further
indicated that the metastable tetragonal phase is
~ ~ ; .. ~ .:.
,~: ~ :: :.. : .
1 3 3 2 1 0 6
-2- K-0933Y
: .
3 obtained by reducing zirconia particle size or through
Z the use of a cubic zirconia stabilization promoter such
as yttria, calcia, magnesia and the rare earth oxides
` in amounts below that required to fully stabilize the
¦ 5 cubic zirconia phase.
~ Cutting tools composed of a variety of
3 compositions containing alumina, zirconia and silicon ;
carbid2 whiskers with or without other additives have
: -:
been proposed (seeZ European Patent Application No.
¦ 10 86107916.8 (published January 21, 1987 as No. 0208910);
YZ United States Patent No. 4,749,667; "Multitoughening
Z~
Zl Ceramic," Techno Japan, Vol. 19, No. 10, October 1986,
Page 78; and European Patent Application No. 86301597.0
published September 17, 1986, as No. 0194811).
i~ 15 None of the foregoing documents teaches or
suggests that, for metalcutting incerts for use in the
high speed roughing of ferrous and nonferrous metals
and alloys, cutting performance can be significan-tly
improved by controlling the alumina based ceramic
composition to within the combination of critical
ranges now discovered by the applicants for zirconia, ~ ~
magnesia, silicon carbide and tetragonal zirconia ~--
contents~
;~ SUMMARY OF THE INVENTION
The present invèntion provides alumina based
ceramic cutting ihserts containing (in volume percent,
v!o) about 1.5 to 37.5 v/o (volume percent) silicon -~
~ carbide whiskers, about 5 to 17.5 v/o zirconia, theZZ
Z~ residueZ of a magnesium oxide or other màgnesiumToxygen
compound addition, and at least 2 v/o tetragonal
zirconia. It has been surprisingly found that, despite
~ ~ the finding that minor magnesia additions act to reduce
Z`~ the amount of tetrag~nal (i.e., metastable tetragonal)
~ zirconia at room temperature, an effective amount of ;~
Z: ~
this addition has a significant positive affect on
cutting edge li~etime in the high speed rough turning
. , .
,
~" ~" " ;~ ~" " ~ Z
1 332 1 06
-3- K-0933Y
of soft steels, such as AISI (American Iron and Steel
Institute) 1045 steel.
It is preferred that magnesia be added in
amounts of about .03 to 3 v/o; more preferably, about
0.03 to 2.0 v/o; and most preferably, about 0.04 to
1 . O v/ o .
Preferably, the alumina based ceramic
composition according to the present invention contains
about 2.5 to 35 v/o and, more preferably, about 5 to
10 32.5 v/o silicon carbide whiskers.
The zirconia content according to the present
invention is preferably 7.5 to 17.5 v/o and, more
preferably, 10 to 15 v/o. In accordance with the
present invention, significant fraction of the zirconia
15 is in the form of tetragonal zirconia and must form at
least 2 v/o of the composition. Preferably, at least
4 v/o; more preferably, at least 6 v/o; and most
preferably, at least 8 v/o of the composition is in the
form of tetragonal zirconia. ~
These and other aspects of the present --
inv~ntion will become more apparent upon review of the
detailed description of the present invention in
conjunctlon with the figures briefly described below;
BRIEF_DESCRIPTION OF THE DRAWINGS
Figure l is an isometric view of an
embodiment of a s~uare cutting insert in accordance
with the present invention.
Y~ Figure 2 shows a graph of the volume percent -
tetragonal zirconia in the composite as a function of
30 the volume percent of magnesia, or yttria, addition.
DETAILED DESCRIPTION OF TH~ INVEN~T0N
In the present invention, as shown in
Figure 1, metalcutting insert 10, preferably of the
indexable type, is provided having a rake face 30, a
35 flank face 50 and cutting edge 70 at the juncture of
the rake and the flank faces. The cutting edge 70 is
preferably in a chamfered condition (e.g., K-land) as
~ 1 332 1 06 :
-4- K-0933Y
shown in Figure 1. The metalcutting insert is composed
of an alumina based ceramic composition containing:
about 1.5 to 37.5 v/o silicon carbide whiskers; about 5
to 17.5 v/o zirconia; and a residue of a magnesium
oxide or other magnesium-oxygen compound addition added
in an amount effective to enhance the metalcutting
lifetime of the cutting edge. The silicon carbide
whiskers, zirconia and residue of magnesia are
substantially homogeneously distributed in an alumina
based matrix.
The silicon carbide whiskers are present at a
level of at least about 1.5 v/o to assure minimal
levels of cutting edge lifetime improvement. More ~-
~; preferably, silicon carbide whiskers are present at ~-
about 2.5 v/o or more and, most preferably, at about 5
v/o or more. The silicon carbide whisker content
preferably should not exceed about 37.5 v/o of the
composition. We believe that silicon carbide whisker
contents above this value result in a significant
decrease in the cutting edge lifetime. Therefore, to
further maximize cuttin~ edge lifetime during high
speed rough turning, it is preferred that the maximum
content of silicon carbide whiskers be held at or below
about 35 v/o and, more preferably, 32.5 v/o of the
alumina based ceramic composition. The optimum silicon ;~
carbide whisker level will depend upon the application.
The silicon carbide whiskers utilized herein
may, for example, be any of the commercially available
brandslwhichihave been used in the past in alumina
based metalcutting inserts for machining nickel base
superalloys.
While less preferred, silicon carbide
particles of a generally equiaxed shape or platelet
shape may be substituted for part of the silicon
carbide whiskers in this invention. -
, The zirconia content is in the range of about
5 to 17.5 v/o of the ceramic cQmposition. Zirconia
: ~ ,
~ .
~.
~ 3321 06
-5- K-0933Y
.
contents outside of this range are believed to provide
compositions having reduced cutting edge lifetime
during the high speed roughing of AISI 1045 steel.
Preferably, to maximize cutting edge lifetime, the
zirconia content should be within the range of about
7.5 to 17.5 v/o and, more p~eferably, about 10 to
15 v/o of the composition. While we believe that the -
concentration of tetragonal zirconia should be
maximized for best cutting performance, it is equally,
if not more, important in our opinion that as much of
the tetragonal zirconia as possible, present at room
temperature, be available for transformation toughening
at, or as near as possible to, the temperatures
z encGuntered at the cutting edge during machining. Itis, therefore, critical to the present invention that
i~ magnesia be present in certain smail, but effective,amounts which have been found critical to the
maximization of cutting edge lifetime. In accordance
with the present invention, and at least 2 v/o of the ~
~- 20 ceramic composition, must be tetragonal zirconia. ~-
Preferably, the tetragonal zirconia forms at least
about 4 v/o of the composition, more preferably, at
least about 6 v/o of the composition, and most
preferably, at least about 8.0 v/o of the composition. `-~
Magnesia additions, despite tne fact that
they decrease the amount of tetragonal zirconia
observed at room temperature, are added in the range of
about .03 to 3 v/o of the composition, preferably,
about~0.03 to 2 v/o; more preferably, about 0.04 to ~
1.0 v/o. Magnesia may be blended in with the alumina
or zirconia just prior to compact pressing, or it may
~' ; be preblended or prealloyed with the alumina or
zirconia. Preblending of the magnesia powd2r is
pre,ferred since it is believed that the preblended
~magnesia is more effective in producing high
temperature metastable tetragonal zirconia, thus -~
allowing a smaller amount of magnesia to be added and ;
:''~
~ -
~ 3 32 1 0~ ~
.
.
-6- K-0933Y
minimizing the deleterious effects of high magnesia
additions (e.g., lower melting point glass and Mg-Al-O
formations). Equivalent amounts of other magnesium-
oxygen compounds, such as magnesium carbonate, which
may require an additional processing step such as
calcination to produce magnesia, may be substituted for
- all or part of the magnesia addition. After sintering
of the blended compositions, the magnesia addition may
not exist as a separate phase but as a residue. This
residue may include, for example, magnesium aluminate,
magnesia alumina solid solution, a magnesia zirconia
solid solution and/or a glass, for example, formed with
silicon dioxide impurities which may have existed as a -
thin coating on the silicon carbide whiskers. ~ ~-
The remainder of the ceramic composition is
essentially alumina and preferably entirely alumina ;
except for impurities. In all cases, the present
alumina based ceramic composition contains at least 40
v/o alumina, and preferably at least 50 v/o alumina.
Titanium carbide, as whiskers and/or ;
substantially e~uiaxed particulate, may be added in an ~
amount of about 2-35 v/o of the composition and, -
preferably, about 10 to 30 v/o. Titanium carbide has a
~ higher thermal expansion coefficient than alumina. It `~
-~ 2S is, therefore, believed that titanium carbide additions
should allow more tetragonal zirconia to be xetained at
room temperature. Titanium carbide whiskers may be
manufactured and harvested by the methods described in ~
A. Kato et al, "Growth Rate of Titanium Carbide ~i
30 Whiskers in Chemical Vapor Deposition," J. Cryst. -~
Growth, 37 tl977), Pages 293-300; and N. Tamari et al,
~' "Catalytic Effects of Various Metals and Refractory
Oxides on the Growth o~ TiC Whiskers by Chemical Vapor
Deposition," J. Cryst. Growth, 4~ (1979), Pages 221-
; 35 237. Titanium carbide whiskers and their incorporation
~ and use in alumina based cutting inserts are disclosed
A :
t 332 1 06
-7- ~ K-0933Y
in ~ehrotra et al Canadian Patent No. 1,308,919 issued
October 20, 1992.
The alumina powders utilized herein should be
high purity alumina (i.e., >99% pure) such as produced
by ALCOA (e.g., grade A16SG), or by Ceralox (e.g.,
grade HPA - 0.5 with or without magnesia), or by
Reynolds Chemicals (grade RC-HP or RC-HP-DBM).
Yttria, calcia, the rare earth oxides, and
other compounds which have, through a reduction in the
tetragonal to monoclinic transformation temperature, an
adverse affect on cutting edge lifetime are preferably
present only as impurities, if present at all.
The foregoing material, in accordance with
the present invention, may be milled, blended, and -~
densified at high temperature to produce at least 98%,
and preferably, at least 99% dense alumina based ~
ceramic compositions having an alumina based matrix, ~ -
which is preferably entirely alumina, in which the
silicon carbide, magne~ia residue from the magnesia
addition, zirconia and titanium carbide, if any, are at ~-
;~ least substantially homogeneously distribu~ed. Hot
pressing temperatures are preferably held below 1750~C,
and more preferably, below 1650C and, most preferably,
below about 1600~C to minimize zirconia particle growth
and thereby maximlze the tetragonal (i.e., metastable -;~
tetragonal) zirconia phase present in the final
product. The average zirconia particle size in the ;~
cutting insert should not exceed about 5 microns,
preferably should not exceed 2 microns, and more ii.~.
30 l preferably, should not exceed 1 micron. Howeverl, the
average zirconia particle should be large enough to
-` allow most tetragonal zirconia to transform to
monoclinic zirconia during use. This minimum size will
depend upon the ceramic composition and is presently
undetermined. - -~
While not wishing to be bound by any
particular theory, applicants offer the following
A
~ t 332 1 06 K-0933Y
explanation of the present invention. In alumina-
silicon carbide whisker-zirconia compositions, the
amount of metastable tetragonal zirconia that can be
obtained at room temperature can be increased by a
reduction in zirconia particle size or the addition of
the so-called cubic stabilizing agents, such as yttria,
calcia and/or the rare earth oxides. (See: Stevens,
"An Introduction to Zirconia--Zirconia and Zirconia
Ceramics," Magnesium Elektron Pub. No. 113, Magnesium
Elektron Ltd., England (1986)). While the literature
commonly includes magnesia among the foregoing list of
stabilizing agents, applicants have found that, when
magnesia is added in the amount of about .03 to 3 v/o
to the present compositions, magnesia decreases the
amount of tetragonal zirconia present at room
temperature. When yttria is added to zirconia, it
~- tends to stabilize the tetragonal and cubic phases of
zirconia to a lower temperature. All the
aforementioned stabilizing agents, and most impurities,
;~ ~ 20 except for magnesia, affect ZrO2 in similar ways (i.e.,
they reduce the temperature at which the tetragonal ~
zirconia phase is stable). At room temperature, some -
of the zirconia may be present as metastable tetragonal
zirconia. Under the action of tensile stress, this
tetragonal zirconia may become monoclinic, giving rise
to trans~ormation toughening. However, as the
temperature increases, tetragonal zirconia becomes
stable and, therefore, unavailable ~or transformation
, to the monoclinic crystal structure. Thus, any I
impurity or additive, such as yttria, which stabilizes
tetragonal zirconia at lower temperatures, is
' unsuitable for metalcutting applications since the
cutting tip temperature may rise to about 1000 to
1200~C in high speed machining. Therefore, in
accordance with the theory of the present invention,
additives which raise the monoclinic to tetragonal
transformation temperature of the zirconia are required
: .-
1 332 1 06 K-0933Y
for high temperature transformation toughening. We
have found that there are only two oxide additives, MgO
and HfO2, which raise the transformation temperature.
Thus, we believe that by keeping the zirconia particle
size small, a large proportion of the zirconia can be
retained as metastable tetragonal zirconia at the high ;
temperature of metalcutting by alloying the zirconia
with magnesia or hafnia. It is our belief that this
helps obtain enhanced cutting edge lifetimes during
metalcutting operations. It should be noted that
zirconia normally contains up to about 2 w/o (weight
percent) hafnia as an impurity. -~
The significant positive impact that magnesia
additions have on metalcutting performance is more
clearly indicated by the following examples which are
purely illustrative of the present invention.
~,~ Six compositions were prepared (Table 1) with ;~
the nominal compositions being Al2O3 - 10 v/o SiCw ~;~
(silicon carbide whiskers) - 10 v/o ZrO2. Small ~;~
~ 20 additions of Y203 an~ MgO were made. In the case of
; ~ Mix No. 6, abput 0.05 w/o (apprQximately 0.06 v/o) ~gO
had already been blended with Al203 by the powder
manu~acturer. This provided Mix No. 6 with a magnesia
content of abou~ .04 v/o. Fifty gram batches of these
powders~ were prepared by first blending Al2O3 and ZrO2
;(and stabilizing additives, if any) slurries (propanol)
in a ~ar mill using Al203 cycloids for one hour.
Sonicated SiCw slurry was then added, and the whole mix
was blended for one hour. A12O3 and ZrO2 slurries had
~ previously been milled to obtain mean particle sizes of
0.5 to 0.6 ~m and 0.6 to 0.8 ~m, respectively
y~ (oorresponding specific surface areas measured by BET
were 10 to 14 m2/g and~20 to 40 m2jg, respectively).
Thèn, the mix was pan driad, screened through 100 mesh
, !'`" ~35 screen, and hot pressed in a one inch diam~ter graphite
`^ die at the temperatures and pressures shown below in ;~
~ Table l for one hour in argon. The resulting billets
32 t O6
-10- K-0933Y
were more than 99% dense, and were cut, ground and
polished for measurement of physical and mechanical
properties. The billets were also cut and ground to
produce indexable cutting inserts for metalcutting ~
tests. -
T~BLE 1: COMPOSI~IONS
Nominal Composition: A12O3 - 10 v/o SiCw - 10 v/o Zro2 -~
A12O3: Alcoa A16SG -~
SiCw: Tokai Carbon Co. (Tokyo, Japan)
TOKAWHISKER (TOKAMAX) Grade 1
(0.3 - 1.0 ~m diameter; 20-50
~m length)
ZrO2: Zircar - unstabilized
MgO: Fisher Scientific Corp. - Reagent
Grade BET = 40.4 m2/g
~` Y2O3 MolyCorp, BET = 15.4 m2/g
Hot Pressinq
Mix No. Temp.(C) Pressure (psi) Composition -
1. 1650 4000 Nominal
2. 1650 4500 Nominal ~ 1 v/o Y2O
3. 1625 4500 Nominal + 1 v/o MgO
4. 1600 4500 Nominal + 1 v/o Y2O3
+ 1 v/o MgO
5. 1625 4500 Nominal, except that
A12O3 used was Ceralox
Grade HPA - 0.5 ~0.5
to 0.7 ~m median
particle size)l
6. 1625 4500 Nominal, except that
A12O3 used was Ceralox
~` ~ Grade HPA - 0.5 with
MgO ( 0-05 w/o)
;; 35
~ .
'~; ,
, ~ ~
1332106 ~ ~
-11- K-0933Y
TABLE 2 - PROPERTIES ~ ~ .
Tetragonal Zro22 ~;
NixFracture Toughness as v/o as v/o of
5 No. RA Hardness KIC (E&C)l (MPa m~) of ZrO2 Composite
:
1 93.3 5.59 84 8.4
~ -
2 93.3 4.97 100 10
3 93.6 5.96 70 7
,
4~ 93.6 4.63 98 9.8
~-~ 5 93.9 5.88 84 8.4
6 93.8 5.88 76 7.6
1 Evans and Charles, "Fracture Toughness ;-~
Determination by Indentation," J. American Ceramic
Society, Vol. 59, No. 7-8, Pages 371, 372, usin~ an
18.5 kg load.
2 As measured by x-ray diffraction of a polished
surface. The remainder of the zirconia is assumed to -~
be monoclinic zirconia. Cubic zirconia, which may be
~present in minor amount~, is included within the
; tetragonal zirconia estimate.
Physical and mechanical properties of the hot
pressed composites are shown ln Table 2.
25 ~ The Porter-Heuer ~Porter et al, J. American
Ceramic Society,- ~ol.; 62, No. 5-6 (1979), pages
298-305) formula~was modified and used to estimate the
fraction of monoclinic ZrO2 (Vm~ from peak intensities ;;
of the 111 reflection of the monoclinic ZrO2 (I~ 13,), ;~
~and 111 reflection of;the tetragonal ZrO2 ~It(lIl)):
1.60 Im (111) ~ It (111)
and Vt - 1-Vm (2) ;
35~ wherè Vt is the fraction of æro2 Which is tetragonal `~-
~: : ' :-' ':
: .
` 133~2106
-12- K-0933Y
The estimated volume fraction of tetragonal
Zr2 in the whole composite is (vt):
vt = Vt vz (3)
where vz is the volume fraction of the total ZrO2 added
in the mix. The above relationship assumes that ZrO2
substantially remains unchanged during consolidation
except for the phase transformations discussed above.
Figure 2 shows the effect of the various
additives on the amount of tetragonal zirconia in the
composites. It can clearly be seen that magnesia
additions lower the amount of tetragonal zirconia
(curve 1), whereas, yttria additions increase the
amount of tetragonal zirconia at room temperature
(curve 2).
TABLE 3: TURNING AISI 1045* STEEL (192-200 BHN)
InsertCutting Edge Life &Average Life
MaterialFailure Mode (minutes)
Mix 1 14. BK 14.7 BK 14.4
Nix 2 8. BK 12.6 ~K 10.3
, j, ,
` MiX 3 15.9 BK 29.2 FW 22.6
Mix 4 1.7 FW - 7.7 FW 4.7
Mix 5 17. FW 7.5 BK 12.3
Mix 6 22.9 DN 32.9 FW 27.9
25 Test Conditions:
lOOO s~m (sur~ace feet/minute)
0.025 ipr (inch/revolution)
0.100 inch doc (depth of cut)
SNGN-453T (American National Standard j
Designation in accordance with ANSI B212.4
1986) indexable cutting insert style
~d (cutting edge preparation:
0.008 inch x 20 K-land)
15 lead;angle (side cutting edge angle)
-5 side rake angle
~ -5 back rake angle
`~ no coolant
~ .
-13- K-0933Y
~332~06
Cuttin~ Edqe Life Criteria:
FW - .015" uniform flank wear
MW - .030" concentrated flank wear
CR - .004" crater wear
DN - .030" depth of cut notch
CH - .030" concentrated wear or chipping
BK - breakage
*AISI 1045 is equivalent to Unified Numbering System
(UNS) Designation - Gl0450.
Indexable insert cutting edge lifetimes in
the high speed roughing of a premachined AISI 1045
steel are shown in Table 3. It can be clearly seen
that a significant improvement in cutting edge life is
achieved by the addition of magnesia, whereas, a :
decrease in tool life occurs when yttria is added :~:
despite the high level of tetragonal zirconia present
in the yttria containing compositions. ::
TABLE 4: COMP SITIONS
Nominal Composition: Al203 - 5 v/o SiCw - 10 v/o ZrO2 ~
Al2O3: Ceralox-HPA-.5 without MgO ;.
SiCw: Tokai Grade l ~:
: ZrO2: Magnesium Elektron (SC15) ~
25 : unstabilized (.5 - .6 ~m particle :
size BET 5 - 8 m2/g) :-
Hot P~essing -::
Mix No. Tem.C P~essure~psi~ Composition .:~
7. 1535C 5000 psi Nominal + .05 v/o MgO :- ~
; 30 8. ~ 1550C 5000 psi Nominal + .25 v/o MgO : .
9. ~ 1550C 5000 psi Nominal + .50 v/o MgO .`- :
10. - 1550C 5000 psi Nominal ~ 1.0 v/o MgO
ll. 1550C 5000 psi Nominal + 3.0 v/o MgO
, . .
A second series of mixes, 7 to ll shown in -~ :
:: Table 4, were made to further demonstrate the effect
~: that the level of the magnesia has on tetragonal
':
-14- 1 3 32 1 06 K-0933Y
zirconia content and cutting edge lifetime. All
samples were processed and hot pressed essentially as
described with respect to the samples produced from
Mixes 1 to 6.
The physical and mechanical properties of the
materials are reported in Table 5. It can clearly be
seen that the tetragonal zirconia content again clearly
decreases with increasing amounts of magnesia addition.
This affect is also shown in Figure 1, curve 3. It can
be seen that Mixes 7 to 11 have a higher tetragonal
zirconia content than that found in the materials
represented by curve 1. The affect is believed to be
due to the lower SiCw content (5 vJo versus 10 v/o)
used in the second group of mixes. Applicants have
observed that generally, as SiC whisker content
increases, the amount of tetragonal zirconia decreases
for a given zirconia content and zirconia particle
size, and everything else being held constant.
TABLE 5: PROPERTIES
Tetragonal
Zr2 as
Mix Fracture Toughness v/o of
No. V/o MgO RA Hardness KIC (E~C) (MPa m~) Composite
__ __
7 0.05 93.5 5.57 8.5
8 0.25 93.4 5.00 8.1
'~ 9 0.50 93.3 5.06 8.0
1.0 93.4 4.75 ! 7.6
11 3.0 93.5 4.98 7.3
Indexable insert cutting edge lifetimes in
~ the hlgh speed roughing of AISI 1045 steel are shown in
-~ Table 6.
~ 35
I~ 15- 1 332 1 06 K-0933Y
TABLE 6: TURNING AISI 1045 STEEL (197-199 BHN)
Insertcutting Edge Life Average Life
Material& Failure Mode (Minutes)
Mix 7 19 bk5 ch/bk 12
8 12 . 6 dn/ch 1 bk 6.8
9 12.3 dn14.1 dn 13.2
9.7 bk7.1 dn 8.4
11 7.0 bk6.8 dn 6.9
The test procedures and conditions and
cutting edge life criteria used to generate the data
shown in Tables 5 and 6 were the same as that described -~
for Tables 2 and 3. ~ ;
In another example in accordance with the
present invention, a composition containing A12O3-2.5
v/o SiCw-10v/o ZrO2-1.05 v/o MgO was made. A fifty ;~
gram batch of this composition was prepared by first
blending an A12O3 (Ceralax Grade ~PA-0.5 with MgO (0.05
w/o)), ZrO2 (Magnesium Elektron SC15) and MgO (Fisher
Reagent Grade) slurry (propanol) in a jar mill using ~-
A12O3 cycloids for one hour. A Sonicated SiCw ~Tokai
Grade 1) slurry was then added and the whole mix was
blended for one hour. The A12O3 and ZrO2 containing
slurries had previously been milled to obtain mean
particle si~ze of about 0.5 - 0.7 ~m and 0.5 to 0.6 ~m,
- ~ 25 respectively. Then the mix was pan dried, screened
through a 100 mesh screen and isostatically compacted ~-
at 30,000 psi at room temperature. Pieces were then
cut from the resulting cold compacted blank and
sintered at 1700 degrees Centigrade for one hourl in one
atmosphere argon followed by hot isostatic pressing at
1600 degrees Centigrade for one hour in 17,000 psi
argon. The resulting samples were greater than 99
percent dense (i.e., fully dense). As described in the
prior examples, samples were then prepared for physical
i! ~
`~35 and mechanical testing and ground into indexable
cutting inserts. I~ was determined that the samples
contained about 6.6 v/o tetragonal zirconia. It is
1 ~
:
1 3~2 1 06
-16- K-0933Y
estimated that material processed in this manner has a
zirconia particle size of about 5 ~m, or less. Cutting
inserts of the style described in Table 3 were tested
under the conditions used in Table 3. Cutting edge
life times of 14.4 (DN failure) and 18.9 (FW ~ CH
failure) minutes were obtained.
It is believed that cutting edge lifetime may
be extended or made more uniform if the cutting edge is
honed and/or the insert surface is lapped or polished
to remove surface material containing a higher
percentage of monoclinic and a lower percentage of
tetragonal zirconia than is characteristic of the bulk
of the material (i.e., a polished surface). It is
known that grinding stresses create a surface layer in
which a portion of the metastable tetragonal zirconia
has been transformed to monoclinic zirconia. It is
preferred that at least those surface areas of the
insert which will encounter high temperatures during --
use have the maximum amount of tetragonal zirconia
available for high temperature transformation.
While the foregoing examples have shown the
value of the present invention in cutting soft steels,
it should be understood that cutting inserts in
~-~ accordance with the present invention may be used for
z5 machining other materials by optimizing the composition
` to provide best results. For example, for high speed
`~ rough turning of nickel base super alloys, compositions
containing about 25 to 37.5 v/o silicon carbide
whiskers may be most appropriate.
I 30 ! ' Other embodiments of the invention will be'
apparent to those skilled in the art ~rom a
consideration of this specification or practice o~ the
invention disclosed herein. It is intended that the ;~
specification and examples be considered as exemplary
A
~,. . :
: -17- 1 3 ~ 2 1 0 6 K-0933Y
only, with the true scope and spirit of the invention
being indicated by the following claims. -
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