Note: Descriptions are shown in the official language in which they were submitted.
~S~V40
B~CXGROUND OF THJ3 INvE~r ION
F IELD OF THE INVEN~ ION
___~__
This invention relates to materials and manufacturing pro-
ces~es for these materials and, more particularly, to an ~nproved
uniformly fine-grain alumina titaniwfl carbide material and a
technique for producing this material, and the lilce.
Dl sc~ oll or 5NII ~1~ AIC'
Alwnina (A1203 ) and alumina compounds have besn used for
high temperat,ure and high strength purpo~es for many years O For ~;
0 example~, in refracto~y applications and in me~alworking tools that
are subjected to high ~peeds and great wear, the~e materials have :~
found wide~pread industrial acceptanceO
It appears, moreov2r, that the ~trength of this material
is in some manner related to its density and crystal si~e, the
more den~e and smaller crystal structures providing ~tronger and
moxe durabIe tools. Consequently, there i9 a great deal of emphasis
on producing ceramic cutt mg tools with the3e characterLstics.
When used as a cutting edge, however, alwmina occa3ionally fracturesO
In general~ these fra ture~ s~em to be related to the pre~ence of
~o relatively large alumina crystals, or "grains"~ in an e~sentially
small cry~tal or "fine" grain ~tructureO Thu~, much of the alumina
re~earch effort has be~n directed to the more specific development
of techniques for large-~cale production o~ a high den~ity ma~erial
with a uniformly fine grain struc~ure.
The crystal gr~wth that occurs when the raw powder material
i~ heated to coalesce (or is "~intered") often i9 retard~d
through the add~tion of magnesiwm oxide (MgO) in an amount o~ -
0O5% or le~s. ~his beating can be accomplished in a vacuum
fuxnace that raises the material temperature to a 1400 to 1550Co
~ 1 ~
rangeO Processes of thi~ 90rt have been reported to provide a
material that has a cry~tal -~ize on the order of 2 to 3 micronsO
To attain this re~ult, however~ heating t~nes in ~xce~ of four
hours during sintering are requiredO
In the interest of efficiency and production ~conomy, it
is clear that: a reduction in heating time is desirable, e3pecially
if the reduced heating tim~3 can be coupled with the production
of a more uniformly fine grain structure. Because of the tendency
for alwmina tools to fracture9 there also is a need for a tech-
nique ~o produce the even smaller crystal sizes that lead to
greater strength.
SUMMARY OF THE I~VENT ION
In accordance with the invention, reduced heating time and
a fine crystal structuxe of significantly improved uniformity
in 5 ize than that which hereto~ore has been available is achieved
through a novel control of th~physical pressure that is applied
to powdar to be sintered and the rate at which the pr~ssurized
powder is heatedO Some material produced through this technique
has compressive and modulus of rupture strengths that are ~igni- ~
~icantly greater than the best available alwmina~ ~`
The process charac~erizing the invention is, essentially,
a form o rate-controlled sintering i.n which a relatively low
pres~ure is applied to the die while the contained powder is
being heatedO In the cour~e of this heating the compacted powder
at ir~t expand~ in volumeO There is a point, howeve~ termed
the "onset o~ shrinkage temperature" or "onset o~ powder shrinkage"
also called the "~reak away point", at which sintering commences
and the volume of the powd~r begins to shrinkO A maxLmum hot
proce~s pressure i5 applied to the powder when this condition
B :
~0510~
is reached. Subsequently, the powder temperatur~ also is increased
;~ to reach the maximum temperature at~ained in the process. Thus, it ~ .
seems that the physical pressure applied to the sintering po~der
lends an additional driving force that not only reduces production
time, but also provides a d~monstrably superior product.
According to the invention, powders are densified into
fine grained sintered ceramics by applying a first compressing ::
force to the powder and heating it at a predetermined first tem-
perature rate during said force application to an onset of powder
shrinkage and then densifying the powder by applying an increased
second physical pressure and a second, lower heating rate thereto.
In accordance with an emhodiment of the invention, a me~hod for ~:
solidifying an alumina- titanium carbide powder comprises tne steps
of working the powder to remove surface gases and to reduce
agglomerates formed in the powder, compressing the powder, heating
-: the powder at a first rate to produce an onset of powder shrinkage,
increasing the temperature of the powder to a process maximum at
a lower second rate of heating to enhance said sintering for
several minutes at a rate of densification that approaches the
theoretical maximum density of the powder, increasing the physical
pressure, and curing said sintering powder at said maximum process
temperature and said inçreased physical pressure for a few minutes
for sintering the powder at a rate of densification that approaches
. the theoretical maximum density of the powder.
In a preferred embodiment of ~he invention, an alumina- ;
titanium carbide material is produced by ball milling a titanium .
carbide powder in alcohol to an average particle size of about
one micron, the ball milled carbide powder is mechanically mixed
with alumina powder, and the resultant powder, havin~ thus been
worked to remove surface gases and to reduce agglomerates in the ~ :
.
powder, is compressed by a constant physical pressure of about
6300 psi, the pressure is reduced to a lower constant value of about ~ ~
1000 psi and the powder is heated at a first predetermined rate of ~ -
s _ 3 _ ~
:
s~
~00 to 1000C per minute until onset of powder shrinkage at
about ~C, the pressure is increased to a maximum in the range
of 3000 to 9000 psi and the heating is increased, at a lower
second rate than said first rate to a temperature of about 1500C
in 6 to 10 minutes to sinter the powder at a rate of densification
that approaches the theoretical maximum density of the powder,
and said maximum pressure and temparature of about 1500C are
held for 2 to 6 minutes for curing the sintering powder.
In accordance with another embodiment, a method for
producing an al~mina -titanium carbide material comprises the
steps of ball milling a titanium carbide powder, mixing alumina
- powder and said carbide powder r compressing the resultant mixed
powder, heating said compressed powder at a ~irst rate of 400 :
to 1000C per minute, applying a oonstant physical pressure to
said powder while the powder is being heated at. said rate, applying
an increased physical pressure to said powder upon reaching an onset
of powder shrinkage and heating said powder at a second rate while
applying said increased physical pressure for sintering the powder ~ `
at a rate of densification that approaches the theoretical maximum
density of the powder until material densification is complete.
BRIEF DESCRIPTION OF THE DRAWINGS
,.
Fig. 1 is a schematic graph of ram displacement versus
time illustrate the "break away point", and
! Fig. 2 is an array of graphs that show pressure, tem-
perature, density and breakaway point as function of time for a
number of materials.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS ;`~
-- -- - ;: . . ,
Fig. 1 graphically illustrates features of the invention
expressed in terms of the movement or displacement of the ram that ~ ~-
is used to compress the powdered material which is being sintered
I as a function of time. The ram displacement 10 is necessary to
I "prepress" the powdered mixture in order to enhance sintering .
and to remove any entrapped gases in the powder between time O and
~ _ 4 ~
`' ~ .
,
s~
t1. After time tl and before time t2, the application of heat ~:
to the precompressed powder leads to a thermal expansiQn dis-
placement 12 of the ram. This step in the process is terminated
.~
'~
~C)5:~04(~
by a "braak away point" 13 at the t~me t20 This "break away
point" is cha:racteri~ed by a change ~rom khe sxpansion o~ the
pr2compressed powder to a contraction l4 that c:ommence~ as
sintering beginsO The contrac'cion culminates at time t3. The
time t3 i~ a t~ of maximum densification and coale~cence o~ :
the sintered powderO The further application o~ heat aftex
tlme t3 produces exce~ive grain growth or "bloatingl' 16 as indi-
cated by the incr~3a3e in ram di~placement . It i3 at this time
t3, befor~ the material start~ bloating, that the proces~
10 tenninated.
Fig. 2 is a graphic repre~entation of ~intered product ~;
temperature, den~ity and "break away points" as a function of
time for the following materials~
Billet Diameter ~aterial
1/4" U2 ; .
1 " A12C)3
Al23 : ~;
1 " A12(~3-Ti~ '
:
: ~ 5 ~l2o3-Tic
For purpo~es of orientation between Figs.l and 2 the initial
timeJ zero, of Fig. 2 corresponds to the time tl in Fig. lo
The pre~ure "hi~tory" 20 for all of these ma~erials is
bounded by ~traight line ~egments that identify a pre~sure in- ; ;~:
crea~e, a~step func~ion from the initial pres~ure to the maximwm
.: :.'::
hot proce~s pres~ure that i~ maintained throughout the remainder
of the proce~sO ;~
The t~mperature history 22 is bounded by straight line
~egmentsO ~hese temperature bounds indicate an increa3ing tem~
perature in respon~e to the initial heating, ~ollowed by a
~ 5 ~
B: ~:
.... . ... .; ..... ... ... ... . . .,; . . ... . . . . .. i . ..... .. . .. ..
`;.. . ........ .... . . .. . .. .. ~ .. .. , . ..... . . ~
4~
minLmum and maxlmum process temperature range for the remainder
of the proce~.
The theoretical maximum density "hi tory" 24 ~ollow paths
to maximum value~ which are represented by a generalized graph
24. ~he theoretical maximum denfiity i9 defined as the closest
pos~ib~e packing of atoms into the crystalline structure of the
co~pound, exclusi~e o~ any and all Lmpuritic~, that will pro-
duce a minimum inter~titial volume between the packed atoms.
~ he ~reak away point~ as a functio~ time 30 vary, moreover
0 with the material and billet size under con~id~rationO
E:xample:
Alpha alumina powder o~ les~ than one micxon, preferably
less than one tenth micron, particle Y ize i~ worked or ball milled
in a dry mill from four to eight hour~0 Preferably, alumina sold
by W, R, G~ace Company under the name ~IGrace-KA 210" (trade mark)
should be u~ed as a raw material for the practice of the inventioll.
~his alwmina powder has a surface area on the order o~ 9 met~rs2/
gram. It iJ, moreover, of very high purity, although it doe~ :
contaln 00}~ addi~ion o~ MgOO Other aluminas al50 can be u~ed,
although exparimental data does seem to indicate that best r~-
sults are achieved with ~he Grace-KA 210 (trade mark) material.
To maintain powder purityJ moreover, th~ ball mill al~o
should be formed from very pure alumina.
Upon completion of the milling step, the powdar i~ baked
for another four to eight hours at 50 to 100C. Baking the ..
powder at 72C. seems to be a pre~erred temperature for this :
step in he processO These ball milliny and drying operations
appear to have the e~ect of removing exce~ surface gase~
to produce a ~iner grained end productO The relation between
- 6 :~
.- - . .. . .
vs~-o~o ::
the ~urface gas and the gxain size of the fully processed material
has not b~en definitely e~tablished. It i po~sible, however,
that the surface gas behaves a~ an ~mpurity phase that cau9e~
severe selective grain growth at high temperaturesO
After outgas~ing, to produca a one-inch diameter billet
of A1203 in accordance with the invention, the powder is screened
through a 200 mesh United States Standard ~ieve to break up any
agglQmera~es that may have formad. The si~ted powder i8 placed
in a high temperature, high strength die. Typically, a graphite ;
die in an iner~, vacuum or reducing atmosphe~e i9 $uitable for ;~
the purpo9eO A compacting pre~sur0 c~ 4000 to 8000 pound~ per
square inah (p~i) is applied to the powder within the dieO This ~ .
pre~sure i~ applied to initial1y compact the powder to 30% to
50% of its maxlmum theoretical den9ityO For this~anple, it has
been foùnd that an initial compacting or "pre~pres~ing" pressure
o~ 5750 p~i }e~ds to the best end product resultsO ~his pr~presq
~orce is then raduced to a ranye of 500 to 1000 psi. Gencrally~
a reduction in pressure to 1000 psi will produce acceptable
result~
~ , .
:~ 20 ~he powder and the die are pl~ced in a hot pre~s or other
: ` :
high temperature and high preBsure sintering deviceO A protective ~` ~``;
atmosphere, moreover, is establi~hed in this 9y8tem in oxder to
preserve the die. A vacuwm, a heliwm or other inert atmospher~
or a mixed a~mosphere of inert gas and 8~ by weight of hydrogen
have been found suitable for this purpose. Furthermore, relative-
ly le~s expensive nitrogen gas may be used for process economyO .
Starting then with the reduc~d pressure on the compacted .
powd~r, the t~mpe~ature of the powder and die is raised by means
of an induction heater at a rate that is bounded by 400 to 1000
-- 7 ~
..... . .. .. ; ~ ....... : : , .
p~r minuteO By proper positioning and ~izing of khe induction
heater and tha billet generally uniorm heating throughout the
powder can be establishedO Within the above ra~gc it appears
that the rate of temperature change can be varied in an almost
random manner until the onset of ~hrinkage o~ "break away point"
13 (FigO 1 ) i9 reached without degrading the quality o the ~inal
product~
With re~pect to the sample und~r consideratio~9numerou~
te~t~ indicate that raising the temperature, wi~hin the above
rate boundaries, of the powder and the die tc, 760 to 815 CO as
measursd with an optical pyrometer will produce the desired re- ;
sultO That i~, the onset of shrinkage or ~break aw~y point" usually
commences as the te~perature reaches about 800Co In accordance
with a feature of the invention, while the temperature i9 being
rai~ed to the illustrative 80oc., to commence shrinkage, the
reduced pressure o~ lOOO p~i also i9 applied to thc powder billetO
This shrinkage may be observed with ~he aid of a linear variable
displacement transducer that is attached to the ra~ that applies
the pre~sure to the ~intering powder.
Af~er the "break away point" is reached, both ~emperature
and pres~ure ar0 increased in order to promote ~he ra~e o~ den-
~ification that i3 inherent or natural to the particular material
a~d billet size. Bo~h pres~ure an~ temperature can be monitored
and adjusted to approximat~ this natural rateO This natural den-
sification rate i8 identified through a series of tests conducted
with s~mple powder~O In each of these te~ts, pressure and ~ :
temperature incr~ase rates are varied to identify the ranges of
pressures 20 (FigO 2) and temperature~ 22 that provide the
close~t approach to the theoretical ~axLmum density 240 It ~hould
t,.,, ~ .
, . . . . . . . . . .
4~)
bs noted in FigO 1 that the natural rate of den~ification change~
a~ the powder i~ sintered into it~ maximum densification as indi-
cated by th~ minimum billet volu~e at tLme t3.
With respect to the above alumina example~ the onset of
: ;
powder shrinlcage i~ accompanied by an application to the now
sintexing billet o~ a physical or ram pre~sure of 3600 p6io Al~
though this a prafarred maximum proces~ pres~ure, ~uitable results
are obtained with pressures in the range of 2000 to 6000 p~
This rapid increa~e in pres~ure i~ reflected in the step-function
pres~ure change that characterizes the pressure graph 20.
As the applieation of this pressure continues, the tem-
perature al~o is incr2ased, but at a lower rate than that which `~
characterized the initial incr~a5e to 800Co Best result~ seem
to be achieved with a temperature of about 1600C. that i8 reached
about eight minutes after the earlier 800Co temperature wa~ ~ :
attained ~ These higher temperature~ alao are obsexved through ~:
an optical pyrometer. This maximum temperature and pre~sure are
.
su~tained for two to six minutes, and pre~erably for threa
minutes, if a maximun proce~ temperature of 1600C. is achieved.
:~ . 20 During this tLme~ the alumina is sintering at its "natural" or
inherent rate of den8i~icationO
The linear change in ram displacement between the time~
t2 and t3 ~hown in FiyO L ig a characteristic feature o~ a
billet tha~ i~ s in bering at this natural rate. Other natural
. :
densi~ic~tion rate indice8 are po3~ible, aLthough ram dlsplacement
is a most conv2nient ~chniqueO
In accordance with the invention, from a broad viewpoint
th~ pressure and t-mperature that are appliQd to the sintering
billet after the "break away point" 13 has been xeached ~ire
~ 9 ~
B
.
.. .. , ~ .. ; .. . .. . . ; . .. ... ..... .~ ,.. . .; , ~ .. - .
. . .. . . .. ~ .. . ,.. . .. . . '. , .. . . ~ .; . .. .. .. .
adiustsd to ~1tabli~3h and maintain ~his natural densification
rateO The natural densification rate will, of cour~e, vary
according to the mate~ial that i9 being proc:e~sedO This natural
rate, moreover, also may vary for different batches of the ~ame
materialO Consequently~ the precise temperature and pressures
that should be applied to the sintering billet for any particular
material can be determined through a number o~ te~ts each par-
formed on a different batch of the material. These tests will
identify tho~e conditions that produce the linear ram displace- ;;
ment 14 (Fig. 1), or other indication~ of the natural den~ifi- ~ ~
cation rate, for the material under consideration. Once these ~ ~ :
sintering oonditions are identi~ied, subsequent billets can be
proce~sed without ram displacement observations and the like.
Thus3 the natural rate of densification can be approximate-
ly identified by applying a f irst physical pres~ure and heating
rate to a first batch o~ the material to begîn the ~intering of
said ~irst batch, applying an increased second phy~ical pressure ;~
and a ~econd heati~g rate to ~aid first batch, recording tha
density of the slntered f:irst batch, applying a physical pressure
20- and heating ra~e to a second batch o~ ~he material to begin the
sintering of ~aid second batch, and applying an increa~ed third
physical pre~sure and a third heating rate to said ~econd batch,
recording the density of said ~intered ~econd batch to identify
~he density of said fir~t and ~econd batches o~ the material that ~.
is closest to the theoretical maximwm density of said materialO
A ~ore detailed con~ideration of FigO 1 indicates that
the ram displacemen~ is not entirely li~ear toward the completion
of sintering 170 Thu~, a~ shown in tbe dra~ing, the rat~ of ram
displacement as a function of time decreases a~ the ~intering
- 10 -
S~ 4~
billet approaches a condition of maximum den~ificationO As
this terminal portion of the sintering proce~ approached,
the pres~ure and temperature applied to the billet i~ stabilized ~ -
for two to YiX minutes to "cure" the now sintered billetO
Thus~ in the process of Fig, 1 to den~ify a metal oxide -
powder into a fine grained sintered ceramic,, during stage 12,
a compressing force i~ applied to the powder and the p~wder i~
heated at a predetermined tempE3xature rate during said force .
.. ..
applicat~ian to an onset of powder shri~age at 13, and the powder
i~ ~ubssquently den~ified (~intersd~ through at least two
~equential decreasing den~ification (sintering) rate~, during the ~ `:
~intering and curing stage 14, by variation of said force. Tha ;;-
densification rate is constant, or linear, until th~ t~rminal
portion at 17 i5 approached when the densification rate i9 non~
~, .. .
linear, the rate o~ intering being controlled by adju~tment o~
the applied pressure. :
Care must be exercised to terminate production conditions
at this point in order to pravent the development of a "bloated"
~ .
~ ~ billet. This:"bloati~g":16 is characterized ~y a reduced density
:~ ;
billet, as indicated through t~e greater bille~ volumie which the
increasing ram di~placement registers. ~`
Turning once more to the complekion o~ ~intering 17, it ~:
i9 possiblQ to more preci~ely promote the natural rate of sinter-
ing, which apparently changes as maxLmwm dsn~ification i~ ap-
proached, by adju~ting the temperature and pressure that i9
appliod to the sintering billet in a manner that will enable the
ram diYplacement to mora nearly apprQximate the preferred curve
illustratea in FigO lo ~ ~
. After the period of curing, or ~ustained heating at the ~:
-- 1 1 -- ~ .
~5~
max~mum proce~3~ temperatur~3 and pressureJ the induction heater,
or other ~ource of heat, is ~urned o~ and th~ pra~ure on ~he
alumina within the die i~s reduced to zero. A cooling perioa o~ ;
one to five minute~ i~ sufficient to enable the die (and the now
sintered alumina) to cool to room temperature for removal from
the press and separation from the die.
Samples of ~intered alumina~ produced in the foxegoing
manner, have shown in carefully sxecuted laboratory tests the
following charactexi~tic~: :
Number ofAvg. Knooplloo*
Samples Hardne~
Grace-KA 100 (trade mark) 8 2045
Grace-XA 210~(txade mark) 21 2334
Commercial Sample A 10 2~77
Commercial Sa~ple -B 10 1952
.
,~,
-* Knoop hardness i8 a mea~ure of the microhardness of a
material by mea~ of a long, narrow, dia~ond ~haped impressionO
The hardness number i~ calcula~ed a~ the ra~io of the indenting
load to the projected area o~ the indentation: THE M~KI~G~
20 SE~PI~ AND TREATING OF STEEIJ, tJNITED ST~TES STEE:~ 8th EDITIO~.
In this conn~ction it ~hould be noted that the te~n "9 tandard
deviation" as used h~rei~ is the squara root of the arithmetic
mean o~ the squares o the deviations of the phy~ical test data
f rom their arithmetic mean.
- 12 -
B
0~
AvgO Cosnp~e~iv e A~gO Modulu~ of
Grace-KA 100 (trade mark) 326,700 44,100 :
13race-KA 210 (trade maxk) 543,200 82,600
Commercial Sample A 321,000 59,500
Commercial ~ampla B 404,300 65,70o
Modulus of P~uptur¢ :
Standard Deviation, ;.
Grace-KA lO0 ( trade mark ) 16, 500
Grace-~ 210 (trada mark) 23,200 ~ ~ .
Commercial Sa~mP1Q A 16~000
~.
Commercial Sample B 11,300 ~ :~
,~ .
C4mpre~sive Strength
Standard Deviation,
. .
~j Grace-KA 100 (trade mark) 115,000
Grace-KA 210 (trade mark) 122,300 ` `~
Commercial Sampl~ A 1}1,600 ~ :
Commercial 5ample B 104, 200
Average Grain Si~e
Grace-KA lO0, (tra~e mark) 206
'~ Grace-KA 210 (trade mark) 0072
Commercial Sample A 1.3
Commercial Sample B 107
The ~uparior properties, on the average, of ~he sin~ered ~ :
: .
al~u~ina that can be obtained i~ the Grace-KA 210 (trad2 mark)
powder i~ used a~ a basic raw materiAl in the process ch~racteriz-
ing the invention i3 apparent . It should be noted that the
- 13 -
.
'~ ~os~
Grace-KA 100 (trada mark) p~wder doe~ not have an added Ool~
MgO cry~tal growth inhibitor. In developing the foregoing te~t
d ta, moreover, ~ample preparation has been found ~o exert a
significant influence. Chemical polishing of the sampl~, for
instance, provides more realis~ic modulu~ of xupture te~t dataO ~.
Mecha~ical polisbing, however, ~eems to be detr~mental ko the
~ctual strength of the sample that is undergoing te~tingO
Studies with a s~anning el~ctron microseope (at a m~gnifica-
tion of 10,000) of the fracture surface~ of repre~entative samplo~
of alumina ceramic billet~ in the 1" to 5" diameter range that
wera produ~e~ in ths manner de~cribed above demonstrate that the
material has a grain si~e di~tribution as follows:
Gra m ~ize Range Percent of Grain Structure ~.
Le~-~ than 0.3 micron 0~ ;
Between 0O3 and 0O5 micron 25~ : :
Between 0.3 and 0.7 micron 54%
Between 0.3 and 0.3 micron 80% .
Botween 0 . 3 and 1. 5 micron 100%
The "break away point" graph 30 in FigO 2 illustrates the
relation between the diameter of the snd product billet and the
proce~s condition9. Thu~ to manufacture a five inch diameter
billet o~ Al203 in accordance with the principles o~ the inven-
tion9 somawhat higher temparature~ and pre~sure~ should be applled
during proces~ing than the3e condition~ which are mention~d above
with respect to the one ~ch diameter billetO It shoula be kept
in mind, however, that a basic ~eature o~ the invention for all
o~ the materials and billet ~iz2s described herein is the appli-
cation o~ an increa~ed proces~ pre~sure, within described boun-
darie~ throughout the sintering process, iOeO after the ~break
- 14 - ~ :~
.~~' ~''.
l(~S~LV4~
away point" (Fi~ o Moreover, a maximwm proce~ pr~ur~, an :
o~served optLmum, i9 identified within the describ~d bound~ ob ~
tained by comparing the pressure "history" of the ~intering bill~k ~ ~;
with the den~ity of the billet~ and may be more conveniently
applied to the billet to provide the desired closest approach to
the theoretical maximum den~ ityO .
Thu~, alwmina ceramics manufactured in accordance with the
principles of the invention have a grain s~ructure that i~
different from those grain S iZ08 that have characteri~ed the
prior ar~0 Crystal~ of much larger ave~ags size, eOgO two or
three micron~, o~dinarily were grown in these prior art aluminaO
Accordingly, a new alu~ina ceramic with a fine grain size and
better grain ~ize distribution that heretofore was unobtainable ~;
is provided through the in~en~ionO
. ~he inventio~,~ moreover, i9 not limited in application to
alumina but also can be used in connection with other metal-oxidesO .
For e~ample, uranium dioxide (U02) pellet fabri~ation can be in~
~:~ p~oved through the practice of the inventionO Typically, a pellet
donslty that is within 1/2~ of the theoretical attainable maxLmum
:can be reached by mean3 of this pressure and temperature rate con-
trolled sinteringO IlLustratively, to achieve 95~ of the theore- ;
tica~ maxLmum den~ity, the powder is subjected to maxLmwm proces~
temperatures that arc on the order to 800 to 900C in an eight : - :
: to nine minute heating cycleO Within thi~ time cycle9 moreover~
phy-.ical pres~ure al~o is applied to the p~wder that is being
. . ~
sintered. There i ~ of cour~e, an initial or prellminary heating
:
~eriod of about one minute, characterized by the on~et of powder
shrinkage, during which tim- the pQwder is rai~ed rapidly to a
higher tamperature and subjected to increasing physical or
- 15 -
~v~ o
mechanical preSsureo The re~ulting uranium dioxide pellet~ donot require grinding or oth~r fini~hing operations because they
are fabricated in die3 o~ correct diameterO ~he elLmination of a
machine finishing operation in the fabrication of uranium dioxide
reactor fuel pellets is espe~ially beneficial because it reduce~
proces~ing costs and elLminates a major ~ource of fis~ionabl~
material manufacturing wa~teO
A further exa;nple of the invention compri~es the ~intering
oP alumina with other carbide~, nitride~ or oxides to improve
further the physical properties of the resulting productO A~ a .-
speci~ic example five inch diameter billets o~ alumina-titanium
carbide (A1203-TiC) were made from 70~ al~nina powder Grace-KA 210,
trade mark and '30~ titanium carbide powderO The original particle
~ize of the titanium carbide p~wder i9 2 to 4 micron3. The par-
~icle size i~ reduced by ball milling for 16 hour~ in alcohol~ to
an average particle size of 1 micronO The ball-milled powder is
mechanically mixed w.ith the alumina powaer for uniform distribution
o~ the two materials in the resulting powd~rO Illustratively,
the alumina and the b~ll milled TiC are blended together in an
20 alcohol mixture in a ball mill for four hour~O These mixed
materials are re~oved from the ball mill~ the alcohol is evaporat-
ed and the resulting powder, having thu~ been worked to remove
surface gase3 and to reduce agglomerates in the powder~ is pre-
pressed or compacked with a pre~sure in the range of 4000 psi to
8000 p~i to achieve a prepres3ed billet that has a density that
is 30% to 50~ of the maxLmwm theoretical density~ For the exa~ple
. . ~ . .
under consideration, the 6300 p~i pre-pressing pre~ure afford3 a
suitable balance between powder packing and the elLmination of en-
trapped gases. ~he applied ram pressure is then reduced to a range
- 16 -
~f! ~ .
4~
of 5Q0 to 1000 p9io While this lowsr pre~ure i8 being applied,
the material i~ heated ~ a rate that is not les~ than 400C per ~ :
minute nor more than 1000C per minute until the onset of ~hrinkage ~: :
commences, u~ua}ly at about 800Co While the material i~ being
heated to this 800C temperature the aforemention~d reduced pressure
iY maintained conRtant to provide bill~t integrity, as not~d aboveO
With the onset of powder shrinkage that occurs at point 31 on the
"break away" graph 30 in Fig. 2, the ram pre~sure on the now .
sint~ring billet i9 increa~ed to 5000 p~i, the preferred maximwm
hot proce~s p~es~ure. Suitable results~ however~ can be obtained
with applied ram pre3sures in the 3000 to 9500 p~i rangeO
A~ the application of ~his pre~sure continues 9 the temper-
ature is increased, but at a lower rate than that which charac-
terized the initial increase to 800Co Thu~J within six to ten
minutes, ~he maximwm process temperature is reached in the range
from 1200C to L800Co Based on available experimental data~ ;~
bs~t results are achieved with a temperature of about 1500Co For
curing thi~ ~ax~mu~ temperature and 5000 psi pressure are ~u~tained
for two to six minute~O Thus, the p~wder i9 sintered at a rate of
densification that approaches the theoretical maximum den~ity of
the powder until material densification is complete~
As ~hown in Table I below, the xe~ulting material is 9up-
erior to chemically ~imilar material~ khat are produced through
prior art proce#se30
Twenty~ ~ive-inch diameter billets o~ alwmina-titanium
carbide were fabricated in accordance wi~h the principles of the
invention to demon#trate proc~3ss reproducibility and the ~uperior
physical characteristic~ of the productO
Tha re~ulting den~ity data for all 20 billets, is 3hown
17 -
in Table I. Th~s average billet density wa~ 4.257 g/cc -0007%,
whereas the prior art den~ity for thi~ material i~ 4.21 g/ccO
The ~arm average9 a~ used herein, i~ the quotient of the arithmetic
sum of the data divided by the number of data value~ used in
calculating the sum
TAB LE I . BILLE:T DE~$ITIES
BILLET ~UM~ER D~ S ¦-I/cc
63 4.249
64 40259
4~ 254
66 4 . 257
67 4 0 256
6~3 40 260
69 4 0 260
4 Q 256
71 ~ 0 254
72 ~0 258
73 40258
4 ~ 260
40 262
76 ~ o 257
7'7 4.258 ~.
78 40258
rg 40256
~ o 260
81 40254 ~ `
82 4 D 260
Average 40257 gm/cc5tandard Deviation 00003 gm/cc
(0007%)
-- 18 --
."i
~:., , : , , -
4~
The billets were ground top and bottom on a BlanchArd
model NQO 11 grinder and dice~ into 21 blank~, each 3/4" ~quare
and 5/16" thicko From each of the 20 billets, two of the 21
blanks were randomly selected for transverse rupture strength
testsO (TRS)o The two ~elected rupture test blanks were each
sliced into three 1/4" by 3/4" by 5/16" parallelepiped~ to pro-
vide a total of six rupture ~pecimen~ for aach billetO The
specLmen~ were surface ground on all ~ide3 for edge sharpn~ss
and size unifornityO
The individual specimen~ war~ te~ted for tran~ver~e rup- :
ture ~trength by a thre~-point loading~ The tran~verse rupture
strength (TRS~ r0sults of these test~ are tabulated in Table
II. In Table II, the average ~RS o~ the 9iX rupture specimens
taken from each billet i~ tabulated below along with the standard
deviation ~or thiu dataO The overall average (the average o~
the average of each group of six samples) and the standard de- :
viation o~ this overall average was found to be I24,333
11,54~ psio
TA8LE II
BILLET TRANSVERSE RUPTURE STRENGTHS
BILLET ~00SAMPLE ~00 T~S (PSI) AVE._ RS (PSI~
63 1 114~379
2 145,899
3 89,388 124,557
4 133,232 +18,75~ :
139,993
6 124~453 ~:~
:
64 1 139~0~2
2 150,663 ;
- 19-
~. , .
cX~, '
. . . .
~os~
BILLET N0. S~MPLE NOo
3 146~399 14~,187
4 137~252 ~ 5~967 ~ `
149~353
6 154,456
1 136,167
2 150,363
3 120~494 113,799
4~ 104,1~9 +28, 545
599861
o 6 111,783
:
~6 1 89,962 : -
2 127,815 :
3 78,107 117J929
4 132,966 +24,59
142,334
6 136,3g2 ~ .
~ .
': '~'~' ''
99570 ` -
2 124,848 `~ ~
3 87,309 97,015 ;`
4 70, 860 +20,93T
1O~J966
6 74,539
68 1 135,543
2 139,812
3 1~5,~72 132,003
4 118,659 ~ 7~904 :~
- 20 - ~ ;
~5~
E~ILLET NOo SAMPLE NO. ~ ~VE.TRS (PSIl ~
131,558 :-
6 140,972
69 1 136,554 ~:
2 106,444
3 89,163 117~195
4 1~8,363 ~21,286
12~,498 ~ :
6 97,147
o 7 1 161~2
2 147,558 ~:
3 81~,116 130,232
4 108,674 ~26,180
'i 5 145,~53
6 133,870 ~:~
. 71 1 141J 234
2 1289750
3 1~5~508 137,381
4 131,122 ~ 6,832
1499525
6 138,146 ~`
. .
:. -.,
: 72 1 1199675
~ 1~6,684
: 3 68,657 108,767
4 1425746 +27,271
86~o72
:: :
6 No Tegt ::
- 21 -
o~ o
BILLE:~ N0. SAMPLE N0., RS (PSIl ~
73 l 144,735
2 146,156
3 132,574 13~,0~
4 151,390 ~12,447 : ~;
~ 133,613 :~
6 113~596
74 l 116,096
2 136,419
3 99J9~ 120,030
4 118,341 14,520 !;
140,984 `
6 108,405 ~
: 75 1 149,731 `~:
2 132,475 ~
3 143,868 1~3,487 ~;
-.,
4 151,635 + 6,244 ~ ; -`
141,792
6 `141,422
76 1 121,54
2 133,975 : `
: 3 79,607 121,824
: 4 139,120 ~19,871 ~ ~
123,104 ~ ~ ;
6 133,589
77 ~ 1 132,863 `
2 141,150
22 -
,~,. ~ .
' ~ ` '
13ILLBT l~Oo S~PL13 1;0. ~ ~
3 107~ o48 130~ 709
l~ 111~856 +15~711
147~ 159
6 144~178
78 1 137~ 132
2 118~460 ;~
3 113~05~ 120~846
4 108> 541 +1~3~356
96 ~ 3~34
6 151 J 509
79 1 103~270
2 118~ 557
3 110,070 120~667
124~957 +11~877
13g7715
6 127~ 435
, ' '
` ~ 80 1 69 ~ 338
2 907804
3 152~226 119~563
4 110~484 +31~824
151~ 916
6 142~611
81 ~140r372
2 ~41~461
3 136~273 130~407
4 138~ 052 ~16 9 gO8
- 23 -
..~..
~)J :
.. .. ... ...... i, ... ' --
- ` ~vs~
BILLET WOO S~MPLE NOo
93,o83
6 133j~00
82 1 141J39~ ~ `
2 78,340 ~ ;
3 139i642 1173042
4 88,848 ~25,437
115,036
6 138,986
~. ..
The broken trani~veri~e rupture specimens were then mounted
and polished for hardne~:s t~sti~gO RockwellA hardne~s te~ts were
discontinued when three of the Rockwell indentors were ruined
~ .
after application to only five billetsO In pa~sing, it should
be noted that a Rockwell te~t i8 a measure o~ hardne~s as mani-
fested by the materials resistance tv the penetratlon o~ an
indentor in rei~ponse to the application of a known load. The
~ubscript, A in thi~ te~t, indica~e3 the load and indentor type
u~ed in the test for this material (THE MRKIN~, SHAPING A~D
T~EATING OF ST L, UNITED STATES STEEL 8th EDITION, 1964)0 Knoop : ~
hardnes~ test~, however~ were psrformed on all twenty billetsO ~
The hardness data a~e tabulated in ~able III~
Although the RockwellA test~are not conclusive due to the
above mentiQned breakage problemJ the average o~ five data point~
indicate~ a Oo8 increa~e in ~he RockwellA hardnes~ over the prior
art. Thi~ Oo8 increasa is a ~ignificant improvement over the ~ :
pxior art because increases of Ool are of practical LnpOrtanCe ln
the industry, eOg., tools are graded by increai~ei~ of Ool in
Rockw~ hardne~
- 24
~5~
T~aL~ III. 3ILLET ~RDNESS
B ILLEr l~t), R . HARD~;SS lK~OOP HA~SS
, ~
63 93075 3557 ;~
64 93078 3557
98083 3557
6~ 93078 3557
67 93.95 3557
68 No Do 3557
69 No Do 3557
No D. 3557 ~:,
71 ND D . 3227
72 N . D 0 3557
73 No Do 3557
74 No Do 3557
No Do 3557
76 ~0 Do 3557
77 No Do 3227 ;-: . .
78 No Do 3557
79 ~ ~ : No Do ~940
~ ~ 80 ~ N. Do 3227
81 No Do 3557
8~ No Do 3557
A~ rage 93082
.,: D. = ~t Da~ermined: ~ ~verage 3477
Two of the six broken tran~verse rupture specLmens rom . ;
each billet were photographed at a ten power (lOX) magnification~
Sampl~ A, for macro-homogeneityJ i.e. visibla di~ferences in :~
the color o~ the sample material under in~pectionO Only one
~pecir~en from all of tha sample~ studied shc~wed an inhomogen~ity ~
- 25 - ~;
O~ '
(a 004 nun equivalent diameter tit~niuun carbide particle) a~
enum~rated in ~able ~V belowO ~he ec~uiv~lent ~ize of the in-
homogeneitie~ listed in Table IV, moreover~ are defined as the
average of the major and minor ~xi~ of the inhomoganeity~
TABLE IV~ BII.LET MACRO-HO~OGENEITY
~qBER OF ~JIS IBLE ~QUIVALE~T :
DIPFERENCES IN 5 IZE OF
li IT~T~ET 1;0 D ~ rOLOR
63 1 o __
2 o __
64 1 o __
2 0 --
1 0 _-
2 0 ~~
66 1 0 __
2 0
67 1 0 __
2 0 ----
68 1 o __
2 0 ---
69 1 0 _-
2 0 __
1 0 --
2 0 ---- :
71 1 0 --
2 0 :
72 1 0 ---
2 0 __
73 1 O __
2 O --
74 1 O --
2 O --
1 1 OO
2 O __
76 1 O __
2 O -_
77 1 O __
2 O __
7~ 1 O --
2 ~~
79 1 O --
2 0
1 0 __
O -- ':~
81 1 0 --
2 0 __
82 1 0 - --
2 0 __
Two of the broken tran~vex~e xupture gpecimen5 f rom
50 the remaining sample~ o~ each billet, Sample B, wera xandomly
- 26-
~ (~5~
select~d for micro-homogeneity. The~e micro-homogenei~y samples W9Le
pvli~hed and photomicrographed at 900X magnificationO The re~ults
tabulated in Table V below indicate that the average large~t titanium
carbide agglomerate was 12 microns, and the average titanium carbide
grain was 4~82 microns. It ~hould be noted that agglomerates are
combinations of two or more grains into one mas~O
TABLE V. BILLET MIC~O-HOMOGE~EITY
,
: LARGEST TiC TiC AGGLOMERATES
AGGLOMER~TE OVER 10 ~
BILLET SAMP~LE EQUIVALE~T EQUTVALE~T LA~GEST TiC
N~. B MO. IAMETBR _~ GR~
63 1 10 ~ 0 6 ~ :
~ 15 ~ 2 55
6l~ 1 9 ~ 0 4
16 ~ 3 5
~5 1 14 ~ 1 5
2 18 ~ 1 5
66 1 9 ~ o 5
2 r ~ 5
67 1 14 ~ 2 5
2 16 ~ ~ 4
68 1 14 1l 1 4 ,u
2 2005~ ~ 4
69 1 12 ~ 1 505~ .
2 12 ~ 1 505
7 1 8 ~ 0 505
2 15 ~ 2 5054
1 14 ~ 1 4
71 1 14 ~ 1
2 14 ~ 2 4
72 1 9 ~ 0 4
2 14 1l 1 4 11
73 1 12 ~ 1 33
3 2 1505~ 1 6
7L~ 1 8 ~ 0 5
2 1005~ 1 5
1 11 ~ 2 L~
2 19 ~ 3 505
76 1 7 ~ 6
2 15 ~ 1 5
77 1 lo ~ 0 55~
2 1005~ 1 T
78 1 9 ~ 0 5 ~ :
2 10 ~ 0 6
79 1 }202~ 1 3-3~ -:
2 70~ ~ 0 5
1 13 ~ 20 3
2 17 ~ 1 5
81 1 1005~ 1 4
2 12 ~ ~ 5.5
82 1 ~5~5~ 2 5
2 1~ ~ 1 4
AV~AGE - 27 - 4~82
~OSi~40
A ~canning electron microscopa indicate~ that the
alumina grain 5 ize of thi~ material i~ of the sama order a~
the grain size ~003 - 105 ~) of the sintered alumina aloneO
~he TiC, however, is on the order of 1 ~, which was the size ~ :~
of the ball millad titanium carbide powder.
As described, the process produces a ~ignificantly im~
proved product in comparison with the prior artO ~he increased
den~ity of alwnina-titanium carbide indicates that the applied
rate controlled sintering technique immediately following khe
10 "break away" point maximizes the den~i~ication o the material
relative to that which was heretofore obtainableO Thi~ pro-
ces~ ~ moreover~ i~ applicable to other powdared materials on~e
the "break away" point i~ determined and the inherent or
natural densi~ication rate for the material in question is
established O
''~ . '
`'`''
,
- 28 - .
