CA1051040A - Technique for producing alumina-titanium carbide material - Google Patents

Abstract

ABSTRACT
A powder, for example a mixture of titanium carbide and alumina powder, is densified into a fine grained sintered ceramic by applying a first constant physical pressure to the powder and heating it at a first predetermined temperature rate to onset of powder shrinkage, and then applying an increased second physical pressure and a second, lower heating rate. To identify approximately the natural rate of densification, the density of a sintered first batch is recorded, and then a second batch is similarly treated by applying a constant pressure and predetermined heating rate thereto to onset of powder shrinkage and then applying an increased third physical pressure and a third heating rate to the second batch and recording the density of the second sintered batch, to identify the density of the first and second batches that is closest to the theoretical maximum density of the material. Conditions for approximating the natural densification rate can thus be identified. After the onset of powder shrinkage the densification rate is sub-stantially constant but it becomes non-linear as the condition of maximum densification is approached.

Description

~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 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.

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 .

'~ ~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 ~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 ~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 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 4~ 254 66 4 . 257 6~3 40 260 71 ~ 0 254 72 ~0 258 4 ~ 260 76 ~ o 257 7'7 4.258 ~.

rg 40256 ~ o 260 81 40254 ~ `

Average 40257 gm/cc5tandard Deviation 00003 gm/cc (0007%) ."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 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 ;

~. , .
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 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 3 1~5~508 137,381 4 131,122 ~ 6,832 6 138,146 ~`
. .
:. -., : 72 1 1199675 ~ 1~6,684 : 3 68,657 108,767 4 1425746 +27,271 86~o72 :: :
6 No Tegt ::

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 ,~,. ~ .
' ~ ` ' 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 79 1 103~270 2 118~ 557 3 110,070 120~667 124~957 +11~877 13g7715 6 127~ 435 , ' ' ` ~ 80 1 69 ~ 338 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 ..~..
~)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~

~5~
T~aL~ III. 3ILLET ~RDNESS
B ILLEr l~t), R . HARD~;SS lK~OOP HA~SS
, ~
63 93075 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 :

li IT~T~ET 1;0 D ~ rOLOR
63 1 o __ 2 o __ 64 1 o __ 1 0 _-2 0 ~~
66 1 0 __ 67 1 0 __ 68 1 o __ 69 1 0 _-2 0 __ 2 0 ---- :

2 0 :

2 0 __ 73 1 O __ 2 O __ 76 1 O __ 2 O -_ 77 1 O __ 2 O __ 7~ 1 O --2 ~~

1 0 __ O -- ':~

2 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 ~ (~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 - .

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An alumina-titanium carbide material the physical attributes of which are characterized by an average transverse rupture strength of 124,333 ? 11,542 psi.

2. An alumina-titanium carbide material according to claim 1, the physical attributes of which are further characterized by an average RockwellA hardness of 93.82.

3. An alumina-titanium carbide material according to claim 1, the physical attributes of which are further characterized by an average Knoop Hardness of 3477.

4, An alumina-titanium carbide material according to claim 1, 2 or 3, the physical attributes of which are further characterized by an average titanium carbide grain size of the order of 1µ and an alumina grain size of the order of 0.3 to 1.5µ .

5. A method for solidifying an alumina-titanium carbide powder comprising the steps of working the powder to remove sur-face gases and to reduce agglomerates formed in the powder, com-pressing 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 densifica-tion 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 increased physical pressure for a few minutes for sintering the powder at a rate of densification that approaches the theoretical maximum density of the powder.

6. A method according to claim 5 wherein said maximum temperature is in the range of 1200° to 1800°C.

7. A method according to claim 5 or 6, wherein said increased physical pressure is in the range of 3000 to 9500 pounds per square inch.

8. A method for producing an alumina-titanium carbide material comprising the steps of ball milling a carbide powder in alcohol to an average particle size of about 1µ , mechanically mixing alumina powder and said ball milled carbide powder, com-pressing said powders by a physical pressure of about 6300 psi, reducing said pressure to about 1000 psi, heating said powders at a rate of 400° to 1000°C per minute, holding said reduced pressure constant during said heating until said powders reach a temperature of about 800°C, increasing said pressure to a maximum in the range of 3000 to 9500 psi, increasing said heating at a lower second rate than said first rate to a temperature of about 1500°C in 6 to 10 minutes, maintaining said maximum pressure during said heating, and holding said maximum pressure and said temperature of about 1500°C for 2 to 6 minutes for sintering the powder at a rate of densification that approaches the theoretical maximum density of the powder.