CA1237449A - Sintered dense silicon carbide - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Submicron sized silicon carbide powders having dispersed therein minor values of carbon and boron may be formed into complex shaped green bodies by conventional ceramic techniques such as slip casting. The green bodies are sintered in an inert atmosphere, preferably nitrogen, at a pressure not greater than atmospheric to a density of at least 85%
theoretical. Articles such as thin walled tubing and gas turbine blades are now readily formable.

Description

- ~ ~ 3~

SINTERED _ENSE SILICON~CARBIDE

The lnvention herein de~cribed w~ made ln the cour~e of or under a contract or ~ubcontract thereunder (or grant) with the Department of the NavyO
The chemical and physic~l properties of ~ilicon S carblde m~ke lt an excellent m~tcri~l or high ~empersture ` structural application~. These propertie~ includ~ good oxldation re~iRtance snd corro8ion beh~vior9 good heat tr~n~fer coefficients~ low thermal expansion coefficient~
high thermal ~hock resist~nce ~nd h~gh tren~th at elevated tempcrature. Thi~ unique combination of pro~
perties sugge~ts the u~e of ~ilicon carbide ~9 componen~
for gas turbines, check valve~ for handling corro~ive liquid~ ning~ of ball mill~, hest exchang~rs ~nd re-fr~ctories for high temperature urnace~, pump~ for die cssting machine~ and combustion tubes.
Heretoore9 hot pre~sing of ~llron carbide wa~
used to produce sm~11 specimens under clo~ely controlled e~ndi~onA. Unfortunately, sil~con carbide i~ no~ e~slly sintered to den~i~1es approaching the theoretlcal den~ity of 3.21 gram~ per cublc cen~imeter. A method of hot pre~slng sil~con carbid~ to uniform den~itie~ on the ord~r - ~ o 98% of ~he theoretlcal density wi~h slight additlons of alumin~m ~nd iron aiding ~n den~ific~tion ~ di~closed by Alliegro et al., J. Ceram. Soc,, Vol. 39, ~I ~November 3~

1956), pages 38~-389.
My Canadi~n applioation entitled HOT PRESSEI) SILICON CARBIDE1 Serial No. 1~7,956 f~ led April ~2 1974, de~cribes ~n improved method of m~king a dense 5 silicon c~rbide ceramic by fonming a homogeneou dis-persion of a submicron powder of silicon carbide and a boron containing additive and hot pre~8ing the disp~rsion at a temperature o about 1900-2000 C. ~nd at a pressure of about 5,000-lO,OOG p~i for ~ su~flcient time to produce a den~e nonporous silicon carbide ceramlc. The advantage of boron a~ a sintering aid, in comparison to o~her ma-terial~ such as alumina, aluminum nltride and other me~allic compounds, is that boron provide~ incre~ed oxidation and corrosion resistance at elevated tesnperature.- Sub~equently~
Prochazka et al, ~n the Canadian application Serial No, 198,393 filed April ~9, 1974 disclo~ed a furthQr im-provement ~n hot pressing siliFon c~rbide by incorporat~ng a carbonaceous additive into the homogeneous dl~perslon ~ of ~ilicon carbide and boron containing additive powders.
20 ~ The addition of the carbon suppres~e~ exaggera~ed grain growth in the microstructure of t~e den~e ~ilicon ~srbide ceramic product and yield~ improved ~trength propertie~.
Howeverg hot press~ng yields excellent materials only in the form-of billets having a ~imple geometric shape ~nd such blllet~ require expensive machining whenev~r a com~
plex shaped p~rt i9 required.

2 -~ .

RD-6~9~

In accord~nce with the present inv~ntion I have discovered a method of m~k~ng a dense silicon c~rblde ceramic by fonming a homogeneous di~per~ion of a Rub-micron powder con8i8tln8 essentially of silicon carbld~, a boron-cont~inlng addltive ~nd a csrbonaceou3 addi~iv~.
The d~spersion is then formed into a shaped gr~en body snd sintered in ~ controlled a~mosphere inert to ~ilicon carbide at a temperature of about 1900 2100 C. to fonm a shaped silicon c~rbide body having a density of at le~t 85% of the theoretical densi~y, The preferred product obtained has a density of at least 98% of the theoretical density. It is suitable ~s an engineering m~terial such as, for example, ln high tempera~ure gas turbine ~ppli cations.
The accompanying drawing, which 1s a flow ~heet of the novel process, while not intended as a deXi~ition es~entially illustr~te he invention~ A full di~cusslon i~ set forth h~reinbelow.
- It i~ essential tha ~ the powder di~perslon i~ a mixture of ~ubmicron particle sized powder~ ln ord~r to obtain the high densities and strength~ upon ~intering.
These may be obtained by different techniques a~, for exs~ple9 by direct ~ynthe3is from the element~, by re~
duction of 811ic3g or ~y pyroly~is o~ compound~ containlng ~5 ~ilicon ~nd carbon. The pyroly~ic techni~u~ i~ particul~rly sdvsnta~eous in that it yield~ a powder havlng ~ controll~d

- 3 RD-67~9 particle size, a predete~ined composltion and i8 com-posed mainly of isolated crystallites. In this process tr~chloromethylsilane vapor ~nd hydrogen or a m~xture of SiC14 vapor and a suit~ble hydroc~rbon vapor, such ~9 toluene, and hydrogen are lntroduced ~nto ~n argon plasma gener~ted between two concentr~c electrod~s. In the hot plasma the compounds decompose into ions and the most stable molecule~, i.e~ 9 SiC and HCI~ fonm on cool~ng the gases. The SiC i~ prepsrad a~ small cry~tal3 typically 001-0.3~ in ~ize. The advantage of thi~ product is th~t the crystallltes are not ~ggregated and that the earbon to ~ilicon ratio c~n be controlled by monitoring the lnitial vspor composition ~o ~hat SiC powders slightly enrlched in carbon can be obtained. Moreover~ BC13 can be further added to the reactants in the desired amounts whereby the SiC powder~ ~re doped with boron which hs~ been dispersed e~.~ent~ally on ~ molecul~r level~
Another proces~ for preparing silicon carbide powder with excellent ~intering propertie~ i8 dlsS~lo~ed by Prener in U.S, patent 3~085,863entitled METHOD OF
~KING SILICON CARBII)E. The p~tent te~che~ a proces8 of making pure ~llicon carbide which includes the ~teps of forming a silica gel in sugar solu~lon~ dehydr~tin~ the gel to decompo~e ~he ~ugAr . and to fonn finely divided mlxture of ~ilic~ ~n~ c~rbon, ~nd he~in8 the mixtur~
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3 7~

~n an inert atmosphere to form ~ilicon carbide. We have found that it i~ pref~rable to modify this procedure by ~ubstituting e~hyl~ilicflte for the ~ilicon tetrachloride to.eliminate the inconvenience of va~t amounts of hydro-chlorlc aeid relea~ed on hydrolysis.
The boron cont~lning ~dditive may be in the form of a submicron ~ized powder and further may be either a0 elemental boron or boron carblde. Alternativ~ly, the boron may be added directly to the siliea gel in the fonm of a boron compound~ ~uch ~s boric acid dur~ng the p~eparatlon of the silicon carbide powder. In order to obta~n denslfi-cation, the ~mount of boron eont3ining additive i9 critical~
the amount of the additive being equivalent to about - 0.3-3.0% by weight of elementsl boron. Experiments on sintering of ~ilicon carbide wlth the boron cont~ining addition lndlcate th~t there i~ a lower ~imit of efi-ciency below which there i8 e~sentially no effec~. This : critic~l concentration appears ~o be equivalen~ to between 0.3-0.4% by weight of boron. A further incre~e ln boron concentration does not brlng out enhancement of den~
cation~ and9 when the amount i~ equivalent to more thsn 3.0% by weight of boron, the oxidatlon re~i~tance of the produc t i8 degradèd.
The optimum amount to be added by powder mixin~
25 procedure3 - i5 ~bout equivalent to one part b~ w~igh~ boron per 100 parts of silicon carbide. This opt~mum ~mount. i8 R~-6799 probably related to the solubility limit o boron in ~ilicon carbide whlch has to be ~pproached or exceeded in order to get segregation of boron at grain bound~rie3 and the resulting effe~t. However, as there are limi-tations to the degree of disper~on of boron in the silicon carbide powder which can be achieved, it i8 ad-vantageous to slightly sxceed ~he lower limit of effective-ness o boron. This brings about ~flfe den~lfication throughout the compact snd eliminate~ isle~ of lower densification which may fonm with low concentrations and incomplete mixing. Thus, for the mo~t part9 an ~mount equiva~ent to 1% by welght of boron i8 the minim~l addition when elemental boron powder ls mechanically mix~d - with silicon carbide powders. On the other h~nd, when boron i8 intr~duced during preparation of silicon c~rbide powders, the most de3irable diRpersion i~ achi~ved and ~n ~dditlon of only an amount equivalent to about 0.4% by weight of boron gives ~atisfactory results.
.In ordèr to obt~in h~gh degre~s of den~ification~
the oxygen content of ~he po~der ha~ ~o be very l~w, iOe~, le~ than 0.1 we~ght percent and a small exce6~ of carbon ls necessary. Thus, Eor ~ns~ance, a powder ~hloh GOn-tained 0.4% by w~igh~ boron and no free carbon exhiblts on firin8 8t 2020-C. a linear 3hrink~ge of only 5~
which corre3ponded to abou~ 70% fin~l th~oretical density. When, ho~ev~r~ ~n addi~lon of c~rbon i~ m~de in RD~6799 the form of ~ soluble carbonaceous compound prior to compacti.ng, the linesr shrinkage increases to 1870 and the density is 96% of the theoret~cal after f~ring un~er the same condition3. Thu~, clearly, some free c~rbon ~ 8 absolutely e~sentisl to the ~intering of S~
The funct~on of csrbon i8 to reduc~ silica which alwsys i~ present in 3ilicon csrbide powders in small amount~ or which forms on heating rom oxygan ad~orbed on the powder surf~ces. Carbon ~hen xeact~ during heating with the silica according to the reaetion:
SiO2 + 3C ~ SiC ~ 2C0. Silica, when present ~n the SiC
powder~ in ~ppreciable amounts 9 halts densificatiQn of sillcon carbide completely so that lit~le or no shrlnkage i8 obtained, There i8 ~n additlonal role of the free carbon~
It will act as ~ gett~r or free silicon if pre~ent ln the powder~ or i it is fonmed by the following r~action during heating up to the ~in~ering temperature:
SiO2 ~ 2SiC ~ 3Si + 2C0. The presence of silicon~ just ~0 as the sillca, te`nd~ to halt or retard densiflca~ion of SiC and must be elimin~ted. The amount o car~on required depends largely upon the oxygen content ln the ~t~rting - SiC powders. Thus, for in~nce, a boron doped powder wi~h an oxygen conten~ of 0006% sinter~ ~a~iLy to 98.5~ of -the theoretical dens~ty wlth an addi~ion of 0.3~ o~r~on. .
AnotheF powder con~aining 0 3% oxygen ~lnter~ to 91%
.

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RD~79g relative density with 0.9% free carbonO A sub~tantialexcess of carbon beyond the necessary amount for de~
oxidation of the SiC is h~rmful. Carbon gener~lly i~
dlfflcult to di~perse and the unre~cted exces3 carbon S tend~}~o form voluminous grains ln the sintered SiC
matrix that fl~t much like perm~nent pore~ and such excess thereby limits the ultimate achievable dens~ty and ~tren~th. Systematic experiments have shown that 0.1 to 1.0 weight percent carbon i8 sufflc~ent to provide sinterability. Powder which does not sinter under these condition~ will not sinter ev~n w~en more carbon wa~
added.
Since carbon in the fonm of a powder ~s ex-tremely difficult to disper~e uniformly on a submicron lS level, it is advantageous to introduce i~ as a ~olution of a carbon~ceous organ~c compound which i8 8ub80quently pyrolyzed into carbon, Certain genersl unctional criteria may thu~ be establi~hed whlch may be u~çd to de~cribe the characteristics of the c~r~onaceous additive.
Flrstly, compounds which re~dily crystallize from ~olutions, ~uch as ~ugar from an aqueou~ solution; will tend to-preci-pitate as cryst~l during ev~poration of the solvent. S~ch crystal~ turn into rel&tively l~rge carbon particles on pyroly~is and ~orm unde3irable lnclu~ions in the mlcro-5 structure of the final produc~. ~ence~ compo~nd~ which- 8 --~23'7~

RD~6799 do not crystallize from solution are preferred. Seccndly;
compounds derlved from aliphatic hydrocarbon~ give low yieldq of csrbon which moreover varies with the rate of heating, so that no exact control m~y be exercised over the carbon sddition. The low yield is therefore anoth~
~erious limitation. For in~tance, acrylic ~esins ~hich yield about 10% carbon on pyroly~is are not effective.
High molecular weight aromatic compounds are the preferred material for ma~ing th~ carbon addition sin~e these give high yield of c~rbon on pyrolysis snd do not crystallize. Thus~ for in~tance9 a phenol-fvrmaldehyde condensate-novolak which i8 soluble in acetone or higher ~lcohols, such as butyl alcohol~ may be u~ed as well ~8 many of the rel~ted cond~ns~tion pro~ucts9 such ~8 re-sorcinolform~ldehyde, aniline-formaldahyde9 cresoI-formaldehyde, etc. Similar compound~ yield ~bout 40~60%
of earbon. Anoth~r sati~fsctory group o compounds are derivatives of polynucle~r aromatic hydroc~rbons con-tained in co~l tar, suoh a~ dibenzanthr~cene, ~hrysene, etc. A preferred group o c~rbonaceous add~tives are polymers of ar~matic hydrocarbon3 ~uch a9 ~olyphsnylene or polymethylphenylene whlch ~re soluble in arom~ic hydrocarbon~ and yield up to 90% of carbon. However, the additlon o elemental carbon direc~ly to ~he sillcon carbide powder is le~s pr~ctlcal~ s~nce i~ i~ very _ 9 ,, difficult to obtain the required degree o d~stribution and, frequently~ large amount~ of carbon inclu~ions are found after s~ntering. Such inhomogenelt~es h~ve9 of course, a detrimental effect on strength because they inltiate ~ractures. `~
An excellent way to introduce carbon into the ~ubmicron silicon carbide powders ig by adding a 801utio~
of the carbonaceous substance which i8 decomposed to car-bon on being heat treated. In m~king the carbon addition, the first step i~ to prepare a solution of the ~elected carbonaceous compound in a convenient ~olvent preferably having a moder~ely high melting point in case fr~eze dry~ng i~ to be u~ed. The powder is then di~per3ed in the desired amount of ~olution cont~ining ~he necessary amount of the organic compou~d. The volume of the $olvent required i8 an amount ~ufficient to yield a thin ~lurry when the ~llicon carbide powder i~ fully dl~perged. The solvent is then evaporated either directly from the liquid dispersion or by freeze drying ~he d~ spersion ~nd ~ub~
~0 liming off the ~olvant in vacuum. Thi~ latter procedur~
has the adv~ntage, that it prevent~ i~homogenei~ie~ ln the distrlbution of the ~dditive which are ~lways in~ro~
duced on dry~ng in the liquid ~ta~e due to Lhe migra~ion of the oolute. In thi~ w~y, a uniform coatlng of t~e organic ~ub~t~nce on the silicon c~rblde cry~t~ es i~ ob~ained which yield~ the desired degree of ca~bon distrlbution.
Another approach to lmproved carbon distri-bution on a submicron p~rticle size level i~ the appli-cation of jet m~lling. The ~illcon c~rbide powder i~
so~ked with a solution of~ for ln~tance~ a novol~k re3in in acetone9 dried in air snd heated up to 500 C. to 1800 C. in nltrogen to pyrolyze the re~l~. The ~ctusl amount of carbon introduced by this proce~ detenmined as weight gain after the pyrolysis or by analysis o ree carbon. The powder wi~h the added carbon iB then ~et milled whlch gre~tly improves ~he distribution of carbon and elimlnate~ major carbon grains in the ~int~red pro~
duct To mold and sh~pe the p~wder into a de3~red fonm, ~ny of the conventlon~l techniqu~ gener~lly used in the fleld of cer~mics may be applied and the proce~slng of the powder mixture is treated aocordingly.
In die pressing, the powder usually rsquires the addition of a small amount of lu~ricants, such a~
1 weight percent of ~tearatesS ~lthough ~ome powders oan be pres~ed lnto ~imple shaE~es withou~ such additions~
Thus, for example~ 300 g. of the SiC powder to which an ~d~ltion o~ boron and carbon i~ rnade on preparationg i~
disper3ed ln 300 cc. o a ~ solution of ~luminum ste~rate in benzene and milled in a pl~tlc ~ar by ~em~nted carb~de ball~ or 5 h~urs. After ~hat the ~llp is gtr~ined through a 200 mesh sieve, and the ~olvent i~ evapor~ted.
The resulting powder may ~hen be pres~ed ~t 5000 psi to ~hapes having a green density of about 55%. The ~ame powder m~y also be ~80~t~tically pressed into more com-plex shapes such as tubes, ~rucibles, etc., by the wet-bag method. The application of 30,000 psi pressure yleld8 a green den~ty corresponding to 59%.
To obtain more c~plex shapes, the green body may be machined by grinding, milling, etc. or if desired 10 , it may irst be prefired at a temperature of about 1600 C~
in an atmosphere of nitrogen or argon to obtain ~re~ter initial streng~h. In any case, shrinkage should be t~ken into account in detenmining the f~nal dimension~0 These dimensions, after ~ring, are of course? the funct~on of the green ~nd fired densities and are es~blished in a conventional manner.
It is also feasible to slip ca8t the silicon carbide powders. A convenient dispersion medium 1~ w~er and the deflocculant i~ speciic of powders prep~red by different procedures previou~ly discu~ed. C~sting 81ip~
with up to 4Q volume percent of solid c~n be prepared by di~persing the powder ln wa~er to which the de10cculant i~ ~dded and b~ll mllling the suspension for ~eversl hours~
The sh~ping is done by casting in pl~ter-ofop~r~ mold~
accordlng to conventional slip efl~ting technique~

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` RD-6799 ~, .
Furthermore, the silicon carbide powder mixture can be extruded or injection molded by the addl~ion o~ a binder to for~ a moldable paste~ There exists a wide selectlon of useful binders which will decompose and evaporate on heating in an inert ~tmosphere without ~n ~ppreciflble re~idue, as exemplified by polyethylene glycol, . or which may be removed by a porous contact~ng media in.
much the ~ame ~ashion as the vehicle i8 removed in 81ip casting.
Firing of the silicon carbide compact~ can be done in conventlonal high temperature furnaa~ provided with means to control the fu~nace atmosphere. It is advantageous particularly with l~rge sh~pe~ to sep~rate . /the firing operation into two steps carried out $n separate furnsce3. Thls is 80 bec~u e the high temper-sture furnaces usually l~ck good temperature control at low temper~tures where the moldln~ additive~ are el~minat~d.
The prefiring is done in an inert atmo~phere such as argon~
hel~um, nitrogen and hydrogen whLch contains le B th~n a~out 10 pp~ oxygen. A temperature of 1500 C. i8 u~ually sufflc~ent to at~ain good strength for further h~ndling9 but somewhst higher or lower temper~tur~s msy be used de-pending upcn the degre~ of ~trength required ~or green . mschin~ng!
The densificat~on o the compact is by pre~sure~
less sintering without the aid of external preasure. Thi~
. - 13 ~37~
RD~6 79g iA distinguished from hot pressing during which a sub-stanti~l external pressure must be applled. The fiLr~
sintering mu~t be performed in an atmo~phere ~Iner~ to SiC such a~ tho~e l~ted above or mixtures thereo$ and S also in vacu~im. However, to aehie~e hi~h densitles9 above 95%, the flring mu~t be done in ni~rogen or a miYcture o nitrogen and a rare gas. Nitrogen ha~ ~ specific eact in that it ~uppresses or retard~ the ~3 to a-(6H) SiC tran~-formation. This tr~fonnation which proceeds in SiC
above about 1600 bring~ about exaggerated grain growth of the a- (6H) phase . Due to this proce~s the SiC po~der co~rsens frequently before the ultlmate density i8 ach~eved and thls coar~ening holds further denslfication ~t some lower final den~i~y typlcally 85 to 90%. N~ trogen, however9 prevent~ thi3 /~oarsen~ng by ~abilization of th~
~-SiC ph~se 80 that high densit~ 2R are achievable.
Ni~rogen al~o 81s)Ws d~wn the rate of ~i~terlng ~o tha~
with higher n~trogen pressure, ~ higher temper~ture have to be ~ppl~ed. Thu~ for instance a ~ilicorl carbid p~wder compac t m~y be fired in 40 ~r~ Hg nitrogen ~t 2020~ C . to 96.5% theoretical density. In 760 ~m.Hg nitrogen, a temperature 2100 C. i~ necessary to obt~in 95%. H~wever, the higher the nl trogen pre~sure, ~he greater th~ gra~n growth control ~nd the optlmum firing condition~ rnay be e~tablished by routine experimentation.

RD 6~99 The temperature schedu~e employed durin~
sintering depend~ cn tha volume of the par~s to be ired.
Small ~pecim~ns weighlng ~ever~l gram3 are gen~rally quite in~en~itive to the temperature program and can be con-veniently brought up to the firing t~perature in about15 minutes. A hold 15 minute~ at the peak temper~ture will bring about the de~ired den~ity. An ex~endgd dwell at high temperature i3 harmful because it brings ~bout coarsening of the mierostructure and con~equently de-gradation of mechsnical properties. Thu~) the shortegtnece3sary hold is preferable.
With large shapes, the firing ~chedule has to bc extended ~o ~llow for nitrogen diffusion throu~h th~ body on heAting up and to avoid therm~l 8r~dients in the fir~d bodle~. Thus~ for in~tance, a 250 g. pres~ng may be pre~
fired at 1500 C. 2nd tr~n ferred ~n~o the high temperatur~
`furnace. In an argon-ni~rogen pro~ective atmo~phere3 the pressing can ba heated up to 1600 CO in 40 m~n. and the temperature then grsdually increased up to 2020 C~ ln 20 80 minO snd held ~here for an ~dditlon~l 60 minUteB.
Coolin~ i~ not eritic~l, because of the high thermal eon~
duc~ivity of ~intered silicon carbide.
The nltrogen atmo~ph~re~ on firing~ ha~ an add~tion~l ~pecific ~fect on th~ ~intered 5iC ln that it ~nducss electrical conductivlty by lntroducing n-type ~¢~1-conductivity. The degree o conductl~ity `~ proportion~l - 15 ~ .

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3'7~ ~

to the nitrogen pre~ure on ~inter~ng but i8 a18Q afected by mlnor amounts of other elements ~nd impur~tles which enter the lat~ice. Thu~, by monltoring the nltrogen pre~sure in the furnace, i~ i~ po~sible to prepare poly- -crystalline SiC with a resi~tivity r~nge from 10 ohm~omtypical or nitrogen free ~in~ering atmo~pheres to 10 ohm-cm typical for an atmo~phere of 760 torr N2.
i My novel process now makes it possible to fabricate c~mplex shsped article~ of ~ high grade single ph~e, polycrystalline ~ilicon carbid~ by convention~l cer~mic techniques. Heretofore9 ~uch complex 3bsped ~rticles could elther not be m~nufac~ured from ~illcon c~rbide ~t all or required expen~ive ~nd tedlous m~chin~ng because of the very nature of the m~terial. Thus, articlc~
such a8 8a~ turbine airfo~l~, imp~rvioug crucibles, thln w~lled tube~, long rodst spheric~l bodies~ ~nd hollow shap~ e.g. gas turbine blsda3 can no~ be obtained directly. The preferred hi8h dèn~ity ~ilicon carbide9 of which the ~rticl~s are formed9 ha8 ~ densi~y of at le~t 95% of theoretical a modulus of rup~ure of ~bout 80,000 psi~ a hi~h resist~nce to oxida~iong 8 hlgh ~esi~tance to creep a~ 1~500 C. and essentially the des~r~bl~ proper~ies o ho~ pre~sed slllcon c~r~id~ ~B reported in the Canadian applicatlon Serial NQ. 1g8,3930 ~OreOVer7 the 81~t~r~d silicon carbide m~y be prepared in ~ch a w~y ~h~ ~he 3 ~

RD-67~9 product ha~ a wide range of electric~1 r~3istance pro-pertie~, .
My invention i~ further illu~trated by the ~ollowing examples:
EXAMPLE I
A submicron silicon carbide powder was prepar~d and characterlzed and the results are listed below:
Chemical:

Oxygen ppm 600 N~trogen ppm ~ 50 Free carbon ppm 6000 Iron ppm 180 Aluminum ppm C 13 Boron ppm 4000 ~15 Specific 3urface area, m /g 16 Mean ~urface average 0.15 crystallite size, ~m X-ray dif~rflction: ~-SiC
trace~ of a~SlC 6H

Two hundred grams of the ~ilicon earbide powder were di~-persed in 200 cc. of a ~olut~on of I g~ aluminum 8 tearàte and l g. oleic ac~d in benzen~ ~nd ball m~lled ~or 2 hrs.
, .
w1.th cemell~ed carbide balls. The ~lurry was strained through a 150 me~h U.S. Standard sieve and freeze dried.
The obtained friable cake w~ broken up and sifted through ` 25 a 42 mesh U.S. Stand~rd ~ieve. Pres~ing of tha re3~1t~ng powder ~n a 2~.5 ~n~ di~meter ~teel die at S000 p~i yi~lded a denslty 1,6S g./cc. which i~ equlvalent to S1~5~ of the , .
.

RD-~799 theoretical. On isostatic repreq~ing o the blank at 25,000 psi the density increased to 1.76 g~/cc~ which ls equivalent to 55% of the theorctical, The pressing was fired in ~ graphi~e resi~tor furnace in flowing nitrogen at 40 mm. Hg pressure wlth the following temperature schedule:

R.T. to 200 C. 10 min.
200C. to 400C. 50 min.
400C. to 1500C, 30 min.
1500 hold 30 min.
1500-1950C. 20 min.
1950-2020C. 30 mln.
2020C. hold 40 min.

After the 40 min. hold ~t the highest temper-15 ature thc furnace was shut off" filled with nitrogen to a tmospheric pre~ ~ure and allowed to coo~ to room temper-ture .
The disc un~erwent 14 . 57" ~hrink~ge (bas~d on the green diameter) and h~d a den3ity of 3.16 g. /cc . which is 20 equivalen~ to 98% of the theoretic~l. Sect~oning and micro~copy revealed th~ had bimodal micro~tructure composed of ~ matr~x about 3 ~m grain ~i~e and large tabular cry~tals up to 200 ~m.
A di~c pressed in a steel di~ only, h~ving green den~ty 51.57~ of the theoretical) ~ired at th~ ~me c~nd~-tions yielded ~ fired den~ity 3.07 g./cc. corr~spondln~
to 96,2% of the theoretic~l. The el~ctrical resistiv~y wa~ 70lacm.

~3~

~D-S~9 E8A~PLE II
A pr~ssing prepared from ~ powder de~er~bed in Ex~mple I ~green density of 5~%3 was fired in flowing nitrogen at atmo~pherlc pressure at a ~lmilar temper~ture time schedule with the peak temperBture incre~sed to 2080 C.
The final den~ity of the body was 915h o the theoretic~l.
Sectionlng revesled ~ refined microstructure with grain~
not exceed~ng 20 ~. Elactrical resistance was 0~2~ cm.

EXAMPLE III
__ A cylinder having a diameter of 5/8 inch ~nd - 1/2 inch long pre~sed at 5000 p~i from the p~wder compo-~ition deserlbed ln Ex~mple I ~green dens~ty 51%~ was fired in flowlng argon ~t 40 mm. Hg at 2080V C. for 15 min, and cooled to room temperature. The final relative density w~s 91.5% and the microstructure W88 coar~e gr~ined, composed of lar~ t~bular cry~tals. The elect~ical resi~tivity was 8 x lQ ~ cmO

EX~PLE IV
A ~pecimen of the same ~iz~ and green den~ty a~ de~crlbed in Example III wa~ fired in ~ vacuum of lO0 microns Mg (the re~idual atmosph~re being compo~ed of N2 snd C0) ~t 2000 C. or 15 m~n. The flnal d~n~ity wa~
93% of the ~heoretlcal and the re~i~tiYity 4 x lO Jlcm.

The specimen's surf~ce wsa covered by c~rbon due to de-~3~7~4~3 RD~6799 composition of Si~ snd volatl~i2ation of silicon.

E _ An aqueous 81ip was prepared fr~m the su~micron SiC characterized in Eæ~m~le I by mixing 400 ~. of the powder with 250 cc. of distilled water and adding 2 cc.
of ~odium ~ilicate ~olution cont~ining 20% Na20~3SiO2 (22 Be). The slip wa8 ball milled for 2 hours with cemented carbide ball~ and 3tralned thr~ugh a 150 me~h s~eve.
Cr~cibles 1-1/2" diameter x l~ " high were then formed from the ~lip by drain c~!~ting into plaa~er-of-parig molds removed rom the dle and dried, The ca~t~ng~
were fired in flowing ni~rogen at 40 mm. Hg in a firing cyele de~cribed in Example I. The final density wae . 95.5% of the theoretical and ~he s~rink~ge w~s 18.5%.

EXAMPLE V-I
~ .
A eommerGial sil~con carbide pawder of slmilar characteristie~ ~ the one dexcribed ln Example I but con~
taining les3 than 20 ppm,o~ boron WR3 proces3ed, pressed - into a 5/8" diameter pelle~ (green density 60%~ ~nd flred at 2020 C. in flowing N~ at 40 mm. Hg for lS ~nu~
- No ~hrinkage or deneiication wa~ observed.

EXAMPLE VIT
To the ~ame p~wder as in ExRmple ~I was ~dded 1% amorphou~ bo~on wh~ch wa~ ~et mllled ~o a particl~

RD-67g9 size~ 2 ~m 50 g. of the powder mixture w~ dispersed in ~enzene and milled with cemented carbide balls for 2 hour~. The 81ip W~S dried ~d the resulting powder pres~ed ir to 5/8 inch diameter pellets hav~ng ~0% green density. Firing of the specimens in flowing nitrogen at 600 torr ~t 2080 C. for 20 mlnutes resulted in 12%
shrinkage~ The fin~l density w~s 93% of the theoretic~

E~LE VIII
Amorphous silica ~nd c~rbon black wer~ mixed in a molar rstio 1/4 ~nd fired in hydrogen at 1600 C. for 2 hours. The product was refired at 700 C. in ~ir for 5 hours untll the unreacted carbon wa~ burned off. The resulting powder W~3 le~ched with 20% hy~rofluoric ~cid9 washed with water ~nd ethyl alcohol and dried. T ~ pro-duct w~ chsr~cterized as pure ~-S~C by x-r~ys ~nd con-t~lned leR~ than 2000 ppm. metallic impuritie~ 0.270 oxygen and 0.08% nitrogen.
The powder was comblned with lX by ~eight boron using the same procedure de~cribed in Example VII
and ~et milled. Pre~ing at 5000 pBi yielde~ pellet~ of 50% relstive density. Firing in flowlng ni~rs:~gen ~t 40 mm. Hg ~nd 20~0 C. rasulted in 3Z ~hrinkage ~nd B
fin~l density of 61%.

EXAMP~ IX
The process~d powder de~cribed in Ex~ple YIII

W8~ dispersed in a ~olution of 1 g. of polymethylphenylene in 100 cc. toluene. The dl~per~ion of 10 g~ of the powder in 10 cc. of the ~olution wa~ dried and reRulted into an approximately 0.97O o carbon Addition on pyroly~ls sf the organic compound.
Thls powder W8~ pre~ed into 5/8 lnch di~meter pellets ~green dcn~i~y 4970) ~nd'fired in flowing n~trogen at 40 mm. Hg and 2020 C. The ~pecimen~ underwent 14.5%
shrinka8e and h~d a final density o ~5%.

EXAMPLE X
. .
SiC powder qpeci~ied in Example VI w~ combined wlth 1~ aluminum metal powder ~nd mixed dry. 20 g. o~
: the mixture wa~ jet-milled u~ing ni~rogen as grinding medium. 10 g. of the obt~lned powder wa~ dispersed in 10 g. of the obtflined po~der wa8 di~per~ed in 10 cc. ~f a 1~ solution of aluminum stearata and dried. Compaction in 5/8" d~ameter ste~l die yielded 55% green den~ity~
The ~pecimen wa~ flred in vacuum (at 100 ~ Hg) at 2020 C. for 15 minute~. The flred cylinder~ ~howed 4 ~hrink~ge and a fin~l density ~bout 65%.

EXAMPLE XI
SiC powder ~pecified in Ex~mple VIII W~8 com-p~cted without sny add~tion at 5000 p~i in ~ ~t~el di~ t~
denslty of 51%. The pellet was fired in l~w pre~sure 22 ~

.

~D-67~9 nitrogen (at 40 mm. Hg) at 2080 C. for 15 minutes. Mo ~hrink~ge W~8 detected in the fired ~pecimen.

It will be appreclsted that ths inven~ion not limited to the specific details ~hown in the example~
and illustrations snd that various modific~tions may be made within the ordinary 8kill in the art without de-p~rtLng from the spirit and scope of the invention.

.

-

Claims (38)

The embodiments of -the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A method of making a pre-shaped dense sintered silicon carbide ceramic article comprising the steps of:
a) forming a homogeneous dispersion of a submicron powder of 1) .beta.-silicon carbide, 2) a boron containing compound in an amount equivalent to about 0.3-3.0% by weight of boron based on said silicon carbide, and 3) an elemental carbon source selected from the group consisting of elemental carbon and a carbonaceous additive in an amount equivalent to 0.1-1.0% by weight of elemental carbon based on said silicon carbide, b) shaping the homogeneous dispersion into a green body substantially of the form of said desired final pre-shaped sintered article and of dimensions larger than those of said desired sintered article by the amount of sintering shrinkage, and c) sintering the green body in an inert atmosphere chemically-inert with respect to silicon carbide at atmospheric pressure or below atmospheric pressure at a temperature of about 1900°-2100°C until the ceramic article has a density of at least 85% of theoretical, said carbonaceous additive being pyrolyzable to produce said element carbon at a temperature below sintering temperature.

2. The method of claim 1 wherein said boron compound is elemental boron.

3. The method of claim 1 wherein said boron compound is boron carbide.

4. The method of claim 1 wherein said green body is formed by slip casting.

5. The method of claim 1 wherein said atmosphere is a member selected from the group consisting of argon, helium, nitrogen and mixtures thereof.

6. The method of claim 1 wherein said atmosphere is nitrogen.

7. The method of claim 1 wherein prior to sintering, the green body is subjected to a prefiring step and the prefired body is machined to shape.

8. A method or making a pre-shaped dense sintered silicon carbide ceramic article comprising the steps of:
a) forming a substantially homogeneous first dispersion of a submicron powder of .beta.-silicon carbide, a boron containing additive in an amount equivalent to about 0.3-3.0% by weight of boron based on said silicon carbide, b) incorporating in said first dispersion a carbona-ceous additive in an amount equivalent to 0.1-1.0% by weight of elemental carbon after pyrolysis based on said silicon carbide, to form a second dispersion, c) pyrolyzing said second dispersion at a temperature which decomposes the carbonaceous additive to elemental carbon, d) shaping the resulting pyrolyzed dispersion into a green body substantially in the form of said desired pre-shaped sintered article and of dimensions larger than those of said desired sintered article by the amount of sintering shrinkage, and e) sintering the green body in an inert atmosphere chemically inert with respect to silicon carbide at atmospheric pressure or below atmospheric pressure at a temperature of about 1900°-2100°C until the ceramic article has a density of at least 85% of theoretical.

9. The method of claim 8 wherein said boron additive is elemental boron.

10. The method of claim 8 wherein said boron additive is boron carbide.

11. The method of claim 8 wherein said first dispersion is formed by the steps comprising forming a silica gel in a solution containing sugar and boric acid, dehydrating the gel to form a finely divided mixture and heating the mixture in an inert atmosphere to form a boron doped silicon carbide powder.

12. The method of claim 8 wherein said carbon-aceous additive is a phenolformaldehyde condensate resin.

13. The method of claim 8 wherein said carbon-aceous additive is polyphenylene.

14. The method of claim 8 wherein said carbon-aceous additive is polymethylphenylene.

15. The method of claim 8 wherein said green body is formed by slip casting.

16. A pre-shaped sintered ceramic article consisting essentially of silicon carbide, about 0.3-3.0%
by weight of boron based on said silicon carbide, and up to 1.0% by weight of elemental carbon based on said silicon carbide, the sintered ceramic article having a density of at least 85% of theoretical.

17. The ceramic article of claim 16 consisting essentially of .beta.-silicon carbide and having a fine grain uniform microstructure.

18. The ceramic article of claim 17 wherein said microstructure has a grain size of less than 10 microns, and wherein said density is at least 95% theoretical.

19. The ceramic article of claim 16, 17 or 18 wherein said elemental carbon is present in an amount of 0.1 to 1.0% by weight.

20. The ceramic article of claim 16, 17 or 18 in the form of a gas turbine blade.

21. The ceramic article of claim 16, 17 or 18 wherein the article has a complex shape.

22. The pre-shaped sintered ceramic article according to claim 16 wherein said elemental carbon is present in an amount of from 0.1 to 1.0% by weight.

23. A pre-shaped sintered ceramic article consisting essentially of .beta.-silicon carbide, about 0.3-3.0%
by weight of boron based on said silicon carbide, and up to 1.0% by weight of elemental carbon based on said silicon carbide, the sintered ceramic article having a density of at least 85% of theoretical and a fine grained uniform microstructure.

24. The pre-shaped sintered ceramic article according to claim 23 wherein said elemental carbon is present in an amount of 0.1-1.0% by weight.

25. The pre-shaped sintered ceramic article of claim 23 wherein said article has a complex shape.

26. The pre-shaped sintered ceramic article of claim 23 wherein said article is a gas turbine blade.

27. A pre-shaped sintered silicon carbide ceramic article consisting essentially of .beta.-silicon carbide, about 0.3-3.0% by weight of boron based on said silicon carbide, and up to 1.0% by weight of elemental carbon based on said silicon carbide, said ceramic article having a density of at least 95% of theoretical and a uniform grain size of less than 10 microns.

28. The pre-shaped sintered ceramic article according to claim 27 wherein said elemental carbon is present in an amount of 0.1 1.0% by weight.

29. A pre-shaped sintered ceramic article in the form of a gas turbine blade consisting essentially of silicon carbide, about 0.3-3.0% by weight of boron based on said silicon carbide, and up to 1.0% by weight of elemental carbon basedon said silicon carbide, the sintered ceramic article having a density of at least 85% of theoretical.

30. A pre-shaped sintered ceramic article in the form of a gas turbine blade consisting essentially of .beta.-silicon carbide having a fine grain uniform microstructure, about 0.3-3.0% by weight of boron based on said silicon carbide, and up to 1.0% by weight of elemental carbon based on said silicon carbide, the sintered ceramic article having a density of at least 95% of theoretical, and wherein said microstructure has a grain size of less than 10 microns.

31. A pre-shaped sintered ceramic article consisting essentially of silicon carbide, about 0.3-3.0%
by weight of boron based on said silicon carbide, and up to 1.0% by weight of elemental carbon based on said silicon carbide, the sintered ceramic article having a density of at least 85% of theoretical, and wherein the article has a complex shape.

32. A pre-shaped sintered ceramic article consisting essentially of .beta.-silicon carbide having a fine grain uniform microstructure about 0.3-3.0% by weight of boron based on said silicon carbide, and up to 1.0% by weight of elemental carbon based on said silicon carbide, the sintered ceramic article having a density of at least 95% of theoretical, wherein said microstructure has a grain size of less than 10 microns, and wherein the article has a complex shape.

33. A method of making a pre-shaped dense sintered silicon carbide ceramic article comprising the the steps of:
(a) forming a homogeneous dispersion of a submicron powder of (1) .beta.-silicon carbide, (2) a boron-containing compound in an amount equivalent to about 0.3-3.0% by weight of boron based on said silicon carbide, and (3) an elemental carbon source in an amount equivalent to 0.1-1.0% by weight of elemental carbon based on said silicon carbide, (b) shaping the homogenous dispersion into a green body substantially of the form of said desired final pre-shaped sintered article and of dimensions larger than those of said desired sintered article by the amount of sintering shrinkage, and (c) sintering the green body in an omert atmosphere chemically inert with respect to silicon carbide at atmospheric pressure or below atmospheric pressure at a temperature of about 1900°C-2100°C until the ceramic article has a density of at least 85% of theoretical.

34. The method of claim 33 wherein said boron-containing compound is elemental boron.

35. The method of claim 33 wherein said boron-containing compound is boron carbide.

36. The method of claim 33 wherein said green body is formed by slip casting.

37. The method of claim 33 wherein said atmosphere is a member selected from the group consisting of argon, helium, nitrogen, and mixtures thereof.

38. The method of claim 33 wherein said atmosphere is nitrogen.