Note: Descriptions are shown in the official language in which they were submitted.
- ~ ~ 3~
RD-6799
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~
RD-6799
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~
-- 4 ~
3 7~
RD-6799
~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
-- 5 --
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
- 6 --
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%
.
7~
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 ,,
RD 6799
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
- 10 -
RD-67
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
RD-6799
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~
~3~7~
` 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.
- 14 -
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 ~ .
.
3'7~ ~
RD-6799
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
- 17 -
,
.
.
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.
- 18 -
~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-
- - 19 -
~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
RD 6799
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.
.
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