Note: Descriptions are shown in the official language in which they were submitted.
~Z561Z6
T)T~NS~3 C131~MIC.5 COMT~INJNG ~ SOL:[D SOLUTION
AND M13TE~OI) FOR M~KING THe SAME
B _ ground_o _the Irlvel!-tion
Fie _ : This invention relates to the field of
ceralnics and particularly to ceramics contailling solid
soLutions containing the elements Si, C, Al, O, and N
(reEerred to by the acronym SiCAlON), and a method for
densiEying such ceramics.
Description of the Prior Art: Silicon b~sed
I() cerarnics are leading candidates for applications in high
temperature environmellts including energy conversion
devices due to their high strength to temperatures on the
, order of 15U0C. .5iC and Si3N4 ceramics also find use in
I applications, over a wide temperature range, where wear and/or chelllical resistance is required.
Pressureless sintering of SiC has been accom-
plished using either B and C (or B4C) or Al (or A12O3) as
sintering aids to obtain nearly single phase SiC with
densities greater than 97% of theoretical. Very active
2() powders having high surface area (mean particle si æ is
! less than 0.5 micrometers) are required to provide the
driving force for sintering. Very little densiEication
!. occurs by pressureless sintering with SiC particle sizes
greater thall 1 micrometer. Since pressureless sintering
al]ows fabrication oE complex shapes economically, it
would thereEore be an improvement in the art if SiC
particles in the 1-10 micrometer range could be densified
without pressure.
Similarly, several metllods have recently been
7~1) develc)~ed Eor the pressureless sintering of Si3N4. One
oE tlle most successEul methods for pressureless sintering
involves the Eormatioll oE a solid solution of aluminum
~25~26
oxynitride itl silicon nitride to form SiAlON (a coined
term consistillg oE the chemical abbreviatioIls for the
elemellts pre~ent in the :~olid solutioll). l'llis family of
mater;als sinter via a traIlsient liquid pllase an~1 has
r, proven successful in malcing cuttirlg tools.
lrl allalogy with the Si7~10Ns, CutLer et al.
(U.S. Patent 4,14L,740 issued Fl3B. 27, 1979 for SOT~ID
soLu~rro~ ANI) PROCESS FOR PROI)UClNG ~ SOLlD SOLUTIOM)
(1iscovered t~lal: a complete solid soLution exists between
1() alpha pllase SiC (2~ polykype), ~lN, and ~l20C. This
Eamily o~ new materials was named usirlg the acronym
SiC7~l0~, analogous to the SiAlON system. Tllese compounds
exhihit tlle same hexagonal wurkzite structure and have
similar lattice parameters.
Subsequellt studies on the SiC-AlN system
indicated that the system had mecharlical properties
similar to silicon carbide. The main advantage of the
solid solutions was that by changillg composition, one
could control properties (i.e., density, hardness,
2() fracture toughlless, Young's modulus and thermaL expansion
co-eEficient) of the den~e cerarnic. This family of
materia]s thus appears to have tlle potential to broaden
the appIications of SiC. It would also appear that the
sol;d solubility rallge in the SiC-~lM-~I20C system is
IllUCh greater l:hall that found in the si1~LoN system, thus
broadening the physical property range where engineering
of properties ls possible.
'r~e Illajor limitation of tlle prior SiCAlON work
was that attempts to pressureless sinter tlle composites
3() were ullsuccess~ul and processing was expensive, requiring
hot pressillg. WlliLe hot pressing resuLted in dense
polycrystalline ceramics, in or~ler to make complicated
shapes econolllic~IIy in large volullles, one must prefoml
the material to a shape simi]ar to that desired for tlle
,,
1~6~
-3- 1
final colnpol1ent and sinter without pressure. As an
exarllp]e of the utility oE sucl1 a tecllniyue, one is
referred to the work of cutler (U.S. Patent 3,960,581,
issued Jul1e 1, 1976, for PROCESS OF PROI~UCING A SOLII)
.', ~SOr.UTTON OF Al.[IMl:N[JM ox:rl)~3 IN Sl:LlCON NITE~ 3 ) where
Si~L()N materials were pressureless sintered. No
techrliqlles heretofore have been reported ti~at allow
sintering (wi~l1out hot-pressing) o compounds in the
Si~ L~ 0C pllase field into a sub~starltially c1ense
1() polycrystaLIir1e ceramic.
In recognitior1 oE the interest in high-strer1gth
ceramic materials havil1g specific physical properties, it
would be an adval1tage in the art to provide an inexpen-
sive metl1od Eor fabricating complex shapes of a solid
solution consisting of silicon carbide, aluminum oxycar-
bide and aLuminull1 nitride.
Sumn!ary of the Ir1verltiorl
~ process for pressureless sintering (i.e.,
sinteril1g in tlle absence oE applied pressure) SiCAlON
2() ceranlics has been invented. In addition, this process
allows mixtures of the solid solution witl1 other
materials to be formed. Specifically, mixtures oE the
soLid so]ution Witl1 SiC and AlN allow improved tilermaL
conductivity as compared to the complete solid solutions,
while retaining the desirable aspects of pressureless
sintering and engineering of properties.
~ metl1od is disclosed for densifying solid
solutions of at least alumir1um oxycarbide and silicon
carbide and/or a]uminum l1itride (i.e., SiC~lON ceramics).
3() ~1aterials can be made whic11 consist of an
intirllate mixture oE SiCA]ON solid solution witll a
distil1ct second pllase oE either Si( or ~lM.
i6~'26
~ tec1~ni.que for pressureless sintering oE
ox;(les, carbi(les and n.itrides of si.licon an~ alun1i1lun1 in
tt1e prese1lce oE a.L-1min~ 1 ancl carbon or alu111;111lm carbide
or alumi1l~l111 oxycarbi.de to for111 a substa11tial:1.y dense
pol.ycrysta1:1.ine body oE virtually any sl1ape has been
discovered.
r1hese ceramic bodies can comprise about 1 to
99~ by vo:1.u111e of a so:1id so.lution consisti11g of aluminu1n
oxycarhi-.~e a11d s;..1.ico11 cArl~i.de a11d/or a1u111i11um nitride
1~) a11d at .Least: one reEractory phase of SiC and/or ~I.N.
Descri~ rl of tlle_)_awi~
FlG. :L is an x-ray diffraction pattern of a
pressure1.ess sinterecl SiC~l()N ceramic described in
~xample 2.
FIG. 2 is an optical micrograph oE the SiCAlON
ceran1ic in F]:G. 1, taker1 at 15~0 magnifications, showing
the existence of two distinct phases. 11he so].id solution
i5 tllerefOre illhOlllOyelleOUS.
Fl(;. 3 is an x-ray diEEraction patter11 illus-
2() trating pea1cs due to t:he solid soLutio1l and SiC.
FIG. ~ is a graphical representat.ion oE the
strengt11 data from pressure:~ess sintered bars of SiC~lON
(contai11i11y a reEractory pl1ase of Si.C) in co1nparison to
pressure:Less sintered SiC (without the solid solutio1l).
FIG. 5 is a graphical representation of the
fracture toug11r1ess data Erom SiC~lON (contai1li1-g a
refractory phase of SiC) in compariso1l to SiC without the
solid solution.
FIG. 6 is a graphical representatiorl of the
3() sinteri1lg behavior of SiC-A12OC without containme1lt when
sinterecl at 2()1)()C in N2 for 5 minutes.
FT~. 7 is a graphica:1. represer1tatio1l of the
sinteri1lg behavi.or of Si.C and SiC~lON (containi1lg a
.s'~ '
L ~
~2~i;6~26
refractory phase of SiC) as a unctioll of tlle starting
SiC partic]e size.
Oescrl~tion of tlle Invention
tn contrast to previou~s work whicl1 required
intimate mi~ctu~es oE reactant.s to form a solid solution
and hot pressing to densify SiCAlON materials, tl)e
present ;nvel1t;on relies upon convelltiollal ceramic
peocessillg. I)ellse polycryst:allille SiCAlON ceramlc
bodies ~or mixtures of Si('AlON witll SiC and/or ~IN) can
1() be made according to tlle instant inventioll by mixillg
certain compoullds containillg the elemel1ts Si, C, Al, (),
and N in tlle proper proportions and malll-er, formillg
sllaped bodies (;ncllldillg complex sllapes) by convel-tional
pres.sing techniques, slip casting, injection molding, and
tlle like, and sinterillg without applied pressure in a
furnace. 'rllere are a nulnber of compoullds which can be
used, some of w~lich are illustrated by the following
reactions:
x(SiC) + z(4~1 + Al203 ~ 3C )~ [xSiC-3z(Al20C)]
2() x(SiC) + z(Al203 ~ Al4C3) --~ [xSiC 3z(Al20C~
x[a SiC. bAlN] + z(Al203 -1- Al4C3) ---~ [xaSiC 3z(Al20C) xbAlN]
(n-l)SiC ~- SiO2 ~ Al4C3 ---~ [nSiC 2Al20C]
Si -~ SiO2 ~ 4Al ~4C --~ 2~SiC-Al20C]
'I'he bracket [ ] is meant to imply that a solid solution
of tlle indicated chemical composition can be formed.
Ilowever, a complete solid solution is not necessarily
formed in the densified ceramic. Several types of solid
, .. .
. . ~
;126
--6--
soluti.ons may exist as discrete particles witlli.rl tlle
cerami.c- bc~(ly that mlght be ricll or deficient ill certain
el.emellts. Tlowever, the overall. or averaged composition
oE the resu.l.tillg body would be es.sentially that of the
brackete(l compoulld.
Cut:Ler and Mi.l.ler (US. Patent 4,l4l,740)
cl.aime~l tllat a complete so:Lid solution e~ists between
~L2~(, S.iC and ~J.M indicatillg tllat tlle variables a, b, n,
x, all(l z vary over tlle entire stoichiometry range.
l~) RaEaniel.. l.o alld Vi.rlcar Eound that the solid so:Lution
betweerl Si.C and AIM varied between 5 and l.()0 weight
perCellt alllllli.llUIII nitri.(3e at 21()()C. Since the forlllatior
of the sol.id solution is aiEfusion limited, the wide
rarlge Eor the solid solution is dependellt on partic.Le
l.S si.ze, .sinterillg temperature and tinle. Since the present
inventioll does not rely on the formatiorl oE a complete
so:Lid so].ution, the only li.mitation on x and z is that
t:llere be enough l.iquid pllase to allow sinterillg. Since
mirlor liquid amounts allow actival:ed sinterirlg to occur,
2() the ratio oE x to z can be as great as 99:1. SiC, ~lN or
otller compounds (i.e., 13eSiN2, MgSiN2, beta ~14SiC4, beta
~lsC3N, Si3~].4N4C3, and tlle like) havil-lg the wurtzite
structure are needed to stabilize ~12OC so ratios oE x to
z are preferably not less than 2:98.
Z5 ~rhe urlique t:echnique oE the present invention
comprises perEorming tlle sinterirlg in sucll a n~nr-er as to
substantially limit decomposition or volatilization of
the powder compact prior to densification. ~ensification
occurs rapidly over a narrow temperature range (between
3(~ :l800 and 2000C). It is tllereEore believed tllat densifi-
cation ta~es place primarily vi.a a liquid phase or tran-
sient liquid pllase mecllallism. ~ liquid pllase is knowrl to
be present at temperatures i.n excess of 1840C in the
~1203-~l4C3 system due to a eutectic reaction between
'126
~1203 arld 7~14C~4C. Microstructural evidence of
solutiorl-precipitatioll confirllls tllat: a liquid phase is
present duritlg tlle reaction. Iiquid pllase sinterillg
tllereEore competes witl~ decomposi tion of some of tlle
reaction constituerlts due to their lligl~ vapor pressure.
necOmpOsitiOrl may be limited by a llumber of
d; fferellt teclln;ques incl ud;rlg 1) using a c]osed c~ucible
corlta;llilly tlle green body (i.e., a grapllite or boron
n; tride crucible ) 2 ) by embedmerlt of tlle green body in a
I 0 loose]y paclced mass of ceramic particles of a substan-
tially simi]ar cllemical composi tion; 3 ) by controlling
the ~eating rate and sintering ti ll~ to limit decomposi-
ti on and promote sinterillg; and 4 ) by controlling the
sinterillg atmosphere so as to suppress the decomposition
J 5 and subsequent volati zation of reaction componerlts . By
suppressillg decompositiorl, sintering to lligll aensities is
possible. Ihere is a minimum temperature at whicll tlle
above reactions take place. ~t temperature extremes in
excess of the mirlimulll temperature required for densif ica-
2~J tion, tllere is evidence of decomposition of tlle reaction
products. Tl~e range of acceptable sinterillg telllperatures
is obviously deperlderlt oll tl~e volume oE tlle liquid pllase,
but ternperatures between l 750 and 2200 C llave been found
to be acceptable.
Ille tecJ~nique is not limited to speciali ty
cllemicals but ratller can utilize commercially available
raw materials witll starting purities preferably greater
thall 98.5%. Starting particle size determines the extent
of tlle solid solution formed. Substantial densificatiorl
~() llas been obtained from starting materials with powders in
the 1-10 micron particle size range. ~rl~e finer the
starting particle size, tlle greater tlle amoullt of soLid
s o L ut i Oll f o rmed .
t~'~
1n eacb oE t11e examples citecl be1ow a met1~od
Eor controLLi11g decomposition and volatiLization is
d; sclosed 1n tl~e absence of controlliny tl~e vapor
pressure of the reaction little or no densiEication
'i takes p1ace.
7~9 Ci. ted abo~1e previou .5 work was limited to
ma1ci11g powders wl~icl~ were colnpLete solid solutions (ll.S.
Patent 4 141 740). The present inve1ltlon in co11trast
per11~ 3 mi xt11re.s oE tlle so1i(l SOllltiOII alld Otller ll.igll
1~ telnF)-?ra1-11rf? re~rac1()ry p~1ases Wl1;CI1 l1ave qnod El1ysica
properties to be made. '1'1~e pressureless sintering
tecl111ique l1erei1 discLosed can be applied to eitl1er
l1o111oge11e~ s soJid soLutions in11o111Oge11eou~s solid
so1utions or mixtures of SiC?~ N and other pl1ases.
IS ~X~MPL13
M~'1'130D F(~ I)r'N~IFYING 1~ E10MOGENEOUS SOLID SOI.UTION
SiC (0.615 grams) made by tlle carbotl1erlllal reduc-
tion o ~ ().255 grams ~1203 (Meller 1).3 micrometers)
and 1).36() gra111s A14C3 made by tl1e ca rbotl1ermic red1lctio11
2(1 Oe 7~12()3 were n1ixed in an agate mortar and pestle ~or 15
mi1-utes. 1~pproxi111ate]y 3 wt. % po LyvinyL pyrrolidone
(PVP) was added as a binder duriny the mixir1g operation
~n(l the powder was r111iaxially pressed at 7() MPa to forln a
J8 m111 cliameter dis1c. '1'he binder was pyrolyzed by slowly
2!; l1eatillg to 900C under N2. Tlle compacted powder was the
placed in a 20 m1n diameter by 20 m1n deep cavity withir1 a
der1se grapllite (Poco graphite) crucible 9 an in diameter
and 1() cm high. The crucible was closed using a graphite
foil seal whic11 mated the crucib1e to a threaded graphite
3() lid. 'l'1~e crucible was tl1en placed in a yraphite resis-
tance heated furnace and heated at a rate of approxi-
mately 75C/mi11ute to 2015C under flowiny N2 and heLd
for 15 minutes. 'l'he disk sintered (16% linear shrir1kaye
.
~S61:26
g
in diameter) to closed porosity and a density of 3.1 g/cc
(greater t~lan 99~ of t~leoretical density). The micro-
structure indicated tl~e presence of a single phase wllen
viewed optically at 1500X rnagnification. X-ray diffrac-
tion also indicated that a homogeneous solid solution had
formed (see 'L'able 1) and conEirmed that the samples had
reacted to form a solid solution consisting of 70 mole
percent SiC and 30 mole percent ~12OC. Since sintering
was dolle in a nitrogen envirollment tllere is no doubt that
Il) tlle solid solutio~l contaills some ~lN.
~rable 1
XRD PFIl~K POSITIONS FOR CU Kd~ RADlATlON
(lC)MPOtlND 2e AT PtlANE
(100) (002) (101) t102) (110) (103) (200) (112)
I_
SiC (2~) 33.5 35.6 38.1 49.8 60.0 65.2 70.~ 72.0
~lN (411) 33.1 36.0 37.9 49.8 59.3 66.0 69.7 71.4
~120C (4EI) 32.5 35.3 37.1 48.6 58.4 64.2 67.9 70.8
~5iC-~20C~ 33.0 35.3 37.6 49.2 59.2 65.2 69.6 71.(
*~xperimentally n~asured value.s for solid solution com-
%() posed of 70 mole percent SiC and 30 mole percent ~120C.
EK~MPLE 2
METI101) FOR VENSIFYING AN INEIOMOG~N~OUS SOLlV SOLU'rION
SiC (150 grams of Starclc BD-10 beta SiC, 17
m2/g containirlg s and C additions), ~1203 (58.19 grams of
Bialcow91ci CR-30), and ~14C3 (87.81 grams, Cerac) were
milled for 10 hours in a poLyetllylene ball milL with 425
ml of 2-propanol and 1 kg. of nigll purity alumilla milling
media to make a uniforlll mixture of tlle powders. ~fter
air drying the 5 gram disk of tlle powders was formed by
uniaxial pressing at 34.5 MPa, followed by isostatic
~S~i~Z6
--10--
pressing at 207 MPa. ~rhe pressed dislc was loaded into
t}le grap}lite cylinder described in example 1 and heated
in N2 at a rate oE approximately 60C per minute to
2000C and lleld there for 1 hour. Upon cooli~lg, it was
~1etermilled that the linear shrinkage was 13.6~ and the
density was 2.93 g/cc or 95% of tlleoretical. X-ray
diffraction s}lowed tllat the SiC 3() mole % A12oC material
was a complete solid solution (F1G. 1). Optical Inicro-
scopy sllowed two d;stinct phases (F:rG~ 2), wllicll were
1() aE~parent:ly Si and ~1 rich SiCAlON so:Lid solutiolls.
~'X~MPTJES 3-7
Fo~MA~l~roN OF A MIXTURL~ OF SiCAlON
Al`11) SiC l~rJD ME'I'IIOD FOR l)RNSIFlCATl(:)N
Commercial grades of SiC (tbiden beta SiC, 17
m2/g arld contains no boron), A12o3 (Biakowski CR-3()), and
~14C3 (Cerac), with weights as givell in Table 2, were
vibratory milLe(l for 15 hours in 1~5~ cc of cyclohexalle
alld 6 kg ~12O3. Tlle powders were air dried and screened
throug)l a ~0 Ine.qtl screen before ulliaxial pressing at 35
2(1 MPa, followed hy isostatic pressing at 2()7 MPa. Tlle
parts were contailled in a graphite crucible as described
in ~xample :I and sintered at the conditions listed in
q~able 3. 'I'he powders sintered to closed porosity with
sllrilll~ages and densities as indicated in 'rable 3. X-ray
diffraction (F'~G. 3) showed that invariably the sintered
samples consisted of SiC and the solid solution (SiCAlON).
nptical microscopy revealed three phases, indicating tllat
tlle solid solution was l~ot holllogelleous. Bar samples were
tested in four poillt belldillg ~FIG. 4) and tlle strengtlls
3~) were comparable witll ~sic. Fracture toughlless of tlle new
materials as determilled ~)y the indelltatiollllletllod was
superior to SiC (Fl(,. 5). Similar results were obtained
with a a wide variety of SiC materials illcluding Starck
.'~
~6~26
BV-10 (beta SiC with B and C additions), Starck B-10
(beta SiC without B and C additions~, Starck AD-10 (alpha
SiC wittl B and C additions), and Starck A-10 (alplla SiC
without B and C additions).
'l'able 2
Composition of ~xan~les 3-7
Example Composition (grams) % SiC in SiC.A12OC
_ SiC ~ ~l~O~ ~laC~ (mole percent),
32~0.0 26.4 37.3 90.0
l() 4240.0 51.0 72.0 80.0
5~.50.0 58.2 87.8 70.0
6120.() 68.0 96.0 60.0
7. 1 ~0.0 ~ 6~.0 96.0 50.0
Table 3
Sintering Conditions and Densification of Examples 3-7
Example Sintering coiiditLons Shrinkage Density
Temp(C) ¦Time a-t Temp(min) I (%) (g/cc)
3 1925 60 14.8 3.12
4 2050 5 14.2 3.15
2() 5 2050 5 14.2 3.15
6 2~50 5 16.3 3.14
7 _ 1925 60 13.0 3.07
~AMPLE 8
j MET~OD FOR SINT~RING SiC-AlN TO FO~ SiCAlON
¦ 25 giC (made by carbotllermal reduction of silica,
j 3.0 grams), AlN-15SiC (made by Cutler process (U.S. Pat.
! 4,141,74U), 3.0 grams), A:l2O3 (Meller, 0.8293 grams), C
i (carboll black, 0.2928 grams), and ~ 325 mesh, 0.8778
! grams) were mixed in a mortar and pestal for 30 minutes
and processed and sintered as in ~xample 1. The sample
sintered to a density near 80~ of theoretical.
~Z5fi~
i
--12--
~X ~MP LE 9
Ml~lllOI) FOR PR~SSUR~I E.5S SINTF:RING SiCAlON VI~ MBEDMENT
SiC (Stark 7~D-10, 90 grams~, ~1203 ~Reynolds
~lP-DBM, 51 grarns ), Al (Cerac, 54 gralns ), and C (Cabot
', Mogul L, 18 grams) were ball milled with 1500 grarns of
l~igl~ purity alumina media in a plastic mill with 500 ml
isopropanol for 12 hours. The powder was pressed into a
pellet as in T~xample 2 and subsequently embedded in its
own powder. The embedded sample was heated to 2000C in
I () 10 rmillutes and l~eld for 5 minutes . Tlle embedded disk
sintered to greater tharl 95% of tlleoretica l density and
~ad an x-ray diEfraction pattern of a mixture of SiC~lOM
and SiC.
FX ~MPLE 1 ()
METHOD FOR PRESSURELESS SINTERING
WITIIOUT CONTAINMENT
I he pressed pellets prepared as in Example 2
were sintered uncontailled in W2 by heatillg from 1()00C in
le.s.s tharl 5 millutes (Fl(~. 6). Tlle SiC- 30 mole ~ ~12OC
disks silltered to closed porosity, being greater tllan 9596
of tl~eoretical density. Tl~e densification occurs quicker
tllan decompo.9iti.0Jl and contai.llment is not necessary.
Since C O is tlle ma jor product of tlle decomE)ositioll
process, control of CO partial pressure will allow tlle
pressureless sinterillg of SiC~lON ceramics at low heating
ra te s wi t llou t con ta i nllle n t .
F.X~MPLE 11
S l:NTER r NG 1- 5 M [ CRON Si C USING Si C~l ON
SiC (( arborulldulll 1500 grit alpha SiC Witllollt 8
3() or C additions, 152.04 grams), A12O3 (Reynold's TIP-I~BM,
58.98 grams), and ~14C3 ((erac, 89.0() grams) were mixed
or 2 hours in llexalle. Tl~e powder was compacted and
. :~
~.~5~
-13-
sinte~ed as described in ~xample 2. Considerable shrink-
age and densification occurred (FIG. 7) whereas little or
no densification occurred when the powder was sintered in
tl~e absence of SiC~lON. Sintering of larger particle
size powders is possible due to the presence of the
liquid phase.
EXAMPLE 12
SIN'rEl~ING SiCAlON USING sio2 AS TH~: OXYGl~:N SOVRCE
SiO2 (M5 Cab-O-Sil, 30 grams), ~1 (Alcoa 123,
1() 54 grams), Si (~tlantic ~quipment, 13.5 grams), and C
(Gulf ~cetylene Black, 24 grams) were ball milled in 600
ml 2-propanol for 10 hours with 1 kg high purity alumina
media. The parts compacted as in Exarnple 2 and sintered
at 1925~C in N2. The disk sintered (13.3% linear shrink-
age) and X-ray diffraction showed SiC~lON and SiC phases.
The present invention is unique in the following
respects:
1. Starting materials may be conventional ceramic
powders in terms of composition (e.g., SiC, A12O3, and
the like), which are of a conventional particle size
(i.e., less than 10 microns in diameter). The starting
particle sizes are preferably 1-5 microns if inhomoge-
neous solid solutions or mixtures of the solid solution
and a refractory phase are desired, or preferably less
than 0.5 microns if a complete solid solution is desired.
2. Complex shapes may be formed in the green state
(using conventional binders) and sintered without the
application of external pressure to form a dense, strong
ceramic body havil)g propertles equivalent to tllose of
,~
