CA1096407A - Sintering of silicon nitride using be additive - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A particulate dispersion of silicon nitride and beryllium additive is shaped into a green body and sintered at a temperature ranging from about 1900°C to about 2200°C in a sintering atmosphere of nitrogen at superatmospheric pressure, producing a pre-shaped polycrystalline silicon nitride sintered body with a density ranging from about 80% to about 100% of the theoretical density of silicon nitride. This method of producing pre-shaped silicon nitride sintered bodies is useful in the production of components for high temperature heat engines.

Description

The present invention relates to a method of producing a pre-shaped polycrystalline silicon nitride sin-tered body having a density of at least about 80%
of the theoretical density of silicon nitride.
Silicon nitride, the stoichiometric formulation for which is Si3N4, is a refractory electrical insulator with high strength, high hardness and high resistance to thermal shock and it consequently has many potential high temperature applications. The characteristics which make it unique among materials are its low thermal expansion coefficient/ its refractoriness and its oxidation stability. Silicon nitride has long been a prime candidate material in the development of components for high temperature heat engines.
~ Silicon nitride parts are currently manufactured by either reaction bonding o~ silicon or hot-pressing.
The first process has inhèrent limitations in achievable densities, and therefore strength, wh]ch exclude it from a number of typical applications. Consolidation by hot-pressing is achieved by using additions of oxides or nitrides of Mg, Be, Ca, Y, La, Ce, Zr to Si3N~ powders. The resulting ceramic is very strong but machining of complex components is very lengthy, difficult and frequently impossible or prohibitiveIy expensive.

~ 7 ~_g~l3 Sln~erillg whlch would overcome t.he ~haping problems has ~190 b~en tried but with limited ra~ults since at tempera~
tures appro~ching 1750~C at atmosplle~ic pres~ure sllLeun nitrlde decomposes rapidly, Silicon nitrlde with 90% densl~
ha~ been obtained by uslng an addltlon o~ 5% magnesia, by G.R, Terwilliger and F',F. Lange~"Pr0ssureless Sinterlng o Si3N4", Journal of Materials S~lence 10~1975~1169, however, weight losses of up to 30% were observed and made the process impractical.
M. Mitomo, "Pressure Sintering o~ Si3N4"~ Jollrnal o~
Materials Science 11~1976)1103-1107, dlscloses the sintering ; of Si3N~ with 5% MgO at 1450 to 1900C under a pressure of 10 atmospheres of nitrogen produc~ng a maximum density of 95%
of the theoretical value, that density and weight loss initially ;15 increased at the higher temperatures, that the density then decreased above a certain temperature because it was deter-mined by two countervaiLlng processes, shrinkage and thermal decompositlon of silicon nitride and that his optlmum -tempera~ure was r~ 1800~C, It is known in the art that the high magnesium oxide additive necessary to induce sintering degrades oxidation resistance and high ~emperature mechanical properties of ~he ~ilicon nitride product, The present invention does not use an oxlde additive. Sp cificallyg in contrast to sintering proce~ses which use metal oxide additives such as ~agneslum oxide which do not ~ecompo~e readily and which therefore retain substantially aLl of the oxygen introduced by the metal oxide

-2 G`~ 9~13 additive, in the presPnt sitltering procas~ oxygen ~s ~lw~y~
lcst rom the sinterlng body in a slgniflcant amoun~, ALso, ln the present proce~,the ~inte.ring body undergoes nc ~ignificallt weight lvss due to the thenmal decompo~itlon of the ~ilicon nitride and thi~ i9 lndlcated by the high den~itie~, of the re~
sulting ~intered product which can range from 80% to 100% of the theoretical density of ~llicon nltride. In addition, the pre-3ent invention make~, it possible to fabricate eomplex shaped arti.cles o silicon nitride directly with little or no m~chining.
Those skilled ln the art will g~in a further and better understanding of the pre~ent invention from the detailed des-cription set forth below,considered in conjunction with the ~re accompanying and forming a part of tha specification whlch shows condltions where spontaneous decomposltion of silicon nitride occurs,i.e, to the left of the heavy solid lin~, conditions where spontaneous decomposition of silicon nitride doe~ not occur,i.e, to the right of the heavy solid line9and conditions necessary for producing the present sintered product,i.eO the shaded area referred to as t~e Region of Sinterability, Brieflv stated,the present process comprises fonming a homogeneous dispersion having an avar~ge particle ~ize which is submicron of ~llicon nitride and ~ beryllium ~dditive, shaping the dl~p~rslon into a green body9and sintering the green body at a temp~rature ranging from about 1900~C to ~bout 2200C in a sln-~ering a~mo~phere of nitrogen at a ~uperatmo~pher~-c pres~ure of nltrogen which at the sinterlrlg t~mp~ra~re ~ ~ff~ ~nt ~ prevent slgnificant ~ermal d~co~p~9i~kn o~ th~s~icon ni~ride ~Ll wh~

1~ E;4~ RD-9413 produce~ a ~I.ntered ~ot3y wlt~ denslty ranging :rom ~ laa~3t ~bout BO% to ~bout 1[)0% of the theoretical den~sity o~ cQn T'dtri-.lP.
By a ~ignlficant thermal decompositl.on of sillcon nitrlde herein it is meant signlfieant we~3Sht loss O:e ~ilicon ni~ride due S to thermal decompositiorl of silicon nitri.de and such significant weight loss of sillcon nitride would be highar than about 3% by we:Lght of the total amount of ~illcon n:l t:rlde in the green bsdy.
Usllally,however~ln the present invention,welght los~ of ~llicon nitx:l de due to thermal decomposition of ~llicon nitrlde is le~s l~ than 1% by weight of the total amount of silicon nltride in ~he green body, The silicon nitride powder used in ~he presen~ process ean be amorphous or crystalline or mixture5 thereof. The cry~talline silicon nitrlde powder can be a~ or ~- iltcon nitr~e or mlxtures 15 thereofO
The pre~ent starting silicon nitride powder can range ~n pur~ty from ~ totally pure silicon nltrida powder to one o cera~
grade, The nece~sary purity of the silicon nitrlde po~der used depends largely on the temperatures and Loads at which the flnal ~intered product will be used at with the highe~t temperatures ~
use generally requiring the mo~t pure powders. Specifically,wikh increa~ingly pure powder the resulting sintered product ~xrea~n~
retain~ its room temperature properties at hlgh temperaturas ~ i . e, the more stable are ~he propertie~ of the sinterad product with increasing temperature~, The presant 8ilicon nitride powder may contain metallîc and non~metallic impuritie Speclflcally, based ~ 7 RD~9413 __.

on tha tota.L composltlon of the starting ~ on nitrlde powder, it~ oxygen content may range up to about 5P by wel~ht, A powder having an o~eygetl content in exce~s o~ about 5% by weight provides no advantage ~ecau~e it i~ likely to produce a slntered product with impa~rad high t~3mpera~ure mec~anical propertie~, Normally the oxygen is pre~ent in the form of silic~, The amou~t of excess elemental s-llicon wh~ch may be presen~ in the powder i~ not crlticaL, providlng it 1~ of submicron size, s~nce during the sinterlng proces~ Pleme~tal silicon is nitrided to orm ~ilicon nitridel and providing ; that the volume increase accompanying nitridation of the . ...
; ~lemental silicon has no signlflcant deleterious ef~ect on the sintered product, Ordinarily, elemental silicon may be present in sillcon nitride powder in amounts rangillg up : 15 to about 4% by weight. Non-metallic impurities ~uch as halogens which evaporate during sintering and which do not significantly deteriorate the propertieg of the si.ntered silicon nitride body may also be present frequently in amounts up to ab~ut 3% by weight o the starting silicon nitride powder, Ceramic grade sllicon nitride powder normally contains metallic impurities such as calcium, iron, and a~uminum which tend ~o form in ~he sintered product intergranular low meltlng phases that have a signiflcantly deleterious e~fect on the product's properties at elevated temper2ture~, In the present :' , t, , .

0'~ D 9413 process, when ceramic grade silicon nitrlde powder is u~ed, tha total amount o such metallie impurities should not be higher ~han th~t typically ~ound ln such powders which i5 about 1% by welght oE the starting powder, Specifically, when such metalllc impuritLes ara pre~ent ln an amount of abnut 1% by weighta the resuLting sintered ~roduct is usually dark grey in color and it is usa~ul ~or application~
at temperatures not h~gher t~an about 1300C and not requiring high load bearing capacity. With decreasing amounts of these metallic impuritie~ 3 the mechanical properties of the resultln~ sintered product at elev~ted temperatures improve, particularly with elimination of calcium and iron.
T~ produce ~ sintered product whioh has signi-flcantly stable mechanical properties at high temperatures, the preerred starting silicon nitride powder has a low oxygen content, i.e. less than about 2% ~y weight of ~he powder and it is free vr substantially free of metallic impuri~ies which form intergranular low melting phasesO
20 Speclfically~ this preferred silicon nitride powder may contain metallic impurlties such as calcium, iron and aluminum in total amount ranging up to about Q~1~lo by welght, and preferably no higher than about 0,05% by weight, o the startlng ~illcon nitride powder Such a powder pro~uees a light tan, light grey or whL~e s~ntered product, However, the most preferred silicon nitride powder of the present gi4~

process for produclng a slntared product with ~b~t~ ially st~ble, l.e~ ~ most stable, mech~nie~Ll prop~rtle~ a~ high temparatures i9 also oxygen ~ree or may contaln oxggen ln an amount ranging up to ab~ut 1% by weiglht o~ the pow~er, 5 Such a pure silicon nitride powder carl ble synthesized, Alternatively, to reduce its oxygen content and EILso remove lts vapor~zable i.mpur~tia~, the siLicon nitrid~ powder can be calcined at a temperature r~nging from about l~OO~C: to about lS00C in a vacuum or in an atmosphere which has no lû signiicant deterioratlng efect on the powder such as helium, nitrogen~ hydrogen and mixture~ thereo~O
Specifically, the preferred silicon nLtride powders can produce in accordance with the present proc~s a sintered product which retains its room ~emperature shape and mechanlcal prop~rties at high temperatures maklng it particularly useul for hlgh temperature structuraL applications such as gas turbine blades, i.e.~ they can produce a sinterad ~roduct which undergoes no sign~icant change in density cr mechanical properties after subs~antlal exposure to air at temperatureR ranging up to about 1400C and after ~ubstantlal exposure in ~n atmosphere in which lt i substantially inert ~uch a~ argon to temperatures abvve 1500C ranging up to about 1700Co The present preferred silicon nitride powder can be produced by a number of processes, For example, in one process .. ., ... . . . ,...... .... ., , . ~ .; . . . .

~ RD-94L3 SiO2 is reduced with carbon in nitrogen below 1400C.
Still other processes react a silicon halide wi-th ammonia or a nitrogen and hydrogen mixture to obtain either Si3N4 d:irectly or via precursors such as Si(NH)2 which are converted to Si3N4 by calcination yielding silicon nitride which usual]y contains oxygen and halogens at a 1~ to 3%
by weight level. The powder can also be sythesized in a plasma from silicon vapor and nitrogen.
Very pure silicon nitride powder can be formed by a process set forth in U.S. Patent No.
dated ~bob~ 19~ in the name of Svante Prochazka and Charles D. Greskovich. Specifically, this U.S. patent discloses reacting silane and an excess amount of ammonia above 600C and calcining the resulting solid at between 1100C to 1500C to obtain amorphous Or crystalline silicon nitride.
In the present process -the beryllium additive is selected from the group consisting of elemental beryllium, beryllium carbide, beryllium nitride, beryllium fluoride, beryllium silicon nitride and mixtures thereof. The known stoichiometric formulations for these additives are Be, Be2C, Be3N2, BeF2, and BeSiN2, Be6Si3N8, Be~SiN4, Be5Si2N6, BellSi5N14 BegSi3N10. In the present process the beryllium ; additive is used in an amount so that its beryllium component is equivalent to from about 0.1~ to about 2.0~ by weight of ~ Rn~9~l3 elemental berylllum~ and preferably from ab~ut 0.5% to about 1,0% by weight of el~mental baryllium, ~ased on the amoull~
o~ siLicon nitride. Amounts o~ the beryllium additive outside the r~nge are not ef~ect-lve in producin~ the pre~ent slntered S bo~y with a density o at least about 80%.
In arrying out the process ~t le~st a slgnlficantly or subs~antially uni:Eorm or homogenevus particulate disper~ion or mixture having an average particLe size which ls submicron of silicon nitride and baryllium ~dditive i~ formed~ Such a disper3ion ls necessary to produce a sintered product with slgnificantly uni~on~ propertLes and having a density o at least 80%, The silicon nitride and berylllum ad~itive powders, themselves, may be of a particle siZP wh~ch breaks down to the desired size in forming the dispersion9 but preferably 15 the starting silicon nitride is submicron and the starting beryllium 2dditive is less than 5 microns in p~rticle size, and preferably submlcron, Generally, the silicon nitride powder ranges in mean surace area from abou~ 2 squara ~e~ers per gram to about 50 square meters per gram which is equivalent 20 to about O . 94 mlcron to O . 04 micron, respectlvely, Preferably, the silicon nitride powder ranges in mean surface area from about 5 square meters per gram to abOLlt 25 square meters per gram which is equivalent tn about 0,38 micrcn to about 0.08 micron, respectively, The sillcon nitridP and beryllium additive powders can be admixed by a numb~r of techniques such as 9 or example, .09_ ~ ~ ~ 6 ~o~7 RD-9413 ball mllllng or ~jet mllling, to produce a ~lgni1cant or substantially uniform or homogeneDu~ dispersion or mix~ure.
The more uniform the dl~perslon, ~h~ more unifonm i8 ~he microstructure, and thereforel th~ propertles of the re~uL~lng ~intared body, Repre~entative of these mixing techniqu~ bal~
milling, preferably with balls of a material such a~ tungsten carbide or silioon ni~ride which has low wear and whlch ~s no signific~nt detrimental eff~ct on the propertie~ des~red in the final product, If desir~d, such milling can also be used to reduce particle siæe, and to distribut any ~mpurities which may be present substantially uniformly throughout the powder. Preferably, milling is carried out in a liquid mixin~ medium which is inert to the ingredients, Typical liquid mixing mediums inciude hydrocarbons such as benzene and chlorinated hydrocarbons. Milling tlme varles widely ~ and depend~ largely on the amount and particle si~e of the : powder and type of milling equipment, In general, milling time ranges rom about 1 hour to about 100 hours~ The re~ult~ng wet milled material can be dried by a number of conventional techniques to remove the liquid medium, Preferably, i t is dried in a vacuum oven maintained just above the boiling point of the llquid mixing medlum, A number of techniques can be used ~o shape the powder mixture, i.e,, partlculate dispersion, into a green body, For example, the pcwder mixture can be extruded, r ' ~ RD~9413 in~ect~on moldedl die-pres~ad, i~ostaticall.y pre~ed or 811p cast to produce the green body o~ desired ~hape, Any lubrlc~nts, binders or slmilar mat~rials used in sh~.ping the dlsperslon should have no significant deteriorating eect on the green body or ~he resulting sintered body~ Such material~ ~re preferably o the typ~ whieh evaporat~ on heating at rel~tively low temperatures, pre~erably helow 200C, leaving no significant res~due/ The green body should have ~ -density of at Least about 35%, and preerably at least about 45% or higher, to promote densi~ication during sintering and achieve attainment of the desired density of at least 80% and higher.
- In the present process, the sintering atmosphere of ni~rogen can be stagnant, but preferably it is ~ 1Owing atmosphere and need only be suf~iciently flowing to remove gaseous products which may be present, normally as a result of contaminants. Generally, the speci~ic ilow rate of nitrooen ga~ depends on the size of the furnace loading and somewhat on sintering temperature, The nitrogen gas used should be free of oxygen or substantiaLly free of oxygen ~; so that there is no significant oxygen pickup by the sintering :~ bod~J.
~ Si.ntering of the green body ls carried out at a ; temperature ranglng ~rom about 1900 C ~o about 2~00 C in a slntering atmosphere of ni~rogen at supera~mospheric pressure ... .... .. ..... - . .... .. . ~ . . . . . .. . .. ~ . .

~ RD~9413 which at the 3in~ering temperatur~ prevents thertnal ~com-position o~ the sllicon nitride and also promote3 shrinkage, i,e~ densi~ication~ of the green body producing a ~intered body with a densi~y of at Least 80% of the theoretical densl.~y of silicon nitride. Sintering temper~tures lower than about 190~C are not effective or producing the present slntered product w~erea~ temperatures higher than 2200C would require nitrogen pressures too high to be practical, Preferably, the sintering temperature ranges from about 2050C to 2150C, The effect of increa5ed nitrogen pressure on the sintering of silicon nitride can be be~t descrlbed by con-siderlng the efect of nitrogen pressure on tha thermal decomposition Si3N~ = 3 Si ~ 2N2 i,e. ~ilicon nitride decomposes in~o silicon and nitrogen, `~
and conse~uently ~here is alway~ a finite pressure o sili.con vapor and nitrogen above a surface of silicDn nltride, Accord-ing to principles of chemlcal equi.librium, the higher the nitrogen pressure the lower the silicon vapor pressure and vice versa. This may be expressed in quantitative terms by ~ Si x PN ~ K~T~
where P~i is partial pressure o~ silicon vaporl PN partial pressure o nitrogen and K i~ the equilibrium const~nt which is calculat~d from avaiLable published thermodynamicaL data~ ~ :
and reers to a specif;c ~empera~ure, Specific~lly, the ~ r7 I~ 9~l3 publiehed tharmodynamical dat~ reliecl on herein i8 dl~cLosed in Still et al, JA~F Thermochemical T~bles, 2nd Ed~, U~S,Dept, of Commeree~ Nat.St~nd.Ref. ~ta Ser, ~ Nat.Bur~
Stand.(U~S,), 373 U,S,Government Prin~ing Office, W~shlngton, (J~ne 197L), Th~se thermodynam~c relation~hips were ealculat~d and are shown in the accompanyin~ 1gure where ~he logarith~
of p~r~ial pressure of ~i.licon ~por and p~rtial pressure of nitrogen were plotted along with ternperature scales and the coexi~ting phases shown.
From the figure it can ba seen that if n~trogen pre~sure above Si3N4 decraa9es at a glven temperature, silicon vapor pressure increases uncil the sa~urated pressure of sllicon vapor at the temperature applied 1~ reached, At this and at lower nitrog~n pressures silicon nitride w~ pontaneously 15 decompose into silicon metal ~llquid or solid) and nitrogen.
In the figure, the heavy solid line, ~rom lower left to upper right delineates the set of conditions e~here silicon ni tride, condensed si.licon, silicon ~apor and nitrogen gas coexistl i,e~
condi'cions where spontaneou~ decomposition of ~ilicon nitride 20 occur~q, Speciieally, at any conditions selected to the left of the heavy solid line determlned by nitro~en pressure and temperature, spontaneous decompositlon o~E Si3N~ excludes sintering. At any conditions sel ected to the right of the hea~Ty solid line, spon~aneous thermal decomposition of silicon 25 nitride doe~ not occur~ However, according to the prasent inventlon 9 only the shaded area in the ~igure re Eerred to as ... ~ ... . . . . .. .. . . . .... .

D-~413 ~he R~gion o~ Sin~erability set~ for~h temperatur~ ~nd corres-ponding pres~ure conclitlons which prevenlt thenmal decompo~ltivn or ~ignifir~nt thermal decomposition of the 3ilit`0rl nitride and also produce the present s~ntered product h~v~ng a denslty of at Least 80%. SpecificallyJ the -flgure lllustrate~
that at ev~ry sintering temperature in the Region of S~nterability, a particular minimum pre3~sure of nitrogen has to be applied and ma~ntained which is suk~tantialLy higher than the minimum pressure of nltrogen neces~ary to prevent spontaneous sllicon nitride decomposition. The minimum sintering pressure of nitrogen is one which at a particular sintering temperature prevents thermal decomposition or sign-ficant thermal decomposition of the sillcon ni~ride and also promotes densification, i,e~ shrinkag2, of the body to produce a sintered product with a density of at least 80~
Ganerally, at a given sinterlng tempera~ure in the Region of Sinterability, an increase in nitrogen presQure will : show an increase in the denslty of the sintered product, i,e,, higher ~itrogen pr~.ssures should produce higher density products, Likewi~e, at a given nitrogen pressure in the ~ .
Regi~n of Slnterability, the higher the sintering temperatur~, the higher should be the density of the ~esulting elntered produc~.
The shaded area referred to a~ th2 Reg~on of ~5 Sinterability in the accompanying figure shows that the ..
particular minimum pressure of nltrogen used in the pre~Pot W14~

~O~ 7 RD 9413 proce~s depends Oll sinterlng temperature ~nd ranges :~rom about 2û atmosp~eres at 1900C to about 130 atmo~pher~s at:
temper~ture ~ 7~00C~ Spec:LicalLy, the fiKure show~ that in accordance wlth the pr&3ent proces~ the mlnimum required pres ur~ of nitrogen at 2000 C i3 about 40 ~tmo3phere~3 ~ and at 2100C it is about 75 atmospheres. l:n the present process pressures of nitrogen higher than the r~qulred minimu~m pres~ure at a particular ~intering temperature are useful to additionally densify the body ~o produce a sin~ered body having a density highcr than 80%~ The preferred ma~imum pressure o nitrogen ls one whlch produces a ~intered body of the highest densi ty at the particular ~intering temperature and 3uch preferred maximum nltrogen pressure i~ determinabla empiricallyO Nitrogen pre~sures higher than the preferred maximum pre~sure are useful but such pressures cause no ~igniicant additional densification of the body~
The present sintered product i~ compri~ed of silicQn nitride and ~ome form of beryllium, It may also contain oxygen in some form in an amount less than about S% by weight ~ 20 of the sintered prcduct since durlng s~ntaring some oxygen ; is always lost. Preferably, for high temperatures applieations, the sintered product contains 02ygen in an amount less than about 2% by weight o~ the ~intered product. For be~9t : result~, and in its preerred form9 the present sintered produet is ~ubstantially ree o~ oxygen or may contain oxygen .~, . ' ;. 1 ~; ! . !:

- -RD-~413 ln ~ome form in an amount les~ than about 1% by welght o the ~ tered produc t .
The silicon nitrlde in ~he pre~erlt produc:~ range~
from the ~ orm to a mix~ura of the l3-and a-:Eorsns w~er~
the ~-form o sllicon nitride is pr~sen~ in an amoun~ of at least abou~ 80% by weight o the total amoun~ of ~ilicon nitride. Preferably, the prasent produc~ i~ comprised of only the ~-~on~ of silicon nitride since it provides the mo~t ~ :
stable properties.
Sinoe during s~ntering a portion o~ the beryllium component of the additlve evaporate~, the sintered product contains beryllium in some form in ~n amount which i5 always les~ than about 2 .0% by weight of the silicon n~ tride~ The amolmt o the beryllium component which evap~rates depends larg~ly on s$ntering temperature and pressure~ i,e, the hlgher ; ~
the temperature and the l.ower the pre~ure the m~re beryllium ~ :
i9 likely to evaporate, U~ually, the amount of beryllium which ev~porates during sinterlng is ~igniicant. Speeiically, the present product will contain beryllium in some orm in an amount ranging from le~s than about 0.1% by welght to less than about 2% by weigh~ of the silicon nitride, The beryllium component of the sintered product is detectable : or determinable by techniques such as emis~ion spectroscopy and chemical analysis, Specii.calLy, the minimum amount o beryllium present in the ~' '~

.

~_9413 pre~ent product 1~ th~t ~rQount detectehle by e~l~s~Dn ap~ctro~coW, The llitlter8d bcdy or prodluct of ~h,e presen~ lnven~iorl has den~i~y ramging rom abou~: BO% to abou~ lOOIv/o of the ~cheor~ic~I
denslty of silicon nltrid~, Wi~h reiEerenoe ~o ~he ~intered body 5 or prodllct of th~ pre~ant irlvetltlon by ~hla ~arm ~ingl~ pha~ or prlmary pha~e it i~ me~nt herein ~he ~illcotl nitrlde pha~e~
the awform or ~-fonn of s:ilicon nicrld~ ~nd mixture3 thereof.
X-ray diractloo aaaly8i8 of ~he sintered prcsduct ~how~ ~h~t wl~h lower amount~ of the beryllium ~dd~tive, it i~ ~ single~ ph~e 10 m~eri~l, bu~ tha~ with higher ~mounts o:E the bsryllium,tr~res of a secondary phase may be detectable~ Ganerally, wh~en the berylli~m additlve i8 used ln amount~ wherein its beryllium component i9 equivalent ~o level~ up to about 1% by weight ~f element~l beryllium,tha sint~red product i~ usually a ~ingle phase ma~erial~, 15 However, when the beryllium additive i8 u~ed in amount~ wherein it~ beryllium component 18 equivalent to levels approAchlng or at about 2% by weight o elemen'cal beryllium, a ~econd~xy beryllium-containlng pha~e may be detec~d in the re~ul~ng ~intered produc~. The ~econdary phase or ph~3e~ ar~ di~crete and distri~
20 buted signifi~antly or ~ubs~antlally uniformly thr~llghout ~he sintered body, ~enerally, ~he gr in~ o the ~econdary phase or pha~e~ are of abou~ ~he s~me ~lze or :Lner than the gr~ins of the primary phase, When a preerred ~ilicon nitEide powder 1~ used, i.e.
25 non~ceramic gradQ powder, i.e, a powder not prepared by ni~rlda~ioFI of ~ilicorl, con~aln~ng oxygen in an am~un~ le~s ~ :L7 -RD~413 le~s than about ~% by weight of the ~tartlng powder, usually the secondary phase is beryllium slllcon nltrlde, DependLng on the partlcular amount of ~eryllium present, the secondary phasP or phases det~ctabl.e in the resulting S sintered product may r~nge in to~al amount from a trace amount which is just detectable by X-ray dLffraction analysis, : l~e. about 2% by volume of the slntered l)ocly, up to about 5%
by volume of the sintered body.
When a ceramic grade ~ilicon nitride powder is used, -~
the metallic impurities thereln may also orm a secondary phase in the sintered product. ~or example, such a powder ;~
may contain metallic impurities such as calcium, iron and aluminum in total amount no higher than about 1% by weight, ~nd oxygen in some form ranging up to about 5% by weight of the starting silicon nltride powder. The amount of secondary -~
pha~e or pha~es formed ln the sintered product in this instance depends largely on the amounts of metallic impurities ~nd oxygen, as well as the amount of berylLium present.
Specifieally, the secondary phase or phases may range up to about 10% by volume of the sint~red body, but it may or may not be detPcted by X ray diffraction analysis depending on the particular secondary phase formed, Due to the particular lmpurities present in ceramic grade powder, the secondary phase may be a glassy phase and not de~ectable by X-ray diffraction analysls, The e~tent and distribution of glassy phase pre~ent is very difficult to determine, and ' lt is usl1ally dolle by selective etchLng of the 9p~0imen and observing the pits formed by the etched out gla~sy pha~e, However, it ls estimated that rom the maximum amounts o metallic impuritle~, oxy~en and ber~llium which may be present herein, the secondary phase or pha8e8 produced may range, ln total amount, up tQ about 10% by volume of the sintered body, Also, in tha present sintering process a significant amount of oxygen is lost, usually in the form of silicon monoxlde. Therefore, the maxlmum amount o cxyg~n wh~ch can be present in the present sintered product is slgnificantly less than 5% by weight of the product.
The present sintered product has a microstructure which ~rgely temperature dependent, The microstructure m~y rang~ from an equiaxed type composad o unifor~, ~inesiæed equiaxed or substantially equiaxed grains which may be of :
the ~-form or a mixture of the ~-and a~forms of silicon nitride, to an elongated type which is compo~ed of nonuniform, elongated, needle-like grains of the ~-form of silicon nitride, This rangs in microstructure includes micro-structures o all ratios or eombinations of the equiaxed and elongated types, The lower sintering temperature of about 1900~C
produces a product with a fine-grained microstructure with uniform or substantially unifcrm grains which normally 7-ange from equiaxed to only slightly elonga~ed and usually ~re : -19-RD-9~13 less than about 2 microns in size. ~lowever, at a sintering temperature of about 2000C, elongated needle-like grains of the ~ -Eorm of silicon nitride appear in t:he still fine-equiaxed-grain uniform microstructure. Specifically, a sintering temperature of about 2000C procluces a product with a microstructure with elongated grains~ typically 1 to 2 microns thick and 3 to 10 microns in length, dis-tributed in a fine-grained matrix typically with grains 1 to 2 microns thick. At sintering temperatures higher than 2000C, the elongated ~ -yrains increase in number per unit volume. Usually, these elongated grains have an aspect ratio ranging from about 5 to about 10 but occasion-ally these elongated grains may range in length up to about microns or longer. Subjecting or annealing the present product to a temperature of about 2000C or higher at the present required nitrogen pressure for such -temperature for a sufficient period of -time converts the entire or at least substantially the entire microstructure to the nonuniform elongated ~ -form. Such annealing can be carried out in a matter of hours, depending on the size of the product and the particular annealing temperatures used. Preferably, such annealing can be carried out by combining the sintering and annealing in a single step, but if desired, the anneal-~ ing can be carried out as a separate step. The elonga-ted `~ needle-like grains of -the ~-form of silicon nitride are desirable because they increase the fracture toughness of the product mal~ing it less brittle as long as they are not grown to a length greater -than about 75 microns.

~D~9413 The pre~ent sintered body having a den~lty o 90%
or hlgher la usually one wherein most of or aLl of tha re~dual pores are closed, l.e, n~n-interconn~c~ing, and ~uch a sintered body i~ preferred slnca it i.~ Impsrviou~ and h~ghly reslstant to lnternal oxidation at eLevated temperature~, A180, the higher the den~ity of the ~intered product, the better are its mechanical properties, The presant invention makes i~ possible to fabricate complex shaped polycrystalline silicon nitrlde ceramic ~rticles directly. Speci~cally, the present sintered ; product can be produced in the fonm of a useful complex ; shaped articLe without machinlng such as an impervlous crucible, a thin wallPd tube, a long rod, a spherical body, or a hollow shaped articl20 The dimensions of the present sintered product differ from those of its green body by the extent of shrinkage, i,e. densification, wh~ch occur~ during sintering. ~lso, the surface quality of the sintered body -~ depend on those of the green body from which it is ormed, i~e, it h~ a substantially smooth ~urface if the green body rom which it is formed ha~ a smooth .Qurface.
In the present invention, unless otherwise stated, : the density of the sintered body as well a~ that of the green body i~ given as a ractional denslty of the theore~ical denælty of silicon nitrlde (3.l8/CC).
The Lnvention is urther illustrated by the following example wherei.n the procedure was as folLows unless otherwise stated:

RD~9413 Sur~ace are~ maa~urement~ wero m~de by a low ~emperature nitrogen absorption ~achnique.
~ interlng wa9 carried out in ~m impervLou~ clo~ed ~nd silicon carbide tube, 1.2 cm, in cliame~er, i.e, sllicon carblde tuba open ~t one end only.
Temparatura wa3 measured by an optical pyrometer ~t the clo~ed end o the ~illcon car.blde tube, corree~ed for furna~e window absorp~ion and a mlrror, At the end of e~ch 3i~terlng run, the power was ~witched of and tha sint~red 5ilicon nitrlde bodlea were furn~ce cooled to room temperature in the nitrog~n atmosphere which w~s slowLy depre~surized to atmospherlc pre3sure, Liquid nitrogen labeled as "High Purity Dry Nitrogen!' :
.. .
havlng les~ than 10 parts per million oxyg~n content wa~ u3ed ~ the 90urce o~ nitrogen g~ for the ~urnace atmo~phere, i,e, the sinterlng atmo~phere, The bulk den~ity of each pre~ing or green body was detenmined from its weight and dimen~lons~
Densl~y of the ~intered prsduct was determlned by water displacement u~ing Archimede~ methodD
Shrinkage given in Table I i~ linear ~hrlnk~ge (%~, and lt i9 the difference in length betwcen the green body and the sin~ered body, aL, divided by the length o the green body Loo This ~hrinkage ~s an indication of th~ extent of densiication~

~2-,` ; .

RD-9~13 ~ Weight loss is the difference in weight between the green and sintered bodies divided by the weight of the green body.
EXAMPLE l For all the runs tabulated in Table I all formulation, mixing and drying was carrled out in nitrogen under dry-box conditions. Also, in all of the runs the only additive used was beryllium nitride poweder which was admixed with the silicon nitride powder in an amount of 2% by weight of silicon nitride powder which corresponds to 1.0% by weight of elemental beryllium. In addition, in all of the tabulated runs the green bodies were of substantially the same size.
E'or Run Nos. l to 5 of Table I in-house silicon nitride powder was used which was prepared as disclosed in U.S.
Patent No- ~ dated ~ ~ ~b~
Specifically, this powder was prepared in furnace which included an open-ended fused silica reaction tube 3.8 cm.
diameter placed in a tube furnace, i.e. except for its open-end portions the reaction tube was located inside the furnace, and connected on the downstream end to a coaxial electrostatic separator operated between 5 and 15 R~ and 0.2 to 0.5 mA.
The outlet of the separator was terminated with a bubbler ; filled with an organic solvent which ensured positlve pressure in the system. A liquid manometer indicated gas ; pressure in the reaction tube. For each run the reaction ,' ~

~ 23 -g413 ~uba wa~ hsated at a lengt~ of 15 incheA to a maxlmum tamperature oE 850C, ~he ~yatem purge~ with puri.:fl~d argon and ~he reactants were therl metered in. Electronlc grade sil~n~ and anhydrous ammonia dried :further by pa~ing t~e ga~ throu~h a column of c~lcium nitride were m~ered in ~epar~tely by coaxi~1 lnlats lnto the reactiorl tube, The ga8. flow rates were ad~u~ted to V.2 st~Lndard cubic feet per hour (SCE~PH~ of SiH4 ~nd 3,5 SCFPH of NH3. A volumirlou~, llght t~n powder wa3 collected in the downstre~m end of tha re~c~ion tube and iLn the attached electro~t 'cic ~epar~tor, After four hours the ga~ flow o reactarlt~ W88 discontinued and the sy~tem wa~ left to cool off to room tempPrature under .~ a flow of 0,5 SCFPH of purified ~rgon. The powder was recovered :
; from the reactsr ~nd sepRra~or and wa~ calcined by hea~ing it in a mullite crucible ln a flowing gas mlxture of 3 par~
: by volume nitrogen/ll parts by volume hydr~gen at ~bout 1450C for 30 minutes. The produc~ wa~ a light tan powder ; amorphou~ to X-rays, had wide absorption band~ in lt~ I,R.
~pectra cen~er~d around 10.5 and 2L.0 microns (ch~racteri~tic f~r ~ilicon-nitrogen bonding), ~nd contained no me~als ~bove 50 ppm-determined by emi~sion ~peotro~copy. The oxygen content of a batch of powder prepared and calcined in ~he same manner wa~ d~ermined by n2u~ron ao~ivation an~ly~is ~o be 2~0%
by weight of the powdar, ~ 25 For Run No, 6 a silicon nitride powder was used - which was prepared by reac~ing ~illcon tetraohloride and ~ 4-.... . . ... ... ....... ... . . ... .. .........

ammonia, The powder was calcined by heati.ng it Ln ~ mulllte crucible the flowlng ga~ mixture of 3 p~rt~ by volume nitrogen and 11 parts by volumQ ~tydro~en at 1450C for 30 minutes, Th powder ~d an oxygen content below 5 ~ 0% by S weight of the powder, For Run Nos. 7 and 8 a purch~sed sil:Lcon nitr:Lde powder wa3 used which was indica~ed to be 95% by weight u and 5.0/~ by w~ight ~i silicon ni~ride. ThL~ powder wa~ indicated by the manufacturRr to be 99.97% pure, except for oxygen, : 10 that it contained molybdenum in an amount of about 0,01% by weight and also oxygeil in an amount of 1.0% by weight of the powder .
In Run Nos. 9 to ll a pwrchased silic~n nltride powder indicated to be 80% by weight a-and 20% ~i-silicon 15 nltride was used. Thiæ powder was indicated by the manu~
acturer to have been prepared by nitridation of silicon and that it contained the following impuritles in weight %
as ollow~: Ca<0,1%, ~Ig<0~1%, Fe~0.4%, Al<0,20~c~ and oxygen ~ 1.5%~ In addition, this powder was ound to contain 20 unreacted elemental sillcon in an amount of about 7 weigh~ %.
In Run Nos. 1 to 5 the silicon nitride powder wa~i admixed with the ~eryllium nitride powder and a sufricien~
amount of 1~/o solution of parafin in benzerle to orm ~ slurry, The mixture was milled with 1/4 inch silicon nitride grinding me~ia at room temperature. After ~bout 6 hours the resulting slurry was ~trained and dried from the solvent agaln undsr ,,,.. .. ,, ., , ,.. ....... , ... O,,, . ... , , ., . . . , ~. . . . .. .

dry-box conditiorls~ The resulting ho~logeneou8 8ubmicron powder m:Lxturc wa~ die-pre3sed into green bodie~, i,e,

3/8 lnch x 3/8 inch cyllnders which were stored in ~
dessLcator above Ca3N2. One of the~e presslngs, i.e. green bodie~, was used for Run No~ 1 o~ Table I.
The P~un No. 1 pressing was pl~ced ~n the 3ilicon carbide sintering tube whieh was in turn placed within a carbon re3istance tube furnace except for its open end w~ich was fitted wlth a pressllre head, The pressing was plac~d so that iL was positioned in the hot zone, iOeO the closed end portion of ~he sintering ~ube, The sin~ering tube was evacuated and then brought up to 800C. At thl6 point the pumping was dlscontinued and the sintering tube was pressurized to 72 atmospheres o~ nitrogen, The s~ntering tube wa~ then brought up to the sLntering ~emperature of 1990C in about 20 minutes, and held at 1990C or 15 minutes, At the end of this time, it was urnace cooled to room temperature and the resulting sintered body was evaluated.
The results are shown in Table I, Th~ procedure used in Run Nos~ 2 to 5 was oubstantially the same as that for Run No, 1 except as indirated in Table I, In Runs 7 and 8 a portion of the ~ilicon nitride powder wa~ admixed with the b~ryllium nI.tride and a sufficient amount of a 1% solutlon of paraffln ln benzene in a mortar .

..... .. .. ... . .... . . .

and pestle to ~o~m a substalltially unlform slurry. The slurry was strained and dried ~rom the solvent. The resulting powder mix~ura, w~ic~ was 8igniflc~ntly homog~neou~ and had an average particle size that was submic.ron~ was pressed into cylinders o~ substantially the same siæe and stored in a dessicator above calcium nitride.
The procedure used Eor Rùn Nc. 6 was substantially the same a~ that for Run No. 7 except that the green body formed by die pressing was then isostatically pressed at a pressure of 120,000 psi, For Run Nos, 9 to 11 all formuLation, mixing and drying was carried out subs~antially in the same Manner as : disclosed for Run No, 1 except that the mixture was milled wi~h the silicon nitride grinding media for about 16 hcurs~
The resulting homogeneous submicron powder mixture was die-pressed into green bodies, i.e. 3/8 inch x 3/3 inch cylinders, which were s~ored in a dessicator above calcium ni~ride.
The green bodies o~ Run Nos, 2 to 11 were sintered ;n the same manner as disclosed ~or Run No. 1 except as indicated in Table I, ~' ~ .
:; ~ '.

:
~ -27~

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~ ~ RD-9~~3 In Table I the slnt~red products of R~m Nos~ 1 to 5 and 6 were llght gr~sy in cnlor~ the produc~s o:E Run~ 7 and 8 were light grey ko grey in color and the pr~cluct~ o ~uns 9 tD 11 were grey to d~rlc grey in color~
Table I show~ tha e~fectivene~:~s of the beryllium additive where ~un Nos~ 4 to 8 ~nd 10 to 11 illu~trate the pre~ent invention, Thes~ runs ~how the ~ub~t~ntlal ~hrlnkage or densification which takes place ~uring sintering resul~ing ~:
in the present highly dense sintered productsO Thesa products were hard and strong, ~-ray di:~fraction ana].ysis of the sintered product of Run No. 5 showed it to be comprised of ~-silicon nitride with a ~race amount of ~eSiN2 of less than 2%, The analysis also showed lines which were not identifiable and th~se ~ lS lines would indicate a trace amount ~ some unknown compound ; believed to b~ some polytype o~ beryllium silicon nitride, The sintered product o ~un No~ 5 was sectioned, poli~hed, etched and a photomicrograph (magni~ied 550 X) was taken thereo~, The photomicrograph showed it to be composed of very uni:form equiaxed and slightly elongated grains up to about 5 microns and had an average grain size - of a~out 2 microns~
The sintered product of Run No~8 was also section~d, polished and etched and a photomiorograpll (magnified 550 X) wa~ taken thereof~ The photomicrograph ~howed elongated ~rai.n~ up to about 25 microns in length distribut2d in a ~9~ :
i 7 RD-94l3 ~lne~ ~rained matrix composad o ~m~ ub~tantially aq~llaxad grain~ of the order o 3 ml ::ron~, Table 1: al~o indlca~e~ ~a~ WeL~h~ J~ a~out con-stant for ~ certaln batoh o powder and ~ refor~ i~ relatæ.s9 mor~ likely, ~o the clhemL~ry o ~e powder than to the ~in~ ;
tarin~ condition~ This ~9 lllu~tr~ated by Run No~O 1-4 w~ich were prepare~ from the sama batch of ~lllcon nitride powder and whlch shaw~d ~ub~tantially t:he ~ame wai~ht lo~. Speolfically, it 1 believed tha~ the weight ~ 3 in Table I r~flects the ~um of ive or 8iX component~: evapora~ion of parafin uaed a~ -binder ~about 3%), los,~ o weight due to oxygen removal in the form of SiO (albout2%~, los~ of weight due to release of amm~nia from the ~morphous sllicon nl~ride powd~r, we~ght 1058 due to the~mal decompositli on of ~13N4 durlng ~int~rlng9 wai~sht lo~s due lto los8 o berylliwn and w~Lght galn due to nitrillation o free s~ licon pr~s~nt ln the ~tar'cing powder 7ompact~ i.e. the green body, A~ a r~sult, welght 1O~s due to ~he~nal decom-positi~n of ~ilicon nitride most llkaly i9 les~ than 0,3%
by w~ight of ~he tot:al amount of slllcDrl nitrlde pre~ent.
2 0 ~LI~ 2 Thi~ exampl~ shows that beryllium is los~ in the pre~erlt sintering proce~, ~: .
A green body was prepared ln the ~ame m~nner ~s disclo~ed or Run NoO 1 o Example l except ~hat 1% by weight of B~3N2 wa~ used along with 2% by w~ght Mg~N~, w~ieh : -~0~
;

~n~ 7 corresponds to 0 . S% by wei~ht o~ elemerlt~l berylli.um ~nd 1~5V/o by waight of elemerlt~l ma~ e~ium~ based on th~ ~mount of silicon nl ~:ride, The green body had a green clensl~y of 46%, I~
S was slntered in the same manner a~ dlscll~sed ~r Run Mo~ 1 o~ Example :L except that a slnterlng temper~ture o l.950C
and ~ nitl~og~n sinterlng pre~sure of 85 atmospheres were used ~nd slntering tcime was 20 minutesO
The sinterecl product was analyzed by emlssion 10 spec~roscopy and was ound to cont:airl 0,1.% by weigh~:
magnesium and 0.3% by weight beryllium based on the amo~mt o silicorl nitrlde~ This indlcates that there ls a ~igni~icant loss of berylllum under the conditions of the prQsen~ intsring ~:
process, ~ .
This e~ampLe also shows that beryllium is losl: in the present sintering process, ~; A green body was prepared ln the same manner a~
disclosed for Run Mo. 7 v Example 1 except that 1% by 20 weight ~f Be3N~ was use~ al~ng wikh 2% by welghk Mg3N2, which corrPsponds to 0,5% by weight of eLemental beryll~um and 1.5% by wQight of elemental ~gnzsium based on the amount o ~ilicon nitride, lChe green body had a green dan~ity of 46%~ It was 25 sintered in the ~me manner as d:l ~clo~e~! or Run No, 7 ~xcept a sin~aring temp ra~ure o 2040C was u~ed~ a siTll;erlng p:ressure of 75 atmospheres was used :~or a period of t;ime o F 15 m-lnutes -The sint~red product was analyzed by emi~;sion spectroscopy and ~as found to cont:ain 0~1% by weight magneslum - ` 5 and 0.2% by weight beryllium based on the amount of sllicon nitride, This indlcate5 that there is a 9igniicant 1QS~
o b~ryll~um in îhe present sintering proce~s, Thi~ example lllustrates one technique o:E determining ~0 weight 1c)S5 due to ~hermal decomposition of ~ilicon nitride.
a reaction bonded silicon ni~ride ~linder, abou~
lf2 lnch in length and about 1/~ inch in diameter, preyared by the :n~ridation o~ a highly pure silicon sintered body was used. . The cylinder had a density of abou~ 70% ~o: the .
: ~ 15 theoretical density of silicon nitride and w~s porous: :~

~h interconnec~ing pore~
. ~ ~: : .
~ The cylinder was weighed and subJected to conditi~ns;
which were sub~tantially the same a~ the 9intering conditions .
~et fDrth in Example 1 except that it was maintained at a 20 temperature 1800C~C under a pressur2 ~f nitrogen o 80 .
a:t~nGspheres or a period of one hour. I t was weighed again and.
.~
then r~-h~ated to a temperature of 2000C and kept at ~000C
under a pressure of 80 atmos:pheres for on~ hour~ It wa~

then we~hed again~
The total wei~ht loss was found to be less than 0, 5% by . .

RD-9~13 s7ei~ht oE the product inclica-tincJ that silicon nitride did not undercJo significan-t the.rmal decomposition under these conditions.

Claims (9)

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

1. A method of producing a pre-shaped polycrystalline body which consists essentially of providing at least a significantly homogeneous dispersion having an average particle size which is submicron of silicon nitride and a beryllium additive, said beryllium additive being selected from the group consisting of beryllium, beryllium carbide, beryllium fluoride, beryllium nitride, beryllium silicon nitride and mixtures thereof, said beryilium additive being used in an amount wherein the beryllium component is equivalent to from about 0.1% by weight to about 2% by weight of elemental beryllium based on the amount of silicon nitride, shaping said dispersion into a green body and sintering said green body at a temperature ranging from about 1900°C. to about 2200°C. in a sintering atmosphere of nitrogen, said nitrogen being at a superatmospheric pressure which at said sintering temperatures prevents significant thermal decomposition of said silicon nitride and produces a sintered body with a density of at least about 80% of the theoretical density of silicon nitride, the minimum pressure of said nitrogen ranging from about 20 atmospheres at a sintering temperature of 1900°C.
to a minimum pressure of about 130 atmospheres at a sintering temperature of 2200°C.

2. A method according to claim 1 wherein said sinter-ing temperature is about 2000°C. and said minimum pressure of nitrogen is about 20 atmospheres.

3. A method according to claim 1 wherein said sintering temperature is about 2100°C. and said minimum pressure of nitrogen is about 75 atmospheres.

4. A method according to claim 1 wherein said beryllium additive is beryllium nitride.

5. A method according to claim 1 wherein said beryllium additive is used in an amount wherein the beryllium component is equivalent to from about 0.25% by weight to about 1% by weight of elemental beryllium based on the amount of silicon nitride.

6. An as-sintered polycrystalline silicon nitride product of complex shape of a preformed compact of particulate material, said as-sintered product having a density ranging from at least about 80% to about 100% of the theoretical density of silicon nitride, said as-sintered product consisting essentially of silicon nitride and beryllium, said silicon nitride ranging from the .beta.-form to at least about 80% by weight .beta.-form and 20%
by weight .alpha.-form based on the total amount of said silicon nitride, said beryllium ranging in amount from less than about 0.1% by weight to less than about 2.0% by weight of said silicon nitride, said as-sintered product according to X-ray diffraction analysis ranging from a single phase body to one comprised of a primary phase and a secondary phase.

7. An as-sintered product according to claim 6 which according to said X-ray diffraction analysis is a single phase body .

8. An as-sintered product according to claim 6 wherein said secondary phase is a beryllium-containing phase.

9. An as-sintered product according to claim 6 which contains oxygen in an amount ranging from less than about 1% by weight to less than about 5% by weight of said product.