Note: Descriptions are shown in the official language in which they were submitted.
' .~11
~39'7
1 ¦SINTERE1) S~1.ICON CARBIDE - ALUMIN~M NITRIDE ARTICLES
2 ¦AND
3METHOI)()I~ MAXING SUCH ARTICLES
~BACKGROUND OF T~IE INVENrION
7Silicon carbide, a crystalline compound of silicon ~nd car~on, has
8long been known for its hardness, its strength, and its e~ce~lent resistanGe to
9 ¦oxidation an.l corrosion. Silicon carbide has a low coefficient of expansion,
0 ¦good heat transfer properties, and maintains lligll strength at elevated tempera-
11 ¦tures. In recent years, the art of producing high density silicon carbide
~2 bodies from silicon carbide powders has been developed. Methods include reac-
13 ¦tion bonding, chemical vapor deposition, hot pressing, and pressureless sinter~
4 ing ~initially formin~ the article and subsequently sintering). `2xamples of
¦these methods are described in U. S. Patents Nos. 3,~53,566; 3,~52,099;
16 3,954,q83; and 3,960~577. The high density silicon carbide bodies so produced
17 ¦are excellent engineering materials and find utility in fabrication of compo-
8 ¦nents for turbines, heat exchange units, pumps, and other equipment or tools
that are exposed to severe wear a~d/or operation un(ler high temperature condi-
tions.
In order to obtain high density and high strength,silicon carbide
22 ceramic materials, ~arious additives ha~e been utilized. For example, a method
23 of hot pressing silicon carbide to densities in order of 98 percent of theore-
24 tical by addition of aluminum and iron as densification aids is disclosed by
Z5Alliegro, etal, J. Ceram. Soc., Vol. 39, No. 11, Nov., 1965, pages 386 to 3~9.
6 They foun(l that a dense silicon carbide could be produced from a powder mixture
~1 containing 1 percent by we;ght of aluminum. ~heir product had a modulus of rup-
28 ture of 54,000 psi at room temperaturc and 70,000 psi at 1371 C.
2~Alumi1lum nitride, a crystalline compound of aluminum and nitrogen, is
widely used as a refractory ma~erial. Aluminum nitride exhibits an e~oellent
11
. ~L3~'7~
1 resistance to molten metals and is an especially useful refrac~ory for molten
2 aluminum. Aluminum nitride has ~ood thermal shock resistance, good stren~th,
3 and excellent rcsistance to most chemicals.
4 A num~er of proposals have been made to cornbine silicon carbide and
aluminum nitride ~o produce an improved high density body which may be fahrica-
6 ted into articles which will withstand severe operatin~ conditions, or into
7 refractory materials which offer the stren~th of silicon carbide and the inert-
8 ness of aluminum nitride. Such combinations have been proposed to improve the
9 electrical conducti~ity of silicon carbide resistance elements and better the
o resistance of silicon carbide to corrosion at elevated temperatures. Examples
ll of such mixtures are found in U. S. Patents Nos. 3,259,509, 3,287,478; and
12 3,492,153. However, $he previously proposed products are mixtures of silicon
3 carbide and aluminum nitride which do not sinter to produce a high density,
~4 co-sintered prodllct. One negative characteristic of aluminum nitride is that it
is soluble in warm water. This characteristic has lessened the use of aluminum
16 nitride in many applications in wllich it would otherwise be utili~ed to an
17 ~ a~lvantage. The mixtures proposed in the prior art have not substantially reme-
8 died the solubility of aluminum nitride in warm water. I~ has now been found
that a co sintered silicon carbi~e - aluminllm nitride product~ containing up to
about 95 percent by weight aluminum nitride, may be produced which has the posi-
21 tive attributes of both silicon carbide and aluminum nitride and which is sub-
2 stantially insoluble in warm water.
23
BRIEF DESCRIPTION OF THE INVENTION
24
The sinterable mixtures of the present invention comprise silicon
26 carbide, carbon an~ aluminum nitride. The mixtures are particularly adapted to
27 use in pressureless sintering operations to produce hard, dense, sintered
28 ceramic l)ro~ucts. The sintered ceramic product of the present invention com-
2~ prises a mixture of co-sintered silico~n carbide and aluminum nitride. The sin-
; 30 tcred ceramic prl)duct contains rrom abolt 5r) to al)out 9'3.5 percent by weight
I~ !IL3~'7~3
l silicon carbide co-sintered with from about 0.5 to about ~15 percent by weight
2 luminum nitride. Although tl1e final sintered product is comprised substan-
3 ially completely oi co-sintered slilicon carbide and aluminum nitride! minor
4 mounts, usually less than about l.0 percent by weight, of excess carbon or
intering aids, or residue from sintering aids, may be present without dele-
6 terious effect.
7 The theoretical density of silicon carbide is 3.21 gm/cc. The sin-
~ tered silicon carbide - aluminum nitride products of the present invention
9 typically have a density g~eater than 75 percent of theoretical and usually
greater than 85 percent. Co-sintered ceramic products having densities over 90
ll percent of theroetical may be prQduced by the presen~ invention. The sintered
12 products undergo a shrinka~e of about lO percent dur~' ng the sintering process.
~3 The sintered products may be utili~.ed in the form or shape in which they are
4 sintere~, or they may be mad~ned or processed into more complicated shapes.
The components utilized to produce the sinterèd products of the
l~ present invention are silicon carbide, aluminum nitri~le, and combinable carbon,
7 or a combinable carbon source material. Sinterin~ aids, such as boron or a
18 boron source material, may also be present to aid in the sintering step. Gene-
19 rally, the components are present in amounts ranging from about 55.0 percent to
2~ about 99.0 percent by weight silicon carbide, from about 0.5 to about 45.0 per-
21 cent by weight aluminum nitride, and from about 0.5 to about 6.0 percent by
22 wei~ht carbon. In preparing ceramic materials use~ul as bricks, crucibles, or
23 furnace components, the starting composition preferably contains higher amounts
2~ of aluminl1m nitride, usually in the range of from about 3.0 to about ~5.0 per-
cent by weight. In preparing a ceramic material useful for the fabrication of
26 articles such as turbine blades or high temperature tools, the starting composi-
2~ tion pre~erably contains higher amounts of silicon carbide, usually in the range
28 of from about 90.0 to about 99.0 percent by weight.
29 The sinterable mixtures of the present invention are particularly
3D adapted to use in pressureless sintering processes. In one mode of the inven-
.,11
~ 3~'7'
1 ¦ tion, the starting components are admixed, the mixture cold pressed to form a
2 ¦ green body, and the green body subsequently sintered to form dense, hard pro-
3 ¦ ducts. In another mode, only the silieon carbide and carbon components are
4 admixed, cold pressecl, and sintered in an atmosphere containing aluminum ni-
¦ tride~ In either mode, the silicon carbide component and the aluminum nitride6 component are co-sintered to produce a hard, dense product ha~ing the property
7 ¦ of being substantially insoluble in warm water.
8 ¦ The silieon carbide starting material may suitable be selected from
alpha or beta phase silicon carbide. Mixtures or alpha and beta phase mate-
0 rials may be utilized. The silicon carbide startin~ material of the present
11 invention does not re~uire separation or purification of phases to obtain a
2 sinterable material. ~inor amounts of amorphous silicon carbide may be ineluded
~3 without ~leleterious effect.
1~ The silicon carbide starting material is preferably utilizcd in
¦ finely-divided form. A suitable finely-divided material may be produced by
16 ¦ grinding, ball milling, or jet milling larger particles of silicon carbide and
17 ¦ subsequently classifying or separating a component suited to use in the present
18 ¦ invention. Preferably, the silicon carbide starting material has a maximum
19 particle size of about 5 microns and an average particle size of about 0.10 to
about 2.50 mierons. It is diffieult to obtain accurate partiele size distrilbu-
tion for silieon earbide powders having a size less than about 1 micron in size,22 and therefore, surfaee area may be considered relevant in determining suitable
23 material. Accordingly, the preferred silicon carbide ~articles for use in the
24 present powders have a surface area of from about 1 to about 100 m2/g. Within
~5 this range, it is more prefo~re~ that the surface area of the particles range
26 botween about 2 an(l about 50 m2/g, and, within that range, a range from about
27 2 to about 20 m~/g has been found em;nently useful.
2a The aluminum nitride oomponent of the present invention may be uti-
2~ li%ed in the form of finely-divided powdor, if it is to be initially mixe(l with
¦ tho o~he coml)ononts. ~rel~rab1y, a partic10 siYe Or 1ess ~han 5 micrnns is
~L~L3~ L
I ¦utilized, ancl, more prcferably, a particle size less than about 2 microns is
2 ¦desirable. If it is to be utili%ed to supply a sintcring atmosphere of aluminum
3 nitride, it may be utilized in the form of pressed pellets or bodies placed in
4 the sinterin~ furnace. Alternatively, aluminum nitride may be produced in situ
in the sintering furnace; however, this method is generally not preferred be-
6 cause of the difficulty in controlling the amount of aluminum nitride in the
7 product an~ the problem of maintaining furnace conditions conducive to both8 sintering and to the production of aluminum nitride. A preferred method of
providing an aluminum nitride atmosphere in the sintcring furnace is by merely
o positioning compacts or pellets of aluminum nitride of a known weight in the
sintering furnace and allowing the aluminum nitride to vaporize ~uring the sin-
2 tering operation. Suitably, aluminum nitride may be produced by nitriding a3 mixture of 75 percent by weight aluminuln powder and ~5 percent by weight alumi-
4 num fluoride at a temperature of 1000 C in an atmosphere of ~0 percent nitrogen
and 20 percent hydrogen. The nitride product is suitably crushed and may sub-
16 sequently be pelleted to obtain a useful source of aluminum nitride.
1 The present compositions also contain excess or combinable carbon in
8 amounts from about 0.5 to about h.0 percent by weight. The carbon component1~ facilitates the subsequcnt sintering operation and aids in reducing the amounts
of oxides that might otherwise remain in the finished sintered product. In pre-
Zl ferred compositions, the combinable carbon is present in amounts between about
22 2.0 and about 5.0 percent by weight of the silicon carbide material. ~he carbon
~3 component may be utilized in any form that facilitates mixing of the carbon com-
24 ponent with the silicon carbide component to obtain a dispersion of carbonthrouyhout the mixture. If the carban component is utilized in fine~y-divided
26 form, it su;tably may be in the form of colloidal graphite. However, a parti-
27 cularly useful form of carbon is a carbon source material, which suitably may be
28 a carbonizable organic material. Such materials may be easily dispersed
2~ -thrpughout the silicon carbide component, utilized as a binder in an initial
cold pressing or forming operation, and subsequently provide the required
I!
I ~L39'7~3~
l ¦excess or combinable carbon by decomposition during the sintering operation.
2 ¦ Of particular use are c~rbonizable organic materials such as phenolic resins,
3 ¦ acrylic resins, and polyphenylene resins. Generally, such carboni~able organic
4 materials will provide from about 30 ~o about 50 percent of their ori~inal
S ¦weight in combinable carbon.
6 In one mode of carrying out the present invention, the silicon carbide
7 ¦component, the carbon component, and the aluminum nitride component are admixed,
¦preferably by dispersing a c`arbon source material throughout the silicon carbide
9 and aluminum nitride components. The mixture is then cold pressed at a pressure
between about 12,000 and about 18,000 psi to form a green body. The green body
11 is subsequently sintered under substantially pressureless sintering conditions
12 ¦to pro~uce a co-sintered product. This mode is particularly adapted to use when
13 the composition contains large, from about 3 to about 45 percent by weight,
14 amounts of aluminum nitride. Such sintered compositions find use in the fabri-
cation of crucibles, refractory bricks, or furnace components.
16 In another mo~e of the invention, the sintered product contains a
1~ ~ lesser amount of aluminum nitride, usually from about 0.5 to about 3.0 percent
18 by weight. Such products are suitably produced by initially mixing the silicon
19 carbide and carbon components, cold pressing to form a green body, and sintering
the green body in an atmosphere of aluminum nitride. The atmosphere of aluminum
21 nitride may be produced by heating a source of aluminum nitri~e to a temperature
22 above its vapori~ation point during the sintering step, or aluminum nitride may
23 be produced in situ during the sintering step. The products of this mode are
24 particularly adap~ed to use in the fabrication of components for equipment or
25 Laols that are to be used un(ler severe wear, high temperature, or corrosive
26 con~litions.
2~ The startin~J mixtures of the present invention may also contain minor
28 amounts of materials that act as sintering aids, for examplel boron or boron-
29 conta;ning compoun-ls. Sintering aids are generally effective in the range of
from about 0.3 to about 3.0 percent by weight of the aid, for example, boron, as
compared to ~he wcight o the silicon carbide componcnt. A boron-containing ;
-t)-
~3~3'7~3.~
l ¦atmosphere may be provided ;n the furnace durin~ sintering to aid in densifica-
2 ¦tion. In such mode, boron gas may be utilized in sintering atmosphere or a3 ¦boron source~ for example, H2~03 or ~ 03, may be placed in ~he furnace and
4 allowed to decompose during the sintcring opera~ion. In either mode~ partial
p.essure of at least about 10 7 atmospheres of boron is preferably maintained
6 durin~J the sintering operation.
7 The sintered ceramic products of the present invention are o high
8 density and high strength, substantially non-por,ous, and eminently useful in
9 en~ineering applications. If desired, the high-density, high-stren~th silicon
o ¦carbide product may subsequently be machined~ by diamond grinding, electro-11 chemical machining, ultrasonic machining, or by electrical discharge machining
12 techniques to provide tools or machine components requiring clase tolerances.
13
14 DETAII~ED DESCRIPTION OF THE INVEllTION
The sintered ceramic products of the present invention are comprised
16 of from about 55 to about 99.5 percent by weight of silicon carbide co-sintered
17 with from about 0.5 to about 45 percent by weight of aluminum nitride. The pro-
ducts are substantially insoluble in warm water. The sintered products have a
1~ bulk density of at least 75 percent of the theoretical density of silicon car-
bide. For many applications, a density of at least ~5 percent of theoretical is
~ dcsirable, and densities of over 90 percent are obtainable with the mixtures of
z the present invention.
23 The silicon carbide startin~ maeerial is preferably utilized in
24 finely-divided form having a particle size of less than about 5 microns and,
more preferably, less than about 2 microns. The silicon carbide starting mate-
26 rial has a surface area greater than ~.0 m2/g, and material having a surface
27 area ~reater than about 20 m2/g is eminently useful.
2~ The carbon component, present in amounts of from about 0.5 to about
2~ 6.0 percent by wei~ht, may be utilized in a fine1y-divided form with a particle
size less than abou~ 5 microns and, preferably, less than about 2.0 microns.
` `I ~L~IL3~7g.~
1 llowcver, ;~ is preferre(l to utilize a carbon source material such as a carboni-
2 zable organic material which scrves the dual purpose of acting as a binder
3 during the cold pressiny operation and subsequently as a source of carbon when
4 it carboniæes during the sinteriny operation. Particularly useful in this mode
are organic resins which provide resi~ual carbon in amounts of from about 30 to6 about 50 percent by weight after carbonizing.
7 The silicon carbide starting material and the carbo~lor carbon source
màterial are thoroughly mixed eO obtain a dispersion of the carbon or carbon
9 source matcrial throughout the silicon carbide material.
0 In one mode of the present invention, particularly where the amount of
~t aluminum nitride desired in the sintered product is between about ~,O!a~d;~b~ut
95.0 percent by weight, based on the total weight of the mixture, the aluminum
13 nitrid~ componen~ in finely-divided form is admixed with the silicon carbicle and
1 14 carbon or carbon source material componen~s. A particle siæe of less than lO
m;crons in preferrecl. Eminently useful is a particle size of less than 5
16 microns, and, for ease of even distribution, a particle size of less than about
17 2 microns is particularly useful.
8 In another mode of -the invention, particularly when the amount of
aluminum nitride desired in the sintered product is bctwecn about 0.5 and about
0 3.0 percent by weight of the mixture, the aluminum nitride component may be
21 added to the silicon carbide - carbon mixture in the vapor state duriny sinter-
22 ing. In this mode, the silicon carbide and carbon or carbon source components
23 are admixed, shapcd by cold pressing, and subsequently sintered in an atmosphere
24 containing betwecn about 5-10 and about l-lO 3 atmospheres of aluminum nitride .
The aluminum nitride atmosphere may suitably be provided by the inclusion of
26 solid aluminum niLride in the furnace which vaporizes duriny the sintering
27 process Alternatively, aluminum ni~ride may be produced in situ in the furnace
28 during sintering.
29 The cold pressing step is suitably carried out in a metal die at
pressllres between ahout 12,000 and about l~,000 psi. Generally, pressures abov
~L~3~3'~
I ahnut 1',500 psi are useful. Pressures above about 1~,000 psi may be utiliæed;
2 however, minimal h~neficial results in the final sintered product are obtained.
3 A second pressing of the present mixture results in an improvement in
4 the density of the final product. In such process, the mixture is initially
cold pressed, crushed to about a ~0 mesh si%e powder, and re-pressed. It is
6 postulated that this process is effective in removing air from the powder par-
7 ticles which yields a higher green body density and, in turn, a higher sintered
8 product density. However, double pressing is not critical to the present
9 invention, and the improvement is not substantial enough to warrant two
o pressings as a procedure to be employed ln all cases.
11 The pressed product, green body, typically has a density ranging
2 between about 1.79 and about 1.95 g/cc. The porosity of the green bodies pro-
3 duced by mixtures of the present invention typically range from about 39.3 to
4 about ~5.8 percent.
The sintering step is suitably carried out utilizing a graphite resis-
6 tance element furnace. Temperatures between about 1900 and about 2250 C. are
11 eminently useful. Usually, when temperatures less than about 1~00 C. are
18 employed, the sintering process does not proceed to produce a desired den6e1~ product, Usually, when temperatures over about 225n C. are employed, deterio-
zo ration of the sintered product may occur.
2~ The sintering step is preferably carried out in an atmosphere that is
2~ inert to the mixtures being sintered. Inert gases such as argon, helium, and
23 nitrogen may be employed. An atmosphere of ammonia may be utilized. A vacuum
24 in the order of about 10-3 torr may also be utilized.
Generally, the mixtures of the present invention sinter under the
26 foregoirlg conditions to prodllce the desired co-sintered product when sintering
21 times between about ~ to about 6 hours are employed. ~sually, sintering times
28 between about ~ to about 2 hours are sufficient.
29 ~he present invention may be more fully illustrated by the following
~o ¦ exal s which mre not ~o be intorpreted as limiting. ~nless othe~wise indi~ ¦
39'791
1 ¦ cated, all parts and percenta~es are by weigh~, and all temperatures are in
2 degrees Celsius.
4 EXAMPLE 1
S A mixture containing 95 parts of alpha phase silicon carbide havin~
6 an average particle size of less than 5 microns, 1 part of aluminum nitride also
7 ¦having an average particle si~e of less than 5 microns, and 4 parts of carbon
8 ¦using a phenolic resin, a product designated as B178 Resin by Varcum Chemical
9 ¦Division, Reichhold Che~icals, Inc., was made by ball milling the components in
o ¦an acetone slurry for a period of 8 hours. The mixture was then dried at room
Il ¦tempe~ature o~er a period of about twelve hours. The dry mixture was then
12 ¦crushed by dry ball milling and screened through a 80 mesh screen~
13 The screened powder was then cold pressed at lS,000 psi using a metal
14 die to produce a green body ~" in diameter and Y2" in height. The green body
¦had a density of 1.8fl g/cc. The body was then allowed to dry at room ~empera-
6 ~ture and subsequently was heated at 110 C. in air for a period of 1 hour to
17 ¦cure the phenolic resin composition.
18 ¦ The green body was then sintered in a graphite resistance element
1~ furnace at a temperature of 2150 C. for a period of 3D minutes in an argon
atmosphere, The co-si'ntered product was found to have undergane a linear
2l shrinkage of 10.93%. The product had a porosity of 2.91% and a bulk density of
22 3.00 ~/cc, which corresponds to 93.30% of the theoretical density of silicon
23 carbide.
24 This example is shown in the following table as Example 1. Examples
26 2 throu~h 10 were conductcd in a similar manner.
21
29
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28
3.1L
l ¦ EXAMPLE ll
2 ¦ A mixture of 96.0 parts alpha phase silicon carbide having an average
¦particle si~.e less than ') microns and 4.0 parts of carbon using a B-~7~1 phenolic
4 ¦resin composition was made by ball milling the silicon carbide and resin compo-
¦nents in a slurry of acetone for a period of eight llours~ Thc ~ixture was dried
6 at room temperature and processed as ;n Æxample l to produce a green body. The
7 green body was placed in the sintering furnace along with a pellet of aluminum
8 nitride. A sintering temperature of 1950 C. was maintained for a period of 30
9 minutes with an argon atmosphere~ The partial pressure of aluminum nitride was
o calculated to be about ~5-lO-~ atmospheres. The sintercd product was found to
have a porosity of 4.25% and a bulk density of 2.62:g/cc, corresponding to
2 ~l.62% of the theoretical density of silicon carbide.
3 Example ll is shown in Table II as Example ll. Examples 12 through
~ 16 we conducted in a similar mann-r.
, 1
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6 ¦ a ~ o o~ n o ~r
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13 1 ~ ~ o o~
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16 l h ~ ~ 3 0~
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1 It w;ll be appreciated ~hat the present invention is not to be consi-
2 ¦dered as limited to the specific examples and embodiments given in the foregoing
3 ~and that various modifications may be ma~e within the ordinary skill of the art
6 withou-t departing from the spirit and scope of the invention.
9 . .
2 .
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1~ .
8 .
26 l
28 .
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