CA1079309A - Beryllium containing silicon carbide powder composition - Google Patents
Beryllium containing silicon carbide powder compositionInfo
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- CA1079309A CA1079309A CA289,906A CA289906A CA1079309A CA 1079309 A CA1079309 A CA 1079309A CA 289906 A CA289906 A CA 289906A CA 1079309 A CA1079309 A CA 1079309A
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
- C04B35/56—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides
- C04B35/565—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
- C04B35/56—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides
- C04B35/565—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide
- C04B35/575—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide obtained by pressure sintering
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Abstract
BERYLLIUM CONTAINING SILICON CARBINE POWDER COMPOSITION
ABSTRACT
Disclosure is made of a suitable silicon carbide mixture which is prepared by mixture finely divided silicon carbide containing between about 0.5 and about 5.0 per cent by weight of excess carbon with a finely divided beryllium containing additive wherein the amount of beryllium in the mixture is equal in between about 0.03 to about 3.0 per cent by weight of the powder.
A dense silicon carbide ceramic product may be prepared from the powder mixture by either but pressing the mixture at temperatures of between about 1900° and about 2200° C. at a pressure between about 1,000 to about 10.000 psi or by pressureless sintering wherein the article is imitially shaped and subsequently sintered at a temperature from about 1950° to about 2300° C.
ABSTRACT
Disclosure is made of a suitable silicon carbide mixture which is prepared by mixture finely divided silicon carbide containing between about 0.5 and about 5.0 per cent by weight of excess carbon with a finely divided beryllium containing additive wherein the amount of beryllium in the mixture is equal in between about 0.03 to about 3.0 per cent by weight of the powder.
A dense silicon carbide ceramic product may be prepared from the powder mixture by either but pressing the mixture at temperatures of between about 1900° and about 2200° C. at a pressure between about 1,000 to about 10.000 psi or by pressureless sintering wherein the article is imitially shaped and subsequently sintered at a temperature from about 1950° to about 2300° C.
Description
N3-&
BERYLLIUM CONTAINING SILICON CARBIDE POWDER COMPOSITION
BACKGROUND OF THE INVEN T ION
Silicon carbide, a crystalline compound o~ metallic sil-icon and non-metallic carbon, has long been known for its hardness, its strength, and its excellent resistance to oxi-dation and corrosion. Silicon car'bide has a low coe~'ficient of expansion, good heat transfer properties and main-tains high strength at elevated temperatures. In recent years, the art of producing high density silicon carbide bodies from silicon carbide powders has been developed. Methods include reaction bending, chemical vapor deposition, hot pressing, and pressureless sintering (initially f7arming the article and subsequently sintering). Examples of these methods are ~' described in U. S. Patents 3,853,566; 3,852,099; 3,954,483; '~ ;
and 3,960,577. The high density silicon carbide bodies so '~
produced are excellent engineering materials and find utility in fabrication of components for turbines, heat exchange '-units, pumps, and other equipment or tools that are exposed to severe wear and/or operation under high temperature con- ' ditions. The present invention relates both ~o silicon car-bide powder mixtures that are adapted to use in the various methods of producing a high densi~y silicon carbide body by hot pressing or sintering~and to the ceramic articles pro-: : :
~ duced therefrom.
~ -~ 25 In order to obtain high density and high strength silicon .
carbide ceramic materials, various additives have been util-ized. For examp'Le, a;~method of hot pressing silicon carbide to denslties in order o~9&/O o~theoretical by addition of aluminum and iron as~densification aids is disclosed by ~Alliegre, et al, J. Ceram. Sec., Vol. 39, No. 11, Nov., 1965, .
~ 3~ ~
Pages 386 to 389. They found that a dense silicon carbide could be produced from a powder mixture containing 1% by weight of aluminum. Their product had a modulus of rupture of 54,000 psi at room temperature and 70,000 psi at 1371C.
More recent advance is the use of boron as a densification additive, usually in the range of between about 0.3 and about 3.0% by weight of -the powder. The boron additive may be in the form of elemental boron or in the form of boron containing compounds, for example, boron carbide. Examples of silicon carbide powders containing boron may be found in U. S. Patents 3,852,099; 3,95~,483; and 3,968,19~.
SUMMARY OF THE Il~VE~TION
It has now been discovered that beryllium may be utilized as a densification aid in the production of sintered silicon carbide material. Beryllium in the range from about 0.03 to about 3.0 per cent by weight of the powder has been found to be eminently useful, more particularly a range :Erom about 0.1 to about 1.0 per cent by weight of the powder has been found to be aptly suited to the promotion of densification of sili-con carbide powder compacts. Beryllium may be utilized asthe densification aid or may be utilized in addition to other densification aids, for example, boron. Thus, mixtures of beryllium and boron or other aids may be utilized. Usually the range of such aids is from about 0.03 to about 3.0 per cent by weight of the silicon powder.
DETAILED DESCRIPTION
The starting silicon carbide powder, containing from about 0.5 to about 5.0 per cent by weight excess carbon, is admixed with finely divided beryllium or a beryllium contain-
BERYLLIUM CONTAINING SILICON CARBIDE POWDER COMPOSITION
BACKGROUND OF THE INVEN T ION
Silicon carbide, a crystalline compound o~ metallic sil-icon and non-metallic carbon, has long been known for its hardness, its strength, and its excellent resistance to oxi-dation and corrosion. Silicon car'bide has a low coe~'ficient of expansion, good heat transfer properties and main-tains high strength at elevated temperatures. In recent years, the art of producing high density silicon carbide bodies from silicon carbide powders has been developed. Methods include reaction bending, chemical vapor deposition, hot pressing, and pressureless sintering (initially f7arming the article and subsequently sintering). Examples of these methods are ~' described in U. S. Patents 3,853,566; 3,852,099; 3,954,483; '~ ;
and 3,960,577. The high density silicon carbide bodies so '~
produced are excellent engineering materials and find utility in fabrication of components for turbines, heat exchange '-units, pumps, and other equipment or tools that are exposed to severe wear and/or operation under high temperature con- ' ditions. The present invention relates both ~o silicon car-bide powder mixtures that are adapted to use in the various methods of producing a high densi~y silicon carbide body by hot pressing or sintering~and to the ceramic articles pro-: : :
~ duced therefrom.
~ -~ 25 In order to obtain high density and high strength silicon .
carbide ceramic materials, various additives have been util-ized. For examp'Le, a;~method of hot pressing silicon carbide to denslties in order o~9&/O o~theoretical by addition of aluminum and iron as~densification aids is disclosed by ~Alliegre, et al, J. Ceram. Sec., Vol. 39, No. 11, Nov., 1965, .
~ 3~ ~
Pages 386 to 389. They found that a dense silicon carbide could be produced from a powder mixture containing 1% by weight of aluminum. Their product had a modulus of rupture of 54,000 psi at room temperature and 70,000 psi at 1371C.
More recent advance is the use of boron as a densification additive, usually in the range of between about 0.3 and about 3.0% by weight of -the powder. The boron additive may be in the form of elemental boron or in the form of boron containing compounds, for example, boron carbide. Examples of silicon carbide powders containing boron may be found in U. S. Patents 3,852,099; 3,95~,483; and 3,968,19~.
SUMMARY OF THE Il~VE~TION
It has now been discovered that beryllium may be utilized as a densification aid in the production of sintered silicon carbide material. Beryllium in the range from about 0.03 to about 3.0 per cent by weight of the powder has been found to be eminently useful, more particularly a range :Erom about 0.1 to about 1.0 per cent by weight of the powder has been found to be aptly suited to the promotion of densification of sili-con carbide powder compacts. Beryllium may be utilized asthe densification aid or may be utilized in addition to other densification aids, for example, boron. Thus, mixtures of beryllium and boron or other aids may be utilized. Usually the range of such aids is from about 0.03 to about 3.0 per cent by weight of the silicon powder.
DETAILED DESCRIPTION
The starting silicon carbide powder, containing from about 0.5 to about 5.0 per cent by weight excess carbon, is admixed with finely divided beryllium or a beryllium contain-
-2-": -. . :, ~...... -. , - .
3~
ing compound. Pre~erably, the particle size of both compon-ents i~s less than 5 microns and, more preferably, less than 2 microns. Exceptionally good distribution is obtained when the components are less than about 1.0 microns. In order to obtain densification, the beryllium or ~eryllium containing additive should be utilized in an amount whereby between about 0.03 and about 3.0 per cent by weight of the powder is beryllium. The use of less than about 0.03 per cent by weight has not been found to substantially increase the density of the sintered prodwct. The addition of more than about 3.0 per cent by weight of beryllium gives little additional densi-fication and is conducive to exaggerated grain growth and loss of strength of the sintered product.
A bulk density of at least 75% of theoretical is required for most applications, and bulk densities o~ at least 85% of theoretical are more often required. A hot pressed or sin-tered produc~ having a density of 85% of theoretical may be obtained by the present invention.
The beryllium additive of the present invention may be utilized alone or may be mixed with other densification aids, the most usual being boron. ~en such mixtures are used, the beryllium component may be completely substituted for the boron or other densification aid or may be substituted for part of the boron or other densification aid. In general, such mixtures, when ready for sintering, will contain from about 0.03 to about 3.0 per cent by weight of total densifi-cation additive.
The silicon carbide source material is preferably a sub- --micron powder having a surface area greater than 8.0 m2/gm and containing from about 0.5 to about 5.0 per cent by weight ... . ..
, ~q)~ 3 ~
of excess carbon. Generally powder compositions having sur-face areas between about 5 and about 20 m2/gm are found emminently useful. ~'he excess carbon may be introduced, for example, during the production process, by the subse~uent addition of carbon or a carbonaceous material or as a binder prior to sintering.
The beryllium or beryllium containing additive starting materials found use~ul are generally less than 50 microns in particle size and preferably less than 10 microns in parti-cle size. Emminently useful is a particle size of less than 5 microns, and, for ease of even distribution of the beryllium or beryllium containing additive with the silicon carbide powder to ob-tain a homogeneous mixture useful in sintering, a particle size less thzn about 1.0 micron is very useful.
Other additives may be utilized but are not necessary for the promotion of densification during the sintering pro~ -cess. Preferably the sintering operation is carried out in an inert gas atmosphere, argon or helium being aptly suited to use as the inert gas. A reducing atmosphere may also be utilized.
; The powder compositions of the present invention may be utilized in hot pressing or pressureless sintering. For example, in hot pressing, a silicon carbide powder containing from about 0.5 to abou~ 5.0 per cent by weight of excess carbon is mixed t:o form a homogeneous mixture with beryllium or a beryllium containing additive so that a total of between about 0.03 to about 3.0 per cent by weight of berylllum is present. The mi~ture is placed in a hot pressing mold and heated to a temperature of between about 1900 and about 2200C under a pressure of between 1,000 and 10,000 psi for ~;
~; 4 ~ 3~
a time sufficient to obtain a silicon carbide product having a density of over 75% of theoretical. More particularly, a silicon carbide powder having a surface area of about 11 m2/gm and containing about 2.0% by weight excess carbon may be ad-mixed with between about 0.1 and about 1.0 per cent by weightof beryllium, added as Be2C, the mixture placed in a graphite pressing mold and hot pressed at about 200C under pressure of about 5,000 psi. The silicon carbide product thus formed typically has a density greater than 85% of theoretical and can be used as formed or may be machined into more complicated shapes.
In pressureless sintering, a silicon carbide powder con-taining from about 0.5 to about 5.0 per cent by weight of excess carbon is mixed to form a homogeneous mixture with beryllium or a beryllium containing additive so that a total of between about 0.03 to about 3.0 per cent by weight of beryllium is present. ~he homogeneous mixture is then shaped into a green product. Suitable additives to increase flow and binding of the particles may be incorporated into the starting mixture. The green product is subsequently sintered in an inert or in a reducing atmosphere at a temperature of between about 1950 and about 2300C for a time sufficient to obtain a silicon carbide product having a density greater than 75% of theoretical. More particularly, a silicon carbide powder having a surface aréa of appro~imately 11 ~2/gm and containing about 2.0% by weight excess carbon may be ad~ixed ~with between about 0.03 and about 1.0 per cent by weigh~ of beryl~ium, suitably added as Be2C, or in elemental form. The resultant mixture may be pressed to a density of about 1.76 .~ .
gm/cm~. Binders may be used to increase the flowability of :
.~ , 3~9 the powder or to increase the green strength o~ the pressed product. The pressed compacted powder is then sintered, preferably in an inert atmosphere, at a temperature of about 2100C for about 30 minutes. After cooling, the sintered product typically has a density of greater than 85% of theoretical.
Tlle invention will IIOW be illustrated by more specific examples:
HOT PRESSING
A silicon carbide powder having the following specifi-cations was utilized as a starting material. The silicon carbide powder had a surface area greater than 8.0 m /gm and the following analysis in per cent by weight:
Oxygenless than 0.8 Ironless than 0.2 Aluminum less than 0.4 Nickelless than 0.1 ~-Titanium less than 0.1 ~
Tungsten less than 0.5 -Free Silicon less than 0.4 Sllicon Carbide greater than 97.5 c~inety-seven and one-half grams of the abo~e powder were mixed with 4.8 gm of a phenolic resin known as Resin No. ~121, a product of Varcum Chemical Company, and 0.5 grams of minus 325 mesh berylLium metal powder. Upon decomposition of the ~ .
phenolic resin during subsequent heating to the hot pressing ~temperature a carbonaceous residue is let behind. The mix-~; ture was blended in a closed environment and then pressed ~ 6 ; ~ '' .~, .. - - . . ~. ; . . . . . .
~ . . . .
3~1 into a 3-inch 91ug at a pressure of about 2000 psi. The slug is suited to be hot pressed by techniques well known in the art, namely by heating to a temperature of about 2000C and a pressure of about 4000 psi. The time required to achieve final density was about 30 minutes. After densiication, the p-ressure was reduced and the temperature allowed to decrease.
Products prepared in accord with the above technique may be expected to have a bulk density in excess of 2.88 gmtcm3 (about 90% of theoretical density).
PRESSURELESS SINTERING
Forty-nine and three-fourths grams of a silicon carbide powder having the characteristics of that described in Exam-ple 1, containing approximately 2% by weight of carbon added in the form of a phenolic resin (Varcum Chemical Company No.
8121)~ was admixed with 0. 25 grams of minus 325 mesh beryllium metal powder. Three per cent of polyvinyl alcohol was added as a binder to impart h;gher green strength to the powder mixture. The preparation of the mixture was carried out in a . .
closed environment. In order to insure a good dispersion of the additives in the powder, a wet mixing technique was used.
The mixture was slurried in a mixture of 80% by volume methyl . .
alcohol and 20% by vol~lme water. After thcrough mixing, the ~ -slurry was evaporated to dryness and the resulting powder mix- -~25 ture pressed into 1-1/8" diameter pellets, weighing about 10 grams each, at a pressure of about 12~000 psi. '~he resultant powder compaot was then placed into a graphike resistance heatlng element furnace, heated slowly to eliminate the vola-tlles, and then~ralsed ln temperature rapidly to about 2100C
.
.
: ~ : ` ` :
:: ~
~ 7_ ~
- . . .. . ..
3allt3 and held at that temperature for 30 minutes.
A typical heating rate schedule is as follows:
room temperature to 150C 30 minutes 150 to 400C 120 minutes 400 -to 800C 60 minutes 800 to 2100C 180 minutes After sintering, the power was shut off and the sintered product allowed to cool with the furnace. An atmosphere of argon was utilized throughout the sintering process. The powdered compacts produced have bulk densities in excess of 2.75 gmlcm (over 85% of theoretical).
EXA~IPLE 3 ADDITIVE MIXTURES
''' '~
A typical silicon carbide powder containing approximately 2% by weight of excess carbon added as a phenolic resin as described in Example 1 was admixed with sufficient boron car- -bide to provide O.lV/o by weight of boron and sufEicient beryl~ :
lium carbide to provide 0.1% by weight of beryllium in a 30 ~ gram powder batch. Both the boron carbide powder and the : 20 beryllium carbide powder had a particle size less than 10 : ~ microns. 5% of a binder containing polyvinyl alcohol and plasticisers was added to the mix in order to improve the ~ -green strength o:E pressed powder compacts. The powder mix-~: ture was prepared as described in Example 2.
The powder was pressed into ~" diameter pellets weighing approximately 0.75 gm each at a presstire of about 16,000 psi. :
, Two pellets were placed in a small crucible made from the : - . .
same powder mixtt.lre as~were the pellets, the ~rucible closed :
and the pellets plus crucible were sintered at a temperature :: ~: ::: : :::
':
3L~7~361~
o~ approximately 2150C for a period o about 30 minutes in an inert atmosphere. The sinterecl pellets, initially having a density before sintering of 1.71 gm/cm3 (53/O of theoretical), were found to have a bulk density of 2.98 gm/cm3 (93% of theoretical).
ADDITIVE MIXTURES
Using the procedure of Example 3, sufficient beryllium carbide was added to provide 0.33% by weight of Be to the pow-der mixture while all other ingredients remained the same asdescribed previously. ~" diameter pellets weighing approxi-mately 0.75 gm each were pressed from this mixture ~nd encap-sulated in a crucible which had been formed from powder having the same composition as the pellets. The pellets plus crucible were sintered at a temperature of approximately 2150~C in an inert atmosphere for a period of about 30 minutes. Before sintering, the density of the pressed powder compacts averaged 1.70 gm/cm (53% of theoretical) while after sintering at 2150C the powder compacts had an average density of 3.02 gm/cm3 (94.1% o theoretical).
-- , ADDITIVE MIXTURES
UsLng the procedure of Example 3, sufficient boron car-bide was added to the sillcon carbide powder containing approx-imately 2% by weight excess carbon in order to provide 0.2%
~ , by weight of boron in the powder mixture. In addition, suf-ficient beryllium carbide was added to provide 0.03% by weight of beryllium in ~he powder mixture. This mixture, :: :
_9_ 3~
composed of 0.08 gram of minus 325 mesh boron carbide, 0.02 gram of minus 325 mesh beryllium carbide and 29.90 grams of a silicon carbide powder having the characteristics of that described in Example 1, and containing 2% by weight of excess carbon added in the form of a phenolic resin was wet mixed together with 5% by weight of a binder system containing polyvinyl alcohol and plasticizers. After drying, the powder was pressed at 16,000 psi into pellets about ~" diameter and weighing about 3/4 gram. Two specimens made in this fashion were placed into a crucible having the same composition as that of the pellets, the crucible covered and the crucible plus specimens sintered at about 2150C in an inert atmosphere for a period of about 30 minutes. The specimens, which ini-tially had a density of about 1.72 gm/cm3 (54% of theoretical), had a bulk density after sintering of approximately 3.15 gm/cm (98% Gf theoretical).
CONTROL
Fifty grams of a silicon carbide powder having the char-acteristics as described in Example 1, containing approxima~ely 2i~ by weight of excess carbon added in the form of a phenolic resin (Varcum Chemical Company No. 8121), was prepared and sintered according to the procedure described in Example 2, ~except that the addition of beryllium powder was omitted.
The powder compact produced was found to have a bulk density of less than about 2.25 gm/cm3 (70% of theoretical).
. , .
. .
ing compound. Pre~erably, the particle size of both compon-ents i~s less than 5 microns and, more preferably, less than 2 microns. Exceptionally good distribution is obtained when the components are less than about 1.0 microns. In order to obtain densification, the beryllium or ~eryllium containing additive should be utilized in an amount whereby between about 0.03 and about 3.0 per cent by weight of the powder is beryllium. The use of less than about 0.03 per cent by weight has not been found to substantially increase the density of the sintered prodwct. The addition of more than about 3.0 per cent by weight of beryllium gives little additional densi-fication and is conducive to exaggerated grain growth and loss of strength of the sintered product.
A bulk density of at least 75% of theoretical is required for most applications, and bulk densities o~ at least 85% of theoretical are more often required. A hot pressed or sin-tered produc~ having a density of 85% of theoretical may be obtained by the present invention.
The beryllium additive of the present invention may be utilized alone or may be mixed with other densification aids, the most usual being boron. ~en such mixtures are used, the beryllium component may be completely substituted for the boron or other densification aid or may be substituted for part of the boron or other densification aid. In general, such mixtures, when ready for sintering, will contain from about 0.03 to about 3.0 per cent by weight of total densifi-cation additive.
The silicon carbide source material is preferably a sub- --micron powder having a surface area greater than 8.0 m2/gm and containing from about 0.5 to about 5.0 per cent by weight ... . ..
, ~q)~ 3 ~
of excess carbon. Generally powder compositions having sur-face areas between about 5 and about 20 m2/gm are found emminently useful. ~'he excess carbon may be introduced, for example, during the production process, by the subse~uent addition of carbon or a carbonaceous material or as a binder prior to sintering.
The beryllium or beryllium containing additive starting materials found use~ul are generally less than 50 microns in particle size and preferably less than 10 microns in parti-cle size. Emminently useful is a particle size of less than 5 microns, and, for ease of even distribution of the beryllium or beryllium containing additive with the silicon carbide powder to ob-tain a homogeneous mixture useful in sintering, a particle size less thzn about 1.0 micron is very useful.
Other additives may be utilized but are not necessary for the promotion of densification during the sintering pro~ -cess. Preferably the sintering operation is carried out in an inert gas atmosphere, argon or helium being aptly suited to use as the inert gas. A reducing atmosphere may also be utilized.
; The powder compositions of the present invention may be utilized in hot pressing or pressureless sintering. For example, in hot pressing, a silicon carbide powder containing from about 0.5 to abou~ 5.0 per cent by weight of excess carbon is mixed t:o form a homogeneous mixture with beryllium or a beryllium containing additive so that a total of between about 0.03 to about 3.0 per cent by weight of berylllum is present. The mi~ture is placed in a hot pressing mold and heated to a temperature of between about 1900 and about 2200C under a pressure of between 1,000 and 10,000 psi for ~;
~; 4 ~ 3~
a time sufficient to obtain a silicon carbide product having a density of over 75% of theoretical. More particularly, a silicon carbide powder having a surface area of about 11 m2/gm and containing about 2.0% by weight excess carbon may be ad-mixed with between about 0.1 and about 1.0 per cent by weightof beryllium, added as Be2C, the mixture placed in a graphite pressing mold and hot pressed at about 200C under pressure of about 5,000 psi. The silicon carbide product thus formed typically has a density greater than 85% of theoretical and can be used as formed or may be machined into more complicated shapes.
In pressureless sintering, a silicon carbide powder con-taining from about 0.5 to about 5.0 per cent by weight of excess carbon is mixed to form a homogeneous mixture with beryllium or a beryllium containing additive so that a total of between about 0.03 to about 3.0 per cent by weight of beryllium is present. ~he homogeneous mixture is then shaped into a green product. Suitable additives to increase flow and binding of the particles may be incorporated into the starting mixture. The green product is subsequently sintered in an inert or in a reducing atmosphere at a temperature of between about 1950 and about 2300C for a time sufficient to obtain a silicon carbide product having a density greater than 75% of theoretical. More particularly, a silicon carbide powder having a surface aréa of appro~imately 11 ~2/gm and containing about 2.0% by weight excess carbon may be ad~ixed ~with between about 0.03 and about 1.0 per cent by weigh~ of beryl~ium, suitably added as Be2C, or in elemental form. The resultant mixture may be pressed to a density of about 1.76 .~ .
gm/cm~. Binders may be used to increase the flowability of :
.~ , 3~9 the powder or to increase the green strength o~ the pressed product. The pressed compacted powder is then sintered, preferably in an inert atmosphere, at a temperature of about 2100C for about 30 minutes. After cooling, the sintered product typically has a density of greater than 85% of theoretical.
Tlle invention will IIOW be illustrated by more specific examples:
HOT PRESSING
A silicon carbide powder having the following specifi-cations was utilized as a starting material. The silicon carbide powder had a surface area greater than 8.0 m /gm and the following analysis in per cent by weight:
Oxygenless than 0.8 Ironless than 0.2 Aluminum less than 0.4 Nickelless than 0.1 ~-Titanium less than 0.1 ~
Tungsten less than 0.5 -Free Silicon less than 0.4 Sllicon Carbide greater than 97.5 c~inety-seven and one-half grams of the abo~e powder were mixed with 4.8 gm of a phenolic resin known as Resin No. ~121, a product of Varcum Chemical Company, and 0.5 grams of minus 325 mesh berylLium metal powder. Upon decomposition of the ~ .
phenolic resin during subsequent heating to the hot pressing ~temperature a carbonaceous residue is let behind. The mix-~; ture was blended in a closed environment and then pressed ~ 6 ; ~ '' .~, .. - - . . ~. ; . . . . . .
~ . . . .
3~1 into a 3-inch 91ug at a pressure of about 2000 psi. The slug is suited to be hot pressed by techniques well known in the art, namely by heating to a temperature of about 2000C and a pressure of about 4000 psi. The time required to achieve final density was about 30 minutes. After densiication, the p-ressure was reduced and the temperature allowed to decrease.
Products prepared in accord with the above technique may be expected to have a bulk density in excess of 2.88 gmtcm3 (about 90% of theoretical density).
PRESSURELESS SINTERING
Forty-nine and three-fourths grams of a silicon carbide powder having the characteristics of that described in Exam-ple 1, containing approximately 2% by weight of carbon added in the form of a phenolic resin (Varcum Chemical Company No.
8121)~ was admixed with 0. 25 grams of minus 325 mesh beryllium metal powder. Three per cent of polyvinyl alcohol was added as a binder to impart h;gher green strength to the powder mixture. The preparation of the mixture was carried out in a . .
closed environment. In order to insure a good dispersion of the additives in the powder, a wet mixing technique was used.
The mixture was slurried in a mixture of 80% by volume methyl . .
alcohol and 20% by vol~lme water. After thcrough mixing, the ~ -slurry was evaporated to dryness and the resulting powder mix- -~25 ture pressed into 1-1/8" diameter pellets, weighing about 10 grams each, at a pressure of about 12~000 psi. '~he resultant powder compaot was then placed into a graphike resistance heatlng element furnace, heated slowly to eliminate the vola-tlles, and then~ralsed ln temperature rapidly to about 2100C
.
.
: ~ : ` ` :
:: ~
~ 7_ ~
- . . .. . ..
3allt3 and held at that temperature for 30 minutes.
A typical heating rate schedule is as follows:
room temperature to 150C 30 minutes 150 to 400C 120 minutes 400 -to 800C 60 minutes 800 to 2100C 180 minutes After sintering, the power was shut off and the sintered product allowed to cool with the furnace. An atmosphere of argon was utilized throughout the sintering process. The powdered compacts produced have bulk densities in excess of 2.75 gmlcm (over 85% of theoretical).
EXA~IPLE 3 ADDITIVE MIXTURES
''' '~
A typical silicon carbide powder containing approximately 2% by weight of excess carbon added as a phenolic resin as described in Example 1 was admixed with sufficient boron car- -bide to provide O.lV/o by weight of boron and sufEicient beryl~ :
lium carbide to provide 0.1% by weight of beryllium in a 30 ~ gram powder batch. Both the boron carbide powder and the : 20 beryllium carbide powder had a particle size less than 10 : ~ microns. 5% of a binder containing polyvinyl alcohol and plasticisers was added to the mix in order to improve the ~ -green strength o:E pressed powder compacts. The powder mix-~: ture was prepared as described in Example 2.
The powder was pressed into ~" diameter pellets weighing approximately 0.75 gm each at a presstire of about 16,000 psi. :
, Two pellets were placed in a small crucible made from the : - . .
same powder mixtt.lre as~were the pellets, the ~rucible closed :
and the pellets plus crucible were sintered at a temperature :: ~: ::: : :::
':
3L~7~361~
o~ approximately 2150C for a period o about 30 minutes in an inert atmosphere. The sinterecl pellets, initially having a density before sintering of 1.71 gm/cm3 (53/O of theoretical), were found to have a bulk density of 2.98 gm/cm3 (93% of theoretical).
ADDITIVE MIXTURES
Using the procedure of Example 3, sufficient beryllium carbide was added to provide 0.33% by weight of Be to the pow-der mixture while all other ingredients remained the same asdescribed previously. ~" diameter pellets weighing approxi-mately 0.75 gm each were pressed from this mixture ~nd encap-sulated in a crucible which had been formed from powder having the same composition as the pellets. The pellets plus crucible were sintered at a temperature of approximately 2150~C in an inert atmosphere for a period of about 30 minutes. Before sintering, the density of the pressed powder compacts averaged 1.70 gm/cm (53% of theoretical) while after sintering at 2150C the powder compacts had an average density of 3.02 gm/cm3 (94.1% o theoretical).
-- , ADDITIVE MIXTURES
UsLng the procedure of Example 3, sufficient boron car-bide was added to the sillcon carbide powder containing approx-imately 2% by weight excess carbon in order to provide 0.2%
~ , by weight of boron in the powder mixture. In addition, suf-ficient beryllium carbide was added to provide 0.03% by weight of beryllium in ~he powder mixture. This mixture, :: :
_9_ 3~
composed of 0.08 gram of minus 325 mesh boron carbide, 0.02 gram of minus 325 mesh beryllium carbide and 29.90 grams of a silicon carbide powder having the characteristics of that described in Example 1, and containing 2% by weight of excess carbon added in the form of a phenolic resin was wet mixed together with 5% by weight of a binder system containing polyvinyl alcohol and plasticizers. After drying, the powder was pressed at 16,000 psi into pellets about ~" diameter and weighing about 3/4 gram. Two specimens made in this fashion were placed into a crucible having the same composition as that of the pellets, the crucible covered and the crucible plus specimens sintered at about 2150C in an inert atmosphere for a period of about 30 minutes. The specimens, which ini-tially had a density of about 1.72 gm/cm3 (54% of theoretical), had a bulk density after sintering of approximately 3.15 gm/cm (98% Gf theoretical).
CONTROL
Fifty grams of a silicon carbide powder having the char-acteristics as described in Example 1, containing approxima~ely 2i~ by weight of excess carbon added in the form of a phenolic resin (Varcum Chemical Company No. 8121), was prepared and sintered according to the procedure described in Example 2, ~except that the addition of beryllium powder was omitted.
The powder compact produced was found to have a bulk density of less than about 2.25 gm/cm3 (70% of theoretical).
. , .
. .
Claims (6)
1. A sinterable powder mixture useful in pressureless sintering operations comprised of a silicon carbide powder having a size less than 5 microns and a surface area greater than 8 m2/gm, containing from about 0.5 to about 5.0 percent by weight of excess carbon and a beryllium or a beryllium containing compound wherein the beryllium comprises from about 0.03 to about 3.0 percent by weight of the powder.
2. The powder mixture of claim 1 wherein the silicon carbide powder has a size less than 2 microns.
3. The powder of claim 1 wherein the powder contains between about 0.03 and about 1.0 per cent by weight of beryllium.
4. The powder of claim 1 wherein the beryllium is elemental beryllium.
5. The powder of claim 1 wherein the beryllium is in the form of beryllium carbide.
6. A method of making a silicon carbide product com-prising the steps of:
a) forming a homogeneous mixture of the powder of claim 1, and b) shaping said mixture into a green product, and c) sintering said green product in an inert atmosphere at a temperature of between about 1950° and about 2300°C., and d) maintaining said temperature for a time suffic-ient to obtain a silicon carbide product having a density greater than 85% of theoretical.
a) forming a homogeneous mixture of the powder of claim 1, and b) shaping said mixture into a green product, and c) sintering said green product in an inert atmosphere at a temperature of between about 1950° and about 2300°C., and d) maintaining said temperature for a time suffic-ient to obtain a silicon carbide product having a density greater than 85% of theoretical.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US74503576A | 1976-11-26 | 1976-11-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1079309A true CA1079309A (en) | 1980-06-10 |
Family
ID=24994976
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA289,906A Expired CA1079309A (en) | 1976-11-26 | 1977-10-31 | Beryllium containing silicon carbide powder composition |
Country Status (6)
| Country | Link |
|---|---|
| JP (2) | JPS5367711A (en) |
| BR (1) | BR7707857A (en) |
| CA (1) | CA1079309A (en) |
| DE (1) | DE2751851A1 (en) |
| GB (1) | GB1558254A (en) |
| SE (1) | SE7713343L (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4144207A (en) * | 1977-12-27 | 1979-03-13 | The Carborundum Company | Composition and process for injection molding ceramic materials |
| CA1122384A (en) * | 1978-08-28 | 1982-04-27 | Richard H. Smoak | Pressureless sintering beryllium containing silicon carbide powder composition |
| JPS6125472Y2 (en) * | 1979-08-03 | 1986-07-31 | ||
| JPS5987893A (en) * | 1982-11-12 | 1984-05-21 | 株式会社日立製作所 | Circuit board, method of producing same and semiconductor device using same |
| US4874725A (en) * | 1984-04-27 | 1989-10-17 | Nippon Tungsten Co., Ltd. | High-density sintered article of silicon carbid |
| DE3840594A1 (en) * | 1988-02-05 | 1989-08-17 | Hoechst Ag | PROCESS METHOD FOR THE PRODUCTION OF HIGH-STRENGTH AND HIGH-DENSITY, POLYCRYSTALLINE SILICON CARBIDE MOLDED BODIES WITH HOMOGENOUS CARBON DISTRIBUTION BY CONTROLLED DIFFUSION |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3993602A (en) * | 1975-11-17 | 1976-11-23 | General Electric Company | Polycrystalline silicon carbide with increased conductivity |
| DE2621932C3 (en) * | 1976-05-17 | 1979-02-01 | Kajetan 8170 Bad Toelz Leitner | Chuck for clamping head and stud bolts during the screwing-in process |
-
1977
- 1977-10-31 CA CA289,906A patent/CA1079309A/en not_active Expired
- 1977-11-18 GB GB4801477A patent/GB1558254A/en not_active Expired
- 1977-11-19 DE DE19772751851 patent/DE2751851A1/en not_active Withdrawn
- 1977-11-24 JP JP14009977A patent/JPS5367711A/en active Granted
- 1977-11-25 BR BR7707857A patent/BR7707857A/en unknown
- 1977-11-25 SE SE7713343A patent/SE7713343L/en unknown
-
1987
- 1987-06-09 JP JP62142399A patent/JPS6325274A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| GB1558254A (en) | 1979-12-19 |
| JPS6253473B2 (en) | 1987-11-10 |
| SE7713343L (en) | 1978-05-27 |
| JPH0253388B2 (en) | 1990-11-16 |
| JPS6325274A (en) | 1988-02-02 |
| JPS5367711A (en) | 1978-06-16 |
| DE2751851A1 (en) | 1978-06-01 |
| BR7707857A (en) | 1978-09-05 |
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