CA1334677C - Silicon carbide sintered body - Google Patents
Silicon carbide sintered bodyInfo
- Publication number
- CA1334677C CA1334677C CA000254996A CA254996A CA1334677C CA 1334677 C CA1334677 C CA 1334677C CA 000254996 A CA000254996 A CA 000254996A CA 254996 A CA254996 A CA 254996A CA 1334677 C CA1334677 C CA 1334677C
- Authority
- CA
- Canada
- Prior art keywords
- silicon carbide
- sic
- boron
- weight
- alpha
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Classifications
-
- 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
Landscapes
- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Ceramic Engineering (AREA)
- Manufacturing & Machinery (AREA)
- Materials Engineering (AREA)
- Structural Engineering (AREA)
- Organic Chemistry (AREA)
- Ceramic Products (AREA)
- Crystals, And After-Treatments Of Crystals (AREA)
- Silicon Compounds (AREA)
Abstract
A silicon carbide sintered body characterized by its microstructural stability at high temperatures is produced by forming a mixture of .beta.-silicon carbide powder, .alpha.-silicon carbide seeding powder, boron additive and a carbonaceous additive into a green body and sintering it to produce a sintered body with a density of at least 80% wherein at least 70% by weight of the silicon carbide is .alpha.-silicon carbide.
Description
m e chemical and physical properties of silicon carbide make it an excellent material for high temperature structural applications. However, silicon carbide is produced in the form of particles or powder from which dense bodies must be formed and it is the formation and properties of these dense bodies that have presented problems.
Hot pressing of silicon carbide powder has been used to produce small dense bodies under closely controlled conditions. However, hot-pressing methods require high pressures and temperatures necessitating expensive energy consuming machinery equipped with graphite dies. Also, hot-pressing yields pressed bodies in the form of billets of simple geometric shape only which require time-consuming machining to produce a complex shaped part.
Silicon carbide powder alone is not sinterable.
However, in my Canadian application Serial No. 208,705, filed September 9, 1974, there is disclosed a method of producing a ~ -silicon carbide sintered body by forming a mixture of submicron powder composed of ~-silicon carbide, a boron additive and free carbon into a green body and sintering it at a temperature of about 1900-2100 C.
me onset of exaggerated growth of large tabular ~-silicon carbide crystals on densification of ~-silicon carbide powders doped with boron is a limitation to obtaining the uniform fine-grained microstructures necessary to withstand fracture, especially at temperatures of the order of about 2000C. m is phenomenon is related to the transformation of ~-silicon carbide into the thermo-dynamically more stable o~Sic phase at temperatures of about 2000C and higher.
While several means to suppress this kind of grain growth on hot pressing have been devised, none of them is currently applicable to sintering. m us, for instance, _ 1 334677 RD-7361 when the hot-pressing temperature is decreased and compensated for by increased pressing pressure, conditions may be found where exaggerated grain growth does not occur. Also various additions such as aluminum, silicon nitride, aluminum nitride, and boron nitride to ~-SiC powder have been proven effective in controlling the growth of tabular ~ crystals on hot-pressing:
these means, however, cannot be used for sintering as they interfere with the densification process and prevent obtaining high densities.
The present process relates to improved grain growth control on sintering of silicon carbide by transforming a substantial mass of the sintering ~-SiC powder to the a form, i.e. into the thermodynamically more stable form, by seeding with an addition of ~-SiC. The ~-SiC nuclei thus provided induce a rapid ~ to ~ transformation during sintering. m e growing ~-SiC grains impinge on each other early in their development, cease to grow and result in a sintered product with a substantially uniform, relatively fine-grained micro-structure wherein at least 70% by weight of the silicon carbide present is composed of ~-SiC in the form of platelets or elon-gated grains which may range in the long dimension from about 5 to 150 microns, and preferably from about 5 to 25 microns.
m e present invention provides a number of advantages.
One advantage is that since the present process provides grain growth control, sintering can be carried out through a wide temperature range which is particularly economical and practical since it eliminates the need for critical temperature controls. The second advantage is that the presentsintered produce has a shape and mechanical properties which do not change significantly through a wide temperature range, i.e.
temperatures ranging from substantially below 0C to temperatures higher than 2300C.
1 334677 R~7361 -m ose skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below, considered in conjunction with th~ figures accompanying and forming a part of the specification, in which:
FIGURE 1 is a photomicrograph (magnified 500 X) of an etched specimen produced according to the present invention but without the addition of ~-SiC, sintered at 2080C
illustrating the uniform microstructure of ~-SiC.
FIGURE 2 is a photomicrograph (magnified 500X) of an etched specimen of the same initial composition as FIGURE 1 sintered at 2150C showing a feather-like morphology of large grains of ~-SiC polytypes in a matrix of ~ -SiC.
FIGURE 3 is a photomicrograph (magnified 500 X) of an etched specimen prepared in accordance with the present invention sintered at 2175C showing a substantially uniform microstructure of ~-SiC.
Briefly stated, the process of the present invention comprises providing a substantially homogeneous particulate dispersion or mixture, wherein the particles are submicron in size, of ~-silicon carbide powder, ~-silicon carbide seeding powder, boron additive and a carbonaceous additive which is free carbon or a carbonaceous organic material which is heat-decomposible to produce free carbon, shaping the mixture into a green body, and sintering the green body at a temperature ranging from about 1950C to 2300 C in an atmosphere in which the green body and resulting sintered body having a density of at least 80% of the~theoretical density for silicon carbide and a significantly uniform microstructure wherein at least 70% by weight of the silicon carbide is ~-SiC.
In the present invention single phase ~ -silicon carbide powder is used having an average particle size ranging , up to about 0.45 micron, and generally from about 0.05 micron to 0.4 micron. A~ a practical matter and for best results the ~-SiC powder preferably ranges in size from an average particle size of about 0.1 micron to 0.2 micron. ~-silicon carbide powder of this size can be prepared by a number of techniques as, for example, by direct synthesis from the elements, by reduction of silica, or by pyrolysis of compounds containing silicon and caxbon. A number of processes which involve the pyrolysis of silicon compounds and organic compounds to produce silicon and carbon are particularly advantageous since they can be controlled to produce ~-silicon carbide of desired submicron particle size composed mainly of isolated crystallites.
Hot plasma techniques are especially preferred for producing the powders useful in the present invention. The final product generally requires leaching, especially with acid to remove any silicon which may be present, to produce a sinterable phase pure ~-silicon carbide powder.
In the present process, to achieve the desired grain growth control, the particle size of the a-SiC seed powder should be at least about twice as large as the average particle size of the ~-SiC. Also, the ~-SiC seed powder is always submicron in size, and generally has a particle size ranging from about 0.1 micron to about 0.6 micron. All polytype compositions of the ~-SiC are operable in the present invention.
The present fine-size ~-SiC can be prepared by a number of techniques. For example, abrasive grade silicon carbide, which is always totally ~-SiC, can be milled and the milled powder admixed with a liquid,such as water to separate fractions of large and fine-sized particles by se~i -~tation. Specifically, the large-sized particles are allowed to settle and the liquid in which the desired 1 334677 R~7361 finer-sized particles float is decanted and evaporated to yield the fine-sized, submicron particle fraction.
The ~-SiC powder is used in amounts ranging from about 0.05% by weight to 5% by weight based on the ~-SiC. The larger the amount of ~-SiC used, the lower is the density of the sintered product. Amounts of a-SiC ranging from 1% by weight to 3% by weight based on the ~ -SiC produce the finest and most uniform microstructures. Amounts of ~-SiC powder larger than 5% by weight are likely to produce a sintered pro-duct having a density lower than 80%, and amounts of ~-SiC
smaller than 0.05% by weight do not provide sufficient nuclei for grain growth control.
The d-sic powder is admixed with ~-SiC powder alone, or with ~-SiC powder containing the boron additive and/or carbonaceous additive to produce a homogeneous dispersion.
Specifically, the 0~SiC should be dispersed through ~ -SiC
powder substantially uniformly in order to produce a sintered product with the desired uniform microstructure.
The~-SiC powder can be admixed with the ~-SiC
powder by a number of techniques such as, for example, ball milling or jet milling, to attain the necessary uniform distribution and produce a substantially homogeneous dispersion.
One technique for introducing ~-SiC powder into ~-SiC powder utilizes milling balls formed of silicon carbide containing C~-SiC in significant amount, i.e. at least 10% by weight. In this technique ~ -SiC powder is milled with the SiC balls which introduce d-SiC seeds by wear due to milling. Milling is preferably carried out in a liquid dispersion. The a~ount of -~-SiC introduced is controlled by controlling milling time. Introduction of the proper amount of ~ -SiC is determinable empirically. For example, in accordance with the present process, the resulting powder can be sintered and the product sectioned and ~x~mined metallographically. The proper amount of ~-SiC has been introduced in accordance with the present invention when the product has a significantly uniform microstructure, contains ~-SiC in an amount of at least 70% by weight of the total amount of SiC and has a density of at least 80% of the theoretical density for SiC.
m e boron additive in the powder mixture from which the green body is shaped is in the form of elemental boron or boron carbide. In order to obtain significant densification - ~ during SinterinQ, the amount of boron additive is critical and is e~uivalent to about 0.3% to about 3.0% by weight of elemental boron based on the total amount of silicon carbide.
The particular amount of boron additive used is determinable empirically and depends largely on the degree of dispersion achieved in the mixture since the more thoroughly it is dispersed the more uniform is the density of the sintered product.
However, amounts of elemental boron below 0.3% by weight do not result in the necessary degree of densification whereas amounts of elemental boron greater than 3.0% by weight produce no significant additional densification and may deteriorate the oxidation resistance of the product. During sintering, the boron additive enters into solid solution with the silicon carbide. In addition, generally when amounts of the additive in excess of that e~uivalent to about 1% by weight of elemental boron are used, a boron carbide phase also precipitates.
The carbonaceous additive is used in an amount equivalent to 0.1% by weight to 1.0% by ~eight of free carbon based on the total amount of silicon carbide. Specifically, the carbonaceous additive is particulate free carbon of submicron size such as, for example, acetylene black, or a ..
carbonaceous organic material which is heat-decomposible to produce particulate free carbon of submicron size in the re~uired amount. In addition, the carbonaceous organic material is a solid or liquid at room temperature and completely decomposes at a temperature in the range of about 50C to 1000C to yield free carbon and gaseous products of decomposi-tion. Also, the carbonaceous organic material is one which has no significant deteriorating effect on the silicon carbide, boron additive or the resulting sintered product.
To produce the present sintered product having a density of at least 80%, the oxygen content of the silicon carbide powder should be less than 1% by weight of the total amount of 9 ilicon carbide used, and preferably, less than about 0.4% by weight. m is oxygen content is determinable by standard techniques and generally, it is present largely in the form of silica.
The function of free carbon in the present process, is to reduce silica which always is present in silicon carbide powders in small amounts or which forms on heating from oxygen absorbed on the powder surfaces. The free carbon reacts during heating with silica according to the reactions:
SiO2 + 3C ~ SiC + 2C0. Silica, when present in the SiC
powders in appreciable amounts, halts densification of silicon carbide completely so that little or no shrinkage, i.e.
densification, is obtained.
The free carbon also acts as a getter for free silicon if present in the powders or if it is formed by the following reaction during heating up to the sintering temperature: SiO2 + 2SiC ~ 3Si + 2C0. The presence of silicon, just as the silica, tends to halt or retard densification of SiC.
The specific amount of submicron free carbon required in the present process depends largely upon the oxygen content in the starting SiC powder and ranges from about 0.1% to 1.0% by weight of the total amount of silicon carbide used. Specifically, green bodies of the present invention which contain about 1% by weight of free carbon that do not sinter also will not sinter with amounts of free carbon significantly in excess of 1% by weight to a density of at least 80%. Also, amounts of free carbon significantly in excess of 1% by weight function much like permanent pores in the sintered product limiting its ultimate achievable density and strength.
Free carbon in the form of a submicron powder can be admixed with the silicon carbide powder by a ~u~ber of conventional techniques such as, for example, jet milling or ball milling in a liquid dispersion.
In carrying out the present process, the carbo-naceous organic material can be introduced by a number of techniques and heat-decomposed before or after the green body is formed. If the carbonaceous organic material is a solid, it is preferably admixed in the form of a solution with the silicon carbide powder and boron additive to substantially coat the particles. The wet mixture can then be treated to remove the solvent, and the resulting dry mixture can be heated to decompose the carbonaceous organic material producing free carbon in situ before the mixture is formed into a green body.
If dèsired, the wet mixture can be formed into a green body and the solvent removed therefrom. In this way, a substantially uniform coating of the organic material on the silicon carbide powder is obtained which on decomposition produces a uniform distribution of free carbon. The green body is then heated to decompose the carbonaceous organic material to produce free carbon in situ and diffuse away gaseous products of decomposition before sintering initiates. The solvent can be removed by a number of techniques such as by evaporation or by freeze drying, i.e. subliming off the solvent in vacuum from the frozen dispersion. Likewise, if the carbonaceous organic material is a liquid, it can be admixed with the silicon carbide powder and boron additive, and the wet mixture heated to decompose the organic material and form free carbon, or the wet mixture can be formed into a green body which is then heated to decompose the organic material to form free carbon in situ and diffuse away gaseous products of decomposition. The heat-decomposition of the carbonaceous organic material should be carried out in an atmosphere in which the components being heated are substantially inert or which has no significant deteriorating effect on the components being heated such as argon or a vacuum. Preferably, the carbonaceous organic material in the green body is heat-decomposed in the sintering furnace as the temperature is being raised to sintering temperature.
High molecular weight aromatic compounds are the preferred carbonaceous organic materials for making the carbon addition since they ordinarily give on pyrolysis the required yield of particulate free carbon of submicron size. Examples of such aromatic compounds are a phenol-formaldehyde condensate-novolak which is soluble in acetone or higher alcohols, such as butyl alcohol, as well as many of the related condensation products, such as resorcinol-formaldehyde, aniline-formaldehyde, and cresolformaldehyde. Another satisfactory group of compounds are derivatives of polynuclear aromatic hydrocarbons contained in coal tar, such as dibenzanthracene and chrysene. A
preferred group of carbonaceous additivès- are polymers of aromatic hydrocarbons such as polyphenylene or polymethylphenylene which a~e soluble in aromatic hydrocarbons and yield on heat-decomposition up to 90% of free carbon.
.~
_ 9 _ 1 334677 R~7361 -Another approach to improved carbon distribution on a submicron particle size level is the application of jet milling. The silicon carbide powder is soaked with a solution of, for instance, a novolak resin in acetone,dried in air and heated up to 500C to 800C in nitrogen to pyrolyze the resin.
The actual amount of carbon introduced by this process is determined as weight gain after the pyrolysis or by analysis of free carbon. The powder with the added carbon is then ~et milled which greatly improves the distribution of carbon and eliminates major carbon grains in the sintered product.
A number of techniques can be used to shape the powder mixture into a green body. For example, the powder mixture can be extruded, injection molded, die-pressed isostatically pressed or slip cast to produce the green body of desired shape. Any lubricants, binders or similar materials used in shaping the powder mixture should have no significant deteriorating effect on the green body or the resulting sintered body. Such materials are preferably of the type which evaporate on heating at relatively low temperatures, preferably below 200C, leaving no significant residue. The green body, preferably, should have a density of at least 45% of the theoretical density for silicon carbide to promote densification during sintering and achieve attainment of the desired density of at least 80%.
Sintering of the green body is carried out in an atmosphere in which it is substantially inert, i.e. an atmosphere which has no significant deteriorating effect on its properties such as, for example, argon, helium or a vacuum. The sintering atmosphere can range from a substantial vacuum to atmospheric pressure.
Sintering is carried out at a temperature ranging from about 1950 C to about 2300C. The particular sintering - temperature is determinable empirically and depends largely on particle size, density of the green body, and final density desired in the sintered product with higher final densities requiring higher sintering temperatures. Also, lower sintering temperatures would be used with sintering atmospheres below atmospheric pressure. Specifically, the smaller the size of the particles in the green body and the higher its density, the lower is the required sintering temperature. Sintering temperatures lower than 1950 do not produce the present sintered bodies with a density of at least 80%. Temperature higher than 2300C can be used since the present process provides sufficient grain growth control but the use of temperatures significantly higher than 2300C provide no particular advantage and bring about evaporation of silicon carbide.
The sintered body of the present invention has a density ranging from ~0% to about 95% of the theoretical density for silicon carbide. m e product is composed of silicon carbide, boron or boron and boron carbide, and free elemental carbon. The composition of the silicon carbide-in the present product ranges from o~sic alone to a composition composed of 70% by weight a-SiC and 30% by weight ~-SiC.
The ~-SiC is present in the form of a substantially uniform microstructure in the form of elongated grains or platelets which, in the long dimension, may range from about 5 microns to about 150 microns with an average length ranging from about 10 microns to 30 microns, and preferably have a grain size of from about 5 microns to 25 microns in the long dimension with an average length of about 10 microns. ~he ~-SiC is present in the form of fine grains ranging from about 1 micron to about 10 microns with an average grain size of about 3 microns. me boron is present in an amount ranging from 0.3% by weight to 3%
by weight based on the total amount of silicon carbide. The boron is in solid solution with the ~-and ~-silicon carbides and also may be present as a boron carbide phase in a very fine-sized precipitated form detectable by X-ray analysis. m e boron or boron and boron carbide are substantially uniformly distributed throughout the sintered body. m e sintered body also contains from 0.1% to 1% by weight of free carbon based on the total amount of silicon carbide. The free carbon is in the form of particles, substantially submicron in size, which are substantially uniformly distributed throughout the sintered body.
Since the present sintered produce has a substantially stable microstructure, it retains its room temperature shape and mechanical properties at high temperatures. Specifically, the sintered product undergoes no significant change in density or mechanical properties after substantial exposure in air to temperatures ranging up to about 1700C and after substantial exposure in an atmosphere in which it is substantially inert such as argon to temperatures above 1700C ranging up to about 2300C. Such properties make it particularly useful for high temperature structural applications such as gas turbine blades.
Although, at temperatures of 2000C or higher, ~-SiC in the present sintered product will transform to ~-SiC, the newly form ~-SiC grains cannot grow significantly because they impinge on and are blocked by the substantial number of d-sic grain already present substantially uniformly throughout the product. As a result, any additional transformation of ~-SiC has no significant effect on shape or mechanical properties of the product.
The present invention makes it possible to fabricate complex shaped polycrystalline silicon carbide ceramic articles directly which heretofore could not be - manufactured or which were produced by expensive and tedious machining because of the hardness of the material. The present sintered product requires no machining and it can be made in the form of a useful complex shaped article, such as a gas turbine airfoil, an impervious crucible, a thin walled tube, a long rod, a spherical body, or a hollow shaped article such as a gas turbine blade. Specifically, the dimensions of the present sintered product differ from those of its green body by the extent of shrinkage, i.e. densification, which occurs during sintering. Also, the surface characteristics of the sintered body depend on those of the green body from which it is formed, i.e. it has a substantially smooth surface if the green body from which it is formed has a smooth surface.
m e invention is further illustrated by the following Examples which, unless otherwise noted, were carried out as followed:
All sintering and firing was carried out in a carbon-element resistor furnace by bringing the furnace up to sintering or firing temperature in about one hour, holding at sintering or firing temperature for 20 minutes, shutting the furnace off and furnace-cooling to room temperature.
~-SiC powder used had an average particle size of 0.17 micron.
~-SiC powder used had an average particle size of 0.32 micron.
The powder dispersion was pressed into a green body in the form of a cylinder, 1.5 cm x 1.5 cm, which had a density of 55% of the theoretical density for silicon carbide.
% Density given herein is fractional % of the theoretical density for silicon carbide.
Sintered and fired products were subjected to metallographic analyses and X-ray analyses.
EX~MPLE 1 A carbon-rich silicon carbide powder prepared by a pyrolytic process was used. Specifically, it was a powder disper~ion, submicron in size, consisting essentially of cubic ~ -silicon carbide with free carbon uniformly and intimately dispersed therein in an amount of 0.35% by weight of the ~ -SiC, The ~-SiC contained 0.17% by weight 2' had an average particle size of 0.17 micron and a surface area of 9.2 m2/g. This powder dispersion was ball milled with particles of amorphous elemental boron of submicron size to produce a uniform powder dispersion containing 0.4% by weight boron baced on the ~ SiC, A portion of the resulting powder dispersion was pressed into a cylinder which was sintered in argon at a temperature of 2020 C, The resulting sintered product was Px~mined and the results are shown as Example 1 in Table I. Specifically, the sintered product had a uniform microstructure of ~-SiC such as that shown in FIGURE 1.
m e sintered product of Example 1 was fired at a temperature of 2080C.
Examination of the resulting product showed that raising the temperature from 2020C to 2080C resulted in a 4%
transformation into (~-sic, which phase appeared in the form of large plates, twenty times larger than the average grain size of the ~-SiC matrix.
Ex~MæLE 3 The product of Example 2 was fired at a temperature of 2150 C.
Examination of the resulting product showed that this further increase in temperature resulted in a high degree of conversionto ~-SiC accompanied by catastrophic exaggerated grain growth of huge ~-grains of feather-like morphology as shown in FIGURE 2. This microstructure development is manifested by a substantial decrease in strength as shown by additional specimens which were prepared in the form of bars 4 mm x 4 mm x 40 mm according to Example 2 and which had at room temperature in three point bending a modulus of rupture, i.e. strength, of 80,000 psi, and which after firing at a temperature of 2150C showed a modulus of rupture of 40,000 psi.
This example illustrates the present invention utilizing ~-SiC.
The procedure used in this example was the same as set forth in Example 1 except as shown in Table I.
To prepare the submicron a-SiC, abrasive grade silicon carbide, 325 mesh grit, was milled in an aqueous dispersion in a steel jar with steel balls for 50 hours. m e product was then repeatedly leached with concentrated HCl and washed with distilled water until all iron cont~m~nation due to ball wear was removed, filtered and dried. The resulting powder was dispersed in water to obtain a 2% dispersion which was stabilized by the addition of 1 cc sodium silicate solution per 500 g. of SiC. m e liquid was left standing for all particles of about one micron or larger to settle. The dispersion was siphoned off and the submicron SiC was recovered from the dispersion by an addition of nitric acid to obtain pH3, filtered and dried. The resulting powder contained particles ranging up to one micron.
m e resulting submicron powder was characterized and found to consist essentially of ~-SiC with 0.2% by weight 2 and 0.2% by weight of free carbon. Th-e d-sic particles had a surface area of 5.5 m2/g, an average particle size of 0.32 micron and X-ray analysis showed it to be composed of ~-SiC polytypes 6H, 15R (4H,3C).
A portion of the powder dispersion prepared in Example 1, which consisted essentially of ~-SiC, and based on the ~ -SiC, 0.35% by weight free carbon and 0.4% by weight boron, was used in this example. To this dispersion there was added the a-SiC having an average size of 0.32 micron in an amount of 0.1% by weight based on the amount of ~-SiC.
The resulting mixture was balled milled in benzene in a plastic jar with cemented balls. After 5 hours of milling, the benzene was removed by evaporation and the resulting powder was pressed into a green cylinder which was sintered at 2080C. The resulting sintered product was examined and found to have a uniform microstructure. The results as shown in Table I
illustrate that the addition of only 0.1% of ~-SiC to the starting powder brought about a high degree of conversion to ~-SiC on sintering at 2080C which ~-SiC phase crystallized in the form of a uniform network of platelet-like grains.
The sintered product of Example 4 was fired at 2180C.
F.x~m; nation of the resulting product showed that although additional ~-SiC phase formed after firing at 2180C, the platelet-like grains of the product of Example 4 grew relatively very little, i.e. to 48 microns, and that the microstructure retained its uniformity.
The product of Example 5 was fired at 2250C.
Examination of the resulting product showed that although still additional ~-SiC phase formed after firing at 2250C, the average d-sic grain size;grew very little, i.e. to 67 microns, which illustrates the stability of the microstructure of the present sintered product to temperature fluctuation.
In these examples, which also illustrate the present invention, the procedure and materials used were the same as set forth in Example 3 except as shown in Table I.
The green body of Example 9 consisted essentially of ~-SiC, 5% by weight of ~-SiC based on the ~-SiC, and based on the total amount of SiC about 0.35% by weight of free carbon and about 0.38% by weight of boron.
,_ o o o O ~ ~ ~ $ ~~/ ~
a~
01 ~
_ P~¦ D
~!41 5 1 1 0 J
r '~ U ~ ~
W I ~ U~ ~ ' '~ ~
Y ' rl J
Hl ~ r ~ D I r ~ r 1 r O ~ ~ 4 r- r rs P~l ~
01 ~ 0 .rl -rl ~1 1 + + h ; N I O O O I t~ ~ D
rl Irl O~1 ~ ~D ~ ~ N ~
Wl-- ------------------____________________________~r Wl ,1 r~ D 1 l N ~ ~ l I ~D r r-l N ~ l >
H¦ ~D
H I ~ O O11 ) d' ~ Oll') 1 tnl~ ~ . . . . .
D .r~ r~lr~l d' ~r~l/ U
¢ C~ 01) 0 CD
~1 ~¦ I ~
H l ~ O O O O O O O O O
5 C~) N Ct) u~ r~l~r l r~l ~t J O O O rl rl N r~r~l rl lr E- ~ ~ N N N N N N ~ N _N__ !~ D r~ r~
H¦ {~ N r o ~, t, 'ql l ~ ~-- .¢ b a`
.~ ~-.'i G ~1 4 O
,_ O o J ~ I ~
' O '~ O
r~-r~ r , I r J U
ID I ID ID ~ ID r~l V
~ ~D '5 ~ '5 D '5~ _1 '5 r r~ /D J raD N ) d' i ~a ~
r; _ ~ h r_~h U~ O O
Q t~ O r~ X r'l X ~ ~r j X r~ X
Cq ~ ~ ~ OU~ E~l ~ Ed O ~
ID -I
, r~l N ~r) d'Ir) ~ ~ C~ ~ * +
-It can be seen from Examples 4 to 9 in Table I, which illustrate the present invention, that the addition of d-SiC results in a lower terminal density of the sintered product which cannot be further increased significantly by increasing the sintering temperature. Table I illustrates that the terminal density obtained on sintering is determined by the amount of C~-sic used for seeding as demonstrated by Examples 4, 7, 8 and 9 where increases in the amount of -SiC
resulted in decreases in terminal densities.
Also, the sintered or fired products of Examples 4 to 9 showed particles of free carbon, substantially submicron in size, and present in an amount of less than about 0.5% by weight of the total amount of silicon carbide distributed substantially uniformly throughout each product. Also, analysis showed the boron to be in solid solution with the silicon carbide substantially uniformly throughout the product. In addition, the sintered cylinders had a smooth surface since the green bodies from which they were formed had a smooth surface.
The ~-silicon powder used in this example was the same as that used in Example 1 except that it contained 0.05%
by weight of free carbon. This powder was mixed with, submicron in size particles and based on the amount of ~-SiC, 0.3% by weight of acetylene black and 0.4% by weight of amorphous boron.
me mixture was ball milled in a solution of 1 g. of polyethylene-glycol per 100 cc of benzene. 200 cc of the solution was used per 100 g. of the powder mi~ture. After 5 hours milling with cemented carbide balls the slurry was spray-dried.
A portion of the resulting ball milled powder was pressed into cylinders which were sintered in argon at 2130C.
The sintered cylinders were subjected to X-ray diffraction analysis and found to consist on the average of 80% by weight ~ -SiC and 20% by weight ~-SiC. The microstructure was characterized by large feather-like ~-SiC crystals in a fine-grained ~-SiC matrix and the density was 95.5% of theoretical.
These SiC sintered cylinders were used as grinding balls with the remaining portion of the ball milled ~ -SiC
powder mixture to introduce ~-SiC into the powder mixture by wear of the cylinders. After 8 hours, the resulting a-SiC
contAi n; ng powder was pressed into pellets which were sintered under the same conditions at 2130 C in argon. m e sintered pellets had a density of 93.5%, a phase composition which was 100% ~-SiC and a substantially uniform microstructure composed of a network of platy ~-SiC crystals.
These sintered pellets were then fired at 2175 C
in argon. The resulting pellets showed the same microstructure as obtained at 2130C. The ~-SiC crystals were about 40 microns long and are shown in FIGURE 3. This illustrates that the exaggerated grain growth observed in the sintered pellets processed with cemented carbide balls containing no ~-SiC
could be eliminated entirely by the present invention.
,. . ~ .,
Hot pressing of silicon carbide powder has been used to produce small dense bodies under closely controlled conditions. However, hot-pressing methods require high pressures and temperatures necessitating expensive energy consuming machinery equipped with graphite dies. Also, hot-pressing yields pressed bodies in the form of billets of simple geometric shape only which require time-consuming machining to produce a complex shaped part.
Silicon carbide powder alone is not sinterable.
However, in my Canadian application Serial No. 208,705, filed September 9, 1974, there is disclosed a method of producing a ~ -silicon carbide sintered body by forming a mixture of submicron powder composed of ~-silicon carbide, a boron additive and free carbon into a green body and sintering it at a temperature of about 1900-2100 C.
me onset of exaggerated growth of large tabular ~-silicon carbide crystals on densification of ~-silicon carbide powders doped with boron is a limitation to obtaining the uniform fine-grained microstructures necessary to withstand fracture, especially at temperatures of the order of about 2000C. m is phenomenon is related to the transformation of ~-silicon carbide into the thermo-dynamically more stable o~Sic phase at temperatures of about 2000C and higher.
While several means to suppress this kind of grain growth on hot pressing have been devised, none of them is currently applicable to sintering. m us, for instance, _ 1 334677 RD-7361 when the hot-pressing temperature is decreased and compensated for by increased pressing pressure, conditions may be found where exaggerated grain growth does not occur. Also various additions such as aluminum, silicon nitride, aluminum nitride, and boron nitride to ~-SiC powder have been proven effective in controlling the growth of tabular ~ crystals on hot-pressing:
these means, however, cannot be used for sintering as they interfere with the densification process and prevent obtaining high densities.
The present process relates to improved grain growth control on sintering of silicon carbide by transforming a substantial mass of the sintering ~-SiC powder to the a form, i.e. into the thermodynamically more stable form, by seeding with an addition of ~-SiC. The ~-SiC nuclei thus provided induce a rapid ~ to ~ transformation during sintering. m e growing ~-SiC grains impinge on each other early in their development, cease to grow and result in a sintered product with a substantially uniform, relatively fine-grained micro-structure wherein at least 70% by weight of the silicon carbide present is composed of ~-SiC in the form of platelets or elon-gated grains which may range in the long dimension from about 5 to 150 microns, and preferably from about 5 to 25 microns.
m e present invention provides a number of advantages.
One advantage is that since the present process provides grain growth control, sintering can be carried out through a wide temperature range which is particularly economical and practical since it eliminates the need for critical temperature controls. The second advantage is that the presentsintered produce has a shape and mechanical properties which do not change significantly through a wide temperature range, i.e.
temperatures ranging from substantially below 0C to temperatures higher than 2300C.
1 334677 R~7361 -m ose skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below, considered in conjunction with th~ figures accompanying and forming a part of the specification, in which:
FIGURE 1 is a photomicrograph (magnified 500 X) of an etched specimen produced according to the present invention but without the addition of ~-SiC, sintered at 2080C
illustrating the uniform microstructure of ~-SiC.
FIGURE 2 is a photomicrograph (magnified 500X) of an etched specimen of the same initial composition as FIGURE 1 sintered at 2150C showing a feather-like morphology of large grains of ~-SiC polytypes in a matrix of ~ -SiC.
FIGURE 3 is a photomicrograph (magnified 500 X) of an etched specimen prepared in accordance with the present invention sintered at 2175C showing a substantially uniform microstructure of ~-SiC.
Briefly stated, the process of the present invention comprises providing a substantially homogeneous particulate dispersion or mixture, wherein the particles are submicron in size, of ~-silicon carbide powder, ~-silicon carbide seeding powder, boron additive and a carbonaceous additive which is free carbon or a carbonaceous organic material which is heat-decomposible to produce free carbon, shaping the mixture into a green body, and sintering the green body at a temperature ranging from about 1950C to 2300 C in an atmosphere in which the green body and resulting sintered body having a density of at least 80% of the~theoretical density for silicon carbide and a significantly uniform microstructure wherein at least 70% by weight of the silicon carbide is ~-SiC.
In the present invention single phase ~ -silicon carbide powder is used having an average particle size ranging , up to about 0.45 micron, and generally from about 0.05 micron to 0.4 micron. A~ a practical matter and for best results the ~-SiC powder preferably ranges in size from an average particle size of about 0.1 micron to 0.2 micron. ~-silicon carbide powder of this size can be prepared by a number of techniques as, for example, by direct synthesis from the elements, by reduction of silica, or by pyrolysis of compounds containing silicon and caxbon. A number of processes which involve the pyrolysis of silicon compounds and organic compounds to produce silicon and carbon are particularly advantageous since they can be controlled to produce ~-silicon carbide of desired submicron particle size composed mainly of isolated crystallites.
Hot plasma techniques are especially preferred for producing the powders useful in the present invention. The final product generally requires leaching, especially with acid to remove any silicon which may be present, to produce a sinterable phase pure ~-silicon carbide powder.
In the present process, to achieve the desired grain growth control, the particle size of the a-SiC seed powder should be at least about twice as large as the average particle size of the ~-SiC. Also, the ~-SiC seed powder is always submicron in size, and generally has a particle size ranging from about 0.1 micron to about 0.6 micron. All polytype compositions of the ~-SiC are operable in the present invention.
The present fine-size ~-SiC can be prepared by a number of techniques. For example, abrasive grade silicon carbide, which is always totally ~-SiC, can be milled and the milled powder admixed with a liquid,such as water to separate fractions of large and fine-sized particles by se~i -~tation. Specifically, the large-sized particles are allowed to settle and the liquid in which the desired 1 334677 R~7361 finer-sized particles float is decanted and evaporated to yield the fine-sized, submicron particle fraction.
The ~-SiC powder is used in amounts ranging from about 0.05% by weight to 5% by weight based on the ~-SiC. The larger the amount of ~-SiC used, the lower is the density of the sintered product. Amounts of a-SiC ranging from 1% by weight to 3% by weight based on the ~ -SiC produce the finest and most uniform microstructures. Amounts of ~-SiC powder larger than 5% by weight are likely to produce a sintered pro-duct having a density lower than 80%, and amounts of ~-SiC
smaller than 0.05% by weight do not provide sufficient nuclei for grain growth control.
The d-sic powder is admixed with ~-SiC powder alone, or with ~-SiC powder containing the boron additive and/or carbonaceous additive to produce a homogeneous dispersion.
Specifically, the 0~SiC should be dispersed through ~ -SiC
powder substantially uniformly in order to produce a sintered product with the desired uniform microstructure.
The~-SiC powder can be admixed with the ~-SiC
powder by a number of techniques such as, for example, ball milling or jet milling, to attain the necessary uniform distribution and produce a substantially homogeneous dispersion.
One technique for introducing ~-SiC powder into ~-SiC powder utilizes milling balls formed of silicon carbide containing C~-SiC in significant amount, i.e. at least 10% by weight. In this technique ~ -SiC powder is milled with the SiC balls which introduce d-SiC seeds by wear due to milling. Milling is preferably carried out in a liquid dispersion. The a~ount of -~-SiC introduced is controlled by controlling milling time. Introduction of the proper amount of ~ -SiC is determinable empirically. For example, in accordance with the present process, the resulting powder can be sintered and the product sectioned and ~x~mined metallographically. The proper amount of ~-SiC has been introduced in accordance with the present invention when the product has a significantly uniform microstructure, contains ~-SiC in an amount of at least 70% by weight of the total amount of SiC and has a density of at least 80% of the theoretical density for SiC.
m e boron additive in the powder mixture from which the green body is shaped is in the form of elemental boron or boron carbide. In order to obtain significant densification - ~ during SinterinQ, the amount of boron additive is critical and is e~uivalent to about 0.3% to about 3.0% by weight of elemental boron based on the total amount of silicon carbide.
The particular amount of boron additive used is determinable empirically and depends largely on the degree of dispersion achieved in the mixture since the more thoroughly it is dispersed the more uniform is the density of the sintered product.
However, amounts of elemental boron below 0.3% by weight do not result in the necessary degree of densification whereas amounts of elemental boron greater than 3.0% by weight produce no significant additional densification and may deteriorate the oxidation resistance of the product. During sintering, the boron additive enters into solid solution with the silicon carbide. In addition, generally when amounts of the additive in excess of that e~uivalent to about 1% by weight of elemental boron are used, a boron carbide phase also precipitates.
The carbonaceous additive is used in an amount equivalent to 0.1% by weight to 1.0% by ~eight of free carbon based on the total amount of silicon carbide. Specifically, the carbonaceous additive is particulate free carbon of submicron size such as, for example, acetylene black, or a ..
carbonaceous organic material which is heat-decomposible to produce particulate free carbon of submicron size in the re~uired amount. In addition, the carbonaceous organic material is a solid or liquid at room temperature and completely decomposes at a temperature in the range of about 50C to 1000C to yield free carbon and gaseous products of decomposi-tion. Also, the carbonaceous organic material is one which has no significant deteriorating effect on the silicon carbide, boron additive or the resulting sintered product.
To produce the present sintered product having a density of at least 80%, the oxygen content of the silicon carbide powder should be less than 1% by weight of the total amount of 9 ilicon carbide used, and preferably, less than about 0.4% by weight. m is oxygen content is determinable by standard techniques and generally, it is present largely in the form of silica.
The function of free carbon in the present process, is to reduce silica which always is present in silicon carbide powders in small amounts or which forms on heating from oxygen absorbed on the powder surfaces. The free carbon reacts during heating with silica according to the reactions:
SiO2 + 3C ~ SiC + 2C0. Silica, when present in the SiC
powders in appreciable amounts, halts densification of silicon carbide completely so that little or no shrinkage, i.e.
densification, is obtained.
The free carbon also acts as a getter for free silicon if present in the powders or if it is formed by the following reaction during heating up to the sintering temperature: SiO2 + 2SiC ~ 3Si + 2C0. The presence of silicon, just as the silica, tends to halt or retard densification of SiC.
The specific amount of submicron free carbon required in the present process depends largely upon the oxygen content in the starting SiC powder and ranges from about 0.1% to 1.0% by weight of the total amount of silicon carbide used. Specifically, green bodies of the present invention which contain about 1% by weight of free carbon that do not sinter also will not sinter with amounts of free carbon significantly in excess of 1% by weight to a density of at least 80%. Also, amounts of free carbon significantly in excess of 1% by weight function much like permanent pores in the sintered product limiting its ultimate achievable density and strength.
Free carbon in the form of a submicron powder can be admixed with the silicon carbide powder by a ~u~ber of conventional techniques such as, for example, jet milling or ball milling in a liquid dispersion.
In carrying out the present process, the carbo-naceous organic material can be introduced by a number of techniques and heat-decomposed before or after the green body is formed. If the carbonaceous organic material is a solid, it is preferably admixed in the form of a solution with the silicon carbide powder and boron additive to substantially coat the particles. The wet mixture can then be treated to remove the solvent, and the resulting dry mixture can be heated to decompose the carbonaceous organic material producing free carbon in situ before the mixture is formed into a green body.
If dèsired, the wet mixture can be formed into a green body and the solvent removed therefrom. In this way, a substantially uniform coating of the organic material on the silicon carbide powder is obtained which on decomposition produces a uniform distribution of free carbon. The green body is then heated to decompose the carbonaceous organic material to produce free carbon in situ and diffuse away gaseous products of decomposition before sintering initiates. The solvent can be removed by a number of techniques such as by evaporation or by freeze drying, i.e. subliming off the solvent in vacuum from the frozen dispersion. Likewise, if the carbonaceous organic material is a liquid, it can be admixed with the silicon carbide powder and boron additive, and the wet mixture heated to decompose the organic material and form free carbon, or the wet mixture can be formed into a green body which is then heated to decompose the organic material to form free carbon in situ and diffuse away gaseous products of decomposition. The heat-decomposition of the carbonaceous organic material should be carried out in an atmosphere in which the components being heated are substantially inert or which has no significant deteriorating effect on the components being heated such as argon or a vacuum. Preferably, the carbonaceous organic material in the green body is heat-decomposed in the sintering furnace as the temperature is being raised to sintering temperature.
High molecular weight aromatic compounds are the preferred carbonaceous organic materials for making the carbon addition since they ordinarily give on pyrolysis the required yield of particulate free carbon of submicron size. Examples of such aromatic compounds are a phenol-formaldehyde condensate-novolak which is soluble in acetone or higher alcohols, such as butyl alcohol, as well as many of the related condensation products, such as resorcinol-formaldehyde, aniline-formaldehyde, and cresolformaldehyde. Another satisfactory group of compounds are derivatives of polynuclear aromatic hydrocarbons contained in coal tar, such as dibenzanthracene and chrysene. A
preferred group of carbonaceous additivès- are polymers of aromatic hydrocarbons such as polyphenylene or polymethylphenylene which a~e soluble in aromatic hydrocarbons and yield on heat-decomposition up to 90% of free carbon.
.~
_ 9 _ 1 334677 R~7361 -Another approach to improved carbon distribution on a submicron particle size level is the application of jet milling. The silicon carbide powder is soaked with a solution of, for instance, a novolak resin in acetone,dried in air and heated up to 500C to 800C in nitrogen to pyrolyze the resin.
The actual amount of carbon introduced by this process is determined as weight gain after the pyrolysis or by analysis of free carbon. The powder with the added carbon is then ~et milled which greatly improves the distribution of carbon and eliminates major carbon grains in the sintered product.
A number of techniques can be used to shape the powder mixture into a green body. For example, the powder mixture can be extruded, injection molded, die-pressed isostatically pressed or slip cast to produce the green body of desired shape. Any lubricants, binders or similar materials used in shaping the powder mixture should have no significant deteriorating effect on the green body or the resulting sintered body. Such materials are preferably of the type which evaporate on heating at relatively low temperatures, preferably below 200C, leaving no significant residue. The green body, preferably, should have a density of at least 45% of the theoretical density for silicon carbide to promote densification during sintering and achieve attainment of the desired density of at least 80%.
Sintering of the green body is carried out in an atmosphere in which it is substantially inert, i.e. an atmosphere which has no significant deteriorating effect on its properties such as, for example, argon, helium or a vacuum. The sintering atmosphere can range from a substantial vacuum to atmospheric pressure.
Sintering is carried out at a temperature ranging from about 1950 C to about 2300C. The particular sintering - temperature is determinable empirically and depends largely on particle size, density of the green body, and final density desired in the sintered product with higher final densities requiring higher sintering temperatures. Also, lower sintering temperatures would be used with sintering atmospheres below atmospheric pressure. Specifically, the smaller the size of the particles in the green body and the higher its density, the lower is the required sintering temperature. Sintering temperatures lower than 1950 do not produce the present sintered bodies with a density of at least 80%. Temperature higher than 2300C can be used since the present process provides sufficient grain growth control but the use of temperatures significantly higher than 2300C provide no particular advantage and bring about evaporation of silicon carbide.
The sintered body of the present invention has a density ranging from ~0% to about 95% of the theoretical density for silicon carbide. m e product is composed of silicon carbide, boron or boron and boron carbide, and free elemental carbon. The composition of the silicon carbide-in the present product ranges from o~sic alone to a composition composed of 70% by weight a-SiC and 30% by weight ~-SiC.
The ~-SiC is present in the form of a substantially uniform microstructure in the form of elongated grains or platelets which, in the long dimension, may range from about 5 microns to about 150 microns with an average length ranging from about 10 microns to 30 microns, and preferably have a grain size of from about 5 microns to 25 microns in the long dimension with an average length of about 10 microns. ~he ~-SiC is present in the form of fine grains ranging from about 1 micron to about 10 microns with an average grain size of about 3 microns. me boron is present in an amount ranging from 0.3% by weight to 3%
by weight based on the total amount of silicon carbide. The boron is in solid solution with the ~-and ~-silicon carbides and also may be present as a boron carbide phase in a very fine-sized precipitated form detectable by X-ray analysis. m e boron or boron and boron carbide are substantially uniformly distributed throughout the sintered body. m e sintered body also contains from 0.1% to 1% by weight of free carbon based on the total amount of silicon carbide. The free carbon is in the form of particles, substantially submicron in size, which are substantially uniformly distributed throughout the sintered body.
Since the present sintered produce has a substantially stable microstructure, it retains its room temperature shape and mechanical properties at high temperatures. Specifically, the sintered product undergoes no significant change in density or mechanical properties after substantial exposure in air to temperatures ranging up to about 1700C and after substantial exposure in an atmosphere in which it is substantially inert such as argon to temperatures above 1700C ranging up to about 2300C. Such properties make it particularly useful for high temperature structural applications such as gas turbine blades.
Although, at temperatures of 2000C or higher, ~-SiC in the present sintered product will transform to ~-SiC, the newly form ~-SiC grains cannot grow significantly because they impinge on and are blocked by the substantial number of d-sic grain already present substantially uniformly throughout the product. As a result, any additional transformation of ~-SiC has no significant effect on shape or mechanical properties of the product.
The present invention makes it possible to fabricate complex shaped polycrystalline silicon carbide ceramic articles directly which heretofore could not be - manufactured or which were produced by expensive and tedious machining because of the hardness of the material. The present sintered product requires no machining and it can be made in the form of a useful complex shaped article, such as a gas turbine airfoil, an impervious crucible, a thin walled tube, a long rod, a spherical body, or a hollow shaped article such as a gas turbine blade. Specifically, the dimensions of the present sintered product differ from those of its green body by the extent of shrinkage, i.e. densification, which occurs during sintering. Also, the surface characteristics of the sintered body depend on those of the green body from which it is formed, i.e. it has a substantially smooth surface if the green body from which it is formed has a smooth surface.
m e invention is further illustrated by the following Examples which, unless otherwise noted, were carried out as followed:
All sintering and firing was carried out in a carbon-element resistor furnace by bringing the furnace up to sintering or firing temperature in about one hour, holding at sintering or firing temperature for 20 minutes, shutting the furnace off and furnace-cooling to room temperature.
~-SiC powder used had an average particle size of 0.17 micron.
~-SiC powder used had an average particle size of 0.32 micron.
The powder dispersion was pressed into a green body in the form of a cylinder, 1.5 cm x 1.5 cm, which had a density of 55% of the theoretical density for silicon carbide.
% Density given herein is fractional % of the theoretical density for silicon carbide.
Sintered and fired products were subjected to metallographic analyses and X-ray analyses.
EX~MPLE 1 A carbon-rich silicon carbide powder prepared by a pyrolytic process was used. Specifically, it was a powder disper~ion, submicron in size, consisting essentially of cubic ~ -silicon carbide with free carbon uniformly and intimately dispersed therein in an amount of 0.35% by weight of the ~ -SiC, The ~-SiC contained 0.17% by weight 2' had an average particle size of 0.17 micron and a surface area of 9.2 m2/g. This powder dispersion was ball milled with particles of amorphous elemental boron of submicron size to produce a uniform powder dispersion containing 0.4% by weight boron baced on the ~ SiC, A portion of the resulting powder dispersion was pressed into a cylinder which was sintered in argon at a temperature of 2020 C, The resulting sintered product was Px~mined and the results are shown as Example 1 in Table I. Specifically, the sintered product had a uniform microstructure of ~-SiC such as that shown in FIGURE 1.
m e sintered product of Example 1 was fired at a temperature of 2080C.
Examination of the resulting product showed that raising the temperature from 2020C to 2080C resulted in a 4%
transformation into (~-sic, which phase appeared in the form of large plates, twenty times larger than the average grain size of the ~-SiC matrix.
Ex~MæLE 3 The product of Example 2 was fired at a temperature of 2150 C.
Examination of the resulting product showed that this further increase in temperature resulted in a high degree of conversionto ~-SiC accompanied by catastrophic exaggerated grain growth of huge ~-grains of feather-like morphology as shown in FIGURE 2. This microstructure development is manifested by a substantial decrease in strength as shown by additional specimens which were prepared in the form of bars 4 mm x 4 mm x 40 mm according to Example 2 and which had at room temperature in three point bending a modulus of rupture, i.e. strength, of 80,000 psi, and which after firing at a temperature of 2150C showed a modulus of rupture of 40,000 psi.
This example illustrates the present invention utilizing ~-SiC.
The procedure used in this example was the same as set forth in Example 1 except as shown in Table I.
To prepare the submicron a-SiC, abrasive grade silicon carbide, 325 mesh grit, was milled in an aqueous dispersion in a steel jar with steel balls for 50 hours. m e product was then repeatedly leached with concentrated HCl and washed with distilled water until all iron cont~m~nation due to ball wear was removed, filtered and dried. The resulting powder was dispersed in water to obtain a 2% dispersion which was stabilized by the addition of 1 cc sodium silicate solution per 500 g. of SiC. m e liquid was left standing for all particles of about one micron or larger to settle. The dispersion was siphoned off and the submicron SiC was recovered from the dispersion by an addition of nitric acid to obtain pH3, filtered and dried. The resulting powder contained particles ranging up to one micron.
m e resulting submicron powder was characterized and found to consist essentially of ~-SiC with 0.2% by weight 2 and 0.2% by weight of free carbon. Th-e d-sic particles had a surface area of 5.5 m2/g, an average particle size of 0.32 micron and X-ray analysis showed it to be composed of ~-SiC polytypes 6H, 15R (4H,3C).
A portion of the powder dispersion prepared in Example 1, which consisted essentially of ~-SiC, and based on the ~ -SiC, 0.35% by weight free carbon and 0.4% by weight boron, was used in this example. To this dispersion there was added the a-SiC having an average size of 0.32 micron in an amount of 0.1% by weight based on the amount of ~-SiC.
The resulting mixture was balled milled in benzene in a plastic jar with cemented balls. After 5 hours of milling, the benzene was removed by evaporation and the resulting powder was pressed into a green cylinder which was sintered at 2080C. The resulting sintered product was examined and found to have a uniform microstructure. The results as shown in Table I
illustrate that the addition of only 0.1% of ~-SiC to the starting powder brought about a high degree of conversion to ~-SiC on sintering at 2080C which ~-SiC phase crystallized in the form of a uniform network of platelet-like grains.
The sintered product of Example 4 was fired at 2180C.
F.x~m; nation of the resulting product showed that although additional ~-SiC phase formed after firing at 2180C, the platelet-like grains of the product of Example 4 grew relatively very little, i.e. to 48 microns, and that the microstructure retained its uniformity.
The product of Example 5 was fired at 2250C.
Examination of the resulting product showed that although still additional ~-SiC phase formed after firing at 2250C, the average d-sic grain size;grew very little, i.e. to 67 microns, which illustrates the stability of the microstructure of the present sintered product to temperature fluctuation.
In these examples, which also illustrate the present invention, the procedure and materials used were the same as set forth in Example 3 except as shown in Table I.
The green body of Example 9 consisted essentially of ~-SiC, 5% by weight of ~-SiC based on the ~-SiC, and based on the total amount of SiC about 0.35% by weight of free carbon and about 0.38% by weight of boron.
,_ o o o O ~ ~ ~ $ ~~/ ~
a~
01 ~
_ P~¦ D
~!41 5 1 1 0 J
r '~ U ~ ~
W I ~ U~ ~ ' '~ ~
Y ' rl J
Hl ~ r ~ D I r ~ r 1 r O ~ ~ 4 r- r rs P~l ~
01 ~ 0 .rl -rl ~1 1 + + h ; N I O O O I t~ ~ D
rl Irl O~1 ~ ~D ~ ~ N ~
Wl-- ------------------____________________________~r Wl ,1 r~ D 1 l N ~ ~ l I ~D r r-l N ~ l >
H¦ ~D
H I ~ O O11 ) d' ~ Oll') 1 tnl~ ~ . . . . .
D .r~ r~lr~l d' ~r~l/ U
¢ C~ 01) 0 CD
~1 ~¦ I ~
H l ~ O O O O O O O O O
5 C~) N Ct) u~ r~l~r l r~l ~t J O O O rl rl N r~r~l rl lr E- ~ ~ N N N N N N ~ N _N__ !~ D r~ r~
H¦ {~ N r o ~, t, 'ql l ~ ~-- .¢ b a`
.~ ~-.'i G ~1 4 O
,_ O o J ~ I ~
' O '~ O
r~-r~ r , I r J U
ID I ID ID ~ ID r~l V
~ ~D '5 ~ '5 D '5~ _1 '5 r r~ /D J raD N ) d' i ~a ~
r; _ ~ h r_~h U~ O O
Q t~ O r~ X r'l X ~ ~r j X r~ X
Cq ~ ~ ~ OU~ E~l ~ Ed O ~
ID -I
, r~l N ~r) d'Ir) ~ ~ C~ ~ * +
-It can be seen from Examples 4 to 9 in Table I, which illustrate the present invention, that the addition of d-SiC results in a lower terminal density of the sintered product which cannot be further increased significantly by increasing the sintering temperature. Table I illustrates that the terminal density obtained on sintering is determined by the amount of C~-sic used for seeding as demonstrated by Examples 4, 7, 8 and 9 where increases in the amount of -SiC
resulted in decreases in terminal densities.
Also, the sintered or fired products of Examples 4 to 9 showed particles of free carbon, substantially submicron in size, and present in an amount of less than about 0.5% by weight of the total amount of silicon carbide distributed substantially uniformly throughout each product. Also, analysis showed the boron to be in solid solution with the silicon carbide substantially uniformly throughout the product. In addition, the sintered cylinders had a smooth surface since the green bodies from which they were formed had a smooth surface.
The ~-silicon powder used in this example was the same as that used in Example 1 except that it contained 0.05%
by weight of free carbon. This powder was mixed with, submicron in size particles and based on the amount of ~-SiC, 0.3% by weight of acetylene black and 0.4% by weight of amorphous boron.
me mixture was ball milled in a solution of 1 g. of polyethylene-glycol per 100 cc of benzene. 200 cc of the solution was used per 100 g. of the powder mi~ture. After 5 hours milling with cemented carbide balls the slurry was spray-dried.
A portion of the resulting ball milled powder was pressed into cylinders which were sintered in argon at 2130C.
The sintered cylinders were subjected to X-ray diffraction analysis and found to consist on the average of 80% by weight ~ -SiC and 20% by weight ~-SiC. The microstructure was characterized by large feather-like ~-SiC crystals in a fine-grained ~-SiC matrix and the density was 95.5% of theoretical.
These SiC sintered cylinders were used as grinding balls with the remaining portion of the ball milled ~ -SiC
powder mixture to introduce ~-SiC into the powder mixture by wear of the cylinders. After 8 hours, the resulting a-SiC
contAi n; ng powder was pressed into pellets which were sintered under the same conditions at 2130 C in argon. m e sintered pellets had a density of 93.5%, a phase composition which was 100% ~-SiC and a substantially uniform microstructure composed of a network of platy ~-SiC crystals.
These sintered pellets were then fired at 2175 C
in argon. The resulting pellets showed the same microstructure as obtained at 2130C. The ~-SiC crystals were about 40 microns long and are shown in FIGURE 3. This illustrates that the exaggerated grain growth observed in the sintered pellets processed with cemented carbide balls containing no ~-SiC
could be eliminated entirely by the present invention.
,. . ~ .,
Claims (5)
1. A method of producing a silicon carbide sintered body having temperature-resistant properties which comprises providing a substantially homogeneous submicron particulate mixture consisting essentially of silicon carbide composed of .beta.-SiC and from 0.05% to 5%
by weight of .alpha.-SiC based on said .beta.-SiC, said .beta.-SiC
particles having an average particle size ranging up to 0.45 micron and said .alpha.-SiC particles having a particle size of at least about twice as large as that of said .beta.
-SiC particles, an amount of boron additive selected from the group consisting of boron and boron carbide equivalent to 0.3% to 3.0% by weight of boron based on the total amount of silicon carbide, and an amount of a carbonaceous additive selected from the group consisting of free carbon and a carbonaceous organic material equivalent to 0.1% to 1.0% by weight of free carbon based on the total amount of silicon carbide, said carbonaceous organic material completely decomposing at a temperature ranging from about 50°C to 1000°C to said free carbon and gaseous products of decomposition, shaping the mixture into a green body, and sintering the green body at a temperature ranging from about 1950°C to 2300°C in an atmosphere in which it is substantially inert at atmospheric pressure or below atmospheric pressure to produce a sintered body having a density of at least 80% of the theoretical density for silicon carbide and containing .alpha.-SiC in an amount of at least 70% by weight of the total amount of silicon carbide.
by weight of .alpha.-SiC based on said .beta.-SiC, said .beta.-SiC
particles having an average particle size ranging up to 0.45 micron and said .alpha.-SiC particles having a particle size of at least about twice as large as that of said .beta.
-SiC particles, an amount of boron additive selected from the group consisting of boron and boron carbide equivalent to 0.3% to 3.0% by weight of boron based on the total amount of silicon carbide, and an amount of a carbonaceous additive selected from the group consisting of free carbon and a carbonaceous organic material equivalent to 0.1% to 1.0% by weight of free carbon based on the total amount of silicon carbide, said carbonaceous organic material completely decomposing at a temperature ranging from about 50°C to 1000°C to said free carbon and gaseous products of decomposition, shaping the mixture into a green body, and sintering the green body at a temperature ranging from about 1950°C to 2300°C in an atmosphere in which it is substantially inert at atmospheric pressure or below atmospheric pressure to produce a sintered body having a density of at least 80% of the theoretical density for silicon carbide and containing .alpha.-SiC in an amount of at least 70% by weight of the total amount of silicon carbide.
2. A method according to claim 1 wherein said green body is prepared by slip casting.
3. A sintered body having a density ranging from at least 80% to less than 95% of the theoretical density for silicon carbide consisting essentially of silicon carbide with boron or boron and boron carbide, and free carbon substantially uniformly distributed throughout said sintered body, said silicon carbide having a composition ranging from .alpha.-silicon carbide to that consisting essentially of 70% by weight .alpha.-silicon carbide-30% by weight .beta.-silicon carbide, said .alpha.-silicon carbine being distributed substantially uniformly throughout the .beta.-silicon carbide, said .alpha.-silicon carbide being present in the form of a significantly uniform microstructure in the form of elongated grains or platelets ranging from about 5 microns to 150 microns in size, said .beta.-silicon carbide being present in the form of a significantly uniform microstructure having a grain size ranging from about 1 micron to about 10 microns, said boron being present in an amount ranging from 0.3% to about 3% by weight based on the total amount of silicon carbide, said boron being in solid solution with said .alpha.-and .beta.-silicon carbides or being in solid solution with said .alpha.-and .beta.-silicon carbides and also being present as a precipitated boron carbide phase, said free carbon being in the form of particles substantially submicron in size present in an mount ranging from 0.1% by weight to 1% by weight based on the total amount of silicon carbide.
4. The sintered body according to claim 3 in the form of a tube.
5. The sintered body according to claim 3 in the form of a hollow gas turbine blade.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US59184075A | 1975-06-30 | 1975-06-30 | |
| US591,840 | 1975-06-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1334677C true CA1334677C (en) | 1995-03-07 |
Family
ID=24368166
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000254996A Expired - Fee Related CA1334677C (en) | 1975-06-30 | 1976-06-16 | Silicon carbide sintered body |
Country Status (9)
| Country | Link |
|---|---|
| JP (3) | JPS526716A (en) |
| CA (1) | CA1334677C (en) |
| CH (1) | CH623018A5 (en) |
| DE (1) | DE2627856A1 (en) |
| FR (1) | FR2316028A1 (en) |
| GB (1) | GB1533675A (en) |
| IT (1) | IT1063441B (en) |
| NL (1) | NL183645C (en) |
| NO (1) | NO143400C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN108558405A (en) * | 2017-03-10 | 2018-09-21 | 成都超纯应用材料有限责任公司 | A kind of preparation method of high-compactness high-purity carborundum substrate material |
Families Citing this family (24)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS526716A (en) * | 1975-06-30 | 1977-01-19 | Gen Electric | Silicon carbide sintered articles |
| DE2744636A1 (en) * | 1977-10-04 | 1979-05-17 | Wolfgang Dipl Ing Boecker | METHOD AND DEVICE FOR MANUFACTURING HIGHLY PURE SILICON CARBIDE POWDER AND ITS USE |
| US4238434A (en) * | 1978-02-16 | 1980-12-09 | Ibigawa Electric Industry Co., Ltd. | Method of producing a silicon carbide sintered body |
| DE2953430C2 (en) * | 1978-12-21 | 1985-07-04 | Evgenij Aleksandrovič Antonov | Process for the production of hollow products by deep drawing and pressing to carry out this process |
| DE3005587A1 (en) * | 1980-02-15 | 1981-08-20 | Kernforschungsanlage Jülich GmbH, 5170 Jülich | METHOD FOR PRODUCING MOLDED BODIES |
| JPS57196769A (en) * | 1981-05-26 | 1982-12-02 | Ibigawa Electric Ind Co Ltd | Manufacture of silicon carbide sintered body |
| JPS57196768A (en) * | 1981-05-26 | 1982-12-02 | Ibigawa Electric Ind Co Ltd | Manufacture of high strength silicon carbide sintered body |
| JPS5974609U (en) * | 1982-11-11 | 1984-05-21 | 東芝機器株式会社 | lighting equipment |
| JPS6046974A (en) * | 1983-08-26 | 1985-03-14 | 信越化学工業株式会社 | Silicon carbide sintered body |
| JPS6064686U (en) * | 1983-10-07 | 1985-05-08 | 富士通株式会社 | Signal interface circuit with optical switch |
| JPS60155572A (en) * | 1984-01-24 | 1985-08-15 | 科学技術庁無機材質研究所長 | Manufacture of high heat conductivity silicon carbide sintered body |
| JPH0625038B2 (en) * | 1985-05-22 | 1994-04-06 | 住友化学工業株式会社 | Method for manufacturing wear-resistant silicon carbide sintered body |
| JPS6256368A (en) * | 1985-09-06 | 1987-03-12 | 株式会社東芝 | Manufacture of silicon carbide sintered body |
| JPH0777987B2 (en) * | 1986-04-14 | 1995-08-23 | 東芝セラミツクス株式会社 | Silicon carbide rod for suspended firing |
| US5192719A (en) * | 1986-09-01 | 1993-03-09 | Ibiden Co., Ltd. | Method of producing high-strength β-type silicon carbide sintered bodies |
| JPS6360158A (en) * | 1986-09-01 | 1988-03-16 | イビデン株式会社 | Manufacture of silicon carbide sintered body |
| DE3718095A1 (en) * | 1987-05-29 | 1988-12-08 | Hutschenreuther | POLYCRYSTALLINE MOLDED BODY FROM SILICON CARBIDE AND METHOD FOR THE PRODUCTION THEREOF |
| DE3736660A1 (en) * | 1987-10-29 | 1989-05-11 | Mtu Muenchen Gmbh | METHOD FOR PRODUCING A POROUS FORM BODY |
| KR910002578B1 (en) * | 1988-01-19 | 1991-04-27 | 닙폰 가이시 카부시키카이샤 | Method for producing a high density sintered body of silicon carbide |
| US5182059A (en) * | 1989-01-17 | 1993-01-26 | Ngk Insulators, Ltd. | Process for producing high density SiC sintered bodies |
| DE3902528A1 (en) * | 1989-01-28 | 1990-10-31 | Sintec Keramik Gmbh | Production of polycrystalline shaped bodies comprising alpha - and/or ss-silicon carbide by pressureless sintering |
| DE4041514C2 (en) * | 1990-12-22 | 1995-05-24 | Forschungszentrum Juelich Gmbh | Process for producing dense sintered workpieces |
| JP2005298280A (en) * | 2004-04-13 | 2005-10-27 | Toto Ltd | Silicon carbide sintered compact and method for producing the same |
| NO335994B1 (en) | 2011-10-13 | 2015-04-13 | Saint Gobain Ceramic Mat As | Process for producing grains useful for the preparation of a silicon carbide-based sintered product, composite grains prepared by the process, and use of the grains. |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB632247A (en) * | 1947-07-04 | 1949-11-18 | Carborundum Co | Improvements in or relating to silicon carbide articles and methods of forming them |
| FR1235791A (en) * | 1958-09-17 | 1960-07-08 | Kempten Elektroschmelz Gmbh | Process for manufacturing articles of specific shape from carbides |
| GB1478898A (en) * | 1973-10-24 | 1977-07-06 | Gen Electric | Silicon carbide ceramic |
| US4312954A (en) * | 1975-06-05 | 1982-01-26 | Kennecott Corporation | Sintered silicon carbide ceramic body |
| JPS526716A (en) * | 1975-06-30 | 1977-01-19 | Gen Electric | Silicon carbide sintered articles |
-
1976
- 1976-05-27 JP JP51060693A patent/JPS526716A/en active Granted
- 1976-06-16 CA CA000254996A patent/CA1334677C/en not_active Expired - Fee Related
- 1976-06-18 GB GB25420/76A patent/GB1533675A/en not_active Expired
- 1976-06-22 DE DE19762627856 patent/DE2627856A1/en active Granted
- 1976-06-25 IT IT24751/76A patent/IT1063441B/en active
- 1976-06-25 CH CH819676A patent/CH623018A5/en not_active IP Right Cessation
- 1976-06-29 NO NO762247A patent/NO143400C/en unknown
- 1976-06-30 NL NLAANVRAGE7607252,A patent/NL183645C/en not_active IP Right Cessation
- 1976-06-30 FR FR7619872A patent/FR2316028A1/en active Granted
-
1982
- 1982-03-24 JP JP57045757A patent/JPS57170877A/en active Granted
- 1982-03-24 JP JP57045756A patent/JPS57170876A/en active Granted
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN108558405A (en) * | 2017-03-10 | 2018-09-21 | 成都超纯应用材料有限责任公司 | A kind of preparation method of high-compactness high-purity carborundum substrate material |
| CN108558405B (en) * | 2017-03-10 | 2021-08-24 | 成都超纯应用材料有限责任公司 | Preparation method of high-density high-purity silicon carbide substrate material |
Also Published As
| Publication number | Publication date |
|---|---|
| DE2627856C2 (en) | 1987-06-11 |
| IT1063441B (en) | 1985-02-11 |
| FR2316028A1 (en) | 1977-01-28 |
| CH623018A5 (en) | 1981-05-15 |
| FR2316028B1 (en) | 1981-02-13 |
| NO762247L (en) | 1977-01-03 |
| JPS6363513B2 (en) | 1988-12-07 |
| JPS5740109B2 (en) | 1982-08-25 |
| JPS526716A (en) | 1977-01-19 |
| NO143400B (en) | 1980-10-27 |
| DE2627856A1 (en) | 1977-01-20 |
| JPS57170876A (en) | 1982-10-21 |
| NO143400C (en) | 1981-02-04 |
| NL7607252A (en) | 1977-01-03 |
| JPS57170877A (en) | 1982-10-21 |
| NL183645B (en) | 1988-07-18 |
| JPS6363514B2 (en) | 1988-12-07 |
| GB1533675A (en) | 1978-11-29 |
| NL183645C (en) | 1988-12-16 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4041117A (en) | Silicon carbide sintered body | |
| CA1334677C (en) | Silicon carbide sintered body | |
| CA1088107A (en) | Silicon carbide-boron carbide sintered body | |
| US4692418A (en) | Sintered silicon carbide/carbon composite ceramic body having fine microstructure | |
| US4525461A (en) | Sintered silicon carbide/graphite/carbon composite ceramic body having ultrafine grain microstructure | |
| US4004934A (en) | Sintered dense silicon carbide | |
| US4354991A (en) | Dense sintered silicon carbide ceramic | |
| US4179299A (en) | Sintered alpha silicon carbide ceramic body having equiaxed microstructure | |
| US3853566A (en) | Hot pressed silicon carbide | |
| US4346049A (en) | Sintered alpha silicon carbide ceramic body having equiaxed microstructure | |
| JPH0131471B2 (en) | ||
| US4455385A (en) | Silicon carbide sintered body | |
| JPH0261435B2 (en) | ||
| JPS6228109B2 (en) | ||
| US4023975A (en) | Hot pressed silicon carbide containing beryllium carbide | |
| US4237085A (en) | Method of producing a high density silicon carbide product | |
| US4005235A (en) | Dense sintered boron carbide containing beryllium carbide | |
| JPH0251863B2 (en) | ||
| EP0178753B1 (en) | Process for producing a sintered silicon carbide/carbon composite ceramic body having ultrafine grain microstructure | |
| US5167887A (en) | SI3 N4 process using polysilane as a binder | |
| US5139719A (en) | Sintering process and novel ceramic material | |
| JPS649269B2 (en) | ||
| US5080844A (en) | SI3N4 process using polysilane or polysilazane as a binder | |
| JPS589785B2 (en) | Manufacturing method of silicon carbide sintered body | |
| JPH0253388B2 (en) |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKLA | Lapsed |