JPH0131471B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Silicon carbide is an excellent material for high temperature structural applications because of its chemical and physical properties.

These properties include excellent oxidation and corrosion resistance behavior, good heat transfer coefficient, low coefficient of thermal expansion, high thermal shock resistance, and high strength at high temperatures. This unique combination of properties means that silicon carbide can be used in gas turbine component elements, check valves for handling corrosive liquids, ball mill linings, heat exchangers and refractories for high temperature furnaces, pumps and combustion for die-casting machines. This suggests that it can be used as a pipe.

Previously, silicon carbide hot presses have been used to make small coupons under strictly controlled conditions. Unfortunately, silicon carbide is not easily sintered to a density close to its theoretical density of 3.21 g/cm ³ . A method of hot-pressing silicon carbide to a uniform density on the order of 98% of the theoretical density with slight additions of aluminum and iron to aid in densification was developed by Alliegro et al. .Ceram.
Soc.), Vol. 39, (November 1956), pp. 386-389.

Currently pending patent application No. 142567 (December 1972)
US Pat. describes an improved method for producing a dense silicon carbide ceramic by hot pressing the dispersion for a sufficient time to produce a dense non-porous silicon carbide ceramic. An advantage of boron as a sintering aid over other materials such as alumina, aluminum nitride and other metallic compounds is that boron provides increased oxidation and corrosion resistance at high temperatures. Subsequently, Patent Application No. 1979-079252 (July 13, 1973)
No. 378,918) discloses another improvement in the hot pressing of silicon carbide by incorporating a carbonaceous additive into a homogeneous dispersion of silicon carbide and boron-containing additive powder. did. The addition of carbon inhibits grain growth coarsening in the microstructure of the dense silicon carbide ceramic product and provides improved strength properties. but,
Hot pressing only produces superior materials in billet form with simple geometries, and such billets require expensive machining when complex-shaped parts are required. .

In accordance with the present invention, a method has been developed for producing high density silicon carbide ceramics by forming a homogeneous dispersion of submicron powder consisting essentially of silicon carbide, a boron-containing additive, and a carbonaceous additive.
This dispersion is then formed into a green excipient and under a controlled atmosphere inert to silicon carbide at a temperature of about 1900-2100°C to form a silicon carbide excipient having a density of at least 85% of the theoretical density. sintered. The preferred product obtained has a density of at least 98% of theoretical density. This is suitable as an industrial material, for example in high temperature gas turbine applications.

In order to obtain high density and strength during sintering, the powder dispersion must be a mixture of fine particles of less than a micron size. These can be obtained by various techniques, such as, for example, by direct synthesis from the elements, by reduction of silica or by thermal decomposition of compounds containing silicon and carbon. Pyrolysis techniques are particularly advantageous in that they produce powders with controlled particle sizes, predetermined compositions, and are comprised primarily of individually disaggregated microcrystalline bodies. In this method, trichloromethylsilane vapor and hydrogen or SiCl ₄
Steam and a suitable hydrocarbon vapor, such as toluene, and hydrogen are introduced into an argon plasma generated between two concentric electrodes. In the hot plasma, the compounds decompose into ions and the gas cools down to very stable molecules, namely SiC and
HCl is formed. SiC is produced as small crystals, typically 0.1-0.3μ in size. The advantage of this product is that the crystals are not agglomerated and the carbon-to-silicon ratio can be controlled by monitoring the initial vapor composition, resulting in a slightly carbon-enriched SiC powder. Additionally, BCl ₃ can be further added in the desired amount to the reactants, thereby doping or infiltrating the SiC powder with boron and substantially dispersing the boron at the molecular level.

Another method of preparing silicon carbide powder with excellent sinterability is disclosed in US Pat. No. 3,085,863. This patent describes the steps of forming a gel of silica in a sugar solution, dehydrating the gel to decompose the sugar and forming a finely divided mixture of silica and carbon, and heating the mixture in an inert atmosphere. and forming silicon carbide. The inventor has found that it is preferable to modify this process by using ethyl silicate instead of silicon tetrachloride, in order to eliminate the disadvantage of the huge amounts of hydrochloric acid released during hydrolysis. I found out.

The boron-containing additive may be in the form of a powder of submicron dimensions and may further be either elemental boron or silicon carbide. Alternatively, boron may be added directly to the silica gel in the form of a boron compound such as boric acid during the preparation of the silicon carbide powder. In order to obtain densification, the amount of boron-containing additive is critical and the amount of additive is 0.3-3.0% by weight.
It is considered to be suitable for the element boron. Experiments with sintering silicon carbide using boron-containing additives have shown that there is a lower limit of efficiency below which there is virtually no effect. This critical concentration appears to correspond to 0.3-0.4% by weight boron. Increasing the boron concentration too much does not result in improved densification, and when the amount corresponds to more than 3.0% by weight boron, the oxidation resistance of the product deteriorates.

The optimum amount to be added by powder mixing method is
This corresponds approximately to 1 part by weight of boron per 100 parts by weight of silicon carbide. This optimum amount is probably related to the solubility limit of boron in silicon carbide, and this solubility limit must be approached or exceeded to obtain boron segregation at grain boundaries and the resulting effects. Must be. However, since there is a limit to the degree of dispersion of boron in silicon carbide powder that can be achieved, it is advantageous to slightly exceed the lower limit of boron effectiveness. This results in guaranteed densification throughout the compact and eliminates isolated zones of densification that can form if the concentration is low or if the mixing is incomplete. Thus, in most cases, the equivalent of 1% by weight boron is the minimum amount added when elemental boron powder is mechanically mixed with silicon carbide powder. On the other hand, when boron is introduced during the preparation of the silicon carbide powder, a very desirable dispersion state is achieved and addition of only an amount corresponding to about 0.4% by weight boron gives satisfactory results.

In order to obtain a high degree of densification, the oxygen content of the powder must be very low, ie below 0.1% by weight, and a slight excess of carbon is required. Thus, for example, a powder containing 0.4 wt% boron and no free carbon exhibits only 5% linear sintering shrinkage upon firing at 2020°C, which translates into a final ring density of approximately 70%. Only equivalent. However, if carbon addition is made in the form of soluble carbonaceous compounds before forming, the linear sintering shrinkage after firing under the same conditions increases to 18% and the density becomes 96% of the theoretical value. Thus, clearly, a small amount of free carbon is absolutely essential for the sintering of SiC.

The action of the carbon is to reduce the silica which is always present in small amounts in the silicon carbide powder or which is formed from the oxygen adsorbed on the powder surface upon heating. In this case, carbon reacts with silica during heating according to the following reaction equation: SiO ₂ +3C
=SiC+2CO When silica is present in an appreciable amount in SiC powder, it completely inhibits the densification of silicon carbide, so that little to no sintering shrinkage is obtained.

Free carbon has an additional role. Free carbon acts as a getter for free silicon, if present. Free silicon may be present in the powder or may be formed during heating to the sintering temperature by the following reaction: SiO ₂ +
2SiC=3Si+2CO The presence of silicon, like silica, prevents or suppresses the densification of SiC and must therefore be eliminated. The amount of carbon required depends largely on the oxygen content in the starting SiC powder. Thus, for example, a boron-doped powder with an oxygen content of 0.06% can be easily sintered to 98.5% of theoretical density even with the addition of 0.3% carbon. Another powder containing 0.3% oxygen also has 0.9% free carbon.
Can be sintered to 91% relative density. A substantial excess of carbon beyond that required to deoxidize the SiC is harmful.
Carbon is generally difficult to disperse, and unreacted excess carbon tends to form numerous grains in the sintered SiC matrix, which act much like permanent pores. In this way, excess carbon limits the density and strength that can ultimately be achieved. Systematic experiments have shown that 0.1-1.0 weight percent carbon is sufficient to provide sinterability. A powder that does not sinter under these conditions will not sinter even if more carbon is added.

Since carbon in powder form is extremely difficult to uniformly disperse at the submicron level, it is advantageous to introduce the carbon as a solution of carbonaceous organic compounds which are then pyrolyzed to carbon.
Thus, some general operational definitions that can be used to describe the properties of carbonaceous additives can be listed as follows. First, compounds that easily crystallize from an aqueous solution, such as sugar, tend to precipitate as crystals during evaporation of the solvent. Such crystals transform into relatively large carbon particles upon pyrolysis and form undesirable inclusions in the microstructure of the final product. Therefore, compounds that do not crystallize out of solution are preferred. Second, compounds derived from aliphatic hydrocarbons give carbon only in low yields, and furthermore, yields vary with heating rate, so precise control over carbon addition cannot be made. Therefore, low yield is another important limiting factor. For example, acrylic resins that yield only about 10% carbon upon pyrolysis are not effective.

High molecular weight aromatic compounds are preferred materials for making carbon additions because they give high carbon yields and do not crystallize upon pyrolysis. Thus, for example, phenols soluble in higher alcohols such as acetone or butyl alcohol
Formaldehyde condensate-novolac can be used with a number of related condensation products such as resorcinol-formaldehyde, aniline-formaldehyde, cresol-formaldehyde, and the like. These similar compounds give carbon yields of about 40-60%. Another satisfactory group of compounds are derivatives of polynuclear aromatic hydrocarbons contained in coal tar, such as dibenzanthracene, chrysene, and the like. A preferred group of carbonaceous additives are polymers of aromatic hydrocarbons, such as polyphenylene or polymethylphenylene, which are soluble in aromatic hydrocarbons and give carbon yields up to 90%. but,
Adding elemental carbon directly to silicon carbide powder
It is very difficult to obtain the required degree distribution and a large amount of carbon inclusions are often found after sintering, making it impractical. Such inhomogeneities, of course, have a useful effect on strength since they serve as fracture initiation points.

An excellent method for incorporating carbon into submicron sized silicon carbide powders is by adding a solution of carbonaceous material that decomposes to carbon when heat treated. When adding carbon,
The first step is to prepare a solution of the selected carbonaceous compound in a convenient solvent, preferably having a fairly high melting point if freeze drying is used. The powder is then dispersed into the desired amount of solution containing the required amount of organic compound. The volume of solvent required is sufficient to provide a thin slurry when the silicon carbide powder is well dispersed. Then the solvent
Evaporation can occur either directly from the liquid dispersion or by lyophilizing the dispersion and removing the solvent by sublimation in vacuo. This latter method has the advantage of preventing inhomogeneities in the distribution of additives, which are always introduced due to solute migration during drying in the liquid state. In this way, a uniform coating of organic material is obtained around the silicon carbide microcrystals, which provides the desired degree of carbon dispersion.

Another strategy for improved carbon distribution at the submicron particle size level is the application of jet milling. The silicon carbide powder is soaked in a solution of novolak resin in, for example, acetone, dried in air, and heated in nitrogen to 500-1800°C to pyrolyze the resin. The actual amount of carbon introduced by this process can be measured as the weight gain after pyrolysis and can be determined by free carbon analysis. The powder with added carbon is then jet milled, which greatly improves the carbon dispersion and eliminates the main carbon grains in the sintered product.

Any of the conventional techniques commonly used in the ceramic industry can be applied and the powder mixture processed accordingly in order to shape or shape the powder into the desired form.

In die press processing, the powder usually requires the addition of a small amount of lubricant, such as 1% by weight stearate. However, it is possible to press into a simple shape without using such additives. Thus, for example, 300 g of SiC powder, to which boron and carbon have been added during the preparation step, is dissolved in a 1% solution of aluminum stearate in benzene.
300 c.c. and milled with cemented carbide in a plastic container for 5 hours. After that, the resulting slip is 200
It is passed through a mesh sieve and the solvent is evaporated. The resulting powder may then be pressed into a shape having a green density of about 55% at 5000 psi. The same powder can also be used in the wet-bag method.
It is also possible to press hydrostatically into complex shapes such as tubes and crucibles depending on the method. Application of 30000 psi pressure results in a green density equal to 59%.

To obtain more complex geometries, the green excipients may be machined by grinding, milling, etc. and, if desired, first treated with nitrogen at a temperature of about 1600°C to obtain even greater initial strengths. Alternatively, preliminary firing may be performed in an argon atmosphere. anyway,
Sintering shrinkage must be considered in determining final dimensions. These dimensions, after firing, are of course a function of the green density and the density after firing, and are established in a conventional manner.

Slip casting of silicon carbide powder is also possible. The preferred dispersion medium is water and the anti-agglomerating agent is inherent in the powders prepared by the various methods already discussed. Casting slips having a solids content of up to 40% by volume can be prepared by dispersing the powder in water to which an anti-agglomerate has been added and ball milling it for several hours. Shaping is accomplished by pouring into a calcined plaster mold according to conventional slip casting techniques.

Additionally, the silicon carbide powder mixture can be extruded or injection molded with the addition of a binder to form a moldable paste. porous materials that decompose and evaporate on heating in an inert atmosphere without leaving appreciable residues, such as those typified by polyethylene glycols, or in much the same way that the vehicle is removed in slip casting. There is a wide selection of useful binders that can be removed by the catalytic medium.

Firing of the silicon carbide compact can be done in a conventional high temperature furnace equipped with means for controlling the furnace atmosphere. Firing work is carried out in separate furnaces2
Dividing into two stages is particularly beneficial for large excipients. This is done because high temperature furnaces do not provide good temperature control on the low temperature side, where molding additives are usually removed. Preliminary firing uses argon, which contains less than 10 ppm of oxygen.
It is done in an inert atmosphere such as helium, nitrogen and hydrogen. A temperature of 1500°C is usually sufficient to obtain good strength for further handling, but slightly higher or lower temperatures may be used depending on the degree of strength required for green machining. It can also be used.

The densification of the compact is due to pressureless sintering without the aid of external pressure. This is different from the hot press method, where considerable external pressure must be applied. Final sintering must be accomplished in an atmosphere inert to SiC, such as the gases or mixtures thereof listed above, and also in vacuum, but to achieve a high density of more than 95%,
Firing must be done in nitrogen or a mixture of nitrogen and noble gases. Nitrogen is β-α(6H)
It has a special effect in suppressing or inhibiting SiC metamorphosis. This transformation, which proceeds in SiC at temperatures above about 1600° C., results in coarse grain growth of the α-(6H) phase. This process often causes SiC powder to coarsen before final density is achieved, and this coarsening stops further densification at a low final density, typically 85-90%. . However, since nitrogen prevents this coarsening by stabilizing the β-SiC phase, high density can be achieved.
Nitrogen also slows the sintering rate, so the higher the nitrogen pressure used, the higher the temperature must be applied. Thus, for example, a silicon carbide powder compact can be fired to 96.5% theoretical density at 2020° C. in 40 mm Hg nitrogen. In 760mmHg nitrogen, 95%
A temperature of 2100°C is required to obtain the theoretical density.
However, the higher the nitrogen pressure, the greater the effect of inhibiting grain growth, and therefore the optimum firing conditions can be established through routine testing.

During the sintering operation, the temperature schedule used depends on the volume of the part being fired. Small specimens weighing a few grams are generally completely independent of the temperature program and can therefore be brought to firing temperature in about 15 minutes without any disadvantage. Holding at maximum temperature for 15 minutes will yield the desired density.
Extended residence times at elevated temperatures are detrimental as it leads to coarsening of the microstructure and consequent deterioration of mechanical properties. It is thus preferable to minimize the required retention period.

For large bodies, firing must be carried out in a systematic manner to allow diffusion of nitrogen throughout the body during heating and to avoid temperature gradients in the body. Thus, for example, a 250 g press is prefired at 1500° C. and transferred to a high temperature furnace. In a protective argon-nitrogen atmosphere, the press is heated to 1600° C. in 40 minutes and then the temperature is increased gradually to 2020° C. over 80 minutes and held at that temperature for an additional 60 minutes. The cooling method is not critical due to the high thermal conductivity of sintered silicon carbide.

Upon firing, the nitrogen atmosphere has an additional special effect on the sintered SiC in that it induces electrical conductivity by introducing n-type semiconductivity. Electrical conductivity is proportional to the nitrogen pressure during sintering, but it is also affected by trace amounts of other elements and impurities that have entered the lattice. Thus, by sensing the nitrogen pressure in the furnace, it can be varied from a typical ¹⁰⁴ Ω-cm for a nitrogen-free sintering atmosphere to a typical ^10-1 Ω-cm for a 760 Torr _N2 atmosphere. It is possible to prepare polycrystalline SiC that can have a resistance range up to -cm.

Thus, the novel method of the present invention allows complex shaped articles to be made of high quality single phase polycrystalline silicon carbide by conventional techniques. Heretofore, articles of such complex shapes either could not be made from silicon carbide at all, due to the nature of the material, or required expensive and tedious machining. In this way, articles such as gas turbine blades, impermeable crucibles, thin-walled tubes, elongated rods, spheres and hollow shapes such as gas turbine blades are directly available. The preferred high-density silicon carbide material from which the article is formed has a density of at least 95% of the theoretical value, a modulus of rupture of approximately 80,000 psi, high oxidation resistance, high creep resistance at 1500°C, and the properties of hot-pressed silicon carbide. It has the desired properties. Additionally, sintered silicon carbide can be prepared in a manner that provides a wide range of electrical resistivities.

The present invention will be further illustrated by specific examples, including the following reference and comparative examples: Example A submicron silicon carbide powder was prepared with the following characteristic results.

Component Oxygen ppm 600 Nitrogen ppm <50 Free carbon ppm 6000 Iron ppm 180 Aluminum ppm <10 Boron ppm 4000 Specific surface area m ² /g 16 Average surface average crystal size μm 0.15 Identification by X-ray diffraction β-SiC Trace amount of α-SiC6H 200 g of silicon carbide powder was dispersed in a 200 c.c. solution of 1 g oleic acid and 1 g aluminum stearate in benzene and ball milled for 2 hours. The slurry was strained through a 150 mesh US standard sieve and lyophilized. The resulting crumbly cake was broken up and transferred through a 42 mesh US standard sieve. 2.5
Pressing the powder at 5000 psi in an inch diameter steel die yielded a density of 1.65 g/cc, which corresponds to 51.5% of the theoretical density. When the stock was hydrostatically repressed at 25000 psi, the density increased to 1.76 g/cc, which corresponds to 55% of the theoretical density.

The press body was fired in a graphite resistance furnace in flowing nitrogen at 40 mmHg pressure, where the temperature schedule was as follows: room temperature to 200 °C 10 minutes 200 °C to 400 °C 50 minutes 400 °C to 1500 °C ℃ 30 minutes 1500℃ hold 30 minutes 1500℃ to 1950℃ 20 minutes 1950℃ to 2020℃ 30 minutes 2020℃ hold 40 minutes After holding at maximum temperature for 40 minutes, the furnace is shut down and filled with nitrogen to atmospheric pressure. It was then allowed to cool to room temperature.

Discs shrink by 19.5% (based on raw diameter)
and had a density of 3.16 g/cc. This corresponds to 98% of the theoretical density. Microscopic examination of the fragments revealed that it had a two-phase microstructure consisting of a matrix of β-silicon carbide with a grain size of about 3 μm and large tabular crystals of α-silicon carbide up to 200 μm in size.

A steel die-pressed disk with a raw density of 51.5% of the theoretical value, upon firing under the same conditions, yielded a fired density of 3.07 g/cc, which corresponded to 96.2% of the theoretical density. Electrical resistivity is 70Ωcm
It was hot.

EXAMPLE Presses prepared from the powders described in the examples (51% green density) were fired at atmospheric pressure in flowing nitrogen on a similar temperature schedule with the maximum temperature increased to 2080°C. The final density of the fired body was 96% of the theoretical value. Fragment tests revealed a thin microstructure with grains not exceeding 20 μm. The electrical resistivity was 0.2Ωcm.

Example 5000 psi from the powder composition described in the example.
A 5/8 inch diameter x 1/2 inch length cylinder (51% green density) was pressed at 2080°C.
Calcined in flowing argon at 40 mmHg for 15 minutes and cooled to room temperature. The final relative density was 91.5% and the microstructure was coarse-grained consisting of large tabular crystals of alpha-silicon carbide. The electrical resistance was 8×10 ³ Ωcm.

Example A specimen of the same dimensions and green density as in the example was placed in a vacuum of 100 μHg (the residual atmosphere was N ₂ and CO
) was baked at 2000℃ for 15 minutes. The final density was 93% of the theoretical value and the resistivity was 4×10 ³ Ωcm. The surface of the specimen was covered with carbon due to the decomposition of SiC and the volatilization of silicon.

EXAMPLE The submicron scale characterized in the example
An aqueous slurry was prepared from the SiC by mixing 400 g of powder with 250 c.c. of distilled water and adding 2 c.c. of a sodium silicate solution containing 20% Na ₂ O.3SiO ₂ . The slip was ball milled for 2 hours using sintered carbide balls and 150
Passed through a mesh sieve.

A 1 1/2" diameter x 1 1/2" high crucible was then formed by pouring the slip into calcined gypsum and removed from the mold and dried. The castings were tested in the firing cycle described in the example.
Calcined in flowing nitrogen at 40mmHg. The final density is 95.5% of the theoretical value and the sintering shrinkage is
It was 18.5%.

EXAMPLE An industrial silicon carbide powder with similar properties as described in the example, but containing less than 20 ppm boron, was processed and pressed into 5/8" diameter pellets (green density 60%) and heated to 2020 °C. The material was fired for 15 minutes in flowing N ₂ at 40 mm Hg. No sintering shrinkage or densification was observed.

EXAMPLE 1% amorphous boron was added to the same powders as in the example and these were ground in a jet mill to particles of size less than 2 μm. 50 g of the powder mixture was dispersed in benzene and milled for 2 hours using cemented carbide balls. The slips were dried and the resulting powder was pressed into 5/8 inch diameter pellets having a 60% green density. Calcination for 20 minutes at 2080°C in flowing nitrogen at 600 Torr resulted in 12
% sintering shrinkage occurred. Final density is 93% of theoretical value
It was hot.

EXAMPLE Amorphous silica and carbon black were mixed in a 1/4 molar ratio and calcined in hydrogen at 1600°C for 2 hours. The product was recalcined in air at 700° C. for 5 hours until the unreacted carbon was burnt off. The resulting powder was soaked in 20% hydrofluoric acid, washed with water and ethyl alcohol, and dried. The product was determined to be pure β-SiC by X-ray diffraction.
and contained less than 2000 ppm metal impurities, 0.2% oxygen and 0.08% nitrogen.

The powder was combined with 1% boron by weight and jet milled using the same method as described in the examples. Pressing process at 5000psi is 50
yielded pellets with % relative density. 40mm
Calcination at 2020℃ in flowing nitrogen in Hg 3%
resulted in a sintering shrinkage of 61% and a final density of 61%.

EXAMPLE The treated powder described in the example was dispersed in a solution of 1 g of polymethylphenylene in 100 c.c. of toluene. 10 in a solution of 10 c.c.
g of the powder dispersion was dried and resulted in the addition of about 0.9% carbon upon pyrolysis of the organic compound.

This carbon is made into 5/8 inch diameter pellets (green density 49
%) and calcined at 2020°C in flowing nitrogen at 40 mmHg. The specimen underwent sintering shrinkage of 14.5% and had a final density of 85%.

EXAMPLE The SiC powder specified in the example was combined with 1% aluminum metal powder and mixed in the dry state. 20 g of this mixture was jet milled using nitrogen as the milling media. 10 g of the resulting powder was dispersed in 10 c.c. of a 1% solution of aluminum stearate and dried. A green powder density of 55% was obtained by forming the powder in a 5/8″ diameter steel die.
Baked for 15 minutes. The fired cylinder exhibited a sintering shrinkage of 4% and a final density of approximately 65%.

EXAMPLE XI The SiC powder specified in Example was compacted to a density of 51% at a pressure of 5000 psi in a steel die without adding any additives. The pellets were calcined for 15 minutes at 2080°C in low pressure nitrogen (40 mmHg). No sintering shrinkage was observed in the fired specimen.

It will be understood that the invention is not limited to the specific conditions or materials set forth herein, but that various modifications may be made within the scope of the invention.

[Brief explanation of drawings]

The drawing is a flow sheet of the method for manufacturing high-density silicon carbide ceramic of the present invention.

Claims (1)

[Claims] 1 Basically, silicon carbide and silicon carbide by weight.
Consisting of 0.1% to 1.0% elemental carbon and 0.3% to 3.0% boron of silicon carbide by weight, it has a theoretical density of 85% or more, and shrinks uniformly from preforming to sintering shrinkage. Shaped silicon carbide ceramic products.