Dense, Self-Sintered Silicon Carbide/Carbon-Graphite Composite and Process for Producing Same

This invention relates to the field of silicon carbide/graphite composite materials and more particularly to dense, self-sintered silicon carbide/graphite composite materials.

Silicon carbide is hard, strong, and exhibits good corrosion and abrasion resistance and high thermal conductivity. It can be used in oxidizing temperatures up to 2500.degree. F. These properties render silicon carbide a useful material in many acid, caustic, corrosive, abrasive, or high temperature environments. Such applications include pump seals and bearings, gas turbine components, mixing nozzles, and flame holders.

Silicon carbide bodies are frequently formed by a sintering process. Sintered silicon carbide has a high hardness, good corrosion resistance, and high thermal conductivity. In sintering, particles of a material bond together when heated to a high temperature, below the material's melting point. In some processes, the material is also subjected to high pressure as well. A self-sintered process is one which does not require application of pressure during the heating step for sintering to occur.

A drawback to silicon carbide is its lack of self-lubricity. A self-lubricating solid is one having low friction in the absence of an additional lubricant. For example, in applications having a high PV (pressure-sliding velocity) limit or dry running applications, parts, such as seals, having a silicon carbide face adjoining a face made of silicon carbide, other ceramics, or steel, will wear excessively due to the forces generated by the high friction. In dry running applications with mating surfaces, special wear surfaces must be provided on at least one of the bodies.

Graphite is a known lubricant and has been incorporated into carbon and silicon carbide materials to impart a self-lubricating property to the material. However, with sintered materials, it has been difficult to incorporate large amounts of a second phase such as graphite into a ceramic matrix without causing cracks to occur in the microstructure or without increasing the material's porosity. Further, adding graphite to silicon carbide is even more difficult, because sintering of silicon carbide already requires stringent conditions, such as fine, high purity powders, sintering aids, and high temperature.

It is known to form a silicon carbide/graphite material by reaction bonding or reaction sintering. However, reaction bonded silicon carbide/graphite material typically has a residual silicon phase which limits corrosion resistance due to reaction with the silicon in some chemical applications. Also, controlling the reaction bonding process to obtain fully reacted and fully dense parts is difficult.

Another known silicon carbide material incorporating graphite is disclosed in U.S. Pat. No. 4,525,461. This material comprises a sintered silicon carbide/graphite/carbon composite ceramic body having a homogeneous fine grain microstructure. At least 50% of the silicon carbide grains are less than 8 .mu.m, and the graphite grains have an average size no larger than that of the silicon carbide grains. However, if the amount of graphite is greater than approximately 8% by weight in this material, the material's density decreases. Less than 8% by weight graphite, while providing a more dense, impervious structure, limits the graphite's lubricating capability in the material.

Thus, there exists a need for a dense, impervious self-sintered silicon carbide body with a greater amount of graphite inclusions to increase the lubricity of the material while maintaining the integrity of the microstructure.

The composite material of the present invention provides a dense, self-sintered silicon carbide/carbon-graphite composite material while incorporating larger amounts of graphite, leading to increased lubricating capability, and maintaining the integrity of the microstructure. The composite material comprises a silicon carbide matrix, between 2 and 30 percent by weight carbon-graphite, and small amounts between 0.1 and 15 percent by weight, of sintering aids such as boron and free carbon. The silicon carbide has an average grain size between 2 and 15 .mu.m, and the carbon-graphite has an average grain size between 10 and 75 .mu.m, the average grain size of the carbon-graphite being greater than the average grain size of the silicon carbide. The composite material has a theoretical density of at least 80 percent and preferably at least 90 percent as determined by the rule of mixtures for a composite material.

The composite body is formed by providing a particulate mixture of silicon carbide, graphite coated with a carbon-precursor binder, and sintering aids. The carbon-bonded graphite comprises at least 5 percent by weight of a carbon-precursor binder, such as a phenolic resin. The silicon carbide may comprise .alpha.-silicon carbide, .beta.-silicon carbide, or a combination of both. A binder, a sintering aid, and other temporary additives are also generally present in the particulate mixture.

The mixture is shaped to form a green body. The green body is heated to carbonize the carbon-precursor binder. The carbonized body is then sintered at a temperature ranging from 1900.degree. C. to 2300.degree. C. in a substantially inert atmosphere at or below atmospheric pressure to produce a sintered body having a theoretical density of at least 80 percent and a microstructure in which the average grain size of the carbon-graphite is larger than the average grain size of the silicon carbide.

The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic illustration of the process for producing a dense, self-sintered silicon carbide/carbon-graphite composite material according to the present invention;

FIG. 2 is a table of test results;

FIG. 3 is an optical photomicrograph at 50.times. magnification of a silicon carbide/carbon-graphite composite material according to the present invention;

FIG. 4 is a further optical photomicrograph at 50.times. magnification of a silicon carbide/carbon-graphite composite material according to the present invention; and

FIGS. 5a, 5b, 5c, and 5d are scanning electron micrographs at 50.times., 500.times., 1000.times., and 2000.times. magnification respectively, of a further silicon carbide/carbon-graphite composite material according to the present invention.

The composite material of the present invention provides a dense, self-sintered silicon carbide/carbon-graphite composite material which also incorporates large amounts of graphite, leading to increased lubricating capability. The integrity of the microstructure, i.e., relative absence of cracks and porosity, is also maintained.

The composite material comprises silicon carbide, between 2 and 30 percent by total material weight carbon-graphite, and between 0.1 and 15 percent by total material weight sintering aids, such as boron and free carbon. The carbon-graphite comprises particles of graphite coated with carbon. The average grain size of the carbon-graphite is greater than the average grain size of the silicon carbide. The silicon carbide has an average grain size between 2 and 15 .mu.m. The carbon-graphite has an average grain size between 10 and 75 .mu.m and preferably between 20 and 30 .mu.m. The composite material has a theoretical density of at least 80 percent, and preferably at least 90 percent, as determined by the rule of mixtures for a composite material.

The process for producing the dense, self-sintered composite material of the present invention is shown schematically in FIG. 1. The composite body is formed from a particulate mixture of a silicon carbide matrix formulation and a carbon-bonded graphite. The carbon binder for the graphite comprises a carbon-precursor resin capable of leaving a carbon residue upon heating, to be described more fully below.

The silicon carbide matrix formulation may comprise a fine-grained, high purity .alpha.-silicon carbide, .beta.-silicon carbide, or a combination thereof. Preferably, at least 0.5 percent .alpha.-silicon carbide relative to .beta.-silicon carbide is present to achieve better densification. A formulation comprising approximately 90 to 91 percent .beta.-SiC, 4 to 5 percent .alpha.-SiC, and 5 percent resin binder has been found to provide satisfactory results. The .alpha.-SiC and .beta.-SiC may be supplied from any commercially available source.

Generally, a slurry of .alpha.-SiC and .beta.-SiC in distilled water is prepared. The SiC is typically provided in powder form and should be fine-grained, having a BET surface area of more than 5 m.sup.2 /g and preferably more than 10 m.sup.2 /g. Also, the powder should be of a high purity, generally at least 95 percent and preferably 97 percent pure. Typically, a sintering aid, such as boron or boron carbide, B.sub.4 C, is added to the SiC suspension. Other known sintering aids, such as aluminum or beryllium or compounds thereof, may be provided if desired. Alternatively, a sintering aid may be introduced at other known steps in the process of forming a sintered composite body. A dispersant, for example, ammonium polymethacrylate, is generally added to the slurry. The dispersant is a temporary additive not forming part of the final composite body. The slurry is mixed, for example, in a ball mill for a period of time, typically about eight hours, sufficient to disperse the ingredients. The slurry is then transferred to a mixing tank.

The carbon-bonded graphite is prepared for addition to the SiC slurry. It comprises at least 5 percent by weight of a carbon binder, the balance comprising a coarse graphite. In the preferred embodiment, a mixture of approximately 70 to 80 percent graphite and 20 to 30 percent binder is provided. A greater amount of binder may be used, although the lubricating capability of the graphite in the final composite body may be reduced.

A coarse graphite such as Lonza KS-150, available from Lonza, Inc., Fairlawn, N.J., in which 55 percent of the particles are larger than 45 .mu.m, is suitable. A suitable binder is a phenolic resin, although other materials which will decompose upon heating to leave a carbon residue, such as furfuryl alcohol, polyester resin, coal tar pitch, or mixtures of these and other materials, may be used. Upon decomposition, the binder causes an increase in the porosity of the graphite particles, as discussed further below.

The binder in powdered form is dissolved in acetone or any other suitable solvent and mixed thoroughly with the graphite to bond the graphite particles. The mixture is dried to evaporate the solvent and crushed to obtain carbon-bonded graphite particles of the desired size. Preferably, the carbon-bonded graphite is passed through a 200-mesh sieve to obtain particles of less than 75 .mu.m. The carbon-bonded graphite is then added to the mixing tank with the SiC slurry. The carbon-bonded graphite generally comprises between 2 and 30 percent by weight of the solids content of the mixture.

A resin solution in distilled water is also added to the mixing tank as a binder for subsequent shaping of the particulate mixture to form a green body. The resin, for example, phenolic resin, typically comprises 5 percent of the total SiC matrix formulation. Also, a die lubricant, such as oleic acid, is generally added in an amount equal to approximately 5 percent of the total SiC matrix formulation (SiC and resin binder). The die lubricant, which is also a temporary additive not forming a part of the final composite body, facilitates subsequent removal from the die in which the particulate mixture is shaped.

The resulting slurry is mixed thoroughly and dried. Typically, the slurry is spray dried to form spherical granules having an average size less than approximately 500 .mu.m. Any other suitable methods for obtaining such granules may be used. For example, the slurry may be pan dried, crushed to obtain a flour, and passed through a sieve to obtain the desired particle size.

A measured amount of the particulate mixture is fed into a die and compacted, typically at pressures ranging between 2 to 20 tons/in.sup.2, to the desired shape to form a green body. Any other suitable method of shaping the mixture may be employed. The shaped green body is carbonized in a non-oxidizing atmosphere above 700.degree. F. In a typical carbonization cycle, the component is heated in an oven from room temperature to 350.degree. F. during a half hour period and allowed to soak at that temperature for another half hour. The temperature is raised to 850.degree. F. over a period of ten hours and held at 850.degree. F. for five hours to carbonize the resin. The component is then cooled to room temperature. Other suitable carbonization cycles may be used.

Next, the carbonized body is sintered at a temperature ranging from 1900.degree. C. to 2300.degree. C. preferably 2000.degree. C. to 2200.degree. C., in a substantially inert atmosphere such as helium or argon at or below atmospheric pressure. Generally, the temperature is raised to the sintering temperature over an eight hour period, although the actual time depends on the particular furnace used. The furnace is held at the peak temperature for one hour and then allowed to cool to room temperature. Other suitable sintering cycles may be used. Additionally, the carbonization cycle and the sintering cycle may be carried out in separate furnaces or in a single furnace.

The process of the present invention results in a sintered composite body comprising between 55 and 97.9 percent by weight silicon carbide, between 2 and 30 percent by weight carbon-graphite, and between 0.1 and 15 percent by weight sintering aids. The material has a density of at least 80 percent, and preferably 90 percent, of the theoretical density of the particular mixture. This density may be achieved, since during carbonization, part of the carbon-precursor binder for the graphite volatilizes, leaving voids, while the remainder forms a coke residue on the graphite. Thus, the carbon-graphite particles at this stage have a greater porosity than graphite alone. Subsequently during sintering, the porous carbon-graphite collapses, allowing greater shrinkage of the SiC matrix despite the presence of the carbon-graphite inclusions. Thus, a relatively dense and impervious composite body results. The resulting composite body has a microstructure in which the average grain size of the carbon-graphite is larger than the average grain size of the silicon carbide. The average grain size of the carbon-graphite ranges between 10 and 75 .mu.m, and the average grain size of the silicon carbide ranges between 2 and 15 .mu.m. Preferably, the carbon-graphite has an average grain size between 20 and 30 .mu.m.

Tests of several different compositions were performed according to the present invention. See FIG. 2. In test bake nos. 1-17, the silicon carbide matrix was provided by a mix consisting of 90.25 percent of a fine-grained .beta.-SiC powder having a BET surface area of 13.5 to 18.5 m.sup.2 /g 4.75 percent of a fine-grained .alpha.-SiC having a BET surface area of 9.0 to 11.0 m.sup.2 /g, and 5.0 percent liquid phenolic resin. In test bake no. 18, the silicon carbide matrix was provided by 100 percent .alpha.-SiC having a BET surface area of 15 m.sup.2 /g. B.sub.4 C was used as a sintering aid, comprising approximately one percent of the total mix. Oleic acid was added as a die lubricant in an amount comprising approximately 5.0 percent of the total mix. These components are readily available from known commercial sources.

Several grades of graphite were tested to determine which provided the best sinterability: a fine graphite in which 50 percent of the particles are smaller than 2.4 .mu.m, a coarse graphite in which 55 percent of the particles are larger than 45 .mu.m, and a coarse, resin-bonded graphite in which 50 percent of the particles are larger than 74 .mu.m. The fine graphite used in the tests was Lonza KS-6, commercially available from Lonza, Inc. The coarse graphite used was Lonza KS-150, also commercially available from Lonza, Inc. The coarse, carbon-bonded graphite was specially mixed as described above and consisted of 20 percent by weight phenolic resin and 80 percent by weight Lonza KS-150.

In the tests, the composite body was formed by mixing a slurry of the silicon carbide with the graphite or carbon-bonded graphite. The mixture was dried and granulated and molded at 2 to 20 tons per square inch The molded green body was carbonized at 850.degree. F. and the carbonized body was sintered at temperatures of 2070.degree. C. and 2090.degree. C.

Test results are shown in FIG. 2. In some test bakes, the graphite was sieved prior to mixing with the silicon carbide to obtain a specific size range. This is indicated in the column "Graphite Size." The entry "Unsized" indicates that the graphite was not sieved. The sieved graphite is indicated by the appropriate size ranges. The specially mixed carbon-bonded graphite is designated Mix No. 1 in the column labeled "Graphite Type."

The tests show that samples containing large amounts of the commercially available non-carbon-bonded graphites, i.e., the fine graphite, Lonza KS6, and the coarse graphite, Lonza KS150, (Test Bake Nos. 3-8) did not provide satisfactory results. The resulting sintered samples were too porous, as indicated by the percent of water absorption, and did not achieve sufficient densification.

The samples using the specially mixed coarse, carbon-bonded graphite (Test Bake Nos. 9-17) indicate that greater densification was achieved with a graphite loading of 15 percent by weight than has been achieved in prior art composite bodies using a comparable loading of non-carbon-bonded graphite. Some lamination in removal from the die, leading to some water absorption, occurred using the larger graphite particles. Lamination did not occur when carbon-bonded graphite particles small enough to pass through the 200-mesh sieve, that is, less than 75 .mu.m, were used.

Silicon carbide/carbon-graphite composite materials according to the present invention are shown in the photomicrographs of FIGS. 3 through 5. The carbon-graphite appears as the darker inclusions among the lighter silicon carbide matrix. FIG. 3 shows an optical micrograph of a polished cross-section of test bake no. 11 magnified 50 times (50.times.). The material appears fully dense and the carbon-graphite is uniformly distributed.

FIG. 4 shows an optical micrograph of a polished cross-section of test bake no. 17 at 50.times. magnification. The material appears fully dense and the carbon-graphite is uniformly distributed.

FIGS. 5a through 5d show scanning electron micrographs at 50.times., 500.times., 1000.times., and 2000.times. magnification respectively of test bake no. 17 polished and etched to show the grain structure. The grain size of the silicon carbide is less than 10 .mu.m, and the carbon-graphite grain size is in the range of 20 to 60 .mu.m.

The invention is not to be limited by what has been particularly shown and described, except as indicated in the appended claims.