Sintered silicon carbide/carbon composite ceramic body having fine microstructure

Claim 2

A process for producing a sintered silicon carbide/carbon composite ceramic body consisting essentially of: a. forming a homogeneous mixture comprising the components of: (1) a carbon-containing additive selected from the group consisting of amorphous carbon, and a solvent soluble, organic material which can be coked at temperatures below about 1000° C. to form amorphous carbon or mixtures thereof, in an amount between 0.5 and 5 percent by weight of uncombined carbon; (2) from about 0.15 to about 15 percent by weight of a sintering aid selected from the group consisting of aluminum, beryllium or boron or compounds containing any one or more of these elements or a mixture of any of the foregoing elements or compounds, said sintering aid containing from about 0.15 to about 5 percent by weight of aluminum, boron or beryllium or a mixture of any of these elements; (3) up to about 25 percent by weight of organic temporary binder; (4) a balance of silicon carbide powder having a surface area of from about 5 to about 100 square meters per gram; b. shaping the mixture in such a way as to produce a microporous shaped object having a density of at least about 45 percent of theoretical; and c. infiltrating the shaped object with a sufficient amount of an organic material which can be coked at temperatures below sintering temperature to form amorphous carbon, in an amount between about 1 and 25 percent by weight of the shaped object; d. firing the infiltrated object in the absence of any substantial externally applied pressure under such conditions of time and temperature in an atmosphere inert to silicon carbide or in a vacuum at a temperature of between about 1900° C. and 2300° C. until a sintered body having a homogeneous microstructure of silicon grains having a maximum size of about 5 microns, said microstructure having carbon substantially uniformly dispersed throughout, is formed.

Claim 3

A process according to claim 1 wherein from about 0.5 to about 17 percent by weight of a solvent-soluble, organic material which can be coked and has a carbon content of from about 25 to about 75 percent by weight of the organic material is employed in forming the homogeneous mixture which further includes from about 25 to about 100 percent of the combined weight of the silicon carbide powder, graphite powder, organic material which can be coked, sintering aid and temporary binder, of a solvent in which the organic material which can be coked is soluble; said mixing being done in such a way as to disperse the organic material which can be coked about the silicon carbide powder and graphite powder and to coat the silicon carbide powder and graphite powder therewith, and drying the mixture in such a way as to evaporate the solvent therefrom prior to shaping.

Claim 4

A process according to claim 1 wherein the silicon carbide of the raw batch is predominately alpha, non-cubic crystalline silicon carbide.

Claim 5

A process according to claim 1 wherein the silicon carbide of the sintered composite body is predominately alpha, non-cubic crystalline silicon carbide.

Claim 6

A process according to claim 1 wherein the body is fired for a time varying from about 10 to about 180 minutes; wherein the temperature is between about 1900° C. and 2300° C.; and said conditions of atmosphere are firing in a vacuum or up to about one atmosphere of pressure of a gas selected from the group consisting of nitrogen, argon, helium, neon and mixtures thereof.

Claim 7

A process according to claim 1 in the level of sintering aid provided in the object being sintered is maintained by enclosing the object being sintered within a sealed container which is impervious to the aluminum, boron or beryllium of the sintering aid at the sintering temperature and which has a volume approximating the size of the object being sintered.

Claim 8

A process according to claim 1 wherein the level of aluminum, boron or beryllium to be derived from the sintering aid and retained in the object being sintered is maintained by containing the object being sintered within a graphite container, which container has been saturated with the sintering aid or elemental aluminum, beryllium or boron by previous exposure to the sintering aid or aluminum, beryllium or boron at or about the temperature of sintering.

Claim 9

A process according to claim 1 comprising the steps of: a. forming a homogeneous mixture by mixing together the components of (1) from about 91 to about 99.35 parts by weight silicon carbide having a surface area of from about 5 to about 100 square meters/gram; (2) from about 0.67 to about 17 parts by weight of organic material which can be coked at temperatures below about 1000° C. to provide amorphous carbon and having a carbon yield of from about 25 to about 75 percent by weight; (3) from about 0.15 to about 15 percent by weight of a sintering aid selected from the group consisting of any one or more of these elements or a mixture of any of the foregoing elements or compounds, said sintering aid containing from about 0.15 to about 5 parts by weight of aluminum, beryllium or boron or a mixture of any of these elements; (4) temporary binder in amount up to 25 percent by total combined weight of the silicon carbide powder, organic material which can be coked and sintering aid; and (5) from about 24 to about 100 percent by total weight of the silicon carbide powder, organic material which can be coked, sintering aid and temporary binder of a solvent in which the organic material is soluble; said mixing being done in such a way as to disperse the organic material about the silicon carbide powder and coat the silicon carbide powder therewith; b. drying the mixture in such a way as to evaporate the solvent from the mixture; c. breaking up the dried mixture so that it will pass an 40 mesh sieve; d. adding graphite powder having an average particle size not in excess of 8 microns and a surface area of at least 5 square meters per gram to the mixture in an amount sufficient to bring the percent by weight of graphite in the resulting mixture in the range of from about 1 to about 48 percent and mixing the graphite powder and the mixture in such a way to disperse the graphite powder substantially homogeneously throughout the resulting graphite-containing mixture; and e. shaping the graphite-containing mixture in such a way as to produce a microporous shaped object having a density of at least about 50 percent of theoretical.

Claim 10

A process for producing sintered silicon carbide/carbon composite ceramic body according to claim 8 wherein the pre-mix comprises from about 0.3 to about 5 parts by weight of B x C wherein " x " is from 2 to 8, said B x C containing from about 0.3 to about 3.0 percent by weight boron.

Claim 11

A process according to claim 1 wherein forming a homogeneous mixture referred to in step a. comprises: a. dissolving an organic material which can be coked in a solvent; and b. mixing the solution so formed with the remaining components, and drying to the mixture prior to shaping.

Claim 12

A process according to claim 1 wherein the temporary binder is used in an amount of from about 1.5 to 5 percent.

Claim 13

A process according to claim 8 wherein the temporary binder is curable; and comprising a step of curing the temporary binder after shaping of the dried mixture but prior to firing of the shaped object.

Claim 14

A process according to claim 1 wherein the shaped object is infiltrated with a phenol-formaldehyde resin and the infiltrated object is thereafter heated to a temperature of about 90° C. to about 150° C. for about 1 to about 2 hours to cure the resin infiltrant.

Claim 15

A process according to claim 8 wherein the shaped body is fired for from about 20 to about 180 minutes at a temperature of from about 2100° C. to about 2300° C. and in up to about one atmosphere of pressure of a gas selected from the group consisting of argon, helium, neon, nitrogen and mixtures thereof.

Claim 16

A process according to claim 14 wherein the gas is approximately one atmosphere of the member selected from the group consisting of argon, helium and neon and the temperature is from about 2050° to about 2200° C.

Claim 17

A process according to claim 8 wherein the silicon carbide comprises predominately alpha, non-cubic crystalline silicon carbide.

Claim 18

A process according to claim 1 wherein the shaped object is infiltrated with an aqueous solution containing from 25 to 100 percent by weight of a phenol-formaldehyde resin having a char yield of about 40 percent by weight.

Claim 19

A process according to claim 1 wherein the resin that has been infiltrated into the shaped object is cured and subsequently the infiltration cycle is repeated.

Claim 20

A process according to claim 1 wherein the sintered composite body is infiltrated with a carbon source resin and subsequently reheated to coke the resin in situ.