JPS6310118B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for making high density ceramics comprising silicon carbide.

The method for producing high-density ceramics made of silicon carbide is known, as described in the periodical "Power Metallurgy International", Vol. 10, No. 2, 1978, pages 87-89. The starting material used in this method is powdered silicon carbide of alpha type. The gasoline-doped material is first treated with hydrofluoric acid and then washed several times to remove the silica and residual oxygen present in the powder. The powder obtained as described above is ground in a steel ball mill and then treated with hydrochloric acid to remove the iron left behind by the grinding. Next, a heat treatment is performed at 700° C. to remove residual free carbon, followed by a second hydrofluoric acid treatment. After heat treatment at 1200° C. under argon, amorphous boron and a carbonaceous product consisting of polyphenylene dissolved in benzene are continuously added to the powder. After removal of the benzene by aeration, the samples are produced by compressing the oleic acid-impregnated powder in a steel matrix at a pressure of 100 N/mm ² .
The sample is then sintered under argon at a temperature on the order of 2100°C.

A disadvantage of the above method is its complexity. actual,
Two lengthy chemical treatments with hydrofluoric acid are required. Moreover, it is not possible to completely remove residual oxygen from the powder by these chemical treatments, which results in a decrease in the density of the ceramic obtained.

The aim of the present invention is to eliminate the above-mentioned drawbacks and to provide a simpler and more efficient method for producing high-density silicon carbide ceramics.

The object of the present invention is to prepare a mixture containing alpha phase silicon carbide, boron and a carbonaceous product;
It is an object of the present invention to provide a method for producing a high density silicon carbide ceramic comprising the step of sintering a sample of said mixture at a temperature on the order of 2100 DEG C. in a neutral atmosphere. The method of the present invention is characterized in that the step of preparing the mixture includes heat treatment of alpha phase silicon carbide powder under vacuum;
This process is carried out at a temperature higher than 1200°C.

Two non-limiting examples of the method of the invention are described below.

In the first embodiment, Acheson
Commercially available powdered silicon carbide of alpha structure type obtained by the ACHESON method is used as starting material. This powder has a specific surface area of 7 m ² /g and an average particle size of about 2 microns. This powder is mainly
Impurities including oxygen less than 4000p.pm and 3000p.pm
Contains various metals bonded to less than carbon or oxygen.

To this powder is added 1% by weight of finely divided crystallized boron dispersed in a volatile organic liquid such as gasoline or cyclohexane.

The suspension obtained by the above method is ground in a steel ball mill. For reference, in the case of a suspension containing 40 g of silicon carbide-boron mixture in 140 ^cm3 of cyclohexane, the mill contains 1.6 kg of balls and the mill is
Rotate at 190t/mm for 4 hours.

After grinding, the suspended product is filtered and dried in a drying chamber.

The dry powder thus obtained contains a significant amount of iron from the steel balls and the mill body. Therefore, the powder is treated with heated dilute hydrochloric acid to completely remove the iron and then washed with deionized water.

The suspension obtained after washing is decanted and then dried in a drying chamber after removing excess water. Under the specific conditions mentioned above, such a dry powder has a specific surface area of
It has 13m ² /g.

This powder is placed in a carbon crucible in the center of a graphite heat-resistant furnace. The furnace is equipped with a vacuum pump, which reduces the pressure inside the furnace to approximately 5.10 ^-6 Torr. The temperature of the oven is then gradually increased, reaching a maximum temperature of between 1200 and 1400° C. after 1 hour.

Outgassing occurs from the powder during temperature rise. This release causes a temporary re-increase in pressure within the furnace.

Maintain maximum temperature for about 1 hour and then turn off the furnace.

After this heat treatment, the specific surface area of the product collected in the crucible is only 4 m ² /g.

Next, an aqueous or alcoholic suspension of this powder is prepared, and a phenol lake is added to this suspension. The dry extract of lake yields 60% carbonaceous residue upon pyrolysis.

The suspension containing the lake is placed in a jar made of plastic material. The jar contains steel balls coated with polytetrafluoroethylene.
The jar is then continued to rotate for 1 hour to effect homogenization.

The suspension is then dried in a rotary evaporator and the powder obtained is sieved.

Next, a sample of the ceramic part to be manufactured is molded.

Such shaping is carried out, for example, by well-known methods of compacting the powder in a steel matrix at pressures of the order of 2000 bar. Molding may be carried out by isostatic compression molding in rubber finger molds, or by casting, in which a suspension of the powder in water to which a peptizing material is added is poured into a plaster mold. good.

Finally this sample was heated under an argon atmosphere at a temperature of approx.
Sinter at 2100℃ for about 30 minutes.

The silicon carbide ceramic part obtained after sintering has, for example, a density of 90 to 98% of the theoretical density of silicon carbide. Such parts have chemical and mechanical properties suitable for manufacturing gas turbine and diesel engine components. 3-point bending strength is 1400℃
is greater than 400MPa.

In a second embodiment of the process of the invention, a commercially available powdered silicon carbide of alpha structure type obtained by the Etchison process is used as starting material. In the example described, this powder has a specific surface area of about 7 m ² /g. This powder mainly contains impurities containing less than 4000 p.pm of oxygen and less than 3000 p.m of various metals and silicon, bound or unbound to oxygen.

This powder is placed in a carbon crucible in the center of a graphite refractory furnace. This furnace is equipped with a vacuum pump. In order to keep costs low, this pump is advantageously a primary vacuum pump capable of obtaining a pressure on the order of 1 Torr, but a secondary vacuum pump capable of obtaining a vacuum on the order of approximately 5.10 ^-6 Torr is preferable. It is also possible to use

The temperature of the furnace is then gradually increased until a maximum temperature of between 1200°C and 1600°C is reached.

When a secondary vacuum pump is used, the pressure inside the furnace is temporarily increased again. This re-rise is caused by gases released from the powder.

Maximum temperature holding time is 5 minutes to 6 hours, depending on temperature, pumping capacity, and amount of impurities present in the treated powder.

Preferably, in order to shorten the duration of the heat treatment, the maximum temperature of the furnace is adjusted to a high value substantially equal to 1600°C. For example, a heat treatment that takes one hour at 1400°C is performed at 1600°C for 5 minutes.

After heat treatment, the powder has a specific surface area of the order of 2 m ² /g and a residual oxygen amount of the order of 500 p.pm. The reduction in the specific surface area of the powder caused by heat treatment actually corresponds to a reduction in the proportion of the finest particles of the powder. Heat treatment causes weight loss. This weight loss amounts to, for example, 6 to 7% by weight of the untreated powder.

Finely ground crystalline boron 1 by mixed dispersion in a volatile organic liquid such as gasoline or cyclohexane
% by weight is added to the powder to be treated.

The suspension obtained as described above is ground in a steel ball mill. For example, in the case of a suspension of 30 g of a silicon carbide-boron mixture in 140 cm ³ of cyclohexane, the mill contains 1.6 kg of balls with a diameter of 3 mm and is rotated for 4 hours at a speed of 300 t/mm.

The dry powder contains a significant amount of iron originating from the balls and the mill body. Therefore, the powder is treated with dilute hydrochloric acid to remove almost all of the iron and then washed with slightly acidic deionized water.

The suspension after washing is decanted and then, after removing excess water, it is dried in a drying chamber.

The powder obtained in the above method has a specific surface area of the order of 9 m ² /g. This value is greater than the specific surface area of the powder (4 m ² /g) obtained from a similar starting powder in the first example of the process. In the first method, the order of heat treatment and pulverization is reversed.

Next, an aqueous or alcoholic suspension of the powder is prepared, and the phenol lake is added to this suspension. Pyrolysis of the dry extract of this lake yields 60% carbonaceous residue.

The suspension containing the lake is placed in a jar made of plastic material. The jar contains steel balls coated with polytetrafluoroethylene. Next, the jar was continued to rotate for 1 hour to achieve homogenization.

The suspension is then dried in a rotary evaporator and the powder obtained is sieved.

Next, a sample of the ceramic part to be manufactured is molded.

This molding can e.g.
This is carried out in a well known manner by compacting at pressures between 500 and 2000 bar. Alternatively, molding may be carried out by isostatic compression molding in a rubber finger mold, or by casting, in which a suspension of the powder in water to which a peptizing material is added is poured into a plaster mold. It's okay.

Finally, the sample was heated to 2100°C under an argon atmosphere.
Sinter for about 30 minutes at a temperature on the order of .

The silicon carbide ceramic parts obtained after sintering have a theoretical density of silicon carbide of 95 to 98, depending on the sample.
% density.

The second embodiment of the method according to the invention has a greater reproducibility of the density of the ceramic parts obtained.

These parts may be used in the manufacture of gas turbine or diesel engine components.

Compared to the prior art method described in said paper,
The advantage of the process of the invention, as shown in the two examples above, is that the starting powder can be prepared by a vacuum heat treatment which is much simpler and more efficient than the chemical treatment with hydrofluoric acid used in the prior art. The removal of oxygen and silica present therein can be carried out.

Furthermore, since the boron addition is carried out before grinding, it is possible to use a coarser-grained and therefore cheaper boron powder, and a mixing process can be omitted.

Furthermore, it is an advantage of the process of the invention that the carbon addition is carried out in the form of phenolic lakes, which are much cheaper than the polyphenylenes used in the prior art.
It is one. This lake is soluble in solvents such as water or alcohol that do not emit toxic vapors like benzene.

Finally, in the method of the present invention, there is no need to incorporate an organic binder into the powder during sample molding.

Claims (1)

Claims: 1. Preparing a mixture containing alpha-phase silicon carbide, boron, and a carbonaceous product, and sintering a sample of said mixture at a temperature on the order of 2100° C. in a neutral atmosphere. A method for producing a high-density silicon carbide ceramic comprising the steps of: preparing the mixture includes heat-treating the alpha phase silicon carbide powder under vacuum, and the treatment is performed at a temperature higher than 1200°C. A manufacturing method for high-density silicon carbide ceramic. 2. The method according to claim 1, characterized in that boron is added in powder form to silicon carbide powder before heat treatment. 3. The method according to claim 2, characterized in that the powder of silicon carbide and boron is mixed with a volatile organic liquid before heat treatment, pulverized in a steel ball mill, and dried in a drying chamber. . 4. The method according to claim 3, wherein the pulverized and dried powder is treated with hydrochloric acid and then washed with water before the heat treatment. 5. The method according to claim 1, wherein the carbonaceous product is a phenol lake, and the lake dissolved in a solvent is added to the heat-treated silicon carbide powder. 6. Process according to claim 1, characterized in that, before sintering, the mixture is kneaded in a ball mill coated with polytetrafluoroethylene, then dried and sieved. 7. The method according to claim 5, wherein the solvent is water. 8. The method according to claim 5, wherein the solvent is ethanol. 9. Process according to claim 1, characterized in that during production, boron in powder form is added after heat treatment of the alpha phase silicon carbide powder. 10. The method according to claim 9, characterized in that silicon carbide and boron are sequentially mixed in a volatile organic liquid, ground in a steel ball mill, and dried in a drying chamber. 11. The method according to claim 10, characterized in that the ground and dried powder is treated with hydrochloric acid and subsequently washed with water. 12 The carbonaceous product is a phenol lake,
12. A method according to claim 11, characterized in that the lake dissolved in a solvent is added after the water washing. 13. The method according to claim 9, characterized in that the temperature of the heat treatment is between 1200 and 1600°C. 14. Process according to claim 13, characterized in that the temperature of the heat treatment is substantially equal to 1600°C. 15. A method according to claim 9, characterized in that the heat treatment is carried out under a primary vacuum of the order of 1 Torr.