Method for the treatment of silicon carbide fibres

Abstract

The invention relates to a method for the treatment of silicon carbide fibers, comprising a step involving the chemical treatment of fibers with an aqueous acid solution containing hydrofluoric acid and nitric acid but free of acetic acid in order to remove the silica present on the surface of fibers and to form a layer of microporous carbon. The invention also relates to a method for the production of a fibrous preform, comprising the formation of a fibrous structure comprising treated silicon carbon fibers and the use of said preform for the production of a part made from composite material.

Claims

1. A process for the treatment of silicon carbide fibers, comprising the step of chemical treatment of the fibers with an aqueous acid solution containing hydrofluoric acid and nitric acid in order to remove the silica present at the surface of the fibers and to form a layer of microporous carbon, wherein said aqueous acid solution does not contain acetic acid and wherein the aqueous acid solution contains a hydrofluoric acid content of between 0.5 and 4 mol/l.

2. The process as claimed in claim 1, wherein the aqueous acid solution contains a hydrofluoric acid/nitric acid molar ratio of less than 1.5.

3. The process as claimed in claim 1, wherein the aqueous acid solution contains a nitric acid content of between 0.5 and 5 mol/l.

4. The process as claimed in claim 1, wherein the chemical treatment step is carried out at ambient temperature and lasts four hours.

5. The process as claimed in claim 1, wherein the fibers obtained have a superficial layer of microporous carbon 100 nm.

6. The process as claimed claim 1, wherein the fibers are covered with a sizing not removable by chemical treatment, and in that the process comprises the prior step of virtually completely removing said sizing by heat treatment.

7. The process as claimed in claim 1, wherein it comprises the additional step of depositing a layer of pyrolytic carbon on the silicon carbon fibers treated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be understood more clearly in the light of the figures and examples which follow.

(2) FIG. 1 represents the atomic concentration of silicon, carbon, nitrogen and oxygen (as %) as a function of the depth (in nm) inside the fiber before treatment (zero corresponds to the surface of the fiber).

(3) FIG. 2 represents the atomic concentration of silicon, carbon, nitrogen and oxygen (as %) as a function of the thickness of the fiber after treatment according to the invention (zero corresponds to the surface of the fiber).

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

Example 1

(4) The process in accordance with the invention was carried out on fibrous textures, and more particularly fabrics made from SiC fibers produced by the Japanese company Nippon Carbon Co. Ltd using a treatment for removal of silica and formation of a layer of microporous carbon by dipping the textures in a bath containing an aqueous solution of hydrofluoric acid (0.7 mol/l) and of nitric acid (3 mol/l) for four hours at ambient temperature.

(5) The fibrous textures obtained are then rinsed with water and dried.

(6) Before this treatment, desizing of the fibrous textures is carried out via the thermal process. The SiC fibers are brought to a temperature of 600 C. in air for 20 seconds.

(7) Comparing FIG. 1 and FIG. 2 attached hereto, it should be noted that the silica has disappeared from the surface of the fibers treated by means of the process according to the invention and has been replaced with a layer of microporous carbon.

Comparative Example 2

(8) The process in accordance with patent application FR 2 640 258 was carried out on fibrous textures, and more particularly fabrics made from SiC fibers produced by the Japanese company Nippon Carbon Co. Ltd using a treatment for removal of silica by dipping the textures in a bath containing an aqueous solution of hydrofluoric acid (at a concentration of between approximately 20 and 30 mol %) for approximately one hour at ambient temperature.

(9) The fibrous textures obtained are then rinsed with water and dried. Before this treatment, desizing of the fibrous textures is carried out by the thermal process, for example at 600 C. in air for 20 seconds.

(10) The fibers before treatment are covered with silica on an average thickness of approximately 0.3 micron. After this treatment, this thickness is reduced to approximately 0.005 micron and no trace of microporous carbon is present at the surface of the fibers.

Example 3

(11) The same process as in Example 1 is carried out, with the exception of the fact that the bath contains an aqueous solution of hydrofluoric acid at 2.3 mol/l and of nitric acid at 4 mol/l, that the treatment time is 75 minutes and that the treatment temperature is 30 C.

(12) The thickness of the layer of microporous carbon obtained at the surface of the fibers is 215 nm.

Example 4

(13) The same process as in Example 1 is carried out, with the exception of the fact that the treatment time is 30 minutes and that the treatment temperature is 30 C.

(14) The thickness of the layer of microporous carbon obtained at the surface of the fibers is 30 nm.

Example 5

(15) The same process as in Example 1 is carried out, with the exception of the fact that the treatment time is 120 minutes and that the treatment temperature is 30 C.

(16) The thickness of the layer of microporous carbon obtained at the surface of the fibers is 100-90 nm.

Example 6

(17) The same process as in Example 1 is carried out, with the exception of the fact that the treatment time is 480 minutes and that the treatment temperature is 10 C.

(18) The thickness of the layer of microporous carbon obtained at the surface of the fibers is at most 70 nm.