Process for depositing a coating on short fibres by calefaction

Abstract

A process for depositing a coating on short fibres of carbon or silicon carbide from a coating precursor, the short fibres having a length of between 50 μm and 5 mm, the process including at least heating the short fibres by placing a mixture including the fibres and a liquid phase of the coating precursor in a microwave field so as to bring the surface of the fibres to a temperature allowing the coating on the fibres from the coating precursor to be formed by calefaction.

Claims

1. A process for depositing a coating on short fibres of carbon or silicon carbide from a coating precursor, the short fibres having a length of between 50 μm and 5 mm, wherein the process comprises: heating the short fibres by placing a mixture comprising said fibres and a liquid phase of the coating precursor in a microwave field so as to bring the surface of the fibres to a temperature allowing the coating on the fibres from the coating precursor to be formed by calefaction, the coating being an interphase coating comprising pyrolytic carbon, boron nitride, silicon carbide, silicon nitride Si.sub.3N.sub.4, or SiBN, a liquid phase of the coating precursor coupling with the microwave field so as to bring it to boiling point so that, in said mixture, the fibers are dispersed in said liquid phase during application of the microwave field.

2. The process according to claim 1, wherein the length of the short fibres is between 100 μm and 300 μm.

3. The process according to claim 1, wherein the short fibres are made of silicon carbide having an oxygen content of less than or equal to 1 atomic percent.

4. A method according to claim 1, wherein the microwave field is obtained from a resonator having a resonant cavity.

5. The process according to claim 1, wherein the short fibres are obtained from fibres which are cut or ground.

6. A process for manufacturing a ceramic matrix composite part comprising a step of forming the matrix around short fibres on which a coating has been deposited by a process according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the present invention will be apparent from the description given below, with reference to the appended drawing, which illustrates an example embodiment without any limiting character. FIG. 1 shows an example of a device for implementing a process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) FIG. 1 shows a device for carrying out a process according to the invention.

(3) The device 1 comprises a resonator 2 with a resonant cavity 4, the resonator is connected to a wave generator 6 by a coaxial cable 8. During operation, a microwave field passes through the resonant cavity 4. A reactor 10 made of a material which is preferably transparent to microwaves (for example quartz) is placed inside the resonant cavity 4. The reactor 10 takes the form of a vial. The reactor 10 contains a liquid phase 12 of a coating precursor in which short fibres 14 of carbon or silicon carbide are present, together forming a mixture 16. The short fibres 14 are dispersed in the liquid phase 12, i.e. they are not bonded together and do not form a part preform at this stage. The reactor 10 can be provided at its opening 18 with means for feeding the liquid phase 12 of the precursor into the reactor, for example an addition bulb, and with means for preventing the escape of the vaporized precursor, for example a reflux arrangement (not shown). The device 1 may additionally comprise conventional means for homogenizing the mixture 16, for example a magnetic stirrer or a quartz propeller (not shown).

(4) In order to deposit a coating on the short fibres by a process according to the invention, the generator 6 is activated, which has the effect of creating a microwave field in the resonant cavity 4 in which the reactor 10 is present. The short fibres 14 are coupled with the microwave field, which has the effect of heating them, in particular by increasing their surface temperature. When the power of the microwave field is sufficient, the liquid phase of the coating precursor 12 vaporizes in the vicinity of the short fibres 14 and the precursor thus vaporized can form the coating directly on the short fibres by calefaction.

(5) As mentioned above, the skilled person can determine the power (and/or frequency if variable) of the microwave field to obtain sufficient coupling and the right temperature for heat deposition. Similarly, the thickness of the deposited coating is, all other things being equal, a function of the residence time of the short fibres and the liquid phase precursor in the microwave field. This allows the skilled person to determine the residence time required to obtain a certain average coating thickness.

(6) The short fibres 14 can be made of carbon or silicon carbide (SiC). For example, such short fibres may be Hi-Nicalon type S fibres marketed by the Japanese company NGS. Silicon carbide short fibres are preferred. Short fibres may have been obtained from continuous long fibres which are cut or ground.

(7) By means of a process according to the invention, an interphase can be formed on the short fibres. For example, a pyrolytic carbon (PyC) interphase can be formed using a coating precursor such as an alcohol, for example ethanol, a polyalcohol or toluene. For example, a silicon carbide interphase may be formed using a coating precursor such as methyltrichlorosilane or dimethylchlorosilane. For example, a boron nitride (BN) interphase may be formed using a coating precursor such as borazine, or a mixture of BCI.sub.3 and NH.sub.3.

(8) The short fibres thus coated with an interphase can then be used, for example, to manufacture a ceramic matrix composite part.

EXAMPLE

(9) A pyrocarbon interphase is deposited on short silicon carbide fibres by an example process according to the invention. For this purpose, a mixture is prepared comprising short fibres of Hi-Nicalon type S silicon carbide having an average length of the order of 200 μm and 10 μm in diameter (average filament diameter), and ethanol. The mixture is placed in a suitable reactor within the resonant cavity of a resonator having a natural frequency of the order of 2.45 GHz and supplied with 300 W power. The mixture is thus subjected to the microwave field for a period of about 10 minutes. Short fibres coated with a homogeneous and continuous pyrocarbon interphase on the surface of the fibres were obtained.