FR2589855A1 - Process for the manufacture of an article made of refractory-refractory composite material - Google Patents

Abstract

In order to form an article made of composite material in which the matrix is based on a refractory chemical compound capable of being obtained by chemical reaction between different components under the effect of a heat treatment, the process comprises the stages which consist in: - impregnating a fibrous reinforcement with a mixture which contains all the said components, at least one of which is in the form of a powder mixed with a resin, - forming the impregnated reinforcement, - thermally degrading the resin of the impregnating mixture in order to release the said components, and - carrying out a heat treatment at a sufficient temperature to permit the formation of the refractory matrix by in-situ reaction between the said components.

Description

PROCESS FOR MANUFACTURING A PART OF COMPOSITE MATERIAL
REFRACTORY-REFRACTORY
The present invention relates to refractory-refractory composite materials, and more particularly to materials of this type comprising a reinforcement of refractory fibers and a matrix based on a refractory chemical compound.

 By refractory fibers is meant here fibers for example made of silicon carbide, carbon, silicon nitride, alumina, alumina-silica, boron nitride ... whereas, by matrices based on refractory compound, we mean here matrices based for example on carbide, nitride, silicide, ...

 The refractory-refractory composite materials are especially usable for the production of parts subjected to significant thermo-mechanical constraints (thermal protections, nozzle elements, brake discs, diesel and turbine engine parts, etc.).

 For such applications, it is well known to use carbon-carbon composites consisting of a carbon fiber reinforcement and a matrix deposited on the fibers, for example by vapor phase infiltration (CVI).

 In some cases, it has been proposed to replace the carbon of the matrix, and possibly of the fibers, by a refractory material, in particular by a material having better mechanical strength and better resistance to oxidation at high temperatures. Thus, French patent No 2 401 888 proposes a method of infiltration of a carbon fiber reinforcement by a matrix of silicon carbide and European patent No 0 032 087 mentions a SiC-SiC composite (reinforcement in carbide fibers silicon and silicon carbide matrix infiltrated in the vapor phase). It has also been proposed to produce composites with a matrix based on one or more oxides.

Thus, European patent No. 0 085 601 describes a process for densifying a fibrous refractory preform by chemical vapor infiltration of an oxide such as alumina or zirconia. Furthermore, United Kingdom patent No 1 353 384 describes the production of a zirconia-zirconia type composite by impregnating a structure of zirconia fibers with a solution containing a zirconium salt and a suspension of refractory oxide followed by heat treatment at a temperature sufficient to transform the zirconium salt into zirconia. However, this latter process, even with repeated impregnation-heat treatment cycles, leads to a matrix which has poor mechanical characteristics and which has a high residual porosity.

 The object of the present invention is to provide a process allowing the production of refractory-refractory composites in which the matrix is a chemical compound put in place faster and more economically than by vapor phase infiltration, while ensuring reduction substantial of the initial porosity of the fibrous reinforcement and by providing a material having good mechanical properties.

 This object is achieved by a process of the type comprising the densification of a reinforcement in refractory fibers by a matrix based on a refractory chemical compound capable of being obtained by chemical reaction between different components under the effect of a treatment. thermal, this process comprising, in accordance with the invention, the steps which consist in - impregnating a fibrous reinforcement with a mixture which contains all of said components, at least one of which is in the form of a powder mixed with a resin, - putting in form of the impregnated reinforcement, - thermally degrading the resin of the impregnation mixture to release said components, and - carrying out a heat treatment at a temperature sufficient to allow the formation of the refractory matrix by in situ reaction between said components.

 Thus, according to the method according to the invention, a preform is first produced by means of the fibrous reinforcement associated with various ingredients serving simultaneously as a temporary matrix and as a precursor of the refractory matrix, the latter being formed during a treatment. subsequent thermal, by in situ reaction between components all preexisting in the provisional matrix.

 The refractory fibers constituting the fibrous reinforcement are chosen from the fibers currently known, for example fibers of SiC, C, Si3N4, A1203, A1203-SiO2, BN, ..... The material of the reinforcing fibers will be chosen according to the refractory matrix sought so as not to react with the constituents thereof during the formation of the matrix. The fibrous reinforcement is formed from continuous fibers or two-dimensional structures such as fabrics or felts which can be used continuously or after cutting.

 When the refractory matrix must be a carbon compound, the resin of the impregnation mixture is chosen from resins having a non-zero coke content, that is to say those giving carbon by thermal degradation, such as resins phenolics, furans and epoxides. The or each other component which must react with carbon to form the desired matrix is in the form of a powder mixed with the carbon precursor resin.

 According to another embodiment of the method, the resin of the impregnation mixture is chosen from thermofugitive resins, that is to say that does not give a residue after thermal degradation, such as polymethylmethacrylates or polyvinyl alcohols. These resins having a zero coke content can in particular be used when the presence of carbon is unnecessary or incompatible with the reaction for formation of the desired matrix. The components of the matrix are all then introduced in the form of powders into the resin.

 The powder or powders of the impregnation mixture are chosen according to the nature of the refractory matrix sought. For example, when the matrix to be formed is a carbide, the powder of the impregnation mixture consists of the chemical element giving the desired carbide by reaction with the carbon obtained by thermal degradation of the resin. In other cases, for example when the matrix to be formed is a silicide, the powders of the impregnation mixture consist of silicon and of the chemical element giving the desired silicide by reaction with the silicon after total degradation of the resin.

 It is possible to introduce into the impregnation mixture non-reactive refractory powders, for example silicon carbide powder in the case of production of a matrix based on silicon carbide, in order to obtain a composite with relatively low porosity. It is also possible to introduce powders of a different nature from that of the matrix in order to modify the properties, for example thermal, electrical, or the resistance to oxidation of the composite.

 In general, the powder or powders introduced into the impregnation mixture are as fine as possible, for example of particle size less than 25 microns.

 The quantities of powders introduced into the impregnation mixtures and intended to react to form the refractory matrix are determined so as to obtain stoichiometric ratios for the components of the matrix.

 The addition of the powder (s) to the resin is carried out so as to obtain a mixture as homogeneous as possible.

 The fibrous reinforcement is impregnated by immersion in a bath constituted by the impregnation mixture. With a fibrous reinforcement in the form of fabric, felt or wire, the immersion can be carried out continuously with spin to adjust the rate of impregnation to the desired value.

 The impregnated fibrous reinforcement is then used to make a preform close to the part to be manufactured. To this end, the impregnated fibrous reinforcement is cut, placed in a mold and molded under pressure at a temperature sufficient to cause the resin to harden in order to form a temporary matrix.

 The next step is thermal degradation of the resin. This treatment depends on the resin used. The conditions to be observed for thermal degradation cycles are known to those skilled in the art; various examples are given below.

 The thermal degradation of the resin is followed by the formation of the refractory matrix within the preform by chemical reaction between its components under the effect of heat.

For reasons of homogeneity of the matrix, limitation of internal stresses within the composites and non-degradation of the fibrous reinforcements, it is preferable to carry out a slow, non-violent reaction, by maintaining a temperature as low as possible while - remaining compatible with a reaction rate satisfying the economic imperatives of industrial manufacturing.

 Diffusion-type reactions are the most suitable. It has in fact been verified that, under these conditions, the molded preforms retain their initial shape and dimensions.

 The thermal degradation operations of the resin and the formation of the refractory matrix can be carried out successively during a single heat treatment operation.

 The composite material obtained after formation of the refractory matrix has a relatively high porosity, but can be used as it is because, as shown in the examples below, it has significant mechanical properties and its insulating power is high.

 The densification of this porous composite material by a refractory material, of the ceramic type, can be continued using conventional densification methods, in order to obtain a denser composite having different properties.

 This densification is achievable by reimpregnation with a ceramic precursor resin followed by thermal degradation treatment.

 It is also possible to carry out densification by chemical vapor infiltration.

 Different examples of implementation of the method according to the invention will now be described.

Example 1
A fabric of silicon carbide was impregnated with a mixture of phenolic resin and silicon powder whose diameter was at most equal to 25 microns. The mixture deposited represented 51% by weight of the impregnated product. The composition of this mixture was as follows: 53% by weight of resin and 47% by weight of Si powder.

These percentages have been calculated - taking into account the carbon content of the resin used - so as to obtain, after thermal degradation treatment, a stoichiometric C-Si ratio for the formation of SiC. Impregnated layers of 100 x 100 mm were cut and stacked for a pressure molding, which was carried out under the following conditions:
- 30 minutes at 130 OC,
- rise from 130 to 200 OC at the speed of 2 OC / minute,
- 20 minutes at 200 OC,
- free cooling in press.

 The molding pressure was 100 bars.

The thickness of the molded plate was 10 -. The volume content of fiber in the plate was 39%.

The thermal degradation of the resin and the formation of the SiC matrix were carried out in a single heat treatment applied to the molded plate
- rise in ambient temperature to 1150 OC at the speed of 300 hours,
- 30 hours at 1150 OC,
- free cooling in the oven.

 After this last treatment, the plate had a correct appearance, it was not cleaved or cracked.

The characterization of the plate gave the following results
- density: 1.80,
- porosity: 35 X volume,
- flexural strength: 120 MPa,
- modulus of elasticity: 56 GPa,
- thermal diffusivity perpendicular to the stratification
1.2.10 6m2.s 1 at room temperature and 0.65.10-6m2.s-1 from 700 to 1250 OC.

- thermal conductivity: 1.5 Wm 1.K 1
- interlaminar shear strength: 10 MPa.

 - specific heat: 0.75 J.g 1.K 1.

 - repeated thermal shocks with an oxyacetylene torch, no appearance of cleavage or cracking on the piece of plate used for the test.

Example 2
A roll of continuous SiC fiber was impregnated with a mixture of epoxy resin, loaded with silicon powder whose average diameter was equal to 2 microns.

 After passing through the impregnation bath, the fiber passes through a steam oven for 20 minutes at 120 ° C.

 After baking, the impregnated fiber contains 46% by weight of resin and Si powder mixture. This mixture consists of 68% by weight of resin and 32% by weight of powder.

 This impregnated fiber was then cut into elements of length equal to 5 mm. The compound thus obtained was molded under press in a punch-die mold in the shape of an engine valve about 15 cm in height.

The molding cycle adopted was as follows
- 30 minutes at 140 OC,
- rise from 140 to 180 OC at the speed of 0.5 C / minute,
- three hours at 180 OC,
- free cooling in press.

 The molding pressure was 70 bars. The volume content of fiber in the room was 42%.

The treatment adopted for the thermal degradation of the resin was as follows
- rise in ambient temperature to 350 OC at the speed of 50 C / hour,
- one hour at 350 C,
- rise from 350 OC to 600 OC at the speed of 50 C / hour,
- one hour at 600 OC,
- free cooling in the oven.

 The high temperature treatment adopted was as follows - rise in the ambient temperature to 1150 ° C. at a speed of 300 ° C./hour, - 30 hours to 1150 ° C., - free cooling in the oven.

 After this treatment, the part had a satisfactory appearance, it was neither cleaved nor cracked and had not undergone appreciable dimensional variations. Its density was equal to 1.35 and its porosity at 53% by volume.

 This part was then densified without any particular problem, by chemical vapor deposition of SiC, to a density equal to 2.4.

Example 3
A silicon carbide cloth was impregnated with a mixture of epoxy resin and silicon powder. The impregnated fabric was then placed for 10 minutes at 120 ° C. in an oven.

The percentages of resin and silicon powder on the fabric were 65% and 35% by weight, respectively.

The impregnate was then cut into 150 x 150 mm layers, and the layers were stacked in a press for molding under the following conditions
- 45 minutes at 140 OC,
- rise from 140 to 180 C at the speed of 1 C / minute,
- one hour at 180 OC,
- rise from 180 to 250 C at the speed of 0.5 C / minute,
- one hour at 250 OC,
- descent of 250 C at room temperature, at the speed of 3 C / minute.

 The cycles of treatment of thermal degradation of the resin, then of treatment at high temperature, which were then applied to the plate, were so by adopting the same conditions as indicated in example 2. At the end of treatment the plate was free from defects.

The characterization carried out on a part of the plate gave the following results
- density: 1.4,
- porosity: 45% by volume,
- flexural strength: 80 MPa,
- modulus of elasticity: 40 GPa,
- thermal diffusivity: 0.72.10 6m2.s 1 at room temperature and 0.5.106m2.s'1 at 800 OC,
- thermal conductivity: 0.76 Wm 1.Kl,
- repeated thermal shocks with an oxyacetylene torch, no appearance of cleavage or cracking.

 The remaining part of the plate was densified by chemical vapor depat of SiC. Weight recovery was 30% in just 80 hours of infiltration.

Example 4
An alumina tissue was impregnated with a mixture of methyl methacrylate and molybdenum and silicon powders. The methyl methacrylate was dissolved in trichlorethylene at the rate of 1 part of methyl methacrylate per 18.33 parts of trichlorethylene.

 The percentages of powders in the mixture were 63.2% by weight of molybdenum powder for 36.8% by weight of silicon powder. These powders represented 25.6% by weight of the impregnation mixture.

 The impregnated fabric was steamed for five minutes at 80 ° C. before cutting into 100 x 100 mm layers.

The conditions adopted for the press molding were as follows
- 30 minutes at 160 OC, under a pressure of 10 bars.

The molded plate was then treated using the following cycle
- rise of ambient 9 1450 OC at a speed of 300 hours,
- 10 minutes at 1450 OC,
- free cooling in the oven.

 After this treatment, the plate had a correct appearance, it was neither cleaved nor cracked.

 An X-ray examination, on a sample of the plate, showed that the matrix of the composite was indeed made up of Moi2, as desired.

Claims (11)

1. Method of manufacturing a part made of refractory-refractory composite material comprising the densification of a refractory fiber reinforcement by a matrix based on a refractory chemical compound capable of being obtained by chemical reaction between different components under the effect of a heat treatment, process characterized in that it comprises the stages which consist of:  - impregnating a fibrous reinforcement with a mixture which contains all of said components of which at least one is in the form of a powder mixed with a resin,  - shaping the impregnated reinforcement,  - thermally degrade the resin of the impregnation mixture to release said components, and  - Perform a heat treatment at a temperature sufficient to allow the formation of the refractory matrix by in situ reaction between said components. 2. Method according to claim 1 for manufacturing a piece of refractory-refractory composite material whose matrix is based on a carbon compound, characterized in that the resin of the impregnation mixture is chosen from that giving carbon by degradation thermal. 3. Method according to claim 2, characterized in that the composition of the impregnation mixture is determined, taking into account the carbon content of the resin, to obtain after thermal degradation of the latter a stoichiometric ratio between the carbon obtained and the other powder component (s) mixed with the resin. 4. Method according to any one of claims 2 and 3, characterized in that the impregnation mixture consists of a resin giving carbon by thermal degradation and silicon powder. 5. Method according to any one of claims 2 to 4, characterized in that the resin is chosen from phenolic, furanic and epoxy resins. 6. Method according to claim 1, characterized in that the resin of the impregnation mixture is thermofugitive, said components being in the form of powders mixed with the resin. 7. Method according to claim 6, characterized in that the resin is chosen from polymethylmethacrylates and polyvinyl alcohols. 8. Method according to any one of claims 1 to 7, characterized in that the powder or powders introduced into the impregnation mixture have dimensions between 0 and 25 microns. 9. Method according to any one of claims 1 to 8, characterized in that the impregnation mixture also contains at least one refractory material in powder form which does not react with said components. 10. Method according to any one of claims 1 to 9, characterized in that the thermal degradation and the formation of the refractory matrix are carried out during the same heat treatment operation. 11. Method according to any one of claims 1 to 10, characterized in that it comprises an additional densification step after the formation of the refractory matrix.