Process for producing a ceramic fiber composite material based on carbon with a proportion of silicon carbide, or based on silicon carbide, as well as corresponding fiber composite materials and components

Abstract

A method for producing a ceramic fiber composite material based on carbon with a proportion of silicon carbide (C / C-SiC), in which a C / C preform comprising a carbon matrix with carbon fibers embedded therein is infiltrated with liquid silicon in order to partially convert the carbon to silicon carbide, characterized in that the silicon used for the infiltration contains a proportion of 3 to 15 mol% boron, and that the infiltration is carried out over a period of 10 to 25 hours, comprising a heating period, a holding period at a maximum temperature and a cooling period, the holding period being between 0 and 1 hour.

Description

[0001] The present invention relates to a method for producing a ceramic fiber composite material based on carbon with a proportion of silicon carbide (C / C-SiC), in which a C / C preform comprising a carbon matrix with carbon fibers embedded therein is infiltrated with liquid silicon in order to at least partially convert the carbon of the matrix to silicon carbide.

[0002] The invention further relates to a ceramic composite material based on carbon with a proportion of silicon carbide, which is produced according to such a method, and to a component made of such a ceramic fiber composite material.

[0003] Ceramic fiber composites are generally characterized by excellent mechanical properties such as high strength, stiffness, and fracture toughness. In particular, the structural strength of these materials is maintained even at high temperatures (e.g., over 1,000 °C), allowing them to be used to manufacture components that must withstand high mechanical and thermal loads (e.g., brake discs for motor vehicles, trains, and aircraft). A further advantage is their significantly lower density compared to metals.

[0004] Compared to pure carbon-based C / C materials, C / C-SiC materials offer additional advantages such as increased oxidation resistance and reduced wear due to abrasion, both due to the proportions of silicon carbide and unreacted silicon. However, the conversion of carbon and silicon to silicon carbide during liquid-phase infiltration (LSI) also has disadvantages, as far as carbon fibers are concerned. Conversion of the carbon contained in the fibers to SiC impairs their load-bearing properties and thus tends to deteriorate the material's mechanical strength.

[0005] With the manufacturing processes known to date, it is not possible to limit the formation of SiC during the infiltration of the C / C preform with liquid silicon to the carbon matrix. Typically, approximately 10 to 50% of the carbon fibers are converted to SiC at the surface of the fiber bundles. This is particularly critical with a small number of carbon fibers per fiber bundle, i.e., with thin or flat bundles, since the conversion rate is correspondingly higher due to the high surface-to-volume ratio of the fiber bundles.

[0006] From the documents US 5 266 236 A, WO 2000 / 018702 A1, FR 3 008 968 A1, EP 1 117 625 A1, RU 2 147 982 C1, WO 2004 / 108 631 A1, FR 2 741 063 A1, FR 3 004 712 A1 and FR 2 996 550 A1, various processes for producing composite materials based on carbon and / or silicon carbide are known.

[0007] The invention is therefore based on the object of proposing a method for producing a C / C-SiC material in which the conversion of the carbon fibers to silicon carbide is reduced.

[0008] This object is achieved according to the invention by the method according to claim 1.

[0009] The effect discovered by the inventors, whereby fiber conversion to SiC can be significantly reduced during liquid-phase infiltration with a mixture of silicon and boron compared to the use of pure silicon, can be attributed to two different effects. Firstly, infiltration can be carried out at a lower temperature, namely below the melting point of silicon (1,410 °C), since a boron content of 3 to 15 mol% leads to a melting point depression. At lower temperatures, the reactivity of silicon towards carbon is reduced. Secondly, the presence of boron itself already leads to such a reduction in reactivity, especially since, according to the invention, a certain effect is achieved even at a constant infiltration temperature compared to pure silicon.

[0010] The boron content in the silicon used for infiltration is preferably in the range of 5 to 12 mol%, more preferably in the range of 7 to 9 mol%. Since the eutectic of the Si / B system is at a boron content of approximately 8 mol% with a melting point of 1,385 °C, this mixing ratio allows the temperature during infiltration to be reduced to the greatest extent.

[0011] Advantageously, the C / C precursor is infiltrated with a binary mixture of silicon and boron, i.e., apart from these two elements, the mixture contains no other components.

[0012] Infiltration is preferably carried out at a maximum temperature below 1,410 °C. In contrast, infiltration with pure silicon according to the state of the art is generally carried out at at least 1,420 °C, i.e., 10 K above the melting point of silicon, but frequently at much higher temperatures of over 1,600 °C. The maximum temperature is the maximum temperature reached after a heating phase.

[0013] In a preferred embodiment of the invention, the maximum temperature during infiltration is in the range of 1,385 to 1,400 °C. This temperature range is significantly below the melting point of silicon and is therefore not feasible when infiltrating with pure silicon.

[0014] The infiltration is carried out over a period of 10 to 25 hours, which includes a heating period, a holding period at a maximum temperature and a cooling period.

[0015] The holding period is between 0 and 1 hour, meaning the cooling period can follow essentially immediately after the heating period. Relatively short holding periods are preferred within the scope of the present invention to further minimize the conversion of the carbon fibers to SiC.

[0016] The heating rate is preferably in the range of 100 to 150 K / h, while the cooling rate is preferably in the range of 200 to 500 K / h. These values ​​can be optimized by a person skilled in the art, as with pure silicon infiltration; they also depend, among other things, on the size of the component to be manufactured.

[0017] The C / C preform is advantageously produced by pyrolysis of a fiber-reinforced plastic material comprising a matrix of a thermosetting resin with embedded carbon fibers. This procedure is known in the art.

[0018] Pyrolysis preferably takes place at a temperature of 900 to 1,600 °C in a vacuum or in an inert gas atmosphere. In particular, purging with an inert gas (e.g., nitrogen or argon) can be used to remove the degradation products formed during pyrolysis.

[0019] The thermosetting resin is preferably selected from phenolic resins, epoxy resins, and acrylic resins, with phenolic resins being particularly preferred. To produce the fiber-reinforced plastic material, the carbon fibers are impregnated with the resin or resin precursor, and the resin is then thermally cured.

[0020] The proportion of carbon fibers in the fiber-reinforced plastic material is preferably in the range of 45 to 70 vol.%, in particular in the range of 50 to 65 vol.%. The individual fibers, which typically have a thickness of 5 to 10 µm, are arranged in bundles (so-called "rovings"), each of which generally comprises 1,000 to 24,000 fibers. The fiber bundles are typically arranged in the form of a non-crimp fabric (e.g., a multiaxial non-crimp fabric), a woven fabric, a braided fabric, or a knitted fabric. Alternatively, short fibers can also be used.

[0021] The present invention also relates to a ceramic fiber composite material based on carbon with a proportion of silicon carbide (C / C-SiC) according to claim 11, which is produced by the process according to the invention.

[0022] Advantages and preferred embodiments of the fiber composite material according to the invention have already been explained using the method according to the invention.

[0023] Due to the lower degree of conversion of carbon fibers to SiC that can be achieved with the present invention, the proportion of unreacted silicon is also increased compared to the prior art (i.e., infiltration with pure silicon). This proportion is above 5 wt.% in the fiber composite material according to the invention, preferably in the range of 10 to 15 wt.%.

[0024] The lower degree of conversion of the carbon fibers usually also leads to an increased thermal conductivity of the fiber composite material, since the thermal conductivity of the fibers is sometimes very high.

[0025] The fiber composite material according to the invention preferably has a density of less than 1.9 g / cm 3 preferably below 1.8 g / cm 3 . The lower density compared to state-of-the-art C / C-SiC materials (usually over 1.9 g / cm 3) represents a further advantage for many applications of the fiber composite material.

[0026] The present invention further relates to a component made of the ceramic fiber composite material according to the invention. The respective geometric shape of the component can be produced by mechanical processing methods, whereby the shaping can take place both before infiltration with silicon, i.e., at the stage of the C / C preform, and after infiltration, i.e., at the stage of the produced C / C-SiC material.

[0027] In a preferred embodiment of the invention, the component is a brake disc or a friction lining, in particular for a motor vehicle, a train or an aircraft.

[0028] A further preferred embodiment of the invention relates to components as structural elements for aircraft and spacecraft, in particular for satellites (e.g., housing structures, antennas, mirror supports). Very thin-walled structural elements can also be produced with particular advantage from the fiber composite material according to the invention, since the C / C-SiC material exhibits excellent mechanical properties due to the low fiber conversion.

[0029] Within the scope of the present invention, it was further found that infiltration with a mixture of silicon and boron is also advantageous in the production of SiC / SiC materials, ie when silicon carbide fibers are used instead of carbon fibers.

[0030] Accordingly, a further aspect of the invention relates to a method for producing a ceramic fiber composite material based on silicon carbide (SiC / SiC) according to claim 14.

[0031] Preferred embodiments of the production method according to the invention concerning the proportion of boron, the temperature and duration of the infiltration as well as the production of the preform by means of resin impregnation of the SiC fibers and pyrolysis also apply accordingly to the production of the SiC / SiC material according to the invention.

[0032] In contrast to the C / C-SiC material described above, however, it is preferred in this case if the carbon of the SiC / C precursor is essentially completely converted to silicon carbide. This results in a ceramic fiber composite material that contains no or only a very small amount of carbon and thus exhibits very high oxidation resistance.

[0033] However, the structure and thus the load-bearing capacity of the SiC fibers are also impaired when a SiC / C preform is infiltrated with pure silicon according to the current state of the art. This essentially results in the dissolution of SiC crystallites in the fibers by the liquid silicon, so that ultimately, parts of the SiC in the fibers are replaced by silicon, and the dissolved SiC crystallizes with the newly formed SiC in the matrix. This essentially corresponds to the well-known process of Ostwald ripening.

[0034] Surprisingly, this effect practically no longer occurs in the process according to the invention, since the reactivity of the mixture of silicon and boron towards the SiC fibers is apparently significantly reduced.

[0035] The invention further relates to a ceramic fiber composite material based on silicon carbide (SiC / SiC) according to claim 16, which is produced by the process according to the invention, as well as components produced therefrom. Due to the largely intact SiC fibers in the material according to the invention, its mechanical properties are significantly improved compared to a material produced by infiltration with pure silicon.

[0036] The components according to the invention made of the SiC / SiC material are preferably used where, in addition to high mechanical strength, very high temperature and oxidation resistance are important, in particular as components of jet engines and stationary gas turbines (e.g. casings, combustion chambers and turbine blades) or as structural components in nuclear reactors.

[0037] These and other advantages of the invention are explained in more detail using the following embodiments. Examples 1. Production of C / C-SiC materials

[0038] To produce carbon / carbon preforms, carbon fibers with a diameter of 5 to 6 µm were arranged in fiber bundles of approximately 6,000 fibers each, impregnated with a phenolic resin, and cured using an autoclave. After curing the resin at 175 °C, the fiber-reinforced plastic material was pyrolyzed at 1,650 °C in a vacuum, converting the resin into a carbon matrix.

[0039] The C / C preforms used for the subsequent step of infiltration with silicon or a mixture of silicon and boron each had dimensions of 100 x 30 x 3 mm.

[0040] To demonstrate the basic effect of the process according to the invention, three examples of infiltration were carried out with the following compositions and maximum temperatures: Example 1: Infiltration with pure silicon; maximum temperature 1,420 °C (not according to the invention) Example 2: Infiltration with 92 mol% silicon / 8 mol% boron; maximum temperature 1,420 °C (according to the invention) Example 3: Infiltration with 92 mol% silicon / 8 mol% boron; maximum temperature 1,395 °C (according to the invention)

[0041] Heating was carried out at a rate of 130 K / h until the maximum temperature was reached, followed by immediate cooling to room temperature at a rate of 500 K / h. Infiltration was carried out in a vacuum.

[0042] SEM images (scanning electron microscopy) were taken of the C / C-SiC materials produced, which for examples 1 to 3 are shown in the corresponding Fig. 1 to 3, each at a magnification of 500x and 1,000x. The center of each image shows fiber bundles with carbon fibers running perpendicular to the image plane. Carbon appears dark gray in the images, silicon carbide appears white, and unreacted silicon appears light gray.

[0043] When comparing the Fig. 1 to 3 clearly show that in non-inventive example 1, the conversion of carbon fibers to SiC is significantly pronounced at the edge of the fiber bundles and along the fractures created during pyrolysis. In inventive example 2, the formation of SiC in these areas is already somewhat lower due to the addition of 8 mol% boron. The effect is even more pronounced in inventive example 3, where the infiltration temperature was lowered to 1,395 °C compared to the other examples.

[0044] While the layer thickness of the fibers converted to SiC in Example 1 (according to the prior art) is up to approximately 15 µm, the thickness of the conversion layer in Example 3 according to the invention can be significantly reduced to approximately 0.5 µm by adding boron and reducing the temperature. Due to the lower conversion to SiC, the carbon fibers retain their load-bearing properties, and the mechanical strength of the ceramic fiber composite material is improved.

[0045] Further examples were used to investigate how the boron content affects the degree of carbon fiber conversion. In these examples, infiltration was carried out at a maximum temperature of 1,400 °C with the following compositions: Example 4: Infiltration with pure silicon (not according to the invention) Example 5: Infiltration with 4 mol% silicon / 96 mol% boron (according to the invention) Example 6: Infiltration with 8 mol% boron / 92 mol% silicon (according to the invention) Example 7: Infiltration with 12 mol% boron / 88 mol% silicon (according to the invention)

[0046] SEM images with a magnification of 2,000 times are available for the C / C-SiC materials according to Examples 4 to 7 in the corresponding Fig. 4 to 7. Here, too, carbon appears in dark gray, silicon carbide in white, and unreacted silicon in light gray.

[0047] A comparison of the Fig. Figures 4 to 7 show that even with a boron content of 4 mol%, the conversion of carbon fibers to SiC can be reduced to a certain extent. However, this effect is much more pronounced at a boron content of 8 mol%, which also represents the eutectic of the Si / B system. Increasing the boron content to 12 mol% leads to a similar result, i.e., a significant increase in the effect compared to 8 mol% is no longer achieved. 2. Production of SiC / SiC materials

[0048] The production of C / SiC preforms was essentially analogous to the production of the C / C molded bodies described above, except that silicon carbide fibers with a fiber diameter of 7 to 10 µm were used instead of carbon fibers. The SiC fibers were purchased from Ube Industries under the designation Tyranno SA3.

[0049] Infiltration with silicon or a mixture of silicon and boron was carried out in a vacuum and at a maximum temperature of 1,415 °C, with a heating rate of 130 K / h and a cooling rate of 500 K / h. The following examples were performed to illustrate the influence of the boron content: Example 8: Infiltration with pure silicon (not according to the invention) Example 9: Infiltration with 4 mol% boron / 96 mol% silicon (according to the invention) Example 10: Infiltration with 8 mol% boron / 92 mol% silicon (according to the invention)

[0050] SEM images of the SiC / SiC materials produced according to Examples 8 to 10 are shown in the corresponding Fig. 8 to 10. The 20,000x magnifications each show the cross-section of a single SiC fiber, the Fig. Figure 10 also shows an image at 200x magnification. The SiC in the fibers appears white, and the surrounding silicon appears black.

[0051] In the Fig. 8 it is clearly visible that the structure of the SiC fiber has been largely dissolved and replaced by silicon, i.e. the infiltration with pure silicon significantly impairs the integrity of the SiC fibers and thus the mechanical properties of the produced fiber composite material.

[0052] In contrast, the Fig. 9 and Fig.10 shows that even at a boron content of 4 mol%, the SiC fibers remain essentially completely intact, with the same result being obtained with 8 mol% boron. The black areas visible here inside the SiC fibers represent residual carbon resulting from the fiber manufacturing process. A similar result was also achieved with 12 mol% boron (not shown).

Claims

[1] Process for producing a ceramic fiber composite material based on carbon with a proportion of silicon carbide (C / C-SiC), in which a C / C preform comprising a carbon matrix with carbon fibers embedded therein is infiltrated with liquid silicon in order to partially convert the carbon into silicon carbide, characterized by that the silicon used for the infiltration contains a proportion of 3 to 15 mol% boron, and that the infiltration is carried out during a period of 10 to 25 hours, comprising a heating period, a holding period at a maximum temperature and a cooling period, the holding period being between 0 and 1 hour. [2] The process according to claim 1, wherein the boron content is in the range of 5 to 12 mol%, preferably in the range of 7 to 9 mol%. [3] A method according to claim 1 or 2, wherein the C / C precursor is infiltrated with a binary mixture of silicon and boron. [4] A process according to any one of the preceding claims, wherein the infiltration is carried out at a maximum temperature below 1,410 °C. [5] The method according to claim 4, wherein the maximum temperature during infiltration is in the range of 1,385 to 1,400 °C. [6] A process according to any one of the preceding claims, wherein the heating rate is 100 to 150 K / h and the cooling rate is 200 to 500 K / h. [7] A method according to any one of the preceding claims, wherein the C / C preform is produced by pyrolysis of a fiber-reinforced plastic material comprising a matrix of a thermosetting resin with carbon fibers embedded therein. [8] The process according to claim 7, wherein the pyrolysis is carried out at a temperature of 900 to 1,600 °C in vacuum or in an inert gas atmosphere. [9] A method according to claim 7 or 8, wherein the thermosetting resin is selected from phenolic resins, epoxy resins and acrylic resins. [10] Method according to one of claims 7 to 9, wherein the proportion of carbon fibers in the fiber-reinforced plastic material is in the range of 45 to 70 vol.%. [11] Ceramic fiber composite material based on carbon with a proportion of silicon carbide (C / C-SiC), produced according to a process according to one of the preceding claims, wherein the proportion of unreacted silicon in the fiber composite material is more than 5 wt.%, preferably in the range of 10 to 15 wt.%. [12] Ceramic fiber composite material according to claim 11, wherein the fiber composite material has a density of less than 1.9 g / cm 3 preferably less than 1.8 g / cm 3 . [13] Component made of a ceramic fiber composite material according to one of claims 11 to 12, wherein the component is preferably a brake disc or a friction lining or a structural element for aircraft and spacecraft, in particular for satellites. [14] A process for producing a ceramic fiber composite material based on silicon carbide (SiC / SiC), in which a SiC / C preform comprising a carbon matrix with silicon carbide fibers embedded therein is infiltrated with liquid silicon in order to convert the carbon at least partially into silicon carbide, characterized by that the silicon used for the infiltration contains a proportion of 3 to 15 mol% boron, and that the infiltration is carried out during a period of 10 to 25 hours, comprising a heating period, a holding period at a maximum temperature and a cooling period, the holding period being between 0 and 1 hour. [15] The method according to claim 14, wherein the carbon of the SiC / C precursor is substantially completely converted to silicon carbide. [16] Ceramic fiber composite material based on silicon carbide (SiC / SiC), produced according to a process according to claim 14 or 15, wherein the proportion of unreacted silicon in the fiber composite material is more than 5 wt.%, preferably in the range of 10 to 15 wt.%. [17] Component made of a ceramic fiber composite material according to claim 16, wherein the component is preferably a component of a jet engine, a stationary gas turbine or a nuclear reactor.

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