DE102005045307B4 - Processing method for an open porous carbonaceous body - Google Patents

Abstract

Processing method for an open-porous carbon-containing body in which an open-porous carbon preform is produced by pyrolysis, which is infiltrated with a medium, wherein the pores are at least superficially closed, then mechanically processed, after which the medium is removed from the pores.

Description

The The invention relates to a processing method for an open porous carbonaceous body.

to Production of SiC ceramics become open porous carbonaceous bodies Subjected to siliconization. Due to the open porosity, the access of the silicon melt in the body allows.

From the DE 600 06 804 T2 there is known a preform used in the manufacture of composite articles having the shape of a generally circular web composed of a fiber assembly located within the confinement of the preform; these boundaries are defined by an upper and a lower circular non-woven surface. The fibers of this arrangement are provided as chords which can have a wide range of orientations across the thickness of the mat.

From the DE 44 38 455 C1 For example, there is known a method of manufacturing a friction unit for frictionally engaging a mating body, wherein a porous carbon body corresponding to the final contour of the friction unit is provided, the pores of that carbon body are infiltrated with liquid silicon, and initiating a chemical reaction to form silicon carbide the body is ceramized. Before the silicon infiltration of the porous carbon body is structured so that cavities and / or recesses for cooling and / or stiffeners are formed in defined inner or outer regions, which are maintained after the ceramization in their shape and size.

From the DE 196 36 223 A1 For example, there is known a method for permanently bonding at least two component components to a molded article, in which a carbonaceous paste is introduced between the component parts to be bonded and a carbon skeleton is heated, and thereafter at a temperature above 1410 ° C with silicon to form silicon carbide is enforced.

Of the Invention is based on the object, a processing method for one open porous carbonaceous body to provide by which the production of ceramic components is simplified.

These Task is inventively characterized solved, that by Pyrolysis made an open porous carbon preform which is infiltrated with a medium, the pores being closed at least superficially be, then mechanically is processed, after which the medium is removed from the pores.

Open porous preforms can basically by Dry machining or wet processing to be edited. However, the high porosity causes a reduced mechanical stability. Processing can cause breakouts, chips or microcracks come. For fiber reinforced intermediate bodies It may be due to the reduced shear strength parallel to the fiber direction come to delaminations.

at the solution according to the invention is the preform temporary stabilized by infiltration with the medium. This can be done a machining and in particular mechanical machining perform at the danger of outbreaks, Chipping or microcracks and delaminations is reduced. As a result, in particular thin-walled and filigree components can be produce.

Of the Infiltrate body let yourself Edit near-net contour. This is at least the scope of the final contour machining a ceramic component reduced; the machining of ceramic components is expensive because diamond tools must be used and long processing times are required. Furthermore, at the Machining of ceramic components with fiber reinforcement the problem occur that fibers be cut and thereby the oxidation behavior of the Ceramic component deteriorates.

The method according to the invention makes it possible to produce components with filigree wall structures. For example, components can be made with wall thicknesses smaller than 5 mm: It can be components for example for plate heat exchangers (such as in the EP 1 544 565 A2 described) or turbine wheels produce.

By the solution according to the invention can be also easy connection joints produce. joints of workpieces have to before the fission be mechanically processed. This can be an open-porous material to outbreaks to be joined Lead bodies. Due to the high capillarity an open-pore Structure can be a significant proportion of applied to a surface Add paste be absorbed by the pore structure and is not more for the coincidence ready. The fact that through the Mediuminfiltrierung closed in particular near-surface pores be, lets go a joining paste easily apply to a workpiece surface to form a film. This can be done a good joint connection between two or more bodies achieve.

A mechanical wet machining of a open porous carbon-containing precursor can lead to a pre-damage of the surface structure of the corresponding workpiece. As a result, voltages can be indicated in particular in further processing steps. For example, microcracks may occur which affect silicon infiltration in the manufacture of a SiC device. In particular, structural defects in the siliconization can increase.

By the solution according to the invention is the rejection rate both during mechanical processing as well as during the siliconization process reduced.

It can also be over the medium introduce dopants into the pores.

Especially becomes the infiltrate body machined to give it a specific contour. In particular, it becomes a final contour of a component to be produced given or he is processed close to the final contour. It is cheap if the Infiltrate body is brought into a defined shape. It is then, for example also no or only a slight mechanical post-processing of a ceramic component necessary.

It can be provided that a or multiple areas of the infiltrate body superficially with a Add paste be provided. In the infiltrate body are, if sufficient Medium quantity is offered, the pores closed on the surface. Thereby can compound paste no over Capillary action are sucked into pores. This is a sufficient Amount of joining paste ready for the addition and there are good joints produce.

Cheap is it if, before providing an area with joining paste a surface layer Will get removed. This can be done achieve that to be joined surface has the structure of a carbon matrix of the preform and not one surface layer with elevated Media content. The surface has then the original one again Carbon structure, which make a connection with the joining paste can.

All it is particularly advantageous if the medium is chosen as that it in liquid Shape into the pores of the pre-body in which pores become solid, with the carbonaceous material preform does not react chemically and is removable from the pores. Thereby let yourself a temporary one mechanical stabilization of the preform for its processing reach, again an open-porous body is produced by the the medium infiltration is only slightly influenced.

It is also cheap if the medium is chosen that way will that the Mass loss by the removal of the medium is at least 80%. Preferably, the mass loss is at least 90%. Most preferably the mass loss is at least 94%. This can be done guarantee, that only a small proportion of the medium after the removal process the pores remain in the corresponding workpiece and that the initial porosity again to a great extent is made.

It is also cheap if the medium is chosen that way will that the Residue when removing at most Carbon or silicon or silicon carbide. This will be the properties of a carbide ceramic through the intermediate manufacturing step of Medium infiltration, especially based on silicon minimally affected.

Especially the medium is removed by decomposition or after phase transformation. decomposable Media are, for example, resins or hard waxes. A hard wax can decompose, for example, by burnout. A resin can be decomposed for example by pyrolysis. An example of a medium that is removable after phase transformation is water. This can in liquid Form are introduced into the pores. By cooling the pre-body can Ice forms in the pores. The corresponding infiltrated preform can then be to edit. By temperature increase let yourself to liquefy the ice again and drive out of the pores.

All very cheap it is when the medium is chosen like that that's it Removable by temperature treatment. By heating the appropriate workpiece let yourself then expel a high mass fraction of the medium out of the pores again. This can be done again an open-pored body produce. The medium is in particular a substance which in the Pores can reversibly or irreversibly solidify or harden.

Especially the medium is a resin (especially carbon based) or a hard wax or a silicon polymer. These substances can be in cure the pores and expelled again by temperature treatment.

It has been considered favorable proven when the medium is an epoxy resin and in particular a cold setting Epoxy resin is. This can be done easily perform a pore infiltration and the medium is also removable from the pores again.

It can be provided that one or more dopants are added to the medium. This allows dopants in the Po bring in the pre-body. A possible material for a dopant is boron. It has been shown that boron promotes crystal growth of SiC in the microstructure. By introducing the doping material through the medium, the dopant can be deposited on an inner pore surface and distributed in the preform.

Especially the medium is burned out or pyrolyzed for removal. The Burning out under air or pyrolysis as decomposition under inert conditions can take place in a particular heating step or during a Pyrolysis process for the carbonaceous body and / or even while heating for take a Silicierungsvorgang. For example, a hard wax heated in air so that it burns out. The temperature must be there are below the oxidation temperature of the carbon body.

For example the removal of the medium takes place under inert conditions such as vacuum conditions or inert gas conditions. This minimally affects the body and leaves it higher ones Temperatures reach because the carbon is not oxidized. The medium is decomposed pyrolytically.

For example the removal of the medium above temperatures of 500 ° C, a high mass loss for to get the medium.

at an embodiment it is envisaged that the Infiltrate body subjected to pyrolysis. In particular, it is for a second pyrolysis step. A first pyrolysis step was previously performed, around the open-porous carbon-containing preform manufacture.

The Medium infiltration can be achieved by immersing the precursor in take a medium bath. This can be a uniform and achieve rapid infiltration.

It can also be provided that a Medium infiltration by brushing the precursor with medium or dripping from medium to the preform he follows.

It is possible, that the Infiltration is carried out at atmospheric pressure. It can also work under vacuum respectively. This can be done in certain circumstances a higher one Infiltration depth at the preform to reach. It can also with overpressure done (after previous evacuation). Depending on the pressurization can the Control infiltration depth. For example, for a grinding or Drilling or for the production of a joint connection often no full impregnation necessary.

It can be provided that the Removal of the medium followed by carbide-former infiltration and in particular a siliconization is carried out. This can be done to produce a carbide ceramic component. This must if the Infiltrate body was processed accordingly, no longer or to a lesser extent be mechanically processed. It can also be another pyrolysis especially if they are large components.

For example If the resulting component is a C-SiC component or C / C-SiC component or SiSiC device or C-SiSiC device with a carbon fiber reinforcement.

Of the open-pored is carbonaceous precursor a carbon preform d. H. a preform, which consists of monolithic carbon, for example in the form consists of graphite.

It is possible, too, that the open-porous carbonaceous preforms a C / C preform with carbon fiber reinforcement is.

Especially is the pore content in the preform between 10% by volume and 90% by volume or between 20% by volume and 80% by volume. This produces the necessary open porosity, to be able to carry out a carbide-forming infiltration and a carbide-ceramic To be able to produce a component.

The The following description of preferred embodiments is used in conjunction with the drawing of the closer explanation the invention. Show it:

1 the mass loss on a C / C body which is infiltrated with the epoxy resin system L285 / H286, with temperature increase relative to the mass of the body before infiltration;

2 a SEM cross section of a C / C preform;

3 a SEM micrograph of a C / C preform after infiltration with an epoxy resin (infiltrate body);

4 a SEM micrograph of an infiltrate body after pyrolysis at 1650 ° C; and

5 a SEM micrograph of a C / C-SiC component, which by siliconation from a workpiece according to 4 is made.

The invention relates to a processing method for an open-porous carbon-containing body. In the open-porous carbonaceous The body is, for example, a carbon body such as a graphite body or a C / C body with carbon fibers embedded in a carbon matrix ( 2 ).

First, will an open-pored carbonaceous precursor produced. For this purpose, for example, a fiber-epoxy resin mixture with previous epoxy hardening pyrolyzed. The resulting open-porous carbon-containing preform (preform) has an open pore fraction, for example, between 20% by volume and 80% by volume.

Of the thus produced open-porous carbonaceous precursors let yourself in principle work mechanically. It can Dry machining processes or wet machining processes used become. For example, machining is done by drilling, turning, milling, grinding. However, it is open-porous preforms due to the porosity mechanically less stable. It can therefore be at the mechanical Processing to outbreaks, Burst or microcracks come.

at fiber reinforced intermediate bodies it may be due to the low shear strength parallel to the fiber direction come to delaminations. The processing is especially difficult in thin-walled Components with wall thicknesses for example, less than 5 mm or filigree components, the for example, have a thread.

According to the invention porosity temporary lifted by the pores are infiltrated with a medium. The medium provides for a mechanical stabilization of the preform, which then better mechanically is workable.

When Medium a material is chosen which in liquid Form penetrate into the pores of the preform can, solidify (in particular harden) there, and which one can Conclusion of the Machining again with low mass residue from the pores removable is and in particular burned out. Preferably, the mass residue in the pores smaller than 10% by mass and favorably smaller than 6 Mass%. Furthermore, the medium should be mixed with the carbonaceous material preform do not react chemically.

If the final product of the manufacturing process is a ceramic component and especially SiC ceramic component, then should advantageously at the most Carbon residues and / or Silicon residues and / or Silicon carbide residues are present.

at the medium is, for example, a liquid epoxy resin, which curable in the pores is. It is basically possible, that this Medium cold-curing is or thermosetting is. A cold curing can be carried out in air under normal pressure. A hot curing can at higher Temperatures are carried out under reduced pressure conditions; A hot curing can be more advantageous be if thicker wall thicknesses available. It is also advantageous in this case if longer infiltration times be set!

For example becomes a cold-curing medium Epoxy resin system used. An example of such an epoxy resin system is the type L285 laminating resin with H286 hardness of MGS resin products GmbH, Stuttgart.

It is also possible, for example, that as a medium a silicon polymer or a (burnable) hard wax used becomes.

The Infiltration of the carbonaceous precursor body for the production of a Infiltrate body takes place, for example, by immersing the preform in a medium bath. It is possible, too, that this Medium on the preform is dropped or that the preform with the medium is coated.

The Infiltration may be at atmospheric pressure or under negative pressure or overpressurization respectively; with negative pressure or overpressurization can be in certain circumstances increase the infiltration depth.

To Production of the infiltrate body, which no longer openly porous is and by that the open-porous preforms mechanically stabilized, can be this particular mechanical edit. For example he will processed close to the final contour.

It is possible, too, several infiltrate bodies over one Add paste to join together. If open-porous preforms with joining paste joined together should be, then basically occurs the problem on that joining paste due to capillary action in pores "disappear" can. In case of an infiltrate body closed the pores, so this Problem does not occur.

It is particularly provided that joints are machined prior to application of the joining paste to give them a defined shape. It is also advantageous if a surface layer after the medium infiltration in a region which is to be provided with joining paste is mechanically removed. It has been shown that in addition to the volume infiltration with the medium also a thin medium layer (medium film), the example by the infiltration process on a carbon-containing precursor body has a thickness of the order of 1 to 2 mm, can form on the component surface. This should be removed, for example by grinding, before the application of joining paste, since the component surfaces to be joined should have the structure of the matrix carbon; this structure is to be exposed by grinding.

To the processing of the infiltrate preform is the medium of this removed to make an open porous carbon-containing body again. The medium removal is carried out by temperature treatment. By the Temperature treatment, depending on the medium, a decomposition or phase transformation solid-liquid.

In 1 a diagram is shown for a C / C preform infiltrated with the L285 / H286 resin system, which is temperature treated. This results in a decomposition with concomitant medium removal. Δ is the mass loss relative to the mass of the carbonaceous precursor before the medium infiltration.

you recognizes from this diagram that at a Temperature treatment (pyrolysis) with temperatures above approx. 500 ° C Residue less than 10 mass% remains in the workpiece.

The Temperature treatment takes place under inert conditions, for example under a vacuum atmosphere or an inert gas atmosphere.

It is possible, that one another (second) pyrolysis stage is performed. For this purpose, the workpiece, for example to a temperature of about 1650 ° C heated under exclusion of air.

The resulting open-porous carbonaceous workpiece can be further processed by, for example, a carbide-forming infiltration and in particular carried out a silicization becomes. During silicification, the pores become liquid silicon infiltrates and SiC can form. The resulting component is a (carbide) ceramic component. For example, it is around a C-SiC component, if the preform is not fiber-reinforced, or to a C / C SiC component, when the preform with carbon fibers reinforced is.

The Ceramic component is over a near net shape treatment of the infiltrate body produced close to the final contour. A mechanical processing of the ceramic component is then not necessary or to a small extent necessary; the mechanical Machining of ceramic components is expensive because diamond tools are used Need to become and long processing times are required. Continue occurs when working on a ceramic component, the problem is that fibers can be cut and thereby the oxidation behavior of the corresponding component deteriorated.

By the solution according to the invention can be in a simple way, a ceramic component of a defined final contour manufacture, wherein the mechanical processing on a mechanical stabilized infiltrate body feasible is. The ceramic component produced can, for example, have wall thicknesses, which are smaller than 5 mm. By the preparation according to the inventive method is an exact fit Final production possible, at most a minimized mechanical post-processing on the ceramic material necessary is (for example, at mating surfaces or functional surfaces).

Farther can be joints with joining paste produce in a simple manner, so that a secure joint connection in a manufactured ceramic component results.

In 2 is an SEM cross section taken from an example of a carbonaceous precursor, namely a carbon fiber C / C precursor embedded in a carbon matrix. This C / C preform is open-pored.

In 3 is a cross-sectional view of an infiltrate body shown, which by medium infiltration (in particular of an epoxy resin) from the preform according to 2 arises. The liquid medium at least partially fills pores and hardens in the pores.

In 4 is the infiltrate body according to 4 shown after pyrolysis at about 1650 ° C. During pyrolysis, the medium decomposes and the pores are released again. Preferably, only a small mass fraction (less than 6 mass%) of the medium remains in the pores.

In 5 a SEM cross-section of a finished ceramic component (C / C-SiC device) after silicon infiltration is shown.

The component according to 5 has substantially the same structure as a component which directly by silicification of the preform according to 2 will be produced.

First concrete embodiment (production of a turbine)

In a first step, a fabric-reinforced CFRP sheet with dimensions of 300 mm × 300 mm × 15 mm was produced in an autoclave process. The fiber volume content was 60%. The open porosity was about 8%. In a second step, pyrolysis of this CFRP preform was carried out guided. In this case, the plate shrank in the thickness direction by about 10% The open porosity of the thus obtained carbonaceous precursor was about 25% by volume.

In a third step was a water jet technique machined out of the plate with a diameter of 100 mm.

In In a fourth step, this disc was made with a mixture of Epoxy resin L285 and hardener H286 infiltrated. The disk was placed in an infiltration tank (Exikator) for about. Evacuated at about 150 to 200 mbar for 0.5 h. The epoxy resin was dripped over the workpiece supplied until it is complete enclosed by the epoxy resin. The holding time in the infiltration tank was about 0.5 h to a complete To ensure degassing. The workpiece was taken from the infiltration vessel after this holding time and at approx. 100 ° C on Air for Hardened for about 8 h.

In a fifth Step was a mechanical processing of this infiltrate body. It was the final contour of the turbine wheel including the Turbine blades made. The turbine wheel had final dimensions of 10 mm thickness, one outside diameter of 72 mm, an internal bore of 8 mm, a diameter up to Shovel foot of 51 mm and lengths of the individual blades of 10.5 mm. The wall thicknesses of the blades were between 0.8 and 2.1 mm. The workpiece thus produced was crack-free and free of delamination.

Subsequently was without additional Weighting of the workpiece carried out a pyrolysis at a temperature of 1650 ° C.

In a seventh step was finally a silicon infiltration of the open-porous C / C body carried out. The body had a mass of 21.6 g. The siliconization was carried out in vacuo at a pressure of less than 3 mbar and a temperature of 1650 ° C. It was while offering a quantity of 32.8 g of Si granules; this amount of silicon was almost complete picked up by the component. After silicon infiltration, this was shown Turbine wheel to a mass of 51.8 g. Another mechanical Editing was not necessary.

Second concrete embodiment (addition of biomorphic Carbon bodies)

In a first step, two circular disks with a mass of approx. 14 g, a diameter of 30 mm and a thickness of 6 mm were made from biomorphic carbon. The density of the circular disks was about 0.8 g / cm ³ . The open porosity was about 70% by volume. The surfaces of this carbon preform were ground plane-parallel.

In a second step was an epoxy resin solution of 71.4% by mass epoxy resin (L285) and 28.6 mass% hardener (H286).

In in a third step, the surface of the carbon precursor was sealed, by the workpiece in air in the epoxy resin solution was immersed. In the thus prepared infiltrate body was due to the low viscosity the epoxy resin solution a plane even distribution possible on the workpiece surface. The workpieces were stored in a laboratory oven at about 150 ° C for 15 h. It was cured the epoxy resin. The mass increase was about 9%.

In In a fourth step, the surfaces were sanded off so far, that this solidified epoxy remained only in the pores, d. H. that no epoxy resin layer on the surface remained.

In a fifth Step (joining step) were the tops of the workpieces with a joining paste, consisting from 68% by mass liquid Phenolic resin, 22% by mass coke powder and 10% by mass beechwood flour with a grain size smaller 50 microns provided. It has thereby a Fügepasteschicht with a thickness of about 1 mm. It then workpieces were joined, with under constant pressure conditions between two metal plates one curing at 135 ° C for 2 h took place.

In a sixth step (combined pyrolysis and siliconation) a high-temperature treatment for burning out the epoxy resin in the infiltrate body and to the siliconization takes place. The joined workpiece with a total weight of 20 g was thereby under vacuum conditions in a graphite crucible with silicon at approx. 1650 ° C liquid infiltrated. For this purpose, 66 g of silicon granules corresponding to 330 were used Mass% offered by the sample bottom. By the temperature treatment the epoxy is removed.

When Result has in the structure cross-section one carbon-free continuous SiSiC layer and a homogeneous SiSiC matrix. The SiSiC layer is from the Fügeschicht between the workpieces emerged. The marriage story as well as the adjacent structural areas were free of pores and crack-free siliconized.

Claims (16)

Processing method for an open-porous carbon-containing body, in which an open-porous carbon preform is produced by pyrolysis, which is infiltrated with a medium, wherein the pores are closed at least superficially, is then mechanically processed, after which the medium is removed from the pores. Processing method according to claim 1, characterized the existence or multiple areas of the infiltrated body superficially a joining paste be provided. Processing method according to claim 2, characterized in that that before providing a region with compounding paste a surface layer Will get removed. Processing method according to one of the preceding Claims, characterized in that the Medium so chosen that's it in liquid form into the pores of the preform in which pores become solid, with the carbonaceous material preform does not react chemically. Processing method according to one of the preceding Claims, characterized in that the Medium is removed by decomposition or after phase transformation. Processing method according to one of the preceding Claims, characterized in that the Medium is a resin or hard wax or silicon polymer. Processing method according to claim 6, characterized that this Medium is an epoxy resin. Processing method according to claim 7, characterized that this Medium a cold-curing Epoxy resin is. Processing method according to one of the preceding Claims, characterized in that the Medium one or more dopants are added. Processing method according to one of the preceding Claims, characterized in that the Removal of the medium is carried out under inert conditions. Processing method according to one of the preceding Claims, characterized in that the Removal of the medium is carried out above 500 ° C. Processing method according to one of the preceding Claims, characterized in that the infiltrated body subjected to pyrolysis. Processing method according to one of the preceding Claims, characterized in that the Infiltration by brushing in the precursor with medium or dripping from medium to the preform he follows. Processing method according to one of the preceding Claims, characterized in that the Removal of the medium followed by carbide-former infiltration carried out becomes. Processing method according to claim 14, characterized characterized in that a Silicification performed becomes. Processing method according to one of the preceding Claims, characterized in that the open-pored preforms fiber reinforced is.