PROCEDURES FOR THE PRODUCTION OF A MEANS CARBON SHORT FIBER STRENGTHENED SILICON CARBIDE OF COMPOSITE MATERIAL

The invention is directed toward a procedure for the production one by means of carbon short fibers of strengthened silicon carbide composite material and/or a molded article made of it.

A goal of the ceramic composite material development is it to lower the brittleness of monolithic powder metallurgical manufactured ceramic(s) to lower i.e. their rigidity and increase their elongation at rupture. A well-known composite material, this characteristic combination in sufficient Maß e exhibits, is a carbon strengthened with carbon fibers (C/C). This material is used therefore e.g. as brake disk and friction lining for Flugzeugoder racing car brakes. Unfavorably at this composite material however the small Oxidationsbeständigkeit of carbon is starting from application uses of above 350°C at air, the one high burn-up and/or Verschleiß the construction units from C/C to the consequence has, if these not under Sauerstoffausschluß (Inert gas or vacuum) to be held can. Applications without inert gas protection are with totally enclosed airplane brakes or as thermal shield because of missiles with short burning duration. The creation of oxidation protective layers on the C/C could solve slight improvements however brought the problem of the continuous oxidation stability not completely.

The materials development concentrated thereupon on the production of with Kohlenstoffoder graphite fibers continuously strengthened (long fiber reinforcement) silicon carbide construction units, in which on the one hand the carbon fibers are better (at least at short notice) protected from oxidation by the surrounding SiC matrix and exhibit on the other hand the fiber integration appropriate defects, so daß during still sufficient reinforcement effect of the carbon fiber the Riß propagation at the boundary surfaces between fiber and matrix by energy consumption is restrained and a damage-more tolerant breaking behavior is obtained. The realization of such a material is valid to be at least partly converted however to today as with difficulty, since silicon and carbon move at higher temperatures during the production very easily by exotherms a reaction to silicon carbide, thus those strengthening working fibers under loss of its reinforcement effect to silicon carbide and because itself production of C-Faserkörpern and the chemical gaseous phase infiltration (CVI) with silicon carbide or with silicon carbide powder for example by Heiß do not press as sufficiently successful and economically proved. Unfavorably with these materials with carbon long fiber reinforcement is also, daß the C-Fasern an extremely anisotropic thermal expansion behavior (in longitudinal axis: 0, in the cross section: 12 x 10-6 K-1) possess, which at high application temperatures to the Riß education in the SiC matrix and thus again to the unhindered burn-up (oxidation) that strengthening working carbon fibers leads.

Improvements promises oneself one in the materials engineering by the coating of the C-Fasern by means of CVDoder CVI procedure (CVD: Chemical Vapour deposition, CVI: Chemical Vapour infiltration) with protective layers from refraktären materials, like boron nitride (BN), Pyrokohlenstoff (PyC), titanium carbide (TIC) or silicon carbide (SiC) before the shaping and/or the infiltration with liquid silicon (SI).

From the DE-39 33 039-C2 is well-known, daß one the protective effect of pyrolytisch separated carbon (PyC) avails oneself, by covering the construction unit from carbon short fibers or carbon fiber felt first with a first layer from pyrolitischem carbon, providing then the construction unit graphitiert and afterwards with a second PyC layer, before the construction unit with liquid metallic silicon is infiltrated. With this procedure the employment of the complex and thus cost-intensive CVDbzw is unfavorable. CVI technology and is the Herstellungsproze&szlig beyond that; with multi-layer PyC coating very lengthily and complex.

In the DE-44 38 456-A1 one proceeds with in special way arranged situations from bundles from carbon continuous fibers, which will surround with a synthetic resin matrix. After the carbon-buildup of the synthetic resin body strengthened with C-fiber bundles on CFK basis (CFK: Carbon fiber-strengthened plastic) exhibits the construction unit due to its special art of manufacturing inside translaminare channels, which are filled up accordingly with the silicon infiltration. The brought in silicon moves then with the carbon matrix essentially to silicon carbide. The structure of the semi-finished material from long fiber clutches of eggs is comparatively complex also here, the construction unit exhibits, depending upon arrangement of the carbon fiber bundles, altogether or in itself lagenweise anisotropic characteristics. In the employment it comes thereby to substantial oxidation effects, if one of the anisotropic layers protecting from oxidation is used up, lies exposed always the layer with C-Fasern, present under it, unprotected and leads inevitably to the burn-up of the entire body.

From the DE-197 10 105-A is a multi-level Prozeß to the production of such oxidation-steady composite materials admits. With this procedure in the first step textile fabric prepregs are woven and/or made of graphite fiber Rovings. The fabric becomes anschließ end with phenolic resin impregnated and in a press under pressure at temperatures above 100°C hardened. After the Preß procedure are verkokt and/or carbonisiert the hardened fabric under inert gas at temperatures by at least 900°C. After the cooling the carbonisierte fabric in the next Proze&szlig becomes; walked again with synthetic resin impregnated and under inert gas at maximum temperatures of 900°C again verkokt. Anschließ end takes place a repeated impregnation of the already several times impregnated and carbonisierten graphite fiber fabrics with coal tar pitch. On this third impregnation step again a temperature treatment takes place for the transformation of the pitch in carbon. Anschließ end the several times impregnated and carbonisierten fabric under inert gas at temperatures by approx. 2.000°C are graphitiert, around Einfluß to take on the reactivity of the assigned and/or produced coal materials. In the next work procedure the fabric becomes Partikelgröß EN of 0 to 2 mm Größ e grind. The gemahlene material is then mixed in a kneading machine with a liquid synthetic resin pitch mixture with high carbon yield. After the mixing process this mixture is filled in into heatable die presses and under appropriate pressure and a temperature of at least 100°C verpreß t and hardened. After releasing from form follows in the next Prozeß a coking of the body walked under inert gas at temperatures from approximately 900 to 1.200°C. The verkokten bodies are graphitiert again after the cooling at temperatures by for instance 2.000°C. The graphitierte body becomes in the next Prozeß walked into a more vakuumoder inert gas-operated furnace transferred, in which a crucible with silicon and porous Dochten is, on which the body in well-known way is positioned. These supports work with the silicon infiltration at temperatures above the fusion point of silicon in well-known way as Dochte for impregnating the silicon from the crucible into the construction unit which can be infiltrated. The infiltrated silicon reacted then more or less with offered carbon from the matrix, the fiber coating produced before and partly the fiber surfaces. For the state of the art is also to the DE-C-197 11,829 are referred, with which the fiber bundles additionally with pyrolysierbarem bonding agent impregnated, and besides no data are given the composition the fiber-simple.

Unfavorably with these well-known procedures is auß erdem, daß the complex produced carbon protective layers with the Mahlprozeß to be partly destroyed and the silicon with the infiltration by the cracked protective coating unhindered with the graphite fiber reinforcement at least to silicon carbide abreact can. With this multi-level and äuß only energy, zeitund thereby cost-intensive procedures can be manufactured with graphitic short fibers strengthened silicon carbide materials with relatively high elongation at rupture and relatively low firmness (< 60 MPa).

As well as task of the invention is the creation of a solution, with the molded articles, plattenund tubular starting material od. such out by means of carbon short fibers strengthened silicon carbide composite materials in a simple procedure is producible thermalmechanically more stable, until 1.800°C is applicable as oxidation-steady ceramic composite material in continous use.

With a procedure of the initially designated kind this task becomes gemäß Requirement 1 of the invention solved.

In principle coated carbon fibers are well-known. The advantage of the available invention consists of using the carbon fiber Roving cuts and/or the carbon fiber bundle sections so in the composite material daß they in a silicon carbide matrix embedded are so daß according to strengthened composite materials with high fracture toughness to be made available know, i.e. carbon fiber-strengthened silicon carbide.

In order to repeat this partial, is referred, da&szlig to it; the available procedure a economical and pollution free Hertellungstechnologie represents. She works for example ausschließ lich with drying resin, i.e. without solvents. Also a Nachimprägnierung is not necessary, since only one manufacture step takes place to the release from formable construction unit, what energieund is time-saving. Is also, da&szlig favourably; with the available invention the Karbonisierung and those anschließ end to SI infiltration and/or conversion to SiC in only a thermal treatment step one accomplishes, which saves again energy, time and costs.

Also an advantage is in it, daß for adjustment at the invention with ceramic fillers (ceramic(s) powders) the special composite material characteristics one works. This leads to the minimization of the organic resin portion, to the improvement Abrasionsund of corrosion behavior and not least also to a better environmental condition.

As a result of the use of drying resins a further advantage, which lies in it, arises daß by the defined Fließ border a Aufspleiß EN of the firmness-increasing fiber bundles is prevented, this concerns a fiber-careful shaping. Since also no grinding is necessary, an appropriate Mahlproze&szlig becomes; saved by fabric semi-finished material, there ausschließ lich with fiber bundles of defined length it is worked whereby the material homogeneity is substantially improved. With meal processes after the well-known procedures it comes inevitably to undefined Faserlängen.

Also an advantage consists of it, daß with a protective layer, in an educated manner from the fiber bundle-simple, to be worked knows, several protective layers is erfindungsgemä&szlig with the available; EN approach dispensable.

Schließ lich that works erfindungsgemäß e procedure only with temperature-steady carbon fiber bundles, which with the state of the art the case is not, since for example after the state of the art the SiC fibers exhibit a maximum temperature stability of 1.000°C. An SI infiltration, as erfindungsgemäß suggested, i.e. at temperatures of > 1.400°C, is not possible there. In addition comes, daß with erfindungsgemäß EN procedure the fiber bundles intentionally not to be impregnated, differently than with the state of the art.

Favourable arrangements of the invention are the subject of the Unteransprüche, whereby the invention beside the pure material has or according to arranged molded articles also the use of these materials to the article.

With that erfindungsgemäß EN procedure for the production of carbon-short-fiber reinforced silicon carbide (C/SiC) with high fracture toughness and isotropic characteristics one proceeds from high-strength carbon fiber Rovings with 1000 single filaments (1K) up to 12000 single filaments (12K), which are coated with fiber-simple from Epoxyharz and/or Furfurylalkohol and/or Glycerin and/or silanes. With strengthening fibers it acts the short-fiber reinforced C/SiC composite material over graphitierte, high-strength carbon fiber bundle cuts (Rovings). These with the simple provided carbon fiber bundles (Rovings) are preferably cut on lengths between 2 and 10 mm, 5 to 8 mm and mixed together with a dry phenolic resin powder and/or phenol Novolak powder and/or - granulates from these arbitrarily and/or isotropically. In the inventive thought other synthetic resin powders and/or granulates, which than carbon donors serve, are naturally miterfaß t. The mixing process should äuß to be only carefully made, so that the short fiber bundles from cut Rovings do not fan out and in ruins into single filaments. This is particularly important, so that at the later matrix infiltration with fusionliquid silicon and the reaction to silicon carbide no firmness-reducing fiber attack takes place. This mixed mass becomes anschließ end in the final construction unit corresponding a form/an outline of arbitrary geometry and Größ e filled in.

The carbon fiber content can between 20 and 80 Gew. - %, preferably 40 - 60 Gew. - %, to be varied. By those erfindungsgemäß e variation option of Faserund matrix content in the semi-finished material is given the possibility, the physical and mechanical characteristics, in particular the firmness, rigidity, fracture toughness, elongation at rupture, hardness and abrasion stability, specific gravity, thermal and electrical conductivity of the short-fiber reinforced C/SiC composite material purposefully at the respective application profiles maß zuschneidern.

That erfindungsgemäß e procedure also plans, daß one with the mixture, consisting of short carbon fiber bundles and dry phenolic resin powder and/or phenol Novolak powder and/or granulates of these, appropriate powders of carbon and/or graphite and/or Ruß and/or silicon carbide and/or silicon or similar materials with the Mischprozeß adds.

For the shaping in principle well-known manufacturing processes are e.g. like the die presses, the ISO-static Preß proceeded and also the so-called vacuum bag shaping procedure suitably. Particularly with the vacuum bag procedure can complicated and groß e of construction units to be problem-free manufactured. The filled form becomes anschließ end for example into a vacuum bag from PP or other suitable foil materials in-wound and under a temperature between 70 and 150°C by means of a vaccum pump on a vacuum printing from < 200 mbar evacuated (vacuum bag shaping procedures). The dry phenolic resin powder and/or phenol Novolak powder and/or the granulates and/or Preß masses from these between the short carbon fiber bundles soften and/or liquefy starting from a temperature from 70°C to 90°C and fließ EN into the pore areas. During further heating above 120°C hardens the now liquid phenolic resin out, fixes the stochastically arranged short carbon fiber bundles in the construction unit structure and leads to a solidified short-fiber reinforced CFK semi-finished material (CFK: carbon fiber-strengthened plastic) with isotropic and/or arbitrary fiber orientation.

A second possibility for the shaping consists e.g. of it, daß one the form trains in such a way, daß one the bodies in a die press under appropriate pressure and simultaneous task of temperature between 70 and 150°C to these CFK molded articles verpreß t and hardens. With the choice assigned Phenolharzund/or phenol Novolak powders is to be paid attention particularly to it, daß one a type of resin with as short a Flie&szlig as possible; away selects. Thus one ensures, daß the fiber bundles during the shaping to be only superficially moistened and no resin between the single filaments of the fiber bundles penetrates. Thus becomes erfindungsgemäß the unwanted Abreaktion that strengthening working fibers prevents. The CFK semi-finished material possess specific weights within the range between 0,8 and 1,4 g/cm3 after the shaping in dependence of the assigned quantities of carbon short fibers and phenolic resin powders and/or phenol Novolak powder portion.

With this procedure is da&szlig particularly favourably; in such a way formed semi-finished material bodies due to the arbitrary and/or isotropic fiber reinforcement with the following thermal Prozeß walked no deformations or shrinking goes through.

By these end form near shaping can be done without usual complex mechanical rework steps of the ceramic composite materials usually completely. Anschließ end the short-fiber reinforced CFK semi-finished material manufactured end form near into a resistance-heated vacuum furnace are positioned, whereby the stoichiometrically needed silicon quantity in the form of silicon granulates and/or silicon powder and/or coated silicon granulates and/or the construction unit angepaß ten silicon granulates molded articles for the production of the silicon carbide matrix in the later C/SiC composite material on the semi-finished material body or inside the semi-finished material body is positioned, and under vacuum and/or inert gas, preferably nitrogen or argon, on temperatures above the fusion point of silicon (> 1.405°C) is heated.

During the heating under Sauerstoffausschluß up to the fusion point of the silicon the superficially adhering fiber-simple carbonisiert consisting of Epoxyharz and/or Furfurylalkohol to the fiber bundle not adhering, but coating carbon case and the phenolic resin matrix between the short carbon fiber bundles and/or - Rovings to carbon. With these Carbonisierung changes itself about 50 to 70 Gew. - % of the assigned phenolic resin quantity to carbon over. The remainder guest as pyrolysis product out and hinterläß t pore area in the short-fiber reinforced C/C Kohlenstoffkörper (C/C: carbon fiber-strengthened carbon) for those anschließ end to silicon infiltration. With reaching of the silicon fusing temperature of 1.405°C the liquid silicon from above or inside the construction unit in the pores and capillaries of the carbonisierten carbon-short-fiber reinforced carbon body produced before penetriert. At further temperature temperature up to maximally 1.800°C the silicon reacts off with offered carbon from the matrix and the fiber bundles coating carbonisierten simple one, which a kind sleeve formed around the fiber bundles, to silicon carbide.

By the transformation the fiber bundles coating carbon case to silicon carbide favourable-proves to in International Telecommunication Union a diffusion barrier for liquid silicon formed and thus an attack and/or a Abreaktion of carbon short-fiber reinforcement to silicon carbide prevented. In addition comes with that erfindungsgemäß EN procedure, daß the distance of the carbon single fibers (filaments in the finished short carbon fiber bundles with 1.000 to 12,000 single fibers, 1K-12K) so small is da&szlig (< 1 µm); a penetration of fusionliquid silicon and the associated and unwanted Abreaktion of the reinforcement fibers to SiC during the infiltration process into the fiber bundles are impossible and are prevented in such a way. At worst those react äuß eren single fibers of the finished fiber bundles and/or fiber ropes off, which however no significant Einfluß on the high fracture toughness of the short-fiber reinforced C/SiC composite material has.

After the cooling a carbon-short-fiber reinforced and/or a short fiber-bundle-strengthened is appropriate silicon carbide composite material (C/SiC) with for high fracture toughness and damage tolerance, an isotropic character, low specific gravity (2.1 to 2.6 g/cm3), high temperature stability (1.800°C) and firmness (> 100 MPa), no porosity, outstanding oxidation, Korrosionsund abrasion stability forwards. The not abreacted silicon portion in the silicon carbide matrix of the ceramic C/SiC composite material with C-short-fiber reinforcement is due to the stoichiometric dosage under or about 5 Gew. - %, preferably under 1 Gew. - %. The elongation at rupture of the short-fiber reinforced C/SiC composite material lies as a function of the intensifying fiber content within the range of 0,2 to 0.6%.

That erfindungsgemäß e short-fiber reinforced C/SiC composite material orders beyond that over further favourable characteristics, like high spalling resistance, high and by variation of the fiber content heat conductivity adjustable in the group, Gasund liquid tightness, and a high hardness and thus abrasion stability. It is particularly as material for applications of high temperatures in oxidative atmospheres suitable with thermalmechanical loads arising at the same time particularly well due to this outstanding characteristic profile.

Erfindungsgemäß also an application of the product manufactured in the procedure is intended, like e.g.

Pot for Glasund of non-ferrous metals, rocket nozzles, burner components, Ofenmuffeln, heat exchangers, turbine construction parts, protective pipes and Meß probes, high speed brake disks and/or friction linings, opto mechanical structures and ballistic protection components, like tank, impact plates od. such.

The invention is represented in the only figure still as block diagram with appropriate inscription.