DE102013114628A1 - Process for producing near net shape shaped silicon carbide ceramics - Google Patents

Abstract

A method is disclosed for producing silicon carbide ceramics by liquid or gas phase siliciding, comprising the steps of: (a) preparing a mixture of thermoplastic binder and a sinterable carbon precursor; (b) shaping the mixture into a green compact by means of a near-net shape shaping process, preferably by means of extrusion or injection molding; (c) pyrolyzing the green compact to produce a porous semi-finished product; (d) infiltrating the porous semi-finished product with silicon by liquid or gas phase infiltration.

Description

 The invention relates to a method for producing silicon carbide ceramics by means of liquid or gas phase siliciding, in which a mixture of a carbon precursor and a binder is first prepared, then the mixture is formed into a green compact, which is subsequently pyrolyzed and then infiltrated with silicon ,

Such a method is known from EP 1 795 513 A1 known.

 Thereafter, first a green compact of carbon powder of uniform sieve fraction, such as charcoal or biochar powder, with a cokbaren binder, for example phenolic resin, pitch or tar, added, then subjected to a heat treatment at a temperature between 40 and 200 ° C for driving off volatiles, then pyrolyzed to produce a porous carbon body and finally infiltrated with silicon, such as by gas phase infiltration. Here, from a charcoal flour and binder-containing mixture, a green compact with near-net shape dimensions by pressing, extrusion, injection molding or other shaping process to be produced. The binder is either in solid form, that is as a powder, or as a liquid. As binders, phenol-formaldehyde resins having a high carbon yield, carbonizable resins, such as, for example, furan resins, ie, thermo-type resins, are disclosed. The temperature program used during the molding process is adapted to the melting and curing behavior of the binder. First, a slow heating to a sufficient for the curing of the binder resin temperature and then a longer hold time at this temperature.

From the EP 1 657 227 A1 Another method for the production of silicon carbide ceramics is known in which first of a starting material comprising cellulosic powder, with the addition of binder in the form of powdered thermosetting phenolic resin, a precursor is prepared with graded phase distribution profile, which is then by pyrolysis in an open-pored carbon body is converted, which is finally siliconized.

From the EP 2 053 029 A1 For example, another method for producing silicon carbide ceramics is known in which a mixture is first prepared from a starting material comprising activated carbon, a curable binder and carbon fibers. This is pressed in a mold and cured the binder. This is followed by liquid siliciding.

 In the molding of such components by the above-mentioned methods, the necessary slow heating to a temperature for curing the binder used is disadvantageous. Also, it is difficult to actually produce suitable mixtures that can be shaped well near net shape by extrusion.

 Commercially available silicon carbide components (also called C-SiSiC components) have hitherto been produced mainly by different sintering methods, such as pressureless sintering (SSiC), pressure assisted sintering (HPSiC), or liquid phase sintering (LPSiC). All of these methods require SiC powders with strong covalent SI-C bonds and sintering aids, such as carbon or boron carbide, as raw materials. Sintering temperatures for SiC produced by powder technology range from over 1800 to about 2300 ° C. Such a production is thus very complicated.

 Against this background, the invention has for its object to disclose a method for producing silicon carbide ceramics, which enables a production of high-quality components of silicon carbide in the simplest possible way using a near-net shape shaping process. Preferably, it should also be possible to produce components with a relatively complex geometry.

• (a) Herstellen einer Mischung aus thermoplastischem Bindemittel und einem sinterfähigen Kohlenstoffprecursor;
• (b) Formen der Mischung zu einem Grünling mittels eines endkonturnahen Formgebungsverfahrens;
• (c) Pyrolisieren des Grünlings zur Herstellung eines porösen Halbzeugs;
• (d) Infiltrieren des porösen Halbzeugs mit Silizium durch Flüssig- oder Gasphaseninfiltration.

This object is achieved according to the invention by a method for producing silicon carbide ceramics by means of liquid or gas phase siliciding with the following steps:

• (a) preparing a mixture of thermoplastic binder and a sinterable carbon precursor;
• (b) shaping the mixture into a green compact by a near-net shape molding process;
• (c) pyrolyzing the green compact to produce a porous semi-finished product;
• (d) infiltrating the porous semi-finished product with silicon by liquid or gas phase infiltration.

 The object of the invention is completely solved in this way.

According to the invention, a thermoplastic binder is used as the binder. This allows for rapid heating up to the processing temperature during molding, without having to consider hardening of the binder. The processing is significantly simplified in this way.

 While thermodur binders, such as phenolic resins, are thermally crosslinked into polymatrix composites (PMC) and subsequently pyrolyzed and siliconized, the degrees of freedom of shape of components are inherently very limited in geometry and size, allowing the use of the present invention thermoplastic binder considerable degrees of freedom in the near-net shape shaping. It can be both an extrusion and injection molding of complicated shapes allow.

 In the context of this application, a thermoplastic binder is understood as meaning a binder which can be deformed in a specific temperature range ("thermo" - "plastic"). The process is reversible, in contrast to the thermodurable material, i. it can be repeated as often as required by cooling and reheating. In contrast, thermo-thermosets or thermosets cure irreversibly, which has a three-dimensional cross-linking effect.

 As the thermoplastic binder, for example, a binder selected from the group consisting of binders based on polyethylene, polypropylene, polyethylene copolymers, polypropylene copolymers, polystyrene, polyethylene waxes, ester waxes, natural waxes, polyethylene glycol, styrene acrylonitrile, acrylic ester copolymers, paraffin waxes is used , Fischer-Tropsch waxes and vinyl acetate copolymers.

 Alternatively, it is also possible to use thermoplastic polyacetal-based binders which can be debindered thermally oxidatively or catalytically in the acidic vapor.

 In a preferred embodiment of the invention, the mixture is additionally added at least one filler.

 This makes it possible to influence the properties of the ceramic produced in a targeted manner.

 The pyrolysis is carried out in a preferred embodiment of the invention with exclusion of oxygen at temperatures above 500 ° C up to a maximum of 1700 ° C.

 Preferably, the pyrolysis is first heated at a low heating rate of preferably 10 K / h to 30 K / h to a temperature above 500 ° C, preferably about 800 to 1000 ° C, and then with a higher heating rate of preferably 50 to 150 K. / h to a temperature of at least 1500 ° C, preferably 1600 to 1700 ° C, in particular 1650 ° C heated.

 This ensures good process control.

According to a further embodiment of the invention, debinding is first carried out after step (b) to remove the thermoplastic binder. This is preferably done in a bath of water or organic solvents at a temperature elevated from room temperature, more preferably in a water bath at 30 to 40 ° C.

 This assumes that the thermoplastic binder is correspondingly soluble in water or organic solvents. Alternatively, it is also possible to use thermoplastic polyacetal-based binders which can be debindered thermally oxidatively or catalytically in the acidic vapor.

 According to an alternative embodiment of the invention, debinding to remove the thermoplastic binder is carried out simultaneously with the pyrolysis in step (c).

 With both variants of the method, it is possible to produce suitable porous semi-finished products which are suitable for the subsequent siliciding.

 Since the thermoplastic binder itself does not play any role in the actual pyrolysis itself, since it has either been previously removed in a debindering step or is thermally driven off at the beginning of the pyrolysis, the sintered carbon precursor is added. This makes it possible to obtain structurally solid, highly porous semi-finished products during pyrolysis.

This may be selected from the group consisting of petrol, coal tar, synthetic pitch, polyaromatic mesophase pitch (PAM pitch) and mixtures thereof.

 It has been found that the use of such carbon precursors is particularly suitable for obtaining a sufficiently stable highly porous carbon precursor for subsequent silicization. A particular advantage is the high carbon yield of these materials and in the expression of a graphite-like crystal structure.

 In a further preferred embodiment of the method according to the invention, a filler in the form of a technical carbon is additionally added to the mixture.

 This may be selected from the group consisting of cokes, natural graphite, synthetic graphite, carbon blacks, activated carbon, carbon fibers, CNTs, graphene, SiC and mixtures thereof.

 The properties of the ceramic thus produced can be selectively influenced in this way within wide limits. The strength of the porous carbon semi-finished product after the pyrolysis and the later absorption capacity in the silicification can be advantageously influenced.

 The carbon precursors and / or fillers are preferably added at least partially in powder form.

 These may be selected from, for example, the group consisting of coked cellulose, coked polysaccharides, PAM pitches, ground petroleum coke, pitch coke, graphene, natural graphite, synthetic graphite, carbon black, flame black, gas black, and mixtures thereof.

 In addition, the carbon precursors and / or fillers may be added at least partially in fibrous form.

 As a result, the strength of the ceramics produced can be influenced in a targeted manner.

 The fibrous carbon precursors and / or fillers may preferably be selected from the group consisting of rayon-based, PAM-based, pitch-based, carbon nanotubes, graphene, SiC fibers, and mixtures thereof. Preferably, these are short fibers.

 According to a further embodiment of the invention, 10 to 30% by volume, preferably 15 to 20% by volume, of sinterable carbon precursor are added.

 If fibrous fillers are additionally used, their proportion of the total fillers added is preferably 10 to 40% by volume, more preferably 15 to 25% by volume.

 According to a further preferred embodiment of the invention, the proportion of the thermoplastic binder in the mixture is at least 35% by volume, preferably at least 40% by volume, more preferably at least 45% by volume. The maximum proportion of the thermoplastic binder in the mixture is preferably at most 65% by volume, more preferably at most 55% by volume.

 It has been found that the production of high-quality C-SiSiC ceramics is possible with such mixing ratios, the properties of which can be influenced within wide limits by the mixing ratios and raw materials used.

 By using a volume fraction of at least 35% by volume, or even at least 40 or 45% by volume of the thermoplastic binder in the mixture, it is possible to produce a particularly readily extrudable mixture. If production is to be effected not by extrusion but by injection molding, the proportion of the thermoplastic binder is preferably chosen to be even higher, up to about 55 or 65% by volume.

 The silicon infiltration is preferably carried out in step (d) as liquid phase infiltration (LSI), which takes place above the melting temperature of silicon, ie above about 1414 ° C.

 This can be done without pressure via capillary forces.

Preferably, however, the liquid phase infiltration is carried out under reduced pressure.

 This is preferably carried out at a temperature of 1450 to 1750 ° C, preferably from 1550 to 1700 ° C, more preferably at 1600 to 1700 ° C, more preferably at about 1650 ° C.

 The siliciding temperature can preferably be heated at a high heating rate, for example at 50 to 200 K / h, preferably at 100 to 160 K / h, particularly preferably at about 130 K / h. After reaching the maximum temperature, this is preferably maintained for at least ten minutes, preferably 20 to 60 minutes.

 As already mentioned above, the shaping in step (b) is preferably carried out by extrusion or injection molding.

 The extrusion is preferably carried out at elevated temperature in a range of 100 to 200 ° C, preferably in the range of 120 to 160 ° C, more preferably at about 140 ° C.

 As an alternative to shaping by extrusion or injection molding, shaping by hot pressing with a pressure of less than 10 MPa is also conceivable. However, molding by extrusion or injection molding has distinct advantages compared to this.

 According to a further feature of the invention, the green compact after step (b) and / or the porous semi-finished product after step (c) are processed in a intermediate processing step before the subsequent sizing with a cutting process, in particular by grinding, milling, drilling.

 The green body or the porous semi-finished product has a sufficiently high strength to allow such intermediate machining with simple tools. In this way, the processing costs that must be driven after the siliconization in order to obtain the final contour can be significantly reduced.

 Through high surface area reactive carbon fillers and a suitable amount of silicon, the reaction mechanism between the reactive elements and the SiC yield after liquid phase infiltration can be controlled.

 The porosity formation during the debinding step and the carbon conversion and morphology of the resulting carbon matrix determine the degree of infiltration of the porous carbon semi-finished product.

 By the mixing ratios of the different starting materials, by the proportion of the thermoplastic binder and by the nature of the materials used can be produced with the inventive method tailored C-SiSiC ceramics, which can be tailored to the desired application.

 A high residual carbon content after the siliconization can advantageously be used, for example, for good tribological properties and, if appropriate, intrinsic lubricity.

 By contrast, components with little or no silicon and carbon surplus are designed for maximum hardness and strength.

 In this way, the production of ceramic composite materials with special functional properties, such as high thermal conductivity, low thermal expansion, and high hardness and modulus of elasticity, and combinations thereof is made possible.

 It is understood that the features of the invention mentioned above and those yet to be explained below can be used not only in the particular combination indicated, but also in other combinations or in isolation, without departing from the scope of the invention.

 Further features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the drawings. Show it:

1 the basic procedure in the production of silicon carbide ceramics according to the inventive method;

2 a thermogravimetric analysis of the raw material components used (thermoplastic binder, carbon filler and stabilizing elements) under nitrogen atmosphere (temperature range 20 to 1500 ° C, heating rate 10 K / min);

3 a thermogravimetric analysis of the mixtures produced in the green state after extrusion with prior debinding and without prior binder removal under a nitrogen atmosphere (temperature range 20 to 1500 ° C, heating rate 10 K / min);

4 the development of porosities starting from the green body after debinding in a water bath and subsequent pyrolysis or direct pyrolysis without prior debindering;

5 an EDS analysis of typical C-SiSiC microstructures after siliconization (T> 1450 ° C);

6 SEM images of polished cross sections after pyrolysis;

7 SEM images of polished cross-sections of typical C-SiSiC material after siliconization;

8th SEM images of unpolished cross sections of the FS-3 green compact after the extrusion step and before the water debinding step;

9 the pore size distribution after the debinding step by water removal and pyrolysis on the various samples FS-1 (a), FS-2 (b and FS-3 (c), measured by mercury infiltration porosimetry DIN 66133 ,

The basic steps of the method according to the invention are in 1 shown schematically. It is a four-step process. In the first stage, a blend is prepared by dry blending raw materials, using binders and carbon precursors, optionally with the addition of fillers.

 This is followed by a near-net shape forming process, followed by debinding and carbon conversion by pyrolysis, followed by finally silicidation.

The first production route investigated here 1 includes molding by extrusion or injection, followed by water debinding in the water bath. This is followed by thermal debinding (pyrolysis) at temperatures below 1650 ° C. Finally, the silicization is preferably carried out by liquid phase infiltration at temperatures above the melting temperature of silicon, and preferably below 1650 ° C.

 In a second production route is dispensed with the previous water debindering and expelled the binder in the course of thermal debinding (pyrolysis).

In an alternative method, which is not examined in more detail here, the shaping takes place by hot pressing with pressures of P <10 MPa (Route 3 ).

Examples:

1. Used materials

Table 1 summarizes the composition of three different mixtures designated FS-1, FS-2 and FS-3. Here, in particular, different mixing ratios of a thermoplastic binder of semi-coke were used as sinterable Kohlenstoffprecursormaterial and fillers of activated carbon and carbon fibers. material binder char Activated carbon C-fibers Vol .-% Vol .-% Vol .-% Vol .-% FS-1 36.0 35.2 9.0 19.8 FS-2 45.0 30.3 7.7 17.0 FS-3 48.3 23.5 11.2 17.0 Table 1

 The thermoplastic binder used was a commercially available thermoplastic binder (Licomont EK 583 G, eMBe Products and Service GmbH, Germany, having a melting temperature of about 100 ° C. The semicarbons used were a carbon tar material based on a sinterable semi-coke (Carbosint Rütgers Chemicals AG, Germany).

As activated carbon, carbon powder with a specific surface area of 1100 m ² / g (RD-90, Chemviron Carbon, Belgium) was used as a further filler. The activated carbon particles had an average diameter of 10 μm and were spherical. Further, as a further filler milled carbon fibers (C-fibers with an average length of 360 microns and an average diameter of 13 microns (Ashland-Südchemie-Kernfest GmbH, Germany) were used.

 The semi-coke served as a sinterable carbon precursor. The activated carbon powder served to aid the reaction mechanism between the reactive elements and the silicon in the later liquid phase infiltration. The carbon fibers served as an additional source of carbon and also as an additional strength enhancing stabilizer.

2. Mix and extrude

First, all materials in powder form according to Table 1 were premixed. The semi-coke was previously premixed for the FS-2 and FS-3 material systems to allow a direct comparison of different particle sizes. To homogenize the materials, a rotary drum mixer was used. The mixing time was ten minutes for each material system.

 The thus premixed compositions were pressed three times through a twin screw extruder (16 mm diameter screw) without a die at 140 ° C to melt the binder and homogenize the masterbatch. Finally, intensively premixed granules were fed into the same extruder and slowly extruded through a rod (12 mm diameter) at 140 ° C. Extrusion speed was limited by the design of the machine. The resulting bars were supported by a V-shape, cooled and cut into pieces for further processing.

3. Pyrolysis and silicization

The production of porous carbon semi-finished products and finally C-SiSiC ceramics requires two processing steps, namely pyrolysis and siliciding, after the green compacts are made by extrusion. The green compacts based on FS-1, FS-2 and FS-3 were first debinded in a water bath at 33 ° C for seven days. In the following, the pyrolysis was carried out (see production route 1 according to 1 ). The pyrolysis was carried out at a heating rate of 15 K / h up to 900 ° C, followed by 100 K / h up to 1650 ° C final temperature under flowing nitrogen.

During pyrolysis, the thermoplastic binder evaporates completely, while the semi-coke reacts at a high rate of carbon and thus forms a carbon matrix which, together with the other fillers, has a high porosity. Carbon conversion is accompanied by mass loss and shrinkage. To quantify the impact of previous water debindering, other samples of green bodies were directly pyrolyzed without prior water debindering (see Route 2 . 1 ).

 In the examples that follow, material configurations that include water debindering are labeled "WD", materials that are derived only from thermal pyrolysis, labeled "TD", and materials that are manufactured based on both steps "WD + TD".

After water debindering or pyrolysis, the obtained porous carbon semi-finished products were infiltrated with liquid silicon in an oven. This was heated to 1650 ° C with 130 K / h and holding at the final temperature for over thirty minutes to complete the silicization. Thereafter, adhered silicon was removed by sand blasting the samples with corundum granules.

4. Characterization and test results

To investigate the thermal behavior of the raw materials and the mixtures produced, gravimetric thermal analyzes (TGA) were carried out under flowing nitrogen (100 ml / min). The heating rate was 10 K / min in the temperature range of 20 to 1500 ° C.

To determine the microstructure density and porosity, a helium pycnometer was used according to ISO 1183 used.

4.1 Characterization of raw materials

In 2 TGA curves of the raw materials used are shown. These were previously dried for 24 hours at 40 ° C under a nitrogen atmosphere.

 The mass loss of the thermoplastic binder began at about 250 ° C and was completed at 550 ° C. The mass loss of activated carbon as well as carbon fibers was below 5% up to a temperature of 1100 ° C. At high temperature, the activated carbon showed a significantly higher mass loss gradient. The resulting mass loss is the result of elimination of organic groups, particularly the thermoplastic binder, vaporized hydrocarbons, carbon oxides and hydrogen. In the end, the mass loss of the raw materials converged asymptotically to their specific mass yield (in wt%), which in turn is an important source of total carbon yield and porosity distribution of the porous carbon semi-finished product.

4.2. Characterization of the mixtures and extrusion

4 shows the thermogravimetric analyzes (TGA) of the materials of the green compacts produced by extrusion under a nitrogen atmosphere at a heating rate of 10 K / min.

 The different materials showed different processing characteristics during extrusion. The material FS-1 was difficult to extrude due to the low level of thermoplastic binder of only 36% by volume. Increasing the binder content to 45% by volume and 48.3% by volume, respectively, for FS-2 and FS-3 allowed significantly easier extrusion with significantly lower shear forces and lower viscosity torque. Even increasing the proportion of activated carbon and simultaneously reducing the content of semi-coke hardly affected the processability of the FS-3 material system.

4 shows that there are similar weight losses for all three materials FS-1, FS-2, FS-3. Considering the different densities of the fillers and the binder, it follows that the water debinding step can remove about 40 to 45% by weight of the binder content. The major weight loss for each material ended at about 550 ° C, which correlates with the weight loss of the thermoplastic binder and indicates its complete degradation. In addition, the weight loss for each material system is almost the same.

4.3. Density, porosity and microstructure during extrusion and pyrolysis

The porosities and pore diameters measured on Samples FS-1, FS-2, FS-3 are summarized in Table 2. material ^a mode pore diameter ^a average pore diameter ^a Open porosity ^b Open porosity [Nm] [Nm] [Nm] [Nm] FS-1 4,169.9 484.2 47.59 38.91 FS-2 4,115.3 820.0 52.76 41,00 FS-3 2,037.4 343.0 52.45 46,00 Table 2 ^a measured by mercury infiltration spectrometry DIN 66133 and calculated according to pore shape model "cylindrical"
^b measured by Archimedes method DIN 51918

The measurements were detected by mercury infiltration porosimetry DIN 66133 and secondly according to the Archimedes method DIN 51918 carried out. The carbon yield of the material systems previously subjected to a water debinding step and then pyrolyzed is about 10% by volume larger compared to the carbon yields in direct pyrolysis. This result corresponds to the TGA measurements according to 2 respectively. 3 , This shows that the decay of the thermoplastic binder does not affect the disintegration mechanism of the other constituents, ultimately leading to an identical density after pyrolysis by route 1 and route 2 leads.

 However, some effects on total density and open porosity can be attributed to the different debindering methods.

4 shows the evolution of the total density and the open porosity in green bodies FS-1, FS-2 and FS-3. Obviously, FS-1 and FS-2 have the highest solids content and the lowest porosity, with a difference in porosity depending on the performance of water debindering. The additional water-debindered materials (WD + TD) have a greater total density and thus a lower porosity than the materials treated exclusively with the pyrolysis.

8th shows SEM photographs of unpolished cross sections of the FS-3 green compact after the extrusion step (a, c) and after the water debinding step (b, d). For the pyrolyzed material with prior water debindering (WD + TD) as well as the direct pyrolyzed material (TD), the open porosity is in the range of 38 to 49 vol% and the corresponding total density is between 0.78 and 0.94 g / cm ³ (cf. 4 ). In general, the open porosity is predominantly determined by the volume content of the thermoplastic binder within the material system.

6 shows the microstructures of FS-1 ( 6a . 6b ), FS-2 ( 6c . 6d ) and FS-3 ( 6e . 6f ) after pyrolysis with prior water debindering.

The pore size distribution for all material systems after pyrolysis with prior water debindering was measured by mercury infiltration porosimetry (cf. 9 ).

9a shows the material system FS-1, 9b shows the material system FS-2 and 9c the material system FS-3. The pore size distributions differ in particular in the case of FS-1 according to 9a , The measured pore size distributions reflect the differences in the material compositions, in particular a variation in the proportion of the activated carbon with a high surface area, which in particular leads to an influence on the pore size distribution in the range of <100 Nm.

 The measured porosities according to Table 2 differ somewhat with respect to the selected measuring method, which is not essential in the result.

4.4. Microstructure of C-SiSiC samples after silicization

In 7 are SEM images of polished cross-sections after silicization of FS-1 ( 7a . 7b ), FS-2 ( 7c . 7d ) and FS-3 ( 7e . 7f ). All C-SiSiC materials show a fairly homogeneous three-phase microstructure consisting of reaction-formed silicon carbide (SiC), residual silicon (Si), and residual carbon (C) from the carbon precursor.

The C-SiSiC material based on FS-1 ( 7a . 7b ) shows a high proportion of unconverted residual carbon (about 6 to 10% by volume). The residual carbon is fairly evenly distributed. Generally, unconverted carbon is an indication of insufficient porosity and not one accessible dense carbon matrix of the carbon precursor. The second effect that affects reactivity is the crystal structure.

The pitch coke formed from the polyaromatic semi-coke precursor and the pitch fibers formed from chemically similar precursors are well crystallized and therefore relatively unreactive. The open porosity and large pore diameters of the FS-1 carbon semi-finished product are the reasons for a C-SiSiC material with high residual carbon content and correspondingly low silicon carbide content (approximately 71 vol.%). In comparison, FS-2 based C-SiSiC materials ( 7c . 7d ) and FS-3 ( 7e . 7f ) has a lower residual carbon content (about 0 to 3 vol.%) and a slightly higher silicon carbide conversion efficiency than FS-1. The FS-2 and FS-3 systems achieved a silicon carbide yield of approximately 75% by volume.

All microstructures showed large interconnected areas of residual silicon after siliconization. This effect was stronger for the FS-2 and FS-3 C-SiSiC materials with higher green binder thermoplastic binder content and thus higher porosity in the debinded material compared to FS-1. The larger binder content causes binder clusters, which in turn cause large pore clusters after the debinding step. These large and compact porosity domains are only sporadically seen in the pore size distribution analyzes shown. No silicon carbide can be formed in these large pores and there are large areas of residual silicon. The major difference in residual carbon and the silicon ratio of the microstructure compositions correlates strongly with the difference in densities as shown in Table 3. C-SiSiC ^a Density based on WD + TD ^b Density based on WD + TD ^a Density based on TD ^b Density based on TD [g / cm ³ ] [g / cm ³ ] [g / cm ³ ] [g / cm ³ ] FS-1 2.88 2.88 2.93 2.94 FS-2 2.97 2.99 2.98 3.04 FS-3 3.01 3.01 2.97 2.99 Table 3 ^a measured by mercury infiltration spectrometry DIN 66133 and calculated according to pore shape model "cylindrical"
^b measured by Archimedes method DIN 51918

In 5 EDS studies of typical C-SiSiC microstructures after siliconization are shown. A bright color indicates the presence of the corresponding elements in the elementary subframes shown below. In addition to the pure silicon carbide matrix (SiC), a two-phase (C-SiC, Si-SiC) and three-phase (C-SiSiC matrix) can be formed. Therefore, only the elements silicon (Si) and carbon (C) are shown.

The formation of the microstructure is strongly influenced by the ratio between the carbon matrix and the open and closed porosity in the carbon precursor. If the volume ratio of carbon and porosity are inconsistent with the silicon carbide volume formed from the elements, there is either residual carbon or residual silicon in the microstructure (cf. 5a ). In 5b the dashed line indicates a carbon fiber completely converted to SiC. The carbon fiber was completely chemically transformed by the formation of silicon carbide. Thus, by the chemical reaction of the carbon fibers and the diffusion of the silicon through the initial silicon carbide edge layer, a part of fine-grained secondary SiC grain which is crystallized in liquid silicon is formed. The residual carbon reacts with the silicon which diffuses into the carbon fiber matrix regions, forming a granular structure.

5. Conclusions

The processability of mixtures by means of thermoplastic extrusion or the injection molding process depends in particular on the relationship between thermoplastic binder and carbon precursor and carbon filler content. The material system FS-1 shows a relatively poor processability by means of the extrusion process, caused by a relatively small volume fraction (36 vol .-%) of thermoplastic binder and correspondingly high viscosity.

On the basis of the carbon precursor additions and fillers used, thermoplastic binder levels above about 40% by volume are preferred (FS-2: 45% by volume and FS-3: 48.3% by volume) to low viscosities receive. This is helpful for the extrusion.

 After extrusion and pyrolysis, the carbon semi-finished products show sufficient strength for further processing by means of liquid siliciding (LSI). The sinterable semi-coke reacts in the pyrolysis to a pitch coke structure, which acts as a permanent binder after removal of the thermoplastic binder and ensures sufficient strength in the carbon skeleton. Partial substitution of semi-coke by activated carbon in FS-3 leads to a significant decrease in the strength of the carbon semi-finished product compared to FS-1 and FS-2.

 The morphology of carbon fillers and the thermoplastic binder content as well as the distribution of the constituents influence the pore distribution and the average pore sizes in the carbon semi-finished products after pyrolysis. The pores in FS-2 and FS-3 are more homogeneously distributed and smaller than in the material system FS-1. This is partly due to the pre-ground semi-cokes in FS-2 and FS-3. In the FS-3 carbon semi-finished product, the pores are significantly smaller (average pore diameter of 343 Nm with open porosity between 46 and 52% by volume), due to the increased amount of activated carbon and thus an increase in nano-size pores leads.

After silicization, all material systems investigated have a three-phase C-SiSiC microstructure. The residual carbon and the residual silicon are embedded in a relatively dense SiC matrix, with SiC grain sizes between 200 Nm and 2 μm. FS-1 shows a high proportion of non-converted residual carbon (6 to 10% by volume) due to the larger semi-coke particles that were used. In comparison, in the case of the ground semi-cokes in FS-2 and FS-3, the residual carbon content is lower. This also affects the SiC yield.

As a result, tailored C-SiSiC ceramics can be produced by the mixing ratios of the various starting materials, by the proportion of the thermoplastic binder and by the type of materials used, which can be tailored to the desired application. A high residual carbon content can advantageously be used, for example, for good tribological properties and optionally self-lubricity. By contrast, components with little or no silicon and carbon surplus are designed for maximum hardness and strength. In this way, the production of ceramic composite materials with special functional properties, such as high thermal conductivity, low thermal expansion, and high hardness and modulus of elasticity, as well as combinations thereof are made possible.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1795513 A1 [0002]
• EP 1657227 A1 [0004]
• EP 2053029 A1 [0005]

Cited non-patent literature

• DIN 66133 [0067]
• ISO 1183 [0084]
• DIN 66133 [0090]
• DIN 51918 [0090]
• DIN 66133 [0091]
• DIN 51918 [0091]
• DIN 66133 [0102]
• DIN 51918 [0102]

Claims (29)

 Method for producing silicon carbide ceramics by means of liquid or gas phase siliciding, comprising the following steps: (a) preparing a mixture of thermoplastic binder and a sinterable carbon precursor; (b) shaping the mixture into a green compact by a near-net shape forming process; (c) pyrolyzing the green compact to produce a porous semi-finished product; (d) infiltrating the porous semi-finished product with silicon by liquid or gas phase infiltration.  The method of claim 1, wherein the mixture additionally at least one filler is added.  Process according to Claim 1 or 2, in which the pyrolysis in step (c) is carried out with exclusion of oxygen at temperatures above 500 ° C up to a maximum of 1700 ° C.  A method according to claim 3, wherein for pyrolysis, first with a low heating rate of preferably 10 K / h to 30 K / h is heated to a temperature above 500 ° C, preferably about 800 to 1000 ° C and then with a higher heating rate of preferably 50 to 150 K / h to a temperature of at least 1500 ° C, preferably 1600 to 1700 ° C, particularly preferably up to 1650 ° C, is heated.  A process according to any one of the preceding claims wherein the thermoplastic binder is selected from the group consisting of polyethylene, polypropylene, polyethylene copolymers, polypropylene copolymers, polystyrene, polyethylene waxes, ester waxes, natural waxes, polyethylene glycol, styrene acrylonitrile, acrylic ester copolymers , Paraffin waxes, Fischer-Tropsch waxes, vinyl acetate copolymers and polyacetal.  Method according to one of the preceding claims, in which, after the step (b), debinding is first carried out to remove the thermoplastic binder, preferably in a bath of water or organic solvents at a temperature elevated from room temperature, more preferably in a water bath at 30 to 40 ° C.  A process according to any one of claims 1 to 5, wherein debinding to remove the thermoplastic binder is carried out simultaneously with the pyrolysis in step (c).  A process according to any one of the preceding claims, wherein a carbon precursor selected from the group consisting of petroleum pitches, coal tar pitches, synthetic pitches, polyaromatic mesophase pitches (PAM pitches) and mixtures thereof is added.  Method according to one of the preceding claims, in which a technical carbon is added to the mixture as filler.  The method of claim 9, wherein a filler selected from the group consisting of cokes, natural graphite, synthetic graphite, carbon blacks, activated carbon, carbon fibers, CNTs, graphene, SiC and mixtures thereof is added.  Method according to one of the preceding claims, in which the carbon precursors and / or fillers are added at least partially in powder form.  A process according to claim 11 wherein powdered carbon precursors and / or fillers selected from the group consisting of coked cellulose, coked polysaccharides, PAM pitch, ground petroleum coke, pitch coke, graphene, natural graphite, synthetic graphite, carbon black, flame black, are added , Gas black and mixtures thereof.  Method according to one of the preceding claims, in which the carbon precursors and / or fillers are added at least partly in fibrous form.  The method of claim 13, wherein fibrous carbon precursors and / or fillers are added selected from the group consisting of rayon-based, PAM-based, pitch-based, carbon nanotubes, graphene, SiC-fibers, and mixtures thereof , A process according to any one of the preceding claims, wherein 10 to 30% by volume, preferably 15 to 25% by volume, of sinterable carbon precursor is added.  A process according to any one of claims 2 to 14 wherein 10 to 40% by volume, preferably 15 to 25% by volume, of the filler is added in fibrous form.  Method according to one of the preceding claims, in which the proportion of the thermoplastic binder in the mixture is at least 35% by volume, preferably at least 40% by volume, more preferably at least 45% by volume.  Method according to one of the preceding claims, in which the proportion of the thermoplastic binder in the mixture is at most 65% by volume, preferably at most 55% by volume.  Method according to one of the preceding claims, wherein in step (d) a liquid phase infiltration above the melting temperature of silicon is carried out without pressure via capillary forces.  A method according to any one of claims 1 to 18, wherein in step (d) liquid phase infiltration is performed above the melting temperature of silicon under reduced pressure.  The method of claim 19 or 20, wherein the liquid phase infiltration at a temperature of 1450 to 1750 ° C, preferably from 1550 to 1700 ° C, more preferably at 1600 to 1700 ° C, more preferably at about 1650 ° C is performed.  Process according to Claim 21, in which the heating rate is from 50 to 200 K / h, preferably from 100 to 160 K / h, more preferably from approximately 130 K / h, to maximum temperature and maintained for at least 10 minutes, preferably 20 to 60 min.  Method according to one of the preceding claims, in which the shaping in step (b) takes place by extrusion or injection molding.  A method according to claim 23, wherein the molding in step (b) is carried out by extrusion at a temperature in the range of 100 to 200 ° C, preferably in the range of 120 to 160 ° C, more preferably at about 140 ° C.  A method according to any one of claims 1 to 22, wherein the molding in step (b) is carried out by hot pressing at a pressure of less than 10 MPa.  Method according to one of the preceding claims, in which the green compact after step (b) or the porous semi-finished product after step (c) is processed in a intermediate processing step prior to the subsequent sizing with a cutting process, in particular by grinding, milling, drilling.  A silicon carbide ceramic component formed from an extruded or injection-molded porous carbon-based semi-finished product infiltrated and reactive with silicon prepared according to any one of the preceding claims.  Use of the silicon carbide ceramic component according to claim 27 with a residual carbon content of preferably more than 1% by volume, particularly preferably more than 5% by volume, as a component with tribological stress and / or intrinsic lubricity and / or good thermal conductivity.  Use of a silicon carbide ceramic component according to claim 27 with low residual carbon content and low residual silicon content of preferably in each case <5% by volume, particularly preferably <2% by volume for applications with high hardness and / or strength.

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