Methods for manufacturing three-dimensional components

Abstract

In this process, a thermoplastic, plastically deformable mass is produced using a thermoplastic polymer or polymer mixture as an organic binder and diamond particles containing 30% to 80% solids by volume (SiC). The organic binder and particles are mixed, and the resulting thermoplastic mass is heated to a viscosity suitable for printing, or a corresponding thermoplastically deformable mass is prepared. Subsequently, the thermoplastic mass is applied to a substrate or component to be coated using a printing process. Afterward, the organic binder is removed and / or decomposed by thermal treatment.Subsequently, by applying contacted silicon or a silicon alloy in which silicon is the main component, solid silicon is infiltrated into the respective component by increasing the temperature to a temperature above the melting point of silicon or the silicon alloy. This infiltration, involving liquefied silicon, reacts with carbon and silicon to form SiC. The SiC and residual silicon then form a SiC matrix in which particles are embedded.

Description

[0001] The invention relates to a method for manufacturing three-dimensional components. In this method, thermoplastic material containing synthetically produced diamond particles is applied layer by layer. Thus, the component production falls within the field of additive manufacturing of diamond-containing materials. Additive manufacturing can also be colloquially referred to as 3D printing. The layers applied by a printing process can be composed of surface, line, or point elements. These deposited elements can be arranged side by side and / or on top of each other. Additive manufacturing can be used to produce very complex as well as simple geometries, and suitable coatings with a predetermined layer thickness appropriate for the respective application.

[0002] The invention relates in particular to the layer-by-layer processing of diamond-containing material with a starting total solids content (inorganic components without binder) of 30 vol.% to 80 vol.%. The particle sizes used according to the invention can be in the range of 0.05 µm to 250 µm. The complementary quantity consists of thermoplastic polymer necessary for shaping, the overall composition of which can also be a polymer mixture of different thermoplastic polymers. The microstructure of the silicon-infiltrated, reaction-bonded component can be significantly influenced by targeted polymer composition. The thermoplastic polymers necessary for shaping can be polymer mixtures with different flow properties when the glass transition temperature is exceeded. The polymers and polymer mixtures can be referred to as binders. In addition to polymer, the material can contain various solids other than SiC or only diamond particles.

[0003] In contrast to robocasting, the inventive method relies on the solidification of the melt to solidify the printed mass. In particular, solidification occurs through cooling and falling below the melting or glass transition point of the thermoplastic material to maintain structural integrity during the additive manufacturing process.

[0004] From WO 20 / 038799 A1, it is known to use a printing process for the production. In addition to diamond particles, silicon carbide (SiC) particles are also used in a binder matrix for printing. The SiC, contained as a solid in the initial solid composition, reacts with liquid silicon to form a SiC matrix in which diamond particles are embedded and thus fixed.

[0005] However, a component manufactured in this way has disadvantages, particularly due to the method of manufacture.

[0006] In contrast to robocasting, the Fused Filament Fabrication (FFF) process offers the advantage of higher build rates due to the absence of drying times. The polymeric binder content present in the green part also improves machinability.

[0007] To achieve high diamond content and thus favorable wear and thermal properties, materials without additional silicon carbide (SiC) not formed during silicification are necessary. Furthermore, adding SiC to the initial mixture leads to higher levels of free silicon. This results, for example, in poorer corrosion resistance and reduced cavitation stability of the material. Therefore, it is essential for the material properties that the manufacturing process does not require the addition of SiC particles.

[0008] To avoid these disadvantages, the purpose of the invention is to provide possibilities for manufacturing correspondingly improved components.

[0009] According to the invention, this problem is solved by a method having the features of claim 1. Advantageous embodiments and further developments can be realized with features specified in dependent claims.

[0010] In the process, a homogeneous, thermoplastically deformable mass is produced in a first step (i). This mass consists of a thermoplastic polymer as a binder and diamond particles, preferably synthetic, uncoated diamond particles, which are present in the mass with a solids content of 30 vol.% to 80 vol.% and which contains no SiC. The binder and particles are homogeneously mixed, and the resulting mass is heated to a viscosity suitable for mixing. This mass can also be formed into semi-finished products (filament, rod, granules) by kneading or extrusion to enable subsequent printing.

[0011] Alternatively, in the first step i) a corresponding polymer or thermoplastically deformable mass can also be provided.

[0012] In a second subsequent step ii), the semi-finished products or the thermoplastic, plastically deformable mass containing diamond particles produced or provided in step i) are applied to a substrate or component to be coated in a predefinable quantity, geometry, and / or layer thickness using a printing process, and the binder hardens. This fixes the applied geometries. These steps can be repeated several times sequentially or in parallel until the required preform is obtained. At this point, or after the next step, the resulting molded body can be removed from the substrate for further processing.

[0013] Subsequently, in at least a third step iii), the organic binder is partially removed by solvent extraction and / or pyrolyzed by thermal treatment at a temperature of at least 600 °C up to a maximum of 1670 °C. This can be carried out in an inert atmosphere or under vacuum conditions, preferably up to 850 °C, below 450 °C in air, and at temperatures above this only in an inert environment. This results in a preform of carbon-bonded diamond with an open porosity.

[0014] Subsequently, in a fourth step (iv), a reaction bond is carried out between the resulting pyrolyzed porous semi-finished product and silicon or an alloy containing silicon as its main component. For this purpose, silicon or a silicon alloy contacting the preform is heated in a furnace process to above its melting point. A temperature above the melting point is advantageous, preferably ≥ 50°C above the melting point of silicon or the silicon alloy. Through contact, which occurs at the latest when the silicon or the alloy is molten, with the preform, the silicon or the silicon alloy flows into the open pores of the semi-finished product. In this process, the silicon or the silicon alloy reacts with the binder carbon present and partially with the carbon on the surfaces of the diamond particles to form silicon carbide. The maximum temperature for the reaction bond should be 1670°C.

[0015] This process can be carried out with a heating and subsequent cooling rate of 0.5 K / min to 100 K / min (preferably 2 K / min - 10 K / min). The solid diamond particles and any other solid components are reactionally bound and dispersed within the SiC matrix.

[0016] The reaction bonding results in a diamond material bound in a silicon carbide matrix as the main component. A small residual silicon content is present as a minor component. Depending on the impurities or alloying elements in the silicon, further carbides, carbonitrides, borides, or silicides may also be present as minor components in the reaction-bonded structure.

[0017] Following appropriate cooling, a component obtained in this way or a component coated in this way can either be further processed and then put to use or immediately.

[0018] The properties of a component or coating can also be further improved or adapted to a specific use if, in the first step, i) a thermoplastic mass is formed which, in addition to synthetic diamond particles, also contains particles of a transition metal carbide (e.g., TiC, ZrC), - oxycarbides (e.g., TiC1-xOy), - carbonitrides (e.g., TiC1-xOy). 1-x N y , except boron carbide), or a mixture of which contains a metal boride (e.g. TiB2, a metal silicide (MoSi2) and / or max phases).

[0019] Examples of organic binders that can be used include polyamide (caprolactam-based as well as laurin-lactam-based), thermoplastic elastomers (TPE), polylactides (PLA), acrylonitrile butadiene styrene copolymers (ABS), acrylonitrile styrene acrylates (ASA), polyolefins (PP; PE, polyolefin waxes), polyethylene terephthalate (PET), polyvinyl butyral (PVB), polycarbonate (PC), phenolic resin (PF), furan resins (FA), lignin, di- or polysaccharide (sugar and starch compounds), PEEK, polyaryletherketones (PEEK, PEKK), polyimides (PEI, PI), polyoxymethylene (POM), or natural, semi-synthetic or synthetic wax, or carboxylic acids as pure substances, modifications or mixtures.

[0020] It is also possible to at least partially crosslink the thermoplastic binder before pyrolysis by using a suitable hardener, so that the thermoplastic polymer transitions into an infusible state. This prevents significant deformations during pyrolysis, which can otherwise occur due to the melting of the thermoplastic polymer and subsequent creep and flow processes during pyrolysis. An example of this is the modification of a novolac with 8 wt% hexamethylenetetramine (HMTA). It is also possible to use self-curing thermoplastic polymers that crosslink into an infusible polymer under the influence of temperature and time without the need for an additional hardener. Examples of these are resoles or furan resins.

[0021] Material extrusion (MEX), as defined in DIN EN ISO 52900, can be used to manufacture the semi-finished product and shape the component. The most prominent example is Fused Filament Fabrication (FFF), in which a thermoplastic filament is primarily used as the semi-finished product. It is fed through an extruder head, heated, partially melted, and extruded in a molten state through a die (e.g., 0.1 mm - 10 mm free cross-section). Temperatures in the range of 40°C to 380°C are used to melt the polymers. Other semi-finished products or feedstocks, such as granules or rods, can also be fed using suitable extruder heads, resulting in similar components.

[0022] The semi-finished products obtained can be used for further shaping or directly as shaped bodies (e.g. to produce rods, wires or profiles, these are then, like the shaped bodies listed above, pyrolyzed and reactively infiltrated during thermal treatment).

[0023] It is also possible to use mixtures of these materials in the initial solid mixture. These particulate solids are also homogeneously embedded in a thermoplastic polymer matrix and together form the semi-finished product for the process used in the invention. This semi-finished product can be used, for example, as granules (diameter: 1 mm - 4 mm, length 1 mm - 4 mm) or strand-like elements, as a filament (diameter: 1 mm - 4 mm; length > 200 mm) or cylindrical rod (diameter: 4-20 mm; length 10-200 mm). Another geometric form of the semi-finished product can be a rectangular or other complex cross-section.

[0024] If layers or layer sequences are applied to an existing component, e.g., made of SSiC or SiSiC, the component to be coated should have sufficient temperature stability corresponding to the maximum temperature to be achieved in step iv). This means that the material may only deform to a limited extent, and under no circumstances should a large portion (or all) of the material become molten. Accordingly, it is also possible to apply layers or layer sequences to printed preforms, pyrolyzed bodies, or infiltrated bodies.

[0025] Process control and the quality of the components or coatings produced in this way can also be improved by using a mixture of at least two different organic thermoplastic polymers as a binder, which is homogeneously mixed with the particles. This allows for better adaptation to the respective printing process and an improvement in properties through a different method of removing organic components during solvent extraction or pyrolysis.

[0026] An example of this is the use of a polymer blend of polyvinyl butyral with a novolac in a mass ratio of 20% to 80%. Polyvinyl butyral (PVB) possesses excellent bonding strength and very good film-forming properties, enabling better adhesion of the diamond particles to the polymeric binder when mixed with the thermoplastic polymers, thus improving printing results. Furthermore, PVB exhibits a very low carbon yield during pyrolysis. In contrast, phenolic resins have very high carbon yields. By blending the two polymers, the binder carbon content after pyrolysis can be precisely varied. This allows the porosity of the pyrolyzed components to be influenced and controlled. The ability to adjust the porosity and binder carbon content can significantly improve process reliability during the bonding reaction.

[0027] As already indicated, in the third step iii), starting with the solidification of the polymeric components with a solvent, some thermoplastic polymer can be removed by dissolution before further portions of the thermoplastic polymer are removed primarily by pyrolysis as the temperature is increased. This simplifies the pyrolysis process (shorter times, no deformation, fewer defects) and, in particular, positively influences the porosity of the resulting body before infiltration begins in the fourth step iv). However, solvent debinding is not mandatory. It is also possible to remove all polymer components thermally (here: primarily by pyrolysis).

[0028] It is also possible, after producing the thermoplastic, plastically deformable mass in the first step i), to manufacture granules or strand-like elements using a suitable forming process. The granules or strand-like elements can then be fed to a printing device via an extruder at a suitable temperature, and the second step ii) can be carried out. This allows for improved adaptation to the respective printing process or simply enables stockpiling. For example, granules can be stored in a container and fed to a heated extruder, plasticized, and then extruded through a nozzle opening for the respective additive manufacturing process.In the case of rod-shaped elements made of particle-filled thermoplastic material, these can, for example, be fed from a magazine and heated for a printing process in a form also known per se.

[0029] As previously mentioned, after shaping, the so-called green parts undergo heat treatment. This process partially or completely removes the organic or inorganic polymers, transforming the components or coatings into dense or porous ceramic components or coated components. To remove the organic components, solvent extraction can be used beforehand. This process, employing organic (e.g., acetone, isopropanol, ethanol, alkanes) or inorganic solvents (water), removes a portion of the polymers before the remaining polymeric components are thermally decomposed during heat treatment. Solvent debinding can take place in the temperature range of 10 °C to 100 °C. Depending on the chosen atmosphere and temperature during heat treatment, some polymer components may remain in the material, either passively or with reactive properties.

[0030] For successful material production, a tailored pore structure and carbon content of the precursor—i.e., the green part and pyrolyzed part—are necessary prior to silication. The carbon balance and pore structure can be precisely controlled by combining partial solvent extraction with thermal debinding under inert conditions. Solvent debinding creates pore channels that then enable the homogeneous pyrolysis of the remaining binder system, resulting in a homogeneous carbon distribution within the precursor. This homogeneous distribution is crucial for achieving both complete infiltration and a uniform silicon content across the cross-section, particularly in components with larger cross-sections (e.g., thicknesses greater than 1 cm).An excessively high concentration of pyrolyzed carbon can lead to residual glassy carbon remaining in the infiltrated material, weakening it. The amount of carbon that reacts after pyrolysis depends on the density and is expressed in [value missing]. Fig. 1 shown.

[0031] The amount of carbon obtained through the organic binders should always be less than the amount in Fig. The levels listed in point 1 should be less than 75 wt.% of these limits. However, the residual carbon content introduced by the binder should preferably not fall below 1-2 wt.%, otherwise the stability of the preform will be too low.

[0032] The density of the preform achieved before infiltration also determines the achievable diamond content in the silicon-infiltrated material. The relationship between the parameters is described in Fig. Figure 2 shows these curves. These curves can be used to adjust the diamond content in the material produced by the process. If second phases are added to the diamond particles as solids, these must be taken into account in the calculation.

[0033] Through infiltration of the components or coatings with, for example, molten doped or undoped silicon, reactions can occur with components of the starting solid or the starting solid mixture and the carbon that has formed during the pyrolysis of the binder.

[0034] Infiltration can be carried out with silicon or silicon alloys. Suitable silicon alloys are those that melt at temperatures ≤ 1550 °C. Transition metals that form stable carbides (transition metals of groups 4, 5, and 6) are particularly preferred. Transition metals of groups 7 and 8 can be present in amounts of < 1 vol%, as they can catalytically influence graphitization. Aluminum should also be limited to a level of < 1 vol%, or preferably < 0.2 vol%, as it tends to form Al₄C₃, which is chemically less stable and can weaken the material. Boron or nitrogen doping can increase the electrical conductivity.

[0035] Due to this fact, various microstructures can reactively form, containing boron carbide, transition metal carbides, carbonitrides, oxycarbides, metal borides, metal silicides, or MAX phases (content < 50 vol.% based on the diamond content) or mixtures thereof, in addition to the main phases diamond and SiC. Besides these phases, residues of silicon or intermetallides may be present in proportions of 1 to 20 vol.%. Iron, nickel, and cobalt should be present only as minor impurities, with their proportions ideally kept below 1 wt.%.

[0036] An alloy can be chosen so that it allows melting to occur at temperatures below 1550 °C.

[0037] An optional embodiment of the inventive process is the use of different and multiple starting solid compositions, which can be processed using additive manufacturing via MEX (material extrusion). A starting composition can also contain components. At least one component is diamond-containing material. Non-diamond-containing components consist primarily of transition metal carbides, carbonitrides, nitrides, or metal borides, or mixtures thereof. This makes it possible to selectively combine properties in a component or coating. However, SiC should not be present in the entire diamond-containing starting solid composition. SiC can, however, be present in one component thereof.

[0038] SiC can also serve as a substrate on which the components are printed.

[0039] An optional embodiment of the inventive process can be carried out by machining the unbound and unheat-treated components in their green state or after pyrolysis. The parts can be machined using HSS, carbide, or diamond tools with defined or undefined cutting edges. Surface roughness, tolerances, or geometric shapes can be selectively introduced or adjusted. Machining can be performed sequentially after completion of the green part production by additive manufacturing (the component is manufactured and then machined) or simultaneously, alternating between the two processes before completion of the additive green part production (one or more layers are additively manufactured, then machined, followed by further additive manufacturing, etc.).

[0040] The silicon-infiltrated reaction-bound materials can be surface-treated by sandblasting or grinding, polishing, lapping, electrical discharge machining (EDM) or by laser beam-based methods.

[0041] A special variant of the material is the possibility of using the preform as a polymer-bound diamond material without pyrolysis and / or metal infiltration.

[0042] After pyrolysis, it would also be possible to infiltrate the material with resins (e.g., epoxy resin (EP), vinyl ester resin, or organometallic polymers such as polysiloxanes, polycarbosilanes, or silicones) instead of reactive silicon infiltration and subsequent thermal treatment to achieve modifications, such as crosslinking of the polymer matrix. This would result in a polymer-bonded diamond material that could be used as a wear-resistant material or as a component with high thermal conductivity. One or more resin infiltrations can also be applied prior to silicon infiltration. This allows, for example, the porosity before silicon infiltration as well as the residual silicon content to be precisely controlled by varying the carbon balance in the pyrolyzed material after silicon infiltration.

[0043] The resulting components, with their diverse geometries, can be used, for example, as wear protection components (e.g., nozzles, bearings and bearing holders, seals, dressing tools, reactor and vessel linings) and also as components in thermal management systems. Their high thermal conductivity and corrosion resistance can be utilized, for instance, in housings, heat sinks, and other applications.

[0044] The following serve as an explanation: Fig. 1 and Fig. 2.

[0045] This shows: Fig. 1. A diagram showing the amount of carbon reacted with silicon in wt.% as a function of the density of the shaped green body after pyrolysis and the desired residual silicon content in the finished material. Fig.2 a diagram showing the dependence of the diamond content in vol.-% in the dense SiC-bonded diamond-containing material on the density after pyrolysis and the desired residual silicon content in the finished material.

[0046] The invention will now be explained in more detail using the following exemplary embodiment: In step i), a bimodal diamond powder mixture is used as the starting solid composition. This consists of a fraction of 70 vol.% with a mean particle diameter d. 50 of 50 µm and 30 vol.% with a particle diameter d 50The powder mixture consists of 10 µm synthetic, uncoated diamond particles. It is combined with a polyamide-like polymer binder mixture as an organic binder in a heated sigma kneader at a temperature of approximately 80 °C and pre-mixed. The solids content was 65 vol%. The binder mixture comprised high- and low-melting polymers as well as wetting agents. To homogenize the mixture of solids and polymer, the material is fed into a twin-screw extruder, which homogenizes the material at a temperature of approximately 110 °C–130 °C. The extruded round strand is then segmented into approximately 3 mm long granules using a cutting blade. This process can be repeated multiple times. The final material feed produces an endless round strand with a diameter of 1.75 mm (±0.05 mm).This resulting filament is cooled on a conveyor belt and wound onto a spool for storage. In step ii), this filament is fed into a fixture suitable for FFF printing to produce the component. A file of the component to be manufactured, generated using a Computer Aided Design (CAD) program, is equipped with data for controlling the manufacturing process using a parameterization program. This includes parameters such as printing temperature, layer height, printing speed, nozzle diameter, and information on the movement and retraction conditions of the various drive motors. This so-called G-code ensures the integration of geometric data and system-specific manufacturing parameters. After the component has been manufactured, quality control is performed. The removal of the binder components takes place in two stages according to step iii). In the first stage, approximately 70% of the binder is removed.-% of the binder content is removed in an acetone bath at approximately 35 °C over approximately 48 hours. The dried components are then pyrolyzed under protective gas up to a temperature of 800 °C in Ar. Post-processing then begins as step four (iv), in which the component is transformed into a dense ceramic component through thermal treatment. Densification is achieved by silicon molten infiltration.

[0047] The pyrolyzed preform is reaction-bonded with excess silicon via a wicking process. This was carried out in a vacuum furnace up to 1575 °C. Heating and cooling were performed at a rate of 5 K / min. For larger components, a slower heating and cooling rate is more advantageous. Above the melting point of silicon (1410 °C), the semi-finished product is infiltrated by capillary action and reacts with the pyrolyzed carbon from the organic binder and partially with the contained diamond. This process forms the microstructure. The component was held at the maximum temperature for 30 minutes.

[0048] After the component was removed from the oven, it was sandblasted to remove any adhering silicon.

[0049] The density of the manufactured materials was 3.30 g / cm³. 3 .

[0050] A homogeneous distribution of the diamond particles embedded in the formed SiC was achieved. The infiltration was complete, and there are only low levels of free silicon in the material produced according to the invention. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 20 / 038799 A1

[0004]

Claims

[1] Method for producing three-dimensional components in which the components contain particles made of diamond, wherein In a first step, i) a thermoplastic, plastically deformable mass is produced, which is mixed with a thermoplastic polymer or polymer mixture as an organic binder and diamond particles, which are contained in the thermoplastic mass with a solids content of 30 vol.% to 80 vol.% and which does not contain SiC, and the organic binder and particles are homogeneously mixed and the resulting thermoplastic mass is heated to a viscosity suitable for printing, or a corresponding thermoplastically deformable mass is provided. and in a second subsequent step ii) the thermoplastic mass containing the diamond particles is applied to a substrate or component to be coated in a predetermined quantity, geometry and / or layer thickness using a printing process, thereafter in at least a third step iii) the organic binder is removed and / or decomposed by means of a thermal treatment at a temperature of at least 600 °C up to a maximum of 1670 °C_and subsequently In a fourth step iv) by applying and / or depositing silicon or a silicon alloy in which silicon is the main component, solid silicon is infiltrated into the respective component by raising the temperature to a temperature above the melting point of silicon or the silicon alloy, thereby carrying out an infiltration with liquefied silicon into the respective component. With carbon and silicon, SiC is formed, and thereby a SiC matrix is ​​formed with the SiC formed and the residual silicon from the silicon infiltration that is not used for SiC formation, in which particles are embedded. [2] Method according to claim 1, characterized by , that in the first step i) a thermoplastic mass is used in which, in addition to synthetic diamond particles, particles of boron carbide, or carbides, oxycarbides, carbonitrides of the transition metals, or their mixture, a metal boride, a metal silicide and / or Max phases are also contained. [3] Method according to any one of the preceding claims, characterized by, that the organic binder used is polyamides (caprolactam- and laurin-lactam-based), polylactides (PLA), acrylonitrile butadiene styrene copolymers (ABS), acrylonitrile styrene acrylates (ASA), polyolefins (PP; PE, polyolefin waxes), polyethylene terephthalate (PET), polyvinyl butyral (PVB), polycarbonate (PC), thermoplastic elastomers (TPE), phenolic resin (PF), furan resins (FA), lignin, di- or polysaccharide) PEEK polyaryletherketones (PEEK, PEKK), polyimides (PEI, PI), polyoxymethylene (POM) or natural, semi-synthetic or synthetic wax or mixtures thereof. [4] Method according to any one of the preceding claims, characterized by , that in the first step i) a mixture of at least two different organic thermoplastic polymers is used as a binder, which is homogeneously mixed with the particles. [5] Method according to any one of the preceding claims, characterized by, that in the third step iii) beginning after solidification of the polymeric components with a solvent, thermoplastic polymer is partially removed by dissolution, before further proportions of thermoplastic polymer are removed and / or removed by pyrolysis upon further temperature increase. [6] Method according to any one of the preceding claims, characterized by , that after the production of the thermoplastic mass in the first step i) a granules, rods or strand-like elements are produced using a suitable shaping process and the obtained granules, rods or strand-like elements are fed to a printing device by means of an extruder at a suitable temperature and the second step ii) is carried out. [7] Method according to any one of the preceding claims, characterized by , that in step iii) a temperature of up to a maximum of 1670 °C is maintained. [8] Method of one of claims 1 to 5 and 7, characterized by , that after the production of the thermoplastic mass in the first step i) a granulate or strand-shaped elements are produced using a suitable shaping process and these are directly subjected to steps iii) and iv) and are used in this form as components. [9] Method according to any one of the preceding claims, characterized by , that the diamond particles are partially or completely coated, a layer thickness between 0.05 µm and 1 µm should be maintained as a benefit. [10] Method according to claim 9 wherein the layers are formed with a transition metal of group 4-6 or its carbides, silicides or carbonitrides or SiC. [11] Method according to any one of the preceding claims, characterized by that the heating and cooling rates are 2-10 K / min. [12] Method according to any one of the preceding claims, characterized bythat the Si content in the silicated body is < 15 vol.% preferably < 5 vol.%. [13] Method according to any of the preceding claims, wherein the amount of carbon produced during binder pyrolysis is limited to a maximum of 75% of the proportion of carbon converted during silicification.

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