Method for preparing a coated substrate, a coated substrate, and its use

Abstract

The present invention relates to a method for preparing a coated substrate, in which an aqueous suspension is first prepared comprising water, at least one aggregate former and at least one refractory metal carbide particle, wherein the at least one refractory metal carbide particle forms aggregates in the aqueous suspension. The at least one aqueous suspension is then applied to a porous substrate. The substrate is then subjected to a sintering treatment. According to the invention, the diameter of each of the aggregates is greater than the pore entrance diameter of each of the pores of the porous substrate. The present invention further relates to a coated substrate that may be or is producible by the method of the present invention, and to the use of such a coated substrate.

Description

Technical Field

[0001] The present invention relates to a method for preparing a coated substrate. In the method, first, an aqueous suspension containing water, at least one agglomerate former, and particles of at least one refractory metal carbide is prepared, wherein the particles of at least one refractory metal carbide form agglomerates in the aqueous suspension. Next, at least one aqueous suspension is applied to a porous substrate. Next, the substrate is subjected to a sintering treatment. According to the present invention, the diameter of each of the agglomerates is larger than the pore inlet diameter of each of the pores of the porous substrate. The present invention further relates to a coated substrate that can be produced or can be produced by the method of the present invention, and the use of such a coated substrate.

[0002] Refractory metal carbides such as tantalum carbide (TaC) are generally characterized by their high mechanical, chemical, and thermal resistance. The use of these materials is primarily focused on high-temperature applications, such as in semiconductor crystal growth, where highly corrosive and aggressive species exist, thus limiting the usability of existing (e.g., graphite) components or significantly reducing their service life. Since it has been demonstrated that producing components of proven volume and complex geometric shapes from refractory metal carbides at low cost using hot-pressure treatments described in the literature is difficult, coatings are preferred. In this treatment, the formation of ceramic layers via hot-pressure is not possible. Coatings are produced, for example, via CVD treatment, which deposits a dense layer of several micrometers onto a substrate via a gas phase. An example of this is a TaC coating with a single-layer structure. However, this cost-intensive method prevents the realization of coating components with arbitrary geometric shapes and sizes, and with arbitrary layer thicknesses. To ensure greater flexibility in these areas, there is the option of applying the coating to the substrate via a wet ceramic treatment (dipping, brushing, or spraying). This can be achieved, for example, via an organic solvent suspension (see, for example, US2013 / 0061800A1). To produce the desired protective coating properties, a sintering process is added downstream of the initial suspension coating process.

[0003] In addition to the creation of a mechanically stable coating (high abrasion and adhesion resistance) through the final sintering process, a high degree of compression is simultaneously required to optimally protect the substrate from corrosive media in high-temperature applications. Beyond the requirement of high compression, it is also necessary to minimize crack formation in the coating after sintering to ultimately ensure the protective coating properties of the refractory metal carbide coating and to maximize protection of the base substrate from corrosive media in high-temperature applications. Cracks can occur during the sintering process, for example, during compression or shrinkage, or during cooling. Shrinkage cracks can be avoided if the applied green sheet exhibits a uniform or homogeneous thickness progression, thus allowing for uniform compression. If there is heterogeneity (such as depressions) in the layer progression, shrinkage cracks can easily occur, which may expand longitudinally or transversely during further stages of the sintering process or later under operating conditions. Crack formation during cooling is due to the release of excessive thermal tensile stress induced by the typically large difference in thermal expansion coefficients between the refractory metal carbide coating and the base substrate.

[0004] However, obtaining a homogeneous coating pattern in a suspension-based coating of porous substrates such as CFC substrates with refractory metal carbide coatings is hindered by the strong penetration behavior of the porous substrate and the resulting penetration of the suspension into the pores, thus resulting in a heterogeneous pattern of the refractory metal carbide coating.

[0005] Based on this, an object of the present invention is to provide a method for preparing a coated substrate that can obtain a substrate with a refractory metal carbide coating that is as homogeneous as possible. Furthermore, an object of the present invention is to provide a coated substrate having a refractory metal carbide coating that is as homogeneous as possible.

[0006] This objective is addressed with respect to a method for producing a coated substrate having the features of claim 1 and with respect to a coated substrate having the features of claim 12. Claim 15 illustrates possible uses of the coated substrate according to the present invention. Dependent claims represent advantageous further developments. [Brief explanation of the drawing]

[0007] [Figure 1] This diagram schematically illustrates aggregate formation. [Figure 2] Photographs of suspensions prepared using various methods are shown. [Figure 3] Photographs of suspensions prepared using various methods are shown. [Figure 4] The image shows a cross-section of the coated substrate. [Figure 5] The image shows a cross-section of the coated substrate. [Figure 6] The image shows a cross-section of the coated substrate. [Figure 7] The image shows a cross-section of the coated substrate. [Modes for carrying out the invention]

[0008] Therefore, according to the present invention, a) Prepare at least one aqueous suspension containing water, at least one aggregate-forming agent, and at least one particle of refractory metal carbide, wherein the particle of at least one refractory metal carbide forms aggregates in the aqueous suspension. b) Apply at least one aqueous suspension to a porous substrate, c) After step b), the substrate is subjected to a sintering process. Each aggregate has a diameter greater than the diameter of each pore opening in the porous substrate. A method for preparing a coated substrate is disclosed. In step a) of the method according to the present invention, first, at least one aqueous suspension is prepared. The at least one aqueous suspension comprises water, at least one aggregate-forming agent, and particles of at least one refractory metal carbide. In this context, the aggregate-forming agent can be understood as a substance that influences the formation of aggregates. Preferably, the at least one aggregate-forming agent is selected from the group consisting of tetrabutylammonium hydroxide, polyvinyl alcohol, and mixtures thereof. The at least one aqueous suspension may consist of water, at least one aggregate-forming agent, and particles of at least one refractory metal carbide. Preferably, the at least one refractory metal carbide is tantalum carbide.

[0009] In step b) of the method according to the present invention, at least one aqueous suspension (prepared in step a) is applied to the porous substrate. The at least one aqueous suspension may be applied to one or more partial areas (e.g., one or more surfaces) of the porous substrate or to the entire porous substrate (or the entire surface of the porous substrate). The at least one aqueous suspension may be applied to the porous substrate in layers. The layers (or layers) of the at least one aqueous suspension applied in this manner may be referred to as green sheets (or layers of green sheets). Preferably, in step b), at least one layer of the at least one aqueous suspension is applied to the porous substrate.

[0010] Preferably, step b) is performed immediately after step a).

[0011] The porous substrate may preferably be a carbon substrate, more preferably a graphite substrate, and most preferably an isotropic graphite substrate. In this context, isotropic graphite is understood to mean graphite produced by isotropic pressure treatment. For example, the porous substrate may be a crucible, preferably a carbon crucible, more preferably a graphite crucible, and most preferably an isotropic graphite crucible.

[0012] Preferably, the pores of the porous substrate have an average pore inlet diameter in the range of 0.1 μm to 5 μm, preferably 0.5 μm to 5 μm (preferably on the surface). The average pore inlet diameter (preferably on the surface) can be determined, for example, by mercury intrusion (DIN 66133:1993-06) or mercury porosimetry (DIN 15901-1:2019-03).

[0013] Preferably, the porous substrate has an open porosity in the range of 5% to 20%. The open porosity can be determined, for example, by the mercury intrusion method (DIN 66133:1993-06).

[0014] After step b), in step c) of the method according to the present invention, the substrate is subjected to a sintering treatment. The sintering treatment can produce at least one protective layer containing at least one refractory metal carbide from at least one aqueous suspension (applied in step b). In other words, the sintering treatment can convert at least one aqueous suspension (applied in step b) into a protective layer containing at least one refractory metal carbide.

[0015] The method according to the present invention enables the creation of a refractory metal carbide coating on a substrate that can function as a high-temperature resistant and abrasion-resistant coating or coating system.

[0016] The method according to the present invention is a wet ceramic treatment for the formation of refractory metal carbide coatings on a substrate. In contrast to coatings prepared via CVD or PVD treatment, coatings prepared via wet ceramic treatment exhibit an isotropic texture with random grain size orientation, resulting in reduced susceptibility to cracking and increased diffusion pathways for harmful species to the substrate. Consequently, coated substrates produced according to the present invention exhibit improved protection against corrosive materials used in high-temperature applications compared to coated substrates produced via CVD or PVD treatment. Furthermore, the wet ceramic treatment according to the present invention is less expensive than CVD or PVD treatment and also offers greater flexibility in the geometric shape and size of the coating components that can be produced, as well as in the thickness of the applied coating or layer.

[0017] Furthermore, the method for producing a coated substrate according to the present invention is based on the use of an aqueous suspension. Compared to the use of an organic suspension, the use of an aqueous suspension has several advantages. Thus, in contrast to organic suspensions, aqueous suspensions are inexpensive, harmless from an environmental and health standpoint, and do not involve the safety-related issues associated with flammable spray mist. Moreover, the use of an aqueous suspension eliminates the need for thermal decomposition to remove organic solvents, which can lead to the undesirable introduction of foreign matter into the coating. Furthermore, in contrast to the use of known organic suspensions, the use of an aqueous suspension allows for controlled application of the suspension. In particular, with spray application of known organic suspensions, controlled application is not possible because the suspension properties can fluctuate due to the evaporation of the solvent during the process, and therefore, a homogeneous layer cannot be obtained over time.

[0018] Due to the sintering process, the protective coating obtained by the method according to the present invention is a mechanically stable coating with high abrasion and adhesion resistance. Furthermore, as a result of the sintering process, a higher degree of compression is achieved compared to the initial density (Green density) after application.

[0019] The presence of at least one aggregate-forming agent in at least one aqueous suspension promotes the controlled formation of aggregates of refractory metal carbide particles in the at least one aqueous suspension. As a main result, the particles do not aggregate in an uncontrolled form. Instead, the aggregates are formed in a relatively narrow size range, i.e., aggregates having all similar sizes. By using the narrow size range of the formed aggregates, it can be ensured that the size of each formed aggregate does not vary excessively from the average size of the formed aggregates. In this way, it can be prevented that the individual formed aggregates have an excessively small diameter.

[0020] Since the size or size range of the aggregates is determined according to the weight percentage of the aggregate-forming agent in the aqueous suspension, it is also possible to specifically control and adjust the size of the aggregates or the size range of the aggregates with the aid of the aggregate-forming agent. The higher the weight percentage of the aggregate-forming agent, the smaller the diameter of the aggregates. Preferably, the at least one aqueous suspension contains from 0.1 to 2% by weight, more preferably from 0.1 to 1% by weight, of at least one aggregate-forming agent based on the total weight of the aqueous suspension. By using such a proportion of the aggregate-forming agent, relatively large aggregates (i.e., aggregates having a relatively large diameter) can be obtained in a relatively narrow size range.

[0021] Depending on the stability of the individual particles in the suspension affected by the targeted addition of the aggregate-forming agent, the aggregate size, which is also measured by the standing time of the suspension, can be specifically adjusted. The longer the standing time of the suspension, the better the individual particles are stabilized and the smaller the aggregates become. Without the specific addition of the aggregate-forming agent, uncontrolled strong aggregation makes it almost impossible or only possible for a very short time to spray the suspension.

[0022] According to the present invention, the diameter of each aggregate is larger than the pore inlet diameter of each pore of the porous substrate. Thereby, it is ensured that no aggregate can enter any of the pores of the porous substrate. As a result, when at least one aqueous suspension is applied to the porous substrate, it is possible to prevent at least one refractory metal carbide from entering the pores of the substrate, and ultimately, the formation of a homogeneous protective layer is brought about.

[0023] When the aggregate of refractory metal carbide or refractory metal carbide particles enters the pores of the porous substrate, this brings inhomogeneity within the layer. If there is such inhomogeneity (depressions, etc.) in the transition of the layer, shrinkage cracks are likely to occur, which may spread longitudinally or transversely in a further process of the sintering treatment or even under service conditions later. The generation of cracks during cooling is due to the release of excessive thermal tensile stress induced by the usually large difference in the coefficient of thermal expansion between the refractory metal carbide coating and the (e.g., carbon-based) base substrate.

[0024] Due to the controlled aggregate formation provided in the method of the present invention, where the diameter of each aggregate is larger than the pore inlet diameter of each pore of the porous substrate, penetration of the refractory metal carbide into the pores of the porous substrate can be prevented here, so that it is possible to obtain a very homogeneous (or uniform) coating. Due to the very homogeneous (or even) transition of the layer, shrinkage cracks within the protective layer can be avoided. This is also true because the uniform or homogeneous transition of the layer enables uniform compression. The fewer the number of shrinkage cracks in the obtained protective layer, the better the substrate is protected by the protective layer (e.g., from corrosive media in high-temperature applications). Therefore, by the treatment according to the present invention, it is possible to obtain a refractory metal carbide protective layer that is very homogeneous and has only a few cracks (or even no cracks), thereby effectively protecting the substrate from external influences (such as corrosive media in high-temperature applications).

[0025] Using SEM images of the fabricated coated substrate, it can be demonstrated that the pores of the coated substrate are not filled and that none of the aggregates of at least one type of refractory metal carbide particles have penetrated the pores of the substrate. From this, it can be concluded that the diameter of each aggregate is larger than the pore inlet diameter of each pore in the porous substrate. Therefore, the SEM images can demonstrate the existence of the characteristic that the diameter of each aggregate is larger than the pore inlet diameter of each pore in the porous substrate.

[0026] Preferably, in step b), at least one layer of at least one aqueous suspension is applied to the porous substrate. The at least one layer of at least one aqueous suspension may be referred to as at least one green sheet. The green sheet may exhibit a uniform or homogeneous thickness.

[0027] The coating substrate produced by the method according to the present invention can be used, for example, as part of a gallium evaporator or gallium evaporator in a VPE GaN reactor that can be used for growing gallium nitride semiconductor crystals, and the layer system obtained by the process according to the present invention in this case functions as a coating for the gallium evaporator.

[0028] A preferred modification of the method according to the present invention is: - The porous substrate contains or consists of a material selected from the group consisting of graphite, preferably isotropic graphite, carbon fiber reinforced carbon (CFC), C / SiC fiber composite, SiC / SiC fiber composite, carbide ceramics, nitride ceramics, oxide ceramics, and mixtures thereof, and / or - At least one refractory metal carbide is selected from the group consisting of titanium carbide, zirconium carbide, hafnium carbide, vanadium carbide, niobium carbide, tantalum carbide, chromium carbide, molybdenum carbide, tungsten carbide, and mixtures thereof, and / or - At least one agglomerating agent is selected from the group consisting of polyvinyl alcohol; polyacrylic acid; polyvinylpyrrolidone; polyalkylene glycol ether; bases, preferably tetrabutylammonium hydroxide, tetramethylammonium hydroxide, polyethyleneimine, inorganic bases (e.g., NaOH, ammonium hydroxide); and mixtures thereof, and at least one agglomerating agent is particularly preferably selected from the group consisting of tetrabutylammonium hydroxide, polyvinyl alcohol, and mixtures thereof. It is characterized by the following:

[0029] Particularly preferred is that at least one refractory metal carbide is tantalum carbide. Tantalum carbide provides a particularly good protective effect for porous substrates.

[0030] The porous substrate may preferably contain or consist of a material selected from the group consisting of graphite, preferably isotropic graphite, carbide ceramics, nitride ceramics, oxide ceramics, and mixtures thereof.

[0031] The porous substrate may preferably contain or consist of a material selected from the group consisting of graphite, preferably isotropic graphite, carbon fiber reinforced carbon (CFC), C / SiC fiber composite, SiC / SiC fiber composite, and mixtures thereof.

[0032] Carbon-based and SiC-based substrates exhibit increased penetration behavior when coated with an aqueous suspension. As a result, the method according to the present invention is particularly suitable for such substrates.

[0033] Most preferably, the porous substrate contains or comprises graphite, preferably isotropic graphite.

[0034] Agglomerating agents selected from the group consisting of polyvinyl alcohol; polyacrylic acid; polyvinylpyrrolidone; bases, preferably tetrabutylammonium hydroxide, tetramethylammonium hydroxide, polyethyleneimine, inorganic bases (e.g., NaOH); and mixtures thereof, are particularly suitable as agglomerating agents in the treatment according to the present invention.

[0035] Particularly preferred flocculants are those selected from the group consisting of tetrabutylammonium hydroxide, polyvinyl alcohol, and mixtures thereof. By using flocculants selected from the group consisting of tetrabutylammonium hydroxide, polyvinyl alcohol, and mixtures thereof, highly controlled aggregate formation can be achieved, where the formed aggregates can have a size within a particularly narrow size range.

[0036] Preferably, the pores of the porous substrate have an average pore inlet diameter of up to 1.5 μm, preferably in the range of 0.1 μm to 1.5 μm, and particularly preferably in the range of 0.5 μm to 1.5 μm. The average pore inlet diameter can be determined, for example, by mercury porosimetry (DIN 15901-1:2019-03) or by mercury intrusion (DIN 66133:1993-06). Such a low average pore inlet diameter can better prevent aggregates of refractory metal carbide particles from penetrating the pores of the porous substrate.

[0037] Preferably, the porous substrate contains or consists of a material having an average particle size of up to 5 μm. The average particle size can be determined, for example, by laser diffraction (DIN 13320:2020-01). The average particle size affects the penetration behavior of the porous substrate. An average particle size of up to 5 μm can better prevent aggregates of refractory metal carbide particles from penetrating the pores of the porous substrate.

[0038] A more preferred modification of the method according to the present invention is: - The diameter of each aggregate is at least 20 μm, preferably at least 10 μm, and especially preferably at least 5 μm, and / or - The pore inlet diameter of each pore in the porous substrate is at most 19 μm, preferably at most 9 μm, and particularly preferably at most 4 μm. It is characterized by the following:

[0039] Preferably, the diameter of the aggregate may be at least 20 μm, and the pore inlet diameter of the porous substrate may be up to 19 μm. Particularly preferably, the diameter of the aggregate may be at least 10 μm, and the pore inlet diameter of the porous substrate may be up to 9 μm. Very preferably, the diameter of the aggregate may be at least 5 μm, and the pore inlet diameter of the porous substrate may be up to 4 μm.

[0040] The diameter of the aggregates and the pore inlet diameter of the porous substrate can be determined, for example, from SEM images.

[0041] The minimum size of the aggregates can also be indirectly determined by determining the pore inlet diameter of the substrate's pores. Since none of the pores are filled, it can be concluded that the diameter of the aggregates is larger than the pore inlet diameter of the substrate's pores. Furthermore, the size of the aggregates can also be estimated from the standing time test of a specifically prepared suspension.

[0042] According to a more preferred modification of the method according to the present invention, in step a), first a mixture containing the components of the aqueous suspension to be prepared is prepared, and then the mixture is left to stand for a period of time from 3 to 30 minutes, preferably from 5 to 15 minutes, and particularly preferably from 5 to 10 minutes, without stirring, thereby preparing at least one aqueous suspension.

[0043] By allowing the mixture to stand for 3 to 30 minutes, preferably 5 to 15 minutes, and especially preferably 5 to 10 minutes, without stirring, a particularly stable suspension can be obtained having aggregates within a desired narrow size range, where the aggregates are almost completely stabilized. In this way, it becomes even easier to prevent the formation of individual aggregates having a diameter smaller than the pore inlet diameter of the pores of the porous substrate.

[0044] At least one aqueous suspension may contain at least one binder selected from the group consisting of polyethylene glycol, polyvinyl butyral, polyurethane, chloroprene rubber, phenolic resin, acrylic resin, carboxymethylcellulose, alginic acid, dextrin, sodium biphenyl-2-yl oxide, polyphenyl oxide, and mixtures thereof, wherein the at least one binder is particularly preferably selected from the group consisting of sodium biphenyl-2-yl oxide, polyphenyl oxide, and mixtures thereof, and where the at least one binder may be present in at least one aqueous suspension at a level of 0.05 to 1% by weight or 0.01 to 5% by weight based on the total weight of the aqueous suspension.

[0045] The binder increases the disintegration point of the particles, and therefore, they remain in the suspension during application. As a result, the aqueous suspension applied to the substrate can dry without penetrating the substrate. This is also a better method for preventing aggregates of refractory metal carbide particles from penetrating the pores of porous substrates.

[0046] According to a more preferred modification of the method according to the present invention, - The surface of the porous substrate on which at least one aqueous suspension is deposited has an average roughness value of up to 3 μm, preferably 0.5 μm to 3 μm, and particularly preferably 1 μm to 3 μm, and / or - The surface has an average roughness depth of up to 12 μm, preferably 1 μm to 12 μm, and particularly preferably 3 μm to 12 μm.

[0047] By using a porous substrate having such an average roughness value and / or such an average roughness depth, a more homogeneous coating can be obtained.

[0048] The average roughness value can be determined, for example, by optical interferometry and evaluation in accordance with DIN EN ISO 25178:2016-12.

[0049] The average depth of roughness can be determined, for example, by optical interferometry and evaluation in accordance with DIN EN ISO 25178:2016-12.

[0050] According to another preferred modification of the method according to the present invention, the average particle size of at least one refractory metal carbide particle is larger than the average pore inlet diameter of the pores in the porous substrate. In this way, it is achieved that the at least one refractory metal carbide particle is already large enough that not all or at least most of it can penetrate the pores. In this way, it becomes even easier to prevent aggregates of refractory metal carbide particles from penetrating the pores of the porous substrate.

[0051] Preferably, the pores of the porous substrate have an average pore inlet diameter of up to 1.2 μm, preferably in the range of 0.1 μm to 1.2 μm, and more preferably in the range of 0.5 μm to 1.2 μm.

[0052] Preferably, the average particle size (d50 value) of at least one refractory metal carbide particle is in the range of 2 μm to 50 μm, preferably in the range of 3 μm to 30 μm.

[0053] The average particle size (d50 value) of particles of at least one refractory metal carbide can be determined, for example, by laser diffraction (DIN 13320:2020-01).

[0054] - The average particle size (d50 value) of at least one refractory metal carbide particles is in the range of 0.2 μm to 5 μm, preferably 0.5 μm to 4 μm, and particularly preferably 0.8 μm to 2.5 μm, and the average pore inlet diameter of the porous substrate is at most 1.2 μm, preferably in the range of 0.1 μm to 1.2 μm, and particularly preferably in the range of 0.5 μm to 1.2 μm, and preferably the substrate contains or consists of a material having an average particle size of up to 5 μm, or - The average particle size (d50 value) of at least one refractory metal carbide particles is in the range of 2 μm to 50 μm, preferably 3 μm to 30 μm, and particularly preferably 5 μm to 20 μm, and the average pore inlet diameter of the porous substrate is greater than 1.2 μm, and preferably the substrate contains or consists of a material having an average particle size greater than 5 μm. It would be even more preferable.

[0055] A more preferred modification of the method according to the present invention is characterized by preparing at least one further aqueous suspension containing at least one particulate refractory metal carbide and water in addition to the at least one aqueous suspension prepared in step a), and then, after step b), applying the at least one further aqueous suspension (for example, in the form of a layer) to the at least one aqueous suspension applied in step b), wherein the particles of the at least one refractory metal carbide contained in the aqueous suspension prepared in step a) have an average particle diameter in the range of 5 μm to 50 μm, and the particles of the at least one refractory metal carbide contained in the further aqueous suspension have an average particle diameter in the range of 0.2 μm to 2 μm. Using such a multilayer coating structure can improve the uniformity of the thickness of the refractory metal carbide layer and avoid potential drawbacks in the sintering process. Therefore, refractory metal carbide particles having a relatively large diameter in the range of 5 to 50 μm in a layer directly deposited on the substrate can more effectively prevent aggregates of refractory metal carbide particles from penetrating the pores of the porous substrate. At the same time, since sintering of refractory metal carbide particles with a large particle size is poorer, refractory metal carbide particles having a relatively small diameter in the range of 0.2 to 2 μm in a further layer can be used to achieve better sintering of these refractory metal carbide particles.

[0056] A further preferred modification of the method according to the present invention is characterized in that the preparation of at least one aqueous suspension in step a) comprises mixing the components of the at least one aqueous suspension to be prepared with the assistance of a dispersion device, wherein the mixing with the assistance of a dispersion device is preferably carried out using a grinding medium and / or over a period of at least 12 hours.

[0057] Optimal mixing of the aqueous suspension can be achieved by mixing the components, preferably using a grinding medium and / or over a period of at least 12 hours, with the assistance of a dispersion device, thereby better avoiding heterogeneity in distribution and, consequently, incompatibility in compression. For example, when mixing with a dispersion device, a rotational speed of up to 1 m / s can be used.

[0058] According to a more preferred modification of the method according to the present invention, the application of at least one aqueous suspension in step b) is carried out by dipping, brushing, or spray application. Particularly preferably, the application of at least one aqueous suspension in step b) is carried out by spray application. Spray application is a preferred choice for producing one or more thin, fast-drying refractory metal carbide coatings having a layer thickness preferably in the range of 20 μm to 80 μm. In this process, a very thin suspension layer can be applied to a surface by passing the components through a spray jet and rotating them at high speed. Depending on the solids content of the suspension, this layer can dry rapidly to very rapidly. The preferred solids content of the refractory metal carbide powder is higher than or equal to 70% by weight of the total suspension. Each individual layer to be applied should preferably exhibit equivalent drying behavior. In principle, fast-drying behavior of the applied suspension layer is preferred because if the drying time of the layer is too long, heterogeneity in the particle distribution may occur due to density differences between the refractory metal carbide and sintering additives.

[0059] According to an exemplary preferred modification, in step b), at least one layer of aqueous suspension having an average layer thickness less than 150 μm, preferably 20 μm to 100 μm, and particularly preferably 30 μm to 80 μm, can be applied to the porous substrate.

[0060] According to an alternative exemplary preferred embodiment, in step b), at least one layer of aqueous suspension having an average layer thickness less than 50 μm, preferably less than 30 μm, can be applied to the porous substrate.

[0061] A more preferred modification of the method according to the present invention is that at least one aqueous suspension is - Based on the total weight of the aqueous suspension, it contains 60 to 90% by weight, preferably 70 to 85% by weight, of particles of at least one refractory metal carbide, and / or - Contains 0.1 to 2% by weight, preferably 0.1 to 1% by weight, of at least one aggregate-forming agent, based on the total weight of the aqueous suspension. It is characterized by the following:

[0062] In a more preferred embodiment, at least one aqueous suspension may preferably contain a sintering additive selected from the group consisting of refractory metal silicides, refractory metal nitrides, refractory metal borides, silicon, silicon carbide, boron nitride, tungsten carbide, vanadium carbide, molybdenum carbide, boron carbide, and mixtures thereof, wherein the sintering additive is particularly preferably selected from the group consisting of silicon, zirconium boride, refractory metal silicides, and mixtures thereof.

[0063] The refractory metal silicide is preferably selected from the group consisting of titanium silicide, zirconium silicide, e.g., zirconium disilicate (ZrSi2), hafnium silicide, e.g., hafnium disilicate (HfSi2), vanadium silicide, e.g., vanadium disilicate (VSi2), niobium silicide, e.g., niobium disilicate (NbSi2), tantalum silicide, e.g., tantalum disilicate (TaSi2), chromium silicide, molybdenum silicide, e.g., molybdenum disilicate (MoSi2), tungsten silicide, e.g., tungsten disilicate (WSi2), and mixtures thereof.

[0064] The refractory metal nitride is preferably selected from the group consisting of titanium nitride, zirconium nitride, hafnium nitride, vanadium nitride, niobium nitride, tantalum nitride, chromium nitride, molybdenum nitride, tungsten nitride, and mixtures thereof.

[0065] The refractory metal boride is preferably selected from the group consisting of titanium boride, zirconium boride, hafnium boride, vanadium boride, niobium boride, tantalum boride, chromium boride, molybdenum boride, tungsten boride, and mixtures thereof.

[0066] These sintering additives have been shown to have at least the same or even better effect in terms of the degree of compression compared to transition metals (e.g., cobalt, nickel, iron, etc.) used as sintering additives in the prior art, due to their properties (e.g., melting point, boiling point, etc.). Therefore, by using them, a high degree of compression of the sintered layer can be achieved, thereby providing very good protection of the substrate from corrosive media in high-temperature applications. Compared with sintering additives used in the prior art, such as cobalt, the sintering additives mentioned are, first and foremost, harmless from a safety and health standpoint. Furthermore, their use and thus the avoidance of certain transition metals such as cobalt, nickel, and iron as sintering additives further prevents these transition metals from remaining in the coating as harmful impurities to the growth atmosphere when the coated substrate is used in high-temperature applications in semiconductor crystal growth.

[0067] The sintering process in step c) is - At a temperature of 2100°C to 2500°C, preferably 2200°C to 2400°C, and / or - Holding time of 1 to 15 hours, preferably 2 to 10 hours, and / or - At a pressure of 0.1 bar (10 kPa) to 10 bar (1000 kPa), preferably 0.7 bar (70 kPa) to 5 bar (500 kPa), and / or - Under an argon atmosphere The method according to any one of the prior claims, characterized in that it is carried out.

[0068] On the other hand, these designs of the sintering process ensure that the resulting protective coating has particularly high mechanical stability, including high abrasion and adhesion resistance. Furthermore, these designs of the sintering process further enhance the stability of the molten phase throughout the entire sintering process.

[0069] The present invention further relates to a coating substrate comprising a porous substrate and at least one layer disposed on the porous substrate which contains or comprises at least one refractory metal carbide, wherein the at least one layer disposed on the porous substrate has a granular structure of isometric grains, and the porous substrate has unfilled pores that are closed by the at least one layer disposed on the porous substrate.

[0070] Equal-length grains are understood to preferably mean grains having an aspect ratio close to or equal to 1. For example, equal-length grains may have an aspect ratio of at least 0.95, preferably at least 0.95, more preferably at least 0.99, and especially 1.

[0071] The granular structure of equal-length grains occurs only in coatings obtained by suspension deposition, and not in coatings obtained by CVD treatment. Subsequently, the coated substrate according to the present invention differs from a substrate coated by CVD treatment due to the granular structure of equal-length grains in at least one layer disposed on the porous substrate.

[0072] SEM images of the coated substrate (or its cross-section) can demonstrate that the porous substrate has unfilled pores that are closed by at least one layer placed on the porous substrate.

[0073] Due to the characteristic that the porous substrate has unfilled pores that are closed by at least one layer placed on the porous substrate, it is possible to obtain at least one layer placed on the porous substrate that is highly homogeneous and only slightly cracked (or even crack-free), thereby allowing the at least one layer placed on the porous substrate to better protect the porous substrate from external influences (such as corrosive media in high-temperature applications).

[0074] A preferred embodiment of the coating substrate according to the present invention is characterized in that at least one layer disposed on a porous substrate has an average layer thickness of at least 20 μm, preferably 20 μm to 150 μm, and particularly preferably 30 μm to 100 μm.

[0075] A preferred embodiment of the coating substrate according to the present invention is characterized in that the standard deviation of the average layer thickness of at least one layer disposed on the porous substrate is at most 6%, preferably in the range of 0.5% to 6%, and particularly preferably in the range of 1% to 6%.

[0076] Very preferably, at least one layer disposed on the porous substrate has an average layer thickness of at least 20 μm, preferably 20 μm to 150 μm, and particularly preferably 30 μm to 100 μm, and the standard deviation of the average layer thickness of at least one layer disposed on the porous substrate is at most 6%, preferably in the range of 0.5% to 6%, and particularly preferably in the range of 1% to 6%.

[0077] The standard deviation of the average layer thickness is a measure of the homogeneity (or uniformity) of the coating layer thickness. The smaller the standard deviation of the average layer thickness of at least one layer placed on a porous substrate, the more homogeneous (or uniform) the layer thickness of at least one layer placed on a porous substrate is.

[0078] The optical path of layers arranged on a porous substrate containing or consisting of at least one refractory metal carbide can be displayed and evaluated in a classical manner by examining the cross-section. Here, visual observation of the cross-section and qualitative classification into homogeneous or heterogeneous layer systems can be performed.

[0079] From the cross-section of the coating substrate, the average layer thickness of at least one layer placed on the porous substrate can also be determined. Therefore, by performing multiple point measurements on the cross-section of the layer, the average layer thickness can be determined, and the standard deviation can be calculated from this, which also provides a quantitative evaluation of the homogeneity of the layer.

[0080] For example, homogeneity can be quantified using the standard deviation of layer thickness as follows: - Preparation of the cross-section of the coating substrate (i.e., layer + substrate) - Measurement of the distance (layer thickness) between interfaces and layer surfaces based on recorded cross-sectional images. - For example, layer thickness analysis in an area with a maximum extent of 4 cm. - The number of individual layer thickness measurements is at least 25 per 1 cm measurement area. - The intervals between individual layer thickness measurements are regular. - Determination of the standard deviation across all individual layer thickness measurements For example, with a standard deviation of 6% or less, the stratum in question can be assumed to be homogeneous.

[0081] This method allows for a rapid qualitative evaluation of layer homogeneity based on top view, without requiring time-consuming cross-sectional preparation.

[0082] Another preferred embodiment of the coating substrate according to the present invention is: - The porous substrate contains or consists of a material selected from the group consisting of graphite, preferably isotropic graphite, carbon fiber reinforced carbon (CFC), C / SiC fiber composite, SiC / SiC fiber composite, carbide ceramics, nitride ceramics, oxide ceramics, and mixtures thereof, and / or - At least one refractory metal carbide is selected from the group consisting of titanium carbide, zirconium carbide, hafnium carbide, vanadium carbide, niobium carbide, tantalum carbide, chromium carbide, molybdenum carbide, tungsten carbide, and mixtures thereof. It is characterized by the following:

[0083] Particularly preferred is that at least one refractory metal carbide is tantalum carbide.

[0084] The porous substrate may preferably contain or consist of a material selected from the group consisting of graphite, preferably isotropic graphite, carbide ceramics, nitride ceramics, oxide ceramics, and mixtures thereof.

[0085] The porous substrate may preferably contain or consist of a material selected from the group consisting of graphite, preferably isotropic graphite, carbon fiber reinforced carbon (CFC), C / SiC fiber composite, SiC / SiC fiber composite, and mixtures thereof.

[0086] Most preferably, the porous substrate contains or comprises graphite, preferably isotropic graphite.

[0087] The porous substrate may preferably be a carbon substrate, more preferably a graphite substrate, and most preferably an isotropic graphite substrate. In this context, isotropic graphite is understood to mean graphite produced by isotropic pressure treatment. For example, the porous substrate may be a crucible, preferably a carbon crucible, more preferably a graphite crucible, and most preferably an isotropic graphite crucible.

[0088] According to another preferred embodiment of the coating substrate according to the present invention, the coating substrate comprises at least one further layer, wherein the at least one further layer is disposed directly on a layer disposed on a porous substrate and comprises or consists of at least one refractory metal carbide.

[0089] A more preferred embodiment of the coated substrate according to the present invention is characterized in that the coated substrate can be produced or is produced by the method according to the present invention.

[0090] Furthermore, the present invention also relates to the use of a coating substrate according to the present invention in semiconductor crystal growth, wherein the coating substrate is preferably a coating crucible.

[0091] Without limiting the present invention to the parameters specifically shown, the present invention will be described in more detail with reference to the following drawings and examples.

[0092] Figure 1 schematically illustrates aggregate formation. Five suspensions containing water and particles of at least one refractory metal carbide are shown, each having a supernatant 1 and an aggregate-containing portion 2, and were prepared under different conditions. The three right-hand suspensions in regions B and C contain at least one aggregate-forming agent (or aggregate stabilizer), with different proportions of the aggregate-forming agent. The two left-hand suspensions do not contain any aggregate-forming agent. The standing time used in the formation of each suspension increases from left to right.

[0093] Region A shows two suspensions in which no agglomerating agent was used in their preparation. Here, even with a standing time of 1 to a maximum of 2 minutes, uncontrolled, random, and strong agglomeration occurs with a wide size distribution. Individual particles are not stabilized at any time.

[0094] Area B shows two suspensions in which different concentrations of aggregate-forming agents were used in their preparation. Individual particles are stabilized over a maximum standing time of 5 minutes. Aggregate formation occurs, particularly intentionally, within a narrow size range, from a minimum standing time of 5 minutes to a maximum of 15 minutes.

[0095] Region C shows a suspension in which an aggregate-forming agent was used at a higher concentration than in the two suspensions in Region B during its preparation. Individual particles are almost completely stabilized for a maximum standing time of at least 15 minutes. From at least 15 minutes of standing time onward, small amounts of aggregate formation occur, specifically intentionally within a very narrow size range.

[0096] Figure 2 shows photographs of suspensions prepared by various methods. The suspensions in the top three photographs contain water and tantalum carbide particles. The suspensions in the middle three photographs contain water and tantalum carbide particles, plus tetrabutylammonium hydroxide as an aggregate-forming agent. The suspensions in the bottom four photographs contain water and tantalum carbide particles, plus a mixture of tetrabutylammonium hydroxide and polyvinyl alcohol as aggregate-forming agents. In each photograph, the suspension on the left contains 70 wt% tantalum carbide particles, and the suspension on the right contains 50 wt% tantalum carbide particles. The standing time used to prepare the suspensions increases from left to right and is shown at the top of the photographs. Depending on the stability of the individual particles in the suspension, which is affected by the targeted addition of the aggregate-forming agent, the aggregate size, measured by the standing time of the suspension, can be specifically adjusted. The longer the standing time of the suspension, the better the individual particles are stabilized, and the smaller the aggregates become. Without the specific addition of an aggregate-forming agent, uncontrolled and strong aggregation makes it almost impossible or only possible for a very short time to spray the suspension.

[0097] Figure 3 shows photographs of suspensions prepared by various methods. The suspensions in the top three photographs contain water and tantalum carbide particles. The suspensions in the middle three photographs contain water, tantalum carbide particles, and tetrabutylammonium hydroxide as an aggregate-forming agent. The suspensions in the bottom four photographs contain water, tantalum carbide particles, and a mixture of tetrabutylammonium hydroxide and polyacrylic acid as aggregate-forming agents. In each photograph, the suspension on the left contains 70 wt% tantalum carbide particles, and the suspension on the right contains 50 wt% tantalum carbide particles. The standing time used to prepare the suspensions increases from left to right and is shown at the top of the photographs. Depending on the stability of the individual particles in the suspension, which is affected by the targeted addition of the aggregate-forming agent, the aggregate size, measured by the standing time of the suspension, can be specifically adjusted. The longer the standing time of the suspension, the better the individual particles are stabilized, and the smaller the aggregates become. Without the specific addition of an aggregate-forming agent, uncontrolled and strong aggregation makes it almost impossible or only possible for a very short time to spray the suspension.

[0098] Example 1 First, a mixture consisting of 80 wt% TaC powder, 0.1 wt% tetrabutylammonium hydroxide, 1 wt% polyvinyl alcohol, and 18.9 wt% water was prepared, and then an aqueous suspension was prepared by allowing the mixture to stand for 12 minutes without stirring. Here, the TaC particles of the TaC powder formed aggregates in the aqueous suspension. The aqueous suspension thus prepared was then placed on a porous graphite substrate (average pore inlet diameter: 0.6 μm, particle size: 2 μm, R a The material was applied in layers of 1.5 μm thickness. Subsequently, the substrate, which was then provided with an aqueous suspension, was subjected to a sintering process at a temperature of 2300°C, a holding time of 10 hours, and a pressure of 1 bar (100 kPa).

[0099] In this way, a coated graphite substrate comprising a porous graphite substrate and a TaC protective layer disposed thereon was obtained. The TaC layer has a granular structure of equal-length grains.

[0100] A cross-section of the coated substrate was prepared for inspection. An image of this cross-section is shown in Figure 4.

[0101] SEM images of the cross-section reveal that the pores of the porous substrate are not filled and are closed by the TaC layer. From this, it can be inferred that none of the aggregates in the aqueous suspension have penetrated the pores of the porous substrate, and therefore, each aggregate is larger than the pore inlet diameter of each pore in the porous substrate.

[0102] Furthermore, the average thickness and standard deviation of the TaC layer were determined using the cross-section. For this purpose, individual measurements were taken at least 25 measurement points per 1 cm measurement area of ​​the layer by placing regular distances between individual measurement points and measuring the distance (layer thickness) between the interface and the layer surface based on the recorded cross-sectional image. In this way, a value of 55.9 μm was determined for the average thickness of the TaC layer. Furthermore, a standard deviation of 2.5 μm (4.5%) was determined across all individual layer thickness measurements.

[0103] Since the standard deviation does not exceed 6%, this TaC layer is therefore a homogeneous layer.

[0104] Example 2 First, a mixture consisting of 80 wt% TaC powder, 0.1 wt% tetrabutylammonium hydroxide, 1 wt% polyvinyl alcohol, and 18.9 wt% water was prepared, and then an aqueous suspension was prepared by allowing the mixture to stand for 12 minutes without stirring. Here, the TaC particles of the TaC powder formed aggregates in the aqueous suspension. The aqueous suspension thus prepared was then placed on a porous graphite substrate (average pore inlet diameter: 0.6 μm, particle size: 3 μm, R a The material was applied in layers of 1.5 μm thickness. Subsequently, the substrate, which was then provided with an aqueous suspension, was subjected to a sintering process at a temperature of 2300°C, a holding time of 10 hours, and a pressure of 1 bar (100 kPa).

[0105] In this way, a coated graphite substrate comprising a porous graphite substrate and a TaC protective layer disposed thereon was obtained. The TaC layer has a granular structure of equal-length grains.

[0106] A cross-section of the coating substrate was prepared for inspection. An image of this cross-section is shown in Figure 5.

[0107] SEM images of the cross-section revealed that the pores of the porous substrate were not filled and were closed by the TaC layer. From this, it can be inferred that none of the aggregates in the aqueous suspension penetrated the pores of the porous substrate, and therefore, each aggregate is larger than the pore inlet diameter of each pore in the porous substrate.

[0108] Furthermore, the average thickness and standard deviation of the TaC layer were determined using the cross-section. For this purpose, individual measurements were taken at least 25 measurement points per 1 cm measurement area of ​​the layer by placing regular distances between individual measurement points and measuring the distance between the interface and the layer surface (thickness) based on the recorded cross-sectional image. In this way, a value of 50.9 μm was determined for the average thickness of the TaC layer. Furthermore, a standard deviation of 2.7 μm (5.3%) was determined across all individual thickness measurements.

[0109] Since the standard deviation does not exceed 6%, this TaC layer is therefore a homogeneous layer.

[0110] Example 3 First, a mixture consisting of 80 wt% TaC powder, 0.1 wt% tetrabutylammonium hydroxide, 0.5 wt% polyacrylic acid, and 19.4 wt% water was prepared, and then an aqueous suspension was prepared by allowing the mixture to stand for 8 minutes without stirring. Here, the TaC particles of the TaC powder formed aggregates in the aqueous suspension. The aqueous suspension thus prepared was then placed on a porous graphite substrate (average pore inlet diameter: 0.6 μm, particle size: 2 μm, R aThe material was applied in layers of 1.5 μm thickness. Subsequently, the substrate, which was then provided with an aqueous suspension, was subjected to a sintering process at a temperature of 2300°C, a holding time of 10 hours, and a pressure of 1 bar (100 kPa).

[0111] In this way, a coated graphite substrate comprising a porous graphite substrate and a TaC protective layer disposed thereon was obtained. The TaC layer has a granular structure of equal-length grains.

[0112] Furthermore, the average thickness and standard deviation of the TaC layer were determined using non-contact layer thickness measurement. In this way, an average layer thickness of 48.6 μm was determined for the TaC layer. Additionally, a standard deviation of 2.1 μm (4.3%) was determined across all individual layer thickness measurements.

[0113] Since the standard deviation does not exceed 6%, this TaC layer is therefore a homogeneous layer.

[0114] Comparative Example 1 First, an aqueous suspension was prepared by creating a mixture consisting of 80 wt% TaC powder, 1 wt% polyvinyl alcohol, and 19 wt% water, and then allowing the mixture to stand for 8 minutes without stirring. Here, the TaC particles of the TaC powder formed aggregates in the aqueous suspension. The aqueous suspension thus prepared was then subjected to a porous graphite substrate (average pore inlet diameter: 1.8 μm, particle size: 10 μm, R a The material was applied in layers of 1.5 μm thickness. Subsequently, the substrate, which was then provided with an aqueous suspension, was subjected to a sintering process at a temperature of 2300°C, a holding time of 10 hours, and a pressure of 1 bar (100 kPa).

[0115] In this way, a coated graphite substrate comprising a porous graphite substrate and a TaC protective layer disposed thereon was obtained. The TaC layer has a granular structure of equal-length grains.

[0116] A cross-section of the coating substrate was prepared for inspection. An image of this cross-section is shown in Figure 6.

[0117] SEM images of the cross-section allowed us to determine that the pores of the porous substrate were partially filled with the material of the TaC layer. From this, we can infer that aggregates of the aqueous suspension had penetrated the pores of the porous substrate, and therefore, each aggregate was not larger than the pore inlet diameter of each pore in the porous substrate. Compared to Embodiments 1 and 2, this is particularly due to the fact that the graphite substrate used in Comparative Example 1 has larger pores and therefore larger pore inlet diameters.

[0118] Furthermore, the average thickness and standard deviation of the TaC layer were determined using the cross-section. For this purpose, individual measurements were taken at least 25 measurement points per 1 cm measurement area of ​​the layer by placing regular distances between individual measurement points and measuring the distance between the interface and the layer surface (thickness) based on the recorded cross-sectional image. In this way, a value of 44.7 μm was determined for the average thickness of the TaC layer. Furthermore, a standard deviation of 5.3 μm (11.8%) was determined across all individual thickness measurements.

[0119] Since the standard deviation exceeds 6%, this TaC layer is therefore heterogeneous.

[0120] Comparative Example 2 First, an aqueous suspension was prepared by creating a mixture consisting of 80 wt% TaC powder, 1 wt% polyvinyl alcohol, and 19 wt% water, and then allowing the mixture to stand for 8 minutes without stirring. Here, the TaC particles of the TaC powder formed aggregates in the aqueous suspension. The aqueous suspension thus prepared was then subjected to a porous graphite substrate (average pore inlet diameter: 3.3 μm, particle size: 20 μm, R a The material was applied in layers of 1.5 μm thickness. Subsequently, the substrate, which was then provided with an aqueous suspension, was subjected to a sintering process at a temperature of 2300°C, a holding time of 10 hours, and a pressure of 1 bar (100 kPa).

[0121] In this way, a coated graphite substrate comprising a porous graphite substrate and a TaC protective layer disposed thereon was obtained. The TaC layer has a granular structure of equal-length grains.

[0122] A cross-section of the coated substrate was prepared for inspection. An image of this cross-section is shown in Figure 7.

[0123] SEM images of the cross-section allowed us to determine that the pores of the porous substrate were partially filled with the material of the TaC layer. From this, we can infer that aggregates of the aqueous suspension had penetrated the pores of the porous substrate, and therefore, each aggregate was not larger than the pore inlet diameter of each pore in the porous substrate. Compared to embodiments 1 and 2, this is particularly due to the fact that the graphite substrate used in Comparative Example 2 has larger pores and therefore larger pore inlet diameters.

[0124] Furthermore, the average thickness and standard deviation of the TaC layer were determined using the cross-section. For this purpose, individual measurements were taken at least 25 measurement points per 1 cm measurement area of ​​the layer by placing regular distances between individual measurement points and measuring the distance between the interface and the layer surface (thickness) based on the recorded cross-sectional image. In this way, a value of 30.7 μm was determined for the average thickness of the TaC layer. Furthermore, a standard deviation of 5.5 μm (18%) was determined across all individual thickness measurements.

[0125] Since the standard deviation exceeds 6%, this TaC layer is therefore heterogeneous.

Claims

1. a) Prepare at least one aqueous suspension, the aqueous suspension containing water, at least one aggregate-forming agent, and at least one particle of refractory metal carbide, wherein the particle of at least one refractory metal carbide forms aggregates in the at least one aqueous suspension. b) Apply the at least one aqueous suspension to the porous substrate, c) After step b), the porous substrate is subjected to a sintering process. The diameter of each of the aggregates is greater than the pore opening diameter of each pore in the porous substrate. The at least one aggregate-forming agent is selected from the group consisting of polyvinyl alcohol; polyacrylic acid; polyvinylpyrrolidone; polyalkylene glycol ether; tetrabutylammonium hydroxide; tetramethylammonium hydroxide; polyethyleneimine; NaOH; ammonium hydroxide; and mixtures thereof, and The porous substrate is made of a material selected from the group consisting of graphite, carbon fiber reinforced carbon (CFC), C / SiC fiber composite, SiC / SiC fiber composite, and mixtures thereof. A method for preparing a coating substrate.

2. - The at least one refractory metal carbide is selected from the group consisting of titanium carbide, zirconium carbide, hafnium carbide, vanadium carbide, niobium carbide, tantalum carbide, chromium carbide, molybdenum carbide, tungsten carbide, and mixtures thereof, and / or - The at least one aggregate-forming agent is selected from the group consisting of tetrabutylammonium hydroxide, polyvinyl alcohol, and mixtures thereof. The method according to claim 1.

3. - The diameter of the aggregate is at least 20 μm, and / or - The maximum diameter of the pore openings in the porous substrate is 19 μm. The method according to claim 1.

4. The method according to claim 1, wherein in step a), first a mixture containing the components of the aqueous suspension to be prepared is prepared, and then the mixture is left to stand for a period of time from 3 to 30 minutes without stirring to prepare at least one aqueous suspension.

5. The method according to claim 1, wherein the at least one aqueous suspension contains at least one binder in an amount of 0.05 to 1% by weight, based on the total weight of the aqueous suspension.

6. The method according to claim 1, wherein the surface of the porous substrate on which the at least one aqueous suspension is deposited has an average roughness value of up to 3 μm as determined by optical interferometry and evaluation in accordance with DIN EN ISO 25178:2016-12, and / or has an average roughness depth of up to 12 μm as determined by optical interferometry and evaluation in accordance with DIN EN ISO 25178:2016-12.

7. The method according to claim 1, wherein the average particle size of the particles of the at least one refractory metal carbide is larger than the average pore inlet diameter of the pores of the porous substrate.

8. The method according to claim 1, wherein the preparation of the at least one aqueous suspension in step a) includes mixing the components of the at least one aqueous suspension to be prepared with the assistance of a dispersion device.

9. The method according to claim 1, wherein in step b), the at least one aqueous suspension is applied by dipping, brushing, or spraying.

10. The at least one aqueous suspension is - Based on the total weight of the aqueous suspension, 60 to 90% by weight of the particles of at least one refractory metal carbide, and / or - Based on the total weight of the aqueous suspension, 0.1 to 2% by weight of the at least one aggregate-forming agent The method according to claim 1, comprising:

11. The sintering process in step c) is performed as follows: - At temperatures of 2100°C to 2500°C, and / or - With a holding time of 1 to 15 hours, and / or - At a pressure of 0.1 bar (10 kPa) to 10 bar (1000 kPa), and / or - Under an argon atmosphere The method according to claim 1, which is carried out.

12. A coating substrate comprising a porous substrate and at least one layer disposed on the porous substrate, wherein the at least one layer disposed on the porous substrate contains or consists of at least one refractory metal carbide, the at least one layer disposed on the porous substrate has a granular structure of isometric grains having a grain aspect ratio of at least 0.95, and the porous substrate has unfilled pores closed by the at least one layer disposed on the porous substrate. The porous substrate is a coating substrate made of a material selected from the group consisting of graphite, carbon fiber reinforced carbon (CFC), C / SiC fiber composite, SiC / SiC fiber composite, and mixtures thereof.

13. - The at least one layer disposed on the porous substrate has an average layer thickness of at least 20 μm, and / or - The relative standard deviation of the average layer thickness of the at least one layer disposed on the porous substrate is at most 6%. The relative standard deviation of the layer thickness is determined as follows: - Preparation of the cross-section of the coated substrate - Measurement of the distance between interfaces and layer surfaces based on recorded cross-sectional images. - Layer thickness analysis at the area with maximum extent - The number of individual layer thickness measurements is at least 25 per 1 cm measurement area. - The intervals between individual layer thickness measurements are regular. - Determination of the relative standard deviation across all individual layer thickness measurements, The coating substrate according to claim 12.

14. Use of the coating substrate according to claim 12 or 13 in semiconductor crystal growth.

Images