METHOD FOR THE PRODUCTION OF COATED SUBSTRATES AND COATED SUBSTRATE AND ITS USE

Description

[0001] The present invention relates to a process for producing coated substrates, in which a first aqueous suspension and a second aqueous suspension are prepared, at least one layer of the first aqueous suspension is applied to a substrate, at least one layer of the second aqueous suspension is applied to the at least one layer of the first aqueous suspension applied to the substrate, and the substrate thus coated is subjected to a sintering process. The first aqueous suspension contains at least one refractory metal carbide, at least one 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, and water, or consists thereof. The second aqueous suspension also contains at least one refractory metal carbide and water.Furthermore, the second aqueous suspension can contain at least one 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 weight percentage of the at least one sintering additive in the second aqueous suspension, based on the total weight of the second aqueous suspension, is less than the weight percentage of the at least one sintering additive in the first aqueous suspension, based on the total weight of the first aqueous suspension. Alternatively, the second aqueous suspension can also contain no sintering additive. The present invention further relates to a coated substrate, which can be produced or manufactured using the process according to the invention, and to 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 focuses primarily on high-temperature applications, for example, in semiconductor crystal growth, where highly corrosive and aggressive species are present, thus limiting the usability of existing components (e.g., made of graphite) or significantly reducing their service life. Since it is difficult to produce a proven, cost-effective, and complex-geometrically complex component from refractory metal carbides using the hot pressing processes described in the literature, coatings are preferred. The production of ceramic layers via hot pressing is not possible due to process limitations. Coatings are produced, for example, using the CVD process.In this process, dense layers just a few micrometers thick are deposited onto a substrate via the gas phase. An example of this would be TaC coatings in a layered structure. However, this cost-intensive method prevents the production of coated components with arbitrary geometries and sizes and with arbitrary layer thicknesses. To ensure greater flexibility in these areas, the layers can be applied to the substrate using a wet ceramic process (dip, brush, or spray). This can be achieved, for example, using a suspension based on organic solvents or aqueous suspensions containing metal carbide particles and dispersants (see, e.g., US 2013 / 0061800 A1 and WO 2019 / 154690 A2). To generate the desired protective layer properties, a sintering process is performed after the deposition process using a starting suspension.

[0003] In addition to producing a mechanically stable coating through the final sintering process (high abrasion and adhesion strength), a high degree of compaction is simultaneously required to optimally protect the substrate from corrosive media in high-temperature applications. Since refractory metal carbides are primarily compounds exhibiting strong covalent bonds and very low self-diffusion, a primary challenge lies in optimizing the sintering process to achieve the maximum possible degree of compaction. A popular method for increasing sintering activity, particularly in pressureless sintering, is the use of sintering aids that transition into a liquid melt phase during the sintering process, thus promoting preferential particle rearrangement (liquid-phase sintering).It is particularly important to ensure that the sintering aid forms a stable melt phase under the given sintering conditions (temperature, pressure, etc.). When using selected sintering additives, it can also be advantageous to adjust the sintering conditions for an ideal sintering result, for example, by increasing the system pressure under an inert gas atmosphere.

[0004] It is known from previously published work (see, for example, US 2013 / 0061800 A1) that the use of certain transition metals as sintering additives, such as cobalt, can maximize the density of TaC coatings during pressureless sintering. However, cobalt is a highly toxic substance that poses environmental, health, and safety problems for suspension-based coating technology. Furthermore, the use of certain transition metals as sintering additives can result in highly flammable powder mixtures, which can also present safety concerns. The use of a refractory metal carbide-based protective layer in semiconductor crystal growth also requires the avoidance of critical impurities, including, in particular, the transition metals cobalt, nickel, iron, etc., known from the literature., under no circumstances may it escape from the reactor interior structure, in which the coating is to be used, into the growing atmosphere.

[0005] Based on this, the object of the present invention was to provide a safer method for producing coated substrates that are particularly well suited for use in high-temperature applications, such as semiconductor crystal growth. Furthermore, the object of the present invention was to provide coated substrates that can be produced in a safer manner and are particularly well suited for use in high-temperature applications, such as semiconductor crystal growth.

[0006] This problem is solved with respect to a method for producing coated substrates with the features of claim 1 and with respect to a coated substrate with the features of claim 10. Claim 15 specifies possible uses of the coated substrate according to the invention. The dependent claims represent advantageous embodiments.

[0007] According to the invention, a method for producing coated substrates is thus provided, in which a) a first aqueous suspension is prepared, which contains or consists of at least one refractory metal carbide, at least one 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, and water; b) a second aqueous suspension is prepared, which contains at least one refractory metal carbide and water, wherein the second aqueous suspension contains at least one 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 weight percent of the at least one sintering additive in the second aqueous suspension, based on the Total weight of the second aqueous suspension,is less than the weight percent of the at least one sintering additive in the first aqueous suspension, based on the total weight of the first aqueous suspension, or contains no sintering additive, c) at least one layer of the first aqueous suspension is applied to a substrate, d) at least one layer of the second aqueous suspension is applied to the at least one layer of the first aqueous suspension applied to the substrate, and e) the (coated) substrate is subjected to a sintering process after step d).

[0008] In step a) of the process according to the invention, a first aqueous suspension is prepared, and in step b) of the process according to the invention, a second aqueous suspension is prepared. Step b) can be carried out before step a), after step a), or (at least partially) simultaneously with step a). The first aqueous suspension and / or the second aqueous suspension can (each) be prepared by mixing the components that are to be contained in the respective suspension. The first aqueous suspension and the second aqueous suspension each contain at least one refractory metal carbide and water. The at least one refractory metal carbide contained in the first aqueous suspension and the at least one refractory metal carbide contained in the second aqueous suspension can be the same refractory metal carbide or different refractory metal carbides.Furthermore, the first aqueous suspension contains at least one 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. The second aqueous suspension may also 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 weight percent of the at least one sintering additive in the second aqueous suspension, based on the total weight of the second aqueous suspension, is less than the weight percent of the at least one sintering additive in the first aqueous suspension, based on the total weight of the first aqueous suspension.Alternatively, the second aqueous suspension may not contain any sintering additive. For example, the second aqueous suspension may not contain any sintering additive selected from the group consisting of, or mixtures thereof. Preferably, the second aqueous suspension consists of at least one refractory metal carbide, water, and optionally at least one sintering additive selected from the group consisting of, or mixtures thereof, 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. The at least one sintering additive contained in the first aqueous suspension and the at least one sintering additive contained in the second aqueous suspension may be the same sintering additive or different sintering additives.

[0009] Preferably, the refractory metal silicides are selected from the group consisting of titanium silicides, zirconium silicides, hafnium silicides, vanadium silicides, niobium silicides, tantalum silicides, chromium silicides, molybdenum silicides, tungsten silicides and mixtures thereof.

[0010] Preferably, the refractory metal nitrides are selected from the group consisting of titanium nitrides, zirconium nitrides, hafnium nitrides, vanadium nitrides, niobium nitrides, tantalum nitrides, chromium nitrides, molybdenum nitrides, tungsten nitrides and mixtures thereof.

[0011] Preferably, the refractory metal borides are selected from the group consisting of titanium borides, zirconium borides, hafnium borides, vanadium borides, niobium borides, tantalum borides, chromium borides, molybdenum borides, tungsten borides and mixtures thereof.

[0012] Preferably, the at least one sintering additive contained in the first aqueous suspension is selected from the group consisting of silicon, zirconium boride, refractory metal silicides and mixtures thereof.

[0013] If the second aqueous suspension contains at least one sintering additive, then this at least one sintering additive contained in the second aqueous suspension is preferably selected from the group consisting of silicon, zirconium boride, refractory metal silicides and mixtures thereof.

[0014] For example, the second aqueous suspension consists of at least one refractory metal carbide, water, and optionally at least one sintering additive selected from the group consisting of silicon, zirconium boride, refractory metal silicides and mixtures thereof.

[0015] It is possible that the second aqueous suspension does not contain a sintering additive. For example, the second aqueous suspension may not contain a sintering additive selected from the group consisting of silicon; zirconium boride; refractory metal silicides, for example TaSi₂, MoSi₂, ZrSi₂; transition metals; boron carbide (B₄C); silicon nitride (Si₃N₄); carbon; and mixtures thereof.

[0016] A sintering additive can be understood as a substance that is specifically added as an aid for the sintering of ceramic systems for the production of ceramic components in order to control the microstructural and spatial development (e.g. shrinkage, grain growth, shape change and / or homogenization).

[0017] In step c) of the process according to the invention, at least one layer of the first aqueous suspension is applied to a substrate. The substrate is preferably a carbon substrate, more preferably a graphite substrate, and most preferably an iso-graphite substrate. Iso-graphite is understood to be graphite produced by isostatic pressing. For example, the substrate can be a crucible, preferably a carbon crucible, more preferably a graphite crucible, and most preferably an iso-graphite crucible. The application of the at least one layer of the first aqueous suspension to the substrate can be carried out, for example, by dipping, brushing, spraying, or a combination thereof.For example, at least one layer of the first aqueous suspension with an average layer thickness of at least 20 µm, preferably from 20 µm to 150 µm, particularly preferably from 30 µm to 100 µm, can be applied to the substrate.

[0018] In step d) of the process according to the invention, at least one layer of the second aqueous suspension is applied to the at least one layer of the first aqueous suspension applied to the substrate in step c). The application of the at least one layer of the second aqueous suspension can be carried out, for example, by dipping, brushing, spraying, or a combination thereof. For example, at least one layer of the second aqueous suspension with an average layer thickness of at least 20 µm, preferably from 20 µm to 150 µm, and particularly preferably from 30 µm to 100 µm, can be applied to the at least one layer of the first aqueous suspension applied to the substrate in step c).

[0019] In step e) of the process according to the invention, the substrate is subjected to a sintering process. This takes place after step d), i.e., after the application of the at least one layer of the second aqueous suspension to the at least one layer of the first aqueous suspension applied to the substrate. The coated substrate subjected to the sintering process in step e) therefore has both the at least one layer of the first aqueous suspension applied in step c) and the at least one layer of the second aqueous suspension applied in step d). Thus, in step e), both the at least one layer of the first aqueous suspension and the at least one layer of the second aqueous suspension are simultaneously subjected to a sintering process.The sintering process can produce at least one first sintered layer containing at least one refractory metal carbide from the at least one layer of the first aqueous suspension, and at least one second sintered layer containing at least one refractory metal carbide from the at least one layer of the second aqueous suspension.

[0020] The inventive method enables the production of refractory metal carbide-based layers on substrates that can serve as high-temperature and wear protection layers or wear protection layer systems.

[0021] The process according to the invention is a wet ceramic process for producing refractory metal carbide-based coatings on substrates. In contrast to coatings produced by CVD or PVD processes, the coatings produced by wet ceramic processes exhibit an isotropic texture with random grain size orientation, which leads to a reduced susceptibility to cracking and an increased diffusion path for substrate-damaging species. As a result, the coated substrates produced according to the invention offer improved protection against aggressive substances used in high-temperature applications compared to coated substrates produced by CVD or PVD processes.Furthermore, the wet ceramic process according to the invention is more cost-effective than CVD or PVD processes and also offers more flexibility in the geometries and sizes of the coated components that can be produced, as well as in the layer thicknesses of the applied coatings or layers.

[0022] Furthermore, the inventive method for producing coated substrates is based on the use of an aqueous suspension. Compared to the use of organic suspensions, the use of aqueous suspensions offers several advantages. For example, aqueous suspensions are cost-effective, environmentally and health-wise harmless, and do not present the safety problem of highly flammable spray mist. Moreover, the use of aqueous suspensions eliminates the need for pyrolysis to remove organic solvents, which can lead to the undesirable introduction of foreign substances into the coating. Finally, unlike known organic suspensions, aqueous suspensions allow for controlled application of the suspension.In particular, when spraying known organic suspensions, controlled application is not possible because the suspension properties can fluctuate during this process due to solvent evaporation, so that homogeneous layers cannot be obtained over time.

[0023] In the process according to the invention, one or more sintering additives are used, 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, preferably selected from the group consisting of silicon, zirconium boride, refractory metal silicides, and mixtures thereof. It has been shown that these sintering additives used according to the invention, due to their properties (e.g., melting points, boiling points, etc.), have at least the same or even a better effect on the degree of compaction than the transition metals (e.g., cobalt, nickel, iron, etc.) used as sintering additives in the prior art.Thus, their use enables a high degree of compaction of the sintered layer produced on the substrate, thereby providing excellent protection for the substrate against corrosive media in high-temperature applications. The sintering additives used according to the invention are distinguished from prior art sintering additives, such as cobalt, primarily by their safety and health-related harmlessness. Furthermore, their use, and thus the avoidance of certain transition metals, such as cobalt, nickel, and iron, as sintering additives, prevents these transition metals from remaining as impurities in the layer. This would be detrimental to the growth atmosphere in high-temperature semiconductor crystal growth applications.

[0024] Because the second aqueous suspension either contains no sintering additive or contains a lower weight percentage of sintering additive than the first aqueous suspension, the coating produced on the substrate using the inventive method has a very advantageous layer structure consisting of at least two different sintered layers.This layer structure comprises at least one first sintered layer arranged on the substrate, which contains or consists of at least one refractory metal carbide and at least one 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, and at least one second sintered layer arranged on the at least one first sintered layer, which contains or consists of at least one refractory metal carbide and optionally at least one 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.The at least one sintering additive can be present, at least partially, in decomposed form and / or as a reaction product of a reaction between the at least one sintering additive and the at least one refractory metal carbide. Since the presence of a sintering additive, or a higher proportion of a sintering additive, in the resulting layer during the sintering process leads to a higher degree of density of the sintered layer, the second sintered layer of the layer structure exhibits a lower relative density, or a lower degree of density, than the first sintered layer. The resulting layer structure thus exhibits a density gradient, in which the degree of density, or the relative density, decreases with increasing distance from the substrate (i.e., from the first sintered layer to the second sintered layer).For example, the at least one first sintered layer can have a relative density of at least 70%, and the relative density of the at least one second sintered layer can be at least 3% lower than the relative density of the at least one first sintered layer.

[0025] Due to the higher proportion of sintering additive in the first aqueous suspension, the first sintered layer of the produced coated substrate (i.e., the first layer of the build-up in direct contact with the substrate) exhibits a comparatively high degree of compaction or a comparatively high relative density, which is why the substrate is very well protected against aggressive media in high-temperature applications. Since the second aqueous suspension contains a lower proportion of sintering additive or no sintering additive at all, the second sintered layer of the produced coated substrate (i.e.,The second layer of the layer structure contains no or only a low amount of sinter additive, thus reducing the risk of sinter additive leaching from the coating into the growth atmosphere and contaminating it when the manufactured coated substrate is used in high-temperature semiconductor crystal growth applications. The at least one second sintered layer can also act as a protective layer, further reducing the risk of sinter additive leaching from the at least one first sintered layer into the growth atmosphere. Furthermore, due to the lower proportion of sinter additive or its absence in the second aqueous suspension, the at least one second sintered layer exhibits a lower degree of compaction or a lower relative density than the at least one first sintered layer.The resulting higher porosity of at least one second sintered layer leads to advantages in the application of the coated substrate in high-temperature applications in semiconductor crystal growth, since this higher porosity provides a larger contact area for the melt used in crystal growth, thereby increasing the evaporation rate and thus also the growth rate.

[0026] The layer system obtained by the inventive method described above makes the coated substrate produced according to the invention particularly suitable for use in high-temperature applications, such as semiconductor crystal growth.

[0027] By selecting the amount of sintering additive in the first and second aqueous suspensions, and optionally adjusting the sintering parameters, the relative density in the individual layers of the coating system can be set to achieve a desired density gradient within the system. A low density gradient is particularly advantageous, as it results in greater thermal stability.

[0028] The coated substrate produced by the inventive method can, for example, be used as a gallium evaporator or part of a gallium evaporator in a XLPE-GaN reactor suitable for growing gallium nitride semiconductor crystals, wherein the layer system obtained by the inventive method then functions as a coating of the gallium evaporator. In this case, the at least one first sintered layer of the layer system (i.e., the sintered layer resulting from the first aqueous suspension) can, due to its higher relative density, particularly well protect the base material of the gallium evaporator from the corrosive gallium melt, whereas the lower relative density and higher porosity of the at least one second sintered layer of the layer system (i.e.,(the sintered layer resulting from the second aqueous suspension) increases the gallium evaporation rate and thus also the gallium nitride growth rate, as there is a larger contact area with the gallium melt.

[0029] Preferably, the at least one coated substrate is not subjected to a sintering process between steps c) and d). This means that the substrate coated with the at least one layer of the first aqueous suspension is preferably not subjected to a sintering process until the at least one layer of the second aqueous suspension has been applied to the at least one layer of the first aqueous suspension on the substrate. By subjecting the coated substrate to a sintering process only after step d), the at least one layer of the first aqueous suspension applied in step c) and the at least one layer of the second aqueous suspension applied in step d) can be sintered simultaneously in a single sintering process. This avoids the need for separate sintering of each of the two layers after their respective application, thus saving a sintering step.As a result, the process is faster and more cost-effective. Furthermore, when applying at least one layer of the second aqueous suspension to an already sintered layer of the first aqueous suspension, the bond can weaken due to a lack of infiltration, thus increasing the likelihood of delamination under thermal stress. This can be avoided by sintering both layers applied in steps c) and d) together in a single sintering process following step d).

[0030] In step c) of the process according to the invention, the at least one layer of the first aqueous suspension can be applied to the entire surface of the substrate or only to one or more sections of the substrate surface. In step d) of the process according to the invention, the at least one layer of the second aqueous suspension can be applied to the entire at least one layer of the first aqueous suspension applied in step c) or only to one or more sections of the at least one layer of the first aqueous suspension applied in step c).If the substrate is a crucible, for example, in step c) the at least one layer of the first aqueous suspension can be applied to the entire surface of the crucible, and in step d) the at least one layer of the second aqueous suspension can be applied only to the section of the at least one layer of the first aqueous suspension that is on the inside or insides of the crucible.

[0031] A preferred variant of the method according to the invention is characterized by the fact that the substrate contains or consists of a material selected from the group consisting of graphite, preferably iso-graphite, carbon fiber reinforced carbon (CFC), C / SiC fiber composites, SiC / SiC fiber composites, carbide ceramics, nitride ceramics, oxide ceramics, and mixtures thereof, and / or the at least one refractory metal carbide (in step a) and / or in step b)) is selected from the group consisting of titanium carbides, zirconium carbides, hafnium carbides, vanadium carbides, niobium carbides, tantalum carbides, chromium carbides, molybdenum carbides, tungsten carbides, and mixtures thereof, and / or the refractory metal silicides (in step a) and / or in step b)) are selected from the group consisting of titanium silicides, zirconium silicides, e.g. zirconium disilicide (ZrSi₂), hafnium silicides, e.g. Hafnium disilicide (HfSi 2 ), vanadium silicides, e.g. vanadium disilicide (VSi 2 ), niobium silicides, e.gNiobium disilicide (NbSi₂), tantalum silicides, e.g., tantalum disilicide (TaSi₂), chromium silicides, molybdenum silicides, e.g., molybdenum disilicide (MoSi₂), tungsten silicides, e.g., tungsten disilicide (WSi₂), and mixtures thereof, and / or the refractory metal nitrides (in step a) and / or in step b)) are selected from the group consisting of titanium nitrides, zirconium nitrides, hafnium nitrides, vanadium nitrides, niobium nitrides, tantalum nitrides, chromium nitrides, molybdenum nitrides, tungsten nitrides, and mixtures thereof, and / or the refractory metal borides (in step a) and / or in step b)) are selected from the group consisting of titanium borides, zirconium borides, hafnium borides, vanadium borides, niobium borides, tantalum borides, Chromium borides, molybdenum borides, tungsten borides, and mixtures thereof. . .

[0032] According to a particularly preferred embodiment of the process according to the invention, the refractory metal silicides (in step a) and / or in step b)) are selected from the group consisting of zirconium disilicide (ZrSi 2 ), hafnium disilicide (HfSi 2 ), vanadium disilicide (VSi 2 ), niobium disilicide (NbSi 2 ), tantalum disilicide (TaSi 2 ), molybdenum disilicide (MoSi 2 ), tungsten disilicide (WSi 2 ), and mixtures thereof.

[0033] Due to their ecologically, health-wise, and safety-related harmlessness, combined with a melting temperature range between 1400 °C and 2200 °C, the refractory metal silicides selected from the group consisting of titanium silicides, zirconium silicides, hafnium silicides, vanadium silicides, niobium silicides, tantalum silicides, chromium silicides, molybdenum silicides, tungsten silicides, and mixtures thereof are ideally suited as sintering additives for the high-temperature sintering process. Their use as sintering additives ensures that the desired melt phase forms during the heating phase and that a favorable rearrangement of the particles occurs, increasing sintering activity before active sintering. The melting temperature of the sintering additive can therefore be lower than the sintering temperature.Furthermore, the boiling point of the sintering additive can be significantly higher than the sintering temperature under the given pressure conditions, thus ensuring a certain stability of the liquid phase throughout the entire sintering process and preventing potential evaporation. Additionally, the melting point of the sintering additives can be below the application temperature of the refractory metal carbide coating (e.g., TaC coating), thereby preventing any potential evaporation of the sintering additive during application.

[0034] It is particularly advantageous if at least one sintering additive is MoSi₂, TaSi₂, or mixtures thereof. With a melting point of approximately 2050°C, these materials exhibit high temperature stability and are therefore particularly suitable for use at high operating temperatures, e.g., in reactor internal structures.

[0035] Tantalum carbide is particularly preferred as at least one refractory metal carbide. Tantalum carbide provides exceptionally good protection for the substrate.

[0036] The substrate may preferably contain or consist of a material selected from the group consisting of graphite, preferably iso-graphite, carbide ceramics, nitride ceramics, oxide ceramics, and mixtures thereof.

[0037] The substrate may preferably contain or consist of a material selected from the group consisting of graphite, preferably iso-graphite, carbon fiber reinforced carbon (CFC), C / SiC fiber composites, SiC / SiC fiber composites, and mixtures thereof.

[0038] Carbon-based and SiC-based substrates exhibit increased infiltration behavior when an aqueous suspension is applied. Consequently, the method according to the invention is particularly suitable for such substrates.

[0039] The substrate most preferably contains or consists of graphite, preferably iso-graphite.

[0040] Another preferred embodiment of the process according to the invention is characterized in that the at least one refractory metal carbide and the at least one sintering additive are each present in particulate form, wherein the mean particle size of the particles of the at least one sintering additive is less than 5 µm and / or smaller than the mean particle size of the particles of the at least one refractory metal carbide. A particle size of less than 5 µm for the particles of the at least one sintering additive prevents larger holes or even structural loss in the coating after the sintering process. A mean particle size of the particles of the at least one sintering additive that is smaller than the mean particle size of the particles of the at least one refractory metal carbide has the advantage that the sintering additive particles can adhere better to the voids in the refractory metal carbide layer.The mean particle size of the particles of the at least one refractory metal carbide can preferably be in the range between 0.2 and 2 µm.

[0041] The mean particle size of the particles of the at least one refractory metal carbide and / or the mean particle size of the particles of the at least one sinter additive can be determined, for example, by laser diffraction (DIN 13320:2020-01).

[0042] According to a further preferred embodiment of the process according to the invention, the at least one refractory metal carbide is present as a powder mixture containing or consisting of powders, preferably of the same refractory metal carbide, that differ in their mean particle size. In other words, the at least one refractory metal carbide used here is a powder mixture containing different refractory metal carbide particles—preferably of the same refractory metal carbide—where these particles differ in their mean particle size. For example, the powder mixture can contain two types of particles—preferably of the same refractory metal carbide—where the mean particle diameter of the particles of the first type is larger than the mean particle diameter of the particles of the second type.For example, to obtain such a powder mixture, different powders of the same refractory metal carbide can be mixed together in specific ratios of nano- and micrometer-sized particles. Because the at least one refractory metal carbide is present as a powder mixture, the arrangement of the refractory metal carbide powder particles on the substrate can be made more compact, as open spaces can be filled more effectively. In this way, the relative density of the produced layer can be increased, regardless of the presence and amount of a sintering additive in the layer.

[0043] Preferably, the at least one refractory metal carbide is present as a powder mixture in the preparation of the second aqueous suspension in step b), containing or consisting of powders of the same refractory metal carbide differing in their mean particle size, wherein the second aqueous suspension preferably does not contain a sintering additive. In this way, the relative density of the layer resulting from the at least one second suspension can be increased (preferably without the use of a sintering additive), resulting in a lower density gradient in the layer structure. This, in turn, increases the thermal stability of the layer structure as well as the quasi-diffusion-inhibiting effect (e.g., with regard to the diffusion of impurities into a growth atmosphere).Thus, a smaller density difference prevents abrupt changes in layer density and the resulting unevenly distributed thermal stresses within the layer system, which can lead to cracks, delamination, or, in the worst case, system failure. The layer system is therefore more thermally stable. Furthermore, the use of refractory metal carbide particles of varying sizes ensures that the second layer also exhibits a comparatively low porosity, allowing it to act as a barrier to the external atmosphere, resulting in a quasi-diffusion-inhibiting effect.

[0044] Another preferred variant of the method according to the invention is characterized in that the first aqueous suspension and / or the second aqueous suspension 60 to 90 wt.%, preferably 70 to 85 wt.%, of at least one refractory metal carbide, based on the total weight of the respective aqueous suspension, and / or 0.1 to 20 wt.%, preferably 0.5 to 10 wt.%, of at least one sintering additive, based on the total weight of the respective aqueous suspension.

[0045] For example, the first aqueous suspension can contain 0.2 to 20.0 wt.%, preferably 0.6 to 10.0 wt.% of the at least one sintering additive, based on the total weight of the respective aqueous suspension, and / or the second aqueous suspension can contain 0.1 to 19.9 wt.%, preferably 0.5 to 9.9 wt.% of the at least one sintering additive, based on the total weight of the respective aqueous suspension.

[0046] According to a further preferred embodiment of the inventive process, the weight percentage of the at least one sintering additive in the second aqueous suspension, based on the total weight of the second aqueous suspension, is 0.1 wt.% to 20 wt.%, preferably 0.5 wt.% to 10 wt.%, lower than the weight percentage of the at least one sintering additive in the first aqueous suspension, based on the total weight of the first aqueous suspension. In this way, a small density gradient can be obtained within the layer structure consisting of the first layer (i.e., the layer resulting from the first aqueous suspension) and the second layer (i.e., the layer resulting from the second aqueous suspension), thereby increasing the thermal stability of the layer structure as well as the quasi-diffusion-inhibiting effect (e.g., with regard to the diffusion of impurities into a growth atmosphere).This smaller density difference prevents abrupt changes in layer density and the resulting unevenly distributed thermal stresses within the layer system, which can lead to cracking, delamination, or, in the worst case, system failure. Consequently, the layer system is more thermally stable. Furthermore, the use of the sintering additive ensures that the second layer also exhibits a comparatively low porosity, allowing it to act as a barrier to the external atmosphere, resulting in a quasi-diffusion-inhibiting effect.

[0047] For example, the weight percent proportion of the at least one sintering additive in the second aqueous suspension, based on the total weight of the second aqueous suspension, can be 0.1 wt.% to 19.9 wt.%, preferably 0.5 wt.% to 9.9 wt.%, smaller than the weight percent proportion of the at least one sintering additive in the first aqueous suspension, based on the total weight of the first aqueous suspension.

[0048] The at least one first aqueous suspension and / or the at least one second aqueous suspension may contain at least one binder selected from the group consisting of polyvinyl alcohols, polyethylene glycol, polyvinyl butyral, polyacrylic acid, polyurethanes, chloroprene rubber, phenolic resins, acrylic resins, carboxymethyl celluloses, alginic acid, dextrins, sodium biphenyl-2-yl oxides, polyphenyl oxide, and mixtures thereof, preferably selected from the group consisting of polyvinyl alcohols, sodium biphenyl-2-yl oxides, polyphenyl oxide, and mixtures thereof, wherein the at least one binder may preferably be present in the at least one first aqueous suspension and / or the at least one second aqueous suspension in a proportion of 0.05 to 1 wt.% or 0.01 to 5 wt.%, based on the total weight of the respective aqueous suspension.

[0049] Another preferred embodiment of the process according to the invention is characterized in that the production of the first aqueous suspension in step a) and / or the production of the second aqueous suspension in step b) is carried out by mixing the components of the suspension to be produced using a dispersing device, wherein the mixing using the dispersing device is preferably carried out using grinding media and / or over a period of at least 12 hours. In this way, optimal mixing of the respective aqueous suspension can be achieved, so that inhomogeneities in the distribution of the sintering additive and thus in the compaction can be avoided. For example, rotational speeds of up to 1 m / s can be used when mixing with the dispersing device.

[0050] Another preferred embodiment of the method according to the invention is characterized in that the application of the at least one layer of the first aqueous suspension in step c) and / or the application of the at least one layer of the second aqueous suspension in step d) by means of dipping, brushing or spraying, and / or with an average layer thickness of less than 150 µm, preferably from 20 µm to 100 µm, particularly preferably from 30 µm to 80 µm.

[0051] Spray application is the preferred method for producing one or more thin, fast-drying refractory metal carbide layers, preferably with layer thicknesses in the range of 20 µm to 80 µm. Rapid rotation of the component through the spray jet allows a very thin suspension layer to be applied to the surface, which can dry quickly to very quickly depending on the solids content of the suspension. Solids contents of the refractory metal carbide powder greater than or equal to 70 wt% of the total suspension are preferred. Each individual layer applied should preferably exhibit comparable drying behavior. In general, fast-drying behavior of the applied suspension layers is preferred, as density differences between the refractory metal carbide and the sintering additive can lead to inhomogeneity in the particle distribution if the layers dry for too long.

[0052] A further preferred embodiment of the process according to the invention is characterized in that at least one additional aqueous suspension is produced, which contains or consists of at least one refractory metal carbide and water, wherein the at least one third aqueous suspension does not contain a sintering additive, and that between steps d) and e), at least one layer of the at least one third aqueous suspension is applied to the at least one applied layer of the second aqueous suspension. In the sintering process step e), a joint (simultaneous) sintering of the at least one layer of the first aqueous suspension, the at least one layer of the second aqueous suspension, and the at least one layer of the third aqueous suspension can then take place. In this way, stable bonds are formed between the layers, which are also stable under thermal stress.The at least one refractory metal carbide contained in the at least one third aqueous suspension can be identical to the refractory metal carbide contained in the first aqueous suspension and / or to the refractory metal carbide contained in the second aqueous suspension. Preferably, the at least one refractory metal carbide contained in the at least one third aqueous suspension is selected from the group consisting of titanium carbides, zirconium carbides, hafnium carbides, vanadium carbides, niobium carbides, tantalum carbides, chromium carbides, molybdenum carbides, tungsten carbides, and mixtures thereof.

[0053] Preferably, several third aqueous suspensions can additionally be prepared, containing or consisting of at least one refractory metal carbide and water, wherein the several third aqueous suspensions do not contain a sintering additive, and between steps d) and e), at least one layer of each of the third aqueous suspensions is applied one above the other onto the at least one applied layer of the second aqueous suspension such that a layer sequence of the layers of the third aqueous suspensions is obtained. In this way, the coated substrate obtained after sintering can comprise a layer sequence of several third sintered layers, wherein the relative density of the several sintered layers within this layer sequence decreases with increasing distance from the at least one second sintered layer.For example, by adjusting the sintering parameters, the relative density in the individual layers of the layer sequence can be set so that a (desired) density gradient can be achieved within the layer sequence.

[0054] Another preferred variant of the method according to the invention is characterized in that the sintering process in step d) at a temperature of 2100 °C to 2500 °C, preferably 2200 °C to 2400 °C, and / or with a holding time of 1 h to 15 h, preferably 2 h to 10 h, and / or at a pressure of 0.1 bar to 10 bar, preferably 0.7 bar to 5 bar, and / or such that after a first time period of the sintering process the pressure is increased, preferably by 3 bar to 7 bar, and / or under an argon atmosphere, This has been done.

[0055] These modifications to the sintering process allow for a particularly advantageous ratio between the degree of compaction in the first layer of the layer structure (i.e., the layer resulting from the first aqueous suspension) and the degree of compaction in the second layer of the layer structure (i.e., the layer resulting from the second aqueous suspension). Furthermore, these modifications to the sintering process increase the stability of the melt phase throughout the entire sintering process.

[0056] By the preferred variant in which the pressure is increased after a first time period of the sintering process in step e), preferably by 3 bar to 7 bar, evaporation of the sinter additive, e.g. at a sintering temperature of 2300 °C, can be prevented, while at the same time the stable liquid phase is optimally distributed between the particles.

[0057] According to a further preferred variant of the inventive method, the substrate is a graphite substrate, preferably an iso-graphite substrate.

[0058] The present invention further relates to a coated substrate, comprising a substrate, at least one first sintered layer arranged on the substrate, which contains or consists of at least one refractory metal carbide and at least one 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, and at least one second sintered layer arranged on the at least one first layer, which contains or consists of at least one refractory metal carbide and optionally at least one 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 at least one first layer has a relative density of at least 70%, and wherein the relative density of the at least one second layer is at least 3% lower than the relative density of the at least one first layer.

[0059] The presence of at least one sintering additive in the first sintered layer and / or in the second sintered layer can be demonstrated, for example, using an elemental analysis method or XRD analysis.

[0060] The relative density can be determined, for example, via SEM cross-section analysis.

[0061] The at least one sinter additive may be present in the first sintered layer and / or in the second sintered layer, at least partially, in decomposed form and / or in the form of a reaction product of a reaction of the at least one sinter additive with the at least one refractory metal carbide.

[0062] The at least one refractory metal carbide contained in the first sintered layer and the at least one refractory metal carbide contained in the second sintered layer can be the same refractory metal carbide or different refractory metal carbides. The second sintered layer can 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 weight percentage of the at least one sintering additive in the second sintered layer, based on the total weight of the second sintered layer, is preferably less than the weight percentage of the at least one sintering additive in the first sintered layer, based on the total weight of the first sintered layer.The at least one sintering additive contained in the first sintered layer and the at least one sintering additive contained in the second sintered layer can be the same sintering additive or different sintering additives. Alternatively, the second sintered layer can also contain no sintering additive.

[0063] Preferably, the at least one sintering additive contained in the first sintered layer is selected from the group consisting of silicon, zirconium boride, refractory metal silicides and mixtures thereof.

[0064] If the second sintered layer contains at least one sintering additive, then this at least one sintering additive contained in the second sintered layer is preferably selected from the group consisting of silicon, zirconium boride, refractory metal silicides and mixtures thereof.

[0065] The coated substrate according to the invention has a highly advantageous layer structure consisting of at least two different sintered layers. Due to the relative density of at least 70% of the at least one first sintered layer, the substrate is very well protected against aggressive or corrosive substances in high-temperature applications, such as the melt in semiconductor crystal growth. The at least one second sintered layer has a relative density or a lower degree of compaction that is at least 3% lower than that of the at least one first sintered layer. The resulting layer structure thus exhibits a density gradient in which the degree of compaction or the relative density decreases with increasing distance from the substrate – i.e., from the at least one first sintered layer to the at least one second sintered layer.

[0066] The resulting higher porosity of the at least one second sintered layer compared to the at least one first sintered layer leads to advantages in the application of the coated substrate in high-temperature applications in semiconductor crystal growth, since this higher porosity provides a larger contact area for the melt used in crystal growth, thereby increasing the evaporation rate and thus also the growth rate.

[0067] This advantageous layer system makes the coated substrate according to the invention particularly suitable for use in high-temperature applications, such as semiconductor crystal growth.

[0068] A preferred embodiment of the coated substrate according to the invention is characterized in that the substrate contains or consists of a material selected from the group consisting of graphite, preferably iso-graphite, carbon fiber reinforced carbon (CFC), C / SiC fiber composites, SiC / SiC fiber composites, carbide ceramics, nitride ceramics, oxide ceramics, and mixtures thereof, and / or the at least one refractory metal carbide (in step a) and / or in step b)) is selected from the group consisting of titanium carbides, zirconium carbides, hafnium carbides, vanadium carbides, niobium carbides, tantalum carbides, chromium carbides, molybdenum carbides, tungsten carbides, and mixtures thereof, and / or the refractory metal silicides (in step a) and / or in step b)) are selected from the group consisting of titanium silicides, zirconium silicides, e.g. zirconium disilicide (ZrSi₂), hafnium silicides, e.g. Hafnium disilicide (HfSi 2 ), vanadium silicides, e.g. vanadium disilicide (VSi 2 ), niobium silicides, e.gNiobium disilicide (NbSi₂), tantalum silicides, e.g., tantalum disilicide (TaSi₂), chromium silicides, molybdenum silicides, e.g., molybdenum disilicide (MoSi₂), tungsten silicides, e.g., tungsten disilicide (WSi₂), and mixtures thereof, and / or the refractory metal nitrides (in step a) and / or in step b)) are selected from the group consisting of titanium nitrides, zirconium nitrides, hafnium nitrides, vanadium nitrides, niobium nitrides, tantalum nitrides, chromium nitrides, molybdenum nitrides, tungsten nitrides, and mixtures thereof, and / or the refractory metal borides (in step a) and / or in step b)) are selected from the group consisting of titanium borides, zirconium borides, hafnium borides, vanadium borides, niobium borides, tantalum borides, Chromium borides, molybdenum borides, tungsten borides, and mixtures thereof. . .

[0069] According to a particularly preferred embodiment of the coated substrate according to the invention, the refractory metal silicides (in the first sintered layer and / or in the second sintered layer) are selected from the group consisting of zirconium disilicide (ZrSi 2 ), hafnium disilicide (HfSi 2 ), vanadium disilicide (VSi 2 ), niobium disilicide (NbSi 2 ), tantalum disilicide (TaSi 2 ), molybdenum disilicide (MoSi 2 ), tungsten disilicide (WSi 2 ), and mixtures thereof.

[0070] Tantalum carbide is particularly preferred as the at least one refractory metal carbide.

[0071] The substrate may preferably contain or consist of a material selected from the group consisting of graphite, preferably iso-graphite, carbide ceramics, nitride ceramics, oxide ceramics, and mixtures thereof.

[0072] The substrate may preferably contain or consist of a material selected from the group consisting of graphite, preferably iso-graphite, carbon fiber reinforced carbon (CFC), C / SiC fiber composites, SiC / SiC fiber composites, and mixtures thereof.

[0073] The substrate most preferably contains or consists of graphite, preferably iso-graphite.

[0074] The substrate can preferably be a carbon substrate, more preferably a graphite substrate, and most preferably an iso-graphite substrate. Iso-graphite is understood to be graphite produced by isostatic pressing. For example, the substrate can be a crucible, preferably a carbon crucible, more preferably a graphite crucible, and most preferably an iso-graphite crucible.

[0075] Furthermore, it is preferred that the at least one first sintered layer and the at least one second sintered layer contain essentially no cobalt, nickel, and iron. "Essentially no cobalt, nickel, and iron" here means that the at least one first sintered layer and the at least one second sintered layer may contain minimal amounts of cobalt, nickel, and / or iron that do not interfere with the use of the coated substrate, e.g., in semiconductor crystal growth. Preferably, the phrase "that the at least one first sintered layer and the at least one second sintered layer contain essentially no cobalt, nickel, and iron" means that the at least one first sintered layer and the at least one second sintered layer contain no more than 1 wt.%, particularly preferably no more than 0.1 wt.%, cobalt, and no more than 1 wt.%.-%, particularly preferably not more than 0.1 wt.%, nickel and not more than 1 wt.%, particularly preferably not more than 0.1 wt.%, iron, based on the total weight of the at least one first sintered layer and the at least one second sintered layer. Particularly preferably, the at least one first sintered layer and the at least one second sintered layer contain no cobalt, no nickel and no iron.

[0076] By using one or more sintering additives 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, the use of cobalt, nickel, and iron as sintering additives can be omitted. Consequently, the at least one first sintered layer and the at least one second sintered layer preferably contain (essentially) no cobalt, nickel, or iron. As a result, cobalt, nickel, and iron cannot leach out of the coated substrate as impurities during an application, such as a high-temperature application in semiconductor crystal growth, and negatively affect the application.

[0077] Furthermore, it is preferred that the grains grow by at least a factor of 1.5, preferably a factor in the range of 1.5 to 10, and particularly preferably a factor in the range of 1.5 to 5, starting from the grain size of the initial spray layer or the sprayed particles.

[0078] Another preferred embodiment of the coated substrate according to the invention is characterized in that the at least one first sintered layer having a relative density of more than 75%, preferably more than 80%, particularly preferably more than 90%, and / or having a permeability of less than 1 e -11< m 2< , preferably less than 1 e -13< m 2< , and / or having an adhesion strength of at least 2 MPa, preferably at least 4 MPa, and / or having an average layer thickness of at least 20 µm, preferably from 20 µm to 150 µm, particularly preferably from 30 µm to 100 µm.

[0079] Furthermore, it is preferred that at least one second sintered layer a relative density that is at least 5%, preferably at least 10%, particularly preferably 10% to 30%, most preferably 15% to 25%, particularly 18% to 22% lower than the relative density of the at least one first sintered layer, and / or has a permeability of less than 1 e -11 < m 2 < , preferably less than 1 e -12 < m 2 < , and / or has an adhesion strength of at least 2 MPa, preferably at least 4 MPa, and / or has an average layer thickness of at least 20 µm, preferably from 20 µm to 150 µm, particularly preferably from 30 µm to 100 µm.

[0080] The relative density can be determined, for example, via SEM cross-section analysis. The mean layer thickness can also be determined, for example, via cross-section analysis. The permeability can be determined, for example, by a setup for measuring the gas flow rate through the sample as a function of a pressure difference across the sample and converting this to Darcy's permeability constant. The adhesion strength can be determined, for example, via tensile tests.

[0081] Preferably, the at least one first sintered layer has a lower permeability than the at least one second sintered layer.

[0082] Preferably, the at least one first sintered layer has a higher adhesive strength than the at least one second sintered layer.

[0083] Preferably, the at least one first sintered layer has a smaller layer thickness than the at least one second sintered layer.

[0084] Particularly preferably, the at least one first sintered layer has a lower permeability, a higher adhesive strength and a lower layer thickness than the at least one second sintered layer.

[0085] According to a further preferred embodiment of the coated substrate according to the invention, the coated substrate comprises at least one third sintered layer arranged on the at least one second sintered layer, which contains or consists of at least one refractory metal carbide, wherein the at least one third sintered layer does not contain a sintering additive, and wherein the relative density of the at least one third sintered layer is at least 5%, preferably at least 10%, particularly preferably 10% to 30%, most preferably 15% to 25%, and in particular 18% to 22% lower than the relative density of the at least one second sintered layer.The at least one refractory metal carbide contained in the at least one third sintered layer can be identical to the refractory metal carbide contained in the at least one first sintered layer and / or to the refractory metal carbide contained in the at least one second sintered layer. Preferably, the at least one refractory metal carbide contained in the at least one third sintered layer is selected from the group consisting of titanium carbides, zirconium carbides, hafnium carbides, vanadium carbides, niobium carbides, tantalum carbides, chromium carbides, molybdenum carbides, tungsten carbides, and mixtures thereof.

[0086] Furthermore, it is preferred that the at least one third sintered layer comprises a layer sequence of several sintered layers, wherein the relative density of the several sintered layers within the layer sequence decreases with increasing distance to the at least one second sintered layer. The relative density can be determined, for example, by SEM cross-section analysis.

[0087] Furthermore, it is preferred that the coated substrate according to the invention can be produced or manufactured using the inventive method for producing coated substrates.

[0088] The present invention also relates to the use of the coated substrate according to the invention in semiconductor crystal growth, wherein the coated substrate is preferably a coated crucible.

[0089] The coated substrate according to the invention can, for example, be used as a gallium evaporator or part of a gallium evaporator in a PEX-GaN reactor suitable for growing gallium nitride semiconductor crystals, wherein the layer system contained in the coated substrate according to the invention then functions as a coating of the gallium evaporator. In this case, the at least one first sintered layer, due to its higher relative density, can particularly well protect the base material of the gallium evaporator from the corrosive gallium melt, whereas the lower relative density and higher porosity of the at least one second sintered layer of the layer system can increase the gallium evaporation rate and thus also the gallium nitride growth rate, since there is a larger contact area with the gallium melt.

[0090] The present invention is explained in more detail with reference to the following figures and examples, without limiting the invention to the parameters specifically illustrated.

[0091] In Fig. 1 A part of an exemplary variant of the method according to the invention is shown schematically. In this example, a (closed) crucible, preferably a graphite crucible, e.g., an iso-graphite crucible, is used as substrate 1, which is to be used in semiconductor crystal growth. In this application, a growth atmosphere is present inside the crucible. Fig. 1The following schematic descriptions are provided: Step A involves applying a first layer 2 to the entire surface of the substrate 1, and Step B involves applying a second layer 3 to the portion of the first layer 2 that is applied to the inner surface of the substrate 1. This results in a targeted layer architecture or a targeted layer structure consisting of two layers. The first layer 2 is obtained by applying a layer of a first aqueous suspension consisting of tantalum carbide, water, and 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, preferably selected from the group consisting of silicon, zirconium boride, refractory metal silicides, and mixtures thereof.The second layer 3 is obtained by applying a layer of a second aqueous suspension consisting of tantalum carbide and water, and thus containing no sintering additive. The application of the first layer 2 in step A and the application of the second layer 3 in step B can each be carried out by spray application. Only after the application of the second layer 3 does sintering of the (coated) substrate take place in step C, so that the first layer 2 and the second layer 3 are sintered together, with the first sintered layer 4 being formed from the first layer 2 and the second sintered layer 5 from the second layer 3. During sintering, a targeted densification of the layer architecture or layer structure occurs.

[0092] The resulting coated substrate exhibits a highly advantageous layer structure consisting of two different sintered layers 5 and 6. Due to the use of the sintering additive in the first layer 2, the sintered layer 4 has a high relative density, which provides excellent protection for the substrate against the corrosive melt used in semiconductor crystal growth. The second sintered layer 5, because the sintering additive is not present in the second layer 3, has a lower relative density than the first sintered layer 4. The resulting layer structure thus exhibits a density gradient, whereby the degree of compaction, or relative density, decreases with increasing distance from the substrate (i.e., from the first sintered layer 4 to the second sintered layer 5).The resulting higher porosity of the second sintered layer 5 compared to the first sintered layer 4 leads to advantages in the application of the coated substrate in semiconductor crystal growth, since this higher porosity provides a larger contact area for the melt used in crystal growth, thereby increasing the evaporation rate and thus also the growth rate.

[0093] In Fig. 2The density distribution within the layer system of an exemplary coated substrate according to the invention is shown graphically, with the relative sintered density being depicted as a function of the layer thickness (or the distance from the substrate surface). The exemplary coated substrate described here has a layer structure with three sintered layers of different relative densities. Point 0 on the x-axis represents the substrate surface, where t indicates the layer thickness of the entire coating, including the first, second, and third sintered layers. The Δx values ​​indicate the distance between two layer layers of different densities, such that Δx1 corresponds to the thickness of the first sintered layer, Δx2 to the thickness of the second sintered layer, and Δx3 to the thickness of the third sintered layer. The difference in density between the adjacent sintered layers is quantified by Δρ.The first sintered layer, i.e., the sintered layer in direct contact with the substrate, has a relative density of at least 70%. In the... Fig. 2The graph shown reveals that the relative density in the sintered layers of the coating system decreases with increasing distance from the substrate. This means that the second sintered layer has a relative density at least 5% lower than the first, and the third sintered layer has a relative density at least 5% lower than the second. This results in a density gradient within the coating system. To determine the values ​​shown graphically, a scanning electron microscope (SEM) image of a polished cross-section can be used to distinguish the area Δx over which a uniform density exists. Based on the determined Δx and the mass of this area, measured during coating, a sinter density can then be calculated for Δx. The difference in density between adjacent layers, with an extent of Δx, is quantified by Δρ. Example 1

[0094] First, a first aqueous suspension consisting of 80 wt% tantalum carbide, 1 wt% silicon as a sintering additive, and 19 wt% water, and a second aqueous suspension consisting of 80 wt% tantalum carbide and 20 wt% water were prepared. The second aqueous suspension therefore contains no sintering additive. The average powder particle size of the TaC particles is preferably in the range between 0.2 and 2 µm. To prepare the aqueous suspensions, the mixtures of the respective components are blended using a disperser (rotational speeds up to 1 m / s) and, if necessary, grinding media. The blending process can take at least 12 hours.

[0095] Subsequently, a layer 2 of the first aqueous suspension is applied to a substrate 1, preferably a graphite substrate, e.g., an iso-graphite substrate. Application is carried out by spraying, with a layer 2 of the first aqueous suspension having an average thickness in the range of 20 to 80 µm. Alternatively, application can also be carried out, e.g., by dipping or brushing.

[0096] Subsequently, a layer 3 of the second aqueous suspension is applied to a section of layer 2 of the first aqueous suspension already applied to the substrate. This application is also carried out by spraying, with a layer 3 of the second aqueous suspension having an average thickness in the range of 20 to 80 µm. Alternatively, application can also be carried out, for example, by dipping or brushing.

[0097] After the application of the two layers 2 and 3, the coated substrate undergoes a sintering process under an argon atmosphere at a temperature of 2300 °C, a holding time of 3 hours, and a pressure of 5 bar. Since all applied layers are sintered in a single pass, all individually applied layers sinter together, resulting in a layer system with stable bonds even under high thermal stress. The first sintered layer 4, resulting from layer 2 of the first aqueous suspension, exhibits a high relative density of over 70% due to the use of the sintering additive. The second sintered layer 5, resulting from layer 3 of the second aqueous suspension, has a lower relative density and thus a higher porosity than the first sintered layer 4, as the second aqueous suspension does not contain the sintering additive.

[0098] A schematic overview of the process steps carried out after the preparation of the aqueous suspensions is shown in Fig. 3 depicted.

[0099] In the fabricated sintered substrate, the first sintered layer 4 exhibits a permeability of less than 1 e⁻¹³ < m², determined by a setup for measuring the gas volume flow rate through the sample as a function of a pressure difference through the sample and conversion to Darcy's permeability constant. The bond strength, determined by tensile tests, of the first sintered layer 4 is greater than 4 MPa. Furthermore, the first sintered layer 4 has an average thickness of 25 µm to 30 µm, determined by cross-sectional analysis.

[0100] By determining the geometric density via the mass and volume of the compacted layers, a comparison can be made between the degree of compaction of the first sintered layer 4 (resulting from the first aqueous suspension with 1 wt% silicon sinter additive) and the second sintered layer 5 (resulting from the second aqueous suspension without sinter additive). The increase in the degree of compaction is quantified in relation to the relative density (ratio between geometric density and theoretical density of TaC with a value of 14.5 g / cm³). The result of the comparison is shown in Fig. 4 The figure shows the relative density of the first sintered layer 4 represented by a triangle and the relative density of the second sintered layer 5 represented by a circle. Fig. 4It is evident that the first sintered layer 4 has a relative density of over 70% and also a significantly higher relative density than the second sintered layer 5. This demonstrates that the use of silicon as a sintering additive results in a higher degree of compaction in the sintered layer. Example 2

[0101] First, a first aqueous suspension consisting of 80 wt% tantalum carbide, 1 wt% molybdenum silicide (MoSi₂) as a sintering additive, and 19 wt% water, as well as a second aqueous suspension consisting of 80 wt% tantalum carbide and 20 wt% water, were prepared. The second aqueous suspension therefore contains no sintering additive. The average powder particle size of the TaC particles is preferably in the range between 0.2 and 2 µm. To prepare the aqueous suspensions, the mixtures of the respective components are blended using a disperser (rotational speeds up to 1 m / s) and, if necessary, grinding media. The blending process can take at least 12 hours.

[0102] Subsequently, a layer 2 of the first aqueous suspension is applied to a substrate 1, preferably a graphite substrate, e.g., an iso-graphite substrate. Application is carried out by spraying, with a layer 2 of the first aqueous suspension having an average thickness in the range of 20 to 80 µm. Alternatively, application can also be carried out, e.g., by dipping or brushing.

[0103] Subsequently, a layer 3 of the second aqueous suspension is applied to a section of layer 2 of the first aqueous suspension already applied to the substrate. This application is also carried out by spraying, with a layer 3 of the second aqueous suspension having an average thickness in the range of 20 to 80 µm. Alternatively, application can also be carried out, for example, by dipping or brushing.

[0104] After the application of the two layers 2 and 3, the coated substrate is subjected to a sintering process under an argon atmosphere at a temperature of 2300 °C, a holding time of 3 hours, and a pressure of 5 bar. Since the sintering of all applied layers occurs in a single sintering pass, all individually applied layer layers sinter together, resulting in a layer system with stable bonds even under high thermal stresses. The first sintered layer 4, resulting from layer 2 of the first aqueous suspension, exhibits a high relative density of at least 70% due to the use of the sintering additive. The second sintered layer 5, resulting from layer 3 of the second aqueous suspension, has a lower relative density and thus a higher porosity than the first sintered layer 4, since the second aqueous suspension does not contain any sintering additive.

[0105] A schematic overview of the process steps carried out after the preparation of the aqueous suspensions is shown in Fig. 3 depicted.

[0106] In the manufactured coated substrate, the first sintered layer 4 exhibits a permeability of less than 1 e⁻¹³ < m², determined by a setup for measuring a gas volume flow through the sample as a function of a pressure difference through the sample and conversion to Darcy's permeability constant. The adhesion strength, determined by tensile tests, of the first sintered layer 4 is greater than 4 MPa. Furthermore, the first sintered layer 4 has an average thickness of 45 µm to 50 µm, determined by cross-sectional analysis.

[0107] Based on the determination of the geometric density via the mass and volume of the compacted layers, a comparison of the degree of compaction of the first sintered layer 4 (resulting from the first aqueous suspension with 1 wt% sinter additive MoSi₂) and the second sintered layer 5 (resulting from the second aqueous suspension without sinter additive) can be made below. The quantification of the increase in the degree of compaction is related to the relative density (ratio between geometric density and theoretical density of TaC with a value of 14.5 g / cm³). The result of the comparison is shown in Fig. 5 The figure shows the relative density of the first sintered layer 4 represented by a cross and the relative density of the second sintered layer 5 represented by a circle. Fig. 5It is evident that the relative density of the first sintered layer 4 is more than 70%. Furthermore, the first sintered layer exhibits a significantly higher relative density than the second sintered layer 5. This demonstrates that the use of MoSi₂ as a sintering additive results in a higher degree of compaction in the sintered layer.

Claims

1. Method for preparing coated substrates, in which a) a first aqueous suspension is prepared, comprising or consisting of at least one refractory metal carbide, at least one 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, and water, b) a second aqueous suspension is prepared, containing at least one refractory metal carbide and water, wherein the second aqueous suspension - contains at least one 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 percentage by weight of the at least one sintering additive in the second aqueous suspension, based on the total weight of the second aqueous suspension, is smaller than the percentage by weight of the at least a sintering additive in the first aqueous suspension, based on the total weight of the first aqueous suspension, or - contains no sintering additive, c) at least one layer (2) of the first aqueous suspension is applied to a substrate (1), d) at least one layer (3) of the second aqueous suspension is applied to the at least one layer (2) of the first aqueous suspension applied to the substrate (1), and e) the substrate (1) is subjected to a sintering process after step d).

2. Method according to the preceding claim, characterised in that - the substrate comprises or consists of a material selected from the group consisting of graphite, preferably iso-graphite, carbon fibre reinforced carbon (CFC), C / SiC fibre composites, SiC / SiC fibre composites, carbidic ceramics, nitridic ceramics, oxidic ceramics, and mixtures thereof, and / or - the at least one refractory metal carbide is selected from the group consisting of titanium carbides, zirconium carbides, hafnium carbides, vanadium carbides, niobium carbides, tantalum carbides, chromium carbides, molybdenum carbides, tungsten carbides, and mixtures thereof, and / or - the refractory metal silicides are selected from the group consisting of titanium silicides, zirconium silicides, hafnium silicides, vanadium silicides, niobium silicides, tantalum silicides, chromium silicides, molybdenum silicides, tungsten silicides, and mixtures thereof, and / or - the refractory metal nitrides are selected from the group consisting of titanium nitrides, zirconium nitrides, hafnium nitrides, vanadium nitrides, niobium nitrides, tantalum nitrides, chromium nitrides, molybdenum nitrides, tungsten nitrides, and mixtures thereof, and / or - the refractory metal borides are selected from the group consisting of titanium borides, zirconium borides, hafnium borides, vanadium borides, niobium borides, tantalum borides, chromium borides, molybdenum borides, tungsten borides, and mixtures thereof.

3. Method according to any one of the preceding claims, characterised in that - the at least one refractory metal carbide and the at least one sintering additive are each present in particulate form, wherein the mean particle size of the particles of the at least one sintering additive is less than 5 µm and / or smaller than the mean particle size of the particles of the at least one refractory metal carbide, and / or - the at least one refractory metal carbide is present as a powder mixture which comprises or consists of powders which differ in terms of the average particle size of the particles, preferably of the same refractory metal carbide.

4. Method according to any one of the preceding claims, characterised in that the first aqueous suspension and / or the second aqueous suspension - comprises 60 to 90% by weight, preferably 70 to 85% by weight, of the at least one refractory metal carbide, based on the total weight of the respective aqueous suspension, and / or - comprises 0.1 to 20% by weight, preferably 0.5 to 10% by weight, of the at least one sintering additive, based on the total weight of the respective aqueous suspension.

5. Method according to any one of the preceding claims, characterised in that the weight percentage of the at least one sintering additive in the second aqueous suspension, based on the total weight of the second aqueous suspension, is less than by 0.1% by weight to 20% by weight, preferably by 0.5% by weight to 10% by weight than the weight percentage of the at least one sintering additive in the first aqueous suspension, based on the total weight of the first aqueous suspension.

6. Method according to any one of the preceding claims, characterised in that the preparation of the first aqueous suspension in step a) and / or the preparation of the second aqueous suspension in step b) is carried out by mixing the components of the suspension to be prepared with the aid of a dispersing device, wherein the mixing is carried out with the aid of the dispersing device, preferably using grinding media and / or over a period of at least 12 hours.

7. Method according to any one of the preceding claims, characterised in that the application of the at least one layer (2) of the first aqueous suspension in step c) and / or the application of the at least one layer (3) of the second aqueous suspension in step d) - is carried out by means of dipping, brushing or spray application, and / or - is carried out with an average layer thickness of less than 150 µm, preferably from 20 µm to 100 µm, particularly preferably from 30 µm to 80 µm.

8. Method according to any one of the preceding claims, characterised in that at least one third aqueous suspension is additionally prepared, the at least third aqueous suspension comprising or consisting of at least one refractory metal carbide and water, wherein the at least one third aqueous suspension comprises no sintering additive, and between steps d) and e), at least one layer of the at least one third aqueous suspension is applied to the at least one applied layer (3) of the second aqueous suspension.

9. Method according to any one of the preceding claims, characterised in that the sintering process in step e) - is carried out at a temperature of from 2100°C to 2500°C, preferably from 2200°C to 2400°C, and / or - is carried out with a holding time of 1 h to 15 h, preferably of 2 h to 10 h, and / or - is carried out at a pressure of 0.1 bar to 10 bar, preferably 0.7 bar to 5 bar, and / or - is carried out such that after a first time segment of the sintering process, the pressure is increased, preferably by 3 bar to 7 bar, and / or - is carried out under an argon atmosphere.

10. Coated substrate comprising a substrate (1), at least one first sintered layer (4) arranged on the substrate (1) which at least one first sintered layer (4) comprises or consists of at least one refractory metal carbide, and at least one 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, and at least one second sintered layer (5) arranged on the at least one first sintered layer (4), which at least one second sintered layer (5) contains or consists of at least one refractory metal carbide and optionally at least one 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 at least one first sintered layer (4) has a relative density of at least 70%, and wherein the relative density of the at least one second sintered layer (5) is at least 3% lower than the relative density of the at least one first sintered layer (4).

11. Coated substrate according to claim 10, characterised in that the at least one first sintered layer (4) - has a relative density of more than 75%, preferably more than 80%, particularly preferably more than 90%, very particularly preferably more than 95%, and / or - has an average layer thickness of at least 20 µm, preferably from 20 µm to 150 µm, particularly preferably from 30 µm to 100 µm.

12. Coated substrate according to claim 10 or 11, characterized in that the at least one second sintered layer (5) - has a relative density which is at least 5%, preferably at least 10%, particularly preferably 10% to 30 %, very particularly preferably 15% to 25%, particularly 18% to 22% lower than the relative density of the at least one first sintered layer, and / or - has an average layer thickness of at least 20 µm, preferably from 20 µm to 150 µm, particularly preferably from 30 µm to 100 µm.

13. Coated substrate according to any one of claims 10 to 12, characterised in that the coated substrate comprises at least one third sintered layer arranged on the at least one second sintered layer (5) and comprising or consisting of at least one refractory metal carbide, wherein the at least one third sintered layer comprises no sintering additive, and wherein the relative density of the at least one third sintered layer is at least 5% lower than the relative density of the at least one second sintered layer (5), wherein the at least one third sintered layer preferably comprises a layer sequence of a plurality of sintered layers, wherein the relative density of the plurality of sintered layers within the layer sequence decreases as the distance from the at least one second sintered layer (5) increases.

14. Coated substrate according to any one of claims 10 to 13, characterised in that the coated substrate can be prepared or is prepared using a method according to any one of claims 1 to 9.

15. Use of a coated substrate according to any one of claims 10 to 14 in semiconductor crystal growth, wherein the coated substrate is preferably a coated crucible.