Composite element

Abstract

A composite element (1) having a main part (2) made of a molybdenum-based material, on at least portions of which a tungsten-carbide-containing coating (3) is formed, the coating (3) having a microstructure that essentially consists of microcrystalline and / or nanocrystalline tungsten carbide grains.

Description

[0001] 1120WO Text Ausland 14.05.2024 COMPOSITE BODY The present invention relates to a composite body comprising a base body made of a molybdenum-based material with a coating. Molybdenum displays excellent chemical resistance to various media. However, in oxygen, molybdenum is only resistant up to approximately 250°C. When molybdenum is used in oxidizing gases and elements, it is generally coated to protect against oxidation. Common coatings, for example, are based on silicon and boron (e.g., SIBOR ^ ). The presence of carbon at high temperatures also limits the resistance of molybdenum, since molybdenum reacts with carbon above approximately 800°C to form MoC and further to Mo2C. Mo2C and molybdenum have different thermal expansion coefficients, whereby thermal stresses lead to spalling and destruction of the surface.Therefore, coatings are also proposed for the use of molybdenum under carburizing conditions. For example, Kardoulaki et al. described the problem of molybdenum reaction in graphitic reactors and discussed various nitride, oxide, and carbide coatings as a possible solution. They propose TiC, ZrC, TaC, NbC, and SiC as protective coatings for molybdenum. See: Kardoulaki, E., Nizolek, T., Luther, E., & Swartz, M. (2021): On the Interactions of Molybdenum and Graphite, a Promising Material System for Microreactors, JOM, 73, 3499–3512; https: / / doi.org / 10.1007 / s11837-021- One of the disadvantages of the coatings mentioned is that the proposed chemical compounds are undesirable for certain thermal processes. The object of the present invention is to provide a composite body with a base body made of a molybdenum-based material which is resistant under carburizing conditions.In particular, the composite body should not lead to any contamination of the atmosphere of a heat treatment facility in which the composite body is used. This object is achieved by a composite body having the features of claim 1. Furthermore, a method for producing a composite body is described. Preferred developments are specified in the dependent claims. By the composite body comprising: a base body made of a molybdenum-based material, on which a coating comprising tungsten carbide is formed at least in sections, wherein the coating has a microstructure consisting essentially of fine-crystalline and / or nano-crystalline grains of tungsten carbide, a composite body is created which is particularly resistant under carburizing conditions. In the context of this application, molybdenum-based material means a material with at least 50 at.% (atomic percent) molybdenum.In particular, the molybdenum content is greater than 90 at.%, more preferably greater than 95 at.%. The base body can consist entirely of the molybdenum-based material. It can also be provided that the base body is designed as a material composite. The substrate carrying the coating and optionally an intermediate layer is in this case a molybdenum-based material. It is provided that the coating comprises tungsten carbide. In particular, the coating consists essentially of tungsten carbide. "Essentially" in this context means that the coating consists of at least 80 vol.% tungsten carbide, more preferably at least 90 vol.% tungsten carbide. Tungsten carbide is defined as:

[0002] Public in the context of the present application is understood to mean in particular the species WC and W2C. With particular preference, the coating consists entirely of tungsten carbide, optionally with usual impurities. The level of usual impurities is less than 1 at.% (atomic percent), preferably less than 0.1 at.%. The microstructure of the coating consists essentially of fine-crystalline and / or nanocrystalline grains of tungsten carbide. “Essentially” here means that the microstructure consists largely of fine-crystalline and / or nanocrystalline grains of tungsten carbide. In particular, at least 80%, preferably at least 90%, more preferably at least 95% of the microstructure is fine-crystalline and / or nanocrystalline grains of tungsten carbide. The percentages refer to a quantitative microstructure analysis. In other words, the coating predominantly has a fine-grained microstructure.In particular, the structure is entirely fine-crystalline and / or nano-crystalline. According to this development, there are no coarse tungsten carbide grains or other structural components. The absence of other structural components, such as carbides of other metals, oxides, etc., is important because it prevents contamination by the coating. In the context of this application, nanocrystalline refers to grains with a grain size of up to 100 nm (nanometers). In this application, fine-crystalline refers to a structure with grain sizes from 100 nm to a few μm, preferably grain sizes from 100 nm to 5 μm (micrometers), in particular grain sizes from 100 nm to 1 μm. In this application, coarse grains refer to grains with a grain size of greater than or equal to 10 μm.

[0003] Public It is therefore preferable for the coating to be particularly fine-grained. This has the advantage, among other things, that even with the smallest layer thicknesses, the coating comprises a large number of grains along its thickness. Another significant benefit of a fine-grained structure of the coating is its high hardness and strength. It is well known that grain refinement is associated with an increase in yield strength. A fine grain therefore leads to an increase in strength and hardness. Furthermore, a coating with a fine-grained structure has high fracture toughness. The fine-grained structure is advantageous with regard to crack propagation. These mechanical properties of the coating are particularly advantageous when the composite body is used under cyclic thermal stress.Preferably, the pore content in the coating is less than 10%, more preferably the pore content in the coating is less than 5%, more preferably less than 3%, even more preferably less than 1.5%. The pore content can be determined using quantitative metallographic methods. An evaluation is proposed such that the area fraction of pores in the coating is determined on a metallographic section of a sample and this area fraction is compared to the image section under consideration. An example of such a determination of the pore content of the coating is explained in the description of the figures. Advantages of the low pore content in the coating according to this development include: - the coating has high strength and hardness - the coating is particularly impermeable. In particular, there are no channels through the coating to the surface of the base body.Diffusion of undesirable species through the coating, especially the diffusion of carbon, is difficult - the coating shows a homogeneous thermal expansion.

[0004] Public - the coating has a high thermal conductivity - heat is transported homogeneously through the layer into the substrate. By producing the coating - directly or indirectly - via a gas phase process, the characteristic of low porosity can be achieved particularly well. It is preferably provided that the coating has a thickness between 0.2 μm and 200 μm, more preferably a thickness between 1 μm and 20 μm. The coating is particularly preferably between 3 and 10 μm thick. Advantages of a coating thickness in the specified range, in particular in the preferred range, include excellent layer adhesion even under cyclic thermal stress of the composite body. Furthermore, a thickness in the specified range promotes a faithful representation of a surface morphology of the substrate (here: the base body) and a substrate geometry such as recesses or steps.In addition, with a thickness in the preferred range, a particularly balanced relationship between manufacturing effort, layer adhesion and a barrier effect, in particular against the diffusion of carbon, is achieved. It is preferably provided that the coating reflects the surface morphology of the base body. In other words, it is preferably provided that the coating is present in the same thickness along the surface of the base body. Any structures of the surface of the base body are thus continued in the coating and reflected in a coating surface. A variation in the thickness of the coating is preferably less than 20%, more preferably less than 10%. In the case of previously known protective layers, such as those produced by pack cementing, slurry coating or powder spraying,

[0005] Publicly, the coating levels out any structures on the surface of the base body. This can result in locally varying layer thicknesses. As the variation in the thickness of the coating is preferably small, preferably less than 20%, more preferably less than 10%, diffusion paths through the layer, for example, are essentially the same length at all locations on the coating. The characteristic of small variation in the thickness of the coating is advantageous for the stability of the coating in use, particularly under cyclic thermal stress and / or diffusion of elements into the coating, in particular the diffusion of carbon. A small variation in the thickness of the coating also means a good representation of the surface morphology of the base body. The characteristic of small variation in the thickness of the coating can be determined, for example, using quantitative microstructure analysis.The thickness of the coating can be determined at several positions on a test specimen transverse to the coating. Edge regions of the base body and areas with steps, flanks, radii or similar are excluded. The characteristic of a variation in the thickness of the coating of less than 20% expresses that the thickness of the coating at the positions considered lies within a range of ± 20% with respect to the average thickness of the coating determined for the positions considered. According to a further development, it can be provided that at least one intermediate layer is formed between the surface of the base body and the coating comprising tungsten carbide. This also means that the coating is not necessarily formed directly on a surface of the base body.An intermediate layer can be beneficial for adhesion of the coating and / or for increasing the barrier effect against the diffusion of carbon.

[0006] Public According to a further development, the intermediate layer consists of tungsten or a tungsten-based alloy. Accordingly, it is provided that a metallic layer made of tungsten or a tungsten-based alloy is formed on the surface of the base body and beneath the coating. A tungsten-based alloy is understood to mean a composition with at least 50 at.% tungsten. In particular, the intermediate layer made of tungsten or a tungsten-based alloy comprises columnar tungsten grains. Columnar grains have a pronounced aspect ratio. Typically, a grain aspect ratio, i.e. a ratio of the extent along a longitudinal axis to a grain width, measured perpendicular to the longitudinal axis, is greater than or equal to two, in particular greater than or equal to five. A columnar microstructure has a significantly higher thermal conductivity in a direction along the longitudinal axis of the columnar grains than perpendicular to it.This can be explained by a smaller number of grain boundaries to be overcome. The formation of the intermediate layer of tungsten in the form of columnar tungsten grains is particularly advantageous. In particular, the columnar tungsten grains have a preferred orientation such that a majority of the columnar tungsten grains are oriented essentially normal to the surface of the base body. "Normal to the surface" means that the grains are oriented at an angle of approximately 90° to the surface of the base body with respect to their longitudinal axis (the axis with the greatest extension of the grain), whereby angular deviations of ± 15° are also included. Preferably, at least 80% of the columnar tungsten grains have such a preferred orientation, based on the total area proportion of columnar tungsten grains.The formation of the tungsten intermediate layer in the form of columnar tungsten grains with such a preferred orientation is particularly advantageous, especially with regard to thermal conductivity. One possible way to produce the intermediate layer from tungsten or a tungsten-based alloy is by sputtering it onto a target of the appropriate composition.

[0007] Public Alternatively, the intermediate layer comprises a mixture of tungsten and molybdenum. The intermediate layer of tungsten and molybdenum creates an additional barrier against the diffusion of carbon into the base body. A solution of tungsten in molybdenum significantly reduces the diffusion constant of carbon compared to diffusion in pure molybdenum. In addition, tungsten, when present as a result of diffusion along grain boundaries of the molybdenum, can act as a carbon getter and absorb harmful carbon diffusion. More preferably, the composition of the intermediate layer is between 1 at.% tungsten and 100 at.% tungsten, with the remainder optionally being molybdenum and usual impurities. According to a further development, the intermediate layer contains grains of a molybdenum-based material with tungsten present at the grain boundaries.It is therefore preferably provided that the grain boundaries of the grains are enriched with a molybdenum-based material, for example by diffusion. Such an intermediate layer is preferably created by a coating with tungsten and a subsequent heat treatment, which causes the tungsten to diffuse into a superficial zone of the base material. More preferably, the intermediate layer is graded. The intermediate layer preferably has a gradient with regard to the tungsten content such that the tungsten content in the intermediate layer decreases starting from the coating towards the base body. If the intermediate layer is graded, the above tungsten contents in the intermediate layer refer to an average composition along the thickness of the intermediate layer. For example, the thickness of the intermediate layer is between 500 nm and 20 μm. The intermediate layer is preferably between 500 nm and 8 μm thick, more preferably between 3 μm and 5 μm thick.

[0008] Public In particular, it is proposed that the base body comprise a molybdenum alloy with carbide components. In particular, carbide components are present as dispersoids. Dispersoids are particles that are finely distributed in a matrix. Dispersoids can be added as particles or obtained by precipitation. A particularly suitable base body is the molybdenum alloy known as TZM with contents of titanium, zirconium and carbon. A typical composition of the alloy is 0.5 wt.% Ti, 0.08 wt.% Zr and 0.01 - 0.04 wt.% C (wt.% = weight percent) with molybdenum as the remainder. The molybdenum alloy TZM contains carbides of zirconium and titanium in particular. Compared to pure molybdenum, TZM is stronger and has a higher recrystallization temperature as well as higher creep strength.The applicant has determined that the adhesion of the tungsten carbide coating to a base body made of a molybdenum alloy with carbide components is further improved compared to a base body made of pure molybdenum. Without committing to a scientific explanation, it is assumed that the bonding of the tungsten carbide coating is further improved by the carbide species in the substrate (here: the base body). As a preferred development, it is therefore provided that the base body consists of a molybdenum alloy with carbide components, in particular TZM. The composite body according to the invention is particularly suitable for applications of molybdenum-based materials in a carburizing atmosphere. In the composite body according to the invention, diffusion of carbon into the base body made of molybdenum or a molybdenum alloy is effectively prevented by the coating comprising tungsten carbide.The coating comprising tungsten carbide forms an effective diffusion barrier against the diffusion of carbon into the base material molybdenum.

[0009] Public The diffusion constant of carbon in tungsten carbide is several orders of magnitude lower than the diffusion constant of carbon in molybdenum. If molybdenum is used unprotected in a carburizing atmosphere, molybdenum carbide, particularly Mo2C, forms on the surface. The compound Mo2C has a higher coefficient of thermal expansion (CTE) than metallic molybdenum. A difference in the thermal expansion coefficients, particularly in connection with a layer and a substrate, is referred to as a "CTE mismatch." Carburization of the base material molybdenum and the described CTE mismatch can, among other influencing factors, lead to spalling and loss of substance in the base material.Applications in which the CTE mismatch is particularly detrimental are processes with cyclic thermal stress, such as the use of structural components made of molybdenum in furnace systems with graphite heaters or the use of molybdenum substrates in carbon-containing plasma, such as in diamond synthesis (production of artificial diamonds). Also relevant are applications in reactor construction, such as heat exchangers or vessels in reactor construction. With the composite body according to the invention, this adverse effect is prevented or at least significantly reduced. A further advantage of the coating is that the vapor pressure on a surface coated in this way is significantly reduced compared to an uncoated base body. Thus, the composite body potentially leads to less introduction of undesirable species into a process space than an uncoated base body.This is particularly noticeable when the composite body according to the invention is used in low-pressure or vacuum applications. Public protection is also sought for the use of a previously described composite body as a component in a high-temperature furnace. This includes, in particular, radiation shields, holders, and charging devices. Protection is also sought for the use of a previously described composite body as a substrate in the production of synthetic diamonds. Also relevant are applications of the composite body as components in chemical reactors. Protection is sought for a method for producing a composite body. The method provides for a coating comprising tungsten carbide to be formed, at least in sections, on a base body made of a molybdenum-based material, for example molybdenum or a molybdenum alloy.The method comprises the following steps: providing a base body made of a molybdenum-based material, a) coating a surface of the base body with metallic tungsten to form a metallic tungsten film on the surface, and then at least partially carburizing the metallic film to tungsten carbide, or b) coating a surface of the base body by depositing tungsten carbide by sputtering from a tungsten carbide target or by reactive sputtering from a tungsten-containing target in a carburizing atmosphere. In both process variants, a layer is applied via a gas-phase process, resulting in the previously explained advantages of the coating. The formation of the tungsten carbide coating can occur indirectly, according to the first alternative, or directly, according to the second process alternative. It would also be conceivable to combine the two process variants.In a further development of the method, an intermediate layer can be formed on a surface of the base body, on which the tungsten carbide coating is subsequently formed. The intermediate layer can be in the form of a layer of tungsten. An intermediate layer made of tungsten can be obtained, for example, if the previously deposited metallic film of tungsten is not completely carburized in the process variant of the indirect production of the tungsten carbide coating. The parameters for this can be determined by a person skilled in the art through experiments. The intermediate layer is in particular made of molybdenum with tungsten. An intermediate layer made of tungsten and molybdenum creates an additional barrier against the diffusion of carbon into the base body.Regarding the production of the intermediate layer made of molybdenum with tungsten, it is preferred to deposit metallic tungsten on the base body, for example, by sputtering from a tungsten target. The coated base body is then subjected to a heat treatment such that tungsten diffuses into the molybdenum-based base body. The process for producing the composite body is explained in more detail below. Production Example 1: Molybdenum and TZM discs measuring 10 mm x 5 mm were first coated with a roughly 5 μm thick metallic tungsten film by sputtering from a tungsten target. The samples were then heat-treated in a tube furnace (manufacturer: Reetz GmbH) under a carburizing atmosphere. The carburizing gas mixture used was Ar / CH4 (argon / methane) with a composition of 99 vol.% argon + 1 vol.% methane at a flow rate of 10 L / h.It was heated at a ramp of 15 K / min to 1000 °C and at 10 K / min to the corresponding test temperature. Cooling took place under the same gas mixture. After a holding time of 15 hours at 1300 °C, complete carburization of the tungsten into tungsten carbide was observed. Public Phase analysis via XRD (X-ray diffraction) revealed the presence of hexagonal WC. At shorter holding times, W2C was also detected in addition to WC. Production example 2: Using an alternative production route, TZM test specimens were coated by sputtering from a WC target. The samples were sandblasted before coating. To compensate for the different sputtering rates of tungsten and carbon, the target was doped with graphite in a WC / C (tungsten carbide to carbon) ratio of 85 / 15 mol% (mol percent). This made it possible to achieve tungsten carbide with the stoichiometry WC in the coating.A coating of cubic WC with a layer thickness of 4.5 μm was created after a coating time of 20 minutes. The coating exhibited a nanocrystalline structure. The layer adhesion was excellent; no pores were detectable. The production of the coating from or at least comprising tungsten carbide using a gas-phase process, directly or indirectly, is associated with significant structural properties and advantages. By producing the coating - directly or indirectly - using a gas-phase process, the coating exhibits particularly low porosity. Furthermore, the layer adhesion is excellent. Because the grains of the coating grow on the surface of the base body in a gas-phase process, a chemical bond exists between the base body and the coating, resulting in a more intimate bond than with purely mechanical entanglement, such as with powder deposition.In particular, there can also be an epitaxial connection, at least in part, between grains of the base body and grains of the coating. A further feature that can be achieved by deposition of the coating via a gas phase is the formation of a microstructure with at least a proportion of columnar grains. Public In summary, the following features and associated advantages exist, among others, of a coating produced via a gas phase: ^ Low porosity: high strength, high density, good barrier effect against diffusion ^ Fine grain size: high strength and toughness ^ Contour accuracy: no leveling of the surface structure of the substrate ^ Layer adhesion: intimate chemical / metallurgical bond in contrast to sintered or sprayed layers ^ Microstructure: homogeneous structure, small variation in grain sizes, if necessary preferred orientation of the grains.A person skilled in the art can easily distinguish a coating produced via a gas phase from sintered or sprayed layers. Further advantages and usefulness of the invention will become apparent from the following description of exemplary embodiments with reference to the attached figures. The figures show: Fig. 1: a composite body in a first exemplary embodiment Fig. 2: a step in the production of a composite body Fig. 3: a next step in the production of a composite body Fig. 4: a further step in the production of a composite body Fig. 5: a scanning electron micrograph of an intermediate stage in the production of a composite body Fig. 6: a scanning electron micrograph of a composite body Fig. 7: a scanning electron micrograph (fracture image) of a composite body with an intermediate layer Fig. 8: a schematic view of the structure of a composite body with an intermediate layer9 a scanning electron micrograph (fracture image) of a composite body in a further embodiment Public Figure 1 schematically shows a composite body 1 in a first embodiment. The composite body 1 comprises a base body 2 made of a molybdenum-based material, on whose surface at least in sections a coating 3 based on tungsten carbide is formed. The size ratios are not shown to scale for reasons of clarity. In reality, the coating 3 is generally much thinner than the thickness of the base body 2. Figure 2 schematically shows a first step in the production of a composite body 1 according to a first method variant. In this case, metallic tungsten is deposited onto a surface 21 of the base body 2 by sputtering a target 5.As an alternative to cathode sputtering, tungsten could also be deposited using CVD (chemical vapor deposition). In the CVD variant, a gaseous, usually organic, precursor is passed over a substrate. For the deposition of tungsten, tungsten hexafluoride (WF6), for example, is used as a precursor. After the precursor has been deposited, the aids used to transport the metallic element (here: tungsten) are desorbed, leaving a metallic film of tungsten on the surface 21 of the base body 2. Using the process step illustrated in Figure 2, the base body 2 shown in Figure 3 can be obtained with a metallic film 51 of tungsten. Figure 4 illustrates a subsequent process step in which the metallic film 51 of tungsten is carburized with carbon ("C").A manufacturing example for converting the metallic film 51 made of tungsten into tungsten carbide is given in the description. By carburizing the metallic tungsten to tungsten carbide, a composite body 1 according to Figure 1 is obtained. The metallic film 51 made of tungsten therefore serves as a precursor layer for the coating 3. Alternatively, the composite body 1 can be obtained by directly forming a coating 3 based on tungsten carbide on a base body 2. This can be done, for example, by cathode sputtering from a target based on tungsten carbide. Optionally, processing steps can be carried out to smooth and / or clean the composite body 1. For example, it can be useful to gently blast or grind the coating 3. Tungsten carbide is preferably used as the blasting material in order to avoid contamination of the coating 3.Figure 5 shows a scanning electron micrograph of a base body 2 made of a molybdenum-based material, in this example TZM, with a metallic film 51 made of tungsten, as shown in Figure 3. The tungsten film 51 was deposited via cathode sputtering. The tungsten-coated sample was fractured as sample preparation. A fracture pattern is shown here. A layer thickness t. Wof the metallic film 51 made of tungsten was approximately 5 μm in this example. Clearly visible are the columnar grains of tungsten, which have a preferential orientation relative to the surface 21 of the base body 2. The columnar tungsten grains are oriented substantially perpendicular to the surface 21. In other words, the longitudinal axes of the columnar tungsten grains run substantially parallel to a surface normal N of the surface 21 of the base body 2. An extension of the columnar tungsten grains parallel to the surface normal N in the present example is such that at least one to two grains along the layer thickness t Ware formed. A grain width of the columnar tungsten grains normal to their longitudinal axis was around 200 nm in the present example. In the present example, the columnar tungsten grains typically have grain lengths that correspond to half the layer thickness of the metallic film 51 made of tungsten. Public A grain aspect ratio of the columnar tungsten grains is therefore around 12.5, which is the ratio of half the layer thickness (2.5 μm) to a grain width (0.20 μm). It can also be seen that the metallic film 51 made of tungsten reflects a morphology of the base body 2. Structures on the surface 21 of the base body 2 continue in the metallic film 51 made of tungsten. The layer thickness tW of the metallic film 51 made of tungsten is essentially constant along the surface 21 of the base body 2. The metallic film 51 made of tungsten has excellent layer adhesion to the base body 2.Figure 6 shows a scanning electron micrograph of a composite body 1 with a base body 2, here made of TZM, with a coating 3 of tungsten carbide. A fracture pattern is shown. The coating 3 in this exemplary embodiment was obtained by carburizing a metallic precursor layer of tungsten. In this exemplary embodiment, the composite body 1 has an intermediate layer 4 of metallic tungsten, which is present between the surface 21 of the base body 2 and the coating 3 of tungsten carbide. A thickness t of the coating 3 in the example was approximately 2 μm, a thickness t. ZWThe thickness of the intermediate layer 4 was approximately 3 μm. By varying the carburizing conditions, the ratio of the thicknesses of the intermediate layer 4 and the tungsten carbide coating 3 can be adjusted to the desired level. The total thickness (t + tZW) of the coating 3 and the intermediate layer 4 exhibits little variation. A variation of less than 20% is desired. In the present example, the variation in the total thickness (t + tZW) of the coating 3 and the intermediate layer 4 was less than 10%, as is particularly preferred. For this purpose, the total thickness (t + tZW) was measured under a scanning electron microscope at ten different locations. The locations were at least one public total thickness apart and the section under consideration lay within 5000 μm. An average total thickness was determined from the ten measurements. The deviation of the individual thicknesses from the average layer thickness was less than 10%. Rod-like grains of tungsten can be seen in the intermediate layer 4.The zone carburized to tungsten carbide, which corresponds to coating 3, has fine-crystalline grains of tungsten carbide. Figure 7 shows a scanning electron microscope image of the same exemplary embodiment as shown in Figure 6; in this case, the image was taken on an etched section. The columnar grains of tungsten in the intermediate layer 4 are clearly visible. The columnar tungsten grains are oriented essentially perpendicular to the surface 21. In other words, the longitudinal axes of the columnar tungsten grains run essentially parallel to a surface normal N of the surface 21 of the base body 2. The phrase “essentially perpendicular” also includes angular deviations of d 15° in terms of their magnitude. It corresponds to a preferred development that columnar tungsten grains have a preferred orientation that is essentially normal to the surface 21 of the base body 2, wherein angular deviations of d 15° are also included.The microstructure of the carburized area, i.e., coating 3, comprises fine-crystalline grains of tungsten carbide. A dashed box highlights an image section "D," where a pore content was measured using quantitative microstructure analysis. A skilled person can identify pores in the microstructure and mark them in a suitable image processing program. The image processing program can add up the marked pore areas. The section shown in Figure 7 could be used for comparison purposes if necessary. Public The total area of ​​the selected image section was approximately 82 μm. 2 . The total area of ​​pores in the selected image section was 0.75 μm 2. The pore proportion results from the ratio of the total area of ​​pores to the total area of ​​the selected image section and was determined to be 0.92% in the present example. This means that the pore proportion in the coating 3 and the intermediate layer 4 was less than 1%. It is preferably provided that the pore proportion in the coating 3 is less than 10%. More preferably, the pore proportion in the coating 3 is less than 5%, more preferably less than 3%, and even more preferably less than 1.5%. The dark areas on the surface 21 are not pores, but residues of blasting material. Figure 8 shows a schematic view of the structure of a composite body 1 with a coating 3 made of tungsten carbide. The composite body 1 has an intermediate layer 4 made of metallic tungsten, which is present between the surface 21 of the base body 2 and the coating 3 made of tungsten carbide.Figure 9 shows a scanning electron micrograph of a composite body 1 with a base body 2, here made of TZM, with a coating 3 made of tungsten carbide. The image shows a fracture pattern. In the present exemplary embodiment, the coating 3 made of tungsten carbide was applied directly by sputtering using a tungsten carbide target. The thickness t of the coating in the exemplary embodiment is approximately 4 μm and exhibits a particularly small variation in thickness. It is clearly visible that the coating 3 reproduces the surface contours of the substrate (here: the base body). The microstructure of the coating 3 is extremely fine and consists of nanocrystalline grains of tungsten carbide. In the exemplary embodiment, the grain size is a few nanometers and cannot be resolved with the scanning electron microscope used in the selected contrast mode. In the present exemplary embodiment, the coating is consistently nanocrystalline.Public The coating 3 formed in this way exhibits excellent adhesion to the substrate, the base body 2. Due to its fine grain, the coating is both extremely strong and tough. Furthermore, the coating is virtually pore-free. No pores 5 could be detected. Such a pore-free layer is—as already described at the beginning—particularly advantageous, among other things, with regard to strength and impermeability. Deviating from the present exemplary embodiment, an additional intermediate layer 10 can also be formed for the variant of the coating deposited from a WC target. Public.

Claims

Claims 1. A composite body (1) comprising a base body (2) made of a molybdenum-based material, on which a coating (3) comprising tungsten carbide is formed at least in sections, wherein the coating (3) has a microstructure consisting essentially of fine-crystalline and / or nanocrystalline grains of tungsten carbide.

2. A composite body (1) according to claim 1, wherein a pore content in the coating (3) is less than 10%.

3. A composite body (1) according to one of the preceding claims, wherein a variation in a thickness (t) of the coating (3) is less than 20%.

4. A composite body (1) according to one of the preceding claims, wherein at least one intermediate layer (4) is formed between a surface (21) of the base body (2) and the coating (3).

5. A composite body (1) according to claim 4, wherein the intermediate layer (4) is made of tungsten or a tungsten-based alloy. 6.The composite body (1) according to claim 4 or 5, wherein the intermediate layer (4) comprises molybdenum and tungsten.

7. The composite body (1) according to any one of claims 4 to 6, wherein the intermediate layer (4) contains grains of a molybdenum-based material, with tungsten present at the grain boundaries.

8. The composite body (1) according to any one of the preceding claims, wherein the base body (2) comprises a molybdenum alloy with carbide components.

9. Composite body (1) according to one of the preceding claims, wherein the composite body (1) is designed as a component of a high-temperature furnace.

10. Composite body (1) according to one of the preceding claims, wherein the composite body (1) is designed as a substrate in the production of synthetic diamonds.

11. Use of a composite body (1) according to one of the preceding claims as a substrate in the production of synthetic diamonds. 12.Method for producing a composite body (1) comprising a base body (2) made of a molybdenum-based material, the method comprising the steps of providing the base body (2) and forming a coating (3) comprising tungsten carbide on a surface (21) of the base body by: ^ gas phase deposition of a film comprising metallic tungsten on the surface (21) of the base body and subsequent at least partial carburization of the metallic film to tungsten carbide, or ^ gas phase deposition of the coating (3) comprising tungsten carbide by means of sputtering from a tungsten carbide target or by reactive sputtering from a tungsten-containing target in a carburizing atmosphere.

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