High-temperature components

Abstract

The present invention relates to a high-temperature component (1) made of a refractory metal or a refractory metal alloy, which has a coating (3) for increasing the thermal emissivity. The coating (3) consists essentially of tungsten and rhenium, i.e., consists of at least 55 wt.% of rhenium and at least 10 wt.% of tungsten, and has at least 35 wt.% of the Re3W phase.

Description

Technical Field

[0001] The present invention relates to high-temperature components composed of refractory metals or refractory metal alloys, and a method for producing high-temperature components. Background Art

[0002] In many components used in high-temperature applications, such as heating conductors for physical or chemical vapor deposition, electrodes of gas discharge lamps, or for rotating X-ray anodes, the heat generated in the components is emitted into the environment by thermal radiation. The energy emitted here is proportional to the thermal emissivity of the radiation surface. This value represents the amount of radiation emitted by an object relative to an ideal black body. The higher the thermal emissivity of the surface, the more thermal radiation output the object can emit through this surface.

[0003] This also applies to the absorption of thermal radiation output: Since the emissivity and absorptivity of an object are proportional, an object with a high thermal emissivity also absorbs more radiation output than an object with a lower thermal emissivity.

[0004] For technical surfaces that transfer heat by radiation, the aim is to achieve the highest possible thermal emissivity. Due to the increased radiation capacity, the same radiation output can be emitted at a lower component temperature. To achieve a given radiation output, a component with a high thermal emissivity can thus be operated at a lower operating temperature than the corresponding component with a low thermal emissivity. The lower component temperature generally has a positive impact on the service life and process stability of the component.

[0005] Various methods for achieving a higher thermal emissivity are known in the prior art. One known method aims to increase the microscopic surface area of the emission region.

[0006] An example of this method is WO2014023414(A1), which describes a heating conductor having a porous sintered coating made of tungsten applied by a slurry process. The porous sintered coating made of tungsten can increase the thermal emissivity to approximately 0.34 in the wavelength range of 1700 - 2500 nm; in contrast, the thermal emissivity of a smooth tungsten surface at room temperature is approximately 0.16 in this wavelength range.

[0007] EP1019948(B1) describes an anode of a high-pressure gas discharge lamp provided with a metal coating having a dendritic structure. The needle-like microcrystals of the dendritic structure also increase the surface area of the anode. It should be able to achieve a thermal emissivity of up to 0.8. However, the dendritic structure is very complex and the production cost is expensive.

[0008] A general disadvantage of the above coating methods with lower micron-range structures is that the coating degrades over the service life. Especially at operating temperatures > 1500 °C, due to the sintering process, the surface area steadily decreases, accompanied by a reduction in the thermal emissivity. Structuring the surface on the scale of several hundred μm, for example by laser, to avoid the sintering process is very expensive.

[0009] In addition to the geometric optimization of the surface, suitable coating materials are being further developed to increase the thermal emissivity.

[0010] WO2018204943(A2) describes a high-temperature component composed of refractory metals, having a coating that contains tantalum nitride and / or zirconium nitride and tungsten with a tungsten content of 0 to 98 wt.% (weight percentage). It should be able to achieve a thermal emissivity of up to 0.8.

[0011] DE102009021235(B4) discloses an electrode for a discharge lamp, having a coating in which tungsten particles are embedded in a ceramic matrix layer.

[0012] The common feature of the above two coatings is that they are not suitable for components that withstand high thermal stresses, such as heating conductors, which are used in coating systems, especially metal-organic chemical vapor deposition (MOCVD) systems, and are exposed to temperatures > 2000 °C during operation.

[0013] US2002 / 0079842(A1) describes an electrode for a high-pressure gas discharge lamp, which is coated with rhenium. Compared with tungsten, rhenium has a higher thermal emissivity, but it is very expensive. For cost reasons, tungsten can be added to rhenium. Compared with pure rhenium, the thermal emissivity of the resulting mixture decreases due to the addition of tungsten. Summary of the Invention

[0014] The object of the present invention is to further develop high-temperature components and provide a method for producing high-temperature components. The high-temperature component should be characterized by a high thermal emissivity and be suitable for operating temperatures of about 2000 °C or higher.

[0015] This object is achieved by the high-temperature component protected by this application and the method protected by this application. Preferred embodiments are specifically described in other advantageous improvements or technical solutions.

[0016] Applications related to this application are those with operating temperatures generally of 1000 - 2500 °C or higher. This particularly includes applications in lighting technology (such as electrodes in high-pressure discharge lamps), furnace technology (such as heating conductors, internal furnace fittings, charging devices, crucibles), and medical technology (such as rotating X-ray anodes).

[0017] In the context of this application, components with high operating temperatures are referred to as high-temperature components.

[0018] Refractory metals or refractory metal alloys are generally used in the above-mentioned high-temperature applications. In the context of the present application, refractory metals are understood to be the metals of Group 4 (titanium, zirconium, and hafnium), Group 5 (vanadium, niobium, tantalum), and Group 6 (chromium, molybdenum, tungsten) of the periodic table of elements, as well as rhenium. A refractory metal alloy is an alloy having at least 50 at.% (atomic percentage) of elements. One of the characteristics of these materials is that they have excellent dimensional stability at high operating temperatures. High-temperature components are based on refractory metals or refractory metal alloys. This means that the high-temperature components are substantially composed of refractory metals, i.e., at least 50 at.%, preferably greater than 95 at.%. The high-temperature components are particularly preferably composed entirely of refractory metals or refractory metal alloys and common impurities. Attachment parts can be connected to the actual high-temperature components. The high-temperature components can be, for example, part of a composite component.

[0019] Particularly preferably, the material used for high-temperature components is tungsten or tungsten alloy due to its heat resistance.

[0020] General high-temperature components have a coating for increasing the heat emissivity. In this case, the coating can be applied to the entire component or only to a part thereof.

[0021] According to the present invention, the coating for improving the heat emissivity is substantially composed of tungsten and rhenium, with the proportion of rhenium being at least 55 wt.%, and the proportion of tungsten being at least 10 wt.% (both including the limit values). In other words, the proportion of rhenium is thus between 55 wt.% and 90 wt.%, and the rest is tungsten; the proportion of rhenium is preferably between 60 wt.% and 85 wt.%, and particularly preferably between 65 wt.% and 80 wt.%.

[0022] Here, "substantially" means that the main components are tungsten and rhenium. The coating can contain a small amount of other components and conventional impurities. Oxides, nitrides, or carbides, as well as metals such as molybdenum, iron, copper, tantalum, and niobium, can exist as impurities. However, the proportion of the main components tungsten and rhenium is preferably greater than 95 wt.%, particularly greater than 98 wt.%.

[0023] According to the present invention, tungsten and rhenium in the coating are at least partially in the form of the cubic Re3W phase, i.e., the coating has at least 35 wt.%, particularly at least 40 wt.%, especially preferably at least 50 wt.%, and more preferably at least 70 wt.% of the cubic Re3W phase.

[0024] Re3W is an intermetallic phase with a cubic crystal system, which is equivalent to the cubic Re 0.75 W 0.25 phase except for the lattice constant. In the context of the present invention, the Re3W phase is also understood to be the cubic Re 0.75 W 0.25 phase.

[0025] The different phases in the tungsten-rhenium binary phase diagram can be seen in Figure 1 . The materials of the phases enclosed by parentheses are in the form of solid solutions, while the phases without parentheses are intermetallic phases. In contrast to solid solutions, they exhibit a lattice structure different from that of the constituent metals, and there are mixed bonds between individual metal atoms consisting of metallic bond components and low-level atomic or ionic bond components. The Re3W phase (or Re 0.75 W 0.25 phase) is denoted by χ in the phase diagram.

[0026] Surprisingly, it has been shown that in the presence of a minimal amount of the Re3W phase, the thermal emissivity increases significantly. At room temperature, in the wavelength range of 1700 - 2500 nm, the thermal emissivity of tungsten with a smooth surface is approximately 0.16, and the corresponding thermal emissivity of rhenium is approximately 0.18. Thus, those skilled in the art would expect, see also US200200779842(A1) in this regard, that the thermal emissivity decreases with decreasing rhenium ratio. However, surprisingly, this is not the case. The thermal emissivity in the range of rhenium concentrations from 55 to 90 wt.% of rhenium is significantly greater than the values expected from the linear interpolation between the thermal emissivities of tungsten and rhenium. If there is a very high proportion of the Re3W phase, for certain rhenium concentrations, a thermal emissivity even greater than that of pure rhenium, i.e., a value greater than 0.18, can be achieved. In the applicant's experiments, the maximum value of the thermal emissivity achieved occurred between approximately 70 and 80 wt.% of rhenium.

[0027] For quantitative phase analysis, representative samples are taken from the coating, ground into powder, and the obtained powder is analyzed by X-ray diffraction (XRD).

[0028] The applicant suspects that the specific characteristics regarding the thermal emissivity are due to the phonon spectrum of the intermetallic Re3W phase.

[0029] Due to the material properties of the coating, the thermal emissivity can be further increased by increasing the microscopic surface area.

[0030] The coating is preferably formed to be porous. Here, porous means that the coating has a considerable pore content, for example, more than 5%. The proportion of pores is understood here as the fractional area of the pores in the total cross-sectional area and is determined based on the representative cross-sectional area of the coating sample. Compared with a pure geometric surface, the pores present in the coating volume increase the surface area of the coating, thereby further increasing the thermal emissivity. A porous coating can be prepared, for example, by powder metallurgy methods.

[0031] Alternatively, compared to a pure geometric surface, the surface of the high-temperature component with the applied coating can already be increased. In this variant, the surface of the high-temperature component below the coating is thus structured and increased. The structuring can be achieved by mechanical, chemical, or heat treatment. In this case, the coating itself does not necessarily have to be porous. The pre-treatment of the surface of the high-temperature component is particularly important for physical vapor deposition (PVD) coating processes.

[0032] According to a preferred embodiment, the coating is formed as a sintered layer. A sintered layer is understood to be a layer obtained by a powder metallurgy coating process. A slurry coating can be taken as an example of a powder metallurgy coating process. After the coating material is actually applied in particulate form, the layered coating is cured by sintering. A sintered layer is generally porous and has a rough surface.

[0033] The coating can also be in the form of a PVD layer. In this case, the coating is formed on the surface of the high-temperature component by an appropriate sputtering target in a physical vapor deposition process. A PVD layer is generally smooth and dense and thus has no pores. To increase the surface area, the surface of the high-temperature component can be structured before coating using mechanical, chemical, or heat treatment.

[0034] PVD and sintered layers can be easily distinguished because the surface finish is very different.

[0035] By production, the thickness of the sintered layer is preferably from 2 μm to 300 μm, more preferably from 3 μm to 100 μm, and particularly preferably from 5 μm to 50 μm.

[0036] In the case of a PVD layer, the thickness can also be significantly smaller. The typical thickness of a PVD layer is between 10 nm and 4 μm.

[0037] The thickness of the coating is not decisive for its function.

[0038] The coating is preferably formed on the outer side of the high-temperature component. This means that the coating forms the outermost layer on the surface of the high-temperature component. In one use of the high-temperature component, this layer is intended to participate in heat transfer by radiation.

[0039] There can be additional layers below it.

[0040] According to a preferred exemplary embodiment, the high-temperature component is formed as a heating conductor. In the present application, a heating conductor refers to a metal resistance heater used in a heat treatment device. The heating conductor can be formed from a metal plate, rod, stranded wire, bundled wire, or wire mesh. In the case of a planar heating conductor, i.e., the basic form of the heating conductor originates from a metal plate, it may be necessary to provide the coating only on the side of the heating conductor facing the inside of the furnace during the operation of the heating conductor.

[0041] When used for heating a conductor, the effect of the coating is that, due to the improved thermal radiation, a specific heating output can be generated at a lower temperature. The lower operating temperature of the heating conductor is advantageous in terms of service life, since it can, for example, reduce the creep of the material.

[0042] Advantageous coatings are of particular interest for heating conductors used in coating systems, in particular MOCVD systems. Due to the operating temperature being higher than 2000 °C, there is a risk of evaporation of the heating conductor material and a risk of contamination during the coating process. According to the prior art, these heating conductors are made of tungsten or rhenium, with the heating conductors subjected to the highest thermal stress being made of rhenium. Both materials have a low vapor pressure at high temperatures, but different thermo-mechanical properties. Thus, in certain applications, the more expensive rhenium is more preferred than the cheaper tungsten. The coating according to the invention can increase the thermal emissivity of a tungsten heating conductor to such an extent and, accordingly, can reduce the surface temperature to such an extent that its field of application can be significantly expanded. According to the invention, a tungsten heating conductor coated with rhenium and tungsten is a very economically attractive alternative to a heating conductor made entirely of rhenium and accordingly expensive. Due to the relatively high thermal emissivity, for a given heating output, it can operate at a relatively low temperature.

[0043] Similarly, it can be conceived that in certain applications, the present invention allows the use of a heating conductor based on molybdenum and coated with rhenium and tungsten according to the present invention to replace the tungsten heating conductor.

[0044] Of course, especially for applications with particularly high temperatures, a rhenium heating conductor with a coating comprising a Re3W phase can also be provided, thereby increasing the thermal emissivity.

[0045] The heating conductor is discussed here, and the suggestions regarding the substrate also apply to other high-temperature components.

[0046] According to another exemplary embodiment, the high-temperature component is designed as an electrode of a high-pressure discharge lamp, in particular as the anode of a high-pressure discharge lamp. Due to the coating of the electrode (in particular the anode) according to the invention, it can emit more heat during operation, which results in a reduction of the component temperature and has a favorable effect on the service life.

[0047] According to another exemplary embodiment, the high-temperature component is formed as a crucible. For example, in the production of single-crystal sapphire, refractory metal crucibles are used to melt alumina. For this purpose, the crucible is placed in a high-temperature furnace and heated by the radiant heat of a heating conductor. The heat transfer mainly takes place through the side surface of the crucible, which absorbs the radiant heat and transfers it to the product to be melted. The coating according to the invention transfers a larger part of the heat released by the heating conductor into the crucible.

[0048] The thermal emissivity of the coating is preferably greater than 0.6, measured in the wavelength range between room temperature and 1700 - 2500 nm, as explained in more detail below. ε

[0049] The present invention also relates to a method for producing a high-temperature component. According to the present invention, the method for producing a high-temperature component comprises the following steps:

[0050] - providing a body of the high-temperature component, and then

[0051] i)

[0052] - optionally increasing the surface area of the body of the high-temperature component,

[0053] - coating the body with tungsten and rhenium by physical vapor deposition using a target material comprising tungsten and rhenium, the target material comprising at least 35 wt.% of the Re3W phase,

[0054] or

[0055] ii)

[0056] - optionally increasing the surface area of the body of the high-temperature component,

[0057] - coating the body with tungsten and rhenium by physical vapor deposition using a target material comprising tungsten and rhenium,

[0058] - heat-treating the coated body in an inert or reducing environment or in a high vacuum at a heat-treatment temperature of at least 500 °C, preferably at least 1000 °C, more preferably above 1800 °C to form the Re3W phase,

[0059] - optionally slowly cooling the coated and heat-treated body from the heat-treatment temperature to 800 °C,

[0060] - rapidly cooling the body to room temperature at a cooling rate greater than 20 K / min to stabilize the Re3W phase,

[0061] or

[0062] iii)

[0063] - coating the body by powder metallurgy with a powder mixture containing rhenium and tungsten in a molar ratio of 25 at.% tungsten to 75 at.% rhenium,

[0064] - heat-treating the coated body in an inert or reducing environment or in a high vacuum at a heat-treatment temperature of at least 500 °C, preferably at least 1000 °C, more preferably above 1800 °C to form the Re3W phase,

[0065] - optionally slowly cooling the coated and heat-treated body from the heat-treatment temperature to 800 °C,

[0066] - The main body is rapidly cooled to room temperature at a cooling rate greater than 20 K / min to stabilize the Re3W phase.

[0067] The main body should be understood to refer to a high-temperature component or a semi-finished product from which a component is manufactured before coating.

[0068] Therefore, three different method variants are proposed. Method variant i) and method variant ii) are based on the PVD process, and method variant iii) is based on the powder metallurgy process.

[0069] According to method variant i) and method variant ii), first, it is preferred to pre-treat the surface of the main body of the high-temperature component so that the surface area is increased compared to the geometric surface. This "roughening" can be achieved by removing material from the surface, for example, by mechanically treating (such as sandblasting), chemically treating (such as etching or pickling), or heat-treating (such as laser structuring) to structure the surface.

[0070] Alternatively or additionally, the surface area can also be increased by a slurry coating. In the case of the slurry method, the powdered components are suspended in a liquid. The component (here the main body of the high-temperature component) can be coated with the obtained suspension by means such as dipping, spraying, or brushing. The suspension usually also contains a binder. After drying, the coating is usually sintered. The coating formed in this way is usually porous and rough. It forms a favorable basis for the subsequent coating. The slurry coating can be based on tungsten powder, for example.

[0071] Subsequently, according to method variant i), tungsten and rhenium are applied to the main body by physical vapor deposition, optionally with an increased surface. A target material containing tungsten and rhenium and having an appropriate composition can be used as the source, where the required Re3W phase is already present in the target material in sufficient quantity. The preferred rhenium content in the layer can be set by appropriately selecting the target composition. In addition to the target material in which there is already a sufficient amount of the Re3W phase and whose composition corresponds to the desired tungsten-rhenium content in the layer to be deposited, alternatively, two or more target materials can also be used, where one target material consists mainly or only of the Re3W phase, and one or more additional target materials composed of tungsten and / or rhenium with appropriate tungsten-rhenium compositions are provided to adjust the desired tungsten-rhenium concentration.

[0072] Preferably, the target material has at least 35 wt.% of the cubic Re3W phase. More preferably, the proportion of the Re3W phase is at least 40 wt.%, particularly preferably at least 50 wt.%, and especially preferably at least 70 wt.%.

[0073] Therefore, the PVD coating has a rhenium content between 55 wt.% and 90 wt.%, the rest being tungsten, where the proportion of the Re3W phase is at least 35 wt.%.

[0074] If component deformation with narrow component tolerances is to be avoided, method variant i) (PVD coating with Re3W phase) may be advantageous. The PVD coating is carried out in particular at relatively low temperatures and does not require any heat treatment of the coating.

[0075] Method variant ii) is also a PVD coating process, which differs from variant i) in that the Re3W phase does not necessarily have to be present in the target material, but the Re3W phase is subsequently formed in the sputtered layer only by heat treatment. For this purpose, the body coated by physical vapor deposition process is annealed at a heat treatment temperature in the phase field of the Re3W phase. The heat treatment temperature is at least 500 °C, technically preferably at least 1000 °C, more preferably above 1800 °C. The duration of the heat treatment depends on the level of the heat treatment temperature.

[0076] At temperatures below 1000 °C, due to insufficient kinetics, the formation of the required phase is very slow, which makes such heat treatment temperatures not satisfactory from a technical point of view.

[0077] Good results can be obtained at a heat treatment temperature of about 1800 °C and a holding time of 20 hours. Those skilled in the art can obtain other suitable combinations of heat treatment temperature and holding time through experiments.

[0078] The regulation when selecting heat treatment parameters is that the Re3W phase content in the coating reaches at least 35 wt.% through heat treatment.

[0079] In particular, the heat treatment should make the Re3W phase content in the coating reach at least 40 wt.%, particularly preferably at least 50 wt.%, and especially preferably at least 70 wt.%.

[0080] An inert environment is provided by an inert gas such as nitrogen or argon at a pressure of about 1 bar, and a reducing environment is provided by, for example, hydrogen. High vacuum means a vacuum with a pressure of 10 -3 to 10 -8 mbar. The heat treatment converts at least part of the tungsten and rhenium in the sputtered layer into the intermetallic Re3W phase.

[0081] After heat treatment, the coated body is preferably slowly cooled from the heat treatment temperature to about 800 °C and quickly cooled from about 800 °C to room temperature. In order to protect the heat treatment system used, it is technically advantageous to slowly cool to a temperature below the heat treatment temperature but still in the phase field of the Re3W phase.

[0082] However, it is also equally possible to directly cool quickly from the heat treatment temperature to room temperature.

[0083] Quick cooling kinetically stabilizes the Re3W phase, which is metastable at room temperature.

[0084] In the present application, slow cooling means cooling over a time scale of several hours, corresponding to a cooling rate between 1 K / min and 10 K / min, typically less than 10 K / min.

[0085] In the present application, rapid cooling means quenching at a cooling rate typically between 20 and 150 K / min, preferably greater than 25 K / min, more preferably greater than 50 K / min, and particularly preferably greater than 100 K / min.

[0086] Re3W is a metastable phase below about 500 °C and is kinetically stabilized by rapid cooling. Method variant ii) has the advantage over method variant i) that it does not require a target material containing Re3W (although of course a target material already containing Re3W can be used). The disadvantage is that an additional heat treatment step is required at a relatively high temperature.

[0087] According to method variant iii), the body is first coated with a powder mixture containing rhenium and tungsten (molar ratio of tungsten to rhenium approximately 1:3) using a powder metallurgy process, and then, similar to method variant ii), heat treatment is carried out (i.e., annealing to form the Re3W phase and rapid cooling to stabilize the Re3W phase).

[0088] The rhenium- or tungsten-containing here means that the powder contains rhenium or tungsten in metallic form. In addition to these two metals, the powder mixture can also include other components, such as a binder. The powder metallurgy process can in particular be a slurry process. The heat treatment cures the powder metallurgy applied layer, and due to the relatively long process duration of the sintering process, about 20 hours (a suitable holding time as an example for a heat treatment temperature of about 1800 °C), part of the tungsten-rhenium particles (the process duration usually used with these metals is 3 to 10 hours) will be transformed into the intermetallic Re3W phase in solid solution form. After heat treatment, the coated body is optionally slowly cooled to 800 °C and then rapidly cooled to room temperature. The parameters of the heat treatment and cooling correspond to those in method variant ii). The rapid cooling kinetically stabilizes the Re3W phase, which is metastable at room temperature.

[0089] Compared with the traditional PVD process, this method variant (powder metallurgy coating) has an advantage in terms of cost. In addition, the layer thickness of the Re3W phase is generally higher, which has a positive impact on the long-term stability of the coating. Description of the Drawings

[0090] The present invention will be explained in more detail below with reference to the following production examples and the drawings.

[0091] Figure 1 : Phase diagram of the binary tungsten-rhenium system,

[0092] Figures 2a - 2d: Scanning electron micrograph (fracture surface) of the cross-section of the coated surface according to the present invention ( Figure 2a and Figure 2c ) and top view ( Figure 2b and Figure 2d ),

[0093] Figure 3 : Values of the thermal emissivity ε of various coatings,

[0094] Figure 4a 、 Figure 4b : X-ray diffraction patterns (XRD) of the layers produced according to the present invention and conventionally produced layers,

[0095] Figure 5 : Schematic diagram of a high-pressure discharge lamp as an exemplary embodiment of a high-temperature component,

[0096] Figure 6 : Heating conductor as an exemplary embodiment of a high-temperature component,

[0097] Figure 7 : Crucible as an exemplary embodiment of a high-temperature component. Detailed Description of the Invention

[0098] Production Example 1:

[0099] To produce a high-temperature component according to Production Example 1, a body made of tungsten is coated with a slurry of different powder mixtures. For this purpose, tungsten powder and / or rhenium powder are first weighed and added to a binder of an ethanol solution of 2 wt.% ethyl cellulose so that the total solid content is 50%. Stirring is carried out for 15 minutes at 1500 rpm using a Netzsch Multimaster device.

[0100] Samples are prepared for the following layer compositions:

[0101] 100 wt.% tungsten

[0102] 10 wt.% rhenium, the remainder being tungsten

[0103] 20 wt.% rhenium, the remainder being tungsten

[0104] 30 wt.% rhenium, the remainder being tungsten

[0105] 40 wt.% rhenium, the remainder being tungsten

[0106] 50 wt.% rhenium, the remainder being tungsten

[0107] 60 wt.% rhenium, the remainder being tungsten

[0108] 70 wt.% rhenium, the remainder being tungsten

[0109] 80 wt.% rhenium, the remainder being tungsten

[0110] 90 wt.% rhenium, the balance being tungsten

[0111] 100 wt.% rhenium.

[0112] The weight percentages given here refer to the weights of the solid components rhenium and tungsten and also correspond to the weight percentages in the layer, since the organic components volatilize during the heat treatment.

[0113] Then spraying was carried out manually on the tungsten sheet at a distance of approximately 20 cm to obtain a target layer mass of 15 mg / cm 2 The drying was carried out in ambient air.

[0114] Then the dried layer was heat treated (annealed). The organic components (such as binders) volatilized due to the heat treatment, and the layer was consolidated. In an argon (Ar) atmosphere, the heat treatment was carried out at 1800 °C for 20 hours each. After the heat treatment, the coated body was gradually cooled down to 800 °C in 10 hours (corresponding to an average cooling rate of 1.67 K / min) and quenched from approximately 800 °C to room temperature in 20 minutes (corresponding to a cooling rate of approximately 40 K / min).

[0115] For comparison, additional samples of 80 wt.% rhenium, the balance being tungsten, were produced in a similar manner to the aforementioned production method, except that they were heat treated in an argon atmosphere at 1600 °C for 6 hours instead of 20 hours.

[0116] At room temperature and in the wavelength range of 1700 - 2500 nm, the thermal emissivity of the layer was measured using a Solar 410 Reflectometer from Surface Optics Corporation, since this infrared wavelength range is particularly relevant for evaluating the thermal radiation of the body.

[0117] In the following table, the measurement results were also compared with the known values of the thermal emissivity of coatings known in the prior art (such as tantalum nitride coatings according to WO2018204943 (A2)).

[0118] Table 1 summarizes the selection of results, Figure 1 and the graph gives a more detailed representation, where the thermal emissivity ε is shown as a function of the rhenium content.

[0119]

[0120]

[0121] Table 1: Comparison of the thermal emissivity of different coatings

[0122] Sample 1, a porous tungsten coating obtained using 100% tungsten paste, has a thermal emissivity of 0.34. Sample 2, a porous rhenium coating obtained using 100% rhenium paste, has a thermal emissivity of 0.36. Sample 3 is a tantalum nitride coating produced according to the details provided by the applicant in WO2018204943. This has a relatively high thermal emissivity of 0.89, but can only be used at temperatures up to 1500 °C. Sample 4 has a coating composed of 80% rhenium and 20% tungsten, which was prepared as described above and was heat-treated at 1600 °C for 6 hours for comparison purposes. As explained in more detail below, this sample mainly has a tungsten / rhenium solid solution, with only a very small amount of the Re3W phase. Its thermal emissivity is 0.35. Sample 5 is a coating of 80% rhenium and 20% tungsten prepared according to the foregoing description (heat-treated at 1800 °C for 20 hours). The proportion of the Re3W phase is approximately 90 wt.%. Its measured thermal emissivity is 0.66.

[0123] Figures 2a to 2d A scanning electron micrograph of Sample 5 is shown. Figure 2a and 2b is an image magnified 1000 times, Figure 2c and Figure 2d is an image magnified 3000 times. Figure 2a and Figure 2c show a fracture surface perpendicular to the surface of the sample, Figure 2b and Figure 2d is a top view of the surface, i.e., the viewing direction is perpendicular to the coating surface. In the fracture surface, the substrate 2 made of tungsten sheet material can be seen in the lower part of the figure. Above it, the porous coating 3 can be seen. The pores increase the microscopic surface area and contribute to a further increase in the thermal emissivity.

[0124] Figure 3 The measured thermal emissivity ε of the series of tests with different rhenium contents mentioned at the beginning is shown graphically. The abscissa is the rhenium content, and the ordinate is the measured thermal emissivity ε. The points in the figure represent the respective measured values. The dashed line ε th (the theoretical value of ε) marks the thermal emissivity values expected when linearly interpolating the thermal emissivity from 100 wt.% tungsten to 100 wt.% rhenium. It can be seen that especially in the range of 50 wt.% to 90 wt.% rhenium, the measured values of the thermal emissivity surprisingly do not extend along this line ε th but are above this line, sometimes significantly higher than this line. The maximum value of the thermal emissivity occurs between 70 wt.% and 80 wt.% rhenium. The applicant's measurements indicate that the favorable increase in the value of the thermal emissivity may be due to the presence of the Re3W phase.

[0125] Table 2 illustrates this. Table 2 shows the detailed quantitative phase analysis results of samples with rhenium contents of 70 wt.% (Sample I) and 80 wt.% (Sample II). To quantitatively determine the phases, a portion of the coating of the corresponding sample was scraped off, ground into powder and subjected to XRD analysis. For comparison, the measured values of samples produced in a conventional manner (i.e., heat treatment duration of 6 hours) (Sample Ia and Sample IIa) are also given.

[0126]

[0127] Table 2: Phase analysis

[0128] (W) and (Re) are both solid solution phases ((W) is a tungsten crystal in which rhenium is dissolved, and similarly, (Re) is a rhenium crystal in which tungsten is dissolved). W 0.5 Re 0.5 is an intermetallic phase, which is also shown as the σ phase in the phase diagram. The amounts in each phase are in wt.%.

[0129] The measurement results show that in the samples with the coatings according to the invention (which have been heat treated for a significantly longer time), the proportion of Re3W is significantly higher than that in the samples produced by the heat treatment time of the powder metallurgy process commonly used for tungsten and rhenium. For both Sample I (70 wt.% rhenium) and Sample II (80 wt.% rhenium), the proportion of Re3W is about 90 wt.%, while the proportion of Re3W in the corresponding conventionally prepared samples is 21.8 wt.% (Sample Ia) or 27.8 wt.% (Sample IIa). The high proportion of Re3W is also associated with a significantly higher thermal emissivity coefficient.

[0130] Figure 4a and Figure 4b shows the X-ray diffraction patterns (XRD) of Sample II ( Figure 4a ) and Sample IIa ( Figure 4b ). In the diffraction pattern, the intensity values are given as a function of the deflection angle 2θ (range from 30 to 65 2θ), and the measured reflections (peaks) are assigned to the existing phases. In Sample II with the coating according to the invention, the proportion of Re3W is dominant.

[0131] Table 3 shows the temperature resistance of the samples according to the invention. The measured values of the thermal emissivity are shown, which is a function of temperature for the samples subjected to the thermal stress test. The samples were annealed at this temperature for one hour.

[0132] Thermal stress test (temperature) Thermal emissivity ε None 0.66 2000 0.65 2200 0.58

[0133] Table 3: Temperature resistance

[0134] A series of tests showed that the thermal emissivity at T = 2000 °C did not decrease significantly (from 0.66 for the sample before the thermal stress test to 0.65), and the coating did not degrade. At T = 2200 °C, a decrease in thermal emissivity of approximately 12% was observed. The material can withstand high thermal stress, but the porous layer started to sinter somewhat. Nevertheless, even at such high temperatures, a relatively high thermal emissivity can be maintained. Thus, the coating according to the present invention can withstand temperatures of 2000 °C and above, and can therefore be used for heating wires in MOCVD systems.

[0135] Production Example 2:

[0136] Another variant for producing the coating is based on physical vapor deposition. In this production example, a tungsten sheet is first coated with a conventional 100% tungsten paste layer. This helps to increase the surface area. A Re3W layer approximately 4 μm thick is sputtered onto this layer using a target material containing approximately 98% Re3W phase. The resulting layer has approximately 75 wt.% rhenium. Since part of the radiative exchange also occurs through the parts of the porous tungsten structure not covered by the PVD coating, the measured value of the thermal emissivity does not fully reach the value in Production Example I.

[0137] Reference Figures 5 to 7 , and application examples of high-temperature components are explained below.

[0138] Figure 5 A high-pressure discharge lamp 6 is schematically shown. During operation, a discharge arc is formed between the electrodes (cathode 5 and anode 4). In the present exemplary embodiment, the anode 4 is a high-temperature component 1 and is provided with a coating 3 according to the present invention. The coating 3 allows the anode 4 to emit a higher thermal radiation output, which reduces its temperature and increases its service life. Similarly, the cathode 5 or both the anode 4 and the cathode 5 can be provided with the coating 3. Obviously, the coating 3 for increasing the thermal emissivity according to the present invention can also be used for other types of lamps.

[0139] Figure 6 A refractory metal heating conductor 7 is shown in an exemplary arrangement as the bottom heater of a high-temperature furnace. The heating conductor 7 is heated by passing an electric current directly through it and heats the interior of the high-temperature furnace by releasing radiant heat.

[0140] In the present exemplary embodiment, the heating conductor 7 forms a high-temperature component 1 and is provided with a coating 3 according to the present invention for increasing the thermal emissivity. When used on the heating conductor 7, the coating 3 allows the heating conductor 7 to produce a given heating output at a lower temperature. This reduces the creep of the heating conductor 7 and extends its service life.

[0141] Figure 7Schematically shows a refractory metal crucible 8. For example, in the production of single crystal sapphire, a refractory metal crucible is used to melt alumina. For this purpose, the crucible is placed in a high-temperature furnace and heated by the radiant heat of a heating conductor. Heat transfer mainly takes place through the side surface of the crucible, which absorbs the radiant heat and transfers it to the product to be melted. In the present exemplary embodiment, the crucible 8 forms a high-temperature component 1 and is provided with a coating 3 according to the invention for increasing the heat emissivity. When used on the crucible 8, the effect of the coating 3 is that a greater proportion of the heat released by the heating conductor is transferred into the crucible 8. The crucible 8 thus reacts more quickly to the heat input from the heating conductor.

[0142] The use of the coating 3 is by no means limited to the example shown here. The coating 3 is generally advantageous for high-temperature components where heat transfer takes place by radiation.

[0143] Reference numerals

[0144] 1 High-temperature component

[0145] 2 Body of the high-temperature component

[0146] 3 Coating for increasing heat emissivity

[0147] 4 Anode

[0148] 5 Cathode

[0149] 6 High-pressure discharge lamp

[0150] 7 Heating conductor

[0151] 8 Crucible

Claims

1. A high-temperature component (1) based on a refractory metal or refractory metal alloy, having a coating (3) for increasing the thermal emissivity, wherein the coating (3) consists essentially of tungsten and rhenium, i.e., consists of at least 55 wt.% of rhenium and at least 10 wt.% of tungsten, and the coating (3) has at least 50 wt.% of the Re3W phase, and the coating (3) has a thermal emissivity ε greater than 0.6 in the wavelength range between 1700 - 2500 nm.

2. The high-temperature component (1) according to claim 1, wherein, The coating (3) is porous.

3. The high-temperature component (1) according to claim 1 or 2, wherein, The surface of the high-temperature component under the coating is structured.

4. The high-temperature component (1) according to claim 1 or 2, wherein, The coating (3) is formed as a sintered layer.

5. The high-temperature component (1) according to claim 1 or 2, wherein, The coating (3) is formed as a PVD layer.

6. The high-temperature component (1) according to claim 1 or 2, wherein, The coating (3) is formed on the top surface of the high-temperature component (1).

7. The high-temperature component (1) according to claim 1 or 2, wherein, The high-temperature component (1) is formed as an electrode (4, 5) of a high-pressure discharge lamp (6).

8. The high-temperature component (1) according to claim 1 or 2, wherein The high-temperature component (1) is formed as a heating conductor (7).

9. The high-temperature component (1) according to claim 1 or 2, wherein, The high-temperature component (1) is formed as a crucible (8).

10. A method for producing a high-temperature component (1) having a coating (3) for increasing the thermal emissivity, comprising the following steps: - Providing a body (2) of the high-temperature component (1), i) - Coating the body (2) with tungsten and rhenium by physical vapor deposition using a target material containing tungsten and rhenium, - Heat-treating the coated body (2) in an inert or reducing environment or high vacuum at a heat-treatment temperature of at least 500 °C to form the Re3W phase, - Cooling the body (2) to room temperature at a cooling rate greater than 20 K / min to stabilize the Re3W phase, wherein the coating (3) contains at least 50 wt.% of the Re3W phase, Or ii) - Coating the body (2) by a powder metallurgy process with a powder mixture containing rhenium and tungsten, the powder mixture having a molar ratio of 25 at.% of tungsten to 75 at.% of rhenium, - Heat-treating the coated body (2) in an inert or reducing environment or high vacuum at a heat-treatment temperature of at least 500 °C to form the Re3W phase, - Cooling the body (2) to room temperature at a cooling rate greater than 20 K / min to stabilize the Re3W phase, wherein the coating (3) contains at least 50 wt.% of the Re3W phase.

11. The method according to claim 10, wherein in sub-step i), the surface area of the body (2) is increased before coating the body (2).

12. The method according to claim 11, wherein in sub-step i), the surface area of the body (2) of the high-temperature component (1) is increased by a slurry coating of the body (2).

13. The method according to claim 11, wherein in sub-step i), the surface area of the body (2) of the high-temperature component (1) is increased by mechanical, chemical or thermal structuring of the body (2).

14. The method according to claim 10, wherein in sub-step ii), the body (2) is coated by a slurry process.

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